Methanol and Hydrogen from Methane, Water, and Light

Introduction Research on the conversion of natural gas (methane) has been an ongoing effort at the National Energy Technology Laboratory (NETL) for over 20 years. A long-term goal of our research team is to explore novel pathways for the direct conversion of methane to liquid fuels, chemicals, and intermediates. Scheme 1: Proposed pathway for the photocatalytic conversion of methane and water Literature reports have indicated that photochemical oxidation of methane may be a commercially feasible route to methanol [1, 2]. In these studies, methane, water and light are reacted at moderate temperatures and pressures. EXPERIMENTAL The tungsten oxide semiconductor photocatalysts were synthesized following a modification of the procedure in the literature [6]. Four dopants, copper, lanthanum, platinum, and a mixture of copper and lanthanum, were selected for study on the tungsten oxide catalyst base. The titania photocatalyst used in this study is a proprietary sol-gel TiO2 catalyst obtained from Attia Corporation. Research in our laboratory [3] has shown that, methane, dissolved in water, at temperatures > 70 °C, with a semiconductor catalyst, can be converted to methanol and hydrogen. The use of three relatively abundant and inexpensive reactants light, water, and methane to produce methanol is an attractive process option. The main advantage of using a photocatalyst to promote the photoconversion of methane to methanol is that the presence of the catalyst, in conjunction with an electron transfer agent, allows reaction to occur with visible light instead of with ultraviolet. This greatly simplifies reactor design and will permit flexibility in the selection of the light source. The products of the reaction of interest, methanol and hydrogen, are both commercially desirable as fuels or chemical intermediates. The limiting factor for conversion of methane appears to be the solubility of methane in water. We hypothesized that if the concentration of methane in water can be increased, conversion should also increase. Methane hydrates might provide a method of increasing the amount of methane dissolved in water, because at standard temperature and pressure (STP), one volume of saturated methane hydrates contains approximately 180 volumes of methane. The 1.0 MPa, a commercially supplied 1-liter quartz photochemical reaction vessel, was fitted to meet the needs of this research. This included use of a Teflon-coated magnetic stirring bar in the reactor, a fritted glass sparger, a nitrogen line used to cool the UV lamp, and an injection port. In a typical 1.00 MPa pressure experiment, the sintered catalyst is suspended, by mechanical stirring, in double-distilled water (~750 mL) containing an electron-transfer reagent, methyl viologen dichloride. A mixture of methane (5 mL/min) and helium (16 mL/min) is sparged through the photocatalytic reactor. The helium is an internal standard for on-line analysis of the reactor effluent. The reaction temperature is maintained at ~371 K by circulation of heated (~393 K) silicone oil in the outer jacket of the reactor. A highpressure mercury-vapor quartz lamp is used as the light source. Methane hydrates were first observed in the laboratory in 1810. It wasn’t until nearly 150 years later that they were observed in nature. Hydrates can occur in permafrost, in sediment where gas exists under moderate to high pressure and low temperatures, and offshore beneath deep water. Hydrates are a problem in the oil and gas production industry because they can form in the well or pipelines, thereby blocking the flow of fuel. Estimates by the U.S. Geological Survey project that world hydrate deposits contain approximately 2 x 10 trillion cubic meters of methane [4]. Estimates of methane hydrate deposits off the coast of the United States is approximately 9 x 10 trillion cubic meters of methane with an additional 17 trillion cubic meters of methane in the permafrost on the north slope of Alaska [1]. All pressurized and hydrate reactions were conducted in a highpressure view cell. The cell is constructed of 316 stainless steel 6.35 cm (2.5 inches) OD and 27.4 cm (11 inches) in length. The internal volume of the cell is ~ 40 mL. The cell is fitted with 2 machined endcaps, one which contains a sapphire window to allow for observation of the contents of the cell using a CCD camera. The cell is fitted with ports to accommodate the fill gas inlet and reaction product outlet, a pressure transducer to monitor the internal pressure of the gas inside the cell, and a thermocouple that terminates inside the cavity of the cell to monitor the temperature of the liquid/hydrate mixture. While the working pressure of the cell is rated at 220 MPa (32,000 psia), all experiments were conducted at 13.8 MPa (2000 psig) or less. The temperature of the cell is controlled by the flow of a glycol/water solution from an external circulating temperature bath through a coil of 0.64 cm (1⁄4 inch) copper tubing that is wrapped around the outside of the cell. Several layers of insulating material are wrapped around the cell to help maintain constant temperature. Our vision is to immobilize methane and water in close proximity by formation of the methane hydrate. The reaction will involve the formation of hydroxyl radical (•OH) within the methane hydrate by photochemical means. The proximity and restricted mobility of the •OH and the CH4 would then favor the formation of CH3OH. Successful demonstration of this principle would then open the possibility of using hydrates to immobilize reactants in a way that favors the desired selectivity. This is the basis of our Patent [5]. A typical experiment involves filling the cell with 40 mL of double-distilled water. A Teflon® coated stir bar is added, followed by portions of the photocatalyst. The endcap is placed on the cell and tightened to specifications. An external magnetic stirrer is used to obtain a high degree of vortex mixing inside the cell. The cell is connected to the gas manifold and purged several times with