New Insights into the Oxidative Coupling of Methane from Spatially Resolved Concentration and Temperature Profiles

Oxidative coupling of methane (OCM) is a high-temperature process involving the transformation of methane into ethane and ethylene, which are valuable intermediates for the chemical processing industry. Despite decades of long research that has resulted in thousands of papers and hundreds of patents, OCM still remains at the research stage. Although many OCM catalysts have been reported, there appears to be an upper limit for the yield of C2 + products of approximately 25 % per reactor pass, for which the kinetic reasons are largely unknown. It has been recognized that to make progress in the OCM an improved quantitative understanding of the underlying detailed chemical kinetic mechanisms (DCKMs) of the coupled surface and gas-phase reactions must be developed and validated over the very broad range of conditions encountered in the process. DCKMs comprise a comprehensive description of chemical transformations in terms of irreducible chemical events or elementary reactions for which independent rate coefficient parameters, frequently expressed in the form k = ATexp( E/RT), are either available from direct measurements or estimated from theoretical considerations. 5–7] DCKMs are then combined with models describing the transport phenomena for the realistic simulation of the performance of the OCM reactors. 5, 7] With the availability of DCKMs, we will then be in a better position to identify improved OCM conditions, superior reactor configurations, and new leads for catalytic materials that are needed to exceed the 25 % limit for C2+ product yields. Validation of DCKMs requires experimental data of high information content because of the presence of a large number of species participating in an even larger number of elementary reactions. Although DCKMs for OCM have been reported in the past, 6] they were all validated by using integral reactor data, that is, reactor exit conditions. However, this is not a particularly demanding test for mechanism validation. In fact, different DCKMs can readily predict similar OCM reactor exit concentrations, as estimated kinetic parameters are used for many of the elementary reactions. Therefore, the performance of more comprehensive validation tests such as the prediction of the absolute concentration profiles of all the major and minor species within the catalytic packed-bed reactors is necessary for the development of truly predictive DCKMs for the OCM process. However, we are not aware of such information-rich data sets in the open OCM literature. Herein, we report, for the first time, the spatially resolved comprehensive species concentrations and temperature profiles in a fixed-bed OCM reactor by using microprobe sampling. Although microprobe sampling techniques have long been used in high-temperature flame-combustion research to obtain spatial temperature and concentration profiles, their adaptation to and use in heterogeneous catalysis is relatively recent. One of the earliest applications of microprobe sampling to catalysis was in the determination of the concentration profiles of species in the catalytic partial oxidation of methane to produce CO and H2 in a Ptand Rh-coated a-Al2O3 foam. [11–13] Experiments were performed by using a fixed-bed tubular reactor system, as shown in Figure 1. The reactor was packed with La2O3–CeO2 nanofiber fabric catalysts that were prepared by electrospinning a viscous solution of polyvinylpyrrolidone (PVP; 1.3 MDa, 0.60 g), water/ethanol ( 1:1 wt ratio, 9.5 g) in which the metal precursors, as La(NO3)3·6H2O and Ce(NO3)3·6 H2O (La/Ce weight ratio = 3, 0.35 g), were dissolved. [14] The electrospun material was calcined at 625 8C in a furnace to form metal oxide nanofiber fabrics. SEM images confirmed that the nanofibers had an average diameter of approximately 90 nm. The fabric also had a low BET area of approximately 20 m g , which suggested that the nanofibers were dense and did not possess internal porosity. This fabric (20 mg) was