Selective oxidation of methane in a trickle bed reactor over a platinum-based catalyst

The direct on-site conversion of methane to methanol could provide a more cost-effective and less energy-intensive utilization of natural gas compared to the industrial two-step syngas based route. Despite the numerous approaches investigated for direct methane to methanol (DMTM) conversion in the last century such as homogeneous gas-phase, homogeneous liquid partial oxidation and heterogeneous catalytic partial oxidation, none of them were deemed successful for commercialization due to either the use of expensive reagents such as H2O2 and H2SO4, low yields and conversions, or inefficient system requirements. This has been mainly attributed to the thermodynamic challenge of breaking the strong C-H bond (bond dissociation energy of 435 kJ/mol) in the highly stable methane molecule and due to the higher reactivity of the product methanol leading to the formation of COx products. Based on density functional theory (DFT) calculations, it was postulated that the selective oxidation of methane to methanol was possible with weakly adsorbed oxygen species over transition metal surfaces such as Ag and Pt at high oxygen and water partial pressures. Recent experimental studies also indicated that the presence of water through site blocking action had beneficial effects on catalytic activity and methanol selectivity. Since the role of liquid water in a single continuous-flow reactor has not been well established, evaluating its effect on catalytic performance and product selectivity in the direct conversion of methane over platinum-based catalysts was the prime focus of this research. Therefore, a specifically designed trickle bed reactor (TBR) system capable of operating continuously and safely at high pressures was constructed to investigate the selective oxidation of methane over platinum-based catalysts in the presence of liquid water. Methane, oxygen and helium were pressurized and flow-metered prior to entering the reactor while the flow of deionized liquid water was controlled through a pump. The reactor assembly consisted of a quartz tube liner sealed at the top with a fluorocarbon O-ring in a stainless-steel shell (545 mm in length and 15.8 mm in inside diameter). A central thermowell fixed along the reactor tube which shielded an internal thermocouple was also quartz-sheathed to prevent undesired side reactions on the metal surface. Pelletized platinum catalyst supported on titania (ca. 1.5 g, dp = 150-250 µm) was packed in the isothermal region (ca. 100 mm) of the reactor with silicon carbide granules (dp ~ 1000 µm) filling the top and bottom void spaces. The reactor body was heated through a five-zone electric furnace. Pressure-controlled argon was allowed to flow between the annular region of the quartz liner and reactor shell at the same pressure as inside the quartz tube liner. The flowrate of argon was set approximately 2-3 times higher than the total gas flowrate through the catalyst bed to completely vaporize the effluent stream. The reactor effluent was throttled to atmospheric pressure and heated via heating cords to ensure no condensation of products prior to analysis via an existing online GC-Polyarc™-FID system. Long-term catalytic experiments were performed on platinum impregnated TiO2(P25) and TiO2(rutile) in the newly constructed trickle bed reactor at 220 °C and 30 bar. The inlet partial pressures of methane and oxygen were kept constant at 0.5 bar and 1.5 bar, respectively, while the feed rate of liquid water was varied from 0 - 0.3 ml/min with intermittent return to baseline condition (i.e., at PH₂O = 6.9 bar) to monitor catalyst activity. The results of this investigation indicated that co-feeding liquid water has beneficial effects on the activity of these catalysts as well as the selectivity towards C1 oxygenated products. Both catalysts were able to selectively convert methane to C1 oxidation products in the trickle bed reactor with the highest activity and selectivity obtained while operating in the flooding region whereby water exists in the liquid phase (PH₂O > 23.1 bar). Pt/TiO2(P25)selectively formed methanol and methoxy methanol with a maximum selectivity of ca. 10% at a methane conversion of 0.6%, corresponding to an average turnover frequency of 0.25 h-1. Carbon dioxide was the major product formed over Pt/TiO2(P25). Conversely, over Pt/TiO2(rutile), formaldehyde was the exclusive selective oxidation product formed with a remarkable selectivity of 90% at a methane conversion of 1%, corresponding to an average turnover frequency of 0.43 h-1. Carbon dioxide and carbon monoxide were also detected in the product spectrum. Thus, under similar operating conditions, Pt/TiO2(rutile) exhibited a significantly higher activity and improved selectivity, greatly favouring the formation of C1 oxygenates compared to Pt/TiO2(P25). The novel trickle bed reactor system has demonstrated great ability in operating continuously at steady state for long-term runs for the selective oxidation of methane in the presence of liquid water at high pressure (< 50 bar) and moderate temperature (< 400 °C). Although the findings in this research indicate that liquid water facilitates the direct conversion of methane to selective oxidation products such as methanol and formaldehyde, the role of liquid water is not still obvious and has not been explicitly studied. Therefore, it is recommended that more catalytic testing with optimised reactor conditions and/or addition of promoters (such as Ag, Ni, Pd, Mo, Co and Cu) should be done in order to improve methane conversion, as well as yield for C1 oxidation products. Furthermore, to enhance the performance of the reactor and better understand the interaction of liquid water with the solid catalysts in the tri-phasic system, hydrodynamics studies should be performed.