261212 Conversion of Hemicellulose to Furfural Using Solid Acid Catalysts in Gamma-Valerolactone, a Biomass-Derived Solvent
Catalytic conversion strategies for the hemicellulose portion are of particular importance, because biological conversion of C5 sugars is not as efficient as the conversion of C6 sugars. In addition, C5 sugars/oligomers are produced as a side stream in the pulp and paper industry. Among the products that can be obtained from C5 sugars, furfural is highly valuable. However, current methods for production of furfural from hemicellulose suffer from drawbacks that do not comply with the principles of green and sustainable biorefining, such as use of corrosive and hazardous mineral acids and/or extractive solvents (in biphasic systems) that cannot be derived effectively from biomass. In addition, the use of solid acids in aqueous environments is problematic in view of catalyst degradation and/or leaching of catalyst components into aqueous solution at elevated temperatures (e.g., 430 K).
These difficulties associated with the conversion of xylose to furfural can be alleviated using γ-valerolactone (GVL) as a solvent in a monophasic system and using solid acid catalysts. Importantly, GVL is a green solvent that can be produced from lignocellulose. Furthermore, by minimizing the amount of water employed in the process, it is possible to use solid catalysts for the conversion of xylose (and xylose oligomers) to furfural, with minimal degradation of the catalyst and without leaching of acid sites into solution, as opposed to the case of using solid acid catalysts in aqueous solutions at elevated temperatures. High yields of furfural (>70%, with the exception of H-ZSM-5 and sulfated-zirconia) can be achieved in GVL at 448 K over a wide range of acid catalysts, including sulfonic acid functionalized catalysts (Amberlyst-70, Nafion SAC-13, sulfonated SBA-15 and carbon catalysts), zeolites (H-ZSM-5, H-mordenite and H-Beta), sulfated inorganic metal oxides (sulfated zirconia), and a homogenous mineral acid (0.02 M H2SO4). H-mordenite was chosen in this work as the solid acid catalyst for more detailed studies, due to high (ca. 80%) furfural yields obtained and its low cost and potential for regeneration with a calcination treatment following deactivation upon deposition of carbonaceous deposits (e.g., humins). The effect of water on xylose dehydration over H-mordenite was investigated by changing the water concentration (0-20 wt%) in the xylose-GVL feed mixture and it was found that as the water concentration was increased, the rate of furfural production decreased, and the maximum yield of furfural began to decrease at water concentrations higher than 10 wt%. The stability of the H-mordenite was investigated by employing this catalyst in a series of xylose dehydration reactions in GVL solvent containing 10 wt% H2O. Importantly, the recycled catalysts showed activities that were similar to that of the fresh catalyst, and the maximum yield of furfural achieved for was still ~80% even after the third recycle. Finally, using GVL as a solvent in the presence of 10 wt% water and H-mordenite, a mixture of monomeric and oligomeric C5 sugars obtained from the hot water treatment of biomass (i.e., poplar wood chips) could also be converted to furfural with high yields (ca. 75%), showing the applicability of this monophasic solvent-solid catalyst system to process the hemicellulose stream obtained at a pulp and paper facility.