Controlling the heterogeneous catalysis of zirconium clusters within a porous SBA-15 scaffold

Abstract

Metal-organic frameworks (MOFs) with Zr-based secondary building units (SBUs) have shown promise as materials for the catalytic degradation of chemical warfare agents (CWAs). The Zr-based SBU within the MOF has been previously determined to be the active site for catalytic hydrolysis reactions within these materials. However, MOF structure dictates access to the SBU active sites with microporous MOFs showing catalysis solely on the surface of the particles of the MOF and MOFs with analogous SBUs exhibiting different reaction rates under the same reaction conditions. The multitude of variables inherent to MOF structures (e.g. pore size, pore structure, connectivity, crystal size, functional groups, defects, and monocarboxylic acid modulators (MCAMs) used in synthesis) complicate the fundamental understanding of the SBU's reactivity in the hydrolysis reaction independent of topological constraints. In this work, we have explored the catalytic activity of a simplified SBU system consisting of Zr₆ and Zr₁₂ clusters decorated with MCAMs varying in size and functionality to simulate the chemical environment of the SBU within the MOF structure. The zirconium clusters were then supported on mesoporous silica (SBA-15) functionalized with either sulfuric or phosphoric acid groups that bind to the zirconium nodes, covalently tethering the clusters to the silica support. These novel porous materials were designed to mimic the porous nature of the MOF structure to determine the effect on hydrolysis reactivity. The final silica-bound zirconium clusters showed enhanced reactivity towards the hydrolysis of dimethyl nitrophosphate (DMNP), a nerve agent simulant, under buffered conditions compared to the bare Zr clusters and showed key differences in the catalytic activity based on the chemical environment imparted by both the MCAM and the modified support. In addition, the use of an acid-modified silica scaffolding allowed for the incorporation of adjacent amine moieties on the SBA-15 support to facilitate hydrolysis of DMNP under neutral aqueous conditions, a benefit over typical Zr-based MOF catalysts that require a buffer for appreciable reactivity.