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Solvent-free ketalization of polyols over germanosilicate zeolites: The role of the nature and strength of acid sites

Publication at Faculty of Science |
2020

Abstract

Isomorphic substitution of silicon for germanium affords germanosilicate zeolites with weak acid centers capable of catalyzing key reactions such as Baeyer-Villiger oxidation of ketones and etherification of levulinic acid. Herein, we show for the first time that UTL (Si/Ge = 4.2) and IWW (Si/Ge = 7.2) germanosilicate zeolites are active and selective catalysts of polyol (e.g., ethylene glycol, glycerol and 1,4 butanediol) ketalization to dioxolanes.

Large-pore IWW outperformed the extra-large-pore UTL zeolite in the ketalization of polyols, thus indicating diffusion limitations in bulky platelet-like UTL crystals. FTIR spectroscopy of adsorbed pyridine revealed the Lewis acidity of the UTL zeolite, whereas the more active IWW catalyst was characterized by water-induced Bronsted acidity.

Increasing the activation temperature (200-450 degrees C) reduced the concentration of Bronsted acid centers in the IWW germanosilicate (i.e., 0.16; 0.07 and 0.05 mmol g(-1) for T-act = 200, 300 and 450 degrees C, respectively) but increased the number of Lewis acid sites in both zeolites. Under optimized reaction conditions (e.g., acetone/glycerol = 25, T-act = 300 degrees C), almost total transformation of glycerol into solketal was achieved within 3 h of reaction time over the IWW zeolite at room temperature (>99% yield of the target product).

The results from the present study clearly show that weak acid centers of germanosilicate zeolites can serve as active sites in ketalization reactions.