Zeolitic nanotubes Nanotubes generally have solid walls, but a low-dimensional version of zeolites now introduces porosity into such structures. Korde et al . used a structure-directing agent with a hydrophobic biphenyl group center connecting two long alkyl chains bearing hydrophilic bulky quaternary ammonium head groups to direct hydrothermal synthesis with silicon-rich precursors (see the Perspective by Fan and Dong). The nanotubes have a mesoporous central channel of approximately 3 nanometers and zeolitic walls with micropores less than 0.6 nanometers. Electron microscopy and modeling showed that the outer surface is a projection of a large-pore zeolite and the inner surface is a projection of a medium-pore zeolite. —PDS

Zeolites are nanoporous aluminosilicates widely used in catalysis and separations applications. Though generally formed as 3D crystals, new synthesis techniques have given access to 2D zeolite nanosheets with small diffusion path lengths and accelerated molecular diffusion. Since most previous research has focused on bulk zeolite crystals, there is little understanding of the surface adsorption and diffusion mechanisms likely involved at such length scales and their contributions to the permeability and selectivity of different species. To enable the systematic examination of such surface properties, we constructed a database of more than 800,000 computation-ready 2D zeolite nanosheets from the full range of known zeolite structures in the IZA database of zeolite structure types. The nanosheet surfaces cover a wide range of orientations and were created via the principle of minimizing the number of bonds broken during the termination of a unit cell. The database consists of two sets of nanosheets: one set with known heights and unrelaxed surfaces and another set with arbitrary heights and relaxed surfaces. As an initial example of the utility of this database, we generated equilibrium Wulff shapes for 203 3D zeolite structure types in the International Zeolite Association (IZA) database.

Most zeolite membranes contain an active layer of intergrown zeolite crystals that are hundreds of nanometers to a few microns thick. For such membranes, the effect of the surface resistances on transmembrane transport is expected to be small. Progress has been made in recent years in synthesizing zeolite nanosheets with unit cell-level thicknesses and converting them into ultrathin zeolitic membranes or catalysts with very small intraparticle diffusion length scales. In such situations, effects from surface resistance are likely to be amplified relative to large zeolite crystals. Little is currently known, however, about the characteristics of these resistances for zeolite nanosheets. In this paper, we use molecular dynamics to measure the surface resistance of different zeolite nanosheets for seven adsorbate species under a range of temperature and pressure conditions. We find that surface resistance dominates the resistance to mass transfer associated with the zeolite phase under almost all conditions we simulated, with low temperature and low-loading conditions exhibiting the highest surface resistance. We show that surface resistance is strongly dependent on the heat of adsorption of the diffusing molecule in the bulk zeolite and use this dependence to predict diffusion and ideal selectivity in nanosheets from six different frameworks using only data collected in the bulk material.