Rigidity with Flexibility: Porous Triptycene Networks for Enhancing Methane Storage.

In the pursuit of advancing materials for methane storage, a critical consideration arises given the prominence of natural gas (NG) as a clean transportation fuel, which holds substantial potential for alleviating the strain on both energy resources and the environment in the forthcoming decade. In this context, a novel approach is undertaken, employing the rigid triptycene as a foundational building block. This strategy is coupled with the incorporation of dichloromethane and 1,3-dichloropropane, serving as rigid and flexible linkers, respectively. This combination not only enables cost-effective fabrication but also expedites the creation of two distinct triptycene-based hypercrosslinked polymers (HCPs), identified as PTN-70 and PTN-71. Surprisingly, despite PTN-71 manifesting an inferior Brunauer-Emmett-Teller (BET) surface area when compared to the rigidly linked PTN-70, it showcases remarkably enhanced methane adsorption capabilities, particularly under high-pressure conditions. At a temperature of 275 K and a pressure of 95 bars, PTN-71 demonstrates an impressive methane adsorption capacity of 329 cm3 g-1. This exceptional performance is attributed to the unique flexible network structure of PTN-71, which exhibits a pronounced swelling response when subjected to elevated pressure conditions, thus elucidating its superior methane adsorption characteristics. The development of these advanced materials not only signifies a significant stride in the realm of methane storage but also underscores the importance of tailoring the structural attributes of hypercrosslinked polymers for optimized gas adsorption performance.

Dibromomethane Knitted Highly Porous Hyper-Cross-Linked Polymers for Efficient High-Pressure Methane Storage.

Hyper-cross-linked polymers (HCPs) with ultra-high porosity, superior physicochemical stability, and excellent cost-effectiveness are attractive candidates for methane storage. However, the construction of HCPs with BET surface areas exceeding 3000 m2 g-1 remains extremely challenging. In this work, a newly developed DBM-knitting method with a slow-knitting rate is employed to increase the cross-linking degree, in which dichloromethane (DCM) is replaced by dibromomethane (DBM) as both solvent and electrophilic cross-linker, resulting in highly porous and physicochemically stable HCPs. The BET surface areas of DBM-knitted SHCPs-Br are 44%-120% higher than that of DCM-knitted SHCPs-Cl using the same building blocks. Remarkably, SHCP-3-Br exhibits an unprecedentedly high porosity (SBET = 3120 m2 g-1) among reported HCPs, and shows a competitive volumetric 5-100 bar working methane capacity of 191 cm3 (STP) cm-3 at 273 K calculated by using real packing density, which outperforms sate-of-art metal-organic framework (MOFs) at comparable conditions. This facile and versatile low-knitting-rate strategy enables effective improvement in the porosity of HCPs for porosity-desired applications.

Metal Doping to Control Gate Opening and Increase Methane Working Capacity in Isostructural Flexible Diamondoid Networks.

Adsorbed natural gas (ANG) systems involve using porous materials to increase the working capacity and/or reduce the storage pressure compared to compressed natural gas (CNG). Flexible metal-organic materials (FMOMs) are particularly interesting in this context since their stepped isotherms can afford increased working capacity if the adsorption/desorption steps occur within the proper pressure range. We report herein that metal doping in a family of isostructural FMOMs, ML2 (M=Co, Ni or Nix Co1-x , L=4-(4-pyridyl)-biphenyl-4-carboxylic acid), enables control over the gate opening between non-porous (closed) and porous (open) phases at pressures relevant to methane storage. Specifically, methane-induced phase transformations can be fine-tuned by using different Ni/Co ratios to enhance methane working capacity. The optimal working capacity from 5 to 35 bar at 298 K (153 cm3  cm-3 ) was found for Ni0.89 Co0.11 L2 (X-dia-1-Ni0.89 Co0.11 ), which is greater than that of benchmark rigid MOFs.

Discovery of High-Performing Metal-Organic Frameworks for On-Board Methane Storage and Delivery via LNG-ANG Coupling: High-Throughput Screening, Machine Learning, and Experimental Validation.

Liquefied natural gas (LNG) gasification coupled with adsorbed natural gas (ANG) charging (LNG-ANG coupling) is an emerging strategy for efficient delivery of natural gas. However, the potential of LNG-ANG to attain the advanced research projects agency-energy (ARPA-E) target for onboard methane storage has not been fully investigated. In this work, large-scale computational screening is performed for 5446 metal-organic frameworks (MOFs), and over 193 MOFs whose methane working capacities exceed the target (315 cm3 (STP) cm-3 ) are identified. Furthermore, structure-performance relationships are realized under the LNG-ANG condition using a machine learning method. Additional molecular dynamics simulations are conducted to investigate the effects of the structural changes during temperature and pressure swings, further narrowing down the materials, and two synthetic targets are identified. The synthesized DUT-23(Cu) and DUT-23(Co) show higher working capacities (≈373 cm3 (STP) cm-3 ) than that of any other porous material under ANG or LNG-ANG conditions, and excellent stability during cyclic LNG-ANG operation.

Computational Identification and Experimental Demonstration of High-Performance Methane Sorbents.

Remarkable methane uptake is demonstrated experimentally in three metal-organic frameworks (MOFs) identified by computational screening: UTSA-76, UMCM-152 and DUT-23-Cu. These MOFs outperform the benchmark sorbent, HKUST-1, both volumetrically and gravimetrically, under a pressure swing of 80 to 5 bar at 298 K. Although high uptake at elevated pressure is critical for achieving this performance, a low density of high-affinity sites (coordinatively unsaturated metal centers) also contributes to a more complete release of stored gas at low pressure. The identification of these MOFs facilitates the efficient storage of natural gas via adsorption and provides further evidence of the utility of computational screening in identifying overlooked sorbents.

Heat and Mass Transfer in an Adsorbed Natural Gas Storage System Filled with Monolithic Carbon Adsorbent during Circulating Gas Charging.

Adsorbed natural gas (ANG) technology is a promising alternative to traditional compressed (CNG) and liquefied (LNG) natural gas systems. Nevertheless, the energy efficiency and storage capacity of an ANG system strongly depends on the thermal management of its inner volume because of significant heat effects occurring during adsorption/desorption processes. In the present work, a prototype of a circulating charging system for an ANG storage tank filled with a monolithic nanoporous carbon adsorbent was studied experimentally under isobaric conditions (0.5-3.5 MPa) at a constant volumetric flow rate (8-18 m3/h) or flow mode (Reynolds number at the adsorber inlet from 100,000 to 220,000). The study of the thermal state of the monolithic adsorbent layer and internal heat exchange processes during the circulating charging of an adsorbed natural gas storage system was carried out. The correlation between the gas flow mode, the dynamic gas flow temperature, and the heat transfer coefficient between the gas and adsorbent was determined. A one-dimensional mathematical model of the circulating low-temperature charging process was developed, the results of which correspond to the experimental measurements.

Tuning Open Metal Site-Free ncb Type of Metal-Organic Frameworks for Simultaneously High Gravimetric and Volumetric Methane Storage Working Capacities.

The design and synthesis of a single metal-organic framework (MOF) material with simultaneously high gravimetric and volumetric methane storage working capacities are still a great challenge. The open metal site (OMS) in MOFs is generally regarded as an advantage to enhance host-guest affinity. However, it is detrimental to the methane storage working capacity to some extent due to the resulting high low-pressure uptake. Moreover, the reported methane storage MOFs are predominately focusing on edge-transitive or low-connected mixed-linker networks. In contrast, high-connected mixed-linker MOFs have been less investigated for methane storage. Herein, three isoreticular nine-connected trinuclear iron-based Fe-ncb-MOFs without OMSs have been judiciously designed and successfully constructed by means of the mixed-linker approach associated with the fixing amide-functionalized pyridyl-carboxylate ligand LP (4-(pyridin-4-ylcarbamoyl)benzoate) and three differing sized dicarboxylate ligands. High-pressure methane adsorption measurements show that, with the isoreticular extension from BDC (1,4-benzenedicarboxylate) to BPDC (4,4'-biphenyldicarboxylate) and ABDC (azobenzene-4,4'-dicarboxylate), three Fe-ncb-MOFs exhibit gradually increasing not only gravimetric but also volumetric storage capacities because of their balancing gravimetric surface area and volumetric surface area, hierarchical pore system, and modest CH4 heats of adsorption. Among them, the Fe-ncb-ABDC demonstrates a rare combination of simultaneously high gravimetric and volumetric CH4 storage working capacities of 0.302/0.37 g g-1 and 196/240 cm3 (STP) cm-3 at 298/273 K and between 80 and 5 bar, respectively, which outperform the 8-c Fe-8T18-ABDC assembled from a shorter pyridyl-carboxylate ligand IN (isonicotinate) and ABDC, due to its limited pore volume, the presence of OMSs, and more confined pore spaces, and place Fe-ncb-ABDC among the best performing MOFs.

Using Helium Pycnometry to Study the Apparent Densities of Metal-Organic Frameworks.

When investigating the gas storage capacities of metal-organic frameworks, volumetric values are often reported based on crystallographic densities. Although it is widely accepted that Langmuir and BET surface areas of a given MOF can vary depending on the exact synthetic conditions used to prepare the materials, it is rare that deviations in density from the optimal crystallographic density are considered. The actual (apparent) densities of these materials are highly variable depending on the presence of defects, impurities, or multiple phases that arise during synthesis. The apparent density of specific samples, which represent an experimentally determined crystallographic density, can be measured with helium pycnometry where the skeletal density measured via pycnometry is easily converted to an apparent density. In the work reported here, apparent density was measured for 46 samples across a series of different structure types where experimentally measured density was consistently lower than crystallographic density, up to 30% in some cases. Subsequently, use of this technique allows for quantification of densities for those materials whose structures have not been crystallographically determined.

Ligand-Conformer-Induced Formation of Zirconium-Organic Framework for Methane Storage and MTO Product Separation.

In pursuit of novel adsorbents with efficient adsorptive gas storage and separation capabilities remains highly desired and challenging. Although the documented zirconium-tricarboxylate-based metal-organic frameworks (MOFs) have displayed a variety of topologies encompassing underlying and geometry mismatch ones, the employed organic linkers are exclusively rigid and poorly presenting one type of conformation in the resultant structures. Herein, a used and semirigid tricarboxylate ligand of H3 TATAB was judiciously selected to isolate a zirconium-based spe-MOF after the preliminary discovery of srl-MOF. Single-crystal X-ray diffraction reveals that the fully deprotonated TATAB linker in spe-MOF exhibits two distinct conformers, concomitant with popular Oh and rare S6 symmetrical Zr6 molecular building blocks, generating an unprecedented (3,3,12,12)-c nondefault topology. Specifically, the spe-MOF exhibits structurally higher complexity, hierarchical micropores, open metal sites free and rich electronegative groups on the pore surfaces, leading to relatively high methane storage capacity without considering the missing-linker defects and efficient MTO product separation performance.

Computational Screening of Trillions of Metal-Organic Frameworks for High-Performance Methane Storage.

In the past decade, there has been an increasing number of computational screening works to facilitate finding optimal materials for a variety of different applications. Unfortunately, most of these screening studies are limited to their initial set of materials and result in a brute-force type of screening approach. In this work, we present a systematic strategy that can find metal-organic frameworks (MOFs) with the desired properties from an extremely diverse and large set of over 100 trillion possible MOFs using machine learning and evolutionary algorithm. It is demonstrated that our algorithm can discover 964 MOFs with methane working capacity over 200 cm3 cm-3 and 96 MOFs with methane working capacity over the current world record of 208 cm3 cm-3. We believe that this methodology can take advantage of the modular nature of MOFs and can readily be extended to other important applications as well.

Thermodynamic Behaviors of Adsorbed Methane Storage Systems Based on Nanoporous Carbon Adsorbents Prepared from Coconut Shells.

The present work focused on the experimental study of the performance of a scaled system of adsorbed natural gas (ANG) storage and transportation based on carbon adsorbents. For this purpose, three different samples of activated carbons (AC) were prepared by varying the size of coconut shell char granules and steam activation conditions. The parameters of their porous structure, morphology, and chemical composition were determined from the nitrogen adsorption at 77 K, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM) measurements. The methane adsorption data measured within the temperature range from 178 to 360 K and at pressures up to 25 MPa enabled us to identify the most efficient adsorbent among the studied materials: AC-90S. The differential heats of methane adsorption on AC-90S were determined in order to simulate the gas charge/discharge processes in the ANG system using a mathematical model with consideration for thermal effects. The results of simulating the charge/discharge processes under two different conditions of heat exchange are consistent with the experimentally determined temperature distribution over a scaled ANG storage tank filled with the compacted AC-90S adsorbent and equipped with temperature sensors and heat-exchanger devices. The amounts of methane delivered from the ANG storage system employing AC-90S as an adsorbent differ from the model predictions by 4-6%. Both the experiments and mathematical modeling showed that the thermal regulation of the ANG storage tank ensured the higher rates of charge/discharge processes compared to the thermal insulation.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials.

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm3/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.

Enhancement of Clathrate Hydrate Formation Kinetics Using Carbon-Based Material Promotion.

Although hydrate-based technology has been considered as a safe and environmentally friendly approach for gas storage and transportation in recent decades, there are still inherent problems during hydrate production, such as a long induction time, slow formation kinetics, and limited hydrate storage capacity. Attempts to resolve these issues have resulted in the development of various kinetics promoters, among which carbon-based materials have become one of the most attractive owing to their unique promotion effect. Herein, results on promotion by bulk wetted carbon materials in the forms of a packed bed, carbon particles in a suspension, and nano-carbon materials in a nanofluid are collected from the published literature. Meanwhile, the promotion mechanisms and influencing factors of the carbon-based promoters are discussed. The purpose of this mini-review is to summarize recent advances and highlight the prospects and future challenges for the use of carbon-based materials in hydrate production.

Design of Zeolite-Covalent Organic Frameworks for Methane Storage.

A new type of zeolite-based covalent organic frameworks (ZCOFs) was designed under different topologies and linkers. In this study, the silicon atoms in zeolite structures were replaced by carbon atoms in thiophene, furan, and pyrrole linkers. Through the adoption of this strategy, 300 ZCOFs structures were constructed and simulated. Overall, the specific surface area of ZCOFs is in the range of 300-3500 m2/g, whereas the pore size is distributed from 3 to 27 Å. Furthermore, the pore volume exhibits a wide range between 0.01 and 1.5 cm3/g. Screening 300 ZCOFs with the criteria towards methane storage, 11 preliminary structures were selected. In addition, the Grand Canonical Monte Carlo technique was utilized to evaluate the CH4 adsorption ability of ZCOFs in a pressure ranging from 1 to 85 bar at a temperature of 298 K. The result reveals that two ZCOF structures: JST-S 183 v/v (65-5.8 bar) and NPT-S 177 v/v (35-1 bar) are considered as potential adsorbents for methane storage. Furthermore, the thermodynamic stability of representative structures is also checked base on quantum mechanical calculations.

Densification-Induced Structure Changes in Basolite MOFs: Effect on Low-Pressure CH4 Adsorption.

Metal-organic frameworks' (MOFs) adsorption potential is significantly reduced by turning the original powder into pellets or granules, a mandatory step for their use at industrial scale. Pelletization is commonly performed by mechanical compression, which often induces the amorphization or pressure-induced phase transformations. The objective of this work is the rigorous study of the impact of mechanical pressure (55.9, 111.8 and 186.3 MPa) onto three commercial materials (Basolite C300, F300 and A100). Phase transformations were determined by powder X-ray diffraction analysis, whereas morphological changes were followed by nitrogen physisorption. Methane adsorption was studied in an atmospheric fixed bed. Significant crystallinity losses were observed, even at low applied pressures (up to 69.9% for Basolite C300), whereas a structural change occurred to Basolite A100 from orthorhombic to monoclinic phases, with a high cell volume reduction (13.7%). Consequently, adsorption capacities for both methane and nitrogen were largely reduced (up to 53.6% for Basolite C300), being related to morphological changes (surface area losses). Likewise, the high concentration of metallic active centers (Basolite C300), the structural breathing (Basolite A100) and the mesopore-induced formation (Basolite F300) smooth the dramatic loss of capacity of these materials.

Molecular Expansion for Constructing Porous Organic Polymers with High Surface Areas and Well-Defined Nanopores.

Construction of porous organic polymers (POPs) with high surface areas, well-defined nanopores, and excellent stability remains extremely challenging because of the unmanageable reaction process. Until now, only a few reported POPs have Brunauer-Emmett-Teller (BET) surface areas (SBET ) exceeding 3000 m2 g-1 . Herein, we demonstrate a molecular expansion strategy to integrate high surface areas, large nanopore sizes, and outstanding stability into POPs. A series of hyper-crosslinked conjugated polymers (HCCPs) with exceptional porosity are synthesized through this strategy. Specially, HCCP-6 and HCCP-11 exhibit the highest surface areas (SBET >3000 m2 g-1 ) and excellent total pore volumes (up to 3.98 cm3 g-1 ) among these HCCPs. They present decent total CH4 storage capacities of 491 and 421 mg g-1 at 80 bar and 298 K, respectively. Meanwhile, they are highly stable in harsh environments. The facile and general molecular expansion strategy would lead to improved synthetic routes of POPs for desired functions.

Formulation of Metal-Organic Framework Inks for the 3D Printing of Robust Microporous Solids toward High-Pressure Gas Storage and Separation.

The shaping of metal-organic frameworks (MOFs) has become increasingly studied over the past few years, because it represents a major bottleneck toward their further applications at a larger scale. MOF-based macroscale solids should present performances similar to those of their powder counterparts, along with adequate mechanical resistance. Three-dimensional printing is a promising technology as it allows the fast prototyping of materials at the macroscale level; however, the large amounts of added binders have a detrimental effect on the porous properties of the solids. Herein, a 3D printer was modified to prepare a variety of MOF-based solids with controlled morphologies from shear-thinning inks containing 2-hydroxyethyl cellulose. Four benchmark MOFs were tested for this purpose: HKUST-1, CPL-1, ZIF-8, and UiO-66-NH2. All solids are mechanically stable with up to 0.6 MPa of uniaxial compression and highly porous with BET specific surface areas lowered by 0 to -25%. Furthermore, these solids were applied to high-pressure hydrocarbon sorption (CH4, C2H4, and C2H6), for which they presented a consequent methane gravimetric uptake (UiO-66-NH2, ZIF-8, and HKUST-1) and a highly preferential adsorption of ethylene over ethane (CPL-1).

Machine Learning Enabled Tailor-Made Design of Application-Specific Metal-Organic Frameworks.

In the development of advanced nanoporous materials, one clear and unavoidable challenge in hand is the sheer size (in principle, infinite) of the materials space to be explored. While high-throughput screening techniques allow us to narrow down the enormous-scale database of nanoporous materials, there are still practical limitations stemming from a costly molecular simulation in estimating a material's performance and the necessity of a sophisticated descriptor identifying materials. With an attempt to transition away from the screening-based approaches, this paper presents a computational approach combining the Monte Carlo tree search and recurrent neural networks for the tailor-made design of metal-organic frameworks toward the desired target applications. In the demonstration cases for methane-storage and carbon-capture applications, our approach showed significant efficiency in designing promising and novel metal-organic frameworks. We expect that this approach would easily be extended to other applications by simply adjusting the reward function according to the target performance property.

MD simulation of methane adsorption properties on pillared graphene bubble models.

Pillared graphene bubble framework is selected as the methane storage vessel in this article. All investigations of methane adsorption are executed by using the MD simulations. The average adsorption energy of methane on different bubble models is between - 4.3 and - 5.2 kcal/mol, which is desirable for absorbing and desorbing gas molecules. The methane adsorption properties of bubble models are obviously different from those of pillared graphene. The effect of graphene interlayer spacing on methane adsorption in selected bubble models can be negligible. Nevertheless, bubble density and temperature have a significant influence on methane adsorption. The amount of adsorbed methane on pillared bubble models at room temperature can reach up to 18.2 mmol/g. This performance of methane adsorption on pillared graphene bubble structures may bring new enlightenment to the investigations of gas storage materials.

Syntheses, Structures and Sorption Properties of Three Isoreticular Trinuclear Indium-Based Amide-Functionalized Metal-Organic Frameworks.

Amide-functionalized metal-organic frameworks (AFMOFs) as a subclass of MOF materials have received great interest recently because of their intriguing structures and diverse potential applications. In this work, solvothermal reactions between indium nitrate and two mixed-linkers afforded two new isoreticular 8-connected trinuclear indium-based AFMOFs of [(In3 O)(OH)(L2)2 (IN)2 ]⋅(solv)x (2-In) and [(In3 O)(OH)(L2)2 (AIN)2 ]⋅(solv)x (NH2 -2-In) (H2 L2=4,4'-(carbonylimino)dibenzoic acid and HIN=isonicotinic acid or HAIN=3-aminoisonicotinic acid), respectively. Moreover, by means of reticular chemistry, an extended network of [(In3 O)(OH)(L3)2 (PB)2 ]⋅(solv)x (3-In) (H2 L3=4,4'-(terephthaloylbis(azanediyl))dibenzoic acid, HPB=4-(4-pyridyl)benzoic acid) was also successfully realized after prolongation of the former dicarboxylate linker and HIN, resulting in a truly 8-connected isoreticular AFMOF platform. These frameworks were structurally determined by single-crystal X-ray diffraction (SCXRD). Sorption studies further demonstrate that 2-In and NH2 -2-In exhibit not only high surface areas and pore volumes but also relatively high carbon capture capabilities (the CO2 uptakes reach 60.0 and 75.5 cm3 g-1 at 298 K and 760 torr, respectively) due to the presences of amide and/or amine functional groups. The selectivity of CO2 /N2 and CO2 /CH4 calculated by IAST are 10.18 and 12.43, 4.20 and 4.23 for 2-In and NH2 -2-In, respectively, which were additionally evaluated by mixed-gases dynamic breakthrough experiments. In addition, high-pressure gas sorption measurements show that both materials could take up moderate amounts of natural gas.