How to make the catalyst reach the most ideal state

For traditional chemical processes, the most worrying is the deactivation of the catalyst. No matter how active and selective it is, it will be ruined after one use, which makes people feel distressed. Therefore, the ideal catalyst not only has excellent activity and selectivity, but also needs to have long-term stability and resistance to deactivation.

However, there are still great difficulties in designing such a catalyst. For example, the copper/zinc oxide-based catalyst commonly used in the industrial process of direct synthesis of dimethyl ether from syngas has high activity, but the "sintering" problem of copper in the long-term reaction process usually reduces its performance.

"Sintering" is a headache for many researchers. "Sintering" refers to the change in the microstructure of the carrier after a period of time when the catalyst is at a high temperature (sometimes in a special atmosphere), such as the collapse of the pore structure, a sharp decrease in surface area, or the loading of active components under high temperature conditions. The phenomenon of crystal grain growth eventually leads to the aging of the catalyst and the gradual loss of activity.

The main reason for the collapse of the pore structure of the carrier is the poor thermal stability of the carrier and its inability to withstand high temperatures, resulting in a change in morphology and a sharp decrease in the specific surface area. The active component crystallites are prone to spontaneously agglomerate together at high temperatures to form a more stable state. Since the catalytic reaction occurs on the surface of the active component, the agglomeration and growth of crystal grains result in the reduction of the active surface area, reducing the active sites, and therefore the activity decreases.

For copper/zinc oxide-based catalysts, in order to solve the "sintering" problem, it is necessary to limit the thermal movement behavior of copper nanoparticles at high temperatures. The copper oxide obtained by the conventional co-precipitation synthesis route will be randomly dispersed into the zinc oxide carrier. At high temperatures, these unconstrained "smart" copper oxide particles are easy to migrate and aggregate.

Recently, the team of Researcher Liu Jian from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences and the team of Professor Liu Shaomin from Curtin University, Australia have conducted a two-step pyrolysis of the zinc-doped metal-organic framework (CuZn-BTC) in nitrogen and air. The copper/zinc oxide catalyst with a new octahedral structure has made the "smart copper" "honest".

Throughout the synthesis process, the octahedral structure of the original metal-organic framework material is retained, and the uniform distribution of copper oxide and zinc oxide is realized in it. The higher zinc content acts as a "partition wall" and limits the growth of copper oxide nanoparticles. The catalyst has excellent stability in the production of dimethyl ether, and the catalytic activity remains basically unchanged after the continuous reaction time exceeds 40 hours. The stable catalytic performance comes from the limitation of copper in the octahedral structure. During the reaction, copper and zinc remain uniformly dispersed without significant particle aggregation.

Although "limiting" is a good strategy to achieve high stability, excessive limitation can also reduce catalyst activity or even deactivate. The nanoreactor with egg yolk-egg shell/core shell structure shows a degree of relaxation in terms of "limitation".

When the active component is used as the core, it can be protected by the outer shell, which prevents the interference of the external environment and inhibits the aggregation and growth of the active component nanoparticles under high temperature conditions. The gap between the core and shell additionally provides free space for active components, so the stability of the catalyst is improved without affecting its activity. In addition, such confined space creates conditions for the accumulation of reactants or products and the restriction of intermediates. The manipulation of the confined space microenvironment can also improve reaction activity and selectivity.