Investigating the influence of acid sites in continuous methane oxidation with N₂O over Fe/MFI zeolites

Abstract

Methane oxidation using N2O was carried out with Fe-MFI zeolite catalysts at 300 °C. Methane conversion over Fe-ZSM-5, Fe-silicalite-1 and Fe-TS-1 indicates that Brønsted acidity is required to support the Fe-based alpha-oxygen active site for the important initial hydrogen abstraction step. Increasing the calcination temperature of Fe-ZSM-5 from 550 to 950 °C showed that the catalyst retained the MFI structure. However, at 950 °C the Brønsted and Lewis acid sites were altered significantly due to the migration of aluminium, which led to a significant decrease in catalytic performance. Over Fe-ZSM-5 the desired partial oxidation product, methanol was observed to undergo a reaction path similar to the methanol to olefin (MTO) process, which predominately produced ethene and subsequently produced coke.Methanol control experiments over Fe-silicalite-1, Fe-ZSM-5, Fe-TS-1 and H-ZSM-5 indicated that with the presence of Brønsted acidity the catalyst were more effective at forming ethene and subsequent aromatic species from DME, which resulted in an increased level of catalyst fouling. The implication of these observations are that the desorption of methanol is crucial to afford high mass balances and selectivity, however, Brønsted acid sites appear to slow this rate. These sites appear to effectively retain methanol and DME under reaction conditions, leading to low mass balances being observed. Our results confirm that to afford efficient and continuous methane oxidation by N2O, the catalytic active site must be extra-framework Fe coordinated to Al.

A Kinetic Study of Methane Partial Oxidation over Fe‐ZSM‐5 Using N2O as an Oxidant

“…Adsorption of CH 3 OH at an adjacent Brønsted acid site is captured to highlight that the formation of CO and CH 3 OH is unlikely to remain equivalent. The determination of CO and C 2 H 4 as primary products in the delplot analysis suggests that cooperativity exists between the active Fe sites for methanol formation and Brønsted acid sites, as has been suggested previously . The formation of C 2 H 6 is thought to occur through hydrogen transfer under the umbrella of the Methanol‐to‐Hydrocarbons chemistry, however, we did not detect any of the expected dienes, trienes or polymethylbenzenes that would form post‐transfer.…”

“…water/no water). This implies that CO 2 forms directly from methane at Fe sites, as we previously reported low selectivity towards CO 2 (<1 %) when methanol and N 2 O was passed over H‐ZSM‐5, Fe‐ZSM‐5, Fe‐TS‐1 or Fe‐Silicalite . Therefore, it is reasonable to postulate that methane can be oxidised to CO 2 via a second pathway and not exclusively via oxidation of CO (Scheme ).…”

“…A further benefit is the reduction in the concentration of C 2 products, specifically C 2 H 6 , which is absent from the reaction effluent. We consider that C 2 H 6 is formed from C 2 H 4 , probably via methyl benzenes, through the hydrocarbon pool mechanism . Accordingly, water has decreased the effective Brønsted acidity of the zeolite surface, thereby limiting C 2 H 6 formation.…”

“…Methane oxidation reactions were conducted over Fe‐ZSM‐5 catalysts in the presence of N 2 O and where reported, co‐fed H 2 O. This catalyst was extensively characterised in our previous report in an effort to deconvolute the influence of the Fe location and framework acidity on CH 4 oxidation with N 2 O. Figure a shows the product distribution of 2 % Fe‐ZSM‐5 catalysed methane oxidation with increasing time‐on‐stream and Figure b shows methane conversion, N 2 O conversion and carbon balance values for the same reaction, which was carried out in the absence of added water vapour.…”

“…The formation of methanol is associated with alpha ‐oxygen, which is generated by decomposition of N 2 O on Fe . However, due to the structure of the ZSM‐5 zeolite, CH 3 OH has been observed to react at Brønsted acidic sites to yield DME and higher hydrocarbons . This may be alleviated by removing or blocking Brønsted acid sites and thereby improve CH 3 OH selectivity and carbon mass balance.…”