Phosphorylated Sporopollenin as a Sustainable Catalyst for Selective 5-Hydroxymethylfurfural Formation in Water: Insights into Phosphate Functionalization, Kinetics, and Mechanism

Metal-free Brønsted acid catalysts in water often lead to undesirable side reactions, complicating the synthesis of 5-hydroxymethylfurfural (5-HMF). As a result, organic heterogeneous catalysts with high selectivity for 5-HMF are highly sought after. Hence, this study uses sporopollenin (exine) as a heterogeneous support. 31P CP/MAS NMR showed that conventional H3PO4 cleaning of spores not only produces empty sporopollenin (ESP) but also functionalizes it with mono- and di-phosphoesters (41:59). This functionalized ESP, termed ESP-Phos, serves as a selective catalyst for the formation of 5-HMF from glucose. By activating ESP-Phos at 200 °C, the di-phosphoester ratio increases to 29:71, improving 5-HMF yield (92%) and selectivity (96%) in H2O. DFT calculations reveal strong glucose interactions with di-phosphoester, explaining enhanced catalysis. Mechanistic studies involving isotopic labelling, 13C NMR, and DFT calculations suggest direct glucose dehydration to form 5-HMF. Despite humin deposition causing deactivation, calcining at 200 °C restores catalytic activity. This method is environmentally friendly and suitable for large-scale 5-HMF production.