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Origin of Zeolite Confinement Revisited by Energy Decomposition Analysis Benteng Song, Yueying Chu, Guangchao Li, JIqing Wang, An-Ya Lo, Anmin Zheng, and Feng Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09059 • Publication Date (Web): 11 Nov 2016 Downloaded from http://pubs.acs.org on November 14, 2016

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The Journal of Physical Chemistry

Origin of Zeolite Confinement Revisited by Energy Decomposition Analysis

Benteng Song,1,2, # Yueying Chu, 1,# Guangchao Li, 1,4 Jiqing Wang, 2 An-Ya Lo, 3 Anmin Zheng, 1, * Feng Deng 1,*

1. State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, National Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China 2. Hunan Key Laboratory of Green-Packaging and Application of Biological Nanotechnology, Hunan University of Technology, Zhuzhou 412008, China 3. Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taiwan 4. Graduate School, the Chinese Academy of Sciences, Beijing 100049, PR China.

*

Corresponding authors: Fax: +86 27 87199291.

E-mail addresses: [email protected] (A. Zheng), [email protected] (F. Deng). [ # ] These authors contributed equally to this work. 1

ACS Paragon Plus Environment


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ABSTRACT: Our previous work has demonstrated that hydrocarbon species can be stabilized in the confined zeolite in the form of ion pair, π complex and alkoxy species. Nevertheless, the interaction mechanism between the different reactants/intermediates and zeolite framework remains undetermined, and thus the origin of zeolite confinement effect has not been thoroughly revealed. In this work, a recently developed energy decomposition analysis (EDA) method was applied to theoretically investigate the energy parameters of a series of hydrocarbon species confined in the zeolitic catalysts with different pore diameters. It is demonstrated that for the carbenium ion intermediates, the electrostatic interaction plays a key role in their stabilization; for the alkoxy species, both orbital and electrostatic interactions are the key factors; while for the neutral hydrocarbons, the dispersion interaction favors their stabilization. In addition, the principal components analysis (PCA) reveals that the dispersion interaction does not play a crucial role in improving the reaction activity due to the same extent of stabilization effect for different reaction species (e.g., reactant, transition state, intermediate or product), and thus the dispersion contribution would be counteracted in a specific zeolite catalytic reaction. In contrast, the difference in electrostatic interaction caused by the variations of charge characteristics of the various confined species, considerably contributes to the decrease of activation barrier and the increase of reaction energy, which in turn largely promotes the catalytic performance of zeolite catalysts.

1. INTRODUCTION In recent years, along with the strengthening of environmental protection consciousness, zeolite catalysts have been extensively applied in the chemical industry due to their environmentally friendly properties, namely non-corrosive, easily to recycle and non-harmful gases formation during the catalytic process compared with the conventional liquid acids.1-3 Zeolites are microporous catalysts comprised of networks AlO4- and SiO4 tetrahedra that form pores of molecular size, which find industrial applications as solid acid catalysts in the petrochemical industry, such as 2


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methanol-to-olefin

(MTO)

conversion,

alkane

activation

reaction,

alkene

polymerization and Beckmann rearrangement reactions etc.4,5 The diversity of zeolitic catalysts with varied pore shapes and dimensions has been explored extensively both by catalytic experiments and theoretical calculations,6-10 and it is illustrated that the confinement effect derived from the zeolite framework influences the structures and stabilities of all the reaction species (e.g., alkene, carbenium ion and alkoxy species), and thus mediates the catalytic performance.11,12 Apparently, the confinement effect is of high importance among the factors that can affect the reactivity because the adsorbed molecules in zeolite pores can exhibit quite different chemical behaviors compared to those in gaseous state. Previous studies have revealed that the pore confinement depends on the dimensional matching between adsorbed molecules and zeolite channels. For example, several hydrocarbon pool (HP) intermediates (such as benzenium cations) that may exist during the MTO conversion process within zeolite cavities Cha, Lev, and Lta were investigated by Yu et al. through density functional theory calculations. Compared to Lev and Lta, the larger Cha cavity matches HP species well, and provides the most suitable confinement to HP species. As a result, the Cha cavity displays the lowest intrinsic free-energy barriers, and leads to a higher reactivity.13 Similarly, it is confirmed that cavity sizes of various zeolites (e.g., SAPO-35, SAPO-34 and DNL-6) control the dimensions

of

HP

species

(e.g.,

polymethylbenzenium

and

polymethylcyclopentadienyl), and then result in different MTO activity and product selectivity.14 In addition, our previous work has demonstrated that, relative to small acetoxime, H-ZSM-5 zeolite can catalyze the cyclohexanone oxime Beckmann rearrangement reaction more effectively due to the perfect-fit properties between the zeolite pores and the size of cyclohexanone oxime.15 Besides the Brønsted acid-catalyzed reactions, the pore dimension also determines the catalytic performances of the Lewis acid-catalyzed sugar conversion reactions over Sn-based zeolitic catalysts. It is revealed that the product distribution is strongly dependent on the size of catalyst pore, that the formation of bulky methyl-4-methoxy-2hydroxybutanoate is favored by mesopores catalysts, such as Sn-MCM-41 and 3



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Sn-SBA-15; whereas microporous catalysts such as Sn-β or Sn-MFI, favor the formation of methyl vinyl glycolate with relatively small size.16 Furthermore, our previous theoretical investigation has illustrated that the charge properties of adsorbates also play a key role in determining the pore confinement effect, and the ion pairs would be more sensitive to the zeolite pore confinement effect than neutral hydrocarbons and alkoxide species.17 We also demonstrated that the ethylene dimerization reactivity could be obviously improved in the zeolite pore structure due to the ionic character (possessing more net charges) of the transition state.4 In addition, it was also shown that the zeolite pore framework causes a surprisingly large reduction in the barrier for ethane cracking, due to having much more net charge of the transition states on organic fragments.18 It is generally accepted that such confinement effect always leads to a lower activation barrier, resulting in a higher catalytic reactivity. It can be seen from above discussions that the confinement effect is mainly determined by the molecular sizes and the charge characteristics of adsorbates. It should be noted that the stabilization of adsorbates imposed by confinement effect was qualitatively explained by electrostatic interactions and vdW dispersion interactions between adsorbed molecules and zeolite frameworks. However, no quantitative analysis has been performed to explore the contributions of each component on the energy stabilization in the zeolitic catalysis so far. Furthermore, the interaction mechanism between the zeolite frameworks and reaction species with different forms (e.g., neutral hydrocarbon, covalent bond alkoxy, or positive carbenium ion) has not been revealed. Apparently, the confinement effect is far from being fully understood on the basis of host/guest interaction mechanism. In recent years, the energy decomposition analysis (EDA) method, which decomposes energy into four components (i.e., electrostatic interaction, dispersion interaction, orbital interaction and Pauli repulsion interaction), begins to attract much more attention, because it allows us to understand host/guest interaction in more detail. Among the four components, electrostatic, dispersion and orbital interactions facilitate the stabilization of adsorbates, while Pauli repulsion interaction is responsible for the steric constraint destabilization. Based on the EDA calculations, Bickelhaupt and 4


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co-workers have thoroughly studied the Diels-Alder cycloaddition reaction between 1,3-butadiene and noble gas endohedral fullerenes. They concluded that the presence of heavier noble gas dimers in the cage induced a prominent change in the electronic structure of the cage, which could be ascribed to the effect of orbital interaction between C60 and Xe.19 Furthermore, the EDA method was also used to explore the host/guest interactions between xenon and members of a cryptophane family, and it was found that the dispersion force was the predominant factor that stabilized the host/guest interactions.20 As aforementioned, it is demonstrated that EDA is a powerful method to analyze the cluster-like organic reactions, and thus can be used in the zeolite catalysis. For physical adsorbed complex, ion pairs and alkoxy species, they were widely involved inside the zeolite acid-catalyzed reactions, acting as reactants, intermediates or products. The present work explores the influence of the confinement effect on the stabilization of these hydrocarbon species confined inside H-ZSM-5 zeolite by using the EDA method. It is noteworthy that the charge distribution properties and molecular size of adsorbates are two main factors that affect the electrostatic, orbital interactions, dispersion and repulsion interactions. Herein, the quantitative relationship between the unique characteristics (charge distribution properties and molecular size) of adsorbed hydrocarbon species and the four decomposed energies of zeolite confinement effect is explored in detail. In addition, Hβ zeolite was also chosen for comparison in order to further clarify the influence of pore sizes on the stabilization of adsorbed molecules. On the basis of the results of EDA calculations, principal components analysis (PCA) was adopted to explore the confinement effect on these hydrocarbon species on the whole. The consequences of confinement on the stabilization of various adsorbates are thoroughly discussed, and thus, it may provide a fundamental insight into the pore confinement effect during zeolite-catalyzed reactions.

2. THEORETICAL METHODS 2.1. Calculational Model 5



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In this work, all of the optimized structures are derived from our previous work.17 The 72T H-ZSM-5 model covers the intersection of 10-membered ring straight and zigzag pore channels with cross sections of 5.3 × 5.6 and 5.1 × 5.5 Å, respectively (Figure 1a). Based on the 72T H-ZSM-5 model, an 8T cluster model only including the local acid site was constructed to represent the catalysts without pore structures (Figure 1b). Furthermore, the 80T Hβ model with larger pore size was also constructed which contains a three-dimensional channel with 12-membered ring apertures with cross sections of 6.6 × 7.7 and 5.6 × 5.6 Å, respectively (Figure 1c). The Amsterdam density functional (ADF 2014) software package21, 22 was used to explore the stability mechanism for different intermediates (neutral adsorbed complex, ion pair and alkoxide species). For all the ADF calculations, the triple-ζ polarized (TZP) Slater-type all-electron basis set was used.22,23 The local density approximation of Vosko–Wilk–Nusair (VWN)24 augmented with the Becke–Perdew25,26 generalized gradient approximation (GGA) was employed for the exchange-correlation functional.27 This level provides very good results for the interaction between adsorbate and zeolite framework.28 Meanwhile, the dispersion-corrected term was introduced to describe the noncovalent interactions, which was crucial to describe the host/guest interactions. Furthermore, scalar relativistic effects were incorporated by applying the zeroth-order regular approximation (ZORA).29, 30 The molecular charges of organic fragments were calculated using natural bond orbital (NBO) analysis.31 The interaction energy (Eint = Ezeolite - hydrocarbon − Ezeolite − Ehydrocarbon ) is defined as the energy difference between the zeolite adsorption complex and the strained fragments (adsorbed molecules and zeolite frameworks), which reflects the host/guest interaction between adsorbed molecules and zeolite frameworks. A Morokuma-type energy decomposition method was used in the EDA calculation. The Eint is further analyzed using the

molecular orbital (KS-MO) model provided

by the

Kohn-Sham,32,33 which is decomposed into the following physically meaningful terms Eint = Ees+ EPauli + Eoi + Edisp 1. The Ees (electrostatic interaction) term corresponds to the classical Coulomb interaction between occupied molecular orbitals of the strained fragments as they are 6


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brought together at their final positions, giving rise to an overall density. The method is based on the assumption that the charge distribution of the constituent fragments is not changed in comparison with the original molecules. 2. The EPauli (Pauli repulsion interaction) term is based on Pauli principle. It comprises the destabilizing interactions between occupied orbitals, and is responsible for any steric repulsion. 3. The Eoi (orbital interaction) term usually involves a reorganization of electron distribution of the constituent molecules. It accounts for electron pair bonding, charge transfer (e.g., HOMO-LUMO interactions) and polarization (empty/occupied orbital mixing on one fragment due to the presence of another fragment). 4. The Edisp (dispersion interaction) term considers the dispersion forces. It is important to describe the host/guest interactions in zeolite-catalyzed reactions. In addition, the noncovalent interaction index approach, developed by Yang et al.34 was also utilized to visualize the noncovalent interactions between adsorbates and zeolite frameworks aiming at further understanding the confinement effect. In this method,

the

reduced

density

gradient

(RDG),

defined

as

s

=

(1/(2(3π2)1/3))((|∆ρ(r)|)/(ρ(r)4/3)), was used to distinguish the covalent and noncovalent interactions. The sign of the second largest eigenvalue (λ2) of the electron density Hessian can be used to distinguish bonded (λ2 < 0) from nonbonded (λ2 > 0) interactions. In general, the different types of noncovalent interactions can be discerned from analysis of the sign of λ2. H-bonding interactions corresponding to sign(λ2)ρ < 0; strong repulsive interactions corresponding to sign(λ2)ρ > 0 and weak van der Waals (vdW) interactions, sign(λ2)ρ ≈ 0. Here intramolecular interactions were eliminated for the calculated RDG function in order to elucidate intermolecular noncovalent interactions between the adsorbates and zeolite frameworks more obviously. Calculation of function RDG and sign(λ2)ρ were performed using Multiwfn software.35 2.2. Multivariate Data Analysis. In order to clarify the characteristics of these intermediates more clearly, the EDA results are further analyzed by the principal components analysis (PCA).36,37 The PCA 7



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method is a multivariate analytical procedure which has been extensively used to explore the correlated nature of the variables within a dataset according to statistical techniques, where the data are transformed into principal components based on the internal relevance and the largest variance in the dataset can be described by the first principal component. Concurrently the maximum of the remaining variance in the dataset will be represented by the following principal components.38 In this work, the PCA method was applied, aiming at (i) exploring the components that could separate the three kinds of intermediates from each other by scores plot; (ii) demonstrating the extent of each of the four decomposed energies affects the two principal components that allow us to separate the three kinds of intermediates by analyzing the loadings plot. Multivariate data analysis was conducted using software package SIMCA-P+ (V. 12.0, Umea, Sweden).

3. RESULTS It is well known that the zeolite pore can provide different stabilized environments for adsorbed hydrocarbons existing as neutral adsorbed complex, alkoxide and carbenium ion.17 Generally, the visualization of isosurfaces of reduced density gradient in real space can effectively describe the noncovalent interactions between the organic fragments and zeolite frameworks.15,39 The isosurfaces of reduced density gradient for ethene and ethoxide confined in ZSM-5 zeolite (see Figure 2a and b) show that the interactions strongly depend on the chemical structures. The physically adsorbed ethene gains pronounced vdW interaction imposed by the zeolite framework (green regions in Figure 2a), while significantly repulsive effect is present for the chemically adsorbed ethoxide species (red regions in Figure 2b). It is well accepted that the electron density difference which is defined as the electron difference between the adsorbed complex and the sum of isolated zeolite and the adsorbate (∆ρ = ρ(zeolite-hydrocarbon) − ρzeolite − ρhydrocarbon), can qualitatively characterize the electron transfer between organic fragment and zeolite fragment in the adsorbed structure.40 Generally speaking, the remarkable electron transfer between zeolite and organic 8


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fragment will result in a stronger electrostatic interaction in host-guest system. As displayed in Figure 2c and d, the relatively obscure electron transfer between the ethene molecule and zeolite framework compared to covalent ethoxide suggests the weaker electrostatic interaction of ethene confined in the zeolite pore. Therefore, the pore confinement effect of the zeolitic catalysts displays different features for neutral adsorbed complex, charged adsorbed species, e.g., alkoxide species. Hence, it is necessary to explore the unique stabilization factors of different reaction species inside the zeolite pores and then reveal their inherent characters (e.g., chemical structure, dimension and charge) that affect the stabilization mechanisms in detail. 3.1 Neutral Adsorbed Complexes in H-ZSM-5 Zeolite Three

types

of

alkenes

(linear

alkene,

polymethylcyclopentadiene

and

polymethylbenzene) illustrated in Scheme 1 are considered as typical neutral organic compounds to explore the olefin adsorption characteristics over H-ZSM-5 zeolite. In this work, 8T and 72T H-ZSM-5 models were applied to elucidate the confinement effect imposed by zeolite framework. Compared with 8T cluster model, the 72T model containing the complete ZSM-5 pore structure can supply additional stabilizing effect on the confined species. Taking ethene as an example, it is found that the two carbon atoms of the C=C bond interact with the acidic bridging proton with almost the similar distances (i.e., the distances of two C…H are 2.09 and 2.11 Å, respectively) in of the 72T model, which leads to the formation of a π complex.17 The EDA results of the 12 olefins over the 72T models are depicted in Figure S1 and the corresponding energies are given in Table 1. Clearly, the terms of Ees, Edisp and Eoi act as the stabilizing components, while the EPauli term constitutes the destabilization factor for adsorbates confined inside the zeolite framework. For the various olefin fragments, the values of energy decomposition terms vary significantly. For linear alkene (1h (ethene) to 3h (isobutene)), the Ees, Edisp and Eoi terms are in the range between -50 and -77 kJ/mol, -47 and -85 kJ/mol, -54 and -79 kJ/mol, respectively. Each energy component is gradually enhanced with the increase of adsorbed alkene size. The remaining destabilizing factor (EPauli) is monotonously increased from 81 to 127 kJ/mol, which is larger than the absolute values of each stabilization components. It is 9



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well known that the protonation affinity (PA) of alkene is an intrinsic indicators of its basicity, and PA value of a hydrocarbon is defined as the released energy of the protonation reaction of a hydrocarbon with an acidic proton, i.e., H+ + R → HR+.41,42 Clearly, a larger PA value of a hydrocarbon corresponds to a stronger basicity. On the basis of the previous DFT calculations, the PA values of 1h (ethene), 2h (propene) and 3h (isobutene) are 686, 743 and 803 kJ/mol, respectively.17 It is indicative that the basicity is gradually increased from 1h to 3h, so that the charge transfer from the zeolite Brønsted acidic protons to hydrocarbon fragments in the π complexes becomes obvious gradually. As a consequence, the interaction strengths of Ees and Eoi (from 1h to 3h) are enhanced as illustrated in Table 1. In the case of Edisp and EPauli components, they depend strongly on the dimension size of hydrocarbon. In generally, the larger size of the organic fragment, the higher the interaction energy, such as the values of Edisp (ranging from -47 to -85 kJ/mol for 1h to 3h) and EPauli (81 to 127 kJ/mol). For polymethylcyclopentadiene fragments with varied methyl groups (from 4h (1, 3-cyclopentadiene) to 6h (1, 2, 3-trimethylcyclopentadiene)), the energy components of Ees, Edisp, Eoi and EPauli are in the ranges of (-59, -116), (-91, -169), (-64, -91) and (117, 237) kJ/mol, respectively. Similar to the linear alkene, each part of the decomposed energies is increased with the numbers of methyl substitutions. In addition, the decomposed energies of polymethylcyclopentadienes are larger than those of linear alkenes, especially for 6h, which can be attributed to its stronger basicity (PA is ca. 918 kJ/mol) and larger dimension size. Among these stabilizing components, it is noteworthy that the Edisp term varies the largest (78 kJ/mol) compared to the terms of Ees (57 kJ/mol) and Eoi (27 kJ/mol), which suggests that the Edisp is much more sensitive to the size of adsorbates inside zeolite pores. On the other hand, on the basis of the absolute energy values, it is found that Edisp is larger than both Ees and Eoi. Apparently, the Edisp mainly contributes to the stabilization of alkene species in the π complexes. The polymethylbenzene species (from 7h (toluene) to 12h (hexamethylbenzene)) are the possible intermediates in the MTO reaction.14,43 Thus, the investigation of their adsorption behavior is of high importance for understanding the MTO reaction 10


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mechanism. As illustrated in Table 1, the values of each decomposed components (i.e., Ees, Edisp, Eoi and EPauli) gradually increase from 7h to 12h, in accordance with the aforementioned results from linear alkene and polymethylcyclopentadiene species. It is noteworthy that the dispersion interaction from the bulky fragment of polymethylbenzene and zeolite framework plays a crucial role in enhancing the stabilization of the host-guest system. For example, Ees, Edisp and Eoi terms of 11h (pentamethylbenzene) are -164, -241 and -98 kJ/mol, and Edisp value is about twice larger than that of Eoi. Certainly, the bulky fragment of polymethylbenzene also will result in an increase of the Pauli repulsion interaction (EPauli), which is significantly increased to 386 kJ/mol (11h) from 150 kJ/mol (7h) as listed in Table 1. For hexamethylbenzene (12h), the EPauli value dramatically increases to 612 kJ/mol. This suggests that the repulsion force between the bulky fragment and the zeolite framework would lead to the destabilization of the confined molecules. The previous work has demonstrated that the strain energy less than 33 kJ/mol was necessary for an organic species stabilized inside zeolitic catalysts.44 However, the strain energy for 12h is 43 kJ/mol inside H-ZSM-5 zeolite (see Table S1 in the Supporting Information). On the other hand, the total interaction energy (Eint) is significantly decreased to -52 kJ/mol for 12h compared to (-112, -135) kJ/mol for the other lower ploymethylbenzene. This indicates that hexamethylbenzene is unlikely present in the MTO reaction inside the severely restricted H-ZSM-5 pores. It is noteworthy that the pentamethylbenzene

hydrocarbon

has

been

identified

as

the

largest

polymethylbenzene that could be persistent in the MTO conversion over zeolite H-ZSM-5 by the GC-MS and in situ 13C MAS NMR analysis.43 In order to further demonstrate the environment effect on the stability of the organic fragment, the EDA calculations over 8T cluster models were also performed. Comparing to the 72T models with the complete double 10-membered ring (10-MR) pores, the 8T models only contain the local active centers, almost ignoring all the confinement effect imposed by the zeolite framework. Similar to the 72T results, all the energy components are synergistically determined by the basicity and dimension size of organic fragments. However, except 12h, all the stabilizing components (i.e., 11



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Ees, Edisp and Eoi) are within -88 kJ/mol, and the destabilizing component of EPauli is in the range of ca. 69-122 kJ/mol. Moreover, the variation in the each energy components is limited (typically, less than 17, 61, 14 and 53 kJ/mol for Ees, Edisp, Eoi and EPauli) in 8T models in comparison with that in 72T cluster models (114, 194, 44 and 305 kJ/mol, respectively) as given in Table 1. Even for the bulky polymethylbenzene (12h), the decomposed energies (i.e., Ees, Edisp, Eoi and EPauli) are not obviously differentiated by the methyl substitutions compared to other polymethylbenzene (7h-11h) on the 8T cluster models (see Table 1). On the basis of the aforementioned discussions, it is illustrated that all the values of decomposed components (i.e., Ees, Edisp, Eoi and EPauli) in the 72T models are much larger than those in the 8T models, especially the Edisp term, indicating that the confinement effect plays an important role in stabilizing the neutral organic complexes (Table 1). This can be reflected in the visualization of isosurfaces of reduced density gradient. The isosurfaces of neutral species 7h and 12h adsorbed on both 8T and 72T cluster models are shown in Figure 3. On the isolated 8T cluster model, the isosurfaces of 7h and 12h complexes are similar and are exclusively located at the local active sites (see Figure 3a and b). While the much larger green regions are observed for 7h confined inside the 72T H-ZSM-5 model (Figure 3c), being indicative of the confinement effect on stabilizing the adsorbed molecules in zeolite pores. Compared to 7h over 72T model, the isosurface of 12h displays much larger red regions (Figure 3d), suggesting the existence of stronger steric repulsions between the bulky fragment and zeolite framework. Thus, this provides evidence that 12h complex is probably unstable in H-ZSM-5 zeolite pores. 3.2 Alkoxy Species in H-ZSM-5 Zeolite It is well known that the protonation of an adsorbed olefin by the Brønsted acid site may result in formation of an alkoxide complex in which the positively charged carbon atom is covalently bound to the zeolite basic oxygen atom . Alkoxide species act as important intermediates in the zeolite acid-catalyzed reactions, such as the alkane cracking, olefin dimerization and MTO reactions.4,45,46 According to the molecular structures, two alkoxide isomers, i-propoxide (secondary alkoxide 12


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formation) and n-propoxide (primary alkoxide formation), exist for the propoxide inside the zeolites. Moreover, three possible isomers (e.g., primary, secondary and tertiary structures) are formed for the butyl-alkoxides. Stabilities of different alkoxides remain under debate. In some cases, the secondary alkoxide is found to be more stable than primary alkoxide, or tertiary structure is much favored over primary and secondary alkoxides. This has been demonstrated in several theoretical and experimental studies.47,48 For example, Rozanska et al. have demonstrated the reaction energy of i-propoxide (-27 kJ/mol) was 10 kJ/mol lower than n-propoxide (-17 kJ/mol) for the propene reaction in acidic chabazite.49 In contrast, Corma et al.50 and Davis et al.51 analyzed the chemisorption of several alkenes on both mordenite zeolite and phosphotungstic acid, such as ethene, propene and isobutylene. They confirmed that the stability of the covalent alkoxides decreases in the order of primary > secondary > tertiary, and, as a rule, lower stability is related to the longer C-O covalent bond distance. Apparently, it remains controversial about which alkoxide species is more stable. Therefore, it is necessary to systematically investigate the interaction mechanism of alkoxide and zeolite framework. In this section, ethoxide (1R), i-propoxide (2R) and tert-butoxide (3R) were represented as typical structures of primary, secondary and tertiary alkoxide species inside the zeolites, respectively (see Scheme 1). Each contribution from electrostatic energy (Ees), dispersion energy (Edisp), orbital energy (Eoi), and repulsion energy (EPauli) to the alkoxides will be discussed in detail. As shown in Table 1, the repulsion interaction energies (EPauli) are in the range of 838-898 kJ/mol for the three alkoxides (1R, 2R and 3R), which are much larger than those of their corresponding neutral adsorbed species (simply alkenes (81-127 kJ/mol for 1h to 3h), polymethylcyclopentadiene (117-237 kJ/mol for 4h-6h) and polymethylbenzene (150-386 kJ/mol for 7h-11h)) (see Table 1). Therefore, it is illustrated that a significant repulsion interaction is present between alkoxide species and zeolite framework (see Figure 2b and Figure S2). Since the alkoxide species are directly bound to the zeolite framework through covalent bond, the distances between the organic fragments and adsorbents are dramatically shortened to ca. 1.5 Å from the 13



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corresponding alkene adsorption (ca. 3.0 Å) on the zeolites. And thus the repulsion interaction is significantly increased for the alkoxide species. Compared to ethoxide (867 kJ/mol), EPauli component of i-propoxide is increased to 898 kJ/mol, in consistent with the increase of molecular size. However, for the tert-butoxide (3R) with the largest dimension size, EPauli is surprisingly decreased to 838 kJ/mol. Based on the previous work, the C-O covalent bond lengths are extended from 1.50 (1R) and 1.52 Å (2R) to 1.58 Å (3R)17. It is noteworthy that the total repulsion energies of host/guest interactions are synergistically determined by the size of adsorbate and the distance between the host and guest fragments. Apparently, the bulky dimension of tert-butoxide (3R) would result in a slight increase of the repulsion energy. However, the relatively largest C-O distance of tert-butoxide (3R)/zeolite would largely weaken the steric repulsions, which results in a decline of the total repulsion energy. Compared to the dramatically enhancement of repulsion interaction (EPauli) for alkoxide (> 800 kJ/mol), Edisp of adsorbed alkoxide species in zeolites is slightly increased from -56 kJ/mol (1R) to -107 kJ/mol (3R) with increasing the molecular dimensions. It is noteworthy that these values are in good agreement with Edisp results of neutral alkene adsorbed complex (ca. -47- -85 kJ/mol for 1h-3h over the 72T H-ZSM-5 model). Apparently, Edisp is not as sensitive as EPauli to the interaction distance between organic fragment and zeolite framework (especially C-O bond). This can be rationalized by the different dependences of Edisp and EPauli on the RC-O bond length in the alkoxide systems that Edisp is associated with six power of distance, while term of EPauli is correlated with 12 power of distance.52 And the detailed relationship between these two energy components and distance will be discussed in the next section. With respect to the terms of Ees and Eoi, which are strongly related to the charge properties of adsorbed species, they are in the range of (-717 and -778) and (-656 and -798) kJ/mol, respectively for the alkoxides (i.e., 1R-3R). While the corresponding values for the neutral hydrocarbons (1h to 3h) are less than -80 kJ/mol. Such large Ees and Eoi are strongly determined by the electronic distribution and charge transfer between organic fragment and zeolite framework in the alkoxide structures. 14


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Compared to the neutrality of adsorbed alkenes (< 0.06 |e|), the alkoxide species hold ca. 0.4-0.5 |e| charges (see Table 2), which will considerably enhance the orbital and electrostatic interactions. This can be seen from the results of ethene and ethoxide as shown in Figure 2c and d. As well-known, orbital interaction accounts for electron pair bonding, charge transfer (e.g., HOMO-LUMO interaction) of host and guest fragments. As aforementioned that the covalent C-O bond was weakened from 1R to 3R, this in turn leads to the decreasing of Eoi energies (Table 1). Generally speaking, Ees component is determined by the charge densities of organic fragments for the host/guest systems. Since the bulkier alkoxide complex possess more ionic character (i.e., the positive charges on the organic fragment are 0.467, 0.486 and 0.516 |e| for ethoxide, i-propoxide and tert-butoxide), the Ees component should exhibit an increasing trend. However, it is noteworthy that the electrostatic interactions are slightly decreased (i.e, Ees are -778, -764 and -717 kJ/mol for 1R, 2R and 3R respectively). Clearly, besides the charge characteristics of the organic fragment, the distance between positive organic fragment and negative zeolite fragment also plays a crucial role in determining the electrostatic interaction. The synergy effect of charge and distance ultimately results in the decline of the total electrostatic interaction of alkoxide. Additionally, it can be seen that decomposed energies of Ees and Eoi are much larger than that of Edisp, demonstrating electrostatic and orbital interactions both contribute to the stabilization of alkoxide species. In terms of the capability of methyl group in spreading the positive charges, the stability of -C(CH3)3 should surpass those of -CH(CH3)2 and -CH2CH3 in liquid.53-55 However, it is not in agreement with the alkoxide stability as illustrated by Sauer et al.56 that it follows the order of ethoxide > i-propoxide > tert-butoxide inside the H-FER zeolite. And our previous work also confirmed that n-propoxide is thermodynamically much more favorable than i-propoxide over the ZSM-5 zeolite.57 Apparently, such differences between the liquid homogeneous catalysis and zeolite heterogeneous catalysis are mainly caused by the rigid framework of zeolites, which dramatically restrict the alkoxy formation through the covalent bond for the bulky organic fragment. In turn, such increasing steric hindrance of the tert-butoxide inside 15



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zeolite would result in the decrease of orbital and electronic interactions. On the other hand, the strain energy of tert-butoxide in H-ZSM-5 amounts to 153 kJ/mol (see Table S1), which is much larger than the strain threshold of hydrocarbon confined inside zeolites (33 kJ/mol).44 It is clearly indicative that the structure tert-butoxide directly bound to the zeolite has been strongly deflected from its balance position in the free state. Thus, the instability of tert-butoxide inside zeolitic catalysts was theoretically predicted. In fact, this bulky alkoxide has not been experimentally captured in zeolite H-ZSM-5 so far. 3.3 Carbenium Ions Compared to alkoxide species, the carbenium ions shown in Scheme 1 act as intermediates with much higher catalytic reactivity in zeolite heterogeneous reactions, cyclopentadienyl and benzenium cations have been confirmed experimentally as co-catalysts in the MTO reaction over ZSM-5, Y, SSZ-13 and Beta zeolites.58-64 13C solid-state

NMR

experiments

have

identified

the

formations

of

1,3-dimethylcyclopentadienyl, tetramethylbenzenium and pentamethylbenzenium ions inside ZSM-5 zeolite.46,58-61 While besides the 1,2,4,5-tetramethylbenzenium and pentamethylbenzenium

ions,

the

relatively

larger

dimension

size

hexamethylbenzenium cation was also observed in the Beta zeolite.62,63 Apparently, the formations of carbenium ions are strongly depended on zeolite pore size and its shape. Despite various carbocation species were experimentally observed in different zeolites, the nature of the host-guest interaction and the stability mechanisms of carbocation adsorbed inside zeolites have not been well understood. As aforementioned that the electrostatic interaction plays a crucial role in the stabilization of alkoxides by the charge characteristics of the organic fragments (0.4-0.5 |e|). Similarly, larger electrostatic interactions (Ees) are generally predicted for the carbenium ions (the charges of organic fragments amount to ca. 0.9 |e|, see Table 2) if the electrostatic interactions are solely determined by the separate charges. However, an inverse trend has been observed that electrostatic interactions are less than -415 kJ/mol for carbocation species (see Figure S3a), which are much lower than those of the alkoxide species (< -717 kJ/mol) (see Table 1). This abnormal phenomenon can be 16


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well explained by the adsorbed structures of alkoxides and carbenium ions over zeolite pores. In contrast to the C-O bond (ca. 1.5 Å) for the alkoxide species, the distances between carbenium ions and zeolite frameworks extend to ca. 3.0 Å, which considerably decrease the electrostatic energy in the carbocation/zeolite systems. Since the orbital interactions also strongly depend on the overlap of HOMO (zeolite framework) and LUMO (carbocation ion), and thus much small contribution of the orbital energy (less than -148 kJ/mol, see Table 1) to the total interaction energy (Eint) is obtained for the relatively larger distance between carbocation and zeolite framework. Besides the charge characteristics, the dimension size of carbocation species is another important factor to determine the interaction mechanism. It is accepted that a bulkier adsorbate generally leads to a larger Edisp and EPauli. As shown in Table 1, the Edisp of polymethylcyclopentadienyl and polymethylbenzenium ions inside H-ZSM-5 zeolite are in the range between -109 (4c) and -245 kJ/mol (11c), indicating that Edisp is mainly determined by molecular sizes. On the other hand, the bulkier carbocation species would lead to a larger steric repulsion between organic fragment and zeolite framework. As listed in Table 1, the repulsion energies (EPauli) are dramatically increased from 161 (4c) to 395 kJ/mol (11c). Especially, the repulsion energy of hexamethylbenzenium (12c) is further increased to 462 kJ/mol, and the Eint begins to decline. It is evident that hexamethylbenzenium (12c) is unstable or can not be formed in H-ZSM-5 zeolite, which is in good agreement with the

13

C NMR experimental

results that bulkier hexamethylbenzenium cation has not been observed inside limited space of H-ZSM-5 zeolites.59-61 In addition, the isosurface of 12c displays a much larger unstable steric exclusion (red regions) compared with the relatively small molecule (see Figure S4a and S4b in the Supporting Information). Fortunately, it is found that such a repulsion interaction as shown in Figure S3b is considerably decreased in Hβ zeolite with a larger cavity (6.62 Å). All the energy decomposition terms (electrostatic energy, orbital energy, dispersion energy and repulsion energy) of hexamethylbenzenium ions are decreased in BEA zeolite compared to ZSM-5 zeolite, and the repulsion energy (EPauli) was decreased by 219 kJ/mol, much larger than the 17



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sum of other three terms (154 kJ/mol), which results in the interaction energy for BEA (-439 kJ/mol) being 65 kJ/mol larger than that in ZSM-5 zeolite (-374 kJ/mol). The isosurfaces of reduced density gradient also indicate that steric repulsions have been effectively released for hexamethylbenzenium confined in Hβ zeolite (see Figure S4d). Furthermore, the strain energy of hexamethylbenzenium cation (12c) is decreased to 4 kJ/mol in Hβ zeolite relative to 33 kJ/mol in H-ZSM-5 zeolite (Tables S1 and S2). At the same time, the Eint value of 12c is -439 kJ/mol in Hβ zeolite, which is similar to that of other polymethylbenzenium cations (see Table 3). Therefore, it is apparently indicative that bulkier 12c cation is ready to form inside Hβ zeolite. Compared to H-ZSM-5 zeolite, the decrease in the repulsion energies and the increase in the total interaction energies are observed for the other ploymethylbenzenium ion in Hβ zeolite as well (Table 3). Our theoretical results are consistent with in situ NMR experiment observations that besides 1, 2, 4, 5-tetramethylbenzenium and pentamethylbenzenium ions, the relatively larger hexamethylbenzenium cation was also observed in Hβ zeolite.62,63 In

addition

to

the

conventional

polymethylcyclopentadienyl

and

polymethylbenzenium carbocations, the tert-butyl cation (3c) has attracted much attention as an important intermediates in zeolite catalytic reactions. Recently Hunger et al.65 and Deng et al.66 confirmed its formation during the isobutene and isobutanol transformation. Therefore, the tert-butyl cation is also considered in this work. As shown in Table 1, the Ees value (-381 kJ/mol) of 3c is much larger than both the Edisp (-97 kJ/mol) and the Eoi (-132 kJ/mol) values, demonstrating that the electrostatic interaction (Ees) plays a critical role in stabilizing the tert-butyl cation, in consistence with

the

results

derived

for

the

polymethylcyclopentadienyl

and

polymethylbenzenium carbocations. It is noteworthy that the Eint values of alkoxy species and carbenium ions are much larger than those of the corresponding neutral complexes (see Table 1). However, the relatively larger Eint values do not mean that the transformation of neutral complexes to the corresponding alkoxide species or carbenium ions is facile in zeolite catalysts. In order to compare the relative stabilization of the intermediates in different forms, 18


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their single point energies should be calculated, which has been done in our previous work.17 Based on the calculated single point energies, it is conclusive that only part of alkoxide species and carbenium ions could be formed in ZSM-5 zeolite from the thermodynamics point of view.

4. DISCUSSION On the basis of the EDA results, it can be seen that the unique properties of hydrocarbons (e.g., dimension size and charge characteristic) differentiate the stabilization mechanisms in zeolite confinement pores. The predominant factor stabilizing neutral hydrocarbon complex is the dispersion interaction; the covalent alkoxide species strongly depend on both orbital and electrostatic interactions; while the electrostatic interaction plays a crucial role in stabilizing carbenium ion inside the zeolite pores. Furthermore, the dispersion interactions usually increase with the increase of dimension size of adsorbed species with similar structures, no matter which kind of hydrocarbons is considered (i.e., neutral hydrocarbon, alkoxy species and carbenium ion). Based on the universal trend as aforementioned, quantitative analysis of the components for the energy stabilization will be explored in detail. The Lennar-Jones interaction (ELJ) proposed by John Lennard-Jones52 is described by the following equation:

ELJ

 R ij = ∑ ε ij    rij ij 

12 6  R    − 2 ij     rij      

(1)

Where εij = (εiεj)1/2 and Rij = (Ri + Rj)/2, Ri is the van der Waals radius, and εi is the characteristic energy for the Lennard-Jones potential; rij is the distance between particles i and j. The first polynomial that associates with 12 power of radius represents steric repulsion (EPauli) between two fragments in eq 1; while the second polynomial is related to dispersion component (Edisp). The dispersion component will be discussed in order to reveal the interactions influenced by different factors. Apparently, the total dispersion energy is the sum of dispersion energy between guest fragment (adsorbed molecule) and host fragment (zeolite framework). 19



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Undoubtedly, the increase of host/guest size will lead to an enhancement of the total dispersion energy. Therefore, an extension of the calculated model from isolated 8T to 72T model H-ZSM-5 zeolite results in an increase of Edisp. As shown in Table 1, the dispersion energies are -27, -44 and -53 kJ/mol for ethene (1h), propene (2h) and isobutene (3h) over 8T model, which are increased to -47, -67 and -85 kJ/mol in 72T model. This provides direct evidence that the zeolite framework can sufficiently stabilize the reactants. On the other hand, the increase of the dimension size of adsorbates can affect the dispersion interaction, as confirmed by the Edisp values of ethene (1h), propene (2h) and isobutene (3h) regardless of calculation models. The Edisp energies of cyclopentadiene (4c) and 1,2,3-trimethylcyclopentadiene (6c) in the 72T H-ZSM-5 models are dramatically increased to ca. -109 and -179 kJ/mol due to their bulky sizes. It can be seen from eq 1 that the dispersion interaction is also related to the distance between organic fragment and zeolite framework, which can be rationalized by the values of 1R (-56 kJ/mol), 2R (-80 kJ/mol) and 3R (-107 kJ/mol). Compared to neutral hydrocarbons, the alkoxide species are directly bound to zeolite framework through covalent C-O bond, and thus the considerably shorter distances (ca. 1.5 Å, which is close to the standard C-O bond length) in the alkoxide systems lead to larger dispersion energies. Especially, the distance decreases with molecular size (M) of adsorbate being increased (Figure 4). Thus, synergy of the adsorbate size and the shortened distance would considerably strengthen the dispersion interaction as for 10h (-214 kJ/mol) and 11h (-241 kJ/mol). According to the above analysis, it is demonstrated that the dispersion interaction in the zeolite systems is mainly determined by the pore structures, the molecular sizes of adsorbed species, and the distances between host/guest fragments. In general, the complete pore structure always results in larger dispersion interaction. On the other side, the dispersion interaction is further enhanced with the increase of molecular size of adsorbed species due to the reduction of their distance to zeolite framework, regardless of the type of adsorbed species (e.g., neutral hydrocarbon, covalent alkoxide, or carbocation). On the basis of eq. 1, the factor that influences the Pauli repulsion interaction is 20


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similar to that of the dispersion interaction, namely the zeolite framework, shorter distance and bulky adsorbate would result in an observable EPauli (see Tables 1 and 3). For instance, the EPauli of 12h in the 72T H-ZSM-5 model dramatically increases to ca. 612 kJ/mol. The large repulsion energy can be explained by the large molecule confined inside relatively small zeolite channels. While the larger EPauli values of smaller alkoxides (larger than 800 kJ/mol for 1R, 2R and 3R) can be attributed to the short covalent C-O bond length (ca. 1.5 Å). It is noteworthy from the EDA results that in all cases the absolute value of repulsion energy is much larger than that of dispersion energy (see Table 1) since the repulsion component is more sensitive to the distance (the dispersion component is related to six power of distance, while repulsion component corresponds to 12 power as shown in eq 1) between organic fragment and zeolite framework. Thus, the steric repulsions are strongly mediated by the distance between two fragments in the zeolite catalytic reaction. Compared to the neutral hydrocarbons, the covalent alkoxides and carbenium ions can result in an apparent charge re-distribution over the zeolite frameworks. Therefore, it is necessary to study the interaction mechanism of the charged adsorbates. The electrostatic part of this interaction is given by67 NA NB

qi q j

i =1 j =1

4πε 0 rij

E = ∑∑

(2)

Where qi and qj is the charges on particles i and j, respectively; rij is the distance between particles i and j; and ε0 is the permittivity of free space. The corresponding net charges of neutral adsorbed complexes, alkoxides and carbenium ions in H-ZSM-5 zeolite are given in Table 2. Obviously, weak π-bond interaction only results in a slight charge migration (less than 0.06 |e|), and thus, the electrostatic interaction is not a dominating factor to determine the stabilization of the neutral hydrocarbons. This can be seen from Table 1 that the dispersion interaction plays prominent role in stabilizing the neutral complexes, especially for bulky polymethylcyclopentadiene

and

polymethylbenzene

fragments

confined

inside H-ZSM-5 zeolite. In contrast to the neutral fragments, the alkoxide and carbenium ion species possess ca. 0.5 and 0.9 |e| net charges respectively (Table 2). As 21



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shown in Table 1, the Ees values of cyclopentadienyl cations are increased to (-356, -375) kJ/mol, much larger than those of the corresponding neutral cyclopentadiene (-59, -116) kJ/mol (see Table 1). Meanwhile, the Ees values of carbenium ions both of polycyclopentadienyl and polymethylbenzenium are much larger than the other components (e.g., Edisp, Eoi and EPauli). This indicates that the stability of carbenium ions can be considerably enhanced by the electrostatic interaction from the zeolite framework, which has been confirmed by the previous theoretical work.39,68 Compared to the bulky carbocations, the alkoxide species have relatively smaller dimensions and less charges, however, the Ees values of alkoxide species are larger than -700 kJ/mol, which are almost twice of those of the carbocations. Such a difference between alkoxides and carbocations can be ascribed to the covalent bond of organic fragement to the zeolite framework oxygen atom. The distance between two fragments can significantly affect the electrostatic interaction (see eq 2). Compared with the distances (ca. 3.0 Å) between the carbocations and zeolite frameworks, the much shorter distances (ca. 1.5 Å) in alkoxide systems would result in a considerable increase of the electrostatic energies (see Table 1). On the other hand, the covalent structure of alkoxide species inside zeolites also strengthens the orbital interaction. The orbital interaction is related to the orbital overlap of the host/guest species, which is useful to discuss the charge transfer between two isolated fragments.69 As shown in Figure 5, the orbital overlap between the HOMO and LUMO of isolated zeolite and ethoxide is present when the ethoxide is bound to the zeolite framework oxygen atom. According to the traditional frontier molecular orbital (FMO) theory,70,71 the orbital overlap (S) generally occurs between LUMO of adsorbates and the corresponding HOMO located on the zeolites. In general, a stronger orbital interaction corresponds to a larger orbital overlap. In terms of quantity, the orbital overlap integral can be described by frag 1 SiInt (r )ϕ jfrag 2 ( r ) dr , j = ∫ ϕi

(3)

Where φi and φj are two molecular orbitals belong to molecular fragment 1 and fragment 2, respectively. 22


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The corresponding S values of the various hydrocarbons (e.g., alkene, alkoxides and carbenium ions) are listed in Table 4. It can be seen that the S values change significantly with varying the adsorbed molecules. For neutral adsorbed fragments, the values are 0.00419 (1h), 0.01195 (2h), 0.00603 (3h), 0.01206 (4h) and 0.00636 (5h); the S values of alkoxides are 0.07394 (1R), 0.05570 (2R) and 0.01830 (3R),; with respect to carbenium ions, the corresponding values are usually lower than 0.01, such as 0.00464 (3c), 0.00875 (4c) and 0.00294 (5c). The results clearly indicate that the alkoxides possess the largest value of orbital overlap in comparison with neutral complexes and carbocations. As shown in Table 4, the orbital interactions for neutral alkene species range from -54 to -81 kJ/mol; and for carbocations, they are smaller than -132 kJ/mol; while in case of alkoxides with largest orbital overlap, the orbital interactions are -798 (1R), -747 (2R) and -656 (3R) kJ/mol. Furthermore, the orbital interactions strongly depend on the length of C-O covalent bond, and the relatively shorter C-O bond for ethoxide (1R) results in a stronger orbital interaction. Apparently, a larger S value corresponds to a stronger orbital interaction in the host/guest systems, and thus an increased stability of adsorbates. This trend can be confirmed by the contribution of orbital interaction in all the stabilizing components as well. It can be seen from Table 4 that the orbital component in alkoxides contributes about 50% to the stabilizing energies. While the orbital component (Eoi values) of both neutral adsorbed fragments and carbenium ions are less than -132 kJ/mol, which contribute less than 40% to the stabilizing energies. Principal components analysis (PCA) acting as a multivariate analytical procedure was widely used in metabonomics and material sciences,36,72-74 in which the data can be described by a few orthogonal components (principal components) that are linear combination of the original variables. Herein, the PCA method is employed on the whole energy components (variables) (Tables 1 and 3) (i) to reveal the primary factors that determine the stabilities of hydrocarbon fragments and (ii) to explore which characteristics of adsorbed hydrocarbons (e.g., chemical structure or charge property) favors the catalytic activity enhancement within zeolite confined environments. On the other hand, the loading plot is applied to explore the extent to which energy terms 23



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affects the two components, where the neutral complexes, carbenium ions and alkoxy species are separated from each other. Generally speaking, the descriptors on the loading plot at the extreme ends of the x or y axes have the most significant impact on the component that defines that axis. While those are closed to the origin have a negligible effect. As shown in Figure 6a, the neutral complexes, carbenium ions and alkoxide species are clearly separated from each other in the scores plot by using the two PCA components. The neutral complexes are separated in the middle-upper part of the score plot, the carbenium ions in the middle-down part, and the alkoxide species are separated in the right part of the scores plot. Component one (x axis) describes the physical component that allows us to separate alkoxide species from neutral complexes and carbenium ions, while neutral complexes are split from carbenium ions from each other on the basis of component two (y axis). The distribution of the three kinds of hydrocarbons can be further explained from the loading plot as depicted in Figure 6b. Apparently, the EPauli component has significant positive loading on the x axis, indicating that the EPauli values would increase in magnitude as we go from neutral complexes and carbenium ions to alkoxide species (emphasized with gray rightward arrow). As already known, the repulsion energies are dramatically increased to ca. 800 kJ/mol in alkoxide systems from the corresponding values for neutral fragments (< 612 kJ/mol) and carbocations (< 462 kJ/mol) (see Tables 1 and 3). It is noteworthy that 12h is obviously separated from other neutral complexes (the repulsion energy for 12h is obviously larger than other neutral species) due to the remarkably steric repulsions in zeolite. It is also found that the Eoi component with the largest negative loading on the x axis is important to the first component, which would decrease on going from neutral complexes (> -131 kJ/mol) and carbenium ion (> -150 kJ/mol) to alkoxide species (< -656 kJ/mol) due to the much shorter C-O bond length in the alkoxide systems. It is generally accepted that the significant improvement of catalytic performance is caused by the dispersion interaction (Edisp) dedicated by zeolite framework. Our results in Section 3 have illustrated that the Edisp can effectively stabilize different adsorbed species (i.e., neutral adsorbed complex, 24


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alkoxide and carbenium ion) inside the zeolite confined spaces. However, it is noteworthy that the Edisp is much close to the origin (Figure 6b), implying that dispersion interaction is not significantly differentiated by the detailed forms of adsorbed species (i.e., neutral adsorbed complex, alkoxide and carbenium ion with the same sizes). For example, the Edisp of toluene (7h, -132 kJ/mol) is similar to that of toluenium (7c, -121 kJ/mol), and thus, the Edisp contribution can be counteracted when neutral adsorbed complex and carbenium ion act as reactant, transition state or product. Therefore, the dispersion interaction does not play a crucial role in improving the reaction activity inside the zeolite pores if the reaction intermediates having almost the same dimension sizes. It’s noteworthy that the transition states, reactants and products are quite different in the structure and size during a reaction, which may lead to the variation in the dispersion interaction between the transition states and the framework.75 On the other hand, the electrostatic interaction (Ees ) has the largest positive loading on the y axis (Figure 6b), which separates the neutral complexes from carbenium ions. Thus, the component two essentially describes the charge character of organic fragments in zeolite catalytic reactions. Furthermore, the Ees values decrease when going from neutral complexes to carbenium ions, which can be mainly ascribed to the large amount of charges on carbenium ions (ca. 0.9 |e|) compared to neutral species (

Origin of Zeolite Confinement Revisited by Energy ... - ACS Publications

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