Stabilizing Single Sites on Solid Supports: Robust Grafted Ti(IV)-Calixarene Olefin Epoxidation Catalysts via Surface Polymerization and Cross-Linking

Abstract

This manuscript develops a surface polymerization and cross-linking approach for the stabilization of single-site catalysts on solid surfaces, which is demonstrated here for grafted Ti(IV)-calixarene Lewis acids on silica. Our approach relies on cationic polymerization that is initiated by an adsorbed B(C6F5)3 and uses styrene as the monomer and diisopropenylbenzene as the cross-linking agent. The mildness of this polymerization method is demonstrated by its lack of blocking micropores and only slight consumption of mesopore internal surface area on the basis of N2 physisorption data at 77 K, both of which are in contrast to previously reported surface-polymerization approaches. Catalysis of samples before and after polymerization and cross-linking was investigated with a probe reaction consisting of the epoxidation of 1-octene with tert-butyl hydroperoxide as oxidant, which is known to be catalyzed by Lewis-acid sites, and a comparison of catalyst hydrolytic stability was performed. Added water in the latter was used as a a trigger to induce site aggregation, as a stress test to determine the effectiveness of site protection by our polymerization approach. Consistent with the N2 physisorption data, catalysis data demonstrate that surface polymerization does not block small-molecule reactant and product access to Lewis-acid sites on the surface, since the conversion remains essentially unchanged before and after surface polymerization and cross-linking. DR UV–vis, TGA, and catalysis data reveal that the grafted Ti(IV)-calixarene sites on silica maintain their catalytic activity even after being treated with corrosive protic stress-test solution. In sharp contrast, grafted sites without the polymer layer leach nearly all of their calixarene and Ti contents during similar stress testing, resulting in the near complete loss of catalytic activity. We hypothesize that the surface polymer acts as a nanoreactor gatekeeper, which prevents the large Ti(IV)-calixarene site from leaching and keeps surface complexes as single sites grafted on the silica surface, by blocking access for the migration of sites from the surface to bulk solution.