(764i) Limitations of Top-Down Synthesis and Chloride Based Bifunctional Polymer Solid Acid Catalysts for Cellulose Hydrolysis
Maksim Tyufekchiev, Michael T. Timko, Marion Emmert,
Worcester Polytechnic Institute, Worcester, MA
Pu Duan and Klaus Schmidt-Rohr
Brandeis University, Waltham, MA
Clark University, Worcester, MA
Solid acid catalysts have been proposed as a viable alternative to liquid acids and enzymes for cellulose hydrolysis for the recovery of glucose and other valuable compounds. The low catalytic activity of materials possessing only acidic groups in their chemical structure has motivated a new approach. A dominant hypothesis in the literature suggests that introduction of specific binding groups to the catalyst structure in addition to strong acidic groups increases the interaction time between cellulose and the catalyst resulting in increase of hydrolysis rate. A typical procedure for synthesis of such bifunctional is top-down modification of insoluble polymer precursors either commercially available or polymerized prior to modification. Materials that possess chloride as a binding group, thought to interact with cellulose by forming a hydrogen bond with a hydroxyl group, have shown remarkable activity towards cellulose hydrolysis to glucose and levulinic acid. However, the mechanism of catalysis involving solid acid and cellulose solid particles has not been elucidated.
In this work, we investigated the literature hypothesis of structure-related catalytic activity by detailed catalyst structure characterization and a series of hydrolysis tests. Commercially available cross-linked chloromethyl polystyrene beads were functionalized by a well-known synthesis procedure to introduce sulfonic acid groups to the structure, while retaining part of the chlorine groups. The catalyst was characterized using NMR, FTIR, Raman microscopy, and EDS. Interestingly, we discovered that part of the chloromethyl groups in the polymer was converted to hydroxymethyl groups, which could also form hydrogen bonds. Hydrolysis in water of cellobiose (soluble) and cellulose (insoluble) at 150 °C and 175 °C was carried out to establish the catalytic activity. The bifunctional catalyst was able to completely convert cellobiose to glucose and cellulose to glucose, levulinic acid, and formic acid. For comparison, a completely sulfonated catalyst, lacking chloromethyl groups that could potentially bind, showed low catalytic activity towards cellulose hydrolysis.
To elucidate the structure of the catalysts, we performed cross-sectional analysis with Raman microscopy and EDS, each with spatial resolution of 1-2 microns. The analysis revealed the chlorine was almost completely lacking from the structure of the bifunctional catalyst, while the sulfonic acid groups were predominantly on the surface. NMR analysis of the catalyst after hydrolysis reaction revealed that the chloromethyl groups were converted to hydroxyl groups in the hydrothermal environment. Such a degradation would result in generation of hydrochloric acid. To confirm this we tested the hydrothermal stability of the bifcuntional catalyst and ion chromatography and pH measurements confirmed chloride leaching to form acid.Based on the results from the catalyst characterization and hydrolysis reactions, we conclude two things: 1) top-down modification of solid material for catalyst preparation does not result in uniform chemical structure and 2) catalysts utilizing chloride as binding groups are not stable in hydrothermal conditions and leach hydrochloric acid. We suggest that benzylic alcohol groups may instead contribute to cellulose binding. This work points to the disadvantages of top-down bifunctional catalyst synthesis and the hydrothermal stability of chlorides, suggesting some new explanations to account for their performance in cellulose hydrolysis.