Phosphorus coordinated Pd single-atom sites on hypercrosslinked polymers for heterogeneous hydroesterification of ethylene

Recently, the development of separable and recycling heterogeneous catalyst for ethylene hydroesterification to methyl propionate (MP) has attracted wide interest of researchers. Herein, a kind of 1, 2-bis(di- tert -butyl phosphinomethyl)benzene (dtbpb)-functionalized hypercrosslinked polymers (HCPs) coordinated with Pd single-atom sites was designed and utilized for heterogeneous hydroesterification of ethylene to MP using CO and methanol. The as-prepared catalyst series were characterized with FT-IR, SEM, XPS, NMR, HRTEM, HAADF-STEM, and XAS, which confirmed the coordination structure of Pd with dtbpb ligand linked to HCPs, as well as the dispersion of Pd in the form of single atoms on the polymer support. The effects of Pd loading and reaction conditions on the ethylene hydroesterification were systematically investigated. As a result, 100 % selectivity of MP with 96.2 % conversion of ethylene were achieved on Pd/dtbpb@HCPs catalyst under the optimal condition and no obvious deactivation behavior was observed during the recycling evaluation. The detailed reaction mechanism of ethylene hydroesterification was proposed by identification of crucial intermediates of Pd-H, Pd-alkyl and Pd-acyl species by employing in-situ DRIFTS. The kinetic study was conducted and the reaction orders of ethylene and CO are corresponding to 0 and 0.5, with the activation energy of 50.7 kJ/mol.