Simulating the Adsorption of Binary and Ternary Mixtures of Linear

Monte Carlo simulations are used to predict the adsorption isotherms at 300 and 600 K for binary and ternary mixtures of linear, branched, and cyclic ...
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J. Phys. Chem. B 2004, 108, 17136-17142

Simulating the Adsorption of Binary and Ternary Mixtures of Linear, Branched, and Cyclic Alkanes in Zeolites Joseph P. Fox* and Simon P. Bates School of Physics, Mayfield Road, Edinburgh UniVersity, EH9 3JZ, United Kingdom ReceiVed: February 27, 2004; In Final Form: May 14, 2004

Monte Carlo simulations are used to predict the adsorption isotherms at 300 and 600 K for binary and ternary mixtures of linear, branched, and cyclic alkanes in silicalite-1, AlPO4-5, and the recently synthesized ITQ22. The theoretical binary and ternary adsorption isotherms predicted by Ideal Adsorption Solution Theory (IAST) agree well with the simulated isotherms. Increasing the temperature altered the adsorption hierarchy, with the adsorption of cyclic molecules increasing as the linear and, to a greater extent, branched molecules decreased. A microscopic analysis of the adsorption locations and molecular conformations provides an explanation for the change in the selectivity of adsorption with temperature.

1. Introduction Real industrial processes that involve the adsorption of hydrocarbons in zeolites will entail mixtures of many different species interacting with the zeolite. Conventional experimental techniques used to study single-component adsorption behavior in zeolites are not easily modified to cope with the increased level of complexity associated with mixtures, because not only must the uptake be measured but the composition of the adsorbed mixture also must be determined. In contrast, properties of mixtures within zeolites can be studied efficiently using Monte Carlo simulations, as evidenced by recent simulation studies of the adsorption of binary mixtures of linear and branched alkanes.1-3 However, mixtures that involve cyclic molecules have not been investigated, to our knowledge. Cyclic molecules have a significant role in many processes and an understanding of their behavior is important, both fundamentally and practically. The simulation of the adsorption of mixtures presents an excellent opportunity to test adsorption theory, which makes predictions about the adsorption of mixtures based on the singlecomponent isotherms. Comparing the theoretical mixture isotherms with the equivalent simulated isotherms allows any discrepancies between the predictions from the two techniques to be highlighted. The theoretical predictions are made at the macroscopic level, whereas the simulations permit exploration at the microscopic level, such as the molecular conformation and adsorption location which is of great use in understanding adsorption. Conventional methods used in Monte Carlo simulations are not applicable for use in the simulation of large molecules, because they are prohibitively slow. Novel techniques, such as the Configurational Bias Monte Carlo (CBMC)4 technique, which grows the molecules within the pore of the zeolite are more efficient but cannot be applied to cyclic molecules. This work uses a modification5 to the CBMC growth algorithm to allow the simulation of cyclic molecules which we have already applied to the study of single-component adsorption of cyclohexane in silicalite-1 and AlPO4-5. * Author to whom correspondence should be addressed. E-mail address: [email protected].

The aims of this work are to explore the adsorption of binary and ternary mixtures of linear, branched, and cyclic alkanes at different temperatures in three different zeolites: silicalite-1, AlPO4-5, and the recently synthesized ITQ-22.6 This choice of zeolites provide a broad range of pore dimensions and geometry: silicalite-1 has been used extensively in simulations of single components and binary adsorption of linear and branched molecules, and, thus, it is natural to extend the focus to include mixtures that involve cyclic molecules. The size of the pores in AlPO4-5 is much larger than that of silicalite-1, which allows the effect of pore size on adsorption to be investigated. ITQ-22 is a new zeolite with unique properties which itself justifies it as the focus of a study to determine its adsorption properties. The molecules chosen in this study are hexane, 2-methylpentane, and cyclohexane (i.e., C6 molecules). Cyclohexane provides an excellent test for the cyclic CBMC growth technique, because it is a large molecule that only fits tightly within the pores of silicalite-1. The effect that temperature has on the adsorption selectivity of the zeolites will be discussed and an explanation will given for the temperature dependence, advanced on the basis of a microscopic analysis of the siting of the adsorbates and on their conformation. Ideal Adsorption Solution Theory7 (IAST) will be used to predict mixture isotherms based on the single-component isotherms. This work is ambitious in its aim and scope for two principal reasons. The first is that multicomponent isotherm predictions of large cyclic, linear, and branched alkanes have not previously been investigated. The second is that the microscopic information afforded by the simulations, across a range of different pore topologies, is used to rationalize the predicted macroscopic adsorption behavior. This, coupled with the validation of macroscopic predictions by comparison with experimental datas where availablesserves to highlight the predictive power of such simulations. The outline of this paper is as follows: the next section describes the simulation model, which is followed by the results for the binary mixtures and a discussion of the IAST predicted isotherms and the ternary mixtures. Finally, the conclusions will be presented.

10.1021/jp0491212 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/07/2004

Simulating the Adsorption of Alkanes in Zeolites

J. Phys. Chem. B, Vol. 108, No. 44, 2004 17137

2. Models and Simulation Details This section outlines the model used to describe the system that is being simulated. As with any model, approximations are made and further details of these (and any consequences of using such approximations) can be found in the relevant references. Monte Carlo simulations, in the Grand Canonical ensemble (constant µVT), making use of the Configurational Bias technique (where appropriate), are used in this work. Following the work of Kiselev,8 the zeolites are modeled as a rigid network of O atoms, which provides a balance of accuracy and efficiency, enabling simulations to be performed in an order-of-magnitude shorter time, compared with the corresponding flexible zeolite simulations. Zeolite flexibility is known to influence the dynamics of large molecules, whose kinetic diameter is commensurate with the size of the pore.9,10 Thus, the rigid zeolite approximation will be less appropriate for large molecules at high loadings. However, it may be expected that the location of the adsorption sites will not be as greatly influenced as the dynamics in using the rigid zeolite approximation. The chain guest molecules (hexane and 2-methylpentane) are modeled as a collection of CH, CH2, and CH3 pseudo-atoms11 with flexible bond angles and torsion angles. Cyclohexane is modeled as a rigid molecule, in the chair conformation, consisting of CH2 pseudo-atoms. There exist several methods12-14 for modeling flexible molecules with strong internal potentials (such as cyclic molecules). However, in this work, the cyclohexane is modeled as a rigid molecule whose conformation remains in the chair state (which has been shown15 to dominate the adsorption process) throughout the simulation. To insert the rigid molecule, the method developed by Errington et al.13 is used, whereby the first three pseudo-atoms are inserted using a configurational bias (on the basis of the bond length and bond angle) and then the remaining pseudo-atoms are inserted in their fixed positions (which are fixed by the rigid bond angle and torsion angle constraints). This technique has been shown5 to predict the adsorption isotherm of cyclohexane in silicalite-1 accurately over a wide range of temperatures and pressures. The nonbonded pseudo-atoms interact with each other via a Lennard-Jones potential; each pseudo-atom has its own parameters. Details of the potentials used can be found in reference,5 which also validates their use for adsorption simulations and the algorithms used for the modified growth technique for cyclic molecules. The structure of silicalite-1 and AlPO4-5 are well-known and have been used in many previous simulation studies.2,4,16-20 However, the recently synthesized zeolite (ITQ-22) has never (to our knowledge) been the focus of a simulation, and so a brief description of the structure of this new zeolite follows; ITQ-22 comprises interconnected 8-, 10-, and 12-memberedring (8MR, 10MR, and 12MR, respectively) channels (as shown in Figure 1), providing it with unique selectivity properties. The 8MR channels are inaccessible to all but the smallest adsorbates at very high pressure. All of the zeolites used in this study are modeled as a network of O atoms; therefore, they all share the same zeolite-adsorbate potential parameters. This widely used20,21 approximation is based on the fact that the O atoms outnumber and are more polarizable than the other zeolite atoms, and, thus, they dominate the zeolite-adsorbate interaction. The IAST7 can be used to predict the adsorption of mixtures from single-component adsorption data. It works by equating spreading pressures for each species in the mixture and comparing the spreading pressure with the original singlecomponent isotherms, predicts the amount of each species which will adsorb when in a mixture. This relatively straightforward

Figure 1. Depiction of the structure of ITQ-22; the 10-memberedring (10MR) sinusoidal channels intersect both the 12-membered-ring (12MR) and 8-membered-ring (8MR) channels.

TABLE 1: Maximum Capacities for Hexane, 2-Methylpentane, and Cyclohexane in Silicalite-1, AlPO4-5, and ITQ-22 at 300 K and 600 K and 104 kPa Maximum Capacity (molecules/unit cell) Silicalite-1 hexane 2-methylpentane cyclohexane

ITQ-22

AlPO4-5

300 K

600 K

300 K

600 K

300 K

600 K

8.00 7.00 4.00

4.52 4.00 4.00

9.55 10.00 12.00

7.64 7.77 10.22

2.00 2.25 4.00

1.90 1.95 3.20

technique is used extensively22-24 to predict the adsorption of mixtures in many different materials without having to perform mixture experiments. One of the key assumptions of IAST is that the adsorption area available to each species in the mixture is equal at all temperatures. This may not be the case for mixtures in zeolites, because some of the mixture species may have access to areas that, due to size constraints, are inaccessible to other components in the mixture. However, despite this constraint, IAST works well in regard to predicting mixture isotherms in zeolites and will be used in this work to compare to the simulated mixture isotherms. Details of the single-component isotherms for silicalite-1 and AlPO4-5 have previously been presented,5 and we note here that they were determined to be in excellent agreement with the experimental data over a large range of temperature and pressure. The maximum loading of hexane, 2-methylpentane, and cyclohexane in silicalite-1, AlPO4-5, and ITQ-22 are given in Table 1 as a point of reference for the subsequent discussion of mixtures. The Henry coefficients and heats of adsorption for the three guest molecules in the three zeolites are given in Table 2. 3. Results 3.1. Binary Mixtures. Figure 2 shows the adsorption isotherm for an equimolar binary mixture of cyclohexane and 2-methylpentane in silicalite-1 at 300 and 600 K. At 300 K, it is clear that (i) the branched molecule dominates the adsorption and (ii) the cyclic molecule is only sparingly adsorbed. Previous studies2,5 have shown that 2-methylpentane adsorbs in silicalite-1 with the branched “head” in the intersection and its “tail” in the straight or zigzag channels. At elevated pressures, 2-methylpentane can also adsorb in the zigzag channels; this is seen at pressures of >100 kPa in the top portion of Figure 2. Cyclohexane is most favorably adsorbed in silicalite-1 at the intersections,5 so there is direct competition for adsorption sites between these two species. Cyclohexane has a much lower Henry coefficient and heat of adsorption (see Table 2), compared

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TABLE 2: Heats of Adsorption (-Qst) and Henry Coefficients (KH) for Hexane, 2-Methylpentane, and Cyclohexane in Silicalite-1, ITQ-22, and AlPO4-5 at 300 K Silicalite-1 hexane 2-methylpentane cyclohexane

ITQ-22

AlPO4-5

KH (mol kg-1 Pa-1)

-Qst (kJ/mol)

KH (mol kg-1 Pa-1)

-Qst (kJ/mol)

KH (mol kg-1 Pa-1)

-Qst (kJ/mol)

2.35 3.25 1.3

70.8 71.4 61.8

0.65 0.88 1.04

61.3 63.0 58.7

0.38 1.06 2.52

53.4 57.2 55.2

Figure 2. Adsorption isotherms for an equimolar mixture of cyclohexane and 2-methylpentane in silicalite-1 at 300 K (top) and 600 K (bottom). Note: In all isotherm figures, the crosses (×) refer to 2-methylpentane, circles (O) refer to cyclohexane, squares (0) refer to hexane, and the dashed lines (- - -) are the IAST predicted isotherms.

Figure 3. Adsorption isotherms for an equimolar mixture of cyclohexane and hexane in silicalite-1 at 300 K (top) and 600 K (bottom).

to 2-methylpentane, and so it may be expected that it would be energetically more favorable for the branched molecule to adsorb in preference to the cyclic one. However, as the temperature is increased to 600 K, the adsorption preference is reversed (see bottom portion of Figure 2). The cyclohexane molecules are now adsorbed in preference to the 2-methylpentane molecules and, when the siting information was examined, it was observed that the 2-methylpentane molecules were no longer able to occupy the zigzag channels and adsorbed only at the intersections. The reversal of selectivity, which was observed in the binary mixture of cyclohexane and 2-methylpentane at high temperatures (Figure 2), is also noted in an equimolar binary mixture of hexane and cyclohexane but not for 2-methylpentane and hexane. Figure 3 shows the isotherms for the mixture of hexane and cyclohexane at 300 and 600 K. At 300 K, the hexane

isotherm is almost unaffected by the presence of the cyclic molecule and the isotherm is very similar to that of pure hexane. The selectivity is reversed at 600 K but the total number of molecules adsorbed remains low, even at high pressures. Analysis of the siting of the molecules shows that, as the pressure increases, the hexane molecules adsorb preferentially in the straight and zigzag channels, whereas the cyclohexane molecules adsorb only in the intersection. However, it is possible for a hexane molecule that is adsorbed in a channel to prevent the cyclohexane molecule from adsorbing at the intersection, because of the repulsion between the two molecules; this is discussed further in the next section. One possible explanation for the reversal in selectivity at high temperatures comes from examining the range of conformations accessible to the three molecular types. At low temperatures, cyclohexane will be in its lowest energy conformation (the “chair” conformation), the hexane molecule is most likely to have, at maximum, one gauche25 bond (the others being trans) and similarly for 2-methylpentane, in which the all trans or one gauche conformers will dominate.26 However, as the temperature increases, the molecules are able explore their higher-energy conformations. In the case of cyclohexane, this means flexing between chair and boat conformations. The change between chair and boat does not significantly alter the shape of the molecule, which still maintains its ringlike structure. In contrast, the shape of the linear and branched molecules can change more dramatically as more than one gauche bond is accommodated. At higher temperatures, the fraction of hexane and 2-methylpentane molecules with one or more gauche bonds will increase. A hexane molecule with all trans bonds will find it much easier to fit into the channels of a zeolite, compared to a hexane molecule with a gauche bond. The same is true of 2-methylpentane, which has even more difficulty in fitting into pores when not in its all-trans form, because it has a branched head, which creates further restrictions on fitting into a zeolite pore. Examining the end-to-end length of hexane and 2-methylpentane molecules at high and low temperatures shows that there is a decrease in length of ∼10% from the long molecules at low temperature (300 K) to shorter, fatter molecules at high temperature (600 K). The same analysis of cyclohexane shows that there is no appreciable difference in molecular size at low and high temperatures. ITQ-22 is a zeolite with slightly larger pores than silicalite1, and, therefore, it may be expected to exhibit adsorption characteristics different from those of silicalite-1. The singlecomponent adsorption capacities (see Table 1) show that cyclohexane is adsorbed in preference to 2-methylpentane and finally hexane. The cyclohexane and 2-methylpentane molecules adsorb at the large intersections between the 12- and 10MR channels and in the 12MR channels, midway between these intersections. The hexane molecules prefer the intersection between the 8- and 10MR channels and in the 12MR channel between the intersections with the 10MR channels. As the temperature increases, the loadings become more localized around the preferred adsorption sites. The single-component adsorption hierarchy in ITQ-22 is reversed, compared to that

Simulating the Adsorption of Alkanes in Zeolites

Figure 4. Adsorption isotherms for an equimolar mixture of cyclohexane and hexane in ITQ-22 at 300 K (top) and 600 K (bottom).

Figure 5. Adsorption isotherms for an equimolar mixture of 2-methylpentane and hexane in ITQ-22 at 300 K (top) and 600 K (bottom).

in silicalite-1, and so it would be reasonable to expect mixtures to behave differently in ITQ-22 than silicalite-1. Indeed, the top portion of Figure 4 shows that the selectivity is reversed for a mixture of cyclohexane and hexane at 300 K, compared with the same mixture in silicalite-1; however, at 600 K, both zeolites exhibit the same selectivity. The same is true for a mixture of cyclohexane and 2-methylpentane at 300 K (not shown): in ITQ-22, the cyclic molecule is preferentially adsorbed. In the mixture of hexane and 2-methylpentane at 300 K in ITQ-22 (see top portion of Figure 5), the branched molecule adsorbs more freely than the linear molecule. However, if the temperature is increased to 600 K, the adsorption hierarchy switches and the linear molecule is preferentially adsorbed, as shown in the bottom portion of Figure 5. The adsorption hierarchy of ITQ-22 is echoed in the zeolite AlPO4-5, which has unconnected straight pores, larger than those in both ITQ-22 and silicalite-1. At 300 and 600 K, the binary mixtures of cyclohexane with hexane and cyclohexane with 2-methylpentane both result in a substantial adsorption of cyclohexane with only limited adsorption of the other mixture component. A microscopic analysis of the molecular orientation shows that the cyclohexane molecules are able to arrange themselves such that the average molecular separation is much less than that of hexane or 2-methylpentane. They do this by packing approximately perpendicular to the channel axis. Thus, more cyclohexane than hexane or 2-methylpentane molecules can occupy a given volume of channel in AlPO4-5 (for further

J. Phys. Chem. B, Vol. 108, No. 44, 2004 17139 details, see ref 5). In the mixture of 2-methylpentane and hexane at 300 K, three times as much 2-methylpentane is adsorbed than hexane. However, as the temperature is increased, the amount of 2-methylpentane that is adsorbed decreases while the hexane increases until, at 600 K, there is only 1.6 times as much 2-methylpentane adsorbed as hexane. An analysis of the results from the binary mixtures in all three zeolites uncovers a consistent influence of temperature on the outcome of the mixture adsorption simulations. As the temperature increases, cyclohexane becomes more favorably adsorbed, regardless of whether it is in a mixture with hexane or 2-methylpentane. Mixtures that involve hexane and 2-methylpentane reveal that, as the temperature increases, the ability of 2-methylpentane to adsorb decreases while that of hexane increases. Thus, it is possible to tailor which species in a mixture preferentially adsorbs within the zeolite by simply varying the temperature of the adsorption process. The temperaturecontrolled selectivity predicted by the simulations is confirmed by experimental work of Funke et al.,27 who measured the adsorption selectivity of silicalite membranes with binary and ternary mixtures of hydrocarbons. The experiments showed that the selectivity of hexane over cyclohexane decreased as the temperature was increased from 369 K to 443 K. Experiments were only conducted up to 443 K; thus, the complete reversal of selectivity predicted by the simulations at 600 K was not observed. However, using the selectivity predicted by our simulations at 300, 400, and 500 K, it is possible to estimate the selectivity at intermediate temperatures. A comparison between the experimental and simulated selectivity at 369 and 443 K show good agreement. The simulated selectivities were determined to be 46 and 4.7, whereas the experimental data from ref 27 was 55 and 1.2 at 369 and 443 K, respectively. It would be very interesting to examine if further experiments at 600 K are able to confirm (or refute) the predictions made by the Monte Carlo simulations. The IAST7 uses single-component isotherms to predict a mixture isotherm at a given temperature (see Section 2 for more details). One of the assumptions of IAST is that the area available to each species of the mixture should be equal at all temperatures. This is not the case for mixtures that contain large molecules in zeolites, because one molecule that is inaccessible to the other molecule may be able to adsorb through the zeolite channels. However, IAST does predict mixture isotherms that are in agreement with the simulations isotherms. Figures 2-5 show the comparison between the simulated mixture isotherms and the theoretical isotherms predicted using IAST. The predicted isotherms are in good agreement with those found from the simulations, confirming that the techniques used to simulate the adsorption of mixtures are correct and such simulations can be used to accurately predict the relative adsorption of components within mixtures. Despite the assumption that the area available to each species of the mixture should be equal at all temperatures, IAST can be used to predict isotherms for mixtures of C6 alkanes in silicalite-1, ITQ-22, and AlPO4-5. However, while IAST can make predictions about the macroscopic nature of the adsorption process, it cannot divulge information about the microscopic behavior of the adsorbates. To explore the siting and conformational details of the adsorbates, other techniques, such as computer simulations, are necessary to compliment IAST. 3.2. Ternary Mixtures. Using the information obtained from the single-component isotherms, coupled with the knowledge of the change in selectivity with temperature, it should be

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Fox and Bates

Figure 6. Adsorption isotherms for an equimolar mixture of hexane, 2-methylpentane and cyclohexane in silicalite-1 at 300 K (top) and 600 K (bottom).

possible to make predictions about equimolar ternary mixtures. In the case of silicalite-1, the single-component loadings and the binary mixture isotherms suggest that, in the ternary mixture at 300 K, hexane would dominate the adsorption, with 2-methylpentane adsorbing in small quantities and cyclohexane adsorbing only sparingly. However, analysis of the hightemperature binary isotherms reveals that cyclohexane is adsorbed in preference to hexane, which adsorbs in slight preference to 2-methylpentane. Thus, it may be expected that, at high temperature, an equimolar ternary mixture in silicalite1scyclohexaneswould adsorb in greater numbers than hexane and finally 2-methylpentane. Figure 6 shows the simulated adsorption isotherms for an equimolar ternary mixture of hexane, 2-methylpentane, and cyclohexane in silicalite-1 at 300 and 600 K, together with the IAST isotherms. As predicted, on the basis of the findings from binary mixture adsorption, at low temperature, the linear alkane dominates the adsorption, with only a tiny fraction of the adsorbed mixture being composed of branched or cyclic molecules. Indeed, the presence of the branched and cyclic molecules does not seem to alter the maximum loading of hexane significantly, whose isotherm is close to that of the pure component. The theoretical (IAST) isotherms are in good agreement with the simulated isotherms and correctly predict both the selectivity and the amount of each component that is adsorbed. A microscopic analysis of the molecular locations shows that, at high pressures, the hexane molecules adsorb in the straight and zigzag channels but not the intersections. 2-Methylpentane molecules adsorb with their branched “heads” in the intersection and their “tails” in the straight or zigzag channels. Cyclohexane preferentially adsorbs at the intersections in silicalite-1 and, yet, there is almost no uptake of cyclohexane molecules in the ternary mixture. This is because the hexane molecules (which dominate the adsorption) do not fit perfectly into the channels, and, thus, they may impinge upon the intersections, preventing cyclohexane from having sufficient space to adsorb. Figure 7 shows a snapshot of the position of the hexane and cyclohexane molecules in silicalite-1 at 300 K and 10-4 kPa. It highlights the way in which the hexane molecules may prevent the cyclohexane molecules from adsorbing in the intersections by encroaching on the intersections from their adsorbed position in the channels. The dashed circles show parts of an intersection that are occupied by the end of the hexane molecules, preventing the adsorption of cyclohexane in that intersection. The solid black circle shows the location

Figure 7. Location of hexane and cyclohexane molecules in silicalite-1 at 300 K and 10-4 kPa; the black circle with a solid boundary shows the location of the cyclohexane molecule, whereas the circles with dashed boundaries highlight the area in the intersection occupied by the hexane molecules.

of the only cyclohexane molecule to adsorb successfully. Note that there are no hexane molecules in that intersection, but they are present in all other intersections. The influence of temperature on the adsorption selectivity was explored in the previous section. The adsorption of cyclohexane was observed to increase with temperature, whereas that of hexane and, to a greater extent, 2-methylpentane, were observed to decrease with temperature. Using these findings to predict the ternary adsorption in silicalite-1 at 600 K suggests that the amount of cyclohexane adsorbed should increase while the amount of hexane adsorbed should be more than that of 2-methylpentane. The bottom portion of Figure 6 shows that, indeed, the amount of cyclohexane adsorbed has increased, at the expense of hexane, which is now the second-most favorably adsorbed species, ahead of 2-methylpentane. An analysis of the siting of the three molecular species shows that, at 103 kPa, cyclohexane and 2-methylpentane both adsorb at the intersections, whereas hexane is equally distributed between the straight and zigzag channels. Once again, the IAST predicted isotherms for the ternary mixture in silicalite-1 (shown in Figure 6) correctly determines the adsorption hierarchy at both low and high temperatures. ITQ-22 has slightly larger pores than silicalite-1 and, as the binary adsorption isotherms show, has different adsorption characteristics, compared to silicalite-1. Based on the binary adsorption isotherms, it would be expected that, at 300 K, the adsorption of an equimolar ternary mixture of hexane, 2-methylpentane, and cyclohexane in ITQ-22 would consist mainly of cyclohexane, followed by 2-methylpentane and finally hexane. At high temperature, the binary mixture isotherms revealed a switch in adsorption preference in the mixture of hexane and 2-methylpentane, with hexane adsorbing in preference to 2-methylpentane. Thus, at high temperature, the ternary mixture would be expected to consist of predominantly cyclohexane, followed by hexane and finally 2-methylpentane. The ternary adsorption isotherms in Figure 8 confirm the predictions and show that, at 300 K, the cyclic molecule is preferentially adsorbed, followed by the branched and finally the straight molecule. The hexane molecules adsorb at the

Simulating the Adsorption of Alkanes in Zeolites

J. Phys. Chem. B, Vol. 108, No. 44, 2004 17141 the hexane and 2-methylpentane molecules, which are longer and whose adsorbed molecular separation is larger than cyclohexane. The IAST predictions are again in good agreement with the simulated isotherms at both temperatures. 4. Conclusions

Figure 8. Adsorption isotherms for an equimolar mixture of hexane, 2-methylpentane, and cyclohexane in ITQ-22 at 300 K (top) and 600 K (bottom).

Figure 9. Adsorption isotherms for an equimolar mixture of hexane, 2-methylpentane, and cyclohexane in AlPO4-5 at 300 K (top) and 600 K (bottom).

straight and zigzag channels, avoiding the large intersections, whereas the 2-methylpentane molecules prefer the center of the 12MR channels, halfway between the intersections with the 10MR. The cyclohexane molecules adsorb at the intersections between the 12MR and 10MR channels and in the 12MR channels halfway between the intersections with the 10MR channels. The 8MR channel is too small to admit any of the molecules and doe snot participate in the adsorption process. At 600 K, the cyclohexane molecules again dominate, whereas the hexane molecules are adsorbed in favor of the 2-methylpentane molecules. The adsorption locations remain as they were for the 300 K mixture. Using the binary adsorption isotherms to predict the ternary isotherm in AlPO4-5 yields similar results to the predictions in ITQ-22. The binary isotherms suggest that, at 300 K, the adsorption order (from most to least) should be cyclohexane, 2-methylpentane, and hexane, whereas at 600 K, the binary isotherms predict a change in order, to cyclohexane, hexane, and 2-methylpentane. Figure 9 confirms these predictions and shows that the uptake of the linear and branched molecules is very small, compared to the cyclic molecule. A microscopic analysis of the molecular positions and orientations shows that the cyclic molecules are able to stack themselves within the pores, allowing a large number of molecules to adsorb in a small volume (see ref 5 for further details). The same is not true for

Configurational bias Monte Carlo (CBMC) simulations have been used to investigate the behavior of binary and ternary mixtures of hexane, 2-methylpentane, and cyclohexane in silicalite-1, AlPO4-5, and ITQ-22 at 300 and 600 K. Ideal Adsorption Solution Theory (IAST) was used to verify the simulated mixture isotherms, which were determined to be in good agreement with the theoretical predictions. In silicalite-1, at 300 K, hexane dominated the adsorption, followed by 2-methylpentane and finally cyclohexane. This order was altered at 600 K, with cyclohexane adsorbing in greater numbers than hexane and, finally, 2-methylpentane. The increase in adsorption of cyclohexane, coupled with a decrease in 2-methylpentane and, to a lesser extent, hexane, with increasing temperature was echoed in AlPO4-5 and ITQ-22. A possible explanation for this is that, as the temperature increases, the shape of the cyclohexane molecule does not change significantly, whereas the hexane and 2-methylpentane molecules decrease in length and increase in width, because of the increased number of gauche bonds present at high temperatures. The short, fat molecules find it harder to fit into the channels of the zeolite, and, because 2-methylpentane is further hindered by the presence of the branched CH3 group, the uptake of 2-methylpentane decreases faster than that of hexane at high temperatures. The change in adsorption selectivity with temperature provides an easy way to control which species in a mixture preferentially adsorbs within a zeolite. A microscopic analysis of the locations of the sorbed molecules shows that, in silicalite-1, hexane prefers to adsorb in the intersections and both the straight and zigzag channels, whereas 2-methylpentane and cyclohexane prefer only the larger intersections. In ITQ-22, which has larger channels than silicalite-1, the cyclohexane and 2-methylpentane molecules can adsorb at the intersections between the 10MR and 12MR channels and midway between the intersections, within the 12MR channels, whereas the hexane molecules can adsorb in either the 10MR or 12MR channels. None of the molecules enter the 8MR channels, which is too small to admit the bulky C6 molecules. In AlPO4-5, which consists of unconnected straight channels, the cyclohexane molecules are able to arrange themselves so that they can adsorb more closely to each other than the 2-methylpentane or hexane molecules. This stacking of cyclohexane molecules explains the domination that it has of the ternary adsorption isotherm. This work has shown that CBMC simulations can accurately predict the binary and ternary adsorption properties of linear, branched, and cyclic alkanes in various different zeolites. Mixture adsorption experiments are extremely difficult to perform, and, thus simulations may provide a practical alternative to assessing the molecular sieving properties of new zeolites, and, with microscopic analysis, provide insight to the adsorption processes that occur in industrial catalysis processes. The simulation of ternary mixtures that involve cyclic molecules makes a significant step toward the simulation of real industrial processes that involve mixtures of many different species. Acknowledgment. J.F. would like to thank Vincent Rooy for his assistance with the code and EPSRC for funding (under Grant No. GR/R23008).

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