There have been significant advancements in small-pore zeolite membranes in recent years. With pore size closely related to many energy- or environment-related gas molecules, small-pore zeolite membranes have demonstrated great potential for the separation of some interested gas pairs, such as CO₂/CH₄, CO₂/N₂ and N₂/CH₄. Small-pore zeolite membranes share some characteristics but also have distinctive differences depending on their framework, structure and zeolite chemistry. Through this mini review, the separation performance of different types of zeolite membranes with respect to interested gas pairs will be compared. We aim to give readers an idea of membrane separation status. A few representative synthesis conditions are arbitrarily chosen and summarized, along with the corresponding separation performance. This review can be used as a quick reference with respect to the influence of synthesis conditions on membrane quality. At the end, some general findings and perspectives will be discussed.

The separation of small molecule gases by membrane technologies can help performance enhancement and process intensification for emerging advanced fossil energy systems with CO₂ capture capacity. This paper reports the demonstration of controlled modification of zeolitic channel size for the MFI-type zeolite membranes to enhance the separation of small molecule gases such as O₂ and N₂. Pure-silica MFI-type zeolite membranes were synthesized on porous α-alumina disc substrates with and without an aluminum-containing thin skin on the outer surface of zeolite membrane. The membranes were subsequently modified by on-stream catalytic cracking deposition (CCD) of molecular silica to reduce the effective openings of the zeolitic channels. Such a pore modification caused the transition of gas permeation from the N₂-selective gaseous diffusion mechanism in the pristine membrane to the O₂-selective activated diffusion mechanism in the modified membrane. The experimental results indicated that the pore modification could be effectively limited within the aluminum-containing surface of the MFI zeolite membrane to minimize the mass transport resistance for O₂ permeation while maintaining its selectivity. The implications of pore modification on the size-exclusion-enabled gas selectivity were discussed based on the kinetic molecular theory. In light of the theoretical analysis, experimental investigation was performed to further enhance the membrane separation selectivity by chemical liquid deposition of silica into the undesirable intercrystalline spaces.

Transport properties of metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) play important roles in their performance in various applications. This Mini Review discusses how transport properties, e.g., diffusivity and transport resistance, might be manipulated in these materials for optimal performance. We focused on three applications, e.g., membrane separation, adsorption, and catalysis. For membrane separation, examples will be given to illustrate different strategies applied to address the limited diffusion selectivity issues, high transport resistance from thick membranes, and the challenges for scaling up high-performance membranes. For adsorption, strategies will be explained on how to achieve ultrahigh adsorption selectivity (molecular sieving), leveraging kinetic selectivity for separations with low equilibrium adsorption selectivity, and addressing the slow adsorption kinetics issues. The importance of pore size engineering will be discussed for applying MOFs and COFs either as the catalysts or catalyst supports. Last, how computational chemistry and machine learning for accelerating materials research will also be touched. This article will give readers a general idea of recent progress on tuning the transport properties of MOFs and COFs and how innovative strategies could be leveraged to achieve the desirable process outcomes.