Adsorption of alkanes in Silicalite

In the previous chapters, we have discussed techniques to calculate thermodynamic properties of chain molecules. In this chapter, we will use these techniques to study the adsorption of linear and branched alkanes in the zeolite Silicalite. 

Fewer experimental data cire available for adsorption of brcinched hydroccirbons adsorbed in Silicalite. The adsorption isotherms of isobutane also shown cin inflection [21,135,136] but for 2-methylpentcine a simple Lcingmuir isotherm was found [137]. Moleculcir simulations have shown that the inflection of isobutane is related to the preferential adsorption of the brcinched alkcines at the intersections of the zigzag and straight channel of Silicalite [21]. 

In this chapter we present the results of computer simulations of lineeir cind branched alkcines in the zeolite Silicalite. We focus on the development of the model and a detailed comparison with experimental data for the linear and brcinched alkcines. In addition we demonstrate that these isotherms can be described quantitatively with a dual-site Langmuir isotherm. 

A schematic drawing of the Silicalite structure is shown in 4.1. Silicalite has two types of channels, straight and zigzag channels which are connected via intersections. 

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Fig. 8.22 Schetnatic structure of the zeolite silicalite showing the straight and zig-zag chaimels. (Figure adapted fron Smit B and JI Siepmann 2994. Simulating the Adsorption of Alkanes in Zeolites. Science 264 1118-1120.)...

The CB-MC method has been used to simulate the adsorption isotherms of various alkanes in silicalite (170, 171). Using potential parameters that were fitted to obtain good agreement with experimental Henry s law coefficients, Smit and Maesen (170,171) have simulated the adsorption isotherms of straight-chain alkanes in silicalite. Good agreement was obtained for ethane and propane in comparison with the different type-I curves measured experimentally. The overall agreement with experimental isotherms was found to be satisfactory with hexane and heptane, and a kink is seen... 

Adsorption and diffusion of linear and branched Ce alkanes in silicalite-1 were investigated by Zhu et al. (34,35). They also developed a mathematical model taking into account the thermodynamical factor for intercrystalline diffusivities, enabling the determination of intracrystalline diffusivity from the uptake curve operated outside the linear adsorption range, van Donk et al. (36) also made transient uptake measurements to investigate the diffusivity of -hexane in Pt/H-mordenite. 

Pascual P, lingerer P, Tavitian B, and Boutin A. Development of a transferable guest-host force field for adsorption of hydrocarbons in zeolites. II. Prediction of alkenes adsorption and alkane/alkene selectivity in silicalite. J Phys Chem B 2004 108 393-398. 

In Table 1, the heats of adsorption of hydrocarbons obtained from polarizability model[9] have been compared with the experimentally measured values[21-25] in the case of ZSM-5, Silicalite, FAU, and mordenite surfaces and data displayed in Fig.l (B). For the carbon number n=l to 6, the heats of adsorption of alkanes on ZSM-5 were reported by Flanigan et al[21], Doelle et al [22] and Abdul Rehman et al [23]. Smith [24] has extended the study for n=8 to 25 who observed that for large number of paraffins, in the temperature... 

Table 2 shows the adsorbed concentrations of the pure components. At a partial pressure of 6.6 kPa the amount of n-hexane is just slightly higher than that of isohexane in silicalite-1, while the linear alkane is obviously adsorbed more strongly than 2-methylpentane in H-ZSM-5 due to the stronger interaction with the acid sites. The maximum loading of each component has been measured by a separate adsorption study. The sorption capacity of n-hexane (7 molecules per unit cell), in agreement with earlier studies [48,59-61] exceeds that of 2-methylpentane (4 molecules per unit cell). The latter value equals the number of channel intersections in the MFI pore system per unit cell. Indeed, the sorption of isohexane molecules at... 

Zhu W. et al. Adsorption of Light Alkanes in Silicalite-1 Reconciliation of Exp. Data and Mol. Simulations. Phys. Chem. Chem. Phys., 2 (2000) pp 1989-1995... 

Figure 2 The heats of adsorption of the alkanes in silicalite as a function of the total number of carbon atoms. The closed symbols are experimental and the open symbols are simulation data from Maginn et al. and Smit and Siepmann ...

Compared to linear alkanes much less experimented data are available on the adsorption of branched alkanes in Silicalite. Adsorption isotherms of isobutane have been reported by Sun et al. [136] and Zhu et al. [135,166], for various hexane isomers by Cavalccinte cind Ruthven [137], and for 2-methylheptane by Eder [160,162,163]. 

The adsorption behavior of branched alkanes in Silicalite is completely different from linear alkanes. Branched alkanes are preferentially adsorbed at the intersections of Silicalite, which is due to larger available space for the branch at the intersections. At a loading of 4 molecules per unit cell Silicalite, all intersections are occupied. Additional molecules will have to reside in the channel interiors. As this is energetically unfavorable, an additioncd driving force is needed to force the molecules into the channel interiors. This is the cause of the inflection in the isotherm, which has also been obtained experimentally for Silicalite. All isotherms can be described well using a dual-site Langmuir isotherm. 

Du, Z., Vlugt, T.J.H., Smit, B., and Manos, G. (1998). Molecular Simulation of Adsorption of Short Linear Alkanes in Silicalite Single Components and Binary Mixtures , AICHE Journal, 44, 1756-1764. 

The density of molecules arrangement of various nature and geometry in pentasil zeolites varies. For example, n.alkanes in silicalite are arranged very dense (end to end) and occupy all adsorption space (9). N.alcohols fill O.b of sorption volume of their channels while benzene only O.b of the volume. (Thus, in one case intermolecular interaction of alcohols with each other through hydrogen bonds prevent their close packing in the channels while in the other case (when benzene is adsorbed) molecular conformation prevent it from penetration into more narrow zig-zag channels. 

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