Grand canonical Monte Carlo simulation study on the catenation effect on hydrogen adsorption onto the interpenetrating metal-organic frameworks

Among recently synthesized isoreticular metal-organic frameworks (IRMOFs), interpenetrating IRMOFs show high hydrogen adsorption capacities at low temperature and under ambient pressure. However, little is known about the molecular basis of their hydrogen binding properties. In this work, we performed grand canonical Monte Carlo (GCMC) simulations to investigate the effect of catenation on the interactions between hydrogen molecules and IRMOFs. We identified the adsorption sites and analyzed the adsorption energy distributions. The simulation results show that the small pores generated by catenation can play a role to confine the hydrogen molecules more densely, so that the capacity of the interpenetrating IRMOFs could be higher than that of the non-interpenetrating IRMOFs.     

Design of new materials for methane storage

One of the strategic goals of the modern automobile manufacturing industry is to replace gasoline and diesel with alternative fuels such as natural gas. In this report, we elucidate the desired characteristics of an optimal adsorbent for gas storage. The U.S. Department of Energy has outlined several requirements that adsorbents must fulfill for natural gas to become economically viable, with a key criterion being the amount adsorbed at 35 bar. We explore the adsorption characteristics of novel metal-organic materials (IRMOFs and molecular squares) and contrast them with the characteristics of two zeolites, MCM-41, and different carbon nanotubes. Using molecular simulations, we uncover the complex interplay of the factors influencing methane adsorption, especially the surface area, the capacity or free volume, the strength of the energetic interaction, and the pore size distribution. We also explain the extraordinary adsorption properties of IRMOF materials and propose new, not yet synthesized IRMOF structures with adsorption characteristics that are predicted to exceed the best experimental results to date by up to 36%.     

Hydrogen storage in porous structures of adamantane‐based nitrogen‐heterocyclic ring with diamond‐like structure

Using the density functional theory and molecular mechanics methods, we calculated the binding energy and parameters about the primitive cell designed by us with the adamantane and the nitrogen heterocyclic ring, the vibrational frequencies about the small complexes. Grand canonical Monte Carlo simulations were performed to predict the capacities for the hydrogen storage and adsorption isotherms. The results show the positive effects of bigger specific surface area and pore volume on hydrogen storage and isosteric heat. The gravimetric hydrogen uptake of adamantane‐based nitrogen‐heterocyclic ring of quaterpyridyl can reach 9.02 wt % at room temperature and 100 bar. But the volumetric H₂ capacities of the four materials are low at T = 298 K because of weak interaction between the materials and H₂ molecule. © 2014 Wiley Periodicals, Inc.     

Storage of hydrogen at 303 K in graphite slitlike pores from grand canonical Monte Carlo simulation

Grand canonical Monte Carlo (GCMC) simulations were used for the modeling of the hydrogen adsorption in idealized graphite slitlike pores. In all simulations, quantum effects were included through the Feynman and Hibbs second-order effective potential. The simulated surface excess isotherms of hydrogen were used for the determination of the total hydrogen storage, density of hydrogen in graphite slitlike pores, distribution of pore sizes and volumes, enthalpy of adsorption per mole, total surface area, total pore volume, and average pore size of pitch-based activated carbon fibers. Combining experimental results with simulations reveals that the density of hydrogen in graphite slitlike pores at 303 K does not exceed 0.014 g/cm(3), that is, 21% of the liquid-hydrogen density at the triple point. The optimal pore size for the storage of hydrogen at 303 K in the considered pore geometry depends on the pressure of storage. For lower storage pressures, p < 30MPa, the optimal pore width is equal to a 2.2 collision diameter of hydrogen (i.e., 0.65 nm), whereas, for p congruent with 50MPa, the pore width is equal to an approximately 7.2 collision diameter of hydrogen (i.e., 2.13 nm). For the wider pores, that is, the pore width exceeds a 7.2 collision diameter of hydrogen, the surface excess of hydrogen adsorption is constant. The importance of quantum effects is recognized in narrow graphite slitlike pores in the whole range of the hydrogen pressure as well as in wider ones at high pressures of bulk hydrogen. The enthalpies of adsorption per mole for the considered carbonaceous materials are practically constant with hydrogen loading and vary within the narrow range q(st) congruent with 7.28-7.85 kJ/mol. Our systematic study of hydrogen adsorption at 303 K in graphite slitlike pores gives deep insight into the timely problem of hydrogen storage as the most promising source of clean energy. The calculated maximum storage of hydrogen is equal to approximately 1.4 wt %, which is far from the United States Department of Energy (DOE) target (i.e., 6.5 wt %), thus concluding that the total storage amount of hydrogen obtained at 303 K in graphite slitlike pores of carbon fibers is not sufficient yet.     

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.     

Low-temperature adsorption of hydrogen on nanoporous aluminophosphates: effect of pore size

Several nanoporous aluminophosphates (AlPOs) have been used to analyze the effect of pore diameter on the hydrogen adsorption characteristics. The heat of adsorption and adsorption capacity per unit micropore volume increase with decreasing pore size. AlPOs with smaller micropores favorably adsorb hydrogen at relatively low pressures. This work demonstrates that small pore size and large micropore volume are beneficial for high hydrogen uptake.     

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.     

Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials

We report the synthesis of zeolite-like carbon materials that exhibit well-resolved powder XRD patterns and very high surface area. The zeolite-like carbons are prepared via chemical vapor deposition (CVD) at 800 or 850 degrees C using zeolite beta as solid template and acetonitrile as carbon precursor. The zeolite-like structural ordering of the carbon materials is indicated by powder XRD patterns with at least two well-resolved diffraction peaks and TEM images that reveal well-ordered micropore channels. The carbons possess surface area of up to 3200 m2/g and pore volume of up to 2.41 cm3/g. A significant proportion of the porosity in the carbons (up to 76% and 56% for surface area and pore volume, respectively) is from micropores. Both TEM and nitrogen sorption data indicate that porosity is dominated by pores of size 0.6-0.8 nm. The carbon materials exhibit enhanced (and reversible) hydrogen storage capacity, with measured uptake of up to 6.9 wt % and estimated maximum of 8.33 wt % at -196 degrees C and 20 bar. At 1 bar, hydrogen uptake capacity as high as 2.6 wt % is achieved. Isosteric heat of adsorption of 8.2 kJ/mol indicates a favorable interaction between hydrogen and the surface of the carbons. The hydrogen uptake capacity observed for the zeolite-like carbon materials is among the highest ever reported for carbon (activated carbon, mesoporous carbon, CNTs) or any other (MOFs, zeolites) porous material.     

Amine functionalised metal organic frameworks (MOFs) as adsorbents for carbon dioxide

Three different porous metal organic framework (MOF) materials have been prepared with and without uncoordinated amine functionalities inside the pores. The materials have been characterized and tested as adsorbents for carbon dioxide. At 298 K the materials adsorb significant amount of carbon dioxide, the amine functionalised adsorbents having the highest CO₂ adsorption capacities, the best adsorbing around 14 wt% CO₂ at 1.0 atm CO₂ pressure. At 25 atm CO₂ pressure, up to 60 wt% CO₂ can be adsorbed. At high pressures the CO₂ uptake is mostly dependent on the available surface area and pore volume of the material in question. For one of the iso-structural MOF pairs the introduction of amine functionality increases the differential adsorption enthalpy (from isosteric method) from 30 to around 50 kJ/mole at low CO₂ pressures, while the adsorption enthalpies reach the same level at increase pressures. The high pressure experimental results indicate that MOF based solid adsorbents can have a potential for use in pressure swing adsorption of carbon dioxide at elevated pressures.     

Molecular simulation of hydrogen diffusion in interpenetrated metal-organic frameworks

In this work a combined molecular dynamics simulation and dynamically corrected transition-state theory (dcTST) study was performed to investigate the effect of interpenetration (catenation) on hydrogen diffusion in metal-organic frameworks (MOFs) as well as their relationships. The results on 10 isoreticular MOFs (IRMOFs) with and without interpenetration show that catenation can reduce hydrogen diffusivity by a factor of 2 to 3 at room temperature, and for the interpenetrated IRMOFs with multi-pores of different sizes, free volume can serve as a measure for hydrogen diffusivity: the bigger the free volume, the larger the hydrogen diffusivity. In addition, the present work shows that dcTST can directly reveal the influence of the MOF structure on hydrogen diffusivity, which is a powerful tool for providing a better understanding of the relationship between gas diffusivity and MOF structure.     

Low-temperature adsorption/storage of hydrogen on FAU, MFI, and MOR zeolites with various Si/Al ratios: effect of electrostatic fields and pore structures

Several zeolites, such as faujasite, mordenite, and ZSM-5, with various aluminum contents have been used to analyze the effect of aluminum or cation concentration (strength of electrostatic field) on hydrogen adsorption at low temperature. Irrespective of the zeolite structure, the adsorption capacity, isosteric heat of adsorption (-DeltaHads), surface coverage, and micropore occupancy increase with increasing aluminum content of a zeolite. Zeolites with a higher amount of aluminum favorably adsorb hydrogen at relatively low pressures. For zeolites with similar aluminum contents, the adsorption capacity, isosteric heat of adsorption, surface coverage, and micropore occupancy are in the order of mordenite>ZSM-5>faujasite, probably due to differing pore sizes and the presence or absence of pore intersections. This work demonstrates that zeolites with strong electrostatic fields and narrow pores without intersections are beneficial for high hydrogen uptake.     

二氧化碳在金属-有机骨架材料中吸附的阶梯现象

采用巨正则系综蒙特卡罗(GCMC)模拟方法, 对二氧化碳在5种具有相同拓扑结构的金属-有机骨架材料(IRMOFs), 即IRMOF-1, -8, -10, -14, -16中吸附产生的阶梯现象进行了详细的研究. 结果表明: 低温条件下, 孔径越大的IRMOFs越容易发生阶梯现象; 发生阶梯现象的转变压力与能够发生阶梯现象的转变温度都与孔径呈线性关系. 此外, 模拟结果进一步验证了二氧化碳分子之间的静电作用力是阶梯现象发生的关键因素. 这些规律将为金属-有机骨架材料(MOFs)的设计和改性以及二氧化碳在混合气体中的吸附分离提供有用的信息.     

BET specific surface area and pore structure of MOFs determined by hydrogen adsorption at 20 K

Low-pressure high-resolution hydrogen adsorption for the metal-organic framework MIL-101 are measured at 19.5 K and pressures below 57 kPa. The BET specific surface area and micropore volume are determined and compared to results from nitrogen adsorption at 77 K. Steps in the hydrogen adsorption isotherm are correlated to the pore structure of MIL-101.     

The potential of organic polymer-based hydrogen storage materials

The challenge of storing hydrogen at high volumetric and gravimetric density for automotive applications has prompted investigations into the potential of cryo-adsorption on the internal surface area of microporous organic polymers. A range of Polymers of Intrinsic Microporosity (PIMs) has been studied, the best PIM to date (a network-PIM incorporating a triptycene subunit) taking up 2.7% H(2) by mass at 10 bar/77 K. HyperCrosslinked Polymers (HCPs) also show promising performance as H(2) storage materials, particularly at pressures >10 bar. The N(2) and H(2) adsorption behaviour at 77 K of six PIMs and a HCP are compared. Surface areas based on Langmuir plots of H(2) adsorption at high pressure are shown to provide a useful guide to hydrogen capacity, but Langmuir plots based on low pressure data underestimate the potential H(2) uptake. The micropore distribution influences the form of the H(2) isotherm, a higher concentration of ultramicropores (pore size <0.7 nm) being associated with enhanced low pressure adsorption.     

Pressure and temperature dependence of hydrophobic hydration: volumetric, compressibility, and thermodynamic signatures

The combined effect of pressure and temperature on hydrophobic hydration of a nonpolar methanelike solute is investigated by extensive simulations in the TIP4P model of water. Using test-particle insertion techniques, free energies of hydration under a range of pressures from 1 to 3000 atm are computed at eight temperatures ranging from 278.15 to 368.15 K. Corresponding enthalpy, entropy, and heat capacity accompanying the hydration process are estimated from the temperature dependence of the free energies. Partial molar and excess volumes calculated using pressure derivatives of the simulated free energies are consistent with those determined by direct volume simulations; but direct volume determination offers more reliable estimates for compressibility. At 298.15 K, partial molar and excess isothermal compressibilities of methane are negative at 1 atm. Partial molar and excess adiabatic (isentropic) compressibilities are estimated to be also negative under the same conditions. But partial molar and excess isothermal compressibilities are positive at high pressures, with a crossover from negative to positive compressibility at approximately 100-1000 atm. This trend is consistent with experiments on aliphatic amino acids and pressure-unfolded states of proteins. For the range of pressures simulated, hydration heat capacity exhibits little pressure dependence, also in apparent agreement with experiment. When pressure is raised at constant room temperature, hydration free energy increases while its entropic component remains essentially constant. Thus, the increasing unfavorability of hydration under raised pressure is seen as largely an enthalpic effect. Ramifications of the findings of the authors for biopolymer conformational transitions are discussed.     

The study of controlling pore size on electrospun carbon nanofibers for hydrogen adsorption

Polyacrylonitrile (PAN)-based carbon nanofibers (CNFs) were prepared by using electrospinning method and heat treatment to get the media for hydrogen adsorption storage. Potassium hydroxide and zinc chloride activations were conducted to increase specific surface area and pore volume of CNFs. To investigate the relation between pore structure and the capacity of hydrogen adsorption, textural properties of activated CNFs were studied with micropore size distribution, specific surface area, and total pore volume by using BET (Brunauer-Emmett-Teller) surface analyzer apparatus and the capacity of hydrogen adsorption was evaluated by PCT (pressure-composition-temperature) hydrogen adsorption analyzer apparatus with volumetric method. The surface morphology of activated CNFs was observed by SEM (scanning electron microscope) images to investigate the surface change through activation. Even though specific surface area and total pore volume were important factors for increasing the capacity of hydrogen adsorption, the pore volume which has pore width (0.6-0.7 nm) was a much more effective factor than specific surface area and pore volume in PAN-based electrospun activated CNFs.     

纳米碳管储能的化学原理与储存容量研究

通过在大温度、压力范围内系统地测定氢在纳米碳管粉末与压片上的吸附等温线和对所得等温线的理论分析,计算出吸附热,并用超临界气体的吸附模型充分地描述了氢在纳米碳管上的吸附行为,证明纳米碳管储氢的原理是超临界吸附;比表面积和储气温度控制着储气容量.甲烷在干纳米碳管上的吸附机理与氢气相同,但在湿纳米碳管中的存储机理在于甲烷水合物的生成,因此孔容控制储气容量.单位质量多壁管的湿储容量是干储容量的5.1倍 ,单壁管可能产生更大的增强存储作用.     

CF4 on carbon nanotubes: physisorption on grooves and external surfaces

We present the combined results of a computer simulation and adsorption isotherm investigation of CF4 films on purified HiPco nanotubes. The experimental measurements found two substeps in the adsorption data. The specific surface area of the sample and the coverage dependence of the isosteric heat of adsorption of the films were determined from the measurements. The simulations, conducted for homogeneous bundles of close-ended tubes, also found two substeps in the first layer data: one corresponding to adsorption on the grooves and a second one, at higher pressures, corresponding to adsorption on the outside surface of the tubes. Our computer simulations are in very good agreement with the experimental data.     

氢在A、X及ZSM-5型沸石上的高压物理吸附

The hydrogen adsorption properties and uptake capacities of the A, X and ZSM-5 types of zeolites were investigated at temperatures of 77, 195 and 293 K and pressures up to 7MPa, using a conventional volumetric adsorption apparatus. All hydrogen adsorption isotherms were basically type I, but the maximum in isotherm,a unique feature of supercritical adsorption, was observed at high pressures of 2-5 MPa at 77 K. The isosteric heats of adsorption were determined from the isotherms and the factors that influence their variations were discussed. Different types of zeolites exhibited remarkably different hydrogen uptake, based on both the framework structure and the nature of the cations present. The highest gravimetric storage capacity of 2.55wt% was obtained for NaX-type zeolite at 4 MPa and 77 K. In CaA, NaX and ZSM-5 types of zeolites,hydrogen uptakes were proportional to the specific surface areas, which were associated with the available void volumes of the zeolites. A threshold in hydrogen adsorption observed in NaA and KA was attributed to a pore blocking effect by large cations in KA. A ratio of the kinetic diameter of adsorbate to the effective opening diameter of zeolite was used to judge the blocking effect for physisorption.     

汉麻杆基活性炭表面织构与储氢性能的研究

以天然汉麻杆为原料,采用KOH化学活化的方法改变活化时间制备出了高比表面积活性炭,并且对其表面进行硝酸氧化处理,研究活性炭表面化学状态对其吸附性能的影响。采用77 K低温氮气吸附和FTIR对样品进行了表征,并在77 K、100 kPa的条件下测定样品的氢气吸附等温线。结果表明,所有样品具有较高的比表面积(2 435.93～3 240.95 m²·g^-1)和总孔容(1.3～1.98 cm³·g^-1),且随活化时间的延长而增加,3.5 h达到最大值,之后由于骨架坍塌有所减小。所有样品的孔径分布较为一致呈多峰型分布,主要以小于2 nm的微孔为主,同时含有少量的中孔和大孔。活化3.5 h样品的吸氢量最大,达到3.28wt%。研究发现,吸氢量受比表面积和孔容等参数影响较大,77 K下不仅小于2 nm的微孔对活性炭吸氢行为贡献较大,中孔也有十分重要的影响。样品经硝酸氧化处理后,BET比表面积和总孔容均在一定程度上减小,而氢气吸附量也有所降低。