Solid-State Ion-Exchanged Cu/Mordenite Catalysts for the Direct Conversion of Methane to Methanol

Abstract

The selective oxidation of methane to methanol is a highly challenging target, which is of considerable interest to gain value-added chemicals directly from fuel gas. Copper-containing zeolites, such as Cu/mordenite, have been currently reported to be the most efficient catalysts for this reaction. In this work, it is shown that solid-state ion-exchanged Cu/mordenites exhibit a significantly higher activity for the partial oxidation of methane to methanol than comparable reference catalysts, i.e., Cu/mordenites prepared by the conventional liquid-phase ion exchange procedure. The efficiency of these Cu/mordenites remained unchanged over several successive cycles. From temperature-programmed reduction (TPR) measurements, it can be concluded that the solid-state protocol accelerates Cu exchange at the small pores of mordenite: those are positions where the most active Cu species are presumably located. In situ ultraviolet–visible (UV-vis) spectroscopy furthermore indicates that different active clusters including dicopper- and tricopper-oxo complexes are formed in the catalyst upon oxygen treatment. Notably after activation of methane, different methoxy intermediates seem to be generated at the Cu sites from which one is preferably transformed to methanol by reaction with water. It is furthermore described that the applied reaction conditions have considerable influence on the finally observed methanol production from methane

Stepwise Methane‐to‐Methanol Conversion on CuO/SBA‐15

“…It is therefore suggested that the formation of smaller CuO clusters in Cu‐AA/SBA led to the better catalytic performance of Cu‐AA/SBA compared to Cu‐OA/SBA. Calculation of H 2 consumption in the TPR measurements further indicated that, after activation in O 2 , the samples indeed contain Cu II species only as found in Cu/zeolites, which were responsible for activation of methane in earlier studies …”

“…The last step for extraction of methanol could be performed with liquid water (off‐line extraction) or steam (on‐line extraction). For comparison, Cu/mordenite with a Cu loading of about 2.6 wt %, prepared by a recently reported solid‐state ion‐exchange reaction between NH 4 ‐mordenite and Cu(acac) 2 , was used as a reference zeolite‐based catalyst …”

“…Dimethyl ether (DME) can be considered to be a side product due to acidic zeolite‐catalyzed in situ dehydration of methanol, as the extraction with steam was performed at elevated temperature (≥135 °C) . However, in our earlier study applying a Cu/mordenite catalyst, methanol and DME were detected at room temperature, and this suggests that different intermediates were generated on the catalyst surface and transformed into methanol and DME, respectively, by treatment of the catalyst with steam . In this study, no DME was produced over CuO/SBA‐15 catalysts, whereas 1.6 μmol of DME was obtained in the catalytic cycle with Cu/mordenite (Table S1 in the Supporting Information).…”

“… Proposed formation of products on the Cu‐based catalyst, based on former studies, and mass spectrometric data.…”

“…It was previously demonstrated that the methanol yield can be significantly improved by increasing the activation temperature and using more water ,. Similar to our previous report on Cu/mordenite, CO 2 was observed when the extraction was performed above 100 °C (Figure ). Notably, the selectivity toward methanol in the reaction with the CuO/SBA‐15 catalysts (>84 %) is comparable to that obtained with Cu/mordenite.…”