N-hexane in silicalite

Table 2 FR parameters of n- hexane in silicalite- 1 derived from the best theoretical fits ...

Around a value of the gas-phase fraction of 2-methylpentane of about 0.83, the influence of the acid sites on the n-hexane diffusivity is not dominant anymore in comparison to the pore occupation of slow-diffusing 2-methyl-pentane. Figure 14 shows the dependence of the diffusivities of both components versus the concentration of adsorbed 2-methylpentane in terms of molecules per unit cell. The diffusivities of n-hexane in silicalite-1 and H-ZSM-5 become nearly equal when the concentration of 2-methylpentane reaches approximately 2.75 molecules per unit cell. For 2-methylpentane we And that the self-diffusivity in silicalite-1 becomes very close to the value in H-ZSM-5 at the same loading. 

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 self-diffusivities of 2-methylpentane and n-hexane in their binary mixtures have been measured as a fimction of the ratio of the hydrocarbon in silicalite-1 at a temperature of 433 K. Figure 8 shows the tracer re-exchange... 

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

Figure 15 displays the loadings of n-hexane and 2-methylpentane in both zeolites. Under similar conditions, the adsorbed concentration of n-hexane is higher than that of 2-methylpentane, especially at lower temperatures. The interaction with n-hexane results in higher loadings for H-ZSM-5 than for silicalite-1. From the temperature dependence of the diffusivity of n-hexane in both zeolites, the apparent activation energy has been deduced and the results are collected in Table 3. Corresponding Arrhenius plots are shown... 

Summarizing, we observe that the presence of acid sites causes a decrease in the self-diffusivity of n-hexane and 2-methylpentane. In H-ZSM-5, we find that the diffusivity of n-hexane in mixtures with its branched isomer is determined by two factors (i) the interaction with acid sites, strong for the linear alkane, which decreases the diffusivity and (ii) the presence of 2-methylpentane which has an order of magnitude lower diffusivity. At low 2-methylpentane loadings the influence of the acid sites is dominating. However, at a loading of about 2.7 molecules per unit cell, the effect of pore blocking by the preferential location of the branched alkane in the intersections dominates. The diffusivities are then more or less equal in silicalite-1 and H-ZSM-5. 

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. 

Liu et al. [42] reported permeation of mixture of hexanes and octanes through silicalite membranes. It was found that the permeances of the mixture could not be predicted by the single-component data. In the separation of alkane isomers, the permeance of 2,2-DMB is significantly reduced in the presence of n-hexane resulting in a permselectivity much higher than the ideal separation factor [7]. 

Hernandez and Catlow (86) recently reported an investigation of n-butane and of n-hexane diffusion in silicalite the work is similar to that of June et al. (85). Many calculation details were the same as in the earlier work, including the assumption of identical Lennard-Jones coefficients of intermolecular dispersion and repulsion. Simulations were performed at different loadings for butane, namely, 2, 4, 5.3, and 8 molecules per unit cell. In addition, simulations were performed at a constant loading and variable temperature (200, 300, and 400 K) for both butane and hexane. These calculations were performed for 1000 ps, twice the length of those of June et al. The zeolite framework was held rigid. 

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. 

There is, as is well known, a close similarity between the crystalline and porous structures of silicalite-1 and silicalite-2. The same similarity therefore exists between TS-1 and TS-2, and it appears logical that they should have very similar catalytic properties. TS-2 has been evaluated as a catalyst for many different reactions, such as Beckmann rearrangement of cyclohexanone oxime with vapor-phase reactants H202 oxidation of phenol, anisole, benzene, toluene, n-hexane, and cyclohexane and ammoximation of cyclohexanone. As described in detail in Section V.C.3, differences that had been claimed between the catalytic properties of TS-1 and those of TS-2 have not been substantiated. Later investigations have shown that, when all the relevant parameters are identical, the catalytic activities of TS-1 and TS-2 are also identical. The small differences in the crystalline structure between the two materials have no influence on their catalytic properties (Tuel et al., 1993a). 

The water and n-hexane adsorption isotherms of the zeolitic mesoporous materials obtained are compared to that of a 4S0 nm colloidal silicalite-1 in Figure 5. The water adsorption isotherms are distinctively type HI, whereas the n-hexane isotherms are type 1. The lowest water isotherm was for the colloidal silicalite-1, where the first point measured for the n-hexane isotherm was already at 80 mg g. The amount of n-hexane adsorbed reached 2S0 mg g at high pressure, which roughly corresponds to the filling of silicalite-1 micropores. 

For mono-methyl paraffm separation, two pulse test techniques, one with and one without iso-octane pre-pulse, were developed (2,3). In each test the feed was a mixture containing equal volumes of 3,3,S-trimethyl heptane, 2,6-dimethyl octane, 2-methyl nonane, n-decane, and I,3,S-trimethyl benzene. The pulse test column had a volume of 70 cc and was held at a temperature of 120 C in the experiments shown. The flow rate through the column was 1.2 ml/min. The adsorbent was silicalite and the desorbent was a 30/30 volume % mixture of n-hexane/cyclohexane. Test I was run without a pre-pulse and test 2 was run with a pre-pulse of 40 ml of iso-octane injected into the test loop immediately before the feed mixture was injected. Iso-octane pre-pulse diluted the n-hexane concentration at the adsorption zone and increased the adsorbent selectivity for mono-methyl paraffin. 

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... 

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. 

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

In the oxyfunctionalization of n-hexane by H2O2 (30 wt %) over titanium silicalites in methanol as a solvent [49a], the parallel-sequential reaction scheme was selected for transformation of the hydrocarbon into hexanol and hexanone ... 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Table 8 also summarizes the activation energies, a> which do not exhibit any apparent dependence on the chain length. This behavior was also stated by Eic and Ruthven [54,55], who found an increasing activation energy for the diffusion of paraffins in silicalite-1 up to n-hexane and a leveling off for carbon numbers greater than six. For n-pentane, these authors determined... 

Fig. 8 TEX-PEP profiles for labeled 2-methylpentane left) and n-hexane (right) at several detection positions in an equimolar mixture of n-hexane and 2-methylpentane in silicalite-1 at a total hydrocarbon pressure of 6.6 kPa and a temperature of 433 K (note that these two graphs have been obtained from two different experiments in which one of the two hydrocarbons was labeled)...

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