Fundamental transport mechanisms, fabrication and potential applications of nanoporous atomically thin membranes

Graphene and other two-dimensional materials offer a new approach to controlling mass transport at the nanoscale. These materials can sustain nanoscale pores in their rigid lattices and due to their minimum possible material thickness, high mechanical strength and chemical robustness, they could be used to address persistent challenges in membrane separations. Here we discuss theoretical and experimental developments in the emerging field of nanoporous atomically thin membranes, focusing on the fundamental mechanisms of gas- and liquid-phase transport, membrane fabrication techniques and advances towards practical application. We highlight potential functional characteristics of the membranes and discuss applications where they are expected to offer advantages. Finally, we outline the major scientific questions and technological challenges that need to be addressed to bridge the gap from theoretical simulations and proof-of-concept experiments to real-world applications.

To the pore and through the pore: thermodynamics and kinetics of helium in exotic cubic carbon polymorphs

Applying pore size analysis, Monte Carlo simulations, and transition state theory, we study the molecular sieving properties of recently discovered crystalline exotic cubic carbon allotropes (Hu et al., J. Phys. Chem. C, 2012, 116, 24233-24238) at 298 K and infinite dilution. The fcc-C10 cubic carbon crystal shows unusual molecular sieving characteristics. The carbon cavities of the fcc-C10 cubic carbon polymorph (with an effective size of ~3.5-4 Å) are kinetically closed to common gaseous contaminants of He fluid (including: Ne, Ar, H2, and CO). Because the sizes of nanowindows connecting carbon cavities are comparable with the effective size of a He atom (~2.556 Å), we predict a significant resistance to self-diffusion of the He in the fcc-C10 crystal. Computed self-diffusion coefficients ~1.3 × 10(-6)-1.3 × 10(-7) cm(2) s(-1) for He inside fcc-C10 fall in the range characteristic of molecular diffusion in zeolites. Infrequent "jumps" of He atoms between neighboring carbon cavities and kinetic rejection of other gaseous particles indicate potential application of the fcc-C10 carbon polymorph for kinetic molecular sieving of He near ambient temperatures. The theoretical results presented here are useful for correct interpretation of the pore volumes of carbon molecular sieves measured from helium porosimetry.

Molecular insight into the high selectivity of double-walled carbon nanotubes

Combining experimental knowledge with molecular simulations, we investigated the adsorption and separation properties of double-walled carbon nanotubes (DWNTs) against flue/synthetic gas mixture components (e.g. CO(2), CO, N(2), H(2), O(2), and CH(4)) at 300 K. Except molecular H(2), all studied nonpolar adsorbates assemble into single-file chain structures inside DWNTs at operating pressures below 1 MPa. Molecular wires of adsorbed molecules are stabilized by the strong solid-fluid potential generated from the cylindrical carbon walls. CO(2) assembly is formed at very low operating pressures in comparison to all other studied nonpolar adsorbates. The adsorption lock-and-key mechanism results from perfect fitting of rod-shaped CO(2) molecules into the cylindrical carbon pores. The enthalpy of CO(2) adsorption in DWNTs is very high and reaches 50 kJ mol(-1) at 300 K and low pore concentrations. In contrast, adsorption enthalpy at zero coverage is significantly lower for all other studied nonpolar adsorbates, for instance: 35 kJ mol(-1) for CH(4), and 14 kJ mol(-1) for H(2). Applying the ideal adsorption solution theory, we predicted that the internal pores of DWNTs have unusual ability to differentiate CO(2) molecules from other flue/synthetic gas mixture components (e.g. CO, N(2), H(2), O(2), and CH(4)) at ambient operating conditions. Computed equilibrium selectivity for equimolar CO(2)-X binary mixtures (where X: CO, N(2), H(2), O(2), and CH(4)) is very high at low mixture pressures. With an increase in binary mixture pressure, we predicted a decrease in equilibrium separation factor because of the competitive adsorption of the X binary mixture component. We showed that at 300 K and equimolar mixture pressures up to 1 MPa, the CO(2)-X equilibrium separation factor is higher than 10 for all studied binary mixtures, indicating strong preference for CO(2) adsorption. The overall selective properties of DWNTs seem to be superior, which may be beneficial for potential industrial applications of these novel carbon nanostructures.