Homogeneous synthesis of cellulose ethers in ionic liquids
The invention is directed to a simple and new method for the homogeneous synthesis of cellulose ethers. Ionic liquids are not only used as solvent, but also as reaction media for the homogeneous etherification of cellulose. The dissolved cellulose is treated with the etherification agent in the absence of organic and/or inorganic bases and in the absence and/or in the presence of moderate amounts of water. The obtained cellulose ethers show new distributions of substitution on the polymer chain, resulting in new properties and applications.
1. A process for the production of cellulose ethers comprising the steps of
dissolving cellulose in a solvent comprising an ionic liquid in the absence of (a) inorganic bases; and (b) organic bases that are not ionic liquids,
adding to the dissolved cellulose at least one etherifying agent without addition of organic or inorganic base to the solution in order to produce a cellulose ether,
adding the reaction mixture to a non-solvent for the cellulose ether obtained in the etherifying reaction, and thereby precipitating the cellulose ether,
separating off the cellulose ether and
purifying the cellulose ether.
2. A process as claimed in claim 1 , wherein the ionic liquid exhibits a melting point in the range of from −100 to +200° C.
3. A process as claimed in claim 2 , wherein the ionic liquid exhibits a melting point in the range of from −50 to +150° C.
4. A process as claimed in claim 2 , wherein the ionic liquid exhibits a melting point in the range of from 50 to 120° C.
5. A process as claimed in claim 1 , wherein the solvent comprising an ionic liquid further comprises a co-solvent.
6. A process as claimed in claim 5 , wherein the co-solvent comprises water, dimethyl sulfoxide, N,N-dimethyl acetamide, dimethoxyethane, chloroform or mixtures thereof.
7. A process as claimed in claim 5 , wherein the solvent comprising an ionic liquid further comprises a co-solvent in an amount of up to less than 50% by weight, relative to the total weight of the solvent.
8. A process as claimed in claim 5 , wherein the solvent comprising an ionic liquid further comprises a co-solvent in an amount of up to 20% by weight, relative to the total weight of the solvent.
9. A process as claimed in claim 1 , wherein the solvent comprises up to 25% by weight of water, relative to the total weight of the solvent.
10. A process as claimed in claim 9 , wherein the solvent comprises up to 15% by weight of water, relative to the total weight of the solvent.
11. A process as claimed in claim 1 , wherein the non-solvent for the cellulose ether is a solvent for the ionic liquid.
12. A process as claimed in claim 1 , wherein the non-solvent for the cellulose ether is an alcohol, a ketone, acrylonitrile, an oligo- or polyglycol, water or mixtures thereof.
13. A process as claimed in claim 12 , wherein the alcohol is a straight-chain or branched (C 1 -C 6 ) alkanol.
14. A process as claimed in claim 1 , wherein the amount of cellulose in the solution comprising an ionic liquid is in the range of from 1 to 35% by weight of cellulose, relative to the total weight of the solution.
15. A process as claimed in claim 14 , wherein the amount of cellulose in the solution comprising an ionic liquid is in the range of from 4 to 20% by weight, relative to the total weight of the solution.
16. A process as claimed in claim 1 , wherein a cation in the ionic liquid comprises one or more cations selected from the group consisting of
wherein
R 1 and R 2 independently from each other represent a (C 1 -C 6 )alkyl or (C 2 -C 6 )alkoxyalkyl group,
R 3 , R 4 , R 5 , R 6 , R 7 , R 8 and R 9 independently from each other represent a hydrogen or halogen atom, a (C 1 -C 6 )alkyl, (C 2 -C 6 )alkoxyalkyl or a (C 1 -C 6 )alkoxy group.
17. A process as claimed in claim 16 , wherein R 1 and R 2 are both (C 1 -C 4 )alkyl groups, and R 3 -R 9 , if present, are hydrogen atoms.
18. A process as claimed in claim 1 , wherein an anion in the ionic liquid comprises one or more anions selected from the group consisting of halide; pseudohalide; perchlorate; (C 1 -C 6 )carboxylate; nitrate; dicyanoamide; (C 2 -C 6 )carboxylate substituted by one or more halogen atoms; (C 1 -C 6 )alkyl sulfonate substituted by one or more halogen atoms; borate; phosphorus hexafluoride, hexafluoroantimonate and dibutylphosphate.
19. A process as claimed in claim 18 , wherein the halide is chloride, bromide or iodide; the pseudohalide is thiocyanate or cyanate; the (C 1 -C 6 )carboxylate is formate, acetate, propionate, butyrate, lactate, pyruvate, maleate, fumarate or oxalate; the (C 2 -C 6 )carboxylate substituted by one or more halogen atoms is trichloroacetate; the (C 1 -C 6 )alkyl sulfonate substituted by one or more halogen atoms is trifluoromethanesulfonate; and the borate is tetrafluoroborate or bis[oxalate(2-)]borate.
20. A process as claimed in claim 1 , wherein the ionic liquid is 1-ethyl-3-methyl-imidazolium acetate, 1-ethyl-2,3-dimethyl-imidazolium acetate, 1-butyl-2,3-dimethyl-imidazolium chloride or mixtures thereof.
21. A process as claimed in claim 1 , wherein the cellulose ether is 2-hydroxy-ethyl cellulose, 2-hydroxy-propyl cellulose, 2-hydroxy-3-isopropoxy-propyl cellulose, 3-allyloxy-2-hydroxy-propyl cellulose, 3-chloro-2-hydroxy-propyl cellulose, chloride of 2-hydroxy-3-trimethylammonium-propyl cellulose or 2-hydroxy-3-phenoxy-propyl cellulose.
22. A process as claimed in claim 1 , wherein the etherifying agent is selected from the group consisting of alkyl halides, alkyl sulfates, epoxides, aziridines, acrylonitriles, acrylamides, vinylsulfonates and diazoalkanes.
23. A process as claimed in claim 1 , wherein a catalyst is present during the etherification reaction and the catalyst is an organic or inorganic acid or a solid-state catalyst.
24. A process as claimed in claim 1 , wherein the ionic liquid is recycled from the non-solvent for the cellulose ether.