Phosphate-Terminated Nb₂C MXene as Efficient Catalyst for the Transformation of Glucose Into 5-Hydroxymethylfurfural...

The production of 5-hydroxymethylfurfural (HMF) from glucose is a reaction of considerable interest in biomass conversion into platform molecules. This reaction suffers, however, in general from low selectivity and the generation of undesirable polymeric byproducts (humins). Herein, we describe the preparation of phosphate-terminated Nb₂C MXene (Nb₂C-PO₄) as a novel heterogenous water-tolerant acid catalyst, able to afford high yields of HMF, while minimizing humin formation. The phosphate-modified Nb₂C has been extensively characterized (X-ray diffractometry (XRD), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM-EDS), X-ray photoelectron spectroscopy (XPS)) in order to prove the retention of the MXene structure and the presence of phosphate groups on the surface, and the acid-base properties have been assessed by NH₃- and CO₂-TPD and pyridine-DRIFTS. Apparently, the effect of phosphate is to mask less selective acid sites, while the remaining ones are more selective to promote HMF formation from glucose. This proposal is also supported by the contrasting behavior of pyridine and 2,6-di-tert-butylpyridine as quenchers of acid sites. Under optimized conditions, 64% yield of HMF can be obtained at 150°C in a monophasic THF/water mixture, using a high glucose concentration (5% w/w) and a low Nb₂C-PO₄ loading (S/C = 10 w/w), reaching a TOF of 3077 h^-1.