Highly Active Silver ion-Exchanged Silicotungstic Acid Catalysts for Selective Esterification of Glycerol with Lauric Acid

Abstract

A series of highly active silver ion-exchanged silicotungstic acid (H4SiW12O40; STA) catalysts, AgxH4−xSiW12O40 (x = 1–4), were successfully synthesized and applied for the production of glycerol monolaurate (GML) via esterification of lauric acid (LA) with glycerol (GL). The physicochemical properties of these Ag-promoted STA were characterized by TGA, FE-SEM, XRD, and FT-IR, and their acidic properties were studied by a solid-state 31P NMR probe molecule method using trimethylphosphine oxide (TMPO) as the probe. Among various catalysts, the Ag2H2SiW12O40 catalyst showed extraordinary activity owing to the strong Bronsted acidity, Bronsted–Lewis acid synergy, as well as the unique Keggin structure possessed by the catalyst. A Box–Behnken design (BBD) based on response surface methodology (RSM) was exploited for the optimization of process variables. A maximum GML yield of 80.2% was achieved for the Ag2H2SiW12O40 catalyst under a GL/LA molar ratio of 4.4, catalyst amount of 4.6 wt%, reaction temperature of 429 K, and reaction time of 153 min, which was in conformity with those predicted by the mathematical model. Further recyclability and kinetic studies confirmed that the Ag2H2SiW12O40 catalyst was reusable, robust, and highly efficient for the synthesis of GML with an activation energy (Ea) of 38.14 kJ/mol. Due to the strong acidity and synergy of B/L acid sites, Ag ion-exchanged silicotungstic acid catalysts show superior activity and durability for the synthesis of glycerol monolaurate.