Partial oxidation of CH 4 and C 2H 6 over noble metalcoated monoliths

Abstract

The direct partial oxidation of hydrocarbons offers promising alternatives to chemical synthesis. By replacing endothermic processes such as steam reforming and steam cracking, fast and exothermic oxidation reactions should require much smaller and simpler reactors. However, direct oxidation reactions are much more difficult to manage because of potential heat release in total oxidation and hazardous because of the possibility of homogeneous reactions which are nonselective and can produce flames and explosions. We describe experiments in which monolith catalysts are used for partial oxidation of CH 4 and C 2H 6 to produce synthesis gas or alkenes by direct oxidation at or above atmospheric pressure in pure O 2 in nearly adiabatic reactors operating at 1000°C with very high flowrates (space velocities of 10 6h −1 and residence times of 10 −3 s). With methane oxidation we obtain over 90% selectivities to synthesis gas (a 2:1 H 2:CO mixture) with > 90% conversion of the methane and complete conversion of O 2 on Rh coated ceramic monoliths with contact times of 10 −3 s. With Pt catalysts under the same conditions, the H 2 selectivity drops to 70%; while with Pd, the catalyst rapidly forms carbon. This process appears to be primarily a surface reaction in which CH4 pyrolyzes on the hot Rh surface and the H atoms dimerize and the carbon is oxidized to CO. A model has been constructed which accurately predicts the conversions and selectivities and the variations between Rh and Pt. With higher alkanes, synthesis gas is produced on Rh with comparable selectivities and conversions on metal-coated monoliths. However, with Pt we observe up to 70% selectivity to alkenes with 80% conversion of alkanes at adiabatic temperatures near 1000°C with approximately 5 ms contact times. These results can be explained as occurring by predominantly surface reactions in which the alkane adsorbs to form the alkyl by H abstraction with adsorbed O atoms. Then the adsorbed alkyls undergo primarily β-elimination reactions on Pt to produce alkenes. These products are therefore far from thermodynamic equilibrium at these very short contact times, even though the temperatures are very high. The use of very short contact times and high temperatures promises to provide new routes to production of partial oxidation products with very small adiabatic reactors and thus opens up new types of reactions and reactors for chemical synthesis.