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

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.