Mikhail V. Luzgin

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.

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to2H NMR, the proton of the acidic ≡Al-OH-Si≡ group of the zeolite is transferred into the CH2= group of the octene-1 molecule. (2) According to13C NMR the13C label inserted into the terminal CH2= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.

n-Butane conversion on sulfated zirconia: the mechanism of isomerization and 13C-label scrambling as studied by in situ 13C MAS NMR and ex situ GC-MS

Abstract Using 13 C MAS NMR, conversion of selectively 13 C-labeled n -butane on sulfated zirconia catalyst has been demonstrated to proceed initially via two parallel routes: scrambling of the selective 13 C label in the n -butane molecule and selective formation of isobutane. The combination of the results obtained by both in situ 13 C MAS NMR and ex situ GC-MS analysis provides evidence for the monomolecular mechanism of the 13 C-label scrambling, whereas isomerization into isobutane proceeds through a pure bimolecular mechanism. Further, the intermolecular mechanism of n -butane isomerization is complicated and turns into conjunct polymerization. Besides isobutane, conjunct polymerization gives also the products of butane disproportionation, propane and pentanes, as well as the stable cyclopentenyl cations; the latter may be in charge of catalyst deactivation.

Formation of Carboxylic Acids from Small Alkanes in Zeolite H‐ZSM‐5

The activation of propane and isobutane in acidic zeolite H-ZSM-5 in the presence of both CO and H2O has been studied by in situ solid-state NMR and GC analysis. Evidence was provided for the conversion of propane to isobutyric acid at 373-473 K by cleavage of the C-C bond; methane and ethane are also produced. Isobutane is transformed into pivalic acid with simultaneous production of hydrogen. The low conversion (1-2%) at this temperature was rationalized by the existence of a small number of sites that are capable of generating carbenium ions which are trapped by CO at this temperature. A formate species was observed when CO and H2O were present on H-ZSM-5. This species disappeared in the presence of the alkane. At 573 K, the generation of large amounts of CO2 indicates a much higher conversion of the alkanes into carboxylic acids which, however, decompose under the reaction conditions.

Olefin as an intermediate in n-butane isomerization on sulfated zirconia. An in situ 13C MAS NMR study of n-octene-1 conversion

By using in situ13C MAS NMR and ex situ GC-MS, the analysis of hydrocarbon products formed from n-octene-1 adsorbed on sulfated zirconia catalyst (SZ) has been performed. It is shown that a mixture of alkanes and stable alkyl substituted cyclopentenyl cations (CPC) is formed as the basic reaction products. Formation of both alkanes and CPC from n-octene-1, a precursor of C8+ cation, the key intermediate in n-butane isomerization via a “bimolecular pathway”, implies that formation of the isomerized alkane occurs by a complex process of “conjunct polymerization”, rather than isomerization itself. CPC deposited on the SZ surface can be in charge of the catalyst deactivation.