Equilibrium modeling of xylene adsorption on molecular sieves

The separation of xylene isomers is an important application in separation processes that is based on their adsorption properties on different adsorbents. In this work, the Price and Danner method was employed with a neural network to investigate the adsorption behavior of binary systems of p-xylene/m-xylene, p-diethyl benzene/m-xylene, and p-diethyl benzene/p-xylene and the ternary system of p-diethyl benzene/m-xylene/p-xylene at 130 and 175 °C. The Redlich–Kister, Wilson, and NRTL models were used to determine the activity coefficients in the adsorbed phase. Comparison with experimental data from the literature indicated that the proposed thermodynamic model would best determine surface excess when it is used along with the Redlich–Kister activity coefficient model.     

Codiffusion and counterdiffusion of para-xylene and ortho-xylene in a zeolite with 10 MR/12 MR interconnected channels. An example of molecular traffic control

Diffusion has a significant effect in zeolite catalysis. Molecular traffic control is a good example. For the study of diffusion in microporous crystalline materials with 10 MR/12 MR interconnected channels, as H-CIT-1, the isomers, para- and ortho-xylene are excellent probes. In this regard, by means of a FTIR technique, codiffusion and counterdiffusion of these isomers were studied. The values of the diffusivities in the codiffusion and counterdiffusion experiments were measured. A correlation in the transport of both isomers, during codiffusion and counterdiffusion was observed. Throughout codiffusion, the more mobile p-xylene molecule hinders the motion of o-xylene. During the counterdiffusion case, when p-xylene diffuses “in” and o-xylene “out” the zeolite, the 10 MR channels, which are blocked for the o-xylene molecules, are free for p-xylene motion. Subsequently, under the experimental conditions of the present study, molecular traffic control, takes place.     

The development of a screening molecularly imprinted polymer optosensor for detecting xylenes in water samples

We describe a rapid, sensitive, fluorescent screening test for xylenes in water samples that avoids more costly time-consuming methods. The screening test is based on a molecularly imprinted polymer and it runs without the need for any pre-concentration step, thus rendering it suitable for routine use in water-quality-control laboratories. The test recognizes contaminated samples rapidly (50 s) and inexpensively with a cut-off level of 10 μg mL^{− 1}, which is the value that the International Organization has laid down in its assessment of the water quality for human consumption. The reliability of the screening test was 23% false positives and 0% false negatives in 30 samples. The applicability was confirmed by analyzing mineral, tap and river water samples.     

Study of [EMim][ESO4] ionic liquid as solvent in the liquid-liquid extraction of xylenes from their mixtures with hexane

The aim of this work is to determine if the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate is a good solvent for the separation of xylenes and hexane by liquid extraction. With this purpose, liquid-liquid equilibrium (LLE) data for the ternary systems {hexane + o-xylene, or m-xylene, or p-xylene + 1-ethyl-3-methylimidazolium ethylsulfate} were determined at T = 298.15 K and atmospheric pressure. Selectivity and solute distribution ratio, derived from the experimental equilibrium data, were calculated and used to determine if this ionic liquid can be used as a potential solvent for the extraction of xylenes from their mixtures with hexane. The experimental LLE data for the ternary systems were correlated using the NRTL and UNIQUAC models.     

Preparation and characteristics of sol-gel-coated calix[4]arene fiber for solid-phase microextraction

5,11,17,23-Tetra-tert-butyl-25,27-diethoxy-26,28-dihydroxycalix[4]arene/hydroxy-terminated silicone oil coated fiber was first prepared and applied for solid-phase microextraction (SPME) with sol-gel technology. The possible sol-gel mechanism was discussed and confirmed by IR spectra. It showed wonderful selectivity and sensitivity to polar (aromatic amines), nonpolar (benzene derivatives, polycyclic aromatic hydrocarbons) and high boiling point compounds (phthalates) and the extraction equilibria were reached quite fast. The coating has high thermal stability (380 degrees C) and solvent stability (organic and inorganic), thus its lifetime is longer than conventional fibers. In addition, it has surprising fiber-to-fiber and batch-to-batch reproducibility. The detection limits were quite low and the linear ranges were pretty broad for all analytes.     

Absorption and velocity of ultrasonic waves in thiophene, o-, p-xylenes and their mixtures

The results of acoustic measurements of velocity and absorption in three pure liquids: thiophene, p-and m-xylenes and their mixtures are presented. The experiments for the mixtures of thiophene were carried out by Eggers' method at frequencies 0.3–5 MHz, and for pure liquids by the pulse method in the frequency range 10 MHz–10 GHz, all at 293.15 K except for thiophene (at 281 and 333 K).

The absorption in thiophene shows that all vibrational degrees of freedom take part in the observed relaxation, caused by the Kneser processes. This process can be described as a vibrational relaxation with one relaxation time. Absorption in the mixtures decreased when increasing the amount of xylenes, as predicted by theory of gases, thus suggesting that the absorption is probably due to the same phenomenon as in gases.     

Monte Carlo and energy minimization studies of binary xylene adsorption in AEL and AFI networks

Adsorption of binary xylene mixtures in AEL and AFI networks was investigated using normal and biased GCMC simulations. Preferential o-xylene adsorption was evidenced in the simulations, as previously reported in single-component experimental data. In contrast to the FAU and MFI sieves, the AEL and AFI networks exhibit surprising azeotropic behavior. The selectivity switches from o-xylene to p-xylene at a gas phase mole fraction of ca. 0.5. Energy minimization was performed in the AlPO₄-11 molecular sieve to determine the energy differences between the adsorption sites. The minimization study showed that AlPO₄-11 has small adsorption energy differences between sites. The azeotropic behavior of the AEL and AFI networks can be explained using the two patch model proposed by Do and Do (Adsorption 5:319–329, 1999).     

Ortho-selectivity in aluminophosphate molecular sieves: A molecular simulation study

Monte Carlo adsorption simulations of xylenes have been performed in aluminophosphate molecular sieve structures. A new force field fitted for o-xylene in AlPO₄-5 was used. It is shown that force fields have good transferability among the aluminophosphate sieves series and the new force field adequately describes the experimentally observed adsorption isotherms for xylene/AlPO₄-5. A previous investigation of adsorption isotherms and structural analysis has been extended to AlPO₄-8 and VPI-5 sieves. In AlPO₄-8, like in AlPO₄-5, the variations in the channels diameters and the corresponding interaction energy of the molecule-crystal lattice drive all molecular positioning. In VPI-5, the modulation between wide and narrow regions becomes negligible due to the larger pore diameter, so no ortho-selectivity was observed. The simulations confirm the ortho-selectivity mechanism proposed to aluminophosphates.     

Effect of addition of a graphitized carbon black trap to a glass beads trap on the cryoconcentration of some non-methane hydrocarbons in ambient air

Carbotrap was added to a glass beads cryotrap in order to increase the retention of the very volatile two-carbon hydrocarbons, ethylene, ethane and acetylene. Indeed the obtained recoveries increased from 2 to 3% for ethylene, the poorest retained compound, from 7 to 20% for ethane and from 23 to 31% for acetylene between the glass beads only and the Carbotrap/glass beads cryotrap. The addition of the Carbotrap, however, decreased the obtained recovery of o-xylene by 22–46%, depending on the employed conditions. The smaller decrease was observed when the longer desorption time was employed. The variation of the temperature of desorption by 40°C had very little effect on the resulting recoveries.