Reactivity of C-1 Surface Species Formed in Methane Activation on Zn-Modified H-ZSM-5 Zeolite

Solid-state C-13 magic angle spinning (MAS) NMR spectroscopy investigations identified zinc methyl species, formate species, and methoxy species as C-1 surface species formed in methane activation on the zeolite Zn/H-ZSM-5 catalyst at T <= 573 K. These C-1 surface species, which are possible intermediates in further transformations of methane, were prepared separately by adsorption of C-13-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 C-13 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 C-1 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 C-1 surface species with rationally designed co-reactants (i.e., probe molecules) under optimized reaction conditions.