Javier Sánchez-Laínez

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.

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.

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.

The fabrication of ultrathin films and their gas separation performance from polymers of intrinsic microporosity with two-dimensional (2D) and three-dimensional (3D) chain conformations

The expansion of the use of polymeric membranes in gas separation requires the development of membranes based on new polymers with improved properties and their assessment under real operating conditions. In particular, the fabrication of ultrathin films of high performance polymers that can be used as the selective layer in composite membranes will allow large reductions in the amount of the expensive polymer used and, hence, the cost of membrane fabrication. In this contribution, two polymers of intrinsic microporosity (PIMs) with very different chain configurations (two-dimensional, 2D, chains or conventional contorted three-dimensional, 3D, conformation) have been compared in their ability to form ultrathin films, showing the relevance of polymer design to obtain compact and defect-free films. Monolayers of the 2D polymer PIM-TMN-Trip can be efficiently deposited onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) to obtain composite membranes with a CO2/N2 selectivity similar to that of the corresponding thick membranes of the same PIM using only a small fraction of the selective polymer (less than 0.1%).

Synthesis of ZIF-93/11 Hybrid Nanoparticles via Post-Synthetic Modification of ZIF-93 and Their Use for H2/CO2 Separation

The present work shows the synthesis of nano-sized hybrid zeolitic imidazolate frameworks (ZIFs) with the rho topology based on a mixture of the linkers benzimidazole (bIm) and 4-methyl-5-imidazolecarboxaldehyde (4-m-5-ica). The hybrid ZIF was obtained by post-synthetic modification of ZIF-93 in a bIm solution. The use of different solvents, MeOH and N,N-dimethylacetamide (DMAc), and reaction times led to differences in the quantity of bIm incorporated to the framework, from 7.4 to 23 % according to solution-state NMR spectroscopy. XPS analysis showed that the mixture of linkers was also present at the surface of the particles. The inclusion of bIm to the ZIF-93 nanoparticles improved the thermal stability of the framework and also increased the hydrophobicity according to water adsorption results. N2 and CO2 adsorption experiments revealed that the hybrid material has an intermediate adsorption capacity, between those of ZIF-93 and ZIF-11. Finally, ZIF-93/11 hybrid materials were applied as fillers in polybenzimidazole (PBI) mixed matrix membranes (MMMs). These MMMs were used for H2 /CO2 separation (at 180 °C) reaching values of 207 Barrer of H2 and a H2 /CO2 selectivity of 7.7 that clearly surpassed the Robeson upper bound (corrected for this temperature).