Beatriz Zornoza

Combination of MOFs and Zeolites for Mixed-Matrix Membranes

Mixed-matrix membranes (MMMs) were prepared by combinations of two different kinds of porous fillers [metal-organic frameworks (MOFs) HKUST-1 and ZIF-8, and zeolite silicalite-1] and polysulfone. In the search for filler synergy, the MMMs were applied to the separation of CO(2)/N(2), CO(2)/CH(4), O(2)/N(2), and H(2)/CH(4) mixtures and we found important selectivity improvements with the HKUST-1-silicalite-1 system (CO(2)/CH(4) and CO(2)/N(2) separation factors of 22.4 and 38.0 with CO(2) permeabilities of 8.9 and 8.4 Barrer, respectively).

Mesoporous silica sphere-polysulfone mixed matrix membranes for gas separation.

A series of mixed matrix membranes were prepared comprising polysulfone Udel matrix and ordered mesoporous silica spheres as filler with loadings varying between 0 and 32 wt %. The interaction between the filler and the polymer was studied by scanning and transmission electron microscopy, thermogravimetry, differential scanning calorimetry and dynamic mechanical analyses, N2 porosity, X-ray photoelectron spectrometry, and attenuated total reflectance Fourier transform infrared spectroscopy. All these characterizations allowed us to infer an optimum interaction based on both the penetration of the polymer chains into the mesoporosity of the silica spheres and the establishment of hydrogen bondings between the hydroxyl-rich surface and the aryl ether groups of the polymer. An optimum loading of 8 wt % was found in terms of H2/CH4 separation performance. In addition, the optimum membrane was tested for CO2/N2 separation.

Beyond the H2/CO2 upper bound: one-step crystallization and separation of nano-sized ZIF-11 by centrifugation and its application in mixed matrix membranes

The synthesis of nano-sized ZIF-11 with an average size of 36 ± 6 nm is reported. This material has been named nano-zeolitic imidazolate framework-11 (nZIF-11). It has the same chemical composition and thermal stability and analogous H2 and CO2 adsorption properties to the conventional microcrystalline ZIF-11 (i.e. 1.9 ± 0.9 μm). nZIF-11 has been obtained following the centrifugation route, typically used for solid separation, as a fast new technique (pioneering for MOFs) for obtaining nanomaterials where the temperature, time and rotation speed can easily be controlled. Compared to the traditional synthesis consisting of stirring + separation, the reaction time was lowered from several hours to a few minutes when using this centrifugation synthesis technique. Employing the same reaction time (2, 5 or 10 min), micro-sized ZIF-11 was obtained using the traditional synthesis while nano-scale ZIF-11 was achieved only by using centrifugation synthesis. The small particle size obtained for nZIF-11 allowed the use of the wet MOF sample as a colloidal suspension stable in chloroform. This helped to prepare mixed matrix membranes (MMMs) by direct addition of the membrane polymer (polyimide Matrimid®) to the colloidal suspension, avoiding particle agglomeration resulting from drying. The MMMs were tested for H2/CO2 separation, improving the pure polymer membrane performance, with permeation values of 95.9 Barrer of H2 and a H2/CO2 separation selectivity of 4.4 at 35 °C. When measured at 200 °C, these values increased to 535 Barrer and 9.1.

Synthesis and characterisation of MOF/ionic liquid/chitosan mixed matrix membranes for CO2/N2 separation

Mixed matrix membranes (MMMs) have been prepared by combining a small amount of highly absorbing non-toxic ionic liquid, [emim][Ac] (IL) (5 wt%), a biopolymer from renewable abundant natural resources, chitosan (CS), and nanometre-sized metal–organic framework (MOF) ZIF-8 or HKUST-1 particles to improve the selectivity of the IL–CS hybrid continuous polymer matrix. The TGA revealed that the thermal stability has been enhanced by the influence of both IL and ZIF-8 or HKUST-1 fillers, while keeping a water content of around 20 wt%, which suggests the potential of such materials for developing high temperature water resistant membranes for CO2 separation. The CO2 and N2 single gas permeation performance was tested at temperatures in the range of 25–50 °C, to compare with the previously reported IL–CS hybrid membranes. The best CO2 permeability and CO2/N2 selectivity performance is obtained for 10 wt% ZIF-8 and 5 wt% HKUST-1/IL–CS membranes, as high as 5413 ± 191 Barrer and 11.5, and 4754 ± 1388 Barrer and 19.3, respectively. This is attributed to a better adhesion and smaller particle size of ZIF-8 than HKUST-1 nanoparticles with respect to the IL–CS continuous matrix, as interpreted by Hansen solubility parameters and Maxwell-based models, modified to account for rigidification, pore blockage and crystallinity of the CS matrix, with very accurate predictions.

Advances in Hydrogen Separation and Purification with Membrane Technology

Hydrogen economy is a situation where hydrogen is used as the major carrier of energy. Proton exchange membrane fuel cells (PEMFCs) are some of the most interesting candidates to cover the necessity of new technologies that use hydrogen as fuel. PEMFCs are fueled by highly pure hydrogen, which is industrially produced as a hydrogen-rich stream mainly via steam reforming of natural gas; it is then further purified to achieve the desired quality. Nowadays, membrane-based processes are considered to be the most promising technologies for the production of high-purity hydrogen. For hydrogen separation, polymeric, metallic, mixed matrix and ceramic membranes have been extensively studied. In this chapter, the principal results and technical advances for hydrogen separation membranes are summarized. H 2 could be selectively removed from the reaction system and higher conversion be obtained even at lower temperatures if the membrane is coupled to catalytic reactors (membrane reactors (MRs)). The use of palladium and silica membranes into MRs is reviewed.

On the chemical filler–polymer interaction of nano- and micro-sized ZIF-11 in PBI mixed matrix membranes and their application for H2/CO2 separation

The evolution of nano- and micro-sized ZIF-11 (nZIF-11 and ZIF-11, respectively) when embedded into a PBI polymeric matrix is studied. The prepared membranes, with loadings up to 55 wt%, have been characterized through several techniques (XRD, SEM, FTIR, TGA, 13C NMR and XPS) and the changes in the morphology of the fillers upon combination with PBI, as well as in the chemical environment of their main atoms (interactions between the linker of the filler and the benzyl rings of the polymeric bIm units) are discussed. All the membranes have been tested at temperatures ranging between 70 and 200 °C to study their H2/CO2 separation performance. The integration of both types of MOF in the polymeric phase improves not only the hydrogen permeability but also the selectivity in comparison with the pure polymer in all cases. H2 permeability increases due to a better diffusion of the penetrants, while CO2 adsorption on the MOF and solution in the polymer decreases. The best result obtained corresponds to the membrane with 55 wt% loading of ZIF-11, with 495 Barrer of H2 permeability and a H2/CO2 selectivity of 7.0.

Tuning the separation properties of zeolitic imidazolate framework core–shell structures via post-synthetic modification

The conversion of ZIF-8 into ZIF-7 via post-synthetic modification with benzimidazole has been monitored by quantifying the liberated 2-methylimidazole by chromatography. The reaction kinetics have been adjusted to the shrinking core model, providing the diffusion coefficient of bIm inside the pores and the reaction kinetic constant (2.86 × 10−7 cm2 s−1 and 1.36 × 10−4 cm s−1, respectively). A wide variety of ZIF-7/8 hybrid core–shell frameworks have been obtained during this reaction. The most promising have been characterized by SEM/TEM, TGA, N2 and CO2 adsorption, FTIR and 13C NMR, showing features of the coexistence of both phases inside the frameworks. Their structures have also been simulated, providing comparable XRD and adsorption results. The hybrid material has been used as a filler for PBI mixed matrix membranes (MMMs) applied to H2/CO2 separation, enhancing the performances of the bare PBI polymer and MMMs containing ZIF-8 or ZIF-7 as a filler, with a maximum H2 permeability value of 1921 Barrer and a H2/CO2 selectivity of 11.8.

Mixed Matrix Membranes from Nanostructured Materials for Gas Separation

This work describes the preparation, characterization and application of mixed matrix membranes made from inorganic phase Nu-6(2) and Matrimid® polyimide. Due to the hydrophobic character of both the zeolitic phase (with a high Si/Al ratio of 45) and the polymer, membranes obtained are free of interfacial voids, as observed by SEM characterization and inferred from the membrane performance. Gas permeation tests carried out for the 50/50 H2/CH4 binary mixture revealed a significant enhancement of H2 separation. H2 permeability and H2/CH4 separation selectivity for the bare polyimide membrane were 32 Barrer and 118, whereas the Matrimid®-Nu-6(2) mixed matrix membrane reached a permeability of 49 Barrer and a selectivity of 206 with a zeolite content of 15%. This superior performance of the hybrid membrane is attributed to the molecular sieve role played by the selected zeolitic phase which has two different types of eight-membered-ring channels with limiting dimensions of 2.4 and 3.2 A.

Ultrathin composite polymeric membranes for CO2/N2 separation with minimum thickness and high CO2 permeance

The use of ultrathin films as selective layers in composite membranes offers significant advantages in gas separation for increasing productivity while reducing the membrane size and energy costs. In this contribution, composite membranes have been obtained by the successive deposition of approximately 1 nm thick monolayers of a polymer of intrinsic microporosity (PIM) on top of dense membranes of the ultra-permeable poly[1-(trimethylsilyl)-1-propyne] (PTMSP). The ultrathin PIM films (30 nm in thickness) demonstrate CO2 permeance up to seven times higher than dense PIM membranes using only 0.04 % of the mass of PIM without a significant decrease in CO2 /N2 selectivity.

Insight into ETS-10 synthesis for the preparation of mixed matrix membranes for CO2/CH4 gas separation

An in-depth study into the synthesis of the titanosilicate ETS-10 has been carried out to obtain crystals with different particle sizes, roughness and porosity. The effect of these parameters on the CO2/CH4 gas separation performance using mixed matrix membranes (MMMs) has been studied. MMMs based on ETS-10 polycrystalline particles of 1–2 μm in size with high surface roughness and porosity gave rise to a good filler dispersion and filler–polymer interaction. The addition of 10 wt% ETS-10 polycrystalline particles into the polysulfone matrix increased the CO2 permeability from 6.1 to 7.8 Barrer and the CO2/CH4 selectivity from 31 to 38. When using the polyimide 6FDA-6FpDA, a glassy polymer with high gas permeability, the addition of 10 wt% ETS-10 polycrystalline particles increased the CO2 permeability from 96 to 125 Barrer, with a decrease in CO2/CH4 selectivity from 56 to 51.