Purification of ethylenically unsaturated alcohol streams, preparation of ethylenically unsaturated aldehydes, in particular pentenal, and compounds derived therefrom

Abstract

Organic bound nitrogen is removed from an ethylenically unsaturated alcohol stream by contacting the alcohol stream with a weakly acidic solid adsorbent. Trace amounts of organically bound nitrogen tend to poisoning the oxidation catalyst during a subsequent oxidation 5 process using the ethylenically unsaturated alcohol stream.

Description

[0001] The present invention relates to a process for purifying ethylenically unsaturated alcohol streams, and more particularly for removing organically bound nitrogen from ethylenically unsaturated alcohol streams, and to a process for preparing ethylenically unsaturated aldehydes from ethylenically unsaturated alcohols, particularly prenals from prenols, in the presence of an oxidant and a catalytically active metal catalyst. The invention further relates to a process for preparing derived 3,7-dimethyl-octa-2,6-dienal (citral), menthol and linalool.

[0002] Ethylenically unsaturated aldehydes such as pentenal (3-methyl-2-butene-1-al) are important chemical intermediates, for example, for the preparation of terpene-based fragrances such as citral and for the preparation of vitamins such as vitamin E. Such ethylenically unsaturated aldehydes are therefore of great technical and economic importance. The literature provides various examples for the preparation of ethylenically unsaturated aldehydes.

[0003] WO 2009 / 106621 A1 describes a process for producing olefinically unsaturated carbonyl compounds, for example by oxidative dehydrogenation of prenol and / or isoprenol. The reaction is carried out at a temperature in the range of 50° C. to 240° C. in an oxygen-containing atmosphere over a supported gold-containing catalyst. For example, p-xylene can serve as solvent.

[0004] WO 2018 / 002040 A1 describes a process for preparing α,β-unsaturated aldehydes by oxidizing alcohols using oxygen or air as an oxidant in the presence of a catalyst comprising platinum on a support. The reaction is carried out in the presence of a liquid phase containing at least 25 wt.-% of water, based on the total weight of the liquid phase.

[0005] WO 2018 / 172110 A1 describes a method for preparing α,β-unsaturated aldehydes (e.g., pentenal) by oxidizing alcohols (e.g., pentenols) in the presence of a liquid phase. The liquid phase contains 0.1 wt.-% to less than 25 wt.-% of water and at least 25 wt.-% of alcohols (e.g., pentenols) and α,β-unsaturated aldehydes (e.g., pentenal). Oxygen and / or hydrogen peroxide can be used as oxidants. Preferably, the oxidation is carried out in the presence of a catalyst of a catalytically active metal selected from platinum, palladium and gold contained on a carrier.

[0006] WO 2019 / 121012 A1 describes a method for preparing α,β-unsaturated aldehydes (e.g., pentenal) by oxidizing an alcohol (e.g., pentenol) in the presence of a liquid phase containing at least 25 wt.-% of water. Oxygen is used as an oxidant. The oxidation is carried out in the presence of a catalyst comprising a catalytically active metal on a support, wherein the catalytically active metal is mainly located in the shell of the catalyst. Preferably, the catalytically active metal is selected from platinum, palladium and gold.

[0007] WO 2019 / 121011 A1 describes a method for preparing pentenal (3-methylbut-2-ene-1-aldehyde) from dimethylvinylmethanol (2-methylbut-3-ene-2-ol) which may optionally further contain pentenol. It is assumed that 2-methyl-3-butene-2-ol isomerized to 3-methyl-2-butene-1-ol, which is then oxidized to 3-methyl-2-butene-1-aldehyde. The method is carried out in the presence of an oxidant and a catalyst. The catalyst comprises a catalytically active metal preferably on a carrier, wherein the catalytically active material is preferably selected from platinum, palladium and gold. In addition, the method is carried out at a pH of less than 7, wherein the pH can be adjusted by adding, for example, an acid or a strongly acidic cation exchanger. Suitably, the method is carried out in the presence of a liquid phase comprising at least 25wt.-% of water.

[0008] These catalysts, which have a noble metal deposited on a support, exhibit good alcohol conversions and excellent selectivity and, depending on the exact nature of the catalyst, may exhibit long lifetimes. However, over time, these catalysts may lose some activity and may occasionally become sufficiently deactivated to render use of the catalyst impractical. At this stage of partial or complete deactivation, the catalyst must be regenerated or replaced.

[0009] The present invention is based on the insight that trace amounts of organically bound nitrogen, for example in the form of amines, tend to poison oxidation catalysts. Isopentenol is produced by chemical condensation of isobutylene and formaldehyde, resulting in further isomerization of isopentenol to prenol. The reaction of isobutylene and formaldehyde can be carried out in the presence of a catalyst such as an amine base, for example hexamethylenetetramine (urotropine), as described, for example, in US 3,574,773. The amine base also traps the formic acid formed by the disproportionation of formaldehyde. Therefore, the obtained prenol and / or isopentenol stream can, for example, contain organically bound nitrogen impurities in an amount of several ppm.

[0010] However, it is not easy to reliably purify ethylenically unsaturated alcohols from traces of organically bound nitrogen on an industrial scale. In general, such ethylenically unsaturated alcohols are reactive compounds. The reactivity can lead to difficulties, for example, when performing purification steps for removing impurities from such ethylenically unsaturated alcohols. For example, isoprene is suspected of forming tertiary carbocations that can cause undesirable side reactions when treated with solid adsorbents. In addition, nitrogen compounds, which are present only in low concentrations, compete with large amounts of alcohol for adsorption sites.

[0011] DE 199 10 504 A1 describes a method for reducing the amine content (e.g. monomethylamine) of amine-contaminated N-substituted lactams (e.g. N-methyl-2-pyrrolidone) by treating the contaminated N-substituted lactams with acidic macroporous cation exchangers. At the same time, the metal cations contained as impurities in the N-substituted lactams may be depleted.

[0012] Therefore, there remains a need for a viable process for removing organically bound nitrogen from feed streams, such as ethylenically unsaturated alcohol streams. Additionally, there remains a need for improved processes for preparing ethylenically unsaturated aldehydes.

[0013] The objects of the present invention are solved by a process for removing organically bound nitrogen from an ethylenically unsaturated alcohol stream by contacting the stream with a weakly acidic solid adsorbent.

[0014] By the process of the present invention, the ethylenically unsaturated alcohol stream is depleted of organically bound nitrogen. Thus, the present invention allows overcoming the above problems in subsequent processes using the ethylenically unsaturated alcohol stream. For example, such subsequent processes include oxidation of the ethylenically unsaturated alcohol.

[0015] Typically, the alcohol stream is contacted with the weakly acidic solid adsorbent in the substantial absence of an oxidant. Typically, the alcohol stream is contacted with the weakly acidic solid adsorbent in the absence of a catalytically active metal catalyst.

[0016] In this context, the term "organically bound nitrogen" is intended to mean 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 of particular importance in practice is hexamethylenetetramine (urotropine). An ethylenically unsaturated alcohol stream, such as a prenol or isopentenol stream, subjected to the process of the invention may contain from about 5 to 30 ppm of organically bound nitrogen.

[0017] The "ethylenically unsaturated alcohol stream" of the present invention comprises alcohols having a carbon-carbon double bond. For example, the ethylenically unsaturated alcohol stream may be an α,β-unsaturated alcohol or a β,γ-unsaturated alcohol.

[0018] Examples of suitable ethylenically unsaturated alcohols include 3-buten-1-ol, 3-penten-1-ol, 3-methylbut-3-en-1-ol, 3-methylbut-2-en-1-ol, 3-hexen-1-ol, 3-methylpent-3-en-1-ol, 3-ethylbut-3-en-1-ol, 2-methylhex-1-en-5-ol, 2-methylhex-1-en-4-ol, 2-phenylbut-1-en-4-ol, 4-methylpent-3-en-1-ol and 2-cyclohexylbut-1-en-4-ol, and mixtures thereof.

[0019] In a preferred embodiment, the ethylenically unsaturated alcohol is selected from 3-methylbut-2-en-1-ol (pentenol), 3-methylbut-3-en-1-ol (isopentenol), and mixtures thereof, and is preferably 3-methylbut-2-en-1-ol (pentenol).

[0020] According to the invention, the alcohol stream is contacted with a weakly acidic solid adsorbent. It has been found that, in the context of the present invention, such solid adsorbents are capable of adsorbing organically bound nitrogen in the presence of large amounts of alcohol while not disturbing reactive carbon-carbon double bonds.

[0021] Weakly acidic adsorbents may include adsorbent materials having sufficient acidity to adsorb organically bound nitrogen from a stream.

[0022] In an embodiment, the solid adsorbent is a cross-linked resin having phosphonic acid functional groups.

[0023] Preferably, the resin polymer is a vinyl aromatic copolymer, preferably crosslinked polystyrene and more preferably polystyrene divinylbenzene copolymer.Other polymers having phosphonic acid functional groups may also be used.

[0024] Preferably, the crosslinked resin having phosphonic acid functional groups is of the macroporous type.

[0025] The preferred solid adsorbent is Purolite S956.

[0026] Resin is typically used in bead form and is loaded into the column. The flow is passed through the column, contacting the resin beads. During contact, the organically bound nitrogen in the flow reacts with the functional group and exchanges, wherein protons are transferred to the nitrogen and ionic bonds are formed to the anionic sites of the resin. Contact is maintained until a threshold level is reached, i.e., a breakthrough concentration. At this breakthrough point, the process reaches equilibrium, wherein additional organically bound nitrogen cannot be effectively removed. The flow is stopped and the column is backwashed with water, preferably deionized water or softened water. By reverse flow, the resin is fluidized, and the solids captured by the beads are loosened and removed.

[0027] In another embodiment, the solid adsorbent is a silica-alumina hydrate.Many silica-alumina catalyst compositions and methods for their preparation are described in the patent literature, see for example US 4,499,197.

[0028] Preferably, the alumina content of the silica-alumina hydrate is from about 10 wt.-% to about 90 wt.-% Al2O3. A preferred range of alumina contents is from about 30 wt.-% to about 70 wt.-% Al2O3.

[0029] The introduction of silica into alumina results in the introduction of acidic centers. The number of acidic centers can be controlled by the amount of silica introduced. The number of acidic centers increases with the amount of silica introduced up to a maximum number of acidic centers and decreases again with a further increase in the amount of silica after the maximum number of acidic centers has been reached.

[0030] An example of a commercially available silica-alumina hydrate is available from Sasol Germany Gmbh, Hamburg, Germany. Based on orthorhombic aluminum oxide hydroxide (boehmite; AlOOH) and doped with SiO2. Available are various Grades: Siral 1 (Al2O3 / SiO2=99 / 1), Siral 5 (Al2O3 / SiO2=95 / 5), Siral 10 (Al2O3 / SiO2=90 / 10), Siral 20 (Al2O3 / SiO2=80 / 20), Siral 28M (Al2O3 / SiO2=72 / 28), Siral 30 (Al2O3 / SiO2=70 / 30), Siral 40 (Al2O3 / SiO2=60 / 40). Siral 40 is particularly preferred.

[0031] In a suitable determination method, the solid adsorbent is characterized by ammonia temperature programmed desorption (TPAD) performed on an apparatus constructed by Raczek Analytical Technique GmbH in Hannover (Germany). For this purpose, the samples are conditioned at a temperature of 400° C. in a helium stream. Afterwards, a mixture of 10% NH3 / He is passed through the sample at 70° C. Physically adsorbed ammonia is removed by flushing with helium at 120° C. for 2 h. Chemically adsorbed ammonia is removed by passing helium through a sample heated to 400° C. at a linear heating rate of 15° C. / min. The integrated value of the peak of the amount of ammonia desorbed from the solid adsorbent is reported as the amount of acidic centers.

[0032] In an example, a stream of ethylenically unsaturated alcohol is passed through a bed of a weakly acidic solid adsorbent.

[0033] Suitably, the step of "passing through a bed" means providing a layer ("bed") of weakly acidic solid adsorbent in a conventional reaction vessel known to the skilled person, for example in a stirred tank reactor, which may preferably be equipped with a stirring device. The ethylenically unsaturated alcohol stream is then introduced into the reaction vessel and directed through the reaction vessel in such a way that it comes into contact with the weakly acidic solid adsorbent.

[0034] Alternatively, the weakly acidic solid adsorbent may be provided in a reaction tube, such as a reaction tube of a tubular reactor, and then the ethylenically unsaturated alcohol stream continuously flows through the reaction tube while contacting with the weakly acidic solid adsorbent.

[0035] In an embodiment, the ethylenically unsaturated alcohol stream contains less than 2 ppm of organically bound nitrogen after contacting the alcohol stream with the weakly acidic solid adsorbent.Herein, "ppm" means wt.-ppm of compounds incorporating organically bound nitrogen relative to the total weight of the ethylenically unsaturated alcohol stream.

[0036] Suitably, the content of organically bound nitrogen in the ethylenically unsaturated alcohol stream may be determined by Kjeldahl analysis. Alternatively, an oxidative combustion method according to DIN 51444 with a chemiluminescence detector may be used.

[0037] The present invention further relates to a process for preparing ethylenically unsaturated aldehydes from ethylenically unsaturated alcohols in the presence of an oxidant and a catalytically active metal catalyst, wherein the ethylenically unsaturated alcohol stream is treated by a process as described above before contacting with the catalytically active metal catalyst.

[0038] The process for preparing ethylenically unsaturated aldehydes is referred to herein as the "alcohol dehydrogenation process."

[0039] In other words, the ethylenically unsaturated alcohol stream is treated by a process for removing organically bound nitrogen from the ethylenically unsaturated alcohol stream by contacting the alcohol stream with a weakly acidic solid adsorbent. After said treatment, the treated ethylenically unsaturated alcohol stream is then reacted in the presence of an oxidant and a catalytically active metal catalyst, as will be described hereinafter.

[0040] This procedure allows to increase the catalyst activity and catalyst life. In addition, the conversion and selectivity of the subsequent oxidation reaction are enhanced.

[0041] The alcohol dehydrogenation process is carried out in the presence of an oxidant. In a preferred embodiment, the oxidant is selected from oxygen and hydrogen peroxide.

[0042] The oxidant may be a gas mixture containing oxygen in a content ranging from 2% to 50% by volume, preferably from 3% to 40% by volume, more preferably from 7% to 18% by volume. In addition to oxygen, the gas mixture may further contain a diluent gas. Suitably, the diluent gas is an inert gas such as nitrogen, argon, carbon dioxide, etc. For example, the oxidant may be air as an easily available oxidizing medium.

[0043] Preferably, undiluted oxygen is used.

[0044] The alcohol dehydrogenation process is carried out in the presence of a catalytically active metal catalyst.

[0045] Suitably, the catalytically active metal may be selected from metals of Groups 8, 9, 10 and 11 of the Periodic Table of the Elements according to IUPAC. Elements of Groups 8, 9, 10 and 11 include iron, cobalt, nickel, copper, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold.

[0046] In a preferred embodiment, the catalytically active metal is selected from platinum, palladium and gold. Platinum is particularly preferred.

[0047] The catalytically active metals may be used in any form (eg unsupported or on a support).

[0048] The catalytically active metals can be used in unsupported form, for example as powder, mesh, sponge, foam or net.

[0049] In a preferred embodiment, the catalytically active metal is deposited on a support, which is preferably selected from carbonaceous materials and oxidic materials.

[0050] For example, the support may include alumina, silica, magnesia, or hydrotalcite.

[0051] In principle, suitable support materials are alkaline, acidic or amphoteric support materials. It has been found that alkaline materials are particularly suitable. It has been found that aluminum oxide, alkaline aluminosilicates or hydrotalcites, preferably aluminum oxide and hydrotalcites, are advantageous in some cases. Support materials based on carbon (e.g. various charcoals) are also suitable.

[0052] In a preferred embodiment, the catalytically active metal is platinum on a support, wherein the support is selected from carbonaceous materials and oxidic materials, and wherein the oxide is selected from oxides of: Al, Ce, Zr, Ti, V, Cr, Zn and Mg, preferably Al, Ce, Zr and Ti.

[0053] In particularly preferred embodiments, the catalyst is selected from platinum / carbon (Pt / C) and platinum / alumina (Pt / Al2O3).

[0054] In case the catalytically active metal is deposited on a support, the total weight of the catalyst herein shall be defined as the sum of the weight of the catalytically active metal and the weight of the support.

[0055] The content of catalytically active metals in the catalyst is not subject to any particular restrictions per se and may range from 0.1 to 20 wt.-%, preferably 0.1 to 15 wt.-%, more preferably 0.5 to 10 wt.-%, based on the total weight of the catalyst.

[0056] Optionally, the catalyst may include one or more promoters that enhance the activity of the catalytically active metal. Examples of such promoters are bismuth, antimony, lead, cadmium, tin or tellurium. For example, the promoter may be present on or in the support, or may be added separately to the process.

[0057] In an embodiment, the process is carried out at a temperature in the range from 1°C to 250°C, preferably 5°C to 150°C, more preferably 20°C to 100°C, most preferably 25°C to 80°C, particularly 30°C to 70°C, in particular 35°C to 50°C.

[0058] In an embodiment, the reaction is carried out in a liquid phase containing at least 25 wt.-%, preferably at least 30 wt.-%, more preferably at least 40 wt.-%, in particular at least 50 wt.-% water, determined at a temperature of 20° C. and a pressure of 1 absolute bar and based on the total weight of the liquid phase.

[0059] Optionally, the liquid phase may further comprise one or more solvents. As used herein, the term "solvent" refers to any component other than reactants, products, oxidant, or water, which is liquid at a temperature of 20°C and a pressure of 1 absolute bar.

[0060] The liquid phase may comprise less than 70 wt.-%, preferably less than 60 wt.-%, more preferably less than 50 wt.-%, most preferably less than 40 wt.-%, in particular less than 30 wt.-%, in particular less than 20 wt.-%, in particular less than 10 wt.-%, determined at a temperature of 20° C. and a pressure of 1 absolute bar and based on the total weight of the liquid phase.

[0061] Preferably, the solvent is an aprotic organic solvent. Available aprotic organic solvents are selected from

[0062] - aliphatic hydrocarbons, such as hexane, heptane, octane, nonane, decane and petroleum ether;

[0063] - aromatic hydrocarbons, such as benzene, toluene, xylene and mesitylene;

[0064] - aliphatic ethers, such as 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (diglyme), diethyl ether, dipropyl ether, methyl isobutyl ether, tert-butyl methyl ether, tert-butyl ethyl ether, dimethoxymethane, diethoxymethane, dimethylene glycol dimethyl ether, dimethylene glycol diethyl ether, trimethylene glycol dimethyl ether, trimethylene glycol diethyl ether and tetramethylene glycol dimethyl ether;

[0065] - cycloaliphatic hydrocarbons, such as cyclohexane and cycloheptane;

[0066] - cycloaliphatic C3-C6-ethers, such as tetrahydrofuran (THF), tetrahydropyran, 2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxolane, 1,4-dioxane and 1,3,5-trioxane;

[0067] - short-chain ketones such as acetone, ethyl methyl ketone and isobutyl methyl ketone;

[0068] - C3-C6-esters, such as methyl acetate, ethyl acetate, methyl propionate, dimethyl oxalate, methyl methoxyacetate, ethylene carbonate, propylene carbonate, ethylene glycol diacetate and diethylene glycol diacetate;

[0069] - C3-C6-amides, such as dimethylformamide (DMF), dimethylacetamide and N-methylpyrrolidone (NMP);

[0070] - sulfoxides, such as dimethyl sulfoxide (DMSO);

[0071] - C3-C6-nitriles, such as acetonitrile and propionitrile; and

[0072] - mixtures thereof.

[0073] Preferably, the solvent has a boiling point above 50°C, more preferably in the range from 50 to 200°C, most preferably 65 to 180°C, especially 80 to 160°C.

[0074] In an embodiment, the liquid phase contains 1 to 75 wt.-%, preferably 1 to 50 wt.-%, more preferably 2 to 45 wt.-%, most preferably 3 to 40 wt.-% of ethylenically unsaturated alcohol, based on the total amount of the liquid phase.

[0075] By this measure, it is ensured that a sufficient amount of starting material (ie ethylenically unsaturated alcohol) is present in the reaction mixture of the alcohol dehydrogenation process.

[0076] The alcohol dehydrogenation process can be carried out in a reaction vessel that is customary for such reactions. For example, the alcohol dehydrogenation process can be carried out in a continuous, semi-batch or batch mode. Suitable reaction vessels are known to those skilled in the art.

[0077] The alcohol dehydrogenation process can be carried out at atmospheric pressure or under pressure. However, preferably, the alcohol dehydrogenation process is carried out at a pressure in the range of greater than 1 to 15 absolute bar, more preferably 1 to 10 absolute bar.

[0078] In the case of using oxygen as the oxidant, the alcohol dehydrogenation process can be suitably carried out at an oxygen partial pressure in the range of from 0.1 to 15 absolute, preferably 0.2 to 10 absolute, more preferably 0.2 to 8 absolute, most preferably 0.2 to 5 absolute, in particular 1 to 3 absolute, in particular 1 to 2.5 absolute, in particular 1.2 to 2 absolute.

[0079] The crude product obtained from the alcohol dehydrogenation process can be subjected to conventional purification measures known to those skilled in the art, including distillation or chromatography, or a combination thereof. Suitable distillation apparatus for purifying one or more products include, for example, distillation columns, such as tray columns optionally equipped with bubble cap trays, sieve plates, sieve trays, packing or filler materials, or rotating belt columns, such as thin film evaporators, falling film evaporators, forced circulation evaporators, wiped film (Sambay) evaporators, etc., and combinations thereof.

[0080] A variety of ethylenically unsaturated aldehydes can be produced by the present invention. In one embodiment, the ethylenically unsaturated alcohol is 3-methylbut-2-en-1-ol (pentenol), and the ethylenically unsaturated aldehyde produced is pentenal (3-methyl-2-butene-1-al).

[0081] Prenol which can be used as a starting material in the present invention can be obtained by reacting a formaldehyde source and isobutylene to obtain 3-methylbut-3-en-1-ol (isopentenol), and subjecting the obtained isopentenol to isomerization.

[0082] In one embodiment, isopentenol is obtained by mixing a formaldehyde source and isobutylene and injecting them into a reactor through at least one nozzle and reacting the formaldehyde source and isobutylene under supercritical conditions. 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° C. to 290° C., and at an absolute pressure of at least 200 bar. The reaction of isobutylene and formaldehyde can be carried out in the presence of a catalyst such as an amine base, for example hexamethylenetetramine (urotropine).

[0083] The formaldehyde may be provided as a liquid, for example as a solution of paraformaldehyde.Preferably, the formaldehyde source is an aqueous formaldehyde solution.

[0084] Further details about reacting a formaldehyde source and isobutylene to obtain prenol can be found in WO 2020 / 049111 A1.

[0085] It is not easy to separate prenol from formaldehyde. This difficulty is caused by the fact that both monomeric formaldehyde (as well as polymeric formaldehyde) forms hydrates with water and hemiformals with prenol. Hydrates and hemiformals with different degrees of polymerization of formaldehyde have mixing boiling points.

[0086] However, it has been found that formaldehyde can be almost completely separated from prenol by distillation at a temperature at which hemiformal is cleaved into formaldehyde and prenol, so that formaldehyde can be easily separated from prenol.

[0087] Therefore, crude isopentenol can be purified by subjecting a crude isopentenol stream containing isopentenol, water and formaldehyde or an isopentenol containing fractions thereof to distillation in a low-boiler separation column operated at a pressure of 2 absolute bar or more, preferably 2.5 absolute bar or more, to obtain a distillate stream containing aqueous formaldehyde and a bottom stream containing isopentenol substantially free of formaldehyde.

[0088] In particular, it has been found that formaldehyde can be separated almost completely from prenol and concentrated aqueous formaldehyde suitable for recycling to the prenol synthesis can be obtained in a distillation train comprising a first distillation carried out at a temperature at which the equilibrium shifts towards the hemiformal of formaldehyde and prenol, so that substantially all the formaldehyde remains in the bottoms of the distillation, and a second distillation carried out at a temperature at which the hemiformal is cracked into formaldehyde and prenol, so that the formaldehyde can be easily separated from the prenol.

[0089] In order to allow the first distillation to be carried out at a temperature lower than the prenol-formaldehyde dissociation temperature and the second distillation to be carried out at a temperature higher than the prenol-formaldehyde dissociation temperature, two low-boiling-point separation towers operating at different pressures are envisaged. Therefore, under the relatively low pressure existing in the first low-boiling-point separation tower, a first distillate containing water and low-boiling-points substantially free of formaldehyde is obtained. Under the relatively high pressure existing in the second low-boiling-point separation tower, almost all formaldehyde is separated from the prenol. Therefore, the method allows the prenol substantially free of formaldehyde to be obtained.

[0090] Therefore, in a more preferred embodiment, the purification method comprises

[0091] (i) directing the stream of crude isopentenol to a first low boiler separation column operated at a pressure of 1.5 absolute bar or less to obtain a first bottom stream containing isopentenol and formaldehyde and a first distillate stream containing water and low boilers;

[0092] (ii) directing the first bottom stream to a second low boiler separation column operated at a pressure of 2 absolute bar or higher to obtain a second distillate stream containing aqueous formaldehyde and a second bottom stream containing isopentenol; and

[0093] (iii) the second bottom stream is directed to a refining column to obtain pure isopentanol as a distillate stream, and a bottom stream containing high boiling substances.

[0094] The second distillate stream constitutes concentrated aqueous formaldehyde suitable for recycling to the prenol synthesis.

[0095] Suitably, the second low boiler separation column is operated at a pressure of 2.5 absolute bar or higher, preferably 2.8 absolute bar or higher, most preferably 2.9 absolute bar or higher. The bottoms temperature of the second low boiler separation column is preferably in the range of 160°C to 200°C, more preferably 170°C to 185°C, most preferably 175°C to 180°C. The top temperature of the second low boiler separation column is preferably in the range of 115°C to 160°C, more preferably 125°C to 145°C.

[0096] In a particularly preferred embodiment, the second low boiler separation column is operated at a pressure in the range of 2.9 to 3.5 absolute bar, a bottoms temperature in the range of 175 to 180°C and a top temperature in the range of 130 to 140°C.

[0097] Further information on methods for recovering essentially formaldehyde-free prenyl alcohol can be found in WO 2022 / 189652 A1.

[0098] The prenol obtained is subjected to catalytic isomerization in order to obtain prenol.

[0099] The isomerization of isopentenol to 3-methyl-2-butene-1-ol (pentenol) can be carried out on a supported noble metal, preferably in the presence of hydrogen. Preferred catalysts are fixed bed catalysts containing palladium and selenium or tellurium or a mixture of selenium and tellurium supported on silicon dioxide. Isomerization is carried out at a temperature of 50 ℃ to 150 ℃ to produce a reaction mixture of prenol and isopentenol. Isopentenol can be recycled. Additional details are provided in WO 2008 / 037693.

[0100] The pentenals produced by the present invention are useful intermediates in the preparation of citral. Citral is a mixture of the isomeric compounds neral and geranal.

[0101] 3,7-Dimethyl-octa-2,6-dienal (citral) can be prepared by obtaining pentenal by the method as described above, which further comprises the steps of: condensing pentenal with pentenol to obtain the diprenyl acetal of pentenal; and subjecting the diprenyl acetal of pentenal to cleavage conditions to obtain citral via pentenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

[0102] In particular, 3,7-dimethyl-octa-2,6-dienal (citral) can be prepared by a method comprising the following steps:

[0103] a) condensing pentenals with pentenols in a reaction column in the presence of at least one catalyst, while simultaneously removing an acetal fraction comprising the dipentenyl acetal of pentenals from the reaction column;

[0104] b) subjecting the acetal fraction to cracking conditions in a cracking column in the presence of at least one catalyst, while withdrawing from the cracking column a cracked fraction containing at least one of pentenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene and optionally citral; and

[0105] c) reacting the cleavage fraction in a plug flow type reactor to obtain citral.

[0106] The overall reaction sequence is illustrated by the following reaction scheme.

[0107]

[0108] In step a), unsaturated acetal 3-methyl-2-butenal-dipentenyl acetal (referred to herein as "dipentenyl acetal of pentenal" or "dipentenyl acetal") is formed from pentenol and pentenal using a catalyst. For this purpose, pentenal is reacted together with pentenol in the presence of a catalytic amount of acid, and the water formed during the reaction is separated in a reaction column. In step b), the resulting 3-methyl-2-butenal dipentenyl acetal (dipentenyl acetal) of step a) is cracked in a cracking column in the presence of a catalyst, and 3-methyl-2-butene-1-ol (pentenol) is removed to give pentenyl (3-methylbutadienyl) ether. Claisen rearrangement of the pentenyl (3-methylbutadienyl) ether obtained gives 2,4,4-trimethyl-3-formyl-1,5-hexadiene, which subsequently undergoes a Cope rearrangement to give 3,7-dimethyl-2,6-octadienal (citral).

[0109] Step a) is carried out in the presence of a catalyst, preferably an acid. In an embodiment, the catalyst in step a) is nitric acid.

[0110] Preferably, in step b), the acetal fraction is continuously subjected to cleavage conditions in a cleavage column. "Cleavage conditions" means reaction conditions selected such that the dipentenyl acetal contained in the acetal fraction is cleaved to pentenyl (3-methylbutadienyl) ether (which can subsequently rearrange to 2,4,4-trimethyl-3-formyl-1,5-hexadiene and citral).

[0111] The acetal fraction comprises dipentenyl acetal as main component.The acetal fraction does not necessarily need to consist of pure dipentenyl acetal but may also comprise pentenol, pentenal and citral building blocks.

[0112] Step b) is carried out in the presence of a catalyst, preferably an acid catalyst. Suitable acid catalysts are selected from non-volatile proton acids such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid.

[0113] Suitably, the continuous cleavage in the cracking column of step b) can be carried out in the lower part or sump of a distillation column acting as a cracking column. Preferably, the acetal fraction and / or the catalyst 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.

[0114] If desired, a high boiling point inert compound can be introduced into the sump of the cracking column in order to ensure a minimum filling level of the sump and the evaporator. Suitable high boiling point inert compounds are selected from liquid compounds that are inert under the reaction conditions and have a higher boiling point than citral and dipentenyl acetal. For example, the high boiling point inert compound can be selected from hydrocarbons, such as tetradecane, pentadecane, hexadecane, octadecane, eicosane; or ethers, such as diethylene glycol dibutyl ether; white oil; kerosene; or mixtures thereof.

[0115] Suitably, distillation conditions are selected so that dipentenyl acetal is mainly retained in the lower part of the distillation column or in the storage tank. During the cleavage reaction, the cracked fraction is continuously discharged from the cracking column, and the cracked fraction contains at least one of pentenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene and optionally contains citral. For ease of reference, pentenyl (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 is an intermediate on the reaction pathway leading to citral and can be converted into citral in the subsequent step c).

[0116] Additionally, the pentenols formed during the cleavage reaction in step b) are continuously removed from the reaction mixture, typically at the top of the cleavage column.

[0117] The cleavage fraction together with the pentenols formed can be discharged at the top of the distillation column.

[0118] Alternatively and preferably, the cracked fraction can also be discharged in liquid or vaporous form at a side draw of the distillation column.

[0119] In step c), the cracked fraction is reacted in a plug flow reactor to obtain citral. To this end, the cracked fraction is guided through a plug flow reactor at a suitable temperature for carrying out the rearrangement reaction to obtain citral. By adopting a combination of a highly back-mixed cracking tower and a plug flow reactor, the selectivity and yield of the cracking reaction can be increased. In step b), preferably all the catalyst required for the cracking reaction is introduced into the cracking tower, and preferably, no catalyst is introduced into the plug flow reactor.

[0120] In an embodiment, the pentenol removed in step b) is recycled to step a). This allows increased yields to be achieved in the process of the invention.

[0121] Therefore, in one aspect, the present invention relates to an improved process for preparing citral (3,7-dimethyl-octa-2,6-dienal), comprising the following steps

[0122] A) reacting a formaldehyde source with isobutylene to obtain 3-methylbut-3-en-1-ol (isopentenol), and subjecting at least a portion of the isopentenol obtained to isomerization to obtain prenol;

[0123] B) preparing pentenal (3-methylbut-2-en-1-al) from 3-methylbut-2-en-1-ol (pentenol) using the method described above, and / or preparing pentenal via isopentenal (3-methylbut-3-en-1-al) prepared from isopentenol using the method described above; and

[0124] C) condensing pentenal with pentenol to obtain dipentenyl acetal of pentenal; and subjecting the dipentenyl acetal of pentenal to cleavage conditions to obtain citral via pentenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

[0125] Step A can be carried out as described above or by other methods known in the art, preferably via distillation at a temperature at which hemiformal is cleaved into formaldehyde and prenol so that formaldehyde can be easily separated from prenol, and more preferably by subjecting a stream of crude prenol containing prenol, water and formaldehyde or a prenol containing fraction thereof to distillation in a low boiler separation column operated at a pressure of 2 absolute bar or more, preferably 2.5 absolute bar or more, to obtain a distillate stream containing aqueous formaldehyde and a bottom stream containing prenol substantially free of formaldehyde.

[0126] Step B comprises the oxidative dehydrogenation of prenol and / or isopentenol.Isopentenol is converted with a catalytically active metal catalyst to form a reaction mixture of 3-methylbut-3-ene-1-aldehyde and 3-methylbut-2-ene-1-aldehyde.Then, the former isomer can be isomerized under base catalysis to give the desired 3-methylbut-2-ene-1-aldehyde.

[0127] Step C can be performed as described above, for example via steps a) to c).

[0128] The citral thus obtained is a useful intermediate for example for menthol or linalool.

[0129] Menthol can be prepared from citral via a process comprising the following steps

[0130] - catalytic hydrogenation of citral to obtain citronellal;

[0131] - cyclizing citronellal in the presence of an acidic catalyst to obtain isopulegol; and

[0132] - Catalytic hydrogenation of isopulegol to obtain menthol.

[0133] The overall reaction sequence is illustrated by the following reaction scheme.

[0134]

[0135] The hydrogenation of citral to obtain citronellal can be achieved by carrying out the hydrogenation in the presence of a rhodium-phosphine catalyst.

[0136] The cyclization of citronellal to isopulegol can be achieved by carrying out the cyclization in the presence of a Lewis acidic aluminum-containing catalyst such as a bis(diarylphenoxy)aluminum compound, which catalyst can be used in the presence of an auxiliary such as a carboxylic anhydride. Isopulegol can be recovered from the reaction product containing the catalyst by distillation separation to give an isopulegol-rich overhead product and an isopulegol-depleted bottom product. The catalyst can be regenerated from the bottom product. The isopulegol obtainable in this way by the cyclization of citronellal can be further purified by suitable separation and / or purification methods (in particular by crystallization) and is at least largely free of undesirable impurities or by-products.

[0137] The hydrogenation of isopulegol can be achieved by carrying out the hydrogenation in the presence of a heterogeneous nickel-containing catalyst, preferably a heterogeneous nickel- and copper-containing catalyst.

[0138] Further details on the reaction sequence from citral to menthol can be found in US 2013 / 46118 A1 (which is incorporated herein by reference).

[0139] Thus, in one aspect, the invention relates to an improved process for preparing menthol by producing citral using the above process and then producing menthol from the citral.Menthol may be prepared as described herein or by other processes known in the art.

[0140] Linalool can be prepared from citral via a process comprising catalytic hydrogenation of citral to obtain nerol and / or geraniol and isomerization thereof.

[0141] The hydrogenation of citral to obtain nerol and / or geraniol can be achieved by hydrogenation in the presence of a supported ruthenium, rhodium, osmium, iridium or platinum catalyst, preferably a ruthenium catalyst supported on carbon black.

[0142] Isomerization of nerol and / or geraniol to obtain linalool can be achieved by carrying out the isomerization in the presence of a tungsten catalyst, in particular a tungsten(VI) dioxide complex. Further details on the isomerization of nerol and / or geraniol can be found in US 7,126,033 B2.

[0143] Thus, in one aspect, the present invention relates to an improved method for preparing linalool by producing citral using the above method and then producing linalool from citral. Linalool may be prepared as described herein or by other methods known in the art.

[0144] The present invention can be further explained and illustrated based on the following figures and examples. However, it will be understood that these figures and examples are included for illustrative purposes only and are not intended to limit the scope of the present invention in any way.

[0145] Figure 1 The oxygen consumption in the liquid phase oxidation of pentenol is shown.

[0146] Figure 2 The conversion and selectivity of the oxidation of pentenols to pentenals are shown.

[0147] Examples

[0148] Example 1 – Removal of Hexamethylenetetramine from a Pentenol Stream Using Different Adsorbents

[0149] Tested the ability of different adsorbents to remove urotropine from pentenols. For this purpose, the relevant adsorbent was mixed with water at ambient temperature and stirred for 15 min for washing. Subsequently, water-wet adsorbent and a small amount of pentenols (nitrogen-free) were stirred for 5 min. Then pentenol-wet adsorbent was transferred to the tower, and untreated pentenol feed was passed through the tower (from top to bottom). The results are shown in Table 1.

[0150] Table 1: Removal of hexamethylenetetramine from pentenol streams using different adsorbents

[0151]

[0152] [1] Weakly acidic macroporous cation exchanger based on acrylic acid

[0153] [2]Polyacrylic acid porous weak acid cation exchange resin

[0154] [3] Polyacrylic acid macroporous weak acid cation exchange resin

[0155] [4] Acid activated bentonite

[0156] [5] Zeolite (aluminosilicate mineral)

[0157] [6] Alumina-based adsorbents

[0158] [7] Activated carbon

[0159] [8] Coconut-based granular activated carbon

[0160] [9] Macroporous cross-linked polymers with phosphonic acid functionality

[0161]

[10] Weakly acidic silica-alumina, pore size 40 μm, high level of surface acidity

[0162] *Comparative Examples

[0163] These examples demonstrate that Purolite S956 and Siral 40 show remarkable behavior in removing hexamethylenetetramine ("organically bound nitrogen") contained in a pentenol stream ("ethylenically unsaturated alcohol stream").

[0164] Example 2 – Removal of Hexamethylenetetramine from a Pentenol Stream Using Siral 40

[0165] The pentenol stream having an initial content of 35 ppm of urotropine was passed through a column of 1 g of Siral 40 (15 mm inner diameter, 250 mm length) at a flow rate as indicated in Table 2. Every 30 min, a sample was taken and the urotropine content of the treated pentenol was determined by an oxidative combustion method using a chemiluminescence detector. The results are shown in Table 2.

[0166] Table 2: Removal of hexamethylenetetramine from pentenol stream using Siral 40

[0167]

[0168]

[0169] As can be seen from the results in Table 2, the process of the invention allows for a significant removal of hexamethylenetetramine from pentenol using Siral 40 as adsorbent without breakthrough during the duration of the test.

[0170] Example 3 - Oxygen consumption in liquid phase oxidation of pentenols

[0171] The reactor was charged with 103 g of Pt / Al2O3 catalyst (0.9 wt.-% Pt). A feed stream containing 97 g / h of pentenol and 7 g / h of water was metered through the reactor using a metering pump. The pentenol specification (urotropine content) was carried out throughout the reaction stages 1 to 7 (see Figure 1 ) changes, as shown in Table 3. The reaction was carried out for 59 days at an inlet pressure of 3.9 absolute bar and an outlet pressure of 3.6 absolute bar, and at an inlet temperature of 45° C. and an outlet temperature of 48° C. A flow meter was used to quantitatively determine the oxygen consumption by reducing the oxygen flow rate (in L / h). The oxygen consumption is a measure of the catalyst activity.

[0172] Table 3: Pentenol specifications throughout reaction stages 1 to 7

[0173]

[0174] [1] As shown in Example 2,

[0175] *Comparative Examples

[0176] Table 3 and Figure 1 The data show that the decrease in oxygen consumption is less significant when using the pentenol stream pretreated in the adsorption step according to the invention (stage 2 and stages 4 to 7) compared to the unpretreated pentenol stream (stage 1 and stage 3).

[0177] Example 4 - Selectivity for Pentenals

[0178] A fixed bed reactor containing a heterogeneous catalyst was filled with an unsaturated alcohol; i) treated with an adsorbent for nitrogen removal or ii) filled with an untreated feed containing up to 35 ppm nitrogen. In both cases, the reaction was run at 80°C at 1.5 bar for several hours in order to compare the results with respect to catalyst activity and selectivity. The results are plotted in Figure 2 In. From Figure 2 It can be seen that when the organically bound nitrogen is removed from the pentenol feed stream prior to the reaction, the conversion to pentenals is higher and the selectivity is higher compared to the untreated pentenol feed stream.

Claims

1. A process for removing organically bound nitrogen from an ethylenically unsaturated alcohol stream by contacting the alcohol stream with a weakly acidic solid adsorbent.

2. The method according to claim 1, wherein: The solid adsorbent is a cross-linked resin having phosphonic acid functional groups.

3. The method according to claim 1, wherein: The solid adsorbent is silica-alumina hydrate.

4. A method according to any one of the preceding claims, wherein: The ethylenically unsaturated alcohol stream is passed through a bed of the weakly acidic solid adsorbent.

5. A method according to any one of the preceding claims, wherein: After contacting the alcohol stream with the weakly acidic solid adsorbent, the ethylenically unsaturated alcohol stream contains less than 2 ppm organically bound nitrogen.

6. A process for preparing ethylenically unsaturated aldehydes from ethylenically unsaturated alcohols in the presence of an oxidizing agent and a catalytically active metal catalyst, wherein the ethylenically unsaturated alcohol stream is treated by a process according to any one of the preceding claims before contacting with the catalytically active metal catalyst.

7. The method according to claim 6, wherein: The catalytically active metal is selected from platinum, palladium and gold.

8. The method according to claim 6 or 7, wherein: The catalytically active metal is deposited on a support.

9. The method according to claim 8, wherein: The support is selected from carbonaceous materials and oxidic materials.

10. The method according to any one of claims 6 to 9, wherein: The oxidizing agent is selected from oxygen and hydrogen peroxide.

11. The process according to any one of claims 6 to 10, which is carried out at a temperature in the range of 1°C to 250°C.

12. A method according to any one of the preceding claims, wherein: The reaction is carried out in the presence of a liquid phase which contains at least 25 wt.-% water.

13. The method according to claim 12, wherein: The liquid phase contains 1 wt.-% to 75 wt.-% of the ethylenically unsaturated alcohol, based on the total amount of the liquid phase.

14. The method according to any one of claims 6 to 13, wherein: The ethylenically unsaturated alcohol is 3-methylbut-2-en-1-ol (pentenol), and the ethylenically unsaturated aldehyde is 3-methylbut-2-en-1-al (pentenal).

15. The method according to claim 14, wherein: Prenol is obtained by reacting a formaldehyde source and isobutylene to obtain 3-methylbut-3-en-1-ol (isopentenol), and subjecting at least a portion of the obtained isopentenol to isomerization.

16. A method for preparing 3,7-dimethyl-octa-2,6-dienal (citral), the method comprising obtaining pentenal by the method according to claim 14 or 15, further comprising the following steps: condensing the pentenal with pentenol to obtain a dipentenyl acetal of pentenal; and subjecting the dipentenyl acetal of pentenal to cleavage conditions to obtain citral via pentenyl (3-methyl-butadienyl) ether and 2,4,4-trimethyl-3-formyl-1,5-hexadiene.

17. A method for preparing menthol, comprising preparing citral by the method according to claim 16, and reacting the citral to obtain menthol.

18. A method for preparing linalool, comprising preparing citral by the method according to claim 16, and reacting the citral to obtain linalool.

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