Unraveling the Activity-Selectivity ‘Tradeoff’ Effect in Catalytic Conversion: Disentanglement

The researchers report an activity-selective crosslinking strategy for the direct conversion of syngas, a mixture of carbon monoxide and hydrogen, into the desirable ethylene, propylene and butylene. These hydrocarbons are known as light olefins and are the most widely used building blocks for plastics.

said Jiao Feng, assistant professor at the Dalian Institute of Chemical Physics at the Chinese Academy of Sciences in Dalian, China. “Selectivity reflects the percentage of products that are desirable; for example, ethylene, propylene and butylene in this case, which determines the economy of the technology.”

For nearly a century, a process called Fischer-Tropsch synthesis (FTS) has been used for direct conversion of syngas using iron or cobalt-based catalysts for the synthesis of chemicals. However, the selectivity of light olefins remained a challenge. An alternative process, called OXZEO and developed by the same research team six years ago using a zeolite metal oxide catalyst, has improved the light olefin’s selectivity well beyond the theoretical limit of FTS. Despite significant progress over the years, activity is still restricted by a selective-activity trade-off.

For example, when using FTS to convert syngas to light olefins, the yield is about 26%. Using traditional animal-based silicon within the OXZEO catalyst concept, light olefins production has reached its maximum yield to date at 27%. These limits arise from a trade-off of activity selectivity, which is a long-standing challenge in catalysis. This can be traced back to the catalytic sites of both target and lateral interactions, which are usually intertwined with technical stimuli.

Now, in a paper published in the journal Sciences On May 18, 2023, a team led by Dr Jiao, Professor Pan, and Professor Bao showed that incorporation of germanium-substituted aluminophosphates within the OXZEO catalyst concept could disentangle a desired target reaction from unwanted secondary reactions. It enhances the conversion of intermediates to produce olefins by creating more active sites and thus generating intermediates but without reducing the selectivity of light olefins. With this new strategy, the researchers achieved simultaneously high CO conversion and selectivity for light olefins, and yields reached an unprecedented 48% under optimized conditions.

To validate the mechanism, the researchers also studied the silicon substituent and the magnesium substituent aluminophosphate and tested them in similar scenarios. The active sites of these two zeotypes cannot efficiently shield the side reaction of hydrogenation and oligomerization, and thus the selective activity trade-off cannot be overcome, despite optimizing acid site density or reaction conditions.

“Separating the active sites of the two major steps of syngas conversion by OXZEO catalysts, increasing the active site density and modifying its properties for intermediate transport kinetics and interactions within animal-type confined pores provides one efficient solution for converting syngas into light olefins,” said Pan Xulian, professor at the Institute of Dalian School of Chemical Physics at the Chinese Academy of Sciences in Dalian, China.”We expect that this will be applicable to similar bifunctional catalysis in other reactions and will be of interest to the further development of zeolite catalysis.”

said Bao Xinhe, professor at the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences in Dalian and president of the University of Science and Technology of China.