Detangling – the path towards efficient and precise catalytic reactions

In the chemical industry, more than 85% of processes rely on catalysts to accelerate reaction rates. But in most cases, the two important parameters that determine the efficiency of the catalytic reaction, the conversion rate of the reactant and the selectivity of the target product, are often entangled with each other, like a “seesaw”, the conversion rate increases, the selectivity decreases, and the trade-off cannot be taken into account at the same time.

How to unravel this “entanglement”, crack the “seesaw” effect, and achieve more accurate and efficient catalysis is an important challenge in basic science and applied research of catalysis, and it is also the direction that catalysis researchers have been working towards.

The research team of Dr. Jiao Feng, researcher Pan Xiulian and academician Bao Xinhe of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences (hereinafter referred to as “Dalian Institute of Chemical Physics”) has made new innovations in the direct conversion of coal into olefin OXZEOR-TO by syngas, which provides an effective scientific method to solve the bottleneck problem of high activity and high selectivity. The findings were published May 19 in the journal Science.

The innovative bifunctional catalyst breaks the active-selective “seesaw” in the synthesis gas to olefin reaction of coal. Photo courtesy of Dalian Chemical Properties

“From the lab to the factory”

Low-carbon olefins (including ethylene, propylene, butene) are an important industrial raw material, and various plastics and resins in daily life are inseparable from it.

At present, low-carbon olefins are mainly obtained from petrochemicals. However, China’s energy resource endowment is characterized by “poor oil and gas relative rich coal”, and exploring the use of coal instead of oil to produce low-carbon olefins is of great significance to ensure China’s energy security. As an important platform compound for carbon resource utilization, coal-based syngas (a mixture of carbon monoxide and hydrogen) has always been a frontier and challenge in international research in this field.

In the traditional Fischer-Tropsch synthesis technology route, metal or metal carbide catalysts are used, and the activation of reactant molecules and the formation of product molecules occur on the same catalytic reaction activity center on the open catalyst surface, so carbon-carbon coupling cannot be accurately controlled, which leads to a wide distribution of carbon chain length of hydrocarbon products. The research team of Dr. Jiao Feng, researcher Pan Xiulian and Academician Bao Xinhe created a catalytic system (OXZEO) for the composite of oxides and zeolite separated by active centers after a lot of research.

OXZEO bifunctional catalyst. Photo courtesy of Dalian Chemical Properties

In this system, the activation and dissociation of the reactants carbon monoxide and hydrogen, as well as the generation of the active intermediate ketene CH2CO, are carried out on the surface of the oxide ZnCrOx, and the intermediate enters the molecular sieve pores through gas phase diffusion, and then the reaction process of carbon-carbon coupling to generate olefins is realized in the molecular sieve confined pores. In this way, the team successfully realized the effective separation of the two active centers of reactant activation and product generation, and achieved the selectivity of low-carbon olefins of 80% when the carbon monoxide conversion rate was 17% for the first time in the world, breaking through the theoretical limit of ASF that the selectivity of low-carbon hydrocarbons in syngas direct olefin reaction cannot be higher than 58%. Moreover, this process eliminates the steps of water gas transformation and synthesis of intermediate products, and in principle opens up a new way of coal conversion with low water consumption and low emissions.

After this result was reported in Science in 2016, it attracted great attention and praise from peers, and then Dalian Chemical Institute cooperated with enterprises to create OXZEOR-TO catalyst, and completed an industrial test with an annual output of 1,000 tons of low-carbon olefins in the factory in 2020, which verified the correctness of the scientific principle of this process and the feasibility of the process. According to statistics, more than 20 research teams at home and abroad have carried out systematic research based on this concept, and the research system has expanded from syngas conversion to efficient utilization of carbon dioxide.

In order to further recognize and understand the mechanism of this innovative reaction and improve the catalytic reaction efficiency of the process, the team did not stop exploring.

Catalytic reactions will also face the “seesaw” problem

However, with the deepening of the research, the team found that the activity and selectivity of direct synthesis gas conversion are always difficult to balance, and the “double high” of conversion and selectivity cannot be achieved.

“The activity and selectivity of the catalyst is like sitting at both ends of the “seesaw”, raising this and not caring about that, we don’t know the reason at first, it is a headache. Jiao Feng introduced.

As a typical selective hydrogenation reaction, the direct production of low-carbon olefins from syngas is limited by the “active-selective” seesaw and cannot obtain high yields. Are “active” and “selective” really as incompatible as fish and bear’s paw? Overcoming this “hard bone” became a new scientific goal for the team to focus on.

In the following six years, the Dalian Institute of Chemicals and the research team of the University of Science and Technology of China worked closely to carry out systematic and in-depth basic research and theoretical analysis, and discovered the secret behind it: the existing molecular sieve active center not only catalyzed the main reaction of converting intermediates into low-carbon olefins, but also catalyzed the side reactions of over-hydrogenation of low-carbon olefins to generate low-value alkanes or over-polymerization into macromolecular olefins, so this common active center is like the fulcrum of the “seesaw”, and the conversion rate is increased at one end. The selectivity at the other end is reduced, and the conversion rate and selectivity cannot be increased at the same time, resulting in the inability to improve the yield of low-carbon olefins.

After figuring out the principle behind the reaction, the team dived into the research to find a solution.

“Accelerating the transport and conversion of intermediates while reducing the occurrence of side reactions in the pores of molecular sieve is an effective way to unravel this “entanglement”. Jiao Feng said.

Having grasped the core of the problem, the team members came together to open up new explorations.

Breaking through the limit relies on active center reconstruction

After many experimental studies, the researchers realized that the zeolite acid site is very important for the OXZEO process. At present, most of the international research on molecular sieve is almost the use of Si-OH-Al acid sites for the catalytic conversion of intermediates.

“Based on previous research, we gradually realized that if we continue to use Si-OH-Al as the active site, no matter how much the catalyst or reaction conditions are optimized, we will not be able to break through the limitations of the existing ‘seesaw’.” Jiao Feng told China Science News, “So we thought that if we can find a new catalytic active site that can accelerate the conversion of intermediates while reducing the occurrence of side reactions of target products, we may be able to unravel the ‘entanglement’ of the main reaction and the side reaction.” ”

The researchers creatively developed germanium ion homocrystalline substituted microporous zeolite (GeAPO-18), which maximizes the pulling ability of the zeolite pores to the active intermediates by increasing the density of the Bronster acid site in the zeolite pores, promotes the formation and conversion of intermediates, and appropriately reduces its acid strength, reduces overhydrogenation and over-polymerization in the carbon-carbon coupling process, thereby reducing the occurrence of side reactions, and improving the catalytic reaction performance in a two-pronged manner. In this way, the conversion and selective “seesaw” that was originally set at both ends of a fulcrum is turned into wings that touch two separate active positions and can soar freely.

Researchers are performing high-throughput fixed-bed heterogeneous catalytic reaction catalyst loading operations. Photo courtesy of Dalian Chemical Properties

Under the optimized reaction conditions, the single-pass conversion rate of carbon monoxide reached 85% under the condition of maintaining the selectivity of low-carbon olefins greater than 80% (up to 83%), achieving the international best level of low-carbon olefin yield of 48%, which is more than double that of the first generation OXZEO catalyst.

“This optimization of reaction intermediates transport and reaction kinetics through active center separation, as well as the regulation of molecular sieve pore channels and acid site density and structural characteristics, breaks the concept of seesaw effect of conversion and selective entanglement in catalytic reactions, and should have universal applicability to similar bifunctional catalytic systems, which will surely promote the further development of the field of zeolite catalysis research from the foundation,” said Prof. Annette Pan, “In the next step, we will strive to develop a new generation of OXZEO catalysts for industrial processes. Accelerate the process of industrial application. ”

Academician Bao Xinhe also put forward higher goals: “In the future, we will further combine with green hydrogen produced from renewable energy to develop China’s original new coal chemical system with low water consumption and low carbon emissions, so as to help ensure the country’s energy, resource security and the realization of the “dual carbon” goal.” (Source: Sun Danning, China Science News)

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