Fructose dehydration to 5-hydroxymethylfurfural over supported acid catalysts: impact of acid chemical species.

Biomass-derived 5-hydroxymethylfurfural (HMF) represents a crucial platform molecule in the advancement of sustainable chemical production, being produced via acid-catalysed dehydration of sugars. This study investigates the impact of three supported organic acids, namely sulphonic (RSO3H/stellate), phosphonic (RPO3H2/stellate), and carboxylic (RCOOH/stellate) acids, grafted on mesoporous silica nanospheres. Deployment of these as catalytic species facilitates the evaluation of acid strength on HMF dehydration and the impact on subsequent undesirable side reactions, including polyfuranic humin formation. While increased acid strength promotes higher fructose conversion, the weaker carboxylic acid catalysts consistently exhibit superior long-term selectivity towards HMF. Maintaining high HMF selectivity is reliant on preventing further reactions of it, with acid-catalysed aldol condensations yielding humins more favourably over stronger acid sites. After 45 min at 165 °C, HMF selectivity is greater for RCOOH/stellate (85% at 53% conversion) relative to RSO3H/stellate (78% at 83% conversion) and RPO3H2/stellate (71% at 91% conversion). As reaction temperature escalates, a trade-off between elevated conversion and diminishing selectivity is apparent for all catalytic materials. At 185 °C, conversion reaches 70% in only 5 min with HMF selectivity of 89%, but at the expense of reduced long-term process selectivity (HMF selectivity ∼23% at 45 min). The catalyst's green credentials are further apparent from a facile reactivation protocol, which restores >85% of the initial catalyst activity.