Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite

A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Akira Oda, Takahiro Ohkubo, Takashi Yumura, Hisayoshi Kobayashi, Yasushige Kuroda

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Oxygenase reactivity toward selective partial oxidation of CH4 to CH3OH 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 CH4. 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 CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temperature. The resultant CH3• species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH 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 CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-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 CH4. 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.

Original languageEnglish
JournalInorganic Chemistry
DOIs
Publication statusAccepted/In press - Jan 1 2018

Fingerprint

Zeolites
Methane
Molecular orbitals
molecular orbitals
methane
reactivity
Chemical activation
activation
zeolites
quantum mechanics
room temperature
catalysts
polarization
Molecular mechanics
Quantum theory
metals
Temperature
hydrogen atoms
selectivity
tuning

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

Cite this

@article{eae741df33e3440785ac759962217a15,
title = "Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-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 CH4 to CH3OH requires an atomic oxygen-radical bound to metal (M-O{\^a}€¢: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH4. 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{\^a}€¢ intermediate. However, no experimental data of M-O{\^a}€¢ have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O{\^a}€¢, its reactivity for CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O{\^a}€¢ in the MFI zeolite. One ZnII-O{\^a}€¢ species does perform H-abstraction from CH4 at room temperature. The resultant CH3{\^a}€¢ species reacts with the other ZnII-O{\^a}€¢ site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH 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 CH4 activation by the free [M-O{\^a}€¢]+ species, the remarkable H-abstraction reactivity of the ZnII-O{\^a}€¢ 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{\^a}€¢ bond and thereby enables to create effectively the highly reactive M-O{\^a}€¢ bond required for low-temperature activation of CH4. 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.",
author = "Akira Oda and Takahiro Ohkubo and Takashi Yumura and Hisayoshi Kobayashi and Yasushige Kuroda",
year = "2018",
month = "1",
day = "1",
doi = "10.1021/acs.inorgchem.8b02425",
language = "English",
journal = "Inorganic Chemistry",
issn = "0020-1669",
publisher = "American Chemical Society",

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T1 - Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite

T2 - A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

AU - Oda, Akira

AU - Ohkubo, Takahiro

AU - Yumura, Takashi

AU - Kobayashi, Hisayoshi

AU - Kuroda, Yasushige

PY - 2018/1/1

Y1 - 2018/1/1

N2 - Oxygenase reactivity toward selective partial oxidation of CH4 to CH3OH 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 CH4. 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 CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temperature. The resultant CH3• species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH 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 CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-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 CH4. 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.

AB - Oxygenase reactivity toward selective partial oxidation of CH4 to CH3OH 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 CH4. 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 CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temperature. The resultant CH3• species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH 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 CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-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 CH4. 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.

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