Pore Selectivity for Olefin Protonation Reactions Confined inside

Article pubs.acs.org/JPCC

Pore Selectivity for Olefin Protonation Reactions Confined inside Mordenite Zeolite: A Theoretical Calculation Study Yueying Chu,†,‡,§ Bing Han,†,‡,§ Anmin Zheng,*,† Xianfeng Yi,†,‡ 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, The Chinese Academy of Sciences, Wuhan 430071, China ‡ Graduate School, The Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: The activity of olefins protonated by Brønsted acid sties in different pore structures (12-MR and 8-MR channel) of mordenite (MOR) zeolite is investigated by the B3LYP+D/6-31G(D,P)//ONIOM(B3LYP/631G(D,P):MNDO) method to reveal the pore selectivity for the protonation reaction. It is demonstrated that for ethene, the size of the molecule is smaller compared with the zeolite pores (both 12-MR and 8-MR channels), the pore confinement effect is weak, and the intrinsic acid strength of the solid acid plays a key role in the reaction, resulting in that the ethene protonation occurs preferentially at the strong acid sites within the 12-MR channel. For propene, the reaction can occur inside both channels (12-MR and 8-MR) due to the reactant being well fitted into the 8-MR channel of MOR zeolite. In this case, the confinement effect that stabilizes the intermediates and transition states compensates the deficiency of acid sites in the 8-MR channel. However, for the bulky isobutene, the protonation reaction occurs selectively in the 12-MR channel as the size of the reactant is larger than the size of the 8-MR channel, which results in a pronounced destabilizing effect due to the steric constraint.

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INTRODUCTION Environmentally friendly zeolite catalysts have been extensively applied in the chemical and petroleum industries due to their special characteristics such as unique channel structures and tunable acidic properties. It has been demonstrated that the activities of zeolite-catalyzed reactions are strongly correlated with the acidic strength of zeolite catalysts.1−3 The influence of acid strength on the reactivities of alkane activations (such as alkane hydrogen exchange, cracking, and dehydrogenation reactions) has been systematically studied in our previous works.4 It was revealed that the activation barriers of alkane activation reactions decreased with the increase of Brønsted acid strength of zeolite catalysts, indicating that the reactivities of alkane activations are enhanced by increasing the acid strength. However, for the Beckmann rearrangement reaction, the zeolites with a weak acid strength (i.e., silicalite-1 and H[B]ZSM-5) show relative higher activities compared with those with a strong acid strength (i.e., H-ZSM-5).5 Besides the acid property, the pore confinement effect (pore size and its shape) of zeolites is another important feature of zeolite catalysts, which plays a key role in catalytic reactions occurring inside zeolite pores at nanoscale.6−11 It was experimentally demonstrated that the limited space of zeolites would affect the mechanism and reactivity of alkane hydrogen exchange reactions, and such a trend was also confirmed by the theoretical calculations.12−15 Unlike the reaction mechanisms found in homogeneous catalytic systems such as liquid superacids,16,17 it was found that the zeolite pore constraints dictate the lower reaction energy observed for the methyl group © 2013 American Chemical Society

compared to the methylene (or methine) group in the alkane hydrogen exchange reaction.12 Therefore, it is necessary to investigate the effect of both acid site and pore confinement on the reaction activity and selectivity. Mordenite (MOR) zeolite consists of 12- and 8-membered ring (12-MR, 8-MR) channels with diameters of 6.5 × 7.0 Å and 2.6 × 5.7 Å, respectively (see Figure 1).18−20 Because of its unique acidic and pore properties, mordenite is one of the most important zeolites that have been extensively studied both theoretically and experimentally.2,21,22 For example, Patrizia Calaminici et al. have theoretically studied the Brønsted acid strength in H-MOR with CO and CH3CN probe molecules, and they found that the Brønsted acid site in the 12-MR main channel has a slightly stronger intrinsic acid strength with respect to that in the 8-MR channel.23 On the other hand, based on the experimental results, Iglesia et al. revealed that alkane cracking and dehydrogenation turnovers occurred preferentially on acid sites within the smaller 8-MR channels in H-MOR, while the rates on acid sites within the 12-MR channels were much lower and often undetectable.2 This phenomenon was also observed in the carbonylation of dimethyl ether to methyl acetate in the zeolite, in which the activity and selectivity are all higher inside the 8-MR channels due to the perfect fit of transition state.24 Therefore, mordenite zeolite can be used as a model to investigate the effect of both Received: November 14, 2012 Revised: January 17, 2013 Published: January 17, 2013 2194

dx.doi.org/10.1021/jp311264u | J. Phys. Chem. C 2013, 117, 2194−2202

The Journal of Physical Chemistry C

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species. In this contribution, the protonation reactions of olefins with varying sizes (ethene, propene, and isobutene) within the 12-MR and 8-MR channels of MOR zeolite have been theoretically investigated. The DFT-D theoretical method was employed to explore the reaction mechanism, aiming for a comprehensive understanding of the effects of acid site (mainly accounting for the local electronic interaction) and zeolite framework confinement (mainly accounting for the long-range dispersion interaction) on the reaction.

2. COMPUTATIONAL METHODS 2.1. Models. Mordenite has an orthorhombic unit cell which consists of 12- and 8-MR channels (see Figure 1). There are four nonequivalent crystallographic tetrahedral sites (T: Si or Al atoms) in the MOR unit cell: T1, T2, and T4 in the 12MR main channel and T3 inside the 8-MR channel (see Figure 1). As demonstrated by Sauer et al.,41 Al is preferentially located in T4 (inside 12-MR) and T3 (inside 8-MR) positions; therefore, they are the active sites for the catalytic reactions. In our theoretical calculations, 112T and 76T cluster models were used to represent the 12-MR and 8-MR structures. As shown in Figure 2, such extended cluster models have contained the

Figure 1. Structure framework showing used T (T = Si, Al) atoms in the 12-MR and 8-MR channels of MOR zeolite.

acid site and pore confinement on the reaction mechanism and the reactivity inside the zeolite pores. It is well-known that the pore confinement effect can be described by long-range weak dispersion interactions between adsorbed molecule and zeolite framework.25 However, it is difficult to extract the weak dispersion interactions experimentally. As a valuable complement to experimental study, theoretical calculation is suitable to offer an atomic-level description of reaction mechanism, including the structures and energies of intermediates and transition states.26−32 Furthermore, the weak host/guest interactions between adsorbed molecules and zeolite framework can be accurately determined with simple fitting functions based on empirical data. Addition of empirical dispersion interaction corrections to DFT, denoted DFT-D (density functional theory-dispersion), can be used to model the bimolecular dispersion interactions with atom pair dispersion coefficients, thus providing a semiempirical means to account for dispersion energies.33,34 The olefins protonation reactions over zeolites are the elementary step in many olefin-participating acid-catalyzed reactions, such as skeletal isomerization, dimerization, polymerization, and formation of active species (cocatalytic hydrocarbon pool) in the MTO reaction.35−38 The protonation mechanism of olefinic hydrocarbons on zeolite catalysts has been extensively studied by using solid-state NMR39 and IR spectroscopy.22,40 As shown in Scheme 1, a weakly hydrogenbonded complex (π complex) is first formed when an olefin adsorbs on a Brønsted acid site of zeolite; then, via a carbenium ion-like transition state (TS), the transfer of an acidic proton from the zeolite to the olefin leads to formation of alkoxy

Figure 2. Representations of the 12-MR and 8-MR channels of MOR zeolite framework by 112T and 76T cluster models. The 8T cluster model (right) represents the acid site of the zeolite which is treated as the high-layer atoms during the ONIOM calculations. (a) and (b) for the 12-MR channel; (c) and (d) for the 8-MR channel.

complete pore structures of 12-MR and 8-MR; thus, they can be used to explore the pore confinement effects on the olefin protonation reactions. In the theoretical calculations, the terminal Si−H was fixed at a bond length of 1.47 Å, oriented along the direction of the corresponding Si−O bond. The combined theoretical model, namely ONIOM(B3LYP/6-31G(d,p):MNDO), was applied to predict the geometries of various adsorption structures and transition states. To preserve the integrity of the zeolite structure during the structure optimizations, only the acid site region (AlSi7O8H, as shown in Figures 2b and 2d) and adsorbed molecule in the high-level layer were relaxed while the

Scheme 1. Reaction Mechanism of Olefin Protonation Catalyzed by Zeolite Solid Acid

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Table 2. Deprotonation Energy (DPE) (kJ/mol−1), Mulliken Charge for H (QH) (|e|), and Main Geometry Parameters (Bond Length, Å; Angle, deg) of 12-MR and 8-MR Modelsa

rest of atoms were fixed at their crystallographic locations. In order to obtain accurate energy results, the single point energy calculations were further refined at the level of B3LYP/631G(D,P). All calculations were performed using the Gaussian 09 package.42 2.2. Dispersion Energy Corrections. It is well-known that the long-range weak dispersion interaction between adsorbed molecules and zeolite framework plays an important role in the catalytic reactions confined inside the zeolite pore structures.15,23,38,43 However, the classic DFT functional is subject to inherent deficiencies when describing theses weak dispersion interactions. It was demonstrated that DFT-D could better describe nonbonding interactions between the reactive species and the zeolite framework.25,33,34 In order to obtain the contribution of dispersion energy (Edisp) in zeolite systems, the Edisp is calculated with the DFT-D method as the sum of atomic pair interactions over all atoms in the system, modeled by the following function: Nat − 1

Edisp = −S6

C6i C6j

Nat

∑ ∑ i=1

j=i+1

R ij6

fdamp (R ij)

geometry parameters

0.000 1.485 1.397 1.364 1.639

⟨AlOSi⟩

1.666 1.674

123.58 123.80

12-MR

Table 1. Dispersion Coefficient (C6) and Radii R0 for Elements H, C, O, Al, and Si in the Calculations of Dispersion Energy23 0.14 1.75 0.70 10.79 9.23

rSi−O

1.819 1.832

π complex

R is the radius of the atom, i and j are the atomic numbers of two atoms in the system, and Rij represents the interatomic distance between the two atoms. The dispersion coefficient (C6) and radii R0 for elements H, C, O, Al, and Si in the calculations of dispersion energy are shown in Table 1.

H C O Al Si

rAl−O

0.974 0.974

Table 3. Main Geometry Parameters (Bond Length, Å; Angle, deg) for Protonation Reactions of Ethene, Propene, and Isobutene in the 12-MR and 8-MR Models

(2)

R0/ Å

rO−H

0.397 0.277

agreement with the previous results (1218−1235 kJ/mol).49 It is noteworthy that the smaller DPE value at T4 site indicates that the Brønsted acid site in the 12-MR channel has a slightly stronger acid strength than that in the 8-MR channel, which is in good agreement with the previous theoretical work using CO and CH3CN as probe molecules.23 3.2. Olefin Adsorption. Table 3 lists the main geometry parameters of various complexes formed by olefins (ethene,

0

C6/(J·nm6/mol)

QH

1218.1 1225.5

DPE is obtained from B3LYP/6-31G(D,P)//ONIOM(B3LYP/631G(D,P):MNDO) calculation.

Nat is the number of atoms in the system, S6 is a global scaling factor for correcting the differences that result from the choice of functional (it is accepted to be 1.05 for the B3LYP functional44), C6 parameters are species-dependent constants inherent to the atoms they represent, and fdamp is a damping function included to diminish the correction at short interatomic distances. fdamp is defined as

element

DPE

12-MR 8-MR a

(1)

fdamp (R ij) = {1 + exp[−α(R ij/R0 − 1)]}−1

channel

r(OaH) r(HCa) r(HCb) r(CaCb) r(CbOb)

0.991 2.187 2.197 1.337 

r(OaH) r(HCa) r(HCb) r(CaCa) r(CbOb)

0.996 2.063 2.220 1.341 

r(OaH) r(HCa) r(HCb) r(CaCb) r(CbOb)

1.001 2.028 2.238 1.346 

TS

8-MR alkoxy

Ethene 1.484  1.210 1.088   1.398 1.508 2.336 1.527 Propene 1.768  1.146 1.090   1.429 1.512 2.681 1.585 Isobutylene 1.436  1.258 1.083   1.395 1.525 3.615 1.615

π complex

TS

alkoxy

1.007 1.777 1.826 1.340 

1.386 1.210  1.391 2.313

 1.090  1.512 1.521

1.019 1.702 1.796 1.342 

1.540 1.208  1.393 2.730

 1.088  1.512 1.545

1.021 1.751 1.712 1.344 

1.364 1.319  1.375 3.078

 1.074  1.523 1.613

propene, and isobutene) adsorbed in H-MOR zeolite. When an olefin molecule is adsorbed on the Brønsted acid site in the 12MR channel, the distances between proton and the carbon atoms of olefin CC double bond (i.e., Ha−Ca and Ha−Cb distances) are nearly identical, indicating that a weakly hydrogen-bonded π complex is formed. For ethene adsorption in the 12-MR channel, the r(Ha−Ca) and r(Ha−Cb) bond distances are 2.187 and 2.197 Å, respectively. In addition, an elongation of the Oa−H bond length from 0.970 to 0.991 Å was observed, and the CC bond length of adsorbed ethene (1.337 Å) is also slightly lengthened by ca. 0.007 Å compared to the free ethene (see Table 3). Similar results were also evident for the adsorption of propene and isobutene in the 12MR channel, and the CC bonds were lengthened by 0.008 and 0.009 Å, respectively. This indicates that the CC double bond of olefins is weakened after their adsorption on Brønsted acid sites.

3. RESULTS AND DISCUSSION 3.1. Acid Strength of Brønsted Acid Sites inside 12MR and 8-MR. As is well-known, the acid strength of zeolite catalysts has a significant influence on the catalytic reactions, such as cracking, hydrogen exchange, and dehydrogenation.3,4,45 For a Brønsted acid site, deprotonation energy (DPE) is a criterion to evaluate the intrinsic acid strength of zeolites and other solid acids. It is defined as the energy required to remove the acidic proton to form an anionic conjugate base (AH → H+ + A−), and a smaller DPE value corresponds to a stronger acidity.46−48 As shown in Table 2, the DPE values for H-MOR at T4 (12-MR) and T3 (8-MR) sites are 1218.1 and 1225.5 kJ/mol, respectively, which is in good 2196



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Table 4. Calculated Electronic Interaction (Eelec),a Dispersion Interaction (Edisp),b and Total Energy (Etotal)c (kJ/mol) of π Adsorption Complex (ads), Transition State (TS), and Alkoxy Species (alk) for Ethene, Propene, and Isobutene in the 12-MR and 8-MR Models π complex 8-MOR

12-MOR

C2H4 C3H6 C4H8 C2H4 C3H6 C4H8

transition state

alkoxy species

Eelec(ads)

Edisp(ads)

Eads

Eelec(TS)

Edisp(TS)

ETS

Eelec(alk)

Edisp(alk)

Ealk

−35.6 −40.5 −41.3 −36.8 −42.8 −46.1

−64.4 −89.1 −45.0 −33.8 −45.3 −67.5

−100.0 −129.6 −86.3 −70.6 −88.1 −113.6

73.2 19.8 53.2 51.0 25.3 7.6

−76.8 −93.6 −45.3 −48.7 −61.4 −72.5

−3.6 −73.8 7.9 2.3 −36.1 −64.9

−71.0 −43.5 −37.7 −58.1 −29.8 21.3

−68.4 −96.1 −52.2 −53.2 −78.1 −104.6

−139.4 −139.6 −89.9 −111.3 −107.9 −83.6

a Eelec is obtained from B3LYP/6-31G(D,P)//ONIOM(B3LYP/6-31G(D,P):MNDO) calculation. bEdisp is obtained from B3LYP-D//ONIOM (B3LYP/6-31G(D,P):MNDO) calculation. cEads = Eelec(ads) + Edisp(ads); ETS = Eelec(TS) + Edisp(TS); Ealk = Eelec(alk) + Edisp(alk).

2−5 kJ/mol less than that of the olefins in the 12-MR channel. This is likely due to the relatively weaker acid strength of acid sites in the 8-MR channel. When considering the confinement effect on olefin adsorption inside the 8-MR channel, some interesting results were found. The Edisp(ads) of propene (−89.1 kJ/mol) is much lower than that of ethene (−64.4 kJ/mol) due to the better fit of propene (2.7 × 3.0 Å) in the 8-MR pore (2.6 × 5.7 Å). However, for the larger isobutene (3.5 × 3.8 Å), the corresponding Edisp(ads) is −45.0 kJ/mol, nearly 44.1 kJ/mol higher than that of propene, indicating that the bulky size of isobutene obviously decreases the stabilization energy imposed by the zeolite framework. The calculated total adsorption energies (Eads) are −100.0, −129.6, and −86.3 kJ/mol for ethene, propene, and isobutene adsorbed in the 8-MR pore, respectively, indicating that the stability of propene in the 8-MR channel is the highest. It has been demonstrated by our previous works that adsorbed molecules can be effectively stabilized if the size of hydrocarbon fragments is comparable to the pore size of the zeolites.7 Therefore the best fit of propene molecule in the 8-MR channel results in the highest adsorption energy. Olefin adsorptions on the zeolite have been extensively investigated by both the experimental methods and theoretical calculations.38,39,51−54 The experimental work of Cant and Hall showed that the adsorption energy of ethene in H−Y zeolite which contains a supercage with dimension of 13 Å (7.4 × 7.4 Å) is 9.0 kcal/mol (or 37.7 kJ/mol).53 Obviously, the calculated adsorption energy ethene inside 12-MR of mordenite (16.9 kcal/mol or 70.6 kJ/mol) is higher than that in H−Y zeolite, and this can be attributed to the stronger acidity of mordenite.41 In addition, the relatively narrower pore dimension of the 12-MR channel (6.5 × 7.0 Å) of mordenite also results in the stronger dispersion interactions (Edisp(ads)) between the ethene and zeolite framework, and thus the larger adsorption energy. Compared with the large 12-MR pore (6.5 × 7.0 Å), when ethene is adsorbed inside the small 8-MR (2.6 × 5.7 Å), the Edisp(ads) is remarkably increased. As shown in Table 4, the dispersion interaction of ethene confined in 8-MR (−64.4 kJ/mol) is almost two times larger than that in 12-MR (−33.8 kJ/mol). Therefore, the adsorption of ethene inside the smaller 8-MR is more exothermic (−100.0 kJ/mol). It is obvious that the alkene can be effectively stabilized if the size of the hydrocarbon fragments is well fit with the zeolite pore size, while the too large or too small pore is unfavorable to stabilizing the adsorption complexes. This trend is also present for isobutene adsorption on mordenite (12-MR) and H-FER zeolites. Since the smaller pore size (4.2 × 5.4 Å) of H-FER

Table 4 shows the adsorption energies of olefins in H-MOR zeolite. It should be noted that the adsorption energy here is defined as the sum of electronic interaction (Eelec(ads)) and dispersion interaction (Edisp(ads)), i.e., Eads = Eelec(ads) + Edisp(ads). The Eelec(ads) is considered as the influence of the local acid site (such as the Brønsted acid strength), while the Edisp(ads) reflects the influence of zeolite pore confinement effect (nonbonded weak interaction) on adsorbed species. As shown in Table 4, the calculated Eelec(ads) values are −36.8, −42.8, and −46.1 kJ/ mol for the π complexes of ethene, propene, and isobutene adsorbed in the 12-MR channel, which is consistent with their basicity order: ethene < propene < isobutene. It is well-known that the weak dispersion interaction can increase the stabilization of the species confined in zeolite pore.11,15,32,43,50 As shown in Table 4, the Edisp(ads) value is gradually increased with the increase of molecular size. The Edisp(ads) values for the ethene, propene, and isobutene adsorbed in the 12-MR channel are −33.8, −45.3, and −67.5 kJ/mol, respectively. For ethene, its molecular size of ethene (1.9 × 2.4 Å) is much smaller than that of the 12-MR channel (6.5 × 7.0 Å); therefore, the confinement effect from the zeolite framework on the stability of ethene is the weakest (−33.8 kJ/mol). However, with the increasing of the size of the adsorbed molecules (e.g., propene (2.7 × 3.0 Å) and isobutene (3.5 × 3.8 Å)), the confinement effect becomes more and more pronounced (−45.3 and −67.5 kJ/mol), which will remarkably enhance the stabilities of the olefins adsorbed in the 12-MR channel. Taking both electronic interaction and dispersion interaction into account, we can obtain the total adsorption energies (Eads = Eelec(ads) + Edisp(ads)), which are −70.6, −88.1, and −113.6 kJ/mol for ethene, propene, and isobutene, respectively. It is indicative that the electronic interaction (acid−base interaction) has an effect on the adsorption energy as aforementioned above (−36.8, −42.8, and −46.1 kJ/mol for ethene, propene, and isobutene), and the total adsorption energy becomes more exothermic with increasing the size of olefin molecules, because of the greater dispersion interaction for larger adsorbed molecules in the larger 12-MR pore (6.5 × 7.0 Å), such as isobutene. For the adsorption of olefin molecules in the 8-MR channel, similar variation trends can be found for the structural parameters. As shown in Table 3, the r(Ha−Ca) and r(Ha− Cb) bond distances are similar, and the acidic Oa−H and the CC bond lengths are in the ranges of 1.007−1.021 Å and 1.340−1.344 Å, respectively. For the local acid site contribution (Eelec(ads)) to the adsorption energies in the 8-MR channel, the Eelec(ads) values of ethene, propene, and isobutene are −35.6, −40.5, and −41.3 kJ/mol, respectively (see Table 4), which is 2197



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Figure 3. Local optimized structures of transition state (TS) for the olefin protonation reaction on the Brønsted acid site models: (a) ethene, (b) propene, (c) isobutene inside 12-MR; (d) ethene, (e) propene, (f) isobutene inside 8-MR. Selected interatomic distances (in Å) are indicated.

Figure 4. Difference charge densities for transition states of ethene (a, d), propene (b, e), and isobutene (c, f) protonation in the 12-MR (upper) and 8-MR (bottom) models. The green represents where the electrons are coming from, and the red represents where the electrons are going.

in the π complex state to 1.398 Å in the transition state, indicating that the CC double bond is nearly turned into a single C−C bond (1.54 Å). Simultaneously, the other carbon atom (Cb) of ethene interacts with the neighboring basic oxygen atom of zeolite to form the ethoxide species. Moreover, the length of Oa−H increases from 0.991 to 1.484 Å and the Ca−H distances is accordingly reduced by 0.997 Å, reflecting the transfer of the acidic proton from the zeolite framework to ethene. Similar variations can be found for propene and isobutene protonation reactions in the 12-MR channel of HMOR zeolite (see Figure 3b and c).

would result in stronger steric repulsion of isobutene with zeolite wall, the adsorption energy of isobutene inside H-FER (−61.9 kJ/mol, or −14.8 kcal/mol)38 is much smaller than that inside 12-MR of mordenite (−113.6 kJ/mol, or −27.2 kcal/ mol). 3.3. Transition States of Olefin Protonation. Figure 3a shows the carbenium ion transition state of adsorbed ethene, in which the π complex is going to convert into ethoxide species inside the 12-MR channel. In this step, the carbon atom of the olefin double bond (Ca) is protonated by the acidic proton, and the double bond of CC of ethene is elongated from 1.337 Å 2198



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4a−c, the host−guest (between zeolite framework and alkyl carbenium) electrostatic interactions are enhancing with the increase of olefin sizes, which follows the order of ethyl carbenium ions < isopropyl carbenium ions < tert-butyl carbenium ions, being consistent with the results from the stabilization energies listed in Table 3. However, for the smaller 8-MR pore, the isopropyl carbenium ions can be well stabilized by the electrostatic interactions compared to the smaller ethyl carbenium ion (see Figure 4d−e). However, for the larger dimension size of tert-butyl carbenium ion, the electron density obviously overflows the limited 8-MR pore space (Figure 4f), which reflects the existence of electrostatic repulsion interaction between the zeolite framework and the tert-butyl carbenium ion. Thus, compared to the smaller ethyl and larger tert-butyl carbenium ions, the isopropyl carbenium ion can be well stabilized by the confined 8-MR pore. 3.3. Alkoxy Species of Olefin Protonation. As is shown in Scheme 1, protonation of an adsorbed olefin by a Brønsted acid site (O−H) may result in the formation of an alkoxy species. The optimized geometries of alkoxide species formed inside 12-MR and 8-MR channels are also summarized in Table 3. It is obvious that the size of alkoxide has an influence on the geometries and stabilities of the alkoxide complexes. Taking the alkoxy species formed in the 12-MR channel as an example, the Cb−Ob bond lengths of ethoxide, isopropoxide, and tertbutoxide are 1.527, 1.585, and 1.615 Å. This suggests the presence of steric repulsive interaction between the methyl groups and zeolite frameworks, which usually leads to destabilization of the formed alkoxide complexes. The calculated electronic energies (Eelec(alk)) are −58.1, −29.8, and 21.3 kJ/mol for ethoxide, isopropoxide, and tert-butoxide, respectively (see Table 4), implying a gradual decrease of the stability of the alkoxides. In contrast, similar to the π complex and TS formed inside the 12-MR channel, the contribution of dispersion interaction to stabilizing the alkoxide species gradually increases. The corresponding Edisp(alk) is −53.2, −78.1, and −104.6 kJ/mol for ethoxide, isopropoxide, and tert-butoxide, respectively. However, the increase of Edisp(alk) is not significant enough to alter the stability order of the alkoxide complexes. As shown in Table 4, the total stabilization energies (Ealk) are −111.3 (ethoxide), −107.9 (isopropoxide), and −83.6 kJ/mol (tert-butoxide), respectively, suggesting that the stability order of the alkoxide is ethoxide > isopropoxide > tertbutoxide, which is in accordance with the results obtained by the previous works that tertiary alkoxide species is much less stable than primary or secondary ones or even nonexistent when bonded to sterically constricted sites.50,58 Furthermore, as shown in Table 5, the reaction energies (Ereac) are −40.7, −19.8, and 30.3 kJ/mol for ethoxide, isopropoxide, and tertbutoxide inside 12-MR, respectively. Obviously, compared with ethoxide and isopropoxide, the formation of tert-butoxide is an endothermic process, which has a ca. 30 kJ/mol higher energy compared with the corresponding π complex. Therefore, it is indicative that the tert-butoxide species is thermodynamically unstable, which would turn into other more stable species, such as isobutyl alkoxide, by isomerization reactions.50,58 As for the 8-MR channel, the corresponding Ereac energies are −39.4, −10.0, and −3.6 kJ/mol for ethoxide, isopropoxide, and tertbutoxide, respectively (see Table 5). Similar to the trend for the 12-MR channel, the reaction energy is decreasing with increasing the size of alkoxide due to the increase of repulsive interaction between the methyl groups and the active site. It is noteworthy that the reaction energies in the 8-MR channel are

The contributions of electronic interaction (Eelec(TS)) to the stability of ethene, propene, and isobutene transition state are 51.0, 25.3, and 7.6 kJ/mol, respectively (Table 4). As aforementioned, the transition states of the olefin protonation involve the carbenium cations properties; thus, the stabilities of TS are in accordance with the charge distribution capacities of the corresponding carbonium ions. Therefore, the reactivities follow the trend: tert−C−H > sec−C−H > prim−C−H, naming the stabilities of TS in this order: tert-butyl carbenium ions > isopropyl carbenium ions > ethyl carbenium ions for ethene, propene, and isobutene.16,17 Since the dimension size of the three carbenium ions is smaller than the diameter of the 12-MR channel, the stabilities of transition states will be enhanced by the confinement effect imposed by the zeolite framework. As shown in Table 4, the Edisp(TS) contributed from pore confinement effect is −48.7, −61.4, and −72.5 kJ/mol for ethyl-, isopropy-, and tert-butyl carbenium ions, respectively, indicating that the host−guest interactions between the carbenium ions and the confined pores increase gradually with increasing the size of carbenium ions. Comparing with the energies of the transition states in the 12-MR channel, both electronic interactions (Eelec(TS)) and dispersion interaction (Edisp(TS)) of the carbenium ion exhibit different trends in the 8-MR channel. As shown in Table 4, the electronic interaction (Eelec(TS)) contribution to the stability of carbenium ions (TS) follows the order of ethyl carbenium ions (73.2 kJ/mol) < tert-butyl carbenium ions (53.2 kJ/mol) < isopropyl carbenium ions (19.8 kJ/mol), which is not consistent with the basicity of olefins: ethene < propene < isobutene. The limited 8-MR pore (2.6 × 5.7 Å) makes the dispersion energy (Edisp) considerably decrease to −45.3 kJ/ mol for the larger tert-butyl carbenium ion (ca. 3.7 × 4.6 Å), which is dramatically higher than those of ethyl and isopropyl carbenium ions in 8-MR pore (−76.8 and −93.6 kJ/mol, respectively), and tert-butyl carbenium ion confined inside 12MR pore (−72.5 kJ/mol). On the basis of our previous studies, it is revealed that the stability of transition state in the zeolites was determined by both electrostatic stabilization and repulsive interactions between the zeolite pores and confined hydrocarbon fragments.7 Therefore, intermediates and transition states can be effectively stabilized if the size of the hydrocarbon fragments is comparable to the pore size of the zeolites. Thus, the well-fit of isopropyl carbenium ion (TS, ca. 2.7 × 3.7 Å) inside the 8-MR channel (2.6 × 5.7 Å) results in that the isopropyl carbenium ion transition state is most effectively stabilized by the 8-MR pore (ETS= −73.8 kcal/mol). As is well-known, the difference charge densities can be used to represent the interaction strength between host and guest molecules.55−57 In Figure 4, difference charge densities of the TS species confined in the 12-MR and 8-MR pores are depicted to further elucidate the zeolite pore confinement effect. The different charge density is defined by the charge density difference between the adsorbed system and the isolated zeolite pore structure (12-MR or 8-MR) plus the hydrocarbon fragments, i.e., Δρ = ρ(zeolite‑hydrocarbon) − ρzeolite − ρhydrocarbon. It is noteworthy that a dramatic electron transfer from the framework oxygen atoms to the transition states is evident, demonstrating the strong electrostatic interaction between the hydrocarbon fragment and zeolite framework. However, the electrostatic interactions are different in 8-MR and 12-MR channels. For the larger 12-MR channel, as shown in Figure 2199



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channel follows the intrinsic base strength order of the olefins (see Table 6: ethene < propene < isobutene). However, when the protonation reactions occur in the constrained 8-MR channels, the reaction activity order is followed as ethene < isobutene < propene. The much smaller size of ethene molecule compared with both 12-MR and 8-MR channels results in a similar confinement effect imposed by zeolite framework. Therefore, the intrinsic Brønsted acid strength of the acid site will play a key role in the reaction. The stronger acid strength of the acid site in the 12-MR channel is responsible for a higher reactivity of ethene protonation inside the 12-MR channel than inside the 8-MR channel. As for propene, although the Brønsted acid strength of acid site is weaker in the 8-MR channel, the activation energy (55.8 kJ/ mol) of propene in the 8-MR channel is much closer to that in the 12-MR channel (52.0 kJ/mol). As mentioned above, the isopropyl carbenium ion is better fitted into the constrained 8MR channels, and the contribution of the pore confinement effect will largely decrease the energy of the transition state, which results in the close activation energies for propene protonation in 8-MR and 12-MR channels. However, for isobutene, the stabilizing effect on its TS (tert-butyl carbenium ion) from the zeolite framework becomes less predominant and the destabilizing effect from steric constraint is increasing remarkably, leading to a higher activation energy (Eact = 94.2 kJ/mol) in the 8-MR channel compared with that in the 12-MR channel (Eact = 48.7 kJ/mol). The activation barriers can be used to estimate the relative reaction rates of the protonation reaction over zeolites based on the transition state theory (TST).59,60 The reaction rate (KTST) can be determined by the following equation:

Table 5. Reaction Energies (kJ/mol) for Protonation Reactions of Ethene, Propene, and Isobutene in the 12-MR and 8-MR Modelsa 12-MR ethene propene isobutene

8-MR

Eelec(reac)

Edisp(reac)

Ereac

Eelec(reac)

Edisp(reac)

Ereac

−21.3 13.0 67.4

−19.4 −32.8 −37.1

−40.7 −19.8 30.3

−35.4 −3.0 3.6

−4.0 −7.0 −7.2

−39.4 −10.0 −3.6

Eelec(reac), Edisp(reac) , and Ereac are computed relative to the π adsorption complexes from Table 4. Eelec(reac) = Eelec(alk) − Eelec(ads); Edisp(reac) = Edisp(alk) − Edisp(ads); Ereac = Eelec(alk) + Edisp(reac). ΔEelec is obtained from B3LYP/6-31G(D,P)//ONIOM(B3LYP/6-31G(D,P):MNDO) calculation, and ΔEdisp is obtained from B3LYP-D// ONIOM(B3LYP/6-31G(D,P):MNDO) calculation. a

smaller than those in the 12-MR channel for ethene (−40.7 kJ/ mol for 12-MR; −39.4 kJ/mol for 8-MR) and propene (−19.8 kJ/mol for 12-MR; −10.0 kJ/mol for 8-MR), which can be ascribed to the much larger steric repulsion in the smaller 8-MR channel (2.6 × 5.7 Å). With respect to the adsorption of π complexes, the formation of tert-butoxide is slightly exothermic (Ereac = −3.6 kJ/mol) in the 8-MR channel, whereas it is endothermic (Ereac = 30.3 kJ/ mol) in the 12-MR channel (see Table 5). However, this does not mean that tert-butoxide is more preferentially formed inside the 8-MR channel of mordenite. As in the aforementioned discussion, stronger steric repulsion is present for isobutene adsorbed inside 8-MR, which will result in the adsorption of isobutene inside 8-MR (−86.3 kJ/mol) much less stably than inside 12-MR (−113.6 kJ/mol) (see Table 4). Therefore, the instability of isobutene adsorption inside 8-MR disfavors the formation of tert-butoxide. 3.4. Pore Selectivity for the Olefin Reactions. As is wellknown, the activation energies can reflect the reactivity of catalytic reactions. Table 6 provides the activation energies

k TST(T ) =

ethene propene isobutene

⎛ ΔE12 ‐ MR − ΔE8 ‐ MR ⎞ ⎛ ΔΔE ⎞ k12 ‐ MR ⎟ = exp⎜ − ⎟ = exp⎜ − ⎝ ⎝ ⎠ k 8 ‐ MR RT ⎠ RT

8-MR

ΔEelec

ΔEdisp

Eact

ΔEelec

ΔEdisp

Eact

87.8 68.1 53.7

−14.9 −16.1 −5.0

72.9 52.0 48.7

108.8 60.3 94.5

−12.4 −4.5 −0.3

96.4 55.8 94.2

(3)

Therefore, the relative reaction rate (k12‑MR/k8‑MR) of olefin reactions in the 8-MR and 12-MR channels of mordenite zeolite can be descried as

Table 6. Activation Barriers (kJ/mol) for Protonation Reactions of Ethene, Propene, and Isobutene in the 12-MR and 8-MR Modelsa 12-MR

⎛ ΔE ⎞ kBT ⎟ exp⎜ − ⎝ h RT ⎠

(4)

where R is the ideal gas constant and T is the reaction temperature. For ethene, the activation energy in the 12-MR channel is 23.5 kJ/mol lower than that in the 8-MR channel. Based on eq 4, the decreased amount (ca. 23.5 kJ/mol) of the activation barrier would result in an increase of reaction rate constant by about 4 orders of magnitude, which implies that the ethene protonation preferentially occurs in the 12-MR channel at room temperature (298 K). For propene, the difference of the activation energy in the 12-MR and 8-MR channels is 3.8 kJ/ mol, leading to a relative reaction rate of 4.6. Therefore, the propene protonation can take place in both 12-MR and 8-MR channels. However, for the bulky isobutene, the activation energy in the 12-MR channel (Eact = 48.7 kJ/mol) is 45.5 kJ/ mol smaller than that in the 8-MR pore (Eact = 94.2 kJ/mol), leading to an increase of reaction rate constant by about 8 orders of magnitude. This indicates that the isobutene protonation might solely occur inside the 12-MR channel.

ΔEelec, ΔEdisp, and Eact are computed relative to the π adsorption complexes from Table 4. ΔEelec = Eelec(TS) − Eelec(ads); ΔEdisp = Edisp(TS) − Edisp(ads); Eact = ΔEelec + ΔEdisp. ΔEelec is obtained from B3LYP/631G(D,P)//ONIOM(B3LYP/6-31G(D,P):MNDO) calculation, and ΔE disp is obtained from B3LYP-D//ONIOM(B3LYP/6-31G(D,P):MNDO) calculation. a

(Eact) of the protonation reactions catalyzed by the acid sites in both 12-MR and 8-MR channels of mordenite zeolite. When taking the pore confinement effect and the local property of the acid site into account, the calculated activation energies (Eact = ΔEelec + ΔEdisp) are 72.9, 52.0, and 48.7 kJ/mol for ethene, propene, and isobutene protonation in the 12-MR channels. With respect to the 8-MR channels, the calculated Eact are 96.4, 55.8, and 94.2 kJ/mol for ethene, propene, and isobutene, respectively. On the basis of our theoretical results, it can be concluded that the reaction activity inside the main 12-MR 2200



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4. CONCLUSIONS In this work, the pore selectivity for the olefin reactions inside mordenite zeolite with different pore structures has been theoretically investigated. The theoretical results based on the 8-MR and 12-MR channels of the MOR zeolite have revealed the effect of both pore confinement and acid site on the protonation reaction. When the size of adsorbed molecule (e.g., ethene) is smaller compared with the zeolite pore, the pore confinement effect would be weak and the intrinsic acid strength of the zeolite will play a leading role in the reaction; therefore, the olefin protonation occurs inside the 12-MR of mordenite. When the reactant is fitted perfectly into the channels or cages of the zeolite (e.g., propene inside 8-MR), the effect of pore confinement plays a key role for decreasing the activation energy and can compensate the weak acid strength of 8-MR; therefore, propene can be transformed inside both 12MR and 8-MR channels. When the size of the reactants is larger than the pore size of 8-MR, the stabilizing effect from 8-MR framework will become less predominant (e.g., isobutene inside 8-MR) and the destabilizing effect from steric constraint is increased remarkably, resulting in that the reaction selectively occurs inside the large 12-MR channel.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], fax +86-27-87199291 (A.Z.); e-mail [email protected], fax +86-27-87199291 (F.D.). Author Contributions §

These authors equally contributed to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21173255, 21073228, and 20933009 and 2121005). The authors are grateful to Shanghai Supercomputer Center (SSC, China) for its support in computing facilities.

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