Effect of NH⁺₄ 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 Q³ signal in the ²⁹Si-NMR spectrum almost disappeared and the amount of H₂O adsorbed decreased due to the NH⁺₄ exchange of Ti-Si-beta. The NH⁺₄ exchange resulted in a higher catalytic activity of the zeolite for cyclohexene epoxidation with H₂O₂ and a high selectivity toward epoxide compared with the H₂SO₄-treated Ti-Si-beta and Al-containing Ti-beta samples prepared in this study. Summarizing these results, the NH⁺₄ exchange of Ti-Si-beta zeolite is found to induce a hydrophobic environment around Ti sites without detitanation from the zeolite frame-work.