P-Xylene sorption

The use of various zeolites for the separation of xylene isomers has received a considerable amount of attention. Numerous patents have been issued for such separations however, equilibrium selectivity factors tend to be relatively low (9). We found virtually no p-xylene selectivity for a dealuminized H-mordenite under equilibrium conditions. H-ZSM-5, however, did exhibit a distinct preference for p-xylene sorption even at equilibrium (Table V). 

The high isomer selectivity observed for ZSM-5 can be further enhanced by imposing kinetic diffusion-al effects upon the thermodynamic selectivity, thereby magnifying the preference for p-xylene sorption (1 ). ... 

Fig. 54. Time-resolved FTIR spectra in polarized IR radiation of p-xylene sorption into sili-calite-1 at 353 K. From bottom to top, the spectra correspond to the uptake of 0,1,2,3 and 4 molecules of p-xylene per unit cell. The insert above the spectra shows schematically a silicaUte-1 single crystal with the two types of straight and zigzag (or sinusoidal) channels (reproduced from [949]).

Peng, J., Wan, A. (1998) Effect of ionic strength on Henry s law constant of volatile organic compounds. Chemosphere 36, 2731-2740. Peng, J., Wan, A. (1998) Effect of ionic strength on Henry s constants of volatile organic compounds. Chemosphere 36, 2731-2740. Pennell, K.D., Rhue, R.D., Rao, P.S.C., Johnston, C.T. (1992) Vapor-phase sorption of p-xylene and water on soils and clay minerals. Environ. Sci. Technol. 26, 756-763. 

The zeolites and catalysts used in this study were prepared as described previously (1,16,18,20). The ortho-xylene sorption rate data, obtained on a computer-controlled Du Pont 951 TGA, were measured at 120°C and P(o-xylene) 3.8 torr. The isomerization and disproportionation data were obtained using a... 

The n-PrOH concentration in the permeate and the normalized permeation rate increased with the increasing CD content in the CD/PVA membrane. The addition of CD in the PVA membrane determined the increasing of the swelling degree and preferential sorption of n-PrOH and p-xylene, due to the fact that the affinity of CD for these isomers was stronger than that for i-PrOH and o-xylene respectively [84],... 

The influence of the CD content in the membrane and the n-PrOH respectively p-xylene content in the feed mixture on the separation factors and sorption and diffusion selectivities of the CD/PVA membranes for the n-PrOH/I-PrOH and p-xylene and o-xylene mixtures by evapomeation are presented in tables 12 and 13. 

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. 

In a typical experiment, two solutions were prepared A - contained 0.3g p-xylene, 0.3g l,3-di(tri-fluoromethylbenzene, and 11.4g 1,3,5-tri-isopropyl-benzene B - contained l.Og n-nonane, l.Og mesitylene and 6.0g 1,3,5-tri-isopropylbenzene. 3.0g of solution A was added to l.Og H-ZSM-5 at room temperature the sorption of p-xylerie was monitored over a period of several hours by gas chromatography. When the sorption had reached a constant value, 0.6g of solution B was added. The resulting desorption of p-xylene and the adsorption of n-nonane was monitored by gas chromatography. In these experiments, the 1,3-di(trifluoro-methyl)benzene and mesitylene behaved exclusively as non-sorbing internal standards. 

Highly selective sorption of aromatic compounds from paraffin-containing solutions has been reported for the faujasites NaX, NaY, and HY under equilibrium conditions (2). Thus, benzene is preferentially adsorbed relative to n-hexane or n-decane, p-xylene relative to n-octane, and naphthalene relative to n-decane. The measured separation factors in these systems are so large (K>700) that essentially only one species exclusively occupies the internal volume of the zeolites. 

In distinct contrast to the faujasites, the intermediate-pore zeolites ZSM-5 and ZSM-11 exhibited a marked preference for n-paraffins relative to aromatics. As can be seen from Table II, both H-ZSM-5 and H-ZSM-11 preferentially sorbed n-nonane from mixtures of nonane and p-xylene dissolved in an inert, non-sorbable solvent, 1,3,5-tri-isopropylbenzene. Selectivity factors greater than 40 were observed, despite the fact that both compounds were readily sorbed when higher zeolite/sorbate ratios were used. Highly selective sorption of n-heptane relative to naphthalene, and n-tetradecane relative to 1-phenyloctane, was also observed with H-ZSM-5. 

Iso-paraffins too can be separated from aromatics on H-ZSM-5, a process that cannot be accomplished with either the small-pore or large-pore zeolites, such as A and Y. While the selectivity is not nearly as large as it is for n-paraffins, selective sorption of 2-methylheptane relative to p-xylene (selectivity factor = 6.7) was nevertheless observed. 

Similarly, Zhao, Little and Cox (2004) examined PUF as a sink and source of indoor VOCs by investigating its interaction with VOCs such as naphthalene, 1,2,4-trimethylbenzene, styrene, p-xylene, ethylbenzene, chlorobenzene, toluene and benzene. The results suggest that the sorption of VOCs by PUF is fully reversible. Thus it is a potential sink and source of indoor VOCs. (More detailed discussion on VOC composition of building products can be found in Chapter 16.)... 

Rao et al. (1990) investigated the effect of nonpolar cosolutes (trichloroethylene, toluene p-xylene), polar cosolutes (1-octanol, chlorobenzene, nitrobenzene, o-cresol) and polar cosolvents (methanol and dimethyl sulfoxide) on sorption of several polycyclic aromatic hydrocarbons (PAHs). The nonpolar cosolutes did not significantly influence PAH sorption, while the polar cosolutes (nitrobenzene, o-cresol), having sufficiently high aqueous solubilities, caused a significant decrease in PAH sorption. 

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. 

All the above mentioned high perm-selectivity of zeolite membranes can be attributed to the selective sorption into the membranes. Satisfactory performance can be obtained by defective zeolite membranes. Xylene isomers separation by zeolite membranes compared with polymeric membranes are summarized in Table 15.4. As shown, zeolite membranes showed much higher isomer separation performances than that of polymeric membranes. Specially, Lai et al. [41] prepared b-oriented silicalite-1 zeolite membrane by a secondary growth method with a b-oriented seed layer and use of trimer-TPA as a template in the secondary growth step. The membrane offers p-xylene permeance of 34.3 x 10 kg/m. h with p- to o-xylene separation factor of up to 500. Recently, Yuan et al. [42] prepared siUcalite-1 zeolite membrane by a template-free secondary growth method. The synthesized membrane showed excellent performance for pervaporation separation of xylene isomers at low temperature (50°C). 

Abstract Infrared spectroscopic methodsfor the measurement of adsorption and adsorption kinetics of some aromatics (benzene, ethylbenzene, p-xylene), pyridine, and paraffins in solid microporous materials such as zeolites (MOR, ZSM-5, silicalite-1) are described as well as the evaluation of the spectroscopically obtained data. The adsorption isotherms are of the Langmuir-Freundlich type. Isosteric heats of adsorption, transport diffusivities, and activation energies of diffusion as deduced from the spectroscopic measurements are compared with literature data as far as available, and they are found to be in reasonable agreement with results provided by independent techniques. Special attention is paid to sorption and sorption kinetics of binary mixtures, especially the problems of co- and counter-diffusion. ... 

Fig. 28 Co-diffusion of p-xylene and benzene. Uptake from a mixture of benzene and p-xylene as a function of the square root of time at 395 K but for two different partial pressures of benzene in the mixture. The overshooting of benzene sorption is clearly demonstrated (see text)...

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