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 are known to facilitate undesired side reactions such as polymers and formic acid synthesis, making it challenging to synthesize 5-hydroxymethylfurfural (5-HMF) from C6 sugars. Therefore, water-tolerant organic heterogeneous catalysts with high selectivity towards 5-HMF are of great interest. In this study, sporopollenin (exine), a natural biopolymer biomass, is employed as a heterogeneous support. We demonstrate for the first time that ortho-phosphoric acid-cleaning of the protoplasmic content on spores produces empty sporopollenin (ESP) functionalized with mono- and di-phosphoesters (41:59), (ESP-Phos) which can be used as a selective sustainable catalyst for the formation of 5-HMF from glucose. Activating ESP-Phos at 200 °C results in a substantial increase in the di-phosphoester ratio of ESP-Phos200 (29:71), as shown by 31P NMR. This results in improved activity (92% yield) and selectivity (96%) for the synthesis of 5-HMF. The DFT calculations further suggest that glucose to di-phosphoester interactions are stronger (–86.1 kJ mol–1) than mono-phosphoester interactions (–72.5 kJ mol–1), explaining the potential enhancement in glucose to 5-HMF catalysis. Kinetics analysis indicated that the catalytic system follows a pseudo-first-order reaction. ESP-Phos200 increased the glucose to HMF formation rate 23-fold (rate constant k = 0.0046 min–1) higher under optimized conditions (180 oC, 12 h) as compared to the humin formation rate. A detailed mechanistic study involving isotopic labelling, 13C NMR, and DFT calculations suggested that ESP-Phos200 followed a direct glucose dehydration mechanism rather than glucose isomerization to fructose and its subsequent dehydration. Although humin polymer deposition was observed after each run (up to 4 cycles), causing deactivation of active sites, subsequent calcining of ESP-Phos200 at 200°C restored the activity. The catalyst and reaction method used here for the synthesis of 5-HMF are environmentally friendly, sustainable, and promising for large-scale production of 5-HMF.