Elucidation of reaction network and kinetics between cellulose-derived 1,2-propanediol and methanol for one-pot biofuel production

Abstract

Cellulose upgrading with methanol (MeOH) to produce fuel-range alcohols involves the formation of α,β-diols like 1,2-propanediol (1,2-PDO) that can undergo C–C scission, C–O scission, and C–C coupling with MeOH. In this work, the reaction network for conversion of 1,2-PDO and MeOH in H₂ flow over CuMgAlO_x has been elucidated. Isotopic ¹³C-MeOH studies show that C–C coupling with 1,2-PDO produces 2,3-butanediol (14% selectivity) and 1,2-butanediol (5.3%), while 1,2-ethanediol (3.3%) forms from retro aldol condensation of a C–C coupling intermediate. Products observed from dehydrogenation, direct C–C scission, and C–O scission of 1,2-PDO include hydroxyacetone (44%), ethanol (15.2%), 1-propanol (5.5%) and 2-propanol (6%). C–C coupling is zero-order with respect to P_Hydrogen and P_1,2-PDO, but has a 1.3 reaction order with respect to P_MeOH. C–O scission is 1^st order in P_Hydrogen and has low/near-zero reaction orders with respect to P_1,2-PDO and P_MeOH. Direct C–C scission has a 0.8–0.7 reaction order with respect to P_Hydrogen, P_1,2-PDO and P_MeOH. Dehydrogenation and C–C coupling have the lowest apparent activation energies of 45.6 and 47.2 kJ mol⁻¹, respectively, while all other pathways have apparent activation energies at least 15 kJ mol⁻¹ higher. Cofeed experiments with formaldehyde show that the rate determining step in C–C coupling is associated with the nucleophilic attack by 1,2-PDO. C–C coupling, direct C–C scission and esterification rates increase with P_{Hydroxyacetone}, while C–O scission and retro aldol condensation rates do not. C–C coupling is observed to occur with other α,β-diols and primary monoalcohols, but not with α,ω-diols or secondary monoalcohols.