Mechanisms of biomass feedstocks conversions and carbon dioxide reduction
Date of Issue2016
School of Materials Science and Engineering
A*STAR Institute of Bioengineering and Nanotechnology
With the growing demand for renewability and sustainability, the conversion of biomass into value-added chemicals has attracted comprehensive attention all over the word. The fundamental challenge of this conversion is that biomass feedstocks are fairly oxygen-rich and hyperfunctionalized. Two feasible approaches have been developed to solve this issue, namely deoxydehydration and dehydration. Oxorhenium-catalyzed deoxydehydration is demonstrated to be an ideal option to remove two adjacent hydroxyl groups from sugars and sugar-derived polyols. To have a better understanding of the mechanism of this catalytic reaction, systematic density functional theoretical calculations have been performed. We have investigated multiple sugar alcohols and derivatives with vicinal or non-vicinal diol structures. The deoxydehydration reactions are significantly exothermic for all alcohols with the oxorhenium reduction being the overall rate-limiting step. Aside from the traditional deoxydehydration for vicinal diols, the 1,2*n (n=2,3..)-deoxydehydration has been proposed for the first time by calculations and further confirmed by experiments. In comparison with three-step Pathway A, the alternative Pathway B is kinetically more favorable for 1,2*n - deoxydehydration. As the most abundant monosaccharide in nature, glucose plays a key role in the synthesis of biofuels and useful chemicals from renewable biomass. Through isomerization and subsequent dehydration, glucose can be transformed into 5-hydroxymethyfurfural which has been listed as one of the top ten value-added bio-based chemicals by U.S. Department of Energy. Catalyzed by ammonium polymer (C6H6-CH2-NH2/C6H6-CH2-NH3+), the overall reaction of glucose isomerization is endothermic while the ring-opening step is the rate-limiting step. In addition, the effect of positive charge has been investigated by comparing the energetic profiles and charge distribution with neutralized system. The results indicate that chloride ion has negligible influence on the reaction. Moreover, the water-assisted pathway is determined to be more kinetically and thermodynamically favorable. In terms of acid catalyzed fructose dehydration into 5-hydroxymethyfurfural, the reaction is exothermic by -15.2 kcal mol-1 with the hydroxyl group elimination sequence being decided by comparing the energetics of protonation on different oxygen sites. In addition to biomass conversions, utilization of carbon dioxide as a renewable source is a hot topic of green chemistry. The mechanism of CO2 splitting catalyzed by N-heterocyclic carbenes has been investigated in the presence of benzaldehyde as the reducing agent. Three channels of pathways have been discussed in details, namely NHC-CO2 pathways, NHC-PhCHO pathways and NHC-CO2/NHC-PhCHO pathways. The Pathway I-4 is proven to be the most favorable one with an overall activation barrier of 24.5 kcal.mol-1. Interestingly, bases (K2CO3/KOtBu) play a vital role in this CO2 splitting reaction.