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.