Methane In Zeolites Research Articles

Determination Of The Effects Of Catalyst Components On The Total Oxidation Of Methane In Zeolite Supported Catalysts Prepared By The Impregnation Method

Determination Of The Effects Of Catalyst Components On The Total Oxidation Of Methane In Zeolite Supported Catalysts Prepared By The Impregnation Method

Detailed Investigation of the Binary Adsorption of Carbon Dioxide and Methane in Zeolite Na-ZSM-5 Studied Using in Situ ATR-FTIR Spectroscopy

The separation of carbon dioxide from methane is an important process when purifying biogas and natural gas. Zeolite membranes and adsorbents are among the technologies suggested for efficient separation of carbon dioxide from these gases. In the present work, the adsorption of carbon dioxide and methane from binary mixtures in a low alumina Na-ZSM-5 zeolite film at various gas compositions and temperatures was studied using in situ ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopy. Adsorbed concentrations were successfully extracted from infrared spectra. The experimental values of the adsorbed phase mole fraction of carbon dioxide and methane were compared to values predicted using the ideal adsorbed solution theory (IAST). The values predicted with the IAST agreed very well with values determined experimentally. The CO2/CH4 adsorption selectivity was determined, and at 35 °C a selectivity of 15.4 was obtained for an equimolar gas mixture. At the highest (0.9) and lowest (0.03) investigated mole fractions of carbon dioxide in the gas phase, the selectivity was higher compared to the other investigated mole fractions. At 35 °C the highest observed selectivity values were 31.1 and 20.4 for the highest and the lowest adsorbed mole fraction, respectively. At compositions closest to those found in biogas and natural gas, there was a decrease in the selectivity at higher temperatures, indicating that separation of carbon dioxide from methane in biogas and natural gas may be more efficient at low temperatures.

Adsorption of Methane on Several Zeolites by Monte Carlo Method

Grand Canonical Monte Carlo (GCMC) method is employed to simulate the adsorption of methane in several nanoporous zeolites. Adsorption isotherms over the temperature 177-398K and the pressure 0-12MPa are simulated. And their adsorption capacities of methane in these zeolites at different temperatures and pressures are also compared. The results show that: (1) the methane uptake is in the order of LTA>MOR>MFI at the same condition. The isosteric heat can support this conclusion: the value of isosteric heat in LTA is the largest, intermediate in MOR and the least in MFI. (2) The effects of the pore volume, channel size and the energetic interactions between zeolite and methane on adsorption amounts are considered. A large pore volume and a suitable channel size near to the kinetic diameter of a methane molecule are very important for improving the storage capacity of zeolites. Based on this, we conclude that LTA zeolite with a large pore volume and a suitable channel diameter exhibit a most efficient methane storage capacity than MOR and MFI zeolites.

Intermolecular potential for methane in zeolite A and Y: Adsorption isotherm and related properties

Based on energies derived from ab initio calculations on methane in zeolite A and Y a more accurate intermolecular potential is proposed for hydrocarbon-zeolite interaction. We then employ the proposed potential POTJI (consisting of sets ABCHA and ABCHY) as well as another potential (termed KDMG) based on the work of Kiselev and Du and Murad and Gubbins in grand canonical Monte Carlo simulations to obtain adsorption isotherms and molecular dynamics simulations to obtain self-diffusion coefficients. For methane in zeolite A we show that the derived potential predicts accurately adsorption isotherm, heats of sorption and diffusion coefficients. For methane in zeolite Y a somewhat less accurate reproduction of adsorption isotherm is obtained. In general, these potentials predict values that are closer to experimentally obtained values than existing potential. Radial distribution function is more structured than for existing KDMG potential. Translational diffusion coefficients are generally lower than KDMG potential while the rotational diffusion coefficients are actually higher than seen for KDMG potential. Further, it is seen that the set of potential parameters derived for zeolites A and Y are similar suggesting that it might be possible to derive one unified set of parameters that would perform reasonably across a large set of zeolite structures. Similar transferability of potentials across the range of hydrocarbons may also be possible. Finally, these results suggest that it may be possible to derive accurate classical potentials for interaction between organic and inorganic systems based on ab initio calculations.

Adsorption isotherm and other properties of methane in zeolite A from an intermolecular potential derived from ab initio calculations.

A classical Lennard-Jones potential is derived from a fit to the ab initio energies obtained from an all-electron mixed-basis calculation for methane in zeolite LTA. The potential predicts the heat of adsorption, adsorption isotherm, and self-diffusivity of methane in excellent agreement with experiment. This study suggests, for the first time, that ab initio energies-in addition to experimental data-can form a good basis for derivation of accurate classical potentials between organic and inorganic elements.

MD ? Simulation of Diffusion of Methane in Zeolites of Type LTA

Using molecular dynamical computer simulations (MD) the dynamics of kinetic processes in zeolites will be discussed on a molecular level. Small changes in lattice parameters can cause dramatical changes in the diffusion coefficient. The presence of cations Na+, Ca2+ also strongly influences the diffusion. Changes of the self-diffusivities will be discussed that appear if a vibrating lattice instead of a rigid one is used. Nonequilibrium simulations show the correlation between transport-diffusion and self-diffusion in zeolites.

The influence of changes in the framework on the diffusion in zeolites. Molecular dynamics simulations

Abstract Some less known methods are described for the analysis of trajectories of guest molecules in porous solids. Such trajectories can be calculated by Molecular Dynamics (MD) computer simulations in order to analyze the interrelations between the structure and the particle behaviour including collective phenomena. Some results obtained with these methods for diffusing methane in zeolites of type LTA are presented. An analytical potential model for LTA type zeolites is given that make extremely long runs or simulations of large lattice regions for systems with rigid lattice possible. Such runs are necessary e.g. to examine questions as the influence of extented lattice defects or the fractal behaviour of the partical trajectories.

Anomalous behaviour of cage-to-cage diffusion of methane in zeolites A and Y

Computer simulation studies of methane in zeolites NaY and NaCaA are reported. Rates of cage-to-cage crossovers are found to be significantly higher through the narrower window of zeolite NaCaA. These results suggest that such behaviour first observed for noble gases may be widely prevalent in many sorbate—zeolite systems and therefore need to be taken into account as an important factor influencing the diffusion coefficient. Variation of the rate of cage-to-cage diffusion with temperature has been obtained. The observed variation with temperature is shown to be consistent with the surface-mediated and centralized diffusion mechanisms previously reported for cage-to-cage diffusion. At high temperatures, the influence of the sorbate-zeolite interactions is minimal.

High-pressure adsorption of methane in zeolites NaX, MgX, CaX, SrX and BaX

Adsorption isotherms were measured for methane in NaX, MgX, CaX, SrX, and BaX zeolites at temperatures from 25 to 70 o C, and at pressures up to 6900 kPa. The effects of activation were studied. It is found that activation under vacuum at 1 o C/min up to 400 o C for 24 h is best for these zeolites. The virial adsorption model, Ruthven's statistical model, and Dubinin's equation were used to correlate the high-pressure absorption data

Determination of the average kinetic energy of methane in NaZSM-5 zeolite by neutron inelastic scattering

Neutron inelastic-scattering spectra of methane adsorbed in NaZSM-5 have been measured at 10 and 100 K. A large recoil effect is observed for the vibrational modes of methane. Average kinetic energy values are derived from the widths of the recoiled bands. It is found that the translational and rotational behaviour of methane in the zeolite is far from the perfect gas. At 100 K, the total kinetic energy is less than for the ideal gas: 162 K compared to 300 K. At 10 K, the translational kinetic energy is however much higher than in the gas: 85 K compared to 15 K.

The siting, energetics and mobility of saturated hydrocarbons inside zeolitic cages: methane in zeolite Y

To understand how reactant hydrocarbons are catalytically converted into more useful products inside the cages of zeolite solids, it is first necessary to evaluate the strength of binding and ease of migration of the reactant within the cages, the inner walls of which house the active sites. As it is not easy to arrive at the properties by direct experiment, especially at elevated temperatures, it is profitable to consider computer simulation, which we show to be a valuable alternative approach for the case of methane inside a model, faujasitic catalyst (Na48 Al48 Si144 O384, abbreviated Na-Y). The information that can be gleaned from the potential-energy distribution functions evaluated by Monte Carlo methods1,2 in the range 10–298 K augurs well for the computational study of bulkier reactants inside other zeolitic catalysts.

A model for the temperature dependence of the incoherent scattering from H2 and CH4 in zeolites

Hydrogen and methane in zeolites have been studied by neutron spectroscopy at temperatures between 25 and 300 K and concentrations of n = 0.5−10 molecules/cage. In this paper, the incoherent elastic scattering intensity from the adsorbed molecules is interpreted in terms of a model which assumes a box-like adsorption potential which is located inside a box-like potential of larger size. In terms of this model, hydrogen in chabazite at low temperature populates bound states in the inner box-like potential of depth U a = 153 meV and size LB = 0.08 nm. With increasing temperature the elastic scattering changes according to an increasing population of states which are extended over a box of size LF = 0.4 nm, representing the free path of H 2 molecules in the chabazite lattice cages.

A model for the temperature dependence of the incoherent scattering from H2 and CH4 in zeolites

Hydrogen and methane in zeolites have been studied by neutron spectroscopy at temperatures between 25 and 300 K and concentrations of n = 0.5−10 molecules/cage. In this paper, the incoherent elastic scattering intensity from the adsorbed molecules is interpreted in terms of a model which assumes a box-like adsorption potential which is located inside a box-like potential of larger size. In terms of this model, hydrogen in chabazite at low temperature populates bound states in the inner box-like potential of depth Ua = 153 meV and size LB = 0.08 nm. With increasing temperature the elastic scattering changes according to an increasing population of states which are extended over a box of size LF = 0.4 nm, representing the free path of H2 molecules in the chabazite lattice cages.

Methane in zeolites A and X studied by neutron scattering: a solid or a fluid phase at ambient temperature?

The motion of CH 4 molecules in the zeolites NaX and NaA has been studied by neutron time-of-flight experiments. Elastic peaks have been found, the intensity of which increases at decreasing scattering angle. The main features of the scattered neutron distributions are the same for different zeolite samples and various amounts of adsorbed methane. The observed neutron scattering law cannot satisfactorily be interpreted in terms of the current solid state models for the adsorbate dynamics. A computation of the neutron scattering law on the basis of a fluid phase model yields good agreement with the experiment.