Have you ever wondered if the gas used to heat your home or cook your food was destined for greater things? With the increased abundance of domestic natural gas, it is more economically feasible to convert natural gas components to higher value chemicals than to break down imported crude oil. These natural gas conversions involve small hydrocarbon molecules (containing only C-H and C-C bonds) that can either be modified, for example to alcohols (which also contain C-O and O-H bonds), or coupled together to make liquid compounds used in transportation fuels. One of the “Holy Grail” conversions currently being sought is the transformation of methane (a hydrocarbon) to methanol (an alcohol), which can subsequently be used as a more sustainably sourced fuel. Scientists at the Inorganometallic Catalyst Design Center (ICDC), an Energy Frontier Research Center, are striving to develop next-generation materials for this transformation and similar energy-related chemical conversions.
Within ICDC, researchers use nano-materials composed of metal atoms or clusters bridged by organic linkers, called metal-organic frameworks (MOFs), as catalysts to enhance the efficiency of natural gas conversions. Catalysts, being materials that are not consumed within a chemical reaction, serve the purpose of transforming chemicals that are less valuable into more useful compounds, in an efficient manner. Therefore, catalysts are desirable for developing economical processes in industry, the cost savings of which then trickle down to lower cost goods and services provided to the general public. Because MOFs are materials composed of both organic and inorganic components, synthetic chemists have an immense toolbox at their fingertips for developing next-generation catalysts. Scientists can select their desired building blocks, targeting specific properties at the nanometer scale (about 1/100,000th the size of a human hair). While being able to selectively choose ideal properties is desirable, this handy toolbox provides endless combinations of building blocks, making synthetic screening a time-consuming process.
Technology to the Rescue! In addition to experimental efforts within ICDC, computational modeling and machine learning allow researchers to identify candidate materials before time-consuming experimental efforts, which can take 10–100x longer than computational efforts and generate waste. With supercomputing resources at their fingertips, researchers perform complex calculations that provide meaningful information that allows experimentalists to narrow their synthetic focus, a prime example of which can be found published in ACS Catalysis. 2
While, in theory, the combined efforts of computational screening and synthetic targeting work well hand-in-hand, having a reliable synthetic method is essential when using MOFs as catalysts. The most common method of catalyst installation is the deposition of catalytic metal species on the nodes of the MOFs, taking advantage of an already synthesized MOF as a robust scaffold. To that end, not only do various experimental conditions, such as temperature or duration, impact the catalyst loading, but using different metal precursors also can drastically affect the physical properties of the MOF, such as surface area, pore volume, crystallinity, etc. For example, using a precursor that installs too little metal catalyst onto the MOF will produce a less efficient material, while installing too much metal can lead to big chunks and nanoparticle agglomerations that block the pores and inhibit the reaction. In addition, higher temperatures may be needed to deposit sufficient amounts of metal catalysts in the MOF, but those temperatures could degrade the MOF structure, thereby losing its desirable properties, such as high surface area and crystallinity.
Iron and Nickel and Copper, Oh My! Using atomic-layer deposition (ALD), a vapor-phase method of depositing metals onto MOFs using reactive metal compounds, researchers developed a database spanning different classes of precursors. Through systematic screening of these precursors by deposition on MOF NU-1000 (NU = Northwestern University), they identified the metal loading, structural uniformity, and porosity of the postmodified MOFs. From iron and nickel to indium and palladium, and many more, each different metal and different class of precursor provided valuable information in the form of their effect on the physical properties of the catalyst material. By identifying which classes of precursors caused more or less degradation of the MOF, the researchers were able to identify certain classes of precursors that provided more ideal materials. In future studies, these materials will be tested for their catalytic activity, based on supporting computational studies.
By establishing this library of precursors, including determination of their effects on physical properties, ICDC researchers have laid the groundwork for identifying trends in successful precursor classes for the ALD-based synthesis of MOF catalysts. While this is not an exhaustive list of precursors, the researchers have begun a systematic study and initiated the discourse on identifying both the challenges and benefits of their modification method, which can feed into the design of next-generation catalysts.
Using the comprehensive experimental and computational toolbox outlined above, as well as the newly developing library, the cohort of talented and inspiring junior and senior researchers within ICDC is paving the way for next-generation catalyst material design for upgrading natural gas to more valuable chemicals.
