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Diffusion of methane, ethane, propane and n-butane was studied within the micropores of several metal-organic frameworks (MOFs) of varying topologies, including the MOFs PCN-14, NU-125, NU-1100 and DUT-49. Diffusion coefficients of the pure components, as well as methane/ethane, methane/propane and methane/butane binary mixtures, were calculated using molecular dynamics simulations to understand the effect of the longer alkanes on uptake of natural gas in MOFs. The calculated self-diffusion coefficients of all four components are on the order of 10 −8 m 2 /s. The diffusion coefficients of the pure components decrease as a function of chain length in all of the MOFs studied and show different behaviour as a function of loading in different MOFs. The self-diffusivities follow the trend D PCN-14 NU-125≈NU-1100>DUT-49. By comparing the diffusion of pure methane and methane mixtures with the higher alkanes, it is observed that the diffusivity of methane is unaffected by the presence of the higher alkanes in the MOFs considered, indicating that the diffusion path of methane is not blocked by the higher alkanes present in natural gas. [ABSTRACT FROM AUTHOR]

The methane deliverable capacity of adsorbent materials is a critical performance metric that will determine the viability of using adsorbed natural gas (ANG) technology in vehicular applications. ARPA-E recently set a target deliverable capacity of 315 cc(STP)/cc that a viable adsorbent material should achieve to yield a driving range competitive with incumbent fuels. However, recent computational screening of hundreds of thousands of materials suggests that the target is unattainable. In this work, we aim to determine whether the observed limits in deliverable capacity (~200 cc(STP)/cc) are fundamental limits arising from thermodynamic or material design constraints. Our efforts focus on simulating methane adsorption isotherms in a large number of systems, resulting in a broad exploration of different combinations of spatial distributions and energetics of adsorption sites. All systems were classified into five adsorption scenarios with varying degrees of realism in the manner that adsorption sites are created and endowed with energetics. The scenarios range from methane adsorption on discrete idealized lattice sites to adsorption in metal–organic frameworks with coordinatively unsaturated sites (CUS) provided by metalated catechol groups. Our findings strongly suggest that the ARPA-E target is unattainable, although not due to thermodynamic constraints but due to material design constraints. On the other hand, we also find that the currently observed deliverable capacity limits may be moderately surpassed. For instance, incorporation of CUS in IRMOF-10 is predicted to yield a 217 cc(STP)/cc deliverable capacity. The modified material has a ~0.85 void fraction and a heat of adsorption of ~15 kJ/mol. This suggests that similar, moderate improvements over existing materials could be achieved as long as CUS incorporation still maintains a relatively large void fraction. Nonetheless, we conclude that more significant improvements in deliverable capacity will require changes in the currently proposed operation conditions. [ABSTRACT FROM AUTHOR]

A metal–organic framework(MOF) with high volumetric deliverablecapacity for methane was synthesized after being identified by computationalscreening of 204 hypothetical MOF structures featuring (Zr6O4)(OH)4(CO2)ninorganicbuilding blocks. The predicted MOF (NU-800) has an fcutopology in which zirconium nodes are connected via ditopic1,4-benzenedipropynoic acid linkers. Based on our computer simulations,alkyne groups adjacent to the inorganic zirconium nodes provide moreefficient methane packing around the nodes at high pressures. Thehigh predicted gas uptake properties of this new MOF were confirmedby high-pressure isotherm measurements over a large temperature andpressure range. The measured methane deliverable capacity of NU-800between 65 and 5.8 bar is 167 cc(STP)/cc (0.215 g/g),the highest among zirconium-based MOFs. High-pressure uptake valuesof H2and CO2are also among the highest reported.These high gas uptake characteristics, along with the expected highlystable structure of NU-800, make it a promising materialfor gas storage applications. [ABSTRACT FROM AUTHOR]

We designed, synthesized, and characterized a new Zr-based metal-organic framework material, NU-1100, with a pore volume of 1.53 ccg−1 and Brunauer-Emmett-Teller (BET) surface area of 4020 m2g−1; to our knowledge, currently the highest published for Zr-based MOFs. CH4/CO2/H2 adsorption isotherms were obtained over a broad range of pressures and temperatures and are in excellent agreement with the computational predictions. The total hydrogen adsorption at 65 bar and 77 K is 0.092 g g−1, which corresponds to 43 g L−1. The volumetric and gravimetric methane-storage capacities at 65 bar and 298 K are approximately 180 vSTP/v and 0.27 g g−1, respectively. [ABSTRACT FROM AUTHOR]

Highlights: [•] The geometric properties of over 137,000 hypothetical MOFs were analyzed. [•] The hypothetical MOFs have diverse geometrical properties. [•] Only 6 different framework nets appear in this database of hypothetical MOFs. [•] Energy minimization provides insights to MOF construction algorithms. [Copyright &y& Elsevier]

A Cu–carborane-basedmetal–organic framework (MOF), NU-135, which containsa quasi-spherical para-carborane moiety, has beensynthesized and characterized. NU-135exhibits a porevolume of 1.02 cm3/g anda gravimetric BET surface area of ca. 2600 m2/g, and thusrepresents the first highly porous carborane-based MOF. As a consequenceof the unique geometry of the carborane unit, NU-135hasa very high volumetric BET surface area of ca. 1900 m2/cm3. CH4, CO2, and H2adsorptionisotherms were measured over a broad range of pressures and temperaturesand are in good agreement with computational predictions. The methanestorage capacity of NU-135at 35 bar and 298 K is ca.187 vSTP/v. At 298 K, the pressure required to achievea methane storage density comparable to that of a compressed naturalgas (CNG) tank pressurized to 212 bar, which is a typical storagepressure, is only 65 bar. The methane working capacity (5–65bar) is 170 vSTP/v. The volumetric hydrogen storage capacityat 55 bar and 77 K is 49 g/L. These properties are comparable to thoseof current record holders in the area of methane and hydrogen storage.This initial example lays the groundwork for carborane-based materialswith high surface areas. [ABSTRACT FROM AUTHOR]

The article offers analysis on the molecular simulations of hydrogen, methane and acetylene storage in metal-organic frameworks (MOFs). It evaluates computational studies of gas adsorption in MOFs using molecular modeling of the three gases. Furthermore, it notes the importance of molecular simulation in understanding adsorption activities in other porous materials and MOFs.

Metal-organic frameworks (MOFs) are porous materials constructed from modular molecular building blocks, typically metal clusters and organic linkers. These can, in principle, be assembled to form an almost unlimited number of MOFs, yet materials reported to date represent only a tiny fraction of the possible combinations. Here, we demonstrate a computational approach to generate all conceivable MOFs from a given chemical library of building blocks (based on the structures of known MOFs) and rapidly screen them to find the best candidates for a specific application. From a library of 102 building blocks we generated 137,953 hypothetical MOFs and for each one calculated the pore-size distribution, surface area and methane-storage capacity. We identified over 300 MOFs with a predicted methane-storage capacity better than that of any known material, and this approach also revealed structure-property relationships. Methyl-functionalized MOFs were frequently top performers, so we selected one such promising MOF and experimentally confirmed its predicted capacity. [ABSTRACT FROM AUTHOR]

Abstract: Incorporation of Li cations into MOFs by either of two methods, chemical reduction or cation exchange, significantly improves the CO2/CH4 selectivity. While the selectivity enhancement by the chemical reduction seems to come from favorable displacement of catenated frameworks, the selectivity improvement by the Li cation exchange comes from enhanced solid–gas interactions. [Copyright &y& Elsevier]

We show that the MOF NU-111 exhibits equally high volumetric and gravimetric methane uptake values, both within ≈75% of the DOE targets at 300 K. Upon reducing the temperature to 270 K, the uptake increases to 0.5 g g−1 and 284 cc(STP) per cc at 65 bar. Adsorption of CO2 and H2 is also reported. Simulated isotherms are in excellent agreement with those obtained from experiments. [ABSTRACT FROM AUTHOR]

A diimide based porous organic polymer (POP) post-synthetically reduced with lithium metal demonstrates a drastic increase in selectivity for carbon dioxide over methane. [ABSTRACT FROM AUTHOR]

Separation of CO2/CH4 mixtures was studied in carborane-based metal–organic framework materials with and without coordinatively unsaturated metal sites; high selectivities for CO2 over CH4 (∼17) are obtained, especially in the material with open metal sites. [ABSTRACT FROM AUTHOR]