Processes for the preparation of isoprenol and downstream products thereof

Abstract

A process for the preparation of isoprenol comprises the steps of a-i) preparing isobutylene by oxidizing isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol. The process allows for providing isobutylene for the preparation of isoprenol and its downstream products, including prenol, prenal, isoprenal and / or 3,7-dimethyl-octa-2,6-dienal (citral), from renewable sources.

Description

[0001] Processes for the Preparation of Isoprenol and Downstream Products Thereof

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a process for the preparation of isoprenol, a process for the preparation of prenol, a process of the preparation of prenal and / or isoprenol, and a process for the preparation of 3,7-dimethyl-octa-2,6-dienal (citral).

[0004] BACKGROUND OF THE INVENTION

[0005] Isoprenol is an important intermediate for the synthesis of scents, vitamins and carotenoids, such as 3,7-dimethyl-octa-2,6-dienal (citral).

[0006] WO 2008 / 037693 discloses a method for producing citral. Said method involves the following steps: a) 3-methyl-3-butene-1-ol (isoprenol) is produced from isobutylene and formaldehyde; b) 3-methyl-2-butenal (prenal) and 3-methyl-3-butenal (isoprenal) are produced from 3-methyl-3- butene-1-ol (isoprenol) by oxidative dehydrogenation by means of an oxygen-containing gas on a silver support catalyst; c) additional 3-methyl-2-butenal (prenal) is produced from a mixture containing 3-methyl-3-butenal (isoprenal) by isomerization; d) 3-methyl-2-butene-1-ol (prenol) is produced from 3-methyl-3-butene-1-ol (isoprenol) by isomerization; e) the unsaturated acetal 3-methyl-2-butenal-diprenylacetal is produced from 3-methyl-2-butene-1-ol (prenol) and 3-methyl-2-butenal (prenal) using an acidic catalyst; and f) 3,7-dimethyl-octa-2,6-diene-al (citral) is obtained from 3-methyl-2-butenal-diprenylacetal by cleavage and subsequently rearranging.

[0007] The preparation of isoprenol from formaldehyde and isobutylene has been widely described in the literature. Isobutylene in turn has been typically produced from non-renewable sources, e.g., via dimerization of ethylene derived from catalytic or steam cracking of fossil feedstocks.

[0008] Light olefins such as isobutylene are important building blocks of modern chemical industries. The search for alternative materials for the production of light olefins has led to the use of renewably-sourced oxygenates such as alcohols, e.g., alcohols from biomass.

[0009] Biomass is considered as a CO2 neutral energy carrier, and is one of the most abundant and renewable of natural resources. In recent years, both as a result of market conditions as well as in response to a variety of governmental initiatives and mandates, biomass transformation to produce biofuel is has attracted significant effort and investment.

[0010] Fusel oils are formed as a by-product of alcoholic fermentation and consist of a mixture of several alcohols comprised mainly of amyl alcohols along with lesser amounts of propanol, n-butanol, and iso-butanol depending upon the purification process employed. Fusel oils are produced by yeast in anaerobiosis from nitrogenous materials.

[0011] Alcohol to olefin conversion processes such as alcohol dehydration are well known. However, in particular with branched alcohols, these conversions often proceed with less than optimum selectivity to the desired olefin. US 2021 / 0040012 A1 describes a process for the preparation of olefin by alcohol dehydration, such as isoamyl alcohol comprised in fusel oil. The dehydration of isoamyl alcohol yields a mixture of C5 olefins, with the examples specifically indicating 2-methylbut-2-ene and trans-2-pentene as major components.

[0012] In the case of butylene from renewable sources, the butylene produced must meet critical purity specifications for downstream applications. This requirement is not easily achieved when the starting materials comprise a mixture of compounds and / or the steps involved in the conversion proceed with less than ideal selectivity.

[0013] It is an object of the present invention to provide isobutylene for the preparation of isoprenol and its downstream products from renewable sources. In particular, it is desirable that the steps involved in the provision of isobutylene proceed via easy-to-separate intermediates and / or with high selectivity towards the desired compounds.

[0014] In a first aspect, the invention provides a process for the preparation of isoprenol, comprising the steps of: a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol.

[0015] In a second aspect, the invention provides a process for the preparation of prenol, comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

[0016] In a third aspect, the invention provides a process for the preparation of prenal and / or isoprenal, comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; b) optionally, isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0017] In a fourth aspect, the invention provides a process for the preparation of 3, 7-d i methyl -octa-2, 6-d I enal (citral) comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene. SUMMARY OF THE INVENTION

[0018] First Aspect - Preparation of Isoprenol

[0019] Preparation of Isobutylene

[0020] In the first aspect of the invention, isobutylene is prepared in step a-i) by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene.

[0021] This includes an embodiment wherein step a-i) comprises feeding an alcohol feedstock comprising isoamyl alcohol at a concentration of 60 to 99 wt.-% into a reaction vessel and oxidizing the alcohol feedstock to an oxidation product comprising isovaleric acid. The alcohol feedstock may be fusel oil.

[0022] In one embodiment, the renewably-sourced isoamyl alcohol of step a-i) is obtained from fusel oil. The term "fusel oil” as used herein refers to products that are formed as a by-product of alcoholic fermentation. Fusel oil is well known in the art and typically comprises a mixture of light alcohols, fatty esters, terpenes and furfural. The alcohols comprised in fusel oil are mainly propanol, butanol, amyl alcohol, isoamyl alcohols and hexanol and optionally heavier linear alcohols such as Cz or Cs alcohols.

[0023] Fusel oils, occasionally referred to as "amyl oils” or “fusels”, have compositions which vary depending on their origin (potato, beet, wheat, barley, etc. musts).

[0024] Fusel oils form colorless or yellowish liquids, which have a characteristic odor. They have a density of about 0.83 g / mL. Their boiling point is far from constant, since they are complex mixtures of substances with a very variable boiling point. Boiling commences at about 80° C and rises to 130 to 134°C. Fusel oils insoluble in water and are usually washed with water and separated out by settling of the phases in order to reduce the amount of ethanol they contain by about 4% to 5%. It should be noted that fusel alcohols are natural alcohols directly produced via biotechnology in distilleries, without any intermediate chemical step.

[0025] Fusel oil may be obtained by several processes well known from the skilled person, e.g. by direct removal in the distillation column and cooling. The removed fraction can be purified e.g. by extraction and decantation. A liquid / liquid extraction by addition of water followed by a decantation leads to the formation of two phases. The upper phase comprises mainly amyl and butyl alcohols, slightly soluble in water. The various fractions of fusel oil may also be separated by using adsorbents, which are regenerated thereafter. Among the tested adsorbents, granulated vegetal activated charcoal is preferred since it is able to adsorb eight times its weight of fusel oil.

[0026] In an embodiment, the fusel oil contains a mixture of linear or branched C5 alcohols, C4 alcohols or C3 alcohols.

[0027] In a preferred embodiment, C5 branched alcohol present in the initial composition is a mixture of isoamyl alcohol and amyl alcohol, i.e. 3-methylbutan-1 -ol (isoamyl alcohol) and 2-methylbutan-1 -ol (amyl alcohol). In a preferred embodiment, the initial composition comprises at least 30 wt.-%, preferably at least 40 wt.-%, more preferably at least 50 wt.-%, more preferably at least 60 wt. %, even more preferably at least 70 wt.-% C5 branched alcohols, based on the total weight of the composition.

[0028] Here and throughout the specification, the terms “wt%”, “wt.-%”, “wt.%”, "weight percent” and "% by weight” are used synonymously.

[0029] C4 alcohols may also be present in the initial composition, for example, butan-1-ol and 2-methylpropan-1- ol. The initial composition may comprise one of these C4 alcohols or both.

[0030] C3 alcohols may also be present in the initial composition, for example, n-propanol. The initial composition may comprise 0.01 to 20 wt.-% of C3 alcohol.

[0031] Fusel oil may further contain hexanol and optionally heavier linear alcohols such as C7 or Cs alcohols.

[0032] Fusel oil typically comprises 5 to 20% of water, 60 to 95% of alcohols mainly consisting of linear or branched alkanols containing from 2 to 5 carbon atoms, and impurities including furfurals, ethers and / or fatty acids.

[0033] In one embodiment, the composition of fusel oil is as follows: ethanol: 5 to 40%,

[0034] 1 -propanol: 1 to 8%,

[0035] 2-propanol: 0 to 1 %,

[0036] 2-methylpropanol: 5 to 15%,

[0037] 1 -butanol: 0 to 1 %,

[0038] 2-methyl-1 -butanol: 10 to 30%,

[0039] 3-methyl-1 -butanol (isoamyl alcohol): 25 to 70%, the combination of alkanols representing 100%.

[0040] In an embodiment, step a-i) comprises feeding an alcohol feedstock comprising isoamyl alcohol at a concentration of 60 to 99 wt.-% into a reaction vessel, oxidizing the alcohol feedstock to an oxidation product comprising isovaleric acid, isolating isovaleric acid from the oxidation product, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene. In an embodiment, the alcohol feedstock comprises isoamyl alcohol at a concentration of 60 to 99 wt.-%, the remainder comprising at least one Cs-Cs-alcohol other than isoamyl alcohol. In a preferred embodiment, the alcohol feedstock is fusel oil.

[0041] In an embodiment, the isoamyl alcohol is obtained from the fusel oil contains the Cs-Ca primary alcohol has a pMC greater than 90 when measured by a method as described in the ASTM norm D6866 (the current version is D6866-22), which defines the concept of "percent Modern Carbon” or pMC. Preferably the pMC is greater than 91 , preferably greater than 93, preferably greater than 95, preferably greater than 96, preferably greater than 97, more preferably greater than 98, even more preferably greater than 99, more preferably about 100.

[0042] In an embodiment, the verification that a feedstock was derived from renewable raw materials is possible according to ASTM D6866 via14C for example. A feedstock shall be regarded as "derived from renewable raw materials” for the purposes of this invention when the carbon-14 (14C) presence therein corresponds substantially (to within not more than 6%) to the ASTM D6866 content of14C in atmospheric CO2.

[0043] The14C content of a material may be determined by determining the decays of14C in this material by liquid scintillation. Such raw materials shall preferably be regarded as derived from renewable raw materials when they have a14C content displaying a radioactive decay of not less than 1 .5 dpm / gC (decays per minute per gram of carbon), preferably 2 dpm / gC, more preferably 2.5 dpm / gC and yet more preferably 5 dpm / gC.

[0044] "Renewably-based” or "renewable” denote that the carbon content of a biofuel precursor and subsequent products is from a "new carbon” source as measured by ASTM test method D 6866-05, "Determining the Biobased Content of Natural Range Materials Using Radiocarbon and Isotope Ratio Mass Spectrometry Analysis”, incorporated herein by reference in its entirety. This test method measures the14C / 12C isotope ratio in a sample and compares it to the14C / 12C isotope ratio in a standard 100% biobased material to give percent biobased content of the sample. "Biobased materials” are organic materials in which the carbon comes from recently (on a human time scale) fixated CO2 present in the atmosphere using sunlight energy (photosynthesis). On land, this CO2 is captured or fixated by plant life (e.g., agricultural crops or forestry materials). In the oceans, the CO2 is captured or fixated by photosynthesizing bacteria or phytoplankton. For example, a biobased material has a14C / 12C isotope ratio greater than 0. Contrarily, a fossil-based material, has a14C / 12C isotope ratio of about 0. The term "renewable” with regard to compounds such as alcohols or hydrocarbons (linear or cyclic alkanes / olefins / alkynes, aromatic, etc.) refers to compounds prepared from biomass using thermochemical methods (e.g., Fischer-Tropsch catalysts), biocatalysts (e.g., fermentation), or other processes, for example as described herein.

[0045] The term "biofuel precursor” refers to an organic molecule in which all of the carbon contained within the molecule is derived from biomass and is thermochemically or biochemically converted from a feedstock into the precursor. A biofuel precursor may be a biofuel in its own right or may be configured for conversion, either chemically or biochemically, into a biofuel with different properties. Biofuel precursors include, but are not limited to, 1 -propanol, 2-propanol, 1 -butanol, 2-butanol, isobutanol, 1 -pentanol, isopentanol (3-methyl- 1 -butanol), 3-pentanol, 2-methyl-1 -butanol, or neopentanol.

[0046] "Carbon of atmospheric origin” as used herein refers to carbon atoms from carbon dioxide molecules that have recently (e.g., in the last few decades) been free in the earth's atmosphere. Such carbon atoms are identifiable by the ratio of particular radioisotopes as described herein. "Green carbon”, "atmospheric carbon”, "environmentally friendly carbon”, "life-cycle carbon”, "non-fossil fuel based carbon”, "nonpetroleum based carbon”, "carbon of atmospheric origin”, and "biobased carbon” are used synonymously herein.

[0047] A small amount of the carbon atoms of the carbon dioxide in the atmosphere is the radioactive isotope14C. This14C carbon dioxide is created when atmospheric nitrogen is struck by a cosmic ray generated neutron, causing the nitrogen to lose a proton and form carbon of atomic mass 14 (14C), which is then immediately oxidized to carbon dioxide. A small but measurable fraction of atmospheric carbon is present in the faun of 14CO2. Atmospheric carbon dioxide is processed by green plants to make organic molecules during the process known as photosynthesis. Virtually all forms of life on Earth depend on this green plant production of organic molecule to produce the chemical energy that facilitates growth and reproduction. Therefore, the14C that forms in the atmosphere eventually becomes part of all life forms and their biological products, enriching biomass and organisms which feed on biomass with14C. In contrast, carbon from fossil fuels does not have the signature 140:120 ratio of renewable organic molecules derived from atmospheric carbon dioxide. Furthermore, renewable organic molecules that biodegrade to CO2 do not contribute to global warming as there is no net increase of carbon emitted to the atmosphere.

[0048] Assessment of the renewably based carbon content of a material can be performed through standard test methods, e.g. using radiocarbon and isotope ratio mass spectrometry analysis. ASTM International (formally known as the American Society for Testing and Materials) has established a standard method for assessing the biobased content of materials. The ASTM method is designated ASTM-D6866.

[0049] The application of ASTM-D6866 to derive "biobased content” is built on the same concepts as radiocarbon dating, but without use of the age equations. The analysis is performed by deriving a ratio of the amount of radiocarbon (14C) in an unknown sample compared to that of a modern reference standard. This ratio is reported as a percentage with the units "pMC” (percent modern carbon). If the material being analyzed is a mixture of present day radiocarbon and fossil carbon (containing very low levels of radiocarbon), then the pMC value obtained correlates directly to the amount of biomass material present in the sample.

[0050] The isoamyl alcohol used in the present invention preferably has a pMC value of greater than 90, preferably greater than 95, preferably greater than 98, more preferably greater than 99, more preferably about 100, inclusive of all values and subranges there-between.

[0051] Oxidation of Renewably-Sourced Isoamyl Alcohol to Isovaleric Acid

[0052] In step a-i), the renewably-sourced isoamyl alcohol is oxidized to isovaleric acid.

[0053] In an embodiment, the oxidation of the renewably-sourced isoamyl alcohol is carried out using an oxidizing agent selected from oxygen, hydrogen peroxide and nitric acid. Preferably, the oxidizing agent is oxygen or nitric acid. More preferably, the oxidizing agent is nitric acid.

[0054] In an embodiment, the weight ratio / molar ratio of the oxidant to that of the isoamyl alcohol is in the range of 10:1 , preferably 8:1 , more preferably 6:1 , even more preferably 5:1.

[0055] When nitric acid is used as the oxidizing agent, the concentration of nitric acid is typically in the range of 30 to 65 wt.-%, preferably 50 to 65 wt.-%, more preferably 60 to 65 wt.-%.

[0056] When oxygen is used as the oxidizing agent, the oxidation reaction is carried out in the presence of a gas stream containing O2 as the oxidizing agent. Oxygen can be used undiluted or diluted. The oxygen can be diluted with other inert gases like N2, Ar or CO2, e.g., in the form of air. In a preferred embodiment of the invention oxygen is used undiluted.

[0057] In an embodiment, the oxidation reaction is carried out in the presence of an O2 stream which has a flow rate of 1 to 10 L / h, preferably, 2 to 8 L / h, more preferably 3 to 7 L / h, more preferably 4 to 6 L / h.

[0058] In a preferred embodiment, the oxidation reaction is carried out in the presence of an O2 stream which has a flow rate of 5 L / h.

[0059] In an embodiment, the oxidation reaction may be carried out in the presence of a gas stream containing O2 as the oxidizing agent and a heterogeneous catalyst comprising metal catalyst.

[0060] The metal catalyst comprises at least one catalytically active metal. The catalytically active metal may selected from the elements selected from the groups 8, 9, 10 and 11 of the periodic table (according to IUPAC nomenclature). The elements of group 8, 9, 10 and 11 of the periodic table comprise iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, copper, silver and gold.

[0061] In a preferred embodiment, the catalytically active metal is selected from elements from the groups 10 and 11 of the periodic table (according to IUPAC nomenclature).

[0062] In a preferred embodiment, the catalytically active metal is selected from elements selected from the group consisting of platinum, palladium and gold or mixtures thereof.

[0063] In a preferred embodiment of the invention the catalytically active metal is platinum.

[0064] The catalytically active metal can be used in any form, e.g., unsupported or on a support. The catalytically active metal can be used in an unsupported form, for example as a powder, a mesh, a sponge, a foam or a net. In a preferred embodiment, the catalytically active metal is on a support.

[0065] In an embodiment, the metal catalyst is on a support selected from active carbon, silica or alumina. Preferably the support is selected from active carbon.

[0066] The catalyst can optionally comprise one or more so called promoters, which enhance the activity of the catalytically active metal. Examples for such promoters are bismuth (Bi), antimony (Sb), lead (Pb), cadmium (Cd), tin (Sn), tellurium (Te), cerium (Ce), selenium (Se) or thallium (Tl).

[0067] In a preferred embodiment, the catalyst comprises at least one promoter selected from the group consisting of bismuth (Bi), antimony (Sb), lead (Pb), cadmium (Cd), tin (Sn) and tellurium (Te). In a preferred embodiment, the catalyst comprises at least one promoter selected from the group consisting of bismuth (Bi), lead (Pb) and cadmium (Cd).

[0068] The promoters can for example be employed as metals, nitrates, acetates, sulphates, citrates, oxides, hydroxides or chlorides and mixtures thereof. In an embodiment, the metal catalyst is carried on inert support, wherein the support is selected from active carbon, silica or alumina which are optionally doped with bismuth cadmium or lead.

[0069] In case a promotor is employed, suitable molar ratios of the catalytically active metal and the promotor are in the range from 1 : 0.01 to 1 : 10, preferably 1 : 0.5 to 1 : 5, more preferably from 1 : 0.1 to 1 : 3.

[0070] The promoters can for example be present on the support or can be added separately to the process.

[0071] The term "on a support” encompasses that the catalytically active metal and / or promoter can be located on the outer surface of a support and / or on the inner surface of a support. In most cases, the catalytically active metal and / or promoter will be located on the outer surface of a support and on the inner surface of a support.

[0072] In case the catalytically active metal is on a support, the catalyst comprises the catalytically active metal, the support and optionally promoters.

[0073] In an embodiment, the oxidation is carried out at a temperature in the range of 70 to 100 °C, preferably in the range of 80 to 100 °C, preferably in the range of 80 to 95 °C, preferably in the range of 80 to 90 °C, preferably in the range of 80 to 85 °C, more preferably the temperature is about 80 °C.

[0074] In an embodiment, the oxidation of the alcohol is performed in the presence of a gas stream of O2, wherein the reaction is carried out for a time period of 120 min to 24 h, 120 min to 12 h, more preferably 120 min to 8 h, even more preferably 120 to 240 min.

[0075] When the oxidation of the renewably-sourced isoamyl alcohol is performed in the presence of nitric acid, the reaction is preferably carried out for a time period in the range of 60 to 240 min, preferably 60 to 180 min, more preferably 60 to 100 min.

[0076] In an embodiment, the oxidation is carried out at a temperature of less than 40 °C, preferably the temperature is less than 35 °C, more preferably the temperature is less than 30 °C.

[0077] In an embodiment, the oxidation is carried out at 30 to 40 °C, such as 35 °C when fusel oil is used as a source of the renewably-sourced isoamyl alcohol.

[0078] In an embodiment, the oxidation is carried out at -5 to 5 °C, such as 0°C, when pure renewably-sourced isoamyl alcohol is used as a reactant.

[0079] In an embodiment, the oxidation is carried out in the presence of a solvent.

[0080] In an embodiment, the solvent is selected from aliphatic hydrocarbons, such as hexane, heptane, octane, nonane, decane and also petroleum ether, or halogenated hydrocarbons such as bromopropane, methylene chloride or dichloromethane, chloroform, tetrachloroethylene, aromatic hydrocarbons, such as benzene, toluene, the xylenes and mesity-lene, aliphatic Ca-Cs-ethers, such as 1 ,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (diglyme), diethyl ether, dipropyl ether, methyl isobutyl ether, tert-butyl methyl ether and tert-butyl ethyl ether, dimethoxymethane, diethoxymethane, dimethylene glycol dimethyl ether, dimethylene glycol diethyl ether, trimethylene glycol dimethyl ether, trimethylene glycol diethyl ether, tetramethylene gly-col dimethyl ether, cycloaliphatic hydrocarbons, such as cyclohexane and cycloheptane, alicyclic C3-C6 ethers, such as tetrahydrofuran (THF), tetrahydropyran, 2-methyltetra-hydrofuran, 3- methyltetrahydrofuran, 1 ,3-dioxolane, and 1 ,4-dioxane, 1 ,3,5-trioxane, short-chain ketones, such as acetone, ethyl methyl ketone and isobutyl methyl ketone.

[0081] In a preferred embodiment, the solvent is dichloromethane.

[0082] In an embodiment, the isovaleric acid obtained by oxidation of the renewably-sourced isoamyl alcohol is isolated by distillation.

[0083] Oxidative Decarboxylation of Isovaleric Acid to Isobutylene

[0084] The obtained isovaleric acid is subjected to oxidative decarboxylation so as to obtain isobutylene.

[0085] In an embodiment, the oxidative decarboxylation is carried out in the presence of a homogenous catalyst, wherein the homogenous catalyst generally comprises at least one metal or its salt or complex and a ligand.

[0086] In an embodiment, the at least one metal is selected from Nickel, Palladium, or Platinum. Preferably the metal is Palladium.

[0087] In an embodiment, the at least one metal or its salt or complex is selected from PdCl2, tetrakis(triphenylphosphine) palladium, dichlorobis(triphenylphosphine) palladium, tris(dibenzylideneacetone) dipalladium [Pd2(dba)3], bis(dibenzylideneacetone) dipalladium [Pd(dba)2], palladium acetate, dichloro(1 ,5-cyclooctadiene) palladium and bis[cinnamyl palladium(ll)] chloride.

[0088] Preferably the metal salt is PdC

[0089] In an embodiment, the homogeneous catalyst includes a ligand selected from the group consisting of 5-(di- tert-buty I phosph! no)- 1 ', 3', 5'-tri pheny 1-1 ' H-1 , 4'-bi py razole, bis(2-methyl-2-propanyl)(2',4',6'-triisopropyl-3,6- dimethoxy-2-bi pheny ly I) phosph I ne, dicyclohexyl (2‘ ,4',6'-tri isopropy I-3, 6-dimethoxy-[1 , 1 '-bi pheny l]-2- yl)phosphine, bis(2-methyl-2-propanyl)(2',4',6'-triisopropyl-2-biphenylyl)phosphine, di-(1-adamantyl)-2- morpholinophenylphosphine, tributylphosphine, butyldi-1-adamantyl phosphine, (5-diphenylphosphanyl- 9,9-dimethylxanthen-4-yl)-diphenylphosphane, (R)-1-[(SP)-2-(diphenylphosphino)ferrocenyl] ethyldicyclohexyl phosphine, dicyclohexyl-[2-[2,6-di(propan-2-yloxy)phenyl]phenyl]phosphane, bis[5-(di(1- adamantyl)phosphino)-T,3',5'-triphenyl-TH-[1 ,4']bipyrazole, trimethylphosphine, triethylphosphine, tripropylphosphine, triisopropylphosphine, tributylphosphine, tricyclohexylphosphine, trimethylphosphine, triethylphosphite, tripropylphosphite, triisopropylphosphite, tributylphosphite, tricyclohexylphosphite, triphenylphosphine, tri(o-tolyl)phosphine, triisopropylphosphine, tricyclohexylphosphine, 2, 2'-bis(d I pheny I phosph! no)-1 , 1 '-binaphthyl (Bl NAP), 1 , 2-bis(di methyl phosph I no)ethane, 1 , 2-bi s(d iethy I - phosphino)-ethane, 1 , 2-bi s(d I propy Iphosph I no) ethane, 4, 5-bi s(d I pheny I phosph I no)-9, 9-di methy l-xanthene (xant-phos), 1 ,T-bis(diphenylphosphino)ferrocene (dppf), bis(2-(diphenyl-phosphino)phenyl)ether [DPE- phos], 1 , 2-bis(d i i sopropy I phosph i no)ethane, 1 , 2-bi s-(di buty Iphosphi no)ethane, 1 , 2-bi s(dicy clohexy I - phosphino)ethane, 1,3-bis(diisopropyl-phosphino)propane, 1 ,3-bis(dicyclohexylphosphino)propane, 1 , 4-bis(d i isopropyl -phosphino)butane, 1 , 4-bi s(dicy clohexy I phosph i no)butane, 1 , 4-bi s(d I pheny Iphosphi no)- butane (bppb), 2,4-bis(dicyclohexylphosphino)pentane and 1,T-bis(diphenylphosphino) ferrocene (dppf), Triphenyl phosphine (PPh3), Xantphos ((9,9-Dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)), SPANphos (4,4,4',4',6,6'-Hexamethyl-3,3',4,4'-tetrahydro-2,2'-spirobi[[1]benzopyran]-8,8'-diyl)bis(diphenyl- phosphane)), 2,2' -Bis-(diphenylphosphino)-benzophenone.

[0090] Preferably the ligand is selected from bis(2-(diphenyl-phosphino)phenyl)ether [DPE-phos], Triphenyl phosphine (PPh3), Xantphos ((9,9-Dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)), SPANphos (4,4,4',4',6,6'-Hexamethyl-3,3',4,4'-tetrahydro-2,2'-spirobi[[1]benzopyran]-8,8'- diyl)bis(diphenylphosphane)), 2,2' -Bis-(diphenylphosphino)-benzophenone.

[0091] More preferably, the ligand is bis(2-(diphenyl-phosphino)phenyl)ether [DPE-phos],

[0092] In an embodiment, the homogenous catalyst is used at a concentration of less than 5 mol-%, preferably less than 3 mol-%, more preferably less than 2 mol-%.

[0093] In an embodiment, the catalyst is used at a concentration in the range of 0.1 to 2 mol-% based on the isovaleric acid.

[0094] In an embodiment, the mole ratio of the ligand to the metal catalyst or its salt or complex is in the range of 5: 1 to 1 : 1 , preferably the ratio of the ligand to the metal catalyst metal catalyst or its salt or complex is 2: 1.

[0095] In an embodiment, oxidative decarboxylation is carried out in the presence of an acid anhydride. In an embodiment, oxidative decarboxylation is carried out in the presence of a carboxylic anhydride different from the coupling product of isovaleric acid, i.e. isovaleric anhydride.

[0096] In another embodiment, the acid anhydride is selected from acetic anhydride, propanoic anhydride, butanoic anhydride, or maleic anhydride. Preferably, the acid anhydride is acetic anhydride.

[0097] In an embodiment, oxidative decarboxylation is carried out at a reaction temperature of less than 150 °C, preferably less than 145 °C, preferably in the range of 130 to 150 °C, more preferably 130 to 145 °C.

[0098] In a further embodiment, oxidative decarboxylation is carried out for a time period in the range of 60 to 300 min, preferably 100 to 300 min, more preferably 120 to 240 min, more preferably 60 to 240 min.

[0099] In an embodiment, the process comprises the steps of:

[0100] - feeding a feedstock comprising renewably-sourced isoamyl alcohol, at a concentration of 60 to 99 wt.- %, into a reaction vessel, oxidizing the isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene, wherein the isoamyl alcohol is converted to isovaleric acid at a yield of at least about 80 wt.-% of the maximum theoretical molar yield.

[0101] The term "yield” as used herein is defined as the amount of product obtained per unit weight of raw material and may be expressed as g product / g substrate. Yield may be expressed as a percentage of the theoretical yield. "Theoretical yield” is defined as the maximum amount of product that can be generated per a given amount of substrate as dictated by the stoichiometry of the metabolic pathway used to make the product. For example, if the theoretical yield for one typical conversion of glucose to isobutanol is 0.41 g / g, the yield of butanol from glucose of 0.39 g / g would be expressed as 95% of theoretical or 95% theoretical yield.

[0102] In an embodiment, the process comprises the steps of:

[0103] - feeding a feedstock comprising renewably-sourced isoamyl alcohol, at a concentration of 60 to 99 wt.- %, into a reaction vessel,

[0104] - oxidizing the isoamyl alcohol to isovaleric acid in the presence of oxidizing agents selected from nitric acid and O2, and in the presence of a heterogeneous material comprising platinum, and

[0105] - subjecting the isovaleric acid to oxidative decarboxylation in the presence of a homogenous catalyst at a concentration of less than 5 mol-% preferably less than 3 mol-%, more preferably less than 2 mol-%, so as to obtain isobutylene, wherein the isoamyl alcohol is converted to isobutylene at a yield of at least about 80 wt.-% of the maximum theoretical molar yield.

[0106] Advantages of the present invention include:

[0107] 1 . The process according to the invention enables the preparation of isobutylene with high yield and high selectivity under mild conditions, both of temperature and pressure, while requiring only moderate to low amounts of catalyst.

[0108] 2. The process can be conducted with no or low amounts of organic solvent, thus avoiding or minimizing environmentally problematic waste stream.

[0109] 3. A further advantage of the process of the invention is that the desired isobutylene is obtained in a high concentration in the reaction mixture, thus minimizing down-stream isolation steps.

[0110] 4. The process of the present invention enables the formation of isobutylene in comparable yield and purity irrespective of the source of the renewably-sourced isoamyl alcohol used as the starting material. (Pure renewably-sourced isoamyl alcohol or renewably-sourced isoamyl alcohol isolated from fusel oil). Thus, the impurities of the starting material do not hamper the yield and purity of the final product (isobutylene in this case).

[0111] 5. The process according to the present invention uses readily available reagents and thereby reduces the overall cost of the process, thereby making it industrially viable process which can be easily scaled up.

[0112] 6. The process according to the present invention, using 3-methyl-1 -butanol from fusel oil as the starting material and using the combination of oxidation of the alcohol to 3-methylbutanoic acid and oxidative decarboxylation to isobutene. Although the individual steps are known reaction, they have not been described for this particular substrate, and they have not been combined as a means of obtaining olefin (isobutene) with a pMC > 90% and thus can be considered to be inventive.

[0113] 7. The isobutylene that is obtained would also have a pMC value greater than 90 which can be deduced from the fact that the input stream comprising isoamyl alcohol has a pMC value greater than 90 (which is obtained in natural sources, namely the fusel oil).

[0114] Reacting a Formaldehyde Source and Isobutylene to Obtain Isoprenol

[0115] In the first aspect of the invention, the isobutylene obtained in step a-i) is reacted in step a-ii) with at least one formaldehyde source to obtain 3-methylbut-3-en-1 -ol (isoprenol). The at least one formaldehyde source and isobutylene are typically reacted in a reactor, in general at elevated temperature and pressure.

[0116] As used herein, "formaldehyde source” refers to any source containing formaldehyde or capable of cleaving off formaldehyde. Formaldehyde sources include aqueous formaldehyde solutions and oligomers or polymers of formaldehyde, like paraformaldehyde.

[0117] In one embodiment, the isoprenol is obtained by introducing, preferably by mixing and injecting, at least one formaldehyde source and isobutylene into a reactor, preferably through at least one nozzle, and reacting the at least one formaldehyde source and isobutylene under supercritical conditions. In order to achieve supercritical conditions, formaldehyde and isobutylene are preferably reacted at a temperature of at least 220 °C, for example in the range of 220 to 290 °C, and an absolute pressure of at least 200 bara. The reaction of isobutene and formaldehyde may be carried out without a catalyst as well as in the presence of at least one catalyst. The reaction of isobutylene and formaldehyde source may also be carried out in the presence of one or more auxiliary chemicals such as ammonia and / or hexamethylenetetramine (urotropin). Conducting this reaction in the presence of such auxiliary chemicals, especially ammonia and / or urotropin, has been described, e.g., in DE 1279014 B.

[0118] The at least one formaldehyde source and isobutylene are preferably introduced into the reactor in a manner which allows for mixing of the reactants so as to obtain an intimate mixture. Introduction methods include injecting, splashing, stirring in and I or spraying into the reactor. Preferably, the at least one formaldehyde source and isobutylene are injected or sprayed into the reactor through at least one nozzle.

[0119] Formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde in methanol. Preferably, the at least one formaldehyde source comprises or is an aqueous formaldehyde solution.

[0120] While initial rapid and intense mixing of reactants is desirable, it may be advantageous to continue and complete the reaction under conditions of limited back-mixing. Thus, the reaction mixture may be passed into a post-reaction chamber disposed after the reactor or in a lower portion of the reactor. In the postreaction chamber, back-mixing is limited.

[0121] In one embodiment, the reactor comprises an upper portion and a lower portion. Introduction of the reactants, in particular by injecting and mixing of the reactants, occurs in a mixing chamber of the reactor disposed in the upper portion, and a fluid comprising formaldehyde and / or isobutylene and / or isoprenol is passed from the mixing chamber into a post-reaction chamber disposed in the lower portion.

[0122] In one embodiment, reacting at least one formaldehyde source and isobutylene comprises introducing, preferably mixing and injecting, the at least one formaldehyde source and isobutylene into an internal loop reactor through at least one nozzle into first conduit(s), the internal loop reactor comprising:

[0123] - a vertically disposed cylindrical vessel comprising a sidewall;

[0124] - at least one draft tube having a tube inlet end and a tube outlet end , arranged vertically within the vessel, the draft tube(s) being arranged concentrically to the nozzle(s) , and having an inner surface and an outer surface, wherein the draft tube(s) provide(s) the first conduit(s) within the draft tube(s), and a second conduit outside of the draft tube(s) and within the sidewall, the first conduit(s) being in fluid communication with the second conduit;

[0125] - reactor fluid outlet means; wherein the inner surface of the draft tube(s) convexly curves so that the first conduit(s) exhibit(s) an annular constriction of the cross-section between the tube inlet end and the tube outlet end; wherein the constriction is located closer to the tube inlet end; wherein the convex curvature of the inner surface of the draft tube(s) extends over at least 70%, preferably at least 80%, most preferably at least 90% of the length of the draft tube; and wherein the outer surface of the draft tube(s) convexly curves so that the draft tube(s) exhibit(s) a circumferential protuberance between the tube inlet end and the tube outlet end, which circumferential protuberance is preferably located closer to the tube outlet end; wherein the convex curvature of the outer surface of the draft tube(s) extends over at least 70%, preferably at least 80%, most preferably at least 90% of the length of the draft tube; and wherein the edges of the draft tube(s) are rounded so that the at least one formaldehyde source and isobutylene introduced through the nozzles travel generally downward in the first conduit(s) to obtain a reacted fluid, the reacted fluid is then diverted in the opposite direction so as to travel through the second conduit and is subsequently back-mixed with the introduced fluid.

[0126] In a preferred embodiment, the nozzles are two-component nozzles. It is especially preferable that a two- component nozzle is designed so as to provide an annular jet of isobutylene around a central jet of the at least one formaldehyde source, and that the velocities upon introduction, for example the injection velocities or spraying velocities, of these two jets are different. In this embodiment, the jet of isobutylene has a large shear surface towards both the central jet of the at least one formaldehyde source and the reaction mixture in the reactor, allowing for favorable fast mixing of the reactants.

[0127] In a preferred embodiment, the loop reactor comprises deflector means arranged between the nozzle and the draft tube, the deflector means being suitable for deflecting fluid travelling in the second conduit in the opposite direction.

[0128] The deflector means suitably comprise a surface which is concave relative to the end of the draft tube which defines the tube inlet end. In a preferred embodiment, the deflector means have a partial toroidal surface. It is especially preferred that the deflector means are provided in the shape of the upper portion of a ring torus bisected in a plane parallel to the toroidal direction. This shape allows for an especially efficient deflection of the fluid travelling in the second conduit. The deflector means may allow for a stabilization of the introduced, for example injected or sprayed fluid stream. This is especially relevant when the flow rate of the fluid travelling in the second conduit is not uniform across the cross section of the reactor, which may lead to an eccentricity of the introduced fluid stream. Such an eccentricity may cause a decrease in circulation ratio if left unattended.

[0129] When the first conduit is downcomer conduit and the second conduit is a riser conduit, it is preferred that the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction, wherein the ring torus is bisected at least 50% of its height, such as at least 55% or 65% of its height. Thus, the upper portion of the ring torus is the same size or smaller than the lower portion of the ring torus. In another preferred embodiment, the shape of the deflector means constitutes the upper portion of a ring torus bisected in a plane parallel to the toroidal direction wherein the ring torus is bisected at at most 85% of its height, for example 80% of its height. In these ranges, the entry of the deflector means is angled especially suitable for fluid deflection.

[0130] Further details regarding aforementioned embodiments concerning loop reactor may be found in WO 2023 / 104863, which herewith is incorporated by reference in its entirety.

[0131] High temperatures are required to obtain a high isoprenol yield in the reaction of formaldehyde with isobutylene. Effective removal of the heat is critical for the product quality and process safety. The heat removed from the isoprenol is used for raising the temperature of isobutylene before it enters the reactor. The stream of the hot isoprenol contains sensible heat from the chemical reaction. The sensible heat is potentially reclaimable energy that can be reused.

[0132] Advantageously, reacting at least one formaldehyde source and isobutylene preferably comprises heatexchanging a stream of hot isoprenol withdrawn from the reactor with a isobutylene stream directed to the reactor; wherein heat-exchanging is performed in one or more shel l-and-tube heat exchangers; each of the heat exchangers comprising a plurality of tubes and a shell-side heat exchange passage; wherein the hot isoprenol is directed through the tubes of the heat exchangers; and the isobutylene is guided through the shell-side passage, and in case of more than one heat exchangers at least two of the heat exchangers are connected in series with regard to both the shell-side flow and the tube-side flow.

[0133] In a group of the preferred embodiments, the heat-exchanging is performed in one shell-and-tube heat exchanger.

[0134] In another group of the preferred embodiments, the heat-exchanging is performed in at least one or more shell-and-tube heat exchangers, wherein the hot isoprenol is directed through the tubes of the heat exchangers; and the isobutylene is guided through the shell-side passage, and in case of at least two of the heat exchangers these are connected in series with regard to both the shell-side flow and the tube-side flow.

[0135] Such configurations allow for prolonging operation intervals between maintenance disruptions in such a method. The term "maintenance disruptions” is intended to mean a shutdown of the process that becomes necessary at recurring intervals in order to clear the tubes of the heat exchanger that have been clogged by fouling. An indicator of a necessity of a maintenance disruption is typically when isobutylene leaving the last heat exchanger is insufficiently pre-heated and that even a subsequent heater is hardly able to put in additional external heat into the isobutylene to bring isobutylene to the required temperature before it enters the reactor. One aspect of the invention is that the pre-heating of the isobutylene stream can be maintained for a longer time at levels high enough so that the desired temperature of the isobutylene can easily be reached before the isobutylene enters the reactor.

[0136] One particular area prone to fouling in conventional shell-and-tube heat exchangers is the tube area near the tube sheet near the inlet where the tube-side fluid leaves the individual tubes. Excessive fouling in this area can cause clogging of individual tubes and fluid stagnation along the entire length of these tubes. The fluid stagnation generally leads to reduced heat-transfer performance.

[0137] As a further consequence of the decreased heat transfer performance caused by fouling, the energy required in a heater to adjust the temperature of the pre-heated isobutylene stream to the desired reaction temperature increases. Consequently, more additional external heat becomes necessary which is detrimental in terms of energy demand and process economy, and often has a negative impact on the carbon dioxide footprint of the product.

[0138] By using two or more heat exchangers, the impact of fouling in individual tubes on the overall heat exchange capacity is reduced in comparison to arrangements where only a single heat exchanger is used. As a consequence, the heat transfer rates are maintained at a desired level for longer periods, hence prolonging operation intervals between maintenance disruptions, and the pre-heating of the isobutylene stream requires less additional external heat compared to a plant with a single heat exchanger in an advanced state of fouling.

[0139] Further details regarding aforementioned embodiments concerning heat exchangers and energy savings and reducing maintenance intervals may be found in WO 2023 / 198714 A1 , which herewith is incorporated by reference in its entirety.

[0140] Second Aspect - Preparation of Prenol

[0141] The second aspect of the invention relates to the preparation of 3-methyl-2-buten-1 -ol (prenol), comprising a) providing isoprenol as described above via steps a-i) and a-ii) according to the first aspect, and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

[0142] As used herein and hereinafter, the term "reactant stream” refers to a stream comprising a reactant or reactants consumed in the course of a chemical reaction. In this sense, the reactant stream may further comprise solvent(s), catalyst(s), additive(s) and / or any other substance involved in the chemical reaction.

[0143] Generally, the isomerization of isoprenol to 3-methyl-2-buten-1-ol (prenol) may be carried out over a supported noble metal, preferably in the presence of hydrogen. A preferred catalyst is a fixed bed catalyst containing palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide.

[0144] The catalyst contains 0.1 to 2.0% by weight of palladium and 0.01 to 0.2% by weight of selenium, tellurium or a mixture of selenium and tellurium, based on the total weight of the catalyst.

[0145] The BET surface area is, for example, in the range of 100 to 150 m2 / g, in particular in the range of 110 to 130 m2 / g. The BET surface area is determined by N2 adsorption according to DIN 66131.

[0146] The pore volume in the pore diameter range from 3 nm to 300 pm is preferably 0.8 to 0.9 cm3 / g, in particular 0.8 to 0.85 cm3 / g. Thereby, 80 to 95%, preferably 85 to 93% of this pore volume is in the pore diameter range of 10 to 100 nm. The pore volume is determined by Hg porosimetry.

[0147] Preferably, the catalyst contains 0.2 to 0.8% by weight, in particular 0.4 to 0.6% by weight of palladium. Preferably, the catalyst contains 0.02 to 0.08, in particular 0.04 to 0.06 wt% selenium, tellurium or a mixture of selenium and tellurium, preferably selenium. In addition to the active components mentioned, other metals may be present on the catalyst in small amounts. Preferably, only palladium, selenium and / or tellurium, in particular only palladium and selenium, are present on the silica support.

[0148] The described isomerization of isoprenol to prenol on a fixed-bed catalyst is also described in EP-A 841 090, to which express reference is made.

[0149] The isomerization is carried out at a temperature in the range of 50 to 150 °C, preferably in the range of 60 to 130 °C, more preferably in the range of 70 to 120 °C to produce a reaction mixture of prenol and isoprenol. The isoprenol can be recycled. Further details are provided in WO 2008 / 037693.

[0150] Generally, a regeneration cycle is performed periodically, to remove accumulated coke from the catalyst. The regeneration cycle can be initiated when the pressure drop increased above a threshold value, or at arbitrary time intervals, for example once a week. A regeneration cycle consists of sending diluted air or air for a defined period of time, for example 6 to 24 h, over the reactor while increasing the salt bath temperature, for example 400 to 450 °C, to allow coke combustion.

[0151] The unreacted isoprenol from the isoprenol isomerization process may be used, i.e. recycled for the isoprenol isomerization.

[0152] Reducing the Content of Aldehydes in the Reactant Stream

[0153] Surprisingly, it has been found that the presence of aldehydes, especially formaldehyde and / or prenal in the reactant stream, is detrimental to the activity and selectivity of the process and may accelerate catalyst deactivation and / or poisoning in the isomerization of isoprenol to prenol.

[0154] As used herein and hereinafter, the term "concentration of aldehydes in the reactant stream” refers to the total concentration of aldehydes existing in the reactant stream. Aldehydes include those intrinsic to the isoprenol preparation process and those formed by oxidation and isomerization. Hence, the aldehydes usually include formaldehyde and prenal. Therefore, if formaldehyde and prenal are the only aldehydes existing in the reactant stream, the concentration of aldehydes in the reactant stream is the sum of the respective concentrations of formaldehyde and prenal.

[0155] It has been found that deterioration of catalyst properties is related to the presence of aldehydes, especially formaldehyde and / or prenal in the reactant stream. Formaldehyde is generally considered to be the most critical of these aldehydes. Catalyst-fouling reactions of condensation and polymerization are believed to be the principal reactions involved in carbon or coke formation on the catalyst. It is thought that this carbon formation involves thermal condensation of aldehydes, for example formaldehyde and / or prenal, or of these aldehydes with the olefinic hydrocarbons isoprenol. In the presence of the catalyst the primary condensation products tend to undergo dehydrogenation and polymerization type reactions and to settle on the catalyst and undergo further dehydrogenation and decomposition until carbonaceous deposits are formed.

[0156] One of the poisoning mechanisms of the catalyst is supposed to involve a catalytic or non-catalytic dehydrogenation of aldehydes, especially formaldehyde and / or prenal to carbon monoxide, which is chemisorbed on the catalyst and blocks the active centers.

[0157] A further cause of catalyst deactivation, which may occur in combination with the previously mentioned cause of catalyst poisoning, is the formation of paraformaldehyde or trioxane which may deposit, in the form of solids, on the catalyst and shield the catalytically active surfaces from the isoprenol being processed. This leads to progressive deactivation of the catalyst.

[0158] In one embodiment, the concentration of aldehydes in the reactant stream is, therefore, maintained at a certain level or less, i.e. less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. As used herein and hereinafter, the term "ppm” refers to parts-per-million (ppm, 10-6).

[0159] In more preferred embodiments, the concentration of aldehydes is maintained at less than 0.2% by weight, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

[0160] Aldehydes, preferably formaldehyde and / or prenal may be removed from the streams comprising isoprenol by a conventional separating method such as distillation, selective adsorption and or selective reaction, in particular by the purification process involving the pressure-swing distillation as described herein.

[0161] Alternatively, it is feasible to mix the unreacted isoprenol stream with an amount of a sufficiently purified fresh feed stream so as to give in the combined stream a desired weight ratio of formaldehyde to isoprenol.

[0162] As used herein and hereinafter, the term "unreacted isoprenol stream” refers to a stream which is derived from an isoprenol isomerization process and comprises unreacted isoprenol of the isoprenol isomerization process. In this sense, the unreacted isoprenol stream may further comprise solvent(s), catalyses), additive(s) and / or any other substance involved in the isoprenol isomerization process.

[0163] As used herein and hereinafter, the term "crude isoprenol stream” refers to a product stream of an isoprenol production process from which unreacted isobutylene has been removed. Removal of aldehydes, such as formaldehyde and / or prenal, is accomplished in a purification unit following the isoprenol synthesis. A preferred method of recovering aldehydes from a crude isoprenol stream to which an unreacted isoprenol stream is admixed, is described in more detail below.

[0164] Preferably, the aldehydes existing in the reactant stream comprise formaldehyde. Also preferably, the aldehydes existing in the reactant stream comprise prenal besides formaldehyde.

[0165] More preferably, the aldehydes existing in the reactant stream consist of prenal and formaldehyde. In certain instances, the aldehydes existing in the reactant stream consist of formaldehyde.

[0166] Preferably, the concentration of aldehydes in the reactant stream is less than 0.5% by weight, or 0.4% by weight, or 0.3% by weight, more preferably less than 0.25% by weight, or 0.2% by weight, even more preferably less than 0.15% by weight, yet even more preferably less than 0.1 % by weight, equal to or less than 0.08% by weight or less than 0.05% by weight, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of aldehydes is less than 0.025% by weight, more less than 0.02% by weight, based on the total weight of the reactant stream. In one embodiment, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. The skilled person will appreciate that any of the upper limits of aldehyde concentration can be combined with any of the lower limits of aldehyde concentration, wherein in certain embodiments the aldehyde is either formaldehyde, prenal, or formaldehyde and prenal.

[0167] Also preferably, the concentration of formaldehyde in the reactant stream is less than 0.5% by weight, or 0.4% by weight, or 0.3% by weight, more preferably less than 0.25% by weight, or 0.2% by weight, even more preferably less than 0.15% by weight, yet even more preferably less than 0.1% by weight, equal to or less than 0.08% by weight or less than 0.05% by weight, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of formaldehyde is less than 0.025% by weight, more less than 0.02% by weight, based on the total weight of the reactant stream, wherein the concentration of formaldehyde in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

[0168] If the aldehydes existing in the reactant stream comprise or consist of formaldehyde, the concentration of formaldehyde in the reactant stream is preferably less than 0.5% by weight, or less than 0.4% by weight, or less than 0.3% by weight, more preferably less than 0.25% by weight, or 0.2% by weight, even more preferably less than 0.15% by weight, yet even more preferably less than 0.1 % by weight, or less than 0.05% by weight, most preferably less than 0.025% by weight, or less than 0.02% by weight, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

[0169] Preferably, the concentration of prenal in the reactant stream is less than 0.3% by weight, more preferably less than 0.2% by weight, even more preferably less than 0.15% by weight, in particular less than 0.1 % by weight, based on the total weight of the reactant stream, but not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

[0170] Therefore, in preferred embodiments, the aldehydes in the reactant stream consist of or comprises formaldehyde, and the concentration of formaldehyde is less than 0.5% by weight, or less than 0.4% by weight, or less than 0.3% by weight, more preferably less than 0.25% by weight, or 0.2% by weight, even more preferably less than 0.15% by weight, yet even more preferably less than 0.1% by weight, equal to or less than 0.08% by weight, or less than 0.05% by weight, based on the total weight of the reactant stream, but at least 10 ppm with respect to the total weight of the reactant stream. In another embodiment, the concentration of formaldehyde is less than 0.025% by weight, more preferably less than 0.02% by weight, based on the total weight of the reactant stream, but not less than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream. In one embodiment, the concentration of formaldehyde is equal to or less than 0.08 % by weight, based on the total weight of the reactant stream, but optionally at least 10 ppm with respect to the total weight of the reactant stream.

[0171] In an embodiment, the aldehydes existing in the reactant stream consist of prenal and formaldehyde, and therefore the concentration of aldehydes in the reactant stream corresponds to the sum of the concentrations of prenal and formaldehyde, wherein the concentration of aldehydes in the reactant stream, i.e. the sum of the concentrations of prenal and formaldehyde is less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight or less than 0.2% by weight, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

[0172] In a group of the preferred embodiments, the weight ratio of aldehydes, preferably prenal and / or formaldehyde, to isoprenol in the reactant stream is adjusted at a certain level or less, i.e. less than 0.04, preferably less than 0.03, in particular less than 0.02, or less than 0.01 . In still more preferred embodiments, the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol is adjusted at less than 0.002, or less than 0.001. In one embodiment, the ratio is lower than 0.0009.

[0173] The terms "maintaining in the reactant stream” and "adjusting in the reactant stream” or "maintained in the reactant stream” or "adjusted in the reactant stream” with respect to the aldehyde levels in the reactant stream are used interchangeably herein.

[0174] Reducing the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol in the reactant stream beyond a certain point, however, reaches a point of rapidly diminishing return. Removal of aldehydes, especially of formaldehyde and / or prenal involves additional equipment and operating costs. An economic balance must be taken between the improvement due to reducing the ratio and the cost of maintaining such a ratio.

[0175] Hence, the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol is preferably not lower than 0.0005 or, in some instances, not lower than 0.0007.

[0176] Since the double-bond isomerization of isomerization of isoprenol to prenol is an equilibrium reaction, a complete conversion of substances is not achieved in a single pass. Instead, a portion of isoprenol always remains, which unreacted isoprenol is suitably separated from the desired prenol. The unreacted isoprenol may be recycled to the isomerization reaction, or may be directed to other isoprenol-consuming reactions.

[0177] Generally, the reactant stream will comprise or consist of a fresh isoprenol stream. The term "fresh isoprenol stream” refers to a stream of isoprenol directly obtained from the purification unit following the isoprenol synthesis, i.e., from a purification unit wherein a crude isoprenol stream from the reaction of isobutene and formaldehyde is purified. The reactant stream may further comprise recycled, unreacted isoprenol, and / or isoprenol from other sources.

[0178] Preferably, the reactant stream comprises or consists of a fresh isoprenol stream. Also preferably, the reactant stream comprises or consists of a mixture of unreacted isoprenol stream and a fresh isoprenol stream.

[0179] In yet another embodiment, the reactant stream consists of a mixture of the unreacted isoprenol stream, and isoprenol from other sources. Other sources of isoprenol are processes other than the reaction of isobutene and formaldehyde, in which isoprenol is obtained as a by-product or target product, or isoprenol from commercial sources.

[0180] The presence of aldehydes, especially formaldehyde and / or prenal in the reactant stream reduces both catalyst activity and selectivity and causes increase in pressure drop and reactor clogging. Besides aldehydes, especially formaldehyde and / or prenal, other impurities which may be present in the reactant stream can cause a decrease in catalyst activity and selectivity. Preferably, the equipment or operations used for maintaining in the reactant stream a certain concentration of aldehydes, preferably formaldehyde and / or prenal, or a certain weight ratio of aldehydes, preferably formaldehyde or prenal to isoprenol is also effective to remove a major portion of these impurities. In preferred embodiments, the concentration in the reactant stream of at least one of the following impurities is kept below the limit indicated, in particular of all of the following impurities:

[0181] Compliance with these limits is particularly important when the reactant stream accommodates isoprenol streams from other sources. Reducing the concentration of aldehydes, preferably formaldehyde and / or prenal in the reactant stream will inherently reduce the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol in the reactant stream. Therefore, the following applies for reducing the concentration of aldehydes, preferably formaldehyde and / or prenal in the reactant stream as well as reducing the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol in the reactant stream.

[0182] The presence of formaldehyde in the reactant stream is due to two main sources. Formaldehyde may be contained in the isoprenol stream sent to the reactor, that is as an impurity originating from the isoprenol manufacture step. In industrial practice, isoprenol is synthesized from isobutene and formaldehyde. All the formaldehyde that cannot be separated in the purification step following the isoprenol synthesis ends up in the reactant stream.

[0183] In addition, formaldehyde is also generated in situ. Part of the isoprenol splits back to isobutene and formaldehyde.

[0184] Since most continuous industrial processes operate at single-pass conversion levels of 50 to 60% and with recycling of the unconverted isoprenol, formaldehyde may be present in the recycling stream of unconverted isoprenol, if no steps to purify the stream containing unreacted isoprenol are taken. The recycle stream of unconverted isoprenol has now been found to typically constitute the biggest source of formaldehyde contamination in the reactant stream. The process is generally carried out at partial conversions, for example at conversions of 30 to 70%, preferably 50 to 60%. An unreacted isoprenol stream is separated from the product stream. The unreacted isoprenol stream is recycled, that is, combined with a fresh feed stream comprising isoprenol (a crude isoprenol stream) to provide the reactant stream. The unreacted isoprenol stream comprises isoprenol as a main constituent, but may also comprise prenal, isoprenal, isoamylalcohol, isovaleraldehyde, isovaleric acid, prenol, formaldehyde. It can also contain traces of other C3 and C2 aldehydes and acids.

[0185] Prenal may be contained in the isoprenol stream sent to the reactor, that is as an impurity originating from the isoprenol manufacture step. The isoprenol stream may further contain traces of ammonia, and / or C5- oxygenates other than prenal besides formaldehyde and / or prenal. All the prenal and / or other impurities that cannot be separated in the purification step following the isoprenol synthesis ends up in the reactant stream.

[0186] Since the double bond isomerization of isoprenol is an equilibrium reaction, conversion is necessarily incomplete. For economic operation of the process, the unconverted isoprenol has to be removed and recycled. Recycling of isoprenol may therefore inadvertently (re)introduce formaldehyde into the isomerization step if no steps to purify the stream containing unreacted isoprenol are taken.

[0187] Reducing the concentration of aldehydes, preferably formaldehyde and / or prenal in the reactant stream or reducing the weight ratio of aldehydes, preferably formaldehyde and / or prenal to isoprenol in the reactant stream can be accomplished in several different ways.

[0188] In a preferred embodiment, the process includes separating an unreacted isoprenol stream from the prenol containing product stream, optionally removing at least some aldehydes, preferably some formaldehyde and / or prenal from the unreacted isoprenol stream, followed by combining the unreacted isoprenol stream with a fresh isoprenol stream to form the reactant stream.

[0189] In another preferred embodiment, the process includes separating an unreacted isoprenol stream from the prenol containing product stream, combining the unreacted isoprenol stream with a crude isoprenol stream containing isoprenol, water and aldehydes, and removing aldehydes, preferably water and aldehydes from the combined stream to form the reactant stream

[0190] As mentioned above, the crude isoprenol stream is generally the product stream of an isoprenol production process from which unreacted isobutylene has been removed. This means that formaldehyde removal is accomplished in the purification unit following the isoprenol synthesis. A preferred method of recovering formaldehyde from a crude isoprenol stream to which an unreacted isoprenol stream is admixed, is described in more detail below.

[0191] Aldehydes, preferably formaldehyde and / or prenal may be removed from isoprenol streams by a conventional separating method such as distillation, selective adsorption and / or selective reaction.

[0192] Removal of aldehydes, preferably formaldehyde and / or prenal by distillation can involve the use of a single distillation column or a train of distillation columns. The towers and columns used may be conventional distillation columns. Suitable types of distillation columns include packed columns, such as columns with random packing or structured packing, plate columns (i.e., tray columns), and mixed columns comprising both packings and trays.

[0193] Suitable plate columns may comprise internals over which the liquid phase flows. Suitable internals include sieve trays, bubble cap trays, valve trays, tunnel trays and Thormann® trays, in particular bubble cap trays, valve trays tunnel trays and Thormann® trays. Random packed columns may be filled with a variety of shaped bodies. Heat and mass transfer are improved by enlarging the surface area by means of shaped bodies, which usually have a size in the range of 25 to 80 mm. Suitable shaped bodies include Raschig rings (hollow cylinders), Lessing rings, Pall rings, Hiflow rings and Intalox saddles. The packing materials may be provided in the column in a regular or irregular manner (as bulk material, i.e. loosely filled). Suitable materials include glass, ceramics, metal and plastics.

[0194] Structured packings are an advancement of regular packings and have a regularly shaped structure. This allows for the reduction of gas flow pressure loss. Suitable types of structured packings include fabric and metal sheet packings.

[0195] Removal of aldehydes, preferably formaldehyde and / or prenal by selective adsorption involves contacting the stream with an adsorbent that exhibits selectivity for low molecular weight aldehydes, especially formaldehyde and / or prenal. Useful adsorbent materials should deliver high selectivity and high adsorption capacity. An additional and critically important requirement is that the adsorbent material should not catalyze or participate in chemical reactions that might lower the recovery of the (iso)prenal and / or render the adsorbent inactive. Adsorbents include ion exchange resins, mesoporous solids, activated carbons, and zeolites. Removal of aldehydes, preferably formaldehyde and / or prenal by selective reaction involves exposing the stream to reaction conditions under which aldehydes, preferably formaldehyde and / or prenal are (is) selectively reacted to products that are less prone to catalyst deactivation and clogging or to products that can be separated from the stream more easily than aldehydes, preferably formaldehyde and / or prenal.

[0196] Preferably, removal of aldehydes, preferably formaldehyde and / or prenal from a stream comprising isoprenol is conducted by distillation, selective adsorption and / or selective reaction, in particular by purification process involving the pressure-swing distillation.

[0197] The above described applies for the reducing the concentration of aldehydes other than formaldehyde or prenal and / or of other impurities in the reactant stream as well as reducing the weight ratio of aldehydes other than formaldehyde or prenal to isoprenol.

[0198] In case of formaldehyde, difficulties arise from the fact that monomeric formaldehyde (as well as polymeric formaldehyde) forms both hydrates with water and hemiformals with alcohols such as isoprenol, which is the reactant of the isoprenol isomerization and may still remain in the product stream as unreacted reactant. The hydrates and hemiformals of varying formaldehyde polymerization degree have intermingling boiling points. The stability of and the equilibrium between hydrates and hemiformals is temperature-dependent. Formals formed in an upper region of a distillation tower may decompose in the hotter bottom of the tower, which adds additional complexity to the separation task.

[0199] In an embodiment, the unreacted isoprenol stream is combined with a crude isoprenol stream containing isoprenol, water and aldehydes, preferably formaldehyde and / or prenal; and removing aldehydes, such as formaldehyde and / or prenal, preferably water and aldehydes, in particular water and formaldehyde and / or prenal, from the combined stream comprises

[0200] (i) directing the combined stream to a first low-boiler separation tower operated at a pressure of 1 .5 bara or lower, to obtain a first bottoms stream containing isoprenol and aldehydes, preferably prenal and I or formaldehyde, and a first distillate stream containing water and low-boilers;

[0201] (ii) directing the first bottoms stream to a second low-boiler separation tower operated at a pressure of 2 bara or higher, to obtain a second distillate stream containing aqueous aldehydes, preferably prenal and I or formaldehyde, and a second bottoms stream containing isoprenol; and

[0202] (iii) directing the second bottoms stream to a finishing tower to obtain a bottoms stream containing high- boilers, and the reactant stream as a distillate stream.

[0203] In order to permit a first distillation at a temperature below the isoprenol-aldehyde dissociation temperature of the respective aldehyde(s) present, for example for formaldehyde the isoprenol-formaldehyde dissociation temperature and a second distillation at a temperature above the isoprenol-aldehyde dissociation temperature, like the isoprenol-formaldehyde dissociation temperature, the invention envisages two low-boiler separation towers operated at different pressures. Hence, at the relatively low pressure prevailing in the first low-boiler separation tower, a first distillate containing water and low-boilers essentially free of aldehydes, preferably formaldehyde and / or prenal is obtained. At the relatively high pressure prevailing in the second low-boiler separation tower, a virtually all aldehydes, preferably all formaldehyde and / or prenal is separated from the isoprenol. The process of the invention thus allows for obtaining isoprenol essentially free of aldehydes, preferably formaldehyde and / or prenal.

[0204] The term "essentially free of aldehydes, preferably formaldehyde and / or prenal” is understood to indicate the absence of significant amounts of aldehydes, preferably formaldehyde and / or prenal in the obtained isoprenol. Thus, the obtained isoprenol preferably comprises less than 0.2 wt.-%, in particular less than 0.15 wt.-%, or less than 0.1 wt.-%, based on the total weight of the obtained isoprenol, of aldehydes, preferably formaldehyde and / or prenal.

[0205] Preferably, the crude isoprenol stream is a liquid stream. The liquid stream can be a single-phase liquid stream or a two-phase liquid stream.

[0206] The crude isoprenol is directed to a first low-boiler separation tower operated at a pressure of 1.5 bara or lower. Any higher pressure of the crude isoprenol stream is preferably released before the same is directed to the first low-boiler separation tower. The crude isoprenol stream is preferably fed to the first low-boiler separation tower as a side stream, defining a rectifying section above the location of the feed and a stripping section below the location of the feed.

[0207] In the first low-boiler separation tower, a first bottoms stream containing isoprenol and aldehydes, preferably formaldehyde and / or prenal, and a first distillate stream containing water and low-boilers are obtained. The term "low-boilers" is understood to refer to organic compounds (other than aldehydes, especially formaldehyde and / or prenal) having a boiling point lower than that of isoprenol, hence a boiling point of lower than about 130 °C, at atmospheric pressure. The most common low-boilers are methanol and / or isoprenyl formate formed as by-products during the process.

[0208] In a preferred embodiment, the first low-boiler separation tower is operated at a pressure of 1.2 bara or lower, preferably 0.5 bara or lower. The bottoms temperature of the first low-boiler separation tower is preferably in the range of 80 to 135 °C, more preferably 90 to 115 °C, most preferably 95 to 105 °C. The temperature at the top of the first low-boiler separation tower is preferably in the range of 45 to 105 °C, more preferably 55 to 80 °C.

[0209] In a particularly preferred embodiment, the first low-boiler separation tower is operated at a pressure in the range of 0.2 to 0.5 bara, a bottoms temperature in the range of 90 to 115 °C and a temperature at the top in the range of 55 to 80 °C.

[0210] The first low-boiler separation tower preferably has from 15 to 65 theoretical plates, more preferably from 25 to 40 theoretical plates. In particular, the stripping section of the first low-boiler separation tower preferably has 10 to 25 theoretical plates. The rectifying section of the first low-boiler separation tower preferably has 5 to 40 theoretical plates.

[0211] The first bottoms stream preferably comprises 75 to 95 wt.-% of isoprenol, more preferably 80 to 90 wt.-%, based on the total weight of the first bottom stream. The first distillate is typically withdrawn at the top of the first low-boiler separation tower in gaseous form and condensed to obtain a liquid two-phase stream. The two-phase stream is preferably allowed to phaseseparate in a separating vessel to obtain an aqueous phase and an organic phase. The aqueous phase is preferably passed to a wastewater stripping column described below. The organic phase is preferably partially returned to the top of the first low-boiler separation tower as a reflux stream. Another part of the organic phase is preferably discarded from the process to avoid the accumulation of water-insoluble low- boilers in the first low-boiler separation tower.

[0212] In a preferred embodiment, at least part of the first distillate stream is directed to a wastewater stripping column to separate low-boilers and entrained isoprenol from water. Preferably, the part of the first distillate stream directed to the wastewater stripping column is an aqueous phase obtained by condensation and phase separation of the first distillate stream, as discussed above.

[0213] In the wastewater stripping column, low-boilers are obtained as the low-boiler distillate stream, and wastewater is obtained as a bottoms stream. Both the low-boiler distillate stream and the wastewater bottoms stream are removed from the process, and each stream may be directed to further processing.

[0214] Moreover, isoprenol is preferably obtained as a side stream in the wastewater stripping column. The isoprenol side stream is typically a two-phase stream and preferably comprises 15 to 40 wt.-% of isoprenol, more preferably 25 to 35 wt.-%, based on the total weight of the isoprenol side stream. The isoprenol side stream is preferably recycled to the first low-boiler separation tower.

[0215] The low-boiler distillate stream preferably comprises 75 to 95 wt.-% of low-boilers, more preferably 80 to 85 wt.-%, based on the total weight of the low-boiler distillate stream. The wastewater bottoms stream preferably comprises less than 1.2 wt.-% of organic matter, more preferably less than 0.6 wt.-%, based on the total weight of the wastewater bottoms stream. The wastewater bottoms stream typically comprises aldehydes, preferably formaldehyde and / or prenal in a concentration of 0.05 to 1.5 wt.-% of aldehydes, preferably formaldehyde and / or prenal, such as 0.3 to 0.9 wt.-%, based on the total weight of the wastewater bottoms stream.

[0216] The wastewater stripping column is preferably operated at a pressure of 1.5 bara or lower, preferably 1.1 bara or lower. The bottoms temperature of the wastewater stripping column is preferably in the range of 95 to 110 °C, more preferably 97 to 103 °C. The temperature at the top of the wastewater stripping column is preferably in the range of 65 to 100 °C, more preferably 75 to 85 °C.

[0217] In a particularly preferred embodiment, the wastewater stripping column is operated at a pressure in the range of 0.95 to 1 .1 bara, a bottoms temperature in the range of 97 to 103 °C and a temperature at the top in the range of 75 to 85 °C.

[0218] The wastewater stripping column preferably has from 6 to 30 theoretical plates, more preferably from 10 to 20 theoretical plates.

[0219] The first bottoms stream obtained in the first low-boiler separation tower is directed to a second low-boiler separation tower operated at a pressure of 2 bara or higher. The first bottoms stream is preferably fed to the second low-boiler separation tower as a side stream, defining a rectifying section above the location of the feed and a stripping section below the location of the feed.

[0220] In the second low-boiler separation tower, a second distillate stream containing or consisting essentially of aqueous aldehydes, preferably formaldehyde and / or prenal, and a second bottoms stream containing isoprenol are obtained. The second bottom stream further comprises high-boilers. The term "high-boilers" is understood to refer to organic compounds having a boiling point higher than that of isoprenol, i.e. higher than about 130 °C, at atmospheric pressure.

[0221] In a preferred embodiment, the second low-boiler separation tower is operated at a pressure of 2.5 bara or higher, preferably 2.8 bara or higher, most preferably 2.9 bara or higher. The bottoms temperature of the second low-boiler separation tower is preferably in the range of 160 to 200 °C, more preferably 170 to 185 °C, most preferably 175 to 180 °C. The temperature at the top of the second low-boiler separation tower is preferably in the range of 115 to 160 °C, more preferably 125 to 145 °C.

[0222] In a particularly preferred embodiment, the second low-boiler separation tower is operated at a pressure in the range of 2.9 to 3.5 bara, a bottoms temperature in the range of 175 to 180 °C and a temperature at the top in the range of 130 to 140 °C.

[0223] The second low-boiler separation tower preferably has from 20 to 60, more preferably from 35 to 60 theoretical plates. In particular, the stripping section of the first low-boiler separation tower preferably has 25 to 45 theoretical plates. The rectifying section of the first low-boiler separation tower preferably has 7 to 20 theoretical plates.

[0224] At the top of the second low-boiler separation tower, an offgas is typically obtained. The offgas primarily comprises nitrogen and may comprise traces of isoprenol, formic acid, water, aldehydes, preferably formaldehyde, prenal and / or decomposition gases.

[0225] The second bottoms stream preferably comprises 82 to 96 wt.-% of isoprenol, more preferably 87 to 91 wt.- %. The relatively high pressure of the second low-boiler separation tower allows for a high degree of separation of aldehydes, preferably formaldehyde and / or prenal, and isoprenol. Thus, the second bottoms stream preferably comprises at most 0.5 wt.-%, more preferably at most 0.1 wt.-%, even more preferably at most 0.008 wt% of aldehydes, preferably formaldehyde and / or prenal, based on the total weight of the second bottoms stream.

[0226] The second distillate stream is an aqueous stream, which preferably comprises 25 to 60 wt.-%, more preferably 40 to 50 wt.-%, in particular 45 to 50 wt.-%, based on the total weight of the second distillate stream, of aldehydes, preferably formaldehyde and / or prenal. The second distillate stream preferably comprises at most 15 wt.-% of isoprenol, more preferably at most 5 wt.-%, based on the total weight of the second distillate stream, of isoprenol.

[0227] Owing to the broad condensation curve of the vapor emerging at the top of the second low-boiler separation tower, it is advantageous to use a condenser with liquid recycling. The direct condensation in a quench with liquid circulation is particularly advantageous. Hence, in a preferred embodiment of the process, a quench section is provided downstream, in vapor flow direction, of the rectifying section of the second low-boiler separation tower. The term "vapor flow direction" relates to the direction of the flow of gaseous components in the separation tower, i.e. upwards, towards the top of the tower. The quench section is preferably provided within the second low-boiler separation tower above the rectifying section.

[0228] The direct condensation in a quench also mitigates fouling caused by various condensation and polymerization mechanisms of aldehydes, for example formaldehyde that may occur at spots of high local aldehyde concentrations, like local formaldehyde concentrations. To avoid the risk of fouling in the second low-boiler separation tower and downstream processes, in particular in the offgas of the second low-boiler separation tower, the concentration of aldehydes, preferably formaldehyde and / or prenal in the second distillate is preferably no higher than 60 wt.-%, more preferably no higher than 55 wt.-% and in particular no higher than 50 wt.-%, based on the total weight of the second distillate stream.

[0229] At the lower end of the quench section, an aqueous liquid is collected. When the quench section is provided within the second low-boiler separation tower, the aqueous liquid may be collected, e.g., at a collecting tray above the rectifying section and beneath of the quench section.

[0230] The aqueous liquid is partially circulated into the quench section through a circulation line and partially withdrawn as the second distillate. Suitably, the part of the aqueous liquid circulated into the quench section is circulated into the top of the quench section. Circulation of the aqueous liquid is typically achieved by use of a pump.

[0231] The circulation of a part of the aqueous liquid into the quench section allows for cooling of vapors rising through the quench section, and absorption of aldehydes, preferably formaldehyde and / or prenal from the vapors into the aqueous liquid. Thus, aldehydes, preferably formaldehyde and / or prenal is quenched from the vapors rising through the quench section.

[0232] Further, the aqueous liquid is partially returned to the rectifying section of the second low-boiler separation tower as a reflux stream. This may be accomplished by a reflux line, or aqueous liquid may be partially returned to the rectifying section as overflow from a collecting tray beneath the quench section.

[0233] The mass flow ratio of the reflux stream to the second distillate is preferably in the range of 2: 1 to 10: 1 , more preferably in the range of 3:1 to 7: 1. In a preferred embodiment, the aqueous liquid is cooled before being circulated into the quench section. Preferably, the part of the aqueous liquid withdrawn as the second distillate is a partial stream of the cooled aqueous liquid.

[0234] The temperature of the aqueous liquid collected at the lower end of the quench section is preferably in the range of 80 to 140 °C, more preferably 125 to 135 °C. The temperature of the cooled aqueous liquid circulated into the quench section is preferably 10 to 80 °C below the temperature of the aqueous liquid collected at the lower end of the quench section. This allows for an energetically favorable process.

[0235] The hot aqueous liquid withdrawn at the lower end of the quench section lends itself to heat-integration. In a suitable embodiment, it is heat-exchanged with the stream of crude isoprenol flowing into the first low- boiler separation tower before being circulated into the quench section. In one embodiment, a scrubbing section is provided downstream, in vapor flow direction, of the quench section and water is introduced at the top of the scrubbing section. Preferably, the scrubbing section is provided within the second low-boiler separation tower above the quench section. The scrubbing section allows for maintaining the aldehydes, preferably formaldehyde and / or prenal concentration in the second distillate below the critical concentrations described above and thus to avoid depositions for example paraformaldehyde deposition in, e.g., offgas lines.

[0236] The mass flow ratio of the water introduced at the top of the scrubbing section to the first bottoms stream obtained in the first low-boiler separation tower is typically in the range of 0.01 :1 to 0.06:1 more preferably in the range of 0.015:1 to 0.03:1.

[0237] The second bottoms stream is directed to a finishing tower, in which pure isoprenol is obtained as a distillate stream. High-boilers are withdrawn via a bottoms stream. As the second bottoms stream comprises essentially no aldehydes, preferably no formaldehyde and / or prenal, the separation task of the finishing tower is significantly less complex than in cases where formaldehyde separation is less efficient in the low- boiler separation section.

[0238] The term "essentially no aldehydes, preferably no formaldehyde and / or prenal” is understood to indicate the absence of significant amounts of aldehydes, preferably formaldehyde and / or prenal in the obtained isoprenol. Thus, the obtained isoprenol preferably comprises less than 0.05 wt.-%, preferably less than 0.01 wt.-%, based on the total weight of the second bottoms stream, of aldehydes, preferably formaldehyde and / or prenal.

[0239] The pure isoprenol distillate stream preferably at least 97.0 wt.-% of isoprenol, more preferably 98.0 wt.-%, such as 98.1 to 99.5 wt.-%, based on the total weight of the pure isoprenol distillate stream. The high-boiler bottoms stream preferably comprises 90 to 99.9 wt.-% of high-boilers, more preferably 99 to 99.8 wt.-%, based on the total weight of the high-boiler bottoms stream. Preferably, the high-boiler bottoms stream comprises less than 0.2 wt.-% of aldehydes, preferably formaldehyde and / or prenal, such as less than 0.05 wt.-%, based on the total weight of the high-boiler bottoms stream, of aldehydes, preferably formaldehyde, and / or prenal.

[0240] In a preferred embodiment, the finishing tower is operated at a pressure of 0.5 bara or lower, preferably 0.25 bara or lower. The bottoms temperature of the first low-boiler separation tower is preferably in the range of 130 to 190 °C, more preferably 150 to 170 °C. The temperature at the top of the finishing tower is preferably in the range of 60 to 90 °C, more preferably 65 to 85 °C.

[0241] In a particularly preferred embodiment, the finishing tower is operated at a pressure in the range of 0.05 to 0.2 bara, a bottoms temperature in the range of 150 to 170 °C and a temperature at the top in the range of 65 to 85 °C.

[0242] The finishing tower preferably has from 6 to 40 theoretical plates, more preferably from 10 to 20 theoretical plates. Unreacted isoprenol from the isomerization of isoprenol to prenol may be used, i.e. directed as feed to an oxidative dehydrogenation step of isoprenol to obtain a stream comprising prenal and / or isoprenal, as described in the following.

[0243] Third Aspect - Providing Prenal and / or Isoprenal

[0244] The third aspect of the invention relates to the preparation of prenal and / or isoprenal, comprising a) providing isoprenol as described above via steps a-i) and a-ii) according to the first aspect, and b) optionally, isomerizing isoprenol obtained in step a) to obtain prenol as described above by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen, according to the second aspect, and providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) as described above to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and c-ii) oxidizing prenol obtained in step b) as described above so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0245] The isoprenol obtained as described above is converted to prenal, involving isomerization and an oxidative dehydrogenation in any order. Thus, it is possible to first isomerize isoprenol to prenol, and subsequently oxidize prenol to prenal; or, to first oxidatively dehydrogenate isoprenol to isoprenal, and optionally isomerize at least part of the isoprenal to prenal.

[0246] Oxidizing Prenol to Prenal

[0247] The prenol obtained as described above may be oxidized so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

[0248] Suitable oxidants include hydrogen peroxide and oxygen, in particular oxygen.

[0249] The oxidation is preferably carried out in the presence of a liquid phase and with oxygen as the oxidant. The liquid phase preferably comprises at least 25 wt.-% of water, more preferably at least 50 wt.-% of water or at least 70 wt.-% of water, based on the total weight of the liquid phase, determined at a temperature of 20 °C and a pressure of 1 bar. It has been found that these conditions allow for a simple and efficient process for preparing prenal from prenol.

[0250] The oxidation is typically carried out in the presence of at least one oxidation catalyst selected from the group consisting of platinum, palladium and gold. Preferably, the at least one oxidation catalyst comprises platinum. In a preferred embodiment, the at least one oxidation catalyst is a supported catalyst. The oxidation is suitably carried out at a temperature of 20 to 100 °C, preferably, 25 to 80 °C, in particular 30 to 70 °C, in particular 35 to 50 °C. In another embodiment the oxidation is carried out at a temperature of 20 to 70 °C. The oxidation is suitably carried out under a partial pressure of oxygen between 0.2 and 8 bar.

[0251] Further details of the oxidation reaction may be found in WO 2023 / 222895 A1 , which herewith is incorporated by reference in its entirety.

[0252] Oxidative Deyhdrogenation of Isoprenol

[0253] Oxidative dehydrogenation of isoprenol typically comprises bringing a reactant stream, in particular a gaseous reactant stream, comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in particular at least one silver-containing heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen. The at least one heterogeneous catalyst may consist of an inert support having a smooth surface having an active layer of silver. Alternatively, massive (full-metal) silver bodies may be used.

[0254] In an embodiment, the non-reacted isoprenol from the isomerization of isoprenol to prenol, which corresponds to the step b), is used as feed to the dehydrogenation step.

[0255] Hence, in an embodiment, the process includes separating an unreacted isoprenol stream from a prenol containing product stream obtained in step b) and directing the unreacted isoprenol stream at least partially to step c-i).

[0256] In an embodiment, oxidative dehydrogenation is carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor comprising

[0257] - a shell-side heat exchange passage for circulating a heat transfer medium and a reaction passage comprising the plurality of reaction tubes;

[0258] - an inlet for introducing the reactant stream to the reaction passage; and

[0259] - an outlet from the reaction passage for recovering an effluent stream from the reaction tubes; wherein the reaction tubes comprise a reactant pre-heating zone adjacent to the inlet, and a reaction zone downstream of the reactant pre-heating zone, the reaction zone having a catalytically active wire matrix insert having silver at least on a part of its surface.

[0260] The term "reactant pre-heating zone” denotes a section of the reaction tube, i.e. a section inside the reaction tube, where essentially no catalytic oxidative dehydrogenation reaction occurs and where the gaseous stream through the reaction tubes is heat-exchanged via the tube wall with the circulating heat transfer medium. The pre-heating zone upstream of the reaction zone involves net heat flow into the reaction tube and ensures that the reactant stream is sufficiently heated up to a temperature close to or at the reaction temperature when it reaches the reaction zone. Upon contact with the catalytic surface, the oxidative dehydrogenation reaction immediately starts. Otherwise, in the event when a "cold” reactant stream reaches the catalytic surface such that the reaction onset temperature of the reaction is not reached, coke formation may occur. Less coke formation advantageously leads to a prolonged reactor operation without the necessity of burning off the coke from the catalytic surface.

[0261] Preferably, the reactant pre-heating zone is adapted to allow for laminar flow of the reactant inside the reactant pre-heating zone. This means, the reactant pre-heating zone is devoid of any obstacles to the reactant flow that triggers a laminar-to-turbulent flow transition. Hence, the reactant pre-heating zone preferably has an essentially free cross section, i.e. the pre-heating zone is empty.

[0262] In the case of an "essentially free cross section”, the reactant pre-heating zone may be empty. Alternatively, the reactant pre-heating zone may accommodate fixtures made of a material having zero or limited catalytic activity, which fixtures have a negligible cross-section in a plane perpendicular to the longitudinal axis of the reaction tube. Said fixtures may be attached to the catalytically active wire matrix which is present in the reaction zone and allow to easily place said wire-matrix insert into or remove the same from the reaction zone. For example, the negligible mounting may be a stainless steel wire or rod.

[0263] This setup allows for heating up only the portion of the entire reactant stream that travels near the hot reaction tube wall. Consequently, the portion of the reactant stream flowing in the center of the reaction tube is not heated to the reaction temperature and blind reactions of the unstable starting materials are thus reduced or even avoided. A "blind reaction” is an unselective oxidative reaction that occurs in the absence of the catalyst. Once the reactant stream reaches the reaction zone, the oxidative dehydrogenation reaction is initiated. Due to the exothermic nature of this reaction, energy is released and the remainder of the reactant stream is rapidly heated to the reaction onset temperature, and the reaction proceeds. This fast heat up of the predominant part of the reaction mixture reduces unwanted side-reactions and thus leads to an increased selectivity.

[0264] Alternatively, the reactant pre-heating zone may have a wire matrix insert having zero or limited catalytic activity. The wire matrix insert may reduce or eliminate temperature gradients without creating any obstruction to flow that would promote turbulent flow characteristics. A wire matrix insert is considered as having zero catalytic activity (or in other words, as being "inert”) if it does not catalyze the gas-phase partial oxidation reaction in question to a significant degree, and the chemical composition of a stream passing the wire matrix insert does not change significantly. Similarly, a matrix insert is considered as having limited catalytic activity if its catalytic activity is less than the activity of a reaction zone. In an embodiment, the wire matrix insert having zero or limited catalytic activity is made of an inert material, preferably stainless steel.

[0265] Herein, the term "reaction zone” denotes a region of the reaction tube where the catalytic gas-phase partial oxidation reaction occurs. The reaction zone comprises a catalytically active wire matrix insert having at least on a part of its surface a catalytically active precious metal. Due to the more open structure of the wire matrix contained in the reaction zone as compared to a packing of individual elements, a larger proportion of the reaction heat is discharged to the reaction tube wall by radiation and does not have to be dissipated by the reactant stream. Due the unique flow characteristic of the reactant stream through the reaction tube with the wire matrix insert in place, heat transfer via the tube wall is improved. Formation of prominent hotspots can be avoided. This in turn, avoids deposition of organic constituents of the reactant stream on the surface of the active catalyst material with concomitant pressure drop. Overall, less regular maintenance in the form of regeneration and / or replacement of the catalyst is required. The number of annual operating hours can be increased and the existing production capacities can be fully utilized, reducing operation cost and increasing profit.

[0266] In contrast to individually present catalyst bodies, the wire matrix inserts can be formed contiguously, or in one piece. Hence, placing the wire matrix inserts in the catalyst containment region of the reaction tubes, and removal therefrom is much facilitated.

[0267] The "reaction zone” may be comprised of a single contiguous reaction zone. Alternatively, the reaction zone may comprise an alternating series of regions having catalytically active wire matrix inserts and regions having an essentially free cross section or having wire matrix inserts having zero or limited catalytic activity.

[0268] A "wire matrix insert” is understood to be a self-supporting skeletal-like structure made of coiled, bent or crimped metal wire which is adapted to be inserted into a reaction tube of a shell-and-tube reactor. The wire matrix insert has a more voluminous structure than a longitudinal wire.

[0269] A fixture such as a stainless steel wire or rod may be attached to the wire matrix insert which allows for easily placing the wire-matrix insert into or removing the same from the reaction zone.

[0270] In an embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, that is, neighboring wire loops have an angular offset. The loops may be formed by helically bending the wire over the length of the wire matrix insert. In view of the ease of manufacture, the elongated core preferably comprises at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings.

[0271] The wire loops may be formed from one wire, or more than one intertwined wire, preferably 4 intertwined wires.

[0272] The wire matrix insert comprised in the reaction zone has silver at least on a part of its surface a catalytically active precious metal. The wire constituting the wire loops may be a massive silver wire, or a wire coated with silver. The core wire may be made of brass alloys, or high-grade steels. The coating layer of silver superimposed on the surface of the core has a thickness of, e.g., 10 pm. In general, however, a massive silver wire has better service life and is preferred. If the wire loops are formed from more than one intertwined wire, at least one of the intertwined wires is made of a massive silver wire, or a wire coated with silver while the other intertwined wires can be made of an inert material.

[0273] A silver wire which is of the same composition throughout its cross section and comprises at least 92.5 wt.-% Ag can suitably be used. The silver wire is helically bent to form wire loops, and combined with at least two longitudinal core wire members, which are twisted around each other to form core wire windings, and the wire loops are accommodated in the core wire windings. The longitudinal core wire members can also be silver wire or inert metal wire. In a preferred embodiment, the catalytically active wire matrix inserts comprise an elongated core having a plurality of wire loops extending from the elongated core, wherein the wire loops are longitudinally arranged and helically shifted, and the wire loops comprise a massive silver wire.

[0274] Further details of oxidative dehydrogenation carried out by passing the isoprenol through a plurality of reaction tubes of a shell-and-tube heat exchange reactor as described above may be found in WO 2023 / 241952 A1 , which herewith is incorporated by reference in its entirety.

[0275] When isoprenol is subjected to oxidative dehydrogenation, it may be favorable to maintain in the reactant stream a weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol of less than 0.04, preferably less than 0.03, in particular less than 0.02, or less than 0.01 . In another embodiment, the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol is maintained at less than 0.002, or less than 0.001 and optionally at least 100 ppm.

[0276] The weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol in the reactant stream may be maintained at a certain level or less. Reducing the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol in the reactant stream beyond a certain point, however, reaches a point of rapidly diminishing return. Aldehydes, preferably prenal and / or formaldehyde removal involves additional equipment and operating costs. An economic balance must be taken between the improvement due to reducing the ratio and the cost of maintaining such a ratio. Hence, the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol is preferably not lower than 0.0005. In an alternative embodiment the weight ratio is not lower than 0.005.

[0277] It has been found that reactor clogging and pressure drop increase are significantly affected by the presence of aldehydes, preferably prenal and / or formaldehyde in the reactant stream. Catalyst-fouling reactions of condensation and polymerization are believed to be the principal reactions involved in carbon or coke formation on the catalyst. It is thought that this carbon formation involves thermal condensation of aldehydes, preferably prenal and / or formaldehyde or of aldehydes, preferably prenal and / or formaldehyde with the olefinic hydrocarbons isoprenol and (iso)prenal. In the presence of the catalyst, the primary condensation products tend to undergo dehydrogenation and polymerization type reactions and to settle on the catalyst and undergo further dehydrogenation and decomposition until carbonaceous deposits are formed.

[0278] The process of the invention according to the third aspect may satisfy the following condition 1), and preferably the following condition 2), or the process meets at least one of the following conditions 1) and 2):

[0279] 1) Step c-i) is characterized by maintaining in the reactant stream a weight ratio of aldehydes to isoprenol of less than 0.04.

[0280] 2) Step b) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reactant stream, and, optionally, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, based on the total weight of the reactant stream. Reducing the weight ratio of aldehydes, preferably prenal and / or formaldehyde to isoprenol in the reactant stream can be accomplished in several different ways. In an embodiment, aldehydes, preferably prenal and / or formaldehyde, are removed from the unreacted isoprenol stream prior to combining the unreacted isoprenol stream with the crude isoprenol stream.

[0281] In an embodiment, the unreacted isoprenol stream is combined with the crude isoprenol stream and aldehydes, preferably prenal and / or formaldehyde is removed from the combined stream.

[0282] Aldehydes, preferably prenal and / or formaldehyde, may be removed from isoprenol streams by a conventional separating method such as distillation, selective adsorption and or selective reaction, in particular by the purification process involving the pressure-swing distillation as described above.

[0283] Pre-Treating (Iso)Prenol by Nitrogen Removal Prior to Oxidative Deyhdrogenation or Oxidation

[0284] Prior to contacting with the at least one oxidative dehydrogenation catalyst or with the at least one oxidation catalyst, respectively, the (iso)prenol may advantageously be treated to remove organically bound nitrogen from the (iso)prenol by contacting the (iso)prenol with a weakly acidic solid adsorbent. In other words, the (iso)prenol may be depleted of organically bound nitrogen by this process.

[0285] The term "organically bound nitrogen” is intended to denote any compound containing at least one nitrogen atom directly bound to one or more carbon atoms. For example, such compounds containing at least one nitrogen atom may be selected from amines, such as ethylamine, trimethylamine, aniline, pyridine or piperidine. An amine particularly significant in practice is hexamethylenetetramine (urotropin). (Iso)prenol may comprise about 5 to 30 ppm of organically bound nitrogen.

[0286] The weakly acidic solid adsorbents have been found to be capable of adsorbing organically bound nitrogen in the presence of abundant (iso)prenol while not interfering with the reactive carbon-carbon double bond.

[0287] The weakly acidic adsorbent may include an adsorbent material having sufficient acidity to adsorb the organically bound nitrogen from the (iso)prenol. In an embodiment, the solid adsorbent is a crosslinked resin having phosphonic functional groups. Preferably, the resin polymer is a vinyl aromatic copolymer, preferably crosslinked polystyrene and more preferably a polystyrene divinylbenzene copolymer. Other polymers having a phosphonic functional group may also be used. Preferably, the crosslinked resin having phosphonic functional groups is of the macroporous type. A preferred solid adsorbent is Purolite S956.

[0288] The resin is typically used in bead form and loaded into a column. The (iso)prenol is passed through the column, contacting the resin beads. During contact, the organically bound nitrogen in the (iso)prenol reacts with the functional group and an exchange occurs where a proton is transferred to the nitrogen and an ionic bond is formed to the anionic site of the resin. Contact is maintained until a threshold level is reached i.e. the breakthrough concentration. At this breakthrough point, the process reaches an equilibrium where additional organically bound nitrogen cannot be removed effectively. The flow is halted and the column is backwashed with water, preferably deionized or softened water. By flowing in reverse, the resin is fluidized and solids captured by the beads are loosened and removed. In another embodiment, the solid adsorbent is a silica-alumina hydrate. Numerous silica-alumina catalyst compositions and processes for their preparation are described in the patent literature, see, e.g., US 4,499, 197.

[0289] Preferably, the alumina content of the silica-alumina hydrate is from about 10 to about 90 wt.-% of AI2O3. The preferred range of alumina content is from about 30 to about 70 wt.-% of AI2O3.

[0290] The introduction of silicon dioxide into aluminum oxide leads to the introduction of acidic centers. The number of acidic centers can be controlled by the amount of introduced silicon dioxide. The number of acidic centers increases with the amount of introduced silicon dioxide up to a maximum number of acidic centers, and decreases again with a further increasing amount of silicon dioxide after having reached the maximum number of acidic centers.

[0291] Examples of commercially available silica-alumina hydrates are Siral® available from Sasol Germany Gmbh, Hamburg, Germany. Siral® is based on orthorhombic aluminum oxide hydroxide (boehmite; AIOOH) and doped with SiO2. Various Siral® grades having different ratios of AI2O3 to SiO2 are available: Siral 1 (AI2O3 / SiO2= 99 / 1), Siral 5 (AI2O3 / SiO2= 95 / 5), Siral 10 (AI2O3 / SiO2= 90 / 10), Siral 20 (AI2O3 / SiO2= 80 / 20), Siral 28M (AI2O3 / SiO2= 72 / 28), Siral 30 (AI2O3 / SiO2= 70 / 30), Siral 40 (AI2O3 / SiO2= 60 / 40). Siral 40 is especially preferred.

[0292] In an embodiment, the (iso)prenol is passed over a bed of the weakly acidic solid adsorbent. Suitably, said step of "passing over a bed” denotes that a layer ("bed”) of the weakly acidic solid adsorbent is provided in a customary reaction vessel known to the skilled person which may preferably be equipped with a stirring device, e.g. in a stirred-tank reactor. The (iso)prenol is then introduced into the reaction vessel and guided through the same in a manner that it gets into contact with the weakly acidic solid adsorbent.

[0293] Alternatively, the weakly acidic solid adsorbent may be provided in a reaction tube, e.g. of a tubular reactor and the (iso)prenol then continuously flows through said reaction tube(s) while getting into contact with the weakly acidic solid adsorbent.

[0294] In an alternative embodiment, the (iso)prenol comprises, after contacting the alcohol stream with a weakly acidic solid adsorbent, less than 2 ppm of organically bound nitrogen. Herein, "ppm” denotes wt.-ppm of compounds incorporating organically bound nitrogen, relative to the total weight of the (iso)prenol.

[0295] For example, the content of organically bound nitrogen in the (iso)prenol may be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method with a chemiluminescence detector according to DIN 51444 may be used.

[0296] Fourth Aspect - Preparation of Citral

[0297] The fourth aspect of the invention relates to the preparation of 3,7-dimethyl-octa-2,6-dienal (citral), comprising the steps of a) providing isoprenol as described above, b) isomerizing the obtained isoprenol to obtain prenol as described above, and c) providing prenal by at least one of steps c-i) and c-ii) as described above, and further d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

[0298] In particular, 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by a process comprising the steps of:

[0299] - condensing the prenal with prenol in the presence of at least one catalyst in a reaction column while withdrawing an acetal fraction comprising the diprenyl acetal of prenal from the reaction column;

[0300] - subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one catalyst while withdrawing from the cleaving column a cleaving fraction containing at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene, and optionally containing citral; and

[0301] - reacting the cleaving fraction in a plug-flow type reactor to obtain citral.

[0302] The overall reaction sequence is illustrated by the reaction scheme below.

[0303] - prenol prenal diprenyl acetal 2,4,4-trimethyl-3- prenyl (3-methyl- formyl-1 ,5-hexadiene butadienyl) ether

[0304] The unsaturated acetal 3-methy l-2-butenal-dipreny I acetal (herein referred to as "diprenyl acetal of prenal” or "diprenyl acetal”) is formed from prenol and prenal using at least one catalyst. For this purpose, prenal may be reacted together with prenol in the presence of catalytic amounts of at least one acid and with separation of the water formed during the reaction in a reaction column.

[0305] It has been found that when the conversion rate of diprenyl acetal of prenal is driven to full conversion, the concentration of by-products increases sharply. Accordingly, it is preferred that the conversion rate of diprenyl acetal of prenal is maintained at above 90% and below 100%. Preferably, the conversion rate of diprenyl acetal of prenal in step b) is maintained equal to or below 99.5%, preferably equal to or below 99%, such as equal to or below 98%, or equal to or below 97.5%, or equal to or below 97%. Preferably, the conversion rate of diprenyl acetal of prenal is maintained above 91 %, such as above 92%, or above or 93%, or above 94%, or above 95%. In suitable embodiments, the conversion rate of diprenyl acetal of prenal in is above 94% and equal to or below 99%, such as above 95% and equal to or below 98%. Lower conversion rates will render the process economically unprofitable or will otherwise necessitate recovery and recycling of unreacted diprenyl acetal. Complete conversion is however undesirable as it results in a drop of yield of citral building blocks and increasing by-products-formation. The conversion rate is governed by various parameters including cleaving temperature, nature and concentration of the catalyst(s) and residence time in the cleaving column.

[0306] The resulting 3-methyl-2-butenal diprenyl acetal (diprenyl acetal) is cleaved in the presence of at least one catalyst in a cleaving column with elimination of 3-methyl-2-buten-1-ol (prenol) to give prenyl (3- methylbutadienyl) ether. Claisen rearrangement of the obtained prenyl (3-methylbutadienyl) ether yields 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene which subsequently undergoes Cope rearrangement yielding 3,7- dimethyl-2,6-octadienal (citral).

[0307] Cleaving is carried out in the presence of at least one catalyst, preferably an acid catalyst. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid. In an embodiment, the catalyst comprises phosphoric acid. In a preferred embodiment, the concentration of the phosphoric acid in the bottoms of the cleaving column is maintained above 100 ppm and below 1500 ppm, preferably above 200 ppm and below 1000 ppm. Higher concentrations of (acid) catalyst may result in reduced yields of citral building blocks.

[0308] Condensation of prenol with prenal is carried out in the presence of at least one catalyst, preferably an acid. The catalyst can be a single catalytic species or a combination of two or more different catalytic species. In an embodiment, the catalyst in is nitric acid. Preferably, the concentration of the nitric acid is below 500 ppm, more preferably in the range of from 100 to 300 ppm, relative to the total amount of the starting materials prenol and prenal. Lower amounts of (acid) catalyst may result in a low conversion in the reaction column. Higher amounts of (acid) catalyst may disadvantageously result in increased formation of by-products and in decreased selectivities.

[0309] Preferably, the acetal fraction is continuously subjected to cleaving conditions in a cleaving column. "Cleaving conditions” denotes reaction conditions selected such that the diprenyl acetal contained in the acetal fraction is cleaved to prenyl (3-methylbutadienyl) ether which may subsequently rearrange to 2,4,4- trimethyl-3-formyl-1 ,5-hexadiene and citral.

[0310] The acetal fraction comprises diprenyl acetal as a main constituent. The acetal fraction does not necessarily need to consist of pure diprenyl acetal, but may also comprise prenol, prenal and citral building blocks.

[0311] Cleaving is carried out in the presence of at least one catalyst, preferably at least one acid catalyst. Suitable acid catalysts are selected from non-volatile protic acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid.

[0312] Suitably, the continuous cleaving in the cleaving column may be carried out in the lower part or the sump of the distillation column acting as cleaving column. Preferably, the acetal fraction and / or the catalyst(s) are introduced into the lower part of the distillation column, into the sump of the distillation column or into the evaporator of the distillation column. If desired, a high-boiling inert compound can be introduced into the sump of the cleaving column in order to ensure a minimum filling level of the sump and the evaporator. Suitable high-boiling inert compounds are selected from liquid compounds which are inert under the reaction conditions and have a higher boiling point than citral and diprenyl acetal. For example, the high-boiling inert compounds may be selected from hydrocarbons such as tetradecane, pentadecane, hexadecane, octadecane, eicosane; or ethers such as diethylene glycol dibutyl ether; white oils; kerosene oils; or mixtures thereof.

[0313] Suitably, the distillation conditions are selected such that the diprenyl acetal is predominantly retained in the lower part or the sump of the distillation column. During the cleaving reaction, a cleaving fraction is continuously withdrawn from the cleaving column, the cleaving fraction containing at least one of prenyl (3- methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene, and optionally containing citral. For the ease of reference, prenyl (3-methyl-butadienyl) ether, 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene and citral are collectively referred to as "citral building blocks”. This is because the former are intermediates on the reaction route to citral and can be converted into citral in the subsequent passage through the plug-flow type reactor.

[0314] Additionally, the prenol formed during the cleaving reaction may be continuously removed from the reaction mixture, generally at the top of the cleaving column.

[0315] The cleaving fraction together with the formed prenol may be withdrawn at the top of the distillation column.

[0316] Alternatively and preferably, it is also possible to withdraw the cleaving fraction in liquid or vaporous form at a side draw of the distillation column.

[0317] The cleaving fraction may be reacted in a plug-flow type reactor to obtain citral. To this end, the cleaving fraction is guided through the plug-flow type reactor at a suitable temperature for carrying out the rearrangement reaction(s) yielding citral. By employing a combination of a highly back-mixed cleaving column and a plug-flow reactor, it is possible to increase the selectivity and the yield of the cleaving reaction. All of the catalyst(s) required for the cleaving reaction is / are preferably introduced into the cleaving column and preferably, no catalyst is introduced into the plug-flow reactor.

[0318] In an embodiment, prenol eliminated in the cleaving reaction is recycled to the condensation reaction. This allows for improved yields to be achieved in the process of the invention.

[0319] In particular, the inventive process may comprise recycling prenol obtained in step e) to step d); wherein the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene of the prenol recycled from step e) into step d) is controlled such that the concentration of 2, 4,4-tri methy l-3-formy 1-1 ,5-hexadiene in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal; and wherein the concentration of citral of the prenol recycled from step e) into step d) is controlled such that the concentration of citral in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal. Further Conversion to Menthol or Linalool

[0320] The thus obtained citral is a useful intermediate for, e.g., menthol or linalool.

[0321] Menthol may be prepared from citral via a process comprising the steps of

[0322] - catalytic hydrogenation of citral to obtain citronellal;

[0323] - cyclization of citronellal to obtain isopulegol in the presence of at least one acidic catalyst; and

[0324] - catalytic hydrogenation of isopulegol to obtain menthol.

[0325] The overall reaction sequence is illustrated by the reaction scheme below.

[0326] The hydrogenation of citral to obtain citronellal may be achieved by hydrogenation in the presence of a rhodium-phosphine catalyst.

[0327] The cyclization of citronellal to isopulegol may be achieved by cyclization in the presence of at least one Lewis-acidic aluminum-containing catalyst, such as a bis(diarylphenoxy)aluminum compound, which may be used in the presence of an auxiliary, such as a carboxylic anhydride. The isopulegol may be recovered from the catalyst-containing reaction product by distillative separation to give an isopulegol-enriched top product and an isopulegol-depleted bottom product. From the bottom product, the at least one catalyst may be regenerated. The isopulegol obtainable in this way by the cyclization of citronellal can be further purified by suitable separating and / or purification methods, in particular by crystallization, and be at least largely freed from undesired impurities or by-products.

[0328] The hydrogenation of isopulegol may be achieved by hydrogenation in the presence of at least one heterogeneous nickel-containing catalyst, preferably at least one heterogeneous nickel- and copper- containing catalyst.

[0329] Further details regarding the reaction sequence from citral to menthol may be found in US 2013 / 46118 A1 , which is incorporated by reference herein.

[0330] In one aspect, the invention thus relates to an improved process for the preparation of menthol by producing citral using the above processes and then producing menthol from the citral. Menthol may be prepared as described herein or by other methods known in the art.

[0331] Linalool may be prepared from citral via a process comprising catalytic hydrogenation of citral to obtain nerol and / or geraniol, and isomerization thereof. The hydrogenation of citral to obtain nerol and / or geraniol may be achieved by hydrogenation in the presence of at least one supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably at least one ruthenium catalyst supported on carbon black.

[0332] The isomerization of nerol and / or geraniol to obtain linalool may be achieved by isomerization in the presence of at least one tungsten catalyst, in particular a dioxotungsten (VI) complex. Further details regarding the isomerization of nerol and / or geraniol may be found in US 7,126,033 B2.

[0333] In one aspect, the invention thus relates to an improved process for the preparation of linalool by producing citral using the above processes and then producing linalool from the citral. Linalool may be prepared as described herein or by other methods known in the art. Further Conversion to Vitamin A

[0334] The obtained citral is also a useful intermediate for the synthesis of vitamin A.

[0335] Vitamin A may be prepared from citral via the reaction sequence illustrated by the reaction scheme below.

[0336] Citral (I) can be converted into pseudoionone (II) in reaction step A. Said pseudoionone can be reacted in synthetic step B to obtain p-ionone (III), which is further transformed into p-vinylionol of formula (IV). Phosphorylation of p-vinylionol of formula (IV) can yield the C15-salt of formula (V), which upon reacting it with the C5-acetate of formula (VI) can yield vitamin A acetate of formula (VII).

[0337] Reaction step A can be realized in the presence of a base selected form metal hydroxides, in particular alkali metal hydroxides and earth alkali metal hydroxides. Said base acts as a catalyst and can be added in one or several portions as e.g. disclosed in EP 0 062 291 A1 and WO 2004 / 041764 A1.

[0338] Cyclisation of pseudoionone (II) into p-ionone (III) in step B is realized in the presence of an acid, preferably in the presence of a mineral acid. A method of realizing step B is disclosed in EP 0 133 668 A2 and in US 3,840,601.

[0339] The vinylionol (IV) can be obtained by reacting the compound of formula (III) with a Grignard reagent.

[0340] The Ci5-salt of formula (V) can be obtained from vinylionol (IV) in the presence of a phosphine. A suitable method of obtaining compound (V) is disclosed in WO 2005 / 058811 A1.

[0341] Vitamin A acetate (VII) can finally be obtained by subjecting the Ci5-salt of formula (V) to Wittig conditions in the presence of the acetate of formula (VI). Details of such a Wittig reaction are disclosed in WO 2005 058811 A1.

[0342] The invention is moreover illustrated by the attached drawings and the examples that follow.

[0343] Fig. 1 shows conversion and selectivity over time on stream of an isoprenol oxidative dehydrogenation reaction.

[0344] Fig. 2 schematically depicts Schemes A to D with different options to accommodate the recycling stream of unreacted isoprenol.

[0345] Fig. 3A indicates the relative isoprenol conversion during the first and second phase of Experiment A as described below, based on online gas chromatography measurements.

[0346] Fig. 3B indicates the relative isoprenol conversion during the first and second phase of Experiment B as described below, based on online gas chromatography measurements.

[0347] Fig. 4 shows the prenol selectivity relative to the isoprenol conversion for Experiment A as described below, including the extrapolation of the observed selectivities for Phase 1 (data points between 52.5% and 55% conversion) and Phase 2 (data points between 56% and 57% conversion) as indicated by the dotted line.

[0348] In accordance with this invention, maintaining in the reactant stream a certain weight ratio of formaldehyde to isoprenol can be accomplished in a number of ways as illustrated in Fig. 2. Scheme A shows how the recycling stream b is typically incorporated in a prior art process. Scheme B shows how the recycling stream b can be present through the isoprenol separation steps of an existing isoprenol manufacturing unit. Scheme C shows the total reactor feed stream going through an additional impurity separation step, for example a distillation step. Scheme D shows only the recycling stream going through an additional impurity separation step, for example a distillation step.

[0349] Examples

[0350] Materials Fusel oil was procured from Crop Energies AG. The composition of the fusel oil used was as follows:

[0351] Ethanol 2.15 %

[0352] Isobutanol 13.38%

[0353] Butanol 0.98 %

[0354] Isoamyl alcohol 77.23%

[0355] 2-Methylbutanol 3.55%

[0356] Furfural 0.21 %

[0357] Hexanol 0.21 %

[0358] Isoamyl acetate 0.13%

[0359] Benzaldehyde 0.01 %

[0360] Ethyl hexanoate 0.09%

[0361] Phenyl ethanol 0.20%

[0362] Phenyl ethyl acetate 0.11 %

[0363] Ethyl decanoate 0.10%

[0364] Ethyl laurate 0.05%

[0365] Hexanoic acid ethyl ester 0.31 %

[0366] Unknown impurities 1.29%

[0367] Likewise, isoamyl alcohol isolated from fusel oil (pure isoamyl alcohol) was procured from Crop Energies AG.

[0368] The reaction pathway from isoamyl alcohol to isobutylene is depicted below. Example 1 : Oxidation of Pure Isoamyl Alcohol Using O2

[0369] Isoamyl alcohol (10 g) was mixed with water (90 g). The catalyst Pt / C (3 g, 5 wt.-% Pt on C, 58 wt.-% water) was added, and the mixture was stirred at 80°C for 24h under a dioxygen atmosphere. After cooling down to ambient temperature, the mixture was diluted with ethyl acetate, filtered and the aqueous phase was extracted with ethyl acetate. The solvent was removed in vacuo to yield isovaleric acid. Yield: 85%

[0370] Example 2: Oxidation of Pure Isoamyl Alcohol Using HNO3

[0371] Nitric acid (55 g, 65%) was cooled down to 0°C using an ice bath. While stirring under nitrogen, isoamyl alcohol (10 g) was added dropwise over 45 min. After complete addition, water (50 mL) was added, and the mixture was stirred for 10 min. The biphasic mixture was diluted with dichloromethane (100 mL) and stirred for 30 min. The phases were separated, and the organic phase was dried over sodium sulphate. The product isovaleric acid was isolated by distillation (190°C oil bath, 120°C top temperature). Yield: 94%

[0372] Example 3: Oxidation of Fusel Oil (containing 70% Isoamyl Alcohol) Using HNO3

[0373] Nitric acid (57 g, 65%) was tempered to 35°C in an oil batch. While stirring under nitrogen, fusel oil (14.5 g, containing 70% isoamyl alcohol) was added dropwise over 45 min. After complete addition, water (50 mL) was added, and the mixture was stirred for 10 min. The biphasic mixture was diluted with dichloromethane (100 mL) and water (100 mL) and stirred for 30 min. The phases were separated, and the organic phase was dried over sodium sulphate. The product isovaleric acid was isolated by distillation (200°C oil bath, 170°C top temperature). Yield: 84%

[0374] Example 4: Decarboxylation of Isovaleric Acid to Isobutylene

[0375] Isovaleric acid (1.87g) was mixed with PdCh (1.76 mol-%) and dis[(diphenylphosphino) phenyl]ether (DPE- Phos, 3.53 mol-%) under nitrogen atmosphere. DMPU (N,N'-Dimethyl propylene urea, 36 g) was added and while stirring, acetic acid anhydride (2.53 g) was added slowly. After stirring for 15 min at ambient temperature, the reaction mixture was heated to 140°C for 4h. While heating, PdCh dissolved completely which was indicated by a yellow coloration of the solution.

[0376] The end of the reaction was indicated by a red coloration of the reaction mixture. The formed isobutylene was collected using a gas collector for analytics and condensation in toluene. Yield: 85-87%

[0377] Example 5

[0378] Isoprenol oxidation is investigated in a mini plant reactor. The catalyst bed consists of a 30 cm packing of silver coated (5 wt%) steatite spheres (diameter 2 mm) in a stainless-steel tube (inner diameter 12mm). The cooling jacket temperature is kept at 380°C. An isoprenol load of 300 g / h was adopted with 30 g / h of water and 92 L / h of air. The reactant stream is quenched through a water cooler. The condensate is analysed offline by gas chromatography. The uncondensed gas stream is analyzed online by gas chromatography. Conversion and (iso)prenal selectivity are calculated using both analyses. An experiment with isoprenol having formaldehyde concentrations increasing from 0, 2, 3 to 4 wt% was carried out.

[0379] Fig. 1 shows conversion and selectivity over time on stream. At day 5, formaldehyde concentration was increased to 2 wt%, on day 14 to 3 wt%. Regeneration cycles are marked with a plus sign at 50% conversion. With increasing formaldehyde concentration, drops in selectivity are observed. At initial time on stream, a selectivity drop of about 1 .5 percentage points is observed per wt% of formaldehyde contained in the feed. The regeneration cycles were more often required with increasing formaldehyde concentration. A still higher frequency of regeneration cycles was necessary at 4 wt% of formaldehyde.

[0380] In accordance with this invention, maintaining in the reactant stream a certain concentration of aldehydes can be accomplished in a number of ways as illustrated in Fig. 2. Scheme A shows how the recycling stream b is typically incorporated in a prior art process. Scheme B shows how the recycling stream b can be resent through the isoprenol separation steps of an existing isoprenol manufacturing unit. Scheme C shows the total reactor feed stream going through an additional impurity separation step, for example a distillation step. Scheme D shows only the recycling stream going through an additional impurity separation step, for example a distillation step.

[0381] Experiments A and B

[0382] Isoprenol batches having varying concentrations of aldehydes, i.e. prenal and formaldehyde were subjected to isomerization. The compositions of the batches of isoprenol, as determined via gas chromatography (GC) analysis, are shown below, except for the amount of prenal and formaldehyde of batch 1 , which was provided via quality control from the isoprenol production.

[0383] Isomerization of the isoprenol batches was conducted as follows: A double-walled glass reactor was filled with 100 mL (48.8 g) of an isomerization catalyst (fixed bed catalyst containing Pd / Se supported on SIO2). The top of the reactor was connected to a phase separator, the bottom of which phase separator was connected to the reactor bottom via an external circulation pump. The reactor temperature was regulated via an external oil jacket. Before each experiment, the catalyst was dried under a nitrogen flow at 120 °C for 16 h, reduced using a 1 :1 (vokvol) mixture of hydrogen and nitrogen at 120°C for 2 h, and then flushed with nitrogen at 70 °C for 16 h.

[0384] During start-up, a feed of the respective isoprenol batch was pumped at 100 g / h from the bottom to the top of the reactor, in the same direction of flow as co-fed hydrogen (6 Nl / h, 1 .2 bara), released through a bubble frit at the bottom of the reactor. A product stream from the top of the reactor was fed to the phase separator. After a 24 h start-up phase, sufficient product had accumulated in the phase separator to begin recycling through the circulation pump. The feed supply was reduced to 70 g / h and the liquid phase of the product stream obtained in the phase separator was recycled to the bottoms of the reactor via at a rate of 150 g / h. At the same time, the reactor pressure was increased to 1 .5 bara and the temperature raised to 80°C. These conditions were maintained for the remainder of the experiment.

[0385] In the course of the experiments, liquid samples were taken from the product stream taken from the top of the reactor and analysed by offline GC. Moreover, online GC measurements were continually performed.

[0386] Two feed-switch experiments were conducted, as indicated below, whereby the reactor was supplied with one batch for about 6 to 10 days (first phase) before switching to the other batch and measuring until a new steady-state was reached (about 2 to 5 days, second phase). In this way, it was possible to directly compare the performance of the two batches.

[0387] Conversion of isoprenol (%) was calculated as 100 x ([wt.-% isoprenol in feed] - [wt.-% isoprenol in product]) I [wt.-% isoprenol in feed]. Selectivity for prenol (%) was calculated as 100 x ([wt.-% prenol in product] - [wt. -% prenol in feed]) I ([wt.-% isoprenol in feed] - [wt.-% isoprenol in product]). The prenol yield was calculated as (prenol selectivity x isoprenol conversion) / 100.

[0388] The mean averages of isoprenol conversion as determined via offline GC are shown in the table below.

[0389] It was found that when high-purity isoprenol of Batch 2 was used as the feed, the catalyst was more active and the conversion of isoprenol was higher than when feeding Batch 1 . Moreover, the prenol yield higher. The higher conversion is also evident from the results of the online GC measurements, as indicated in Fig. 3A and 3B. Said figures also show that the rate of deactivation was greater when feeding Batch 1, suggesting that the catalyst lost its activity more rapidly in the presence of this batch of isoprenol.

[0390] Further, it was found that the prenol selectivity when using high-purity isoprenol of Batch 2, when extrapolated relative to the isoprenol conversion, is higher than for the isoprenol of Batch 1 , as depicted in Fig. 4.

[0391] Fig. 4 shows the Prenol selectivity as a function of isoprenol conversion, as determined by offline GC data for Experiment A. Square dots represent the isoprenol batch 1 (normal quality isoprenol); circle dots the isoprenol batch 2 (the high-purity isoprenol). The offline GC analyses performed when feeding the isoprenol batch 1 (red points in Fig. 4) can be well fitted to a linear trendline. This trendline shows that, under these conditions, as isoprenol conversion decreased with extended function of time on stream (TOS), the prenol selectivity increased at a rate of 2.5% of selectivity per 1 % of conversion. Extrapolating this trendline to the higher conversions achieved with the high-purity feed reveals a predicted selectivity towards prenol of -86%.

[0392] However, prenol selectivities of >89% were recorded with the isoprenol batch 2, which is the high-purity feed (circle dots in Fig. 4), showing that a more selective catalysis was achieved after switching to the higher purity feed.

[0393] Interestingly, using batch 2 with lower aldehyde values compared to batch 1 during the start-up phase resulted in a higher activity level of the catalyst right from the start, and hence the conversion rate after approximately 24 h and before the first phase and during the first phase was higher when batch 2 was used from the start, compared to the experiment when batch 1 was used from the start through the start-up and first phase.

Claims

Claims1 . Process for the preparation of isoprenol, comprising the steps of: a-i) preparing isobutylene by oxidizing isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; and a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol.

2. Process for the preparation of prenol, comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; and b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen.

3. Process for the preparation of prenal and / or isoprenal, comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; b) optionally, isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; and c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; and c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase.

4. Process for the preparation of 3, 7-di methy l-octa-2, 6-dienal (citral) comprising the steps of: a) providing isoprenol by a-i) and a-ii): a-i) preparing isobutylene by oxidizing renewably-sourced isoamyl alcohol to isovaleric acid, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene; a-ii) reacting at least one formaldehyde source and the isobutylene obtained in step a-i) to obtain isoprenol; b) isomerizing isoprenol obtained in step a) to obtain prenol by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous isomerization catalyst, preferably in the presence of hydrogen; c) providing prenal by at least one of c-i) and c-ii): c-i) subjecting isoprenol obtained in step a) to oxidative dehydrogenation so as to obtain prenal and / or isoprenal by bringing a reactant stream comprising isoprenol into contact with at least one heterogeneous oxidative dehydrogenation catalyst, in the presence of molecular oxygen, and optionally isomerizing at least part of the isoprenal to prenal; c-ii) oxidizing prenol obtained in step b) so as to obtain prenal by bringing a reactant stream comprising prenol into contact with at least one oxidant and at least one oxidation catalyst, preferably in the presence of a liquid phase; d) condensing prenol obtained in step b) with prenal obtained in step c) to obtain diprenyl acetal of prenal; and e) subjecting diprenyl acetal of prenal obtained in step d) to cleaving conditions to obtain citral via prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene.

5. The process according to any one of the preceding claims, wherein step a-i) comprises feeding an alcohol feedstock comprising isoamyl alcohol at a concentration of 60 to 99 wt.-% into a reaction vessel, oxidizing the alcohol feedstock to an oxidation product comprising isovaleric acid, isolating isovaleric acid from the oxidation product, and subjecting the isovaleric acid to oxidative decarboxylation so as to obtain isobutylene.

6. The process according to any one of the preceding claims, wherein the renewably-sourced isoamyl alcohol has a pMC greater than 90, as measured according to ASTM D6866.

7. The process according to claim 6, wherein the renewably-sourced isoamyl alcohol is obtained from natural sources or from a fermentation process.

8. The process according to any one of the preceding claims, wherein step a-i) comprises oxidizing the renewably-sourced isoamyl alcohol in the presence of an oxidizing agent selected from O2, hydrogen peroxide and nitric acid.

9. The process according to any one of the preceding claims, wherein step a-i) comprises subjecting the isovaleric acid to oxidative decarboxylation in the presence of a homogenous catalyst, in particular a homogenous catalyst comprising at least one metal or its salt or complex, and a ligand.

10. The process according to claim 9, wherein the at least one metal is preferably selected from nickel, palladium, or platinum.11 . The process according to any one of the preceding claims, wherein step a-ii) comprises introducing the at least one formaldehyde source and isobutylene into a reactor and reacting the formaldehyde source and isobutylene under supercritical conditions.

12. The process according to any one of the preceding claims, wherein isoprenol obtained in step a) is purified by subjecting a stream of crude isoprenol containing isoprenol, water and formaldehyde, or an isoprenol containing fraction thereof, to distillation in a low-boiler separation tower operated at a pressure of 2 bara or higher, preferably 2.5 bara or higher, to obtain a distillate stream containing aqueous formaldehyde and a bottoms stream containing isoprenol essentially free of formaldehyde.

13. The process according to any one of claims 2 to 12, wherein step b) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reactant stream, wherein the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, with respect to the total weight of the reactant stream.

14. The process according to any one of claims 3 to 13, wherein in step c-i) the reactant stream is gaseous and at least one heterogeneous oxidative dehydrogenation catalyst is a silver-containing heterogeneous oxidative dehydrogenation catalyst.

15. The process according to any one of claims 3 to 14, wherein step c-i) is characterized by maintaining in the reactant stream a weight ratio of aldehydes to isoprenol of less than 0.04, and optionally, step b) is characterized by maintaining in the reactant stream a concentration of aldehydes of less than 0.5% by weight, preferably less than 0.4% by weight, in particular less than 0.3% by weight, or less than 0.25% by weight, based on the total weight of the reactant stream, and, optionally, the concentration of aldehydes in the reactant stream is not lower than 10 ppm, preferably not lower than 25 ppm, in particular not lower than 50 ppm, or not lower than 100 ppm, based on the total weight of the reactant stream.

16. The process according to any one of claims 4 to 15, wherein step d) comprises continuously condensing prenol with prenal in the presence of at least one condensation catalyst in a reaction column while continuously withdrawing an acetal fraction comprising diprenyl acetal of prenal from the reaction column.

17. The process according to any one of claims 4 to 16, wherein step e) comprises continuously subjecting the acetal fraction in a cleaving column to cleaving conditions in the presence of at least one cleaving catalyst while continuously withdrawing from the cleaving column a cleaving fraction containing at least one of prenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1 ,5- hexadiene, and optionally containing citral; and reacting the cleaving fraction in a plug-flow type reactor to obtain citral.

18. The process according to claim 17, comprising recycling prenol obtained in step e) to step d); wherein the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene of the prenol recycled from step e) into step d) is controlled such that the concentration of 2,4,4-trimethyl-3-formyl-1 ,5-hexadiene in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal; and wherein the concentration of citral of the prenol recycled from step e) into step d) is controlled such that the concentration of citral in step d) is below 1 wt.-%, relative to the total weight of prenol and prenal.

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