Molecular sieving mixed matrix membranes embodying nano-fillers with extremely narrow pore-openings

Molecular Dynamics Simulations of Gas Transport Properties in Cross-Linked Polyamide Membranes: Tracing the Morphology and Addition of Silicate Nanotubes

This study employs molecular dynamics (MD) simulations to fundamentally provide insight into the role of cross-link density in the CO2 separation properties of interfacially polymerized polyamide (PA) membranes. For this purpose, two atomistic models of pure polyamide membranes with different cross-link densities are constructed by MD simulations to conceptually determine how the fractional free volume of polyamide affects the gas separation performance of the membrane. The PA membrane with a lower cross-link density (LCPA) shows a higher gas diffusion coefficient, a lower gas solubility coefficient, and a higher gas permeability than the PA membrane with a higher cross-link density (HCPA). Moreover, the pristine and modified silicate nanotubes (SNTs) as the fast gas transport channels are incorporated into the polyamide membranes to assess the effect of the SNT/PA interface chemistry on the CO2 separation properties of the membranes. SNTs are systematically modified by three modifying agents with different CO2-philic groups and different interfacial interaction energies with the polyamide matrix. The results of MD simulations demonstrate that the incorporation of silicate nanotubes into the PA matrix increases the gas diffusivity and permeability and decreases the CO2/gas selectivity. Moreover, the membranes containing modified SNTs possessing high CO2-philicity and high SNTs/PA interfacial interactions show a high CO2 separation performance.

ZIF-95 as a filler for enhanced gas separation performance of polysulfone membrane

Metal-organic frameworks (MOFs) are found to be promising porous crystalline materials for application in gas separation. Considering that mixed matrix membranes usually increase the gas separation performance of a polymer by increasing selectivity, permeability, or both (i.e., perm-selectivity), the zeolitic imidazole framework-95 (ZIF-95) MOF was dispersed for the first time in polysulfone (PSF) polymer to form mixed matrix membranes (MMMs), namely, ZIF-95/PSF. The fabricated ZIF-95/PSF membranes were examined for the separation of various gases. The characterization of solvothermally synthesized ZIF-95 was carried out using different analyses such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), porosity measurements, etc. ZIF-95 was mixed with PSF at 8%, 16%, 24%, and 32% weight percent to form different loading MMMs. SEM analysis of membranes revealed good compatibility/adhesion between the MOF and polymer. The permeability of He, H₂, O₂, CO₂, N₂, and CH₄ were measured for the pure and composite membranes. The ideal selectivity of different gas pairs were calculated and compared with reported mixed matrix membranes. The maximum increases in permeabilities were observed in 32% loaded membrane; nevertheless, these performance/permeability increases were at the expense of a slight decrease of selectivity. In the optimally loaded membrane (i.e., 24 wt% loaded membrane), the permeability of H₂, O₂, and CO₂ increased by 80.2%, 78.0%, and 67.2%, respectively, as compared to the pure membrane. Moreover, the selectivity of H₂/CH₄, O₂/N₂, and H₂/CO₂ gas pairs also increased by 16%, 15%, and 8% in the 24% loaded membrane, respectively.

Synthesis of ZIF-11 Membranes: The Influence of Preparation Technique and Support Type

Due to its structural features, ZIF-11 is one of the most interesting materials for gas separation applications. Herein, we report a systematic study on the synthesis of ZIF-11 as a supported membrane. For this, we adapted optimized conditions for the ZIF-11 powder synthesis, identified in our previous works, to form ZIF layers on symmetric and asymmetric stainless-steel and asymmetric α-Al₂O₃ supports. Different techniques were investigated for the challenging layer formation, namely, in situ crystallization (ISC), multiple in situ crystallization (MISC), and the seeding and secondary growth (SSG) method. It was possible to deposit ZIF-11 on different supports by ISC and MISC, although it was difficult to obtain complete layers. SSG, in turn, was more effective in forming dense and well-intergrown ZIF-11 layers. This agrees well with the generally accepted fact that seeding considerably facilitates layer formation. Systematic studies of both individual steps of SSG (seeding and secondary growth) led to a basic understanding of layer formation of ZIF-11 on the different supports. The best membranes prepared by rub seeding and secondary growth achieved Knudsen selectivity. Improved gas separation performance is expected if the formation of defects can be avoided.