Adsorption of molecular hydrogen on ultrafine microporous/mesoporous materials

The comparative processing of H₂ adsorption isotherms obtained at 77 K is demonstrated to be applicable to the investigation of the microtexture of the ultrafine oxide materials MCM-41 and ZSM-5 and their mechanical mixtures. The H₂ sorption method allows the micropore volume to be determined correctly for mixed ultrafine microporous/mesoporous materials.     

Quantum effect induced kinetic molecular sieving of hydrogen and deuterium in microporous materials

We report here our investigations using Monte Carlo and molecular dynamics (MD) simulations, as well as quasi-elastic neutron scattering experiments, to study the adsorption and diffusion of H₂ and D₂ in zeolite Rho. In the simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. At low temperatures, we observe a reversal of kinetic molecular sieving in which D₂ diffuses faster than H₂. Based on fits of bulk data, we suggest new set of potential parameters for hydrogen, with the Feynman-Hibbs variational approach used for quantum corrections. The transport properties obtained from MD simulations are in excellent agreement with the experimental results, with both showing significant quantum effects on the transport at low temperature. The MD simulation results on two different structures of zeolite Rho clearly demonstrate that the quantum effect is very sensitive to pore size. High transport flux selectivity is noted at low temperatures, suggesting feasibility of kinetic isotope separation.     

Synthesis of microporous boron-substituted carbon (b/c) materials using polymeric precursors for hydrogen physisorption

This paper discusses a new synthesis route to prepare microporous boron substituted carbon (B/C) materials that show a significantly higher hydrogen binding energy and physisorption capacity, compared with the corresponding carbonaceous (C) materials. The chemistry involves a pyrolysis of the designed boron-containing polymeric precursors, which are the polyaddition and polycondensation adducts between BCl3 and phenylene diacetylene and lithiated phenylene diacetylene, respectively. During pyrolysis, most of the boron moieties were transformed into a B-substituted C structure, and the in situ formed LiCl byproduct created a microporous structure. The microporous B/C material with B content > 7% and surface area > 700 m2/g has been prepared, which shows a reversible hydrogen physisorption capacity of 0.6 and 3.2 wt % at 293 and 77 K, respectively, under 40 bar of hydrogen pressure. The physisorption results were further warranted by absorption isotherms indicating a binding energy of hydrogen molecules of approximately 11 kJ/mol, significantly higher than the 4 kJ/mol reported on most graphitic surfaces.     

Features of the adsorption of hydrogen by various types of microporous materials

A comparative investigation was made of the adsorption of N₂ and H₂ by silicate and phosphate zeolites (Zt) and carbon materials with micropores of various dimensions, shapes, and volumes. It was found that the adsorption of hydrogen has significant sensitivity to the shape of the pores, the chemical composition of the zeolite framework, and the nature of the surface of the adsorbents. In particular, the adsorption is increased in the presence of constrictions (the cationic forms of zeolite LTA) or channels (MFI) with sizes close to hydrogen molecules in the micropores and the ions of divalent metals included isomorphously in the framework of the aluminophosphates. There is also increased affinity to the carbon surface. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 67–73, March–April, 2008.     

Quantum effects on adsorption and diffusion of hydrogen and deuterium in microporous materials

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.     

Guest-host encapsulation of microporous zeolites in ordered mesoporous materials by molecular simulations

The present work provides the first study of ordered mesoporous materials SBA-15 coated with microporous zeolites ZSM-5 using molecular simulations. Several model structures with characteristics such as periodic arrangement of mesopores, randomly arranged micropores, surface hydroxyls and bulk deformations of SBA-15 were used. Cartesian coordinates of ZSM-5 unit lattice were obtained from the literature and the 100 face of H-ZSM-5 unit cell was then placed on the surface of SBA-15 and the entire structure was equilibrated to obtain final configuration. The resulting structure was characterized using simulated small angle and wide angle X-ray diffraction, Connolly surface area (to compare BET area), accessible pore volume for nitrogen molecules (to compare with t-plot volume of micro and mesopores) and methane adsorption at 303 K. The orientation of ZSM-5 on the SBA-15 had no effect on the surface area, pore volume or adsorption capacity. In order to find out if the addition of microporous ZSM-5 should increase the total methane adsorption capacity due to addition of micropores, we studied adsorption on bare and coated SBA-15. However, total adsorption capacity was found to decrease, while the number of methane molecules adsorbed per unit cell of the SBA-15 structure increased. An existing experimental method (J. Am. Chem. Soc., 2004, 126, 14324) of the synthesizing hybrid ZSM-5/SBA-15 structure was studied using accessible micropore volume (by t-plot). It was found that the procedure made all the micropores inaccessible. A modification of the method or use of other host materials is suggested to use the benefits of narrow micropore distribution in ZSM-5.     

微孔碳分子筛表面含氧基团对H_2O_2催化分解的动力学影响

多孔碳的表面含氧官能团对其吸附及催化性能的影响受到越来越多的关注[1-2],表面含氧基团对活性炭催化醇脱水[3]、乙苯脱氢[4]、O2还原为过氧化氢[5]、Fe(II)与O2氧化反应[6]已有报道.     

Zeolite-templated carbon materials for high-pressure hydrogen storage

Zeolite-templated carbon (ZTC) materials were synthesized, characterized, and evaluated as potential hydrogen storage materials between 77 and 298 K up to 30 MPa. Successful synthesis of high template fidelity ZTCs was confirmed by X-ray diffraction and nitrogen adsorption at 77 K; BET surface areas up to ~3600 m(2) g(-1) were achieved. Equilibrium hydrogen adsorption capacity in ZTCs is higher than all other materials studied, including superactivated carbon MSC-30. The ZTCs showed a maximum in Gibbs surface excess uptake of 28.6 mmol g(-1) (5.5 wt %) at 77 K, with hydrogen uptake capacity at 300 K linearly proportional to BET surface area: 2.3 mmol g(-1) (0.46 wt %) uptake per 1000 m(2) g(-1) at 30 MPa. This is the same trend as for other carbonaceous materials, implying that the nature of high-pressure adsorption in ZTCs is not unique despite their narrow microporosity and significantly lower skeletal densities. Isoexcess enthalpies of adsorption are calculated between 77 and 298 K and found to be 6.5-6.6 kJ mol(-1) in the Henry's law limit.     

Manometric submicro determination of carbon and hydrogen in organic materials

A method is described for the determination of carbon and hydrogen in quantities of organic material ranging from 5 to 50 μg. The method involves catalytic combustion of the sample in oxygen and measurement of the pressures of the resulting carbon dioxide and water. Special attention has been paid to the reduction of the effect of water adsorption in the apparatus. The present submicro method can be applied to non-volatile, non-hygroscopic samples and is free from interferences by nitrogen, sulphur and halogens (except fluorine). The standard deviation in the 50-μg sample range is 0.13% for carbon and 0.10% for hydrogen. The average duration per analysis when carried out in series is 20 min.     

Determination of transport coefficients in microporous solids

Macroscopic transient methods are reviewed with respect to their applicability to the investigation of molecular transport in microporous sorption systems. Various levels of sophistication of data evaluation for nonequilibrium sorption results obtained by means of batch methods are identified and characterised. Special attention is paid to the characterisation ofFickian (intracrystalline) diffusion as well as to the identification and quantification of additional rate mechanisms that, in general, may simultaneously occur in molecular sieve systems. A state-of-art determination of transport coefficients is exemplified for the systems benzene/microporous gallosilicate of MFI-type, n-hexane/silicalite-I and p-ethyltoluene/ZSM-5. Their sorption rate behaviour can be understood either byFickian diffusion or byFickian diffusion and intracrystalline molecular immobilisation/mobilisation and surface barrier penetration, respectively. To analyse complex sorption rate patterns in microporous systems, the method oftotal curve fitting with full parameter region consideration becomes mandatory.     

Density functional theory study of hydrogen bonding in ionic molecular materials

Crystal structures are usually described in geometric terms. However, it is the energetics of intermolecular interactions that determine the chemical and physical properties of molecular materials.(1) In this paper, we use density functional theory (DFT) in combination with numerical basis sets to analyze the hydrogen bonding interactions in a family of novel ionic molecular materials. We find that the calculated binding energies are consistent with those of other ionic hydrogen bonded systems. We also examine electron density distributions for the systems of interest to gain insight into the nature of the hydrogen bonding interaction and investigate the effects of different aspects of the crystal field on the geometry of the hydrogen bond.     

Kinetic isotope effect for H2 and D2 quantum molecular sieving in adsorption/desorption on porous carbon materials

Adsorption and desorption of H(2) and D(2) from porous carbon materials, such as activated carbon at 77 K, are usually fully reversible with very rapid adsorption/desorption kinetics. The adsorption and desorption of H(2) and D(2) at 77 K on a carbon molecular sieve (Takeda 3A), where the kinetic selectivity was incorporated by carbon deposition, and a carbon, where the pore structure was modified by thermal annealing to give similar pore structure characteristics to the carbon molecular sieve substrate, were studied. The D(2) adsorption and desorption kinetics were significantly faster (up to x1.9) than the corresponding H(2) kinetics for specific pressure increments/decrements. This represents the first experimental observation of kinetic isotope quantum molecular sieving in porous materials due to the larger zero-point energy for the lighter H(2), resulting in slower adsorption/desorption kinetics compared with the heavier D(2). The results are discussed in terms of the adsorption mechanism.     

3D hydrogen bond thermodynamics (HYBOT) potentials in molecular modelling

A new approach is proposed to more accurately estimate the energies of H-bond interactions in three-dimensional (3D) molecular modelling. The approach is based on the use of H-bond acceptor and donor enthalpy factor values calculated by means of program HYBOT, the use of a sigmoid relationship to determine the optimum H-bond distances and established force-field methods to determine distance and angle dependencies. The base-pair interactions in a short A-form RNA double-helix are presented as an example of enthalpy calculations of hydrogen bonding for a model system.     

Adsorption of carbon dioxide on microporous carbon adsorbents

Adsorption isotherms of carbon dioxide on microporous carbon adsorbents prepared by activation with potassium sulfide in water vapor were measured. The measurements were carried out in the pressure interval from 1 Pa to 0.1 MPa at temperatures from 216.2 to 293.15 K. Based on the theory of volumetric filling of micropores, the main structural and energetic parameters of the microporous carbon adsorbents were calculated. The adsorption isosters of carbon dioxide were calculated from the adsorption isotherms in the same pressure and temperature ranges and approximated by linear dependences. The plots of the differential mole isosteric heats of adsorption vs amount adsorbed were constructed by using the adsorption isosters.     

A heterogeneous model for gas transport in carbon molecular sieves

A dual resistance model with distribution of either barrier or pore diffusional activation energy is proposed in this work for gas transport in carbon molecular sieve (CMS) micropores. This is a novel approach in which the equilibrium is homogeneous, but the kinetics is heterogeneous. The model seems to provide a possible explanation for the concentration dependence of the thermodynamically corrected barrier and pore diffusion coefficients observed in previous studies from this laboratory on gas diffusion in CMS. The energy distribution is assumed to follow the gamma distribution function. It is shown that the energy distribution model can fully capture the behavior described by the empirical model established in earlier studies to account for the concentration dependence of thermodynamically corrected barrier and pore diffusion coefficients. A methodology is proposed for extracting energy distribution parameters, and it is further shown that the extracted energy distribution parameters can effectively predict integral uptake and column breakthrough profiles over a wide range of operating pressures.     

Adsorption-stimulated deformation of microporous carbon adsorbent

Deformation of the ACC microporous carbon adsorbent during adsorption of carbon dioxide, nitrogen, and argon in the temperature interval from 243 to 393 K and at pressures of 1—5·10⁶ Pa was studied. The effect of adsorbent contraction was found in the initial temperature interval at relatively low pressures. However, the negative value of relative linear deformation L/L smoothly transforms into positive values with the pressure increase. Only the effect of adsorbent expansion is observed at high temperatures in the whole pressure interval. The dependence of the deformation effects for different systems on the adsorbent nature was revealed.     

Metal alloys and carbon nanomaterials as potential hydrogen storage materials

Comparative analysis of metals (as well as their alloys and intermetallic compounds) and various carbon nanomaterials as working substances for hydrogen storage and transportation systems has been performed. It has been shown that, because of the fundamental difference in the nature of interaction with hydrogen of these two large classes of compounds, the fields of their application (as well as their performance) are certainly different. Some theoretical calculations and concepts concerning the hydrogen capacity of the materials under consideration have been critically surveyed.     

Computer assisted study of nanostructured microporous materials

We report here the results of our computational studies on porous catalysts to bring out the catalytic role played by nanostructures. We present two typical case studies where the molecular dynamic (MD) and quantum chemical (QC) techniques have revealed the important structural aspects involved in the functioning of nanostructured microporous materials. The central role played by the exchanged metal cations of zeolite A in the molecular sieving of nitrogen and oxygen was studied by MD calculations. The results indicated that the mobility of the exchanged cations which are dependent on temperature causes the separation of nitrogen and oxygen molecules. The real time visualization of the dynamic behavior of the exchanged cations during the MD process aids the understanding of this intriguing process occurring inside the micropores of the zeolites. The controlled pore opening of hydrated VPI-5 molecular sieve by careful removal of water leads to a large one dimensional channel. The possibility of anchoring organometallics, namely porphyrins with ‘enzyme-like’ active sites were studied using QC calculations. The analysis of the 3-d contours of electron density and molecular electrostatic potential maps corresponding to various porphyrin systems and the cluster models representing VPI-5 framework brought out the probable locations for porphyrins inside VPI-5.     

Ion transport in the microporous titanosilicate ETS-10

Impedance spectroscopy was used to investigate ion transport in the microporous crystalline framework titanosilicate ETS-10 in the frequency range from 1 Hz to 10 MHz. These data were compared to measured data from the microporous aluminosilicate zeolite X. Na-ETS-10 was found to have a lower activation energy for ion conduction than that of NaX, 58.5 kJ/mol compared to 66.8 kJ/mol. However, the dc conductivity and ion hopping rate for Na-ETS-10 were also lower than NaX. This was found to be due to the smaller entropy contribution in Na-ETS-10 because of its high cation site occupancy. This was verified by ion exchanging Na(+) with Cu(2+) in both microporous frameworks. This exchange decreases the cation site occupancy and reduces correlation effects. The exchanged Cu-ETS-10 was found to have both lower activation energy and higher ionic conductivity than CuX. Zeolite X has the highest ion conductivity among the zeolites, and thus the data shown here indicate that ETS-10 has more facile transport of higher valence cations which may be important for ion-exchange, environmental remediation of radionucleotides, and nanofabrication.