TY - JOUR
T1 - Theoretical Investigation of Methane Hydroxylation over Isoelectronic [FeO]2+- and [MnO]+-Exchanged Zeolites Activated by N2O
AU - Mahyuddin, M. Haris
AU - Shiota, Yoshihito
AU - Staykov, Aleksandar
AU - Yoshizawa, Kazunari
N1 - Funding Information:
This work was supported by KAKENHI Grants JP24109014, JP15K13710, and JP17H03117 from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the MEXT Projects of “World Premier International Research Center Initiative”, “Integrated Research Consortium on Chemical Sciences”, and “Elements Strategy Initiative to Form Core Research Center”, and JST-CREST Grant JPMJCR15P5. M.H.M. gratefully acknowledges an Indonesia Endowment Fund for Education, the Ministry of Finance of Indonesia, for scholarship support.
Publisher Copyright:
© 2017 American Chemical Society.
PY - 2017/9/5
Y1 - 2017/9/5
N2 - While the most likely structure of the active site in iron-containing zeolites has been recently identified as [FeO]2+ (Snyder et al. Nature 2016, 536, 317-321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d4 isoelectronic systems, [FeO]2+ and [MnO]+ zeolites. We found that [FeO]2+ zeolites favor the direct-radical-rebound mechanism with low CH4 activation energies, while [MnO]+ zeolites prefer the nonradical mechanism with higher CH4 activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and Oα (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO]+ zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for N2O decomposition over [Fe]2+-MFI and -AEI zeolites were also performed.
AB - While the most likely structure of the active site in iron-containing zeolites has been recently identified as [FeO]2+ (Snyder et al. Nature 2016, 536, 317-321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d4 isoelectronic systems, [FeO]2+ and [MnO]+ zeolites. We found that [FeO]2+ zeolites favor the direct-radical-rebound mechanism with low CH4 activation energies, while [MnO]+ zeolites prefer the nonradical mechanism with higher CH4 activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and Oα (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO]+ zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for N2O decomposition over [Fe]2+-MFI and -AEI zeolites were also performed.
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U2 - 10.1021/acs.inorgchem.7b01284
DO - 10.1021/acs.inorgchem.7b01284
M3 - Article
C2 - 28809113
AN - SCOPUS:85028928329
SN - 0020-1669
VL - 56
SP - 10370
EP - 10380
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 17
ER -