Novel Single-Site Titanosilicates with Targeted Connectivity and Nuclearity of Titanium(IV): Synthesis, Characterization and Catalytic Properties in Alkylphenol Oxidation

Titanosilicates are a family of porous materials that have shown excellent catalytic activity in olefin epoxidation and many other selective oxidation reactions. These materials have been extensively studied in the past few decades. One of the central questions in these investigations has been to define the catalytically active centers. The traditional synthesis of wet impregnation followed by calcination often produces more than one type of catalytic center within the same matrix, i.e. different coordination, connectivity and nuclearity. Each type potentially possesses unique catalytic activity. Co-existence of multiple catalytic centers makes it impossible to establish the relationship between the structure of titanium center and catalytic activity.

The goal of this research was to establish a structure-function relationship in titanosilicate materials through targeted synthesis of single-site nanostructured catalysts, in which titanium centers are isolated from one another while possessing the uniform structure. A building block synthetic methodology was utilized to prepare a series of catalysts, namely 2-connected (2C), 3-connected (3C), 4-connected (4C) and tetranuclear (Ti₄ [subscript 4]) titanium catalysts. These materials were characterized in detail via gravimetric analysis, solid state NMR, diffuse reflectance UV spectroscopy, infrared spectroscopy, BET surface area and X-ray absorption spectroscopy (XANES and EXAFS). Catalytic activity of each catalyst was examined in the oxidation of 2, 3, 6-trimethylphenol with aqueous hydrogen peroxide to give the corresponding benzoquinone. Under identical conditions, high to mediocre catalytic activity has been observed in a sequence of Ti₄ [subscript 4] > 2C > 3C ≈ 4C, in terms of both conversion and selectivity. All synthesized catalysts showed excellent stability and recyclability with aqueous hydrogen peroxide at elevated temperature. A structure-function relationship was therefore developed through targeted synthesis of novel single-site titanosilicate catalysts.