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Computer Simulation of the Adsorption of Thiophene in All-Silica Y and Na-Y Shen-gui Ju,* Yong-ping Zeng, Wei-hong Xing, and Chang-lin Chen College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Post Code 210009, China ReceiVed January 19, 2006. In Final Form: May 16, 2006 A grand canonical ensemble Monte Carlo simulation is performed to investigate the adsorption, heat of adsorption, and distributions of thiophene in all-silica Y and Na-Y zeolites. Biased particle insertions and deletions were implemented to allow the computation of equilibrium adsorption isotherms of such molecules. The calculated number of absorbed thiophene molecules in these zeolites is in good agreement with the experimental data. The calculated results show that the number absorbed of thiophene molecules in Na-Y is much greater than that in all-silica Y over the range of pressure. The calculated heat of adsorption is in good agreement with experimental results. The Na-Y zeolite, rather than all-silica Y, preferentially adsorbs the thiophene. A distribution analysis of the adsorbed phase structure reveals a different adsorption site in the zeolites.
Introduction The adsorption of thiophene in zeolitic microporous materials is of great scientific interest in the context of separation and catalysis processes. For instance, the separation of thiophene (and derivatives) from gasoline (a mixture of hydrocarbon molecules) is performed using selective adsorption in synthetictype zeolites.1,2 However, the mechanism of adsorption is not yet understood at the molecule level. Zeolites are microporous materials that have found widespread use in several technological fields. These materials have outstanding properties due to their regular structure and high internal surface areas, and they have been used as catalysts, ion exchangers, and adsorbents.3,4 In addition, these materials are capable of tuning their structures to each particular application by choosing zeolites with cavities or channels or both of suitable shape and dimension and by introducing appropriate nonframework cations within the pores. Some silica becomes catalytically active zeolites by the substitution of trivalent aluminum for tetravalent silicon into the silica framework. These substitutions create a net negative charge in the framework, which is compensated for by a cation. The cation sites influences the adsorption and the catalytic properties of the materials.5 Although crystalline silica structures are well determined, the introduction of aluminum induces chemical disorder. Unfortunately, except for ionic radii, information on the cation sites and the charges has not been well determined. This requires a detailed microscopic description of the distribution of Al atoms in the zeolite framework for the different Si/Al ratios, which is difficult to obtain from the experimental data. Hence, they are often treated as fitting parameters in molecular simulations of adsorption. However, the strong effects of these parameters on adsorption are well established. * Corresponding author. E-mail:
[email protected]. (1) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79-81. (2) Velu, S.; Ma, X.; Song, C. Ind. Eng. Chem. Res. 2003, 42, 5293-5304. (3) Schenk, M.; Calero, S.; Maesen, T. L. M.; van Benthem, L. L.; Verbeek, M. G.; Smit, B. Angew. Chem., Int. Ed. 2002, 41, 2499-2502. (4) Schenk, M.; Smit, B.; Vlugt, T. J. H.; Maesen, T. L. M. Angew. Chem., Int. Ed. 2001, 40, 736-739. (5) Vitale, G.; Bull, L. M.; Morris, R. E.; Cheetham, A. K.; Toby, B. H.; Coe, C. G.; MacDougall, J. E. J. Phys. Chem. 1995, 99, 16087-16092.
Molecular simulations, in conjunction with experiments, have played an important role in developing our understanding of the relation between microscopic and macroscopic properties of confined molecular fluids in zeolites.6 The need to gain detailed insight into the behavior of zeolite/sorbate systems on the molecular scale has inspired some molecular simulation studies. Monte Carlo simulations are capable of predicting the adsorption of guest molecules in zeolites.7-10 However, diffusion is a “mechanical” phenomenon, and molecular dynamics techniques have become feasible in the past decade, making it possible to study the details of the diffusive processes of simple molecules adsorbed in the micropores of zeolites.11 FAU-type zeolites are widely used in catalysis and separation processes.12-14 Computer simulation studies on the adsorption of molecules in cation-free FAU-type silica have also been reported.6 There have been a few simulations on the adsorption behavior of hydrocarbons in the sodium form of FAU-type zeolites (“Na-FAU”) focusing on halocarbons9,15-16 and aromatics.17,18 In this study, however, the model system was not reported previously, and it is interesting because it is a good example of the ability of zeolites to separate a polar compound, thiophene. In this article, we have two goals. The first is to determine whether we can use simulation to make good quantitative predictions of the adsorption characteristics of thiophene in FAU-type zeolites. (6) Fuchs, A. H.; Cheetham, A. K. J. Phys. Chem. B 2001, 105, 7375-7383. (7) Pellenq, R. J.; Tavitian, B.; Espinat, D.; Fuchs, A. H. Langmuir 1996, 12, 4768-4783. (8) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs, A. H. J. Chem. Soc., Faraday Discuss. 1997, 106, 307-323. (9) Mellot, C. F.; Cheetham, A. K.; Harms, S.; Savitz, S.; Gorte, R. J.; Myers, A. L. J. Am. Chem. Soc. 1998, 120, 5788-5792. (10) Schenk, M.; Vidal, S. L.; Vlugt, T. J. H.; Smit, B.; Krishna, R. Langmuir 2001, 17, 1558-1570. (11) Demontis, P.; Suffritti, G. B. Chem. ReV. 1997, 97, 2845-2878. (12) Hattori, H. Chem. ReV. 1995, 95, 537-558. (13) Hulme, R.; Rosensweig, R. E.; Ruthven, D. M. Ind. Eng. Chem. Res. 1991, 30, 752-760. (14) Nagai, M.; Suta, T.; Oshikawa, K.; Hirano, N.; Omi, S. Catal. Today 1999, 50, 29-37. (15) Mollot, C. F.; Davidson, A. M.; Eckert, J.; Cheetham, A. K. J. Phys. Chem. B 1998, 102, 2530-2535. (16) Mellot, C. F.; Cheetham, A. K.; Harms, S.; Savitz, S.; Gorte, R. J.; Myers, A. L.; Myers, A. L. Langmuir 1998, 14, 6728-6733. (17) Gener, I.; Buntinx, G.; Bremard, C. Microporous Mesoporous Mater. 2000, 41, 253-268. (18) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs, A. H. Faraday Discuss. 1997, 106, 307-323.
10.1021/la0601861 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/22/2006
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Figure 1. (Left) View of the structure of the Y zeolite. (Right) Positions of Na+ (purple spheres).
The second is to understand how extraframe ions play a role. To achieve these goals, we perform a detailed study on a model system, thiophene, in all-silica Y and Na-Y. Zeolite Models. The skeletal structure of the Y zeolite is the same as that of naturally occurring faujasite. Each unit cell contains 192 (Si, Al)O4 tetrahedra. The presence of aluminum atoms introduces charge defects that are compensated for by nonframework cations. The faujasite zeolites display cubic crystalline lattices. The microporous network is made of cuboctahedral sodalite cages with a diameter of about 6.5 Å. These cages are linked together in a tetrahedral arrangement by six oxygen atoms rings and form large cavities called supercages. The supercages have a diameter of about 12.5 Å. They are interconnected in a tetrahedral arrangement by windows of 7.5 Å diameter (Figure 1, left). One cubic unit cell contains eight sodalite cages and eight supercages. The ratio of silicon to aluminum atoms and thus the number of cations vary from one faujasite to another. A faujasite is named Y (or X) when it has a Si/Al ratio greater than (or less than) 1.5. A substantial number of diffraction studies have examined the location of extraframework cations in faujasitetype zeolites, including hydrated and dehydrated forms of zeolites X and Y. However, in the case of monovalent cation forms of zeolite X, a significant proportion of cations are often undetected, and the precise location of site III slightly differs from one author to another. The cation sites in FAU zeolites are given in Figure 1 (left). Sites I, I′, and II are not exposed and are not available for interaction with adsorbate molecules, with the possible exception of water, which could fit in the 6-oxygen ring (with an opening of 2.8 Å). Thus, nearly one-half of the cations are not available for adsorption. Sites II and III are exposed to the cavity and are associated with, respectively, the 6- and 4-oxygen rings. Site I is associated with the 6-ring and is displaced into the ellipsoidal cavity. Site II is located near the center of the ellipsoidal cavity. Site III is found at the center of the hexagonal prism, and site IV is near the 8-ring window. Thus, with the exception of cations at site III, all cations are exposed to the ellipsoidal cavity and are available for interaction with adsorbate molecules. The zeolite models are built in light of the work of Fitch et al.19 The space group is Fd3m with a lattice parameter that is typically 25.028 Å. Some authors20-22 explicitly distinguish Si (19) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 13111318.
and Al atoms through the different types of framework oxygen atoms, whereas others8,23-24 assume that the zeolite structure is rigid and average T(Si,Al) site models apply. In our simulation, the zeolite structure was considered to be rigid, and average T-site models were applied. Molecular simulations applying flexible structures showed that a flexible lattice can potentially influence the diffusion properties. To diffuse inside a zeolite, the molecules had to pass energy barriers posed by channels and intersections. In a flexible zeolite framework, fluctuations can affect the size of the channels and intersections and thereby the height of these energy barriers. However, our study focuses on the low-energy equilibrium configurations, so the fluctuations in the higher-energy configurations in flexible zeolite models are negligible.25 For the sodium ions, the three positions found correspond to (i) the I site, of which there are four in each supercage, at 32e; (ii) the I′ site, of which there are four in each sodalite cage, also at 32e; and(iii) the I site at the center of the hexagonal prism at 16c.The fractional occupancies of these sites are 100(2), 58(2), and 44(2)%, respectively.15 The positions of Na+ are shown in Figure 1 (right). Potentials. The thiophene-thiophene interaction is described with a united-atom model. The inclusion of pseudorotation in Jorgensen’s simulation of liquid tetrahydrofuran (THF) had a negligible effect on the results in comparison to the results for planar THF.26 The torsional motions for the five-membered rings are constrained by their limited flexibility. Thiophene is also modeled as a rigid planar molecule consisting of a CH united atom and a S united atom. The bond lengths of C-C and S-C are fixed at 1.40 and 1.72 Å, respectively. The thiophene and zeolite are assumed to interact through the pairwise additive potential between atoms of the adsorbed molecules and atoms of the zeolite. The site-site interactions (20) Jaramillo, E.; Auerbach, S. M. J. Phys. Chem. B 1999, 103, 9589-9594. (21) Jaramillo, E.; Grey, C. P.; Auerbach, S. M. J. Phys. Chem. B 2001, 105, 12319-12329. (22) Calero, S.; Dubbeldam, D.; Krishna, R.; Smit, B.; Vlugt, T. J. H.; Denayer, J. F. M.; Martens, J. A.; Maesen, T. L. M. J. Am. Chem. Soc. 2004, 126, 1137711386. (23) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs, A. H. Langmuir 1999, 15, 8678-8685. (24) Pellenq, R. J. M.; Tavitian, B.; Espinat, D.; Fuchs, A. H. Langmuir 1996, 12, 4768-4783. (25) Vlugt, T. J. H.; Schenk, M. J. Phys. Chem. B 2002, 106, 12757-12763. (26) Chandrasekhar, J.; Jorgensen, W. L. J. Chem. Phys. 1982, 77, 50735079.
Adsorption of Thiophene in All-Silica Y and Na-Y
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Table 1. Short Range LJ Parameters for Thiophene-Zeolitea σ (Å)
σ (Å)
(Å)
(Å)
Thiophene-Zeolite Short-Range LJ Parameters30 CHsp2-O 3.70 68.00 CHsp2-Na 2.80 320.65 S-O 3.30 88.47 S-Na 3.30 422.06 Thiophene-Thiophene Short-Range LJ Parameters CHsp2-CHsp2 3.800 50.800 S-S 3.55 125.8 S-CHsp2 mixing rules mixing rules a
For the thiophene-zeolite potentials, we have employed the previous parameters for CH-O and S-O; however, CH-Na and S-Na are fitted to the experimental data.39
Pacc(N f N + 1) ) fVβ exp -β[(U(N + 1) - U(N))] min 1, (N + 1)
{
uij ) 4ij
[( ) ( ) ] σij rij
-
σij rij
6
+
qiqje rij
2
(1)
where rij is the distance between sites i and j, qi and qj are the partial charges on the sites, and ij and σij are LJ parameters. The LJ potentials were truncated at 13.0 Å. The long-range electrostatic interactions were calculated using the Ewald summation technique.27 The potentials between a sorbent atom and the zeolite are tabulated on a 3D grid throughout the asymmetric unit of the zeolite, and then the during the simulations, the cubic hermite polynomial interpolation algorithm is determined.28,29 The LJ interaction parameters are listed in Table 1. Intercation parameters between different united atoms i and j were calculated by using Lorentz-Berthelot mixing rules. We performed DFT calculations (B3LYP/6-311++G(d,p))30 to estimate the dipole moment, which is in good agreement with the experiment values,31 and extracted the partial charges from the electronic density analysis. Partial charges are placed on sulfur and CH of thiophene (Table 2). The partial charges that are placed on the atoms of the all-silica Y and Na-Y can be also found in Table 2. Details of the Calculation. Grand canonical ensemble Monte Carlo (GCMC) is the most common technique for the prediction of zeolite adsorption.32,33 In this technique, there are three types of moves: a molecule is displaced or rotated, a molecule is destroyed, and a molecule is created at a random position in the fluid.34 The displacement or rotation step was handled by the normal Metropolis method. The molecule is displaced or rotated by the ratio of the probabilities as follows
Pacc(s f s′) ) min{1, exp[-β(U(s′) - U(s))]}
(2)
where β is the inverse temperature 1/kT, in which k is the Boltzmann constant, and U is the potential energy. A new molecule is inserted into the system at a randomly chosen position. The inserted configuration is accepted with probability (27) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Claredon Press: Oxford, England, 1987. (28) June, R. L.; Bell, A. T. Theodorou, D. N. J. Phys. Chem. 1990, 94, 8232-8240. (29) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 13742-13752. (30) Ju, S.; Zeng, Y.; Xing, W.; Chen, C. Submitted for publication. (31) Pasterny, K.; Wrzalik, R.; Kupka, T.; Pastern, G. J. Mol. Struct. 2002, 614, 297-304.
(3)
where V is the volume of the system, N is the number of molecules present before the attempted insertion, f is the fugacity of the sorbate, and β is the inverse temperature, 1/kT. In the deletion step, a molecule is randomly chosen to be deleted, and the deletion is accepted with probability
Pacc(N f N - 1) )
{
}
N exp[-β(U(N - 1) - U(N))] (4) min 1, fVβ
are simulated with a Lenard-Jones (LJ) potential plus pointcharge potential 12
}
It has been proven that statistical properties of simple sorbates in some zeolites can be calculated by GCMC.35-37 Attempts to apply the normal GCMC to larger or more complex systems, such as long-chain alkanes, in which the guest molecules fit very tightly into the pores of zeolites are frustrated by the low acceptance rates of creation and the deletion steps that need to be sampled in the grand canonical ensemble. For normal GCMC, the attempted insertions and deletions are accepted at very low probability during the simulation in our research. Therefore, to get a rational probability accepted, we applied an energy-bias GCMC to sample the insertions; this has been introduced by Snurr et al.38 In this article, we still attempt to apply the insertions described by Snurr et al. that are biased so that more attempts are obtained in the energetically favorable regions of the zeolites. In the simulation, the simulation volume is discretized into small cubic regions, Ncub, and each cubelet n was assigned a weight, w(n). The weight function is determined with
w(n) )
exp[-βU(n)] (5)
Ncub
exp[-βU(j)] ∑ j)1
where U(j) is the sorbate-zeolite energy of a molecule of sorbate at the center of cubelet j. An insertion is performed by randomly choosing a cubelet according to the weighting w(n) and placing a molecule into its center. The probability of acceptance for insertion is min(1, Pacc)
Pacc )
{
1 Vcub fV exp[-β∆U] w(n) V (N + 1)kT
}
(6)
where Vcub is the volume of the cubelet, V is the volume of the simulation box, N is the number of molecules of sorbate in the zeolite, f is the fugacity of the sorbate, β is the inverse temperature, 1/ , and ∆U is the change in the potential energy of the inserted kT molecule. Because the insertions are biased, there will be some changes in the acceptance probability for deletion moves to preserve the (32) Gupta, A.; Chempath, S.; Sanborn, M. J.; Clark, L. A.; Snurr, R. Q. Mol. Simul. 2003, 29, 29-46. (33) Woods, G. B.; Panagiotopoulos, A. Z.; Rowlinson, J. S. Mol. Phys. 1988, 63, 49-63. (34) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications; Academic Press: New York, 1996. (35) Goodbody, S. J.; Watanabe, K.; Macgowan, D.; Walton, J. P. R. B.; Quirke, N. J. Chem. Soc., Faraday Trans. 1991, 87, 1951-1958. (36) Karavias, F.; Myers, A. L. Langmuir 1991, 7, 3118-3126. (37) Tassel, P. R.; Davis, H. T.; McComick, A. V. J. Chem. Phys. 1993, 98, 8919-8929. (38) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1993, 97, 13742-13752.
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Table 2. Partial Charges for the Thiophene-Zeolite Potential T(Si, Al) siliceous Y Na-Y thiophene
+2.0|e| 1.589|e|
Si192O384 Na56Si136Al56O384 CH(sp2
site 1. ) site 2. CH(sp2) site 3. CH(sp2) site 4. CH(sp2) site 5. S
{
-1.0|e| -0.8279|e|
Na +1.0|e|
0.22|e| -0.14|e| -0.14|e| 0.22|e| -0.16|e|
microscopic reversibility. A deletion is performed by randomly choosing one of the existing molecules and deleting it with probability min(1, Pacc)
V Nβ Pacc ) w(n) exp[-β∆U] Vcub fV
O
}
(7)
Orientation-biased GCMC was also implemented as in Snurr et al. The details can be seen ref 38. The initial configuration was taken from a large, standard, random configuration. All simulations were carried out at 363, 393, and 453K. The number of MC cycles was 4 × 106, and 2 × 106 cycles were discarded before equilibrium. To ensure faster equilibration, the last configuration of each run was used as the initial configuration of the next run. The probabilities of the fraction translations, fraction insertion, fraction deletion, and fraction rotation moves are all 25%. The statistical uncertainties in the results were estimated by dividing each simulation run into 10 blocks and calculating the standard deviation of the block averages.
Figures 2-5. In our simulation, for all-silica Y we apply an idealized silica model. In reality, Takahashi et al. use the H-US-Y in their experiments. The ratio of Si/Al is 195. We assume that it is all-silica Y at that high ratio of Si/Al(195), and we compare it with our simulation results for all-silica Y. For Na-Y, the ratio of Si/Al(2.43) is equal to that of Takahashi et al. The curves in the Figures are determined by fitting the experimental data according to the parameters of Takahashi et al. The maximal loading for Na-Y at 393 and 453 K is about 2.7 mmol/g. This value can be compared with the experimental data of Takahashi et al.39 The simulated values, which are shown in Figures 2-5, are in good agreement with the experimental data of Takahashi et al. The adsorptive amount of thiophene for all-silica Y is lower than that for Na-Y over the range of pressure
Results and Discussion Adsorption Isotherm. GCMC simulations were carried out to determine the adsorption of thiophene on all-silica Y and Na-Y. The simulations were performed at 363, 393, and 453 K over a pressure range from 0.001 to 50 kPa. The calculated adsorption isotherms of thiophene in all-silica Y and Na-Y at different temperatures are shown in Figures 2-5. The simulated
Figure 3. Simulated and experimental adsorption isotherm for thiophene in all-silica Y at 393 K. The inset shows the adsorption of the high-pressure parts on a semilog scale.
Figure 2. Simulated and experimental adsorption isotherms for thiophene in all-silica Y at 363 K. The inset shows the adsorption of the high-pressure parts on a semilog scale.
values are compared to their experimental data at the same temperature,39 and the experimental data are also presented in (39) Takahashi, A.; Yang, F. H.; Yang, R. T. Ind. Eng. Chem. Res. 2002, 41, 2487-2496.
Figure 4. Simulated and experimental adsorption isotherms for thiophene in Na-Y(2.43) at 493 K.
Adsorption of Thiophene in All-Silica Y and Na-Y
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Figure 5. Simulated and experimental adsorption isotherms for thiophene in Na-Y(2.43) at 453 K.
in the simulations. These results are clearly due to the strong interactions between the cations in zeolites and thiophene molecules. Ng and Rahman et al.40 investigated the adsorption of thiophenic sulfur compounds on Na-Y and US-Y zeolites. In their studies, the ratio of Si/Al(Na-Y) is about 2.29, and the Na2O content of US-Y is 0.24 wt %. The saturation adsorption isotherms were also determined. The saturation adsorption for thiophene in hexadecane is about 1.89 mmol S/g of Na-Y at 328 K. There are some differences among the experiment results of Thakhashi and Rahman. Considering that the ratio of Si/Al in Takahashi’s work39 is higher than that in Ng et al.,40 the corresponding content of Na+ should be lower, so the available void of adsorption for thiophene molecules should be much more than that reported by Ng et al.40 Heat of Adsorption. The heat of adsorption is an important parameter in the design of an adsorption process. The heat of adsorption also indicates the strength of interaction of the S compound with the sorbent and will provide useful information on removing sulfur for the design of selective adsorbents. The energy of adsorption is generally reported in terms of the isosteric heat of adsorption, Hst, which is defined by
Hst ) R
( ) ∂ ln P ∂1/T
(8)
N
The form is not convenient for the analysis of the simulation results; however, by applying Razmus and Hall’s41 methods and assuming that the gas is ideal and that the adsorbed phase is much denser than the gas, Hst can be related to the differential heat of adsorption, HD, by
Hst ) HD + RT
(9)
where
( )
HD ) -
∂〈U〉 ∂〈N〉
T,V,A
It can be calculated from the ensemble-averaged fluctuations42
( ) ∂〈U〉 ∂〈N〉
T,V,A
)
〈UN〉 - 〈U〉〈N〉 〈N2〉 - 〈N〉2
(40) Ng, F. F. T.; Rahman, A.; Ohasi, T.; Jiang, M. Appl. Catal., B 2005, 56, 127-136.
Figure 6. Calculated heats of adsorption for all-silica Y at different temperatures.
Figure 7. Calculated heats of adsorption for Na-Y(Si/Al ) 2.43) at different temperatures. Table 3. Experimental Isosteric Heat of Adsorption system Na-Y H-US-Y
Hst (kJ/mol) (Ng et al.40)
Hst (kJ/mol) (Takahashi et al.39)
20.9
79.84-81.93 33.02-46.82
where A is the surface area of the zeolite and 〈 〉 refers to the average over the simulation run, N is the number of molecules, and U is the energy. Figures 6 and 7 show the evolution of the isosteric heat of adsorption in all-silica Y and Na-Y with coverage at 363, 393, and 453 K. Ng et al.40 described exploratory research on measuring the heat of adsorption of sulfur compounds in a liquid phase using flow microcalorimetry. Heats of adsorption determined experimentally by Takhashi et al.,39 who used two types of Na-Y(Si/Al ) 2.43) and US-Y(Si/Al ) 195), are given for comparison. Their experimental results are listed in Table 3. This value43 of the apparent heat of adsorption is very low compared with a calculated value of about 80.7 kJ/mol for the heat of adsorption of thiophene in Na-Y in the gas phase, using (41) Razmus, D. M.; Hall, C. K. AIChE J. 1991, 37, 769-779. (42) Vuong, T.; Monson, P. A. Langmuir 1996, 12, 5425-5432. (43) Jiang, M.; Ng, F. F. T.; Rahman, A.; Patel, V. Thermochim. Acta 2005, 434, 27-36.
8358 Langmuir, Vol. 22, No. 20, 2006
Figure 8. Snapshot of the density distribution of thiophene in allsilica Y at 6 kPa and T ) 393 K in the y-z plane: black spheres represent carbon atoms, and yellow spheres represent sulfur atoms.
the Clausius-Clapeyron equation based on the isotherms obtained at different temperatures.39 Ng claim that the result does not represent the actual heat of adsorption of S compounds on the sorbent, but only an overall heat measured in the sorption process when the sorbent was already wetted with hexadecane, so the heat obtained from flow calorimetry is not the true heat of adsorption for the S compound, it only provides a relative measure of the heat of adsorption of the S compound with a wetted sorbent. Our simulation results shown in Figures 6-7 are close agreement with the experimental data of Takahashi et al. Distribution of Thiophene Molecules. The distribution of positions occupied by thiophene in all-silica Y and Na-Y are shown in Figures 8 and 9. The lines are the zeolite structures. These distributions are constructed by plotting the position of the thiophene molecules in the simulation box at fixed intervals throughout the simulation. The density distributions of the molecules are a measure of the probability of finding the particular molecule at a given position. At a high Si/Al(all silica) ratio (Figure 8), we see that thiophene is mostly located in 12-membered-ring channels and not in the supercage cavities. The distribution in Figure 8 shows that thiophene molecules located in such a region have a considerably lower energy than thiophene molecules in the central supercage cavities. Because the size of 12-membered-ring channels is less than that of supercages, the oxygen atoms can exert a stronger influence on adsorption for thiophene molecules than in supercages. This is in agreement with the observation. Because the Si/Al ratio is 2.43, the supercage cavities are effectively located by the sodium ions (Figure 9). This clearly shows that most thiophene molecules are all adsorbed inside supercage cavities not at the 12-membered-ring channels (which are secondary adsorption sites for thiophene). There is a much more accessible void in which to locate the thiophene molecules, whereas the existence of Na+ strengthens the adsorption for thiophene molecules. We suggest that this introduction of the sodium ions explains the observed decrease in the heat of adsorption at high Si/Al ratios (Figures 6 and 7).
Ju et al.
Figure 9. Snapshot of the density distribution of thiophene in Na-Y at 0.1k Pa and T ) 393 K in the y-z plane: black spheres represent carbon atoms, yellow spheres represent sulfur atoms, and purple spheres represent sodium atoms.
Conclusions In this article, we have performed GCMC simulations on a system of thiophene molecules in all-silica Y and Na-Y. These biased algorithms resulted in an improvement in the efficiency of the simulations compared with that of traditional GCMC, making it possible to study the adsorption of systems such as thiophene in faujasites at high loading that could not otherwise have been investigated with GCMC. The calculated adsorption quantity of equilibrium is in very close agreement with the available experimental data. From the simulation results, it can be seen that the parameters selected are appropriate in our simulation. In the lower pressure range (lower than 10 kPa), the adsorption quantity for thiophene in Na-Y is much greater than that in all-silica Y. These results are clearly due to the strong interactions between the cations in zeolites and thiophene molecules. The heats of adsorption determined from the simulation results are in good agreement with the experimentally determined heats of adsorption reported by Takahashi et al. This shows that the existence of Na+ strengthens the adsorption of thiophene molecules. The distribution of thiophene shows that the existence of Na+ has an effect on the adsorption of thiophene molecules. Acknowledgment. This research was supported by the National Natural Science Foundation of China (grant no. 20436030) and the Doctoral Fund of Nanjing University of Technology (BSCX200507). Z.-p.Y. is indebted to Professor Snurr for his advice and was motivated by reading Professor Smit’s articles. Note Added after ASAP Publication. This article was released ASAP on August 22, 2006. On the y axis of Figure 2, the units were changed to mmol/g. The correct version was posted on August 31, 2006. LA0601861
