CuZnOx Active Sites Anchored on the Silanols of Hollow Silicalite-1 Zeolite Enhance CO2 Hydrogenation to Methanol

Abstract

The confinement effect of zeolites has proven to be an effective method for enhancing the catalyst stability and activity. Herein, we employed an in situ defect-capture strategy to encapsulate CuZnO_x species within the cavities of hollow silicalite-1 (H-S-1) zeolite. Initially, alkali etching of silicalite-1 crystals generated unsaturated silicon species, which then captured metal oxides (i.e., CuO and ZnO) on the zeolite surface. These metal oxides, carried by unsaturated silicon, subsequently migrated into the zeolite cavities of the hollow S-1 crystals, while a pure silicon shell was formed on the external surface of zeolite. The metal species were anchored on the silanol sites, leading to the reconstruction of ultrasmall bimetallic nanoparticles (∼ 2.2 nm) and preventing their aggregation. The resulting catalyst, CuO-ZnO_x@H-S-1, exhibited high metal dispersion (37.1%) with loadings of 5.8 wt % Cu and 5.0 wt % Zn. In the CO₂ hydrogenation to methanol reaction at 240 °C and 3 MPa, this catalyst maintained an 85% selectivity toward methanol with a CO₂ conversion rate of 9.6%, achieving a methanol yield per unit mass of Cu of 1.6 g_MeOH g_Cu^–1 h^–1. Moreover, the CuO-ZnO_x@H-S-1 catalyst demonstrated high stability without deactivation over 200 h. In situ infrared spectroscopy confirmed that methanol formation followed the formate reaction pathway, with highly dispersed Cu and ZnO_x increasing the abundance of the active CuZnO_x interface, thereby promoting the rapid conversion of HCOO* to H₃CO* intermediates. This study presents an approach for preparing high-loading, bimetallic catalysts within zeolites, offering an effective strategy for stabilizing metals under harsh reaction conditions.