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...

Advances in theory and their application within the field of zeolite chemistry

“…The most detailed study addressing these problems was reported by To et al, 440,441 who used QM/MM techniques to model Ti substitutional centers in silicalite. The most detailed study addressing these problems was reported by To et al, 440,441 who used QM/MM techniques to model Ti substitutional centers in silicalite.…”

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

“…Regarding the degree of complexity reached by the real catalytic systems (either at the industrial scale or the laboratory scale), it is often mandatory to go beyond the ideal surface approach at low coverage. This is particularly true when defects cannot be described by specific cleavage orientations of the bulk materials, such as for low-coordinated sites of nano-aggregates, extra-framework species or vacancies, [52][53][54][55][56][57][58][59][60][61][62] and when they dominate the surface reactivity of solids. It is also mandatory to address the complexity of the system for supported catalysts when support effects are suspected to influence directly or indirectly the reactivity.…”

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

“…As already reported in ref. [26], a single inversion actually stabilizes the hydrolyzed structure. However the hydrolysis-and-inversion systems are less stable than the system in which water is bonded at the titanium site as a ligand.…”

“…[25] Moreover, it has been reported recently that a TS-1 model with a hydrolyzed TiÀOÀSi bridge is energetically disfavoured with respect to a non-hydrolyzed TiÀOÀSi bridge plus a gas phase water molecule. [26] However, stabilization of the hydrolyzed site may occur via the so-called hydrolysis-and-inversion mechanism, [26] whereby hydrolysis of a TiÀOÀSi bridge is followed by a rotation of the titanium-centred tedrahedron (see ref. [26]).…”

“…[26] However, stabilization of the hydrolyzed site may occur via the so-called hydrolysis-and-inversion mechanism, [26] whereby hydrolysis of a TiÀOÀSi bridge is followed by a rotation of the titanium-centred tedrahedron (see ref. [26]). Besides a structure characterized by simple hydrolysis of a TiÀOÀSi bridge (1W H ), hydrolyzed systems characterized by both a single inversion (1W Hinv ) and a double inversion (1W Hinv2 ) were optimized.…”