Cellulose Valorization in Ionic Liquids

Cellulose and some other polysaccharides can be used, in principle,as renewable sources of valuable chemicals and energy. However,many classical solvents cannot be used as reaction media for processing of these biopolymers. Relatively recently a new class of solvents, ionic liquids, has been discovered as suitable solvents for different types of biopolymers. For instance, the ionic liquid 1-butyl-3-methylimidazolium chloride can dissolve up to 25 wt. % of cellulose. This discovery has opened new perspectives for cellulose valorization. In this dissertation, different approaches for valorization of cellulose and of some other biochemicals are studied. The first approach was the reductive splitting of cellulose in the presence of hydrogen gas. Initial experiments were performed with model substrates. The study was started with the hydrogenolysis of the ketal 1,1-diethoxycyclohexane in solvent-free systems (since this ketal is not soluble in the ionic liquids of interest) over heterogeneous metal catalysts. Secondly, cellobiose was splitted and reduced in 1-butyl-3-methylimidazolium chloride in the presence of the homogeneous precatalyst HRuCl(CO)(PPh3)3. For the splitting of cellulose itself in 1-butyl-3-methylimidazolium chloride the combination of the heterogeneous metal catalyst Rh/C and the homogeneous precatalyst HRuCl(CO)(PPh3)3 proved to be the best choice. One of the possible roles of the ruthenium compound is to enhance the transfer of hydrogen molecules from the atmosphere inside the reactor into the reaction mixture. Cellulose could be fully converted under relatively mild conditions to sorbitol as the dominant product, in 74 % yield. The second approach was the production of alkylglycosides out of cellulose in the presence of an acidic catalyst in 1-butyl-3-methylimidazolium chloride. The substrate was alkylated by the primary alcohols n-butanol and n-octanol. The acidic resin Amberlyst 15DRY proved to be the optimum acidic catalyst: it catalyzes the hydrolysis of the β (1→4) links in the polymeric chain of cellulose aswell as the alkylation of the hydroxyl groups at the C1 position of the glucose intermediate. Cellulose was fully converted under mild conditions. In the reaction with n-butanol the obtained yield of butylglycosides was 86 %. The possibility of transalkylation was also studied. The third approach was the production of α-D-glucose pentaacetate out of cellulose in 1-butyl-3-methylimidazolium chloride. The transformation comprised two steps: first a hydrolysis reaction, catalyzed by the acid catalyst Amberlyst 15DRY, and secondly an acetylation reaction. The product yield was 70 %. α-DGlucose pentaacetate could be quantitatively isolated by simple liquid – liquid extraction from the ionic liquid to dibutyl ether. The solvent and the catalyst were successfully recycled and reused. In the fourth part, new ways towards the hydrolysis of cellulose and xylan in ionic liquids have been studied. Cellulose hydrolysis was carried out in 1-butyl-3-methylimidazolium chloride with α-D-glucose as the major product in 45 % yield. This reaction was performed in the presence of the solid acid catalyst Smopex-101, which can be recycled without significant loss of activity, in contrast with the reference catalyst Amberlyst 15DRY. Xylan was hydrolyzed in 1-ethyl-3-methylimidazolium chloride in the presence of paratoluenesulfonic acid to xylose, with a yield of 42 %. Up to 84 % yield of furfural was obtained after dehydration of xylan in the presence of tungstophosphoric acid in the same solvent. A one-pot conversion of cellulose and xylan into alkylhexosides and alkylpentosides respectively was also performed.nrpages: 150status: publishe