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.

MOF Surface Barriers

Today's paper summary comes from Simon Pang.

Often, model systems can provide insight that would not otherwise be observable into the workings of actual systems. For metal-organic frameworks (MOFs), this can constitute creation of thin film systems that offer nearly 2D analogs to the particle systems that many of us deal with. In their recent paper, Heinke et al. (Nat. Commun., DOI: 10.1038/ncomms5562) take this one step further, constructing a film of HKUST-1 in situ on a quartz crystal microbalance without exposure to atmospheric conditions, creating a pristine surface MOF (“SURMOF”). By varying the thickness of the SURMOF, they are able to show that mass transfer is essentially limited by intracrystalline diffusion – that is, there are no external barriers to mass transfer. However, the rate of mass uptake was significantly decreased after exposure to humid air or water vapor without any change in bulk crystallinity, indicating that exposure to these conditions primarily affected the outer surface of the crystal.

This suggests the formation of surface defects which are certainly well known to the EFRC. The paper doesn’t offer suggestions for what the nature of the surface defects might be, but instead only suggests that the defects are created upon exposure to water vapor that would be present under ambient processing conditions. Importantly, these defects are not necessarily an intrinsic property of MOFs, offering the possibility that modification prior to exposure to water vapor could help prevent these defects.

Molecular Sieve Nanosheets

Today's paper summary comes from Meijun Li.

Metal-organic frameworks (MOFs), which are constructed from transition metal ions and bridging organic ligands, are a new family of nanoporous molecular sieves. MOFs with different microstructures and morphologies such as 1D, 2D, 3D, core-shell MOFS, etc. have been attractive for CO2 capture and separation. New discoveries are still being made constantly as the field is growing quickly. Yang et al. recently reported the preparation of 1-nanometer-thick sheets (molecular sieve nanosheets, (MSNs)) with large lateral area and high crystallinity from layered MOFs  (Yang et al. Science 346, 1356 (2014)).  They demonstrate their use in fabricating ultrapermeable membranes that have excellent molecular sieving properties for H2/CO2 separation. The paper reported for the first time  the synthesis of 1 nm thick MSNs from MOFs. Built upon the MSNs, 5-nm membranes exhibit  an anomalous proportional relationship between the permeance and selectivity for H2/CO2. They achieved a simultaneous increase in both permeance and selectivity by suppressing lamellar stacking of the nanosheets. It is interesting that Lamellar ordering of nano-sheets would block the permeation pathway for H2, but have only a slight effect on CO2 leakage.

Related to our EFRC center, we may adapt/modify the synthesis method reported in this paper to make 2D MOFs that can be used as a model system in both Thrust 2 and Thrust 4 for understanding how the novel 2D materials interact with acid gases. The rich tunability of the pore structure and surface functionalities of the 2D MOFs offers tremendous opportunities for future study in adsorption and separation areas.

Ordered silicon vacancies in a high silica zeolite

Today's paper summary comes from David Sholl.

Far more is known about defects in zeolites than about defects in MOFs, although the structural similarities between ZIFs and zeolites strongly hint that defects should be common in ZIFs and, by extension, other kinds of MOFs. Defects are typically thought of as introducing disorder or randomness into crystals, but this does not always have to be the case. Baerlocher and colleagues give a nice example of a high silica zeolite structure that includes an ordered set of defects in the crystal structure of the zeolite SSZ-74 (Baerlocher et al. Nature Materials, DOI: 10.1038/nmat2228) .  Much of their paper deals with the technical details of solving the crystal structure of this complicated zeolite, but the main conclusion is that the structure includes a set of ordered Si vacancies in each unit cell. It is important to note that a perfectly plausible model of a defect-free silica material with the same framework could be constructed, so the existence of the ordered vacancies is a consequence of the complex crystallization process that generates SSZ-74.

This paper doesn't give a direct example of how the Si vacancies in the structure affect the performance or properties of the material. There are examples, however, of point defects such as oxygen vacancies giving zeolites properties such as luminescence that are not possible with defect-free materials (Bai et al., J. Luminesence, DOI: 10.1016/j.jlumin.2013.08.01).

Welcome to the Uncaged blog

UNCAGE-ME is a DOE-funded Energy Frontier Research Center (EFRC) led by Georgia Tech with partnerships at Oak Ridge National Lab, Washington University in St. Louis, University of Alabama, University of Florida, University of Wisconsin, and Lehigh University. More information about this Center is available from www.efrc.gatech.edu

This blog will function as a virtual journal group for researchers in this EFRC and for others at Georgia Tech and elsewhere interested in porous materials and catalytic surfaces. Posts will typically be a brief summary of a published paper of interest to members of the Center, and discussion will take place through the blog's comments. Most posts will be written by students and postdocs from the Center, and volunteers or ideas for future posts are welcome. If you are interested in writing a post for the blog, please contact Steph Didas.