Ketalization mechanism of glycerol to biobased glycerol levulinate ketal over HZSM-5: A combined experimental and theoretical study

Methyl levulinate ketal (MLK) is a novel biobased chemical derived from glycerol via ketalization, and exploring its synthesis mechanism is of great significance for the high-value utilization of biomass resources. In this study, the ketalization reaction of glycerol was evaluated using the MLK yield calculated by the GC internal standard method. The acidity of HZSM-5 was determined by NH₃-TPD. The catalyst screening experiments demonstrated that HZSM-5 in zeolites and AlCl₃ in metal salts exhibited high catalytic activity in the synthesis of MLK. Furthermore, the kinetic results indicated that the HZSM-5 zeolite catalyst exhibited a lower apparent activation energy in catalyzing the synthesis of MLK compared to the metal salt catalyst AlCl₃. The ketalization reactions of levulinate and glycerol catalyzed by different types of acidic sites in HZSM-5 were then investigated using the density functional theory method. A comprehensive catalytic path, free energy changes, intermediates, and transition state structures were proposed. Furthermore, the free energy barriers of ethyl levulinate ketal and butyl levulinate ketal catalyzed by HZSM-5 were calculated, and the difference between the steric hindrance effect and the electronic effect of alkyl in levulinate was compared. This study enhances the understanding of ketalization reactions and will be useful in future catalyst design studies. 

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