Configurational Entropy Effects during Sorption of Hexane Isomers in Silicalite (original) (raw)
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Entropy effects during sorption of alkanes in zeolites
Chemical Society Reviews, 2002
Monte Carlo (CBMC) techniques allow the accurate calculation of the sorption isotherms for alkanes, and their mixtures, in various zeolites. The CBMC simulations give new insights into subtle entropy effects affecting mixture adsorption. Three types of entropy effects can be distinguished. (1) Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the 'gaps' within the zeolite matrix at high molecular loadings. (2) Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and branched alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules 'pack' more efficiently within, say, the intersecting channel topology of MFI zeolite. (3) Length entropy effects comes into force for sorption of linear and branched alkanes within the cylindrical channels of say AFI and MOR zeolites; here the double branched alkane has the shortest length and can be packed more efficiently within the channels. We demonstrate that CBMC simulations allow the efficient screening of zeolite structures for a given separation duty and aid the development of novel separation processes exploiting entropy effects. This journal is
Physical Chemistry Chemical Physics, 2001
We have developed a novel concept for separating alkane mixtures, in the 5È7 carbon atom range, into three separate fractions consisting of linear, mono-methyl and di-methyl alkanes by adsorption on silicalite. We make use of the fact that with increased degree of branching, the "" packing efficiency ÏÏ within the silicalite matrix is signiÐcantly lower. This conÐgurational entropy e †ect comes into play when the loading exceeds 4 molecules per unit cell, when all the intersection sites are occupied, and results in the following hierarchy of sorption strengths : linear alkanes. To demonstrate the feasibility alkanes A mono-methyl alkanes A di-methyl of the entropy-based separation concept we carried out conÐgurational-bias monte carlo simulations for a variety of binary, ternary and quaternary mixtures of alkanes,
Investigation of entropy effects during sorption of mixtures of alkanes in MFI zeolite
Chemical Engineering Journal, 2002
We have carried out a comprehensive study of sorption of mixtures of alkanes, in the 1-7 C atom range, in MFI zeolite using configurational-bias Monte Carlo (CBMC) simulations. The isotherm characteristics of various binary, ternary and quaternary mixtures have been investigated. Our studies show that two types of entropy effects have a significant influence on mixture sorption:
Langmuir, 2001
We discuss and develop an entropy-driven principle for separating isomers of alkanes in the five to seven carbon atom range by adsorption on silicalite-1. The normal alkanes are preferentially adsorbed because of configurational entropy effects; they "pack" more efficiently within the channel structures of silicalite. To demonstrate the separation principle we carried out CBMC simulations to determine the isotherms of various mixtures of linear and branched alkanes in silicalite-1. We show that the configurational entropy effects manifest at loadings greater than 4 molecules/unit cell and the sorption favors the linear alkanes while the branched alkanes are virtually excluded from the silicalite matrix. Validation of the entropybased separation principle is obtained by analyzing the silicalite membrane permeation data published in the literature.
The Journal of Physical …, 1995
The low-occupancy adsorption thermodynamics of n-alkanes ranging in length from Cq to C25 in the zeolite silicalite is predicted from molecular simulations. A bias Monte Carlo (MC) technique is described which permits these calculations to be carried out with modest computational expense. In addition, a general, systematic coarse-graining methodology is developed which enables the location and shape of chains of arbitrary length to be accurately described using a small number of degrees of freedom. By coupling this methodology with the bias Monte Carlo technique, the free energy of sorbed chains is calculated as a function of the coarse-grained configuration of chains. The results indicate that, at high temperature, n-alkanes probe all the accessible regions of the zeolite pore network, favoring high-entropy conformations that access more than one type of channel environment. As temperature decreases to room temperature, short chains continue to populate all regions of the zeolite, while chains longer than n-octane align along the straight channels in highly localized low-energy configurations. Free energy profiles of this type are also used to gain insight into probable diffusion mechanisms for the long n-alkanes at low occupancy. Macroscopic thermodynamic results, such as Henry's law constants and isosteric heats of adsorption, are calculated and compared to experimentally obtained values. The agreement between simulation and experiment is generally good. The results presented here show that there is a strong thermodynamic driving force for adsorption at all temperatures and chain lengths studied.
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.
The Journal of Physical Chemistry B, 1999
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.
Sorption-Induced Diffusion-Selective Separation of Hydrocarbon Isomers Using Silicalite
The Journal of Physical Chemistry A, 1998
In this paper we demonstrate a new principle for separation of linear and branched (2-methyl)alkanes, in the five to seven carbon atom range, by means of permeation through a silicalite membrane. The permeation selectivity relies on subtle interplay between sorption and diffusion. The required sorption isotherms for the pure components and mixtures are generated using configurational-bias Monte Carlo (CBMC) simulations. The CBMC simulations of the mixture isotherm show a curious maximum in the loading of 2-methyl alkane; this loading decreases to almost zero with increased pressures. The high sorption selectivity for the linear alkane is due to entropic effects; the linear alkane has a higher "packing" efficiency than the branched alkane within the zeolite structure. Calculations for a 50-50 mixture of n-hexane (n-C 6 ) and 2-methylpentane (2MP), for example, show that the higher sorption selectivity for the linear alkane has the effect of enhancing the flux of n-C 6 through the silicalite membrane by up to a factor of 60 above that of 2MP. Experimental evidence to support our new separation principle is provided by permeation data of Funke et al. 2
The Journal of Physical Chemistry, 1996
Results from calculations using a novel Monte Carlo method to simulate the sorption of n-butane to n-decane in various all-silica zeolites are analyzed to obtain information on the location and the conformation of the sorbed molecules. In general, the framework topology determines the conformation of the sorbed molecules. In mordenite, we find that butane is able to adsorb relatively unhindered, whereas longer chains are oriented parallel to the main channel direction and become less kinked with increasing carbon number. In ferrierite, butane molecules are distributed over both the 8-and 10-ring channels, while pentane and longer molecules are restricted to an all-trans conformation in the larger 10-ring channel. Faujasite appears to only slightly perturb the distribution of alkane conformations, compared to those found for gas phase alkanes. In zeolites rho and A, all alkanes are sorbed in a highly coiled conformation inside the R-cages of these structures.
Sorbate Immobilization in Molecular Sieves. Rate-Limiting Step for n-Hexane Uptake by Silicalite-I
The Journal of Physical Chemistry, 1994
Sorption uptake of hydrocarbons by molecular sieves with nonuniform micropore systems such as MFI-type zeolites may be governed by a complex of mechanisms instead of pure intracrystalline diffusion. In the particular case of sorption kinetics of n-hexane on silicalite-I, processes occur on the microcrystal level which comprise both Fickian diffusion and sorbate immobilization/mobilization. The rate processes connected with the immobilization of the sorbing species are due to both geometrical constraints and differences in the interaction potential topology between straight and sinusoidal channels within the zeolite crystals. A full quantitative description of this complex transport phenomenon has been derived.. A strategy has been developed to reduce the three-parameter problem to that with one parameter only, which is the prerequisite of a practical parameter-fitting procedure. In this way, rate coefficients of the particular composite processes were calculated on the basis of experimental uptake data. The latter were fitted by use of a Volterra integral equation technique. The coefficient of intracrystalline diffusion of the system n-hexaneIMFI structure at 323 K amounts to 5 x 10-10 m 2 /s, which is a value independent of loading (as the product of the immobilization and mobilization rates is). It is impossible to interpret the measured uptake curves utilizing a model that encompasses intracrystalline diffusion only (i.e., neglecting the presence of sorbate immobilization). Neglecting the strong deviation in uptake curve shape by utilizing equations for pure intracrystalline diffusion (e.g., the method of statistical moments), diffusivities were obtained that are lower by up to 3 orders of magnitude.