WO2016032403A1

WO2016032403A1 - Synthesis of aliphatic polycarboxylic acid

Info

Publication number: WO2016032403A1
Application number: PCT/SG2015/050287
Authority: WO
Inventors: Yugen Zhang; Xiukai LI
Original assignee: Agency For Science, Technology And Research
Priority date: 2014-08-28
Filing date: 2015-08-28
Publication date: 2016-03-03

Abstract

The present invention provides a method for synthesizing an aliphatic polycarboxylic acid, the method comprising the steps of: (a) dehydrating a polyhydroxycarboxlic acid using a rhenium-based catalyst to produce an unsaturated polycarboxylic acid precursor; and (b) performing a hydrogenation reaction on the unsaturated polycarboxylic acid precursor to produce the aliphatic polycarboxylic acid.

Description

Title of Invention : Synthesis of Aliphatic

Polycarboxylic Acid

Technical Field

The present invention generally relates to synthesizing aliphatic polycarboxylic acids.

Background

The production of bulk chemicals and fuels from renewable bio-based feedstock is of significant importance for the sustainability of human society. Adipic acid (hexanedioic acid), as one of the most demanded "drop in" chemicals from bioresource, is used primarily for the large volume production of nylon-6,6 polyamide. The global demand for adipic acid is growing at 3-3.5% annually and is expected to reach 3.3 million metric tons in 2016. Currently, commercial adipic acid is mainly derived from the petroleum-based cyclohexane, through which process a nitric acid oxidation is involved. Besides the nonrenewable feedstock cyclohexane source used in this synthetic route, the emission of large amounts of nitrous oxides (N₂0, NO, and N0₂) during the oxidation process is also a significant environmental concern. It is therefore highly desirable to develop a sustainable and environmentally friendly process for the production of adipic acid from renewable feedstock.

There have been attempts to synthesize adipic acid from biomass feedstocks such as glucose, glucaric acid, furan-2,5-dicarboxylic acid (FDCA), and 5-hydroxymethylfurfural (HMF). However, harsh reaction conditions are employed and the efficiencies are low. Adipic acid synthesis routes starting from mucic acid have been reported wherein mucic acid is first converted to muconic acid esters by deoxydehydration (DODH) and then to adipic acid esters by a subsequent hydrogen transfer reaction. However, esterified products are generally observed as major products in these DODH reactions. Consequently, only adipic acid esters are obtained by the subsequent hydrogen transfer reaction. Additional hydrolysis would then be required to obtain free adipic acid which may be time-consuming and adds on to the cost of production.

There is therefore a need to provide a method for synthesizing free aliphatic polycarboxylic acid without undesirable side products which ameliorates one or more of the disadvantages described above. Summary

According to a first aspect, there is provided a method for synthesizing an unsaturated polycarboxylic acid precursor for use in a method to synthesize an aliphatic polycarboxylic acid, the method comprising the step of (a) dehydrating a polyhydroxycarboxlic acid using a rhenium-based catalyst.

Advantageously, the use of a rhenium-based catalyst in the dehydration of polyhydroxycarboxlic acid produces a higher yield of an unsaturated polycarboxylic acid precursor. Advantageously in some embodiments the yield of unsaturated polycarboxylic acid precursor has been more than 50% by weight, more advantageously more than 80% by weight and yet more advantageously more than 90% by weight. Advantageously, the use of the rhenium-based catalyst in the dehydration of polyhydroxycarboxlic acid results in less or no byproducts being produced, such as esterified by-products. The obtained unsaturated polycarboxylic acid precursor can then be used to produce aliphatic polycarboxylic acid in greater yield when compared to conventional methods.

Further advantageously, the disclosed method may be highly selective to free acid products which may be achieved by tuning the acidity of the rhenium-based catalyst.

According to a second aspect, there is provided a method for synthesizing an aliphatic polycarboxylic acid, the method comprising the steps of:

(a) dehydrating a polyhydroxycarboxlic acid using a rhenium-based catalyst to produce an unsaturated polycarboxylic acid precursor; and

(b) performing a hydrogenation reaction on the unsaturated polycarboxylic acid precursor to produce the aliphatic polycarboxylic acid.

Advantageously, the disclosed method allows for a highly efficient conversion of a polyhydroxycarboxylic acid to an unsaturated polycarboxylic acid precursor via a deoxydehydration (DODH) reaction catalyzed by a deoxydehydration catalyst. By combining DODH with a hydrogenation reaction, a polyhydroxycarboxylic acid may be successfully converted to an aliphatic polycarboxylic acid in excellent yield. Further advantageously, the reaction proceeds under mild conditions. As such, the disclosed method may simplify the synthetic process of aliphatic polycarboxylic acids such as adipic acids and succinic acids from polyhydroxycarboxylic acids such as mucic acid and tartaric acid, as the reaction conditions are milder and more time- and cost-efficient when compared to conventional methods.

Advantageously, almost quantitative yields may be achieved by converting the polyhydroxycarboxylic acid to the unsaturated polycarboxylic acid and then to the aliphatic polycarboxylic acid.

In a third aspect, there is provided adipic acid synthesized by the method of the second aspect.

In a fourth aspect, there is provided succinic acid synthesized by the method of the second aspect.

In a fifth aspect, there is provided a method for synthesizing adipic acid, the method comprising the steps of: (a) dehydrating mucic acid using a rhenium-based catalyst to produce muconic acid; and

(b) performing a hydrogenation reaction on muconic acid to produce adipic acid.

In a sixth aspect, there is provided a method for synthesizing succinic acid, the method comprising the steps of:

(a) dehydrating tartaric acid using a rhenium-based catalyst to produce maleic acid; and

(b) performing a hydrogenation reaction on maleic acid to produce succinic acid.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term "aliphatic", for the purposes of the present disclosure, refers to an organic compound wherein the carbon and hydrogen atoms are arranged in saturated or unsaturated straight or branched chains, including alkanes, alkenes and alkynes, wherein representative alkanes, alkenes, and alkynes are provided in the definition of the term "alkyl" herein.

The term "alkyl", for the purposes of the present disclosure, refers to Cl-20 inclusive, e.g., an alkyl group of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbons, linear (i.e., "straight-chain"), branched, or cyclic, saturated or unsaturated (i.e., alkenyl and alkynyl) hydrocarbon chains, including for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, octyl, ethenyl, propenyl, butenyl, pentenyl, hexenyl, octenyl, butadienyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, and allenyl groups. "Branched" refers to an alkyl group in which a lower alkyl group, such as methyl, ethyl or propyl, is attached to a linear alkyl chain. "Lower alkyl" refers to an alkyl group having 1 to about 8 carbon atoms, e.g., an alkyl group of 1, 2, 3,4, 5, 6, 7 or 8 carbons (i.e., a Cl-8 alkyl). "Higher alkyl" refers to an alkyl group having about 10 to about 20 carbon atoms, e.g., alkyl groups of 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbons. In some embodiments, "alkyl" refers, in particular, to Cl-8 straight-chain alkyls, e.g., straight-chain alkyls of 1, 2, 3, 4, 5, 6, 7 or 8 carbons. In other embodiments, alkyl refers, in particular, to Cl-8 branched-chain alkyls, e.g., branched- chain alkyls of 1, 2, 3, 4, 5, 6, 7 or 8 carbons.

The term "consecutive", for the purposes of the present disclosure, refers to performing one reaction after the other, in a chronological sequence. The term "consecutively" should be construed accordingly.

The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

The terms "dehydration" or "dehydrating", for the purposes of the present disclosure, refers to a chemical reaction that converts an alcohol into its corresponding alkene. This term may be used interchangeably with the term "deoxydehydration" or "DODH". The term "optionally substituted" as used herein means the group to which this term refers may be unsubstituted, or may be substituted with one or more groups other than hydrogen provided that the indicated atom's normal valency is not exceeded, and that the substitution results in a stable compound. Such groups may be, for example, halogen, hydroxy, oxo, cyano, nitro, alkyl, alkoxy, haioaikyl, haioaikoxy, ary!-4-aikoxy, alkylthio, hydroxyaikyi, alkoxyalkyl, cycloalkyi, cycioaikylalkoxy, alkanoyi, alkoxycarbonyi, alkylsulfonyl, alkylsulfonyloxy, alkylsulfonylaikyl, aryisulfonyl, arylsulfonyloxy, arylsulfonylalkyl, alkylsulfonamido, alkylamido, alkylsultbnamidoalkyl, alkylamidoalkyl, arylsulfonamido, arylcarboxamido, arylsulfonamidoalkyl, arylcarboxamidoalkyl, aroyl, aroyl-4-alkyi, arylalkanoyi, acyl, aryl, arylalkyi, alkyiaminoaikyl, a group R^xR^yN-, ROCO(CH₂)_m, R^xCON(R^y)(CH₂)_ffi, R^xR^yNCO(CH₂)_m, R^xR^yN80₂(CH₂)_m or R^xS0₂NR^y(CH₂)_m (where each of R^x and R^y is independently selected from hydrogen or alkyl , or where appropriate R^xR^y forms part of carbocylic or heterocyclic ring and m is 0, 1, 2, 3 or 4), a group R^*R^yN(CH₂)_p- or R^xR^yN(CH₂)_pO- (wherein p is 1 , 2, 3 or 4); wherein when the substituent is R^xR^yN(CH₂)_p- or R^xR^yN(CH₂)_P0, R with at least one CH₂ of the (CH₂)_P portion of the group may also form a carbocyclyl or heterocyclyl group and R^y may be hydrogen, alkyl.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Certain embodiments may also be described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the disclosure. This includes the generic description of the embodiments with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein. Disclosure of Optional Embodiments

Illustrative, non-limiting embodiments of a method of synthesizing an aliphatic polycarboxylic acid will now be disclosed.

There is provided a method for synthesizing an aliphatic polycarboxylic acid, the method comprising the steps of:

The rhenium-based catalyst may be comprised of a rhenium-oxo catalyst or adduct thereof. The rhenium-oxo catalyst may be selected from the group consisting of alkyltrioxorhenium and adducts thereof, HRe0₄, Re₂0₇, perrhennate salts and combinations thereof.

The rhenium-based catalyst may be comprised of an adduct of alkyltrioxorhenium with an electron donor ligand.

The electron donor ligand may be selected from the group consisting of imine, halogen, amine, diamine, triamine, alkylamine, ammonia, alkyl, cyano, nitro, SCN, hydroxyl, alkoxy, phenoxy, oxalate, alcohol, alkylthio, thiol, t iolate, phosphite, β-diketone, alkylthio, phosphine, alkylnitrile, nitrite, nitrate, isocyanide, isocyanate, azide, and an aromatic group. The aromatic group may be an optionally substituted heteroaryl.

The optionally substituted heteroaryl may be aromatic monocyclic or multicyclic ring system comprising about 5 to about 14 ring atoms, preferably about 5 to about 10 ring atoms, in which one or more of the ring atoms is an element other than carbon, for example nitrogen, oxygen or sulfur, alone or in combination. Non-limiting examples of suitable heteroaryls include pyridyl, pyrazinyl, furanyl, thienyl, pyrimidinyl, pyridone (including N-substituted pyridones), isoxazolyl, isothiazolyl, oxazolyl, thiazolyl, pyrazolyl, furazanyl, pyrrolyl, pyrazolyl, triazolyl, 1 ,2,4-thiadiazolyl, pyrazinyl, pyridazinyl, quinoxalinyl, phthalazinyl, oxindolyl, imidazo[l,2-a]pyridinyl, imidazo[2,l-b]thiazolyl, benzofurazanyl, indolyl, azaindolyl, benzimidazolyl, benzothienyl, quinolinyl, imidazolyl, thienopyridyl, quinazolinyl, thienopyrimidyl, pyrrolopyridyl, imidazopyridyl, isoquinolinyl, benzoazaindolyl, 1 ,2,4-triazinyl, benzothiazolyl and the like. The term "heteroaryl" also refers to partially saturated heteroaryl moieties such as, for example, tetrahydroisoquinolyl, tetrahydroquinolyl and the like.

The optionally substituted heteroaryls may be substituted with a halogen atom. The halogen atom may be fluorine, chlorine, bromine or iodine.

The electron donor ligand may be selected from the group consisting of pyridine, pyridine N-oxide, bromopyridine, 2,2'-bipyridine, and pyrazole.

The alkyltrioxorhenium or adduct thereof may be selected from the group consisting of:

The perrhennate salt may be selected from the group consisting of NH₄Re0₄, AgRe0₄,

KRe0₄, CsRe0₄ and (ra-C₄H₉)₄NRe0₄.

The polyhydroxycarboxylic acid may have the formula HOOC-(CHOH)n-COOH. n may be any integer between 2 and 10. The polyhydroxycarboxylic acid may be a C2 to CIO polyhydroxycarboxylic acid. The polyhydroxycarboxylic acid may be mucic acid or tartaric acid.

Deoxydehydration may be a reaction that simultaneously removes oxygen and hydrogen from a compound. Deoxydehydration may facilitate complete or partial dehydroxylation of a compound. The deoxydehydration reaction may remove hydroxyl groups from a compound. In the disclosed method, the deoxydehydration catalyst in step (a) comprises a rhenium-based catalyst. The rhenium catalyst may comprise rhenium acid, methyltrioxorhenium or rhenium(VII) oxide.

The hydrogenation reaction may be the addition of hydrogen (H₂) to a molecule from a gaseous H₂ source, or a source other than gaseous H₂. The reaction may be mediated by a catalyst.

The hydrogenation reaction in step (b) may be performed in the presence of a hydrogenation catalyst.

The hydrogenation catalyst may be a metal-on-carbon catalyst with the metal being selected from the group consisting of platinum, palladium, ruthenium and any mixture thereof.

The hydrogenation catalyst may be present in the range of about 0.1 mol% to about 10 mol%, about 0.2 mol% to about 10 mol%, about 0.4 mol% to about 10 mol%, about 0.6 mol% to about 10 mol%, about 0.8 mol% to about 10 mol%, about 1 mol% to about 10 mol%, about 2 mol% to about 10 mol%, about 3 mol% to about 10 mol%, about 4 mol% to about 10 mol%, about 5 mol% to about 10 mol%, about 6 mol% to about 10 mol%, about 7 mol% to about 10 mol%, about 8 mol% to about 10 mol%, about 9 mol% to about 10 mol%, about 0.1 mol% to about 9 mol%, about 0.1 mol% to about 8 mol%, about 0.1 mol% to about 7 mol%, about 0.1 mol% to about 6 mol%, about 0.1 mol% to about 5 mol%, about 0.1 mol% to about 4 mol%, about 0.1 mol% to about 3 mol%, about 0.1 mol% to about 2 mol%, about 0.1 mol% to about 1 mol%, about 0.1 mol% to about 0.8 mol%, about 0.1 mol% to about 0.6 mol%, about 0.1 mol% to about 0.4 mol%, about 0.1 mol% to about 0.2 mol%, about 0.2 mol% to about 9 mol%, about 0.4 mol% to about 8 mol%, about 0.6 mol% to about 7 mol%, about 0.8 mol% to about 6 mol%, about 1 mol% to about 5 mol%, about 2 mol% to about 4 mol%, about 10 mol%, about 9 mol%, about 8 mol%, about 7 mol%, about 6 mol%, about 5 mol%, about 4 mol%, about 3 mol%, about 2 mol%, about 1 mol%, about 0.8 mol%, about 0.6 mol%, about 0.4 mol%, about 0.2 mol% or about 0.1 mol% of the reaction mixture. The hydrogenation catalyst may comprise up to 5 mol% of the reaction mixture. Optionally, the said hydrogenation catalyst may be selected from the group consisting of 5 mol% Ru/C, 5 mol% Pd/C, 5 mol% Pt/C and any mixture thereof.

The method may further comprise the use of an alcohol solvent. The alcohol solvent may be selected from the group consisting of propanol, butanol, pentanol, hexanol, heptanol, octanol and any mixture thereof. The alcohol solvent may be 2-propanol, 1- butanol, 3-pentanol, 3-octanol or any mixture thereof.

Step (a) and step (b) may be performed consecutively. The dehydration reaction in step (a) may comprise the use of an alcohol solvent. The alochol solvent in step (a) may be selected from the group consisting of propanol, butanol, pentanol, hexanol, heptanol, octanol and any mixture thereof. The alcohol solvent in step (a) may be 2-propanol, 1- butanol, 3-pentanol, 3-octanol or any mixture thereof.

Step (a) may be performed at a temperature in the range of about 90 °C to about 180 °C, about 100 °C to about 180 °C, about 110 °C to about 180 °C, about 120 °C to about 180 °C, about 130 °C to about 180 °C, about 140 °C to about 180 °C, about 150 °C to about 180 °C, about 160 °C to about 180 °C, about 170 °C to about 180 °C, about 90 °C to about 170 °C, about 90 °C to about 160 °C, about 90 °C to about 150 °C, about 90 °C to about 140 °C, about 90 °C to about 130 °C, about 90 °C to about 120 °C, about 90 °C to about 110 °C, about 90 °C to about 100 °C, about 100 °C to about 170 °C, about 110 °C to about 160 °C, about 120 °C to about 150°C, about 130°C to about 140 °C, about 90 °C, about 100 °C, about 110 °C, about 120°C, about 130 °C, about 140 °C, about 150 °C, about 160 °C, about 170 °C, or about 180 °C.

Step (b) may be performed at room temperature in the range of about 20 °C to about 35 °C, about 20 °C to about 33 °C, about 20 °C to about 31 °C, about 20 °C to about 28 °C, about 20 °C to about 26 °C, about 20 °C to about 24 °C, about 20 °C to about 22 °C, about 22 °C to about 35 °C, about 24 °C to about 35 °C, about 26 °C to about 35 °C, about 28 °C to about 35 °C, about 31 °C to about 35 °C, about 33 °C to about 35 °C, about 22 °C to about 33 °C, about 24 °C to about 31 °C, about 26 °C to about 28 °C, about 20 °C, about 22 °C, about 24 °C, about 25 °C, about 26 °C, about 28 °C, about 30 °C, about 31 °C, about 33 °C, or about 35 °C.

Step (a) may be performed for a period of about 4 hours to about 24 hours, about 4 hours to about 22 hours, about 4 hours to about 20 hours, about 4 hours to about 18 hours, about 4 hours to about 16 hours, about 4 hours to about 14 hours, about 4 hours to about 12 hours, about 4 hours to about 10 hours, about 4 hours to about 8 hours, about 4 hours to about 6 hours, about 6 hours to about 24 hours, about 8 hours to about 24 hours, about 10 hours to about 24 hours, about 12 hours to about 24 hours, about 14 hours to about 24 hours, about 16 hours to about 24 hours, about 18 hours to about 24 hours, about 20 hours to about 24 hours, about 22 hours to about 24 hours, about 4 hours, about 6 hours, about 8 hours, about 10 hours, about 12 hours, about 14 hours, about 16 hours, about 18 hours, about 20 hours, about 22 hours, or about 24.

Step (b) may be performed for a period of about 4 hours to about 24 hours, about 4 hours to about 22 hours, about 4 hours to about 20 hours, about 4 hours to about 18 hours, about 4 hours to about 16 hours, about 4 hours to about 14 hours, about 4 hours to about 12 hours, about 4 hours to about 10 hours, about 4 hours to about 8 hours, about 4 hours to about 6 hours, about 6 hours to about 24 hours, about 8 hours to about 24 hours, about 10 hours to about 24 hours, about 12 hours to about 24 hours, about 14 hours to about 24 hours, about 16 hours to about 24 hours, about 18 hours to about 24 hours, about 20 hours to about 24 hours, about 22 hours to about 24 hours, about 4 hours, about 6 hours, about 8 hours, about 10 hours, about 12 hours, about 14 hours, about 16 hours, about 18 hours, about 20 hours, about 22 hours, or about 24.

The disclosed method may further comprise, before step (a), the step of (aa) synthesizing the polyhydroxycarboxlic acid from a carbohydrate.

The synthesis of polyhydroxycarboxlic acid from carbohydrate may comprise treating an aqueous, basic solution of a carbohydrate having at least one oxidizable functionality with elemental halogen in the presence of an oxoammonium catalyst and halide co-catalyst.

The carbohydrate may be selected from the group consisting of the D or L forms of ribose, arabinose, xylose, lyxose, ribulose, xylulose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, psicose, fructose, sorbose, tagatose, maltose, lactose, cellobiose, gentibiose, sucrose, raffinose, melezitose, a cyclodextrin, cellulose, hemicellulose, amylose, amylopectin, dextran, fructan, mannan, xylan, arabinans, agar, pectins, alginic acid, gum Arabic, hyaluronic acid, chitin, murein, and glucos aminoglucan.

The rhenium-based catalyst may be immobilized on a substrate. The substrate may be a polymer and may be selected from the group consisting of N-containing polymers, poly(4-vinylpyridine), poly-benzylamine and poly(melamine-formaldehyde).

The aliphatic polycarboxylic acid may be selected from the group consisting of an aliphatic dicarboxylic acid, adipic acid and succinic acid.

In one embodiment, the unsaturated polycarboxylic acid precursor may be a dicarboxylic acid. The dicarboxylic acid may be muconic acid or maleic acid.

The polyhydroxycarboxlic acid may be selected from the group consisting of polyhydroxy dicarboxylic acid, mucic acid and tartaric acid.

In one embodiment, when

, the yield of unsaturated polycarboxylic acid precursor of step (a) may be in the range of about 70% to about 99%, about 72% to about 99%, about 74% to about 99%, about 76% to about 99%, about 78% to about 80%, about 82% to about 99%, about 84% to about 99%, about 86% to about 99%, about 88% to about 99%, about 90% to about 99%, about 92% to about 99%, about 94% to about 99%, about 95% to about 99%, about 96% to about 99%, about 97% to about 99%, about 98% to about 99%, about 70%, about 72%, about 74%, about 76%, about 78%, about 80%, about 82%, about 84%, about 86%, about 88%, about 90%, about 92%, about 94%, about 95%, about 96%, about 97%, about 98%, or about 99%, more than about 70%, more than about 72%, more than about 74%, more than about 76%, more than about 78%, more than about 80%, more than about 82%, more than about 84%, more than about 86%, more than about 88%, more than about 90%, more than about 92%, more than about 94%, more than about 95%, more than about 96%, more than about 97%, more than about 98%, or more than about 99%, preferably 95% to 99%.

An adipic acid synthesized by the method as defined above is also described.

A succinic acid synthesized by the method as defined above is also described.

A method for synthesizing adipic acid, the method comprising the steps of: (a) dehydrating mucic acid using a rhenium-based catalyst to produce muconic acid; and (b) performing a hydrogenation reaction on muconic acid to produce adipic acid, is also described.

A method for synthesizing succinic acid, the method comprising the steps of: (a) dehydrating tartaric acid using a rhenium-based catalyst to produce maleic acid; and (b) performing a hydrogenation reaction on maleic acid to produce succinic acid, is also described.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig.lA

[Fig. 1A] is a kinetic study of NH₄Re0₄ catalyzed DODH reactions of tartaric acid: L-(+)- tartaric acid 150.0 mg (1.0 mmol), NH₄Re0₄ 13.4 mg (0.05 mmol), 3-pentanol 20.0 ml, 120 °C.

Fig.lB

[Fig. IB] is a kinetic study of NH₄Re0₄ catalyzed DODH reactions of mucic acid: mucic acid 210.0 mg (1.0 mmol), NH₄Re0₄ 13.4 mg (0.05 mmol), 3-pentanol 20.0 ml, 120 °C. Examples

Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Materials

All starting materials are commercially available and were used as received, unless otherwise indicated. Mucic acid (98%), 3-pentanol (98%), and pyridine (99.5%) were purchased from Merck; L-(+)-tartaric acid (99.5%), poly(4-vinylpyridine) (25% cross-linked with divinylbenzene), (chloromethyl)polystyrene, 2-bromopyridine (99%), MTO (98%), Re₂0₇ (99.99%), AgRe0₄ (99.99%), KRe0₄ (99.99%), HRe0₄ (80% in water), NH₄Re0₄ (> 99%), Cs₂C0₃ (99.9%), and 5%Pt/C were purchased from Aldrich. Tetrabutylammonium hydroxide (40% in water) was purchased from Fluka. 2,2'-bipyridine was purchased from Alfa. Ammonium hydroxide (29% in water) was purchased from Fisher Scientific. Other regents involved were from Sigma or Merck. ' t and ¹³C NMR spectra were obtained using a Brucker AV-400 (400 MHz) spectrometer. Chemical shifts are reported in ppm with reference to tetramethylsilane with the solvent resonance as the internal standard.

Example 1: Dehydration of polyhydroxycarboxlic acid

There have been attempts to synthesize aliphatic polycarboxylic acid from polyhydroxycarboxlic acid through deoxydehydration (DODH) reactions. However, esterified products are generally observed as major products in these DODH reactions. Consequently, additional hydrolysis is required to obtain the free aliphatic polycarboxylic acid (see Scheme

Scheme 1

Scheme 1 shows former attempts to synthesize aliphatic polycarboxylic acid from polyhydroxycarboxlic acid through deoxydehydration (DODH) reactions. The polyhydroxycarboxlic acid (1) undergoes a DODH reaction to form esters of unsaturated polycarboxylic acids (2 and 3). The esters of unsaturated polycarboxylic acids (2 and 3) subsequently undergo a hydrogen transfer reaction to form esters of saturated polycarboxylic acids. Further hydrolysis is then required to form the free polycarboxylic acid.

Unlike these previous attempts, the method of the present disclosure advantageously results in less or no by-products being produced, such as esterified by- products 2 and 3 (see Scheme 2). The method of the present disclosure advantageously leads to a simplified reaction and greater yield of free polycarboxylic acid.

Scheme 2

As can be seen in Scheme 2, the polyhydroxycarboxlic acid (1) undergoes a DODH reaction to directly form free unsaturated polycarboxylic acid. The free unsaturated polycarboxylic acid subsequently undergoes a hydrogenation reaction to directly form the free polycarboxylic acid. Advantageously, undesired ester by-products may be avoided using the disclosed method.

The present disclosure relates to a highly selective DODH process to free acid products which may be achieved by tuning the acidity of the rhenium-based catalyst.

To demonstrate this, the DODH reactions of both mucic acid (1) and tartaric acid (8) were performed with the presently disclosed rhenium-based catalysts and high selectivities to muconic acid (6) and maleic acid (9) (up to 97%) were achieved.

Example la: General procedure for deoxydehydration (DODH) reactions

A mixture of polyhydroxycarboxlic acid, rhenium-based catalyst and solvent were refiuxed in a 50 ml flask under flowing air or N₂. The mixture was initially a first colour and then changed to a second colour after several hours upon formation of unsaturated polycarboxylic acid.

For example, a mixture of mucic acid (1.0 mmol, 210.0 mg) or tartaric acid (1.0 mmol, 150.0 mg), rhenium-based catalyst (0.05 mmol), and 3-pentanol (20.0 ml) were refiuxed (120 °C) in a 50 ml flask under flowing air or N₂. The mixture was initially a white suspension (mucic acid cases) or colorless solution (tartaric acid cases) and then changed to a brown and transparent solution after several hours. For kinetic studies, 1 ml of reaction mixture was taken at certain time interval and dried for NMR analysis. A known amount of mesitylene was added as an internal standard.

Example 2: General procedure for the conversion of mucic acid to muconic acid

A mixture of mucic acid (1.0 mmol, 210.0 mg), NH₄Re0₄ (0.05 mmol, 13.4 mg), and 3- pentanol (20.0 ml) were refiuxed (120 °C) in a 50 ml flask under N₂. After 24 hours, the reaction mixture was cooled to room temperature, filtrated through Celite, and evaporated to dry solid under reduced pressure. The crude product was dispersed in 10 ml «-hexane by sonicate. The solid part was collected by centrifuge separation and vacuum dried at 50 °C overnight to get muconic acid (112 mg, 79% yield). The «-hexane solution was evaporated to dryness and muconic acid monopentylester was then obtained (30 mg, 14% yield).

Example 3: General procedure for the conversion of mucic acid to adipic acid

In the first step of the DODH reaction, a mixture of mucic acid (2.0 mmol, 420.0 mg), NH₄Re0₄ (0.1 mmol, 26.8 mg), and 3-pentanol (40.0 ml) were refluxed (120 °C) in a 100 ml flask under N₂. After 8 hours, the reaction mixture was cooled to room temperature. The unconverted mucic acid was recovered by filtration, and vacuum dried at 50 °C overnight (42 mg, 20% of the initial amount). The filtrate was evaporated to dryness under reduced pressure. The solid was extracted by 20 ml «-hexane 2 times, and then vacuum dried at 50 °C overnight to get muconic acid (204 mg, 72% yield). In the subsequent hydrogenation step, a mixture of muconic acid (100 mg, 0.7 mmol), 5.0% Pt/C (10.0 mg) and H₂0 (10.0 ml) was charged into a Parr reactor. The reactor was sealed, purged with N₂ for 3 times and then pressurized with H₂ (100 psi). The reactor was stirred at room temperature (24 °C) for 8 hours with overhead stirring before it was depressurized. The catalyst was separated by filtration, the solvent was removed by evaporation, and adipic acid was obtained as a white solid (92 mg, 92% yield).

Example 4: Rhenium-based catalysts

Advantageously, the use of the presently disclosed rhenium-based catalyst results in the unsaturated polycarboxylic acid precursor being synthesized with minimal or no byproducts being produced. Hence the unsaturated polycarboxylic acid can then be used to ultimately synthesize an aliphatic polycarboxylic acid. The advantage of this is that an ultimate higher yield of aliphatic polycarboxylic acid can be produced due to the minimal production of by-products when the unsaturated polycarboxylic acid precursor is synthesized.

As described above, the present highly selective DODH process to free acid products may be achieved by tuning the acidity of the rhenium-based catalyst.

The acidity of methyltrioxorhenium (MTO) can be manipulated by the formation of adducts with electron donor ligands such as pyridine, pyridine N-oxide pyrazole and alkyl amines. Without being bound by theory, it is hypothesized that the electron donor ligands could block the acidic site of the rhenium center.

MTO/pyridine adducts (Scheme 3) were used for the present DODH reactions.

Scheme 3

The MTO/pyridine adducts were in situ generated in 3 -pentanol before adding the reactants.

Scheme 4 below shows the DODH of tartaric acid and mucic acid over pure MTO catalyst versus pyridine modified MTO catalysts. As shown in Scheme 4(c), only muconic acid esters (monoester (2) and diester (3)) were produced with pure MTO catalyst (Scheme 4).

(9)

fit 28 isH

Scheme 4. DODH of tartaric acid and mucic acid over MTO and pyridine modified MTO catalysts.

Mucic acid has poor solubility in 3 -pentanol solvent. Kinetic studies indicate that mucic acid was first converted to the muconic acid monoester and then to the diester. Tartaric acid has a shorter chain structure than mucic acid but better solubility in 3 -pentanol solvent. Under similar reaction conditions, maleic acid (78% yield) and maleic acid monopentylester (10) (19% yield) were obtained in 12 hours with pure MTO catalyst (see Scheme 4(a)). It is clear that MTO catalyzed DODH reaction and acidic rhenium catalyzed esterification reaction are parallel and competitive ones. The DODH reaction is also more sensitive towards the substrate solubility. When the MTO catalyst was modified by pyridine, the DODH reaction for tartaric acid was slower but the selectivity to the free maleic acid was remarkably improved. Amazingly, up to 97% yield of maleic acid was obtained in 24 hours, meanwhile only 2% yield of maleic acid monopentylester was formed (Scheme 4, Table 1).

Table 1. DODH of tartaric acid and mucic acid over MTO and modified MTO catalysts.

Time Conv. Yield

Entry Substrate Catalyst/ligand Product

(h) (%) (%)^[b]

tartaric acid MTO 12 100 97(78+19) (9+10) tartaric acid MTO/pyridine 12 42 37(35+2) (9+10) tartaric acid MTO/pyridine 24 100 99 (97+2) (9+10) tartaric acid MTO/2-bromopyridine 24 100 95 (69+26) (9+10) tartaric acid MTO/2,2'-bipyridine 24 30 29 (26+3) (9+10) mucic acid MTO 24 100 99 (35+64) (2+3) mucic acid MTO/pyridine 24 100 99 (74+25) (6+2) mucic acid MTO/2-bromopyridine 24 100 81 (43+38) (2+3) mucic acid MTO/2,2'-bipyridine 24 16 15 (10+5) (6+2)

[a] Reaction conditions: L-(+)-tartaric acid 150.0 mg (1.0 mmol) or mucic acid 210.0 mg (1.0 mmol), MTO 12.0 mg (0.05 mmol), pyridine/2-bromopyridine/2,2'-bipyridine 0.05 mmol, 3- pentanol 20.0 ml, 120 °C. [b] Yield and conversion were determined by ¾ NMR with internal standard.

When MTO/pyridine was employed for mucic acid, the DODH reaction went smoothly and the product distribution was very different from the reaction with sole MTO catalyst. About 74% yield of free muconic acid was obtained together with 25% yield of muconic acid monopentylester. It is a sharp contrast to the reaction with pure MTO catalyst where only muconic acid esters were produced under the same reaction conditions. 2- bromopyridine has much weaker interaction with MTO and therefore does not influence the reaction rate and the product distribution significantly. When bidentate 2,2'-bipyridine was used as the ligand for MTO, the catalytic activity dropped significantly for both tartaric acid and mucic acid, probably because of the strong interaction and the steric effect.

HRe0₄ (or Re₂0₇) and some perrhennate salts are also efficient in catalyzing the present DODH reaction. For the transformation of mucic acid to muconic acid, HRe0₄ (or Re₂0₇) is more active than MTO, presumably because the protonic acidity not only promoted the esterification of mucic acid but also accelerated the extrusion of the olefin intermediate from the oxorhenium complex. For the DODH of tartaric acid, the selectivity toward free maleic acid was decreased with Re₂0₇ catalyst as compared to MTO (Table 2). However, when ammonium perrhennate (NH₄Re0₄) was used as the catalyst, up to 96% yield of free maleic acid was obtained in 24 h, while only 4% yield of maleic acid monopentylester was formed (Table 2). When NH₄Re0₄ was employed for the DODH of mucic acid, 82% yield of free muconic acid was produced together with 16% yield of muconic acid monopentylester. Due to the very different polarities, muconic acid and muconic acid monopentylester could be easily separated by extraction with isolated yields of 79% and 16%, respectively. With the high activity and high selectivity of ammonium perrhennate, various perrhennate salts were also screened for mucic acid/tartaric acid DODH reactions and the results are summarized in Table 2. AgRe0 is more efficient than NH₄Re0₄ but less selective to the free acid products. KRe0₄, CsRe0₄, and (n- C H₉)₄NRe0 showed much lower activity for the DODH reaction of two substrates. Table 2. DODH of tartaric acid and mucic acid over perrhennate salts.

Time( Conv. Yield

Entry Substrate Catalyst Product h) (%) (%)^[b]

1 tartaric acid Re₂0₇ 8 100 99(73+26) (9+10)

2 tartaric acid NH₄Re0₄ 24 100 100 (96+4) (9+10)

3 tartaric acid AgReC-4 12 100 99 (78+21) (9+10)

4 tartaric acid KReC-4 12 20 20 (17+3) (9+10)

5 tartaric acid CsRe0₄ 24 38 38 (35+3) (9+10)

6 tartaric acid (ra-C₄H₉)₄NRe0₄ 24 30 30 (27+3) (9+10)

7 mucic acid Re₂0₇ 8 100 96 (39+57) (2+3)

8 mucic acid NH₄Re0₄ 24 100 98 (82+16) (6+2)

9 mucic acid AgRe0₄ 12 100 98 (43+55) (6+2)

10 mucic acid KRe0₄ 24 N R - -

11 mucic acid CsRe0₄ 24 N R - -

12 mucic acid («-C₄H₉)₄NRe0₄ 24 N R - -

[a] Reaction conditions: L-(+)-tartaric acid 150.0 mg (1.0 mrnol), mucic acid 210.0 mg (1.0 mmol), rhenium catalyst (0.05 mmol), 3-pentanol 20.0 ml, 120 °C. [b] Yield and conversion were determined by NMR with internal standard.

Kinetic studies on tartaric acid and mucic acid DODH reactions with NH₄Re0 catalyst were conducted. Nearly 90% of tartaric acid was converted to maleic acid and maleic acid monopentylester in the initial 8 hours (Fig. 1A). The full conversion of tartaric acid was attained in 24 hours, but the selectivity to maleic acid monopentylester remained at a level less than 4% during the reaction. For mucic acid transformation, more than 65% of reactant was converted in the first 8 hours and the selectivity to free muconic acid is as high as 98% (Fig. IB). The unconverted mucic acid largely remained as undissolved white solid and therefore could be easily recovered by filtration. After 8 hours, the reaction continually proceeded and the total yield of muconic acid and its monoester gradually increased but with more muconic acid monoester in the products. Thus, it is possible to get free muconic acid at high selectivity by using the NH₄Re0 catalyst and controlling the reaction time.

As the high selectivity to free maleic acid and muconic acid were achieved with the MTO/pyridine or NH₄Re0 catalysts, the further hydrogenation reactions toward adipic acid and succinic acid were also demonstrated. From tartaric acid, 91% isolated yield of maleic acid was obtained in 24 hours with NH₄Re0₄ catalyst. Subsequently, hydrogenation of as synthesized maleic acid with Pt/C catalyst and H₂ (100 psi) at room temperature to give 95% isolated yield of succinic acid (11) in 4 h (Scheme 5). For mucic acid, the NH₄Re0 catalyzed DODH reaction was terminated at 8 hours. Muconic acid was isolated in 72% of yield while another 20% of unreacted mucic acid was recovered by filtration (Scheme 6). Thus, on the basis of converted mucic acid, muconic acid was isolated in 90% yield. The hydrogenation of muconic acid was carried out under the same conditions and 92% isolated yield of adipic acid was obtained in 8 hours. It should be noted that here, the hydrogenation reaction was conducted in water instead of organic solvent. Although muconic acid was not dissolved in water in the initial stage, the hydrogenation reaction still could proceed as the product adipic acid can be well dissolved in water. Maleic acid shows better solubility in water, thus short reaction time is required for the full conversion.

HQ PH _NH_4Re0_{4 (5} moL%) ^ 5%Pt/C , _ΗΟ∞^_∞ΟΗ

HOOC COOH 3-pentanol ^H°°C COOH H₂ 100 psi

120 °C, 24 h (9) H₂0, RT 4h (11)

91% yield 95% yield

Scheme 5. Tartaric acid to maleic acid by DODH and then to succinic acid by hydrogenation.

OH OH o ₀

OH OH O ³-P^e _o ^ntan0'

120 °C, 8 h _m (6) Ϊ H₂ 1M ^HO

° _{psi m} (7) J O

(1) 72% yield _{RT 8h} 92% yield

20% of (1) recovered

Scheme 6. Mucic acid to muconic acid by DODH and then to adipic acid by hydrogenation.

The heterogeneous catalysts have pronounced advantages over the homogenous ones as they are recyclable and can be easily separated from the products. Inspired by the results that the MTO/pyridine adduct and NH₄Re0₄ are efficient and selective catalysts for the current DODH reactions, rhenium catalysts were immobilized onto N-containing polymers. Poly(4- vinylpyridine) (PVP), poly-benzylamine (P-Bn) and poly(melamine-formaldehyde) (PMF) were therefore selected as the supports for MTO and perrhennate. There could be different binding models for the active Re catalysts and the polymer supports (Scheme 7). In the case of MTO/PVP, the pyridine unit in PVP acts as ligand to coordinate to MTO. For the HRe0₄/P-Bn system, ionic interaction holds Re0 anion. While for the mesoporous PMF polymer, both MTO and HRe0₄ can be immobilized onto it via coordination or ionic interactions.

PVP P-Bn PMF

Scheme 7. Polymer immobilized rhenium catalysts.

The loading capacities of polymers are summarized in Table 3. The initial amounts of Re catalyst and polymer were both 100.0 mg and the weight for all polymers increased apparently after Re loading. The HRe0 /P-Bn sample showed the best activity for the DODH of both tartaric acid and mucic acid and high selectivity to give free maleic acid (90% yield) and muconic acid (74% yield). HRe0 /PMF and MTO/PVP showed moderate to good activity, while MTO/P-Bn, MTO/PMF and HRe0₄/PVP showed rather low activity. Those samples with good performances were tested for the recyclabilities (Table 4). Although less amount (48 mg) of HRe0 /P-Bn was loaded for the reaction of tartaric acid, the catalyst was recovered and recycled 3 times without any loss of efficiency and selectivity. For the DODH of mucic acid, the activity of HRe0 /P-Bn decreased in the second run, however full conversion could be achieved as the reaction time was extended to 48 hours. Slight decreases in the efficiency were observed for the DODH of tartaric acid over MTO/PVP and the DODH of muconic acid over

SUBSTITUTE SHEETS (RULE 26) HRe0₄/PMF probably due to the leaching of rhenium during the reaction cycles.

Table 3. DODH of tartaric acid and mucic acid over polymer immobilized rhenium catalysts.

Time Conv.

Entry Substrate Catalyst (mg) Yield (%)^[b]

(h) (%)

1 tartaric acid 32%MTO/PVP (148) 24 90 80 (71+9)

2 tartaric acid 40%MTO/P-Bn (167) 24 20 19 (18+1)

3 tartaric acid 37%MTO/PMF (160) 24 41 38 (33+5)

4 tartaric acid 49%HRe0₄/PVP (196) 24 35 28 (26+2)

5 tartaric acid 48%HRe0₄/P-Bn (191) 24 100 96 (90+6)

6 tartaric acid 37%HRe0₄/PMF (160) 24 100 74 (68+6)

7 mucic acid 32%MTO/PVP (148) 24 88 54 (40+14)

8 mucic acid 48%HRe0₄/P-Bn (191) 24 100 98 (74+24)

9 mucic acid 37%HRe0₄/PMF (160) 24 100 91 (79+12)

[a] Reaction conditions: L-(+)-tartaric acid 150.0 mg (1.0 mmol), mucic acid 210.0 mg (1.0 mmol), 3-pentanol 20.0 ml, 120 °C. [b] Yield and conversion were determined by NMR with internal standard. For Entries 1-6, the products are (9 + 10), for Entries 7-9, the products are (6 + 2).

Table 4.DODH of tartaric acid and mucic acid over polymer immobilized rhenium catalysts.

Recycle Time Conv.

Entry Substrate Catalyst (mg)^[a] Yield (%)^[b] sequence (h) (%)

1 Tartaric acid 45%HRe0₄/P-Bn (48) 1 24 100 100 (92+8)

2 24 100 100 (94+6)

3 24 100 100 (95+5)

2 Tartaric acid 32%MTO/PVP (148) 1 24 90 80 (71+9)

2 24 71 71 (66+5)

3 24 75 74 (68+6)

3 Mucic acid 45%HRe0₄/P-Bn (48) 1 24 100 95 (61+34)

2 24 68 60 (50+10)

48 100 97 (71+26)

3 24 65 53 (44+9)

48 100 100 (72+28)

4 Mucic acid 37%HRe0₄/PMF (160) 1 24 100 91 (79+12)

2 24 87 64 (55+9)

3 24 86 62 (52+10)

[a] Reaction conditions: L-(+) -tartaric acid 150.0 mg (1.0 mmol), mucic acid 210.0 mg (1.0 mmol), 3-pentanol 20.0 ml, 120 °C. [b] Yield and conversion were determined by NMR with internal standard. For tartaric acid, the products are (9 + 10), for mucic acid, the products are (6 + 2). It is therefore shown that a highly efficient and selective protocol to convert mucic acid/tartaric acid to adipic acid and succinic acid by oxorhenium complex-catalyzed DODH reaction followed by Pt/C catalyzed hydrogenation reaction under mild conditions was demonstrated. The acidity of the oxorhenium catalysts determined the selectivity of the free acid products in the DODH reactions. With modified rhenium catalysts, mucic acid was converted to muconic acid and then to adipic acid with 98% selectivity and similarly, tartaric acid was converted to maleic acid and then to succinic acid with more than 96% selectivity. In addition, MTO and HRe0₄ catalysts were successfully immobilized onto nitrogen-containing polymers such as PVP, P-Bn, and PMF. The immobilized catalysts are efficient, selective, and recyclable for the DODH reactions of mucic acid and tartaric acid.

Example 5: General procedure for the preparation of polymer supports

Example 5a: Poly(melamine-formaldehyde) (PMF)

Melamine (0.378 g, 3.0 mmol) and paraformaldehyde (1.8 eq, 0.162 g, 5.4 mmol) were added to a 15-ml Teflon container with a magnetic stir bar, and 3.36 ml (2.5 M) of dimethyl sulfoxide (DMSO) was added. The Teflon container with reaction mixture was stirred on a magnetic plate at 120 °C for 1 hour to obtain a homogeneous solution. The solution was then heated in the oven to 170°C for 72 hours. The reaction was allowed to cool to room temperature, and the solid obtained was crushed, filtered, and washed with DMSO, acetone (3x), tetrahydrofuran (THF) (3x) and CH₂C1₂. The resulting white solid was dried under vacuum at 80°C for 24 hours.

Example 5b: Poly-benzylamine (P-Bn)

Pre -dried poly-benzylchloride (2.0 g) was stirred in ammonium hydroxide (80 ml, 29% aqueous) in a sealed vial at 120 °C for 48 hours. The product was filtered out, washed thoroughly with deionized (DI) water, and then vacuum dried at 50 °C overnight.

Example 6: General procedure for the preparation of rhenium-based catalyst

Example 6a: MTO/pyridine adducts

The MTO/pyridine adducts were in situ generated before DODH reaction. In a typical procedure, MTO (0.05 mmol, 12 mg) was dissolved in 2.0 ml 3-pentanol, and then equal mole amount of pyridine (or 2-bromopyridine, 2,2'-bipyridine) was added. The mixture was stirred at room temperature for 1 hour before adding other reagents for the DODH reaction. Example 6b: CsRe0₄ and («-C₄H₉)4NRe04

Cs₂C0₃ (5.32 ml, 0.15 mol L ) or tetrabutylammonium hydroxide (1.0 ml, 40% aq.) was added to HRe0₄ (2 ml, 0.8 mol L ) under stirring and a white precipitate was observed immediately. The mixture was further stirred at RT for 30 minutes. The solid was separated by centrifuge, washed thoroughly with DI water and then vacuum dried at 50 °C overnight.

Example 6c: MTO/polymer catalysts

The polymer support (PVP, P-Bn, or PMF, 100 mg) was added to a MeOH (1.0 ml) solution of MTO (100 mg) and stirred at room temperature for 16 h. The product was filtered out, washed thoroughly with MeOH, and then vacuum dried at 50 °C overnight. The MTO loading was calculated based on weight gain.

Example 6d: HRe Vpolymer catalysts

Re₂0₇ (100 mg) was dissolved in 10% H₂0/MeOH (1.0 ml) first and then the polymer support (PVP, P-Bn, or PMF, 100 mg) was added. The mixture was stirred at room temperature for 16 h for the loading of HRe0₄. The solid was filtered out, washed thoroughly with MeOH, and then vacuum dried at 50 °C overnight. The Re loading was calculated based on weight gain.

Industrial Applicability

The disclosed method is useful in synthesizing aliphatic polycarboxylic acids from polyhdroxycarboxylic acids.

The disclosed method may be used to convert mucic acid to adipic acid, which is used commonly as a monomer precursor for the production a variety of polymers including nylon and polyurethane. Adipic acid may also be used in medicine, such as in controlled - release formulation matrix tablets to obtain pH-independent release of both weakly basic and weakly acidic drugs. In addition, small but significant amounts of adipic acid may be used in food as a flavorant or gelling aid. The disclosed method may therefore be useful in the industrial-scale production of adipic acid for the above applications.

The disclosed method may simplify the synthetic process of aliphatic polycarboxylic acids such as adipic acids from polyhydroxycarboxylic acids such as mucic acid, as the reaction conditions are milder and more time- and cost-efficient compared to conventional methods.

The disclosed method may be highly selective to free acid products wherein less or no by-products are produced, such as esterified by-products.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A method for synthesizing an aliphatic polycarboxylic acid, the method comprising the steps of:

2. The method of claim 1, wherein the rhenium-based catalyst comprises a rhenium- oxo catalyst or adduct thereof.

3. The method of claim 2, wherein the rhenium-oxo catalyst is selected from the group consisting of alkyltrioxorhenium and adducts thereof, HRe0₄, Re₂0₇, perrhennate salts and combinations thereof.

4. The method of any one of claims 1 to 3, wherein the rhenium-based catalyst comprises an adduct of alkyltrioxorhenium with an electron donor ligand.

5. The method of claim 4, wherein the electron donor ligand is selected from the group consisting of imine, halogen, amine, diamine, triamine, alkylamine, ammonia, alkyl, cyano, nitro, SCN, hydroxyl, alkoxy, phenoxy, oxalate, alcohol, alkylthio, thiol, thiolate, phosphite, β-diketone, alkylthio, phosphine, alkylnitrile, nitrite, nitrate, isocyanide, isocyanate, azide, and an aromatic group, and optionally substituted heteroaryl.

6. The method of any one of claims 4 or 5, wherein the electron donor ligand is selected from the group consisting of pyridine, pyridine N-oxide, bromopyridine, 2,2'- bipyridine, and pyrazole.

7. The method of any one of claims 1 to 6, wherein the rhenium-based catalyst is selected from the roup consisting of:

8. The method of claim 3, wherein the perrhennate salt is selected from the group consisting of NH₄Re0₄, AgRe0₄, KRe0₄, CsRe0₄ and («-C₄H₉)₄NRe0₄.

9. The method of any one of claims 1 to 8, further comprising a step (aa) prior to step (a) comprising synthesizing the polyhydroxycarboxlic acid from a carbohydrate.

10. The method of claim 9, wherein step (aa) comprises treating an aqueous, basic solution of a carbohydrate having at least one oxidizable functionality with elemental halogen in the presence of an oxoammonium catalyst and halide co -catalyst

11. The method of any one of claims 9 or 10, wherein the carbohydrate is selected from the group consisting of the D or L forms of ribose, arabinose, xylose, lyxose, ribulose, xylulose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, psicose, fructose, sorbose, tagatose, maltose, lactose, cellobiose, gentibiose, sucrose, raffinose, melezitose, a cyclodextrin, cellulose, hemicellulose, amy lose, amylopectin, dextran, fructan, mannan, xylan, arabinans, agar, pectins, alginic acid, gum Arabic, hyaluronic acid, chitin, murein, and glucosaminoglucan.

12. The method of any one of claims 1 to 11, wherein the rhenium-based catalyst is immobilized on a substrate.

13. The method of claim 12, wherein the substrate is a polymer selected from the group consisting of N-containing polymers, poly(4-vinylpyridine), poly-benzylamine and poly)melamine-formaldehyde).

14. The method of any one of claims 1 to 13, wherein the aliphatic polycarboxylic acid is selected from the group consisting of an aliphatic dicarboxylic acid, adipic acid and succinic acid.

15. The method of any one of claims 1 to 14, wherein the unsaturated polycarboxylic acid precursor may be a dicarboxylic acid. The dicarboxylic acid may be muconic acid or maleic acid.

16. The method of any one of claims 1 to 15, wherein the polyhydroxycarboxlic acid is selected from the group consisting of dihydroxycarboxylic acid, mucic acid and tartaric acid.

17. The method of any one of claims 1 to 16, wherein the hydrogenation reaction of step (b) is performed in the presence of a hydrogen transfer catalyst.

18. The method of claim 17, wherein the hydrogenation catalyst is a metal-on-carbon catalyst containing a metal.

19. The method of claim 18, wherein the metal is selected from the group consisting of platinum, palladium, ruthenium and any mixture thereof.

20. The method of any one of claims 17 to 19, wherein the hydrogenation catalyst is selected from the group consisting of Ru/C, Pd/C, Pt/C and any mixture thereof.

21. The method of any one of claims 1 to 20, wherein the method further comprises the use of an alcohol solvent.

22. The method of claim 21, wherein the alcohol solvent is selected from the group consisting of propanol, butanol, pentanol, hexanol, heptanol, octanol and any mixture thereof.

23. The method of any one of claims 21 or 22, wherein the alcohol solvent is selected from the group consisting of 2-propanol, 1-butanol, 3-pentanol, 3-octanol and any mixture thereof.

24. The method of any one of claims 21 to 23, wherein the alcohol solvent is 3- pentanol.

25. The method of any one of claims 1 to 24, wherein step (a) is performed at a temperature in the range of 90 °C to 180 °C.

26. The method of any one of claims 1 to 25, wherein step (b) is performed at a temperature in the range of 20°C to 35°C.

27. The method of any one of claims 1 to 26, wherein step (a) is performed for a duration of 4 hours to 24 hours.

28. The method of any one of claims 1 to 27, wherein step (b) is performed for a duration of 4 hours to 24 hours.

method of any one of claims 1 to 28, wherein the rhenium-based catalyst is

and the yield of unsaturated polycarboxylic acid precursor of step (a) is in the range of 70% to 99%.

30. The method of claim 29, wherein the yield of unsaturated polycarboxylic acid precursor of step (a) is in the range of 95% to 99%.

31. Adipid acid produced by the method of any one of claims 1 to 30.

32. Succinic acid produced by the method of any one of claims 1 to 30.

33. A method for synthesizing adipic acid, the method comprising the steps of:

(a) dehydrating mucic acid using a rhenium-based catalyst to produce muconic acid; and

(b) performing a hydrogenation reaction on muconic acid to produce adipic acid.

34. A method for synthesizing succinic acid, the method comprising the steps of: