(574f) Comparative Studies of Grafted Fe(III) and Ti(IV) Active Sites on Silaceous Supports:  Crystalline Delaminated Zeolite Vs. Amorphous Silica

Silica has been widely used as a catalyst support with tunable surface area, porosity, and microscopic structure¹, making it ideal for supporting metal catalysts with high dispersion and high active site loading, while avoiding internal diffusion limitations. In contrast, zeotype materials have limited use as catalyst supports in many applications, since the majority of their surface area resides in micropores, making it inaccessible to precursors necessary for dispersing metal catalysts and to reactants and products relevant to their catalytic applications. Delamination of zeolite precursors can significantly improve accessible external surface area of zeotype materials and provides a strategy for their use as a support for dispersed metal catalysts². In particular, removal of framework boron or aluminum and substitution for a different catalytically active cation has been used as a successful approach for synthesizing heterogeneous catalysts which, in contrast to amorphous silica supported metals, contain active sites embedded into well defined, crystalline, zeolitic framework positions³.

The consequences of altering the coordination environment of a grafted cation from an amorphous support such as silica, to a crystalline framework site with well-defined structure, such as that of a zeolite, remain poorly understood. In this work, two case studies will be explored, involving Fe(III) and Ti(IV) cations grafted onto zeolite framework positions, which are compared to their grafting on amorphous silica. While grafting on amorphous silica results in cations covalently attached onto the support surface, grafting on delaminated zeolite supports results in cations that are incorporated into the crystalline framework. This incorporation is achieved by removal of framework B from delaminated zeolite supports, generating vacant tetrahedral framework positions â?? silanol nests. These vacancies are then repopulated with other cations, resulting in heteroatoms that are incorporated into the crystallographic position previously occupied by B, a coordination environment different from that of amorphous silica.

During catalyst synthesis, our data demonstrate significantly higher uptake of aqueous-phase Fe(III) precursors by the delaminated zeolite framework relative to amorphous silica. DR-UV spectra demonstrate the synthesis of two different types of highly dispersed Fe(III) sites in both of these catalysts, which are both active for liquid-phase oxidation of adamantane by H₂O₂. However, after reaction, zeolitic Fe(III) active sites remain nearly unchanged, whereas Fe(III) sites on amorphous silica aggregate, as evidenced by DR-UV spectroscopy. These data support the hypothesis that zeolitic Fe(III) sites are more robust compared to their amorphous silica counterparts, presumably because grafted Fe(III) is stabilized when embedded into the zeolite framework, as opposed to simply grafted onto the silica surface.

We also investigated isolated Ti(IV) cations grafted on the same two supports, as relevant models of industrial catalysts for the epoxidation of olefins with organic hydroperoxides. In order to further facilitate a direct comparison between the two supports, we defined the coordination environment from the top of the Ti(IV) cation with a calix[4]arene macrocycle and from the bottom with either a crystalline zeolitic or amorphous silica support. We investigate both catalysts for the epoxidation of demanding terminal olefins with tert-btuyl hydroperoxide as oxidant, and compare with calcined variants. Regardless of whether there is a calix[4]arene ligand present or not, our results demonstrate significant catalytic consequences that favor the zeolitic support over the amorphous silica support in terms of rate. This in turn suggests that the degree of connectivity to the support is less important in determining rate than the actual support type.

Altogether, these two examples provide insights into the importance of the support crystallinity, which dictates the molecular coordination environment of a grafted metal cations, in establishing the catalytic properties of atomically dispersed supported catalyst.

(1) Soled, S. Science 2015, 350 (6265), 1171â??1172.

(2) Corma, A. J. Catal. 2003, 216 (1-2), 298â??312.

(3) Ouyang, X.; Hwang, S.-J.; Xie, D.; Rea, T.; Zones, S. I.; Katz, A. ACS Catal. 2015, 5 (5), 3108â??3119.