Selective Tandem Catalysis for the Synthesis of 5-Hydroxymethylfurfural from Glucose over In-situ Phosphated Titania Catalysts: Insights into Structure, Bi-functionality and Performance in Flow Microreactors

5-Hydroxymethylfurfural (HMF) synthesis from glucose over in-situ phosphated titania catalysts is presented. Phosphates were incorporated into titania framework forming a titanium phosphate surface layer, where the coordinatively unsaturated tetrahedral TiO4 units act as water-tolerant Lewis acid site (LAS) and the adjacent protonated phosphate as Brønsted acid site (BAS), together forming Lewis-Brønsted acid pairs at molecular-level proximity. Glucose turnover and HMF selectivity were enhanced due to the rapid fructose transfer from LAS to the adjacent BAS for its dehydration to HMF, facilitating LAS liberation for another glucose turnover. Reactions in a water-2-methyltetrahydrofuran biphasic system in packed-bed microreactors gave 66% HMF yield (from 1 M glucose at 150 °C), where the HMF space time yield is about two orders of magnitude higher than that in batch and the literature work. Phosphate leaching from the catalyst is rather limited, whereas the catalyst deactivated mainly by humin deposition and could be regenerated by calcination.