Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.@en

Mixed-matrix membranes comprising NH₂-MIL-53(Al) and Matrimid or 6FDA-DAM have been investigated. The metal organic framework (MOF) loading has been varied between 5 and 20 wt%, while NH₂-MIL-53(Al) with three different morphologies, nanoparticles, nanorods, and microneedles has been dispersed in Matrimid. The synthesized membranes have been tested in the separation of CO₂ from CH₄ in an equimolar mixture. At 3 bar and 298 K for 8 wt% MOF loading, incorporation of NH₂-MIL-53(Al) nanoparticles leads to the largest improvement compared to nanorods and microneedles. The incorporation of the best performing filler, i.e., NH₂-MIL-53(Al) nanoparticles, into the highly permeable 6FDA-DAM has a larger effect, and the CO₂ permeability increases up to 85% with slightly lower selectivities for 20 wt% MOF loading. Specifically, these membranes have a permeability of 660 Barrer with a CO₂/CH₄ separation factor of 28, leading to a performance very close to the Robeson limit of 2008. Furthermore, a new non-destructive technique based on Raman spectroscopy mapping is introduced to assess the homogeneity of the filler dispersion in the polymer matrix. The MOF contribution can be calculated by modeling the spectra. The determined homogeneity of the MOF filler distribution in the polymer is confirmed by focused ion beam scanning electron microscopy analysis.

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The preparation and performance of mixed matrix membranes (MMMs) based on polybenzimidazole (PBI) and ZIF-8 nanoparticles of different average sizes (50, 70 and 150 nm) as filler are reported. MMMs containing 10 and 20 wt% of ZIF-8 were tested for H₂/CO₂ separation (pre-combustion CO₂ capture) at 150 °C and feed pressures from 3 to 6 bar. The addition of ZIF-8 resulted in a clear improvement in membrane performance. Embedding 20 wt% of ZIF-8 resulted in a H₂ permeability increase of six times and the H₂/CO₂ selectivity increased nearly by 55% compared to the bare PBI polymer membrane. Both permeability and selectivity improved as the filler size increased, due to the lower degree of agglomeration of the largest particles, that may be less active owing to their smaller external surface area. MMMs synthesized using dry 150 nm ZIF-8 filler showed a better performance than those containing wet filler. Apart from agglomeration concerns favoring wet filler handling as evidenced by infrared characterization, the MMM preparation with wet filler is simpler than with dry filler. Finally, the reproducibility of the membranes was confirmed by a European interlaboratory Round Robin test involving three different institutions.

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