Adsorption of C2−C8 n -Alkanes in Zeolites † (original) (raw)
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Chemical Engineering Journal, 2002
Inverse gas chromatography has been used to evaluate the adsorption parameters ( H, S and G) of some probes, each representing a class of organics (n-hexane, cyclohexane and benzene) on 4A and 13X zeolites. The adsorption parameters of the probes on 4A were determined in the finite concentration region, and those on 13X were determined in the infinite dilution region. The interactions between the probes and the surface were discussed in the light of determined thermodynamic parameters of adsorption. It was found that the adsorption isotherms for 4A conform with the Langmuir equation and benzene exhibits more negative H than for n-hexane and cyclohexane on both 4A and 13X. Also, interactions of the benzene and n-hexane with 13X were found to be stronger than that on 4A.
Physical Science International Journal, 2019
Different gas equilibrium adsorption models (or isotherms) with various theoretical frameworks have been applied to quantify adsorbed volume (V) of gas (or fluid) through pressure-volume behaviour at a constant temperature. Most often, Langmuir isotherm (representing Type I Isotherm) has been used in modelling monolayer adsorption even though it yields over-estimation at higher pressures thus contradicting the description of Type I isotherm. Here, higher pressures refer to pressures above the adsorption saturation pressure(Ps) . Hence, in this work, a new Type I adsorption isotherm involving pressure(P), adsorption saturation pressure(Ps) , maximum adsorbed volume and adsorbate-adsorbent resistance parameter was developed using kinetic approach. The developed adsorption isotherm is V= and it shows that Vmax is attained when pressure increases to Ps , above which no further gas adsorption occurs. The developed isotherm can be used to model all cases of monolayer adsorptions of gas...
Microporous and Mesoporous Materials, 2004
The configurational-bias Monte Carlo (CBMC) technique has been used for computing the adsorption isotherms for linear and branched 2-methylalkanes on silicalite. The carbon numbers of the alkanes ranged from four to nine. For branched alkanes inflection behavior was observed for all carbon numbers studied. The inflection was found to occur at a loading of four molecules per unit cell. Below this loading the branched alkanes are seen to be located predominantly at the intersections of the straight and zigzag channels. To obtain loadings higher than four, the branched alkane must seek residence in the channel interiors which is energetically more demanding and therefore requires disproportionately higher pressures; this leads to the inflection behavior. Linear alkanes with six and more carbon atoms also were found to exhibit inflection behavior. Hexane and heptane show inflection due to commensurate "freezing"; the length of these molecules is commensurate with the length of the zigzag channels. This leads to a higher packing efficiency than for other linear alkanes. Available experimental data from the literature are used to confirm the accuracy of the predictions of the CBMC simulations. Furthermore, the temperature dependency of the isotherms are also properly modeled. For purposes of fitting the isotherms we found that the dual-site Langmuir model provides an excellent description of the simulated isotherms for linear and branched alkanes. In this model we distinguish between two sites with differing ease of adsorption: site A, representing the intersections between the straight and zigzag channels, and site B, representing the channel interiors. CBMC simulations of isotherms of 50-50 binary mixtures of C 5 , C 6 , and C 7 hydrocarbon isomers show some remarkable and hitherto unreported features. The loading of the branched isomer in all three binary mixtures reaches a maximum when the total mixture loading corresponds to four molecules per unit cell. Higher loadings are obtained by "squeezing out" of the branched alkane from the silicalite and replacing these with the linear alkane. This "squeezing out" effect is found to be entropic in nature; the linear alkanes have a higher packing efficiency and higher loadings are more easily achieved by replacing the branched alkanes with the linear alkanes. The mixture isotherms can be predicted quite accurately by applying the appropriate mixture rules to the dual-site Langmuir model. This model allows the mixture isotherm to be predicted purely on the basis of the parameters describing the isotherms of the pure components. The sorption selectivity exhibited by silicalite for the linear alkane in preference to the branched alkane in mixtures of C 5 , C 6 , and C 7 hydrocarbon isomers, provides a potential for the development of a novel separation technique based on entropy-driven sorption selectivity.
Quantum Chemical Study of Low-pressure Adsorption in Zeolitic Materials
The Journal of Physical Chemistry B, 2001
Zeolites are mainly used as heterogeneous catalysts, with an increasing use as gas-separation adsorbents. Their properties as adsorbents are described by adsorption isotherms and isosteric heats of adsorption. Two methods are presented that predict isosteric heats of adsorption of gaseous molecules at low coverages. Both methods are based on the quantum mechanical (QM) evaluation of the interaction between the molecule adsorbed and the cationic site treated as an embedded cation. The first method uses Boltzmann statistics, whereas the other method assumes that adsorption at low pressure can be described with a Langmuir model. These two procedures yield very comparable isosteric heats for N 2 and O 2 in Ca-A and Ca-LSX. Their comparison allows us to underline the strong heterogeneity of N 2 adsorption in Ca-LSX and the effect of temperature on the equilibrium constant of adsorption.
Equilibrium isotherms studies for light hydrocarbons adsorption on 4A molecular sieve zeolite
Journal of Petroleum Science and Engineering, 2013
Equilibrium adsorption isotherms were studied for pure methane, pure ethane, and methane, ethane, and propane mixture on 4A molecular sieve zeolite at 301K. The constant-volume method was used for measuring the pure and the multi-component experimental equilibria data. Various isotherm models were utilized for correlation of pure and multi-component isotherm data. The pure component experimental data agreed well with the Langmuir and Freundlich equations. Both the extended Freundlich and modified extended Langmuir equations correlated the multi component experimental data fairly well. The results of this study showed that ethane was more selectively adsorbed than methane on 4A molecular sieve zeolite, while propane was slowly adsorbed on this zeolite at the studied temperature.
Computing Adsorption Isotherms for Benzene, Toluene, and p-Xylene in Heulandite Zeolite
Industrial & Engineering Chemistry Research, 1999
When computer simulations were performed in the grand canonical ensemble, adsorption isotherms for benzene, toluene, and p-xylene in Heulandite zeolite were constructed. Nitrogen adsorption was simulated to test a feasible computational strategy. Simulations were performed at three temperatures (200, 298, and 473 K), at pressures ranging from 0 to 200 kPa, and at water contents ranging from 0% to 4%. It was found that the adsorption of the organic species was not significantly affected by increasing the pressure over 10 kPa. Also, increasing the water content of the zeolite reduced the adsorption of these aromatics significantly. On the other hand, as the temperature was increased the amount of adsorbed material was only slightly affected. To access adsorption selectivity information, various mixtures of the aromatics were studied. Results from the simulations show that adsorption of benzene was higher than that of toluene, and toluene adsorption was higher than that of p-xylene. A relation between the electronic environment of the molecular species and the amount of adsorbed material was established. The results obtained are compared with experimental data available on other synthetic and natural zeolites.