Gas permeation in poly(ether imide) nanocomposite membranes based on surface-treated silica. Part 1: Without chemical coupling to matrix

Interfacial Tailoring of Polyether Sulfone-Modified Silica Mixed Matrix Membranes for CO2 Separation

In this work, in situ polymerization of modified sol-gel silica in a polyether sulfone matrix is presented to control the interfacial defects in organic-inorganic composite membranes. Polyether sulfone polymer and modified silica are used as organic and inorganic components of mixed matrix membranes (MMM). The membranes were prepared with different loadings (2, 4, 6, and 8 wt.%) of modified and unmodified silica. The synthesized membranes were characterized using Field emission electron scanning microscopy, energy dispersive X-ray, Fourier transform infrared spectroscopy, thermogravimetric analyzer, and differential scanning calorimetry. The performance of the membranes was evaluated using a permeation cell set up at a relatively higher-pressure range (5-30 bar). The membranes appear to display ideal morphology with uniform distribution of particles, defect-free structure, and absence of interfacial defects such as voids and particle accumulations. Additionally, the CO₂/CH₄ selectivity of the membrane increased with the increase in the modified silica content. Further comparison of the performance indicates that PES/modified silica MMMs show a promising feature of commercially attractive membranes. Therefore, tailoring the interfacial morphology of the membrane results in enhanced properties and improved CO₂ separation performance.

Understanding Gas Transport in Polymer-Grafted Nanoparticle Assemblies

We rationalize the unusual gas transport behavior of polymer-grafted nanoparticle (GNP) membranes. While gas permeabilities depend specifically on the chemistry of the polymers considered, we focus here on permeabilities relative to the corresponding pure polymer which show interesting, "universal" behavior. For a given NP radius, R_c, and for large enough areal grafting densities, σ, to be in the dense brush regime we find that gas permeability enhancements display a maximum as a function of the graft chain molecular weight, M_n. Based on a recently proposed theory for the structure of a spherical brush in a melt of GNPs, we conjecture that this peak permeability occurs when the densely grafted polymer brush has the highest, packing-induced extension free energy per chain. The corresponding brush thickness is predicted to be h max = 3 R c , independent of chain chemistry and σ, i.e., at an apparently universal value of the NP volume fraction (or loading), ϕ_NP, ϕ_NP,max = [R_c/(R_c + h_max)]³ ≈ 0.049. Motivated by this conclusion, we measured CO_-2 and CH₄ permeability enhancements across a variety of R_c, M_n and σ, and find that they behave in a similar manner when considered as a function of ϕ_NP, with a peak in the near vicinity of the predicted ϕ_NP,max. Thus, the chain length dependent extension free energy appears to be the critical variable in determining the gas permeability for these hybrid materials. The emerging picture is that these curved polymer brushes, at high enough σ behave akin to a two-layer transport medium - the region in the near vicinity of the NP surface is comprised of extended polymer chains which speed-up gas transport relative to the unperturbed melt. The chain extension free energy increases with increasing chain length, up to a maximum, and apparently leads to an increasing gas permeability. For long enough grafts, there is an outer region of chain segments that is akin to an unperturbed melt with slow gas transport. The permeability maximum and decreasing permeability with increasing chain length then follow naturally.

Effect of Nanoscale Confinement on Polymer-Infiltrated Scaffold Metal Composites

Most research on polymer composites has focused on adding discrete inorganic nanofillers to a polymer matrix to impart properties not found in polymers alone. However, properties such as ion conductivity and mechanical reinforcement would be greatly improved if the composite exhibited an interconnected network of inorganic and polymer phases. Here, we fabricate bicontinuous polymer-infiltrated scaffold metal (PrISM) composites by infiltrating polymer into nanoporous gold (NPG) films. Polystyrene (PS) and poly(2-vinylpyridine) (P2VP) films are infiltrated into the ∼43 nm diameter NPG pores via capillary forces during thermal annealing above the polymer glass transition temperature (T_g). The infiltration process is characterized in situ using spectroscopic ellipsometry. PS and P2VP, which have different affinities for the metal scaffold, exhibit slower segmental dynamics compared to their bulk counterparts when confined within the nanopores, as measured through T_g. The more attractive P2VP shows a 20 °C increase in T_g relative to its bulk, while PS only shows a 6 °C increase at a comparable molecular weight. The infiltrated polymer, in turn, stabilizes the gold nanopores against temporal coarsening. The broad tunability of these polymer/metal hybrids represents a unique template for designing functional network composite structures with applications ranging from flexible electronics to fuel cell membranes.

Tuning Selectivities in Gas Separation Membranes Based on Polymer-Grafted Nanoparticles

Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.

The effect of nanoparticles on gas permeability with polyimide membranes and network hybrid membranes: a review

Polyimide membranes and network hybrid membranes exhibit high permeability despite good thermal and chemical stability, and high selectivity in gas mixture separation. In this study, the effect of nanoparticle distribution on the network polymer network, and changes in permeability, selectivity, and structure of the composite lattice membrane are investigated. According to the obtained permeability results, this increase in permeability was due to the increase of polymer network free volume and the formation of cavities in the nanoparticle-polymer interface. The significant results were that the permeability growth of gases with larger molecular size such as methane and nitrogen was higher than other gases. A comparison of the permeability growth of gases with the increasing volume fraction of nanoparticles confirms the dominance of the molecular sieve mechanism and the type of membrane transport mechanism change over polyimide and network Hybrid.

Pebax® 2533/Graphene Oxide Nanocomposite Membranes for Carbon Capture

In this work, the behavior of new GO-based mixed matrix membranes was tested in view of their use as CO₂-selective membrane in post combustion carbon capture applications. In particular, the new materials were obtained by mixing of Pebax^® 2533 copolymer with different types of graphene oxide (GO). Pebax^® 2533 has indeed lower selectivity, but higher permeability than Pebax^® 1657, which is more commonly used for membranes, and it could therefore benefit from the addition of GO, which is endowed with very high selectivity of CO₂ with respect to nitrogen. The mixed matrix membranes were obtained by adding different amounts of GO, from 0.02 to 1% by weight, to the commercial block copolymers. Porous graphene oxide (PGO) and GO functionalized with polyetheramine (PEAGO) were also considered in composites produced with similar procedure, with a loading of 0.02%wt. The obtained films were then characterized by using SEM, DSC, XPS analysis and permeability experiments. In particular, permeation tests with pure CO₂ and N₂ at 35°C and 1 bar of upstream pressure were conducted for the different materials to evaluate their separation performance. It has been discovered that adding these GO-based nanofillers to Pebax^® 2533 matrix does not improve the ideal selectivity of the material, but it allows to increase CO₂ permeability when a low filler content, not higher than 0.02 wt%, is considered. Among the different types of GO, then, porous GO seems the most promising as it shows CO₂ permeability in the order of 400 barrer (with an increase of about 10% with respect to the unloaded block copolymer), obtained without reducing the CO₂/N₂ selectivity of the materials, which remained in the order of 25.

Modeling gas transport in polymer-grafted nanoparticle membranes

We use coarse-grained molecular dynamics simulations to study gas diffusion within nanocomposites consisting of matrix-free polymer-grafted nanoparticles. We compare the transport of gas penetrants in systems using polymer models with and without an angle potential and show that gas diffusion enhancement occurs in nanocomposite systems only with the angle potential. This enhancement is related to the free volume in the system, but the cage size experienced by the gas penetrant seems to be a more relevant indicator of gas diffusion enhancement. The enhancement seen in our simulations is smaller than that observed in experiments.

Role of Filler Porosity and Filler/Polymer Interface Volume in Metal-Organic Framework/Polymer Mixed-Matrix Membranes for Gas Separation

Metal-organic frameworks (MOFs) and inorganic fillers are frequently incorporated into mixed-matrix membranes (MMMs) to overcome the traditional trade-off in permeability ( P) and selectivity for pure organic polymer membranes. Therefore, it is of great interest to examine the influence of porous and nonporous fillers in MMMs with respect to the possible role of the polymer-filler interface, that is, the void volume. In this work, we compare the same MOF filler in a porous and nonporous state, so that artifacts from a different polymer-filler interface are excluded. MMMs with the porous MOF aluminum fumarate (Al-fum) and with a nonporous dimethyl sulfoxide solvent-filled aluminum fumarate (Al-fum(DMSO)), both with Matrimid as polymer, were prepared. Filler contents ranged from 4 to 24 wt %. Gas separation performances of both MMMs were studied by mixed gas measurements using a binary mixture of CO₂/CH₄ with gas permeation following the theoretical prediction by the Maxwell model for both porous and nonporous dispersed phase (filler). MMMs with the porous Al-fum filler showed increased CO₂ and CH₄ permeability with a moderate rise in selectivity upon increasing filler fraction. The MMMs with the nonporous Al-fum(DMSO) filler displayed a reduction in permeability while maintaining the selectivity of the neat polymer. A linear dependence of log P versus the reciprocal specific free fractional volume (sFFV) rules out a significant contribution from a void volume. The sFFV includes the free volume of the polymer and the MOF, but not the polymer-filler interface volume (so-called void volume). The sFFV for the MMM was calculated between 0.23 cm³/g for a 24 wt % Al-fum/Matrimid MMM and 0.12 cm³/g for a 24 wt % Al-fum(DMSO)/Matrimid MMM. The negligible effect of an interface volume is supported by a good matching of theoretical and experimental density of the Al-fum and Al-fum/(DMSO) MMMs which gave a specific void volume below 0.02 cm³/g, often even below 0.01 cm³/g.

Mixed matrix membranes based on silica nanoparticles and microcellular polymers for CO2/CH4 separation

Mixed matrix membranes made from silica nanoparticles and microcellular polymers were prepared from Matrimid® 5218 combined with tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane via the sol–gel method. The nanoparticles were prepared in situ during membrane casting yielding a homogeneous distribution inside a foamed polyimide structure. Mixed matrix membranes with SiO2 contents up to 16% wt. were treated at 60℃, 100℃, 150℃, and 200℃. Thermal gravimetric analysis and Fourier transform infrared spectroscopy analyses were performed providing information on chemical composition and thermal stability, while the porous structure (average cell diameter and cell density) was studied by scanning electron micrograph. Also, dynamic mechanical analysis was used to determine the glass transition temperature (Tg) and elastic modulus. Finally, the gas transport properties were studied in terms of treatment temperature, feed pressure, SiO2 loading, and testing temperature. CO2 permeability was found to increase by a factor of 3–4 at 3% SiO2 content using tetraethoxysilane in Matrimid, while ideal selectivity for CO2/CH4 separation was constant. Finally, the plasticization effect was practically eliminated by the introduction of SiO2 nanoparticles.

Postextraction Separation, On-Board Storage, and Catalytic Conversion of Methane in Natural Gas: A Review

In today's perspective, natural gas has gained considerable attention, due to its low emission, indigenous availability, and improvement in the extraction technology. Upon extraction, it undergoes several purification protocols including dehydration, sweetening, and inert rejection. Although purification is a commercially established technology, several drawbacks of the current process provide an essential impetus for developing newer separation protocols, most importantly, adsorption and membrane separation. This Review summarizes the needs of natural gas separation, gives an overview of the current technology, and provides a detailed discussion of the progress in research on separation and purification of natural gas including the benefits and drawbacks of each of the processes. The transportation sector is another growing sector of natural gas utilization, and it requires an efficient and safe on-board storage system. Compressed natural gas (CNG) and liquefied natural gas (LNG) are the most common forms in which natural gas can be stored. Adsorbed natural gas (ANG) is an alternate storage system of natural gas, which is advantageous as compared to CNG and LNG in terms of safety and also in terms of temperature and pressure requirements. This Review provides a detailed discussion on ANG along with computation predictions. The catalytic conversion of methane to different useful chemicals including syngas, methanol, formaldehyde, dimethyl ether, heavier hydrocarbons, aromatics, and hydrogen is also reviewed. Finally, direct utilization of methane onto fuel cells is also discussed.

Enhancing Separation and Mechanical Performance of Hybrid Membranes through Nanoparticle Surface Modification

Membranes with selective gas transport properties and good mechanical integrity are increasingly desired to replace current energy intensive approaches to gas separation. Here, we report on the dual enhancement of transport and mechanical properties of hybrid cross-linked poly(ethylene glycol) membranes with aminopropyl-modified silica nanoparticles. CO₂ permeability in hybrid membranes exceeds what can be predicted by Maxwell's equation and surpasses values of the pure polymer. Furthermore, dynamic mechanical and thermogravimetric analyses reveal increases in both the storage modulus and thermal stability in hybrid membranes, with respect to silica nanoparticle loading.

Engineering Synergy: Energy and Mass Transport in Hybrid Nanomaterials

An emerging class of materials that are hybrid in nature is propelling a technological revolution in energy, touching many fundamental aspects of energy-generation, storage, and conservation. Hybrid materials combine classical inorganic and organic components to yield materials that manifest new functionalities unattainable in traditional composites or other related multicomponent materials, which have additive function only. This Research News article highlights the exciting materials design innovations that hybrid materials enable, with an eye toward energy-relevant applications involving charge, heat, and mass transport.