Benzene in MFI

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Adsorption and Separation Mechanism of Thiophene/Benzene in MFI Zeolite: A GCMC Study Hui Fu, Yajun Wang, Tianhao Zhang, Chaohe Yang, and Honghong Shan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07796 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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The Journal of Physical Chemistry

Adsorption and Separation Mechanism of Thiophene/Benzene in MFI Zeolite: A GCMC Study Hui Fua,b*, Yajun Wanga, Tianhao Zhangb, Chaohe Yanga and Honghong Shana,* a

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, Qingdao, Shandong 266580, PR China b

Department of Chemistry, College of Science, China University of Petroleum, Qingdao, Shandong 266580, PR China

ABSTRACT:

Selective removal of thiophene from aromatic components is one of

the key challenges facing the petrochemical industry. The adsorption and separation mechanism from the molecular viewpoint can guide and upgrade the relative adsorption based technology. Therefore, we performed grand canonical ensemble Monte Carlo (GCMC) simulation to investigate the adsorption performance and mechanism of competitive adsorption. Density distribution and radial distribution functions (RDF) analysis give a more detailed description for the adsorption sites. For pure component adsorption, donut-shaped adsorption sites were obtained for both benzene and thiophene from the straight channel point. From the viewpoint of the zigzag channel, the sorbates follow the straight line shape distribution at low loading and the S shape distribution at high loading. As for the binary component adsorption, more benzene adsorb in the zeolite than thiophene at low pressure, however, thiophene competes successfully at high pressure. This can be explained by the key factor, at low pressure, the size effect plays an important role. While as the pressure increases, the interaction energy dominates the process. Analyzing RDFs of the binary adsorption, when benzene compete with thiophene, the preferential adsorption sites do not change, however, the emergence possibility of benzene gets smaller.

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INTRODUCTION Sulfur compounds can contribute to SOx air pollution, poison catalysis and increase volatile organic compounds. In order to minimize the negative healthy and environmental effects of these exhaust emissions, the strict regulations have been issued.1,2 Under the pressure of these rigorous regulations, refines are forced to produce much cleaner fuels.3 Thus, worldwide concerns over environment have stimulated increasing interest both in academic and industry for deep desulfurization process. Conventionally, hydrodesulfurization (HDS) process is the commonly used technology for removing thiols and thioether. However, saturation of olefins in the hydrotreating process reduces the octane number of gasoline significantly and H2S produced during the process inhibits the deep desulfurization. Besides, HDS is not efficient in removing thiophene species.4 With the need for capital-avoiding technologies that can remove sulfur without obvious octane loss and without high hydrogen consumption, non-HDS methods are gradually developed. Selective adsorption process5-7 and liquid-liquid extraction process8 are entering researchers’ horizons. Adsorptive separation has many advantages: smaller foot print, less exorbitant materials of constructions for equipment and lower pumping costs. Separation processes performed by an adsorption process prefer to choose zeolite materials, which can provide suitable adsorption selectivity and capacity for the desired applications. The structural and chemical stability of zeolite under different operating conditions makes them a potential adsorbent for separation.9 In general, transportation fuels consist of many compositions, depending on the raw oil and production process. Alkanes with different chains occupy the largest percent about 70-80%, aromatic components like benzene and its derivations 20-30%.10 Main classes of organosulfurs contained include mercaptans in the low boiling fraction, thiophene and its derivatives in the medium and heavy gasoline.11 Therefore, removing the sulfur components means that the desulfurization adsorbents need to have significant higher affinity for thiophene than aromatics and alkanes. Qi 2


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et al.11 used polydimethylsiloxane (PDMS) based membrane to separate the binary and multicomponent alkane/thiophene mixtures by pervaporation. The carbon number, alkane concentration, feed temperature were considered for separation efficiency. According to their experimental results, increasing of the lighter alkanes can decrease selectivity to thiophene. Molecular simulation of adsorption phenomena in zeolites is emerging as a powerful tool to understand the microscopic performance. Grand canonical Monte Carlo simulations (GCMC) are appropriate for establishing a correlation between the microscopic behavior of the zeolite and adsorbate with the macroscopic properties. These simulations have provided valuable insight into adsorption phenomenon and demonstrated that the technique is a promising tool in the study of adsorption in zeolites. Shah et al.12 used Monte Carlo simulation to selectively remove hydrogen sulfide (H2S) from sour natural gas mixtures. In their work, the adsorption behavior of binary mixtures containing H2S and methane in seven different zeolites was studied at two temperatures under the pressure form 1 to 50 bar. The results demonstrate MFI zeolite has the highest selectivity and the most favorable enthalpy of adsorption for H2S due to favorable sorbate-sorbate interactions. Besides, about separating aromatic/thiophene system, many investigations have been studied not only from experimental aspects but also in simulation field. The desulfurization process of fuels with zeolites is always a major concern in experiments. Yang group6 found Cu+ and Ag+ modified Y can adsorb sulfur compounds from commercial fuels selectively, but in their study the adsorption mechanism was not analyzed deeply. While Vuel et al.13 and Li et al.14 inferred that S-M bond interaction and π-complexation mechanism were beneficial to sulfur removing from jet fuel on their experimental base. Zhao group15-17 have been working on adsorption mechanism study of benzene/thiophene on zeolite system recently. By Monte Carlo simulation, they found single component adsorption preferred to go through two different stages over the whole loading range in FAU zeolite, the ideal adsorption and insertion adsorption. As for the competitive process, results showed that the adsorption mechanism also transferred from ideal-displacement adsorption to insertion-displacement adsorption with the increasing loading. The competitive 3



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mechanism is due to the interaction energy. In 2015, Zeng et al18 simulated benzene/thiophene binary adsorption on nine kinds of zeolites with focus on the influence of zeolite pore size, not mentioning adsroption mechanism. So far, there are few systemic theoretical study about binary adsorption of this system. Summary of previous studies, one-sided research oriented. Some pay attention to the adsorption mechanism, and some focus on adsorption properties. In fact, the two are complementary and support each other. Thus, it is necessary to combine the two sides together, because mastering the adsorption mechanism helps us to better understand the adsorption performance. The goal of our work is to investigate the adsorption and separation mechanism of benzene and thiophene in MFI zeolite. First of all, we upgraded the LJ parameters and the set of LJ parameters was adjusted to the experimental data. Then the adjusted parameters were applied to simulate the adsorption and separation capabilities of the selected zeolites for single components as well as for the binary components. RDFs analysis help us to confirm the accurate adsorption sites for benzene and thiophene with the effect of loadings and phase compositions, which make it better to understand the mechanism of adsorption and separation process. The results in this work are beneficial to deep understanding of the competitive adsorption performance of benzene and thiophene in MFI.

MODELS AND CALCULATION DETAILS Models. The one-unit-cell siliceous MFI zeolite model was available in the structure database of Material Studio. Supercells containing 8 (2× 2 × 2) lattices with periodic boundaries were constructed from one unit cell for the adsorption study. The MFI zeolite has orthogonal 10-ring pores, one straight pore of 5.4 × 5.6 Å and one

zigzag pore of 5.4 × 5.1 Å. The pores meet at intersections with a lager diameter than

the pores themselves. Based on its special molecular configuration, MFI zeolite has been the subject of considerable fundamental of separation applications, thus, it was 4


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chosen in this work.19 MFI structure was assumed to be rigid in simulations, as it has been shown that the flexibility of the framework has a small effect on the adsorption properties of the zeolite.20 As a result, the Metropolis Monte Carlo (MMC)21 was chosen. For the framework, the partial charge of a silica atom was 2.05e, and that for oxygen atom was -1.025e, which were obtained by Calero and his coworkers.22 Thiophene and benzene were described with rigid five-site and nine-site model respectively as shown in Figure 1. For the partial charge of thiophene model, the sulfur atoms were 0.023e, CH united atoms bonded with sulfur were -0.034e, and the remaining two CH united atoms were 0.0225e.23 For the nine site benzene model, a partial charge of 2.42e was located in the plane of the molecule on the 6-fold axis, and the other two charges of -1.21e were located along the 6-fold axis to represent the π-electron clouds.24 Zeng et al. used the two models and obtained good agreement between experimental and simulated results of thiophene and benzene adsorption in silica zeolites.18

Figure 1. Model of the silica MFI zeolite, consisting of silicon and oxygen atoms (left). Oxygen: red; Silica: yellow. Models of the thiophene and benzene molecules (right).

Simulation

Methods.

Adsorption

and

separation

characteristics

of

thiophene/benzene were calculated using Monte Carlo simulations in the grand canonical ensemble with Materials Studio.25 Simulations were carried out on Sorption and Forcite modules. Monte Carlo simulation methods were used for the computation 5



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of adsorption data. The Sorption module was used to construct the crystal structures of MFI zeolite and simulate the adsorption performance of thiophene and benzene. The Forcite module was chosen to optimize the zeolite cluster and analyze the radial distribution functions (RDF). As for interatomic interactions between zeolites and adsorbates, there are two types of potentials: the short-range dispersion potential and the long-range electrostatic potential. The short-range potential energy is described by a pair-wise Lennard-Jones potential, while the long-range potential calculates the electrostatic interactions. As a result, parameters in the Lennard-Jones potential and partial charges play an important role in describing the adsorption behavior. The interactions of guest-host and guest-guest were modeled with a Coulombic and Lennard-Jones potential as shown in the following part: = 4

− +

!"#$

(1)

Where % and are the LJ parameters, is the distance between i and j sites,

& and & are the charges on the interaction center and ' is the permittivity of vacuum. TraPPE23, 26 is a common forcefield for adsorption description of thiophene and benzenes, so the corresponding LJ parameters were taken from TraPPE. In order to get more accurate results compared with the experimental results, we upgraded the LJ parameters of thiophene molecule and oxygen atoms. Dubbeldam and his coworkers22 have done the parameter optimization work for the alkane adsorption system. According to their parameter optimization process, it can be derived that a higher strength parameter ϵ induces an increased loading (for a fixed pressure), while the amount of inflection is controlled by the size parameter σ. We changed the values of ϵ and σ according to their conclusion, until the simulated adsorption isotherm can accurately reflect experimental results. The LJ parameters of thiophene molecule and oxygen atoms are shown in Table 1.The upgrade parameters were applied in the Forcite module for confirming its reliability. Interactions between unlike pseudoatoms are calculated with the Lorentz-Berthelot combining rules.27 6


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% = (% + %

)/2

(2)

+ = ,% %

(3)

The Ewald summation methods were applied for the Coulombic adsorbate-adsorbate and adsorbate-adsorbent interactions. The cutoff distance was fixed at 12 Å with an atom based calculations and cubic spline cells. The cubic spline truncation was set to 1 Å with a buffer of 0.5 Å. For electrostatic contributions, the accuracy of the Ewald and group calculation was 1 × 10./ kcal/mol with at least 10 equilibration steps and 100 productive Monte Carlo steps were performed.

Table 1. Intramolecular Force Field Parameters (B: benzene; Th: thiophene). Parameter

σ (Å)

ϵ/kB (K)

CH-B

3.74

53.5

CH-Th

3.56

50.30

S-Th

3.39

231.37

Oz

3.01

101.58

Adsorption thermodynamics were further investigated by analyzing the isosteric heats. The values of the isosteric heats of adsorption can represent the adsorption capabilities of the adsorbate in an adsorbent framework. During the GCMC simulation of unary adsorption, the isosteric heat was calculated, using23 345 = 67 −

〈9:〉.〈9〉〈:〉 〈:< 〉.〈:〉
() =

?@: (,B@)CD !" < @: :

(5)

Where i, j stands for two particles, r is the distance between i and j, V is the system volume. ΔNij (r,r+Δr) represents the possible average number of j in a spherical shell at a distance between r and r+Δr from the particle i. 7



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RESULTS AND DISCUSSION

Adsorption Performance of Unary Component and Binary Components. In order to validate the models and interaction parameters applied in our work, we calculated the adsorption isotherms for pure components at 373 K and compared them with the reported data (see Figure 2). The adsorption isotherm of thiophene was compared with available experimental data obtained by Lai et al.28 and Chica et al.29, and theoretical data obtained by Zeng and his coworkers.18 The predicted adsorption isotherms revealed good agreement with the theoretical results but some difference with the experimental results. Comparison of simulation and experiment is difficult because MFI-type zeolite exits in three different forms as pointed out by Zeng et al.18 They have proven that the structure difference can cause the adsorption capacity difference. Thus it is difficult to give a conclusion about the possibility of our model to reproduce the adsorption performance. On the other hand, the all silica zeolite was hardly used in experiment, which may also provide the disagreements. Here, in this study, the MFI structure is imported from the Material Studio databases, which are derived from experimental structure30. Comparing our simulated results with the experimental result in MFI with different Si/Al ratio obtained by Lai et al.28 and Chica et al.29 , it can be seen that the adsorption amount decreases with the increasing Si/Al ratio. Considering our all silicon model, the experimental data at higher Si/Al ratio (50) shows better agreement with our simulation results, while the obvious difference exits at lower Si/Al ratio.

8


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Figure 2. Adsorption isotherm at 373 K of thiophene and benzene. The dotted lines are shown to guide the eye.

Experimental isotherms for benzene have also been reported by several authors.31,32 Song et al.31 studied the adsorption and diffusion behavior of benzene in silicalite at different temperatures. At high temperature (373 K), the experimental isotherm follows a Langmuir shape and levels off. At low temperature (323 K), there are two steps for adsorption performance. The adsorption isotherm will reach a plateau at the first step and then increase again in the second step, as showing VI isotherm behavior. Pressure range of their research was from 0 kPa to 10 kPa. Seen from Figure 2, the isotherm of our work shows the similar trend with them, when the pressure is below 9



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10 kPa. Then the adsorption isotherm increases again and the slope decreases as the pressure is above 40 kPa. Besides, our simulated adsorption isotherms revealed good agreement with the theoretical results obtained by Zeng et al18. Isosteric heats of adsorption are calculated and compared with the experimental results. Takhashi et al.33 obtained the adsorption heat at about 77.79-80.29 kJ/mol for thiophene

and

69-74.85

kJ/mol

for

benzene on Na-ZSM-5 (Si/Al=10)

experimentally. Smaller calculated results (54.6 kJ/mol for thiophene and 50.4 kJ/mol for benzene) based on all silicon zeolite model have been got in our work, which is similar with previous theoretical result (55 kJ/mol for thiophene).34 The difference is obvious but reasonable because the presence of Na+ on ZSM-5 increase the interaction between adsorbates and adsorbents. Garcia et al.35 demonstrated that the vibration of bands of adsorbed thiophene observed are attributed to vibration of thiophene molecules strongly interacting with the Na+ cations, which can give an explanation. Besides, as discussed above, for thiophene, zeolites with Si/Al ratio of 50 show similar loading with our simulated results and zeolites with lower Si/Al ratio show higher loading. In Takhashi’s study, their Si/Al ratio is 10, and that is the other part of reason leading to the high loading. After confirming the relative parameters, let us focus on the adsorption performance. From Figure 2, we can see the adsorption amount of benzene in low pressure is larger than that of thiophene. The phenomenon may be relative with two factors. One is size matching, meaning size of benzene better matching with zeolite structure than thiophene. The other is interaction energy, which can be equivalent to Qst. In Zhao’s studies,15, 16 the Qst of benzene at low loading (9.5 kcal/mol) is larger than that of thiophene (7.5 kcal/mol). This is accord with the adsorption amount of benzene and thiophene. However, the saturated adsorption amount of benzene is lower than that of thiophene. Density and concentration distribution of the pure component adsorption was investigated (Figure 3). For the unary adsorption performance of benzene, the adsorbates prefer to adsorb in the intersection channels at 0.01 kPa. With the increasing pressure, the straight channel and zigzag channel begin to hold the benzene molecule. For thiophene molecules, at low pressure, thiophene 10


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can exist in all three channels. However, the density in the straight channels or zigzag channels is obviously smaller than that in intersection channels from the color difference. Therefore, the preferential adsorption sites of thiophene in MFI are the intersection channels. Then, the density of thiophene in straight or zigzag channels increase significantly with the elevated pressure. Comparing the adsorption performance of the two sorbates, the distribution area of thiophene is wider than that of benzene, which is accounted by the size effect.

Figure 3. Density distribution for thiophene and benzene molecules in MFI under different pressures (pressures are 0.01, 10, 100 kPa from left to right). Top: benzene, bottom: thiophene

The isosteric heats of adsorption (Qst) for sorbate molecules on zeolites can be used as a function of adsorption capacity. Focusing on our theoretical results (Figure 4), the Qst of thiophene increases with the elevated pressure (larger loadings). However, the Qst of benzene remains constant at 53.94 kJ/mol under low pressure and then shows a slight increase at high pressure. We decomposed the Qst values into the interaction energy between sorbate and sorbate and the interaction energy between sorbate and the framework of zeolites to explain the change tendency of Qst. According to the 11



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Figure S1, the interaction energy between sorbates and sorbents (Uads-zeo) remains almost constant in thiophene adsorption while decreases with the increasing pressure in benzene, as pressure is above 10kPa. However, as for interaction energy between sorbate and sorbate (Uads-ads), its values keep climbing up. Combining the two kinds of energies, the increased sorbate-sorbate interactions will fully compensate for the decreased interaction with the framework atoms, leading to the increasing tendency of Qst, which is consistent with the phenomenon studied by Shah et al.12 and Zhao et al15. The Uads-zeo of benzene is much larger than that of thiophene, thus, the Qst of benzene is larger than that of thiophene in low pressure. However, the Uads-ads of thiophene increases significantly faster than that of benzene when the pressure is below 10 kPa. Thus, there is a intersection in the Qst figure. Vlugt et al.24 performed adsorption experiments using TEOM and theoretical calculations for confirming the isosteric heat of benzene in silicalite. They obtained the Qst of 53 kJ/mol with the loading from 0 to 4 molecules per unit cell, which is consistent with our results (Figure 4). We speculate the invariable Qst value is due to monolayer adsorption. For the increasing tendency, it can be explained by the multilayer adsorption. The increasing sorbate-sorbate energy lead to higher isosteric heat. On the other hand, the slope of thiophene is larger than that of benzene, indicating Qst of thiophene can be influenced more significantly than that of benzene. The stronger interaction energy is in favor of centralization of thiophene molecules, which makes it advantageous in competitive adsorption performance and helps to explain the wider distribution area of thiophene than benzene except the size effect.

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Figure 4. Isosteric heats of adsorption for thiophene and benzene under different pressure. The dotted lines are shown to guide the eyes.

The above discussion is about the adsorption performance of pure component. In experiment, the measurement of mixture adsorption isotherm is more complicated than that of pure component. The adsorption performance is influenced by not only pressure but also the mixture composition. Therefore, adsorption performance of mixtures is also investigated. Figure 5 shows the adsorption isotherm of thiophene and benzene in an equimolar (50/50) mixture at 373 K, focusing on the pressure influence. It can be seen that at low pressure, the contribution of benzene and thiophene to total loading is similar. With the increasing pressure, the loading of benzene reaches a peak and then decrease. While for thiophene, it keeps increasing continuously and progressively replaces the benzene molecules at high pressure, presenting high contribution. Considering the density distribution of pure component shown in Figure 3, at lower pressure, benzene and thiophene both can be adsorbed in the intersection channels, however, with increasing loading, thiophene tend to occupy more straight or zigzag channels than benzene. Thus in mixtures it shows competitive adsorption at high pressure.

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Figure 5. Calculated adsorption isotherms of equimolar thiophene/benzene mixtures in zeolites at 373 K. The dotted lines are shown to guide the eyes.

Figure 6. Density distribution for competitive adsorption between thiophene and benzene molecules under different pressures. Red: thiophene; Green: benzene. 14


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Among the whole pressure range, we chose density distribution under four different pressures to give an intuitive explanation. We can see at low pressure, benzene molecules occupies in the intersection channels, while thiophene molecules are adsorbed in all three types of channels in MFI, which is consistent with the unary adsorption. With the increasing pressure, benzene molecules are gradually squeezed out by thiophene. Like the density distribution under 10 kPa and 100 kPa shown in Figure 6, there are less areas occupied by benzene molecules. In summary, the adsorption performance and mechanism can be influenced by zeolite structures and sorbate loadings. To fully understand the mechanism of adsorption and separation, we analyzed RDFs to make a detailed investigation about the adsorption sites. Therefore, the following discussion mainly focus on RDFs of benzene, thiophene and binary mixtures.

Adsorption Mechanisms of Unary Adsorption. RDFs can be analyzed to identify the relative locations of particles by calculating the possibility of the presence of other particles around a particular one. The function g(r) in a system of particles (atoms, molecules, colloids, etc.), describes how density varies as a function of distance from a reference particle. In simplest terms it is a measure of the probability of finding a particle at a distance of r away from a given reference particle. To know the possible adsorption sites of sorbate molecules, we need to fix the location of the first molecule, and confirm the sites of the second one. Based on the above analysis, the intersection channel is the main adsorption site for both thiophene and benzene. As a result, we fix the first molecule at the center of the intersection channel (CSC) of MFI zeolite and mainly analyze RDFs of CSC-COM (the center of sorbate molecules). Figure 7 shows the RDFs of CSC-COM of benzene with the loading from 8 to 52 molecules per unit cell. We combined zeolite structure (Figure 8) to determine the specific location, thus, to find out the adsorption sites. As shown in Figure 8, the perpendicular channel stands for the straight channel, while horizontal channel represents the zigzag channel. They intersect with each other, producing the 15



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intersection channels. The relative distance will be analyzed one by one. 11.5 Å is the distance between the neighboring intersection channels connected by zigzag channels, and 12.65 Å and 13.38 Å both stand for the distance between the neighboring but not interlinked intersection channels. As for the neighboring intersection channel connected by the straight channels, it has the distance of 10.01 Å. Then combining these distances, the RDFs of COM-CSC were analyzed. Firstly, the primary main peak is at r=0-2 Å, indicating the huge possibility of existing in the intersection channels. Peaks at 4.5 Å and 6 Å stand for the locations in straight channels and zigzag channels. As well as it can be seen, at low loading, g(r) is small or even zero and then it gets larger as the increasing loadings. That means the sorbates will adsorb at the intersection channels firstly, after the intersection channels get saturated, the sorbates will get into the straight channel and zigzag channel.

Figure 7. COM-CSC of benzene molecules with different loadings (molecules per unit cell).

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Figure 8. Local amplification figure of MFI structure.

Then, we focus on the magnified circles which corresponds to the distances of 11.5 Å and 12.65 Å. We can see at these two distances, g(r) always intersect at one point, showing uninfluenced by the changing loadings. It means the possibilities of the particles located at these sites are the same over the entire loading range we have investigated, indicating benzene molecules will adsorb in these two sites firstly. As Figure 8 shown, 11.5 Å and 12.65 Å are the distances between the intersection channels. Therefore, we can draw the conclusion that the preferential adsorption sites of benzene in MFI zeolite are the intersection channels. As discussed above, distances at 10.01 Å and 13.38 Å also stand for the intersection channel, however, at these two distances, g(r) do not show a cross point. We suspect the peaks g(r) of these two distances not only show the adsorption possibility in the relevant intersection channel, but also include possibilities in other adsorption sites. It is also interesting that there is not a large peak at 11.5 Å, but it is divided into two peaks at 11.2 Å and 12 Å, respectively. That indicates the adsorption sites of benzene in intersection channel are not at the intersection center but like a donut-shaped (toroid) distribution as shown in Figure 9. The gray part (Figure 9) is the projection of zigzag channel and there are straight channels intersecting with it. We measured the distance between A (the center of the intersection channels) and the points of the inner circle and outer circle of the 17



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toroid. The distances between A and the inner circle changes from 11.2 to 12 Å and distances between A and outer circle changes from 10.01 to 13.38 Å. Therefore, fixing the first particle at the intersection center (A), 11.2 Å is the side nearest to A of the inner circle while 12 Å is the side farthest to A of the inner circle. Studying on RDFs at the distance of 11.2 Å and 12 Å, we can see g(11.2) decreases with the increasing loading while g(12) increases. At low loading, benzene sorbates prefer to adsorb in the side near A. At high loading, benzene molecule will move to the farther side. We pieced the near side and far side together as shown by the blue line and black line (Figure 9). It can be seen that at low loading, sorbates follow a straight line shape distribution (blue) while at high loading, they follow an S shape distribution (black). Besides, the adsorption sites at 10.01 Å and 13.38 Å are discussed. Summing up the above discussion, we can see 10.01 Å not only refers to the distance between the zigzag channels, but also stands for the distance between A and its nearest side of the outer circle. Likewise, 13.38 Å not only refers to the distance between two neighboring but not interlinked straight channels, but also the distance between intersection center and the farthest side of the outer circle. Observed Figure 7, the peaks at the two distances both slightly increase and obviously shift. The peaks of g(10.01) moves to smaller r while g(13.38) shifts to larger r with increasing loading, which is accounted by the movement of the sorbates, from the inner circle to the outer circle. Therefore, we have inferred the preferential adsorption sites and the movement of benzene in zeolite channels with the effect of loadings.

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Figure 9. Adsorption sites distribution rules of benzene in MFI with different loadings.

RDFs of thiophene were also investigated as shown in Figure 10, with the loading from 16 to 96 molecules per unit cell. Obvious peaks appeared at 0-2 Å and 4-6 Å and the peaks get higher as loading increases. It demonstrates that thiophene can get into the straight channel, zigzag channel and intersection channel even at low loading, which is consistent with the density distribution figure (Figure 3). We can also see the cross point at 11.5 Å and 12.65 Å, indicating the preferential adsorption sites of thiophene are also the intersection channels. In addition, the donut-shaped adsorption sites are also obtained in thiophene. Similarly, at low loading, thiophene follows the straight line shape distribution and at high loading follows the S shape distribution in zigzag channels. It can be seen all the above results are similar with benzene, thus there are no more detailed description here. Comparing the RDFs of benzene and thiophene, the peak of g(r) of benzene is thinner than that of thiophene, due to the size effect. The size of thiophene is smaller than that of benzene, as a result, the distribution area is wider and can be adsorbed in straight channels, zigzag channels and intersection channels. On the other hand, the interaction energy can also affect the distribution. We can see that before 100 kPa in Figure S1, the Uads-zeo of benzene decreases while that of thiophene keeps constant. The Uads-ads of thiophene is larger than that of benzene. So, it is advantageous for multilayer adsorption of thiophene. 19



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Thus, we can clearly understand the adsorption behavior of benzene and thiophene in MFI with the effect of loadings. The adsorption mechanism (line distribution and S shape distribution) with the influence of loading can be supported by the density distribution (Figure S2). Density distribution of thiophene is similar with that of benzene, thus, we take thiophene as the example. Area between two black lines is the first layer of zigzag channel and the density distribution in the circle shape stands for the first layer of density dots. At low pressure (low loading), the distribution area in straight channel is close to zigzag channel, combing with the density distribution in zigzag channel, the adsorption distribution is consistent with the line distribution. At high pressure (high loading), the distribution area in straight channel is far away from the zigzag channel, combing with the density distribution in zigzag channel, the adsorption distribution is consistent with the S shape distribution.

Figure 10. COM-CSC of thiophene molecules with different loadings.

Adsorption Mechanism of Binary Adsorption. To fully understand the competitive process of the two molecules, we investigated the influence of phase composition. Three different compositions are studied (the ratio of benzene and thiophene is 3:7, 5:5 and 7:3) with the total loading of 50 molecules. That means 20


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when the adsorption loading of benzene is 15, 25 and 35, the corresponding amount of thiophene is 35, 25 and 15, respectively. RDFs of COM-CSC are shown in Figure 11. For RDFs of benzene, the peak at 11.5 Å is divided into two peaks at 11.2 Å and 12 Å. The peaks at 12 Å get larger while peaks at 11.2 Å get smaller with increasing benzene composition. The above phenomenon is similar with RDFs in the unary adsorption. Therefore, the alike adsorption sites (donut-shaped adsorption) and adsorption performance (movement under the effect of loading) were obtained in binary competitive adsorption. For COM-CSC of thiophene, there is no obvious intersection point at 11.5 Å, indicating that thiophene prefer to be adsorbed following the S shape distribution. Focusing on g(r) in competition adsorption, the two sorbates show different performance. Decreasing composition of benzene, g(r) at 4 Å and 6 Å of benzene shows the tendency g(15) < g(25) < g(35) obviously while that of thiophene shows no obvious changes. It can be concluded, with the increasing composition of thiophene, the possibility of the benzene adsorbing in the straight channel and zigzag channels gets smaller. It means benzene was gradually extruded by thiophene in the straight and zigzag channels. We speculate the interaction energy between sorbates and sorbent plays an important role in the above process. The interaction energy of thiophene-MFI is stronger than that of benzene-MFI.

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Figure 11. RDFs of COM-CSC for competitive adsorption between thiophene and benzene with different compositions (molecules).

To investigate the deep mechanism of the competitive adsorption, we discussed the key factors that control the process. As we discussed above, the adsorption heat of thiophene in zeolite is higher than that of benzene and it can be influenced more significantly with the increasing loading. Besides, benzene has more atoms and its kinetic diameter of 5.8 Å is slightly larger than that of thiophene (4.6 Å).36 Combining the competitive adsorption process as Figure 5 shown, at lower pressure, the loading of benzene is higher than that of thiophene. We attribute this result to the bigger size of benzene than that of thiophene. The size of benzene can better fit the zeolite channel, so that the volume filling becomes favorable for benzene at low pressure. A similar trend was obtained by Zeng et al.18, when they investigated the adsorption 22


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performance of benzene and thiophene in MFI. Though the size of thiophene is smaller than that of benzene, the uptake of benzene is higher due to its better matching size. However, with the increased pressure, the interaction energy between adsorbates and zeolite becomes the key factor. Thiophene molecules adsorbed more strongly than benzene, which suggests MFI can potentially separate thiophene and benzene.

CONCLUSIONS Adsorption and separation of benzene/thiophene in MFI zeolite was simulated by GCMC. For pure component adsorption, RDFs and density distribution analysis reveal that benzene and thiophene both follow donut-shaped distribution in zeolite channels. Furthermore, the adsorption performance with the effect of loadings has been studied. Benzene and thiophene show same distribution in zigzag channels: straight line distribution at low loading and S shape distribution at high loading. For binary adsorption, according to the adsorption isotherm, the amount of benzene is higher than thiophene at low pressure because of the better size match between benzene and zeolite channel. With the increasing pressure, the loading of benzene reaches a peak and then decreases, indicating that benzene molecules are extruding from the channel gradually. When benzene compete with thiophene, RDFs analysis tell us that changes are the adsorption possibility at one specific location rather than the adsorption sites. Competitive mechanism can be induced as follows: at low pressure, the size effect plays an important role, while as the pressure increases, the interaction energy dominates the process. This work provides useful macroscopic and microscopic information for a deep understanding of adsorption and separation mechanism of benzene/thiophene in MFI zeolite.

ASSOCIATED CONTENT Supporting information Detailed comparison of interaction energy (Eads-zeo, Eads-ads) over different pressures 23



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between benzene and thiophene; density distribution of thiophene under different pressures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENTS

This work was supported by the Key Subject of Petrochemical Unite Funds (U1462205), Shandong Province Natural Science Foundation (ZR201702180183), the National Nature Science Foundation of China (Grant No.21203250) and the Fundamental Research Funds for the Central Universities (17CX02068). We also acknowledge the support from the State Key Laboratory of Heavy Oil Processing (China University of Petroleum).

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