An important task confronting catalytic chemists is how to realize direct conversion of methane to versatile fuels and valuable chemicals by building up the desired C–C (or C–O) bond. Thermodynamic constraints on the reactions in which all four C–H bonds of CH4 are totally destroyed, such as CH4 reforming into synthesis gas or CH4 decomposition into carbon and hydrogen, are much easier to overcome than the reactions in which only one or two of the C–H bonds are broken under either oxidative or nonoxidative conditions [110-117]. Direct conversion of CH4 with the assistance of oxidants is thermodynamically more favourable than that under nonoxidative conditions. Therefore, the direct conversion of CH4 under the aid of oxidants has received much more attention than that under nonoxidative conditions, especially when considering the production of fuels and valuable chemicals from CH4 [118-123].
With the urge to quest for renewable energy and cleaner fuels, it is recognized that hydrogen energy will inevitably replace fossil fuel energy in the near future due to the fact that the burning of hydrogen is pollution free. However, it is a practical way to produce H2 from CH4
due to its high H/C atomic ratio and great abundance in reserves. Therefore, the direct conversion of CH4 under nonoxidative conditions into H2 and/or H2 accompanied with basic chemicals is closely related to the effective utilization of CH4-containing resources and thus to sustainable progress and development of the living conditions of humankind [124-127].
The direct conversion of CH4 under nonoxidative conditions is thermodynamically unfavorable. Nevertheless, as an alternative approach, it has still attracted the attention of many researchers. In heterogeneous catalysis, various metals have been discovered that can chemisorb CH4 at moderate temperatures and that can decompose CH4 to C and H2 at higher temperatures [128-137].
Amariglio and co-workers reported a “two-step” process on Pt, Ru, and Co in isothermal experiments [128-130]. In a series of publications, the authors suggested that C–C bonding could take place between H-deficient and CHx formed during the first step of methane chemisorption, while H2 saturated the alkane precursors in the second step and removed them from the surface. In view of the fact that hydrogenation at a temperature lower than that of CH4 chemisorption is favorable for lessening hydrogenolysis. The authors reported a nonoxidative conversion of methane to higher hydrocarbons through a dual temperature two-step reaction on Pt/SiO2 and Ru/SiO2 catalysts. Indeed, when chemisorption of methane was set at a fixed temperature (usually lower than 320°C), the selectivity to heavier alkanes increased with the lowering of hydrogenation temperature on both catalysts. On the other hand, when the hydrogenation temperature was less than 120°C, hydrogenolysis was negligible, and thus the variations of the products can only be attributed to the changes affected by the adlayer formed during the chemisorption of methane at a certain temperature.
It was discovered that the products of C2+ hydrocarbons at every hydrogenation temperature displayed a maximum versus the methane chemisorption temperature on both catalysts. In the case of the Pt/SiO2 catalyst, mainly C2H6 and n-C5H12 were produced during the first minute of the reaction. This illustrates that C–C bonds could form during CH4 adsorption, and the authors assumed a surface intermediate of C5 precursor bonded on dispersed and coordinately unsaturated Pt atoms.
Van Santon et al. suggested that CH4 first dissociated on a precious metal to form carbide and H2. Then, the carbide was hydrogenated by H2 to produce higher hydrocarbons. C–C bonds were supposed to be created during the hydrogenation step. Since the reactivity of the CHx surface intermediates formed from CO and CH4 was quite similar. The authors suggested that the chain-growth probability would depend on the metal–carbon bond strength and that the mechanism of C–C bond formation in the two-step route should be related to that occurring in the Fisher–Tropsch reaction. They also demonstrated that the homologation of olefins (C2H4, C3H6, etc.) with methane could occur over Ru/SiO2 and Co/SiO2 catalysts [131, 132].
The two-step route is also feasible over a number of oxide- or zeolite-supported transition metal and bimetal catalysts. Solymosi and Cserenyi illustrated that over a Cu-promoted Rh/SiO2 catalyst, the enhanced formation of C2H6 and higher hydrocarbons could be observed in the two-step process [133, 134].
Guczi et al. reported that the chemisorption of CH4 at 250°C and the subsequent hydrogenation of the CHx species at 250°C over Co–Pt/NaY and Co–Pt/Al2O3 performed the best of all the catalysts tested. The chemisorbed CHx species had the highest concentration, and all CHx species were hydrogenated in the second step, giving a selectivity of C2+ close to 84%. They found that there was a correlation between the hydrogen content of the surface CHx species (the optimum value of x being around 2) and the chain length of the hydrocarbons produced in the hydrogenation step in their mechanistic study of the two-step process [135]. Later, they reported that the two-step process could be simplified into a one-step process with a C2+ hydrocarbons production higher than that obtained in the two-step process over Co–Pt/NaY bimetallic catalyst. These results could be obtained if the CH4 was pulsed with H2/He mixture at 250°C [136].
Bradford reported the results of the isothermal, nonoxidative, two-step conversion of CH4 to C2+ hydrocarbons over supported and unsupported Pt and Ru catalysts at moderate temperatures and elevated pressures. It was shown that an increase in reaction pressure increased the branching and molecular weight distribution of the product [137].
Several researchers suggested the preparation of a multifunctional catalyst to avoid the use of a two-step process. Furthermore, it has been reported that dehydrogenative coupling of CH4
without any oxidant could be carried out over Pt–SO4/ZrO2 catalysts. A steady conversion of 0.2% (the equilibrium conversion of CH4 into C2H6 and H2 is estimated to be 0.6%) was observed after the catalyst was reduced in H2 at 500°C [138, 139].
On the other hand, in order to overcome the thermodynamic limit and to enhance the reactivity for obtaining high yields in direct conversion of CH4 under nonoxidative conditions, plasma excitation has also been attempted. The product distribution is dependent on the method by which plasma excitation is produced. For example, in pulsed corona discharges at atmospheric pressure, C2 hydrocarbons (mainly C2H2) were obtained with a high selectivity of around 70 to 90%. In microwave plasmas, the product distribution shifted from C2H6 to C2H4
and finally to C2H2 with an increase in power density. By introducing a proper catalyst into the microwave plasma reactor, CH4 could be converted to higher hydrocarbons at atmospheric pressure. In addition, with a CH4 and H2 mixture as the feed gas, the selectivity to C2H2 was 88% and that to C2H4 was 6% at a CH4 conversion of 76% [140, 141]. Here, again, the main
drawback is the low energy efficiency to drive this thermodynamically unfavorable reaction.
Thermodynamically, the transformation of CH4 under nonoxidative conditions is more favorable to aromatics than to olefins. The direct conversion of CH4 to aromatics was tested on several catalysts in either a pulse or a flow reactor. Wang et al. reported on the dehydroaromatization of methane (MDA) for the formation of aromatics (mainly C6H6) and H2 under a nonoxidizing condition in a continuous flow reactor on Mo/HZSM-5 catalysts [142]. More detailed studies on the reaction revealed that the channel structure and acidity of the HZSM-5 zeolite, as well as the valence and location of the Mo species, are crucial factors for the catalytic performance of the Mo/HZSM-5 catalysts. In addition, W/HZSM-5 and Re/HZSM-5 are also reported to be active elements for MDA [142-144].
Solymosi and co-workers [147-152] and Lunsford and co-workers [153-155] characterized the Mo/HZSM-5 catalyst by means of XPS and found that during the initial induction period, the original Mo6+ ions in the zeolite were reduced by CH4 to Mo2C, accompanied by the depositing of carbonaceous cokes. They suggested that Mo2C provides active sites for C2H4
formation from CH4, while the acidic sites catalyze the subsequent conversion to C6H6. The Mo2C species probably are highly dispersed on the outer surface, and some of them reside in the channels of the zeolite. Meanwhile, the spectra of Mo/HZSM-5 samples reacted with CH4
at 700°C for one and 24 hrs were basically identical to the Mo2C reference spectrum, except for a partial contribution from the Mo oxide, given rise by MoOxCy. These authors claimed that the Mo oxide species dispersed in the HZSM-5 framework might migrate onto the external surface of the HZSM-5, be converted by CH4 to Mo2C, and disperse on the support surface. Therefore, the carbonaceous deposits created in MDA are in various forms and play different roles. First, Mo2C and/or MoOxCy, which are possibly active species for CH4
activation, are formed during the induction period. Second, the formation of the active intermediates, the CHx species, follows the activation of CH4 on Mo2C and/or MoOxCy. The last one to be formed is coke leading to the deactivation of the catalyst. It is understandable that there are some similarities between the carbonaceous species formed in MDA and those formed in the first step of the two-step process, since both reactions are carried out under nonoxidative conditions, and Mo2C shows some precious metal-like properties.
In spite of the fact that the reaction is thermodynamically unfavorable under pressurized conditions and that 10% CO2 added to the feed totally suppresses the activity of the 2 wt%
Mo/HZSM-5 catalyst. Ichikawa and co-workers found that an increase in CH4 pressure and the addition of small amounts of CO and CO2 (less than 3%) to the CH4 feed enhanced the catalyst stability in the reaction [156, 157]. By increasing the CH4 pressure, the formation
rates of C6H6 and hydrocarbons could be moderately increased. This kind of pressure relationship may be related to a sufficient supply of H2 from CH4 and a suitable concentration of surface carbon species CHx for the formation of aromatic products. By using a CO and CH4
mixture as the feed to conduct the reaction, the authors suggested that CO dissociated on the Mo sites to form the active carbon species CHx. The dissociated oxygen species [O] from CO might react with the surface inert carbon species to regenerate CO, resulting in the suppression of coke formation on the catalyst. These results imply that although the Brönsted acid sites are necessary, excess Brönsted acid sites are detrimental for the reaction, since severe coke formation will occur on them [157-159].
Considerable efforts have been devoted to developing active and selective catalysts and understanding the bifunctionality of Mo/HZSM-5 catalysts and the nature of carbonaceous deposits formed during the reaction. However, neither new active and selective catalysts nor a thorough understanding of the mechanism of the reaction has been achieved.
However, despite all substantial research efforts into nonoxidative two-step or one-step CH4
homologation, its low efficiency is the main problem to further developing it as a commercial process. In any case, these studies enhanced our knowledge in direct conversion of CH4 under nonoxidative conditions, particularly methane dehydroaromatization, and stimulated chemists to explore new methane conversion processes.