How Can We Make Chemical Manufacturing More Sustainable?
To make a difference in global sustainability via chemical manufacturing, we want to prioritize. Which chemicals offer the best opportunities? That depends on the scale at which they are produced and how much associated CO2 and other pollutants that production puts in the air.
The production of ammonia and ethylene has a significant impact on global CO2 emissions. These compounds, along with sulfuric acid, are among the most highly produced commodity chemicals globally. The scales are massive, as much as or greater than 200 million metric tons each! Ammonia is primarily used for fertilizer. Ethylene is a crucial industrial feedstock, used to make polyethylene for plastics, and converted to other precursors to make vinyl products, PVC piping, synthetic rubber, and foam insulation, to name just a few.
In my lab, we think about new ways to make ammonia and ethylene, and other challenges in chemical synthesis that can advance how we make more complicated molecules, such as drug molecules used in medicines. Along with our collaborators, we aim to develop catalysts that can, ultimately, enable carbon-neutral production of ammonia and ethylene using renewably sourced electricity, water, nitrogen gas from the air, or industrial CO2 waste streams.
In current ammonia production, known as the Haber-Bosch process and practiced much the same way for more than 100 years, methane derived from natural gas reacts with steam and a catalyst to generate hydrogen gas. The hydrogen gas is then combined with nitrogen and an iron catalyst at high temperature and pressure to create ammonia. The high heat and pressure required is supplied by burning fossil fuels, as is the energy required to make the hydrogen gas.
My lab is working on a new, different way of producing ammonia, through electrochemical conversion of nitrogen, the main component of air. After years of fundamental research, we now have exciting science that demonstrates how to achieve electrochemical ammonia generation with molecular catalysts. We, along with others in the field, are also excited by the promise of potentially more durable heterogeneous catalysts.
Electrochemical conversion using renewable energy would make fertilizer production far more sustainable. Even better, if the conversion gets so efficient and cheap that it makes sense to burn the ammonia in an engine, then one has a zero-carbon fuel. This is already being pursued by some fertilizer production companies at pilot scale using renewably sourced energy to drive production via Haber-Bosch.
An early opportunity to test ammonia as a fuel is in maritime shipping, where it could replace the super-polluting, high-sulfur sludge-like diesel used worldwide. Ammonia is easily liquified and stores huge energy density, and can be handled and transported safely from decades of experience.
When you burn ammonia, it releases only water, nitrogen, and a small amount of polluting nitrogen oxides, which we know how to minimize and scrub. One can also crack ammonia back to hydrogen and nitrogen, if hydrogen is instead the on-site fuel that is desired.
We would like to make ethylene with a process based on renewably sourced electricity and waste carbon dioxide. This would be a disruptive approach. Working closely with my colleague and Caltech chemistry professor Theo Agapie, we are exploring with a small team how to modify copper electrodes to enable them to convert CO2 to products like ethylene. Our collaboration is supported by the DOE Liquid Sunlight Alliance and was earlier supported by the DOE's Joint Center for Artificial Photosynthesis (JCAP) at Caltech. Of course, even if we can provide ethylene from CO2, water, and renewably sourced electricity, there are separate and important downstream issues of managing plastic disposal, mitigating single-use plastic, and enabling better recycling to improve plastic's sustainable use as a material.
My lab is also collaborating with Professor Fu's group at Caltech to explore new, more sustainable and non-toxic catalytic approaches to constructing chemical bonds that are essential to the generation of more complex molecules. For example, we're using light-driven copper catalysis to discover new types of carbon-nitrogen bond constructions. Such bonds are among the most prominent types of bonds in important molecules for medicines.
If we can invent low-carbon or even carbon-neutral methods to produce ammonia and ethylene, the potential is very exciting. Relatedly, while cleaning up the production of pharmaceutical chemicals is not in the realm of the existential crisis of climate change, greening industry is essential if we are to be responsible stewards of our environment and resources.
—Jonas Peters, Bren Professor of Chemistry and director of the Resnick Sustainability Institute
You can submit your own questions to the Caltech Science Exchange.