Designing Heterogeneous Catalysts by Incorporating Enzyme-Like Functionalities into MOFs

Abstract

Everyone who works within the field of catalysis draws inspiration from the amazing functionality of nature's catalysts, the enzymes. It is particularly the mild conditions that these catalysts are able to operate at and the selectivity that they demonstrate that make these materials dream targets for scientists involved in the art of synthesizing homogeneous and heterogeneous industrial catalysts. But enzymes also have their weak points; in particular their low thermal stability and their often too slow reaction rates for an economical industrial process are problems that have to be overcome. The obvious solution would be to copy the catalytic active center into a robust open framework. A key property of an enzyme is its selectivity; this property is partly regulated by steric constraints surrounding the catalytically active site. The microporous zeolite based catalysts in some cases show impressive selectivity based on the geometrical constraints imposed by the size and shape of the regular channels in these crystalline silicate and alumino-phosphate based structures, and enzyme-like properties have been claimed but the pure inorganic nature of the selective internal surface in these materials makes it impossible to mimic many important enzymatic properties. The new generation of microporous materials, Metal Organic Frameworks (MOFs) are hybrids of organic and inorganic structures.This dualistic nature offers an unprecedented flexibility in the possibility to incorporate both organic and metallic functional groups into the ordered crystalline lattice and thereby opening up for a much greater possibility to copy structural motifs known from enzymes into much simpler but also more stable open structures. Several groups are working on development of new catalysts by this approach. Here we will illustrate this approach with structures that mimic anhydrase and C-H activation.

H2storage in isostructural UiO-67 and UiO-66 MOFs

“…11,12,64,65,[87][88][89][90][91][92][93][94][95][96][97][98][99][100][101] Conversely, to the best of our knowledge, only Schaate et al 101 have so far been able to reproduce the synthesis of UiO-67. This material has consequently still to be considered as a prototype material, synthesis and framework stability and properties of which have to be checked and confirmed by the scientific community.…”

Aerobic Epoxidation of Olefins Catalyzed by the Cobalt‐Based Metal–Organic Framework STA‐12(Co)

“…Several groups have reported the targeted biomimetic modification of MOFs as an additional strategy. [24,25] Their main advantage compared with zeolites consists in their substantially higher metal loading, which offers the opportunity to significantly reduce the overall amount of catalyst, provided that the internal metal centers are accessible to the substrates. MOFs usually have high porosity and specific surface areas that make them adequate candidates for catalytic applications both as high-surface-area supports and as intrinsic catalysts.…”

Metal–Organic Framework Regioisomers Based on Bifunctional Ligands

“…[7][8][9][10][11][12][13] In these multifunctional MOF materials, the relative abundance of different ligands (and hence different functional groups) can be controlled, but not the distribution nor spatial orientation of the functional groups with respect to each other. To truly achieve the next level of tailored, multi-purpose materials, [14] control over the relative position of different functional groups would be required. Herein, we describe the first class of bifunctional MOF "ligand regioisomers" and show that even these subtle changes can result in materials with dramatically different physical properties.…”