Method for the direct hydrogenation of carboxylic acids to alkanes
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-02
- Publication Date
- 2026-03-18
AI Technical Summary
Existing methods for the hydrogenation of carboxylic acids to alkanes are hindered by the use of expensive metals like palladium and rhenium, require high pressures and temperatures, and are not economically viable due to catalyst instability and recovery issues, especially when dealing with free fatty acids.
The process employs a supported ruthenium catalyst on titanium dioxide or carbonaceous materials, operating within specific pressure-temperature conditions (180°C - 230°C and 30 bar - 120 bar) to directly convert free fatty acids into linear saturated alkanes, allowing for easy catalyst separation and recycling, and avoiding the use of costly metals.
This method achieves high selectivity and yield for alkanes while being economically viable, using moderate pressures and temperatures, and is stable in the presence of free fatty acids, enabling the reuse of the catalyst.
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Abstract
Description
[0001] 202300069 Foreign Countries 1
[0002] Method for the direct hydrogenation of carboxylic acids to alkanes
[0003] Background
[0004] The present invention relates to a process for direct hydrogenation of free fatty acids in the presence of a catalyst in order to produce linear saturated alkanes. The catalyst used is a supported ruthenium catalyst.
[0005] In the prior art, numerous methods for hydrogenation of carboxylic acids have already been described, but these differ in essential aspects from the method of the present invention and therefore have significant disadvantages.
[0006] Breit (ACS Catal. 2018, 8, 785-789) describes the hydrogenation of carboxylic acids using PdRe / C catalysts. Depending on the temperature, either the corresponding alcohols or alkanes are obtained. Temperatures above 160°C result in the formation of alkanes. A disadvantage of this method is the use of very expensive palladium. In addition, rhenium must also be used, which is presumably lost in the product solution during the reaction.
[0007] Hydrogenation using homogeneous ruthenium catalysts: in the prior art (e.g. Angew. Chem. Int. Ed. 2015, 54, 10596 -10599) homogeneous ruthenium catalysts are also described, which appear to be suitable in principle for the reaction. However, the separation and recycling of these catalysts is extremely difficult such that, for economic reasons, their use is only likely to be considered for a few applications.
[0008] Rhenium-based catalysts have impressive selectivity resulting in alcohol as the only product (Broadbent, H. Smith; Bartley, William J., Journal of Organic Chemistry (1963), 28(9), 2347-2350). However, relatively high pressures and temperatures are necessary and the need to use rhenium makes the method commercially unattractive.
[0009] More recently, rhenium on titanium dioxide (Chem. Eur. J. 2017, 23, 1001 - 1006) has been used for the hydrogenation of fatty acids to alcohols. However, the processing of the used catalysts and the recovery of rhenium remains to be clarified. In addition, the use of rhenium renders the method commercially unattractive. Furthermore, so-called copper chromite catalysts (Adkins catalysts) are known, which are available for producing fatty alcohols from fatty acid esters by hydrogenation at high temperatures (see Shigeo Nishimura HANDBOOK OF HETEROGENEOUS CATALYTIC HYDROGENATION FOR ORGANIC SYNTHESIS, John Wiley & Sons, 2001 , page 392). The alcohols are obtained from the corresponding esters at temperatures of 250°C and pressures of 274 bar. The disadvantage of this method is that esters must be used, since the catalysts are not stable in the presence of free fatty acids and dissolve during the reaction. In addition, chromium is used as a feedstock, which must be avoided at all costs for environmental and health reasons.
[0010] Hydrogenations of fatty acids with pure copper catalysts are also known: Guyer, A.; Bieler, A.; Jaberg, K. Helv. Chim. Acta 1947, 30, 39 describes the hydrogenation of fatty acids to alcohols using copper catalysts at 250 bar H2 and 300°C. However, extremely high pressures and temperatures are required, which results in high costs. In addition, the pure copper catalysts of this method are not stable in the presence of free fatty acids and dissolve during the reaction.
[0011] Hydrogenation using modified ruthenium catalysts: various modified ruthenium catalysts are known in which the ruthenium is used in combination with other metals. The addition of other metals serves to increase the selectivity for alcohol formation.
[0012] W02014066062A1 describes the use of bimetallic ruthenium-rhenium and ruthenium-tin catalysts, and also trimetallic ruthenium-rhenium-tin catalysts for the hydrogenation of carboxylic acids. The alcohols are formed selectively here.
[0013] Zhao et al. (Green Chem., 2019, 21 , 3059-3064) describe an example of the use of ruthenium in combination with tin on nitrogen-modified carbon support materials. In this case, fatty alcohols are formed from various fats and fatty acids.
[0014] Rodiansono et al. (RSC Adv., 2022, 12, 13319-13329) describe the direct hydrogenation of free fatty acids using titanium dioxide-supported ruthenium catalysts which have been additionally modified with molybdenum. The catalysts exhibit moderate selectivity for the alcohol dodecanol in the hydrogenation of lauric acid at 170°C and 40 bar, and result in only low formation of alkanes. As a control experiment, Rodiansono et al. also describe the hydrogenation of lauric acid with an unmodified, titanium dioxide-supported ruthenium catalyst at 170°C, partial H2 pressure 40 bar, over a period of 7 h, which gives 65% dodecanol and only 4% dodecane (cf. Rodiansono et al., Table 2). Moreover, CN108993495A discloses a process for producing alkanes from fatty acids by hydrogenation in the presence of a ZrC^ / A Ch-supported ruthenium catalyst. At 140°C and partial H2 pressure 40 bar, alkane yields above 90% are obtained.
[0015] In view of the prior art, the object of the present invention consisted of providing a novel method for producing linear saturated alkanes by direct hydrogenation of carboxylic acids, which avoids the disadvantages of the aforementioned methods.
[0016] The present invention
[0017] Experimental studies associated with the present invention have shown that the known methods can indeed be distinctly improved. By selection of suitable pressure-temperature conditions, even with an unmodified titanium dioxide-supported ruthenium catalyst, free fatty acids in good yields can be converted directly to the corresponding alkanes. This was not to be expected on the basis of the results from Rodiansono et al. (RSC Adv., 2022, 12, 13319-13329). Moreover, for good yields, it is also unnecessary to use mixed oxide supports as proposed by the authors of CN108993495A. Furthermore, the results underlying the present invention show that carbonaceous material can also be used as support material of the catalyst for this conversion, such that reuse of the ruthenium is enabled in a simple manner.
[0018] According to the present invention, alkanes are produced from fatty acids by a process comprising the following step a: a. hydrogenation of a fatty acid F in the presence of a catalyst K, wherein catalyst K is a supported ruthenium catalyst, and fatty acid F is a linear fatty acid having 4 to 24 carbon atoms, present in free form, and either TiC>2 is used as support material for catalyst K at the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar, or a carbonaceous material is used as support material for catalyst K at the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar.
[0019] The method according to the invention allows simple separation of the catalyst from the reaction mixture by means of conventional filtration methods and thus easy recycling thereof. In contrast to numerous catalysts of the prior art, the catalyst K of the present invention is stable with respect to free fatty acids. Furthermore, the method according to the invention affords good yields and has high selectivity for alkane products. In addition, the method of the present invention does not use prohibitively expensive metals and can be carried out under relatively moderate pressure and temperature conditions.
[0020] The fatty acids F are free, linear carboxylic acids having a carbon chain length in the range of C4- C24. They are obtained from renewable raw materials such as coconut, palm or palm kernel oil (triglycerides) by processes known per se. The triglycerides are converted to the free fatty acids and glycerol by so-called fat splitting. During splitting, animal and vegetable fats are continuously cleaved into a fatty acid fraction and glycerol. Depending on the crude fat, the fatty acid fraction comprises both saturated and unsaturated fatty acids. After purification, unsaturated fatty acids can be converted to saturated fatty acids by hydrogenation. After separation of the glycerol and saturation of the unsaturated fatty acids by reaction with hydrogen, the free fatty acids can be hydrogenated directly using the method according to the invention. Depending on the raw material, the free fatty acids have different chain lengths, e.g. C4-C18 (butter fat), Cs-Cie (coconut oil), Cie-C (sunflower, olive, rapeseed, nut and soya oils).
[0021] In the method according to the invention, linear, saturated alkanes having 4 to 24 carbon atoms are obtained according to the following reaction scheme:
[0022] The resulting alkanes are linear and usually have the same number of carbon atoms or one carbon atom less than the fatty acids used as raw material. Lower alkanes are also formed as by-products: for example, lauric acid is converted to a mixture with the main components dodecane and undecane. In addition, alkanes having a lower number of carbon atoms, such as decane, nonane, octane, etc., are also formed. The resulting linear, saturated alkanes are used, for example, as additives for producing cosmetic preparations such as creams, gels, lotions, emulsions, waxes / fats or powders and ointments. Typically, linear saturated alkanes having a carbon number of 4 to 24 carbon atoms are used. Examples of such compounds are n-nonane, n-undecane, n-tridecane or n-heptadecane. Usually these linear alkanes are also used as mixtures, for example as alkane mixtures in cosmetic formulations.
[0023] Linear, saturated alkanes are also constituents of diesel and kerosene fuels. The main constituents of diesel fuel are predominantly alkanes, cycloalkanes and aromatic hydrocarbons, each having about 9 to 22 carbon atoms per molecule and a boiling range between 170°C and 390°C.
[0024] Kerosene consists of a complex mixture of alkanes, cycloalkanes, aromatics and olefins. Jet A, for example, comprises almost exclusively compounds having 9 to 17 carbon atoms, with the emphasis (19.5% mass) on C12 compounds. The products of the method according to the invention can be used accordingly as additives for diesel and kerosene.
[0025] The catalyst K used in the method according to the invention is a heterogeneous ruthenium catalyst, which is characterized in that the active ruthenium is deposited on a support.
[0026] Ruthenium is used as a catalyst for the hydrogenation of a large number of functional groups and aromatic rings (Morris Freifelder, Practical Catalytic Hydrogenation, Wiley and Sons 1971).
[0027] Normally, the reactions are carried out at 70-100°C and 60-70 bar hydrogen pressure. Ruthenium is generally less sensitive to poisoning than other comparable catalysts. Examples are the hydrogenation of aniline to cyclohexylamine, substituted benzenes and pyridines to ring- hydrogenated products, carboxylic acids and hydroxycarboxylic acids to the corresponding alcohols and diols, and the reduction of ketones and aldehydes. For instance, ruthenium on activated carbon is used for the hydrogenation of sugars to the corresponding sugar alcohols.
[0028] The ruthenium catalysts are prepared according to known methods as described, for example, in US2014243562A1 . Ruthenium catalysts are also commercially available, e.g. Evonik Noblyst®P3 series, such as Noblyst®P3060 5% Ru on activated carbon. In addition, it is also possible to use TiC>2-supported ruthenium catalysts.
[0029] Support materials used in the catalysts according to the invention may be various powder materials. In addition to pulverulent support materials, it is also possible to use moulded supports such as extrudates, tablets, spheres or other moulded bodies. The ruthenium may either be applied to carbonaceous support materials or to titanium dioxide supports. Activated carbon, for example, may be used as carbonaceous support material. It is possible to use activated carbon from different source materials such as peat, wood, coconut, bamboo etc., which have been activated by different processes (e.g. by steam or by chemical activation using phosphoric acid, zinc chloride etc.).
[0030] Graphite, carbon black, carbon nanotubes, carbon fibres and carbon aerogels may also be used as suitable support materials based on carbon. In addition, it is possible to use titanium dioxide as support material.
[0031] In preferred embodiments of the present invention, the proportion of the mass of ruthenium in the total mass of catalyst K is in the range between 1 % and 10%.
[0032] The method according to the invention can be carried out, for example, as a suspension process. In this case, the reaction is carried out in a stirred pressure vessel. The catalyst and the fatty acid are initially charged in the reactor without solvent or dissolved in a suitable solvent and reacted with hydrogen at elevated temperature and pressure. In this case, the substrate can be fully initially charged in the reactor or be added continuously or discontinuously during the reaction. After the reaction is complete, apparent by the fact that no more hydrogen is taken up, or after a defined reaction time, the reaction is terminated, the reaction mixture is separated from the catalyst (for example by filtration) and optionally purified. The catalyst can then be reused.
[0033] Alternatively, the method can also be carried out in continuous mode, as is well known to those skilled in the art, for example in a continuous stirred tank reactor, or in a cascade of stirred tank reactors, in loop reactors, in bubble columns, or, for example, in plug flow reactors. Tubular reactors in which the catalyst is permanently installed in the form of a tubular reactor, or is used as a moving bed, are particularly suitable for the use of the catalyst as a moulded body.
[0034] In preferred embodiments of the present invention, activated carbon is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 90 bar.
[0035] In further preferred embodiments of the present invention, a TiC>2-containing material is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar.
[0036] In particularly preferred embodiments of the present invention, the carbonaceous support material used for catalyst K is activated carbon, graphite, carbon black, carbon nanotubes, carbon fibres, carbon aerogels, or a mixture of two or more of these materials. In further particularly preferred embodiments of the present invention, a carbonaceous material is used as support material for catalyst K, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 90 bar.
[0037] In further preferred embodiments of the present invention, the solvent used for the hydrogenation of the fatty acid F in the presence of catalyst K is an alkane having 4 to 24 carbon atoms.
[0038] In particularly preferred embodiments of the present invention, the solvent used for the hydrogenation of a fatty acid F having XC carbon atoms in the presence of catalyst K is an alkane having XC carbon atoms, where XC is an integer from 4 to 24. Methods according to this embodiment have the advantage that, after the catalyst has been removed, the reaction product can be used again as solvent for the hydrogenation.
[0039] The present invention further encompasses the use of a catalyst K for producing alkanes from fatty acids by hydrogenation, wherein catalyst K is a supported ruthenium catalyst, and wherein fatty acid F is a linear fatty acid having 4 to 24 carbon atoms, present in free form, and either TiC>2 is used as support material for catalyst K at the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar, or a carbonaceous material is used as support material for catalyst K at the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar. Examples
[0040] The following examples serve to provide a more detailed elucidation, but are not intended to limit the disclosure content of the invention.
[0041] Example 1 :
[0042] Conversion of a C12 carboxylic acid to the corresponding alkanes
[0043] 1 .7 g of dodecanoic acid dissolved in 15.3 g of heptane and 400 mg of a 5% ruthenium catalyst supported on titanium dioxide were initially charged in a 65 ml pressure vessel. The air in the reactor was then replaced by flushing three times with nitrogen and then flushing three times with hydrogen. The pressure was adjusted to 50 bar with hydrogen at room temperature and slowly heated to an internal reactor temperature of 150°C. Subsequently, the internal reactor pressure was increased to 90 bar. The stirrer was then switched on and the reaction initiated. After a reaction time of 5 hours at 90 bar, the reactor was cooled and the pressure released. The catalyst was separated off and the reaction solution was investigated analytically by gas chromatography. GC analysis revealed the following composition: 16% dodecane, 81% undecane and 3% lower alkanes. No alcohol was observed.
[0044] Examples 2-7:
[0045] Conversion of a C12 carboxylic acid to the alkanes at different temperatures and pressures
[0046] In analogy to Example 1 , in the further Examples 2-7, the reaction conditions and catalysts shown in Table 1 were employed. 1 .7 g of dodecanoic acid dissolved in 15.3 g of heptane and 400 mg in each case of the catalyst used in Table 1 were initially charged in a 65 ml pressure vessel. The air in the reactor was then replaced by flushing three times with nitrogen and then flushing three times with hydrogen. The pressure was adjusted to 50 bar (in the case of final pressure 30 bar only to 10 bar) with hydrogen at room temperature and heated gradually to the internal reactor temperature shown in Table 1 . Subsequently, the internal reactor pressure was increased to the final pressure shown in Table 1 . The stirrer was then switched on and the reaction initiated. After a reaction time of 5 hours, the reactor was cooled and the pressure released. The catalyst was separated off and the reaction solution was investigated analytically by gas chromatography. A GC analysis gave the conversions and compositions of the reaction products shown in Table 1. Table 1 :
[0047] Examples 8-14:
[0048] Pressure and temperature dependence of the conversion of dodecanoic acid with ruthenium catalyst supported on activated carbon in a 100 ml pressure reactor
[0049] 3.5 g of dodecanoic acid dissolved in 31 .5 g of heptane and 875 mg of a 5% ruthenium catalyst supported on activated carbon were initially charged in a 100 ml pressure vessel. The air in the reactor was then replaced by flushing three times with nitrogen and then flushing three times with hydrogen. The pressure was adjusted with hydrogen to 25 bar below the target pressure from Table 2 at room temperature and heated to the internal reactor temperature shown in Table 2 within 30 minutes. Subsequently, the internal reactor pressure was increased to the reaction pressure shown in Table 2. The stirrer was then switched on and the reaction initiated. After a reaction time of 6 hours under the reaction conditions shown in Table 2, the reactor was cooled and the pressure released. The catalyst was separated off and the reaction solution was investigated analytically by gas chromatography. The results of the GC analysis are shown in Table 3.
[0050] Table 2:
[0051] Table 3:
[0052]
[0053] Example 16:
[0054] Reuse of the Ru / C catalyst in dodecane
[0055] (A) 8.95 g of dodecanoic acid dissolved in 21.1 g of dodecane and 1.6 g of a 5% ruthenium catalyst supported on activated carbon were initially charged in a 100 ml pressure vessel.
[0056] (B) The air in the reactor was then replaced by flushing three times with nitrogen and then flushing three times with hydrogen. The pressure was adjusted to 30 bar with hydrogen at room temperature and slowly heated to an internal reactor temperature of 180°C. Subsequently, the internal reactor pressure was increased to 90 bar. The stirrer was then switched on and the reaction initiated. After a reaction time of 6 hours at 90 bar and 180°C, the reactor was cooled and the pressure released. The catalyst was separated off and the reaction solution was investigated analytically by gas chromatography.
[0057] The reuse of the catalyst was performed as follows: the reaction solution was separated from the catalyst by centrifuging. The catalyst was retained as completely as possible in the 100 ml pressure vessel. Thereafter, 9.6 g of dodecanoic acid was dissolved in 22.4 g of the centrifugate obtained from the preceding experiment and transferred into the pressure vessel. The rest of the process was effected as in Example 16-B.
[0058] Table 4 shows the results of the GC analysis of the product solutions from the recycling experiments. The lauric acid was fully converted in all cases. In analogy to the individual batch experiments from Table 4, what is formed is predominantly undecane, the concentration of which rises with increasing degree of recycling.
[0059] Table 4:
[0060] Example 17:
[0061] Reuse of the Ru / C catalyst in a mixture of undecane and dodecane
[0062] A) 11.5 g of dodecanoic acid dissolved in a mixture of 24.11 g of undecane and 2.67 g of dodecane (ratio of 9:1) together with 2.35 g of a 5% ruthenium catalyst supported on activated carbon were initially charged in a 100 ml pressure vessel.
[0063] B) The air in the reactor was then replaced by flushing three times with nitrogen and then flushing three times with hydrogen. The pressure was adjusted to 30 bar with hydrogen at room temperature and slowly heated to an internal reactor temperature of 180°C. Subsequently, the internal reactor pressure was increased to 90 bar. The stirrer was then switched on and the reaction initiated. After a reaction time of 5 hours at 90 bar and 180°C, the reactor was cooled and the pressure released. The catalyst was separated off and the reaction solution was investigated analytically by gas chromatography.
[0064] The reuse of the catalyst was performed as follows: 30 ml of the product solution was centrifuged, and 11 .5 g of the centrifugate was taken for GC analysis. The catalyst remaining in the centrifuge vessel was suspended and returned to the pressure vessel. Subsequently, 11.5 g dodecanoic acid was weighed into the 100 ml pressure vessel, continuing as described in Example 17-B.
[0065] The GC analysis of the reuse experiments is shown in Table 5. The lauric acid is almost fully converted, with constantly increasing lauric esters, which could suggest a certain deactivation or loss of catalyst in the reuse. By addition of fresh catalyst as shown in Recycling 5 and 6, the activity can be recovered again for the most part.
[0066] Table 5:
[0067] * with addition of 7.37 g of undecane, 0.86 g of dodecane and 0.6607 g of fresh catalyst (compensation for loss of mass resulting from the reuse procedure)
[0068] ** with addition of 3.48 g of undecane, 0.39 g of dodecane and 0.3086 g of fresh catalyst (compensation for loss of mass resulting from the reuse procedure)
Claims
Claims1 . Method for producing alkanes from fatty acids comprising the following step a: a. hydrogenation of a fatty acid F in the presence of a catalyst K, wherein catalyst K is a supported ruthenium catalyst, and fatty acid F is a linear fatty acid having 4 to 24 carbon atoms, present in free form, and either TiC>2 is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar, or a carbonaceous material is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar.
2. Method according to Claim 1 , wherein catalyst K consists of ruthenium deposited on one of the following support materials: activated carbon, graphite, carbon black, carbon nanotubes, carbon fibres, carbon aerogels, mixtures of these materials.
3. Method according to Claim 1 , wherein catalyst K consists of ruthenium deposited on titanium dioxide (TiC>2).
4. Method according to any of Claims 1 to 3, wherein the proportion of the mass of ruthenium in the total mass of catalyst K is in the range between 1% and 10%.
5. Method according to any of Claims 1 to 4, wherein the partial hydrogen pressure during hydrogenation is in the range between 30 bar and 90 bar.
6. Method according to any of Claims 1 to 5, wherein the solvent used for the hydrogenation of the fatty acid F in the presence of catalyst K is an alkane having 4 to 24 carbon atoms.
7. Method according to any of Claims 1 to 5, wherein the solvent used for the hydrogenation of a fatty acid F having XC carbon atoms in the presence of catalyst K is an alkane having XC carbon atoms, where XC is an integer from 4 to 24.
8. Use of a catalyst K for producing alkanes from fatty acids by hydrogenation, wherein catalyst K is a supported ruthenium catalyst, and wherein fatty acid F is a linear fatty acid having 4 to 24 carbon atoms, present in free form, and either TiC>2 is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar, or a carbonaceous material is used as support material for catalyst K and the temperature during the hydrogenation is in the range between 180°C and 230°C, and the partial hydrogen pressure during the hydrogenation is in the range between 30 bar and 120 bar.