Kinetic modeling of methane dehydroaromatization over a Mo2C/H-ZSM5 catalyst: Different deactivation behaviors of the Mo2C and H-ZSM5 sites
Graphical abstract
Introduction
Advanced techniques have been developed to exploit alternative energy sources and gradually replace the rapidly exhausted resources of crude oil [1]. Natural gas is one of the most promising alternatives to bridge the gap between fossil fuels and renewable technology [2]. Because the recent advances with respect to discovery and recovery technologies contributed to the increase in the abundance and decrease in the cost of natural gas, there is an increased interest in the use of natural gas. New breakthroughs with respect to C1 chemistry and related technology will expand its utilization both as raw material for liquid fuels and as alternative feedstock for the petrochemical industry [1,3,4].
The conversion of methane, which is the main constituent of natural gas, into added-value products, is one of the most interesting and challenging topics in the field of natural gas exploitation [4,5]. The catalytic conversion of methane proceeds via direct or indirect routes. The latter involve the production of synthesis gas (CO/H2 mixture) and conversion into desired products via Fischer–Tropsch synthesis (FTS) or methanol production [6,7], while the former includes the oxidative coupling of methane (OCM) to ethylene, selective oxidation of methane to methanol, decomposition of methane to hydrogen and carbon nanotubes, and aromatization of methane under non-oxidative conditions [8]. Methane can be more easily converted into CO and CO2 during deep oxidation under oxidative conditions, which leads to a lower selectivity of the products, which can be avoided by the conversion of methane under non-oxidative conditions.
Because it is difficult to activate the stable and symmetrical methane molecule, which contains four CH bonds with a bond energy of 435 kJ/mol, without entirely decomposing it, its transformation to aromatics in the absence of oxygen is thermodynamically favorable compared with its conversion to alkenes under oxidative conditions. In other words, a high selectivity towards higher hydrocarbon products can in principle be obtained [9]. Following an intensive study of the OCM to more valuable hydrocarbons such as ethene in 1982 [10], non-oxidative aromatization of methane into aromatics was first documented in 1993 [11]. It was discovered that methane can be converted to aromatics using a Mo/H-ZSM5 catalyst in a continuous flow packed-bed reactor, demonstrating the production of H2 and C6H6 under non-oxidative conditions at elevated temperatures and atmospheric pressure. The main aromatic products, including benzene, toluene, and xylenes, are important intermediates in the chemical industry and can be easily separated from the methane feed. In addition, valuable H2 is formed as a byproduct under typical aromatization conditions, that is, 973 K and 101,325 Pa [1,9,12]. Although the methane dehydroaromatization (MDA) is thermodynamically limited, a high C6H6 selectivity can be obtained. Since Wang et al.’s work [11], many researchers reported the influence of the reaction conditions [8,[13], [14], [15]] as well as the catalyst functionality [[16], [17], [18], [19]] on the selectivity and yield. For a recent review of current MDA technology and future prospects, please refer to the references [20] and [21].
The MDA process is based on bifunctional transition metal-incorporated zeolite catalysts. Although a variety of transition metals (e.g., Mo, W, Re, V, and Ga) can be incorporated into zeolite structures (e.g., H-ZSM5, ZSM-8, ZSM-11, MCM-41, and FSM-16), Mo/H-ZSM5 reportedly shows the best MDA performance in terms of the benzene selectivity [3,11,[22], [23], [24]]. Zhang et al. [25] reported that H-type silica–alumina zeolites, such as ZSM-5, ZSM-8, and ZSM-11, with a two-dimensional channel structure and a pore diameter approximating the kinetic diameter of benzene are promising support materials.
Typical catalysts, such as Mo/HZSM-5 and Mo/HMCM-22, exhibit a low stability and severe deactivation caused by serious coking [[9], [10], [11], [12], [13]], which is one of the major obstacles for their industrial application [14,23,24,26,27]. The unavoidable catalyst deactivation by carbon deposits motivated practical MDA processes with a typical operating cycle. For this purpose, the mechanism of catalyst deactivation and the nature of the condensation products that form on the outer surface, in zeolite channels, and at Mo-containing sites, should be studied [3,12,18,[28], [29], [30]]. However, few kinetic studies of the MDA reaction have been reported. In this study, a kinetic model, which simultaneously considers the activation, reaction, and deactivation, was established to evaluate the effects of the operating conditions on the catalytic performances.
Section snippets
Catalyst preparation
The Mo/H-ZSM-5 catalyst was prepared using a conventional wet impregnation method. The parent H-ZSM-5 was obtained by calcination of the purchased NH4-ZSM-5 (SiO2/Al2O3 = 30, 3024E, Zeolyst; Brunauer–Emmett–Teller (BET) surface area: 404.5 m2 g−1; micropore volume: 0.14 cm3 g−1) at 600 °C for 2 h to convert NH4+ into H+ before impregnation. Ammonium heptamolybdate ((NH4)6Mo7O24 · 4H2O, Samchun) was used as Mo precursor. After Mo impregnation, the impregnated powder was dried at 80 °C and
Reaction mechanism and rates
Fig. 1 shows the overall reaction mechanism considered in this work. The Mo/HZSM-5 is a bifunctional transition metal-incorporated zeolite catalyst with two types of active sites. The activation of the CH bonds in methane and the formation of the initial CC bonds occur in the Mo species (Mo2C site, denoted s1), followed by the oligomerization of C2 species and cyclization of heavier intermediates to aromatics at the Brønsted acid sites of the zeolite (HZSM-5, denoted s2) [3,4]. The formation of
Conclusions
Considering the experimental observations for the MDA over the Mo2C/H-ZSM5 catalyst, that is, that the reaction and deactivation took place at the same time during the catalyst activation (carburization), the initial time for the simulation was specified to be the time at which the activation was completed and the initial activities under each operating condition were estimated. After the completion of the activation, both the reaction and deactivation were considered and the corresponding
Acknowledgement
This research was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning [NRF-2018M3D3A1A01055765 (J. Jong and M.-J. Park), NRF-2018M3D3A1A01055748 (A. Hwang, Y. T. Kim, D.-Y. Hong)] and by the Human Resources Development of the KETEP grant funded by the Ministry of Trade, Industry & Energy of the Korean Government [No. 20154010200820].
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