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We investigated eight representative metal–organic frameworks for methane storage using molecular simulation. Validated force fields were used to calculate the amount adsorbed for pure methane and its mixtures with CO2 and H2O at 5.8 and 65 bar at 298 K within the composition limits specified for natural gas. Within the analyzed concentrations, the MOFs without open metal sites were minimally influenced by the presence of CO2 and H2O. However, for the MOFs with open metal sites, the presence of these species proved to be harmful. We found that concentrations as low as 25 ppm of water can reduce the delivered volume of methane by more than 20%. A detailed analysis of the adsorption mechanisms leading to this poisoning is presented. These results highlight the possible limitations of MOFs with open metal sites for use in natural gas storage. [ABSTRACT FROM AUTHOR]

Adsorption hysteresis is a phenomenon related to phase transitions that can impact applications such as gas storage and separations in porous materials. Computational approaches can greatly facilitate the understanding of phase transitions and phase equilibria in porous materials. In this work, adsorption isotherms for methane, ethane, propane, and n-hexane were calculated from atomistic grand canonical Monte Carlo (GCMC) simulations in a metal-organic framework having both micropores and mesopores to better understand hysteresis and phase equilibria between connected pores of different size and the external bulk fluid. At low temperatures, the calculated isotherms exhibit sharp steps accompanied by hysteresis. As a complementary simulation method, canonical (NVT) ensemble simulations with Widom test particle insertions are demonstrated to provide additional information about these systems. The NVT+Widom simulations provide the full van der Waals loop associated with the sharp steps and hysteresis, including the locations of the spinodal points and points within the metastable and unstable regions that are inaccessible to GCMC simulations. The simulations provide molecular-level insight into pore filling and equilibria between high- and low-density states within individual pores. The effect of framework flexibility on adsorption hysteresis is also investigated for methane in IRMOF-1. Developing computational tools to study the phase behaviour of adsorbate molecules in complex pore architectures can greatly facilitate our understanding of phase transitions and phase equilibria in porous materials. Here, molecular simulations are used to study the hysteresis and phase equilibria of n-alkane adsorbate molecules in a metal–organic framework with both micropores and mesopores, and simulations in the canonical ensemble with Widom insertions are shown to be a powerful complementary approach to grand canonical Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

• VO x and CeO 2 surface speciation on Al 2 O 3 depends on the ALD growth sequence. • The presence of CeO 2 does not impact the electronic structure of the V O bond. • VO x and CeO 2 on Al 2 O 3 migrate at elevated temperatures under reducing conditions. • CeO 2 clusters on Al 2 O 3 and ceria surface sites in VO x /CeO 2 favor total oxidation. • Different active sites on CeO 2 show varying reactivity towards alkane and alkene. Alumina-supported VO x -CeO 2 materials are synthesized by atomic layer deposition (ALD) and evaluated in cyclohexane oxidative dehydrogenation (ODH). The order of deposition of the two metal oxides by ALD influences the size, geometry and oxidation state of the resulting surface species as indicated by Raman spectroscopy coupled with density functional theory (DFT) calculations and XPS. The individual contributions of VO x and CeO 2 surface sites to the cyclohexane ODH reaction mechanism are investigated through a comparison of the catalytic activity of the supported mixed metal oxide materials, VO x /Al 2 O 3 , VO x /CeO 2 and a bare CeO 2 support. The highest activity is observed for CeO 2 , and different surface active sites are distinguished by introducing cyclohexane and cyclohexene in the reactant feed mixture. The catalytic behavior is dependent on the nature of V O S (S, Support) bonds, and small clusters of CeO 2 on Al 2 O 3 favor total oxidation, similar to exposed CeO 2 sites in VO x /CeO 2. [ABSTRACT FROM AUTHOR]

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]

Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems. [ABSTRACT FROM AUTHOR]

Polymorphism in metal − organic frameworks (MOFs) means that the same chemical building blocks (nodes and linkers) can be used to construct isomeric MOFs with different topological networks. The choice of topology can substantially impact the pore network of the MOF, changing the sizes and shapes of the pores, which has implications for adsorption and separation applications. In this work, we look at the influence of topology in 38 polymorphic MOFs on the separation of linear and branched C4–C6 alkane isomers, a separation of great importance to the petrochemical industry. We find that the MOF Cu 2 (1,4-benzenedicarboxylate) in nbo topology (nbo-Cu 2 BDC) has particularly high affinity for linear alkanes due to its small pore size, which excludes the branched isomers. Upon studying this MOF in further detail, we find that it can take either of two conformations: a cubic conformation, which is typical of nbo MOFs, and a unique star conformation that contains 1D triangular and hexagonal channels. The determination of which conformation the MOF will adopt depends on steric effects between the nodes and linkers. Image 1 [ABSTRACT FROM AUTHOR]

Six first-row transition metal cations (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chemistry of C C bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: 1) single transition metal cations supported on zirconium oxide nodes of the metal–organic framework NU-1000 and 2) small metal hydroxide clusters with two metal atoms (M 2 ) that could be grown by atomic layer deposition on a support exhibiting isolated hydroxyl groups. Using scaling relations, the free energies of co-adsorbed hydrogen and ethene (i.e., (H/C 2 H 4 )*) and adsorbed ethyl (i.e., C 2 H 5 *) were identified as descriptors for ethene dimerization catalysis. Using degree of rate control analysis, it was determined that the rate controlling steps are either ethene insertion (C C bond forming) or β-hydride elimination (C H bond breaking), depending on the metal. Using degree of catalyst control analysis, it was determined that activity on all the catalysts studied could be improved by tuning the free energy of C 2 H 5 *. [ABSTRACT FROM AUTHOR]

The prospect of using copper oxide nanoclusters grown by atomic layer deposition on a porphyrin support for selective oxidation of methane to methanol was examined by means of density functional theory (DFT) calculations. Ab initio thermodynamic analysis indicates that an active site in the form of Cu(μ-O)Cu can be stabilized by activation in O 2 at 465 K. Furthermore, a moderate methane activation energy barrier (E a  = 54 kJ/mol) is predicted, and the hydrogen abstraction activity of the active site could be attributed to the radical character of the bridging oxygen. Methanol extraction in this system is limited by a thermodynamic barrier to desorption of ΔG = 57 kJ/mol at 473 K; however, desorption can be facilitated by the addition of water in a “stepped conversion” process. Overall, our results indicate similar activity between porphyrin-supported copper oxide nanoclusters and existing Cu-exchanged zeolites and provide a computational proof-of-concept for utilizing functionalized organic linkers in metal-organic frameworks (MOFs) for selective oxidation of methane to methanol. [ABSTRACT FROM AUTHOR]

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths. [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 kinetic analysis of gas-phase cyclohexene epoxidation by H 2 O 2 over mesoporous TS-1 was performed. The production of cyclohexene oxide was very stable with high selectivity. Based on the kinetic analysis, the gas-phase mechanism is proposed to be similar to that of the liquid-phase reaction: an Eley–Rideal type mechanism, in which the reaction between a Ti–OOH intermediate and the physisorbed alkene is the rate-determining step. When the partial pressure of water or H 2 O 2 was varied, a compensation effect was observed. Based on the kinetic model, the compensation effect is attributed to variations in the surface coverage of intermediates, specifically the competitive adsorption of water and H 2 O 2 at the Ti active sites. A meaningful activation energy can only be obtained at high surface coverages of H 2 O 2 and was determined to be 40 ± 2 kJ/mol. [ABSTRACT FROM AUTHOR]