Symposium EN04 : Advanced Materials for Carbon Capture and Other Important Gas Separations

Functional materials using or in the presence of ionic liquids represents a burgeoning direction in materials chemistry. Ionic liquids are a family of non-conventional molten salts that can act as both functional materials and solvents. They offer many advantages, such as negligible vapor pressures, wide liquidus ranges, good thermal stability, tunable solubility of both organic and inorganic molecules, and ion conductivity. The unique solvation environment of these ionic liquids provides new reaction and separation media for controlling many energy-related processes. We have recently developed a class of ionic liquids with intrinsic porosities based on nanoscopic building blocks.¹ Challenges and opportunities in synthesizing and using these porous ionic liquids in energy-related applications will be discussed.

Reference

(1) Zhang, J.; Chai, S.-H.; Qiao, Z.-A.; Mahurin, S. M.; Chen, J.; Fang, Y.; Wan, S.; Nelson, K.; Zhang, P.; Dai, S. Porous Liquids: A Promising Class of Media for Gas Separation. Angew. Chem.-Int. Edit. 2015, 54, 932-936. 10.1002/anie.201409420

Ionic liquids (ILs) present intriguing possibilities for removal of carbon dioxide from a wide variety of different gas mixtures, including post-combustion flue gas, pre-combustion gases, air, and raw natural gas streams. Even by physical absorption, many ILs provide sufficient selectivity over N₂, O₂, CH₄ and other gases. However, when CO₂ partial pressures are low, the incorporation of functional groups to chemically react with the CO₂ can dramatically increase capacity, while maintaining or even enhancing selectivity. Previously, we have shown how the reaction stoichiometry can be doubled over conventional aqueous amine solutions to reach one mole of CO₂ per mole of IL by incorporating the amine on the anion, how we can virtually eliminate any viscosity increase upon complexation of the IL with CO₂, by using aprotic heterocyclic anions (AHA ILs) that eliminate the pervasive hydrogen bonding and salt bridge formation that is the origin of the viscosity increase, and how the process energy can be further reduced by using ‘phase change’ ionic liquids, which are AHA ILs whose melting points when reacted with CO₂ are more than 100 °C below the melting point of the unreacted material. Here, we show how mass transfer challenges of the relatively viscous ILs can be overcome by increasing the gas/liquid contact area by encapsulation of the ILs in silicone based shells. In particular, we show that the polymeric shells do not provide significant resistance to transport of CO₂ and the capacity of the AHA ILs is maintained in the shells. The absorbent material in the shells can be cycled repeatedly without loss of capacity. Moreover, we show that the reaction of CO₂ with the AHA ILs in the presence of water, which involves some reprotonation of the anion and formation of bicarbonate, is completely reversible and can be cycled for both the neat material and when it is encapsulated in the shells. Results on both the equilibrium and the rates of these reactions will be presented. Finally, we will show how the encapsulated phase change IL material works effectively in a fluidized bed reactor.

Membranes offer improved energy efficiency in separations processes such as CO₂ capture from combustion point sources, natural gas sweetening, syngas processing and air separation. To this end, a number of advanced polymer, inorganic and hybrid materials have been developed in recent years. Several distinct material classes have emerged, each with its respective set of benefits and limitations. Specifically, polyimides, ionic liquids (ILs), polymers of intrinsic microporosity (PIMs), metal-organic frameworks (MOFs) and thermally rearranged (TR) polymers are at the forefront of advanced membrane materials. We have identified a versatile approach which incorporates the benefits of these structurally diverse materials into a single platform through the synthesis of hybrid polyimide-ionene architectures, or “aromatic ionic polyimides”. This presentation will detail the design of these materials and their performances as gas separation membranes using both experimental data and computational studies.

Most of the world’s energy is presently derived from the burning of fossil fuels, which releases vast quantities of carbon dioxide (CO₂) into the environment and results in undesirable climate change. Practical and cost-efficient methods of CO₂ separation and capture would thus solve one of the most challenging problems today. This presentation summarizes our efforts on the development of novel polymer membranes functionalized with CO₂-philic groups for high flux CO₂ separation. Our strategy focuses on tuning solubility selectivity in addition to diffusivity selectivity for achieving high permeability membranes combined with good selectivity. Various synthetic techniques including ROMP, thiol-ene click reaction, and post functionalization were used and the careful design permits to prepare well-defined novel high permeable polymers containing CO₂-philic groups. This study demonstrated the addition of CO₂-philic groups (e.g. amidoxime and PEO) significantly increased the solubility selectivity of CO₂ over N₂. The membrane performance is also highly dependent on the balance of gas/functional group interaction, intra/inter- molecular interaction (H-bonding etc.) of membrane, packing, and polymer dynamics (degree of crosslinking). Tuning the balance of the interaction and dynamics enables to achieve the CO₂ separation performance over the Robeson upper bound, e.g. CO₂ permeability 6800 Barrer and CO₂/N₂ selectivity 19, or CO₂ permeability 820 Barrer and CO₂/N₂ selectivity 39. The structure-property relationships especially on CO₂ uptake, CO₂ and N₂ permeability, CO₂/N₂ selectivity to the polymer structure, as well as the effort on the selective layer coating will be discussed. Moreover, the other gas pair separation such as CO₂/CH₄, He/CH₄ and He/N₂ via tailoring solubility selectivity and diffusivity selectivity will also be discussed. Our membranes are rubbery-based and thus have no ageing issues, in contrast to glassy membranes.

Gas and water vapor separation properties of a systematic series of sulfonyl-containing poly(benzimidazole)s are reported as a function of pressure and temperatrue. Gas separation properties are reported over temeratures ranging from ambient up to 190C. Additionally, separation properties of PBIs blended with polyimides are also reported, with some blends exhibiting transport properties above the 2008 upper bound.

Saudi Aramco has a strategic interest in recovering valuable higher hydrocarbons (C₃₊). These higher hydrocarbons, also known as natural gas liquids (NGLs), can be used to reduce the Kingdom’s reliance on liquid fuel for power generation and can also be used as a feedstock for downstream chemical production. The conventional separation of NGLs from natural gas is typically accomplished through energy intensive refrigeration processes. Polymeric membranes that are selective for C₃₊ (e.g. propane and butane) relative to methane could provide a lower energy alternative to accomplish such a separation. Currently, polydimethysiloxane (PDMS), a commercially-available silicon-based rubbery membrane material, exhibits prohibitively low C_3+//methane selectivities under rigorous testing conditions, hindering their application in industrial gas processing plants. To achieve significant recovery of NGLs from natural gas while reducing capital and operating expenditures, more efficient membranes with improved selectivity are required. This work describes progress to improve the C₃₊/methane separation efficiency of modified PDMS by incorporating bulky hydrocarbons into the polymer matrix. The effect of polymer backbone modifications and crosslinking agents on membrane physical properties and permeation performance is investigated. Modified PDMS membranes show enhanced gas permeation performance compared to commercial PDMS under industrially-relevant feed streams and testing conditions, including with multicomponent C₁-C₄ hydrocarbon mixtures up to 800 psi and in the presence of aggressive contaminants, including benzene, toluene, ethylbenzene, and xylene (BTEX). The results of this study aim to advance the rapid development of novel rubbery membrane materials for enhanced NGL recovery from natural gas.

Carbon nanotubes (CNTs) were incorporated in polyethersulfone hollow fiber mixed matrix membranes (P HFMs) to improve the gas separation performance. The P-CNTs (PC) HFMs were successfully developed using an indigenous spinning pilot plant. PC HFMs showed the improved thermal stability and mechanical strength as compared to that measured for the pristine P HFMs. The pure gas permeability of CO₂, CH₄, O₂, and N₂ gases for the developed HFMs were measured at 3 bar feed pressure and room temperature in a lab-scale gas permeation setup. It was observed that the presence of CNTs in PC HFMs significantly improved the CO₂ permeability by four-fold to that measured for the pristine P HFMs. Furthermore, the ideal gas selectivity for CO₂/CH₄, O₂/N₂, and CO₂/N₂ gas pairs was also remarkably enhanced by almost 7-times, 1.5-times and 8-times, respectively, to that measured for P HFMs. The gas separation performance was better than or comparable to that of the literature-reported carbon nanomaterials-based membranes. Remarkably, the separation performance crossed or was almost closer to the upper bound curves drawn by Robeson in 2008 for these gas pairs. The improved separation performance can be attributed to the positive effect of doping of CNTs in HFMs, which resulted in selective transport of gases. Thus, the results, obtained in this study, experimentally demonstrated that the developed PC HFMs are a potential membrane material for industrially relevant CO₂/CH₄, O₂/N₂, and CO₂/N₂ gas separations.

Membranes that permeate hydrogen and reject CO₂ at temperatures above 150 °C are of great interest for low cost CO₂ capture in pre-combustion processes. The current leading polymeric material for this application is poly[2,2’-(m-phenylene)-5,5’-bisbenzimidazole] (PBI), which exhibits good thermal stability, strong size-sieving ability and good H₂/CO₂ selectivity. In this study, we demonstrate that H₂/CO₂ selectivity in PBI can be significantly enhanced by doping with phosphoric acid (H₃PO₄). H₃PO₄ can bond strongly with imidazole rings in PBI, and the complexes are thermally stable up to 200 °C under vacuum. We prepares a series of H₃PO₄ doped PBI samples by immersing PBI thin films (12 µm) in solutions containing H₃PO₄ and methanol. The H₃PO₄ concentration can be varied from 0.05 wt.% to 1.0 wt.% to achieve different doping levels in the PBI films (0.3 - 1.0, defined as the molar ratio of H₃PO₄ to PBI repeating unit). Increasing the doping level increases H₂/CO₂ selectivity and decreases H₂ permeability. For example, pure PBI exhibits pure-gas H₂ permeability of 27 Barrers and H₂/CO₂ selectivity of 14 at 150 °C, while the PBI with a doping level of 0.44 exhibits H₂ permeability of 6.1 Barrers and H₂/CO₂ selectivity of 59 at 150 °C. As the doping level increases to 1.0, the PBI shows an impressive H₂/CO₂ selectivity of 136, though the H₂ permeability decreases to 1.3 Barrers at 150 °C. This performance is above the upper bound in the Robeson’s plot for H₂/CO₂ separation. This presentation will also discuss the effect of H₃PO₄ doping on structural changes such as free volume, as well as CO₂ sorption and diffusion, and the structure/property relationships in these H₃PO₄ doped PBI thin films.

We recently developed a new non-equilibrium molecular dynamics simulation method [1] in order to simulate the permeation of pure fluids and mixtures through membranes, namely "Concentration Gradient Driven Molecular Dynamics" (CGD-MD). This new method works by employing bias forces to fix the concentration of fluids (pure or mixture) at the inlet and outlet of a membrane in order to maintain a concentration gradient and drive the diffusion of the molecules through the membrane. This is aimed at mimicking membrane separation experiments; for instance, high concentration/pressure at the feed side and vacuum at the permeate side. CGD-MD addresses two main shortcomings of previous non-equilibrium MD methods used for simulating membrane separation processes at the molecular scale. First, it avoids the feed depletion issue and allows running steady state and continuous simulations for unrestricted simulation times. Second, it maintains the feed composition at a target value without the need of any complex Monte Carlo-MD coupling. We demonstrate the new method for the separation of various gas mixtures in a MOF membrane as well as a composite MOF/Polymer mebrane.

[1] Ozcan, A., Perego, C., Salvalaglio, M., Parrinello, M., Yazaydin, O.*; "Concentration gradient driven molecular dynamics: a new method for simulations of membrane permeation and separation", Chemical Science, 2017, 8, 3858–3865.

Polymer membranes are the material of choice for many gas separations owing to their high performance, scalable synthesis, and easy processing. However, they suffer from a fundamental permeability/selectivity tradeoff that defines the “upper bound” limitations of this technology.¹ Membranes that perform at or above the upper-bound can be achieved through the thoughtful design of polymers containing significant microporosity and rigid backbone chemistries.²

Thermally rearranged polybenzoxazoles excel in this regard.³ Produced through high-temperature solid-state reactions that alter the chain packing of a precursor polyimide, these polymers display enhanced porosity and narrow pore size distributions, placing their performance at or above the upper bound for many relevant gas pairs.

Here, we demonstrate the synthesis and gas separation performance of a novel family of thermally rearranged polybenzoxazoles containing a backbone contortion of exceptional rigidity. The design of these polymers incorporates chemical functionalities featured in other state-of-the-art gas separation membranes into polyimides with ortho-functional groups (PIOFGs), which are subsequently thermally rearranged. Synthesis of the precursor polyimides and the pore network evolution during thermal rearrangement will be discussed. Additionally, we will examine gaseous analyte-polymer matrix interactions in the context of gas separation performance. These findings offer new insights into the design of gas separation membrane polymers and provide a path forward for further optimization of thermally rearranged polymers.

1) Robeson, L. M. J. Membr. Sci. 2008, 320, 390–400.
2) Freeman, B. D. Macromolecules 1999, 32, 375–380.
3) Robeson, L. M.; Dose, M. E.; Freeman, B. D.; Paul, D. R. J. Membr. Sci. 2017, 525, 18–24.

Polybenzimidazole is well known for having a superior heat resistance and has good mechanical properties as good as heat resistance. Due to an excellent various properties, PBI has been used a lot of fields such as gas separation, OSN and solvent-resistant nano filtration. PBI parts, which are produced by using a compression molding process, are promising materials in extreme environments. This work is focused on fabrication of meta-Polybenzimidazole(m-PBI) and control of morphology according to concentrations or manufacturing conditions. The membranes were prepared via the phase inversion method from casting solutions with predetermined amounts PBI, dimethylacetamide (DMAc) and tetrahydrofuran (THF). The polymer solutions were cast on a clean glass plate by using casting knife. The membrane was immersed in IPA to remove the residual solvent and dried in vacuum oven for 24h. Also, this membrane can be prepared by electro-spinning. Electro-spinning can make highly porous non-woven fabrics consisted of nanofibers with the small diameters and overall porous structure. Also, electrospun fabrics have high specific surface area. To observe morphology, Scanning electron microscope (SEM) was used. In case of gas transport properties, H₂, CO₂ Permeability and selectivity of this membrane was measured via pure gas.

Various key gas/vapors separations are accomplished using energy intensive processes as exemplified by the olefin/paraffin separation, an essential separation in chemical industry.
Here we present our progress in the development of functional metal-organic frameworks (MOFs) to address some energy-intensive separations. Successful practice of reticular chemistry had afforded the fabrication of a chemically stable fluorinated MOF adsorbent materials (NbOFFIVE-1-Ni, also referred to as KAUST-7 and AlFFIVE-1-Ni, also referred to as KAUST-8).
KAUST-7 fully split propylene from propane. The bridging of Ni(II)-pyrazine square-grid layers with (NbOF₅)^2- pillars permitted the construction of a 3-dimensional MOF, enclosing a periodic array of fluoride anions in a contracted square-shaped channels. The judicious selection of the bulkier pillar (NbOF₅)^2- caused the looked-for hindrance of the previously free-rotating pyrazine moieties, delimiting the pore system and dictating the maximum opening of the pore aperture-size. The restricted MOF window resulted in the selective molecular exclusion of propane from propylene at atmospheric pressure, as evidenced by multiple cyclic mixed-gas adsorption and calorimetric studies. Remarkably, KAUST-7 maintains its distinctive separation properties in the presence of water as a result of its high chemical and hydrolytic stability.¹
The development of suitable storage and refining processes makes natural gas an excellent alternative fuel, but before its transport and use, natural gas must first be dehydrated. Conventional dehydration agents are energy intensive. KAUST-8 selectively removes water and requires just 105°C for regeneration of the dehydrating agent.²
The deliberate control of the pore aperture-size of various selected MOFs and its impact on various separations will be discussed.

Reference:
1. Cadiau, A.; Adil, K.; Bhatt, P. M.; Belmabkhout, Y.; Eddaoudi, M. “A metal-organic framework-based splitter for separating propylene from propane” Science, 2016, 353, 137-140.
2 Cadiau, A.; Belmabkhout, Y.; Adil, K.; Bhatt, P. M.; Pillai, R. S.; Shkurenko, A.; Martineau-Corcos, C.; Maurin, G.; Eddaoudi, M. “Hydrolytically stable fluorinated metal-organic frameworks for energy-efficient dehydration” Science, 2017, 356, 731-735.

Owing to their high surface areas, tunable pore dimensions, and adjustable surface functionality, metal-organic frameworks (MOFs) can offer advantages for a variety of gas storage and gas separation applications. In an effort to help curb greenhouse gas emissions from power plants, we are developing new MOFs for use as solid adsorbents in post- and pre-combustion CO₂ capture, and for the separation of O₂ from air, as required for oxy-fuel combustion. In particular, MOFs with diamine-functionalized metal sites are demonstrated to operate via an unprecedented cooperative insertion mechanism, leading to high selectivities and working capacities for the adsorption of CO₂ over N₂ under flue gas conditions. Multicomponent adsorption measurements further show these compounds to be effective in the presence of water, while calorimetry and temperature swing cycling data reveal low regeneration temperatures compared to aqueous amine solutions. In addition, a new spin transition mechanism will be elaborated as a means of achieving cooperative CO adsorption.

Membrane separation has emerged as a promising alternative to cryogenic distillation or amine-based wet scrubbing for the CO₂ capture, with potentially high efficiency, lower energy consumption, ease of scale-up and environmental friendliness. However, these membranes typically derived from polymers suffer from an inherent trade-off between permeability and selectivity. To enhance their performance, composite membranes (or mixed matrix membrane, MMM) which consist of filler particles dispersed into an organic polymer phase were proposed since they potentially combine the gas transport and separation properties of the incorporated particles with the good processability and mechanical properties of the polymers. Metal Organic Frameworks (MOFs) were recently proposed as fillers since they present a high separation performance owing to their size/shape exclusion or selective adsorption of gas molecules. However, MOF-based MMMs still pose limitations, which are mainly related to the low MOF loading for numerous MMMs (< 30 wt%). While the permeability of such MOF-based MMMs is usually improved in comparison to pure polymer membranes, an improvement of selectivity is only rarely observed. This mainly results from the fact that the selectivity of this system is driven by the polymer matrix which is the dominant component. In addition, a possible physico-chemical mismatch between MOFs and polymers may lead to the aggregation of MOFs fillers in the polymer matrix and interphase defects (macro or nanovoids). Such voids provide new bypasses through the MMMs that reduce the separation efficiency and compromise performance. There is thus a need to design nanoparticles of MOFs with a good control of the surface chemistry and morphology (diameter and shape) for the shaping of MMMs.
This communication deals with the synthesis of microporous and water stable MOFs nanoparticles for the processing of MMMs. We focused mainly our attention on the aluminum trimesate MIL-96(Al) and the zeolitic imidazolate ZIF-8 which are attractive for the selective capture of CO₂. The synthesis of MIL-96(Al) crystals of different morphology and diameter was achieved by using water as the main solvent. Monodisperse MIL-96(Al) nanoparticles were combined to polymers for the preparation of defect-free MMMs. The microstructure of MMMs and compatibility of MOFs nanoparticles within polymers were also investigated through a complete characterization of MOFs/polymer colloidal solutions by combining complementary experimental (DLS, SAXS, TEM, HAADF-STEM) and computational tools.[1] This study has shown the influence of the surface chemistry of MOFs and the properties of polymers on the physico-chemical matching between MOFs and polymers.
[1] M. Benzaqui, R. Semino, N. Menguy, F. Carn, T. Kundu, J. M. Guigner, N. B. McKeown, K. J. Msayib, M. Carta, E. Malpass-Evans, C. LeGuillouzer, G. Clet, N. A. Ramsahye, C. Serre, G. Maurin, N. Steunou, ACS Appl. Mater Interfaces, 8, (2016), 27311-27321

Flexible metal-organic framework materials are of great interest for gas separation problems as the response of the framework to sorption or other stimuli can influence the further sorption of gas species. We use first-principles density functional theory GGA+U plus empirical van der Waals calculations to investigate the stability of one particular flexible system: the pillared metal-organic framework material Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)₄] (Ni₂(bpene)(CN)₄ or PICNIC-60 for short) as a function of cell volume, bpene orientations and CO₂ content. We find two types of stable orientations for individual bpene molecules: along the monoclinic axis (b) and perpendicular to it (c), with a rotational transition barrier of about 0.4 eV. The most stable pattern of bpene orientations for empty PICNIC-60 changes from all (b) to alternating (b) and (c) as the molar volume increases from 440 ų to 720 ų, with the transition at about 500 ų. The addition of CO₂ changes the picture, with yet a third pattern of bpene orientations- all along (c)- favored for CO₂ concentration greater than 3 CO₂ per mole of bpene. A particularly stable arrangement of 5 CO₂ per mole of bpene is found, beyond which CO₂ sorption becomes much less favorable. The experimental hysteresis observed in CO₂ sorption isotherms is explained in terms of the energy barriers to bpene reorientation and to CO₂-CO₂ interactions. Preliminary results are presented for other sorbant species, including comparisions of the binding energies and geometries for N₂, H₂ and CH₃ .

Metal-organic framework materials (MOFs) are among the most fascinating families of solid state materials, because of their highly tunable compositions, structures, and properties. In this presentation, strategies for the synthesis of new porous MOFs will be discussed, with the focus on the use of different metallic elements and their various combinations. The talk will cover our recent efforts on functionalizing MOF for enhancing gas sorption. The pore space of MOF can be engineered by using extra-framework ligands or nested cage-in-cage configurations to tune the gas sorption properties. The development of the optimum pore architectures for enhanced gas sorption will be covered. To maximize the pore space utilization, we are pursuing a unique synthetic paradigm that is based on the delicate pore space partition through nested pore architecture by using complementary coordination properties of multitopic ligands. In addition to focus on the application in gas adsorption, the potential application of these materials in energy related areas will also be covered.

Metal-organic frameworks (MOFs) have emerged as a special class of hybrid nanoporous materials. The variation of metal oxides and the vast choice of controllable organic linkers allow the pore size, volume and functionality of MOFs to be tailored in a rational manner for designable architectures. MOFs thus provide a wealth of opportunities for engineering new functional materials and are considered as versatile candidates for storage, separation, sensing, catalysis, drug delivery, and other important applications. With ever-growing computational resources and advance in mathematical techniques, molecular simulations have become an indispensable tool for materials characterization, screening, and design. At a molecular level, simulations can provide microscopic insights from the bottom-up and establish structure-function relationships. An overview is presented here on how molecular modeling can a powerful tool in the intelligent design of new smart porous materials for CO2 capture and hydrocarbon separation.


References:
A. Sharma, A. Malani, and R. Babarao, Journal of Physics D: Applied Physics, 2017, 50, 46.
R. Huang, M. R. Hill, R. Babarao, N.V. Medhekar, Journal of Physical Chemistry C, 120, 16658 - 16667, 2016.
R. Babarao, M. R. Martinez, M. R. Hill, A.W. Thornton, Journal of Physical Chemistry C, 2016, 120, 13013 – 13023.
S. Mukherjee, B. Manna, A. V. Desai, Y. Yin, R. Krishna, R. Babarao, and S. K. Ghosh, Chem. Commu., 2016, 52, 8215 – 8218 (Highlighted as Back Cover).
A.W. Thornton, R. Babarao, A. Jain, F. Trousselet, and F- X. Coudert, Dalton Trans., 2016, 45, 4352 – 4359, 2016.
W. Liang, R. Babarao, M, Michael and D. D’Alessandro, Chemical Communication, 2015, 51 (56), 11286-11289.

Molecular sieve membrane-based separation is a potentially energy-efficient alternative to conventional separation processes (e.g., distillation). In particular, the all silica zeolite with structure type MFI has been of interest, as, in addition to its superior thermal and chemical stability, its pore dimensions are close to the sizes of many valuable chemicals. The fabrication of high-performance zeolite MFI separation membranes requires control of microstructure that can be achieved by the secondary growth of a MFI nanosheet coating.

We have developed a direct synthesis method, which readily provides MFI nanosheets with increased lateral dimensions (~2 μm) and similar nano-scale thicknesses (5 nm) compared to the prior stste-of-the-art.² This was enabled by seeded growth with bis-1,5(tripropyl ammonium) pentamethylene iodide (denoted as dC5) as a structure-directing agent. This dC5 SDA was reported to be able to yield high-aspect-ratio MFI crystals with thin dimensions along their b-axes.³ However, the formation of rotational intergrowths prohibits the formation of high aspect ratio flat nanosheets.⁴ We utilized a seeded growth method to suppress the rotational intergrowths in MFI crystals and to grow the MFI crystal in a single nanosheet morphology. Time-dependent growth investigations with TEM established that a nanosheet forms from a corner of the seed crystal and encircles the seed crystal. A single rotational intergrowth appears to trigger a morphology transition from a cylinder to a nanosheet. No additional rotational intergrowth was observed, allowing to achieve high-aspect-ratio MFI nanosheets with ~2-μm lateral dimensions. Upon the fabrication of seed coating with a filtration method, followed by a gel-free secondary growth, the membranes exhibit unprecedented combination of high p-xylene permeance (~5×10^-7 mol Pa^-1 s^-1 m^-2) and separation factor (2,000) for xylene isomer separation.² The separation factor of the membrane was further improved (>10,000), when the membrane was fabricated from dense and uniform nanosheet coatings prepared from the monolayer transfer method.⁵

References
(1) Varoon et al., Science 2011, 334, 72–75.
(2) Jeon et al., Nature 2017, 543, 690–694.
(3) Bonilla et al., Chem. Mater. 2004, 16, 5697–5705.
(4) Chaikittisilp et al., Angew. Chem. Int. Ed. 2013, 52, 3355–3359.
(5) Kim et al., under review.

Porous graphene holds great promise as a one-atom-thin, high-permeance membrane for gas separation and water desalination, but to precisely control the pore size down to a few angstroms proves challenging. Here we demonstrate from classical molecular dynamics simulations that an ion-gated graphene membrane comprising a monolayer of ionic liquid coated porous graphene can dynamically modulate the pore size to achieve selective gas separation. This approach enables the otherwise non-selective large pores on the order of 1 nm in size to be selective for gases whose diameters range from three to four angstroms. We find that the anion serves as a gate to control gas permeation through the pore, while the strong cation-graphene and cation-anion interactions hold the gate in place.

Recently, a few-layered graphene oxide (GO) has been extensively investigated as a membrane material for gas and liquid separation. Although GO membrane is considered as one of promising membranes, it has a limitation to apply for practical application because of low gas permeability and low stability under dry condition. In general, small molecules such as gas, water, and ion are passed along the layered structure, which lead to increase of diffusion path. As such, in this study, we prepared porous GO sheets by generating and decorating the holes on the basal plane of GO sheets using simple modification method without any chemical or thermal reduction, as a result, maintaining original GO functional groups such as epoxy, hydroxyl and carboxyl groups. After stacking the porous GO sheets, we have found that empty holes could be formed between stacked GO layers. The empty holes (or pores) can be a route to reduce diffusion penalty for small molecules and then, it can lead to an increase of mass transport rate than that of pristine GO membranes with high aspect ratio. Especially, GO membrane derived from individual porous GO sheets have a ‘nanopocket’, which can contain large amount of water or CO₂, surrounded by amine functionalized edges around nanopockets. From these reasons, the CO₂ permeability was significantly increased by approximately 30 times compared to the pristine GO membrane and CO₂/N₂ selectivity was increased up to as high as 30. These results have the potential to open up new approaches for an improvement in the transport and absorption properties of porous two-dimensional membrane material for use in next-generation CO₂-selective membranes.

High CO₂ permeability polymer-based membrane materials can address large-scale separations such as CO₂ removal from flue gas and natural gas purification [1]. We have explored several approaches to designing membranes having both high gas permeability and high permselectivity. The alignment of filler materials creates fast and selective gas transport channels to improve membrane performance. Two approaches we explored are:
The vertical alignment of highly CO₂-permeable covalently-anchored Montmorillonite (MT) clay fillers is achieved by interspersing the filler with polymer. The resulting mixed matrix membrane is anchored to a supporting substrate, creating a membrane with transport channels that are selective for carbon dioxide [2]. A high CO₂ permeance is achieved combined with high mixed CO₂/gas selectivity that is stable over a period of 600 h and is independent of water content in the feed gas.
The horizontal alignment of graphene oxide (GO) by deposition of a thin layer on membrane supports allows fast CO₂ transport between the layers, when CO₂ sorbing materials lie within the interlayer spaces [3,4]. These materials are also used to crosslink the GO spaces, thereby fixing the interlayer spacing and affording greater stability.

References
[1] Review: Advances in high permeability polymer-based membrane materials for CO₂ separations, S. Wang, X. Li, H. Wu, Z. Tian, Q. Xin, G. He, D. Peng, S. Chen, Y. Yin, Z. Jiang, M. D. Guiver, Energy & Environ. Sci. 9, 1863 – 1890 (2016).
[2] A highly permeable aligned montmorillonite mixed-matrix membrane for CO₂ separation, Z. Qiao, S. Zhao, J. Wang, S. Wang, Z. Wang, M. D. Guiver, Angew. Chem. Int. Ed., 55, 9321 – 9325 (2016).
[3] A highly permeable graphene oxide membrane with fast and selective transport nanochannels for efficient carbon capture, S. Wang, Y. Wu, N. Zhang, G. He, Q. Xin, X. Wu, H. Wu, X. Cao, M. D. Guiver, Z. Jiang, Energy & Environ. Sci. 9, 3107 – 3112 (2016).
[4] Graphene oxide membranes with heterogeneous nanodomains for efficient CO₂ separations, S. Wang, Y. Xie, G. He, Q. Xin, J. Zhang, L. Yang, Y. Li, H. Wu, Y. Zhang, M. D. Guiver, Z. Jiang, Angew. Chem. Int. Ed., in press DOI: 10.1002/anie.201708048

Carbon dioxide is a major anthropogenic greenhouse gas produced primarily from the combustion of fossil fuels, automobile emission and from industrial exhausts that have posed a huge danger to the environment in recent times. One such way to capture CO₂ in porous materials is by adsorption process. In recent times, a wide range of materials such as zeolites, porous carbon, organic polymers, metal−organic frameworks (MOFs) etc. have been extensively studied for CO₂ capture. Porous carbonaceous materials are especially attractive because of their large specific surface area, regular porosity, and ease of synthesis, abundant availability and thermal stability. An effective CO₂ adsorbent must possess high adsorption capacity, high selectivity, moderate heat of adsorption, fast adsorption and desorption kinetics and excellent chemical and mechanical stability. The nitrogen content of the samples also plays very important role in CO₂ adsorption. One of the most important reasons is that the N content improves the electron density of the carbon framework and thereby increases the basicity of the framework. As a result, it becomes easier for the electron deficient C atom of CO₂ to get anchored in the framework by Lewis acid (CO₂)-Lewis base (N) interactions. Ionic liquids (ILs) are molten salts mainly composed of organic cations and organic or inorganic anions having melting point lower than 100 °C. Room temperature ionic liquids exhibit some significant properties such as low vapor pressure, high chemical and thermal stability, tunable properties, good ionic conductivity and high CO₂ solubility. The interaction of IL with CO₂ is quite fast that effectively increases the sorption kinetics. The present work basically focuses on a solvent-free facile synthesis of ionic liquid functionalized nitrogen doped porous carbon by a simple thermal decomposition technique. This ionic functionalized micro-mesoporous nanocomposites sample showed large CO₂ adsorption capacity along with fast adsorption kinetics when compared to only porous carbon. The ionic liquid used in this particular work is 1-Butyl-3-Methylimidazolium bis(trifluoromethyl sulfonyl)imide ([BMIM][TFSI]). It has already been reported that surface functionalization with ionic liquids basically increases the anchoring sites which leads to a higher CO₂ adsorption capacity. The CO₂ adsorption-desorption studies were carried on at different temperatures and in both high as well as sub-ambient pressure conditions. These studies show that the nanocomposite exhibit excellent cyclic stability in storage performance. In addition, thermodynamic studies suggest that the adsorption takes place mainly by physicochemical adsorption mechanism and therefore easy regeneration process is also possible which is considered as another critical property of an adsorbent. Thus it can be concluded that this particular material has the potential to serve as a good CO₂ adsorbent both in pre as well as post-combustion CO₂ capture.

Porous nanosheets made of effectively aligned zeolitic imidazolate framework-8 (ZIF-8), dubbed ZPGO, with an exceptionally high surface area (2170 m² g⁻¹) were demonstrated. This composite was prepared by growing ZIF-8s on highly porous graphene oxide (PGO) and the mixed matrix membrane with such a nanofiller showed drastically improved size-selective CO₂ transport even at very low filler concentration (0.02 wt%) especially under mixed gas conditions where both selectivity (CO₂/N₂ ∼57) and CO₂ permeability (∼163 barrer) were significantly enhanced. In addition to this, ZPGO effectively suppressed CO₂-plasticization, which indicates great potential in real operations. This new concept of nanofillers is expected to maximize the filler effect in other fields benefitting from a high surface area as well.

Since their discovery over a decade ago, thermally rearranged polymers have become established as a new class of glassy polymer membrane with superior permeability-selectivity combinations for mixed gas transport compared to conventional glassy polymers. Permeability-selectivity combinations surpassing Robeson’s upper bound have been attributed to the porosity that is developed during thermal rearrangement resulting in nanospace reminiscent of bottlenecks connecting adjacent chambers, such as those found in nature in the form of ion channels and aquaporins. Applications of these polymer membranes for clean energy include CO₂ separation to make existing power generation industries cleaner. In research designed to understand and optimize the performance of these membranes we have measured the development of free volume in thermally rearranged polymers using positron annihilation lifetime spectroscopy and small angle X-ray scattering. In addition, we have performed a theoretical study, based on the free volume elements, in order to explain why these membranes perform so well. The hour-glass shaped pores are shown to provide a series of size-selective necks connected with larger flux-assisting cavities. Remarkably, with the correct neck and cavity size, the upper bound can be exceeded for many different gas mixtures. The ability to measure and tailor free volume at the Angstrom scale is shown to be critical for prediction of membrane performance.

Porous organic cages (POCs) are individual molecules that are porous and soluble in certain common solvents. Compared to their extended framework counterparts, such as zeolites and metal-organic frameworks (MOFs), POCs offer the advantage of solution processability. Moreover, when fabricated into mixed matrix membranes, the soluble POC molecules have the potential to exhibit molecular-level intimate mixing with matrix polymer. The incorporation of POCs into mixed matrix membrane is still in its infancy and lacks demonstration of comprehensive improvement of membrane performance. In this work, we utilized vertex functionalized amorphous scrambled porous organic cages (ASPOCs) in mixed matrix membranes to study a series of key questions in this field. The dispersion of ASPOCs possessing different crystallization tendencies within a polymer matrix are probed using Raman imaging and Energy Dispersive X-Ray (EDX) mapping. Gas permeation experiments of N₂, CO₂, CH₄ and SF₆ were carried out as a function of ASPOC loading and crystallization tendency. A 4-8 fold of permeability increase was observed for N₂, CO₂ and CH₄ compared to pure polymer membrane. Moreover, a clear molecular sieving effect was observed for SF₆, resulting in 2-4 fold of increase of N₂/SF₆ selectivity compared to the pure polymer membrane. The membranes were further examined in organic solvent nanofiltration experiments using a cross-flow permeation approach. The Molecular Weight Cut Off (MWCO) of the membranes were calculated based on the polystyrene permeation tests. Overall, these membranes demonstrated homogeneous mixing between the POC molecules and the polymer matrix, and showed potential to be used in molecular separation processes.

Adsorption is considered a promising technology for capturing CO₂ and appropriate adsorbent is one of key factors for successful development of adsorption method. In this study, CO₂ adsorption using polyethylene terephthalate (PET) plastic-derived porous carbon materials was investigated from both equilibrium and kinetic perspectives. The PET plastic was utilized for developing CO₂ adsorbents by carbonization and chemical activation processes. The carbonization process was conducted under 600 °C and N₂ atmosphere within 1 h. Both the KOH and NaOH were selected for activating the carbonized PET plastic, and the activation temperature was varied from 700 to 1000 °C. Varying the activation temperature had a dramatic effect on the textual properties of the prepared adsorbents. The adsorption isotherms were well fitted by the Langmuir isotherm model (R² > 0.999), the experimental CO₂ adsorption data were well described by the pseudo-second-order kinetic model (R² > 0.999), compared with the Elovich and intra-particle-diffusion models, and CO₂ adsorption appeared to be mainly controlled by physisorption. The PET-KOH-700 adsorbent exhibited the highest CO₂ adsorption uptake of 4.75 mmol g^-1 at 25 °C and 1 atm. The outcome of this study revealed that the PET-KOH-700 adsorbent, synthesized from easy recycling and obtainable PET plastic, showed remarkably high CO₂ adsorption uptake, good CO₂/N₂ selectivity at relatively low CO₂ pressures, excellent recyclability, easy regeneration, and rapid adsorption-desorption kinetics, which will be very promising for industrial CO₂ separation applications.

Catalytic steam gasification has been considered as one of the most promising clean technologies that could convert the solid fuel to obtaining an H₂-rich syngas, which has long been an important source of hydrogen for various industrial sectors. K₂CO₃ was utilized as a catalyst due to that it has the ability to significantly increase gasification reaction rates and good mobility to disperse throughout the sample under gasification operating conditions. After conducting the K₂CO₃ catalyzed steam gasification of petroleum coke, the highest H₂ content reached 19.38 vol.% (67.37 vol.% of N₂) under 10 wt% K₂CO₃ loading and gasifying temperature of 900 ^oC, obtaining the carbon conversion of 92.91%. In this study, the CO₂, mainly caused the hottest global warming issue, was also captured from the produced syngas for improving the H₂ concentration in the final H₂-rich syngas. Adsorption is considered a promising technology for capturing CO₂ and appropriate adsorbent is one of key factors for successful development of adsorption method. N-doped microporous carbons derived from petroleum coke was developed for investigating the CO₂ capture from the syngas, from both equilibrium and kinetic perspectives. The N-doped and KOH activated petroleum coke showed high CO₂ capacity, excellent CO₂/N₂ selectivity, easy regeneration, and rapid adsorption-desorption kinetics, which will play a significant role in the petroleum coke-to-H₂ rich syngas system and also be very promising for industrial applications.

In industrial electric production, cogeneration and fossil fuel power plants occupy a high proportion, resulting in excessive carbon dioxide emissions in the atmosphere. As a result, environmental issues such as global warming have emerged as major problems of our society. Various studies have been conducted in order to solve this problem. Among them, a method of utilizing magnesium oxide (MgO) as an adsorbent to capture carbon dioxide has been extensively studied. MgO has several advantages such as excellent theoretical CO₂ adsorption capacity, thermally stable MgCO₃ form, and low regenerative energy. However, MgO has also disadvantages such as slow reaction kinetic and loss of basic site on MgO surface during regeneration, large lattice energy and irregular particle size. Thus it is difficult to exert excellent performance in capturing CO₂ with MgO alone. Recently, it has been found that the promoter can utilize MgO toward CO₂ adsorption by dissociating it into Mg^{2 +} and O^2-. In addition, hydrotalcite as a support can function as a framework for forming MgCO₃ with high stability, CO₂ capture ability, and economic efficiency, thereby further enhancing the CO₂ adsorption ability of MgO. Fundamentally, CO₂ capture on MgO begins with surface chemisorption and ends up with forming MgCO₃ throughout the MgO body. Adsorption-mediated carbonation can be promoted due to the strong solvation effect when the promoter and support are mixed. However, the CO₂ adsorption mechanism on neither pure MgO nor the MgO complex of promoter and support has not been explicitly described for the formation of surface specific MgCO₃ (on pure MgO) and bulk MgCO₃ (on MgO with promoter and support). In this study, adsorbents were prepared by using promoter (NaNO₃) and hydrotalcite (MgO: Al₂O₃, the weight ratio of 30:70 and 70:30) as a promoter for dispersion of MgO and stability of adsorbent. Adsorption performance of hydrotalcite used as a support was investigated by non-isothermal test according to weight ratio of MgO: Al₂O₃. Adsorbent characteristics were analyzed by XRD, cyclic test, fixed bed adsorption test, and TGA. Also, adsorbents were subjected to in-situ transmission electron microscopy (TEM) for the direct observation of fundamental CO₂ adsorption in a complex of MgO, promoter and support under non-vacuum (CO₂ atmosphere) and high temperature (> 250^oC) conditions. Morphological and crystallographical changes at the interface of synthesized mixture were observed in nano-scale by using SAED and EDS. The performance of the adsorbent and the surface change of complex by analysis of CO₂ adsorption mechanism will be discussed in detail. This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRT-2016R1C1B2008694).

For the carbon dioxide capture and separation, lots of researches have been carried out. Most mature process for CO₂ separation is an absorption technology consisting of an amine based absorbent and a pact column. Though this has high absorption efficiency, amine base absorbent is highly corrosive and degradable at operation temperature. Also it requires high energy consumption to regenerate absorbent and degassing CO₂ from the absorbent. Pact column for CO₂ absorption requires high building cost for its huge CO₂ absorbing column. To overcome these problems, combination of membrane contactor and ionic liquid system for CO₂ separation is having great attention. Because, these can make more compact and cost-effective system. Ionic liquid has an advantage of non-corrosive, non-volatile and low degradation sensitive nature compared with conventional amine based absorbent. And membrane contactor also has larger effect surface area than pack column system. To retain CO₂ absorbing efficiency, membrane between liquid absorbent and CO2 gas should be intact without smearing from an absorbent to gas phase and without swelling of membrane by an absorbent. To prevent smearing and swelling, it needs to control membrane surface properties. Especially membrane should be hydrophobic to prevent wetting by hydrophobic ionic liquids. In this study, we used polypropylene (PP) membrane for a flat sheet membrane contactor(MSMC). Also PP membrane surfaces were modified using organosilanes or fluorosilanes and modified surfaces were confirmed using FT-IR and FE-SEM. Hydrophobicity of modified PP membrane was compared with water contact angle. Also contact angle between Ionic liquid and PP membrane was also investigated for the application to CO₂ absorbing membrane contactor.

Hydrogen is a potential sustainable and clean source of energy that can decrease reliance on hydrocarbons. Although advances in renewable sourcing of hydrogen are encouraging, safe storage for hydrogen gas remains a significant hurdle for widespread use in a hydrogen economy. Currently, hydrogen gas must be substantially compressed at low temperatures for safe storage, which poses a large problem for wide spread use. A simple solution lies in metal and covalent organic frameworks (MOF and COF): highly porous materials with promising gas storage capabilities at decreased pressure and higher temperature compared to traditional methods. Hydrogen uptake and operability for these frameworks do not yet meet the U.S. Department of Energy (DOE) 2020 targets of 0.03 kg H₂/L of system and operating temperatures of -40-60°C. To reach these goals, key parameters to optimize are MOF/COF ligand design and surface area, as these correspond to improved hydrogen uptake. We have developed new COF ligands that result in high surface area COFs with potential enhanced hydrogen storage properties. We show hydroxy and fluorine groups on the linkers enhance hydrogen binding within the framework, while judicial solvent choice boosts surface area. Future work will test for metal integration into the framework to further augment hydrogen bonding capabilities. Progress in designing functional COFs with high surface area, preferential hydrogen bonding groups, and metal incorporation improves hydrogen’s potential as a clean, widespread energy source.

Density functional theory combined with the non-equilibrium Green’s function formalism is used to study the adsorption and gas-sensing properties of H₂ gas molecule on pristine and doped ZnO nano-ribbons (NRs). Substitutional doping of oxygen site with C, N and F have been tested versus adsorption of H₂ molecule and other molecules (e.g., N₂, O₂, H₂O, H₂S). The results of relaxation show chemisorption to occur only on C-and N-doped samples. While all these molecules exhibit chemisorption on C-doped ZnO-NR, only H₂ and O₂ molecules are chemisorbed on N-doped ZnO-NRs. The chemisorption of O₂ is associated with the breaking of one π-bond and thus desorption in reversal process is plausible. However, the chemisorption of H₂ is associated with a complete dissociation and introduces donor states into the gap (i.e., it plays a role of n-type dopant) and consequently enhancing the conductivity. These characteristics made N-doped ZnO-NRs have high sensitivity and selectivity towards the detection of H₂ gas. Furthermore, the calculated IV-curves have paved the way for estimating the sensitivity and consolidated our results. Since the change of conductance is one of the main outputs of sensors, our findings will be useful in developing Hydrogen-based solid-state sensors.

Molecules confined in the nanoscale regions usually exhibit intriguing physical and chemical properties. The present paper reports on the direct observation of charge transfer behaviors in spatial confined xenon (Xe) atoms. Individual Xe atoms are trapped at 300 K in nano-cages consisting of silica hexagonal prisms forming a two-dimensional (2D) array on a planar surface. In-situ synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) reveals that the charge transfer takes place at the Xe atoms which are extremely confined between the 2D framework and the Ru(0001) support, in which the interfacial distance can be controlled by the coverage of chemisorbed oxygen on Ru(0001). Density functional theory (DFT) calculations further corroborate these significant charge transfer interactions between the trapped Xe atoms and silica/Ru(0001) systems, opening exciting opportunities for the study of individual noble gas atoms in confinement.

To solve the problem of dehydration in natural gas purification, several MOFs and zeolite molecular sieve membranes has been proposed. Behaviors of adsorption and diffusion for H2O in Natural Gas on HKUST-1 or LTA and T-Type Zeolite Molecular Sieve Membrane were investigated by grand canonical Monte Carlo (GCMC) method. It is found that the classical force fields are not suitable for the system of adsorptions of H2O in Natural Gas on these membranes. The optimized force field parameters were obtained by fitting the experimental data. Results showed that strong competitive adsorption behavior existed in HKUST-1 or LTA and T-Type zeolite for H2O/CH4 mixture. Water loading increased with water content of feed, it was in agreement with the trend of permeation flux for pervaporation dehydration. The diffusivity of pure water molecules was greater than that of pure CH4 . Furthermore, the diffusion behavior of H2O/CH4 mixture through membrane was prominently affected by the interaction between the two components. The CH4 molecules could slow down diffusion of water molecules significantly, thus it would affect permeation flux of these membranes.

The development of effective technologies or strategies for capturing or eliminating CO₂ produced from various processes in order to minimize its influence on climate change remains an urgent and challenging task. To address this issue, in this present study, we have designed and synthesized a novel nitrogen-rich heteroaromatic tetranitrile, namely 4,4',4'',4'''-(1,4-phenylenebis(pyridine-4,2,6-triyl))tetrabenzonitrile, to prepare a set of nitrogen-rich CTFs as high-performance platforms for selective gas sorption and storage. The influence of several parameters such as the ZnCl₂/monomer ratio and reaction temperature on the structure and porosity of the resulting frameworks was systematically examined. Remarkably, the CTF material obtained using 20 molar equiv. of ZnCl₂ at the reaction temperature of 400 ^oC exhibits an excellent CO₂ adsorption capacity (3.48 mmol/g at 273 K and 1 bar) as well as a significant high H₂ uptake (1.5 wt% at 77 K and 1 bar). These values are among the top levels for all the CTFs measured under identical conditions to date. In addition, the obtained CTFs also present a relatively high CO₂/N₂ selectivity (up to 36 at 298 K) making them promising adsorbents for gas sorption and separation.

Power plant fossil fuel combustion is a major source of anthropogenic CO₂ emissions, and as such, much current research has focused on mitigating these CO₂ emissions. As part of our continued research into porous materials we have begun investigating a series of porous polymer network (PPN) sorbents for post-combustion carbon capture. Our leading sorbent series, the PPN-150 series, are mesoporous melamine-formaldehyde resins which, upon loading with amines, can act as highly efficient sorbents for CO₂ capture under flue gas conditions. The high nitrogen content of the parent polymer greatly improves the amine tethering ability of the material compared to porous carbon while still not requiring the extensive synthesis seen in amine functionalized sorbents. The tethering ability can also be tuned by adding in dopants, functionalized small molecules that are dispersed throughout the polymer structure and provide separate sites for amine tethering or CO₂ chemisorption. By adjusting the loaded amines and dopants identities the sorbents are capable of CO₂ uptakes greater than 0.1 g/g sorbent. In addition, cycling studies have been performed with these sorbents showing loss of CO₂ uptakes less than 4.5% over 33 cycles under dry gas conditions, with no noticeable loss in cycling performance observed for wet gas cycling. The PPN-150 series can be produced at the bench scale, taking advantage of the low-cost reagents required for the synthesis, while still maintaining high gravimetric CO₂ uptakes.

Membrane-based gas separation processes have been implemented for the past three decades in a variety of large-scale industrial applications, such as carbon dioxide removal from natural gas, hydrogen recovery from petrochemical off-gas streams and nitrogen production from air. Interestingly, only a few membrane types are currently in use made mostly from commercially available polymers, such as cellulose triacetate, bisphenol-A polysulfone, polyimides, poly(phenylene oxide) polydimethylsiloxane, polyether-polyamide block copolymers etc.
Intrinsically microporous polyimides (PIM-PIs) with pore size < 20 Å were recently introduced into by incorporating highly contorted building blocks to provide polymers with significantly enhanced permeability while maintaining acceptable selectivity and all other desirable properties of high-performance polyimides, such excellent mechanical strength and high thermal stability; however, development of these PIM-PIs is currently limited by the availability of building blocks, such as spirobisindane, ethanoanthracene and triptycene-based monomers. In this work, novel membrane materials for gas separation processes were custom-designed from binary polyimide blends of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) reacted with a contorted hydroxyl-functionalized diamine and a carboxyl-functionalized diamine. These PIM-PI blends exhibited exceptional gas separation properties, specifically extremely high gas-pair selectivity values with improved permeabilities compared to commercial glassy polymer membrane materials. This extraordinary performance resulted from strong hydrogen-bonding-induced interactions between the hydroxyl and carboxyl groups, which generated an ultra-microporous polymeric molecular sieve structure. XRD spectra of resulting blends confirmed creation of average interchain d-spacing of smaller than ~3.5 Å, which made these blends materials highly selective for small gases. An optimized blend displayed a permselectivity of 136 for CO₂/CH₄, 11.4 for O₂/N₂ and 636 for H₂/CH₄ with diffusivity selectivities of 48.5 and 11 for CO₂/CH₄ and O₂/N₂, respectively.

Porous polymers have received an increased interest as CO₂ sorbents due to combining the properties of both porous structure and polymers. High internal phase emulsion (HIPE) as a template by Oil-in-Water (O/W) system and photopolymerization technique was used to prepare crosslinked Acrylamide-co-Acrylic acid (AAM-co-AAC) hydrogel monolith. Also, photopolymerization followed by freeze granulation was utilized for the fabrication of the AAM-co-AAC hydrogel granules. The structural analysis of synthesized scaffolds confirmed crosslinked copolymer. The scanning electron microscopy (SEM) micrographs presented porous structure for monolith consists of a “skeleton copy” of the O/W HIPE where granules revealed many severe wrinkles on the surface. Monoliths and granules exhibited BET surface areas around 24 m²/g and 28 m²/g respectively. The CO₂ uptake ability of synthesized scaffolds for dry scrubbing as well in aqueous media (wet scrubbing) was investigated. The CO₂ adsorption capacity of monoliths and granules reached to saturation at 25 kPa to 0.5 and 0.8 mmol/g, respectively which were higher than adsorption values reported for amide-based porous polymers [1,2]. The electron-rich groups like NH₂, C=O, and R-O-R in the structure of block copolymer can interact with CO₂ molecules and give rise to the higher adsorption capacity. The CO₂ and N₂ adsorption isotherms for granules and monoliths indicated the selectivity of AAM-co-AAC copolymer for CO₂ gas. Furthermore, the granules were able of capturing CO₂ in aqueous media where the absorption of CO₂ on water-swollen granules increased with increasing the amount of water. The CO₂ capture capacity reached to 1.8 mmol/g in completely swollen granules. The uptake of CO₂ in aqueous solution contributes from the physical dissolution of CO₂ in water along with interactions between the functional groups of the hydrogels and gas molecules. The results revealed the CO₂ dry and wet scrubbing ability of the synthesized hydrogel copolymers [3].

References
1. S. Zulfiqar, et al., RSC Adv. 4 (2014) 52263-52269.
2. F. U. Shah, et al., Magn. Reson. Chem. 54 (2016) 734-739.
3. D. Nikjoo, et al., J. CO₂ Utilization 21 (2017) 473–479.

The removal of CO₂ from combustion streams has become a global challenge with significant environmental implications. While a number of approaches have been explored to separate CO₂ from mixed streams, membranes have emerged as a promising technology because of their small footprint, reliability and energy efficiency. Hybrid, or mixed matrix, membranes combine the advantages of a porous material with the stability and reliability of a dense matrix to form an advanced membrane material with enhanced permeance and selectivity. Hollow carbon spheres with a microporous shell incorporated into a polymer matrix will be presented as a unique strategy to fabricate high performance membrane materials. The empty space within the hollow spheres promotes fast gas transport while the symmetry of the carbon shell along with the strong interaction between the carbon surface and a unique block co-polymer allows for the formation of a mechanically robust membrane with enhanced permeance and selectivity compared to the dense polymer. In addition, graphene- and graphene-oxide-based hybrid membranes provide a distinct opportunity to enhance membrane performance through unique architectures. Incorporating ionic liquids into graphene and graphene-oxide membranes can lead to novel gas transport mechanisms that improve permeance and selectivity leading to a new class of ultrathin, hybrid membranes.

There is a growing need to advance CO₂ capture materials and technology to mitigate the impact of climate change especially due to the escalating levels of CO₂ that is strongly tied to the rapid pace of urbanization. Cities are reportedly responsible for more than 80% of global greenhouse gases (GHG) and CO₂ is the most prominent component of anthropogenic GHG emissions. The elevated CO₂ levels leads to higher absorption and thermal trapping which contributes partly to the urban heat island effect. This has led to increased research efforts to remove CO₂ directly from the atmosphere using technical means such as direct air capture, to keep up with the pace of urbanization. Since the efficiency of CO₂ capture is proportional to the purity of the gas stream, key challenges of direct air capture are the relatively low atmospheric CO₂ concentration and low air circulation in most dense urban environments. Amongst the existing forms of carbon capture materials (CCMs), such as liquid sorbents or bulky ceramic-based honeycomb structures, mixed matrix polymeric composites are attractive material system that allow us to tailor the carbon capture performance and yet remain in a form that is versatile and be easily adapted for eventual implementation. The separation performance and properties of the mixed matrix composites depends on intrinsic properties of filler and matrix, filler loading and filler-matrix interaction.
In this work, we will discuss the effects of surface modification and processing strategies on the carbon capture performance of polyethylenimine (PEI)-modified porous silica sorbents (PEI-SiO₂) that were dispersed in an elastomeric Pebax® matrix. PEI functionalization of high surface area silica is required for CO₂ capture by harnessing the high interaction affinity between the amine groups of PEI with CO₂. In this work, we will discuss how the PEI molecular weight and solvents used during the modification affect the PEI-SiO₂ interfacial interactions and its impact on the CO₂ capture capacity. By controlling the PEI-SiO₂ interfacial interactions, we have optimized our PEI-SiO₂ sorbent chemistry to achieve a CO₂ capture capacity of 167 mg of CO₂/g material (75°C), which is comparable to existing reported benchmarks of 132 to 141 mg of CO₂/g material (75°C). These PEI-SiO₂ sorbents were next dispersed within Pebax® to form free-standing films to facilitate eventual wide implementation of these CCMs. The composite formulation and filler dispersion was subsequently tailored to control the capture capacity, CO₂ permeability and CO₂/N₂ selectivity of mixed matrix composite. Our preliminary studies that show successful ambient CO₂ capture in a simulated indoor environment. The captured CO₂ was subsequently recovered through desorption by purging the system using an inert gas such as N₂. The recovered CO₂ could be used as an alternative feed source for other carbonation chemistries.

Two-dimensional (2-D) metal-organic frameworks (MOFs) combine the tunable structure and chemical functionality of MOF materials with the high aspect ratio of the 2-D morphology, which is beneficial for a wide range of applications, such as gas separation, surface sensing, and catalysis. In the field of gas separation, the presence of 2-D MOFs in MOF/polymer mixed-matrix membranes will lead to selective transport of preferably adsorbed species in MOFs, such as CO₂, but not other gases, resulting in improved selectivity with uncompromised permeance. However, challenges still exist in the facile synthesis of 2-D MOFs due to the isotropic nature of most MOF structure building units. In this work, we demonstrated a one-step room-temperature synthesis of 2-D MOFs based on copper paddlewheel units, where intrinsically anisotropic building units were utilized to control the morphology of MOF nanoparticles. This synthesis method was successfully extended to functionalized 2-D MOFs by selecting ligands with functional groups targeted for CO₂ separation applications. To illustrate the advantage of using 2-D MOFs, the synthesized 2-D MOFs were incorporated into polymer matrices and the resultant mixed matrix membranes (MMMs) exhibited improved CO₂/CH₄separation performance with ideal selectivity increased by approximately 400%. The mechanisms underlying the increased selectivity of MMMs can be attributed to the greatly enhanced sorption toward CO₂ molecules as a result of the incorporation of functionalized 2D MOFs, which overwhelmed the impeded gas diffusion owing to reduced chain mobility. Due to the high tunability of MOF structure with retained 2-D morphology, this approach of making 2-D MOFs and its composite materials will be further developed in the separation of other industrially and environmentally important gases.

To be effective in practical applications, advanced gas sorbent materials must selectively adsorb large quantities of the target gas usually from a flowing gas stream containing multiple gases. The capacity to adsorb, e.g., CO₂ under different pressure and/or temperature conditions can frequently be linked to changes in sorbent structure, which must be understood if sorbents are to be improved and optimized by material development. In this connection, metal-organic frameworks (MOFs) and MOF-like materials show considerable promise and potential for future development. Obviously, a lack of selectivity to adsorb only the target gas will result in other gases or water molecules being adsorbed and occupying sites within the sorbent that cannot then be occupied by the target gas. Selective adsorption cannot simply be established by measuring sorption isotherms for different single gases; it is necessary to determine whether one gas being adsorbed (e.g., CO₂), also lets in a second gas by some collective gate-opening effect. Such sorption selectivity (or lack of it) can frequently be related to sorbent structure and microstructural or structural changes during the adsorption/desorption cycle. Building on previous work to elucidate these issues, ^1-4 we have carried out a range of in situ operando small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron diffraction (XRD & ND) studies on different MOF-like sorbent systems during adsorption and desorption of CO₂ and CO₂ gas mixtures under realistic pressure conditions. (Recent experimental advances actually allow the combined SAXS & XRD data to be obtained together with a time resolution of less than 5 minutes.⁵) Pressure conditions include both static and flowing gas environments, and also supercritical CO₂ conditions. By combining these results with density functional theory (DFT) calculations, we have gained new insights in connecting features in the isotherm curves to underlying changes in sorbent microstructure or structure.

[1] K.L. Kauffman, J.T. Culp, A.J. Allen, L. Espinal, W. Wong-Ng, T.D. Brown, A. Goodman, M.P. Bernardo, R.J. Pancoast, D. Chirdon & C. Matranga; Angew. Chem. Int. Ed., 50, 10888-10892 (2011).
[2] L. Espinal, W. Wong-Ng, J.A. Kaduk, A.J. Allen, C.R. Snyder, C. Chiu, D.W. Siderius, L. Li, E. Cockayne, A.E. Espinal & S.L. Suib; J. Am. Chem. Soc., 134, 7944-7951 (2012).
[3] W. Wong-Ng, J.T. Culp, Y.S. Chen, P. Zavalij, L. Espinal, D.W. Siderius, A.J. Allen, S. Scheins & C. Matranga;” CrystEngComm, 15, 4684-4693 (2013).
[4] A.J. Allen, L. Espinal, W. Wong-Ng, W.L. Queen, C.M. Brown, S.R. Kline, K.L. Kauffman, J.T. Culp & C. Matranga; J. Alloys and Compounds, 647, 24-34 (2015).
[5] J. Ilavsky, F. Zhang, R.N. Andrews, I. Kuzmenko, P.R. Jemian, L.E. Levine & A.J. Allen; J. Appl. Cryst., submitted (2018).

Understanding co-adsorption in nanoporous materials such as metal organic framework (MOFs) is important for most applications since they are rarely used solely for pure gases and they are typically subject to gas contamination. Co-adsorption, however, leads to a variety of processes that complicate the analysis, such as molecular competition for adsorption and diffusion. Due to a lack of in situ characterization within these 3D nanoporous structures, these processes remain largely unknown. We report here a novel synergistic effect involving co-adsorption in a prototypical metal organic framework (MOF) material, i.e. MOF-74. We find that the addition of NH₃ or H₂O to MOF-74 previously loaded with a variety of small gases (e.g., CO, CO₂, SO₂) first displaces a certain amount of molecules always and then prevents the removal of the remaining small gases upon evacuation. We further show, with support of first-principles modeling, that this phenomenon is not due to guest-guest binding as usually being regarded as “cooperative binding effect”, but instead to an increase in diffusion barrier for these small molecules. Combining in situ infrared spectroscopic measurements and first-principles modeling, we demonstrate that hydrogen bonding is primarily responsible for the large increase of the diffusion barrier (by a factor of ~7 for CO and CO₂) along the transport pathway. These relevant findings contribute to fundamental understanding of co-adsorption in porous materials and to dispel common assumptions. For instance, H₂O and NH₃ are usually regarded as impurities that are detrimental to the performance of gas adsorption and capture by poisoning active sorption sites. This is clearly not the case here. Therefore, the effect of H₂O and NH₃ (or other hydrogen containing molecules prone to hydrogen bonding) on the gas adsorption performance of MOF materials needs to be reevaluated.

Advanced porous materials as represented by metal–organic frameworks (MOFs) and porous organic polymers (POPs) represent a new class of materials, and one of their striking features lies in the tunable, designable, and functionalizable nanospace, which allows designed incorporation of different functionalities for targeted applications, such as gas storage/separation, sensing, drug delivery, catalysis, conductivity. We will show how MOFs and POPs can be task-specifically designed and functionalized for applications in hydrocarbon separations, particularly paraffin/olefin separations.

The rapid increase in global industrialization necessitates technology shifts in energy production, manufacturing, and carbon management techniques. Large energy costs in refineries, power plants, and manufacturing facilities using traditional separation techniques are currently a major opportunity for innovation. Approximately 10% of global energy use can be attributed to separation processes, with the vast majority of separations being “thermal” in nature (e.g., distillation). Significant energy and cost savings can be realized using advanced separation techniques such as membranes and sorbents. One of the major barriers to acceptance of these techniques remains linking engineering materials to actual processes that are effective in the presence of aggressive industrial feeds.
The creation of robust materials-enabled advanced separators and their manufacturing into low-cost, energy-efficient devices to meet this global challenge will be the focus of the talk. Engineering novel materials—such as zeolitic imidazolate frameworks, polymers of intrinsic microporosity, and carbon molecular sieves—into hollow fiber separation devices shows promise for emerging separation applications. These include natural gas liquid fractionation, olefin/paraffin separation, carbon capture, and organic solvent purification. Specifically, a highly heat integrated sub-ambient pressure swing adsorption process that enables ultra-high swing capacities using robust metal-organic frameworks will be discussed. Synthesis and formation of advanced composite materials, mass transfer of small molecules through these materials, and an outlook for energy- and cost- efficient separations will be discussed. The dual advance of novel materials engineering and scalable separation device manufacturing will enable use of membranes and sorbents in critical industrial separation processes.

Most membranes show carbon dioxide perm-selectivity at low temperatures (such as the room temperature). High temperature carbon dioxide permeable membranes offer advantages for effective carbon dioxide capture from fossil fuel burning power plants at high temperatures. The paper will focus on a new group of dual-phase membranes that permeate only carbon dioxide at high temperatures (>500^oC). These dual-phase membranes consist of a metal or oxygen ionic conducting oxide phase and a molten carbonate phase which conduct respectively electrons or oxygen ions and carbonate ions. The paper will summarize key properties of the dual-phase membranes, including the effects of the ionic conductivity of materials, pore structure of the ceramic phase, membrane thickness and the operation conditions on the permeance of carbon dioxide. Applications of these membranes in membrane reactors for water gas shift reaction or dry-reforming of methane for pre- or post-combustion carbon dioxide capture will be discussed. Finally, the paper will address the key issues of the stability of the membranes in industrially related atmospheres.

Pore density and pore size are two factors for gas molecular permeation and separation in nanoporous membranes. The nanopores on the membranes can be tuned to control gas molecular permeation and separation. But how the pore density can influence the permeation and how the pore size of multilayer nanoporous membrane can be tuning to influence the selectivity have not been fully addressed from a computational simulation. Here we use molecular dynamics (MD) simulation to investigate gas molecular permeation and adsorption behaviors by changing the pore densities of a nanoporous graphene membrane from 0.01 nm^-2 to 1.28 nm^-2 and adjusting the effective pore size by tuning the offset of bilayer graphene membrane. We find that the higher pore density leads to higher permeation for both CO₂ and He, but the increase rate slows down only for CO₂. And We also find that offset can influence the effective pore size, which can be used to build bilayer gas separation membrane flexibly.

An efficient propylene/propane separation is a very critical process for saving the cost of energy in the petrochemical industry. For separation based on the pressure-swing adsorption process, we have screened ~1 million crystal structures in the Cambridge Structural Database and Inorganic Crystal Structural Database with descriptors such as the surface area of N₂, accessible surface area of propane, and pore limiting diameter. Next, grand canonical Monte-Carlo simulations have been performed to investigate the selectivities and working capacities of propylene/propane under experimental process conditions. Our simulations reveal that the selectivity and the working capacity have a trade-off relationship. To increase the working capacity of propylene, porous materials with high largest cavity diameters (LCDs) and low propylene binding energies (Q_st) should be considered; conversely, for a high selectivity, porous materials with low LCDs and high propylene Q_st should be considered, which leads to a trade-off between the selectivity and working capacity. In addition, for the design of novel porous materials with a high selectivity, we propose a porous material that includes elements with a high crossover distance in their Lennard-Jones potentials for propylene/propane such as In, Te, Al, and I, along with the low LCD stipulation.

In the last two decades different lithium and sodium ceramics have been proposed as potential high temperature CO₂ captors, under different physicochemical conditions. Moreover, in the last five years, it has been evidenced that some of these alkaline ceramics can be used as possible bifunctional materials for CO₂ capture and subsequent catalytic conversion to added value products. This kind of alkaline ceramics have been tested in different catalytic reactions (where CO₂ has been previously chemically trapped) such as methane reforming and water gas shift processes. These processes (capture reactions and the subsequent methane reforming process) are environmentally important, as CO₂, CO and CH₄ are catalytically converted into an added value product, the syngas. Moreover, it has been evidenced that the same alkaline ceramics can be used for CO oxidation-capture process, which would be highly useful in hydrogen enrichment process. Therefore, the aim of this presentation is to show the most recent advances obtained about the next three different aspects: 1) CO₂ capture on alkaline ceramics; 2) the use of these ceramics as bifunctional materials for the CO oxidation-capture process; and 3) The methane reforming reaction for the syngas production, using the CO₂ chemically trapped in the alkaline ceramics.

In recent years, sorbent-based CO₂ capture technologies using metal oxides have been extensively studied. Among various metal oxides which can capture CO₂ via adsorption, magnesium oxide (MgO) is among the most widely investigated because of its potential for pre-combustion CO₂ capture applications such as coal gasification and natural gas reforming [1]. While MgO has a theoretical CO₂ capture capacity of ~24 mmol^.g^-1, its reaction with CO₂ gas molecules is usually restricted by the surface product layer which limits its actual capacity to < 1 mmol^.g^-1 [2]. In the last decade, much attention has been paid to improving the properties of MgO to realize its potential as a CO₂ sorbent. Herein, we report a significant promoting effect of eutectic (binary, ternary and quaternary) alkali salt mixtures on the CO₂ capture performance of MgO. The sorbents were prepared by wet impregnation of MgO with varying loadings of the eutectic mixtures. The physicochemical properties were characterized by various techniques such as BET, ICP-OES, XRD, FT-IR, CO₂-TPD, SEM and TEM, and the capture capacity was evaluated using thermogravimetric analysis (TGA). The results suggest that the nature of the salt and its melting temperature (T_m) are crucial factors in the performance of the sorbent. The sorbent modified with NaNO₃ (T_m ≈ 302 °C) exhibited the highest CO₂ uptake of 11.9 mmol^.g^-1 (523.7 mg CO₂ per 1 g of sorbent) after carbonation for one hour at 300 °C under ambient pressure while those modified with LiNO₃ and KNO₃ did not perform well. Interestingly, the use of eutectic mixtures of these salts not only improved the CO₂ uptake but also broadened the operating temperature window. In the case of the (LiNaK)NO₃- and (LiNaK)NO₃-NaNO₂-modified sorbents, the CO₂ uptakes were better than that of the NaNO₃-modified sorbent at temperatures below and above 300 °C. The results of the FT-IR spectroscopy reveal the presence of two types of carbonates (surface and bulk) while the XRD confirms the formation of well-crystallized MgCO₃ after CO₂ sorption. Based on these findings, a mechanism that involves a gas-liquid-solid interface as an alternative pathway for the reaction of gaseous CO₂ and the solvated [Mg^2+...O^2-] ionic pairs will be discussed.

1. MacDowell, N., et al., An overview of CO₂ capture technologies. Energy & Environmental Science, 2010. 3(11): p. 1645.
2. Wang, J., et al., Recent advances in solid sorbents for CO₂ capture and new development trends. Energy & Environmental Science, 2014. 7(11): p. 3478-3518.