Synergistic Lewis—Brønsted Bifunctional Carbonaceous Acid Catalyst for the Sustainable Synthesis of Fuel-additive from Glycerol

Abstract

Biodiesel production generates glycerol as a byproduct, which cannot be directly used as fuel due to its high viscosity, density, and immiscibility with diesel. Therefore, glycerol valorisation into high-value products offers a promising route to improve the economic sustainability of biodiesel production. Consequently, significant research efforts are focused on converting biodiesel-derived glycerol into solketal because of its wide applications as a fuel additive. In this study, a novel, cost-effective, and easy to synthesized carbonaceous Lewis-Brønsted bifunctional acid catalyst employed to develop the solvent-free acetalization of glycerol with acetone to selectively produce solketal. Characterization revealed the presence of strong Lewis acidic (Zr4+) and Brønsted acidic (-SO3H) sites that promote acetalization, while Lewis basic (O2-) sites enhance glycerol adsorption and solketal selectivity. Owing to the synergistic interaction of these active sites, the catalyst exhibited excellent catalytic activity and universality, and the reaction conditions were optimized by varying catalyst loading, glycerol-to-acetone molar ratio, temperature, reaction time, and solvent effect to achieve maximum solketal yield and selectivity. Under the optimal conditions at 70 oC for 2h, the Zr-GHTC-SO3H catalyst achieved 96.2% conversion of glycerol and 96.0% yield of solketal using solvent-free conditions with a 1/6 molar ratio of glycerol/acetone demonstrating the catalyst potential for sustainable glycerol upgrading. The Zr-GHTC-SO3H catalyst demonstrated consistent performance across five successive cycles. The detailed kinetic analysis and mass transfer studies of the reaction system revealed that it follows pseudo-first-order kinetics with rate constant value 0.0193 min-1, and exhibited strong temperature dependence consistent with the Arrhenius equation having activation energy 4.32 kJmol-1, indicating that the process is governed by intrinsic surface kinetics rather than external film diffusion or internal pore diffusion limitations. The catalyst exhibits a straightforward and cost-effective synthesis with strong catalytic activity and high selectivity toward solketal formation under mild conditions, providing a promising route for converting industrial waste glycerol into value-added chemicals via acetalization. The sustainable green chemistry metrics show the practical feasibility of this catalytic approach for C-O condensation of industrial waste glycerol. Moreover, the process supports principles of a circular economy, green chemistry and carbon neutrality, offering a sustainable and greener pathway for glycerol valorization.