(169d) Oxidative Coupling of Methane (OCM) In a Microchemical Reactor

Understanding radical behavior is of great significance in homogeneous-heterogeneous reaction systems, such as high-temperature catalytic oxidation of hydrocarbons. Microreactor technology, with its precise control of fluid and temperature fields, improved reactant mixing, and large surface-to-volume ratio, is ideally suited for the study of such reaction systems. Most significantly, the pathways of homogeneous-heterogeneous coupling can be better understood as critical reactor dimensions are reduced into micrometer range, where diffusive flux starts to play an equally important role as convective transport.

Here, we are reporting a novel modular microreactor system based on the use of silicon-based thin-film catalysts, which allows stable operation during thermal cycling up to 800^oC. Precise control and adjustment of the critical dimension, i.e. the height of reaction chamber, offers the ability to steer the relative importance of gas- and catalytic-phase chemistries due to transport of reactants between the gas phase and the catalytic walls. Additionally, a moveable thermocouple and quartz-glass capillary (connected to a mass spectrometer) allow the in-situ measurement of temperature and composition profiles in the reaction chamber. By combining these two capabilities, the system thus provides an efficient and sensitive way to investigate the interplay between gas phase and catalytic chemistries, and to evaluate the catalytic performance of thin-film catalysts.

The reactor system was tested using oxidative coupling of methane (OCM) as model reaction. OCM has been studied intensively for many decades and offers a potentially highly efficient path for direct upgrading of methane to higher-value C₂ products. The reaction is also well known to include catalytic steps in the generation of methyl radicals (as well as in undesired methane combustion) and homogeneous reaction steps in which methyl radicals are coupled to form the desired C₂ products (C₂H₆ and C₂H₄). It thus forms an ideal test system for the above described microreactor system.

A La-based thin-film catalyst was deposited onto a silicon chip via dip-coating, characterized, and inserted into the microreactor. The effect of major reactor operating parameters, such as temperature, flow rate, C:O feed ratio, and, most importantly, the surface-to-volume ratio were tested in detailed experimental studies using both reactor outlet and spatially-resolved concentration profiles. The results show a strong decrease of C₂ production rates with decreasing microreactor channel height (680 to 460 and 330 ), in agreement with the established OCM reaction mechanism. Detailed FEM reactor simulations including gas phase and catalytic reaction steps are currently under way to validate the experiments and gain deeper insights into the coupling pathways.