Tailoring Brønsted–Lewis Acidity of Manganese phosphate Based Catalysts for Selective 5-Hydroxymethylfurfural Production

Manganese orthophosphate (Mn_3(PO4)2) and manganese pyrophosphate (Mn_2P2O7) catalysts were successfully synthesized through a facile, rapid, and cost-effective precipitation route at ambient conditions, followed by calcination at 350, 600, and 800 °C. The crystalline and textural evolutions of the catalysts were systematically investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), pyridine-adsorbed FTIR, N₂ adsorption–desorption, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). Structural analysis revealed that both catalysts were amorphous at 350 °C and transformed into monoclinic crystalline phases upon calcination at 600 and 800 °C. Acidic characterization demonstrated the coexistence of Brønsted and Lewis acid sites, with Brønsted acidity predominating, particularly in manganese pyrophosphate calcined at 350 °C. The surface area reached 13.727 m^2/g for manganese orthophosphate and 5.861 m^2/g for manganese pyrophosphate at 350 °C. SEM micrographs displayed distinct morphological transitions from flakes to ellipsoids for orthophosphate, and from porous plate-like to compact plate structures for pyrophosphate with increasing calcination temperature. Catalytic evaluations in the dehydration of cellulose, glucose, and fructose to 5-hydroxymethylfurfural (5-HMF) highlighted manganese pyrophosphate (350 °C) as the most active catalyst, affording yields of 25.77%, 50.87%, and 80.19%, respectively. These findings underscore the pivotal role of Brønsted acid sites in governing the catalytic performance, offering new insights into the design of phosphate-based catalysts for biomass valorization.