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Oxidative Coupling of Methane by Nanofiber Catalysts

The direct utilization of methane, the main component in natural gas (NG), as an alternate chemical feedstock to petroleum is a highly desirable but difficult goal in industrial catalysis. Many direct and indirect methods have been proposed and studied to convert CH4 into more-useful products, including olefins (e.g. C2H4, C3H6) and higher-molecular-weight hydrocarbons and liquids (e.g. benzene and gasoline), as discussed in a recent review. The production of ethylene (C2H4) from NG represents a particularly desirable process because of its massive worldwide use as an intermediate in the production of plastics, such as polyethylene and polyvinylchloride (PVC). In addition, ethylene can be oligomerized into liquid hydrocarbons, thereby enabling the efficient utilization of natural gas in remote parts of the world. The global production rate of ethylene is over 100 million tons per year, which represents an annual business in excess of $110 billion (July 2012). All indirect NG-conversion routes utilize a high temperature, endothermic, and costly steam-reforming process as the first step, from which synthetic gas (H2/CO mixtures) is produced. This step is followed by the synthesis of useful products through various catalytic processes. Although direct methods avoid the use of costly syngas, they remain uneconomical, owing, in part, to low yields of C2+ compounds, high temperatures, and low throughputs. High temperatures are particularly detrimental because they result in catalyst deactivation and create problems for reactors. In the oxidative coupling of methane (OCM), CH4 is directly converted into C2H6, C2H4, and water in the presence of O2 and a suitable catalyst. The first step involves the abstraction of H from CH4 by the catalyst to form methyl radicals (CH3C). [2, 3] The coupling of two CH3C radicals creates C2H6, followed by its dehydrogenation to afford C2H4. Some C3 hydrocarbons are also formed by the addition of a CH3C radical to C2H4. [4] However, undesirable surface reactions and gas-phase combustion reactions also lead to CO and CO2 (COx). Because high temperatures promote homogeneous gas-phase free-radical reactions, which are detrimental for C2+ products, the development of new catalysts that can operate at low temperatures is crucial for the economic viability of the OCM. Since the pioneering works of Keller and Bhasin, Hinsen and Baerns, and Ito and Lunsford, the OCM has received immense global attention, as evidenced by the large number of catalysts that have been investigated for this transformation: The oxides of almost all of the metals on the periodic table, either individually or in various combinations, have been considered and analyzed as OCM catalysts. Even the best catalysts have been reported to require feed-gas temperatures (Tf) of 700–850 8C and reaction times of 0.2–5.5 s, with C2+ yields of less than 25 %. Gas temperatures within the catalytic zones were found to be 100–200 8C higher than this range, owing to the exothermicity of this process. The actual catalyst-surface temperature is likely to be even higher still because of heattransfer considerations. An important common feature of all of these OCM catalysts is that they are based on quasi-spherical nanoparticles (powders) 2] and, thus, are prone to metal dispersion, agglomeration, and sintering. All of these problems retard catalytic activity. Herein, we report a new fabric catalyst that is comprised of La2O3 CeO2 nanofibers; this catalyst is capable of negating these problems and promoting the ignition of the OCM at a Tf value of 470 8C. The nanofibers were prepared by electrospinning. An analogous La2O3 CeO2 powder catalyst was also prepared by co-precipitation for comparison. Powders of this binary system have previously been studied by Dedov et al. and have been reported to show OCM activity at significantly higher Tf values (715–830 8C). Shown in Figure 1 (left) is the calcined La2O3 CeO2 nanofiber fabric (La/Ce, 15:1 w/w) that was used in the OCM experiments. A SEM image of this fabric (Figure 1, right) shows the formation of highly uniform La2O3 CeO2 nanofibers with diameters in the range 50–75 nm. This fabric had a low BET area of about 26 m g , which suggested that the nanofibers were dense and did not possess any internal porosity. The SEM image also shows the presence of large voids between the fibers, which enhances fabric diffusivity and decreases sintering. The XRD data for La2O3 CeO2 nanofibers and powders, both before and after the test conditions, are presented in Figure 2 (A, B and C, D), along with those for individual La2O3 (E) and CeO2 (F) fibers and powders. From these data, several interesting features are revealed: First, both the La2O3 fibers (Fig-