Article pubs.acs.org/IECR
Loading Dependence of the Adsorption Mechanism of Thiophene in FAU Zeolite Shanqing Dang, Liang Zhao,* Jinsen Gao, and Chunming Xu State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), 18 Fuxue Road, Beijing 102249, P. R. China ABSTRACT: In desulfurization-related researchers, thiophene is widely studied in adsorption, separation, and catalysis processes as a typical sulfur-containing compound. However, the adsorption behavior of thiophene for the very first step of all processes still remains ambiguous. In this study, we proposed the loading dependence of the adsorption mechanism of thiophene in siliceous faujasite (FAU) zeolite using Monte Carlo simulations combined with the research of adsorption isotherms, density distributions, concentration profiles, radial distribution functions, and interaction energies. The results revealed that the thiophene adsorption mechanism in the whole loading range could be divided into two parts: “ideal adsorption” and “insertion adsorption”, with the inflection point of the loading at 40 molecules/UC, which was similar to the adsorption of monoaromatics in zeolite. Below the inflection point, adsorbed thiophene distributed broadly and mainly occupied S and W adsorption sites ideally; after the inflection point, newly adsorbed thiophene molecules entered into the space near the center of the supercage with no influence on previously adsorbed ones. As the loading of thiophene increased, the adsorption amount on the S sites went up consistently over the entire loading range. By contrast, the adsorption amount on the W sites grew first and then dropped gradually for loadings below the inflection point. Finally, it decreased noticeably with loading close to saturated adsorption. In addition, the occurrence of the inflection point was due to the change of the dominated interaction energy.
1. INTRODUCTION Because of their regular structure, high internal surface, and high hydrothermal stability, faujasite (FAU) zeolites have been broadly employed in practical applications such as adsorption, separation, and catalysis, especially as desulfurization adsorbents in the process of producing clean fuels.1−3 Among all sulfides in the fuels, most are thiophenic sulfurs. For instance, the number of thiophenic compounds accounts for about 80% in fluid catalytic cracking (FCC) gasoline.1,2 Because adsorption is the very first step for removal of sulfides via zeolites,4,5 clarifying the adsorption mechanism of these thiopheneic sulfurs is a prerequisite for further promoting the desulfurization efficiency as well as exploring other applications of FAU zeolites. However, the adsorption mechanism of thiophene, as the simplest thiophenic sulfide, remains unclear. Limited studies still focus on the adsorption properties and adsorption sites of thiophene in zeolites. Richardeau et al.6 found that thiophene oligimerized rapidly at high concentration (27.7 wt %) in FAU zeolites at room temperature. Nevertheless, at dilute concentration, thiophene only adsorbed on adsorption sites without any oligomers. Chica et al.7 revealed that the adsorption amount of thiophene (per alumnium) was independent of the aluminum content in HY zeolite (Si/Al = 6−85). Later, Ju et al.8 employed grand canonical ensemble Monte Carlo (GCMC) simulations to study the adsorption behavior of thiophene in siliceous Y zeolite at dilute concentration. They reported most thiophene molecules positioned in 12-membered ring channels rather than supercages. However, adsorption at high loadings, where the © 2016 American Chemical Society
system can be relatively complex due to the pore-filling effects, was rarely explored. This complexity possibly rises with an increase of loading, such as anomalous adsorption of cyclic hydrocarbons in silicatite-1 at high loadings.9 Recently, our studies10−12 on the adsorption mechanisms of aromatics, which are similar to thiophene in molecular structure, revealed the intriguing change with loading and demonstrated the importance of a good understanding of the adsorption mechanism at the molecular level. Two- and three-stage adsorption mechanisms were respectively found for mono- and diaromatics in FAU zeolites over the whole loading range.12 In addition, a growing body of data reflected the adsorption properties of aromatics such as benzene,13 1,3,5-trimethylbenzene,14 and toluene,4 which were similar to thiophene. Therefore, it is possible for the adsorption mechanism of thiophene to change with loading. Clarifying the adsorption mechanism of thiophene can shed light on further studies of the adsorption mechanisms of other thiophenic sulfides, which contributes to improving the desulfurization rate in petroleum industries. In this work, the adsorption mechanism of thiophene in siliceous FAU zeolite was explored for all loadings by Monte Carlo simulations, with a focus on the reasons for the emergence of a shift in the adsorption mechanism with increasing loading. Received: Revised: Accepted: Published: 11801
August 16, 2016 September 23, 2016 October 24, 2016 October 24, 2016 DOI: 10.1021/acs.iecr.6b03135 Ind. Eng. Chem. Res. 2016, 55, 11801−11808
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Industrial & Engineering Chemistry Research
2. MODELS AND METHODS 2.1. Models. The one-unit-cell siliceous FAU zeolite model, with its cell length of 24.8536 Å, was constructed based on the International Zeolite Association database of zeolite structures. The chemical composition of this zeolite model was Si192O384. This one-cubic-unit-cell zeolite model consisted of eight supercages with diameter of around 12.5 Å, which were connected via 12-membered-ring windows with diameter of about 7.4 Å. In this study, the siliceous FAU zeolite was used because the Si/Al ratio of the zeolite had little effect on the adsorption mechanism according to our previous study.11 Moreover, only one cubic unit cell was applied in this study because no size effect was noticed for the system of concern.10,11 Also, the framework of this model was rigid because of the minor impact of the flexibility of the zeolite frameworks on the static properties.10 For this framework, the partial charge of a silica atom was 1.6+, and that for an oxygen atom was 0.8−.15 During the simulations, the inaccessible solidate cages of siliceous zeolite were artificially blocked by field segregation, which is shown in Figure 1a.
frames were saved. Each of the steps was referred to the attempt to move every adsorbate once. For every step, the trial movement was chosen randomly with a fixed probability, which was 20% guest molecule translation, 20% guest molecule rotation, 55% guest molecule exchange, and 15% guest molecule regrowth. The Ewald summation method19,20 was used to calculate the electrostatic potential energy with the calculation accuracy of 4.184 kJ/mol. The cut-off distance was 12 Å.21 Through the entire simulation process, the periodic boundary conditions were used in three-coordinate conditions.21,22 The variance of the adsorption energy was less than 2% typically. The calculation equations used in the adsorption process are summarized briefly. First, the adsorption energy is in terms of the isosteric heat of adsorption (Qst) generally, which was calculated as follows: ⎛ ∂(Ead − E intra) ⎞ Q st = RT − ⎜ ⎟ ∂Nad ⎝ ⎠V , T
(1)
where Nad is the loading of thiophene, Eintra is the intramolecular energy of thiophene molecules, Ead is the sum of all interactions among adsorbate molecules (Eads−ads) and all adsorbate interactions with the framework of the zeolite (Eads−zeo).23 Besides, radial distribution functions (RDFs) are referred to as gij(r), which is calculated from gij(r ) =
⟨ΔNij(r , r + Δr )⟩V 4πr 2ΔrNN i j
(2)
where i and j refer to two species, r is the distance between these two species, V is the volume of the system, ΔNij(r,r+Δr) means the ensemble-averaged number of the species j around i within a shell of Δr, and Ni and Nj are the number of i and j species.
Figure 1. (a) Model of an all-silica FAU zeolite, consisting of silicon and oxygen atoms. The oxygen atoms are red, and the silica atoms are yellow. The gray lines are the lattice. The green area is the accessible region for adsorbates. (b) Model of a thiophene molecule. The sulfur atom is yellow, hydrogen atoms are light gray, and the carbon atoms are dark gray.
3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherm and Isosteric Heat. Adsorption isotherms of thiophene in an all-silica FAU zeolite model at different temperatures ranging from 300 to 393 K were depicted in Figure 2. Overall, the curves at different temperatures show the same trend that the uptake at pressure lower than 0.1 kPa was
2.2. Simulation Details. Commercial software Materials Studio from Accelrys Inc. was utilized in this simulation. For the whole simulation, a COMPASS force field, which has been successfully applied to investigate the adsorption properties of adsorbates in FAU zeolites,16,17 was used to identify the nonbanding interactions between adsorbates and the framework of the zeolite model. The adsorbate of a thiophene molecule, with a kinetic diameter of about 4.6 Å, was constructed in the software, depicted in Figure 1b. Its charges were implemented in a COMPASS force field. For this thiophene model, carbon atoms near the sulfur atoms were named as C1 atoms, while other carbon atoms were labeled as C2 atoms. The simulation scheme conducted in this study was as follows. First, the models of the zeolite and thiophene molecule went through geometry optimization, according to the Smart minimizer method.18 Second, GCMC simulations were applied to perform the adsorption isotherms for the thiophene/FAU system at various temperatures. This process consisted of 4 × 107 equilibration and 4 × 107 production steps. Finally, the fixed loading simulations were performed at 300 K for thiophene, with the loading of thiophene varying from 1 to near-saturation concentrations of 52 molecules/UC (molecules in per unit cell). During this process, the equilibrium steps of 1 × 107 were carried out, followed by other production steps of 1 × 107. For further study of the properties of adsorption, 2 × 105 configuration
Figure 2. Adsorption isotherms comparison: simulation results of allsilica FAU zeolite model (solid symbols) and experimental studies (open symbols). 11802
DOI: 10.1021/acs.iecr.6b03135 Ind. Eng. Chem. Res. 2016, 55, 11801−11808
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Industrial & Engineering Chemistry Research small but then sharply increased with the pressure, increasing until it reached saturated adsorption. This is in accordance with the physical adsorption behavior in HY zeolite.18,24,25 Specifically, the saturated adsorption amounts for three temperatures were different. Saturated adsorption at 300 K reached the highest with around 4.5 mmol/g, compared to 4.09 mmol/g at 363 K and 3.78 mmol/g at 393 K. Higher temperature corresponded to a lower saturation adsorption amount, which was also observed in the results from Ju et al.8 In contrast with the adsorption isotherms, the isosteric heats (Q) at different temperatures (Figure 3) were almost identical because the isosteric heat was
Figure 4. Contour maps of the density of thiophene molecules in an allsilica FAU zeolite model at (a) 1 molecule/UC, (b) 2 molecules/UC, (c) 4 molecules/UC, (d) 8 molecules/UC, (e) 12 molecules/UC, (f) 16 molecules/UC, (g) 20 molecules/UC, (h) 24 molecules/UC, (i) 28 molecules/UC, (j) 32 molecules/UC, (k) 36 molecules/UC, (l) 40 molecules/UC, (m) 44 molecules/UC, (n) 48 molecules/UC, and (o) 52 molecules/UC.
300 K at loadings ranging from 1 to 52 molecules/UC (0.09− 4.51 mmol/g). Gray lines are the framework of the zeolite, and dots in colors ranging according to the color band are the adsorption regions of thiophene, where colors represent the densities of adsorbed thiophene molecules. At a loading below 40 molecules/UC (3.47 mmol/g) for thiophene, the colored dots almost covered all supercages in the zeolite, suggesting that adsorbed thiophene molecules dispersed in the supercages. However, as the loading continued to grow, the colored area decreased noticeably, illustrating that the distribution of adsorbed thiophene molecules was increasingly localized. This distribution transformation from dispersion to localization can also be seen in the experiments performed by Richardeau et al.6 and Chica et al.7 This increasing centralization in the distribution of thiophene molecules might be ascribed from the decrease in the adsorption amount of some adsorption sites. For HY zeolite, two kinds of adsorption sites were discovered for thiophene and aromatics: W sites referred to the 12-membered-ring windows through supercages and the more preferred S sites, which were adsorption sites excluding W sites.4,10,27−32 The adsorption amount in these adsorption sites was reflected in the density contour planes of thiophene molecules passing through the center of the supercage of all-silica zeolite (Figure 5). Similarly, the increasing localization of the adsorbed thiophene also appeared at the loadings increasing from 40 to 52 molecules/UC. With an increase of the loading, the amount of occupation in the S sites for thiophene went up gradually according to the increase in red areas. For W sites, however, the adsorbed amount decreased at loadings above 40 molecules/UC because of the decrease in areas covered by colors through the 12-membered-ring windows, indicating that S sites were more favorable than W sites for thiophene adsorption, which agreed with the reported results.4 The trend of the occupation of W sites over the whole loading range can be further clearly observed in Figure 6. The adsorption amount in W sites went up first; nevertheless, it decreased gradually at loadings ranging from 8 to 40 molecules/UC. Finally, it dropped noticeably with increasing loading, based on the change of colors inside the 12-membered-ring windows. Variation of the thiophene adsorption amount in the W sites revealed that not all of the adsorption sites in the zeolite were occupied for the entire loading range. Even though the loading reached saturated adsorption, some adsorption sites, such as W sites, were not totally occupied.
Figure 3. Isosteric heat data comparison: simulation result of all-silica FAU zeolite model (black squares) and results from references (open symbols).
not greatly affected by the temperature of the adsorption system as a natural property. Note that the slope of the curve at 300 K was almost zero with a small adsorption amount at pressure below 0.01 kPa. However, it jumped significantly, resulting in an uptake reaching around 3.4 mmol/g. At the beginning value of about 3.4 mmol/g, the slope decreased until the uptake reached about 4.5 mmol/g. This change in the slope represents that the rate of growth in the uptake increased first and then slowed down, illustrating the possible transformation in the adsorption behavior for thiophene occurring at a loading of 3.4 mmol/g. For validation, our simulation results of the adsorption isotherms were also compared with the experimental data,5,8,26 which are shown via open symbols in Figure 2. It can be seen that the simulated adsorption isotherms are in good agreement with the experimental information at both 363 and 393 K over the entire pressure range from 0.1 to 9000 Pa. Additionally, it can be seen from Figure 3 that our simulated Q at 363 K with 8.58−9.02 kcal/mol also agreed with Monte Carlo simulations from Ju et al.8 Moreover, the simulated Q in 0Al model around 7.8−8.2 kcal/mol (at 0.1−0.3 mmol/g) was in agreement with the experimental data ranging from 7.9 to 11.2 kcal/mol, which was calculated by Takahashi et al.26 using the Clausius−Clapeyron formula from their experimental adsorption isotherms for thiophene in H-USY (Si/Al = 195). 3.2. Density and Concentration Distribution. This possibility of transformation for the adsorption behavior could be not only seen in the adsorption isotherms but reflected in the distribution of thiophene molecules. Figure 4 depicts the contour maps of the density for thiophene in an all-silica zeolite model at 11803
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Figure 6. Density contours of thiophene on the representative 12membered-ring window in an all-silica FAU zeolite model at (a) 1 molecule/UC, (b) 2 molecules/UC, (c) 4 molecules/UC, (d) 8 molecules/UC, (e) 12 molecules/UC, (f) 16 molecules/UC, (g) 20 molecules/UC, (h) 24 molecules/UC, (i) 28 molecules/UC, (j) 32 molecules/UC, (k) 36 molecules/UC, (l) 40 molecules/UC, (m) 44 molecules/UC, (n) 48 molecules/UC, and (o) 52 molecules/UC.
Figure 7. Concentration profile of thiophene along the [1 0 0] direction of the unit cell of an all-silica FAU zeolite model at loadings of 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, and 52 molecules/UC.
Figure 5. Density contour planes of thiophene molecules passing through the center of the supercage of one all-silica FAU zeolite unit cell at (a) 1 molecule/UC, (b) 2 molecules/UC, (c) 4 molecules/UC, (d) 8 molecules/UC, (e) 12 molecules/UC, (f) 16 molecules/UC, (g) 20 molecules/UC, (h) 24 molecules/UC, (i) 28 molecules/UC, (j) 32 molecules/UC, (k) 36 molecules/UC, (l) 40 molecules/UC, (m) 44 molecules/UC, (n) 48 molecules/UC, and (o) 52 molecules/UC.
the inflection point, the shift could be related to the mechanism of adsorption thiophene molecules. To investigate this adsorption mechanism, RDFs that can clearly identify the relative locations of particles by calculating the possibility of the presence of other particles around a particular one were employed. The function g(r) represents the possibility of emergence, and the value of r is the distance between two particles. Figure 8 displays RDFs of COM−COM (COM refers to the mass of the center of a thiophene molecule) in all-silica zeolite at 300 K, with the loading of thiophene varying from 1 to 52 molecules/UC. In general, the position of the main peaks over the whole loading range remained at 5.5 Å, which represents that the locations of thiophene molecules adsorbed before kept their original positions when new molecules came into the zeolite. That means the molecules that adsorbed later had no effect on the relative location of molecules that existed before. This can also be confirmed by RDFs between sulfur atoms (Figure 9), where the main peak always remained at 5.8 Å. For loadings lower than 40 molecules/UC, only one peak at about 5.5 Å was observed at each loading, which was below 40
Therefore, on the basis of the thiophene adsorption variations observed from the previous results, we concluded that the inflection point in the adsorption behavior for the thiophene dependence of loading was 40 molecules/UC (3.47 mmol/g). Also, this inflection point can be further confirmed by the concentration profile along the [1 0 0] direction (Figure 7), which can reflect the adsorption amount at different positions intuitively. As observed, the shapes of the red curves, which were at loadings below 40 molecules/UC, were identical, in contrast with the shapes above 40 molecules/UC. 3.3. RDFs. Although a shift did exist in the adsorption behavior when the adsorption amount of thiophene increased in the zeolite, the reason for this transformation remains unclear. Because the concentration of the adsorption sites changed after 11804
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behavior, with the same relative distance away from each other. Furthermore, at most five thiophene molecules may adsorb inside a supercage, on average, because a unit cell of the zeolite has eight supercages. So, the number of adsorption sites (W and S) was enough for adsorbates, resulting in the fact that the entering thiophene molecules could occupy wherever they preferred in the supercage ideally and randomly. This stage can be named as “ideal-adsorption” for adsorbates ideally positioned at adsorption sites. According to our previous study,11,12 monoand diaromatic adsorption in FAU zeolite also experienced this ideal-adsorption stage, in spite of different adsorbates with different loadings in a unit cell zeolite. By contrast, at loadings above 40 molecules/UC, new peaks at 8.3 Å (Figure 8) were observed more and more significantly with an increase of the loading. Meanwhile, it can be seen from RDFs between sulfur atoms (Figure 9) that the first peak at 3.6 Å grew noticeably at loadings above 40 molecules/UC compared to the drop for loadings below the inflection point. This again verifies the inflection point of 40 molecules/UC and the shift in the adsorption behavior. To further evaluate the reasons for the appearance of these newly observed peaks, RDFs of COM-12C (12C refers to the mass center of the 12-membered-ring window) were conducted (Figure 10a). As loadings approached the saturated adsorption amount of 52 molecules/UC, the first peak dropped significantly in contrast with the increase in g(r) for peaks at about 4.4 Å. This illustrates that, above the inflection point, utilization of the 12membered-ring windows (W sites) decreased, which can also be observed in Figures 5, 6, and 10b. Hence, these newly observed peaks above 40 molecules/UC in Figures 8 and 9 should not be attributed to increasing utilization of 12-membered-ring windows (W sites). Therefore, these peaks should be aroused from later-adsorbed thiophene molecules, which had no impact on the adsorbed ones, away from the framework of the zeolite at the same time. Particularly, the first peak at a loading of 52 molecules/UC jumped noticeably in Figure 10 and may be ascribed to, at this saturation adsorption, too many molecules inside the supercages. However, the specific position of the newly adsorbed thiophene at loadings above 40 molecules/UC is still unclear. To figure out it, RDFs of COM-centerSC, C-centerSC, C1centerSC, C2-centerSC, and S-centerSC (-centerSC refers to the
Figure 8. RDFs of COM−COM in an all-silica FAU zeolite model at loadings ranging from 2 to 52 molecules/UC.
Figure 9. RDFs of S−S (S refers to sulfur atoms) in an all-silica FAU zeolite model at loadings ranging from 2 to 52 molecules/UC.
molecules/UC in RDFs of COM−COM (Figure 8), illustrating that the adsorbed thiophene molecules had identical adsorption
Figure 10. RDFs of (a) COM-12C and (b) S-12C in an all-silica FAU zeolite model at loadings ranging from 1 to 52 molecules/UC. 11805
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Figure 11. RDFs of (a) COM-centerSC, (b) S-centerSC, (c) C-centerSC, (d) C1-centerSC, and (e) C2-centerSC in an all-silica FAU zeolite model at loadings ranging from 1 to 52 molecules/UC.
its position identical (Figure 11a). This peak was also in accordance with the observed main peak at around 4.0 Å in Figure 11b−d. This finding further illustrates that, before the inflection point, adsorbed thiophene molecules were in the possession of the adsorption sites, which had an equal distance from the centers of the supercages with 3.9 Å. By contrast, above
center of the supercage) for the entire loading range were performed, as shown in Figure 11. First, from Figure 11a, at r = 0, almost no peaks were observed for all loadings. This means that these centers of supercages were unfavorable for adsorption. Second, below 40 molecules/UC, only one peak at about 3.9 Å was observed for each loading, with 11806
DOI: 10.1021/acs.iecr.6b03135 Ind. Eng. Chem. Res. 2016, 55, 11801−11808
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Industrial & Engineering Chemistry Research the inflection point, a new shoulder at about 2.2 Å in RDFs between sulfur atoms (Figure 9) and the center of the supercage (Figure 11b) was observed, corresponding to the shoulder in the same position in Figure 11c−e. This shoulder at some 2.2 Å grew intensely with the increase of the loading and developed into a peak above the inflection point. This illustrates that the newly adsorbed thiophene molecules located at the positions where they were nearer to the center of the supercage compared to the molecules adsorbed before, just as shown in Figure 12.
Figure 13. Comparison of the calculated interaction energy between adsorbates (Eads-ads) and the interaction energy between adsorbates and the framework of zeolite (Eads−zeo) over the entire loading range of 1−52 molecules/UC.
interaction energy of adsorption. As seen, because of the heterogeneity of the adsorption sites,24,27 the figure for Eads−zeo decreased compared to the growth in Eads−ads. For loadings from 8 to 40 molecules/UC, both Eads−ads and Eads−zeo almost remained constant because two kinds of influences were kept in balance. For Eads−zeo, on the one hand, new entering molecules pushed the adsorbed ones close to the framework, confirmed by the decreasing intensity of the main peaks before the inflection point (Figure 11), leading to an increase of the energy; on the other hand, the increasing occupation of the less preferred W sites (Figures 5, 6, and 10) contributed to the drop of the energy. For Eads−ads, the increase of the loading resulted in the growth of Eads−ads, whereas the newly adsorbed thiophene pushed the old ones to the surface of the zeolite, leading to a decrease of the energy. On the basis of the analysis above, we believe that, below the infection point, the identical adsorption mechanism and distribution of thiophene, which can be observed in these discussed RDFs and density contours for thiophene, were mainly because of this constancy of Eads−ads and Eads−zeo. By contrast, when the loading became higher than 40 molecules/UC, Eads−ads increased noticeably because of too many molecules adsorbed in the cage, whereas Eads−zeo decreased mainly because the rising Eads−ads dragged the molecules close to each other and far away from the framework of zeolites at the same time. This increasing Eads−ads leads to the increasingly localized distribution of thiophene molecules with the rise in the loading, as shown in Figure 5. Additionally, the change of the adsorption mechanism was due to an increase in Eads−ads and a decrease in Eads−zeo noticeably above the inflection point.
Figure 12. Low-energy snapshot of a representative equilibrium geometry of thiophene in the supercage of an all-silica FAU zeolite model at a loading of 48 molecules/UC. The black thiophene molecule is the inserted one.
In addition, compared to the exactly identical value of r for the main peak below the inflection point, the distance between the main peak and the center of supercages became increasingly longer above the inflection point, as shown in Figure 11, which might be ascribed to new inserted molecules pushing adsorbed ones close to the framework of the zeolite. Furthermore, the figure for g(r) of the main peaks in Figure 11 grew higher above the inflection point, compared to the counterparts below the inflection point. This represents more localized and narrower distribution of adsorption, confirmed by increasingly localized density contour planes of thiophene molecules at loadings above 40 molecules/UC, in contrast with low loading counterparts, as shown in Figures 4 and 5. In summary, the stage above the inflection point can be referred to as “insertion adsorption” occurring in the adsorption of aromatics in FAU zeolite, for the newly adsorbed thiophene molecules only inserted into the position close to the center of the supercage with no effect on other adsorbed ones. This stage was in accordance with monoaromatic counterparts.11,12 3.4. Interaction Energy. For this thiophene/FAU system, the interactions between the framework and adsorbates as well as among adsorbates are essential to the adsorption behavior because of its physical adsorption properties,24 possibly resulting in the transformation of the adsorption mechanism. Consequently, the interaction dependence of loading is well worth the study. Figure 13 demonstrates the variation of the interaction energy against the increase of loading. The red lines are the interactions between the framework of the zeolite and the thiophene molecules, labeled as Eads−zeo, whereas the black lines are the interactions among adsorbates, named as Eads−ads. Both Eads−ads and Eads−zeo are referred to collectively as the total
4. CONCLUSIONS In this study, the adsorption mechanism of thiophene dependence on the loading in all-silica FAU zeolite at 300 K was evaluated by Monte Carlo simulations. With an increase of the loading, the mechanism changed from “ideal adsorption” to “insertion adsorption” with the inflection point of 40 molecules/ UC, confirmed by the adsorption isotherms, density distributions, concentration profiles, and RDFs. For the “ideal adsorption” stage, thiophene molecules ideally occupied the S 11807
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Industrial & Engineering Chemistry Research
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and W sites. As the loading of thiophene increased, the adsorption amount on the S sites went up consistently. By contrast, the adsorption amount on the W sites grew first and then dropped gradually for the “insertion adsorption” stage because of the change of the dominated interaction energy. Accordingly, the adsorption mechanism of thiophene also experienced a transformation similar to monoaromatics counterparts. This further elaborated the reasons for their competitive adsorption in FAU zeolites. Simulation results revealed that thiophene molecules preferred S sites over aromatics. Because bringing metal cations or aluminum atoms into FAU zeolites might have a positive effect on the increasing utilization in S sites for thiophene,8,13 it is of great importance to further study those factors based on this study, which can shed light on the design of desulfurizer adsorbents with higher selectivity for thiophenic sulfides from aromatics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (L.Z.). Tel: 86-10-89739078. Fax: 86-10-69724721. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from the National Natural Science Foundation of China (Grants 21336011, 21476260, 21236009, and U1162204) and the Science Foundation of China University of Petroleum, Beijing (Grant 2462015YQ0311).
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REFERENCES
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DOI: 10.1021/acs.iecr.6b03135 Ind. Eng. Chem. Res. 2016, 55, 11801−11808
