Catalytic Conversion of Biomass-Derived Polyols to Value-Added Chemicals: Catalysis and Kinetics

Abstract

Replacing fossil-based feedstocks with biomass to produce renewable fuels and chemicals is one of the major sustainability challenges facing human society. In this context, catalytic upgrading of non-food bio-derived polyols, including glycerol, erythritol, xylitol, sorbitol and mannitol, on heterogeneous catalysts attracts increasing attentions, because it will provide alternative routes for the production of fuels and chemicals. However, several issues are plaguing current technologies: (a) high oxygen contents in these C3~6 polyols demand several difficult steps of deoxygenation, which require elevated reaction temperature (T = 220~300 oC) and high operating pressure of hydrogen; (b) conversion under such harsh conditions involving multi-phase, multi-step and multi-component reactions results in low selectivity towards desired products, loss of large quantities of carbon to less valuable wastes and (c) fast deactivation of catalysts due to poor intrinsic activity and stability. The present work successfully demonstrates that, by rational design of multi-functional metal-based catalytic systems, conversion of various biopolyols to valuable megaton everyday chemicals, such as 1,2-propanediol, ethylene glycol, lactic acid and alcohols, can occur in one pot process under significantly milder reaction conditions with improved efficiency. Detailed investigation on C-C/C-O cleavage revealed possible reaction pathways and mechanism of polyols on metal based catalysts. Therefore design of multi-functional metal catalysts was achieved. It was for the first time to demonstrate that Cu catalysts exhibited an excellent C-C and C-O cleavage activity by immobilizing active sites for retro-aldol, dehydration and hydrogenolysis on one single catalyst, leading to 98% yield towards liquid products. Studies on reaction parameters and surface characterization enabled the establishment of activity-performance correlation for polyol conversion. Further, by rational combining hydrogen generation and hydrogenolysis functionality to one metal catalyst, conversion of biopolyols occurred at only 115~160 oC even without adding external hydrogen, with 95%+ overall atom efficiency. Detailed kinetic modeling revealed that the reaction potential for hydrogen generation and hydrogenolysis is much lower on Pt/C catalyst. This is a significant advancement compared with conventional technologies. In collaboration with material scientists, mono and bimetallic Cu-based catalysts with predominant active [111] surface plane were also designed via lattice match engineering. The Cu nanocatalysts exhibited more than five-fold enhancement in activity compared to traditional ones and selectivity promoted dehydrogenation thus lactic acid was favorably formed in our system. The methodologies and achieved results in this work will provide insights on the further studies on rational design of biomass conversion as well as other chemical processes.