Metal-organic frameworks (MOFs) are a class of hybrid porous crystalline materials comprising of metal centers coordinated to organic linkers. Owing to their well-defined pores and cavities in the scale of molecules combined with abundant surface chemistry, MOFs offer unprecedented opportunities for a wide range of applications including membrane-based gas separations. It is not straightforward (often requiring multiple steps) to prepare membranes of MOFs due to the fact that the heterogeneous nucleation and growth of MOF crystals on porous supports are not generally favored. Furthermore, the performance of polycrystalline MOF membranes strongly depends on the membrane microstructure, in particular, the grain boundary structure. Here we report a simple one step in situ method based on a counter-diffusion concept to prepare well-intergrown ZIF-8 membranes with significantly enhanced microstructure, resulting in exceptionally high separation performance toward propylene over propane.

Propylene/propane separation is one of the most challenging separations, currently achieved by energy-intensive cryogenic distillation. Despite the great potential for energy-efficient membrane-based separations, no commercial membranes are currently available due to the limitations of current polymeric materials. Zeolitic imidazolate framework, ZIF-8, with the effective aperture size of ∼4.0 Å, has been shown to be very promising for propylene/propane separation. Despite the extensive research on ZIF-8 membranes, only a few reported ZIF-8 membranes have displayed good propylene/propane separation performances presumably due to the challenges of controlling the microstructures of polycrystalline membranes. Here we report the first well-intergrown membranes of ZIF-67 (Co-substituted ZIF-8) by heteroepitaxially growing ZIF-67 on ZIF-8 seed layers. The ZIF-67 membranes exhibited impressively high propylene/propane separation capabilities. Furthermore, when a tertiary growth of ZIF-8 layers was applied to heteroepitaxially grown ZIF-67 membranes, the membranes exhibited unprecedentedly high propylene/propane separation factors of ∼200 possibly due to enhanced grain boundary structure.

Here we report a rapid and simple microwave-assisted seeding technique for the synthesis of high-quality ZIF-8 membranes with an average propylene-propane selectivity of about 40. The new seeding method was found to be applicable to other ZIFs including ZIF-7 and SIM-1.

ZIF-67, a Co-substituted ZIF-8 structure, is investigated as a candidate for the industrially highly demanding propylene/propane separation, with the use of computational techniques for the first time. A new force field for the ZIF-67 framework based on density functional theory calculations is reported along with a recently developed force field for ZIF-8. The new force field is validated through comparison with structural data for ZIF-67 from the literature. Molecular dynamics simulations are reported for ZIF-67, showing a dramatic increase of propylene/propane corrected diffusivities ratio when compared to ZIF-8, implying a huge improvement in the separation of the mixture. The sieving mechanism of ZIF frameworks is investigated, and the results yield a dependency of the swelling motion of the gates from the bonding of the metal atom with its surrounding atoms. The presence of Co in the modified framework results in a tighter structure with a smaller oscillation of the gate opening, which leads to a narrower aperture. The results from the simulations and experiments in ZIF-67 place this new structure at the top of the candidates for propylene/propane separation.

Poly(ethyleneimine)-impregnated sorbents are prepared using a hierarchical silica support with bimodal meso-/macroporosity. The sorbents behave unexpectedly during CO 2 adsorption from simulated air and flue gases (400 ppm and 10% CO 2 ) at a fixed temperature, as compared to systems built on commonly studied mesoporous materials. The results demonstrate that (i) impregnation methods influence the efficacy of sorption performance and (ii) the sorbents show almost similar uptake capacities under 400 ppm and 10% dry CO 2 at 30 °C, exhibiting step-like CO 2 adsorption isotherms. These unusual observations are rationalized via control experiments and a hypothesized sorption mechanism. While the sorption performance near room temperature is unexpectedly identical under 400 ppm and 10% CO 2 conditions, there is an optimal temperature at each gas concentration where the uptake is maximized. The maximum sorption capacities are 2.6 and 4.1 mmol CO 2 /g sorbent at the optimized sorption temperatures using 400 ppm and 10% dry CO 2 , respectively. The presence of water vapor under 400 ppm CO 2 conditions further improves the sorption capacity to 3.4 mmol/g sorbent, which is the highest capacity under direct air capture conditions among known amine sorbents impregnated with a similar polymer, to the best of our knowledge.