Process for depolymerization of lignin

Abstract

The present invention discloses a process for depolymerization of lignin to yield substituted phenolic monomers using Brnsted ionic liquid as catalyst under mild reaction conditions to obtain an overall yield of monomers up to 97%.

Claims

1. A process for the depolymerization of lignin to obtain aromatic products comprising the steps of: a. adding dealkaline lignin and Brnsted ionic liquid having SO.sub.3H group in the range of 1:0.25 to 1:1 to a mixture of water and methanol wherein the ratio of methanol to water is in the range of 0:1 to 5:1 to obtain a reaction mixture; b. stirring the reaction mixture obtained in step (a) at a temperature range of 100 to 170 C. for a period of 0.5 to 6 hrs to afford the aromatic products with 95-97% yield and a mass balance of >90%.

2. The process according to claim 1, wherein the Brnsted ionic liquid having SO.sub.3H group used in step (a) is selected from the group consisting of (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium 4-methylbenzenesulfonate), (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrogensulfate), and (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium chloride).

3. The process according to claim 2, wherein the Brnsted ionic liquid having SO.sub.3H group used is (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrogensulfate).

4. The process according to claim 1, wherein the aromatic products are phenolic monomers obtained with m/z in the range of 100 to 300.

5. The process according to claim 4, wherein the phenolic monomers are THF soluble.

6. The process according to claim 4, wherein the phenolic monomers are selected from the group consisting of 3,5 di ter-butyl,4-methylphenol, 2-ter-butyl, 4-methylphenol, 4-acetyl benzoic acid, butyl 2-(acetyloxy) benzoate, 4-methoxy,2-(prop-2-en-yl) phenol, 3,6 dimethylbenzene-1,2,4 triyl triacetate, and (4-ter butylphenyl)methanol.

7. The process according to claim 1, wherein the process comprises a step of isolation of aromatic products and recovery of the Brnsted ionic liquid.

8. The process according to claim 7, wherein recovery of the Brnsted ionic liquid comprises isolating the aromatic products; treating the aromatic products with THF to isolate THF soluble aromatic products subsequently treating the insoluble part of THF with water to selectively dissolve ionic liquids from other insoluble products followed by addition of HCl to obtain a mass and finally treating the mass with ethanol or methanol to isolate ionic liquid followed by evaporating ethanol to recover ionic liquid.

9. The process according to claim 8, wherein the recovery of ionic liquid is carried out at ambient reaction conditions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 IR of different ionic liquids

(2) FIG. 2 Synthesis of different Brnsted acidic ionic liquids (IL's)

DETAILED DESCRIPTION OF THE INVENTION

(3) In view of above, the present invention provides a process for the depolymerization of lignin using acidic ionic liquids as catalysts to yield substituted phenolic compounds in high yields.

(4) The present invention provides a process for the depolymerization of lignin comprising the steps of: a. adding dealkaline lignin and Brnsted ionic liquid having SO3H group to mixture of water and methanol to obtain a reaction mixture; b. stirring the reaction mixture obtained in step (a) at a temperature in the range of 100 to 170 C. for a period of 0.5 to 6 hrs to afford the aromatic product in high yields.

(5) The Brnsted ionic liquid having SO.sub.3H group is selected from the group consisting of (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium 4-methylbenzenesulfonate) [C.sub.3SC.sub.1IM] [PTSA], (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrogensulfate) [C.sub.3SC.sub.1IM] [HSO.sub.4], (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium chloride) [C.sub.3SC.sub.1IM] [Cl].

(6) One preferable Brnsted ionic liquid having SO.sub.3H group according to the invention is (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrogensulfate).

(7) The lignin depolymerization is optionally be conducted in presence of acidic ionic liquid such as 1-butyl, 3-methyl imidazolium chloride.

(8) The depolymerisation of lignin is carried out using the following conditions:

(9) Lignin (0.5 g), [C.sub.3SC.sub.1IM] [HSO.sub.4] (0.5 g), methanol and water (30 mL, ratio 5:1), 120 C., 1 h.

(10) 1st Run: 97% (THF soluble phenolic monomers)

(11) In the second run due to loss of IL [C.sub.3SC.sub.1IM] [HSO.sub.4] during recovery, for maintaining similar Lignin to catalyst ratio following condition are applied,

(12) Lignin (0.25 g), [C.sub.3SC.sub.1IM] [HSO.sub.4] (0.25 g), methanol and water (15 mL, ratio 5:1), 120 C., 1 h.

(13) 2nd Run: 95% (THF soluble phenolic monomers)

(14) The present invention provides a process for the synthesis of acidic ionic liquids in accordance with J. Mater. Chem., 2012, 22, 17140 comprising the steps of: a. refluxing equimolar mixture of 1-methyl imidazole and 1,3-propane sultone in toluene for overnight to obtain a white precipitate (zwitter ion). b. subjecting the white precipitate obtained in step (a) with equimolar solution of Brnsted acid without any solvent to afford the desired acidic ionic liquid.

(15) The Brnsted acid used for the synthesis of acidic ionic liquids is selected from HCl, sulphuric acid or para-toluene sulfonic acid monohydrate.

(16) The present invention provides the phenolic monomers formed by depolymerization of lignin are selected from the group consisting of 3,5 di ter-butyl,4-methylphenol, 2-ter-butyl,4-methylphenol, 4-acetyl benzoic acid, butyl 2-(acetyloxy) benzoate, 4-methoxy,2-(prop-2-en-yl) phenol, 3,6 dimethylbenzene-1,2, 4 triyl triacetate, and (4-ter butylphenyl)methanol as shown below:

(17) ##STR00001##

(18) The invention provides a process for recovery of ionic liquids with Brnsted acidity which comprises the steps of: a) removing the mixture of water and methanol from the reaction mixture to obtain solids; followed by treating the solids with THF to isolate THF soluble aromatic products; b) treating the insoluble part of THF with water to selectively dissolve ionic liquids from other insoluble products followed by addition of HCl to form sodium contamination as NaCl which is also soluble in water along with ionic liquid c) finally treated this mass with ethanol/methanol to isolate ionic liquid (soluble in solvent) from NaCl contamination (insoluble in solvent) followed by separation of solvent containing ionic liquid and then evaporating ethanol to recover ionic liquid.

(19) The recovery may be carried out substantially under ambient conditions.

(20) According to the above method, after the completion of the reaction, the solvent mix, water and methanol were removed by rotavap from the reaction mixture, to obtain solids. To this solid, THF was added to remove the organic compounds (obtained from lignin depolymerization). Since ionic liquid (IL) is not soluble in THF and stuck to round bottom flask, the THF was decanted and thus IL was separated from THF soluble organic compounds. To the insoluble part of THF (containing IL and other insoluble part) water was added to selectively dissolve ionic liquid and filtered the water soluble ionic liquid to separate the same from other organic compounds. To this solution, HCl was added to form NaCl from the Na contamination in IL (as lignin contains ppm level of Na). This solution was stirred at room temperature for 2 h. then rotavap was done to obtain IL with NaCl. To this semi viscous solid, ethanol/methanol was added to selectively isolate IL from NaCl as the same is not soluble in ethanol/methanol and filtered to separate the IL. The IL dissolved in ethanol then recovered after removing ethanol by rotavap and used for next reaction. The recovery of IL was confirmed by NMR, CHNS analysis, TGA etc.

(21) Following are the examples given to further illustrate the invention and should not be construed to limit the scope of the present invention.

EXAMPLES

Example 1

Synthesis of Acidic Ionic Liquids

(22) The synthesis of acidic ionic liquids is carried out in two step reaction. In the first step, equimolar mixture of 1-methyl imidazole and 1,3-propane sultone refluxed in toluene for overnight 16 hours at 115 C. giving white precipitate (zwitter ion). In the second step reaction the white precipitate is reacted with equimolar solution of HCl without any solvent giving acidic ionic liquid 1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium chloride for 16 hours at 105 C.). Similarly other acidic ionic liquids are synthesized by reaction of zwitter ion with equimolar solution of sulphuric acid and para-toluene sulfonic acid monohydrate.

Example 2

Characterization of Catalyst

A) (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium 4-methylbenzenesulfonate)

(23) .sup.1HNMR (200 MHz, D.sub.2O) 8.59 (s, 1H), 7.55 (d, 2H), 7.36 (s, 1H), 7.30 (s, 1H), 7.27 (d, 2H), 4.11 (t, 2H), 3.76 (s, 3H), 2.83 (t, 2H) 2.28 (s, 3H), 1.90 (m, 2H)

(24) .sup.13CNMR 142.42, 139.43, 136.09, 129.40, 125.31, 123.72, 122.14, 47.69, 47.17, 35.66, 25.05, 20.44

(25) Microanalysis: the approximate % of element by microanalysis is C (43), H (5.3), N (7.5) and S (15).

B) (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium hydrogensulfate) [C3SC1IM] [HSO4]

(26) .sup.1HNMR (200 MHz, D2O) 8.66 (s, 1H), 7.42 (s, 1H), 7.35 (s, 1H), 4.26 (t, 2H), 3.79 (s, 3H), 2.82 (t, 2H), 2.21 (m, 2H)

(27) .sup.13CNMR 136.16, 123.74, 122, 47.69, 47.19, 35.66, 25

(28) Microanalysis: the approximate % of element by microanalysis is C (30.56), H (7.31), N (11.63) and S (23.89)

C) 1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium chloride

(29) .sup.1HNMR (200 MHz, D.sub.2O) 8.63 (s, 1H), 7.40 (s, 1H), 7.32 (s, 1H), 4.24 (t, 2H), 3.77 (s, 3H), 2.80 (t, 2H), 2.19 (m, 2H)

(30) .sup.13CNMR 136.19, 123.76, 122.18, 47.72, 47.18, 35.7, 25.08

(31) Microanalysis: the approximate % of element by microanalysis is C (35.10), H (6.02), N (13.16) and S (14.25)

D) 1-butyl, 3-methyl imidazolium chloride

(32) .sup.1HNMR (200 MHz, D.sub.2O) 8.82 (s, 1H), 7.67 (s, 1H), 7.61 (s, 1H), 4.35 (t, 3H), 4 (s, 3H), 1.91 (m, 2H), 1.40 (m, 2H), 0.94 (t, 3H)

(33) .sup.13CNMR 123.76, 122.18, 49.32, 35.7, 31.2, 19.12, and 12.34

(34) Microanalysis: the approximate % of element by microanalysis, is C (55), H (9) and N (16)

Example 3

Depolymerisation of Lignin Using Following Conditions

(35) Dealkaline Lignin (0.5 g), [C.sub.3SC.sub.1IM] [HSO.sub.4] (0.5 g) taken in methanol and water (30 mL, ratio 5:1) and stirred the reaction mixture at a temperature of 120 C. for 1 h and the reaction products are analyzed for the THF soluble phenolic monomers.

(36) Yield: 97% (THF soluble phenolic monomers).

(37) TABLE-US-00001 S.N. Name of compound m/z 1 p-cymene 134 2 1-(2-acetylphenyl)hexan-1-one 218 3 1-(p-tolyl) ethan-1-one 134 4 2-methoxyphenol 124 5 (Z)-2-methoxy-4-(prop-1-en-1-yl)phenol 164 6 4-hydroxy-3-methoxybenzaldehyde 152 7 methyl 4-hydroxy-3-methoxybenzoate 182 8 1,4-dimethoxybenzene 138 9 2-allyl-4-methoxyphenol 164 10 2-(tert-butyl)-4-methylphenol 164 11 4-acetylbenzoic acid 164 12 methyl 4-hydroxy-3,5-dimethoxybenzoate 182 13 m-cresol 108 14 4-(methoxycarbonyl)-3,6-dimethyl-1,2-phenylene diacetate 280 15 (4-(tert-butyl)phenyl)methanol 164 16 3,5-di-tert-butyl-4-methylphenol 220

(38) Catalytic reaction results are summarized in the following tables.

(39) ##STR00002##
Solvent Ratio Study:

(40) Methanol to water ratio was changed and the results are presented below in table 1

(41) TABLE-US-00002 TABLE 1 Methanol:water THF soluble products yield (%) 0:1 9 1:5 27 1:1 33 5:1 97

(42) Reaction condition, Lignin (0.5 g), [C.sub.3SC.sub.1IM] [HSO.sub.4] (0.5 g), methanol+water (30 mL), 120 C., 1 h.

(43) From the above, it is evident that the methanol/water ratio of 5 gives better product yield.

(44) The effect of acidic ionic liquid on depolymerisation of lignin is shown in table 2.

(45) TABLE-US-00003 TABLE 2 Yield, % Mix Sr. Temperature, THF EtOAc solvent No. Catalyst C. Time, h soluble soluble soluble 1 Without 150 1 0.5 1.2 2.5 catalyst 2 [C.sub.3SC.sub.1IM] 150 1 18 15 17 [PTSA]

(46) Lignin (0.2 g), catalyst (0.05 g), solvent (water+MeOH=2+10 mL)

(47) Mix solvent: THF+EtOAc

(48) From the above, it is evident that the use of ionic liquid improves the yield of phenolic monomers.

(49) The effect of catalyst quantity on the depolymerisation of lignin is shown in table 3.

(50) TABLE-US-00004 TABLE 3 Yield, % Mix Sr. Catalyst THF EtOAc solvent No. Catalyst wt., g Time, h soluble soluble soluble 1 Without catalyst 0 1 0.5 1.2 2.5 2 [C.sub.3SC.sub.1IM] 0.05 1 18 15 17 [PTSA] 3 [C.sub.3SC.sub.1IM] 0.20 1 69 68 75 [PTSA]

(51) Lignin (0.2 g), temperature (150 C.), solvent (water+MeOH=2+10 mL)

(52) Mix solvent: THF+EtOAc

(53) Form the above it is evident that the ratio of lignin to catalyst in 1:1 gives better yields.

(54) The effect of time on depolymerisation of lignin is depicted in table 4.

(55) TABLE-US-00005 TABLE 4 Yield, % Mix Sr. Temperature, THF EtOAc solvent No. Catalyst C. Time, h soluble soluble soluble 1 Without 150 1 0.5 1.2 2.5 catalyst 2 Without 150 2 6.0 4.0 5.0 catalyst 3 [C.sub.3SC.sub.1IM] 150 1 69 68 75 [PTSA] 4 [C.sub.3SC.sub.1IM] 150 2 78 55 83 [PTSA] 5 [C.sub.3SC.sub.1IM] 150 6 61 60 61 [PTSA]

(56) Lignin (0.2 g), catalyst (0.2 g), solvent (water+MeOH=2+10 mL)

(57) Mix solvent: THF+EtOAc

(58) From the above table, it is clear that the reaction period of 2 hrs is sufficient to yield better results when compared to extended reaction period of 6 hrs.

(59) The reduction in the yield may be attributed to a possible decomposition of the products or repolymerisation of products.

(60) The effect of reaction temperature on depolymerisation of lignin is shown in table 5.

(61) TABLE-US-00006 TABLE 5 Yield, % Mix Sr. Temperature, THF EtOAc solvent No. Catalyst C. Time, h soluble soluble soluble 1 [C.sub.3SC.sub.1IM] 100 2 54 53 60 [PTSA] 2 [C.sub.3SC.sub.1IM] 120 2 60 59 64 [PTSA] 3 [C.sub.3SC.sub.1IM] 150 2 78 55 83 [PTSA]

(62) Lignin (0.2 g), catalyst (0.2 g), solvent (water+MeOH=2+10 mL)

(63) Mix solvent: THF+EtOAc As is evident from the above study, although the reaction proceeds smoothly even at a temperature of 100 C. gave up to 60% yield, however, the yields are improved up to 83% with increase in temperature.

(64) The effect of IL's on depolymerisation of lignin is shown in table 6.

(65) TABLE-US-00007 TABLE 6 Yield, % Mix Sr. Temperature, THF EtOAc solvent No. Catalyst C. Time, h soluble soluble soluble 1 [C.sub.3SC.sub.1IM] 150 1 69 68 75 [PTSA] 2 [C.sub.3SC.sub.1IM] 150 1 97 53 83 [HSO.sub.4] 3 [C.sub.3SC.sub.1IM] 150 1 54 38 50 [Cl]

(66) Lignin (0.2 g), Catalyst (0.2 g), solvent (water+MeOH=2+10 mL)

(67) Mix solvent: THF+EtOAc

(68) From the above table it is evident that although the reaction proceeds smoothly with different anions of (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium, however, the best results are achieved with (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium with (HSO.sub.4) anion. The phenolic monomers with an yield of 97% are obtained with the use of Brnsted ionic liquid (having SO.sub.3H group), i.e., (1-methyl-3-(3-sulfopropyl)-1H-imidazol-3-ium with SO.sub.3H group.

(69) The catalytic reaction mixture was analyzed by GC and GCMS to assess the percent yield of phenolic monomer products.

(70) The Characterization of acidic Ionic Liquids was done with .sup.1H and .sup.13C NMR, CHNS analysis, IR and TGA.

Example 4

(71) The recycled catalyst was used in the second run. Due to loss of IL [C.sub.3SC.sub.1IM] [HSO.sub.4] during recovery process, for maintaining similar Lignin to catalyst ratio an additional amount of Lignin (0.25 g) and the IL catalyst [C.sub.3SC.sub.1IM] [HSO.sub.4] (0.25 g) along with methanol and water (15 mL) to maintain the ratio of 5:1. The reaction mass was stirred at a temperature of 120 C. for 1 h and the reaction mass was analyzed for THF soluble fraction.

(72) Yield: 76% (THF soluble phenolic monomers)

Example 5

Method for Recycling of Catalyst

(73) After the reaction, from the reaction mixture, solvent (water+methanol) was removed by rotavap to obtain solids. To this solid, THF was added to remove any organic compounds (obtained from lignin depolymerization). Since ionic liquid (IL) is not soluble in THF and very sticky it was stuck to round bottom flask. Afterwards THF was decanted and thus IL was separated from THF soluble. To the insoluble part of THF (containing IL and other insoluble part) water was added. IL dissolves in water and other organic compounds are not. After filtration water soluble IL was separated out. To this solution HCl was added to form NaCl since Na was a contamination in IL (as lignin contains ppm level of Na). This solution was stirred at room temperature for 2 h. then rotavap was done to obtain IL with NaCl. To this semi viscous solid, ethanol was added. Since NaCl is not soluble in ethanol but IL is soluble in ethanol, separation of NaCl and IL was done using filtration. The IL dissolved in ethanol then recovered after removing ethanol by rotavap. This IL was used for next reaction. The recovery of IL was confirmed by NMR, CHNS analysis, IR, TGA etc.

Advantages of Invention

(74) a. Depolymerization of lignin compounds under mild conditions

(75) b. Aromatic monomers having molecular weight <300 obtained

(76) c. Yields monomers formation is up to 97%.