Silicalite sorption

Usually, the zeolite inner surface characteristics are rather complex as a consequence of the (3D) character of the porous topologies of most of the zeolite types. The porous framework is a (3D) organization of cavities connected by channels. Inner surfaces are composed of several sorption sites characterized by their local geometry and curvature. Illustrative examples of such inner surface complexity are represented on Figures 1 and 2 they concern the Faujasite and Silicalite-I inner surfaces respectively. 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

Mentzen, B.F. and Gelin, P. (1995) The silicalite/p-xylene system part 1-flexibility of the MFI framework and sorption mechanism observed during p-xylene pore-filling by X-ray powder diffraction at room temperature. Mater. Res. Bull., 30, 373-380. 

June et al. (85) presented united-atom calculations for butane and for hexane in silicalite, whereby the bond and dihedral angles of the alkanes were allowed to vary. In addition, the calculation of hexane took account of an additional intramolecular Lennard-Jones potential for nonbonded atoms more than three bonds apart (which prevents the alkane crossing over itself). The interaction parameters for the alkane molecules were taken from Ryckaert and Bellmans (3), and those governing the interaction of the alkanes with the zeolite from a previous study of the low-occupancy sorption of alkanes in silicalite (87). Variable loadings of alkanes were considered from 1 to 8 molecules per unit cell were considered, and calculations were allowed to run for 500 ps for diffusion at 300 K. 

Hope et al. (116) presented a combined volumetric sorption and theoretical study of the sorption of Kr in silicalite. The theoretical calculation was based on a potential model related to that of Sanders et al. (117), which includes electrostatic terms and a simple bond-bending formalism for the portion of the framework (120 atoms) that is allowed to relax during the simulations. In contrast to the potential developed by Sanders et al., these calculations employed hard, unpolarizable oxygen ions. Polarizability was, however, included in the description of the Kr atoms. Intermolecular potential terms accounting for the interaction of Kr atoms with the zeolite oxygen atoms were derived from fitting experimental results characterizing the interatomic potentials of rare gas mixtures. In contrast to the situation for hydrocarbons, there are few direct empirical data to aid parameterization, but the use of Ne-Kr potentials is reasonable, because Ne is isoelectronic with O2-. 

In Silicalite. A variety of papers are concerned with sorption of methane in the all-silica pentasil, silicalite. June et al. (87) used a Metropolis Monte Carlo method and MC integration of configuration integrals to determine low-occupancy sorption information for methane. The predicted heat of adsorption (18 kJ/mol) is within the range of experimental values (18-21 kJ/ mol) (145-150), as is the Henry s law coefficient as a function of temperature (141, 142). Furthermore, the center of mass distribution for methane in silicalite at 400 K shows that the molecule is delocalized over most of the total pore volume (Fig. 9). Even in the case of such a small sorbate, the channel intersections are unfavorable locations. 

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... 

June et al. investigated the sorption and spatial distribution of butane and three hexane isomers within the pores of silicalite, using a Metropolis MC method (87) and MD simulations (85). Perturbations of conformation as a result of confinement within the pore were also reported. Heats of adsorption and Henry s law coefficients were found to be in good agreement with experimental values for butane (48-51 kJ/mol) (142,148,150,163-165) and n-hexane (70-71 kJ/mol) (163, 166, 167). The heats of sorption of the other two hexane isomers, 2- and 3-methylpentane, were predicted to be 5 kJ/mol lower than that of n-hexane. 

The configuration-bias Monte Carlo (CB-MC) technique (112) has also been extensively applied to characterize the sorption of alkanes, principally in silicalite (111, 156, 168-171) but also in other zeolites (172-174). Smit and Siepmann (111, 168) presented a thorough study of the energetics, location, and conformations of alkanes from n-butane to n-dodecane in silicalite at room temperature. A loading of infinite dilution was simulated, based on a united-atom model of the alkanes and a zeolite simulation box of 16 unit cells. Potential parameters were very similar to those used in the MD study of June et al. (85). As expected, the static properties (heat of adsorption, Henry s law coefficient) determined from the CB-MC simulations are therefore in close agreement with the values of June et al. The... 

The efficiency of the CB-MC technique has been used by Maginn et al. (769), who considered the low-occupancy thermodynamics of sorption of alkanes as long as C25 in silicalite. The locations of such long molecules are no longer correctly predicted by considering the end-to-end vector and the chain midpoint. To overcome this problem, a coarse-graining technique was used to describe both the adsorbate and the zeolite, allowing for accurate microscopic characterization. 

Vetrivel et al. (154) applied similar EM methods to investigate the sorption of methanol and ethene in silicalite and ZSM-5. Calculation parameters were taken from an earlier structural study (152), and a portion of the silicalite lattice comprising 120 atoms was allowed to relax. The sorbate... 

Sorption energies in silicalite were found to be spread over a relatively small range, 18 and 9 kJ/mol, for full- and reduced-charge models, respectively. In each case, the preferential binding order was the same 1-butanol adsorbs the most strongly, then 2-butanol, and finally butyl alcohol. The locations of adsorption were also found to be similar for all isomers 1-butanol prefers to adsorb in the sinusoidal channels, whereas 2-butanol... 

The complexity of modeling the adsorption of benzene in silicalite has already been discussed in the section concerned with diffusion. A TST study by Snurr et al. (106) led to the identification of 27 unique sorption minima in the asymmetric unit. Given this result, it is unsurprising that there have been relatively few simulation studies of this system. However,... 

Additionally, the microwave treatment during the crystallization process at high temperature may cause the metastable mesophase to collapse into the denser or amorphous phase in synthetic mixture as well as provide the favorable condition for the formation of silicalite-1. A summary of parameters obtained by nitrogen sorption is shown in Table 2. In Table 2, pore diameters of major peaks ( ) for sample II-IV are increased from 2.5 to 2.87 nm as extending the microwave irradiation. It implied that the additional space created in the mesoporous channels, as a consequence of the pore size enlargement, that is filled by extra water [16]. 

Li J.-M. and Talu O., Effect of structural heto-ogeneity on multicomponent adsorption benzene and p-xylene mixture on silicalite, in M. Suzuki (ed.) Proc. IV Int. Corf, on Fundamentals of Adsorption, (Elsevio", Amsto dam, 1993) pp. 373-380. Meininghaus C. K.W. and Prins R., Sorption of volatile organic compounds on hydrophobic zeolites, Microporous and Mesoporous Materials 35-36 (2000) pp. 349-365. 

Influence of Hydrothermal Treatment on Sorption and Diffusion Properties of a Silicalite... 

P. Graham, A.D. Hughes, and L.V.C. Rees, Sorption of binary gas mixtures in zeolites I. Sorption of nitrogen and carbon dioxide mixtures in silicalite. Gas Sep. A Purif. 3 56 (1989). 

Type-IV sorption isotherm is a combination of type-11 sorption at low concentration and type-111 sorption at high concentration. Type-V isotherm as dehned here (Figure 5.1) is exhibited by glassy polymers/polymers containing adsorptive fillers. This type of sorption is dehned as dual mode sorption, which is a combination of Henry s type and Langmuir type. The former applies to the bulk polymer and the latter to the hller/micro-voids in the polymer. Netke et al. [35] have studied the permeation of acetic acid-water mixmres in silicalite hlled PDMS. Equations governing type-V isotherm are... 

Eigure 5.1 shows the sorption isotherm data of Netke et al. [35] for acetic acid-water-PDMS containing 20% silicalite SA-5 (UOP) converted to the form given by Equation 5.7. 

Removal of VOCs from water Silicalite-fllled polyfsiloxane imide) membranes The sorption selectivity of the PS I membranes for chloroform/water solutions was investigated. The silicalite-filled membrane with 120 p.m thickness exhibit a high total permeation flux of 280 gm /h with separation factor of 52.2 for 1.2 wt% of the chloroform/water mixture [109]... 

E. J. Maginn, A. T. Bell, and D. N. Theodorou,/. Phys. Chem., 99, 2057 (1995). Sorption Thermodynamics, Siting, and Conformation of Long n-Alkanes in Silicalite as Predicted by Configurational-Bias Monte Carlo Integration. 

The sorption kinetics of n-hexane in MFI-type zeolites of different sizes have been measured by means of micro-FTIR spectroscopy. To check for an influence of the Si/Al ratio, nsj/Ai, on the sorption characteristics, a sample of silicalite was also investigated. The measured transport diffiisivities show ndther a dependence on the crystal size nor on the Si/Al ratio. The temperature dependence is shown to follow an Arrhenius-type law. The results of this study compare well with literature data obtained by different techniques. 

In this study, we present the results of experiments performed on the sorption of n-hexane in HZSM-5 single crystals of different sizes. To examine the influence of the Si/Al ratio, ngj/Ai. on the sorption properties, a silicalite sample has additionally been studied. 

The results described in this report compare well with data of Van-Den-Begin et al. [15] obtained on silicalite samples with an equivalent radius of 31pm by means of Single-Step Frequency-Response. The authors report a self diffusion coefficient for n-hexane of about 2 10 cmVs at a temperature of444 K. However, it has to be considered that, due to the shape of the sorption isotherm, the self-diffusion coefficient will be somewhat smaller than the transport diffusion coefficient. Caro et al [16] report a transport diffusion coefficient of 1.8 10 cmVs for the system n-hexane/HZSM-5 at 298 K, determined gravimetrically. The crystals used in that study were of prismatic shape, the dimensions being 330 pm (z-axis), 110 pm... 

Al-free Sn-silicalites with MFI, MEL and MTW structures have been synthesized hydrothermally and characterized by XRD, FT-IR, MAS-NMR and sorption... 

The surface areas determined from the N2 adsorption Isotherms in the low partial pressure region (uptp p/po = 0.05) are in the range of 500 m g-1 for Sn-MFI and Sn-MEL samples and 300 m g" for Sn-MTW samples (Table 1). It is estimated that meso pore areas (determined form the t-plots at higher p/po values) contribute roughly to 10% of the total area. The amount of H2O, cyclohexane and n-hexane adsorbed by the samples at 298 K and at p/po of 0.5 are included in Table 1. From the amount of H2O adsorbed, it may be concluded that the Sn-silicalites are more hydrophilic than the parent Sn-free silicalites. The sorption capacities for n-hexane and cyclohexane in all the samples show that the micropore volumes are maintained and that occluded Sn02 type of species may not be present in them. 

Henry constants and saturation concentrations obtained from the gravimetric sorption measurements on silicalite particles and diffusion constants obtained by the membrane method of methane and n-butane. After Vroon et al. [72-74]... 

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