Gas Storage Applications Research Articles

Could Mesophases Play a Role in the Nucleation and Polymorph Selection of Zeolites?

Zeolites and mesoporous silicas are porous materials with important applications in catalysis, gas storage, and separation. Zeolite crystals form in the presence of cationic surfactants that act as structure directing agents (SDAs). The way SDAs control the nucleation and polymorphs selection in zeolites is not fully understood. The formation of mesoporous silica is templated by liquid crystalline mesophases that result from frustrated attraction between silicates and long-chain SDAs. Experiments indicate that surfactants C nH2 n+1(CH3)3N+ with n > 6 yield mesoporous silicas, and the one with n = 6 produces a zeolite. This suggests that the driving force toward mesophase formation is also present for small organocations, but is overcome by the ability of silica to wrap a crystal lattice around them. Here we use molecular dynamics simulations to investigate whether the existence of metastable mesophases can play a role in the nucleation and polymorph selection of zeolitic crystals. As a proof of concept, we investigate the phase behavior of simple mesogenic mixtures of SDAs and a network former T that favors tetracoordinated crystals. We represent the network-former T by Stillinger-Weber models of water and silicon, in lieu of silica, because a computationally efficient silica potential that would allow for the spontaneous nucleation of zeolites in molecular dynamics simulations is not yet available. The mixtures of T and SDA produce a rich phase diagram that encompasses the sII clathrate and at least six zeolites, including sigma-2 (SGT). We find that the nucleation of SGT is not assisted by a mesophase. The nucleation of the other five zeolites of this study, however, is facilitated by the existence of metastable mesophases that decrease the nucleation barriers and direct the selection of the crystal polymorph. Together with the experimental support for mesophases in mixtures of silicates and SDAs, our results for model systems suggest that metastable mesophases could play a prominent role in promoting the nucleation and polymorph selection of some zeolites.

High-mobility band-like charge transport in a semiconducting two-dimensional metal–organic framework

Metal-organic frameworks (MOFs) are hybrid materials based on crystalline coordination polymers that consist of metal ions connected by organic ligands. In addition to the traditional applications in gas storage and separation or catalysis, the long-range crystalline order in MOFs, as well as the tunable coupling between the organic and inorganic constituents, has led to the recent development of electrically conductive MOFs as a new generation of electronic materials. However, to date, the nature of charge transport in the MOFs has remained elusive. Here we demonstrate, using high-frequency terahertz photoconductivity and Hall effect measurements, Drude-type band-like transport in a semiconducting, π-d conjugated porous Fe3(THT)2(NH4)3 (THT, 2,3,6,7,10,11-triphenylenehexathiol) two-dimensional MOF, with a room-temperature mobility up to ~ 220 cm2 V-1 s-1. The temperature-dependent conductivity reveals that this mobility represents a lower limit for the material, as mobility is limited by impurity scattering. These results illustrate the potential for high-mobility semiconducting MOFs as active materials in thin-film optoelectronic devices.

Flat-Histogram Monte Carlo as an Efficient Tool To Evaluate Adsorption Processes Involving Rigid and Deformable Molecules.

Monte Carlo simulations are the foundational technique for predicting thermodynamic properties of open systems where the process of interest involves the exchange of particles. Thus, they have been used extensively to computationally evaluate the adsorption properties of nanoporous materials and are critical for the in silico identification of promising materials for a variety of gas storage and chemical separation applications. In this work we demonstrate that a well-known biasing technique, known as "flat-histogram" sampling, can be combined with temperature extrapolation of the free energy landscape to efficiently provide significantly more useful thermodynamic information than standard open ensemble MC simulations. Namely, we can accurately compute the isosteric heat of adsorption and number of particles adsorbed for various adsorbates over an extremely wide range of temperatures and pressures from a set of simulations at just one temperature. We extend this derivation of the temperature extrapolation to adsorbates with intramolecular degrees of freedom when Rosenbluth sampling is employed. Consequently, the working capacity and isosteric heat can be computed for any given combined temperature/pressure swing adsorption process for a large range of operating conditions with both rigid and deformable adsorbates. Continuous thermodynamic properties can be computed with this technique at very moderate computational cost, thereby providing a strong case for its application to the in silico identification of promising nanoporous adsorbents.

Selective hydrogen adsorption on a buckled carbon nitride sheet: first-principles calculation

We investigate the structural, electronic and magnetic properties of three configurations of buckled heptazine supercell with adsorbed hydrogen, namely, and using DFT approach. The calculated adsorption energy for the three structures is very disparate. The structure is energetically more stable than structures. The less stable structures which contain H atoms adsorbed in vertical direction induced a total magnetic moment of whereas for the case, the adsorbed H atom formed a dangling bond (in the horizontal direction) with N having or -like hybridized structure that leaves no unpaired electrons. A different electronic character is found for the three configurations. Specifically, appears to be semiconducting with a narrow band gap whereas the other two configurations, and are half-metallic and metallic respectively. In general, our findings present theoretical insights of the most stable buckled heptazine with adsorbed H atom structure for the applications in hydrogen gas storage.

Combining continuous flow oscillatory baffled reactors and microwave heating: Process intensification and accelerated synthesis of metal-organic frameworks

We have constructed a continuous flow oscillatory baffled reactor (CF-OBR) equipped with a homogeneous and controllable microwave applicator in an entirely novel design. This affords a new route to chemical production incorporating many of the principles of process intensification and allows, for the first time, investigation of the synergistic benefits of microwave heating and CF-OBRs such as; faster and continuous processing; improved product properties and purity; improved control over the processing parameters; and reduced energy consumption. The process is demonstrated by the production of a metal-organic framework (MOF), HKUST-1, a highly porous crystalline material with potential applications in gas storage and separation, catalysis, and sensing. Our reactor enabled the production of HKUST-1 at the 97.42 g/h scale, with a space time yield (STY) of 6.32 × 105 kg/m3/day and surface area production rate (SAPR) of 1.12 × 1012 m2/m3/day. This represents the highest reported STY and fastest reported synthesis (2.2 s) for any MOF produced via any method to-date and is an improvement on the current SAPR for HKUST-1 by two orders of magnitude owing to the superior porosity exhibited by HKUST-1 produced using our rig (Langmuir surface area of 1772 compared to 600 m2/g).

(Invited) Progress Towards the Design of Metal-Organic Frameworks with Targeted Thermal Conductivities

In practice, the usefulness of metal–organic frameworks (MOFs) for many gas storage applications depends on their ability to rapidly dissipate the heat generated during the exothermic adsorption process. MOFs can be precisely designed to have a wide variety of architectures which can allow tuning their thermal conductivity. In this work, we use molecular dynamics simulations to investigate the effect of pore geometry, gas loading, and interpenetration on the thermal conductivity of MOFs. We find that larger pores and increased gas loading reduce thermal conductivity. We also find that the addition of a parallel thermal transport pathway yields a thermal conductivity nearly the sum of the two constituent frameworks. This relationship holds for a variety of interpenetrating MOFs with different atomic masses and a wide range of interframework interaction parameters. We show that both the strength and range of interactions between constituent frameworks play a significant role in framework mobility as well as framework coupling which can result in deviation from this relationship. We propose a simple model to predict thermal conductivity of the interpenetrated framework based on thermal conductivities of individual frameworks and an interframework interaction parameter which can account for this deviation.

In situ visualization of loading-dependent water effects in a stable metal–organic framework

Competitive water adsorption can have a significant impact on metal-organic framework performance properties, ranging from occupying active sites in catalytic reactions to co-adsorbing at the most favourable adsorption sites in gas separation and storage applications. In this study, we investigate, for a metal-organic framework that is stable after moisture exposure, what are the reversible, loading-dependent structural changes that occur during water adsorption. Herein, a combination of in situ synchrotron powder and single-crystal diffraction, infrared spectroscopy and molecular modelling analysis was used to understand the important role of loading-dependent water effects in a water stable metal-organic framework. Through this analysis, insights into changes in crystallographic lattice parameters, water siting information and water-induced defect structure as a response to water loading were obtained. This work shows that, even in stable metal-organic frameworks that maintain their porosity and crystallinity after moisture exposure, important molecular-level structural changes can still occur during water adsorption due to guest-host interactions such as water-induced bond rearrangements.

Homochiral Metal-Organic Cage for Gas Chromatographic Separations.

Metal-organic cages (MOCs) as a new type of porous material with well-defined cavities were extensively pursued because of their relative ease of synthesis and their potential applications in host-guest chemistry, molecular recognition, separation, catalysis, gas storage, and drug delivery. Here, we first reported that a homochiral MOC [Zn3L2] is explored to fabricate [Zn3L2] coated capillary column for high-resolution gas chromatographic separation of a wide range of analytes, including n-alkanes, polycyclic aromatic hydrocarbons, and positional isomers, especially for racemates. Various kinds of racemates such as alcohols, diols, epoxides, ethers, halohydrocarbons, and esters were separated with good enantioselectivity and reproducibility on the [Zn3L2] coated capillary column. The fabricated [Zn3L2] coated capillary column exhibited significant chiral recognition complementary to that of a commercial β-DEX 120 column and our recently reported homochiral porous organic cage CC3-R coated column. The results show that the homochiral MOCs will be very attractive as a new type of chiral selector in separation science.

Design and synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) materials are crystalline polymer networks constructed by organic building blocks. Their tunable structure, low density, well-defined pores, high surface areas and high thermal stability render them have promising applications in gas storage and separations, energy storage, catalysis, sensing and optoelectronic. This review focuses on the design principle of COF based on the structure and functionality, and summarizes the recent synthesize method of COF bulk powder, single layer COFs and COF films. The design principle of COFs is classified into the structural-oriented and functional-oriented protocols. For structural design, the basic principle is reticular chemistry. The geometry of the building units and the symmetry of the reaction group determine the dimension, topology, structure, and the pore size and shape of COFs. The reaction types of COF are usually based on thermodynamic controlled reversible reaction. Moreover, the functionality of COFs can be easily modulated via three strategies, including directly using functionalized building blocks to construct COFs, post-synthetic modification of the pre-prepared framework, and encapsulation of functional molecules into the channel of COFs. Along with the structural and functional design, various COF materials have been reported and greatly boosted the developments of this field. Then we summarized the synthetic methodologies of COFs. In general, the developed synthetic methodologies follow three aspects: (1) fast and efficient; (2) for large-scale industrial production; (3) for processing shaped-body of COF. Recently, there are several available methods to fabricate COF into free-standing films including the liquid-liquid interfacial polymerization, vapor-assisted conversion and baking the molecular precursors. These approaches indicate a way forward for accessing COF into films and enable applying to real industrial applications. Although there has been a significant expansion of COF chemistries over the past 15 years, yet this field is far from mature, the article ends with a perspective on the future developments in this growing field and discusses the challenges remain for these materials. To date, it is still hard to obtain single crystals, thus advanced characterization techniques and theoretical calculation methods are highly demanded to be developed. The mechanisms for the nucleus formation and growth are still needed to be deeply investigated. In addition, exploring novel topologies and linkages of COF are necessary from the chemistry perspective. Moreover, in order to precisely construct and tune the functions of COF materials, further studies on the structure-property correlation via the directly design functional building blocks and encapsulation of functional molecules into the pores are still required. In terms of the processing COF from the particle to shaped bodies and to devices would be critically important in many practical applications, like gas separation, catalysis and organic light-emitting diodes, thus further development towards the efficient synthesis of COF films or membranes with large area, high orientation, no defect, and uniform thickness would be of great interest. For COF materials, advances in the fast, efficient and economic fabrication methods and the exploration of new functionalization would make them become the commercialized material for industrial application in the fields of energy, health, environment and so on.

Progress in the Physisorption Characterization of Nanoporous Gas Storage Materials

Abstract Assessing the adsorption properties of nanoporous materials and determining their structural characterization is critical for progressing the use of such materials for many applications, including gas storage. Gas adsorption can be used for this characterization because it assesses a broad range of pore sizes, from micropore to mesopore. In the past 20 years, key developments have been achieved both in the knowledge of the adsorption and phase behavior of fluids in ordered nanoporous materials and in the creation and advancement of state-of-the-art approaches based on statistical mechanics, such as molecular simulation and density functional theory. Together with high-resolution experimental procedures for the adsorption of subcritical and supercritical fluids, this has led to significant advances in physical adsorption textural characterization. In this short, selective review paper, we discuss a few important and central features of the underlying adsorption mechanisms of fluids in a variety of nanoporous materials with well-defined pore structure. The significance of these features for advancing physical adsorption characterization and gas storage applications is also discussed.

Characterization of Undercoordinated Zr Defect Sites in UiO-66 with Vibrational Spectroscopy of Adsorbed CO

The highly studied metal–organic framework (MOF), UiO-66, has been touted for its high surface area, stability, and tunability of structural defects for particular applications in catalysis and gas storage. However, atomic-level characterization and quantification of defects remain challenging for traditional methods. Motivated by this knowledge gap, coordinatively unsaturated zirconium (Zrcus) defect sites within UiO-66 have been determined through infrared spectroscopic studies of the highly sensitive probe molecule, carbon monoxide. A characteristic blue shift of the vibrational frequency for CO adsorbed at nodes within the MOF was used to identify the presence of Zrcus defects. The identification of these coordinatively unsaturated metal sites was further corroborated via exposure of the MOF to the Lewis base, D2O, which was shown to block CO binding to the Lewis acidic Zrcus sites. The infrared spectroscopic signature for CO binding at the Zrcus sites was further employed to track the emergence of mi...

Effect of rheological properties of mesophase pitch and coal mixtures on pore development in activated carbon discs with high compressive strength

We measured the rheological properties of mesophase pitch + coal mixtures at temperatures up to 873 K and used quench tests during carbonization to observe the effects of coal particles on foaming and pore development in activated carbon discs (ACD). We observed that high ratios of coal to pitch increased the mixture's viscosity in a low pressure foaming process, which restricted growth of large bubbles during foaming and produced stronger carbon monoliths. The highest strength ACD, with compressive strength of 56 ± 3 MPa, was obtained using a pitch to coal mass ratio of 1:2 (bulk density = 0.91 g·cm−3 and surface area = 579 ± 37 m2·g−1). The adsorption capacities of CO2, CH4, and N2 of the ACDs prepared with a mesophase pitch to coal ratio of 1:2, measured on a gravimetric adsorption apparatus, at 298 K and pressures up to 4000 kPa were 3751 mol·m−3 CO2, 2107 mol·m−3 CH4, and 1691 mol·m−3 N2. These adsorption results suggest the activated carbon discs have potential as structured adsorbents for gas separation or storage applications.

Silane-based hyper-cross-linked porous polymers and their applications in gas storage and water treatment

A series of novel silane-based hyper-cross-linked porous polymers was prepared through the Friedel–Crafts alkylation reaction of dimethyldiphenylsilane as the monomer and formaldehyde dimethyl acetal as the external cross-linker. The structures of the polymers were confirmed by FTIR and solid-state cross-polarization magic-angle spinning 13C NMR. We investigated the porous polymers with different ratios of dimethyldiphenylsilane to formaldehyde dimethyl acetal. Porous properties were determined by nitrogen, carbon dioxide and hydrogen adsorptions. The results revealed that the porous materials possessed surface areas from 742 to 1205 m2 g−1 with the increasing amount of formaldehyde dimethyl acetal. Moreover, this material exhibited high thermal stabilities, good CO2 capture, and H2 storage capacities. The material was also used as adsorbent for organic dyes in aqueous solution and showed an excellent adsorption capacity of 952 mg g−1 for Congo red dye. These results suggest that the silane-based hyper-cross-linked porous polymers are promising materials for CO2 capture, H2 storage, and water treatment.

Microporous Polycarbazole Materials: From Preparation and Properties to Applications.

Polycarbazoles with rigid skeletons and electron-rich conjugated systems are beneficial to form permanent porous materials with intrinsic photoelectric properties. In recent years, the research related to porous polycarbazoles becomes a hot topic in the field of porous organic polymers. The preparation methods for porous polycarbazoles are diverse, commonly represented by the oxidation coupling reaction and the Friedel-Crafts reaction. Other methods such as nitrile-based trimerization, electrochemical polymerization, and classic C-C coupling reactions are also reported. Porous polycarbazoles generally possess large specific surface area, permanent pore structure, and high thermal and chemical stability, which show potential applications in gas storage and separation, catalysis, sensors, and electronics. In this review, the recent research progress on the synthetic methods, functions, and applications of porous polycarbazoles is summarized.

Imide-Based Polymers of Intrinsic Microporosity: Probing the Microstructure in Relation to CO2 Sorption Characteristics.

A range of microporous, imide-based polymers were newly synthesized using two-step poly-condensation reactions of bis(carboxylic anhydride) and various aromatic diamines for CO2 gas capture and storage applications. In this report, we attempted to assess the relative significance of molecular structural aspects through the manipulation of the conformational characteristics of the building blocks of the polymeric structures, the spiro-containing acid anhydride and the aromatic amines, to induce greater intrinsic microporosity and higher surface areas for the resulting solids. Results obtained from this study were thus used to outline a working relationship between the structural diversity of the constructed porous solids and their performance as CO2 sorbents.

Phase Equilibria and the Crystallographic Properties of TBAB–CO2 Semiclathrate Hydrates

Understanding the crystal properties of semiclathrate hydrates is important for their potential application in gas storage and separation. In this work, the phase equilibria of TBAB–CO2 hydrates were measured and hydrates formed from the solutions where TBAB concentrations ranged from 0.01 to 0.32 mass fraction were analyzed using Raman and powder X-ray diffraction. Results showed that the equilibrium pressure of TBAB–CO2 hydrate was more sensitive to temperature than that of simple CO2 hydrate, and the equilibrium temperature also got increased with a rise in TBAB concentration. By measuring the hydrate samples formed at 274 K and 3 MPa, the simple CO2 hydrates could be found distributed randomly and discontinuously in the samples, while the TBAB–CO2 hydrate crystals were relatively large. The simple CO2 hydrate was found to coexist with TBAB–CO2 hydrate, and the TBAB·38H2O was the only semiclathrate hydrate structure regardless of the initial concentrations of TBAB. When the TBAB concentration was high,...

Are the amino acids thermodynamic inhibitors or kinetic promoters for carbon dioxide hydrates?

Abstract: This study deals with the carbon dioxide (CO2) hydrate formation in aqueous solutions, containing 0.5 wt% amino acids (L-valine, L-phenylalanine, L-cysteine, L-methionine and L-threonine) under isochoric conditions. Systematic experiments were conducted in both stirred (300 rpm) and non-stirred configurations. The hydrate formation requires higher sub-cooling than the pure CO2 system. The gas uptake in the hydrates, formed under stirred conditions, in l-val, l-cys and l-met systems is ∼20% higher than the pure system, while in other two (l-phe & l-thr) it is comparable to the pure system. Further, there is no appreciable gas uptake in pure, l-phe and l-thr systems under non-stirred conditions. On the other hand, the gas uptake is higher and faster, similar to stirred conditions, in other three systems, i.e., l-val, l-cys and l-met. Thus, the aqueous solutions consisting of these three amino acids are useful in the CO2 gas capture and storage applications.

Stable Metal-Organic Frameworks: Design, Synthesis, and Applications.

Metal-organic frameworks (MOFs) are an emerging class of porous materials with potential applications in gas storage, separations, catalysis, and chemical sensing. Despite numerous advantages, applications of many MOFs are ultimately limited by their stability under harsh conditions. Herein, the recent advances in the field of stable MOFs, covering the fundamental mechanisms of MOF stability, design, and synthesis of stable MOF architectures, and their latest applications are reviewed. First, key factors that affect MOF stability under certain chemical environments are introduced to guide the design of robust structures. This is followed by a short review of synthetic strategies of stable MOFs including modulated synthesis and postsynthetic modifications. Based on the fundamentals of MOF stability, stable MOFs are classified into two categories: high-valency metal-carboxylate frameworks and low-valency metal-azolate frameworks. Along this line, some representative stable MOFs are introduced, their structures are described, and their properties are briefly discussed. The expanded applications of stable MOFs in Lewis/Brønsted acid catalysis, redox catalysis, photocatalysis, electrocatalysis, gas storage, and sensing are highlighted. Overall, this review is expected to guide the design of stable MOFs by providing insights into existing structures, which could lead to the discovery and development of more advanced functional materials.

Design and synthesis of porous polymeric materials and their applications in gas capture and storage: a review

The design and synthesis of porous organic materials that have increasing physical and chemical characteristics have been discussed. For example, a variety of porous organic polymers, metal–organic frameworks, conjugated microporous polymers, and polymers of intrinsic microporosity have been designed and synthesized using simple and efficient procedures. Such materials have unique gas adsorption properties and can be used in gases separation and storage. In addition, they have high surface area, porosity, and selectivity towards carbon dioxide compared to other gases such as nitrogen and methane.

Research progress of metal-organic frameworks in environmental chemistry

In the past decades, environmental problems have become more and more prominent and the development of new materials to solve environmental problems is imperative. Metal-organic frameworks (MOFs), assembled by organic bridging ligands and inorganic metal centers, are emerging as a relatively new class of porous materials. Comparing to conventional porous materials, MOFs possess high surface area, uniform aperture and adjustable structure, which endow them enormous potential applications in the field of environmental chemistry. Here, we introduce the recent progress in the theoretical and practical research of MOFs related to environmental chemistry, specifically in the applications of catalysis, gas storage, gas adsorption and separation, gas sensing and heavy-metal removal. The challenge and prospective investigation trends of MOFs in environmental chemistry are also proposed and discussed.