Ga/ZSM-5 catalyst improves hydrocarbon yields and increases alkene selectivity during catalytic fast pyrolysis of biomass with co-fed hydrogen

Abstract

Catalytic fast pyrolysis using the zeolite ZSM-5 is an attractive process for converting lignocellulosic biomass into fuels and chemicals. Ga-modified ZSM-5 has demonstrated improved hydrocarbon yields compared to ZSM-5 due to additional functionality imparted by Ga; however, there is little knowledge of the active Ga species and its role in the catalytic mechanisms. Here, we employ micropyrolyzer – GC-MS experiments and theoretical calculations to demonstrate that a hydrogen-pretreated Ga species (Ga*/ZSM-5) and a reductive environment are critical towards upgrading pine pyrolysis vapors into high yields of alkenes and aromatic hydrocarbons at near atmospheric pressure. The total carbon yield (g C in product per g C in pine) in alkenes and aromatic hydrocarbons under these conditions was 37% compared to 25% for the parent ZSM-5 catalyst. The corresponding carbon yield was only 19% for Ga*/ZSM-5 under inert conditions indicating that both the hydrogen-pretreated Ga* species and reducing atmosphere are required to obtain high hydrocarbon yields. The ratio of carbon in alkenes to carbon in aromatic hydrocarbons increased to 2.5 with Ga*/ZSM-5 under a reductive environment vs. 0.4 for ZSM-5. The carbon yield of alkenes increased with Ga loading; in contrast, increasing catalyst acidity promoted aromatic hydrocarbon production. Experiments conducted with isopropanol demonstrated high selectivity to propene over Ga*/ZSM-5 under reductive environments, indicating enhancement of dehydration reactions. A computational mechanism study was conducted to identify the active Ga species ([GaH₂]⁺, [GaO]⁺, [Ga(OH)₂]⁺ or [GaH(OH)]⁺) using the dehydration of isopropanol as a model reaction. Theoretical calculations suggested that [Ga(OH)₂]⁺ and [GaH(OH)]⁺ are the most likely species responsible for dehydration with 39.7 and 38.8 kcal mol⁻¹ activation energy barriers, respectively, and based on thermodynamic analysis, their ratio in the catalyst is dictated by H₂ partial pressure and temperature. The model compound studies and computational results provide mechanistic support for the observed biomass experiments showing increased alkene selectivity.