MoO2/biochar catalysts for oxidative desulfurization toward ultra-low sulfur diesel via carbothermal reduction of pistachio shell waste

Abstract

The requirement for ultra-low-sulfur diesel (ULSD) quality necessitates the use of heterogeneous catalysts with high oxidative desulfurization (ODS) activity, which are also robust and inexpensive to produce. In this work, we describe a new type of molybdenum-functionalized biochar catalyst (MoPSBC) prepared from pistachio shell (PS) agricultural waste via impregnation followed by carbothermal reduction, providing a novel means of simultaneously valorizing agricultural waste and preparing well-dispersed monoclinic MoO₂ nanoparticles with a well-developed carbon framework, as evidenced by SEM analysis. Physicochemical characterization using XRD, FTIR spectroscopy, SEM-EDX, and DSC revealed that 10 wt% Mo is the optimal loading, maximizing active-site availability while maintaining high metal–support interactions and dispersion efficiency. Under optimal reaction conditions (1 h, 70 °C, and H₂O₂/S molar ratio = 6), the 15MoPSBC catalyst exhibited high DBT conversion (>99.3%) in 10 000 ppm DBT model fuel. When tested on real gas oil with 1715 ppm sulfur, formic acid-promoted 10MoPSBCF, where formic acid acts as an in situ oxidant precursor to boost the Mo active site turnover, reduced the total sulfur content to about 89 ppm (94.6% sulfur removal), showing practical applicability to real-world feedstocks. Mechanistic studies revealed that the reversible Mo⁴⁺/Mo⁶⁺ redox cycle produced highly electrophilic oxoperoxo-molybdenum species as primary active sites, while the graphitic biochar surface enabled π–π stacking interactions for refractory dibenzothiophene (DBT) sequestration, as revealed by FTIR and SEM characterization. Kinetic studies validated pseudo-first-order kinetics with respect to the concentration of DBT. These results demonstrate the potential of MoPSBC as a cost-effective, biomass-derived catalytic platform for advanced fuel purification, providing a scalable solution for ULSD production.