Metal-Organic Frameworks as Catalysts: The Role of Metal Active Sites

Today's paper summary comes from Josh Howe.

Article:  Metal-organic frameworks as catalysts:  the role of metal active sites

P. Valvekens, F. Vermoortele, and D. De Vos, Catal. Sci. Technol.,3, 1435-1445 (2013).

The authors present a brief review of various roles MOFs can play in catalysis, highlighting their potential over other nanoporous materials in applications involving encapsulation of catalysts in pores and in having catalytically active metal nodes, while finding that other classes of nanoporous materials are likely preferable for embedding of metallic nanoparticles and post-synthetic modification.  The focus of this review, however, is on catalytically active metal sites in MOFs.

The authors present a selection of cases of MOFs having catalytic activity for a variety of reactions in which the metal nodes of MOFs are thought to play an active role.  Ultimately, they identify a number of unique mechanisms that are able to impart catalytic activity at metal sites within MOFs.  Coordinatively unsaturated sites (open-metal sites), selective substitution of metal-bound linkers by incoming nucleophilic reactants, local coordination destruction with formation of hydroxyl groups, reversible expansion of the coordination sphere, missing link defects, and termination of the lattice in a surface have all been given as reasons for catalytic activity in MOFs. 

All of these mechanisms depend upon the metal site, and all of them depend upon the ability of molecules to interact with the metal site.  While some depend upon the existence of a coordinatively unsaturated metal site (whether from the crystal structure and activation or through defects), others involve the guest molecules inducing the decoordination of the linker from the metal site through preferential binding of the guest molecule.

There are two primary points from this article that I think are of high interest to this EFRC.  First, this supports the notion that metal sites, whether coordinatively unsaturated or fully saturated, are likely sites for attack and destruction of MOFs by strongly binding guest molecules and potentially acid gases.  This point is intuitive, but this article provides some specific examples of this phenomenon from the literature.  Second, it highlights the role of defects (both point bulk defects and surfaces) in MOF activity for catalysis.

Surface Chemistry of MOFs

Today's paper summary comes from Rebecca Han.

A recent feature paper (Fischer and Forgan, 2015, DOI: 10.1039/c4cc04458d) investigates how surface modifications might be used to tune bulk properties in a variety of popular MOFs and ZIFs. Three types of modifications were examined: 1) use of coordination modulation in synthesis to stabilize and controlling size/growth of MOF crystals, 2) post-synthetic modifications, and 3) epitaxial growth of material. 

It was found that adding a modulator during MOF synthesis is an effective way to tune crystal size distribution, growth rate, and growth direction, but functionality is usually confined to the surface and not the bulk material. Zr carboxylate MOFs (e.g. UiO-66) are an exception; they have highly coordinated frameworks and missing linker defects – where fragments of a modulator ligand are incorporated in lieu of linker units – can be induced during synthesis, increasing pore volume and surface area. Post-synthetic modifications likewise do not permeate into the bulk, but surface functionalization can render a MOF more water-stable. Epitaxial growth offers the most potential to deliberately control the bulk structure as seen in previous MOF-5/IRMOF-3 core-shell synthesis. Careful design of surface chemistry could facilitate self-assembly of hybrid structures, or layer-by-layer deposited MOF thin films.

The EFRC is already familiar with and studying various methods of functionalizing MOF/ZIF surfaces for increased water stability, but there may be other simple post-synthesis modification that can enhance chemical selectivity of certain acid gases. Designing hybrid MOFs is also of interest to the EFRC, and tuning surface chemistry could be a way to efficiently propagate an intergrowth defect through a bulk material.