Zeolite methane sorption

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study. 

Figure 1. Methane sorption isotherms measured at T = 77.35 K on four different AlP04-5 zeolite samples S(l), S(2), S(3) and S(4).

In an MD study of methane sorption and diffusion in silicalite, Nicholas et al. (67) identified favorable sites for sorption. From the MD calculations, the time-averaged position of the center of mass of the methane molecule was plotted. Energy minimization calculations were then performed, locating the methane molecule at positions where the MD calculations predicted they spent the most time. Each channel intersection region was found to contain two sites that are minima for methane-zeolite interactions. These two sites are separated by a translation parallel to the straight channel... 

In this paper we report experimental and theoretical results on the sorption of methane and krypton on 5A zeolite. The sorption of methane in the 5A cavity is reported to be non-localized (9.), whereas that of krypton is localized at a cavity site and window site (10). The multicomponent form of the isotherm of Schirmer et al. is used to interpret the experimental data and to predict mixture equilibria at other concentrations. 

The same study has been performed concerning the methane / ZSM-12 zeolite system. The neutron diffractograms, measured for different methane loadings of ZSM-12 zeolite, are represented on figure 9. In addition, a methane calibration sorption isotherm... 

Demontis et al. (94) reported an early MD study of the sorption and mobility of benzene in zeolite NaY. The zeolite was modeled with a Si/Al ratio of 3.0, as in previous calculations for Xe and methane. The zeolite and benzene molecules were treated as rigid. The authors supported the assumption of a rigid zeolite lattice by quoting structural studies (95), in which the cell parameter of NaY zeolite was found to contract little upon uptake of benzene. It is, however, more than possible that the lattice undergoes substantial deformation without an overall change in volume quantum chemical calculations (96) have shown that the Si-O-Si bending potential is very soft. When these calculations were performed, the assumption of a rigid lattice was more a matter of computational necessity than it is today. 

Preferential sorption in the sinusoidal channels was confirmed by Nicholas et al. (67) in an MD study of methane and propane adsorption. This preference was most noticeable at infinite dilution at a loading of 12 molecules per unit cell the distribution of molecules over the channels was found to be close to that expected from the relative volumes of the channel segments. The propane molecules were predicted to spend more time in the intersections than the straight channel at infinite dilution. This result is rationalized by considering the slow motion of the molecules and the conformational changes necessary to move from one channel type to another via an intersection. The distribution of propane backbone bond angles was predicted to be similar to that of gas-phase propane, indicating the rather minor effect of the zeolite on the internal coordinates of propane. 

An intriguing aspect of these measurements is that the values of D determined from NMR and from sorption kinetics differ by several orders of magnitude. For example, for methane on (Ca,Na)-A the value of the diffusion coefficient determined by NMR is 2 x 10 5 cm2 sec-, and the value determined for sorption rates only 5 x 10"10 cm2 sec-1. The values from NMR are always larger and are similar to those measured in bulk liquids. The discrepancy, which is, of course, far greater than the uncertainty of either method, remained unexplained for several years, until careful studies (267,295,296) showed that the actual sorption rates are not determined by intracrystalline diffusion, but by diffusion outside the zeolite particles, by surface barriers, and/or by the rate of dissipation of the heat of sorption. NMR-derived results are therefore vindicated. Large diffusion coefficients (of the order of 10-6 cm2 sec-1) can be reliably measured by sorption kinetics... 

The system methane-krypton 5A was selected for study because previous pure component studies for each of these sorbates on Linde 5A zeolite indicate that the sorption mechanisms are significantly different. 

Pure component experimental data for sorption of methane and krypton on 5A zeolite at 238, 255. and 271K, and in the pressure range of 0 to 97.36 kPa were also obtained during this work (shown in Figures 3 and U). Further sorption data for methane on 5A zeolite (10, 13, 1 0, and for krypton on 5A zeolite (10. 15) are also plotted for other temperatures, all of which appear to be consistent. These experimental data were used to derive the energy and entropy parameters in equation U for the isotherm model of Schirmer et al. by a minimization of a sum of squares optimization procedure. 

The resulting optimized parameters and for sorption of methane and krypton on 5A zeolite are shown in Figure 5 and are presented in Table 1. The calculated energy parameters -22000 Joules/mole for methane and - 16,725-0 Joules/mole for krypton were independent of the amount adsorbed and agree with... 

Figure 5. Variation of integral molar energy of sorption with coverage for (x) methane and ( ) Kr on 5A zeolite (EU data of Rolniak for CHt (--------) theo-...

We have examined whether a simple non-bonded potential can be developed to be (i) transferable from one zeolite to another and (ii) to simulate without parameter adjustment isosteric heats at different temperatures and sorption uptake isotherms. The sorption of methane into Na- and K- zeolite X, and Na-and K-clinoptilolites was considered. Models for Na-X and K-X were constructed based on the averaged crystallographic results. The non-bonded parameters in a Lennard-Jones potential were iteratively adjusted so as to best reproduce the experimental isosteric heats in Na-X and K-X over a small temperature range. Methane-methane interaction parameters were taken from earlier work [89] and a final iteration was made so as to better fit the experimental sorption isotherms in clinoptilolite. This single and simple non-bonded potential parameter set then reproduces to a reasonable degree... 

Initially acetic acid desorbed unaltered. However, as the tenperature increased, products of thermal decomposition and further reaction were observed. At 240°C there was a large 002 peak, some ethene, methane, water and acetone. Integration of the desorption peaks, followed by scaling by their respective sensitivity factors, gave the following mole percentages of products desorbed CO2 39, ethene 15.4, H2O 25.8, CH4 9.9 and acetone 9.9. A mass balance of the C, 0 and H atoms evolved gave a ratio of C 0 H of 109 114 212, approximately the ratio for acetic acid (2 2 4). This implies that the products desorbed were from the decomposition of acetic acid only. This supports the hypothesis that acetic acid was sorbed intact (i.e. equation 2 rather them equation 1). If water had been eliminated upon sorption on an acid site then the desorption products could not include acetic acid, as any water evolved would have rapidly desorbed from the zeolite at 150°C. 

A customized Cu(I)Y zeolite is employed as a sorbent to actively collect CO in air samples to measure the concentration of CO in ambient air.The interaction is selective to CO only, but not to N2, O2, and CO2. The sorption process is facilitated by formation of Cu(l)-CO complexes, while CO can be desorbed at 300°C under helium flow for 2 min. Before the gas chromatographic analysis, a methanizer is used to reduce CO to CH4, which can then be quantified by FID. Detection limit of methane by this method is approximately 0.2 ppm. The laboratory data shows the capacity of the Cu(I)Y zeolite sorbent as 2.74 mg CO/g of sorbent. For a typical sorbent tube containing 0.5 g of treated zeolite sampling at the PEF of 50 ppm with a nominal flow rate of 100 ml/min, sampling can last as long as approximately 4 h before a breakthrough point is reached. Furthermore,... 

ABSTRACT. An isosteric sorption system has been used to study the sorption of methane, ethane, ethene, propane, N2 and CO2 and some of their binary mixtures in silicalite-1. Isotherms of some of these sorbates have been determined at equilibrium pressures up to 20 atmospheres. Isosteric heats of sorption have been obtained from the slopes of the isosteres. Separation factors calculated from the Henry s Law constants determined from the initial slopes of the single conq>onent isotherms are found to be in good agreement with experim tal separation factors. The Langmuir-Freundlich equation has be used to lit the single component data and the Ideal Adsorbed Solution theory has be used to predict a binary sorption isotherm from the respective single component data. Comparison of the sorption behaviour of the hydrocarbons in silicalite-1 and NaY zeolites has been made. 

---