Heteroatom-Substituted Delaminated Zeolites as Solid Lewis Acid Catalysts

Abstract

This manuscript represents a comparative study of Lewis acid catalysis using heteroatom-substituted delaminated zeolites, which are synthesized using an approach that obviates the need for surfactants and sonication during exfoliation. The comparison involves heteroatom substitution into silanol nests of delaminated zeolites consisting of DZ-1 and deboronated UCB-4. Diffuse reflectance ultraviolet (DR-UV) spectroscopy demonstrates framework heteroatom sites, and the Lewis acidity of these sites is confirmed using infrared spectroscopy of adsorbed pyridine. The enhanced catalytic accessibility of these Lewis acid sites is confirmed when performing Baeyer–Villiger oxidation of substituted 2-adamantanones with hydrogen peroxide as the oxidant. Comparison of delaminated Sn-DZ-1 with three-dimensional Sn-Beta for this reaction shows that the delaminated zeolite is more active for bulkier ketone substrates. The role of the two-dimensional crystalline framework of the delaminated zeolite on catalysis is highlighted by comparing delaminated zeolites Sn-DZ-1 with Sn-UCB-4. The former exhibits a significantly higher activity for Baeyer–Villiger oxidation, yet when comparing Ti-DZ-1 with Ti-UCB-4, it is the latter that exhibits a significantly higher activity for olefin epoxidation with organic hydrogen peroxide, whereas both delaminated zeolites are more robust and selective in epoxidation catalysis compared with amorphous Ti/SiO_2.

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