Formation of Heteroatom Active Sites in Zeolites by Hydrolysis and Inversion

Abstract

Following the discovery of the titanosilicalite zeolite TS-1 and its remarkable catalytic properties in selective oxidation reactions with aqueous H 2 O 2 , [1] there have been several reports on the possible use of other heteroatom-substituted zeolites. Of particular interest are tin and zirconium silicalites, which are efficient catalysts in the hydroxylation of phenol with aqueous H 2 O 2 .[2] Germanium-containing silicalites, as well as a number of Ti-Ge-Si zeolites, showing catalytic activity toward oxidation with H 2 O 2 , have also been successfully synthesized. [3] Other important examples are leadcontaining zeolites with framework Pb II species; these compounds are promising photocatalysts in denitrification (de-NO x ) reactions. [4,5] Despite the considerable effort that has been devoted to the synthesis and characterization of these interesting materials, there is still no well-defined model to explain the observed catalytic properties. As the heteroatoms are postulated to be the active sites for the oxidation reactions, accurate information on their coordination environment and electronic structure is of fundamental importance for understanding the catalytic behavior of the heteroatom-substituted zeolites and for designing new materials with predetermined properties.Herein, we propose a new model for the formation of active sites in silicalite, which is based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. We demonstrate that the inversion mechanism can help to stabilize the active sites, as well as increase their accessibility to guest molecules in the zeolite pores. This model is in agreement with early experimental studies on TS-1, [6] which suggested that hydrolysis of a TiÀOÀSi bridge forces the titanium atom to move away from its tetrahedral configuration to a new more "external" relaxed position. In contrast, current models make a strong distinction between the modes of incorporation of titanium atoms: in zeolites they occupy tetrapodal sites within the framework, while in mesoporous materials they are grafted onto the silica surface and have a tripodal coordination environment.[7] Our calculations show that both configurations are realized within zeolites and that the hydrolysis-plus-inversion mechanism is the missing link between them.The characteristic features of this model are closely related to a number of structures in silica. For example, it has been postulated that the hydrolysis step during silica synthesis occurs through an S N 2 mechanism with inversion of a silicon tetrahedron.[8] A similar mechanism was reported for the formation of positively charged oxygen-vacancy defects (E' centers) in a-quartz and amorphous silica.[9] It was suggested that the E' center undergoes a distortion to a stable puckered configuration, which is accompanied by a large relaxation of the silicon atoms adjacent to the oxygen vacancy. Recently, Sokol et al. advanced the hydrolysis-plusinversion model for the stabilization of intrinsic defects, for example, vicinal disilan...

QM/MM modelling of the TS-1 catalyst using HPCx

“…Recently, to address this problem, we investigated a number of possible structures of the active sites in TS-1, which could occur under dry and aqueous conditions. 3 In particular, we identified the hydrolysis and inversion of Ti-O-Si bridges as the key mechanism, which helps to stabilise Ti within the framework of the zeolite. Here we validate and extend that study by applying a larger structural QM/MM model with a more extensive basis set and a more rigorous approach to the choice of the embedded QM cluster.…”