Bolaform surfactant-directed synthesis of TS-1 zeolite nanosheets for catalytic epoxidation of bulky cyclic olefins

Abstract

Hierarchical titanium silicalite-1 nanosheets (HTS-1) were hydrothermally synthesized by using a bolaform surfactant [C₆H₁₃–N⁺(CH₃)₂–C₆H₁₂–N⁺(CH₃)₂–(CH₂)₁₂–O–(p-C₆H₄)₂–O–(CH₂)₁₂–N⁺(CH₃)₂–C₆H₁₂–N⁺(CH₃)₂–C₆H₁₃] [OH⁻]₄ as the structure-directing agent. The resultant zeolite particles possessed not only a superior interlayer stability but also a unique house-of-cards-like structure by the 90° rotational boundary connectivity of TS-1 nanosheets directed by the π–π stacking interaction from the biphenyl group in the bolaform surfactant as well as content controllability of coordinated Ti species in the zeolite framework. The obtained HTS-1 samples were used as catalysts for the epoxidation of bulky cyclic olefins (cyclohexene and cyclooctene) and exhibited improved performance and superior recyclability in comparison with the conventional solely microporous TS-1 (CTS-1) catalyst as well as the mesoporous TS-1 (MTS-1) catalyst directed by the commercial organosilane surfactant TPOAC, due to their exoteric interlayered mesopores and enlarged external surface areas providing more accessible Ti active sites for the bulky molecule reactants. Moreover, the optimized Ti content for the HTS-1 catalysts was proposed by fully taking into account the conversion and turnover frequency (TOF) values. In addition, the recyclability and stability of the HTS-1 catalysts in the epoxidation reaction and the post fluoride treatment to enhance their hydrophobicity as well as epoxidation activity were further discussed.