Binding energies of hydrogen molecules to isoreticular metal-organic framework materials

Recently, several novel isoreticular metal-organic framework (IRMOF) structures have been fabricated and tested for hydrogen storage applications. To improve our understanding of these materials, and to promote quantitative calculations and simulations, the binding energies of hydrogen molecules to the MOF have been studied. High-quality second-order Moller-Plesset (MP2) calculations using the resolution of the identity approximation and the quadruple zeta QZVPP basis set were used. These calculations use terminated molecular fragments from the MOF materials. For H2 on the zinc oxide corners, the MP2 binding energy using Zn4O(HCO2)6 molecule is 6.28 kJ/mol. For H2 on the linkers, the binding energy is calculated using lithium-terminated molecular fragments. The MP2 results with coupled-cluster singles and doubles and noniterative triples method corrections and charge-transfer corrections are 4.16 kJ/mol for IRMOF-1, 4.72 kJ/mol for IRMOF-3, 4.86 kJ/mol for IRMOF-6, 4.54 kJ/mol for IRMOF-8, 5.50 and 4.90 kJ/mol for IRMOF-12, 4.87 and 4.84 kJ/mol for IRMOF-14, 5.42 kJ/mol for IRMOF-18, and 4.97 and 4.66 kJ/mol for IRMOF-993. The larger linkers are all able to bind multiple hydrogen molecules per side. The linkers of IRMOF-12, IRMOF-993, and IRMOF-14 can bind two to three, three, and four hydrogen molecules per side, respectively. In general, the larger linkers have the largest binding energies, and, together with the enhanced surface area available for binding, will provide increased hydrogen storage. We also find that adding up NH2 or CH3 groups to each linker can provide up to a 33% increase in the binding energy.     

多孔金属有机骨架材料储氢性能分子模拟

采用优化的DREIDING力场参数, 通过巨正则系综蒙特卡洛(GCMC)模拟方法对H₂在IRMOF-1、IRMOF-61和IRMOF-62共3种金属有机骨架(MOFs)材料中的吸附平衡性能进行了比较研究. 结果表明, 该力场能够在全压力范围内很好地复制H₂在IRMOF-62材料中的等温吸附曲线; 但对低压下H₂在IRMOF-61中的等温吸附曲线预测出现低估. 与IRMOF-1相比, 具有互穿骨架结构的IRMOF-61和IRMOF-62材料在常温下的储氢能力并无明显提高. 进一步比较77 K时100 kPa、3.0 MPa下H₂在上述MOFs材料中达到吸附平衡时的几率密度分布发现, H₂会优先吸附在Zn₄O骨架附近靠近苯环的位置;对具有互穿结构的MOFs材料而言,由于其孔腔尺寸缩小, 使得H₂优先吸附位区域零散化. 适当长度的有机配体形成的互穿骨架结构能增强与H₂分子之间的相互作用, 具备较高的储氢能力; 而有机配体尺寸过长则会增加骨架结构中H₂吸附死角, 对H₂的吸附能力反而出现下降.     

On the nature of the interaction between H2 and metal-organic frameworks

The mechanism of adsorption of molecular hydrogen (H₂) on IRMOF-1 is studied at the MP2 level. The role of the two principal MOF components, the inorganic connector and the organic linker, for H₂ adsorption is evaluated. Correlation methods and large basis sets are necessary to describe correctly the weak interactions (London dispersion) and to account for the polarisability of H₂. We proof that the electrostatic interactions have a negligible contribution to the interaction energy and the adsorption mechanism is governed by London dispersion (3–5 kJ mol^?1).     

Characterization of H2 binding sites in prototypical metal-organic frameworks by inelastic neutron scattering

The hindered rotor transitions of H(2) adsorbed in the chemically related and prototypical porous metal-organic frameworks IRMOF-1, IRMOF-8, IRMOF-11, and MOF-177 were studied by inelastic neutron scattering to gain information on the specifics of H(2) binding in this class of adsorbents. Remarkably sharp and complex spectra of these materials signify a diversity of well-defined binding sites. Similarities in the spectral features as a function of H(2) loading and correlations with recent crystallographic studies were used to assign transitions ranging in rotational barrier from <0.04 to 0.6 kcal/mol as corresponding to localized adsorption sites on the organic and inorganic components of these frameworks. We find that binding of H(2) at the inorganic cluster sites is affected by the nature of the organic link and is strongest in IRMOF-11 in accord with our adsorption isotherm data. The sites on the organic link have lower binding energies, but a much greater capacity for increases in H(2) loading, which demonstrates their importance for hydrogen uptake by these materials.     

Potential Function and Thermodynamic Property of UO (cited: 2)

Potential energy scan for uranium oxide (UO) was performed by ab initio configuration inter-action (CI) method and density functional theory methods at the PBE1 and the B3LYP levels in combination with the (ECP80MWB_AVQZ+2f) basis set for uranium and 6-311+G* foroxygen. The dissociation energies of UO, after being corrected for the zero-point vibrational energy, are 2.38, 3.76, and 3.31 eV at the CI, PBE1, and B3LYP levels, respectively. The calculated energy was fitted to potential functions of Morse, Lennard-Jones, and Rydberg. Only the Morse function is eligible for the potential. The anharmonicity constant is 0.00425. The anharmonic frequency is 540.95 cm^-1 deduced from the PBE1 results. Thermodynamic properties of entropy and heat capacity at 298.2-1500 K were calculated using DFT-UPBE1 results and Morse parameters. The relationship between entropy and temperature was es-tablished.     

Ab initio rate constants from hyperspherical quantum scattering: application to H + CH4 --> H2 + CH3

A general and practical procedure is described for calculating rate constants for chemical reactions using a minimal number of ab initio calculations and quantum-dynamical computations. The method exploits a smooth interpolating functional developed in the hyperspherical representation. This functional is built from two Morse functions and depends on a relatively small number of parameters with respect to conventional functionals developed to date. Thus only a small number of ab initio points needs to be computed. The method is applied to the H + CH4 --> H2 + CH3 reaction. The quantum scattering calculations are performed treating explicitly the bonds being broken and formed. All the degrees of freedom except the breaking and forming bonds are optimized ab initio and harmonic vibrational frequencies and zero-point energies for them are calculated at the MP2(full) level with a cc-pVTZ basis set. Single point energies are calculated at a higher level of theory with the same basis set, namely CCSD(T, full). We report state-to-state cross sections and thermal rate constants for the title reaction and make comparisons with previous results. The calculated rate constants are in good agreement with experiments.     

Potential energy surfaces for small alcohol dimers. II. Propanol, isopropanol, t-butanol, and sec-butanol

Potential energy landscapes for homogeneous dimers of propanol, isopropanol, tert-butanol, and sec-butanol were obtained using 735 counterpoise-corrected energies at the MP2/6-311+G(2df,2pd) level. The landscapes were sampled at 15 dimer separation distances for different relative monomer geometries, or routes, given in terms of the yaw, pitch, and roll of one monomer relative to the other and the spherical angles between the two monomer centers (taken as the C atom attached to the O). The resultant individual energy surfaces and their complex topographies were also regressed using a site-site pair potential model using a modified Morse potential that provides a mathematically simple representation of the landscapes suitable for use in molecular simulations. Generalized Morse parameters were also obtained for this model from a composite regression of these energy landscapes and those previously reported for methanol and ethanol. The quality of fit for all these energy landscapes suggests that these site parameters have transferability for possible use on other alcohols.     

Adsorption of RDX and TATP on IRMOF-1: an ab initio study

The adsorption of 1,3,5-trinitro-s-triazine (RDX) and triacetone triperoxide (TATP) on representative fragments of metal organic framework (IRMOF-1) was studied at the B3LYP/6-31G(d) level of theory. For examined adsorbates several possible adsorption positions toward the IRMOF-1 fragments were found. The adsorption strength of the adsorbate on IRMOF-1 is largely affected by the geometry of the active site of IRMOF-1 which controls the orientation of the target molecule with respect to the IRMOF-1 fragment. The calculations show that the adsorption on these fragments occurs due to the formation of hydrogen bonds between the molecular C–H groups and the oxygen atoms of IRMOF-1. The RDX and TATP molecules are the most strongly adsorbed on the linker fragment of IRMOF-1. This type of adsorption results in the polarization of RDX and TATP on the IRMOF-1 fragments. The interaction energy of two most stable RDX-, and TATP-IRMOF-1 adsorption systems are ?9.8 and ?12.8 kcal/mol, respectively. It can be concluded that the 1,4-benzenedicarboxylate site of IRMOF-1 shows the stronger molecular adsorption of RDX and TATP than the site containing [Zn₄O(CO₂)₆] and also it is characterized by higher reactivity than the other considered sites. The binding of studied explosive molecules to IRMOF-1 consists of interplay between attractive interactions between the target molecule and MOF as well as the shielding by the IRMOF-1 fragment induced by the molecular adsorption. The relative importance of these effects depends on the chemical nature, the size, and the shape of the molecule and MOF. Small-size molecules require smaller space for the adsorption and also they are less shielded by the sizeable adsorbent. So they interact better when adsorbed on larger IRMOF-1 fragment. On the other side, larger molecules show higher adsorption strength with small fragments of IRMOF-1.     

Thermochemistry of Lewis adducts of BH3 and nucleophilic substitution of triethylamine on NH3BH3 in tetrahydrofuran

The thermochemistry of the formation of Lewis base adducts of BH(3) in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane by triethylamine are reported. The dative bond energy of Lewis adducts were predicted using density functional theory at the B3LYP/DZVP2 and B3LYP/6-311+G** levels and correlated ab initio molecular orbital theories, including MP2, G3(MP2), and G3(MP2)B3LYP, and compared with available experimental data and accurate CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or the B3LYP geometries reproduce the benchmark results usually to within ~1 kcal/mol. Energies calculated at the MP2/aug-cc-pVTZ level for geometries optimized at the B3LYP/DZVP2 or B3LYP/6-311+G** levels give dative bond energies 2-4 kcal/mol larger than benchmark values. The enthalpies for forming adducts in THF were determined by calorimetry and compared with the calculated energies for the gas phase reaction: THFBH(3) + L → LBH(3) + THF. The formation of NH(3)BH(3) in THF was observed to yield significantly more heat than gas phase dative bond energies predict, consistent with strong solvation of NH(3)BH(3). Substitution of NEt(3) on NH(3)BH(3) is an equilibrium process in THF solution (K ≈ 0.2 at 25 °C). The reaction obeys a reversible bimolecular kinetic rate law with the Arrhenius parameters: log A = 14.7 ± 1.1 and E(a) = 28.1 ± 1.5 kcal/mol. Simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical S(N)2 mechanism.     

Diffusion and separation of CO2 and CH4 in silicalite, C168 schwarzite, and IRMOF-1: a comparative study from molecular dynamics simulation

Recently we have investigated the storage and adsorption selectivity of CO(2) and CH(4) in three different classes of nanoporous materialssilicalite, IRMOF-1, and C(168) schwarzite through Monte Carlo simulation (Babarao, R.; Hu, Z.; Jiang, J. Langmuir, 2007, 23, 659). In this work, the self-, corrected, and transport diffusivities of CO(2) and CH(4) in these materials are examined using molecular dynamics simulation. The activation energies at infinite dilution are evaluated from the Arrhenius fits to the diffusivities at various temperatures. As loading increases, the self-diffusivities in the three frameworks decrease as a result of the steric hindrance; the corrected diffusivities remain nearly constant or decrease approximately linearly depending on the adsorbate and framework; and the transport diffusivities generally increase except for CO(2) in IRMOF-1. The correlation effects are identified to reduce from MFI, C(168) to IRMOF-1, in accordance with the porosity increasing in the three frameworks. Predictions of self-, corrected, and transport diffusivities for pure CO(2) and CH(4) from the Maxwell-Stefan formulation match the simulation results well. In a CO(2)/CH(4) mixture, the self-diffusivities decreases with loading, and good agreement is found between simulated and predicted results. On the basis of the adsorption and self-diffusivity in the mixture, the permselectivity is found to be marginal in IRMOF-1, slightly enhanced in MFI, and greatest in C(168) schwarzite. Although IRMOF-1 has the largest storage capacity for CH(4) and CO(2), its selectivity is not satisfactory.     

An assessment of density functional methods for studying molecular adsorption in cluster models of zeolites

The performance of six density functional theory (DFT) methods has been tested for a zeolite cluster containing three tetrahedral atoms (3T) and the complexes it forms with water and methanol molecules. The DFT methods (BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91) give results in good agreement with second-order perturbation theory (MP2). The results in this paper provide evidence of the suitability of DFT methods for studying hydrogen-bonded adsorption complexes in zeolites. Generally, the hybrid DFT methods are in closer agreement with experiment and MP2 than the pure DFT methods for geometrical parameters. The only exception is the Z⁻ geometry, perhaps due to its anionic character. All DFT methods give results in good overall agreement with MP2 for intramolecular geometrical parameters of the adsorption complexes, intramolecular vibrational frequencies, and adsorption energies. The B3LYP method gives intermolecular geometries and intermolecular vibrational frequencies which are closest to those obtained from the MP2 method. Thus, the B3LYP method seems to be the best choice for a density functional treatment of molecular adsorption in zeolite systems.     

含羧酸镁(钙)官能团的多孔芳香骨架材料储氢性能的预测

用MP2方法,TZVPP基组以及基组重叠误差(BSSE)校正计算了氢分子与修饰在多孔芳香骨架(PAF)上的羧酸镁、羧酸钙官能团的相互作用,并建立了描述这一相互作用的分子力学力场.在此基础上用巨正则系综蒙特卡洛(GCMC)模拟预测了氢气在该种新型PAF材料上的吸附等温线.量子化学计算结果表明,每个羧酸镁、羧酸钙官能团分别可以提供13、14个氢分子吸附位点,与每个氢分子的平均结合能在8kJ·mol-1左右.通过比较不同温度和压力下材料的绝对吸附量和超额吸附量发现,在PAF骨架中引入羧酸镁、羧酸钙官能团可以显著提高材料的综合储氢性能,达到并超过了美国能源部提出的2015年储氢标准.同时该工作还揭示了氢吸附量与材料的表面积、空腔体积和分子作用强度间的复杂关系.     

Storage and separation of CO2 and CH4 in silicalite, C168 schwarzite, and IRMOF-1: a comparative study from Monte Carlo simulation

Storage of pure CO2 and CH4 and separation of their binary mixture in three different classes of nanostructured adsorbents--silicalite, C168 schwarzite, and IRMOF-1--have been compared at room temperature using atomistic simulation. CH4 is represented as a spherical Lennard-Jones molecule, and CO2 is represented as a rigid linear molecule with a quadrupole moment. For pure component adsorption, CO2 is preferentially adsorbed than CH4 in all the three adsorbents over the pressure range under this study, except in C168 schwarzite at high pressures. The simulated adsorption isotherms and isosteric heats match closely with available experimental data. A dual-site Langmuir-Freundlich equation is used to fit the isotherms satisfactorily. Compared to silicalite and C168 schwarzite, the gravimetric adsorption capacity of pure CH4 and CO2 separately in IRMOF-1 is substantially larger. This implies that IRMOF-1 might be a potential storage medium for CH4 and CO2. For adsorption from an equimolar binary mixture, CO2 is preferentially adsorbed in all three adsorbents. Predictions of mixture adsorption with the ideal-adsorbed solution theory on the basis of only pure component adsorption agree well with simulation results. Though IRMOF-1 has a significantly higher adsorption capacity than silicalite and C168 schwarzite, the adsorption selectivity of CO2 over CH4 is found to be similar in all three adsorbents.     

Treating dispersion effects in extended systems by hybrid MP2:DFT calculations--protonation of isobutene in zeolite ferrierite

We propose use of a hybrid method to study problems that involve both bond rearrangements and van-der-Waals interactions. The method combines second-order M?ller-Plesset perturbation theory (MP2) calculations for the reaction site with density functional theory (DFT) calculations for a large system under periodic boundary conditions. Hybrid MP2:DFT structure optimisation for a cluster embedded in the periodic model is the first of three steps in a multi-level approach. The second step is extrapolation of the MP2 energy to the complete basis set limit. The third step is extrapolating the high-level (MP2) correction to the limiting case of the full periodic structure. This is done by calculating the MP2 correction for a series of cluster models of increasing size, fitting an analytic expression to these energy corrections, and applying the fitted expression to the full periodic structure. We assume that, up to a constant, the high-level correction is described by a damped dispersion expression. Combining the results of all three steps yields an estimate of the MP2 reaction energy for the full periodic system at the complete basis set level. The method is designed for a reaction between a small or medium sized substrate molecule and a very large chemical system. For adsorption of isobutene in zeolite H-ferrierite, the energies obtained for the formation of different structures, the pi-complex, the isobutoxide, the tert-butoxide, and the tert-butyl carbenium ion, are -78, -73, -48, and -21 kJ mol(-1), respectively. This corresponds to corrections of the pure DFT (PBE functional) results by -62, -70, -67, and -29 kJ mol(-1), respectively. Hence, the MP2 corrections are substantial and, perhaps more importantly, not the same for the different hydrocarbon species in the zeolite. Coupled-cluster (CCSD(T)) calculations change the MP2 energies by -4 kJ mol(-1) (tert-butyl cation) or less (below +/-1 kJ mol(-1) for the other species).     

Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites

Br?nsted acidity of H-chabazite (CHA) zeolites (Si : Al(2) = 4.2) was investigated by means of ammonia infrared-mass spectrometry/temperature-programmed desorption (IRMS-TPD) methods and density functional calculations. Four IR bands were observed at 3644, 3616, 3575 and 3538 cm(-1), and they were ascribable to the acidic OH groups on four nonequivalent oxygen sites in the CHA structure. The absorption band at 3538 cm(-1) was attributed to the O(4)H in the 6-membered ring (MR), and ammonia adsorption energy (DeltaU) of this OH group was the lowest among the 4 kinds of OH groups. The other 3 bands were assigned to the acidic OH groups in 8MR. It was observed that the DeltaU in 8 and 6MR were 131 (+/-3) and 101 kJ mol(-1), respectively. On the other hand, the density functional theory (DFT) calculations within periodic boundary conditions yielded the adsorption energies on these OH groups in 8 and 6MR to be ca. 130 and 110 kJ mol(-1), respectively, in good agreement with the experimentally-observed values.     

Effects of functionalization, catenation, and variation of the metal oxide and organic linking units on the low-pressure hydrogen adsorption properties of metal-organic frameworks

The dihydrogen adsorption isotherms of eight metal-organic frameworks (MOFs), measured at 77 K up to a pressure of 1 atm, have been examined for correlations with their structural features. All materials display approximately Type I isotherms with no hysteresis, and saturation was not reached for any of the materials under these conditions. Among the six isoreticular MOFs (IRMOFs) studied, the catenated materials exhibit the largest capacities on a molar basis, up to 9.8 H(2) per formula unit. The addition of functional groups (-Br, -NH(2), -C(2)H(4)-) to the phenylene links of IRMOF-1 (MOF-5), or their replacement with thieno[3,2-b]thiophene moieties in IRMOF-20, altered the adsorption behavior by a minor amount despite large variations in the pore volumes of the resulting materials. In contrast, replacement of the metal oxide units with those containing coordinatively unsaturated metal sites resulted in greater H(2) uptake. The enhanced affinities of these materials, MOF-74 and HKUST-1, were further demonstrated by calculation of the isosteric heats of adsorption, which were larger across much of the range of coverage examined, compared to those of representative IRMOFs. The results suggest that under low-loading conditions, the H(2) adsorption behavior of MOFs can be improved by imparting larger charge gradients on the metal oxide units and adjusting the link metrics to constrict the pore dimensions; however, a large pore volume is still a prerequisite feature.     

Interaction model for the adsorption of organic molecules on the silver surface

Adsorption of organics on a silver surface is simulated. An Embedded Atom Model is used for the metal, a standard force field for the organics, and a combination of the charge equilibration model and the Morse potential for their electrostatic and nonbonding interactions. The only adjustable parameters of this approach appear in the Morse potential. They are tuned to reproduce experimental and high level quantum chemical data. The adsorption energies of 13 molecules on the Ag(111) surface are obtained with an average error of less than 1 kcal mol(-1). The model should be transferable to molecules with the same chemical groups used in regressing the potential parameters when physisorption or weak chemisorption, i.e., no bond breaking, occur, and also to other Ag surfaces. When used to simulate perylene tetracarboxylic acid dianhydride (PTCDA) on Ag(111), correct geometry of mono- and multilayers are observed in molecular dynamics simulations at room temperature.     

Molecular simulation of the hydration of ethene to ethanol using ab initio potentials and free energy curves

Molecular dynamics simulations of aqueous solutions at infinite dilution of the reaction of water with ethene: H2O + CH2CH2 --> CH3CH2OH were performed using Lennard-Jones 12-6-1 potentials to describe the solute-solvent interactions, and TIP3P to describe the water-water interactions. The Morokuma decomposition scheme of ab initio interaction energies at the SCF level and the dispersion component at the MP2 level were used to reproduce the molecular parameters of the solute-water interaction potentials. The results show that the functions that use the EX-PL-DIS-ES interaction model to describe the solvation of the reactant, transition state, and product systems lead to good values of the reaction (Delta G) and acceptable values of the activation (Delta G#) free energy as compared with those from using AMBER-derived parameters, using the available theoretical and experimental data as referents.     

Computational methods in organic thermochemistry. 1. Hydrocarbon enthalpies and free energies of formation

Standard state enthalpies and free energies of formation can be computed with reasonable accuracy (usually within 4 and often 2 kJ/mol) using high level model chemistries. A comparison set of nearly 300 organic compounds ranging from 1 to 10 carbon atoms having a variety of functional groups for which enthalpy and free energy literature values are available has been examined using G2, G2MP2, G3, G3MP2, G3B3, G3MP2B3, CBS-QB3, and density functional (B3LYP/6-311+G(3df,2p)) model chemistries. G3 gives an average mean absolute deviation of 3.0 and 13.4 kJ/mol for the enthalpies and free energies, respectively, using the atomization method and 3.1 and 3.7 kJ/mol when bond separation reactions are employed. G3 and G3B3 are the most accurate overall; the related G3MP2 and G3MP2B3 are nearly as accurate and can compute larger molecules. CBS-QB3 was also found to be accurate but is more limited in the size of molecules that can be computed. The density functional energies were found to have large deviations from the literature values using either the atomization or the bond separation method. Regardless of the model employed, the free energies are increasingly underestimated by computation as the size of the molecule increases. A series of corrections applied to the aliphatic hydrocarbons is presented, which usually reduces the deviations to less than 4 kJ/mol regardless of the size of the molecule.     

CO2 absorption in aqueous solutions of alkanolamines: mechanistic insight from quantum chemical calculations

DFT and high-level ab initio calculations (among them B3LYP and G3MP2B3) have been used to describe molecular reactions relevant for CO2 absorption in aqueous (alkanol)amine solutions. Reaction mechanisms for various reactions of CO2 with ammonia, monoethanolamine (MEA), and diethanolamine (DEA) to carbamic acid and ion pair products have been investigated and interpreted in light of experimental observations. Additional water, ammonia, MEA, and DEA molecules have also been added to the molecular complexes to simulate microsolvation effects. These extra molecules may act as catalysts for the desired reactions, and in several cases they have a large impact on activation and reaction energies. Solvent effects were estimated by applying electrostatic continuum models for selected systems. Our calculated transition state energies agree well with experimental activation energies.