Liquid-phase process for the production of acrylic acid esters from glyceric acid

Abstract

The present invention relates to the use of solid Rhenium (Re) catalysts for the conversion of bio-based glyceric acid (2,3-dihydroxypropanoic acid), glycerate esters or salts into acrylic acid and propanoic acid and their esters in a batch stirred slurry reactor or continuous fixed-bed reactor. Glyceric acid (derivatives) in the total amount of 0.001 – 20 wt.-% (with regards to the total amount of reaction mixture) is dissolved in a primary or secondary alcohol, e.g. methanol, and a solid, Re-containing catalyst, e.g. mono-, bi- or multi-metallic Re-based cataysts on a support material such as activated carbon, is added in amounts of 0.001 – 10 wt.-%. Furthermore, the reaction mixture may consist of up to 50 wt.-% other solvents, e.g. water, and may contain an additional acid co-catalyst, e.g. an acidic polymer resin and / or an adsorber, e.g. molecular sieve, in amounts up to 50 wt.-% (with regards to the total amount of reaction mixture). This reaction mixture is proceeded at a temperatures in the range of 50 – 250 °C under inert, e.g. pure N2, or reducing, e.g. H2-containing, atmosphere for 0.1 – 500 h to yield dehydroxylated products (acrylic acid, propionic acid and their esters). Further esterification converts the remaining free carboxylic acids into the respective esters.

Description

[0001] Liquid-phase process for the production of acrylic acid esters from glyceric acid

[0002] Description

[0003] FIELD OF THE INVENTION

[0004] The present disclosure relates generally but not exclusively to a novel catalytic process for the production of acrylic acid, propanoic acid and their esters from bio-based glyceric acid (derivatives) in an alcohol over a solid catalyst in an inert atmosphere without further reducing agents. The catalysts required for this transformation consists of the element Rhenium (Re) in metallic or oxidized form and a solid support material (e.g. carbon, SiC>2 or TiC>2), thus being insoluble in the liquid reaction medium.

[0005] This is the first report of using the Re-catalysed deoxydehydration reaction over heterogeneous catalysts for the selective dehydroxylation of glyceric acid (derivatives). Since glyceric acid can be obtained by oxidation of glycerol, this process can be part of a novel route to obtain acrylic acid (esters) from the bio-based feedstock glycerol. The dehydroxylation reaction is the key step in this transformation cascade and is implemented as a heterogeneously catalysed process.

[0006] This invention is characterized by (I) the use of a solid catalyst, which facilitates catalyst recycling and the development of a continuous process, (II) the omission of additional reducing agents, in particular hazardous H2 gas, and (III) the combination of dehydroxylation and esterification, which results in the formation of facilely recoverable and industrially demanded acrylate esters as main products.

[0007] BACKGROUND OF THE INVENTION

[0008] Acrylic acid and its esters are invaluable building blocks of the chemical industry, in particular for the production of polymers. To satisfy this growing demand without having to rely on the conventional, fossil resource-based propylene oxidation pathway, a plethora of investigations have been conducted and reported regarding the use of more renewable alternatives, such as the production of acrylic acid from the bio-based feedstock glycerol as shown in recent review works [1-3], A well-investigated and -patented route is the acid-catalysed dehydration of glycerol to acrolein [4,5] and its subsequent catalytic oxidation [6], Moreover, the catalytic deoxydehydration, preferably over Re catalysts, has been used to convert the feedstock glycerol into allyl alcohol [7-10], which can be converted into acrylic acid by catalytic oxidation [11 ,12], Acrylate esters are obtained via catalytic esterification of acrylic acid

[0013] ,

[0009] Herein, we report a novel, alternative process to acrylic acid (esters) based on the Recatalysed dehydroxylation of glyceric acid or glyceric acid derivatives. The feedstock glyceric acid has been shown to be available from glycerol through selective oxidation [14-20], preferably over solid catalysts, or through biotechnological processes [21-24], Thus, the pathway via glyceric acid presents different and promising pathway to acrylates.

[0010] At present, only one alternative process for this particular transformation of glyceric acid into acrylic acid is found in existing literature, a patent application by Weiran et al.

[0025] , In this two- step process, glyceric acid reacts with HI in the presence of a metal-based catalyst under H2 atmosphere. The initially formed 3-iodopropionic acid is extracted and through base- or acid- catalysed hydrolysis transformed into acrylic acid. In total, >99 % yield are reported. However, there is still considerably benefit in finding a more efficient and environmentally benign alternative.

[0011] Compared to Weiran et al.’s process

[0025] , several distinct differences and improvements are apparent of the invention disclosed here. Most crucially, the novel process does not require an intermediate extraction step with organic solvent (n-hexanol) and stoichiometric or excess amounts of halogen compounds (HI). In addition, no hazardous H2 gas is required in the new process. Furthermore, the combination of dehydroxylation and esterification in a single-step, one-pot process prevents intermolecular side reactions, likely to occur at high reactant concentrations, and results in the formation of acrylate esters, which are more volatile and can be isolated more easily than acrylic acid.

[0012] The invention disclosed here is the first report of applying heterogeneous Re-containing catalysts for the selective deoxydehydration of glyceric acid, glycerate salts or glycerate esters to form acrylic acid and acrylic acid esters as primary products. Previously, this type of reaction has primarily been reported for different reaction systems, feed compounds and products, including the aforementioned production on allyl alcohol from glycerol [7-10], the production of muconates and adipates from aldaric acids [26-28] or others [29-33],

[0013] The most similar study to the disclosed invention, which is mentioning the deoxydehydration of glyceric acid, was reported by Boucher-Jacobs and Nicholas

[0034] , However, it is based entirely on the homogeneously catalysed reaction using methyltrioxo rhenium as the catalyst in combination with, importantly, indoline as reducing agent. After 4 h at 170 °C in n-butanol as the solvent, 35 % of n-butyl acrylate were obtained with the reducing agent indoline being oxidised in parallel. When the inorganic salt ammonium perrhenate was used, no desired catalytic reaction was observed. Overall, the authors conclude that the yield of deoxydehydration products is low compared to other reactants in their study, which is probably due to unspecified competing reactions also in the case of methyltrioxo rhenium as the catalyst

[0034] , Thus, the invention disclosed here marks a significant improvement compared to this previous, homogeneously catalysed attempt. Not only are yields of deoxydehydration products in the range of up to 92% observed under suitable conditions. Moreover, the use of a range of inexpensive alcohols compared to indoline as reducing agent is of particular advantage as well as the fact that a series of different acrylate esters is accessible. The most crucial advantage of our novel process is the use of supported Re catalysts which is the basis for a heterogeneously catalysed reaction. On the one hand, this makes separation of catalyst and product more facile. On the other hand, this allows for catalyst recycling as well as the development of a continuously operated process.

[0014] Beyond the deoxydehydration of glyceric acid (derivatives) to unsaturated acrylic acid (derivatives), the process also allows for control of the subsequent double bond hydrogenation reaction, which results in the formation of propanoic acid and methyl propanoate. By choosing a suitable catalyst and reaction conditions, product selectivity can be steered, similar to the previously patented and reported analogous process of adipic acid production by Hocevar et al. [26,27],

[0015] Moreover, through the esterification reaction of the carboxylic function of the glyceric acid, acrylic acid ad propanoic acid, which is occurring in parallel to the dehydroxylation reaction, the respective esters can be obtained. Using an additional acidic co-catalyst can further enhance the yield of acrylate esters as primary target products of this one-pot process. Besides the industrial demand for acrylate esters, their formation is also favourable in terms of product separation, which is, especially in case of methyl acrylate, facilitated by the low boiling point of the product. Moreover, esterification protects the carboxylic functionality by preventing intermolecular side reactions.

[0016] SUMMARY OF THE INVENTION

[0017] The present invention relates to a novel and sustainable catalytic route from glyceric acid (derivatives), obtained from the bio-chemical glycerol, to industrially demanded, sustainable dehydroxylation products, in particular acrylic acid, propanoic acid and their respective esters. This process is the first to implement the Re-catalysed deoxydehydration reaction over heterogeneous catalysts, which does not require hazardous reducing agents such as H2, in this particular reaction, as shown in the previous section. Moreover, by controlling the catalytic hydrogenation of the unsaturated deoxydehydration products and the parallel esterification with the alcohol solvent in acrylic acid esters or propanoic acid esters can be formed highly selectively and in high yields.

[0018] The crucial step of the invention relating to the dehydroxylation reaction is the catalytic deoxydehydration, which requires the presence of a suitable Re-containing catalyst. Due primarily to the facile separation of liquid reaction medium an solid catalyst, which is also advantageous considering the development of a continuously operated process, the development of a heterogeneously catalysed process using a solid Re catalyst is especially advantageous. Herein, different types of solid Re-containing catalysts have been successfully implemented. While the dehydroxylation is feasible over non-supported, solid Re compounds (e.g. perrhenate salts), the use of supported catalysts, especially Re / C, was particularly promising and yields of dehydroxylated products (acrylic acid, propanoic acid and / or their respective esters) of at least 90 % are feasible under different reaction conditions. Further increase in catalytic activity was observed over supported bi-metallic, Re-containing catalysts, e.g. Pt-Re / C, which also enhances hydrogenation of acrylic acid to propanoic acid.

[0019] Of particular importance is the alcohol used as the solvent, i.e. in excess amounts. It acts as a mild reducing agent and / or hydrogen donor in the deoxydehydration reaction, thus making the use of more expensive or hazardous reducing agents, such as H2 gas, unnecessary. In addition, since glyceric, acrylic and propanoic acid all undergo esterification in parallel to the dehydroxylation reaction, esters are the primary products of the invented process. Acrylate esters are of particular industrial relevance and the esterification is efficiently occurring in a one-pot process. Moreover, the separation of the volatile esters is beneficial compared to the free acids and, importantly, esterification protects the carboxylic group from undergoing undesired side reactions. Esterification can be promoted by additives such as acid co-catalysts and / or water adsorbers, such as zeolites or carbon-based solid material. In addition, with different alcohols (methanol, ethanol, propanol, etc.) a spectrum of acrylates and propanoates can be targeted.

[0020] Overall, the heterogeneously catalysed deoxydehydration over solid Re catalysts presents itself as a suitable reaction to selectively convert glyceric acid derivatives into acrylic acid (esters) or propanoic acid (esters). Using, e.g. a Re / C catalyst (and zeolite H-Y as co-catalyst) at 150 °C in methanol under inert N2 atmosphere yields methyl acrylate in yields > 45 % after 72 h. On the other hand, methyl propanoate can be selectively formed in yields >60 % over Pt- Re / C under the same conditions (but without co-catalyst). Particularly high methyl acrylate yields can be obtained over Re / C when the ester methyl glycerate is used as reactant. Yields >90 % already after 6 h. In summary, the characteristics of the heterogeneously catalysed process, its versatility and efficiency as well as its environmental benefits constitute a significant improvement of the prior state of the art. DESCRIPTION OF THE FIGURES

[0021] Figure 1 : Proposed reaction scheme of glyceric acid derivatives (1-3) into dehydroxylated products (4-7). Glyceric acid, glycerate salts and esters exist as L- and D- isomers. Deoxydehydration yields the unsaturated products acrylic acid (4) or acrylates (5). Subsequent hydrogenation results in the formation of propanoic acid (6) and propanoates (7), respectively. Reducing agent and / or hydrogen source of the catalytic reductions (deoxydehydration, (transfer) hydrogenation) is the alcohol solvent. In addition, as shown in the vertical direction, each free acid can undergo esterification with the solvent resulting in an equilibrium between each pair.

[0022] Figure 2: Gas chromatograms (FID signal, black line) of representative product mixtures.

[0023] GC oven temperature is also displayed (pink line). Figure 2a) shows the analysis of product mixture obtained after 72 h over Pt-Re / C under conditions as described in example 2. Figure 2b) shows the analysis of the product mixture obtained after 72 h over Re / C under conditions as described in example 8. The following compounds were identified: A - methylal, oxidation product of methanol; B - methyl acrylate (5); C - methyl propanoate (7); D - acrylic acid (4); E - propanoic acid (7); F - methyl glycolate; G - methyl 3- methoxypropanoate.

[0024] Figure 3: Mass spectrum of the product peak associated with methyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0025] Figure 4: Mass spectrum of the product peak associated with methyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0026] Figure 5: Mass spectrum of the product peak associated with acrylic acid (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0027] Figure 6: Mass spectrum of the product peak associated with propanoic acid (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound. Figure 7: Mass spectrum of the product peak associated with ethyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0028] Figure 8: Mass spectrum of the product peak associated with ethyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0029] Figure 9: Mass spectrum of the product peak associated with / so-propyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0030] Figure 10: Mass spectrum of the product peak associated with / so-propyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0031] Figure 11 : Mass spectrum of the product peak associated with n-propyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0032] Figure 12: Mass spectrum of the product peak associated with n-propyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0033] Figure 13: Mass spectrum of the product peak associated with n-butyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0034] Figure 14: Mass spectrum of the product peak associated with n-butyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0035] Figure 15: Mass spectrum of the product peak associated with n-pentyl acrylate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0036] Figure 16: Mass spectrum of the product peak associated with n-pentyl propanoate (target) compared to the corresponding NIST 17 library spectrum (below) confirming the identity of the product compound.

[0037] DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel and sustainable catalytic route from the (bio-)glycerol- derived feedstock glyceric acid (derivatives) to industrially demanded, sustainable dehydroxylation products, in particular acrylic acid, propanoic acid and their respective esters. Due to (I) the biomass-derived feedstock, (II) the use of alcohols as mild reducing agents rather than reactive, hazardous and expensive molecules such as gaseous H2, and (III) the heterogeneous catalytic reaction, which enables not only batch but also continuous operation, the process is particularly environmentally friendly and advantageous to conventional fossilbased and other competing processes.

[0038] The process requires the presence of a solid, Re-containing catalyst capable of catalysing primarily the deoxydehydration of the vicinal diol functionality of the reactant molecules (glyceric acid, glycerate salts, and glycerate esters). This results in the formation of the unsaturated products, i.e. acrylic acid and its derivatives. Moreover, the process allows for control of the subsequent catalytic double bond hydrogenation reaction, through which saturated products, i.e. propanoic acid and its derivatives, can be obtained. Finally, the dehydroxylation reaction is accompanied by the (acid-catalysed) esterification of glyceric acid, acrylic acid and propanoic acid with the alcohol used as solvent and reducing agent. The esterification is occurring in parallel to the other reactions. Thus, primarily the respective esters are obtained as main products. Due to their use in the polymer industry and their more facile separation (compared to the free acids) this presents an additional advantage.

[0039] In the simplest case, the process requires only a mono-metallic Re catalyst, e.g. Re / C, and a feed solution containing (aqueous) glyceric acid dissolved in excess amounts of an alcohol such as methanol. Catalyst and feed solution are brought into contact at elevated temperature, e.g. 150 °C, under inert atmosphere. The reaction may be carried out in any high pressure reactor or closed autoclave; however stainless steel reactors (batch / slurry or continuous / fixed- bed) are in particular suitable. Having separated the catalyst after the reaction phase, a product solution containing methyl acrylate as the main product in a yield of 40.8 % after 72 h of reaction in a stirred batch reactor was obtained as well as acrylic acid (12.1 %) and some methyl propanoate (2.8 %). Products were identified and quantified by GC-MS and GC-FID, respectively. This observed products prove the feasibility of the controlled dehydroxylation of glyceric acid combined with its esterification to obtain acrylate esters relying on methanol as the reducing agent.

[0040] Furthermore, the process may be manipulated and optimised by choosing and adapting several independent variables beyond the outlined, basic process. In particular, the nature of the solid, Re-containing catalyst is one crucial influencing factor as well as the process conditions, e.g., concentrations of catalyst(s) and reactant(s), temperature, reaction time, atmosphere and pressure, etc. In addition, additives such as co-catalyst or adsorbers may enhance the desired catalytic reactions. Moreover, using different alcohols as solvent does not only broaden the product spectrum but also influences the observed reactions. Lastly, factors such as water content in the reaction mixture and the use of other feed molecules such as glycerate esters or salts have an impact on the outcome of the process. The influence of the most relevant parameters is shown in the following paragraphs (based on the examples described below).

[0041] The process relates to the use of solid, Re-containing catalysts, which may contain Re in oxidation states from 0 to +7. The use of solid catalysts enables a heterogeneously catalysed reaction, which facilitates separation of catalyst and reaction solution and is the prerequisite for a continuously operated process. One group of suitable catalysts the invention relates to are solid salts of Re, such as Re oxides, Re halides or methyltrioxorhenium. Over (NH4)ReO4, as an example of a solid Re salt, 13.4 % yield of methyl acrylate were obtained after 72 h at 150 °C under inert N2 atmosphere in methanol. While Re salts clearly act as dehydroxylation catalysts (molar ratio of glyceric acid to Re is ca. 25), supported Re-containing catalysts are more preferable.

[0042] The invention further relates to dehydroxylation process using mono-metallic supported Re catalysts may contain of Re in amounts of 0.01 - 30 wt. %, more preferably 0.5 - 10 wt. %, which is dispersed, e.g. by impregnation, precipitation a on a suitable, preferably high-surface- area support material, e.g. a metals oxide like SiC>2, TiC>2, ZrC>2, AI2O3, CeC>2, zeolites or carbon materials. Due to dispersion effects, various metal-support interactions, stabilization of active species, mass transfer properties and / or possible bi-functional reaction mechanisms the choice of support material strongly influences catalytic activity. While the underlying factors are subject of further investigation, significant differences in catalytic activity have been observed for Re on different support materials. Over Re / TiO2, e.g., only 3.2 % of methyl acrylate were obtained whereas over the aforementioned Re / C catalyst (both ca. 5 wt.-% Re) in total > 50 % of dehydroxylated products were formed under comparable process conditions. The catalytic performance may be enhanced or tuned by catalyst pre-treatment under suitable conditions, e.g. reductive or oxidative pre-treatment protocols at elevated temperatures.

[0043] A third group of suitable solid Re catalysts this invention relates to are supported bi- and multi- metallic catalysts. Besides Re in the aforementioned amounts, they may contain one or more additional metal, preferably noble transition metal, most preferably Au, Ag or Pt, each in amounts from 0.01 - 30 wt.-%, most preferably 0.2 - 5 wt.-%. Re and the additional metal(s) are preferably co-dispersed on a suitable support material, e.g. the aforementioned ones. Due to synergistic effects as well as the intrinsic catalytic (hydrogenation) activity of these additional metals in combination with Re and the respective support material this may in case of suitable combination result in enhanced catalytic activity and, in particular, improved control of the double bond hydrogenation reaction. The combination of Re and Pt on a carbon support material, i.e. a bi-metallic Pt-Re / C catalyst, significantly increases dehydroxylation activity. Moreover, the hydrogenation reaction is considerably facilitated by the Pt promotion, even at lower reaction temperature of 120 °C. As a result of both effects, 61.8 % yield of methyl propanoate and 10.4 % yield of propanoic acid were obtained after 72 h. Other metals, which do not facilitate the hydrogenation reaction to such a high degree, such as Au, are more suitable to selectively enhance the dehydroxylation reaction, thus, to target unsaturated acrylic acid (derivatives).

[0044] The dehydroxylation process for glyceric acid (derivatives) disclosed here relates to, besides the presence of a suitable solid, Re-containing catalyst a set of reaction conditions favourable to the desired catalytic transformation. Moreover, by selecting corresponding conditions, the process conditions may be adjusted to steer the set of reactions occurring during the process. One crucial parameter is the reaction temperature, which is in the range of 50 - 250 °C, preferably 90 - 160 °C to enable the desired reactions. At lower reaction temperature, e.g. 120 °C, the reaction proceeds much slower (<10 % of dehydroxylation products after 72 h) over Re / C than in the initially described case at 150 °C (ca. 55 %, primarily methyl acrylate). Expectedly, higher temperature strongly enhances dehydroxylation activity and, in particular, also hydrogenation. Thus, methyl propanoate is the main product at 180 °C in yields >35 % after 72 h. While increasing temperature strongly accelerates overall conversion and total product formation, targeting unsaturated acrylic acid (derivatives) is problematic due to the fast saturation of the double bond. Thus, there is a compromise between increasing the overall reaction rate and product selectivity, which indicates that lower reaction temperature in combination with longer reaction time in general leads to higher acrylate (derivative) yields.

[0045] Another process parameter that can be expected to have considerable influence is the composition of the gas phase in combination with the pressure. The invention disclosed here relates to the process carried out under either inert (e.g. N2 or Ar) or reducing, i.e. Flcontaining, atmosphere with a total pressure of 1 - 500 bar. In the aforementioned cases inert N2 atmosphere (5 bar) was applied. Since the dehydroxylation via the deoxydehydration reaction is a reduction, the reducing agent in these cases under inert atmosphere is the alcohol used as the solvent, such as methanol. It may directly act as reducing agent or hydrogen donor or decompose under H2 formation. Conducting the process under reducing H2 atmosphere (5 bar) does neither affect overall catalytic activity or product selectivity significantly. While considerably higher H2 pressure may result in considerable effects, such as enhancing the double bond hydrogenation reaction in particular, there is no indication that the presence of externally added H2 in the initial stage of the process is of particular benefit. Furthermore, considering also the safety issues and the additional expenses associated with the use of H2 gas, its omission in the dehydroxylation process, i.e. conducting the process under inert atmosphere, is preferred.

[0046] The invention further relates to the dehydroxylation process in the presence of additional additives, such as acid co-catalysts to enhance the esterification reaction(s) and / or solid adsorbers to remove water from the liquid reaction mixture. On or more of additives such as zeolite-, carbon- or polymer-based solids materials may be present in amounts up to 50 wt.-%, preferably up to 5 wt.-%. Using, e.g. the resin Amberlyst®-45 as solid additive, 53.9 % methyl acrylate and 15.2 % of methyl propanoate were obtained after 72 h, indicating that a considerably increase in the yields of dehydroxylated esters as the main products are achievable with such additives. Most notably, the additives are not considered to catalyse the dehydroxylation reaction itself but have a rather indirect effect on the process.

[0047] The invention relates to the dehydroxylation process of feeds containing glyceric acid as well as glyceric acid derivatives, which are preferably derived from a renewable source such as bio-glycerol, in concentrations in the range of 0.001 - 20 wt.-%, preferably 0.001 - 2 wt.-%. In the aforementioned cases a mixture of L- or D-glyceric acid (2,3-dihydroxypropanoic acid) as a 20 wt.-% mixture in water was used, which was further diluted in the main solvent, i.e. the respective alcohol such as methanol. Moreover, other feedstocks containing glyceric acid derivatives such as esters of glyceric acid, preferably esters with short-chain alcohols, as well as glycerate (pseudo) alkali or earth alkali metals salts may be used as alternatives. The process tolerates either type of alternative feedstock as well as the presence of water as a cosolvent, which may be present in amounts up to 50 wt.-%. In case of a salt such as L-glyceric acid calcium salt dehydrate being the reactant, the dehydroxylation reaction over Re / C particularly benefits from the presence of the aforementioned co-catalyst Amberlyst®-45. The methyl propanoate yield of 67.6 % indicates that, possibly due to the lower water content in the reaction medium, double bond hydrogenation is particularly promoted. Directly using glycerate esters, such as (S)-methyl 2,3-dihydroxypropanoate, eliminates the influence of the esterification occurring in parallel to the dehydroxylation reaction and may prevent side reactions of the carboxylic group. This is particularly beneficial for the deoxydehydration reaction and allows to shorten process time. After only 2 h, methyl acrylate yields >90 % are achievable over Re / C.

[0048] The invention relates to the dehydroxylation process using alcohols as solvents for several crucial reasons. One important role is to act as the primary solvent transferring the reactant molecules, which are in the solid state in case of pure glyceric acid and glycerate salts, and diluting the reactant to mitigate intermolecular side reactions. Furthermore, the alcohols are also the reducing agent and / or hydrogen donor in the deoxydehydration reaction. Thirdly, the glyceric acid, acrylic acid and propanoic acid all undergo esterification with the solvent, which protects the carboxylic group by preventing side reactions. The thus accessible acrylate and propanoate esters are particularly interesting target molecules, for which considerable industrial demand exists, and which can be facilely separated due to their lower boiling point compared to the respective acids. Thus, the alcohol is particular beneficial and crucial for the overall deoxydehydration process. Besides the aforementioned methanol, any primary or secondary alcohol with up to 10 carbon atoms, preferably a short-chain aliphatic alcohol, may be used. Successful application of different linear alcohols, in particular methanol, ethanol, n- propanol, n-butanol and n-pentanol is provided and in all cases primarily the respective esters of acrylic acid and, to a lesser degree, propanoic acid are obtained. Overall, only a slight decrease in process efficiency is apparent with increasing alcohol chain length, however not below 30 % after 72 h. Secondary alcohols, such as / so-propanol, on the other hand, are more efficient reducing agents and / or hydrogen donors than primary alcohols, which generally enhances dehydroxylation and promotes the hydrogenation reaction in particular. While >40 % yield of saturated products propanoic acid and / so-propyl propanoate was obtained, less than 10 % esterified products indicate that esterification is more efficient with primary alcohols. In general, a broad spectrum of different acrylate and propanoate esters is accessible through the newly invented dehydroxylation process disclosed here.

[0049] Overall, the present disclosure relates generally but not exclusively to a novel catalytic process for the production of acrylic acid, propanoic acid and their esters from bio-based glyceric acid (derivatives) in an alcohol over a solid catalyst in an inert atmosphere (no reductive gas required). The catalyst required for this transformation consists necessarily of rhenium in its metallic and / or oxidized form, which is preferably dispersed on a solid support (e.g. carbon, alumina, silica, titania), thus it is insoluble in a reaction medium. Using bi- or multi-metallic supported catalysts are in some cases beneficial.

[0050] This is the first report of applying the Re-catalysed deoxydehydration reaction using heterogeneous catalysts for the selective dehydroxylation of glyceric acid (derivatives). Moreover, using a solid catalyst presents a major benefit, as it is easy to separate from the products by a simple filtration or sedimentation and allows its use in a continuous (fixed-bed) operation.

[0051] The use of alcohols (e.g. methanol, ethanol) as a solvents shortens and simplifies the downstream (separation, purification) process, since esterification leads to the formation of volatile products, which can be isolated by distillation. In the continuous process the solvent can be easily recycled with an additional distillation unit and reused.

[0052] A further advantage of using alcohols is their additional role as a mild reducing agent in the catalytic dehydroxylation reaction, which makes the use of additional reducing agents unnecessary. In particular, the use of gaseous H2, known to pose safety risks to the catalytic process, can be omitted entirely.

[0053] Some specific aspects of the invention are summarized in the embodiments below:

[0054] (1) The process for the production of acrylic acid, propanoic acid and / or their corresponding esters generally comprises the following steps:

[0055] (a) Feeding glyceric acid, glyceric acid salts or glyceric acid esters (either as a pure as the reactant(s), an alcohol as the solvent and at least one solid, Recontaining catalyst into a batch or continuous reactor, while further optional constituents of the mixture like acid co-catalysts, adsorbents and / or co-solvents may be added in addition.

[0056] (b) Heating the initial mixture as described in (a) to temperatures between 50 - 250 °C.

[0057] (c) Contacting the reaction mixture with the Re-containing catalyst for 0.1 - 500 h until the conversion of the glyceric acid derivatives is complete or until the maximum concentration(s) of the desired product(s) is reached while the reaction mixture is agitated, preferably by stirring at 100 - 10000 min-1in a batch reactor or by letting the liquid mixture flow through a fixed bed of the solid catalyst in a continuously operated reactor.

[0058] (d) Separating the catalyst from the liquid phase by filtration, sedimentation or deposition and / or fixation.

[0059] (e) Isolating the product(s), in particular acrylic acid, propanoic acid and / or their esters, by evaporation of the solvent and volatile by-products and / or (fractionated) distillation.

[0060] (2) The process according to embodiment (1), wherein the alcohol solvent used in the mixture in step (a) is a primary or secondary alcohol with up to 10 carbon atoms, preferably methanol, ethanol, / so-propanol or n-butanol.

[0061] (3) The process according to any of the previous embodiments, wherein the process is carried out under either inert (e.g. N2 or Ar) or reducing, i.e. ^-containing, atmosphere with a total pressure of 1 - 500 bar.

[0062] (4) The process according to any of the previous embodiments, wherein the initial feed mixture comprises glyceric acid (derivatives) in amounts of 0.001 - 20 wt.-% while it may contain in addition also non-alcohol co-solvents, such as water, in amounts of 0 - 50 wt.-%. (5) The process according to any of the previous embodiments, wherein additives such zeolite-, carbon- or polymer-based solids material are present, which may act as water adsorbers and / or acid co-catalysts to promote the desired reactions.

[0063] (6) The process according to any of the previous embodiments, wherein amounts of 0.001 - 10 wt.-% of the solid, Re-containing catalyst are used.

[0064] (7) The process according to any of the previous embodiments, wherein the solid, Recontaining catalyst used to facilitate the dehydroxylation of glyceric acid (derivatives) is a solid Re salt, e.g. a Re oxide, a Re halide or methyltrioxorhenium, preferably perrhenate salts.

[0065] (8) The catalyst according to any of the previous embodiments, wherein the solid, Recontaining catalyst used to facilitate the dehydroxylation of glyceric acid (derivatives) is a supported catalyst comprising Re comprising a metal oxide- or carbon-based support material, such as e.g. activated carbon, SiC>2, TiC>2 or ZrC>2, on which Re in metallic or oxidized form is dispersed in amounts of 0.01 - 30 wt.-%.

[0066] (9) The catalyst according to any of the previous embodiments, wherein the supported, Re-containing catalyst (as described in embodiment (8)) comprises, besides Re, at least one additional, co-dispersed transition metal, preferably Au, Ag or Pt, in amounts of 0.01 - 30 wt.-% to further facilitate the dehydroxylation of glyceric acid (derivatives).

[0067] (10) The catalyst according to any of the preceding embodiments, showing in the dehydroxylation of glyceric acid (derivatives) a yield of desired products (acrylic acid, propanoic acid and their respective esters) of >40 %.

[0068] EXAMPLES

[0069] The following examples are included to further illustrate various embodiments of the presently disclosed subject matter. However, those of ordinary skill in the art should, in light of the present disclosure, appreciate that many changes can be made in specific embodiments which are disclosed and still obtain like or similar results without departing from the spirit and scope of the presently disclosed subject matter.

[0070] EXAMPLE 1

[0071] Catalysts preparation

[0072] The preparation of the supported Re catalysts has involved the impregnation of catalyst supports using the incipient wetness impregnation method with solutions of rhenium (III) chloride (61.4 - 65.9 % Re, Sigma Aldrich). To prepare, for example, Re / C, 1.8 mL of 0.1 M aqueous ReC solution were slowly added to 1 g of dry activated carbon (C3345, Sigma Aldrich). The sample was heated-up to 50 °C and stirred vigorously until the suspension turned into slurry. Then samples were dried overnight at 110 °C and reduced at 400 °C under a flow of pure H2 for 3 hours.

[0073] The preparation of the supported bi- and multi-metallic catalysts has involved the further impregnation of supported Re-containing catalysts using the incipient wetness impregnation method with solutions of additional metal precursor solutions. To prepare, for example, Au- Re / C, 1 .8 mL of 0.1 M aqueous HAuCk (99.9 %, Sigma Aldrich) solution were slowly added to 1 g of dry mono-metallic Re / C catalyst. The sample was heated-up to 50 °C and stirred vigorously until the suspension turned into slurry. Then samples were dried overnight at 110 °C and reduced at 400 °C under a flow of pure hydrogen for 3 h.

[0074] EXAMPLE 2

[0075] Catalytic dehydroxylation of glyceric acid over Re / C in methanol

[0076] Catalytic dehydroxylation was conducted in a high-pressure high-temperature batch autoclave (Parr 5000 Multi Reactor System; 75 mL total reactor volume) equipped with pressure gauge, rupture disc, gas release valve and liquid-phase sampling line. Temperature in the reactor was followed and regulated using a thermocouple and a magnetic stirrer was used.

[0077] The initial feed mixture comprising 0.5 g of aqueous glyceric acid (LD-2,3-dihydroxypropanoic acid; 20 - 22 wt.-% in water, TCI Chemicals), 0.14 g of carbon-supported Re catalyst (>98 %, ca. 5 wt.-% Re, Riogen Inc.) and 35 g of methanol (>99.8 %, J.T. Baker) was weighed in and fed into the autoclave reactor vessel. The reactor was closed, sealed and purged thrice with N2 (5.0, Messer) before setting a N2 pressure of 5 bar. The stirring speed was set to 600 min-1. The reactor was heated to 150 °C within ca. 30 min and kept isothermal for 72 h. Afterwards, the reactor was cooled down to room temperature, depressurized, purged with N2 and opened to collect the product mixture. This liquid product mixture, as well as liquid samples taken during the reaction time, were analysed by gas chromatography to confirm the identity and quantify the concentrations of the products.

[0078] Liquid samples were analysed with a Shimadzu GCMS-QP 2010 Ultra gas chromatograph equipped with a flame ionization detector (FID) and a quadrupole mass spectrometry (MS) detector in combination with a Zebron ZB-5MS capillary column (60 m x 0.25 mm x 0.25 pm). The detailed method was reported previously [26,27], As shown in Figure 2, besides intermediately formed methyl glycerate (retention time (r.t.) = 10.77 min), the product mixture was containing at least one of the following four dehydroxylated products when methanol was used as the alcohol: Acrylic acid (r.t. = 6.53 min), methyl acrylate (r.t. = 5.46 min), propanoic acid (r.t. = 6.48 min) and methyl propanoate (r.t. = 5.64 min) were identified unambiguously using the mass spectrum of each compound (cf. Figures 3-6) and, in case of the former two, by confirming the retention times with the respective commercially available reference sample. The concentration of each compound was calculated from the FID peak area based on external calibrations using the respective reference compound. The respective yield of each product was calculated as the quotient of obtained amount of substance of the respective compound and the initially present amount of substance of glyceric acid. In later examples, the use of different alcohols results in the formation of the respective esters of acrylic acid and propanoic acid, which were identified and quantified in analogous fashion (cf. Figures 7-16).

[0079] The dehydroxylation of glyceric acid over Re / C in methanol primarily resulted in the formation of unsaturated products, i.e. acrylic acid and methyl acrylate. After the full reaction time of 72 h, a methyl acrylate yield of 40.8 % and an acrylic acid yield of 12.1 % were obtained in the product mixture. In addition, 2.8 % yield of methyl propanoate was observed, indicating that the hydrogenation of the double bond is not favourable at the given process parameters. The corresponding gas chromatogram is shown in Figure 2a. Earlier sampling, e.g. after 2 h, showed that initially primarily methyl glycerate (12.0 %) was formed by esterification as well as acrylic acid (14.6 %). Both compounds are intermediates of two competing to the main product methyl acrylate, parallel reaction pathways, which are apparently both occurring during the process. The yields of both intermediates decrease towards the latter stages of the reaction. However, while no methyl glycerate remained in the product mixture after 72 h, which indicates that all compounds with vicinal hydroxyl groups underwent dihydroxylation primarily to deoxydehydration products, the esterification of acrylic acid was not complete. Since the esterification is governed by a chemical equilibrium, it appears unlikely that longer reaction time can increase the yields of methyl acrylate with regards to the free acid.

[0080] EXAMPLE 3

[0081] Dehydroxylation over other supported Re catalysts

[0082] Catalytic dehydroxylation of glyceric acid was conducted according to example 2 with the only difference being that the Re-containing catalyst was Re / TiC>2, Re / SiC>2, Re / AhCh or Re / H-ZSM- 5 (>98 %, ca. 5 wt.-% Re, Riogen Inc.) instead of Re / C. Over all aforementioned supported Re catalysts the catalytic dehydroxylation of glyceric acid could be observed and the support material significantly influences the catalytic activity. Total yield of dehydroxylated products was up to ca. 2 % after 72 h over Re / SiC>2, Re / AhCh and Re / H-ZSM-5, while over Re / TiC>2 3.2 % yield of methyl acrylate were obtained.

[0083] EXAMPLE 4

[0084] Dehydroxylation over unsupported Re compounds

[0085] Catalytic dehydroxylation of glyceric acid was conducted according to example 2 with the only difference being that the Re-containing catalyst was an unsupported Re compound Re2O? (>99.9 %, Aldrich), (NH4)ReO4(>99 %, Aldrich), or KReO4(99 %, Sigma Aldrich), of which 10 mg were used instead of 140 mg of a supported catalyst in the previous examples. The dehydroxylation is also catalysed over these unsupported, solid Re compounds. The main dehydroxylation product was methyl acrylate with product yields after 72 h being 8.5 % (Re2O?) and 13.4 % ((NH4)ReO4), respectively. Over KReO4, 4.1 % methyl propanoate yield were observed as well as 0.9 % methyl acrylate.

[0086] EXAMPLE 5

[0087] Dehydroxylation over Re / C at different reaction temperature

[0088] Catalytic dehydroxylation of glyceric acid was conducted over Re / C according to example 2 with the difference being that the process was conducted isothermally at varied temperature. Catalytic dehydroxylation activity strongly depends on this process parameter and both deoxydehydration and hydrogenation reactions are enhanced with increasing temperature. At higher temperature of 165 °C, yields of 25.7 % methyl acrylate and 24.1 % methyl propanoate were obtained. Furthermore, at 180 °C yields are 5.7 % and 37.8 %, respectively indicating that the higher temperature significantly promotes hydrogenation of the unsaturated products methyl acrylate to methyl propanoate. At lower reaction temperature of 120 °C, 6.1 % methyl acrylate yield were observed with no hydrogenated products being present, confirming the described trends.

[0089] EXAMPLE 6

[0090] Dehydroxylation over Re / C under H2 atmosphere

[0091] Catalytic dehydroxylation of glyceric acid was conducted over Re / C according to example 2 with the difference being that the process was conducted under reducing H2 atmosphere. After loading and sealing the reactor, it was initially purged thrice with N2 before purging thrice with H2 (5.0, Messer) and setting a H2 pressure of 5 bar. Besides, the procedure was as stated in example 2. Under reducing atmosphere, the product yields over the Re / C catalyst were 54.4 % of methyl acrylate, 4.2 % acrylic acid and 3.8 % methyl propanoate after 72 h. Comparison with example 2 shows that the catalytic activity and product distribution are hardly affected, which indicates that the reducing atmosphere affects neither dehydroxylation nor hydrogenation reactions.

[0092] EXAMPLE 7

[0093] Dehydroxylation over bi-metallic Au-Re catalyst

[0094] Catalytic dehydroxylation of glyceric acid was conducted according to example 2 with the only difference being that the Re-containing catalyst contained also gold (Au). In particular, the process was conducted with 140 mg of bi-metallic Au-Re / C prepared according to example 1. Over Au-Re / C, 34.1 % yield of methyl acrylate and 4.0 % yield of acrylic acid were obtained after 72 h.

[0095] EXAMPLE 8

[0096] Dehydroxylation over bi-metallic Pt-Re catalyst

[0097] Catalytic dehydroxylation of glyceric acid was conducted according to example 2 with the only differences being that the reaction was conducted at 120 °C and the Re-containing catalyst contained also platinum (Pt). In particular, the process was conducted with 140 mg of bimetallic Pt-Re / C prepared according to example 1. Over Pt-Re / C, 61.8 % yield of methyl propanoate and 10.4 % yield of propanoic acid were obtained after 72 h, while no unsaturated products were presents. The corresponding gas chromatogram is shown in Figure 2b. Considering also the low reaction temperature of 120 °C it is apparent that, on the one hand, the bi-metallic catalyst Pt-Re / C is considerably more active than mono-metallic Re / C. On the other hand, not only the dehydroxylation but also the hydrogenation of the unsaturated products is promoted as the product distribution shows.

[0098] EXAMPLE 9

[0099] Dehydroxylation over Re / C in presence of solid additives

[0100] Catalytic dehydroxylation of glyceric acid was conducted according to example 2, however in addition to the Re-containing catalyst a solid additive was present in the initial reaction mixture. 50 mg of either the acidic resin Amberlyst®-45 (DuPont) or zeolite H-Y (CBV 720, Zeolyst int.) were tested as potential acid co-catalysts and / or water adsorbers. In case of zeolite H-Y, product yields after 72 h were 47.5 % methyl acrylate, 2.4 % methyl propanoate and 1 .4 % acrylic acid. When Amberlyst®-45 was used, 53.9 % methyl acrylate and 15.2 % of methyl propanoate were obtained after 72 h. These results, in particular compared to example 2, show the beneficial effect of the additives, which are added to enhance primarily the esterification, on the overall process.

[0101] EXAMPLE 10

[0102] Dehydroxylation of glyceric acid salts over Re / C

[0103] Catalytic dehydroxylation was conducted according to example 9, i.e. in the presence of Re / C and the acid co-catalyst Amberlyst®-45. In contrast to the previous example 9, in the initial reaction mixture (aqueous) glyceric acid was substituted as the reactant by L-glyceric acid calcium salt dehydrate (>97 %, Sigma-Aldrich). The experiment was conducted with 125 mg of the solid reactant. After 72 h, 67.6 % yield of methyl propanoate were obtained. Apparently the glyceric acid salt is more prone to dehydroxylation resulting in saturated products. It is likely that the absence of water in the initial reaction mixture is a major factor resulting in the changed product selectivity.

[0104] EXAMPLE 11

[0105] Dehydroxylation of glyceric acid esters over Re / C

[0106] Catalytic dehydroxylation was conducted according to example 2 over Re / C, however in the initial reaction mixture (aqueous) glyceric acid was substituted as the reactant by methyl glycerate ((S)-methyl 2,3-dihydroxypropanoate, 95 %, ABCR), of which 110 mg were used. After the typical reaction time of 72 h, 59.1 % yield of methyl acrylate and 17.6 % methyl propanoate were obtained. However, with the ester as starting reactant the dehydroxylation is considerably faster compared to aqueous glyceric acid (diluted in methanol) in previous examples, in particular example 2. Thus, already after shorter reaction time near-complete dehydroxylation was observed, reaching 92.0 % yield of methyl acrylate after 6 h with <1 % methyl propanoate. This further indicates that the deoxydehydration is preferably conducted under water-free conditions, which are present here due to the water-free feedstock and the avoidance of H2O formation as side product of esterification by using glyceric acid ester as the reactant. EXAMPLE 12

[0107] Dehydroxylation over Re / C in other alcohols

[0108] Catalytic dehydroxylation of glyceric acid was conducted according to example 2 over Re / C. However, the as a solvent methanol was substituted with other primary and secondary alcohols. Most importantly, this resulted in the formation of the corresponding esters of acrylic acid and propanoic acid as the primary products of the process. When ethanol (99.9 %, J.T. Baker) was used as the solvent, which also acts as reducing agent and / or hydrogen donor, the respective ethyl esters were identified as main products (Figures 7 and 8) and the following yields were obtained after 72 h: 20.2 % ethyl acrylate, 10.4 % ethyl propanoate and 31.2 % acrylic acid. When / so-propanol (>99.8 %, Merck) was used as the solvent, the respective isopropyl esters were identified as main products (Figures 9 and 10) and the following yields were obtained after 72 h: 3.5 % / so-propyl acrylate, 6.0 % / so-propyl propanoate and in total ca. 70 % of acrylic acid and propanoic acid. When n-propanol (>99.5 %, Sigma-Aldrich) was used as the solvent, the respective n-propyl esters were identified as main products (Figures 11 and 12) and the following yields were obtained after 72 h: 20.8 % n-propyl acrylate and 22.6 % n- propyl propanoate. When n-butanol (>99.9 %, Honeywell) was used as the solvent, the respective n-butyl esters were identified as main products (Figures 13 and 14) and the following yields were obtained after 72 h: 27.2 % n-butyl acrylate and 5.4 % n-butyl propanoate. When n-pentanol (>99 %, Sigma-Aldrich) was used as the solvent, the respective n-pentyl esters were identified as main products (Figures 15 and 16) and the following yields were obtained after 72 h: 28.7 % n-butyl acrylate and 3.8 % n-butyl propanoate. While for linear alcohols the product yield of the esters slightly decreases with increasing chain length, their behaviour is in general comparable. The secondary alcohol / so-propanol, on the other hand, enhances the dehydroxylation reaction and especially also the hydrogenation. However, esterification is considerably less pronounced as indicated by the product yields: / so-propyl acrylate 3.5 %, / so-propyl propanoate 6.0 % and, as the main products, acrylic acid and propanoic acid (combined ca. 70 %; due to peak overlap no separate quantification possible), of which at least half is saturated propanoic acid. References

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Claims

CLAIMS1. A process for the production of acrylic acid, propanoic acid and / or esters thereof, from glyceric acid or a derivative thereof, preferably derived from renewable sources, in particular from biomass-derived glyceric acid or derivatives thereof, comprising the following steps:(a) feeding a solid Re-catalyst, glyceric acid or a derivative thereof, in particular a glyceric acid salt or glyceric acid ester, and an alcohol into a reactor, or feeding the glyceric acid or derivative thereof, and the alcohol into a reactor containing the solid Re-catalyst, in particular a batch reactor or continuous reactor;(b) heating the reaction mixture obtained in (a) under inert- or reducing-atmosphere at a temperature in the range of 50 - 250°C;(c) separating the Re-catalyst from the liquid phase; and(d) isolating reaction products comprising acrylic acid, propanoic acid, and / or esters thereof from the liquid phase, preferably by evaporation of the solvent and volatile by-products and / or (fractionated) distillation.

2. The process according to claim 1, wherein step (b) comprises agitating the reaction mixture, preferably by stirring at 100 - 1000 rpm, preferably 300 - 2000 rpm and most preferably 600 - 1000 rpm or by letting the reagent mixture flow through a fixed bed comprising the Re-catalyst.

3. The process according to claim 1 or 2, wherein the temperature in step (b) is in a range of 90-160°C, preferably 120-180°C, more preferably about 150°C, the initial pressure at room temperature is 0 - 500 bar, preferably 1 - 50 bar, and / or heating is carried out for a time period from 0.1 to 500 h, preferably 1 to 80 h.

4. The process according to any one of the preceding claims, wherein temperature, pressure and / or atmospheric composition during step (b) is altered continuously or in at least one step during the progress of the reaction.

5. The process according to any one of the preceding claims, wherein step (b) is carried out under inert atmosphere selected from N2, Ar, and He or mixtures thereof, most preferably under N2, or wherein step (b) is carried out under reducing atmosphere containing H2, either undiluted or as a mixture with inert gas.

6. The process according to any one of the preceding claims, wherein the alcohol added in step (a) is a primary or secondary alcohol comprising 1 - 10 carbon atoms, preferably an aliphatic alcohol selected from the group consisting of methanol, ethanol, / so- propanol, n-propanol, n-butanol and n-pentanol, most preferably methanol.

7. The process according to any one of the preceding claims, wherein the reaction mixture in step (a) comprises glyceric acid or derivatives thereof in an amount of 0.001 - 20 wt.-% and wherein the glyceric acid or derivative thereof is preferably selected from the group consisting of L- or D-glyceric acid (2,3-dihydroxypropanoic acid), an ester of L- or D-glyceric acid with primary, secondary or tertiary alcohols, preferably methanol, ethanol, n-propanol, / so-propanol, n-butanol, terf-butanol, or n-pentanol, and salts of L- or D-glyceric acid with (pseudo) alkali or earth alkali metals such as Na, K, (NH4), Li, Mg, and Ca and mixtures of the aforementioned including mixtures of respective isomers; preferably racemic glyceric acid or methyl glycerate.

8. The process according to any one of the preceding claims, wherein the solid Recatalyst is used in an amount of 0.001 - 10 wt.-%, preferably 0.01 - 1 wt.-% based on the total weight of the reaction mixture, and wherein the solid Re-catalyst is preferably provided as a fixed catalyst bed mounted in a (semi-)continuously operated fixed-bed reactor or as a slurry in a (semi-) continuously operated batch reactor.

9. The process according to any one of the preceding claims, wherein the reagent mixture in step (a) further comprises one or more additional components selected from cocatalysts, adsorbents, co-solvents and combinations thereof, wherein additional components are preferably present in an amount of 0.001 - 50 wt.-% based on the total weight of the reaction mixture.

10. The process according to claim 9, wherein the additional components comprise(i) a non-alcoholic co-solvent, most preferably water, in an amount in the range from 0 - 50 wt.-% based on the total weight of the reaction mixture,(ii) a solid adsorbent to bind water, preferably selected from the group consisting of SiC>2-, zeolite-, carbon- or polymer-based adsorbent materials, most preferably molecular sieves, in amounts in the range of 0 - 50 wt.-% based on the total weight of the reaction mixture, and / or(iii) an acid co-catalyst, preferably a mineral acid or solid acid, most preferably an acidic polymer resin, a functionalized carbon material or a zeolitic material, preferably in amounts in the range from 0.001 - 10 wt.-%, most preferably 0.01- 1 wt.-%, based on the total weight of the reaction mixture.

11. The process according to any one of the preceding claims, wherein the solid Recatalyst comprises a solid Re salt comprising Re in oxidation states from +1 to +7, preferably selected from the group consisting of Re oxides, Re halides and methyltrioxorhenium, most preferably perrhenate salts such as (NH^ReC or NaReC .

12. The process according to any one of the preceding claims, wherein the solid Recatalyst is(ii) a supported catalyst comprising Re and a support material, in particular a metal oxide- or carbon-based support material on which Re in metallic or oxidized form in oxidation states from 0 to +7 is dispersed in amounts of preferably 0.01- 30 wt.-%, most preferably 0.5 - 10 wt.-%, based on the total weight of the catalyst, or(iii) a supported bi-metallic or multi-metallic catalyst comprising Re, one or more additional metals, preferably transition metals in oxidation states from 0 to +7, most preferably Au, Ag or Pt, and a support material on which Re and the one or more additional metals are dispersed in amounts from 0.01 to 30 wt.-%, most preferably 0.2 - 5 wt.-%, based on the total weight of the catalyst.

13. The process according to claim 12, wherein the support material is selected from the group consisting of SiC>2, TiC>2, ZrC>2, AI2O3, CeC>2, MgO, CaO, carbon materials or mixtures thereof in the form of powder or shaped particles, more preferably SiC>2, TiC>2 or activated carbon, most preferably highly porous activated carbon.

14. The process according to any one of the preceding claims, wherein during step (b) the reaction is interrupted at least once to add acid co-catalyst before continuing the process step.

15. The process according to any one of the preceding claims, wherein separating the catalyst from the liquid phase includes filtration, sedimentation or fixation of the catalyst.

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