Scientists develop method for direct conversion of methane to methanol or acetic acid under mild conditions
Researchers at Argonne National Laboratory, Tufts University and Oak Ridge National Laboratory have shown that mononuclear rhodium species, anchored on a zeolite or titanium dioxide support suspended in aqueous solution, can catalyze the direct conversion of methane to methanol and acetic acid using oxygen and carbon monoxide under mild conditions.
In a paper in the journal Nature, they report that the two products form through independent pathways, allowing the tuning of the conversion. Three-hour-long batch-reactor tests conducted at 150 ˚C, using either the zeolite-supported or the titanium-dioxide-supported catalyst, yielded around 22,000 micromoles of acetic acid per gram of catalyst, or around 230 micromoles of methanol per gram of catalyst, respectively, with selectivities of 60–100%. The researchers said that while still too low for commercialization, these unusually high activities may guide the development of optimized catalysts and practical processes for the direct conversion of methane to methanol, acetic acid and other useful chemicals.
An efficient and direct method of catalytic conversion of methane to liquid methanol and other oxygenates would be of considerable practical value. However, it remains an unsolved problem in catalysis, as typically it involves expensive or corrosive oxidants or reaction media that are not amenable to commercialization. Although methane can be directly converted to methanol using molecular oxygen under mild conditions in the gas phase, the process is either stoichiometric (and therefore requires a water extraction step) or is too slow and low-yielding to be practical.
Methane could, in principle, also be transformed through direct oxidative carbonylation to acetic acid, which is commercially obtained through methane steam reforming, methanol synthesis, and subsequent methanol carbonylation on homogeneous catalysts. However, an effective catalyst for the direct carbonylation of methane to acetic acid, which might enable the economical small-scale utilization of natural gas that is currently flared or stranded, has not yet been reported.—Shan et al.
The team prepared the supported rhodium catalysts using relatively simple procedures. Rhodium loadings of 0.5 wt% and 0.6 wt% were used on the ZSM-5 and titanium dioxide supports, respectively. The catalysts were suspended in water and tested in a batch reactor under a total pressure of CH4, CO, and O2 of less than 30 bar. The reactions were immediately activated and catalytic, as measured by catalyst turnover.
The light-off temperature—the temperature at which the catalytic reaction was initiated—was approximately 110 °C in the case of Rh-ZSM-5.
Methanol is a key feedstock for the production of chemicals, some of which are used to make products such as plastics, plywood and paints. Methanol also could potentially fuel vehicles or be reformed to produce high grade hydrogen for fuel cells.
The decades-long interest in finding efficient ways to convert methane to methanol has grown even stronger in recent years thanks to the abundance of methane found in US-based natural gas.
However, the current method for producing methanol from methane involves a multi-step process that is neither efficient nor economical in small-scale applications.
In a commentary in Nature based on the study, Ive Hermans, chemistry professor at the University of Wisconsin-Madison, noted that the research “links homogeneous organometallic chemistry ... with solid-phase (heterogeneous) catalysis, and illustrates the importance of understanding catalysts at the atomic scale.”
Junjun Shan, Mengwei Li, Lawrence F. Allard, Sungsik Lee & Maria Flytzani-Stephanopoulos (2017) “Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts” Nature 551, 605–608 doi: 10.1038/nature24640