Reactivity of C₁ Surface Species Formed in Methane Activation on Zn‐Modified H‐ZSM‐5 Zeolite

Abstract

Solid‐state 13C magic angle spinning (MAS) NMR spectroscopy investigations identified zinc methyl species, formate species, and methoxy species as C1 surface species formed in methane activation on the zeolite Zn/H‐ZSM‐5 catalyst at T≤573 K. These C1 surface species, which are possible intermediates in further transformations of methane, were prepared separately by adsorption of 13C‐enriched methane, carbon monoxide, and methanol onto zinc‐containing catalysts, respectively. Successful isolation of each surface species allowed convenient investigations into their chemical nature on the working catalyst by solid‐state 13C MAS NMR spectroscopy. The reactivity of zinc methyl species with diverse probe molecules (i.e., water, methanol, hydrochloride, oxygen, or carbon dioxide) is correlated with that of organozinc compounds in organometallic chemistry. Moreover, surface formate and surface methoxy species possess distinct reactivity towards water, hydrochloride, ammonia, or hydrogen as probe molecules. To explain these and other observations, we propose that the C1 surface species interconvert on zeolite Zn/H‐ZSM‐5. As implied by the reactivity information, potential applications of methane co‐conversion on zinc‐containing zeolites might, therefore, be possible by further transformation of these C1 surface species with rationally designed co‐reactants (i.e., probe molecules) under optimized reaction conditions.

NMR‐Spectroscopic Evidence of Intermediate‐Dependent Pathways for Acetic Acid Formation from Methane and Carbon Monoxide over a ZnZSM‐5 Zeolite Catalyst

“…In this mechanism, the acylium cation was generally considered as the precursor of acetic acid, however, it is too active to be observable on zeolite-based catalysts [17] and only stabilized in superacid systems in the absence of water. Decomposition of HCOOH produces H 2 and CO 2 , [31] which in turn generates water molecules through the reverse water gas shift reaction that usually occurs in the process of methanol synthesis from CO/ CO 2 /H 2 . This suggests that water can accelerate the reaction by readily trapping the transient acylium cation to generate acetic acid.…”

“…At the high reaction temperature, protons from either the Brçnsted acid sites or from the heterolytic dissociation of methane can react with formate species to form HCOOH. Decomposition of HCOOH produces H 2 and CO 2 , [31] which in turn generates water molecules through the reverse water gas shift reaction that usually occurs in the process of methanol synthesis from CO/ CO 2 /H 2 . [32] Although two reaction pathways proceed in parallel on the ZnZSM-5 catalyst, one of them can be preferred, depending on which reaction intermediate is predominant.…”

C−C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM‐5 Zeolite

“…The signal pairs at 172.5 ppm ( 13 C) and 8.3 ppm ( 1 H) in the upper panel of Figure a are attributed to formyl group (‐CHO) bonding to an oxygen atom on the catalyst . Those at 25 ppm ( 13 C) and 2.8 ppm ( 1 H) are assigned to the methylene group (‐CH 2 ‐).…”

Framework Effects on Activation and Functionalisation of Methane in Zinc‐Exchanged Zeolites

“…Reactivity of the resulting zinc methyl fragment with other small molecules has been explored, in the context of stoichiometric reactions as well as potential catalytic applications. Addition of dioxygen to [Zn‐CH 3 ] + /ZSM‐5 at ambient and elevated temperature has been shown to result in the formation of zinc methoxy and zinc formate species, as monitored through NMR spectroscopic studies . On this basis, it has been shown that the chemical reactivity of [Zn‐CH 3 ] + within ZSM‐5, with molecules such as CO, CO 2 and H 2 O, has been found to be very similar to that of organozinc compounds …”