Sergei S. Arzumanov

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.

Significant influence of Zn on activation of the C-H bonds of small alkanes by Bronsted acid sites of zeolite.

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.

In situ high temperature MAS NMR study of the mechanisms of catalysis. Ethane aromatization on Zn-modified zeolite BEA

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.

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.

H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA

Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D(2)) has been studied by (1)H MAS NMR spectroscopy in situ within the temperature range of 383-548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed.

Competitive pathways of methane activation on Zn2+-modified ZSM-5 zeolite: H/D hydrogen exchange with Brønsted acid sites versus dissociative adsorption to form Zn-methyl species

To clarify the pathways of methane activation on Zn-modified high-silica zeolites, the kinetics of both dissociative adsorption of the alkane C–H bond to form Zn-methyl species and H/D hydrogen exchange between the alkane and Bronsted acid sites (BAS) have been analyzed for Zn2+/H-ZSM-5 containing exclusively Zn2+ cations (no ZnO species in the zeolite) and BAS. Analysis of the kinetics was performed by 1H MAS NMR spectroscopy in situ at 410–540 K. In spite of the activation barrier for H/D hydrogen exchange (68 kJ mol−1) being larger than that for Zn-methyl formation (46 kJ mol−1), the rate of H/D hydrogen exchange has been found to be one order of magnitude higher than the rate of the formation of Zn-methyl species within the studied temperature range. This implies that Zn-methyl species cannot be involved in the reaction of H/D hydrogen exchange as the intermediate responsible for facilitation of this reaction due to the presence of Zn2+ cations in the zeolite (J. Catal., 2008, 253, 11). A new mechanism has been suggested for C–H bond activation in methane on the zeolite modified with Zn2+ cations. It includes first the formation of a transient molecular complex of methane with Zn2+ cations. The complex either is further involved in the reaction of H/D hydrogen exchange or evolves toward the formation of Zn-methyl species and BAS.