(58b) Sulfur-Tolerant Molybdenum Carbide Catalysts Enabling Low-Temperature Stabilization of Fast Pyrolysis Bio-Oil

Fast pyrolysis is a cost effective lignocellulosic biomass liquefaction technology. However, the liquid product, bio-oil, is not suitable for direct use as transportation fuels or as a crude oil substitute due to undesirable qualities such as low heating value, corrosiveness, and instability directly related to its high oxygen and water content. The quality of bio-oils can be improved by catalytic upgrading (e.g., hydroprocessing, cracking) to eliminate oxygenated functionalities.

Developing catalytic bio-oil upgrading technology has proven challenging due in large part to short catalyst lifetime. Bio-oil contains a large quantity of carbonyl compounds which, due to high tendency toward polymerization reactions, lead to rapid catalyst fouling and bed plugging. Stabilization of bio-oil by converting reactive carbonyls prior to higher temperature upgrading is therefore critical. Low-temperature hydrogenation of carbonyl fractions has been recognized as a promising approach to bio-oil stabilization. While reduced precious metal catalysts such as ruthenium are effective in low-temperature hydrogenation reactions, they are highly sensitive to sulfur species present in biomass-derived feedstocks. Therefore, to address rapid performance loss due to S-poisoning of active sites, sulfur management strategies need to be implemented (e.g., sulfur guard, catalyst regeneration) which entails increased process cost and complexity.

Molybdenum carbides have shown potential in various biomass conversion reactions especially with high selectivity towards C-O bond cleavage. In this work, we have assessed Mo carbides in low-temperature hydrogenation of bio-oil with particular emphasis on understanding their sulfur tolerance. Our study revealed that molybdenum carbides are effective in low-temperature conversion of carbonyl compounds. Furthermore, carbides showed excellent sulfur tolerance, and structural robustness with respect to morphology, crystallography, and bulk composition. These properties enabled carbides to maintain catalytic performance in both aqueous-phase furfural (380 ppm S, 150 °C, 120 bar, batch reactor) and real bio-oil (52 ppm S, 140 °C, 124 bar, flow reactor, 100 h TOS) reactor feeds. Due to surface bifunctionality (metal-acid), carbides were able to catalyze both hydrogenation and C-C coupling reactions retaining most of carbon atoms in liquid products as more stable and higher molecular weight compounds. These findings (sulfur tolerance, durability, H and C atom economy) indicate that molybdenum carbides can provide a catalyst solution for the development of commercially viable bio-oil stabilization technology.