Catalytic selective oxidation of methane and ethane to oxygenates in gas and liquid phase
Creators: name: Alshihri, Saeed
Publisher: name: Imperial College London
Year: 2016

description: Direct catalytic oxidation of methane and ethane, which are the main components of natural gas, to oxygenates is one of the most attractive and difficult challenges in catalysis. Although there has been intensive work on methane and a little on ethane, no breakthroughs has been obtained using heterogeneous catalysts. The main aim of this study is to investigate commercial ZSM-5 with different Si/Al ratio and Au bimetallic catalysts supported on nanostructured oxides for the selective oxidation of methane and ethane to oxygenates at mild conditions. The direct selective oxidation of methane and ethane in aqueous hydrogen peroxide over ZSM-5 catalysts at mild conditions was performed using a stainless-steel autoclave (Parr reactor) containing a non-proprietary glass liner (50 ml) and glass coated stir bar. ZSM-5 (50), which had the highest Fe content, was found to be the most active catalyst. A parametric study has been carried out on methane and ethane using ZSM-5(50) in aqueous H2O2. For the methane oxidation, it was found that methylhydroperoxide was first formed, followed by its decomposition and further oxidation to mainly formic acid, which is in good agreement with the previous studies. MeOOH was found to produce HCHO, which postulated to be a key reaction intermediate that can lead to MeOH by reduction and formic acid by oxidation. The reaction of MeOOH and HCHO over ZSM-5(50) using aqueous H2O2 was conducted under the same conditions to confirm these results. Another finding in the apparent polymerization of HCHO to produce low levels of water soluble polyoxomethylene under mild reaction conditions. Under the same conditions, the oxidation of ethane over ZSM-5(50) showed high productivity (64.4 moloxygentaes.Kgcat-1.h-1) with similar product distribution to the previous work. An interesting finding of the oxidation of methane–ethane mixtures is an increase of polyoxomethylene, which we speculate might be due to the formation of mixture of polymers. Various nanostructures, mainly titanate nanotube (TiNT) and ceria nanorods (CNR) have been prepared by the hydrothermal method and used as support for Au-Pd bimetallic particles prepared by incipient wetness impregnation (IWI) and adsorption methods. Conventional supports (silica gel, synthetic amorphous silica, CeO2 nanoparticles and TiO2 anatase have been used to prepare Au-Pd catalysts by the impregnation method for comparison purposes. The catalysts have been characterized by a range of techniques including: XRD, XRF, BET, TEM and TGA. The catalysts have been tested on methane oxidation in both liquid and gas phases. Au-Pd over ceria nanorods (CNR) was found to be the most effective catalyst in both liquid and gas phase, giving a yield of oxygenates which were very low. In the gas phase, the direct partial oxidation of methane was carried out over the bimetallic Au-Pd catalysts in a continuous flow reactor using H2/O2 mixtures. The extraction of methylhydroperoxide as a primary product from a gas-solid reaction is reported for the first time.