PROCESS FOR PRODUCING LEVULINIC ACID

Abstract

A process for producing levulinic acid includes a step of catalytic conversion of a pentose (in particular xylose or arabinose) into furfural in an organic solvent having a boiling temperature from 60° C. to 220° C., followed by a step of reduction of furfural to furfuryl alcohol, in the presence of a Lewis acid as catalyst and a protic solvent. Eventually, furfuryl alcohol is converted into levulinic acid directly or indirectly, by preliminary conversion into a levulinic acid ester and its subsequent hydrolysis. This process has a reduced environmental impact and guarantees satisfactory process yields on an industrial scale. In particular, the process allows to reduce as much as possible the formation of humins, which require complex and costly purification processes and involve a considerable reduction in the levulinic acid yields.

Claims

1. A process for producing levulinic acid, the process including the following steps: (a) converting a pentose into furfural, in the presence of an acid catalyst and of an alkali or alkaline earth metal halide, in an organic solvent having a boiling temperature from 60° C. to 220° C., at a temperature from 120° C. to 200° C., (b) reducing furfural to furfuryl alcohol, in a protic solvent and in the presence of a Lewis acid as catalyst, and (c) converting furfuryl alcohol into levulinic acid, either directly or by converting furfuryl alcohol into an alkyl levulinate and subsequent hydrolysis of alkyl levulinate with formation of levulinic acid.

2. The process according to claim 1, wherein the acid catalyst of step (a) is an organic acid.

3. The process according to claim 1, wherein the acid catalyst of step (a) is a Brønsted-Lowry inorganic acid or a Lewis acid.

4. The process according to claim 1, wherein the acid catalyst of step (a) is an acid zeolite.

5. The process according to claim 1, wherein in step (a) the organic solvent is selected from the group consisting of: γ-butyrolactone (GBL), γ-valerolactone (GVL), tetrahydrofuran (THF), dioxane, and dimethylsulfoxide (DMS).

6. The process according to claim 1, wherein in step (a) the organic solvent is in admixture with water.

7. The process according to claim 1, wherein in step (a) the halide is an iodide.

8. The process according to claim 1, wherein step (b) is carried out in heterogeneous phase, i.e. the Lewis acid is substantially insoluble in the protic solvent.

9. The process according to claim 8, wherein in step (b) the Lewis acid is selected from the group consisting of: zirconium oxide hydroxide (ZrO(OH).sub.2), zirconium hydroxide (Zr(OH).sub.4), zirconium oxide (ZrO.sub.2), aluminum hydroxide (Al(OH).sub.3), titanium hydroxide (Ti(OH).sub.4), tin hydroxide (Sn(OH).sub.4), and magnesium hydroxide (Mg(OH).sub.2).

10. The process according to claim 1, wherein the protic solvent of step (b) is a C.sub.1-C.sub.6 alcohol.

11. The process according to claim 1, wherein step (b) is carried out at a temperature from 30° C. to 150° C.

12. The process according to claim 1, wherein in step (c) the furfuryl alcohol is directly converted into levulinic acid in the presence of an acid catalyst in an aqueous medium, in a homogeneous or heterogeneous phase.

13. The process according to claim 12, wherein the acid catalyst is a Brønsted-Lowry acid.

14. The process according to claim 12, wherein the acid catalyst is a Lewis acid, for example AlCl.sub.3, FeCl.sub.3, ZrCl.sub.4, or a Brønsted-Lowry acid, for example sulphonic silica (SiO.sub.2—SO.sub.3H), ion exchange resins, acid zeolites, sulphonated activated carbons (AC—SO.sub.3H).

15. The process according to claim 1, wherein step (c) is carried out at a temperature from 80° C. to 160° C.

16. The process according to claim 1, wherein in step (c) furfuryl alcohol is converted to levulinic acid in two steps: (c1) esterifying furfuryl alcohol with a C.sub.1-C.sub.4 alcohol to obtain an alkyl levulinate (levulinic acid alkyl ester), and (c2) hydrolyzing the alkyl levulinate with formation of levulinic acid.

17. The process according to claim 16, wherein step (c1) is carried out in the absence of water.

18. The process according to claim 16, wherein step (c2) is carried out in an aqueous environment.

19. The process according to claim 16, wherein both steps (c1) and (c2) are carried out in the presence of an acid catalyst, at a temperature from 80° C. to 160° C.

20. The process according to claim 1, wherein in step (a) the pentose is selected from arabinose and xylose.

21. The process according to claim 1, which comprises a preliminary step (a0), preceding step (a), wherein a hexose selected from the group consisting of: sucrose, fructose, and glucose, or mixtures thereof, is subjected to removal of a carbonaceous unit in the presence of a zeolite to obtain arabinose, which is converted in situ to furfural.

Description

DETAILED DESCRIPTION OF THE DISCLOSURE

[0019] For purposes of the present disclosure, in the description and claims that follow, the term “comprising” also includes the terms “which essentially consists of” or “which consists of”.

[0020] Step (a) of the process according to the disclosure is carried out in the presence of an acid catalyst and an alkali or alkaline-earth metal halide. The acid catalyst can be of the homogeneous type, i.e. soluble in the reaction environment, or can be of the heterogeneous type, i.e. insoluble in the reaction environment.

[0021] In the first case, it is preferable to use an organic acid, in particular methanesulfonic acid CH.sub.3—SO.sub.2—OH (MSA).

[0022] Alternatively, an inorganic Brønsted-Lowry acid (for example H.sub.2SO.sub.4, HCl) or a Lewis acid (for example AlCl.sub.3, FeCl.sub.3, ZrCl.sub.4) can be used.

[0023] MSA is particularly advantageous, as it guarantees particularly high yields (around 80%). This acid is also advantageous from an environmental point of view, as it is a biodegradable product, halogen-, nitrogen- and phosphorus-free, and having high thermal stability and low corrosiveness. Furthermore, MSA can be easily recovered from the reaction environment by vacuum distillation.

[0024] In the case of a heterogeneous acid catalyst, this can consist in particular of an acid zeolite, preferably a βH zeolite.

[0025] Step (a) is carried out in an organic solvent having a boiling temperature from 60° C. to 220° C., preferably from 100° C. to 200° C. This solvent allows to operate at relatively high temperatures (from 120° C. to 200° C.) and this allows to significantly reduce the quantity of humins that can form due to secondary reactions involving the starting pentose when it is treated at high temperatures in an acidic environment.

[0026] Preferably, the organic solvent is selected from: γ-butyrolactone (GBL), γ-valerolactone (GVL), tetrahydrofuran (THF), dioxane, dimethylsulfoxide (DMS). More preferably, the organic solvent is γ-butyrolactone (GBL). It is a product with eco-compatibility characteristics, that is obtained through non-polluting processes and per se free from harmful effects on the environment.

[0027] Preferably, the organic solvent is at least partially miscible with water, so as to allow the reaction to be carried out in the presence of water. The presence of water minimizes the formation of humins and favors conversion to furfural. The organic solvent removes furfural from the aqueous environment, which, in the presence of the catalyst, could decompose, also forming humins. Preferably, the organic solvent is in admixture with water with a weight ratio of organic solvent to water from 50:50 to 98:2, more preferably from 80:20 to 95:5.

[0028] As for the halide, this is a halide of an alkali or alkaline earth metal. The halide is preferably chloride, bromide or iodide, more preferably iodide. The alkali metal is preferably selected from: lithium, sodium and potassium, while the alkaline earth metal is preferably selected from: magnesium, calcium, strontium, barium.

[0029] Without wishing to bind to an interpretative theory, it is believed that step (a) occurs according to the following mechanism (which is shown starting from xylose as initial pentose):

##STR00001##

[0030] The halide substantially acts as a weak base via proton transfer in the first enolization step, which is the slow sub-step of step (a). During this sub-step the effectiveness of the halide is as follows: Cl>Br>I.

[0031] In the following three dehydration sub-steps, it is believed that the halide substantially has the function of stabilizing the two intermediate transition steps that are formed. In this case, the effectiveness of the halide is as follows: I>Br>Cl. Therefore, it may be convenient to use a mixture of two different halides, the first (for example a chloride) more effective in the first sub-step, the second (for example an iodide) more effective in the second sub-step. To avoid the use of a chloride, which poses disposal problems and is therefore not preferable from an environmental point of view, it is particularly convenient to use an iodide, which represents the best compromise of effectiveness for the different sub-steps of step (a).

[0032] Preferably, the acid catalyst is used in an amount of from 2% to 40% by weight, more preferably from 5% to 30% by weight, with respect to the weight of pentose.

[0033] Preferably, the halide is used in an amount of from 15% to 60% by weight, more preferably from 25% to 50% by weight, with respect to the weight of pentose.

[0034] Step (b) of the process is carried out in a protic solvent in the presence of a Lewis acid as catalyst. Preferably, step (b) is carried out in a heterogeneous step, i.e. the Lewis acid is substantially insoluble in the protic solvent.

[0035] Lewis acid is preferably selected from: zirconium oxide hydroxide (ZrO(OH).sub.2), zirconium hydroxide (Zr(OH).sub.4), zirconium oxide (ZrO.sub.2), aluminum hydroxide (Al(OH).sub.3), titanium hydroxide (Ti(OH).sub.4), tin hydroxide (Sn(OH).sub.4), magnesium hydroxide (Mg(OH)). Preferably, the Lewis acid is in the form of nanoparticles.

[0036] Preferably, the Lewis acid is used in an amount of from 5% to 80% by weight, more preferably from 10% to 70% by weight, with respect to the weight of furfural.

[0037] Lewis acid can optionally be supported in order to facilitate its recovery, for example on an ion exchange resin, such as an Amberlyst resin (acid sulphonic resin).

[0038] The protic solvent is preferably a C.sub.1-C.sub.6 alcohol, more preferably a C.sub.3-C.sub.6 secondary alcohol. Particularly preferred are isopropanol and isobutanol. Secondary alcohols act not only as solvents but also as reducing species (hydride donors), oxidized to the corresponding ketone, which is easily removed from the reaction environment.

[0039] Step (b) is preferably carried out at a temperature from 30° C. to 150° C., more preferably from 50° C. to 100° C.

[0040] Step (b) is believed to occur according to the Meerwein-Ponndorf-Verley (MPV) mechanism (see, for example, Boronat M. et al, J. Phys. Chem. B 2006, 110, 42, 21168-21174-doi.org/10.1021/jp063249x).

[0041] This step (b) is particularly advantageous as it allows reduction of furfural to furfuryl alcohol to be achieved without using the common reducing agents based on hydrides and molecular hydrogen, ensuring a safe and sustainable process from an environmental point of view.

[0042] As for step (c), furfuryl alcohol can be directly converted into levulinic acid. Preferably, this conversion is carried out in the presence of an acid catalyst in an aqueous medium, in a homogeneous or heterogeneous phase.

[0043] In the case of homogeneous acid catalysis, it is preferable to use a Brønsted-Lowry acid in an aqueous medium, preferably selected from phosphoric acid (H.sub.3PO.sub.4) and methanesulfonic acid (MSA). MSA is particularly preferred, as it guarantees particularly high yields (around 80%).

[0044] Step (c) is preferably carried out at a temperature from 80° C. to 160° C., preferably from 100° C. to 150° C.

[0045] Preferably, the acid catalyst is used in an amount of from 30% to 120% by weight, more preferably from 70% to 100% by weight, with respect to the weight of furfuryl alcohol.

[0046] In the case of heterogeneous catalysis, a Lewis acid, for example AlCl.sub.3, FeCl.sub.3, ZrCl.sub.4, or a Brønsted-Lowry acid, for example sulphonic silica (SiO.sub.2—SO.sub.3H), ion exchange resins, such as an Amberlyst resin (acid sulphonic resin), acid zeolites (e.g. βNH.sub.4.sup.+ zeolite, βH zeolite, ZSM-5 zeolite), sulphonated activated carbons (AC—SO.sub.3H).

[0047] As an alternative to direct conversion of furfuryl alcohol to levulinic acid in an aqueous medium, it is possible to obtain levulinic acid from furfuryl alcohol via a two-step process: [0048] (c1) esterifying furfuryl alcohol with a C.sub.1-C.sub.4 alcohol to obtain an alkyl levulinate (levulinic acid alkyl ester); [0049] (c2) hydrolyzing alkyl levulinate with the formation of levulinic acid.

[0050] Step (c1) is preferably carried out in the absence of water, so as to favor the formation of the ester and avoid secondary reactions that can lead to the formation of humins. Preferably, the C1-C4 alcohol is selected from: methanol, ethanol, propanol, butanol. Step (c2) is instead carried out in an aqueous environment, being a hydrolysis reaction.

[0051] Both steps (c1) and (c2) are preferably carried out in the presence of an acid catalyst (in homogeneous or heterogeneous phase), at a temperature from 80° C. to 160° C., more preferably from 100° C. to 150° C. The acid catalyst (the same or different for the two steps) can be selected from those indicated above for the direct conversion reaction of furfuryl alcohol to levulinic acid.

[0052] As regards the pentose used as initial substrate for the process of the present disclosure, this can be preferably selected from arabinose and xylose. These are products widely available in nature, constituting the basic monomer units of hemicellulose, a constituent of wood together with cellulose and lignin. Indicatively, in dry wood, cellulose represents 30-45%, lignin 20-30% and hemicellulose 10-25%.

[0053] It is also possible to obtain the pentose starting from a hexose or a mixture of hexoses. For example, the arabinose can be obtained from glucose, fructose and/or sucrose by removing a carbonaceous unit in the presence of PH zeolites, producing arabinose and formaldehyde as a by-product (see for example Jinglei Cui et al, Green Chem., 2016, 18, 1619-1624 and Luxin Zhang et al, Chem. Eng. J., 2017, 307, 868-876). The βH zeolite itself is able to dehydrate arabinose in situ to produce furfural, which leads to obtain levulinic acid in accordance with the process of the present disclosure.

[0054] Therefore, the process according to the present disclosure preferably comprises a preliminary step (a0), preceding step (a), in which a hexose selected from sucrose, fructose and glucose, or mixtures thereof, is subjected to removal of a carbonaceous unit in the presence of a zeolite (in particular a βH zeolite or a βFe zeolite) to obtain arabinose, which is converted in situ to furfural. The latter is then subjected to the subsequent steps (b) and (c).

[0055] Preferably, step (a0) is carried out in an organic solvent selected from those indicated above for step (a), optionally mixed with water. Particularly preferred is γ-butyrolactone (GBL). The reaction temperature is preferably between 140° C. and 200° C., more preferably between 160° C. and 190° C.

[0056] Step (aG) allows avoiding the formation of HMF (2,5-(hydroxymethyl)furfural) which is normally formed from hexoses and produces large quantities of humins, which, as already pointed out, constitute a significant problem from the operational point of view with considerable reductions in final yield.

[0057] The process according to the present disclosure can be summarized by the following general scheme, specifically referring to the two preferred pentoses, namely arabinose and xylose:

##STR00002##

[0058] The following examples are intended for illustrative and not limitative purposes of the present disclosure.

Example 1: Conversion of Xylose to Furfural

[0059] (a) Homogeneous Catalysis.

[0060] A steel batch reactor equipped with magnetic stirrer was loaded with xylose (1.00 g), γ-butyrolactone (GBL, 16.8 mL), distilled water (1.0 mL; GBL/H.sub.2O 95/5 w/w), potassium iodide (250 mg, 25% w/w) and methanesulfonic acid (200 mg, 20% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 150° C. for 3 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (80%) and conversion (95%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0061] (b) Heterogeneous Catalysis.

[0062] A steel batch reactor equipped with magnetic stirrer was loaded with xylose (1.00 g), γ-butyrolactone (16.8 mL), distilled water (1.0 mL; GBL/H.sub.2O 95/5 w/w), potassium iodide (500 mg, 50% w/w) and zeolite-3H (100 mg, 10% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 180° C. for 2 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (52%) and conversion (98%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0063] The scheme of the two reactions is as follows:

##STR00003##

Example 2: Reduction of Furfural to Furfuryl Alcohol (Heterogeneous Catalysis)

[0064] A steel batch reactor equipped with a magnetic stirrer was loaded with furfural (1.00 g), isopropanol (20 mL) and zirconium oxide hydroxide (500 mg, 50% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 80° C. for 24 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (100%) and conversion (100%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0065] The scheme of the reaction is as follows:

##STR00004##

Example 3: Direct Conversion of Furfuryl Alcohol to Levulinic Acid

[0066] (a) Homogeneous Catalysis.

[0067] A steel batch reactor equipped with magnetic stirrer was charged with furfuryl alcohol (1.00 g), THF (20 mL), distilled water (5.0 mL; THF/H.sub.2O 4/1 v/v), and methanesulfonic acid (1.00 g, 100% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 140° C. for 12 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (82%) and conversion (100%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0068] (b) Heterogeneous Catalysis.

[0069] A steel batch reactor equipped with magnetic stirrer was charged with furfuryl alcohol (1.00 g; 0.125 M), distilled water (81.6 mL) and Amberlyst 15 (24.5 g, 12 equiv. of a resin with 5 mmol/g loading). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 120° C. for 1.5 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (48%) and conversion (100%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0070] The scheme of the two reactions is as follows:

##STR00005##

Example 4: Conversion of Furfuryl Alcohol to Ethyl Levulinate

[0071] (a) Homogeneous Catalysis.

[0072] A steel batch reactor equipped with a magnetic stirrer was charged with furfuryl alcohol (1.00 g), ethanol (30 mL) and methanesulfonic acid (1.00 g, 100% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 120° C. for 12 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (95%) and conversion (100%) were determined by analyzing a sample of the reaction mixture by quantitative GC method.

[0073] (b) Heterogeneous Catalysis.

[0074] A steel batch reactor equipped with a magnetic stirrer was charged with furfuryl alcohol (1.00 g), ethanol (30 mL) and βNH.sub.4.sup.+ zeolite (2.00 g, 200% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 120° C. for 8 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (78%) and conversion (100%) were determined by analyzing a sample of the reaction mixture by quantitative GC method.

[0075] The scheme of the two reactions is as follows:

##STR00006##

Example 5: Hydrolysis of Ethyl Levulinate to Obtain Levulinic Acid

[0076] (a) Homogeneous Catalysis

[0077] A steel batch reactor equipped with a magnetic stirrer was charged with ethyl levulinate (1.00 g), distilled water (30 mL) and methanesulfonic acid (153 mg, 15.3% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 120° C. for 18 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (95%) and conversion (95%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0078] (b) Heterogeneous Catalysis.

[0079] A steel batch reactor equipped with a magnetic stirrer was loaded with ethyl levulinate (1.00 g), distilled water (30 mL) and βH zeolite (153 mg, 15.3% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 120° C. for 18 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (90%) and conversion (95%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0080] The scheme of the two reactions is as follows:

##STR00007##

Example 6: Conversion of Sucrose into Furfural Through Arabinose Formation (Heterogeneous Catalysis)

[0081] A steel batch reactor equipped with magnetic stirrer was loaded with sucrose (1.00 g), γ-butyrolactone (16.8 mL), distilled water (1.0 mL; GBL/H.sub.2O 95/5 w/w), potassium iodide (500 mg, 50% w/w) and pH-zeolite (100 mg, 10% w/w). Subsequently the reactor was sealed and the reaction mixture heated under stirring (200 rpm) at 180° C. for 2 h. Once the reaction was complete, the reactor was cooled to room temperature and the reaction mixture centrifuged (5000 rpm) and filtered (porosity 0.45 μm) to obtain a clear solution. Molar yield (65%) and conversion (88%) were determined by analyzing a sample of the reaction mixture by HPLC method.

[0082] The scheme of the reaction is as follows:

##STR00008##