Catalytic aqueous-phase conversion of sugars and biomass to hydroxy acids: Role of weak acid sites on zirconia-supported metal oxides

Abstract

Lactic and glycolic acids are essentialplatform molecules for producing biodegradable plastics and other valuablepharmaceutical chemicals. This work addresses the key challenge of convertingbiomass and derived sugars into these hydroxy acids via catalytic aqueous-phasereaction, avoiding the use of corrosive or contaminant compounds, therebycontributing to more sustainable processes. The deposition of humins on acidicsolids was identified as a major factor contributing to catalyst deactivationand limiting the maximum achievable yields. In this context, the influence ofacid sites of different strengths in enhancing the performance ofhigh-surface-area mesoporous hydrated zirconia (ZH) was investigated. To thisend, a series of metal oxides exhibiting distinct acid-basic behavior (MoO₃,ZnO, BaO, CeO₂, MgO, and SnO₂) were incorporated onto ZH and evaluated in theconversion of a sugars mixture at 180 ºC. A clear linear correlation betweenthe density of weak acid sites and the target acids yields was identified,confirming their crucial role in promoting the desired reaction pathways.Surface analysis by XPS revealed electronic environment modifications thatenabled a suitable acid-base balance. Among the tested systems, ZnO/ZHexhibited the most significant improvement, reaching molar selectivitiestowards lactic and glycolic acids above 80 %. Furthermore, the catalystdelivered promising results when applied to complex biomass feedstocks,including wheat bran, with molar yields of up to 59 %. This study providesvaluable insights into designing efficient, environmentally friendly solidcatalysts for biomass valorization, highlighting the need for strategies tominimize side reactions, particularly humins formation.