Effect of NH4+ exchange on hydrophobicity and catalytic properties of Al-free Ti-Si-beta zeolite

Al-free Ti-Si-beta was synthesized by a steam-assisted crystallization (SAC) method using tetraethylammonium hydroxide as a structure-directing agent. It was noted that Na in a parent gel was essential for synthesizing highly crystalline Ti-Si-beta by means of the SAC method. An absorption band at 225 nm, assigned to isolated Ti species, and another band at around 270 nm appeared in the UV spectra of the Ti-Si-beta. The latter absorption band decreased upon ion exchange of as-synthesized materials with 1 M aqueous NH4NO3 solution, and this treatment did not change the SiO2/TiO2 molar ratio in Ti-Si-beta. The Q(3) signal in the Si-29-NMR spectrum almost disappeared and the amount of H2O adsorbed decreased due to the NH4+ exchange of Ti-Si-beta. The NH4+ exchange resulted in a higher catalytic activity of the zeolite far cyclohexene epoxidation with H2O2 and a high selectivity toward epoxide compared with the H2SO4-treated Ti-Si-beta and AI-containing Ti-beta samples prepared in this study. Summarizing these results, the NH4+ exchange of Ti-Si-beta zeolite is found to induce a hydrophobic environment around Ti sites without detitanation from the zeolite framework. (C) 2001 Academic Press.