Preparation of hydrophobic Ti-beta catalyst derived from Al-rich zeolite beta synthesized without using organic structure-directing agent and its enhanced catalytic performance for phenol oxidation

Abstract

Crystalline microporous titanosilicates with multi-dimensional 12-ring channels are potentially useful as catalysts for the oxidation of phenol with H2O2. Among them, titanosilicate version of zeolite beta (Ti-beta), which has three-dimensional, 12-ring micropores, is one of the most promising catalytic materials. The Ti-beta catalysts are preferably with Al-free and Na-free chemical compositions, and they can be conventionally crystallized via direct hydrothermal synthesis with the aid of a dipiperidinium-type organic structure-directing agent (OSDA) or via the so-called fluoride method using tetraethylammonium in a fluoride-containing medium. Another route involves a post-synthesis treatment such as the isomorphous substitution of Ti for Al starting from Al-rich zeolite beta. One of the Al-rich beta was synthesized in the absence of OSDA. We succeeded in converting it to Ti-beta, denoted as (Ti)-beta_OSDAF, for the first time via deep dealumination along with framework stabilization followed by Ti-incorporation using TiCl4. The (Ti)-beta_OSDAF was much more effective catalyst for the phenol oxidation with H2O2 than the directly crystallized Ti-beta materials prepared in the conventional techniques. The high efficiency of phenol oxidation was ascribed to an appropriate balance between hydrophilicity and hydrophobicity realized by increasing hydrophobicity of titanosilicate catalysts. The bulk hydrophobicity reflects that of the vicinity of active Ti sites. The enhanced affinity of hydrophobic moiety of phenol molecules with the hydrophobic surface around the vicinity of active Ti sites should facilitate the diffusion of phenol molecules to the active sites inside the micropores. The hydrophobicity of Ti-beta samples prepared by the various techniques in this work differed substantially, depending on the amount of silanol groups within the zeolitic framework. Positive effect of EtOH as an additive in the reaction mixture on enhancing the shape-selectivity of the (Ti)-beta_OSDAF catalyst was obvious, probably due to the increase in steric restriction by the EtOH molecules inside the pores of the catalyst. Moreover, the phenylsilylation of (Ti)-beta_OSDAF catalyst drastically increased its hydrophobicity, which was found to be effective for enhancing para-selectivity and turnover number.