Title : An investigation of the phosphate functionalization, kinetics, and mechanistic aspects of phosphorylated sporopollenin as sustainable catalyst for selective 5-hydroxymethylfurfural formation in water
Abstract:
The tremendous use of non-renewable fossil fuels causes global problems such as CO2 emission, hence it is imperative to switch to a renewable biomass resource. Nevertheless, utilizing biomass directly as a fuel poses significant challenges due to its intricate structure and low energy density. However, an alternative strategy involves the utilization of platform chemicals i.e., 5-HMF derived from cellulose, a degraded product of lignocellulosic biomass, which can be further processed for commercialization into liquid fuels, solvents, polymers, and bioplastics. Metal-free Brønsted acid catalysts in aqueous solution are recognized for their tendency to trigger unwanted side reactions, such as polymerization and formic acid synthesis, thereby posing challenges in the synthesis of 5-hydroxymethylfurfural (5-HMF) from C6 sugars. Consequently, there is significant interest in water-tolerant organic heterogeneous catalysts that exhibit high selectivity towards 5-HMF. In this investigation, sporopollenin (exine), a natural biopolymer biomass, serves as a heterogeneous support. We present, for the first time, the use of orthophosphoric acid to remove protoplasmic content from spores, resulting in empty sporopollenin (ESP-Phos) functionalized with mono- and di-phosphoesters (41:59). This material proves effective as a selective and sustainable catalyst for glucose conversion into 5-HMF. Notably, activation at 200°C leads to a significant increase in the phosphodiester ratio of ESP-Phos200 (29:71), observed through 31P NMR analysis. This augmentation correlates with enhanced activity (yield of 92%) and selectivity (96%) for 5-HMF synthesis. DFT calculations reveal stronger interactions between glucose and di-phosphoesters (–20.58 kcal mol–1) compared to mono-phosphoesters (–17.32 kcal mol–1), underscoring the synergistic effect of phosphodiesters in expediting 5-HMF catalysis. Kinetic analysis indicates a pseudo-first-order reaction mechanism. ESP-Phos200 under optimized conditions (180 oC, 12 h) increased the glucose to HMF formation rate 23-fold (rate constant k = 0.0046 min–1) higher than humin formation rate. Isotopic labelling, 13C NMR, and DFT calculations collectively support a glucose dehydration mechanism over fructose isomerization
for ESP-Phos200. Although humin polymer deposition leads to catalyst deactivation after multiple cycles (up to 4 cycles), subsequent calcination of ESP-phos200 at 200°C restores its activity. The catalyst and methodology employed in this study for 5-HMF synthesis offer environmentally friendly, greener, and sustainable alternatives with promising potential for large-scale production.
