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

Karoline Kvande,Unni Olsbye,Carlo Buono,Silvia Bordiga,Matteo Signorile,Stian Svelle,Gloria Berlier,Michael Dyballa,Elisa Borfecchia,Kirill A Lomachenko,Pablo Beato,Bjørnar Arstad,Dimitrios K Pappas

doi:10.3390/catal10020191

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.

Highlights

Methane has become increasingly abundant as a carbon resource in recent years, and can be found in numerous sources [1]
The introduction of Cu was performed with Liquid Ion Exchange (LIE), as described in the experimental section
It has been observed that there are large differences in the Cu speciation of the two CHA topology materials - SAPO-34 and SSZ-13 - as observed from X-ray absorption spectroscopy (XAS), Raman and IR spectroscopies

Summary

Introduction

Methane has become increasingly abundant as a carbon resource in recent years, and can be found in numerous sources [1]. Catalysts 2020, 10, 191 this intermediate gas requires large facilities, due to the economy of scale, and transportation from remote areas is costly. It is of high interest to find a direct route for converting methane into liquid products on-site for the utilization of such stranded gas. Some promising liquids for further utilization are methanol and dimethyl ether (DME). DME has been extensively investigated the last decades as a more sustainable alternative to regular diesel fuel and LPG (both products from oil refining). This is because the properties of DME are quite similar to those of diesel and LPG, and only a few modifications are necessary to the already existing infrastructure [6,7]. The main problem, is the challenge of overcoming the over-oxidation of methanol to COx [3]

Methods

Results

Conclusion