Miren Etxeberría-Benavides

Polybenzimidazole (PBI) is a promising and suitable membrane polymer for the separation of the H₂/CO₂pre-combustion gas mixture due to its high performance in terms of chemical and thermal stability and intrinsic H₂/CO₂selectivity. However, there is a lack of long-term separation studies with this polymer, particularly when it is conformed as hollow fibre membrane. This work reports the continuous measurement of the H₂/CO₂separation properties of PBI hollow fibres, prepared as mixed matrix membranes with metal-organic framework (MOF) ZIF-8 as filler. To enhance the scope of the experimental approach, ZIF-8 was synthesized from the transformation of ZIF-L upon up-scaling the MOF synthesis into a 1 kg batch. The effects of membrane healing with poly(dimethylsiloxane), to avoid cracks and non-selective gaps, and operation conditions (use of sweep gas or not) were also examined at 200°C during approximately 51 days. In these conditions, for all the membrane samples studied, the H₂permeance was in the 22–47 GPU range corresponding to 22–32 H₂/CO₂selectivity values. Finally, this work continues our previous report on this type of application (Etxeberria-Benavideset al. 2020Sep. Purif. Technol.237, 116347 (doi:10.1016/j.seppur.2019.116347)) with important novelties dealing with the use of ZIF-8 for the mixed matrix membrane coming from a green methodology, the long-term gas separation testing for more than 50 days and the study on the membrane operation under more realistic conditions (e.g. without the use of sweep gas).

The elimination of the additional defect healing post-treatment step in asymmetric hollow fiber manufacturing would result in a significant reduction in membrane production cost. However, obtaining integrally skinned polymeric asymmetric hollow fiber membranes with an ultrathin and defect-free selective layer is quite challenging. In this study, P84® asymmetric hollow fiber membranes with a highly thin (~56 nm) defect-free skin were successfully fabricated by fine tuning the dope composition and spinning parameters using volatile additive (tetrahydrofuran, THF) as key parameters. An extensive experimental and theoretical study of the influence of volatile THF addition on the solubility parameter of the N-methylpyrrolidone/THF solvent mixture was performed. Although THF itself is not a solvent for P84®, in a mixture with a good solvent for the polymer, like N-Methyl-2-pyrrolidone (NMP), it can be dissolved at high THF concentrations (NMP/THF ratio > 0.52). The as-spun fibers had a reproducible ideal CO₂/N₂ selectivity of 40, and a CO₂ permeance of 23 GPU at 35 °C. The fiber production can be scaled-up with retention of the selectivity.

Electrocatalytic membrane reactors are becoming a viable solution for the treatment of wastewater contaminated with persistent organic pollutants and compounds. The development of suitable membrane elements made of abundant and stable electrocatalytic materials remains a challenge to enable durable and large-scale operation. In this study, novel nickel (Ni) and nickel-copper (NiCu) alloy hollow-fiber (HF) membranes were synthesized for the first time and used for the electrocatalytic degradation of small organic molecule pollutants from model wastewater effluents. The novel porous metal HFs were characterized and tested for their capacity to degrade salicylic acid (SA) molecules by electrochemical oxidation. The degradation of SA was monitored in situ and over time using an ultraviolet-visible (UV/vis) quartz cell. The HF membranes were found to be highly stable and reusable while the kinetics of SA electro-oxidation were 9 to 20 times greater than with pure platinum wire electrodes. The high performance of the HF materials was attributed to the interconnected pore structure combined with the natural surface reactivity and excellent electron transport properties of the Ni metal and bare NiCu alloy.

The sintering of metal powders is an efficient and versatile technique to fabricate porous metal elements such as filters, diffusers, and membranes. Neck formation between particles is, however, critical to tune the porosity and optimize mass transfer in order to minimize the densification process. In this work, macro-porous stainless steel (SS) hollow-fibers (HFs) were fabricated by the extrusion and sintering of a dope comprised, for the first time, of a bimodal mixture of SS powders. The SS particles of different sizes and shapes were mixed to increase the neck formation between the particles and control the densification process of the structure during sintering. The sintered HFs from particles of two different sizes were shown to be more mechanically stable at lower sintering temperature due to the increased neck area of the small particles sintered to the large ones. In addition, the sintered HFs made from particles of 10 and 44 μm showed a smaller average pore size (<1 μm) as compared to the micron-size pores of sintered HFs made from particles of 10 μm only and those of 10 and 20 μm. The novel HFs could be used in a range of applications, from filtration modules to electrochemical membrane reactors.

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.

Zeolite A/poly (1‐trimethylsilyl‐1‐propyne) (zeoliteA/PTMSP) and [emim][Ac]/chitosan (IL/CS) are mixed‐matrix membrane (MMM) materials with enhanced CO₂/N₂ permselectivity even at higher temperature. The scalability to asymmetric flat and hollow‐fiber geometry by a simple dip‐coating method was analyzed. The CO₂/N₂ separation performance was evaluated at different temperatures. The resulting composite membranes exhibit a significantly enhanced CO₂ permeation flux because the MMM layer thickness is reduced by 97 % from flat to hollow‐fiber geometries in IL‐CS composite membranes, while the selectivity is maintained similar to the self‐standing membranes, thus proving that compatibility between the membrane component materials leads to a defect‐free composite membrane, regardless the geometry and temperature.

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.