4.7 Article

Reactivity descriptors of diverse copper-oxo species on ZSM-5 zeolite towards methane activation

Journal

CATALYSIS TODAY
Volume 338, Issue -, Pages 108-116

Publisher

ELSEVIER
DOI: 10.1016/j.cattod.2019.05.007

Keywords

Cu-oxo-species; Zeolite; Spin density; Methane activation; Descriptors

Funding

  1. National Natural Science Foundation of China [21522310, 21473244, 91645112]
  2. Natural Science Foundation of Hubei Province of China [2014CFA043]
  3. Key Research Program of Frontier Sciences, CAS [QYZDB-SSW-SLH026]
  4. Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) [U1501501]
  5. Sinopec Corp. [417012-4]

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Copper-containing ZSM-5 zeolite can facilitate the direct oxidation of methane to form methanol under mild condition with high selectivity. However, due to the limitation of the experimental characterization, the identification of the catalytic sites in Cu-exchanged zeolites still remains unclear. Herein, the reactivity of different [CuxOy] (x = 1, 2, 3; y = 1, 2, 3)- and [CuOH](+)-ZSM-5 active sites towards methane partial oxidation process has been elucidated by using density functional theory (DFT) calculations. It is indicated that i) C-H activation over five different active sites obeys the radical mechanism, and the reactivity follows the sequence: copper-oxyl [CuO](+) (S = 1) > tricopper [Cu3O3](2+) (S = 3/2) > bis-(mu-oxo)-dicopper [Cu2O2](2+) (S = 1) > mono-(mu-oxo)-dicopper [CuOCu](2+) (S = 1) > mono-copper [CuOH](1)(+) (S = 1/2); ii) The rate constants for C-H bond cleavage of methane at the reaction temperature are faster at the higher temperature; iii) The descriptors for C-H activation of methane on [CuxOy] (x = 1(similar to)3, y = 1 3)- and [CuOH](+)-ZSM-5 have been established, for which the active site with more spin density and less negative charge will induce the lower activation barriers for C-H activation of methane. Meanwhile, the stronger hydrogen affinity of the active site is, the lower the activation barriers of methane C-H activation will be. Taken the stability and activation barrier together, the catalytic performance of copper-oxyl site has been enhanced by the presence of water.

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