Methane is the main constituent of natural gas. The direct oxidation of this compound into methanol at low temperatures has traditionally been a holy grail.
Now, a research team headed by
Tufts University chemical engineers have discovered a new method to achieve this by using a cheap molecular oxygen and a heterogeneous catalyst. The results of the study have been published in the journal, Nature.
Aberration-corrected HAADF/STEM images of as-synthesized Rh-ZSM-5. Single rhodium cations are circled in white with proposed ball-stick model of the structure. (Lawrence F. Allard, co-author and researcher at Oak Ridge National Laboratory )
Methanol is a main feedstock used in the production of chemicals. Some of these chemicals are used for making several products such as paints, plywood, and plastics. Additionally, methanol can be used as fuel in vehicles or can be reformed to create high-grade hydrogen for fuel cells.
In current methods, methanol is produced from coal- or methane-derived synthesis gas but this is a multi-step process and hence not efficient and cost-effective in small-scale applications. According to the U.S. Energy Information Administration, methane emissions from oil wells account for 210 billion cubic feet of natural gas per year.
In the interim, the growth of hydraulic fracturing, also known as fracking, and the resultant use of shale gas whose main component is methane have considerably boosted the natural gas supply in the US and expedited the desire to convert methane into more valuable chemicals, such as through carbonylation to acetic acid or oxidation to methanol.
Consequently, researchers have been searching for cheaper and more efficient means to convert methane with a process that employs low-cost molecular oxygen in mild conditions, i,e., where fairly low pressure and temperatures are utilized. This provides immense potential benefits. Earlier in 2000, the availability of low-priced shale gas represented only 1% of American natural gas supplies, but today it represents over 60%.
The Tufts-led research team discovered that molecular oxygen and carbon monoxide can be used to directly convert methane to methanol, catalyzed by supported mononuclear rhodium dicarbonyl species which were anchored either on the zeolites’ internal pore walls or on the surface of titanium dioxide supports suspended in water under temperature (110 to 150 °C) and mild pressure (20 to 30 bar).
Acetic acid is also produced by the same catalyst through another reaction scheme, where methanol is not involved as an intermediate. Carbon monoxide is important to the heterogeneous catalytic reaction. By properly controlling the operating conditions, the reaction can be adjusted to either acetic acid or methanol particularly the acidity of the support. The study found that even after many hours of reaction, there was no leaching of the catalyst in the water.
Maria Flytzani-Stephanopoulos, Ph.D., the paper’s senior author, a Distinguished Professor and the Robert and Marcy Haber Endowed Professor in Energy Sustainability in the School of Engineering at Tufts University, stated that the team was quite surprised to find that for methanol production carbon monoxide has to be used in the gas mixture.
We attributed this to retaining the active site carbonylated, i nterestingly, our catalyst does not carbonylate methanol. Instead, it carbonylates methane directly to acetic acid, which is a most exciting finding. Although more study is needed, we are encouraged that this process holds promise for further development. Not only could it be effective in producing methanol and acetic acid directly from methane, it also could do so in a more energy efficient and environmentally friendly way than current processes.
Maria Flytzani-Stephanopoulos, Ph.D., the paper’s senior author, Distinguished Professor and the Robert and Marcy Haber Endowed Professor in Energy Sustainability, School of Engineering, Tufts University
First authors of the paper – postdoctoral fellow JunJun Shan and doctoral student Mengwei Li – prepared supported Rh catalysts using relatively simple synthesis procedures. Atomic dispersion of rhodium species was the main focus of the work and this was accomplished through a unique heat treatment procedure on the zeolite support and by anchoring the rhodium precursor species on reduced titania aided by UV-irradiation. Shan informed that the atomic rhodium state is needed to trigger the reaction.
Lawrence F. Allard, Ph.D., the paper’s co-author and distinguished research staff member at Oak Ridge National Laboratory, added that aberration-corrected electron microscopy was vital in supporting the study.
The ‘direct’ imaging of single atom dispersions coupled with more standard ‘indirect’ chemical and spectroscopic methods has been a powerful combination of capabilities that allow these studies to be so successful,” Allard said.
Flytzani-Stephanopoulos directs the Tufts Nano Catalysis and Energy Laboratory, in the Department of Chemical and Biological Engineering, which explores innovative catalyst materials to the production of “green” chemicals and hydrogen. Her lab has performed pioneering work, which showed the use of heterogeneous single metal atom catalysts for target reactions to fuel processing, and to value-added chemicals and commodity production, with reduced carbon footprint and improved yields, whilst utilizing precious metals in a sustainable and more efficient way.
Besides Allard from ORNL and the Tufts researchers from the Department of Chemical and Biological Engineering, Sungsik Lee, Ph.D., another co-author of the paper and staff scientist at Argonne National Laboratory, helped with the X-ray absorption spectroscopy (XAS) work that was utilized to reveal the catalysts’ structural state.
The study was supported by the U.S. Department of Energy (DOE)/ARPA-E program. The XAS research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory. Aberration-corrected electron microscopy at Oak Ridge National Laboratory was sponsored by the DOE, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, and the Propulsion Materials Program. Aberration-corrected electron microscopy at Oak Ridge National Laboratory was sponsored by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office’s Propulsion Materials Program.