Researchers from the Dalian Institute of Chemical Physics at the Chinese Academy of Sciences in Dalian, China, have reported a new strategy for the direct conversion of syngas to light olefins. Syngas is a mixture of carbon monoxide and hydrogen, and light olefins are the most commonly used building blocks for plastics. The two primary indexes of a successful catalyst for chemical reactions are activity and selectivity. Higher activity means higher efficiency in converting feedstock to products, thereby reducing energy consumption. Selectivity reflects the percentage of the desired products, which determines the economy of the technology.
The Fischer-Tropsch Synthesis (FTS)
For almost a century, the Fischer-Tropsch synthesis (FTS) was used for direct syngas conversion with iron or cobalt-based catalysts for the synthesis of chemicals. However, selectivity for light olefins remained a challenge. When FTS is used to convert syngas to light olefins, the yield amounts to around 26%.
The OXZEO Process
An alternative process, named OXZEO and developed six years ago by the same research team using metal oxide-zeolite catalyst, improved light-olefin selectivity far beyond the theoretical limit of FTS. Despite the significant progress over the years, the activity is still limited by the activity-selectivity tradeoff. Using traditional silicon containing zeotypes within the OXZEO catalyst concept, the light-olefins yield has so far maxed out at 27%. These limits originate from activity-selectivity tradeoff, a long-standing challenge in catalysis.
The New Strategy
In a paper published in the journal Science on May 18, 2023, a team led by Dr. Jiao, Prof. Pan, and Prof. Bao has shown that incorporating germanium-substituted aluminophosphates within the OXZEO catalyst concept can disentangle the desired target reaction from the undesired secondary reactions. It enhances the conversion of the intermediates to produce olefins by creating more active sites and in turn generation of intermediates but without degrading the selectivity of light olefins. With this new strategy, researchers simultaneously achieved high CO conversion and light-olefins selectivity, and the yield reached an unprecedented 48% under optimized conditions.
To validate the mechanism, researchers also studied silicon-substitute and magnesium-substitute aluminophosphates and tested them in similar scenarios. The active sites of these two zeotypes cannot efficiently shield the side reaction of hydrogenation and oligomerization, thereby the activity-selectivity tradeoff cannot be overcome, despite optimizing the acid site density or reaction conditions. The researchers expect that this new strategy can be applicable to analogous bifunctional catalysis in other reactions and will be of interest for further development of zeolite catalysis. If it can be incorporated with green hydrogen energy technology in the future, it will make a significant contribution to the goal of carbon neutrality.