Fernando Cacho-Bailo

Positive thermal expansion coefficients (TECs) of 52 × 10^–6 and 35 × 10^–6 K^–1 were experimentally calculated in the −116 to 250 °C range for the III-phases of zeolitic imidazolate frameworks (ZIF) ZIF-9(Co) and ZIF-7(Zn), respectively, by means of the unit cell dimensions and volume of the materials in the monoclinic crystal system calculated from the XRD patterns. The unit cell dimensions and volume showed a significant expansion phenomenon as the temperature increased, by as much as 5.5% for ZIF-9-III in the studied range. To exploit the advantages of such thermal behavior, a new approach to the fabrication of ZIF-9-III membranes on thin, flexible, and highly porous nickel hollow fiber (Ni HF) supports by a versatile and easy-controllable microfluidic setup is herein reported. These Ni HF supports result from the sintering of 25-μm Ni particles and display very positive mechanical properties and bending resistance. As compared to the traditional polymer-based HF membranes, the ZIF metal-supported membrane exhibited good durability and robustness throughout its operation in a wide temperature range and after heating and cooling cycles. These benefits derive from (1) the pore-plugging membrane configuration resulting from the high porosity of the support and (2) the similarity between the TECs of the ZIF and the metallic support, both positive, which enhances their mutual compatibility. An increase in the H₂/CO₂ separation selectivity at low temperatures (as high as 22.2 at −10 °C, along with 102 GPU permeance of H₂) was achieved, in agreement with the structural variations observed in the ZIF material.

A modification in the gas separation performance of zeolitic imidazolate framework (ZIF)-supported hollow fiber (HF) membranes by means of an imine-condensation functionalization reaction carried out by microfluidics is reported. The accommodation of voluminous amine molecules in the SIM-1, Zn(4-methyl-5-imidazolecarboxaldehyde)₂, also known as ZIF-94, sod structure during the functionalization reaction caused the ZIF atoms to be rearranged in a less dense rho structure, with a wider pore diameter and a diminished CO₂ affinity. These changes had effects on the membrane performance, resulting in an enhanced CO₂ permeance while maintaining a good permeance–selectivity balance. ZIF aldehyde-containing SIM-1 membranes were earlier prepared on the inner side of polymeric P84® HF using a microfluidic approach. The SIM-1 membranes displayed very interesting results in the separation of gas mixtures of great relevance to the natural gas field. High selectivities in the separation of He/CH₄ (160), H₂/CH₄ (136) and CO₂/CH₄ (38) mixtures were achieved, and these are the first SIM-1 membranes with such a high separation performance to the best of our knowledge. These SIM-1 membranes were in situ stepwise functionalized with long-chain amine solutions, namely, hexyl- and nonylamine. Microfluidics allowed the easy sequential implementation of this post-reaction step in the membrane fabrication procedure. An imine-condensation reaction took place between the aldehyde groups in the 4-methyl-5-imidazolecarboxaldehyde ligand forming SIM-1 and the corresponding amines. The extent of the reaction was analyzed by FTIR, TGA and XRD, together with the changes in the textural properties and the adsorption capacities.

Double-layered zeolitic imidazolate framework (ZIF) membranes were fabricated inside polyimide P84 hollow fibers by a step-synthesis conducted by microfluidic technology and applied to pre-combustion gas separation. Our hypothesis, based on the information provided by a combination of molecular simulation and experiments, is that a CO₂ adsorption reduction on the surface of the ZIF-9 would enhance the molecular sieving effect of this ZIF-9 layer and therefore the selectivity in the H₂/CO₂ mixture separation of the entire membrane. This reduction would be achieved by means of a less-CO₂-adsorptive methylimidazolate-based ZIF-67 or ZIF-8 layer coating the ZIF-9. ZIF-8/ZIF-9 and ZIF-67/ZIF-9 double-layered membranes were prepared and characterized by XRD, FTIR, SEM, FIB, TEM and EDS. This unprecedented strategy led to a H₂/CO₂ separation selectivity of 9.6 together with a 250 GPU H₂ permeance at 150 °C, showing a significant improvement with respect to the pure ZIF-9 membrane. Double-layered membranes also showed higher apparent CO₂ activation energies than single-layered membranes, attributable to a diminished adsorption.

Polymeric membranes are important tools for intensifying separation processes in chemical industries, concerning strategic tasks such as CO₂ sequestration, H₂ production, and water supply and disposal. Mixed‐matrix and supported membranes have been widely developed; recently many of them have been based on metal–organic frameworks (MOFs). However, most of the impacts MOFs have within the polymer matrix have yet to be determined. The effects related to thermal behavior arising from the combination of MOF ZIF‐8 and polysulfone have now been quantified. The catalyzed oxidation of the polymer is strongly affected by the MOF crystal size and distribution inside the membrane. A 16 wt % 140 nm‐sized ZIF‐8 loading causes a 40 % decrease in the observed activation energy of the polysulfone oxidation that takes place at a temperature (545 °C) 80 °C lower than in the raw polymer (625 °C).

Thermal annealing, a commonly used procedure for improving the performance of polymeric membranes, is in this work exploited in the presence of a metal–organic framework (MOF) supported layer. MOFs and polymers are materials with a common organic character, suggesting an enhanced affinity between them when used together in membrane separation. Zeolite-like imidazolate frameworks (ZIFs) ZIF-8 and ZIF-93 with sod and rho structures and pore apertures of 0.34 and 0.36 nm, respectively, have been grown inside 356 μm OD co-polyimide P84 hollow fibers by microfluidics, leading to continuous supported membranes. When these membranes were thermally in situ annealed below the glass transition temperature, while monitoring both H₂ and CH₄ permeances, the MOF–polymer adhesion was enhanced. Thus the gas separation selectivity increased without any significant reduction in the gas permeance, and H₂/CH₄ and CO₂/CH₄ maximum selectivities of 103 and 18 (ZIF-8) and 101 and 20 (ZIF-93) were respectively measured. The good compatibility between MOF and polymer made improvements possible in the annealing of the membrane once it was prepared. If the annealing of the polymer was carried out before the MOF synthesis, the polymer chain rearrangement and surface smoothing prevented an optimum MOF–polymer interaction and the separation performance worsened. These results proved the compatibility between both materials and their synergistic contribution to gas selective transport.

Aldehyde-functionalized ZIF-93 and P84 copolyimide facilitated a hollow fiber supported membrane with enhanced permselective properties in H ₂ –CH ₄ and CO ₂ –CH ₄ separation processes.