Comparing the Nature of Active Sites in Cu-loaded SAPO-34 and SSZ-13 for the Direct Conversion of Methane to Methanol

Abstract

On our route towards a more sustainable future, the use of stranded and underutilized natural gas to produce chemicals would be a great aid in mitigating climate change, due to the reduced CO2 emissions in comparison to using petroleum. In this study, we investigate the performance of Cu-exchanged SSZ-13 and SAPO-34 microporous materials in the stepwise, direct conversion of methane to methanol. With the use of X-ray absorption spectroscopy, infrared (in combination with CO adsorption) and Raman spectroscopy, we compared the structure–activity relationships for the two materials. We found that SSZ-13 performed significantly better than SAPO-34 at the standard conditions. From CH4-TPR, it is evident that SAPO-34 requires a higher temperature for CH4 oxidation, and by changing the CH4 loading temperature from 200 to 300 °C, the yield (μmol/g) of SAPO-34 was increased tenfold. As observed from spectroscopy, both three- and four-fold coordinated Cu-species were formed after O2-activation; among them, the active species for methane activation. The Cu speciation in SAPO-34 is distinct from that in SSZ-13. These deviations can be attributed to several factors, including the different framework polarities, and the amount and distribution of ion exchange sites.

Copper-zeolites Prepared by Solid-state Ion Exchange - Characterization and Evaluation for the Direct Conversion of Methane to Methanol

“…If this is the case, it could indicate that the higher Cu-loading has led to more monomeric Cu active sites, also for the other zeolites, and not only FAU. A similar effect with methane consumption being at higher temperatures due to different framework properties was also shown comparing two different CHA frameworks (SAPO-34 vs. SSZ-13) in a previous publication by Kvande et al [27]. Another possibility is that the higher amount of Brønsted sites in 0.30Cu I -MOR aids in the reactivity towards methane, and that this is the reason behind the clear difference in CH 4 consumption temperature between the two MOR we observe herein, and not necessarily the formation of different types of Cu-oxo sites.…”

“…To fully study the effect of this SSIE synthesis method for the MTM reaction, the samples were subjected to elemental analysis, and investigated with CO-adsorption coupled with Fourier Transform-Infrared (FT-IR) spectroscopy to understand if the increased amount of Cu led to changes in the Cu speciation. Furthermore, CH 4 -TPR experiments, shown to be a helpful tool in optimizing the MTM reaction both in terms of setting the reaction parameters as well as understanding material properties, were performed [27,28]. Finally, the samples were tested in the methane to methanol reaction and compared to previously obtained results with the same reaction protocol in order to assess their performance.…”

Microcalorimetry on Cu-MCM-22 Reveals Structure–Activity Relationships for the Methane-to-Methanol Reaction

“…Looking at both samples, it is clear that as CO is dosed initially, two bands appear at 2158 and 2151 cm –1 . While only one band is usually observed for Cu + -monocarbonyl species in Cu-zeolites, for Cu-loaded MCM-22, two distinct bands are growing. ,− This has also been seen for Cu-BEA zeolites . The two distinct bands indicate that, in MCM-22, there are at least two structurally different Cu-sites, which is coherent with the sample having at least three major positions for Cu to be positioned, namely, the sinusoidal 10-ring channel, within the supercage, or on the external surface in the supercage half-cups.…”

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

“…The reaction protocol follows our previously described methodology; 13 however, the CH 4 loading was shortened to 3 h without significantly affecting the total yield of the reaction. 63 In our previous study, investigating copper exchanged on the commercial MOR highlighted that a Cu/Al ratio of <0.2 was optimal in terms of the MeOH productivity. 13 We therefore focused our study on two series of Cu-MOR across a range of copper loadings on the tetrahedral Al (i.e., Cu/Al (IV) ).…”

“…Next, we assessed the performance of these Cu-MOR zeolites in the stepwise partial selective oxidation of CH 4 . The reaction protocol follows our previously described methodology; however, the CH 4 loading was shortened to 3 h without significantly affecting the total yield of the reaction …”