Phase transition and structural heterogeneity; Benzene adsorption on silicalite

The adsorption behaviour of benzene in silicalite-1, AlPO 4 -5 and EU-1 has been investigated using gravimetric techniques and molecular simulation methods. For the one-dimensional, 12-membered ring (MR) channels of AlPO 4 -5 and the unidirectional, 10-MR channels with 12-MR side pockets of EU-1, the isotherms of benzene show simple type I behaviour. For the three dimensional 10-MR channels of silicalite-1, an anomalous behaviour of the benzene molecules sorbed has been observed. Two steps at loadings of ca. 4 and 6 molecules per unit cell [m.(u.c.) −1 ], respectively, and an hysteresis loop between loadings from 6 to 8 m.(u.c.) −1 can be found in the isotherms of this system. These stepped isotherms can be classified as showing type VI isotherm behaviour but in this system the reasons behind the steps are of a new and novel nature. These abnormal adsorption properties have been ascribed to the subtle interplay of increased sorbate-sorbate interactions and decreases in the entropy of sorption due to the energetically heterogeneous surfaces which are present in silicalite-1. The composition and structure of the silicalite-1 samples also play an important role on the adsorption properties of this system.

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“…Fig. −1 , which is consistent with previously reported data (Flanigen et al, 1978;Wu et al, 1983;Guo et al, 1989). Table 1).…”

Section: Resultssupporting

confidence: 93%

Benzene Adsorption in Microporous Materials

et al. 2005

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“…for both (Wu et al, 1983; their intersections. Unfortunately, statistically rigor Thamm, 1987a, b;Guo et al, 1989;.…”

Section: Introductionmentioning

confidence: 99%

Adsorption equilibrium of benzene—p-xylene vapor mixture on silicalite

Talu

1994

Chemical Engineering Science

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Abstract-Ads()rpti()n equilibrium ()f benzene-p-xylene vapor mixture ()n silicalite is measured at 70¢C with a specially designed cyclic volumetric apparatus at pressure levels of 2.53 and 1.20 kPa. The isobaric isotherms are S-shaped, and selectivity curves at different pressures cross over; the p-xylene selectivity at 2.53 kPa is higher than that at 1.20 kPa over a certain c()mposition range. The heterogeneous ideal adsorbed solution (HIAS) model implemented on two patches qualitatively predicts these highly unusual behavior. The observed extraordinary phenomena are attributed to structural heterogeneity, a result of the light-fit of sorbate molecules in silicalite pores. The success of HIAS attests to tlte paramount importance of adequate representation of heterogeneity in adsorption models.

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“…To see the limitation of this assumption, let us consider the transition which is observed in the adsorption of benzene or xylene isomers in silicalite. [209][210][211] Olson et al observed a step in the adsorption isotherm for p-xylene at 70°C, a plateau at a loading of four molecules per unit cell with saturation at six molecules per unit cell (see Figure 22). Van Koningsveld showed that at four molecules per unit cell a structural transition of the zeolite framework from the ortho to the para structure occurs.…”

Section: Flexible Latticementioning

confidence: 99%