Pore Size Dependence of Adsorption and Separation of Thiophene

Jun 4, 2015 - Currently, the desulfurization processes are achieved by catalysis with increasing pressures or temperatures. Hydrodesulfurization (HDS)...
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Pore Size Dependence of Adsorption and Separation of Thiophene/ Benzene Mixtures in Zeolites Yongping Zeng,*,†,‡ Peyman Z. Moghadam,‡ and Randall Q. Snurr‡ †

College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States

‡

S Supporting Information *

ABSTRACT: Grand canonical ensemble Monte Carlo simulations were applied to study the adsorption and separation of thiophene from binary and ternary mixtures of benzene and n-alkanes in nine types of zeolites. Adsorption of thiophene is important for removing sulfur from gasoline. The computed pure component adsorption isotherms are in good agreement with experimental results for FAU zeolites. The calculated Qst for thiophene in FAU were found to be 33−46 kJ/mol depending on uptake, in good agreement with the experimental data. The adsorption heats as a function of zeolite pore diameter suggest high thiophene affinity when the pore diameter is between 4.6 and 5 Å. Binary mixture simulations at two different compositions show that FER is very selective toward thiophene over benzene at 373 K. The thiophene selectivity shows a significant dependence on pore size in binary mixtures, and the smaller thiophene molecule can replace benzene at elevated pressures due to size entropy effects for some zeolites. For ternary mixtures of thiophene, benzene, and alkanes, the results show that the selectivity is low in AET, MWW, BOG, and FAU zeolites, while the presence of n-hexane has insignificant effects on the thiophene selectivity in FER and AEL zeolites.

1. INTRODUCTION Owing to worldwide environmental regulations, refiners are faced with the challenge of producing much cleaner fuels.1 The U.S. Environmental Protection Agency has reduced the allowable sulfur contents for diesel fuel to 15 ppmw and for gasoline to 30 ppmw in 2006.2 Thus, the removal of noxious sulfur-containing compounds is an increasingly vital process in the petroleum industry. Currently, the desulfurization processes are achieved by catalysis with increasing pressures or temperatures. Hydrodesulfurization (HDS) has been applied to remove thiols and thioether effectively but is not effective for the removal of the thiophenic species.3 It is difficult to remove thiophenic compounds, especially 4,6-dimethyldibenzothiophene, which is unreactive due to the methyl groups, which hinder the ability of hydrodesulfurization catalysts to adsorb the sulfur atom of dibenzothiophene. Therefore, some sulfides which remain in the fuels currently are mainly thiophenic compounds. In addition, the H2S produced during HDS is the inhibitor for deep hydrodesulfurization of unreactive compounds. For hydrodesulfurization to meet the current government regulations, the HDS reactors would need to have larger volumes (depending on the H2 partial pressure) than those used currently.3 This makes HDS an unsuitable solution due to the severe operational conditions, and this has motivated researchers to develop selective adsorption processes to remove these difficult sulfur compounds.4−9 A crucial issue in developing adsorption technologies is to identify adsorption materials which provide the suitable adsorption selectivity and capacity for the desired applications. © XXXX American Chemical Society

Zeolite materials have well-defined channels which are accessible to different adsorbates, and they have been widely implemented in industrial applications.10 The excellent thermal stability and high internal surface give the zeolites special properties that make them suitable for molecular sieving or catalysis. The composition of fuels varies depending on the raw oils, the product specification, and refining processes. The alkanes with different chain length are main components of some fuels, about up to 70−80%. The aromatic components are mainly benzene and its derivatives, around 20−30%.11 This means that the desulfurization adsorbent needs to have high affinity for thiophene molecule compared to aromatics and alkanes. Some experiments and simulations have reported the energetics of alkane adsorption as a correlation of zeolite pore size. For instance, Stach et al. reported the heats of adsorption for butane increases logarithmically as the pore size decreases in some zeolites.12 Ndjaka et al. compared simulated and experimental alkanes adsorption isotherms in various types of high-silica zeolites with the different pore diameters, probing favorable adsorption sites inside the studied zeolites.13 Applying the configurational-biased Monte Carlo (CBMC) techniques, Bates et al. studied the energies of normal alkanes with different chain length in the various types of all-silica zeolites.14 To the best of our knowledge, however, the comprehensive study of Received: April 1, 2015 Revised: June 1, 2015

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potentials are used to describe the torsion energies. The nonbonded intramolecular interactions and guest−guest interactions are defined with the Lennard-Jones (LJ) potential for n-hexane. The interactions of guest−host and guest−guest were modeled with a Coulombic and Lennard-Jones potential:

thiophene and benzene adsorption in zeolites with various pore sizes has not been performed. Thiophene molecule has a slightly stronger polarity than benzene molecule, which should promote adsorption of thiophene over benzene. However, benzene has more atoms and is slightly larger (kinetic diameter of 5.8 Å) than thiophene (kinetic diameter of 4.6 Å), which would favor adsorption of benzene due to dispersion interactions.15 The goal of this paper is to study the effects of zeolites channel nature and pore size on selective adsorption for thiophene over benzene. We also explore possible effects of nalkanes on thiophene adsorption. To investigate the zeolite pore size dependence of thiophene adsorption, we selected nine types of zeolitesone-dimensional zeolites: AEL, MOR, and AET; two-dimensional zeolites: MWW, FER, and BOG; and three-dimensional zeolites: MFI, BEA, and FAU. We performed grand canonical ensemble Monte Carlo (GCMC) simulations to explore adsorption and separation capabilities of the selected zeolites for thiophenic compounds as well as binary and ternary systems with the presence of n-alkanes and benzene. The results provide insights for a model system, which may ultimately lead to better ways to separate thiophenes from aromatics in the presence of alkanes.

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij uij(rij) = 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ 4πε0rij ⎣⎝ ij ⎠

where σij and εij are the Lennard-Jones parameters, rij is the distance between i and j sites, qi and qj are the charges on the interaction centers, and ε0 is the permittivity of vacuum. The LJ parameters between CH of thiophene and the oxygen atoms of zeolites were taken from Ban et al.18 The interaction parameters between sulfur and the zeolite oxygen atoms were determined by Lorenz−Berthelot (LB) mixing rules. The benzene−zeolite and hexane−zeolite interaction parameters are taken from the work of Ban et al.18 and Vlugt et al.,20 respectively. The LJ parameters for interactions between other unlike atoms were determined with the LB mixing rules. The LJ interactions of the zeolite silicon atoms with adsorbate species were not considered. LJ parameters for all intermolecular interactions are listed in Table S1. All zeolite structures were assumed to be rigid in our simulations, as it has been shown that the flexibility of framework has a small effect on the adsorption properties of zeolites.21 The Ewald summation methods were applied for the Coulombic adsorbate−adsorbate and adsorbate−zeolite interactions. All intermolecular LJ interactions were shifted and truncated at a cutoff of 12.0 Å without the tail correction. The atomic charges of zeolites are qO = −1.025e and qSi = 2.05e, which are obtained from Calero et al.22 All partial atomic charges are listed in Table S2. Table S3 shows the details of the boxes in the simulations that were applied for the different zeolites. The atomic positions of zeolite frameworks are taken from the crystallographic information provided by the International Zeolite Association.23 The crystal structures of the zeolites studied can be found in Figures S1−S3. GCMC simulations of adsorption were performed using the RASPA code.24 The simulations consisted of at least 106 initialization cycles for pure components and 1.2 × 106 cycles for mixtures. After initialization, 5 × 105 production cycles were carried out to calculate average thermodynamic properties. For each running cycle, the Monte Carlo (MC) moves consist of attempts to insert, delete, translate, rotate, or regrow a guest molecule. For mixture simulations, we used identity change moves to speed up equilibrium. Conventional GCMC simulations could be suffered with the low acceptance rates of insertion moves for some systems where the adsorbates interact tightly with atoms of the pores. In order to improve the insertion and deletion acceptance rates, we used the energybiased scheme25 to bias the insertion for benzene and thiophene molecules toward energetically favorable sites. For the alkanes, configurational-bias moves were applied.26

2. MODELS AND CALCULATION DETAILS Thiophene−thiophene interactions were described with a rigid five-site model from the transferable potentials for phase equilibria (TraPPE) force fields.16 For this model, the bond for S−C and C−C are rigid and the lengths are fixed at 1.71 and 1.40 Å, respectively. Lubna et al. used the CHELPG method to obtain the charges for thiophene and obtained charges of 0.023e on the sulfur atom, −0.034e on the two CH united atoms bonded with sulfur, and 0.0225e on the remaining two CH united atoms. Because the charges are small, they neglected partial charges for thiophene in their phase equilibrium calculations. Nevertheless, we still applied the partial charges due to the polar environment in the confined zeolite pores. The TraPPE force fields were also applied for benzene molecule. In order to mimic the quadrupole moment, the 9-site model used three extra partial charges.17 A partial charge of +2.42e is located in the plane of the molecule on the 6-fold axis, and the other two charges (−1.21e) are located along the 6-fold axis at a distance of 0.785 Å from the plane of the molecule to represent the π-electron clouds (Figure 1). Ban et al. used this 9-site model and found very good agreement between experimental and simulated results of benzene adsorption in all-silica zeolites.18 n-Hexane was defined with a flexible, 6-site united-atom model, with CH2 and CH3 groups treated as the single interaction site.19 The stretching and bending of bond are defined as a harmonic potential, whereas the TraPPE dihedral

3. RESULTS AND DISCUSSION 3.1. Adsorption of Pure Components. In order to validate the models applied in our work, we calculated the adsorption isotherms in all-silica FAU for pure component thiophene at 363 and 393 K and compared them with the

Figure 1. Nine-site benzene model. The red and green spheres are positive and negative charges, respectively. B

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Figure 2. Calculated and experimental27 thiophene adsorption isotherms on FAU zeolite at 363 and 393 K. The lines show fits to the experimental data with the Langmuir−Freundlich equation.

Figure 4. Calculated and experimental27 benzene adsorption isotherms on FAU zeolite at 363 and 393 K.

three different silicalite (MFI) crystal structures to calculate thiophene adsorption isotherms, as shown in Figure 3. The three silicalite structures tested are the ORTHO structures from van Koningsveld et al.29 and Olson et al.30 and the PARA structure from van Koningsveld et al.31 Figure 3 also shows experimental adsorption isotherms from two different research groups, Chica et al.32 and Lai et al.33 in HZSM-5. The two sets of experimental data show decreasing adsorption amount with increasing Si/Al ratio. This suggests that the aluminum atoms and extra-framework hydrogen atoms have a significant effect on thiophene adsorption at low pressure. At pressures of less than 1 kPa, the amount adsorbed predicted by simulations for the three types of all-silica MFI are all lower than the experimental data for low ratio of Si/Al (13 and 25). For high ratio of Si/Al (40 and 50), the simulated adsorption loadings agree better with the experimental data. A number of NMR and XRD studies have shown that at a specific loading of aromatics the MFI-type zeolite frameworks undergo a phase transformation from ORTHO structure to PARA structure.34,35 It is not clear if this also occurs for thiophene adsorption. Given the uncertainty about the crystal structure, it is difficult to draw a conclusion about the ability of our model to reproduce thiophene adsorption in MFI. From the simulations, the PARA structure exhibits similar saturation capacity as the ORTHO structure from Olson et al. and higher saturation capacity than the ORTHO structure from van Koningsveld. It has been shown in the literature that p-xylene and benzene also have higher saturation adsorption capacities in the PARA structure.25 Ban et al. used the nine-site model to simulate benzene adsorption in all-silica MFI.18 The predicted adsorption isotherms revealed good agreement compared to experimental data. To test this benzene model for other zeolites, we calculated the adsorption isotherms for FAU zeolite and compared them with the experimental data from the reference.27 As shown in Figure 4, we also observed a good agreement between experiments and simulations at low pressure for isotherms at 363 and 393 K. From comparing Figures 2 and 4, it can be deduced that FAU has a very similar affinity to benzene and thiophene.

Figure 3. Calculated at 373 K and experimental32,33 thiophene adsorption isotherms on MFI zeolite. The lines are shown to guide the eyes.

available experimental data for high-silica (Si/Al = 195) Y zeolite (H-USY).27 As observed in Figure 2, the simulated isotherms are in good agreement with the experimental data for both temperatures over the entire pressure range from 1 to 8000 Pa. While the FAU-type zeolite has high capacity for thiophene at higher pressure, thiophene is only weakly adsorbed at lower pressures due to the large nature of the pores (i.e., the supercages have a size of around 12.5 Å interconnected by windows of 7.5 Å). We also compared the simulated adsorption isotherms of pure thiophene in MFI zeolite with experimental data from the literature. Some groups have studied MFI-type zeolite for thiophene/benzene separation.4,28 However, comparison of simulation and experiment is difficult because MFI-type zeolite exists in three different forms. They are the monoclinic (MONO) and the orthorhombic with the Pnma (ORTHO) and P212121 (PARA) symmetry.25 In our work, in order to compare the effects of these structural differences, we used C

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Figure 5. Calculated adsorption isotherms for (a) thiophene and (b) benzene at the temperature of 373 K in various zeolites. The lines are drawn to guide the eyes. The MFI results are from the ORTHO structure of Olson et al.

Figure 6. Calculated isosteric heats of adsorption for (a) thiophene and (b) benzene at 373 K in various zeolites. The dotted lines are drawn to guide the eyes.

Figures 5a and 5b show the adsorption isotherms for thiophene and benzene in different zeolites at 373 K. The full geometric characterization of the zeolites studied can be found in the Supporting Information. AEL, MOR, and AET consist of straight, one-dimensional channels. AEL has a framework structure that consists of main channels with 10-membered rings which are connected via 6-membered ring building units. Since the 6-membered rings are too small for benzene or thiophene to pass through, the zeolite is effectively onedimensional. The MOR framework consists of one-dimensional main channels with 12-membered rings and side pockets with 8-membered ring coming off of the main channels. The side pockets are too small to accommodate benzene or thiophene. The AET framework consists of main channels with 14membered rings, which are connected via 6-membered ring building units; as in AEL, these 6-membered rings are too small for benzene or thiophene. Figure 5 shows that the saturation uptakes of thiophene are slightly higher than those of benzene in the one-dimensional zeolites. As expected, both thiophene and benzene are adsorbed in the straight channels as shown in the density profiles in Figures S4 and S5. MWW zeolite has two independent channel systems of 10membered ring, with dimensions of 4.1 × 5.1 Å and 4.0 × 5.5

Figure 7. Heats of adsorption for thiophene and benzene as the function of the average zeolite pore diameter. The dashed lines are shown to guide the eyes.

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Figure 8. Calculated adsorption isotherms of equimolar thiophene/benzene mixtures in zeolites at 373 K. The dotted lines are shown to guide the eyes.

channel B. Because the size of benzene matches well with the channel dimensions in MWW zeolite, the slope of the adsorption isotherms at low uptakes is very steep, and because thiophene is smaller, its uptake is lower than benzene at low pressure. At high pressure, thiophene adsorption is higher since more molecules can be closely packed in the pores. In contrast, in FER there is preferential adsorption of thiophene compared to benzene at lower pressures. The FER zeolite has straight 10membered ring channels parallel to the [001] direction that are connected by cages with 8-membered ring windows in the [010] direction. The 10-membered ring channels have the pore size of 4.2 × 5.4 Å. The size of the cages is around 7 Å, while the ellipse-shaped windows of 8-membered ring for the cages have dimensions of ∼3.5 × 4.8 Å.36 The pore limiting diameter in FER is ∼5.3 Å along the [001] direction.37 Lin et al. reported that tetrahydrofuran (THF) can sit in the small channels along the [010] direction of FER.38 Our simulations predict that thiophene molecules can also fit in these channels due to their smaller molecular size. Therefore, the slope of adsorption isotherms of thiophene at low uptake is very steep. However, the bigger benzene molecules cannot enter these channels (Figures S6 and S7). BOG contains intersecting 10- and 12-ring channels, with pore sizes of 5.0 × 5.3 and 6.9 Å, respectively.39 Both pores in BOG are sufficiently large to host both thiophene and benzene. The benzene adsorption isotherm is slightly steeper than that of thiophene due to benzene’s tighter fit in the pores. At high pressure, BOG can adsorb more thiophene than benzene because of the smaller thiophene size.

Figure 9. Selectivities for equimolar thiophene/benzene mixtures in different zeolites at 373 K.

Å. One of these channel systems, which we will refer to as channel A, is defined by intersecting sinusoidal channels that create a two-dimensional network. The other channel system has large supercages with pore size of 7.1 Å defined by a 12membered ring. The supercages are connected to one another through 10-membered apertures, creating a second, independent, two-dimensional channel network, which we will call E

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Figure 10. Snapshots of equimolar thiophene/benzene mixtures (x−y plane) in MWW at 373 K and different pressures: (a) 10, (b) 100, and (c) 10 000 Pa. The cyan spheres are carbon atoms, and the yellow spheres connected with carbons are sulfur atoms. The channel surfaces are shown by the gray shadow. The representative regions of channel A and the cages of channel B are marked with the blue oval and rectangle, respectively, in the left-hand image.

Figure 11. Snapshots of equimolar thiophene/benzene mixtures (x−y plane) in FER at 373 K and different pressures: (a) 10, (b) 100, and (c) 10 000 Pa. The color style is similar to Figure 10. FER consists of straight 10-ring channels parallel to the z-axis which are connected by cages with the windows of 8-membered ring in the y-direction. The representative regions of 10-membered ring channel and cage connected with the windows of 8-membered ring are marked with the blue oval and rectangle, respectively, in the left-hand image.

Figure 12. Adsorption isotherms of ternary thiophene/benzene/n-hexane mixtures (1:1:98) in zeolites at 373 K.

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Figure 13. Calculated adsorption isotherms for ternary thiophene/benzene/n-butane mixtures (1:1:98) in (a) MWW and (b) FER at 373 K.

Figure 14. Simulated adsorption isotherms for ternary thiophene/benzene/n-pentane mixtures (1:1:98) in (a) MWW and (b) FER at 373 K.

The isosteric heats of adsorption (Qst) for thiophene and benzene in the zeolites studied are shown in Figure 6. Takhashi et al.27 calculated the adsorption heats using the Clausius− Clapeyron formula from experimental adsorption isotherms. They determined the experimental Q st for thiophene adsorption in H-USY (Si/Al = 195) and found values ranging from 33 kJ/mol (at 0.1 mmol/g uptake) to 46 kJ/mol (at 0.3 mmol/g uptake) at 363 K. The simulated Qst for FAU is about 35.4 kJ/mol at low loading in good agreement with the experimental results. At low uptake, Qst for thiophene is highest for FER and lowest for FAU. For benzene at low loading, Qst is highest for MWW and lowest for FAU. Interestingly, for zeolites with larger pores (i.e., AET, BOG, BEA, and FAU), similar increasing trends of Qst with respect to loading can be found for thiophene and benzene. This is due to favorable guest/guest interactions. Comparing thiophene and benzene, we see that the larger benzene molecule has a higher Qst for all of the zeolites except FER, where thiophene can access the 8membered ring channels along [010] direction and benzene cannot (Figures S6 and S7). In contrast to other zeolites, there is a sharp drop in the Qst for MWW at higher uptake. Both thiophene and benzene molecules are mainly located in channel A of MWW at low loading and are then adsorbed in the 12membered ring supercages of channel B with increasing

For MFI, benzene molecules are preferentially adsorbed in the relatively wide channel intersections instead of the straight or zigzag channel interiors at the pressure range studied (Figure S9). Experiments show that benzene adsorption isotherms in MFI zeolite have an inflection at about 4 molecules/unit cell, corresponding to the 4 channel intersections per unit cell of MFI.40,41 The adsorption plateau for the benzene isotherm in Figure 5b is found to be in good agreement with experiments42 and other simulation reports.43 At low pressures, thiophene and benzene molecules both prefer to locate at the intersections of channels in MFI zeolite (Figures S8 and S9). At higher pressure, thiophene molecules start to adsorb in the zigzag channel interiors of MFI. Therefore, the adsorption loading of thiophene increases gradually with increasing pressure. BEA (zeolite β) with chiral pore intersections consists of three sets of mutually perpendicular 12-membered ring channels. The pore sizes are 6.6 × 6.7 Å along [100] and [010] and 5.6 × 5.6 Å along [001]. The micropore volume of BEA is about 1.2−1.3 times larger than that of MFI.44 Therefore, BEA exhibits higher adsorption capacity compared to MFI zeolite for both thiophene and benzene. FAU is also a large-pore zeolite and has 12 Å supercages connected with 7.4 Å windows. This results in low uptakes in the Henry region of the isotherm and high saturation capacities for both benzene and thiophene. G

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Figure 15. Adsorption selectivities for ternary thiophene/benzene/n-hexane mixtures at (a) 20:20:60 at 373 K, (b) 1:1:98 at 373 K, and (c) 1:1:98 at 423 K.

pressure. We, thus, attribute the drop in Qst for MWW to the heterogeneous environment of the zeolite. In order to study the effects of zeolite pore size on the Qst values, we also determined the average pore diameter by averaging the pore dimensions for each zeolite following the work of Bates et al. (see Table S3).14 Zeolites having channels of different dimensions along different axes consist of more than one pore diameter. In these cases, we chose the smaller channel for calculating the average pore diameter because guest molecules adsorb more strongly in smaller pores. Figure 7 shows the computed Qst for thiophene and benzene as a relation of the average pore size of zeolites. The maximum adsorption strength for thiophene in these allsilica zeolites is achieved in zeolites with the average pore size between 4.6 and 5 Å, such as FER. The Qst decreases as the pore size increases from 5 to 8.4 Å. Similar trends can be observed for benzene. The results show that zeolites like FER may be potential adsorption materials to maximize the Qst of thiophene. 3.2. Adsorption of Mixtures. In order to study the competitive adsorption for thiophene and benzene molecules in the studied zeolites, equimolar mixture simulations of thiophene/benzene were also carried out at 373 K (Figure 8). For the one-dimensional zeolites (i.e., AEL, MOR, and AET), the preferential adsorption toward thiophene decreases with increasing pore size. AEL shows strong adsorption of

thiophene, and over the whole pressure range thiophene can compete with benzene. The adsorption of benzene exhibits a maximum at intermediate pressure. The replacement of benzene by the smaller thiophene at elevated pressures occurs because of size entropy effects. This means that the smaller thiophene can easily fit into the void space of the pores. This type of effect has been found in adsorption simulations of alkane mixtures in porous materials.45−47 The long chain alkane uptake increases at the beginning and then decreases with the increase of pressure, while the short chain alkane uptake increases continuously and gradually displaces the long chain alkane at higher pressure because of the effect of size entropy. For AET zeolite, the uptake for thiophene and benzene almost overlap for the pressure range studied, meaning that it has no selectivity for thiophene over benzene. MOR zeolite is a slightly weaker adsorbent than AEL but stronger than AET. Thiophene is preferentially adsorbed over benzene in MOR zeolite at high pressures. For the two-dimensional zeolites, MWW preferentially adsorbs benzene over the pressure range investigated, FER preferentially adsorbs thiophene, and BOG shows little selectivity at low pressure and preferential adsorption of thiophene at higher pressure. As explained earlier, MWW consists of two independent channel systems. Both thiophene and benzene prefer to adsorb in channel A and at higher pressures in the larger cages of channel B. The size of benzene H

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thiophene due to the configurational entropy effect. For FER zeolite (Figure 11), thiophenes can adsorb both in the 10membered ring channels and cages connected with 8membered channels, whereas benzene molecules cannot enter these cages at low pressure. With increasing pressure, thiophene molecules gradually occupy the 10-membered ring channels, while benzene molecules also begin to sit in the channels, leading to decreasing thiophene selectivity in FER zeolite. In order to assess the effects of alkanes on thiophene and benzene adsorption in the studied zeolites, we choose n-hexane as a representative alkane and calculated ternary isotherms for thiophene/benzene/n-hexane mixtures at 373 K (Figure 12). In general, a concentration of sulfur compounds less than 0.5% is considered low-sulfur oil, and a concentration greater than 2% is considered high-sulfur oil. In this work, we examined 1% thiophene as a typical concentration. For most of the zeolites examined, at a gas-phase composition of 1:1:98 (thiophene:benzene:n-hexane) n-hexane is adsorbed preferentially over the entire pressure range because of the much higher partial pressures of n-hexane. The two exceptions are MWW, which shows preferential adsorption of benzene, and FER, which shows preferential adsorption of thiophene. These two zeolites also display a maximum in the adsorption of one component as pressure increases. Similar trends were observed in the binary mixture simulations (Figure 8), indicating that the presence of n-hexane has only a small effect on the benzene and thiophene adsorption in MWW and FER zeolites. In contrast, for the rest of the zeolites, the presence of n-hexane significantly reduces the adsorption of thiophene and benzene. To gain some insights into possible influence of chain length of alkane on adsorption in MWW and FER zeolites, we also compared the adsorption of thiophene/benzene/n-butane (Figure 13) and thiophene/benzene/n-pentane (Figure 14) mixtures in both zeolites. Compared to n-hexane, there is only a small change in the benzene and thiophene adsorption in FER and MWW for the shorter chain alkanes, suggesting that the conclusions from Figure 12 should hold also in complex mixtures of alkanes. In order to investigate the influence of hexane mole fraction and the effects of temperature on thiophene adsorption, we carried out ternary mixture simulations at fixed mole fractions of thiophene/benzene/n-hexane of 20:20:60 and 1:1:98 at 373 K and 1:1:98 at 423 K (Figure 15). The results show that the selectivity is lower than 1 for AET, MWW, BOG, and FAU. This means that these zeolites cannot selectively adsorb thiophene from model fuels in the presence of hexane. Interestingly, the selectivities for FER and AEL are greater than 1 for the whole pressure range, similar to the binary case (Figure 9). Therefore, the presence of n-hexane has a minimal effect on the selectivity for FER and AEL. Van Well et al. observed that the short chain alkanes up to n-pentane can enter the two-dimensional pores of FER zeolite and the longer chain n-hexane molecules adsorb in the 10-membered ring channels and not in the cages with the 8-membered ring.48 This explains the insignificant effect that n-hexane has on the adsorption of thiophene in FER. Figure 15c shows the selectivities at higher temperature (423 K). Comparing Figures 15b and 15c shows that the thiophene selectivity in the FER and MFI zeolites depends strongly on the temperature. The temperature has a small effect on the adsorption selectivity for the other zeolites. However, at high temperature, the adsorption selectivity for thiophene in FER increases, and the selectivity for thiophene in MFI decreases. The results imply that changing the process temperature could be an interesting

matches well with the MWW channel dimensions, resulting in a high adsorption in MWW at low pressure. However, thiophene adsorption increases with increasing pressure because of the effect of size entropy. The difference in the uptakes for benzene and thiophene in MWW is also evident for the pure component adsorption isotherms, with benzene showing higher uptake than thiophene in the low pressure region (see Figure 5). The FER has the straight 10-membered ring channels that are connected by the cages through the windows of 8-membered ring. Thiophene molecules fit easily in both channels due to their smaller molecular size. However, the bigger benzene molecules cannot enter the 8-membered ring channels. For BOG, the pores are sufficiently large to easily fit both thiophene and benzene. The benzene adsorption isotherm is greater than that of thiophene due to benzene’s better fit in the pores. At high pressures, the thiophene loading continues to increase, but the benzene loading decreases in BOG zeolite due to size entropy effects. The three-dimensional channel zeolites MFI, BEA, and FAU show a similar trend as BOG, where adsorption of benzene initially increases with increasing pressure, reaches the maximum loading, and then decreases. The adsorption of thiophene, on the other hand, increases continuously and progressively displaces the benzene molecules at high pressures. The effect is strongest in MFI. In MFI, at low loading the thiophene and benzene adsorb in the straight or zigzag channel interiors and channel intersections, respectively, and thus do not compete for adsorption sites. However, with the increase of pressure, thiophene molecules start to displace the benzene molecules from the channel intersections due to entropy effects (see Figure S10). We also computed the selectivity for adsorption of different compositions of benzene and thiophene in the studied zeolites. The selectivity of adsorption is calculated by S = (q1/q2)/(p1 /p2 )

where pi and qi are the partial pressure and uptakes of species i, respectively. Figure 9 shows the simulated adsorption selectivities of equimolar thiophene/benzene mixtures in various zeolites at 373 K, and Figure S11 shows the results for mixtures with a gas phase composition of 33:67 thiophene:benzene. A selectivity greater than 1 means preferential adsorption toward thiophene. Figure 9 shows that the selectivity in most zeolites is greater than 1 at higher pressures, and the best performers at high pressures are MFI (ORTHO structure from Olson et al.), FER, and AEL. At low pressure, most of the zeolites are selective for benzene. The exceptions are FER and AEL. In particular, FER shows a selectivity over 10 at low loading. MWW shows the highest selectivity for benzene over the whole pressure range studied. Comparing Figure 9 and Figure S11, we see that changing the gas-phase composition has little effect on the selectivity. It is interesting to compare FER, which is highly selective for thiophene, with MWW, which is highly selective for benzene. Figures 10 and 11 show typical configurations of the adsorbed molecules at different pressures in these two zeolites for equimolar mixtures. The snapshots indicate that benzene molecules are preferentially adsorbed at channel A at low pressure and then the channel B at higher pressures in MWW (Figure 10). With increasing pressure, thiophene molecules also first occupy the channels and then the cages. However, benzene prefers to adsorb in the channel, resulting in the lower selectivity toward I

DOI: 10.1021/acs.jpcc.5b03156 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Yangzhou University and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

way to enhance the selective adsorption of thiophene in different zeolites.

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4. CONCLUSIONS In this work, we studied the adsorption for thiophene, benzene, and their mixtures in nine types of zeolites using grand canonical ensemble Monte Carlo simulations. The accuracy of the force fields was validated by comparing the calculated adsorption isotherms of thiophene and benzene with available experimental data for FAU and MFI zeolites. Simulation results for pure component thiophene and benzene at low loadings revealed that in general the adsorption heats decrease with increasing pore size. We found that the maximum adsorption heat for thiophene can be obtained for pore sizes ranging between 4.6 and 5 Å, suggesting that small-pore zeolites like FER may be suitable as potential adsorbents for thiophene. This finding can shed light toward design and construction of other classes of porous adsorbents such as metal−organic frameworks (MOFs),49 which have also been studied for selective adsorption of aromatic compounds50,51 due to their pore size and shape tailorability. For mixtures of thiophene and benzene, the energetic contributions prevail at low pressures and benzene has slight preferential adsorption; however, the effect of size entropy gets more important at high pressure, and the smaller thiophene gradually diplaces the larger benzene in some zeolites. FER is quite selective for thiophene over benzene because the different species occupy different adsorption sites in the channels. For ternary mixtures of thiophene, benzene, and n-alkanes, the results show that the selectivity is lower than without the alkanes in many zeolites, but the presence of n-hexane has only a very small impact on the thiophene/benzene selectivity for FER and AEL. The thiophene selectivity in the FER and MFI zeolites depends strongly on temperature. The results show that it is more favorable to selectively remove thiophene from the mixtures in MFI at lower temperature and, however, more favorable in FER at high temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Force field parameters, structural information for zeolites, and density plots. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b03156.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Y.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Emmanouil Tyllianakis and Ki Chul Kim for helpful suggestions. This research work was partially supported by the U.S. Army Research Office (grant W911NF-12-1-0130). Y.Z. acknowledges funding from the National Natural Science Foundation of China (Grant No. 20806064) and Natural Science Foundation of Jiangsu Province, China (BK20131227) and the State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (KL11-11). Generous allocations of computer time were provided by the ScGrid plan of Supercomputing center of CAS and Testing Center of J

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