Understanding Methane Aromatization on a Zn‐Modified High‐Silica Zeolite

Methane is the principle constituent of natural gas and also the most inert of the saturated hydrocarbons. Its conversion into more commercially useful chemicals and liquid fuels represents one of the most important challenges in modern catalysis. Coaromatization of methane and light hydrocarbons (paraffins and olefins) at 700–800 K is one of the alternative methods for the conversion of methane. It has been reported recently that the conversion of methane during coaromatization with higher alkanes or alkenes (C2–C6) at 670–870 K in the presence of bifunctional catalysts (mainly, high-silica ZSM-5 or ZSM-11 zeolites, modified with gallium or zinc) may reach 20–40%. However, previous experiments in which C-labeled methane was used did not confirm the presence of the C-labeled atoms from the methane in the aromatization products. This result gave rise to scepticism as to whether methane-involved aromatization occurred at all. Herein we report that transfer of isotopically C-labeled atoms from methane into the aromatic products does occur to a high degree during the co-conversion of methane and propane on the Zn-modified high-silica zeolite BEA. We have identified the nature of the intermediates formed during the activation of methane and established how the conversion of methane into aromatic compounds occurs. Figure 1 shows the C CP/MAS NMR spectra of the products (in their adsorbed state on the zeolite catalyst) which are formed from methane and propane at 823–873 K. The spectrum of the products formed from unlabeled CH4 and C3H8 exhibits only a weak signal at d = 8.5 ppm from methane (Figure 1a). When unlabeled CH4 was replaced with CH4, the spectrum of the reaction products showed two new signals, which undoubtedly belong to hydrocarbons containing the C labels from the CH4 (Figure 1b). The carbon atoms of the C-labeled methane molecules are incorporated into both methyl groups (signal at d = 20 ppm) and aromatic rings (d = 130 ppm) of the methyl-substituted aromatic compounds (Figure 1b,c). According to GC-MS analysis of the products extracted from the zeolite, a mixture of benzene and toluene, as well as mand p-xylenes (BTX) with C enrichment is formed from CH4 and unlabeled propane at 773–823 K (Figure 2). The presence of singly (C1), doubly ( C2), and triply ( C3) labeled molecules of BTX (Figure 2b) provides proof for the incorporation of C-labeled methane into both the methyl groups and the carbon atoms of the aromatic rings of BTX. Neat propane converts on Zn/H-BEA into a mixture of aromatic products and methane at lower temperature (573– 723 K; Figure 1d). According to the H MAS NMR spectra, approximately 1.6–1.7 methane molecules are produced per reacted propane molecule. The possible overall reaction which describe the aromatization of propane can be described by Equation (1). Figure 1. C CP/MAS NMR spectra of products in the adsorbed state formed from methane and propane on zeolite Zn/H-BEA: a) from CH4 and C3H8 at 823 K for 15 min; b,c) from CH4 and C3H8 at 823 K for 15 min (b) and at 873 K for 15 min (c); d) from [1-C]C3H8 at 723 K for 15 min. Asterisks (*) in Figures 1, 3, and 4 denote the spinning side bands.