Al-modified heteropolyacid facilitates alkyl levulinate production from cellulose and lignocellulosic biomass: Kinetics and mechanism studies

Abstract

Efficient production of fuels and chemicals from lignocellulosic biomass is the main goal of biorefinery processes. Herein, the metal-modified heteropolyacids with tunable Lewis and Brønsted acid sites were designed as the monolithic multifunctional catalyst for valorizing cellulose and lignocellulosic biomass to alkyl levulinate, a versatile platform molecule. Among them, Al-exchanged two protons of H4SiW12O40 (Al2/3H2SiW12O40) was found to be uniquely effective. The kinetic behavior describing the conversion of cellulose to ethyl levulinate was investigated to monitor the process, and compared between parent H4SiW12O40 and Al2/3H2SiW12O40 to understand the mechanistic role of Al3+. The introduction of Al3+ substantially facilitated cellulose depolymerization and glucose-to-fructose isomerization, leading to a high rate and product selectivity. The estimated activation energies for cellulose degradation and glucopyraoside alcoholysis decreased by 50.6 kJ/mol and 40.9 kJ/mol, respectively. 50–72% yields of ethyl levulinate were accomplished from cellulosic biomasses such as pine wood, eucalyptus, bamboo, bagasse and Napier grass. Al2/3H2SiW12O40 allowed cellulose conversion in various linear alcohols producing high-yield alkyl levulinate, and could be reused multiple times with a good stability. This study opens a reliable and promising avenue for one-pot transformation of cellulose and raw lignocellulosic biomass to ester compounds.