(532o) Elucidation of Reaction Kinetics between Supercritical Methanol and Cellulose-Derived 1,2-Propanediol for Biofuel Production

Supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) of cellulose over CuMgAl catalysts results in the formation of C_2-10 monoalcohols with >98% conversion. α,β-Diols are observed as the major products at low reaction times, and methanol (MeOH) incorporation is detected in all liquid products. The selectivity to C₅₊ alcohols cannot be improved without a better understanding of the reactions between α,β-diols and MeOH. In this work, we performed kinetic experiments at 300 °C and 1000 psig in a packed bed reactor with an observed intermediate from cellulose: 1,2-propanediol (1,2-PDO). Isotopic ¹³C-MeOH studies show that 1,2-PDO dehydrogenates to hydroxyacetone, which then undergoes C-C coupling with MeOH to form 2,3-butanediol and 1,2-butanediol. 1,2-PDO also undergoes C-O scission to form 1-propanol and 2-propanol, and C-C scission to form ethanol. Dehydrogenation and C-C coupling have the lowest apparent activation energies of 45.6 and 47.2 kJ/mol, while all other pathways are least 15 kJ/mol higher. From partial pressure experiments, C-C coupling was found to have a zero-order dependence on P_Hydrogen and P_1,2-PDO, but is 1^st order with respect to P_MeOH. C-O scission has a 1^st order dependence on P_Hydrogen but has near-zero reaction orders with respect to P_1,2-PDO and P_MeOH. Cofeed experiments with formaldehyde and hydroxyacetone show that the rate determining step in C-C coupling is the nucleophilic attack by 1,2-PDO on MeOH. We conclude that the selectivity to C-C coupling at low conversions can be increased by carrying out the reaction at lower temperatures (280 – 300 °C), higher P_MeOH, and lower P_Hydrogen and P_MeOH. Lastly, we demonstrate that C-C coupling can occur between other α,β-diols and primary monoalcohols, but not with α,ω-diols or secondary monoalcohols. These advances in quantifying the fundamental kinetics involved in biomass conversion with supercritical MeOH will allow for higher selectivity to desired C₅₊ alcohols and better process design.