Pei Xue, Meng Zheng, Longwei Wang, Liyuan Cao, Liang Zhao, Jinsen Gao, Chunming Xu

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China

Keywords:Competitive adsorption mechanism Cyclohexene Thiophene FAU zeolite Molecular simulation Selectivity

A B S T R A C T Competition of hydrocarbon compounds with sulfides in gasoline has caused a not very high selectivity of sulfides in adsorption desulfurization so far, resulting in a reduction of catalyst lifetime as well as more sulfur oxide emissions.Tostudy the whole competitive process changing with the increase of the loading,the dynamic competition adsorption mechanism of cyclohexene and thiophene in siliceous faujasite(FAU) zeolite was analyzed by the Monte Carlo simulation. The results showed that with the increase of the loading, thiophene and cyclohexene had different performances before and after the inflection point of 40 molecule/UC. The adsorbates were distributed ideally at optimal sites during the stage that occurred before the inflection point, which is called the ‘‘optimal-displacement adsorption” stage.When approaching the inflection point,the competition became apparent and the displacement appeared accordingly,some thiophene molecules at S sites(refers to the sites inside the supercages)were displaced by cyclohexene. After the inflection point, the concentration of adsorbates at W sites (refers to the 12-membered ring connecting the supercages) was significantly reduced, whereas the adsorbates at S sites got more concentrated.The stage some cyclohexene molecules displaced by thiophene and inserted into the center of the supercage can be named as the‘‘insertion-displacement adsorption” stage, and both the adsorption behavior and the competitive relationship became localized when the adsorption amount became saturated. This shift in the competitive adsorption mechanism was due to the sharp increase of interaction energy between the adsorbates.Besides,the increase in temperature and ratio of Si/Al will allow the adsorbates, especially thiophene molecules to occupy more adsorption sites, and it is beneficial to improve the desulfurization selectivity.

1. Introduction

As environmental problems have become more serious, more and more attention is paid to gasoline desulfurization technology.The reduction of sulfur content in gasoline can not only reduce the emission of acid gas,but also reduce the damage to the catalyst[1].Traditional hydrodesulfurization(HDS)technology is an important gasoline desulfurization method in industrial applications [2,3].However,only adopting HDS technology will bring a large loss of octane number[4]. Desulfurization technologies such as oxidative desulfurization [5-8], extractive desulfurization [9-11], reactive adsorption desulfurization [12,13], and adsorption desulfurization[14,15] are constantly being widely used in recent years. Among them,adsorption desulfurization has broad development prospects due to its advantages such as mild conditions and low octane loss.So far, various types of adsorbents have been reported for deep desulfurization, and faujasite (FAU) zeolite is found to be very effective because of its unique channel structure and excellent physical and chemical properties [16-18].

The bottleneck encountered in the current adsorption desulfurization technology is its low adsorption selectivity for sulfur compounds and the core challenge of this technology is to increase the desulfurization selectivity in the presence of other hydrocarbon compounds.The main composition of sulfides in gasoline are thiophene compounds,and many studies have shown that the presence of aromatics and olefins can greatly reduce the removal efficiency of thiophene compounds. For example, Qinet al.[19]experimentally explored the effects of aromatics and olefins on thiophene adsorption in cerium containing Y zeolites prepared by liquid- (L-CeY)and solid-(S-CeY)state ion exchange from NaY and HY.They found that the coexistence of benzene and cyclohexene reduces the thiophene adsorption capacity of these zeolites. Li [20] also found the influence of olefins and aromatics on NiY and KNiY. Shiet al. [21]studied the effect of aromatics on desulfurization and found that toluene harmed thiophene adsorption over both NaY and Ce(IV)Y.Wanget al.[22]studied the effect of 1-octene on the deep desulfurization of Ce(IV)Y zeolite and found that as the 1-octene concentration increased, the thiophene adsorption selectivity decreased significantly.Hanet al.[23]also pointed out that olefin and aromatic can largely reduce the breakthrough sulfur capacity of thiophene on NiY and NiPdY.The above studies have shown that the presence of olefins and aromatics can inhibit the selectivity of thiophenes,and this is also the main reason why sulfides are difficult to remove.

Accordingly, to solve the current problem of ADS technology, a lot of effort has been put into overcoming the bottleneck,including the study of adsorption mechanisms and influencing factors. The influence of aromatics has been deeply studied,whereas the influence of olefins is still not clear. In previous studies in this group,Zheng [24] proposed the two-stage adsorption mechanism for the benzene/HY system using Metropolic Monte Carlo(MMC)simulations at loadings below and above an ‘‘inflection point”. Dang[25] discovered the same rule when studying the loading dependence of the adsorption mechanism of thiophene in FAU zeolite,and also gave the changes in the amount of adsorption at the S and W sites. For two-component adsorption, Dang [26] proposed the competitive adsorption mechanism of benzene/thiophene in FAU zeolite. Up to now, the adsorption of benzene, the adsorption of thiophene, and the competition between the two are relatively clear,but the mechanism of competition between olefins and thiophene is still little known. As for the essential reason for the competitive relationship of adsorbates, Dang [26] believed that the interaction energy is the reason for the change of adsorption mechanism. Similarly, Duanet al. [27] used Intelligent Gravimetric Analyzer(IGA)and Dynamic Sampling Mass Spectrometry to study the adsorption capacity of the adsorbent and the kinetics of multicomponent adsorption. They proposed that the adsorption selectivity and capacity of FAU adsorbents for organic sulfur compounds mainly depended on the adsorption interaction. In addition to the interaction energy, Chret al. [28] found that the polarity of the molecules would affect the adsorption selectivity.As for the exploration of external factors other than internal reasons, Dang [26]pointed out that higher thiophene concentration, adsorption temperatures, and ratios of Si/Al are beneficial to the improvement of adsorption selectivity for thiophene. In this point of view, the clarification of both the competitive adsorption mechanism and its influencing factors can not only shed light on the design of the catalyst but also promisingly contribute to the improvement of the desulfurization selectivity.

Therefore, as an olefin with a structure similar to benzene and thiophene, cyclohexene is one of the adsorbates studied in this paper. The adsorption behavior and the competitive mechanism of cyclohexene/thiophene changing with the loading were revealed by the Monte Carlo simulation at the molecular level. Also, the internal reason for the changes in behavior and mechanism was further explored. The transformation of the mechanism applies to the loading range from infinite dilution to saturated adsorption capacity. Besides, the influence of temperature on the concentration of different adsorption sites and adsorption selectivity was also well studied.

2. Models and Methods

2.1. Models

In this paper,one-unit-cell HY zeolite models were constructed according to the International Zeolite Association database of zeolite structures. The chemical composition of the models is Si192-xAlxHxO384, whenxis 0, 28, and 56, it represents 0Al, 28Al,and 56Al,respectively.The structures of zeolite models and adsorbate models were displayed in Fig. 1. As shown in Fig. 1, the eight supercages(SCs)with a diameter of around 1.25 nm were connected by 12-membered-ring windows whose diameter was 0.74 nm.Moreover,because the flexibility of the zeolite framework had little effect on static properties,the framework of the model was considered to be rigid.One cubic unit cell was applied in this paper attributed to simulation calculations not significantly affected by the model size.More details on building HY zeolite models can refer to our previous work[24-26].

Fig.1. (a)The FAU zeolite models of 0Al,28Al,56Al.Green dots are the areas of blocked solidate cages.The lines in red and yellow are the framework of zeolite.Green lines are the lattice borderline. (b) The adsorbate models of cyclohexene (left) and thiophene (right).

2.2. Simulation details

In this work, the Monte Carlo (MC) simulation was performed by a commercial software Materials studio.The simulation scheme was summarized as follows.

Firstly,HY zeolite models,cyclohexene,and thiophene molecule were optimized in the COMPASS force field. The COMPASS force field was applied in entirely simulations because it could accurately predict the small molecules diffusion and adsorption in different materials included HY zeolites [29]. Secondly, the MC method,based on its unique advantage,was adopted to study the adsorption properties.There were 4×107equilibration and 4×107production steps included in each MC simulation[26],the steps set in the software refered to Zhang’s work[30],who adopted 106steps for equilibration and production.And after a lot of attempts,we found that the set of 107steps is a better choice for the convergence and the final stable state, which is the same setting as in Zheng’ paper[31]. The cutoff distance for van der Waals potential energy was set to be 1.2 nm to avoid adsorbate-adsorbate interactions.Thirdly,in the GCMC simulation, the probability of molecular rotation is 20%,the probability of molecular translation is 20%,the probability of molecular regrowth is 15%, and the probability of molecular exchange is 25%. Besides, Ewald summation can be used for calculating electrostatic interactions and its accuracy was 0.001 kcal·mol-1(1 kcal=4.186 kJ).The canonical ensemble(Abbreviated as NVT, means that it has a certain number of particles (N),volume(V),and temperature(T)),and periodic boundary conditions were used throughout the simulations.

The used calculation equations in this paper are listed as follows. The adsorption heat can be used to measure the strength of the adsorption function of the adsorbent,the larger the adsorption heat, the stronger the adsorption capacity.

Here,Eintralis the intramolecular energy of adsorbates,Eadis the sum of the interaction energy,including the interaction energy between the adsorbate molecules (Eads-ads) and between the adsorbate molecules and the framework of zeolite (Eads-zeo).Nadis the total loading of the adsorbates.

Radial distribution functions(RDFs)can reveal the relative locations of molecules by calculating the possibility of the presence of other particles around a particular particle.RDFs are referred to asg(r), its calculation formula is as follows.

The value ofg(r) is the possibility of emergence for molecules inside the supercage and ther represents the distance among speciesiandj,Vis the volume of the system, ΔNij（r，r+Δr） is the ensemble-averaged number of the speciesjaroundiwithin a shell of Δr,NiandNjare the numbers ofiandjspecies.

For adsorption desulfurization performance,the desulfurization selectivity can be used to indicate. Specifically, the selectivity of thiophene sulfur can be calculated by

whereqiandpiare the loadings and the partial pressure of speciesi.

3. Results and Discussion

3.1. Adsorption isotherms

To understand the influence of the presence of cyclohexene on thiophene,we first need to know the competitive relationship and adsorption behavior when two adsorbates coexist. As a tool for understanding the adsorption behavior of adsorbates in microporous zeolites,adsorption isotherms can be used to verify the accuracy of model establishment and parameter selection, at the same time, it can also reflect the degree of competition between adsorbates intuitively. Taking all-silicon zeolite as an example, the experimental and simulated adsorption isotherms of thiophene at 363 K and 393 K are showed in Fig. 2(a). The simulation results are in good agreement with the experimental values[32],verifying the correctness of the model and parameters. Fig. 2(b) shows the adsorption isotherms of pure adsorbates and the adsorbates in mixtures in the 0Al model at 300 K. With the increase of pressure,the adsorption isotherms all suddenly increased sharply and then gradually reached saturation.This is the same trend as the adsorptive isotherm of thiophene on all-silica Y zeolite studied by Ju[33].Moreover,the adsorption isotherms all showed an inflection point,the change in curvature increased first and then decreased to zero, which should be related to the change of the adsorption mechanism.

Furthermore, when the temperature was kept the same (at 300 K), thiophene reached its saturated adsorption with less than 1 mmol·g-1, which is much lower than the adsorption of 4.47 mmol·g-1of the pure component, and even lower than the adsorption amount of 3.29 mmol·g-1of cyclohexene in the mixture.Similarly,the amount of cyclohexene adsorbed in the mixture is lower than its saturation when adsorbed alone.Therefore,when thiophene and cyclohexene coexisted in the zeolite,the adsorption amount of thiophene decreased significantly, but the adsorption amount of cyclohexene did not decrease much. This competitive relationship between cyclohexene and thiophene was also discovered in Shi’s research[34].From this,we can speculate that cyclohexene is more competitive,but there are still some similarities in the trend of the adsorption curve of cyclohexene and thiophene.However, we still know very little about the adsorption behavior and competitive relationship between cyclohexene and thiophene,which requires further investigation through density distribution.

3.2. Density distribution

The specific adsorption behavior of the adsorbates can be reflected in the concentration of the adsorbates that changes with the loading, and the concentration profile is a good way to reflect the microscopic distribution behavior and the amount of adsorption at different positions. Fig. 3 depicts concentration profiles of the center of mass of adsorbates in their mixtures (the molar ratio is 1:1) along [1 0 0] in the 0Al model separately.According to Fig. 3, the shape of the curves of cyclohexene and thiophene are not the same from 4 to 48 molecule/UC.The relative concentration curves of the two adsorbates below 40 molecule/UC appear to be relatively gentle, but there is a certain difference in the shape of the curve at the same position. This means that the distribution of adsorbates is more dispersed, and the adsorption behavior of cyclohexene and thiophene on the same adsorption sites are not completely the same. However, the adsorption of cyclohexene and thiophene showed new peaks when the loading was above 40 molecule/UC. These new peaks represented new sites where adsorption was concentrated, and the new peaks of cyclohexene erewere different from that of thiophene.At the same time, the shape of the concentration curve above 40 molecule/UC was also completely different from that below 40 molecule/UC.Changes in the shape of the curve may indicate changes in the adsorption behavior and competitive relationship of the adsorbates. The above discussion shows that 40 molecule/UC is likely to be the inflection point of the adsorption behavior.Such a transformation can be further confirmed in the following contour map of density.

Fig. 2. (a) Adsorption isotherms comparison: simulation results and experimental studies for pure thiophene at 363 K and 393 K and (b) adsorption isotherms of pure components and cyclohexene and thiophene in mixtures at 300 K.

Fig. 3. Concentration profile of cyclohexene and thiophene for cyclohexene/thiophene along [1 0 0] of 0Al model at various loadings through MC simulations.

Fig.4 presents the contour maps of density of cyclohexene/thiophene system (the molar ratio is 1:1) in the 0Al model at 300 K at loadings ranging from 4 to 48 molecule/UC. The 0 Al model and temperature of 300 K were selected because 300 K is the temperature that can be achieved in actual production, and a higher Si/Al ratio of zeolite is closer to the ideal adsorption state which can maintain good crystallinity and high product yield [35]. The distribution area of thiophene and cyclohexene did not completely overlap below 40 molecule/UC,and the distribution of cyclohexene seemed to be wider than that of thiophene.At this stage,the adsorbates can be ideally dispersed and adsorbed on different adsorption sites because of sufficient adsorption space and sites. It can be inferred that when the adsorption loading is less than 40 molecule/UC, the competition between adsorbates is relatively weak.But when approaching 40 molecule/UC, the distribution boundary of the two adsorbates no longer became obvious, the competition relationship was gradually strengthened due to the limited adsorption sites and the increasing number of adsorbate molecules.However, after 40 molecule/UC, the density distribution of adsorbates began to become narrow and localized as the loading gradually became saturated. This is consistent with the conclusion obtained in Fig. 3. Fitch [36] used Powder Neutron Diffraction to study the distribution of benzene in NaY, it was also found that benzene was evenly distributed under low loading, but aggregated under high loading, which is the same as what we found. The contour maps of density of cyclohexene and thiophene in the mixtures are given in Supplementary Material Fig. S1 and S2 as supporting information to confirm that 40 molecule/UC is the inflection point of the adsorption behavior and competitive relationship of the adsorbates,and the colored parts,W sites(red)and S sites(green),are another way to represent different adsorption sites.

From the change in density distribution, we can find that the inflection point of competitive adsorption in the 0Al model at 300 K is 40 molecule/UC. And we can get the general change process of the concentration with the increase of the loading, that is,the process from dispersed to concentrated distribution.However,the change of adsorption mechanism must be related to the change of concentration at different adsorption sites, and the specific adsorption sites of adsorbates need to be obtained by radial distribution function as follows.

3.3. Radial distribution functions (RDFs)

For the Y-type zeolite, the adsorption sites mainly include two types: W site (a twelve-membered ring window connected to the supercages) and S site (other adsorption sites besides W site)[24-26,37-40],where S site has a more obvious energy advantage.In previous studies, Dang [26] studied the competitive adsorption of benzene and thiophene in FAU zeolite,and found that the mechanism followed ‘‘ideal-displacement adsorption” to ‘‘insertiondisplacement adsorption”. When approaching the inflection point of the adsorption mechanism, the newly adsorbed thiophene on the S sites would be displaced by benzene and migrated to the W sites.This shows that the concentration of the adsorbates at different adsorption sites will vary with the increase of loading.Considering the similarity of molecular structure, cyclohexene/thiophene may follow the same competitive adsorption mechanism proposed by Dang. To figure out the distribution of adsorbates at specific adsorption sites, radial distribution functions(RDFs) were analyzed to reflect the relative locations of molecules and the probability of emergence.

Fig.4. Contour maps of density of cyclohexene/thiophene system in the 0Al model at 300 K.The number in each figure is the total loading,molecule/UC.Grey lines represent the framework of zeolites. Red dots are the area of adsorbed cyclohexene distributions, and green dots are the region of adsorbed thiophene distributions.

As shown in Fig.5,the RDFs of comcyc-comcyc and comS-comS for cyclohexene/thiophene system (the molar ratio is 1:1) were calculated at 300 K. Andrin Fig. 5 represents the mutual distance between particles (comcyc refers to the mass center of cyclohexene, and comS refers to the mass center of the sulfur atom), whileg(r) stands for the degree of the order for adsorbate molecules inside the supercages.When the total loading was below 40 molecule/UC, there was only one main peak at around 0.58 nm. This manifests that the distance among cyclohexene and thiophene was identical and the adsorbates were relatively evenly distributed. When the loading gradually increased to the inflection point, the value ofg(r) showed a downward trend, indicating that the order of distribution was reduced. This is mutually confirmed with Fig. 4 above, the adsorbate molecules are ideally distributed on the sites that can be preferentially adsorbed and the distribution is gradually dispersed. This adsorption process is similar to the ideal adsorption stage of thiophene single component, thiophene and benzene two components in FAU zeolite. For loadings above 40 molecule/UC,g(r) gradually went up with the loading approached to saturation, which indicated that the distribution order of cyclohexene and thiophene was increasing. This can also be seen from Fig. 3 above that the distribution of cyclohexene and thiophene became more localized.

Noticeably,shoulder peaks were observed at around 0.76 nm in Fig.5(a)and around 0.39 nm in Fig.5(b)respectively.These shoulder peaks appeared because the original S and W sites had been filled, and the new molecules could adsorb on the new sites with higher energy. The adsorption energy of different sites has been studied in Zheng’s[41]research on the stability of adsorption sites in HY zeolite and the effect of framework protons on adsorption energy. When the loading was less than 40 molecule·UC-1, the adsorbates preferentially adsorbed at S and W sites with lower energy, which represented the position of the main peak in Fig. 5, and as the adsorbates increased, the distribution was not concentrated anymore, resulting in the reduction of order degree.However, after the inflection point, the adsorption sites with low energy in the supercage were occupied. The appearance of new peaks means that the new adsorbates can only be adsorbed at the sites near the center of the supercage with higher energy,and the adsorption position at this stage is more fixed and limited.This conclusion is the same as the localization in the contour maps of density. Such new peaks can also be seen in Supplementary Material Fig. S3, the change trends in comthi-comcyc and comthi-cmthi(comthi refers to the mass center of thiophene)also confirm that the insertion of new molecules into the center of the supercage leads to an increase in the distance between the adsorbate molecules.The relative position and density of molecules can be understood by the radial distribution function. However, the density difference at different sites must be shown from the cross-sectional view.

Fig. 5. RDFs of (a) comcyc-comcyc and (b) comS-comS for 0Al at 300 K in cyclohexene/thiophene system at loadings ranging from 4 to 48 molecule/UC, 1 ?=0.1 nm.

Fig. 6. Density contour planes of thiophene in cyclohexene/thiophene system passing through the center of supercage in the 0Al model at 300 K.Numbers in each figure are the total loading, molecule/UC. Regions in colors represent the density contour planes.

Fig. 7. Density contour planes of cyclohexene in cyclohexene/thiophene system passing through the center of supercage in the 0Al model at 300 K.Numbers in each figure are the total loading, molecule/UC. Regions in colors represent the density contour planes.

To further explain the changes in the adsorbate behavior at the S and W sites, a cross-section through the center of the supercage was selected to observe the changes in the adsorbate concentration at different sites. As shown in Figs. 6 and 7, the distribution area resembling the letter‘‘D”is inside the cavity of the supercage,and the intersection area connecting the letter ‘‘D” is the twelvemembered ring connecting adjacent supercages. In this way, the S and W sites can be distinguished.When the loading was less than 40 molecule/UC, the utilization rates of W and S sites increased with the increase of loading,but the distribution density of adsorbates on S sites was higher. The distribution range of cyclohexene and thiophene was relatively wide at low loadings, but the distribution of cyclohexene at S sites was more dispersed, while thiophene was more concentrated. Meanwhile, at the W sites, the concentration of cyclohexene increased more than thiophene.Overall, cyclohexene occupied more adsorption sites than thiophene and was more competitive at low loadings. When the total loading was close to 40 molecule/UC, the adsorption amount of cyclohexene at S sites increased, in contrast to the decrease in the concentration of thiophene. The concentration of the two adsorbates at W sites did not decrease yet. Danget al.[26]explained the concentration change of S sites with the mechanism of displacement and migration. Eneet al. [42] also discovered the alternating adsorption process of benzene and oxygen in Cu/HZSM5 zeolite, that is, benzene can be displaced by oxygen. Sun[43] studied the adsorption of methane and carbon dioxide and found that the residual adsorbed CH4can be replaced by CO2,and the displacement efficiency is related to the bulk pressure.Similar displacement mechanisms have also been discovered in other studies [44,45]. Therefore, it can be considered that the reduced thiophene at S sites may be displaced by newly adsorbed cyclohexene molecules,this again shows that cyclohexene is more competitive below the inflection point. The distribution of adsorbates at low loadings was ideal and uniform, and the competition was relatively weak,but once the displacement process began,the competition for adsorbates became apparent.

However, after 40 molecule/UC, the concentration showed differently from that below the inflection point. The distribution of cyclohexene and thiophene are more concentrated and localized,this can also be obtained from the contour maps of density of cyclohexene and thiophene in Supplementary Material Fig. S1 and S2. As the loading approached saturation, although the adsorbate concentration at W sites continued to decrease,there was still more thiophene at W sites than cyclohexene.The comcyc-12Cand comthi-12Cin Supplementary Material Fig. S4 also give such a trend(where 12Crefers to the center of the twelve-membered ring window). Judging from the color of the regions, the concentration at S sites was increasing, but the areas where cyclohexene and thiophene were concentrated did not completely overlap. Moreover, the amount of thiophene at some S sites that originally adsorbed cyclohexene increased, compared with the decrease in cyclohexene concentration. At this stage, the adsorption amount of thiophene was higher than that of cyclohexene, which was also reflected in the adsorption process in Fig. 2(b) from 0.5 to 8 kPa,indicating that the competitiveness of thiophene has been significantly improved after the inflection point. Since the total adsorption sites are limited, an increase in the adsorption amount at some sites will inevitably cause displacement and competition.However,the overlapping area of the adsorbate distribution above the inflection point also became smaller, indicating that competition only occurred at some sites, and the competition relationship has become localized.

The cross-sectional view of the density distribution can clearly show the adsorbate distribution at the S and W sites, and the distribution above the inflection point needs to be further elaborated by the RDFs of comcyc-center and comthi-center(centerSCrefers to the center of the supercage) in Fig. 8. Below the inflection point,both comcyc-centerSCand comS-centerSChad only one main peak at 0.37nm and 0.42nm, respectively. This means that the adsorption sites of cyclohexene and thiophene did not overlap, and each had its ideal adsorption sites. The preferential adsorption sites of cyclohexene and thiophene are both around 0.4nm from the center of the supercage, rather than the center of the supercage, which requires higher adsorption energy. When approaching the inflection point, it can be seen from the above that the displacement of adsorbates would occur, with the competition becoming more obvious. The cyclohexene molecules displaced some of the thiophene molecules at S sites, so there were little newly adsorbed thiophene molecules. This stage can be labeled as ‘‘optimal-dis placement” adsorption below 40 molecule/UC, the adsorbates can be distributed on the optimal W and S sites.When the loading continued to increase above the inflection point, the positions of the main peaks in both figures shifted to the right by about 0.02 nm.It shows that the supercage became more crowded after the entry of new molecules so that the previously adsorbed molecules moved closer to the framework of zeolite, but it did not affect the original main adsorption position.However,when the adsorption amount reached saturation,cyclohexene showed a small peak at around 0.33 nm from the center of the supercage.It can be found from Figs. 6 and 7 that the displacement process also occurred when the loading was close to saturation. But the displacement process of the adsorbates was opposite to that below the inflection point. Thiophene was more competitive under high loadings, so some cyclohexene molecules adsorbed at S sites were displaced by thiophene and inserted into the unfavorable sites closer to the center of the supercage. The stage above the inflection point can be defined as ‘‘insertion-displacement” adsorption.

Fig. 8. RDFs of (a) comcyc-centerSC and (b) comS-centerSC for cyclohexene/thiophene at 300 K in the 0Al model at loadings ranging from 4 to 48 molecule/UC.

So far, it can be found that due to the similar molecular structure, the competitive adsorption mechanism of cyclohexene and thiophene is almost the same as that of benzene and thiophene,and it is consistent with the adsorption process of the thiophene/FAU system with the increasing loading. From Figs. 4, 6 and 7,and the above discussion, it can be concluded that as the loading increases,the degree of competition changes from weak to strong,and the competition area ranges from wide to the final localized competition. At the stage of ‘‘optimal-displacement” adsorption,the adsorbates were ideally dispersed on the optimal adsorption sites. When approaching the inflection point, the thiophene molecules that continued to be adsorbed at S sites were displaced by cyclohexene, and the growth rate of the thiophene adsorption amount decreased. Cyclohexene occupied more S and W sites and was more competitive; At the stage of ‘‘insertion-dis placement” adsorption, the utilization rate of the W sites decreased, while the localized distribution of the S sites increased significantly. Thiophene became more competitive and occupied more sites. Some of the cyclohexene molecules which were originally adsorbed on the S sites were displaced by thiophene and inserted near the center of the supercage.The adsorption behavior and competition relationship between thiophene and cyclohexene had become narrow and localized when the loading approached saturation. Fig. 9 shows the adsorption process of cyclohexene and thiophene at full loadings, including two main stages and the final distribution state.

The above cross-sectional density map and radial distribution function can show the distribution of adsorbate at different sites,and at the same time intuitively reflect the degree of utilization of the center of the supercage and the twelve-member ring. It can be seen that there were always more molecules adsorbed on the S site than the W site. And with the loading changed from low to high,the adsorbate at the S sites continued to cluster while the adsorbate at the W sites showed a trend of increasing first and then decreasing.There is almost no obvious competition under low loadings,but the competition between cyclohexene and thiophene near the inflection point becomes tense, and after the inflection point, the adsorption behavior changes from wide-area to localized. All these external behaviors are related to the internal dynamics of the interaction energy.

3.4. Interaction energy

According to the mechanism and adsorption process proposed above,the essential cause of the transformation needs to be further explained by the interaction between different adsorbates and between the adsorbate and the zeolite. According to Fig. 10(a),Eads-zeowas always larger thanEads-ads(Eads-zeorefers to the interaction energy between adsorbates and zeolite, while Eads-ads refers to the interaction energy between different adsorbates).Overall,Eads-zeodeclined slowly whileEads-adscontinued to increase,and the decrease ofEads-zeowas far less than the increase ofEads-ads. Especially near the inflection point,Eads-zeoremained stable, then the change of adsorption behavior was mainly due to the sharp rise of the interaction energy between the adsorbates.Also, Fig. 10(b) shows the interaction energy between different adsorbates and zeolite.Since the molecular polarity of cyclohexene was stronger, the interaction energy with zeolite was higher than that of thiophene at loadings ranging from 4 to 48 molecule/UC,which was why the adsorption capacity of cyclohexene was greater than that of thiophene. Yan [46] used molecular simulations to investigate the adsorption of toxic gas by all-silica zeolite, compared the adsorption energy of different adsorbates,and found that the adsorption energy was positively proportional to the adsorption capacity,which was the same as our finding.When approaching the inflection point, the interaction energy between cyclohexene and thiophene and zeolite fluctuated greatly, and the competition was most obvious at this time. However, after the inflection point,the interaction energy of thiophene and zeolite showed an increasing trend, and the competitive advantage of thiophene gradually appeared. Besides, as can be seen from isosteric heat of adsorption in Supplementary Material Fig. S5, cyclohexene owned a higher ability to adsorb because of its higher adsorption heat, which also partly proved the competitive advantage of cyclohexene.

3.5. Effect of temperature on competitive adsorption

From the perspective of experimental research,the temperature has a certain effect on the competitive adsorption process. What kind of changes will the temperature increase bring to the adsorption behavior of the adsorbate can be seen from the RDFs diagram below. Fig. 11 shows the RDFs of comcyc-center and comthicenterat 300 K, 500 K, and 800 K in the 0Al model. It can be seen from the figure that as the temperature increased, theg(r) value of the main peak decreased, the position of the main peak shifted to the left, and the main peak became wider. This shows that the distribution of adsorbates was more dispersed and closer to the center of the supercage.The increase in temperature moved adsorbate molecules more active, and the adsorption sites became less concentrated. Similarly, comcyc-12Cand comthi-12Cat different temperatures are shown in Supplementary Material Fig. S6, and both cyclohexene and thiophene’s utilization of the twelvemembered ring increased with increasing temperature. From the above conclusions, it can be found that with the increase of the temperature, the distribution range of the adsorbates became wider, cyclohexene and thiophene would occupy more S and W sites. Santikary [37] also found that as the temperature increased,the distribution of adsorbate would become delocalized.Similarly,Bakhtiari[47]found that an increase in temperature would lead to an increase in the mass transfer area. Both of these are consistent with our conclusions. Fig. S7 and S8 in Supplementary Material also saw the trend of the inflection point moving backward,meaning that the localization process was shortened. As more adsorption sites are utilized, the change in the selectivity of adsorption desulfurization will be further explored.

Fig. 9. The whole competitive adsorption process for cyclohexene/thiophene in the 0Al model.

In this paper, the selectivity of sulfide can be used to measure the desulfurization effect of the adsorbent and the calculation method of selectivity has been introduced above. The selectivity for thiophene (S) at 300 K, 500 K, and 800 K is shown in Fig. 12.Selectivity fluctuated at minimal pressure owing to the selection process for the adsorbent,and as the pressure gradually increased,it remained stable. The adsorption selectivity reached the highest at around 0.48 at 800 K, meanwhile, the selectivity reached 0.26 and 0.29 at 300 K and 500 K, respectively. This shows that the higher the temperature,the better the selectivity,but the influence on the selectivity is opposite to the capacity. It can be seen from the adsorption isotherms at different temperatures in Supplementary Material Fig.S9 that as the temperature increased,the adsorption capacities of thiophene and cyclohexene both decreased significantly. This discovery was also explained in Rahmati’s [48]research. The thermal motion of the molecules strengthened as temperature increased, thus more adsorbate molecules left the zeolite surface. The increase in temperature means more adsorption sites and higher selectivity but leads to a smaller capacity.Although it is difficult to achieve a high temperature in actual operation, it is indeed conducive to the improvement of desulfurization selectivity.

Fig. 10. (a) Interaction energy inside the supercage for cyclohexene/thiophene system and (b) interaction energy between cyclohexene and zeolite and between thiophene and zeolite in the 0Al model at loadings ranging from 4 to 48 molecule/UC (1 cal =4.186J)

Fig. 11. RDFs of comcyc-center and comthi-center in cyclohexene/thiophene system at 300, 500 and 800 K in the 0Al model (1?=0.1nm)

Fig.12. Selectivity for thiophene in cyclohexene/thiophene system at 300 K,500 K,and 800 K in the 0Al model.

3.6. Effect of ratios of Si/Al on competitive adsorption

In addition to the temperature,the ratio of silicon to aluminum of the adsorbent also has a certain influence on the adsorption process. The adsorbents with relatively high aluminum content are more common,while the ideal zeolite hardly exists in real production. Fig. 13 exhibits the influence of the ratio of Si/Al on the utilization of the supercage center. With the decrease of the siliconto-aluminum ratio, the main peak positions of cyc-center and thi-center are little affected. Supplementary Material Fig. S10 showed the utilization of twelve-membered ring, the possibility of thiophene being close to the W sites increases with the increase of Si/Al, indicating that more thiophene can use the twelvemembered ring. However, it is obvious that the utilization rate of cyclohexene for W sites is higher, reaching the highest in 28Al,but it is basically the same in 0Al and 56Al zeolite. From this, we can infer a higher ratio of Si/Al is beneficial to the utilization of more adsorption sites for thiophene. This conclusion is also confirmed in the Supplementary material Fig.S11 and S12.The distribution of cyclohexene and thiophene in 28Al and 56Al zeolite tended to be localized when the loading was above 36 molecule·UC-1, compared with 40 molecule·UC-1in 0Al. Interestingly, in the above discussion, the increase in temperature shifted the inflection point to high loading,which shows the same effect as the increase in the ratio of Si/Al, bringing out the result that the number of adsorbable sites for thiophene increased,and the selectivity was thus improved.

Fig. 13. RDFs of comcyc-center and comthi-center in cyclohexene/thiophene system in the 0Al, 28 Al, and 56 Al model at 300 K. (1?=0.1nm)

4. Conclusions

As a promising desulfurization technology, the desulfurization selectivity has always been a major problem that plagues adsorption desulfurization technology. From the research in this paper,the existence of cyclohexene did have a clear competitive relationship with thiophene and a two-stage adsorption mechanism with the inflection point of 40 molecule/UC for cyclohexene/thiophene system was proposed by MC simulations. At the first stage of‘‘optimal-displacement” adsorption, the adsorbates were dispersedly adsorbed at the optimal S and W sites. The competitive relationship at this stage was relatively weak because there were enough adsorption sites. But when approaching the inflection point,the competition became fierce and there was a phenomenon of displacement. Part of the thiophene molecules adsorbed on S sites were displaced by cyclohexene,and thiophene was more concentrated on a few S sites.Cyclohexene,therefore,occupied more S and W sites and was more competitive at this stage. Above the inflection point, the adsorption amount of adsorbates at the W sites continued to decline, but the adsorption at the S sites was more concentrated and localized, the concentration of adsorbates at the S sites was higher. At this stage, some cyclohexene molecules were displaced by thiophene and inserted to a position closer to the center of the supercage and thiophene seemed to be more competitive after the inflection point. This stage was labeled the‘‘insertion-displacement adsorption”. When the adsorption amount was close to saturation, the range of adsorption sites for cyclohexene and thiophene competition was also narrowing. The transformation of the mechanism in the dynamic adsorption process was due to the change in the interaction energy, especially the interaction energy between adsorbates and the isosteric heat of two adsorbates. In addition to the above studies, the effect of temperature on the competitive adsorption process was also investigated.The adsorption inflection point would move backward due to the increase of temperature and the increase of Si/Al,the distribution of adsorbates would be more dispersed,the utilization rate of S and W sites would be enhanced due to the increased temperature, but higher Si/Al only leads to higher utilization of W sites,and has little effect on S sites. The high temperature would bring improved desulfurization selectivity, but it would also reduce the adsorption amount due to the intensification of molecular motion.In short, higher temperatures and higher Si/Al may be more conducive to the adsorption selectivity of thiophene.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the support from the National Natural Science Foundation of China (21822810, 21838011)and the National Key Research and Development Program(2018YFC1902603).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.12.024.