Keeping the catalyst “honest”

“I am not afraid of honest people not being smart, but I am afraid of smart people not being honest”. Coincidentally, the same applies to the field of catalysis. For traditional chemical processes, the biggest worry is the deactivation of catalysts. Even if the activity is high and the selectivity is strong, it is heartbreaking if it is destroyed after one use. Therefore, the ideal catalyst needs to have not only excellent activity and selectivity, but also long-term stability and resistance to deactivation.

However, there are still great difficulties in designing such catalysts. For example, the copper/zinc oxide based catalysts commonly used in the industrial process of direct syngas to dimethyl ether are highly active, but the “sintering” of copper during long reaction times usually degrades their performance.

In the case of Cu/ZnO-based catalysts, the thermal behavior of copper nanoparticles at high temperatures needs to be limited in order to solve the “sintering” problem. The copper oxide obtained by conventional co-precipitation synthesis routes is randomly dispersed into the ZnO carrier, and these unconstrained “smart” copper oxide particles tend to migrate and aggregate at high temperatures. Recently, Liu Jian’s research team at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Shaomin Liu’s team at Curtin University, Australia, have prepared a new copper/zinc oxide catalyst with a novel octahedral structure by two-step pyrolysis of a zinc-doped metal-organic backbone (CuZn-BTC) in nitrogen and air, allowing the “smart copper The “smart copper” became “honest”. The octahedral structure of the initial metal-organic skeleton material was retained throughout the synthesis process, and a uniform distribution of CuO and ZnO was achieved in it. The high zinc content acts as a “wall” to limit the growth of copper oxide nanoparticles (Figure 1). The catalyst showed excellent stability in the production of DME, with the catalytic activity remaining essentially unchanged after more than 40 h of continuous reaction time. The stable catalytic performance comes from the confinement of copper in the octahedral structure, and the copper and zinc remain uniformly dispersed during the reaction without significant particle aggregation.

Morphological characterization and reaction performance

Although “confinement” is a good strategy to achieve high stability, excessive confinement can also lead to reduced activity or even deactivation of the catalyst. The yolk-egg-shell/core-shell structured nanoreactors (YCSNs) exhibit a moderate degree of “confinement” in this respect (Figure 2).

When the active component acts as the nucleus (i.e., yolk), it is protected by the shell (i.e., eggshell), which hinders the interference of the external environment and inhibits the aggregation and growth of the active component nanoparticles under high temperature conditions. And the space between the core and shell additionally provides free space for the active components to move, thus improving the catalyst stability without affecting its activity. Moreover, such confined space creates conditions for the accumulation of reactants or products and the confinement of intermediates, and the manipulation of the confined space microenvironment can also improve the reaction activity and selectivity. Considering the multiple advantages of YCSNs, it is predicted that YCSNs will excel in hydrogenation reactions. Based on the previous research, Jian Liu’s team and Qihua Yang’s team have systematically reviewed the progress of YCSNs synthesis methods and the scientific challenges of YCSNs application in various gas-phase and liquid-phase hydrogenation reactions, and discussed the structure, properties, catalytic performance, conformational relationships, reaction mechanism and outstanding issues of YCSNs. Rational “limiting” of catalyst active components provides a new idea to improve the stability and selectivity of catalysts. In addition, this strategy may inspire researchers to create new materials with ideal structures to obtain maximum hydrogenation performance.