Catalytic Wood Fractionation into Chemicals in Supercritical Ethanol and n-Heptane: Potential and Limitations

Abstract

Direct selective wood fractionation into chemicals is an approach that has attracted recent attention. The application of sub- and supercritical (SC) alcohols to fractionate wood into solid cellulose and liquefy phenolic monomers is a process now widely known as “lignin first”. It is justified to study the potential of other SC organic solvents of variable polarities. Herein, we compare the abilities of SC ethanol and SC n-heptane to fractionate pine wood near their critical point. While near-critical ethanol has more affinity for lignin fraction, we show that near-critical n-heptane has preference for carbohydrate deconstruction. If SC ethanol favors biooil formation which contains important ethyl/ethoxy groups, the alkane greatly favors solid carbon products. The impact of addition of heterogeneous catalysts (acid, basic and Cu-based catalysts) on wood fractionation and light chemicals formation was investigated deeply in SC ethanol. In SC ethanol, catalysts favor light liquid products such as esters at the expense of biooil with a total oxygenates yield of 33 wt% relative to carbohydrates over β zeolite. However, we show that depending on the catalysts’ nature, wood components fractionation was completely changed, and this is particularly true with solid acid catalysts which promote cellulose deconstruction and the formation of solid carbon products. It is proposed that liquid products’ accumulation in the autoclave, in particular water, is at the origin of the wood fractionation changes which preclude its control by the choice of the SC organic solvent and conditions. Moreover, all the catalysts underwent severe leaching, which also contributed to the wood component fractionation changes.