Oxidation of Methane to Methanol by Water Over Cu/SSZ‐13: Impact of Cu Loading and Formation of Active Sites

Abstract

Cu/SSZ-13 catalysts with different Cu loadings (1-5 wt.%) were prepared by conventional impregnation via a slow step-dry process for the direct conversion of methane to methanol. Catalytic performance of the catalysts is found to depend on the Cu loading in both stepwise and continuous processes and 2 wt.% Cu presents the best performance. CuO nanoparticles, Cu x O y clusters and monomeric [CuOH] + species are found on Cu/SSZ-13 based on HRTEM, HAADF-STEM, DR UV-vis and FTIR spectroscopies. The high coverage of CuO nanoparticles on 5 wt.% Cu/SSZ-13 results in the loss of catalytic reactivity in spite of the existence of active Cu x O y and [CuOH] + sites. Besides, it is proposed that the formation of [Cu 2 O 2 ] 2 + and [Cu 2 O] 2 + active sites involves [CuOH] + intermediates. We further found that improved catalytic performance in cyclic stepwise process involves the re-dispersion of CuO nanoparticles and/or large clusters as well as the re-oxidation of Cu(I) cations from self-reduction of [CuOH] + species.

How changes in Cu-SSZ-13 catalytic oxidation activity via mild hydrothermal aging influence sulfur poisoning extents

“…For example, if SO 2 oxidation is strongly inhibited by product SO 3 , as NO oxidation is by NO 2 , 60 then it is possible the immediate formation of H 2 SO 4 from SO 3 and H 2 O would diminish the SO 3 inhibition. Yet another possibility is the role of H 2 O itself as an oxidant, as has been proposed for CH 4 conversion to CH 3 OH over Cu-zeolite catalysts, 61,62 which might influence the extent of SO 2 or surface sulfur species oxidation.…”

Investigation of the Active-Site Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Using In Situ UV–Vis Spectroscopy

“…The chemical looping process shows high selectivity for CH 3 OH; however, it has not yet met the criteria for industrial application because of the low productivity of CH 3 OH in the multistep process. 12 A catalytic reaction in which CH 4 is converted to CH 3 OH directly and continuously is attractive because it offers a simpler process for CH 3 OH production. However, as described above, the selective oxidation of CH 4 to CH 3 OH is challenging.…”

“…42,43 On the other hand, 6MR-Cu, 8MR-CuOH + H 2 O, and 8MR-CuOH+2H 2 O themselves have been considered not active. 12,20,47 A part of 6MR-Cu, 8MR-CuOH + H 2 O, and 8MR-CuOH+2H 2 O might slowly diffuse and form active species such as mono-(μ-oxo) species in the reaction conditions. 12,18,20,48 Thus, the group A catalysts are composed mainly of 6MR-Cu, 8MR-CuOH + H 2 O, and/or 8MR-CuOH + 2H 2 O, which is not a highly active species for the catalytic CH 4 oxidation.…”

“… − As an effective process for the direct conversion, a multistep process, i.e., a chemical looping process, has been developed using Cu zeolites in particular. − In the chemical looping process, the direct CH 4 oxidation process is broken down into three steps: the preparation of active oxygen species on solid oxides such as Cu zeolites, the partial oxidation of CH 4 , and the extraction of CH 3 OH from the surface of solid oxides. The chemical looping process shows high selectivity for CH 3 OH; however, it has not yet met the criteria for industrial application because of the low productivity of CH 3 OH in the multistep process . A catalytic reaction in which CH 4 is converted to CH 3 OH directly and continuously is attractive because it offers a simpler process for CH 3 OH production.…”

Impacts of framework Al distribution and acidic properties of Cu-exchanged CHA-type zeolite on catalytic conversion of methane into methanol followed by lower hydrocarbons

“…[12][13][14][15] Very recently, the role of zeolite lattice constraints on methane activation over the Cu-O-Cu active site has been proposed. 16 However, the catalytic performance is still insufficient in terms of CH 4 conversion and CH 3 OH yield. Apart from the high stability of CH 4 molecule, one of the reasons is that produced CH 3 OH is very easily over-oxidized to carbon dioxide under oxidative atmosphere; there is a trade-off between CH 4 conversion and CH 3 OH selectivity.…”