Stability of the Reaction Intermediates of Ethylbenzene

Article pubs.acs.org/JPCC

Stability of the Reaction Intermediates of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies: A Theoretical Investigation Xianfeng Yi,† Youngchul Byun,‡ Yueying Chu,† Anmin Zheng,*,† Suk Bong Hong,*,‡ and Feng Deng*,† †

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Center for Magnetic Resonance, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China ‡ National Creative Research Initiative Center for Ordered Nanoporous Materials Synthesis, School of Environmental Science and Engineering and Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea S Supporting Information *

ABSTRACT: The strain energies of the main reaction intermediates (i.e., monoethylated diphenylethane (mEDPE) and diethylated diphenylethane (dEDPE) derivatives), which can be formed during ethylbenzene (EB) disproportionation over six 10-ring zeolites with different framework topologies, as well as over the large-pore zeolite Y, were determined by the density functional theory calculations in order to more precisely investigate the effects of the pore structure of medium-pore zeolites on their formations. It was found that while the strain energies of mEDPE and dEDPE intermediates in zeolite Y, MCM-22 and TNU-9, were always lower than 19.6 kJ mol−1, some of them were characterized by considerably higher energies (>32.8 kJ mol−1) when positioned in the intersection channels of ZSM-5 and ZSM-57. As expected, in addition, all the mEDPE and dEDPE derivatives embedded in TNU-10 and ZSM-22 with narrower 10-ring channels were strongly distorted, giving them much higher strain energies (>37.7 kJ mol−1), which were in excellent agreements with our recently reported experimental results (J. Phys. Chem. C 2010, 115, 16124). This led us to conclude that the size and shape of void spaces in the medium-pore zeolites play a crucial role in governing the type of mEDPE and dEDPE formations during the EB disproportionation. Our work also shows that the strain energies of various reaction intermediates confined within zeolites with different pore topologies could be regarded as a useful quantitative means in better understanding the shape-selective nature of this important class of microporous crystalline catalysts.

1. INTRODUCTION Disproportionation of ethylbenzene (EB) to benzene (B) and three diethylbenzene (DEB) compounds over zeolite-based catalysts has gained tremendous attention recently due to the wide applications of these compounds in a variety of industrial processes.1 Karge and co-workers studied the EB disproportionation and observed an induction period in which the conversion of EB increased with the time on stream until the maximum conversion was reached.2,3 However, when mediumpore zeolites were used, no induction period was present. Therefore, the EB disproportionation has long been claimed to safely distinguish between medium- and large-pore zeolites, and was recommended as a standard reaction for characterizing the acidic properties of zeolites by the Catalysis Commission of the International Zeolite Association (IZA) in 2002 because the EB conversion over the large-pore zeolites is typically characterized by an induction period accompanied by a DEB deficit, unlike that over medium-pore ones.4−9 However, we have recently demonstrated that some medium-pore zeolites, especially those containing 12-ring cavities/channels accessible only through © 2013 American Chemical Society

10-ring windows, exhibit noticeable DEB deficit, as well as the induction period in the EB conversion.10 To date, there are three major types of reaction pathways proposed for the acid-catalyzed EB disproportionation: (i) monomolecular ethyl transfer, (ii) bimolecular diphenylethanemediated, and (iii) dual-cycle mechanisms (Scheme 1).11−15 In the case of the latter two pathways (Scheme 1, b and c), monoethylated diphenylethane (mEDPE) and/or diethylated diphenylethane (dEDPE) species have been proposed to play a crucial role as reaction intermediates during this particular type of aromatic transalkylation over medium-pore zeolites, as well as over large-pore ones.14,15 Apparently, the availability of intrazeolitic void spaces that can accommodate such bulky transition states may be the key to governing the mechanism of EB disproportionation over zeolite catalysts. Received: April 16, 2013 Revised: October 17, 2013 Published: October 18, 2013 23626

dx.doi.org/10.1021/jp4089386 | J. Phys. Chem. C 2013, 117, 23626−23637

The Journal of Physical Chemistry C

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Scheme 1. (a) Monomolecular Ethyl Transfer, (b) Bimolecular Monoethylated Diphenylethane-Mediated, and (c) Dual-Cycle Mono- and Diethylated Diphenylethane-Mediated Reaction Mechanisms for the Zeolite-Catalyzed EB Disproportionation

methanol-to-olefin process (MTO).29−31 Recently, we also theoretically explored the stability of carbenium ions in H-Y, HZSM-5, and H-Beta to investigate the host−guest interactions between the reaction intermediates and the zeolite frameworks.21 Here, we report the theoretical results for the EB disproportionation over six medium-pore zeolites (i.e., MCM22, TNU-9 (TUN), ZSM-5, ZSM-57 (MFS), TNU-10 (STI), and ZSM-22 (TON)) with different framework topologies, as well as over the large-pore zeolite Y. In an attempt to comprehensively understand the zeolite confinement effects on the formations of mEDPE and dEDPE derivatives during this reaction, the results obtained are compared with the experimental ones already reported.10,15

As a complementary tool of an experimental approach, theoretical calculations have been extensively used not only to more precisely determine the structure and physicochemical properties of zeolites and adsorbed molecules but also to better understand the mechanisms of many hydrocarbon conversions catalyzed by this important class of microporous aluminosilicate solids.16−25 In particular, the energy parameters such as adsorption energy, reaction energy, and activation energy derived from the theoretical approach are quite useful for elucidating the effects of zeolite topology on the product distribution and reaction pathway at the molecule level.26 For example, Dominguez-Soria et al. have used the host−guest binding energies between the basic probe molecule like CO or CH 3 CN and the Brønsted protons to determine the distributions of weak and strong acid sites inside H-mordenite (framework type MOR).20 Limtrakul and co-workers have calculated the adsorption energies of ethene and benzene molecules in various zeolites including H-Y (FAU), H-MCM22 (MWW), and H-ZSM-5 (MFI), and demonstrated that short-range intermolecular interactions, i.e., van der Waals (vdW) interactions, are essential for the adsorption of reactant molecules in zeolites.18,19 Besides the theoretical calculations of the adsorption structures and energies, theoretical modeling is a highly useful tool to explore the reaction mechanisms and the activation barriers about the individual reaction steps occurring inside confined zeolites pores. Limtrakul et al. also used the theoretical calculations to study the effect of the zeolite pores on methane activation in gold cation-exchanged zeolites. On the basis of the calculated activation barriers, it is demonstrated that the Au-MCM-22 with the largest pores exhibited the highest activity because of a better stabilization of its transitionstate structure.27 Clark et al. theoretically identified the existence of benzenium-type carbenium ions in m-xylene disproportionation and demonstrated that the product isomers were not dependent on the transition-state selectivity but the product shape selectivity.16,28 Waroquier et al. theoretically investigated the catalytic reaction cycles responsible for olefin formation and catalyst deactivation mechanism in the

2. COMPUTATIONAL METHODS The 84T, 96T, 64T, 72T, 121T, 70T, and 126T models were used to represent zeolite Y, MCM-22, TNU-9, ZSM-5, ZSM57, TNU-10, and ZSM-22 structures, respectively, to explore the effects of the zeolite frameworks on the EB disproportionation in our theoretical calculations. These extended models include the complete pore structures of the zeolites (Figure 1). Hence, the zeolite confinement effects caused by differences in their framework topologies on the adsorbed reaction intermediates have been theoretically taken into account. Their terminal Si atoms were capped with H atoms at a Si− H bond length of 1.47 Å oriented along the direction of the corresponding Si−O bond. A combined theoretical model, i.e., the ONIOM (ωB97XD/6-31G(d):MNDO) method, was applied to predict the geometries of all possible mEDPE and dEDPE derivatives, where the ωB97XD method is the hybrid meta DFT developed by Chai and Head-Gordon.32 This method implicitly accounts for empirical dispersion and can describe long-range dispersion interactions well with respect to the traditional DFT methods. In our ONIOM simulations, the adsorbed organic species and the atoms of the zeolite framework surrounding the adsorbate (24T for Y, 24T for MCM-22, 26T for TNU-9, 16T for ZSM-5, 30T for ZSM-57, 23627



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Figure 1. Structures of p-mEDPE embedded in zeolites with different framework topologies optimized at the ONIOM (ωB97XD/631G(d):MNDO) level of theory: (a) zeolite Y, (b) MCM-22, (c) TNU-9, (d) ZSM-5, (e) ZSM-57, (f) TNU-10, and (g) ZSM-22.

adsorbed molecules. In this contribution, the strain energy is defined as the sum of strain energy of the zeolite and the adsorbate.34

34T for TNU-10, and 35T for ZSM-22) were treated as high level, and they were allowed to relax during the calculations, while the rest of atoms of the zeolite framework was treated as the low level and kept fixed at their crystallographic locations. During the structural optimization, all the possible configurations and the locations of the organic species confined inside zeolites were taken into account and the most stable configuration was selected to further calculate the strain energy. The single-point energy calculations, e.g., strain and adsorption energies, were further refined at the ωB97XD/631G(d,p) level of theory. All calculations were performed using the Gaussian 09 software package.33 The confinement effect of the zeolite pore structure can result in the distortion of the zeolite framework and the

zeo ads Estrain = ΔEstrain + ΔEstrain

(1)

zeo c ΔEstrain = Ezeolite − Ezeolite

(2)

ads c ΔEstrain = Eadsorbate − Eadsorbate

(3)

(Eczeolite)

Herein, the energies of the zeolite and the adsorbate (Ecadsorbate) are calculated with the geometries found in the adsorption complexes without further optimization; Ezeolite and Eadsorbate refer to the energy of optimal zeolite model and 23628



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Table 1. Structural Features of Zeolites with Different Pore Topologies Studied in This Work zeolite

IZA code

H-Y H-MCM-22 H-TNU-9 H-ZSM-5 H-ZSM-57 H-TNU-10 H-ZSM-22

FAU MWW TUN MFI MFS STI TON

pore sizea (Å)

pore topology 3D, 2D, 3D, 3D, 2D, 2D, 1D,

12-rings 10-rings + large cages 10-rings + 12-ring cavities 10-rings 10 and 8-rings 10 and 8-rings 10-rings

7.4 4.0 5.6 5.1 5.1 4.7 4.6

× × × × × × ×

7.4 5.5, 4.1 × 5.1 5.5, 5.4 × 5.5 5.5, 5.3 × 5.6 5.4 (3.3 × 4.8) 5.0 (2.7 × 5.6) 5.7

channel intersection dia or cavity dimensionb (Å) 12.0 × 12.0 × 12.0 7.0 × 7.0 × 18.7 (7.1 × 7.1 × 18.2) 5.3 × 10.9 × 15.7 8.6 11.7 8.4 −

a The values in parentheses are the size of 8-ring channels in the corresponding zeolite. bCalculated using the CrystalMaker Software. The values in parentheses are those reported in the literature.36

ads organic species in the free state. ΔEzeo strain and ΔEstrain represent the strain energy of the zeolite and the adsorbate, respectively. The adsorption energy of adsorbed molecules arises from two contributions:34,35 the stabilization energy derives from the dispersive interactions with the zeolite walls, and the repulsive energy comes from the distortions of both the zeolite host and the adsorbate. Calculating the different contributions separately would provide a more detailed insight into the interaction between the zeolite framework and adsorbed molecules. Thereby, the total adsorption energy is given by

ΔEads = ΔE inter + Estrain

(4)

c c ΔE inter = Ecomplex − Ezeolite − Eadsorbate

(5)

bi- and monomolecular, respectively.10 Hence, we have chosen these two zeolites as theoretical representatives of medium-pore materials with two intersecting 10- and 8-ring channels. Finally, we simply selected H-ZSM-22 as the representative of 1D 10ring zeolites because of its structural similarity compared with H-ZSM-23. Table 1 lists the structural features of all zeolites with different pore topologies employed in this study, and Figure 2

where ΔEinter is the interaction energy between the adsorbate and the zeolite, which describes the dispersive interactions of the adsorbate with the zeolite walls (the contributions due to deformation of the zeolite host and the adsorbate are excluded).

3. RESULTS AND DISCUSSION 3.1. Experimental Background. The zeolite catalysts employed in our recent gas chromatography−mass spectroscopy (GC-MS) studies on the mechanism of EB disproportionation10,15 include not only the large-pore H-Y but also a total of 13 medium-pore zeolites that possess the following structural features:36 (i) they contain large 12-ring cavities/channels accessible only through 10-ring windows (TNU-9, MCM-22, NU-87 (NES), and EU-1 (EUO)) and consist of (ii) mutually intersecting 10-ring channels only (ZSM-5, IM-5 (IMF), and ITQ-2), (iii) intersecting 10- and 8-ring channels (ZSM-57, TNU-10, SUZ-4 (SZR), and ferreirite (FER)), and (iv) onedimensional (1D) 10-ring channels only (ZSM-22 and ZSM-23 (MTT)). We were able to confirm the formations of both mEDPEs and dEDPEs in all members of the first subgroup of medium-pore zeolites, as well as in H-Y, which led us to choose H-TNU-9 and H-MCM-22 as their representatives for the theoretical investigation on the formations of reaction intermediates of EB disproportionation. Quite similar results were also observed for the second subgroup of materials, except for H-ITQ-2 which consists of two intersecting 10-ring channels with relatively small pore dimension (4.1 × 5.1 Å). Because H-IM-5 has a very complex connectivity of its 10-ring channels which imparts the character of two-dimensional (2D) pore system with restricted diffusion to this three-dimensional (3D) medium-pore zeolite, we selected H-ZSM-5 as the only representative of the second subgroup of medium-pore zeolites to save computational cost. Among the members of the third subgroup of zeolites, by contrast, the prevailing reaction mechanism of H-ZSM-57 and H-TNU-10 was found to be

Figure 2. GC-MS total intensities of the CH2Cl2 extracts from seven zeolites with different framework structures after EB disproportionation at 523 K and 5.2 h−1 WHSV. Adapted from refs 10 and 15.

compares the GC-MS total ion chromatograms of the CH2Cl2 extracts from these seven zeolite catalysts after EB disproportionation at 523 K for 30 h on stream, which were adapted from refs 10 and 15. It is clear that the mEDPE and dEDPE derivatives are observed not only on the large-pore zeolite H-Y but also on the medium-pore materials H-TNU-9 containing 12 ring cavities accessible through 10-ring windows and H-ZSM-5 with mutual intersection of 10-ring channels only. We should also note here that the GC-MS chromatogram from H-ZSM-57 with two intersecting 10- and 8-ring channels is characterized by one only weak peak assignable to p-mEDPE, which is indicative of the bimolecular diphenylethane-mediated reaction mechanism.10 On the other hand, none of DPE species were found in H-TNU-10 with the same pore topology as that of ZSM-57 but with fairly smaller 10-ring channel, as well as in the 1D 10-ring pore zeolite H-ZSM-22, suggesting that the reaction pathway of bimolecular EB disproportionation is no longer 23629



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Figure 3. (a) Three mEDPE and (b) six dEDPE derivatives that can be formed in zeolites as the reaction intermediates of EB disproportionation. Their molecular dimensions in the gas state were calculated at the ωB97XD/6-31G(d) level.

Table 2. Bond Angles of mEDPE and dEDPE Derivatives Embedded in Zeolites with Different Framework Topologies Obtained at the ONIOM (ωB97XD/6-31G(d):MNDO) Level of Theory bond angle (deg) zeolite host reaction intermediate o-mEDPE

m-mEDPE

p-mEDPE

o,o-dEDPE

m,o-dEDPE

p,o-dEDPE

m,m-dEDPE

p,m-dEDPE

p,p-dEDPE

bond

gas state

Y

MCM-22

TNU-9

ZSM-5

ZSM-57

TNU-10

ZSM-22

C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4 C3−C1−C4 C2−C1−C3 C2−C1−C4

111.7 109.5 114.6 111.2 114.5 110.1 111.7 114.3 109.9 114.4 115.0 110.2 112.3 114.4 109.0 112.5 114.6 109.7 111.1 114.4 109.8 111.3 109.8 114.4 111.9 114.2 109.7

111.3 109.6 114.7 110.1 114.5 109.8 109.2 114.2 111.2 113.6 114.9 110.8 110.3 114.8 109.3 112.8 114.2 109.2 109.1 112.2 109.3 111.4 109.9 114.4 111.8 113.4 111.0

112.2 109.0 114.6 109.1 114.5 111.6 109.5 114.1 112.5 115.4 114.6 109.6 110.7 114.8 109.4 111.2 114.7 109.1 107.2 114.5 112.8 108.5 111.8 114.5 107.7 114.4 112.8

110.6 115.6 113.0 110.9 114.2 109.9 109.7 114.9 112.6 111.4 114.6 113.3 108.9 114.5 111.5 111.5 115.1 109.5 105.1 115.0 114.6 108.1 114.9 112.7 109.6 114.6 113.2

104.5 115.1 114.1 104.8 112.5 116.4 111.1 109.5 113.5 111.6 116.6 110.6 105.8 113.6 110.1 106.8 115.4 110.3 106.1 117.8 110.1 109.4 113.9 109.8 112.0 113.8 112.0

112.1 116.2 104.5 116.1 105.9 114.3 110.4 109.5 116.3 119.3 108.0 102.3 116.7 105.2 110.4 108.7 108.7 121.4 115.4 115.6 106.6 120.0 111.6 105.8 123.7 108.2 107.9

118.4 106.5 110.6 119.1 112.1 102.8 122.8 102.0 109.5 124.2 112.8 97.1 122.1 103.3 108.0 118.9 110.9 105.7 120.0 112.4 103.5 119.8 103.0 112.6 121.6 106.6 109.2

129.3 98.8 113.6 119.3 110.8 106.2 121.7 104.9 109.9 123.9 107.4 109.6 129.9 110.3 101.1 137.6 104.5 98.7 119.4 110.1 106.1 120.2 110.0 109.4 127.8 105.0 107.8

identify the positions of the ethyl groups of reaction intermediates generated during the EB disproportionation over the six different medium-pore zeolites. 3.2. Zeolite Y and MCM-22. Although internal void spaces of these two zeolites are large enough to accommodate any of three DEB isomers, the main products of EB disproportionation, and the formation of bulky reaction intermediates, should

possible, probably due to their much smaller internal spaces. It is worth noting that the ethene selectivity of H-TNU-10 and HZSM-22 is much higher than that of the other medium-pore zeolites studied.10 This has previously led us to conclude that their prevailing reaction mechanism is monomolecular in nature.10 Due to the lack of the authentic mEDPE and dEDPE derivatives, however, we were not able to exactly 23630



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Figure 4. Structures of gas-state (a) p-mEDPE, (b) o,o-dEDPE, and (c) p,p-dEDPE species optimized at the ωB97XD/6-31G(d) level.

Figure 5. Structures of the p-mEDPE isomer in (a) zeolite Y, (b) ZSM-5, and (c) ZSM-22 optimized at the ONIOM (ωB97XD/6-31G(d):MNDO) level. The closest distances (Å) between the hydrogen atoms of p-mEDPE and the oxygen atoms of each zeolite framework are also given.

interaction between p-mEDPE and zeolite Y is shown in Figure 5a with the closest distances between the hydrogen atoms of pmEDPE and the oxygen atoms of zeolite Y framework. It is noteworthy that the structure of zeolite Y employed here includes two supercages (13 Å) connected via 12-membered ring window with the free aperture of 7.4 × 7.4 Å, and it is already confirmed by the previous study that this 84T model can take into account all the zeolite framework effects.40 As shown in Figure 5a, p-mEDPE locates inside the middle of the supercage of zeolite Y through vdW interaction between the organic molecule and zeolite framework. All the atoms in the pmEDPE are farther than 2.64 Å from the zeolite framework, which indicates that the interaction between organic molecule and the supercages of zeolite Y is slightly weak. Compared with the p-mEDPE in the gas state, we can identify that the structure of p-mEDPE confined in the supercage of zeolite Y changed slightly; all the bond angles around C1 atom are slightly changed when compared with the gas state (Figure 4 and Table 2). Even for o,o-dEDPE, which has the largest dimension size among the nine isomers, the maximum change of bond angles

be strongly influenced by their size and shape. Therefore, we applied the theoretical method to explore the host−guest interactions occurring between the mEDPE and dEDPE species and the zeolite frameworks at the molecular level. There are three possible mEDPE isomers and six possible dEDPE ones that could be formed as reaction intermediates during the EB disproportionation (Figure 3). When their geometries in the gas state were optimized through the DFT calculations at the ωB97XD/6-31G(d) theoretical level, it was not difficult to find that the tetrahedral bond angles of all of their sp3 carbons would always be 109.5°.37 As listed in Table 2, however, the bond angles of sp3 carbons in p-mEDPE, o,odEDPE, and p,p-dEDPE isomers are deviated by 2−6° from the ideal angle of sp3 hybridization because of the steric hindrance caused by neighboring phenyl rings (Figure 4).38,39 Steric strain exists in the molecules adsorbed in the limited spaces and should also cause the abnormal bond angles of the confined molecules. Therefore, as a next step, all of mEDPE and dEDPE isomers embedded in zeolites were optimized to determine the confinement effect. The optimized structure by the host−guest 23631



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Figure 6. Structures of p-mEDPE (left), o,o-dEDPE (middle), and p,p-dEDPE (right) species embedded in (a) zeolite Y, (b) MCM-22, (c) TNU-9, (d) ZSM-5, (e) ZSM-57, (f) TNU-10, and (g) ZSM-22. The angles of their C3−C1−C4 bonds marked in red are also given. Each zeolite framework was removed after structural optimization at the ONIOM(ωB97XD/6-31G(d):MNDO) level.

is 0.8° (∠C3−C1−C4). MCM-22, with a large 7.0 × 7.0 × 18.7 Å cavity, showed similar changes of bond angles with the zeolite Y; the largest change of the bond angle of o,o-dEDPE is 1.0° (∠C3−C1−C4). This informs us that mEDPE and dEDPE isomers are slightly distorted in the supercage of zeolite Y and the large cavity of MCM-22. The distorted extent of all mEDPE and dEDPE isomers inside the various zeolite frameworks can be quantitatively analyzed using the geometry optimizations (Table 2 and Figure 6). The strain energies of all isomers are shown in Table 3.

Those in zeolite Y and MCM-22 are lower than 8.1 and 19.6 kJ mol−1, respectively, which furthermore confirmed that both the supercage of zeolite Y and the large-cavity of MCM-22 slightly influence on the structures of the mEDPE and dEDPE isomers, and zeolite Y accommodates the DPEs more than those in MCM-22. Therefore, both of the mEDPE and dEDPE isomers are little constrained and can be diffused inside the zeolite Y and MCM-22. The theoretical results are in excellent agreement with the experimental ones (Figure 2) which shows that both mEDPE and dEDPE are highly observed in 23632



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Table 3. Strain Energies of mEDPE and dEDPE Derivatives in Zeolites with Different Framework Topologies Calculated at the ωB97XD/6-31G(d) Level of Theory strain energies (kJ/mol) zeolite Y

MCM-22

TNU-9

ZSM-5

ZSM-57

TNU-10

ZSM-22

a

a Δzeo strain ads b Δstrain Estrainc Δzeo strain Δads strain Estrain Δzeo strain Δads strain Estrain Δzeo strain Δads strain Estrain Δzeo strain Δads strain Estrain Δzeo strain Δads strain Estrain Δzeo strain Δads strain Estrain

o-mEDPE

m-mEDPE

p-mEDPE

o,o-dEDPE

m,o-dEDPE

p,o-dEDPE

m,m-dEDPE

p,m-dEDPE

p,p-dEDPE

2.0 0.5 2.5 1.1 1.0 2.1 3.7 1.9 5.6 7.9 26.7 34.6 15.3 23.9 39.2 15.1 32.1 47.2 43.5 123.5 167.0

2.9 1.1 4.0 1.9 2.3 4.2 2.3 5.1 7.4 5.1 23.8 28.9 14.7 18.1 32.8 2.8 34.9 37.7 11.8 35.8 47.6

2.9 2.6 5.5 2.2 4.8 7.0 2.5 5.5 8.0 3.6 17.3 20.9 14.2 15.9 30.1 10.5 39.8 50.3 14.4 50.8 65.2

5.1 3.0 8.1 2.5 17.1 19.6 4.5 27.4 31.9 19.3 97.2 116.5 41.2 89.5 130.7 29.0 165.6 194.6 66.5 255.2 321.7

2.9 1.7 4.6 0.6 7.5 8.1 4.9 13.0 17.9 15.8 36.2 52.0 25.6 39.1 64.7 25.6 94.9 120.5 40.3 154.1 194.4

3.3 3.4 6.7 0.3 7.9 8.2 1.3 10.2 11.5 16.8 37.0 53.8 11.9 44.8 56.7 14.6 49.2 63.8 40.3 151.7 192.0

1.9 4.7 6.6 1.8 9.4 11.2 0.4 18.7 19.1 5.6 34.2 39.8 14.3 27.5 41.8 5.8 42.8 48.6 12.3 45.1 57.4

1.8 2.0 3.8 1.5 5.8 7.3 3.5 13.0 16.5 8.1 19.1 27.2 12.3 28.1 40.4 4.7 46.9 51.6 17.1 50.0 67.1

2.4 4.6 7.0 1.3 6.3 7.6 2.3 11.4 13.7 0.2 27.7 27.9 9.3 39.5 48.8 9.6 42.2 51.8 19.3 76.1 95.4

b ads c c c zeo ads Δzeo strain = Ezeolite − Ezeolite. Δstrain = Eadsorbate − Eadsorbate. Estrain = Δstrain + Δstrain.

structures and strain energies. This result is consistent with the experimental observation (Figure 2). TNU-9 contains two different types of cavities as a mediumpore zeolite: a 12-ring channel runs parallel to the [001] direction interconnected through 10-ring channels, and the pore size is 5.5 × 5.6 Å and 5.4 × 5.5 Å for the 12- and 10membered ring channels, respectively, with the interconnected cavity dimension at 5.3 × 10.9 × 15.7 Å (Table 1). Figure 1c shows the optimized structure of p-mEDPE inside the TNU-9. It is predicted that the organic intermediates in the cavity of TNU-9 should be considerably more distorted than those in the supercage of zeolite Y and in the large cavity of MCM-22 because the pore size of TNU-9 is smaller than their ones. It can be identified by the bond angles listed in Table 2 that the change of bond angle (∠C2−C1−C3) in the cavity of TNU-9 for the p-mEDPE is 0.6° while those in the zeolite Y and MCM-22 are 0.1° and 0.2°, respectively. The change of bond angle (∠C3−C1−C4) for o,o-dEDPE is as large as 3.0° since the adsorbed molecule has the most bulky dimension among the nine derivatives in the TNU-9 (Figure 6). The large changes of the bond angles have demonstrated that mEDPE and dEDPE derivatives are more distorted than those in the zeolite Y and MCM-22. As listed in Table 3, the strain energy for o,o-dEDPE in the cavity of TNU-9 is 31.9 kJ mol−1, which is obviously larger than those in Y (8.1 kJ mol−1) and MCM-22 zeolite (19.6 kJ mol−1). However, it is noteworthy that the strain energies of all the mEDPE and dEDPE are lower than the threshold barrier energy (32.8 kJ mol−1), and therefore, we can expect that they may be realized inside the TNU-9. This result is also consistent with the experimental one that both mEDPE and dEDPE can be observed in TNU-9 zeolite (Figure 2). ZSM-5 and TNU-9 have similar 3D interconnected channel systems, but the pore sizes of ZSM-5 are 5.1 × 5.5 and 5.3 × 5.6 Å, being slightly smaller than those of TNU-9 (5.5 × 5.6 and 5.4 × 5.5 Å) (Table 1). Therefore, it is predictable that the

the GC-MS chromatogram of zeolite Y and the mEDPE is also highly observed in MCM-22. 3.3. ZSM-57, TNU-9, and ZSM-5. We also investigated the strain energies of all mEDPE and dEDPE isomers in these three zeolites. The pore sizes of ZSM-57 are 5.1 × 5.4 Å and 3.3 × 4.8 Å for 10- and 8-ring channels, respectively, which are smaller than those of the ZSM-5 (Table 1). As shown in Figure 1e, p-mEDPE molecule is mainly located in the straight 10-ring channel through the interconnected cavities of ZSM-57. In this structure, the phenethyl group is almost stabilized in the middle of 10-ring channel and another phenyl ring enters into the 8ring channel of ZSM-57. Even for the p-mEDPE isomer having the most minimum size dimension among all the isomers of mEDPE, the constrained space of ZSM-57 results in the bond angle derivation by ca. 6° (∠C2−C1−C4) and the corresponding strain energy of 30.1 kJ mol−1, which is considerably higher than those in zeolite Y and MCM-22 zeolites. It is noteworthy that only one mEDPE isomer can be observed inside ZSM-57 (Figure 2), and the strain energies for the three mEDPE isomers inside the zeolite are 39.2, 32.8, and 30.1 kJ/mol. Therefore, we select 32.8 kJ/mol as the threshold barrier of the strain energy. If the strain energies of mEDPE and dEDPE isomers are lower than 32.8 kJ mol−1, their structures can be realized in the zeolite framework, and vice versa. On the basis of this value, the strain energies of all possible mEDPE and dEDPE isomers inside zeolite Y and MCM-22 are much less than 32.8 kJ/mol, and therefore all of the mEDPE and dEDPE isomers are observable inside the two zeolites. However, for all of the possible dEDPE derivatives inside the ZSM-57 (Figure 3b), their strain energies are higher than 40.4 kJ mol−1, which indicates that the dEDPE isomers are distorted to a considerable degree. We concluded that, based on the strained energies obtained here, all of the dEDPE derivatives are quite difficult to be formed inside the ZSM-57 because of the limited pore size which influences dramatically their 23633



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Figure 7. (a) Adsorption (ΔEads) and (b) interaction energies (ΔEinter) of mEDPE and dEDPE derivatives in seven zeolites with different framework topologies.

formations of mEDPE and dEDPE derivatives are more steric hindered inside the ZSM-5 than those inside the TNU-9. As shown in Figure 5a,b, the closest distances between the hydrogen atoms of p-mEDPE and the oxygen atoms of zeolite framework are decreased from 2.64 Å in zeolite Y to 2.17 Å in the ZSM-5, which indicates that the interaction between the organic molecule and the ZSM-5 framework is much stronger than that between the organic molecule and the Y framework. Although the strain energies of m-mEDPE, p-mEDPE, p,mdEDPE, p,p-dEDPE (28.9, 20.9, 27.2, and 27.9 kJ mol−1, see Table 3) are not too high to overcome the barrier threshold (32.8 kJ mol−1), they were increased by about 11.1−21.5 kJ mol−1 as the pore size was decreased. A strong interaction will occur for o,o-dEDPE. As shown in Figure 6 and Table 2, a considerably distorted structure is present for the o,o-dEDPE confined inside the ZSM-5 with the strain energy being dramatically increased from 8.1 and 31.9 kJ mol−1 inside zeolite Y and TNU-9, respectively, to 116.5 kJ mol−1 inside ZSM-5. At the same time, it was found that the strain energies of m,odEDPE, p,o-dEDPE, and m,m-dEDPE are higher than the threshold barrier energy of 32.8 kJ mol−1, which indicates that the formations of them are difficult, while m-mEDPE, pmEDPE, p,m-dEDPE, and p,p-dEDPE may be formed because their strain energies are lower than the threshold barrier energy. These results are consistent with our experimental ones shown in Figure 2 that only weak signals of mEDPE and dEDPE are observed in the ZSM-5. 3.4. ZSM-22 and TNU-10. We can speculate that the decrease of the pore size of zeolite will result in the increase of repulsive interaction between both the mEDPE and dEDPE derivatives and zeolite framework. ZSM-22 contains 1D 10-ring channel at the pore size of 4.6 × 5.7 Å parallel to [001] (Table 1), which is smaller than other zeolites studied above. As shown in Figure 5c, p-mEDPE is tightly confined inside the narrow pore of ZSM-22, and the closest distance between the hydrogen atoms of p-mEDPE and the oxygen atoms of ZSM-22 framework is lower than 2.03 Å. Therefore, the adsorbed

structures of mEDPE and dEDPE derivatives were considerably distorted inside the ZSM-22. As shown in Figure 6, in order to accommodate p-mEDPE in the 10-ring channel of ZSM-22, the bond angle (∠C3−C1−C4) was increased from 111.7° in the gas state to 121.7°, almost from the sp3 hybridization to the linear sp2 hybridization. Such distorted structures will clearly result in higher strain energy (65.2 kJ mol−1) than other zeolites, and therefore, p-mEDPE cannot be formed inside the ZSM-22. It is the same case for the other mEDPE and dEDPE isomers with the strain energies higher than 47.6 kJ mol−1, especially o,o-dEDPE which has the largest dimension size among the nine isomers, results in the strain energy of 321.7 kJ mol−1. Therefore, the limited pore size of ZSM-22 strongly inhibits the formations of the mEDPE and dEDPE derivatives. TNU-10 is composed of the interconnected channel by 10-ring (4.7 × 5.0 Å) and 8-ring (2.7 × 5.6 Å) channels) (Table 1). Similar to the ZSM-22, neither mEDPE nor dEDPE will be formed inside the channel of TNU-10 with the strain energies in the range of 37.7−194.6 kJ mol−1. The calculated results have excellent consistency with the experimental results as shown in Figure 2 that no mEDPE and dEDPE can be formed inside the ZSM-22 and TNU-10 zeolites. 3.5. Adsorption Energy in the Zeolites. The adsorption energies are also determined by the theoretical calculation at ωB97XD/6-31G(d,p) level as shown in Figure 7a. However, it is noted that the adsorption energies cannot be used as a standard to explain the experimental results. Only o-mEDPE, and o,o-dEDPE, adsorbed inside ZSM-22 are endothermic with the adsorption energies of 3.1 and 197.6 kJ mol−1, respectively. However, it does not make sense that other mEDPE and dEDPE isomers can be formed inside the ZSM-22. It is wellknown the total adsorption energy comes from the interactions of the adsorbate and zeolite wall and the distortion of the zeolite framework and the adsorbate (see eqs 4 and 5). It is clear that the structure distortion usually decreases the adsorption energy (see eqs 1, 2 and 3), while the host−guest dispersion interaction between adsorbed molecule and zeolite 23634



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framework always stabilizes the adsorbed structure (see eq 5). Most of the distorted mEDPEs and dEDPEs in the mediumpore zeolites would be tightly adsorbed inside the zeolite frameworks; therefore, the confinement effect that comes from the dispersive interaction will dramatically enhance the stability of them comparing with zeolite Y. Since the adsorption energy is the sum of the strain energy and pore confinement interaction, the interaction energy of the confinement effect could be obtained approximately, as shown in Figure 7b. The interaction energies of p-mEDPE are −132.6 and −243.9 kJ mol−1 in zeolite Y and ZSM-22, respectively. The dispersion interaction inside narrower ZSM-22 is obviously almost 2 times larger than that inside the supercage of zeolite Y; and it is the same case for the other mEDPE and dEDPE isomers. Therefore, the interactions between organic molecules and zeolite frameworks are so strong that the diffusions of the intermediates are also strongly limited inside the ZSM-22. This is another important clue to understand the case that mEDPE and dEDPE cannot be observed in the EB disproportionation reaction catalyzed by TNU-10 and ZSM-22. Generally, the adsorption energy cannot be used to determine whether the reaction occurs or not. In the case of highly confined adsorption systems, such as inside the small pore zeolites (ZSM-22 and TNU-10), the dispersion interactions are very strong, which can dramatically stabilize some distorted molecules. In principle, the activation energy derived from the reaction transition state can be used to determine the reactivity and selectivity of catalytic reactions. However, our catalytic system includes nine diphenylethane intermediates and seven zeolites with different framework topologies. These results in the calculations of activation energies too complicated. Compared with the activation energy, the strain energy of reaction intermediates inside zeolites is another parameter that can be used to determine the possibility of catalytic reactions. Here, we studied the influence of zeolite confined voids on the stability of reaction intermediates by calculating their strain energies. Apparently, if an intermediate is much bulkier than the zeolite confined void, a large distortion of its structure and thus a high strain energy will be expected. As a result, such a bulky intermediate cannot be formed in the confined zeolite pores. 3.6. Comparisons with Experimental Results. The theoretical strain energies of both mEDPE and dEDPE isomers have been consistent with our experimental GC-MS spectrum; Figure 8 shows the experimental relative intensities of GC-MS for both mEDPE and dEDPE and the theoretically calculated strain energies over six medium-pore zeolites and large-pore zeolite Y. If the reaction of EB disproportionation follows the bimolecular DPE-mediated mechanism, mEDPE will be formed inside the zeolite. However, both of the dEDPE and mEDPE will be formed when the reaction follows the dual-cycle mechanism. The structures of all the mEDPE and dEDPE derivatives are slightly distorted when they are confined inside the 13 Å supercage of zeolite Y and the large cavity of MCM-22; their strain energies are far less than 32.8 kJ mol−1, which indicates that both the mEDPE and dEDPE isomers can be readily formed inside the zeolite Y and MCM-22. It is also found that mEDPE and dEDPE preferentially adsorbed at the interconnected cavities of 12-ring and 10-ring pores in TNU-9 and ZSM-5 because the strain energies of the mEDPE (m-mEDPE and p-mEDPE) and dEDPE (p,m-dEDPE and p,p-dEDPE) are less than 32.8 kJ mol−1, even though their strain energies are

Figure 8. (a) Relative intensities of GC-MS signals of mEDPE and dEDPE species observed after EB disproportionation at 523 K and 5.2 h−1 WHSV over seven zeolites with different framework topologies and (b) calculated strain energies of mEDPE (top) and dEDPE (bottom) isomers embedded in zeolites studied here.

considerably higher than those in the zeolite Y and MCM-22. We have concluded on the basis of our theoretical results that both mEDPE and dEDPE isomers can be possibly formed inside the zeolite Y, MCM-22, TNU-9, and ZSM-5 (Figures 2 and 8). This conclusion also implies that the dual-cycle mechanism is possibly occurred during the EB disproportionation catalyzed by zeolite Y, MCM-22, TNU-9, and ZSM-5. Although 8-ring and 10-ring are present in zeolite ZSM-57, the strain energies of the mEDPE (o-mEDPE, m-mEDPE, and p-mEDPE) are in the range of 30.1−39.2 kJ mol−1, which are lower than those inside TNU-10 (37.7−50.3 kJ mol−1). In addition, all the strain energies of dEDPEs are in the range of 40.4−130.7 kJ mol−1, which are still higher than the barrier threshold energy (32.8 kJ mol−1). Therefore, although mEDPE can be found inside the ZSM-57, bulky dEDPE cannot be formed due to their larger dimensions, which is in good agreement with the experimental results that only weak mEDPE peaks were observed. It is also indicative that the monomolecular pathway will be favored in the ZSM-57. In contrast to zeolites having large- and medium-pores, our theoretical calculations demonstrated that neither mEDPE nor dEDPE can be accommodated inside TNU-10 and ZSM-22. As shown in Figures 2 and 8a, there are almost no signals attributed to either mEDPE or dEDPE to be observed in the GC-MS results. It was found by the theoretical calculation that 23635



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when all the mEDPE and dEDPE isomers were confined inside the TNU-10 and ZSM-22 they are strongly distorted in the relative limited zeolite cages, which results in their strain energies higher than 37.7 kJ mol−1. Therefore, the monomolecular ethyl-transfer reaction pathway though surface ethoxy group intermediates should be exclusively followed. Surface ethoxy group intermediates can be also transformed to ethene; therefore, ethene selectivity of TNU-10 and ZSM-22 is much higher than that of other catalysts as shown in the experimental observations.41

ASSOCIATED CONTENT

S Supporting Information *

The adsorbed energies and XYZ coordinates of ethylbenzene absorbed inside zeolites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS Here, the adsorbed structures and strain energies of mEDPE and dEDPE derivatives as the reaction intermediates during the EB disproportionation have been theoretically investigated to elucidate the intermediate shape selectivity. The theoretical study provides clear-cut evidence that the formations of the reaction intermediates are strongly controlled by the dimension and size of zeolite void spaces. Zeolite Y and MCM-22 have very large cages so that the adsorbed molecules are less distorted and their strain energies were thus calculated to be quite low. As a consequence, all types of mEDPE and dEDPE isomers can be formed within these two zeolites. While inside the narrower pores of TNU-10 and ZSM-22, the mEDPE and dEDPE derivatives are strongly distorted with the strain energies higher than 37.7 kJ mol−1. Therefore, they cannot be formed inside the TNU-10 and ZSM-22. Some mEDPE and dEDPE derivatives with the strain energies less than 32.8 kJ mol−1 can be found in the intersected channels of TNU-9, ZSM-5, and ZSM-57. Consequently, it is expected that mEDPE and dEDPE could be formed. Our theoretical data are obviously in good agreement with the experimental GC-MS relative intensities. These lead us to conclude that the zeolite void spaces play a crucial role in governing the formations of intermediates in the EB disproportionation. Also, theoretical calculations can offer the accurate strain energies of confined organic intermediates inside the zeolite void spaces, which reveal quantitatively the intermediates shape selectivity.

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ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21173255, 21073228, 20933009, and 21210005) and by the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea. 23636



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Stability of the Reaction Intermediates of Ethylbenzene

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