Influence of Framework Protons on the Adsorption Sites of the

Aug 18, 2014 - For HY zeolite models containing different numbers of protons with the same ratio of H1:H2, the amount of the most stable adsorption si...
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Influence of Framework Protons on the Adsorption Sites of the Benzene/HY System Huimin Zheng, Liang Zhao,* Qing Yang, Jinsen Gao, Baojian Shen, and Chunming Xu State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing, 102249 China S Supporting Information *

ABSTRACT: Monte Carlo (MC) simulations were performed to study the influence of framework protons on the adsorption sites of the benzene molecule in HY zeolite with different Si:Al ratios. Eleven types of adsorption sites were observed including five reported sites (H1, H2, U4, U4(H1), and W) and six newfound sites (W(2H1), U4(2H1), H1(H2), U4(H1,H1), H1(H2,H1), and U4(H1,H1,H1)), which were “supersites” with more than one proton. The stability order of the sites found in the 28Al model can be expressed as U4(H1,H1,H1) > U4(H1) > H1(H2,H1) > W(2H1) > U4(H1,H1) > H1(H2) > H1 > H2 > U4 > U4(2H1) > W. Increasing number of zeolite protons resulted in an increasing proportion of supersites, which enhanced adsorption energies of sites. For HY zeolite models containing different numbers of protons with the same ratio of H1:H2, the amount of the most stable adsorption sites containing H1 proton increased, while the amount of the most stable adsorption sites containing H2 decreased, with increasing number of protons. adsorption energy of 2H1 varies between −56.7 and −44.1 kJ/ mol, which is close to the adsorption energy of the H1 site (varying between −58.8 and −45.3 kJ/mol). Jirapongphan et al.13 performed a supercage-based docking simulation, using the Monte Carlo minimization algorithm19 coupled with the COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field model to provide further insight into the adsorption sites of benzene in zeolite. Seven adsorption sites of benzene in HY were detected at the loading of 4 molecules/UC, including four adsorption sites reported by Jousse.12 They observed that the proton on the rings adjacent to the U4 site (adsorption of benzene on the unprotonated 4-T ring) would result in a new adsorption site (named U4(H1)). The tested adsorption energy of U4(H1) is approximately 4 kJ/mol larger than that of U4. Therefore, it is found that the number and position of zeolite protons may to some extent have an influence on the adsorption energy of sites for the benzene/HY system, which has never been fully discussed yet. This article further explores the adsorption sites of benzene on HY zeolite by force field based Monte Carlo (MC) simulations, with a focus on evaluating the influence of the framework protons on the stability and distribution of adsorption sites.

1. INTRODUCTION Zeolites play an important role in the oil refining process and the petrochemical industry due to their high surface area, high hydrothermal stability, and appropriate acidity.1 Zeolite Y is the main component of commercial fluid catalytic cracking (FCC) catalysts.2 It is known that adsorption of reactants on adsorption sites is the very first step for all catalytic reactions.3 Meanwhile, the distribution of protons in HY zeolite is related to the density and strength of acidity, which largely affects the subsequent reactions and product distribution. A great number of experiments4−8 and simulations9−11 have been performed to study adsorption sites of all kinds of zeolites. More information such as preferred adsorption sites and adsorbate configurations, as well as kinetic and thermodynamic parameters of the adsorption sites, were obtained.12,13 For example, the neutron diffraction,14 IR spectra,15 and NMR16 studies have revealed two types of adsorption sites for the benzene/NaY system at low temperatures. One energetically favorable site is facially coordinated to the Na+ ions, and the other one is centered in the 12-membered (12-T) ring (W site). Su et al.17 investigated the proton sites for the benzene/HY system by IR spectra, and found two types of proton sites, H1 and H2 (H1 and H2 are protons connected with O1 and O2 atoms in the HY zeolite framework, respectively), at very low coverage. Later, Vitale et al.7 confirmed the H2 site, and also found the W site in the benzene/HY system by neutron diffraction. Simulations could give more detailed information about adsorption sites. Since the HY−benzene complex is of physisorbed type, it is considered that a force field based on computational techniques is suitable for studying the adsorption of the benzene/HY system. Jousse et al.12 used the molecular docking procedure described in ref 18 to study the location of benzene in HY zeolite (Si:Al ratio = 2.43) at a loading of 1 molecule/unit cell (UC). They reported three experimentally proven H1, H2, and W sites and the newly observed 2H1 site. The 2H1 site is a “supersite” created by adsorption of benzene on two adjacent H1 protons. The © 2014 American Chemical Society

2. MODELS AND METHODS The 1 × 1 × 1 unit cell of Y zeolite was constructed according to the IZA (International Zeolite Association) Database of zeolite structures, and then a certain amount of Si atoms were randomly replaced with Al atoms in order to get HY zeolite models with considered Si:Al ratios. Then protons with corresponding amounts can be introduced. In this study, the Received: Revised: Accepted: Published: 13610

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chemical composition of the HY zeolite Si192−xAlxHxO384 was considered with x = 58, 28, and 14, namely 56Al, 28Al, and 14Al, respectively. Two rules govern the Si and Al distributions: Löwenstein’s rule,20 avoiding Al−O−Al linkages, and Dempsey’s rule,21 requiring maximum separation of the Al atoms for a given Si:Al ratio. After substitution, the resulting negative charges were compensated by protons (named Hz). Figure 1

Table 1. Detailed Information of HY Models

a

model

Si:Al ratio

chem composn

H1a

H2a

H3a

56Al 28Al 14Al

2.43 5.86 12.71

H56Al56Si136O384 H28Al28Si164O384 H14Al14Si178O384

30 15 7

10 5 3

16 8 4

Number of Hz atoms.

blocked artificially by field segregation, as shown in Figure 2. The framework was considered to be rigid during the simulations because adsorption involving low energy equilibrium configurations and framework flexibility has a marginal effect on the adsorption at infinite dilute loading. The commercial software Materials Studio 5.5 from Accelrys, Inc., was used for geometry optimization and MC simulations. Prior to energy related calculations, the COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field was assigned to all particles in order to define the interactions between the atoms.24,25 The Smart Minimizer Method provided by Materials Studio software, which is a combination of the steepest descent, adjusted basis set Newton−Raphson, and quasi-Newton methods in a cascade, was used for geometry optimization.26 Here is a brief summary of the simulation scheme. First, the structures of benzene molecules and the 14Al, 28Al, and 56Al models were optimized. Then MC simulations in the canonical ensemble were carried out for the three models to obtain the interaction energies and locate the adsorption sites of benzene at the loading of 1 molecule/UC at 300 K in the models. The equilibration MC steps are 5 × 106 followed by another 5 × 106 MC steps for production.27 In this study, one MC step is defined as the attempt to move each of the adsorbates, once. A typical mix of MC moves included 50% regrowth, 25% translation, and 25% rotation with no creation and deletion step for fixed loading simulations. As a check on convergence, several points of our simulations were recomputed with 50 × 106 equilibration MC steps and 50 × 106 MC production steps. The variance of the energy of adsorption was recorded, and this was typically found to be less than 2% of the adsorption energy. The electrostatic potential energy was calculated by the Ewald summation method, with a calculation accuracy of 4.184 J/mol. The partial charges placed on the atoms of benzene and HY zeolite models was taken from ref 12, which can be found in Supporting Information (Table S1). The cutoff distance for the calculation of the van der Waals (vdW) potential energy is 1.2 nm. Periodic boundary conditions were applied in three coordinate directions to form an infinite sorbent structure without open surfaces.28 For the benzene/HY system, both electrostatic (Eelc) and vdW interactions (Evan) are referred to collectively as the interaction energy of adsorption (Ead):29

Figure 1. Framework structure of zeolite Y and a cut of the framework.

shows the HY zeolite framework and a cut of the framework with the position of four kinds of Hz. Among them, H1, H2, and H3 have been detected22 with a reasonable proportion as H1:H2:H3 ∼ 29:10:15;8,23 only H1 and H2 protons were accessible for benzene. For the same kind of Hz atoms, a relatively even distribution of the same kind of Hz atoms was believed to be representative and typical. It is worth mentioning that, after the determination of the number and position of Al atoms, the percentages of H1, H2, and H3 atoms, as well as a relatively even distribution for the same kind of Hz atoms, the local charges of the frameworks can still be different based on the relative distribution of different kinds of Hz atoms. For example, for the 14Al model with seven H1 atoms and three H2 atoms in the framework, the relative positions of H1 and H2 atoms can be numerous. However, based on the experimental studies, there are no specific rules for the distribution of Hz atoms. In order to avoid an unrealistic imposing of the positions of protons, which may easily reflect the creation and stability of the sites, the distances between different kinds of Hz atoms were maximized in our simulations. Detailed information on HY models is illustrated in Figure 2 and Table 1. The procedure used in this study allows the underlying field to be segregated into many subfields and MC calculations could be carried out for restricted pores in the zeolite. In our simulations, the sodalite cages of HY zeolite framework, which are inaccessible for benzene molecules, were

Figure 2. Models of 14Al, 28Al, and 56Al zeolites with blocked sodalite cages. 13611

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Table 2. Benzene Adsorption Sites in HY Zeolite group

site

Si:Ala

A A A A A B B B B B B

H1 H2 U4 U4(H1) W W(2H1) U4(2H1) H1(H2) U4(H1,H1) H1(H2,H1) U4(H1,H1,H1)

allb allb allb allb allb 28, 56 28, 56 14,28 28 56 56

description benzene benzene benzene benzene benzene benzene benzene benzene benzene benzene benzene

facially coordinated to one H1 proton facially coordinated to one H2 proton located over unprotonated 4-T ring without the presence of zeolite protons close to the benzene molecule located over unprotonated 4-T ring with an H1 proton close to the benzene molecule facially framed by a 12-T ring window facially coordinated to two H1a protons in the same 12-T ring window facially coordinated to two H1a protons in the same 4-T ring facially coordinated to one H1 proton with an H1 proton close to the benzene molecule located over unprotonated 4-T ring with two H1 protons close to the benzene molecule facially coordinated to one H1 proton with H1 and H2 protons close to the benzene molecule located over unprotonated 4-T ring with three H1 protons close to the benzene molecule

a Corresponding proton density (14, 28, 56) of the HY zeolite models (14Al, 28Al, and 56Al) to each of the adsorption sites. bSites can be found in all models (14Al, 28Al, and 56Al).

Figure 3. Examples of five benzene adsorption types: (a) H1 from 14Al, (b) H2 from 28Al, (c) U4 from 14Al, (d) U4(H1) from 56Al, and (e) W from 56Al. Light gray lines are zeolite frameworks with the aluminum atoms and hydrogen atoms shown as black and gray balls, respectively. The dashed lines denote the distance for the center of mass of benzene (COMben) and the center of the 4-T ring as well as distances for COMben−Hz. Distances are in nanometers.

Table 3. Average Adsorption Heat of Each Site (in kJ/mol) this work ref 12

H1

H2

U4

U4(H1)

W

U4(2H1)

W(2H1)

2H1

−54.1 −52.1 ± 7

−53.8 −47.9 ± 9

−51.5 −

−55.0 −

−49.5 −49.5

−50.9 −

−54.7 −

− −50.1 ± 6

Ead = Evan + Eelc

3. RESULTS AND DISCUSSION 3.1. Types of Adsorption Sites. The adsorption of benzene in the 14Al, 28Al, and 56Al models at the benzene loading of 1 molecule/UC observes 11 equivalent adsorption sites: H1, H2, U4, U4(H1), W, W(2H1), U4(2H1), H1(H2), H1(H2,H1), U4(H1,H1), and U4(H1,H1,H1). Their descriptions are shown in Table 2. The first five sites (hereafter referred to as group A) can be generally detected from all three zeolite models; however, the observation of the other sites (hereafter referred to as group B) strongly depends on the number of protons. Figures of the sites can be found in the Supporting Information, with groups A and B shown in Figures S1 and S2, respectively. A separate simulation in a larger box

(1)

Radial distribution functions (RDFs) are commonly given as gαβ(r), and can be calculated from gij(r ) =

⟨ΔNij(r , r + Δr )⟩V 4πr 2ΔrNN i j

(2)

where r is the distance between species i and j, ΔNij(r,r + Δr) is the ensemble averaged number of species j around i within a shell from r to r + Δr, V is the system volume, and Ni and Nj are the numbers of species i and j. 13612

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Figure 4. Representative snapshots of benzene molecule on 2H1 sites: (a) U4(2H1) with two protons in the same 4-T ring from 28Al, (b) W(2H1) with two protons in the same 12-T ring from 56Al, and (c) W site from 56Al. The dashed lines denote the distance for COMben and the center of the 4-T ring as well as distances for COMben−Hz. Distances are in nanometers.

with 2 × 2 × 2 unit cells was performed and gave very similar results for both site types and energy of adsorption for each site (within statistical error), implying a negligible size effect. The geometries of adsorption sites in group A (H1, H2, U4, U4(H1), and W) have been fully explored in the literature,12,13 and are also confirmed by our calculations, as shown in Figure 3. Meanwhile, the adsorption energy of each site is calculated from 20 configurations with the lowest energy and compared with the published data,12 as described in Table 3. The adsorption sites in the order of stability are U4(H1) > H1 > H2 > U4 > W. This agrees with the literature.12 It is worth mentioning that the geometries of U4(H1) and U4 are almost identical; however, the energy difference between the U4 and U4(H1) sites is as large as 3.5 kJ/mol, in accordance with the results (approximately 4 kJ/mol difference) carried out by Jirapongphan.13 The difference of U4(H1) compared with U4 is the involvement of an H1 proton approaching the benzene molecule, indicating a significant effect of proton distribution on the adsorption site. All geometries of the adsorption sites in group B (W(2H1), U4(2H1), H1(H2), U4(H1,H1), H1(H2,H1), and U4(H1,H1,H1)) are first observed in our calculation. Among them, W(2H1) and U4(2H1) belong to the 2H1 site,12 which is separated into two sites based on different distributions of two H1 atoms involved. The configurations of benzene for the U4(2H1), W(2H1), and W sites are shown in Figure 4. Two H1 protons of U4(2H1) locate in the same 4-T ring (Figure 4a), with a d(H1−H1) value around 0.57 nm. This configuration is found to be similar to that of the H1 site; that is, the energy difference of them must be attributed to different proton distributions. Two H1 protons of W(2H1) are attached to the O atoms separated by a single O atom in the same window (Figure 4b), with the distance of two H1 protons (d(H1−H1)) around 0.39 nm. According to Löwenstein’s rule,21 the d(H1−H1) of W(2H1) is the smallest in HY zeolite. Although both W(2H1) and W are adsorption types involving a 12-T ring, they can be distinguished easily from the configuration of benzene. For the W site (Figure 4c), benzene is facially coordinated to the 12-T ring, leading to oxygen− hydrogen interactions roughly in the plane of the benzene. For the W(2H1) site (Figure 4b), the local geometry is more similar to that of the H1 site, resulting in interactions perpendicular to the molecular plane. The RDFs of d(H1−H1) could give some evidence for the existence of U4 (2H1) and W(2H1). As shown in Figure 5, the distances of H1−H1 are calculated for the three models. The first peak is around 0.75, 0.39, and 0.38 nm for 14Al, 28Al, and 56Al, respectively. Clearly, there are no U4(2H1) and W(2H1) sites in the 14Al model with d(H1−H1) > 0.75 nm. The

Figure 5. RDFs of framework H1−H1 distances for 14Al, 28Al, and 56Al models.

shortest d(H1−H1) values for 28Al and 56Al are both shorter than 0.4 nm, indicating the possibility of finding U4(2H1) and W(2H1) sites. Actually, there are one U4(2H1) and four W(2H1) sites conceivable in 28Al, five U4(2H1) and eight W(2H1) sites conceivable in the 56Al model, and no U4(2H1) and W(2H1) sites in the 14Al model. These results confirmed that the presence of U4(2H1) and W(2H1) sites are related to the proton density of zeolites, considering an even distribution of Hz protons in our models. The adsorption energy values of U4(2H1) and W(2H1) are shown in Table 3 and compared with that of 2H1 reported by Jousse.12 The reported adsorption energy of 2H1 (varying between −56.7 and −44.1 kJ/mol) is similar to that of H1 (varying between −58.8 and −45.3 kJ/mol). Based on our simulation, the difference of adsorption energy between W(2H1) and H1 is only 0.6 kJ/mol, however, the adsorption energy of U(2H1) is 3.2 kJ/mol higher than that of H1, indicating that the adsorption type mainly considered for the 2H1 site reported by Jousse12 is W(2H1) rather than U4(2H1). In addition, the energy difference of W(2H1) and U4(2H1) is as large as 3.8 kJ/mol, comparable with the energy difference of U4 and U4(H1) (3.5 kJ/mol difference). As described above, we can define two adsorption sites U4(2H1) and W(2H1), and we proved the adsorption types and energies of sites are closely related to the distribution of protons. The finding of other four sites of group B is based on analysis of the effect of protons on the adsorption sites, which will be discussed in section 3.2. 3.2. Effect of Protons. Proton distributions of 10 sites with the lowest adsorption energies are analyzed for the three 13613

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Table 4. Adsorption Sites and Heff in the Three Models 14Al

28Al

56Al

site

Heff (d(COMben−Heff))

site

Heff (d(COMben−Heff))

site

H1 H1 H1 U4 U4 U4 H2 H2 H2 U4(H1)

H2 (0.46) H2 (0.42) − − − − − − − −

W(2H1) W(2H1) W(2H1) W(2H1) U4 U4 U4 H1 U4(H1) U4(H1)

− − − − − − − H2 (0.52) H1 (0.41) −

U4(H1) U4(H1) U4(H1) U4(H1) U4(H1) U4(H1) U4(H1) W(2H1) W(2H1) H1

models to further investigate the effect of protons on the adsorption sites. For each configuration, if the distance between H1/H2 and the center of mass of benzene (COMben) is less than 0.6 nm (d(COMben−Hz) < 0.6 nm), the H1/H2 will be defined as the “effective proton”. To simplify the notion, from here on the “effective proton” is referred to as Heff, with Heff1 referring to H1 and Heff2 referring to H2. Like U4(2H1) and W(2H1), sites with more than one Heff involved can be considered as a “supersite”, which makes the benzene molecule able to shift between one proton and the others.12 Table 4 shows the identified adsorption sites (H1, H2, U4, W, U4(H1), U4(2H1), and W(2H1)), as well as the Heff protons with d(COMben−H). For U4(H1), U4(2H1), and W(2H1), the Heff1 protons of the sites are not shown. For the 14Al model, four sites are presented with the ratio H1:U4:H2:U4(H1) = 3:3:3:1. For three H1 sites, two of them are named H1(H2) due to the finding of one Heff2 proton. The snapshot of H1(H2) is shown in Figure 6. The finding of two

Heff (d(COMben−Heff)) H1 H1 H1 H1 − − − − − H1

(0.43), (0.40), (0.40), (0.55),

H1 H1 H1 H1

(0.58) (0.58) (0.56) (0.58)

(0.54), H2 (0.40)

Figure 7. RDFs of framework H1−H2 distance for 14Al, 28Al, and 56Al models.

supersites such as H1(H2) are likely to be found in the 28Al and 56Al models as well. For the 28Al model, four sites, W(2H1), U4, H1, and U4(H1), are found with the proportion of 4:3:2:1; the latter two sites have Heff protons. As expected, one of the H1 sites is found to be H1(H2), with a d(COMben−Heff2) value around 0.52 nm. The adsorption energy of H1(H2) (1.2 kJ/mol) is higher than that of the most stable site, W(2H1), in 28Al. The vdW interactions of W(2H1) and H1(H2) are similar to each other ( H1(H2) > H1 > H2 > U4. As we can see in Figure 6, the d(H1−H2) of two protons in H1(H2) is about 0.58 nm. The RDFs of d(H1−H2) are shown in Figure 7, and the first peak is around 0.58 nm for 14Al, corresponding with the H1(H2) site. Both 28Al and 56Al have similar first peak locations at 0.46 nm, apparently smaller than the d(H1−H2) of H1(H2). This result indicates that new 13614

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(H1,H1,H1) is 1.6 kJ/mol, which is larger than that of U4(H1). That is mainly contributed by the electrostatic interaction of U4(H1,H1,H1) (1.4 kJ/mol larger than U4(H1)). Their stability decreases as U4(H1,H1,H1) > U4(H1) > H1(H2,H1) > W(2H1). For all the sites discovered until now, the adsorption energy decreases as U4(H1,H1,H1) > U4(H1) > H1(H2,H1) > W(2H1) > U4(H1,H1) > H1(H2) > H1 > H2 > U4 > U4(2H1) > W. This result shows the stability of sites increases with the decreasing Si:Al ratio. We also find that the contribution of H1 proton promotes benzene adsorption while H2 lessens this process under the condition of an equal proportion of H1 and H2 for the three models. For the 14Al model, the number of H1 and H2 sites is equivalent (three of each). For the 28Al model, six sites involving H1 proton and only one site involving H2 (i.e., H1(H2)) are found, which has five H2 protons. The proportion of sites containing H1 and H2 protons is 6:1. For the 56Al model, 7 out of 10 sites have at least one Heff1. However, none of the 10 H2 protons in the 56Al model is attached by benzene. Although the proportion of H1 and H2 is set at the same ratio (H1:H2 ∼ 29:108,23) for the three models, their contribution would be largely influenced by the Si:Al ratio. The RDFs could offer further explanation. Figure 11 shows the

Figure 8. Representative snapshot of benzene molecule on U4(H1,H1) site from 28Al. The green dashed lines denote the distance for COMben−Hz, and the black dashed lines denote the distance between two Hz protons. Distances are in nanometers.

system can be reproduced, as long as the zeolite framework contains the relative locations of H1 and H2 atoms as shown in Figures 6 and 8. The stability order of the sites found in the 28Al model can be expressed as U4(H1) > W(2H1) > U4(H1,H1) > H1(H2) > H1. For the 56Al model, three sites, U4(H1), W(2H1), and H1, are found with the proportion of 7:2:1. Two of them having Heff protons are U4(H1) and H1. For seven U4(H1) sites, four of them involve two additional Heff1 protons, namely U4(H1,H1,H1), and their d(COMben−Heff1) values vary from 0.40 to 0.58 nm, as shown in Figure 9. The H1 site has an Heff1

Figure 11. RDFs of distances for COMben−H1 and COMben−H2 for 14Al, 28Al, and 56Al models.

Figure 9. Representative snapshot of benzene molecule on U4(H1,H1,H1) site from 56Al. The green dashed lines denote the distance for COMben−Hz, and the black dashed lines denote the distance between two Hz protons. Distances are in nanometers.

RDFs of COMben−H1 and COMben−H2 taken from 100 000 snapshots of the configuration (output every 50 MC production steps) for the three models. The shortest distances of COMben−H1 and COMben−H2 are 0.24 and 0.33 nm. This is consistent with the reported distance of COMben−H2 (0.34 nm) from the experimental study of Vitale et al.30 The first peaks of COMben−H1 and COMben−H2 are attributed to benzene adsorption on the H1 and H2 sites, respectively. As we can see, the peak height in the g(r) slightly decreases for H1, and distinctively declines for H2 with decreasing Si:Al ratio. This suggests that the distance between benzene and H2 protons could be further reduced compared with H1 protons. Because the H2 protons locate in the plane with the 6-T ring, and do not protrude observably from the framework like the H1 protons, the spread of the H1 protons all over the zeolite framework with a relatively low Si:Al ratio would cause a decrease of the accessibility of H2 for benzene.

and an Heff2, and is named as H1(H2,H1), which can be seen in Figure 10. This site is very similar to H1(H2), but with an additional Heff1. The stability of this site is between those of U(H1) and W(2H1). The adsorption energy of U4-

4. CONCLUSIONS Monte Carlo simulation was performed to study the influence of the framework protons on the adsorption sites of benzene in HY zeolite models (14Al, 28Al, and 56Al) at the loading of 1 molecule/UC. Eleven types of adsorption sites were observed

Figure 10. Representative snapshot of benzene molecule on H1(H2,H1) site from 56Al. The green dashed lines denote the distance for COMben−Hz, and the black dashed lines denote the distance between two Hz protons. Distances are in nanometers. 13615

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including five reported sites (H1, H2. U4, U4(H1), and W) and six newfound sites (U4(2H1), W(2H1), H1(H2), U4(H1,H1), H1(H2,H1), and U4(H1,H1,H1)), which are supersites with more than one involved proton. The supersite can increase the electrostatic interaction; however, there is no confirmed evidence showing that it can be more stable than other sites due to the uncertainty of vdW interaction. The stabilities of all the observed sites in descending order are U4(H1,H1,H1) > U4(H1) > H1(H2,H1) > W(2H1) > U4(H1,H1) > H1(H2) > H1 > H2 > U4 > U4(2H1) > W. These results indicate that the most stable site types are greatly influenced by the number of protons. Increase in the number of protons results in increased proportion of supersites, which enhances the adsorption energies of the sites. The contributions of H1 and H2 protons to the 10 most stable sites were analyzed. For HY zeolite models containing different numbers of protons with the same H1/H2 ratio, the amount of the most stable adsorption sites involving H1 increased, while the amount of the most stable adsorption sites involving H2 decreased, with increasing number of protons. This is because the distances between benzene and H2 protons were further enhanced compared with H1 protons. Keeping in mind that HY zeolites used in the industry have Si:Al ratios similar to that of the 56Al model, the contribution of H2 atoms might be overestimated if one takes the ratio of H1:H2 as the only consideration. It is generally accepted that the selectivity and reaction predictions rely on both the Si:Al ratio of the zeolite framework and the details of the location and energetics of the available molecular adsorption sites. The gained insights in our simulation illustrated a detailed influence of the Si:Al ratio and the distribution of Hz atoms on the number and type of adsorption sites, which are important to a more rational design of industrial separation and reaction processes.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed information on partial charges of atoms for both adsorbate and HY zeolites, as well as figures for each site observed in this article. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-10-89739078. Fax: 8610-69724721. 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 21176253, 21036008, and 21236009) and PetroChina.

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Industrial & Engineering Chemistry Research

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