Impact of Sub-Ambient Temperature on Aging Rate and Gas Separation Properties of Polymers of Intrinsic Microporosity.

Aging in polymers of intrinsic microporosity has slowed exploitation due to a decay in performance over time since densification makes them unsuitable for industrial applications. This work aimed to study the impact of the operation and storage temperature on the gas separation properties and aging rates of PIM-1 self-standing films. The permeability, diffusivity, and solubility of the tested membranes were monitored through permeation tests for pure carbon dioxide and nitrogen at a maximum upstream pressure of 1.3 bar for temperatures ranging from -20 °C to 25 °C. This study found significant benefits in the operation of glassy polymeric membranes at low temperatures, resulting in a favourable trade-off in separation performance and a reduction in the aging rate by three orders of magnitude. This brings new opportunities for the industrial application of PIMs in innovative carbon capture processes.

Highly-Controlled Soft-Templating Synthesis of Hollow ZIF-8 Nanospheres for Selective CO2 Separation and Storage.

Global warming is an ever-rising environmental concern, and carbon dioxide (CO2) is among its major causes. Different technologies, including adsorption, cryogenic separation, and sequestration, have been developed for CO2 separation and storage/utilization. Among these, carbon capture using nano-adsorbents has the advantages of excellent CO2 separation and storage performance as well as superior heat- and mass-transfer characteristics due to their large surface area and pore volume. In this work, an environmentally friendly, facile, bottom-up synthesis of ZIF-8 hollow nanospheres (with reduced chemical consumption) was developed for selective CO2 separation and storage. During this soft-templating synthesis, a combined effect of ultra-sonication and low-temperature hydrothermal synthesis showed better control over an oil-in-water microemulsion formation and the subsequent growth of large-surface-area hollow ZIF-8 nanospheres having excellent particle size distribution. Systematic studies on the synthesis parameters were also performed to achieve fine-tuning of the ZIF-8 crystallinity, hollow structures, and sphere size. The optimized hollow ZIF-8 nanosphere sample having uniform size distribution exhibited remarkable CO2 adsorption capability (∼2.24 mmol g-1 at 0 °C and 1.75 bar), a CO2/N2 separation selectivity of 12.15, a good CO2 storage capacity (1.5-1.75 wt %), and an excellent cyclic adsorption/desorption performance (up to four CO2 adsorption/desorption cycles) at 25 °C. In addition, the samples showed exceptional structural stability with only ∼15% of overall weight loss up to 600 °C under a nitrogen environment. Therefore, the hollow ZIF-8 nanospheres as well as their highly controlled soft-templating synthesis method reported in this work are useful in the course of the development of nanomaterials with optimized properties for future CO2 capture technologies.

Models for Facilitated Transport Membranes: A Review.

Facilitated transport membranes are particularly promising in different separations, as they are potentially able to overcome the trade-off behavior usually encountered in solution-diffusion membranes. The reaction activated transport is a process in which several mechanisms take place simultaneously, and requires a rigorous theoretical analysis, which unfortunately is often neglected in current studies more focused on material development. In this work, we selected and reviewed the main mathematical models introduced to describe mobile and fixed facilitated transport systems in steady state conditions, in order to provide the reader with an overview of the existing mathematical tools. An analytical solution to the mass transport problem cannot be achieved, even when considering simple reaction schemes such as that between oxygen (solute) and hemoglobin (carrier) (A+C⇄AC), that was thoroughly studied by the first works dealing with this type of biological facilitated transport. Therefore, modeling studies provided approximate analytical solutions and comparison against experimental observations and exact numerical calculations. The derivation, the main assumptions, and approximations of such modeling approaches is briefly presented to assess their applicability, precision, and flexibility in describing and understanding mobile and fixed site carriers facilitated transport membranes. The goal is to establish which mathematical tools are more suitable to support and guide the development and design of new facilitated transport systems and materials. Among the models presented, in particular, those from Teramoto and from Morales-Cabrera et al. seem the more flexible and general ones for the mobile carrier case, while the formalization made by Noble and coauthors appears the most complete in the case of fixed site carrier membranes.

Permeability and Selectivity of PPO/Graphene Composites as Mixed Matrix Membranes for CO2 Capture and Gas Separation.

We fabricated novel composite (mixed matrix) membranes based on a permeable glassy polymer, Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and variable loadings of few-layer graphene, to test their potential in gas separation and CO2 capture applications. The permeability, selectivity and diffusivity of different gases as a function of graphene loading, from 0.3 to 15 wt %, was measured at 35 and 65 °C. Samples with small loadings of graphene show a higher permeability and He/CO2 selectivity than pure PPO, due to a favorable effect of the nanofillers on the polymer morphology. Higher amounts of graphene lower the permeability of the polymer, due to the prevailing effect of increased tortuosity of the gas molecules in the membrane. Graphene also allows dramatically reducing the increase of permeability with temperature, acting as a "stabilizer" for the polymer matrix. Such effect reduces the temperature-induced loss of size-selectivity for He/N2 and CO2/N2, and enhances the temperature-induced increase of selectivity for He/CO2. The study confirms that, as observed in the case of other graphene-based mixed matrix glassy membranes, the optimal concentration of graphene in the polymer is below 1 wt %. Below such threshold, the morphology of the nanoscopic filler added in solution affects positively the glassy chains packing, enhancing permeability and selectivity, and improving the selectivity of the membrane at increasing temperatures. These results suggest that small additions of graphene to polymers can enhance their permselectivity and stabilize their properties.

Effect of relative humidity on the gas transport properties of zeolite A/PTMSP mixed matrix membranes.

Increasing the knowledge of the influence of water vapor in new mixed matrix membranes (MMMs) could favor the integration of novel membrane materials in the recovery of CO2 from wet industrial streams. In this work, the water vapor effect on the N2, CH4 and CO2 permeability through MMMs comprised of 20 wt% hydrophilic zeolite 4A in hydrophobic PTMSP polymer were investigated in the relative humidity range 0-75%. While in the pure PTMSP membranes, the permeability of all gases decreases with water vapor activity, with almost unchanged CO2/N2 and CO2/CH4 selectivities, in zeolite A/PTMSP MMMs, the CO2 permeability increases with increasing water content in the system up to 50% R.H., resulting in an increase in CO2/N2 and CO2/CH4 selectivities with respect to pure PTMSP. Gas sorption was studied so that the effect the residual humidity in the zeolite 4A has on the sorption of the different gases helped explaining the permeability observations. The sorption and humid permeation behavior were evaluated by a simple model equation based on the NELF theory, taking into account the multicomponent gas sorption and diffusion in the presence of humidity, as well as the counteracting effects of the hydrophobic PTMSP and hydrophilic zeolite A in a very accurate way.