Bioinspired Cu(II) Defect Sites in ZIF-8 for Selective Methane Oxidation

Abstract

Activating the C−H bonds of alkanes without further oxidation to more thermodynamically stable products, CO and CO 2 , is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, we synthesized a Cu-doped ZIF-8 metal−organic framework with 25% Cu and 75% Zn in the nodes and activated it by heating to 200 °C and dosing in a stepwise fashion with O 2 , methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle, the selectivity improves to the extent that no CO 2 can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV−vis, SEM, and XAS (XANES and EXAFS). The reaction is postulated to proceed at opencoordination copper sites generated by defects, and the mechanism of methanol production was explicated by density functional calculations with the revMO6-L exchange−correlation functional. The calculations reveal a catalytic cycle of oxygen-activated Cu I involving the conversion of two molecules of CH 4 to two molecules of CH 3 OH by a sequence of hydrogen atom transfer reactions and rebound steps. For most steps in the cycle, the reaction is more favored by singlet species than by triplets.

Optimizing Nickel Orbital Occupancy via Co‐Modulation of Core and Shell Structures to Accelerate Alkaline Hydrogen Electro‐Oxidation Reaction

“…[ 19 ] In Figure 3h, the XANES of Ni 93 Cu 7 @NC at Cu K‐edge aligns with Cu foil but slightly shifts to lower binding energy, indicating the metallic nature of Cu with partly negative charges. [ 20 ] Combining all XANES spectral fingerprints and XPS results, it is evident that Ni 93 Cu 7 @NC predominantly exists in the metallic form of Ni and Cu, with Ni exhibiting partial positive charges and Cu displaying negative charges. Simultaneously, there is electron transfer from the Ni 93 Cu 7 nanoparticles in the core to the carbon shell, signifying a strengthened metal‐support interaction (SMSI).…”

Spin Density Studies of Tetrahedral Cu(II) Ions Doped into Porous Zeolitic Imidazolate Frameworks

“…Efremov and co-workers speculated that Cu 2+ ions might be incorporated as defect sites or extraframework complexes inside the ZIF-8 cavity, and they assigned this species to Cu 2+ in an octahedral environment. Insertion of Cu ions has been confirmed by UV–vis data and electron paramagnetic resonance (EPR) spectroscopy, but the question of whether this is substituting a Zn 2+ framework site or a type of surface species anchored to the framework remains open. ,, …”

Unidentical Twins: Geometrically Similar but Chemically Distinct Metal‐Free Sites in Boron Oxide for Methane Oxidation to HCHO, CO and CO2

“…Inspired by these natural enzymes, scientists have designed iron-and copper-based biomimetic catalysts to convert M2M. [9,16,17] The metal-organic frameworks and zeolites contain-ing these transition metals have also attracted significant interest. [18][19][20] The main focus of the oxidation of methane using synthetic catalysts is to form and react Fe-and Cu-oxygen species akin to those found in biological systems.…”