Synthesis, characterization and catalytic properties of V-, Ti- and Cr- MCM-41 mesoporous molecular sieves

First row transition metal containing silicalite zeolites are important oxidation catalysts. However, there is a need to develop new catalyst systems which retain uniform pore size distribution but would allow molecules too large for the silicalite structure to reach the active sites within the pores. In the present research, several series of Ti, V and Cr containing mesoporous molecular sieves of the MCM-41 type have been synthesized by different strategies, and by systematically varying the pore size and metal loading. The uniform mesoporous structure was confirmed by X-ray diffraction and physisorption and pore size distribution has been calculated by Non-local Density Functional Theory. To obtain information on the local structure of the active species, the catalysts have been characterized by using DR UV-visible, solid state NMR, and X-ray absorption spectroscopies. The UV-visible absorption edge energy may be correlated with the domain size (local symmetry) represented by the average bond length. The active site density of the catalysts was determined by means of in-situ oxygen uptake under reaction conditions. The catalytic behavior was then investigated by utilizing both gas and liquid phase oxidations with molecular oxygen and {dollar}rm Hsb2Osb2,{dollar} respectively. Turnover frequencies were normalized by oxygen uptake. The variation of the catalytic activity may be interpreted in terms of the different pore size and local environment of active species in these materials. A strong effect of pore size on the catalytic activity was observed in the form of a "volcano curve" and may be correlated with a variation in the local bond angle of Si-O-M (M = Ti, V). Considering this effect, the catalytic activity may be adjusted to a maximum by varying the pore size. This interpretation of the pore size effect is consistent with X-ray absorption near edge structure measurements.