Single-Atom Zn Catalyst Derived from ZIF-8 for One-Step CO Hydrogenation to Dimethyl Ether

The development of a stable and selective catalyst for CO hydrogenation is of utmost importance for the chemical industry. Single-atom materials have recently received attention owing to their distinct atomic and electronic properties compared with nanoparticle counterparts. Herein, we report a Zn-NC single-atom catalyst derived from the pyrolysis of ZIF-8, featuring a high loading of Zn in a N-doped amorphous carbon matrix, used for one-step transformation of CO to dimethyl ether (DME). The Zn-NC single-atom catalyst presents highly exposed active sites in favor of the overall activity, reaching a site time yield of 32.4 mmol(CO)/g(Zn)/h during the reaction, 2-4 times higher than the common activated carbon-supported ZnO catalyst (ZnO/AC). With the Zn-N-3 structure dominating in the catalyst demonstrated by EAXFS, the catalyst exhibits a DME selectivity of 95.6%, while the byproducts of methane/ethane are less than 5%. A comprehensive reaction mechanism has been put forward based on the active site of Zn-N-3 by using DFT calculations, and the coupling reaction of CH3O + CHO has been found as the key step in the direct formation of DME. The terminal O atom is captured by CO to form CO2. The two-step route of methanol formation followed by dehydration is not favored kinetically in this catalyst, which accounts for the considerable release of CO2 instead of H2O as the byproduct.