The Laboratory of Advanced Separations (LAS) is engaged in material chemistry of two-dimensional nanoporous materials leading to high-performance membranes that significantly exceed performance limit of the polymeric membranes, making the separation processes for gases and vapors highly energy-efficient. Since the membrane separation process is usually diffusion-controlled, the synthesis of these membranes amounts to synthesizing the thinnest-possible molecular-selective barriers that are stable in the operating conditions. Naturally, a two-dimensional (2d) film hosting molecular-selective pores are the ultimate membranes because the flux through the membrane is inversely proportional to the membrane thickness.
To realize these 2d membranes, LAS has established a multidisciplinary program in chemistry, material science and chemical engineering, focusing on the synthesis, processing, and characterization of inorganic, nanoporous 2d materials (single-layer graphene, nanosheets derived from layered nanoporous materials). LAS is studying several synthetic routes (chemical vapor deposition (CVD), molecular layer deposition (MLD), hydrothermal and solvothermal synthesis, etc.), crystallization mechanism (induction time, nucleation density, anisotropic growth rates, substrate effects, grain-boundaries, etc.), post-synthetic modification including surface functionalization, ion-exchange, and exfoliation. On the theoretical front, an improved understanding of structure-property relationship is being investigated by ab-initio molecular modeling (density functional theory), as well as by modeling adsorption and diffusion across the nanoporous 2d film. Overall, LAS is developing three distinct membrane platforms, a) single-layer nanoporous graphene films, b) ultrathin polycrystalline MOF films, and c) exfoliated nanoporous 2d nanosheets.
Current projects in LAS deal with
- Nanoporous single-layer graphene with a resolution in molecular differentiation reaching 0.1 Å
- Two-dimensional nanoporous nanosheets capable of separating molecules by size-sieving (two-dimensional silicates, g-C3N4, etc.).
- Crystal and defect engineering of metal-organic frameworks (MOF) membranes.
An overview of research activities at LASspace
Nanoporous single-layer graphene
Atom thick graphene film comprised of size-selective nanopores has the potential to be the ultimate membrane with the highest possible molecular throughput. We have developed a scalable fabrication technique (centimeter length-scale) for nanoporous graphene that demonstrates gas pair selectivity and temperature activated transport. Given the importance of nanopores in determining the ultimate separation performance, we are interested in developing synthesis strategies to control size, shape, functionality, and number density of nanopores to target a wide-range of separation problems. For etching controlled pore size in graphene, we are studying kinetics of carbon gasification. As of 2019, we have achieved 1.0 Å resolution in molecular differentiation (H2/CH4 separation, Science Advances 2019, Nature Communications 2018) and are pursuing to improve this to 0.1 Å.
Another direction to obtain selectivity from graphene nanopore is to decorate graphene surface with functional groups that are selective to molecules of interest. We recently demonstrated this by functionalizing the lattice of nanoporous graphene with CO2-selective polymer chains surpassing the performance target for carbon capture (Energy & Environmental Science 2019). We are now looking to expand the list of molecules that can be functionalized on the surface of graphene.
Two-dimensional nanoporous nanosheets
Nanometer-thick crystalline nanosheets possessing a high density (1014 pore/cm2) of precise size-sieving nanopores are an ideal building block of inorganic membranes (Science Advances 2020). We are working on challenges related to the exfoliation of layered precursors; synthesis of dispersed suspension, and finally fabrication of pinhole-free nanosheet film. A special focus is given on molecular transport mechanism where we study the contribution of transport from nanopores as well as the intersheet gallery spacing. So far, we have demonstrated two different nanoporous 2d nanosheets platforms (2d silicate, and a crystalline g-C3N4 (PTI)) for this concept. Both were exfoliated to a single-layer and could be assembled by filtration, leading to attractive H2/CO2 and H2/N2 selectivities reaching as high as 100.
Crystal and defect engineering of metal-organic frameworks (MOF) membranes
MOFs are an interesting candidate for membrane fabrication because one can synthesize a number of membranes with varying pore-size attributing to the versatile reticular chemistry of MOFs. Recently, we reported a novel electrophoretic technique simplifying the heterogenous nucleation of MOF films leading to selective layer thickness less than 0.5 micron on a number of porous and nonporous substrates (Advanced Functional Materials 2018). We have now developed a rapid post-synthetic treatment protocol, which incorporates a small number of defects in the MOF lattice, making the lattice rigid. As a result, CO2/N2 and CO2/CH4 selectivities up to 40 could be achieved (Advanced Materials 2019). Targeting scale-up of MOF films, we have developed a rapid growth protocol for ZIF films, hindering the Ostwald ripening phenomena, leading to synthesis of high-performance ZIF-8 membranes in under 10 minutes at room temperature (Journal of Material Chemistry A 2020).