The paper in Nature Communications is here: https://www.nature.com/articles/s41467-019-09881-9
Synergistic catalysis, as widely adopted by enzymes, is ubiquitous in biological systems. Enzymes achieve the synergy via “induced fit”, for which the pocket featuring multiple binding/recognition sites (preconcentration effects) and conformational flexibility are essential (orientation effects). Extensive yet continuous efforts have been dedicated to mimic enzymes for synergistic catalysis, and significant progress has been accomplished in creating active centers with binding/coordination environment similar to enzymes. Nonetheless, it remains a challenge, particularly in the solid state, to combine both binding pocket and flexibility into one system to function like enzymes that feature high specificity, selectivity, and efficiency. It is needed to prepare catalysts that are rigid but not too rigid, as well as flexible but not too flexible.
Metal-organic gels (MOGs), a class of soft-hybrid polymer materials are functional porous aerogels that have characteristics of low density, versatile porosity, and high internal surface area. To achieve synergistic activity in MOGs, The Ma research group proposed herein the construction of hierarchically porous MOGs that can be combined to show similar coordination found in metal-organic frameworks (MOFs), capable of mimicking the binding pocket/active center of enzyme yet show greater conformational flexibility similar to organic gels. Additionally, a high density of binding sites combined with pre-orientations can engender a catalytic binding pocket that works concertedly to intensify the formation of relevant intermediate and, in turn, enhance catalytic performance including both conversion and chemo/stereo selectivity.
Figure 1 | Schematic representation of the formation of Co-MMPG and its activation energy diagram with traditional catalysis and synergistic catalysis.
From a structural point of view, the key component in porphyrin assemblies for synergistic catalysis is the bonding connectivity between porphyrins. Because porphyrin materials in gel form can be designed with multiple and precisely spaced Lewis acid sites at a very high local concentration and exhibit conformational flexibility, they are an ideal platform to engineer as recyclable solid catalysts for synergistic catalysis. In this contribution, they reported a hierarchically porous metal-metalloporphyrin aerogel matrix by utilizing Co(II) Tetrakis(4-carboxyphenyl)porphyrin (Co-TCPP) as the building unit, named Co-MMPG (Figure 1). Co-MMPG demonstrates superior catalysis performance in terms of both activity and selectivity. In addition, the hierarchical porous structure of Co-MMPG with interconnected micropores and mesopores can facilitate the substrate transport thereby further promoting the catalytic processes.
This work thereby not only provides a new enzyme-mimic design approach to prepare highly active MOG-based catalysts, but also lays a solid foundation for the development of MOG as a new type of highly efficient heterogeneous catalyst. This can merge synergistic catalysis and porous materials to carry out chemo/stereo-selective chemistry.