Titanosilicates are highly versatile zeolite derivatives, with industrially significant applications in heterogeneous catalysis, ion-exchange, and as membranes. The physical structure of titanosilicates can be customized via temperature dependent phase transformations. Understanding the physicochemical consequences of such transformations is key in the effective synthesis of novel and useful titanosilicates. It is well-known that catalytic activity of titanosilicates is brought about by tetrahedrally coordinated, isolated Ti4+ sites, connected to the silica matrix via Ti-O-Si linkages. However, formation of Ti-O-Ti oligomers and TiO2 crystalline phases is unavoidable, and is highly undesirable, as they are catalytically inactive in oxidative catalysis. Formation of crystalline silica phases is also known, however, it is not completely understood whether these have any effect on catalytic activity of titanosilicates. A novel mesoporous titanosilicate was developed and optimized via the Doehlert matrix statistical approach, as an effective and selective catalyst for the epoxidation of cyclohexene. During the synthesis optimization, it was revealed that, although the material is mostly amorphous, the most catalytically active samples contained small amounts of crystals, identified to be cristobalite. In order to understand the effect of the cristobalite phase in heterogeneous catalysis, different samples of the titanosilicate was synthesized and calcined at various temperatures ranging from 650 to 950 Celsius, to gradually increase the content of cristobalite. Cristobalite formation was found to be directly induced by Ti4+, without any addition of alkali metals, which had not been previously reported in the literature. Catalytic experiments conducted with titanosilicate samples calcined at different temperatures with corresponding pure silica samples show that the presence of a cristobalite phase correlates with catalytic activity of the titanosilicate, although this does not prove any causality on activity. The materials were extensively characterized using XRD, XPS, BET, TEM and SEM analysis to help interpret the catalytic experiments. The spectroscopic and textural analysis results reveal key structure-function relationships, which are of critical importance in understanding the factors that affect catalytic activity of titanosilicates, as well as provide insights into the importance of controlling calcination conditions in industrial synthesis.

Titanium(IV)-induced formation of cristobalite in titanosilicates and its potential effect on heterogeneous catalysis: Induced Impact or Spectator?

254th National Meeting (2017)

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