Xylene, adsorption

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

Fig. 4. (Left) Sb location in W SbiOv catalysts (a) in L layer (b) in 2nd layer (Right) Effect of Sb on p-Xylene adsorption over (a) WO3 catalyst surface and (b) W-Sb oxide catalyst surface.

It has also been shown that the selectivity features of para-selective catalysts can be readily understood from an interplay of catalytic reaction with mass transfer. This interaction is described by classical diffusion-reaction equations. Two catalyst properties, diffusion time and intrinsic activity, are sufficient to characterize the shape selectivity of a catalyst, both its primary product distribution and products at higher degrees of conversion. In the correlative model, the diffusion time used is that for o-xylene adsorption at... 

The simulated moving bed operational mode involves four distinct functional zones, the adsorption, purification, desorption and buffer zones. These zones are described in detail in other parts of this book. We now examine the function of each zone as it applies to p-xylene adsorption and which can be extrapolated to the other aromatics separations. 

The complexity of xylene adsorption over zeolites is too high to predict the selectivity from the chemical properties of the zeolite only (electronegativity of the cations, charge of the framework oxygens). The interactions between xylenes and the zeolite must necessarily be considered, which explains the important development of molecular simulation methods. This is supported by the work of V. Lachet et al. (18) who succeeded in reproducing the inversion of selectivity between KY and NaY with Grand Canonical Monte Carlo Simulations. 

Table 4.11 a helps to compare the economics of the most important p>xylene adsorption and crystallization processes. 

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

E. de Vos Burchart, B. E. van der Linden, H. van Bekkum, and B. van de Graaf, Collect. Czech. Chem. Commun., 57, 675 (1992). Molecular Mechanics Studies on MFl Type Zeolites. 1. Effect of p-Xylene Adsorption on the Zeolite Structure. 

Lachet, V., Boutin, A., Tavitian, B., and Fuchs, A.H. (1999). Molecular simulation of p-xylene and m-xylene adsorption in Y zeohtes. Single components and binary mixtures study. Langmuir, 15, 8678—85. 

Let us take one example to try to visualize more precisely the above discussion. The details of a part of the XRD pattern are shown in fig. 3 for the as synthesized (SYN with tetrapropylammonium template), hydrated (WAT) and four xylene molecules adsorbed per unit-cell (XYL-1) of B-ZSM-5 sample from ref. 35 and 36. The calcined form exhibits a monoclinic structure (P2l/n space group) whereas SYN and WAT forms correspond to orthorhombic phases (Pnma space group). The striking differences in XRD patterns are seen in fig. 3. The p-xylene adsorption is known (27) to take place in two steps. The first one, up to four molecules per u.c. and designated XLY-I, corresponds to the formation of a low coverage complex and the second one (XYL-II) to a maximum of xylene adsorption with 8 molecules per u.c. The location of p-xylene could even be determined in the ZSM-5 channel by careful analysis of powder X-ray diffraction patterns using the trail error method (35) as shown in fig. 4. 

The effect of p-xylene adsorption at ambient temperature on the Si MAS NMR spectrum of highly siliceous ZSM-5 as a function of concentration is... 

The crystallographical findings described above indicate two mechanisms which play the most importante role in the p-xylene adsorption on ZSM-5 ... 

Para and meta-Xylene adsorption has been performed in a mlorobalance connected to a vacuum manifold (ultimate vacuum 10 mbar). Before the adsorption measurement each sample has been treated in situ at 573K up to constant weight. The temperature has been then lowered to 303K, the xylene Isomer vapour dosed on the sample and the weight Increase due to the adsorption recorded. 

Fig. 2. p-xylene adsorption by TG analysis a) selected TG curves b) variation of adsorption capacity (calculated from the weight loss in the expressed as volume of liquid p-xylene) with... 

In Fig. 5c the dependence of the lratio upon temperature treatment is illustrated. The following can be noticed i) unlike the previous case, the ratio is not going to zero as temperature approaches 1573K this can be explained with the formation of an amorphous phase still containing substitutional titanium. ii) like the m-xylene adsorption capacity, the 970/ 550 ratio decreases already at temperature lower than 1173K this means that substitutional Ti is going into extraframework position before the occurrence of dramatic changes in sample crystallinity... 

It is most remarkable that the XRD and IR 550 cm band data, which are crystallinity parameters, are strictly correlated with Nj and p-xylene adsorption. This fact indicates that the letters are mainly dependent upon the overall crystallinity. 

On the contrary the m-xylene adsorption correlates well with the IR band at 970 cm and the UV Vis peak at 40000 om which are structural Ti parameters. Ti extraction from the framework is occurring at temperature as low as 1173K and is accompanied by a small relaxation of the framework whith subsequent change the channels dimensions (without significant modification of the total pore volume). m-Xylene, having a larger kinetic diameter with respect to p-xylene and Nj (ref. 18), is a sensitive probe of this transformation consequently it correlates very well with the structural Ti parameters. 

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