J. Phys. Chem. B 2005, 109, 8845-8851
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The Adsorption and Localization of Mixtures of C4-C7 Alkane Isomers in Zeolites by Computer Simulation Linghong Lu, Qi Wang,* and Yingchun Liu Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: NoVember 18, 2004; In Final Form: February 28, 2005
Grand canonical Monte Carlo and configurational-bias Monte Carlo techniques were employed to simulate the adsorption of binary mixtures of C4-C7 alkane isomers in ISV and MOR zeolites at 300 K, and the results were compared to that in MFI. Unlike in MFI, the amount of adsorption of the linear and branched alkanes all increases with pressure increasing in ISV and MOR for 0.5-0.5 gas-phase mixtures. The location of alkane isomers is astatic, and it does not exhibit obvious orientation in ISV and MOR. The interaction energy of 2-methylpropane-zeolite is obviously higher than that of n-butane-zeolite in MFI. As to ISV and MOR, the interaction energy between 2-methylpropane and zeolite is a little lower than that between n-butane and zeolite. It can be found that the zeolite MFI behaves quite differently in adsorption from ISV and MOR.
Introduction Zeolites are important microporous materials; they have been widely used in adsorption, separation, and catalytic processes because of their large surface, confinement, adsorption, and molecular sieve properties. Three-dimensional reticulation of zeolite is the perfect location to separate petrochemicals, for instance, the separation of alkane isomers. Traditionally, people can seek a zeolite for a separation process from a large number of known zeolites. Yet it will cost too much to gain adsorption data by experiments. Moreover, the detailed processes of the adsorption happening in zeolite framework are somewhat complicated. Some microscopic processes of adsorbates in zeolites are generally difficult or impossible to be determined by the experiments. Therefore, it is highly desirable to predict the adsorption and the transport properties of adsorbates from the fundamental knowledge of the structure of a zeolite-sorbent system. The molecular simulation techniques, including molecular mechanics,1,2 molecular dynamics,3-8 and Monte Carlo simulations,9-11 have been widely used to explore the diffusion processes in the zeolite cages. By using molecular simulations and molecular graphic techniques, the researchers can visualize and determine the adsorption and diffusion behaviors of fluids in the available space inside the pores and cages of zeolites. Alkane isomers are important in hydrocarbons processing. Branched hydrocarbons are preferred to straight-chain hydrocarbons as ingredients in petrol because the branched hydrocarbons burn more efficiently and have a higher octane number. Catalytic isomerization is widely used to convert straight-chain hydrocarbons to their mono- or di-branched structures. However, the product of catalytic isomerization is a mixture of linear and branched hydrocarbons that are in thermodynamic equilibrium, and the separation of linear hydrocarbons from their branched isomers becomes necessary.12 Because the micropores are uniform and in the same size range as small molecules (312 Å), zeolites can exhibit specificity and selectivity in adsorbing or rejecting molecules based upon differences in molecular shape, size, and polarity.13,14 A variety of zeolites offer a possibility for hydrocarbon isomer separations relying on differences in their sorption capacities.15 Vlugt et al. performed molecular simulations employing the configurational-bias Monte Carlo (CBMC)16,17 techniques in the Grand canonical ensemble
TABLE 1: Parameters of Lennard-Jones Interactions Used in This Work CH4-CH4 CH3-CH3 CH2-CH2 O-CH4 O-CH3 O-CH2
/kB [K]
σ [Å]
148.0 98.1 47.0 96.5 80.0 58.0
3.73 3.77 3.93 3.60 3.60 3.60
to study the adsorption of linear and branched alkanes in the zeolite silicalite-1.18 However, in this work, we reported the adsorption isotherms of alkane isomer binary mixtures in ISV, and MOR zeolites, and then compared the selectivities of MFI, ISV, and MOR zeolites to alkane isomers. The localization of alkane molecules in zeolites was also discussed. The grand canonical Monte Carlo (GCMC)19 approach was used to calculate the adsorption of the alkane isomer mixtures in zeolites. Alkane molecules were generated by the CBMC method. Potential Model and Simulation Details In previous work, we employed GCMC combined with CBMC technique to simulate short linear alkanes and their mixtures in different zeolites, and we found that our model and programs are able to reproduce the experimental data well.20 In this work, we will use the same model and simulation technique to study binary mixtures of C4-C7 alkane isomers. Potential Model. As to alkane-alkane interactions, the united atoms (UA) representation21 was employed, and the intermolecular interaction between two united atoms is described by a Lennard-Jones potential. The frameworks of zeolite were assumed to be rigid,22 and the alkane-zeolite interactions are dominated by dispersive interactions, which are described by a Lennard-Jones potential as well. For a detailed review of potential model, the reader is referred to ref 20. Because the size and polarizability of Si atoms in zeolites are much smaller than those of O atoms, the contribution of Si atoms to the total potential is small and is taken into account in the O-alkane interaction parameters. The potentials at a series of grid points were calculated and stored in advance. During simulations, the potential at a given position in the zeolite can be calculated by
10.1021/jp0447439 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/12/2005
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Figure 1. Adsorption isotherms of 0.5-0.5 mixtures of n-butane-2-methylpropane (a), n-pentane-2-methylbutane (b), n-hexane-2-methylpentane (c), and n-heptane-2-methylhexane (d) at 300 K in ISV.
interpolation using this grid.23 The parameters of Lennard-Jones interactions were taken from Vlugt et al.17 and are listed in Table 1. Interaction parameters between the different united atoms i and j are calculated by using Jorgensen’s mixing rules:24
ij ) xiijj
(1)
σij ) xσiiσjj
(2)
Simulations. The GCMC method, which is convenient to simulate adsorption, was employed in this work. The linear molecules were inserted in the simulation procedures by using the CBMC technique proposed by Smit et al.12 The result shows that the insertions were effectual. For branched alkanes, the segments at the branch were inserted simultaneously. In the simulation scheme, five types of trial moves were included: moving a molecule, rotating a molecule, partly regrowing a molecule, inserting/removing a molecule, and changing the molecule identity. For a detailed review of these moves, the reader is referred to ref 20. In this way, at least 1.5 × 105 MC cycles were performed for alkane isomer. For long-chain alkanesm the number of cycles will be increased. Additionally, we started from the end configuration of a simulation at a lower chemical potential. The former 1.0 × 105 cycles were performed for equilibration, and the subsequent cycles were used to calculate the statistical properties. The number of trial moves in a MC cycle is equal to the number of adsorbed molecules. Results and Discussion Comparison with the Experimental. The adsorption isotherms of linear and branched alkanes in MFI zeolite were
calculated by Smit and co-workers.17,25 It was found that the agreement of the simulation results with the experimental data is reasonably satisfactory. It demonstrates that the simulation model can well describe the adsorption behaviors of straightchain and branched alkanes. To further verify the model and the programs used in this work, we reproduced the adsorption data in MFI zeolite, and as compared to the experimental data of Sun et al.26,27 and Abdul-Rehman et al.,28 as well as the results of Smit et al.,17,25 satisfactory results were obtained. We then used the model and simulation techniques for the adsorption studies of alkane isomers in ISV and MOR zeolites. Adsorption Isotherms. Most of the experimental data were performed at moderate pressures. It is interesting to simulate the adsorption isotherms of mixtures at various pressures. Experimentally, the measurement of an isotherm is more complicated for mixtures than for a pure component.25 One has to measure both adsorption as a function of pressures and changes in composition of the gas phase. Because the simulation data of pure components were in good agreement with the experimental data, we used the same model and simulation techniques to perform the simulation of isotherms for the isomer mixtures. In these simulations, we fixed the composition of the gas mixtures with 0.5-0.5 (the molar fractions of two components are all 0.5 in gas phase) for all mixture systems, and then changed pressures at 300 K, and the simulations were performed in different zeolites of ISV and MOR. Sixteen unit cells of MOR zeolites were used to construct the simulation box (2 × 2 × 4 cells), and for ISV zeolite the simulation box is composed of 3 × 3 × 2 cells, and periodic boundary conditions were applied in three-dimensions to simulate an infinite (macroscopic) system. A cutoff of 13.8 Å was applied to the Lennard-Jones interac-
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Figure 2. Adsorption isotherms of 0.5-0.5 mixtures of n-butane-2-methylpropane (a), n-pentane-2-methylbutane (b), n-hexane-2-methylpentane (c), and n-heptane-2-methylhexane (d) at 300 K in MOR.
tions, and the missing part of the long-range part of the potential was calculated using the even density model.29 GCMC calculations were carried out in the condition of 300 K, and a series of simulations were performed to predict the adsorption isotherms. The adsorption isotherms for linear and branched alkanes mixtures in MFI zeolite have been simulated by Vlugt et al.25 It shows that for all mixtures at low pressures the adsorbed amount of branched alkanes is close to that of linear ones, but with pressure increasing the adsorption of the branched alkanes component reaches a maximum and then decreases, and the adsorption of the linear alkanes component increases with increasing pressure until saturation is reached. It seems as if the branched alkanes are squeezed out by the linear alkanes at higher pressures. To compare the adsorption behaviors with the zeolite MFI, in this work the adsorption isotherms for linear and branched alkanes mixtures in zeolites ISV and MOR were calculated by molecular simulations. The isotherms of n-butane-2-methylpropane, n-pentane2-methylbutane, n-hexane-2-methylpentane, and n-heptane2-methylhexane at 300 K in ISV are shown in Figure 1. It is observed that for all mixtures the adsorbed amounts of linear and branched alkanes are all increasing with increasing pressure. The linear ones at higher pressures do not squeeze the branched alkanes out; on the contrary, the adsorbed amounts of branched alkanes are larger than the linear ones. Figure 2 shows the adsorption isotherms for mixtures of n-butane-2-methylpropane, n-pentane-2-methylbutane, n-hexane-2-methylpentane, and n-heptane-2-methylhexane at 300 K in MOR zeolite. The case is similar to that in ISV. It is interesting why the MFI shows selectivity for alkane isomers that is different from the ISV and MOR. We consider
Figure 3. Selectivity of mixtures of n-butane-2-methylpropane, n-pentane-2-methylbutane, n-hexane-2-methyl-pentane, and n-heptane2-methylhexane isomers (0.5-0.5 gas phase) at 300 K, 100 kPa in MFI, ISV, and MOR.
that it was determined by their special pore structure. MFI is a kind of medium-sized pore zeolite with 10-membered rings; it has two types of channels (see Figure 4a): one is the straight channel, which is perpendicular to the x-z plane, and another is the zigzag channel, which is in the x-z plane. The pore size of the straight channel is 5.3 × 5.6 Å2, and the zigzag channel is 5.1 × 5.5 Å2. The size is similar to the size of the cross section of alkane molecules. But ISV and MOR are large-sized pore zeolites with 12-membered rings. MFI has good shape selectivity to alkane isomers. We believe that the MFI prefer adsorbing 2M-alkane to adsorbing normal alkane at higher pressure because of their appropriate size of channels. Yet in
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Figure 4. A schematic drawing of the channels structure of zeolites MFI (a), ISV (b), and MOR (c).
Figure 5. Adsorption probability distribution of 0.5-0.5 mixtures of n-butane and 2-methylpropane at 300 K and at 5.0 × 10-3 kPa (a), 1.0 × 10-1 kPa (b), and 5.0 × 101 kPa (c) in ISV; the lines are the zeolite structure ISV.
ISV or MOR the effect of shape is weak. A further discussion is in the section below about localization. Selectivity. As to separation process, the most interesting and important is the selectivity of the zeolites for different components. The selectivities of mixtures of methane-ethane have been measured isothermally at constant loading by a microcalorimeter by Dunne et al.30 Certainly, the experimental measurements are time-consuming and expensive. We simulated the adsorption of binary mixtures of n-butane and 2-methylpropane, n-pentane and 2-methylbutane, n-hexane and 2-methylpentane, and n-heptane and 2-methylhexane isomers at 100 kPa, and the simulations were performed for 0.5-0.5 gas phase in different zeolites of MFI, ISV, and MOR. In the process of separation, the norm to estimate the selectivity of adsorbent is the relative selectivity (R) of this adsorbent.31 In this work, the relative selectivity is defined as:
RA/B ) (xSA/xSB)(yGA/yGB)
(3)
where xSA, xSB are molar fractions of components A and B in the adsorption phase, respectively, while yGA, yGB are molar fractions of A and B in the gas phase.
Figure 3 shows the selectivity of different zeolites for mixtures of 0.5-0.5 gas phases at 300 K and 100 kPa. Here, we designate that A is the component of larger adsorption amount in the zeolite, so A is normal alkanes and B is the isomeric ones in MFI, but A is isomeric alkanes and B is the normal ones in ISV and MOR. From Figure 3, it can be observed that for 4n-4i and 7n-7i systems these zeolites have close values of R, but for 5n-5i and 6n-6i systems the selectivity of MFI is much better than ISV or MOR. Localization. Figure 4 is a schematic drawing of the channels structure of zeolites investigated in this work. Because different zeolites have different selectivity to alkanes, it is necessary to search the location of adsorbate molecules in zeolites. Figures 5 and 6 are the probability distribution of 0.5-0.5 mixtures of n-butane and 2-methylpropane at 300 K and at different pressures in ISV and MOR, respectively. The lines are the zeolite structure. In every 100 MC cycles, the center-of-mass of a molecule is computed, and at this position a dot is drawn in the figures; this procedure is repeated until 10 000 dots have been plotted. In this way, Figures 5 and 6 were obtained. The red dots represent the center-of-mass of n-butane, and the white dots represent the center-of-mass of 2-methylpropane. It was
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Figure 6. Adsorption probability distribution of 0.5-0.5 mixtures of n-butane and 2-methylpropane at 300 K and at 5.0 × 10-3 kPa (a), 1.0 × 10-1 kPa (b), and 5.0 × 101 kPa (c) in MOR; the lines are the zeolite structure MOR.
reported that in MFI all of the 2-methylpropane molecules are located at the intersections between the straight channels and the zigzag channels but the n-butane molecules are located everywhere in MFI zeolites.23,25 Yet from Figure 5, it can be observed that the n-butane and the 2-methylpropane molecules are all located everywhere despite that ISV also has intersections between the channels (see Figure 4b). The reason is that the ISV is a large-sized pore zeolite, we considered. The sizes of the channels in ISV are 6.1 × 6.5 Å2 along 〈100〉 and 5.9 × 6.6 Å2 along [001]. They are, unlike the size of MFI channels, larger than the size of the cross section of alkane molecules. In MOR zeolite, there are 12-membered ring channels (6.5 × 7.0 Å2) and eight-membered ring channels (3.4 × 4.8 Å2, 2.6 × 5.7 Å2), but the C4-C7 alkane molecules are not small enough to enter the eight-membered ring channels, so the channels available for the molecules are only 6.5 × 7.0 Å2 channels, which are large pore channels (see Figure 4c). The n-butane and the 2-methylpropane molecules are all located everywhere in the 12-membered ring channels of MOR. So we considered that there must be a strong repulsion between the 2-methyl alkane molecules in MFI except for the intersections, also, because of the reason the branched chain alkanes are squeezed out by the linear chain alkane at high pressure when the linear chain molecules occupy the intersections. If assuredly there are strong repulsions in MFI to 2-methyl alkane, the average interaction energy between the 2-methyl alkane and the MFI zeolite must be higher than those between other adsorbates and the zeolite. To explore that, we push a test molecule into a channel from one side to another side and calculate the average interaction energy between the alkane molecule and the zeolite. The molecule was put into a channel of zeolite, and the energy was minimized at each point of Ly (a series of proportionally specified points; see Figures 7-10) along the channel to get the optimized configuration by MC sampling at 300 K. At each point, the configuration of the alkane molecule was optimized by 1 × 105 MC cycles, and a lowest energy configuration was obtained. In this way, one could get a series of configurations. Finally, an optimal configuration was selected from these configurations by comparing the energy.
Figure 7. Skeletal drawings of the framework structure of MFI (a), ISV (b), MOR (c), and (1) schematic view of the x-z plane and (2) of the x-y plane. The interaction energy of the black points (the centerof-mass of a molecule) in the drawings was calculated (shown in Figures 8-10).
The configuration of the molecule was then fixed; 1 × 105 MC cycles of rotation perturbation were performed for the alkane molecule at each point. In every cycle, the interaction energy was recorded to calculate the averaged interaction energy at the end. The averaged interaction energy was plotted as a function of Ly in Figures 8-10. Figure 7 is a schematic view demonstrating the location of the center-of-mass of alkane molecule; from one side to another side in a channel, the molecule walked a line in a plane or only one point in the other plane. Figure 8 shows the average adsorbate-zeolite potential for n-butane molecule and 2-methylpropane molecule in MFI. The x-coordinate represents the position of the center-of-mass of the molecule in the channel, and the same for Figures 9 and 10. The energy curves are periodically wavy; the curve of 2-methylpropane-zeolite
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Figure 11. A schematic drawing of the localization of n-butane and 2M-propane molecules in MOR. Figure 8. The average adsorbate-zeolite (MFI) potential experienced by individual molecules for n-butane and 2-methylpropane.
Figure 9. The average adsorbate-zeolite (ISV) potential experienced by individual molecules for n-butane and 2-methylpropane.
Figure 10. The average adsorbate-zeolite (MOR) potential experienced by individual molecules for n-butane and 2-methylpropane.
interaction energy is very sharp, but the curve of n-butanezeolite is flat. Moreover, the peak of the former is obviously higher than the latter. So we can understand why the branched chain alkane located at the intersection and was squeezed out by the linear chain molecules. The low energy of 2-methylpropane-zeolite in the simulated box is at 3.78, 6.36, 13.53, and 16.32 Å, respectively. It is just the intersections. That is the reason the 2-methylpropane molecules are all located at the
intersection where the repulsion of zeolite to 2-methylpropane is the smallest. The results indicate that even the lowest interaction energy (-45.74 kJ/mol) of the 2-methylpropanezeolite curve is higher than the largest interaction energy (-45.88 kJ/mol) of the n-butane-zeolite curve. So one can see the n-butane molecules squeeze the 2-methylpropane molecules out when the pressure increases and more molecules take part in the competition. However, in Figure 9, the results indicate that the linear and branched adsorbates bear similar energetic states in ISV zeolite framework. There is an analogy between the shapes of the two curves. The average interaction energy between 2-methylpropane and zeolite is a little lower than that between n-butane and zeolite. So the n-butane and the 2-methylpropane molecules are all located everywhere in ISV, and the adsorbed amount of linear and branched alkanes increases with increasing pressure. In MOR zeolite, the shape of the average interaction energy curve of 2-methylpropane is just the inverse of that of n-butane. The average interaction energy between 2-methylpropane and zeolite is also a little lower than that between n-butane and zeolite (see Figure 10). Because the curves are not steep, we did not find obvious orientation of location. The adsorbed amount of linear and branched alkanes increases with increasing pressure in MOR, the same as in ISV. An interesting result is, in zeolite MOR, the positions that are energy minima for n-butane are maxima for 2M-propane and vice versa. We considered that it is originated from the cavities inside the zeolite MOR. There are many cavities formed by 12-membered rings, four-membered rings, and five-membered rings between the parallel 6.5 × 7.0 Å2 channels (see Figure 11). These cavities connect channels to channels. Although the space in the cavity is not wide enough to contain the alkane molecule, the cavity must affect the molecules in the channels. The cavities on one side of the channel are not located against the cavities on another side of the same channel. In other words, if a cavity is on one side of the channel, on the opposite side there must be a block. 2M-propane prefers locating there (between a cavity and the opposite block), but n-butane prefers locating between a cavity and the cavity on the opposite side of the channel (a little departure from the location of 2M-propane). It is the reason the curves in Figure 10 show a particular shape, minima for n-butane but maxima for 2M-propane and vice versa. From the discussion above, it can be found that the average interaction energy curve of every zeolite has a particular shape. Probably, one can predict the adsorption behavior of alkanes in zeolite by plotting the interaction energy curve. Conclusions For all mixtures of alkane isomers investigated in the same zeolite, the same trends were observed. The adsorbed amounts
Adsorption and Localization of C4-C7 Alkane Isomers of linear and branched alkanes are all increasing with pressure increasing in ISV and MOR, but the adsorbed amounts of branched alkanes are larger than linear alkanes at higher pressures. It was quite different from that in MFI. For n-butane-2-methylpropane (4n-4i) and n-heptane2-methylhexane (7n-7i) systems, the zeolites MFI, ISV, and MOR have close values of relative selectivity, but for n-pentane2-methylbutane (5n-5i) and n-hexane-2-methylpentane (6n-6i) systems, the selectivity of MFI is much better than that of ISV and that of MOR. The localization of alkane isomers in zeolites ISV and MOR is different from that in zeolite MFI. There is no visible orientation of localization of alkane isomers in ISV and MOR. The interaction energy curves are periodically wavy; the curve of the 2-methylpropane-zeolite interaction energy is sharp, but the curve of n-butane-zeolite is flat to MFI, and the interaction energy of 2-methylpropane-zeolite is obviously higher than that of n-butane-zeolite. As to ISV and MOR, the interaction energy between 2-methylpropane and zeolite is a little lower than that between n-butane and zeolite. It can be found that the zeolites ISV and MOR behave quite different in adsorption from the zeolite MFI because of the different sizes of channels and the different structures. Acknowledgment. This work was supported by the National Natural Science Foundation of China (through Grant No. 20176048). References and Notes (1) Horsley, J. A.; Fellmann, J. D.; Derouane, E. G.; Freeman, C. M. J. Catal. 1994, 147, 231. (2) Klein, H.; Kirschhock, C.; Fuess, H. J. Phys. Chem. 1994, 98, 12345. (3) Demontis, P.; Suffritti, G. B.; Alberti, A.; Quartieri, S.; Fois, E. S.; Gamba, A. Gazz. Chim. Ital. 1986, 116, 459. (4) Yashonath, S. Chem. Phys. Lett. 1991, 17, 54. (5) Yashonath, S.; Demontis, P.; Klein, M. L. J. Phys. Chem. 1991, 95, 5881.
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