Room-Temperature Activation of the C–H Bond in Methane over Terminal Zn^II–Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Abstract

Oxygenase reactivity toward selective partial oxidation of CH 4 to CH 3 OH requires an atomic oxygen-radical bound to metal (M−O • : oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C−H bond in CH 4 . Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M−O • intermediate. However, no experimental data of M−O • have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M−O • , its reactivity for CH 4 , and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH 4 over Zn II −O • in the MFI zeolite. One Zn II −O • species does perform H-abstraction from CH 4 at room temperature. The resultant CH 3• species reacts with the other Zn II −O • site to form the Zn II −OCH 3 species. The H 2 Oassisted extraction of surface methoxide yields 29 μmol g −1 of CH 3 OH with a 94% selectivity. The quantum mechanics (QM)/ molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol −1 . On the basis of the findings in gas-phase experiments regarding the CH 4 activation by the free [M−O • ] + species, the remarkable H-abstraction reactivity of the Zn II −O • species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M−O • bond and thereby enables to create effectively the highly reactive M−O • bond required for low-temperature activation of CH 4 . The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

Advanced X‐ray Absorption Spectroscopy Analysis to Determine Structure‐Activity Relationships for Cu‐Zeolites in the Direct Conversion of Methane to Methanol

“…combined spectroscopic and theoretical results to identify Zn‐oxyl species (Zn II −O . ) in MFI as the active species for CH 4 activation at RT, while Zn‐OCH 3 was observed by Infrared spectroscopy (IR) as the reaction intermediate …”

DFT Mechanistic Study of Methane Mono-Esterification by Hypervalent Iodine Alkane Oxidation Process

“…[28] There is a variety of other recent literature showing methane partial oxidation to methanol, syngas, or other products. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] Our work on the partial oxidation of methane has focused on the search for processes efficient under less acidic environments, in particular trifluoroacetic acid (TFAH). [44] For instance, we have characterized a number of rhodium complexes with C-H activation ability in TFAH.…”

How to Constrain Metal–Oxyl Bonds on a Solid Surface? Lesson from Isovalent Zn(II)–Oxyl and Ga(III)–Oxyl Bonds Isolated in Zeolite Matrix

“…Oxyl bound to a Zn II ion has been recently identified in the zeolite catalyst. 27,28 Despite the poor ESR sensitivity due to the fast relaxation behavior, the Zn II −oxyl bond shows a vibronic progression that provides an electronic structure description of the Zn II −oxyl σ bond. 27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms.…”

“…27,28 Despite the poor ESR sensitivity due to the fast relaxation behavior, the Zn II −oxyl bond shows a vibronic progression that provides an electronic structure description of the Zn II −oxyl σ bond. 27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms. 27,28 Considering the charge balance, it has been suggested that a monovalent ion-exchangeable site (Scheme 1, left) acts as the anchor site to constrain the monovalent Zn II −oxyl bond.…”

“…27,28 The oxyl electrophilicity was evidenced by a unique reversible reaction with O 2 via generation/decomposition of the side-on Zn II −ozonide species through the O−O radical coupling/decoupling mechanisms. 27,28 Considering the charge balance, it has been suggested that a monovalent ion-exchangeable site (Scheme 1, left) acts as the anchor site to constrain the monovalent Zn II −oxyl bond. 27,28 In this study, we newly observe an isovalent Ga III −oxyl bond and define its geometric and electronic structures based on the vibronically resolved spectrum coupled with the DFT calculations.…”