Pre-combustion gas separation by ZIF-8-polybenzimidazole mixed matrix membranes in the form of hollow fibres-long-term experimental study.

Polybenzimidazole (PBI) is a promising and suitable membrane polymer for the separation of the H2/CO2 pre-combustion gas mixture due to its high performance in terms of chemical and thermal stability and intrinsic H2/CO2 selectivity. 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/CO2 separation 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 H2 permeance was in the 22-47 GPU range corresponding to 22-32 H2/CO2 selectivity values. Finally, this work continues our previous report on this type of application (Etxeberria-Benavides et al. 2020 Sep. 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).

Green Preparation of Thin Films of Polybenzimidazole on Flat and Hollow Fiber Supports: Application to Hydrogen Separation.

This work shows the preparation of thin films, with thickness from 70 nm to 1 µm, of meta-polybenzimidazole (m-PBI) on polyimide P84 supports. Ethanolic solutions of m-PBI were used to coat flat and hollow fiber supports of asymmetric P84 with m-PBI in a process where the coating and drying was performed at room temperature. A solution of NaOH in EtOH allowed the dissolution of the m-PBI powder, providing the perfect coating solution to build thin films of m-PBI without damaging the polymeric support. It also meant a green alternative, avoiding the use of toxic solvents, such as dimethylacetamide. The resulting membranes have been tested for the separation of H2 mixtures at high temperature at different setups to allow checking their reproducibility. With 100 nm thickness the membranes showed their best gas separation performance. For flat membranes at 180 °C and 3 bar feed pressure a H2 permeance of 48.5 GPU was obtained, with respective H2 /CO2 and H2 /N2 selectivities of 33.3 and 55.8. Besides, the hollow fibers under a feed pressure of 6 bar and tested at the same temperature showed near 90 GPU of H2 with a H2 /CO2 selectivity of 13.5 in the one-fiber module and over 39 GPU of H2 with a H2 /CO2 selectivity of 20.2 in the five-fiber module. Finally, the stability of the membranes was proved for 22 days at 180 °C.

Fabrication of Defect-Free P84® Polyimide Hollow Fiber for Gas Separation: Pathway to Formation of Optimized Structure.

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.

Preparation of Porous Stainless Steel Hollow-Fibers through Multi-Modal Particle Size Sintering towards Pore Engineering.

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.

Structural Contraction of Zeolitic Imidazolate Frameworks: Membrane Application on Porous Metallic Hollow Fibers for Gas Separation.

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.

On the molecular mechanisms for the H2/CO2 separation performance of zeolite imidazolate framework two-layered membranes.

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.

High selectivity ZIF-93 hollow fiber membranes for gas separation.

Zeolitic imidazolate framework-93 (ZIF-93) continuous membranes were synthesized on the inner side of P84 co-polyimide hollow fiber supports by microfluidics. MOFs and polymers showed high compatibility and the membrane exhibited H2-CH4 and CO2-CH4 separation selectivities of 97 (100 °C) and 17 (35 °C), respectively.

Metal-organic framework based mixed matrix membranes: a solution for highly efficient CO2 capture?

The field of metal-organic framework based mixed matrix membranes (M(4)s) is critically reviewed, with special emphasis on their application in CO2 capture during energy generation. After introducing the most relevant parameters affecting membrane performance, we define targets in terms of selectivity and productivity based on existing literature on process design for pre- and post-combustion CO2 capture. Subsequently, the state of the art in M(4)s is reviewed against these targets. Because final application of these membranes will only be possible if thin separation layers can be produced, the latest advances in the manufacture of M(4) hollow fibers are discussed. Finally, the recent efforts in understanding the separation performance of these complex composite materials and future research directions are outlined.