Preliminary Optimization and Kinetics of SnCl2-HCl Catalyzed Hydrothermal Conversion of Microcrystalline Cellulose to Levulinic Acid

Abstract

Levulinic acid (LA) is a platform biorefinery chemical from biomass which can be converted to green solvents, plasticizers, polymer precursors, bio-based cleaning agents, fuels and fuel additives. This study assessed the potential of SnCl₂-based mixed acid systems as catalyst in the hydrothermal conversion of microcrystalline cellulose to levulinic acid. Maximum LA yield of 36.2 mol% was achieved using 0.2 M SnCl₂ concentration at test conditions of 3 h, 180°C and 1% w/v cellulose loading. To reduce precipitate formation and further improve LA yield, the strategy employed was to combine SnCl₂ (a Lewis acid) with conventional mineral acids (Bronsted acids). Evaluation of the catalytic performance of SnCl₂-HCl, SnCl₂-H₂SO₄, SnCl₂-HNO₃, and SnCl₂-H₃PO₄ (1:1 molar ratio, 0.2 M total acid concentration) were done with highest LA yield of 47.0 mol% obtained using the SnCl₂-HCl system at same test conditions. Response surface methodology optimization employing Box-Behnken design generated a quadratic model with a high coefficient of determination (r²) of 0.964. A maximum LA yield of 63.5 mol% can be achieved at 0.17 M catalyst concentration, 198°C, and 5.15 h reaction time. Rate constants were estimated using nonlinear regression, while activation energies were determined using Arrhenius equation. Cellulose hydrolysis was determined to be the rate-limiting step in the overall process. Low activation energy of 63.3 kJ/mol for glucose dehydration to hydroxymethylfurfural supports the action of SnCl₂ as Lewis acid in the mixed-acid system. LA yield simulations for plug flow reactor (PFR) and continuous stirred tank reactor (CSTR) were done suggesting a similar PFR-CSTR configuration with the established Biofine process. Lastly, a reaction scheme was presented to explain the synergy between SnCl₂ and HCl in LA production from cellulose.