Researchers at the Dalian Institute of Chemical Physics at the Chinese Academy of Sciences in Dalian, China, have developed a groundbreaking strategy to overcome the tradeoff between activity and selectivity in the direct conversion of syngas into light olefins. Syngas, a mixture of carbon monoxide and hydrogen, is a valuable feedstock for the production of ethylene, propylene, and butylene, which are essential building blocks for plastics.
Activity and selectivity are crucial factors in catalyst design for chemical reactions. Higher activity implies improved efficiency in converting raw materials into products, thereby reducing energy consumption. On the other hand, selectivity reflects the percentage of desired products, such as ethylene, propylene, and butylene in this case, which directly influences the economic viability of the technology.
For nearly a century, the Fischer-Tropsch synthesis (FTS) process has been used with iron or cobalt-based catalysts for the direct conversion of syngas. However, achieving high selectivity for light olefins has proven challenging. Six years ago, the research team developed an alternative process called OXZEO, which utilized metal oxide-zeolite catalysts and significantly improved the selectivity for light olefins, surpassing the theoretical limit of FTS. Despite this progress, the activity of the catalyst was still limited by the activity-selectivity tradeoff.
In their recent publication in the journal Science, Dr. Jiao Feng, Prof. Pan Xiulian, and Prof. Bao Xinhe led a team that demonstrated a novel approach to address this challenge. By incorporating germanium-substituted aluminophosphates into the OXZEO catalyst concept, the researchers were able to disentangle the desired target reaction from undesired secondary reactions. This integration resulted in the creation of more active sites for the conversion of intermediates into olefins, without compromising the selectivity for light olefins. The optimized conditions achieved an unprecedented yield of 48% for light olefins, while maintaining high carbon monoxide conversion.
To validate their mechanism, the researchers also investigated the use of silicon-substitute and magnesium-substitute aluminophosphates under similar conditions. However, these zeotypes failed to efficiently suppress the side reactions of hydrogenation and oligomerization, making it impossible to overcome the activity-selectivity tradeoff, despite attempts to optimize acid site density or reaction conditions.
Prof. Pan Xiulian explained that the separation of active sites involved in the key steps of syngas conversion within OXZEO catalysts, along with increased active site density and modulation of their properties for intermediate transport and reactions within the zeolite’s confined pores, presents an effective solution for achieving high selectivity and conversion of syngas to light olefins. This breakthrough may also have implications for bifunctional catalysis in other reactions, making it a promising area for further advancements in zeolite catalysis.
In addition, Prof. Bao Xinhe emphasized the potential impact of incorporating this strategy with green hydrogen energy technologies in the future. The synergy between the new catalyst design and carbon neutrality goals could lead to significant contributions in sustainable energy and the reduction of carbon emissions.
Overall, this research represents a significant advancement in catalyst development, offering a pathway to enhance the production of light olefins from syngas while simultaneously improving activity and selectivity.
Source: Chinese Academy of Sciences