Integrated Membrane System for Liquid Phase Partial Oxidation of Methane in Mild Conditions

Methane, the main component of natural gas, represents the most abundant and low cost energetic reserve on earth. Since many of natural gas reserves are located in remote areas, methane conversion to liquid fuels is a key topic. The conversion is mainly done through an energy intensive two step process. Thus, in last years, considerable efforts have been made in the area of direct and selective methane conversion to methanol either in gas phase or in liquid phase medium. Methanol yield in gas phase is low because a high temperature promotes thermodynamically favoured over oxidation reactions. Instead, in liquid phase, the use of a very aggressive medium under harsh operating conditions is required because of the intrinsic limitation of the heterogeneous catalytic process: the substrate, the catalyst and the oxidant are in different phases, and the poor methane solubility in the liquid medium represents a key limitation. The use of a gas–liquid membrane contactor could overcome this limitation.In the present work an integrated catalytic system - membrane for liquid phase partial oxidation of light hydrocarbons such as methane in mild operating conditions has been studied and tested. Water was chosen as reaction medium because of its chemical stability. The poor methane solubility in water was enhanced by working under moderate pressure (0.4 MPa) and then integrating the catalytic system with a gas–liquid membrane contactor. Preliminary catalytic tests, using the Fenton reagent (Fe2+, H2O2), were carried out in a batch system. Best results were obtained under the following operating conditions: [Fe2+] = 2.70 mmol L−1; [H2O2] = 54 mmol L−1; T = 25 °C; pH = 2.98. GC–MS analyses evidenced production of dimethyl peroxide (DMP), that can be potentially converted to methanol (two moles per mole of DMP). These conditions were applied carrying out some catalytic tests in the integrated membrane system evidencing the influence of both inside membrane diameter and material type on system performance. Three different membranes were tested for assembling the membrane contactor: i) polypropylene fibers; ii) polypropylene capillary membranes; iii) tubular ceramic membrane (inorganic, 0.2 m pore size). This latter one gave the best performance avoiding problems related to membrane degradation and permitting to obtain the best observed catalytic activity maximizing the apparent selectivity and minimizing the oxidation of methane to CO2.