Facile and Rapid Preparation of Ag@ZIF-8 for Carboxylation of Terminal Alkynes with CO2 in Mild Conditions.

Metal-organic frameworks (MOFs) are promising hosts for catalytic active sites due to their adjustable porosity and framework chemistry. Strategies to improve synergistic effects between the installed sites and the parent MOF are highly desired. Herein, a facile and rapid method for the preparation of xAg@ZIF-8 materials was reported. The materials were systematically characterized and used as catalysts for carboxylation of terminal alkynes via direct insertion of CO2 to the C(sp)-H bond (CTACO2). It was found that the integrity of the ZIF-8 structure could be retained upon Ag loading, but short-range crystalline ordering was modified. Two types Ag species could be installed, namely, highly dispersed Ag(I) in the backbone (AgHD) and aggregated Ag(0) nanoparticles on the outer surface (AgNP). The AgNP sites are highly effective for the activation of terminal alkynes due to its high accessibility, while the AgHD-modified ZIF-8 framework worked as a CO2 reservoir with enhanced affinity. Combination of these factors translated to high activity in the CTACO2 process, the measured turnover frequency and time yield are among the highest among most heterogeneous catalysts.