Efficient Hydrolytic Hydrogenation of Cellulose on Mesoporous HZSM-5 Supported Ru Catalysts

Abstract

Hydrolytic hydrogenation of cellulose requires both functionalities of a metal and acid site. HZSM-5 has strong acid sites inside the micropores. However, those sites are not easily accessible to large molecules derived from cellulose. In this work, NaOH treatment of ZSM-5 was applied to introduce mesopores to the crystallite of ZSM-5. Characterizations showed that mesopores of 4–20 nm were successfully created on the surface of ZSM-5, and mesopore volume and total acid density (particularly Lewis acid density) increased as the severity of treatment increased. Loading of Ru on the mesoporous HZSM-5 resulted in a highly active and selective catalyst for conversion of cellulose to hexitols. The optimized hexitols yield of 39.4 % is 6 times higher than that of Ru supported on the parent HZSM-5. Model reaction studies (hydrolysis of cellobiose and hydrogenation of glucose) indicated that the enhanced acid density (particularly Lewis acid in mesopores) promotes the hydrolysis of oligosaccharides from primary depolymerization of cellulose. Furthermore, the uniform dispersion of smaller Ru nanoparticles due to the increased surface area caused by the mesopore enhanced the activity of the hydrogenation reaction. Consequently, a high yield of hexitols with reduced yield of small polyols was achieved on the optimized mesoporous Ru/HZSM-5.