(177d) Investigation of Factors That Induce Cristobalite Formation during Titanosilicate Synthesis and Their Potential Impact on Heterogeneous Catalysis

Titanosilicates are industrially relevant zeolite derivatives with applications in heterogeneous catalysis, ion-exchange, and membranes in petrochemical, pharmaceutical, food and water purification fields.¹ They are potent and selective catalysts for alkene epoxidation, ammoximation and aromatic diol production, therefore, synthesis of new types of titanosilicates, targeting specialized industrially-relevant reactions, is an ever expanding field of research. However, their synthesis is challenging, due to requirement of specialized conditions and extreme sensitivity to changes in the synthesis environment, as well as occurrence of structural heterogeneities in the form of crystalline phases during synthesis. This, in turn, results in low catalytic performance. The physical structure of titanosilicates can be customized via temperature dependent phase transformations. Understanding the physicochemical consequences of such transformations is a key factor in the effective synthesis of novel and useful titanosilicates. The 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 during synthesis.² Formation of such phases is highly undesirable, as they are inactive in oxidative catalysis. Formation of crystalline silica phases such as quartz and cristobalite has also been reported. However, it is not completely understood whether or not these have any effect on the catalytic activity of titanosilicates.

A novel mesoporous titanosilicate was developed and optimized via the Doehlert matrix statistical approach,³ 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. The crystalline phase was identified to be predominantly ᵦ-cristobalite, the lesser known, high-temperature crystalline form of silica, (the other being quartz), which can, however, exist in a metastable state at temperatures lower than 1470 ^oC. In order to understand the effect of this cristobalite phase in heterogeneous catalysis, different samples of the titanosilicate were synthesized and calcined at various temperatures ranging from 650 áµ’C to 950 áµ’C, to gradually increase the content of cristobalite. Control experiments were conducted with pure silica, at the same conditions. It was found that cristobalite formation is 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 and the corresponding pure silica samples show that the presence of a cristobalite phase correlates with catalytic activity of the titanosilicate. The materials were extensively characterized using XRD, XPS, BET, TEM and SEM analysis to help interpret the catalytic experiments. The effect of cristobalite on catalysis, as to whether it induces an impact or is merely a spectator species is discussed. Investigation of such structure-function relationships 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.

1. A. S. Perera and M.-O. Coppens, in Catalysis, Volume 28, ed. J. J. Spivey, RSC Publishing, 2016, vol. 28, ch. 05, pp. 119-143

2. M. Baca, W. J. Li, P. Du, G. Mul, J. A. Moulijn and M.-O. Coppens, Catalysis Letters, 2006, 109, 207-210.

3. D. H. Doehlert, Applied Statistics, 1970, 19, 231–239.