New catalyst makes propylene production more efficient

Michail Stamatakis, UCL

A SINGLE-atom catalyst has been developed that that can lower the carbon footprint of propylene production through increased selectivity and lower temperatures.

Propylene is used for the production of plastics, fabrics, and other chemicals and has a demand of around 100m t/y. It is typically a byproduct of oil production, but the shift to shale gas has limited the availability of propylene. While propane in shale gas can be converted to propylene, it requires the use of catalysts that are only around 90% selective which means that 90% of the proportion of reactions at the surface create the desired product. These catalysts are also prone to becoming deactivated through a buildup of carbonaceous deposits on the surface of the catalyst, known as coking. The catalyst then needs to be regenerated, which disrupts the process and is not cost-efficient.

Researchers from University College London (UCL), Tufts University, University of Cambridge, and University of California at Santa Barbara have now developed a new catalyst to tackle these problems. They used a single-atom alloy (SAA) catalyst to convert the propane to propylene. A conventional catalyst – which in the case of processes like propylene production is usually platinum-based – is made up of combinations of metals with a random, complex structure. The metals are “trapped” inside and therefore can’t participate in the reactions. An SAA catalyst consists of individual metal atoms that have been dispersed onto a second metal that is used as a support. This means that the atoms can better participate in reactions. The new catalyst for propylene production consists of single rhodium atoms embedded in a copper support.

Designing new catalysts is done via calculations to determine how chemicals might interact with the catalyst surface; the team used quantum chemical simulations on a supercomputer to identify the best SAA catalyst.

Michail Stamatakis, Associate Professor in Chemical Engineering at UCL, said: “Improvement of commonly-used heterogeneous catalysts has mostly been a trial-and-error process. The single-atom catalysts allow us to calculate from first principles how molecules and atoms interact with each other at the catalytic surface, thereby predicting reaction outcomes. In this case, we predicted rhodium would be very effective at pulling hydrogens off molecules like methane and propane – a prediction that ran counter to common wisdom but nevertheless turned out to be incredibly successful when put into practice. We now have a new method for the rational design of catalysts.”

The catalyst was synthesised and tested, revealing that it has a 100% selectivity to propylene and it is resistant to coking. Charles Sykes, the John Wade Professor in the Department of Chemistry at Tufts University, said: “That level of efficiency could lead to large cost savings and millions of tons of carbon dioxide not being emitted into the atmosphere if it’s adopted by industry.”

It can also operate in lower temperatures than the traditional Pt-based catalysts. “Our flow-reactor studies showed that the RhCu SAA catalyst is already active at around 180^oC (light-off temperature), while for the conventional Pt-based catalyst to reach the same activity, the temperature has to exceed 200^oC,” Stamatakis told The Chemical Engineer. “Moreover, at around 215^oC, the RhCu SAA catalyst exhibits a ~6-fold higher activity than the Pt-based catalyst. Crucially, the activity of the RhCu SAA remains stable at even higher temperatures and long times, due to the resistance to deactivation via coking that this material exhibits."

“On the other hand, the Pt catalyst deactivates fairly quickly; for instance, at 350^oC, Pt loses more than 60% of its activity after 15 hours on stream, while the RhCu SAA catalyst retains its activity even after 50 hours on stream. Thus, aside from the energy savings due to the lower operating temperature of the RhCu SAA catalyst, the stability thereof also means less frequent disruptions to reactor operation for the purpose of replacing or regenerating the catalyst.”

Producing the catalyst at industrial scale would be more-or-less straightforward said Stamatakis, as it could be synthesised using the same methods currently used to make Cu-based catalysts at industrial scale. The SAA catalyst is also expected to work with current processes and reactors without the need for significant modifications.

Sykes said: “This work further demonstrates the great potential of single-atom alloy catalysts for addressing inefficiencies in the catalyst industry, which in turn has very large economic and environmental payoffs.”

Science https://doi.org/gkth4s