Stability and Reactivity of Intermediates of Methanol Related

Article pubs.acs.org/JPCC

Stability and Reactivity of Intermediates of Methanol Related Reactions and C−C Bond Formation over H‑ZSM‑5 Acidic Catalyst: A Computational Analysis Zhihong Wei,† Yan-Yan Chen,† Junfen Li,† Wenping Guo,‡ Sen Wang,†,§ Mei Dong,† Zhangfeng Qin,† Jianguo Wang,† Haijun Jiao,*,†,∥ and Weibin Fan*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China National Energy Center for Coal to Liquids, Synfuels China Co., Ltd., Huairou District, Beijing 101400, China § University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China ∥ Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany ‡

S Supporting Information *

ABSTRACT: On the basis of density functional theory including dispersion correction [ωB97XD/6-311+G(2df,2p)//B3LYP/6-311G(d,p)], the thermodynamics and kinetics of the reactions of CH3OH and CH3OCH3 over HZSM-5 have been systematically computed. For the reaction of the methylated surface (CH3OZ) with CH3OH, CH3OCH3 formation is kinetically controlled and the competitive formation of CH2O + CH4 is thermodynamically controlled, in agreement with the observed desorption temperatures of CH3OH, CH3OCH3, and CH2O under experimental conditions. For the reaction between ZOCH3 and CH3OCH3, the formation of the framework stabilized (CH3)3O+ is kinetically controlled, consistent with the NMR observation at low temperature, and the competitive formation of surface CH3OCH2OZ + CH4 is thermodynamically controlled. On the basis of the thermodynamically more favored CH2O and CH3OCH2OZ, there are two parallel routes for the first C−C bond formation, from the coupling of CH3OCH2OZ with CH3OH and CH3OCH3 as well as from the coupling of CH2O with CH3OH and CH3OCH3. The most important species is the methylated surface (CH3OZ), which can react with CH3OH and CH3OCH3 to form the corresponding physisorbed CH2O and chemisorbed CH3OCH2OZ, and they can further couple with additional CH3OH and CH3OCH3 to result in first C−C formation, verifying the proposed formaldehyde (CH2O) and methoxymethyl (CH3OCH2OZ) mechanisms.

1. INTRODUCTION

olefins trapped inside the zeolite pores under steady state. Haw et al.18 reported that traces of organic impurities, like high alcohols/hydrocarbons in CH3OH feed, catalysts, or carrier gases, can accelerate reactions. Theoretical studies showed that the intermediates in the proposed direct mechanisms, such as carbene, oxonium ylide, or methane-formaldehyde, are either very unstable or have high formation barriers,19 and they might not participate the processes of C−C bond formation. Therefore, how the first C−C bond is formed in the induction period remains intriguing.2,17 Recent studies implied that the direct route, which takes place at a very low rate during the induction period,20−22 can be eclipsed by traces of impurities. Recent experimental and theoretical study proposed a new mechanism over H-SAPO-34 including methoxylmethy cation (CH3O CH2+) intermediate.23,24 Indeed this new proposal is a modification of the methaneformaldehyde mechanism.15,24 As shown in Scheme 1, both

1−4

The conversion of methanol-to-olefins (MTO), which occurs over acidic microporous catalysts, such as H-SAPO-34 and H-ZSM-5, is an attractive commercial route to produce alkenes and basic chemicals. As methanol (CH3OH) can be produced from almost any gasifyable carbon-based feedstock, e.g., natural gas, coal, biomass, and waste, methanol-based conversion processes can alleviate CH3OH overproduction, and the shortage of transport fuels. How to maximize conversion and selectivity of light-olefins while keeping catalyst deactivation to a minimum remains challenging in industrial MTO process,1−3 and these tasks require a comprehensive understanding into the detailed reaction mechanisms.1−4 In the past decades, mechanistic studies have posed a challenge to scientists since many reactions occur competitively and consecutively in the whole MTO process.1−3,5−8 On the basis of experimental and theoretical studies, two types of mechanisms, the direct one1,9−15 and the hydrocarbon pool (HCP) one,7,8 have been proposed to explain C−C bond formation. The dominant one is the HCP mechanism2,16,17 in which olefin formation occurs through repeated methylation and/or cracking of hydrocarbons, including aromatics and © 2016 American Chemical Society

Received: January 8, 2016 Revised: February 24, 2016 Published: March 3, 2016 6075

DOI: 10.1021/acs.jpcc.6b00211 J. Phys. Chem. C 2016, 120, 6075−6087

Article

The Journal of Physical Chemistry C Scheme 1. Schematic Representation of the Proposed C−C Bond Formation Mechanisms

mechanisms involve the reaction between methyl donor (ZOCH3) and H donor (CH3OH or CH3OCH3). Apart from ZOCH3, the adsorbed C1 species, including CH3OH and CH3OCH3 as well as the ion pairs of oxonium ions, such as trimethyloxonium ions (CH3)3O+, can also supply CH3 groups to form CH2O or CH3OCH2+ intermediate, which may further initiate first C−C bond formation. Since all of these C1 species could act as a methyl group donor, especially CH3OCH3 and (CH3)3O+ having two and three methyl groups in a single species, their specific role in direct C−C coupling should be examined. The interaction of the ion pairs of oxonium ions including CH3OH2+, (CH3)2OH+, and (CH3)3O+ with acidic zeolites and their role in C−C coupling are still under debate.25−35 In MTO process, the first step is CH3OH dehydration to CH3OCH3 and the formation of ZOCH3. Subsequently, (CH3)3O+ ion might be generated through one of the three ways in the induction period,5 (i) nucleophilic attack of ZOCH3 by a free CH3OCH3; (ii) reaction between one adsorbed and one free CH3OCH3, and (iii) reaction between one adsorbed CH3OH and one free CH3OCH3. Consequently equilibrium mixture of C1 species, including CH3OH, CH3OCH3, ZOCH3, (CH3)3O+···−OZ, and H2O, can be established. Experimentally,32,36−39 the formation of CH3OH2+ from CH3OH protonation could not be directly observed during CH3OH adsorption on H-ZSM-5, while stable (CH3)2OH+ from CH3OCH3 protonation is observed on HZSM-5 at 373−473 K, which results in ZOCH3 above 473 K in FTIR experimental study.39 Stable (CH3)3O+ can be formed from the interaction of CH3OCH3 with H-ZSM-5 at 233−423 K as observed by using NMR techniques.32

Theoretical study on the basis of small model clusters showed that the CH3OH2+···−OZ ion pair is a transition state of H shift for the adsorbed CH3OH rather than an energy minimum on the potential energy surface (PES).34 Firstprinciple studies on the basis of a periodic model showed the formation of the stable CH3OH2+···−OZ ion pair for one CH3OH molecule adsorbed at the acidic site of an eight-ring pore of chabazite40 and H-ZSM-58.25 On the basis of larger model clusters, the stable (CH3)3O+···−OZ ion pair, which is additionally stabilized by the framework, has been optimized and characterized.27,41 Since small model clusters cannot adequately describe the electrostatic interaction of the framework, especially for the interaction between oxonium ions and framework, it is necessary to find more reasonable structural models to describe such interaction and also to study the potential roles of oxonium ions in the process of C−C bond formation.5 As the proposed mechanisms were difficult to be identified and evaluated by using experimental methods, computational methods can be used to analyze all these individual steps for getting insights into the underlying mechanisms. In this study, we carried detailed computations into the possible formation of oxonium ion as well as CH2 species (CH2O and CH3O CH2+), initiated by different methyl donor (e.g., ZOCH3, adsorbed CH3OH, CH3OCH3, and oxonium ion) and H donor (e.g., adsorbed CH3OH and CH3OCH3) over H-ZSM-5. In addition, we also studied the possible route for the formation of the first C−C bond on the basis of two CH2 species, e.g., CH2O and CH3OCH2+···−OZ/CH3O−CH2−OZ, and three C1 species, e.g., CH4, CH3OH, and CH3OCH3. 6076


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The Journal of Physical Chemistry C

2. COMPUTATIONAL DETAILS 2.1. Model. To model H-ZSM-5, we employed a 33T cluster, which includes two characteristic channels and their intersection region. In this model, the active site is assumed to be the T12 site located in the intersection region, which has a larger cage (the diameter of the largest included sphere is 6.3 Å42) to host reactants and intermediates. The proton is located on the O24 site, which is accessible to the adsorbates at the intersection of the straight and sinusoidal channels. This site has been also frequently used to model reaction mechanisms for clusters in different sizes.43−45 The dangling Si atoms were terminated with H atoms along the bond direction of the next lattice oxygen with the Si−H distance fixed at 1.498 Å.46 2.2. Methods. DFT calculations were performed at the B3LYP/6-311G(d,p) level of theory with the Gaussian09 software package.47 In the 33T cluster, the local structure of 5T around the Al center and the reactants were optimized, and the rest is fixed as taken from the crystallographic data.48 Transition state (TS) was guessed by OPT = TS and confirmed by the quasi-internal reaction coordinate approach to verify that each transition state was connected with the corresponding reactants and products. The nature of the stationary point was verified by frequency calculation (partial Hessian vibrational analysis) at the same level either as an energy minimum with only real frequencies or a transition state with only one imaginary frequency along the intrinsic reaction coordinates. Furthermore, we carried out single-point energy calculations at the ωB97XD/6-311+G(2df,2p) level on the B3LYP/6-311G(d,p) optimized structures. The ωB97XD functional, a long-rangecorrected hybrid functional for dispersion,49 was recently found to perform very well for the description of adsorption and reactions on zeolites.3,25,50−52 For our energetic discussion and comparison we used Gibbs free energies (ΔG), which are obtained from the ωB97XD/6-311+G(2df,2p) computed total electronic energies and the thermal correction from the B3LYP/6-311G(d,p) frequency calculations at 673.15 K. Rate constant (k) is obtained by using the standard transition state theory in eq 1,53,54 where kB is the Boltzmann constant, h is the Planck constant, ΔG‡ is the changes of standard molar Gibbs free energy between transition and initial states. k TST =

kBT −ΔG‡ / R T e h

distances are negligibly small; the maximal change for CH3OH is 1.02% for the acidic O−H bond; and the maximal change for CH3OCH3 is 1.58% for the distance between the acidic H and the oxygen atom of CH3OCH3. On the basis of the optimized geometries we further compared the adsorption energies (Table S3). For CH3OH, the computed adsorption at ωB97XD/6-311+G(2df,2p)// B3LYP/6-311G(d,p) and ωB97XD/6-311+G(2df,2p)// ωB97XD/6-311G(d,p) level are very close (−91 vs −96 kJ/ mol). Both values are close to the experimental results (−115 ± 5 kJ/mol);43 however, the computed adsorption energy without dispersion correction at B3LYP/6-311+G(2df,2p)//B3LYP/6311G(d,p) level (−68 kJ/mol) is much lower. For CH3OCH3, the computed adsorption energies using two procedures of ωB97XD/6-311+G(2df,2p)//B3LYP/6-311G(d,p) and ωB97XD/6-311+G(2df,2p)//ωB97XD/6-311G(d,p) are very close (−94 vs −98 kJ/mol). Both values are close to the experimental value (−90 kJ/mol),55 while the computed adsorption energy without dispersion correction at B3LYP/6311+G(2df,2p)//B3LYP/6-311G(d,p) level (−56 kJ/mol) is much lower. These results demonstrate that the ωB97XD/6311+G(2df,2p)//B3LYP/6-311G(d,p) approximation is reasonable in both structural and energetic parameters.

3. RESULTS AND DISCUSSION 3.1. Formation of CH3OH2+, (CH3)2OH+, and (CH3)3O+ (Reactions A1−A3). As mentioned above, CH3OH should be adsorbed onto the acidic site prior to dehydration. There are many studies about CH3OH adsorption, and the central issue is the adsorption state, either physisorbed complex through Hbonding interaction or chemisorbed ion pair complex through proton transfer. Indeed, the adsorption nature of CH3OH depends on several factors,25 e.g., topological feature of framework, strength of catalyst, and CH3OH loading. To the best of our knowledge, there are no reports about the stabilization of protonated CH 3 OH [CH 3 OH 2 + ] and CH3OCH3 [(CH3)2OH+] by the lattice oxygen atoms around the Si centers of the framework. Starting from ZOCH3 (Scheme 2) we computed the formation of the corresponding ion pair complex of the oxonium ions from the attack of H2O, CH3OH, and CH3OCH3 for the understanding into the detailed PES. These reactions have been computed on small (3T and 5T) or large clusters with ONIOM approach over H-ZSM-5.5,19,27,56 Only the chemisorbed ion pair complex of (CH3)3O+ has been reported from experiment32,36−38 and computational studies on the basis of large ONIOM cluster models.27 Within our 33T cluster, we located both physisorbed and chemisorbed ion pair complexes of CH3OH (A1) and CH3OCH3 (A2) as energy minimums. The optimized initial state (IS), transition state (TS), and final state (FS) are depicted in Figure 1. The relevant bond distances are summarized in Table S4. The Gibbs free energies and rate constants at 673.15 K are presented in Table 1. It clearly shows that the transition state structures for A1−A3 are quite similar since they can be considered as typical substitution reactions. For the reaction of H2O (A1) and CH3OH (A2) with ZOCH3, the transition states, [H2O··· CH3···OZ]‡ and [CH3(H)O···CH3···OZ]‡, are located, and the distance (O−H···OL) of additional hydrogen bonding between the hydroxyl group and the framework lattice oxygen atom (OL) is 2.190 and 2.471 Å, respectively. In [H2O···CH3···OZ]‡, the forming and breaking O−C distances are 1.949 and 2.069

(1)

To validate our 5T relaxed model within the 33T cluster, we tested the fully relaxed 33T model, where all atoms are relaxed and only the terminal Si−H dangling bonds are fixed in space to avoid unrealistic distortions of the model during the geometry optimizations (Table S1). Using this fully relaxed model, the computed adsorption energy of CH3OH is −92 kJ/ mol; which is nearly the same as that (−91 kJ/mol) by using the 5T relaxed model (Table S1). In addition we also tested the Gibbs free energy barriers for reactions C1 and E2 on these two models. The Gibbs free energy barriers for reactions C1 and E2 are very close, i.e., 150 vs 156 kJ/mol for reaction C1 as well as 161 vs 162 kJ/mol for reaction E2. Such negligible differences validate our 5T relaxed model clearly. Furthermore, we also validated our B3LYP/6-311G(d,p) method in geometry optimization. At first we compared the optimized structural parameters for the adsorption of CH3OH and CH3OCH3 at B3LYP/6-311G(d,p) level as well as at ωB97XD/6-311G(d,p) level including dispersion force. As shown in Figure S1 and Table S2, the changes of the bond 6077


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oxonium ions, CH3OH2+···−OZ and (CH3)2OH+···−OZ, are located, and the corresponding O−H···OL distance is 1.500 and 1.593 Å, respectively, much shorter than those in the transition states. For the reaction of CH3OCH3 with ZOCH3 (A3), the transition state, [(CH3)2O···CH3···OZ]‡, is located, and the forming and breaking O−C distances are 1.990 and 2.000 Å. The computed Gibbs free energy barrier is 117, 107, and 91 kJ/ mol for the reaction with H2O (A1), CH3OH (A2), and CH3OCH3 (A3), respectively, and the rate constant at 673.15 K is 1.1 × 104, 7.3 × 104, and 1.3 × 106 s−1, respectively. The formation of CH 3 OH 2+ ··· − OZ, (CH 3 ) 2 OH + ··· −OZ, and (CH3)3O+···−OZ ion pair complexes is endergonic by 41, 7, and 21 kJ/mol, respectively, while the formation of physisorbed CH3OH (A1) and CH3OCH3 (A2) is exergonic by 58 and 53 kJ/mol, respectively. Physisorbed complex is thermodynamically more stable than the ion pair complex by 99 kJ/mol for CH3OH and 60 kJ/mol for CH3OCH3. Considering the Gibbs free energy difference (ln K = −ΔG/RT) between the physisorbed CH3OH and CH3OH2+, as well as between the physisorbed CH3OCH3 and (CH3)2OH+, the equilibrium constant K is 4.8 × 10 7 for physisorbed CH 3 OH (K[CH3OH/CH3OH2+]) and 4.6 × 104 for physisorbed CH3OCH3 (K[CH3OCH3/(CH3)2OH+]). Both CH3OH2+ and (CH3)2OH+ are thermodynamically less stable, but the existing probability of (CH3)2OH+ is three orders of magnitude higher than that of CH3OH2+ (K[CH3OCH3/(CH3)2OH+]/ K[CH3OH/CH3OH2+] = 1.0 × 103). Indeed, (CH3)2OH+···−OZ ion pair complex is observed by using IR spectroscopy at 373−473 K.39 One can conclude that the formation probability of the protonated CH3OH and CH3OCH3 is rather low during the

Scheme 2. Proposed Mechanisms of Oxonium Ion Formation (A1−A3)

Å. In [CH3(H)O···CH3···OZ]‡, the forming and breaking O−C distances are 1.979 and 1.990 Å. Starting from these two transition states along the intrinsic reaction coordinates, the

Figure 1. Optimized structures of the initial states (IS), transition states (TS), and final states (FS) of reactions A1−A3 (red for oxygen atom at Al center, light salmon for oxygen atom at Si center, yellow for Si atom, pink for Al atom, gray for C atom, and white for H atom). 6078


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Table 1. Computed Gibbs Free Energy Barriers (ΔG‡, kJ/mol) and Reaction Free Energies (ΔGr, kJ/mol) as well as Rate Constant (k, s−1) for Reactions A1−A3 at 673.15 K ΔG‡ H2O (A1) CH3OH (A2) CH3OCH3 (A3) a

117 107 91

k‑a

ΔGra

ΔGrb

k+

41 7 21

−58 −53

1.1 × 10 7.3 × 104 1.3 × 106 4

k‑b

1.7 × 10 2.5 × 105 5.3 × 107 7

3.6 × 10−1 5.5

For chemisorbed ion pair complex. bFor physisorbed complex.

Scheme 3. Direct C−O Bond Activation and C−H Bond Cleavage Reactions for CH3OH (B1−B4) and CH3OCH3 as H Donor (C1−C4)

of CH3OH to ZOCH3 forms CH4 and CH2O···HOZ (or CH2OH+); and they further couple to form CH3CH2OH, while the acidic site is regenerated for the next step reaction. This process was proved to be accessible to generate the initial CH4. Instead of CH3OH, the attack of CH3OCH3 to ZOCH3 forms CH4 and CH3OCH2+···−OZ (or CH3O−CH2−OZ) intermediate, which further reacts with CH4, CH3OH, or CH3OCH3 to result in first C−C bond formation in methyl ethyl ether (CH3CH2OCH3), 2-methoxyethanol

CH3OH dehydration equilibrium stage. The only stable ion pair complex is (CH3)3O+···−OZ, in agreement with the experimental observation from CH3OCH3 on H-ZSM-532 by using NMR method. Therefore, we used the physisorbed CH3OH and CH3OCH3 as well as the (CH3)3O+ ion pair complex as methyl donor to study the formation of the CH2 OH+ for CH3OH and CH2OCH3+ for CH3OCH3 for general comparison. 3.2. Formation of CH2 OH + and CH 3 OCH 2 +. Following the proposed mechanism (Scheme 1), the attack 6079


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Figure 2. Optimized structures of the initial states (IS), transition states (TS), and final states (FS) of reactions B1−B4 (red for oxygen atom at Al center, light salmon for oxygen atom at Si center, yellow for Si atom, pink for Al atom, gray for C atom, and white for H atom).

Lesthaeghe et al.19 showed that the computed energy barrier for the reaction of ZOCH3 with CH3OH (B1) was around 148.2− 189.1 kJ/mol depending on cluster sizes (1T, 3T, and 5T) and computational methods. In our 33T model cluster under the consideration of the lattice oxygen stabilization, the reaction between ZOCH3 and CH3OH forms CH4 and CH2OH+···−OZ (B1, Figure 2); and this differs from the earlier theoretical results. It is noted that the metastable CH2OH+···−OZ once formed can transfer to the thermodynamically more stable species CH2 O···HOZ. The Gibbs free energy barrier is 161 kJ/mol, and the reaction is slightly exergonic by 8 kJ/mol for the formation of

(CH3OCH2CH2OH), or 1,2-dimethoxyethane (CH3OCH2CH2OCH3).23 Considering the physisorbed complexes of CH3OH and CH3OCH3 as well as the chemisorbed ZOCH3 and ion pair complex of (CH3)3O+···−OZ, four methyl donors may react with CH3OH or CH3OCH3 to form CH2O···HOZ (B1− B4) or CH3OCH2+···−OZ (C1−C4) (Scheme 3). The optimized structures of the IS, TS, and FS are shown in Figures 2 and 3. The relevant bond distances are summarized in Table S5. The Gibbs free energies and rate constants at 673.15 K are shown in Table 2. 3.2.1. Reaction with CH3OH (Reactions B1−B4). Earlier theoretical work of Blaszkowski et al.,5 Tajima et al.,15 and 6080


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Figure 3. Optimized structures of the initial states (IS), transition states (TS), and final states (FS) of reactions C1−C4 (red for oxygen atom at Al center, light salmon for oxygen atom at Si center, yellow for Si atom, pink for Al atom, gray for C atom, and white for H atom).

Table 2. Computed Gibbs Free Energy Barriers (ΔG≠, kJ/mol) and Reaction Free Energies (ΔGr, kJ/mol) as well as Rate Constants (k, s−1) for Reactions B1−B4 and C1−C4 at 673.15 K CH3OH ZOCH3 (B1) CH3OH (B2) CH3OCH3 (B3) (CH3)3O+···−OZ (B4)

ZOCH3 (C1) CH3OH (C2) CH3OCH3 (C3) (CH3)3O+···−OZ (C4) a

ΔG‡

ΔGra

ΔGrb

161 209 212 180

−8 106 167 16

−95 −48 −58 −84

4.7 8.2 × 10−4 5.2 × 10−4 1.7 × 10−1 CH3OCH3

1.1 1.3 × 105 4.5 × 109 2.9

G‡

ΔGra

ΔGrc

k

k‑a

17 15 79 8

−43 4 15 −63

3.0 × 10 1.2 × 10−3 1.7 × 10−2 3.6

150 207 192 162

k−a

k

1

5.9 1.7 2.4 1.4

× × × ×

k‑b 2.0 1.7 1.6 5.0

× × × ×

10−7 10−7 10−8 10−8

k‑c 2

10 10−2 104 101

1.5 2.3 2.4 4.4

× × × ×

10−2 10−3 10−1 10−5

For chemisorbed ion pair complex CH2OH+···−OZ or CH3OCH2+···−OZ. bFor physisorbed CH2O···HOZ. cFor CH3O−CH2−OZ.

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Scheme 4. Concerted C−C Coupling Reactions for CH2O···HOZ (D1-D3) and H2COCH3+ (E1-E3) as the CH2 Species

metastable CH2OH+···−OZ and CH4 and strongly exergonic by 95 kJ/mol for the most stable CH2O···HOZ and CH4. On the basis of the physisosrbed CH3OH (B2) as the methyl donor, a second CH3OH attacks the first adsorbed CH3OH resulting in H2O, CH4, and CH2OH+···−OZ. The Gibbs free energy barrier is 209 kJ/mol, and the reaction is endergonic by 106 kJ/mol for the formation of H2O, CH4, and CH2 OH+···−OZ and exergonic by 48 kJ/mol for the formation H2O, CH4, and CH2O···HOZ. On the basis of the physisosrbed CH3OCH3 (B3) as the methyl donor, CH3OH attacks the adsorbed CH3OCH3 resulting in CH3OH, CH4, and CH2OH+···−OZ. The Gibbs free energy barrier is 212 kJ/mol, and the reaction is endergonic by 167 kJ/mol for the formation of CH3OH, CH4, and CH2OH+···−OZ; and exergonic by 58 kJ/mol for the formation of CH3OH, CH4, and CH2O···HOZ. For using the ion pair of (CH3)3O+···−OZ as the methyl donor (B4), the reaction products are CH3OCH3, CH4, and CH2OH+···−OZ. The Gibbs free energy barrier is 180 kJ/ mol, and the reaction is endergonic by 16 kJ/mol for the formation of CH3OCH3, CH4, and CH2OH+···−OZ and exergonic by 84 kJ/mol for the formation of CH3OCH3, CH4, and CH2O···HOZ. The computed rate constant k should be 4.7, 8.2 × 10−4, 5.2 × 10−4, and 1.7 × 10−1 s−1 for ZOCH3, CH3OH, CH3OCH3, and (CH3)3O+···−OZ, respectively. On the basis of the kinetic and thermodynamic parameters, the most effective reaction path is the B1 route, followed by the B4 route, while the B2 and B3 routes are much less favorable. For the reaction between ZOCH3 and CH3OH, it is found that the formation of CH3OCH3 (A2) is much more favorable kinetically than that of CH2O···HOZ (B1) (107 vs 161 kJ/ mol, and 7.3 × 104 vs 4.7 s−1), while the formation of

CH3OCH3 (A2) is much less favorable thermodynamically than that CH2O···HOZ (B1) (−53 vs −95 kJ/mol). This reveals that these two parallel reactions might be competitive, i.e., low temperature favors formation of CH3OCH3 (A2), while high temperature favors the formation of CH4 and CH2O. Indeed, these trends are in agreement with detected desorption temperatures under the reaction condition, i.e., CH3OH at 300−600 K, CH3OCH3 at 450−650 K, and CH4 and CH2O at above 650 K.57,58 3.2.2. Reaction with CH3OCH3 (Reactions C1−C4). Apart from CH 3 OH, we also computed the reactions with CH3OCH3. The reaction of ZOCH3 with CH3OCH3 (C1, Figure 3) results in the formation of CH4 and CH3O CH2+···−OZ, and this ion pair complex can be transferred to the thermodynamically more stable CH3O−CH2−OZ species. The Gibbs free energy barrier is 150 kJ/mol, and the reaction is endergonic by 17 kJ/mol for the formation of CH3O CH2+···−OZ and CH4 or exergonic by 43 kJ/mol for the formation of CH3O−CH2−OZ and CH4. The reaction of physisorbed CH3OH with CH3OCH3 (C2) results in the formation of H2O, CH4, and CH3O CH2+···−OZ. The Gibbs free energy barrier is 207 kJ/mol, and the reaction is slightly endergonic by 15 kJ/mol for the formation of CH3OCH2+···−OZ or by 4 kJ/mol for the formation of CH3O−CH2−OZ. The reaction of physisorbed methyl donor of CH3OCH3 with CH3OCH3 (C3) results in the formation of CH3OH, CH4, and CH3OCH2+···−OZ. The Gibbs free energy barrier is 192 kJ/mol, and the reaction is endergonic by 79 kJ/mol for the formation of CH3OH, CH4, and CH3OCH2+···−OZ or by 15 kJ/mol for the formation of CH3O−CH2−OZ, CH3OH, and CH4. 6082


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Figure 4. Optimized structures of the initial states (IS), transition states (TS), and final states (FS) of reactions D1−D3 (red for oxygen atom at Al center, light salmon for oxygen atom at Si center, yellow for Si atom, pink for Al atom, gray for C atom, and white for H atom).

The reaction of the (CH3)3O+···−OZ ion pair with CH3OCH3 (C4) results in the formation of CH3OCH3, CH4, and CH3OCH2+···−OZ. The Gibbs free energy barrier is 162 kJ/mol, and the reaction is endergonic by 8 kJ/mol for the formation of CH3OCH3, CH4, and CH3OCH2+···−OZ or exergonic by 63 kJ/mol for the formation of CH3O−CH2−OZ, CH3OCH3, and CH4. The computed rate constant k is 3.0 × 101, 1.2 × 10−3, 1.7 × 10−2, and 3.6 s−1 for ZOCH3, CH3OH, CH3OCH3, and (CH3)3O+···−OZ, respectively. On the basis of the kinetic and thermodynamic parameters, the most effective reaction path is the C1 route, followed by the C4 route, while the C2 and C3 routes are much less favorable. From the reaction between ZOCH3 and CH3OCH3, it is found that the formation of (CH3)3O+···−OZ (A3) is much more favorable kinetically than that of CH3O−CH2−OZ (C1) (91 vs 150 kJ/mol, and 1.3 × 106 vs 3.0 × 101 s−1), while that of (CH3)3O+···−OZ (A3) is much less favorable thermodynamically than that CH3O−CH2−OZ (C1) (21 vs −43 kJ/mol). The kinetic preference for the formation of (CH3)3O+···−OZ (A3) is consistent with NMR observation at low temperature.32 3.3. Initial C−C Bond Formation (Reactions D1−D3 and E1−E3). On the basis of the formed CH2O···HOZ and CH3OCH2+···−OZ (or CH3O−CH2−OZ), we computed the first C−C bond formation between the CH2 group and C1 species during the induction period. For CH4, CH3OH, and CH3OCH3 as C1 species in the induction period, three direct

C−C bond coupling reactions were considered (Scheme 4). The optimized structures of the IS, TS, and FS are shown in Figures 4 and 5; and the relevant bond distances are summarized in Table S6. The Gibbs free energies and rate constants at 673.15 K are shown in Table 3. On the basis of the optimized TS, the C−C coupling is accompanied by a concerted double H transfer, one from the acidic site to the oxygen atom of the physisorbed formaldehyde and another one from the methyl group of the C1 species to the basic oxygen center to regenerate the acidic site. The computed Gibbs free energy barrier is 196, 181, and 197 kJ/ mol for CH4 (D1), CH3OH (D2), and CH3OCH3 (D3) as C1 species, respectively. The reaction for CH4 (D1) with the formation of CH3CH2OH is endergonic by 21 kJ/mol, while that for CH3OH (D2) and CH3OCH3 (D3) with the formation of HOCH2CH2OH and CH3OCH2CH2OH is exergonic by 39 and 38 kJ/mol, respectively. The rate constant k is 9.1 × 10−3, 1.2 × 10−1, and 7.9 × 10−3 s−1 for CH4, CH3OH, and CH3OCH3, respectively. Although CH3O−CH2−OZ is much more stable than CH3OCH2+···−OZ, it is not possible to use CH3O−CH2− OZ to couple with the C1 species by the concerted mechanism, and thus we used the less stable CH3OCH2+···−OZ to search the TS for the first C−C bond formation. On the basis of the optimized TS, the C−C coupling is associated with a concerted H transfer from the methyl group of the C1 species to the basic oxygen center to regenerate the acidic site. In our discussion we 6083


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Figure 5. Optimized structures of the initial states (IS), transition states (TS), and final states (FS) of reactions E1−E3 (red for oxygen atom at Al center, light salmon for oxygen atom at Si center, yellow for Si atom, pink for Al atom, gray for C atom, and white for H atom).

Table 3. Computed Gibbs Free Energy Barriers (ΔG‡, kJ/ mol) and Reaction Free Energies (ΔGr, kJ/mol) as well as Rate Constant (k, s−1) for Reactions D1−D3 and E1-E3 at 673.15 K

respectively, and the reaction is exergonic by 8, 80, and 58 kJ/ mol, respectively. The apparent rate constant k is 8.0 × 10−2, 4.5, and 2.0 × 10−1 s−1 for CH4, CH3OH, and CH3OCH3, respectively. Taking energy barriers and rate constants into account, the reaction paths via the most stable physisorbed CH2O···HOZ acting as the CH2 species (D1−D3) are less favorable kinetically and thermodynamically than that with the more stable CH3O−CH2−OZ (E1−E3). The ratio of the rate constant is 9 for E1/D1, 38 for E2/D2, and 25 for E3/D3. This reveals that the main reaction routes should be E1−E3, and the reaction routes D1−D3 should also be possible.

CH2O···HOZ ΔG‡ CH4 (D1) CH3OH (D2) CH3OCH3 (D3) ΔG‡app CH4 (E1) CH3OH (E2) CH3OCH3 (E3)

184 [140] 161 [110] 179 [125]

ΔGr

k‑

k+ −3

196 21 9.1 × 10 4.0 × 10−1 −1 181 −39 1.2 × 10 1.1 × 10−4 −3 197 −38 7.9 × 10 8.9 × 10−6 CH3O−CH2−OZ [CH3OCH2+···−OZ] ΔGr,app −8 [-51] −80 [-131] −58 [-112]

kapp

k‑

−2

2.0 × 10−2

8.0 × 10 [1.8 × 102] 4.5 × 10° [4.5 × 104] 2.0 × 10−1 [3.1 × 103]

4. CONCLUSION For the understanding into the elusive reaction mechanisms of methanol to olefins conversion over acidic zeolite catalysts, we have carried out systematic density functional theory computation on the stability and reactivity of the reaction intermediates of methanol over H-ZSM-5 as well as on the formation of the first C−C bond during the induction period. In order to include all possible interaction as much as possible we used a 33T model cluster, which contains two characteristic channels and their intersection region; and the T12 site located in the intersection region, which has a larger cage to host the reactants and intermediates, was taken as the active site. All intermediates were optimized and characterized at the B3LYP/6-311G(d,p) level. To take the dispersion forces

3.0 × 10−6 6.3 × 10−6

used the energy difference between the more stable CH3O− CH2−OZ and the corresponding transition state as the apparent Gibbs free energy barriers, and the energy difference between the more stable CH3O−CH2−OZ and the corresponding products as the apparent reaction free energies. On the basis of the more stable CH3O−CH2−OZ, the apparent Gibbs free energy barrier for the formation of CH 3 OCH 2 CH 3 (E1), CH 3 OCH 2 CH 2 OH (E2), and CH3OCH2CH2OCH3 (E3) is 184, 161, and 179 kJ/mol, 6084


The Journal of Physical Chemistry C into account, we used the long-range-corrected ωB97XD/6311+G(2df,2p) hybrid functional to calculate the energies on the B3LYP/6-311G(d,p) optimized geometries. At first we computed the adsorption state of H2O, CH3OH, and CH3OCH3 on the methylated ZSM-5 (CH3OZ). For the formed CH3OH and CH3OCH3 from H2O, CH3OH, it is found that the most stable adsorption configuration is the physisorbed CH3OH and CH3OCH3 over H-ZSM-5, and the protonated forms, CH3OH2+ and (CH3)2OH+, are much less stable. For the reaction with CH3OCH3, only the framework stabilized ion pair complex (CH3)3O+···−OZ is located. On the basis of these stable intermediates we computed their further reactions with CH3OH and CH3OCH3 and determined their most favorable routes. For the reaction between ZOCH3 and CH3OH, it is found that the formation of CH3OCH3 + HOZ is much more favorable kinetically than that of CH2 O···HOZ + CH4, while the formation of CH3OCH3 + HOZ is much less favorable thermodynamically than that of CH2O··· HOZ + CH4. Therefore, the formation of CH3OCH3 + HOZ is kinetically controlled, while the formation of CH2O···HOZ + CH4 is thermodynamically controlled. These trends are in agreement with detected desorption temperatures under the reaction condition, i.e., CH3OH at 300−600 K, CH3OCH3 at 450−650 K, and CH4 and CH2O at above 650 K. For the reaction between ZOCH3 and CH3OCH3, the formation of the framework stabilized (CH3)3O+ is kinetic controlled, consistent with NMR observation at low temperature, and the competitive formation of surface CH3OCH2OZ + CH4 is thermodynamically controlled. On the basis of the thermodynamically more favorable physisorbed CH2O and chemisorbed CH3OCH2OZ, there parallel route for the formation of the first C−C bond are identified, one from the coupling of the physisorbed CH2O with CH4, CH3OH, and CH3OCH3 as well as one from the coupling of the chemisorbed CH3OCH2OZ with CH4, CH3OH, and CH3OCH3. The later one is more favorable kinetically and thermodynamically than the former one. Our computations demonstrated clearly that the most important intermediate is the methylated surface (CH3−OZ), which can react with CH3OH and CH3OCH3 to form the corresponding physisorbed CH 2 O and chemisorbed CH3OCH2OZ, and they can further react with additional CH4, CH3OH, and CH3OCH3 to result in the formation of the first C−C bond. These verify clearly the proposed formaldehyde (CH2O) and methoxymethyl (CH3OCH2OZ) mechanisms.

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ACKNOWLEDGMENTS

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REFERENCES

We are grateful for the financial support of the National Basic Research Program (2011CB201403 and 2011CB201406), the National Natural Science Foundation of China (21573270, 21103216, 21273263, and 21273264), the Natural Science Foundation of Shanxi Province of China (2013021007-3, 2015021003), the Coal Based Low Carbon Joint Foundation of NSFC and Shanxi Province of China (U1510104), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00211. Testing results of model and method; selected bond distances for reactions A1−A3, B1−B4, C1−C4, D1− D3, and E1−E3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest. 6085


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Stability and Reactivity of Intermediates of Methanol Related

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