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.