Bringing the promises of microreactors and gold catalysis to lignocellulosic biomass valorization: A study on oxidative transformation of furfural

Valorization of lignocellulosic biomass is a key to a sustainable chemical production industry. Furfural has been recognized as a major biomass-derived platform compound and a precursor of various value-added chemicals including alkyl furoates with a wide range of applications. The conventional production of alkyl furoate utilizes concentrated acids and strong oxidants, making it an environmentally taxing process. A greener alternative is the one-pot direct conversion of furfural to alkyl furoate in an alcohol medium through oxidative esterification using molecular oxygen. However, the explosive mixtures of O₂ and alcohols pose a major challenge to this process. To circumvent this issue, for the first time in the literature, we employed a catalytic microchannel reactor that enabled us to carry out the reaction over a wide range of reaction conditions, including the conventionally explosive regions. The conversion of furfural to methyl 2-furoate (MF) was carried out over a silica-supported Au catalyst coated on the inner wall of a microcapillary. A thorough experimental investigation of the reaction was conducted to obtain kinetic information while accounting for possible mass transfer effects on the process. The barely addressed mechanism of gold-assisted oxidative esterification is then investigated taking advantage of ab initio density functional theory to examine plausible mechanisms. Experimental and computational evidence suggested that surface regeneration by O2 and decomposition of methanol to methoxy species are the crucial steps in the reaction pathway, while furfural was found to have no involvement in the rate-determining step. This work attempts to provide further insights into the reaction engineering and mechanistic aspects of furfural oxidative esterification to potentially contribute to rational catalyst development and process design of the conversion.