New Theoretical Insights into the Contributions of Poly(methylbenzene

Subscriber access provided by University of Pennsylvania Libraries

Article

New Theoretical Insights into the Contributions of Polymethylbenzene and Alkene Cycles to Methanol-to-Propene in H-FAU Zeolite Yingxin Sun, Dan Zheng, Supeng Pei, and Dongli Fan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01991 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

New Theoretical Insights into the Contributions of Polymethylbenzene and Alkene Cycles to Methanol-to-Propene in H-FAU Zeolite

Yingxin Sun*†, Dan Zheng†, Supeng Pei†, Dongli Fan*†

†

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai, 201418, China

*Corresponding authors: Yingxin Sun and Dongli Fan

Tel: +86-21-6087-7214

E-mail address: [email protected] and [email protected]

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The contributions of the polymethylbenzene (polyMB) and alkene cycles to the methanol to propene (MTP) process in H-FAU zeolite have been investigated by a two-layer ONIOM method, which is important to understand the nature of formation of propene in zeolite with large pore sizes. The calculated results demonstrate that the different pathways in polyMB cycle occur in the following order of reactivity: methyl transfer pathway > spiro pathway > direct internal H-shift > paring pathway. The polyMB cycle is more competitive than alkene cycle for the MTP in H-FAU, which is different from the previous results on H-ZSM-5. Introduction of Li+ and Ag+ cations into FAU zeolite does not reduce the free energy barriers of the methylation steps involved in polyMB and alkene cycles, indicating that the experimental efforts to improve propene selectivity probably should focus on the physical effect of Li+ and Ag+ cations. For the step of formation of propene in both cycles, the DCDs suggest a clear electron transfer between propene fragment to aromatic ring or propoxy group. Decomposing of ONIOM energy barriers into QM and MM contributions suggests that the stabilizing effect of zeolite environment on TSs mainly originates from the van der Waals interactions for the spiro and methyl transfer pathways in polyMB cycle, but electrostatic interactions for the alkene cycle. Generally speaking, the formation step of propene is entropy-increased. The direct internal H-shift and paring pathways are entropy-decreased. The entropy effect in the alkene cycle is larger than that in the polyMB cycle due to the larger entropic barriers.


Page 2 of 52

Page 3 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION The conversion of methanol to olefins (MTO) on microporous zeolite catalysts is a key step in the synthesis of polyolefins from methanol.1-2 In recent years, with the increasing demand of propene for polypropene use, the methanol-to-propene (MTP) has attracted extensive attention.3-4 The MTO process includes the MTP. As for the reaction mechanisms of MTO, in the past decades, most efforts focused on “direct” mechanisms that proposes formation of ethene only from methanol and C1 derivatives.5 Recently, the hydrocarbon pool (HP) mechanism is more in accordance with experimental observations and has received wide recognition.6,7 In this mechanism, organic species trapped in the zeolite pores interact with the zeolite framework and undergo repeated methylation steps for elimination of olefins. To date, the general and systematic conclusions in different types of zeolites for this complicated mechanism are still elusive. There is a consensus that polymethylbenzenes (polyMBs) are active hydrocarbon pool species in the HP mechanism. Side-chain and paring mechanisms have been proposed for light olefin formation.8,9 In the side-chain mechanism, olefin elimination may take place through three pathways, the direct internal H-shift, indirect spiro and methyl shift ones. The paring reaction was hypothesized in 1961 by Sullivan et al. to account for the higher conversion of hexamethylbenzene to isobutane on nickel sulfide in a H2 gas stream.9 This reaction leads to a ring carbon from the methylbenzene being incorporated into the formed olefin molecules. Together, catalytic cycles from all the abovementioned polyMBs-based mechanisms can be called polyMB cycle. In addition, Dessau,10 Svelle,11 Bjørgen,12 and Wang et al.13,14 pointed out that alkene cycle on the basis of methylation and cracking reactions of C3 alkenes was an alternative mechanism to obtain light olefins such as propene from methanol. With regard to the priority of different pathways mentioned above, many key experimental and theoretical works have been done in the literature. Sassi et al. experimentally

studied

the

reactions

of

several

methylbenzenes

including

pentamethylbenzene (PMB) and hexamethylbenzene (HMB) on H-Beta zeolite; the side-chain methylation is a predominant pathway for olefin production and the paring reaction is a possible minor pathway.15 Lesthaeghe et al. theoretically investigated side-chain growth of methylbenzene in ZSM-5 for MTO employing the ONIOM approach; they found that the side-chain growth occurred relatively easily in ZSM-5.16 Xie et al. investigated the MTO conversion on H-SAPO-34 using DFT method. Their



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

calculations suggested that the indirect spiro pathway is energetically more favorable than direct internal H-shift one; the side-chain mechanism rather than the paring one is more predominant.8,17 Wang et al. studied the contributions of polyMB and alkene cycles to the MTO in H-ZSM-5 and concluded that the contribution of alkene cycle is larger than the polyMB cycle.13 They further investigated the catalytic roles of various pores over H-MCM-22 and found that propene could be produced more favorably in the supercages through both polyMB and alkene cycles.14 Many studies have shown that the presence of water and methanol in the reaction conditions has a significant effect on the MTO selectivity. Marchi et al. experimentally found that water content led to a higher alkene production in MOR.18 Dehertog et al. reported that the use of water as a diluent did not result in different catalytic behavior from nitrogen as a diluent in ZSM-5.19 Wang et al. theoretically found that water could act as a bridging species to complete the proton shift between methylbenzenes and zeolitic framework in H-SAPO-34.8 Wispelaere and co-authors investigated the side-chain pathway in H-SAPO-34; their DFT calculations suggested that the presence of water could reduce the free energy barrier of deprotonation reaction.20 Recently, Wang et al. theoretically observed that methanol and water molecules might act as a bridge in proton transfer step in the MTO; water are more useful proton transfer reagent for zeolites with stronger acidities.21 In addition, many experimental studies have reported that metal modification (such as Ca, Cu, Ni, Mn, and Co) into zeolites can also improve the propene selectivity.22 FAU zeolite is a typical effective catalyst and has wide applications on such as catalytic cracking of paraffins or olefins in the petrochemical industry.23-25 Although a great deal of works have been done in several important zeolites such as H-ZSM-5, H-SAPO-34, and H-MCM-22, the FAU has a completely different topological structure from these reported zeolites. Then, can the conclusions in these zeolites be applied to the FAU? The following theoretical questions still remain unresolved. (1) For the polyMB and alkene cycles, which is more competitive for the formation of propene in H-FAU? (2) What roles do the water, methanol molecules, and metal cations (such as Li+ and Ag+) play in the MTP? Herein, we carried out a two-layer ONIOM study on the MTP mechanism in proton-exchanged FAU zeolite (i.e., H-FAU) to address these interesting questions. The elucidation of the reaction mechanisms provides insights into the fundamental steps of the reactions and can help to optimize the reaction conditions and design new catalysts for industrial production.


Page 4 of 52

Page 5 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. COMPUTATIONAL MODELING AND METHODS The crystal structure of the acidic FAU zeolite is represented by a 156T (155 tetrahedral atoms of Si and one tetrahedral Al atom) nanocluster model, covering the active region of the H-FAU zeolite in this work. The atomic coordinates of this model were taken from the crystal structure of FAU with cubic Fd-3m space group (a = b = c = 24.258 Å, α = β = γ = 90.0°).26 This employed 156T model covers two supercages connected to each other through a 12-membered-ring (12MR) window, as illustrated in Figure 1. One silicon atom is replaced with an aluminum atom to generate the Brønsted acid site. The acidic proton is added to the bridging oxygen atom bonded directly to the Al atom, conventionally called the O1 position. The active region is described by colored balls in Figure 1. The terminal of this model is saturated by H atoms from cutting the dangling bonds of Si atoms along the Si-O bonds of the FAU framework, with the Si-H bond distances fixed at 1.470 Å. The ONIOM calculations have been carried out with a two-layer scheme. The total energy of the system is calculated by the following equation:27

EONIOM = EMM( real ) − EMM( model ) + EQM( model )

(1)

Herein, EMM( r e al ) is the MM energy of the entire system, called the real system. The real system contains all the atoms and is calculated only at the molecular mechanics (MM) level. The model region is the chemically important part of the real system and is described at a high-level quantum mechanics (QM) approach. Both QM and MM calculations need to be performed for the model region. In the ONIOM scheme, the nonbonded van der Waals (VDW) and long-range electrostatic interactions were treated by the MM energy terms. In this study, the electrostatic energies in MM calculations describe the strength of the classic charge-charge interactions, different from the electrostatic interactions in pure QM models, unless specifically noted. We employed the DFT method with M06-2X hybrid meta-GGA functional and universal force field (UFF) as default to describe the inner and outer regions, respectively.28,29 It is well-known that B3LYP functional has many shortcomings,30 such as (1) it systematically underestimates activation barrier heights of transition states (TSs), and (2) it is not enough to treat medium-range VDW interactions, for example, aromatic-aromatic stacking energies. Recently, Zhao et al. developed new density functionals, called M06-class, for a variety of databases.31-33 The M06-2X is a hybrid meta functional and has excellent performances for accurately predicting main-group thermochemistry, electronic excitation energies, and aromatic-aromatic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

stacking interactions. This functional could be considered to describe the noncovalent interactions as reliably as the Møller-Plesset second-order perturbation theory (MP2) with the affordable computational costs for very large systems. To save computational time, the 18T model region was divided into two parts. One 12T subregion includes the active center of zeolite and reacting molecules and is treated using the 6-31G(d,p) basis set. The remaining part of the 18T model cluster comprises 6T atoms and is described with 3-21G basis set. The total zeolite model was then called ONIOM(M06-2X(12T(6-31G(d,p)):6T(3-21G)):UFF) or ONIOM (18T:156T). From the previous theoretical works, the 18T QM size is reliable enough in the ONIOM model for studying such a complicated MTP reaction network in the bulk catalytic system.34 During structural optimization, only the basic 5T cluster, [(≡SiO)3Al(OH)Si≡] within the abovementioned 12T model subregion, and the organic intermediates were allowed to relax, whereas the rest of the 156T model was fixed to the crystallographic coordinates. The frequency calculations were performed at the same level of theory to ensure that each transition state structure has only one imaginary frequency and none for each intermediate. Instrinsic reaction coordinate (IRC) calculations were also performed for the TSs both forward and reverse directions to determine two relevant minima.35 Single-point energy calculations were carried out at the M06-2X(12T(6-311+G(2df,2p)):6T(6-31G(d,p))) levels for the 18T model region using the abovementioned M06-2X optimized structures in order to obtain more reliable interaction energies. This combination rule was called ONIOM(M06-2X//M06-2X:UFF). Since the frequencies related to the soft modes contribute at most to the vibrational entropy but are also most affected by numerical errors, an effective approach is used to improve numerical accuracy by adopting normal-mode coordinates for structure optimization and numerical frequency calculations.36-39 The complicated calculations by this approach is outside the scope of the current work. Instead, in our study, both the first and second derivatives of energy are computed analytically, not numerically. It is well-known that adsorption energy is always an overestimation due to the basis set superposition error (BSSE), if the adsorption energy is obtained as the difference between the total energy of the complex (AB) and the sum of the energy of the separated molecules (A and B). In this work, interaction energies were corrected using the BSSEs by means of counterpoise (CP) scheme of Boys and Bernardi.40 The CP scheme gives an estimation of the BSSE as the following equation:


Page 6 of 52

Page 7 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BSSE = E ( A )A -E ( A )AB +E ( B )B -E ( B )AB

(2)

where E(A)A and E(B)B are the total energies of separated A and B molecules respectively. E(A)AB is the total energy of fragment A at its equilibrium structure of the complex AB. The E(A)AB is calculated with a basis set including all basis functions of fragments A and B without electrons and nuclei of fragment B. Note that in the ONIOM scheme, the BSSE should be calculated at the QM approach only for the model region. The interaction energy of two molecules A and B can be computed in the following form:

E(interaction) = EONIOM ( AB ) -EONIOM ( A ) -EONIOM ( B ) +BSSE

(3)

where EONIOM(A), EONIOM(B), and EONIOM(AB) are the total ONIOM energies of A, B, and AB molecules respectively. The rate constant (k) at 673.15 K was calculated by using the classic transition-state theory (TST) from the equation:41

k=

kBT kT exp ( −∆G ≠ / RT ) = B exp ( ∆S ≠ / R ) × exp ( −∆H ≠ / RT ) h h

(4)

where kB is the Boltzmann constant, h is the Planck constant, and ∆G ≠ , ∆H ≠ , and −T ∆S ≠ are the Gibbs free energy barrier, enthalpy barrier, and entropy loss at 673.15 K for the optimized TSs, respectively. We selected the temperature of 673.15 K (i.e., 400 °C) because the typical reaction temperature in the industrial MTP process varies between 250 and 400 °C.42 The final ONIOM(M06-2X//M06-2X:UFF) energy values with the thermal corrections at 673.15 K were used when the energies were discussed, unless specifically noted. For the Brønsted acid catalyzed reactions, the transition states in zeolite channels are cationic in nature. In this case, electrostatic interactions between cation-like TSs and anion-like zeolitic frameworks are important to ensure the accuracy of the calculations. However, the charge parameters in the UFF force field were not optimized to reproduce the electrostatic potentials (ESP) in zeolites.29 In current work, this problem is solved by calculating the ESP energy of a 116T optimized pure silica zeolite cluster at the M06-2X/6-31G(d,p) level and fitting the ESP energy to obtain the MM atomic charge values. The charge values of QM region atoms were taken directly from the ESP charges by the DFT calculations with M06-2X functional using the ChelpG scheme.43 Charge neutrality constraint was imposed for the QM and MM regions. All the calculations were performed using the Gaussian 09 code.44



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 52

The difference charge densities (DCDs) can be employed to describe the interaction strength between host and guest molecules.45,46 In current work, difference charge densities of the transition states confined in FAU pore are used to elucidate the migration of electrons for different elementary steps. The following equation is defined to demonstrate the electrostatic interaction between the zeolite framework and hydrocarbon fragment in a given transition state structure.

∆ρ = ρ zeolite −hydrocarbon − ρ zeolite − ρ hydrocarbon

(5)

The ρ zeolite − hydrocarbon , ρ zeolite , and ρ hydrocarbon are the electron densities of TS, isolated zeolite, and the hydrocarbon fragment in the TS, respectively. Multiwfn software is used to obtain the DCD values (∆ρ).47 Classic molecular dynamics (MD) simulations have been carried out to calculate the diffusion properties of several important intermediates in zeolite pores. A unit cell consisting of intermediate molecules was prepared for FAU model. Periodic boundary conditions were applied in all three dimensions.48 The NVT (number of particles, volume, and temperature) ensemble was used to investigate self-diffusivities of guest molecules at typical experimental temperatures (523.15, 573.15, 623.15, and 673.15 K). We used the velocity Verlet algorithm for time integration with a time step of 0.001 ps. Temperature was held constant by Nosé-Hoover thermostat. At each temperature, a 1000-ps pre-equilibrium stage was followed by a 1000-ps production stage to sample the diffusion properties of interest. The zeolite framework was assumed to be rigid and the adsorbed molecules were fully mobile. All MD simulations were performed with the LAMMPS program package.49 The self-diffusion coefficients of intermediates were obtained from the slope of mean-square-displacement (MSD) versus time plot using the Einstein relation:50

Dself =

1 d lim 6 N t →∞ dt

N

∑ ( r (t ) − r ( 0)) i

i

2

(6)

i =1

where ri ( 0 ) and ri ( t ) are the positions of molecule i at time 0 and t respectively, 1 N

N

∑ ( r (t ) − r (0)) i

i

2

is the MSD, and the bracket represents an ensemble average.

i =1

We used a truncated and shifted potential (rcut = 12.0 Å), and tail corrections were not used. A truncated and shifted potential without tail corrections was more suitable and widely applied for zeolite systems in molecular simulations.51-53 Lennard-Jones (LJ) parameters of Si, Al and O atoms were taken from the


Page 9 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DREIDING force field and the other LJ parameters from OPLS force field.48,54 The charges on Si and O atoms were obtained by calculating and fitting the ESP energy of the abovementioned 116T pure silica zeolite cluster at the M06-2X/6-31G(d,p) level. The charge of Al atom is taken directly from the ESP charges by the DFT calculations with M06-2X functional using the ChelpG scheme. The charge values of atoms for intermediate molecules are taken from OPLS force field. The long-range electrostatic interactions were calculated by Ewald summation technique.55,56 The LJ-12-6 potential and Lorentz-Berthelot combination rule were used for calculating VDW interactions and constructing parameters for all the different atom pairs, respectively:

 σ ij E LJ (rij ) = 4ε ij    rij 

σ ij =

σi +σ j 2

12 6   σ ij    −    r     ij  

, ε ij =

ε iε j

(7)

(8)

where the σ ij and ε ij represented the VDW radius and energy well depth for two nonbonded atoms i and j.

3. RESULTS AND DISCUSSION We have proposed the possible reaction mechanisms for the MTP process on H-FAU zeolite based on the previous published works.13,14 The corresponding pathways are given in Schemes 1-6 and Supporting Information (SI, Scheme S1). All related TSs and intermediates are identified in the elementary steps. All energy values were obtained as follows: for each intermediate or TS, energy data were calculated relative to the total energy of the initial reactant molecules (methanol, PMB, H2O, propene) and H-FAU zeolite at infinite separation; the activation barrier of one single step for each TS was calculated as the energy difference between the TS and its previous intermediate. The activation barrier values of all TSs are listed in Tables 1-4. All other data are displayed in Tables S1 and S2. The whole catalytic cycle for MTP can be divided into polyMB and alkene cycles (Scheme 1). In the polyMB cycle, direct internal H-shift pathway (Scheme 2), paring one (Scheme 3), indirect spiro (Scheme 4), and methyl shift ones (Scheme 4) are involved. The side-chain mechanism includes three pathways in Schemes 2 and 4. Water and methanol can serve as the proton transfer reagents in the indirect spiro and methyl transfer pathways. The main product, propene can be eliminated from the side



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

chain of the polyMBs (side-chain mechanisms) or through aromatic ring contraction with subsequent propene removal (paring pathway). Interestingly, the propene can be methylated by methanol leading to the formation of new propene through the alkene cycle (Schemes 5 and S1). Benzene molecules may be produced via an aromatization process of higher intermediates from alkene cycle (Scheme 6) and participate in the polyMB cycle. So propene on H-FAU could be yielded through both polyMB and alkene cycles and these two cycles are promoted reciprocally during the reaction process. It should be pointed out that INT-3 is the initial species in the paring pathway (see Scheme 3); the INT-3 can be formed through repeated methylation and deprotonation steps of PMB as illustrated in Scheme 2. Thus, the whole paring pathway should include all elementary steps from the initial co-adsorption of PMB with methanol to the formation of the INT-3, and from the INT-3 to the last step of paring pathway. For simplicity, the same steps (from complex-1 to INT-3 in Scheme 2) are omitted in Scheme 3. Similarly, INT-7 is the initial species in the spiro and methyl shift pathways (see Scheme 4); the INT-7 can also be formed through a series of steps from the complex-1 (see Scheme 2). So the same steps (from complex-1 to INT-7) are omitted in Scheme 4.

3.1. Direct Internal H-Shift Pathway in Side-Chain Mechanism. Scheme 2 gives the direct internal H-shift pathway in H-FAU zeolite. This pathway begins with the capture of methanol by the Brønsted acid site of H-FAU through hydrogen bonds, as shown in Figure S3. Two different adsorption modes, complex-1 and complex-1’, are formed. In the complex-1, there are two hydrogen bonds, a strong one between the acidic proton of zeolite and oxygen in methanol (Zeo-OH···O(H)CH3, 1.287 Å) and a weak one between the hydroxyl proton in methanol and oxygen in zeolite (Zeo-O···HO-CH3, 2.123 Å). In the complex-1’, there is only one hydrogen bond of Zeo-OH···O(H)CH3 with a bond length of 1.337 Å. The zeolite O-H bond lengths of 1.115 and 1.088 Å in two adsorption modes are longer than the O-H bond length of 0.970 Å in isolated H-FAU zeolite. The small elongations of zeolite O-H bond lengths (less than 0.15 Å) suggest that the methanol molecule is not protonated, which is in accord with the previous reported studies over H-ZSM-5 and H-beta.42,57 The BSSE corrected adsorption enthalpies of methanol are calculated to be -87.38 and -79.15 kJ mol-1 for complex-1 and complex-1’ respectively at the


Page 10 of 52

Page 11 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ONIOM(M06-2X//M06-2X:UFF) with thermal corrections at 673.15 K. The adsorption enthalpy of methanol for complex-1 in H-FAU is comparable to the values of -104 (periodic DFT method, H-FAU, 300 K)58, -97.27 (DFT-D theory, H-ZSM-5, 400 K)59, and -115 ± 5 kJ mol-1 (experimental value, H-ZSM-5, 400 K)60 reported by Nguyen, Speybroeck, and Lee respectively. Although the adsorption enthalpy of methanol is negative (-87.38 kJ mol-1), the BSSE corrected adsorption Gibbs free energy is positive (17.54 kJ mol-1; the BSSE uncorrected value is 8.70 kJ mol-1, see Figure 2). This suggests that the methanol molecule does not tend to adsorb on acidic site at high temperatures, but desorb from the adsorbed state. Although the positive adsorption Gibbs free energy indicates that the adsorption of methanol is unfavorable at the experimental conditions, the methanol can migrate in the zeolitic pores with high speed at high experimental temperatures and be frequently close to the acidic site of H-FAU zeolite. This leads to the subsequent methylation step of other molecules such as HMB and propene by methanol on the acidic sites. In the energy profiles, we employed the BSSE uncorrected adsorption free energies because of the very long computational time for all the BSSE corrections of adsorption energies, unless specifically noted. Both PMB and HMB are important hydrocarbon pool species for the growth of the C-C chain at the initial stage of MTP. The PMB molecule can be co-adsorbed with the methanol to form complex-2 and subsequently methylated through an SN2-type TS1 leading to the formation of hexamethylbenzenium (hexaMB+, INT-1) and H2O molecule. An activation free energy barrier of 175.36 kJ mol-1 at 673.15 K must be surmounted for this methylation step, which is in accord with the reported value (160 kJ mol-1 at 673 K) for methylation of p-xylene over H-ZSM-5 by Wang et al.13 The entropy loss is 0.17 kJ mol-1, suggesting that the methylation step of PMB is controlled by the enthalpy barrier, rather than entropic effect. Figure S4 gives the structure of TS1; the breaking O-C bond of methanol and forming C-C between methanol and PMB molecules are 2.112 and 2.107 Å respectively in length. The HMB can be formed by losing the proton of the resulting hexaMB+ from the aromatic ring to the zeolite oxygen atom (TS2 in Scheme 2). This deprotonation process needs to overcome a free energy barrier of 96.90 kJ mol-1 with a rate constant of 4.24 × 105 s-1. Clearly, the deprotonation step is kinetically faster than the methylation step. A free energy barrier of 168.00 kJ mol-1 is needed for the methylation of HMB (TS3), leading to the formation of heptamethylbenzenium ion (heptaMB+, INT-3) and water molecule. The configuration of TS3 is similar to that of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 52

TS1 (Figure S4) and the activation barrier is slightly lower than that of TS1. Our calculated energy barrier value for methylation of HMB is comparable to theoretically reported values on H-Beta (144 kJ mol-1), H-ZSM-5 (126 kJ mol-1), and H-CHA (60.8 kJ mol-1) at the ONIOM level.16 A previous experimental study verified that the heptaMB+ ion (INT-3) can be converted

into

hexamethylmethylenecyclohexadiene

SAPO-type molecular sieve, DNL-6.

61

(HMMC,

INT-4)

in

a

In current work, a deprotonation step of the

INT-3 can occur through transition state TS4 to produce INT-4. The corresponding activation free energy barrier and reaction energy from INT-3 to INT-4 are 54.60 and 43.55 kJ mol-1, respectively. The calculated rate constant is large, 8.13 × 108 s-1. Clearly, the small activation barrier and the reaction energy indicate that heptaMB+ ion and HMMC are in a fast state of equilibrium, which is in accord with abovementioned experimental observation.61 The HMMC will proceed through a methylation step (TS5) followed by a deprotonation step (TS6) to form INT-6. These two processes need to overcome free energy barriers of 118.04 and 50.81 kJ mol-1, respectively. Then, the reaction proceeds via the methylation of INT-6 to form INT-7 with an energy barrier of 128.46 kJ mol-1 (TS7). Finally, the propene as the product is produced through an internal H-shift step of INT-7 with the energy barrier of 220.37 kJ mol-1 (TS8) and the rate constant of 1.11 × 10-4 s-1. At TS8 in Figure 3, the breaking C-H bond of the CH3 group is elongated to 1.688 Å and the C-C distance between the isopropyl group and the ring carbon is 1.704 Å. The propene molecule is almost formed. The whole catalytic cycle of direct internal H-shift pathway is completed via three successive internal CH3-shift steps (TS9, TS10, and TS11) of INT-8. Finally, the INT-1 is produced and participates in the next cycle. The structural parameters of TS9-TS11 are given in Figure S6. The distances between the methyl group and the nearest-neighbor aromatic carbon atom range from 1.842 to 1.927 Å. For each transition state, the methyl group is located between the acidic site of zeolite and the PMB fragment of TS9–TS11. The activation barriers of internal CH3-shift steps are small and in the range of 38-58 kJ mol-1, which is lower than the reported values (65-97 kJ mol-1) in H-SAPO-34 zeolite by Wispelaere et al. probably due to the smaller spatial hindrance of FAU zeolite channel.20 The rate-determining step for the direct internal H-shift pathway can be obtained by dividing the whole reaction sequence into several different sections. Murdoch pointed out that each section starts with an intermediate and terminates with another


Page 13 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intermediate, which is more stable in energy than the previous intermediate.62 One should compute the energy difference between the transition state with the highest energy and the initial intermediate in each section. The largest energy difference will contain the rate-determining step. In current work, six sections, reactants to INT-2, INT-2 to INT-7, INT-7 to INT-8, INT-8 to INT-9, INT-9 to INT-10, and INT-10 to INT-1, are involved in Figure 2. The largest energy difference is obtained in the third section (from INT-7 to INT-8 via TS8); the free energy barrier is calculated to be 220.37 kJ mol-1. Therefore, the rate-determining step for the direct internal H-shift pathway belongs to the internal H-shift step of INT-7, which is in accord with our previous result in H-Beta,63 but higher than those for propene elimination from p-xylene on H-ZSM-5 (170.57 kJ mol-1),21 H-MOR (159.81 kJ mol-1),21 and

H-MCM-22 (162 kJ mol-1).14 Aromatics, specifically polymethylbenzenes, play an important role in MTP catalysis. Isotopic labeling experimental studies of methanol co-fed suggest that aromatic methylation reactions involve sequential methylation steps.64 In current study, four methylation steps take place from PMB to INT-7 and the activation barriers are in the range of 118-175 kJ mol-1 that is higher than the range of 50-97 kJ mol-1 for three deprotonation steps (Table 1). A careful investigation of these four methylation steps suggests that methylation of the exocyclic double bond (TS5 and TS7) is easier than that of the aromatic ring carbon atom (TS1 and TS3), which is consistent with the conclusion in H-Beta.63 The calculated activation barriers are higher than the reported values (57-98 kJ mol-1, ONIOM(M06-2X:PM3)) by Wispelaere et al. for the methylation of HMB in H-SAPO-34 zeolite.20 Wang et al. reported free energy barriers of 144, 111, and 130 kJ mol-1 required for three successive methylation steps of 1,2,4,6-tetramethylbenzene at 673 K in the supercages of H-MCM-22 zeolite.14 An experimental estimation of activation barrier for toluene methylation over H-ZSM-5 is in the range of 130-160 kJ mol-1.65,66 Wispelaere et al. pointed out that the free energy barriers for the deprotonation steps could be effectively decreased with the assistance of H2O molecules in H-SAPO-34 zeolite.20 Wang and co-workers reported that the energy barriers of deprotonation steps assisted with H2O are only about 3-8 kJ mol-1 lower than those without assistance of water.13 In current work, the calculated results have already suggested that all deprotonation steps without assistance of water are not rate-determining in all pathways involved in polyMB and alkene cycles. Thus, we did not investigate the assistance of water in the deprotonation process based on the fact



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

that this step is generally fast to occur. Decomposing the ONIOM activation free energy barriers into QM and MM contributions is an important way to investigate the effect of zeolite framework on the reaction mechanisms. The MM contributions can be further decomposed in terms of van der Waals and electrostatic interaction energies. Two remarks can be obtained from the energy data, as listed in Table 1. First, all the ONIOM free energy barriers are mainly attributed to the QM contributions. Second, the MM energy values are negative for most of the methylation steps (TS1, TS3, and TS5) and all the internal CH3-shift steps (TS9-TS11), indicating that the zeolite environment have a stabilizing effect on these processes. Interestingly, the MM energy decomposition suggests that this stabilizing effect of zeolite framework mainly originates from the classic electrostatic interactions (i.e., charge-charge ones) for the methylation steps but VDW interactions for the internal CH3-shift steps, respectively (Table 1). For other steps, the MM energy values are positive, showing that the zeolite framework disfavor the occurrence of these reaction steps.

3.2. Paring Pathway. For simplicity, we omitted the same steps (from complex-1 to INT-3 in Scheme 2) in Scheme 3. The paring pathway begins with the methylation of HMB to form heptaMB+ cation (INT-3). Subsequently, a bicyclic intermediate, heptamethylbicyclo [3.1.0] hexenyl cation (INT-paring-1), is formed through a ring contraction step (TS-paring-1) with a free energy barrier of 143.25 kJ mol-1 and a rate constant of 1.07 × 102 s-1. From the INT-paring-1, an intramolecular hydride shift step (TS-paring-2) leads to the formation of hexamethylbicyclo [3.2.0] heptenyl cation (INT-paring-2) with an energy barrier of 250.32 kJ mol-1 and a small rate constant of 5.27 × 10-7 s-1. This is followed by a C-C bond breaking step (TS-paring-3) to form propene and INT-paring-3. The propene elimination needs to overcome an energy barrier of 101.71 kJ mol-1 with a moderate rate constant of 1.80 × 105 s-1. The calculated free energy barriers of ring contraction (143.25 kJ mol-1) and intramolecular H-shift (250.32 kJ mol-1) steps are comparable to the reported results (133.15 and 215.17 kJ mol-1) in H-SAPO-34 zeolite by Wang et al.17 Furthermore, Wang et al. predicted that no transition state could be located in the elimination of propene from the bicyclic INT-paring-2 in H-SAPO-34. In current work, the TS-paring-3 is located in H-FAU with moderate energy barrier (101.71 kJ mol-1) and verified by frequency and IRC calculations, which reflects the effect of different


Page 14 of 52

Page 15 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


zeolite environment on catalytic pathways. In TS-paring-3, the propene is produced through the breaking of both C-C bonds in INT-paring-2; the corresponding breaking C1-C4 and C2-C3 bond distances are 2.801 and 2.391 Å, respectively, indicating that the propene molecule is almost formed (see Figure 3). Figure S8 shows the structural parameters of TS-paring-2. At TS-paring-2, one of proton in methyl group (C1 atom) attached to the three-membered ring shifts to the adjacent C2 atom; the breaking C1-H and forming C2-H bond lengths are 1.638 and 1.186 Å, respectively. Simultaneously, the three-membered ring is broken, and the breaking C2-C4 and forming C1-C4 bond lengths are 2.454 and 2.688 Å, respectively. This step needs to surmount a large free energy barrier (250.32 kJ mol-1) since the TS-paring-2 is a primary carbocation (labeled as the C1 atom). Two steps, deprotonation (TS-paring-4) and protonation (TS-paring-5) from INT-paring-3 occur to produce the third bicyclic species, polymethylbicyclo [3.2.0] hexenyl cation (INT-paring-4). These two steps need to overcome activation free energy barriers of 4.66 and 171.60 kJ mol-1, respectively. It is clear that the deprotonation of INT-paring-3 with such a low free energy barrier (4.66 kJ mol-1) in H-FAU is virtually barrierless. The corresponding energy barrier for this step at 0 K is calculated to be 11.27 kJ mol-1; Wang et al. predicted an energy barrier of 9.65 kJ mol-1 at 0 K for the same step in H-SAPO-34.17 The ring expansion of INT-paring-4 and deprotonation of INT-paring-5 occur to form complex-paring-2 (TMB). The PMB is produced through a methylation and subsequent deprotonation steps of TMB. Finally, the catalytic cycle of paring pathway is closed. The rate-determining step for the paring pathway should be analyzed by adding the elementary steps from complex-1 to INT-3 in Figure 2 into Figure 4. According to the study reported by Murdoch,62 four sections in paring pathway are involved: reactants to INT-2, INT-2 to INT-paring-5, INT-paring-5 to complex-paring-2, and complex-paring-2 to PMB. The corresponding energy differences between transition state of the highest energy and their respective initial intermediate for each section are calculated to be 182.94 (TS1), 301.96 (TS-paring-2), 16.36 (TS-paring-7), and 152.91 kJ mol-1 (TS-paring-8), respectively. So the intramolecular hydride shift step of INT-paring-1 via TS-paring-2 in the second section should be rate-determining. We should note the difference between overall energy barrier of the TS with the highest energy in one given section and energy barrier of one single step for the same TS in the same section. In the second section, the overall energy barrier and energy barrier of the single step (i.e., from INT-paring-1 to TS-paring-2) for TS-paring-2 are 301.96



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and 250.32 kJ mol-1, respectively. The overall energy barrier for the rate-determining step in the direct internal H-shift pathway is 220.37 kJ mol-1, which indicates that the direct internal H-shift pathway is more favorable than paring pathway. This is consistent with the reported experimental results on H-beta zeolite15 and theoretical calculations on H-SAPO-34 zeolite.17 Table 2 lists the QM and MM contributions to the ONIOM free energy barrier. Generally speaking, the ONIOM energy barriers are mainly attributed to the QM contributions. But the MM energy values give a substantial contribution to the ONIOM energy barriers for the deprotonation steps (TS-paring-4, TS-paring-7, and TS-paring-9); especially, the MM energy is slightly larger than the QM energy for the TS-paring-4, indicating the importance of the classic nonbonded interactions in the deprotonation of INT-paring-3.

3.3. Indirect Spiro and Methyl Transfer Pathways in Side-Chain Mechanism. In the direct internal H-shift pathway, H2O molecules are formed as a by-product. According to indirect spiro and methyl transfer pathways mentioned in Scheme 4, the water and methanol molecules as the proton transfer reagents play an important role in the proton shift process between the reactants and zeolite framework. In this section, indirect spiro and methyl transfer pathways have been investigated in detail and compared with the direct internal H-shift pathway. For simplicity, we omitted the same steps (from complex-1 to INT-7 in Scheme 2) in Scheme 4. In the spiro pathway, one water or methanol molecule is co-adsorbed onto the INT-7. Then, an intermediate, INT-spiro-1, is formed through a deprotonation step with activation free energy barriers of 130.40 and 138.50 kJ mol-1 for TS-spiro-1-H2O and TS-spiro-1-methanol respectively. The energy barrier values are consistent with the reported results (138.02 and 135.52 kJ mol-1 for H2O and methanol as proton transfer regents respectively) in H-ZSM-5 zeolite by Wang et al.21 The INT-spiro-1 features a side spiro structure with the angles of the three-membered ring being about 60°. Due to the imposed strain of the small three-membered ring, the INT-spiro-1 is relatively unstable; its energies are 17.02 and 24.64 kJ mol-1 higher than those of INT-7 for INT-spiro-1-methanol and INT-spiro-1-H2O, respectively. Figure S10 gives the corresponding structures of TS-spiro-1-CH3OH and TS-spiro-1-H2O. Propene is yielded through the protonation step of INT-spiro-1 with the free energy barriers of 110.01 and 124.09 kJ mol-1 for H2O (TS-spiro-2-H2O) and methanol (TS-spiro-2-methanol) as proton transfer reagents, respectively, which


Page 16 of 52

Page 17 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


suggests that the H2O molecule is more favorable than methanol to promote the propene elimination. Wang et al. pointed out that water as proton transfer reagent was more efficient than methanol for propene elimination over zeolites like H-MOR and H-Beta with stronger acidity; over H-MCM-22 with relatively weaker acidity, methanol is more favorable than water.21 Zhu et al. reported the experimental order of acidity: FAU > Beta > MCM-22 ≈ ZSM-5.67 Thus, one can predict that water is more favorable than methanol for formation of propene in the spiro pathway over H-FAU zeolite, which is validated by the abovementioned theoretical prediction. After desorption of propene, the resulting another intermediate, INT-8, can reacts with three successive internal CH3-shift and deprotonation steps to form the HMB species, and the indirect spiro pathway is completed. Figure 5 gives the structures of TS-spiro-2-H2O and TS-spiro-2-methanol. It should be noted that the enthalpy barrier and entropy loss in the Gibbs free energy barrier are 116.76 and -6.75 kJ mol-1 for the TS-spiro-2-H2O, which is comparable to the corresponding values for the TS-spiro-2-methanol (117.72 and 6.37 kJ mol-1). The difference of free energy barriers between these two transition states can thus be attributed to the entropic effect. However, we did not find a reasonable explanation of the positive entropy loss (6.37 kJ mol-1) for the TS-spiro-2-methanol from the structural difference between these two different transition states, probably because the entropy loss is small in value. Instead, we investigated the QM and MM contributions in ONIOM energies and found that the difference of free energy barriers between two transition states is due to the slightly higher QM and MM energy barriers (see Table 3). Indirect methyl transfer pathway can also take place for formation of propene as described in Scheme 4. One methyl group of the INT-7, in para position with respect to ring carbon where an isopropyl group is connected, migrates by three successive internal CH3-shifts to form an intermediate, INT-ch3trans-3 with a sp3 hybridized ring carbon atom. Three transition states, TS-ch3trans-1 to TS-ch3trans-3, are needed for this process with activation free energy barriers of 58.57, 44.34, and 101.98 kJ mol-1, respectively. Figure S11 shows the structural parameters of three TSs. Two key C1-C2 and C1-C3 bond distances are in the range of 1.85-1.95 Å. The larger spatial hindrance between methyl and isopropyl groups in TS-ch3trans-3 results in a larger activation energy barrier than those in TS-ch3trans-1 and TS-ch3trans-2 (Table 3). A water or methanol molecule can be co-adsorbed on the INT-ch3trans-3 (Figure S11(d)). A subsequent propene elimination occurs through TS-ch3trans-4 with



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

activation barriers of 51.39 and 30.27 kJ mol-1 at 0 K for TS-ch3trans-4-H2O and TS-ch3trans-4-methanol, respectively. The activation free energy barriers for H2O and methanol at 673.15 K are calculated to be 11.88 and -6.38 kJ mol-1. We further calculated the corresponding free energy barriers for methanol at 298.15 (4.32 kJ mol-1) and 523.15 K (-2.09 kJ mol-1). These negative energy values indicate that the activation barrier for methanol as proton transfer reagent decreases with increasing temperatures. The calculated enthalpy barriers (12.70, 12.90, and 12.81 kJ mol-1) and entropy losses (-8.38, -14.99, and -19.19 kJ mol-1) at 298.15, 523.15, and 673.15 K respectively indicate that the propene elimination at different temperatures are controlled by the entropic effect. Figure 5(c) gives the structural parameters of TS-ch3trans-4-methanol. The methyl transfer pathway is completed when propene and HMB are produced. The entropy loss for TS-ch3trans-4-H2O is negative (-19.59 kJ mol-1, see Table 3), which indicates that the propene elimination step is entropy-increased. This can be explained from the TS structure. At TS-ch3trans-4-H2O (see Figure 5(d)), the isopropyl group is leaving the ring carbon C4 and a proton of methyl group belonging to the isopropyl group is migrating toward the active oxygen of H2O. Simultaneously, the hydrogen atom of H2O is migrating toward the zeolite acidic oxygen O2. The breaking C2-C4 and C1-H bond distances are 3.043 and 1.296 Å, respectively. It is clear that the propene is almost formed in the TS-ch3trans-4-H2O and thus the entropy value of transition state increases, which leads to a negative entropy loss and a low free energy barrier as mentioned above (see Table 3). The rate-determining steps for the spiro and methyl transfer pathways can be obtained by adding the elementary steps from complex-1 to INT-7 in Figure 2 into Figure 6. Three sections in spiro pathway are involved: reactants to INT-2, INT-2 to INT-7, and INT-7 to INT-8-propene. Five sections in the methyl transfer pathway are involved: reactants to INT-2, INT-2 to INT-7, INT-7 to INT-ch3trans-1, INT-ch3trans-1 to INT-ch3trans-2, and INT-ch3trans-2 to HFAU-HMB-propene-H2O. The overall free energy barriers for spiro pathway in each section are 182.94 (TS1), 197.02 (TS5), and 171.98 kJ mol-1 (TS-spiro-2-CH3OH). For methyl transfer pathway, they are 182.94 (TS1), 197.02 (TS5), 58.57 (TS-ch3trans-1), 44.34 (TS-ch3trans-2), and 101.98 kJ mol-1 (TS-ch3trans-3). Therefore, the methylation step of HMMC (INT-4) for TS5 is the rate-determining step for both the spiro and methyl transfer pathways; the methyl transfer pathway is more favorable than the spiro one. From Figures 2, 3, and 5, one can conclude that the different pathways in polyMB cycle


Page 18 of 52

Page 19 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


occur in the following order of reactivity: methyl transfer pathway > spiro pathway > direct internal H-shift > paring pathway. So the indirect methyl transfer pathway is the most favorable in the polyMB cycle in H-FAU, which is in line with the previous theoretical investigation in H-ZSM-5.13 Three viewpoints can be obtained from a close analysis of the energy components of QM and MM as listed in Table 3. First, the stabilities of TSs are mainly due to the QM contributions; the exception is TS-ch3trans-4-H2O with the larger MM energy, showing the importance of the MM term in the ONIOM energy scheme (see equation (1)). Second, the MM contributions are generally larger in the spiro pathway than those in the methyl transfer pathway, which indicates that the effect of zeolite environment is larger on the spiro pathway. Furthermore, the decomposition of MM energies suggests that the effect of zeolite environment mainly originates from the VDW interactions. Third, it is interesting that the MM contributions in Table 3 are favorable for TS-spiro-1-methanol but unfavorable for TS-spiro-1-H2O. The side views of these two TSs in Figure S12(a, b) can give a reasonable explanation of the difference of MM effect. It is clear that the organic fragment of TS-spiro-1-H2O partly enters the 12-membered-ring window, leading to a larger spatial hindrance and thus higher VDW (8.57 vs. -13.48 kJ mol-1) and electrostatic repulsion energy barriers (3.00 vs. 2.84 kJ mol-1) with regard to the TS-spiro-1-CH3OH (see Table 3). So, the MM contribution to the free energy is more stabilizing for TS-spiro-1-CH3OH. We calculated the DCDs of the transition states to elucidate the migration of electrons for the formation of propene in the direct internal H-shift, indirect spiro, methyl transfer, and paring pathways (i.e., TS8, TS-spiro-2-H2O, TS-ch3trans-4-H2O, and TS-paring-3). The DCD for the formation of propene is obtained from the following equation: TS ∆ρ = ρ TS − ρ zeolite − ρ TS − hydrocarbon( nopropene ) propene( gas )

(9)

TS where ρ TS is the electron density of TS. ρ zeolite and ρ TS are − hydrocarbon ( nopropene ) propene( gas )

the electron densities of TS without propene fragment and isolated gas propene molecule respectively at their equilibrium structures of the TS. For all these four TSs, the behaviors of DCDs are the same. For example, for TS-spiro-2-H2O (Figure 7), the electron density in propene fragment decreases but increases in aromatic ring, which reflects a clear electron transfer from propene fragment to aromatic ring.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 52

3.4. Alkene Cycle. The proposed alkene cycle for MTP is presented in Schemes 5 and S1. Four types of reaction steps are involved: methylation, deprotonation, hydride shift, and β scission. The alkene cycle can be divided into the C6+ and C7+ pathways, depending on

the

precursors

for

cracking

process.

Higher

intermediates,

such

as

2-methyl-4-pentyl alkoxide and 2,4-dimethyl-4-pentyl carbenium ion, can undergo β scissions to form propene. In both C6+ and C7+ pathways, the cycle begins with the co-adsorption of methanol with propene and terminates with the formation of new propene molecules. The optimized structures of TSs are shown in Figures 8 and S13-S16. In the C6+ pathway, the methylation steps for TS-alkene, TS-C6-2, and TS-C6-4 need to overcome free energy barriers of 174.61, 161.66, and 144.34 kJ mol-1 respectively, comparable to the range of 118-175 kJ mol-1 in the direct internal H-shift pathway. The deprotonation steps for TS-C6-1 and TS-C6-3 occur easily because they only need a free energy barrier lower than 84 kJ mol-1. The propene molecule can be produced through a β scission step of 2-methyl-4-pentyl alkoxide (TS-C6-6); the free energy barrier and rate constant for this step are 112.55 and 2.59 × 104 s-1, respectively. In TS-C6-6 as shown in Figure 8, a sp2 hybridized propoxy group appears between the propene and active oxygen atom of zeolite; the lengths of forming C-C and breaking C-O bonds are 2.63 and 2.43 Å, respectively. A DCD analysis suggests a clear electron transfer from propene fragment to propoxy group (Figure S21). The propene can also be formed from the propoxy group by a deprotonation step with a free energy barrier of 68.41 kJ mol-1 for TS-C6-7 (Figure 9). In the C7+ pathway, the methylation steps for TS-alkene, TS-C7-2, TS-C7-4, and TS-C7-6 have free energy barriers of 174.61, 168.33, 169.62, and 172.57 kJ mol-1, slightly higher than those in the C6+ pathway, with rate constant of 3.96 × 10-1, 1.21 × 100, 9.64 × 10-1, and 5.70 × 10-1 s-1, respectively. The deprotonation steps of TS-C7-1, TS-C7-3, and TS-C7-5 are fast, as they need a free energy barrier below 95 kJ mol-1. The β scission of 2,4-dimethyl-4-pentyl carbenium ion produces isobutene and a propoxy group that will be rapidly deprotonated to form propene; this cracking step required an energy barrier of 84.50 kJ mol-1 with a rate constant of 3.89 × 106 s-1. The rate-determining step for the alkene cycle can be obtained from Figures 9 and S1 by dividing the whole reaction sequence into different sections. A careful analysis suggests that the methylation step of propene (TS-alkene) is rate-determining for both C6+ and C7+ pathways and the overall free energy barrier is calculated to be


Page 21 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


240.05 kJ mol-1. The indirect methyl transfer pathway is the most favorable in the polyMB cycle and the methylation step of HMMC (INT-4) for TS5 is rate-determining with the free energy barrier of 197.02 kJ mol-1. This indicates that the polyMB cycle is more competitive than the alkene cycle for the MTP process in H-FAU, which is different from the result on H-ZSM-5 zeolite.13 The FAU with large pore size probably can stabilize the aromatic TS structures in the polyMB cycle more effectively than the other types of zeolites. Wang et al. pointed out that the polyMBs molecules could be produced by aromatization from higher intermediates formed in alkene cycle.13 The resulting polyMBs could participate in the polyMB cycle, which yields new propene molecules. The new propene could promote the next alkene cycle. Therefore, both the polyMB and alkene cycles occur in the H-FAU and interact with each other. It is necessary to analyze the communication between these two cycles. Here, we investigated the aromatization of 2-methyl-3-pentyl alkoxide (INT-C6-4 or INT-C7-4) produced from alkene cycle; the aromatization process is given in Scheme 6. In Scheme 6, after successive hydride shift and internal methyl shift from 2-methyl-3-pentyl

alkoxide,

2-hexyl

alkoxide

(INT-arom-4)

is

produced.

Subsequently, the 2-hexyl ion undergoes a series of deprotonation, hydride transfer from methoxide, and 1,6-cyclization to form the cyclohexane alkoxide (INT-arom-8). The cyclohexane ion is finally converted to benzene molecule by repeated deprotonation and hydride transfer steps. The methoxy species is here chosen as the representative of hydride acceptor, because methanol can be easily dehydrated to form methoxy group in acidic zeolites.68 The methoxy group is more reactive than other alkoxide species in the zeolite pores due to its smaller molecular size. A careful analysis of the calculated results suggests that the free energy barriers for hydride shift, deprotonation, and hydride transfer steps from methoxide are in the range of 65-135 kJ mol-1, 33-88 kJ mol-1, and 164-182 kJ mol-1, respectively. The internal methyl transfer and 1,6-cyclization steps need to overcome free energy barriers of 114.72 and 81.60 kJ mol-1, respectively. All these energy barrier values indicate that the hydride shift, deprotonation, internal methyl transfer, and 1,6-cyclization steps can take place easily. Thus, the rate-determining step for aromatization is hydride transfer from methoxide. Our conclusion is in accord with the previously reported result over H-ZSM-5 zeolite.13 Figure 8 gives the optimized TS structures for hydride transfer from methoxide, TS-arom-6, TS-arom-10, and TS-arom-12. Their structures are similar to each other, which leads to the similar



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

energy barriers as mentioned above (164-182 kJ mol-1). A DCD analysis indicates a dramatic electron transfer from the organic carbon chain to the methyl group. The electron density in the organic carbon chain decreases but increases in the methyl group (e.g. TS-arom-6 in Figure S22). Benzene molecules are produced through the aromatization of INT-C6-4 cations. Then, the benzene molecules can be converted into polyMBs by successive methylation and deprotonation steps. The resulting polyMBs can serve as the new active centers for the polyMB cycle, which produces the new propene molecules for new alkene cycle. Higher alkoxides such as INT-C6-4 (or INT-C7-4) can be formed from the new alkene cycle and participate in aromatization to produce new benzene molecules. Therefore, both the polyMB and alkene cycles take place in the supercages of H-FAU; the aromatization step acts as a communication between these two cycles.13 The intermediates, alkyl species with primary and secondary carbon atoms (i.e., carbon atoms of C-O bonds) as displayed in Scheme 5, are alkoxides with C-O bond lengths of about 1.53 Å between organic fragments and zeolitic framework. In Schemes 6 and S1, both the INT-arom-1 and INT-C7-7 with tertiary carbon atoms have two types of intermediates, alkoxides and carbenium ions (Figure S23); the tertiary carbenium ions are slightly more stable in Gibbs free energy at 673.15 K than tertiary alkoxides by 4.97 and 14.78 kJ mol-1 for INT-arom-1 and INT-C7-7, respectively. This calculated difference of stability in H-FAU is comparable to the previously reported results in H-MOR69 and H-theta-1.70 Now we investigate the effect of zeolite environment by analyzing the QM and MM contributions to the ONIOM energy barriers, as listed in Tables 4, S1, and S2. First, all the ONIOM energy barriers in the alkene cycle and aromatization process are mainly attributed to the QM contributions. Second, the electrostatic energies in the MM energy values for almost all the TSs are negative, suggesting that the electrostatic interactions are favorable for the formation of TS structures. Third, the electrostatic contributions to the MM energies for all energy barriers are generally below 5 kJ mol-1 and often smaller than the van der Waals energies. Hence, it is expected that the electrostatic contributions to the activation free energies are of lesser importance. The enthalpy barrier and entropy loss are two key factors to determine the activation free energy barrier. Several interesting remarks can be obtained from Tables 1-4, S1, and S2. First, almost all the free energy barriers are controlled by the enthalpy barriers. Second, the formation step of propene is generally an


Page 22 of 52

Page 23 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


entropy-increased process (TS-paring-3, TS-spiro-2-H2O, TS-ch3trans-4-H2O, and TS-C6-6); the TS8 is an exception, but the entropy loss (5.70 kJ mol-1) is negligible relative to the large enthalpy barrier (214.67 kJ mol-1, see Table 1). Third, in the direct internal H-shift and paring pathways, the entropy losses are almost positive, indicating that these two pathways are generally entropy-decreased process. Fourth, three deprotonation steps (TS-paring-4, TS-paring-7, and TS-paring-9, see Table 2) in the paring pathway are controlled by the entropic effects. Finally, the entropy effect in the alkene cycle (Tables 4 and S1) is generally larger than that in the polyMB cycle (Tables 1-3) due to the larger entropic barriers in the alkene cycle.

3.5. The Role of Metal Cations in the MTP Process. Many experimental studies have shown that introduction of active metal particles into zeolite catalyst such as H-SAPO-34, H-ZSM-5, and H-Beta has high potential to improve MTO efficiency and increases catalyst lifetime.22 In current work, we investigated the effect of Li+ and Ag+ cations on MTP. Li-FAU and Ag-FAU are simply ion exchanged H-FAUs and contain Li+ or Ag+ as counter-ions. It should be noted that the Li+ and Ag+ cations do not contain active metal particles and thus can not participate in the protonation, deprotonation, and intramolecular steps such as ring expansion and methyl transfer. Therefore, we put the efforts into elucidate the effect of Li+ and Ag+ on the methylation steps because this step occurs frequently in most of pathways involved in the MTP. In the direct internal H-shift pathway, there are four methylation steps for TS1, TS3, TS5, and TS7; the free energy barriers are in the range of 118-175 kJ mol-1 in the H-FAU. In the Li-FAU, the corresponding energy barriers are calculated to be 270.04, 306.47, 241.29, and 203.50 kJ mol-1 with rate constants of 1.55 × 10-8, 2.32 × 10-11, 2.65 × 10-6, and 2.27 × 10-3 s-1, respectively. In the Ag-FAU, the corresponding energy barriers are 249.56, 259.55, 260.16, and 207.13 kJ mol-1, respectively. A comparison of free energy barriers between H-FAU and Li-FAU (or Ag-FAU) shows that both Li-FAU and Ag-FAU are unfavorable for methylation steps. The transition states are displayed in Figures S24 and S25. The Li-Omethanol, Omethanol-Cmethanol, and Cmethanol-Caromatic bond distances are about 1.75-1.78, 2.14-2.37, and 1.81-1.91 Å, respectively. In Ag-FAU, the corresponding values are about 2.11-2.13, 2.10-2.32, and 1.85-1.92 Å, respectively. The DCD analysis suggests a clear electron transfer from the aromatic ring to the H3Cδ+ and metal cations (Figures S26 and S27). In the alkene cycle, six TSs for methylation steps, TS-alkene, TS-C6-2, TS-C6-4,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TS-C7-2, TS-C7-4, and TS-C7-6, are involved; the free energy barriers are in the range of 144-175 kJ mol-1 in the H-FAU. However, the ranges of energy barriers are 262-297 and 197-286 kJ mol-1 in the Li-FAU and Ag-FAU, respectively, which again indicates that Li-FAU and Ag-FAU disfavor the methylation steps. The optimized transition states in Li-FAU and Ag-FAU are displayed in Figures S28-S30. In Li-FAU, the Li-Omethanol, Omethanol-Cmethanol, and Cmethanol-Calkene bond distances are about 1.73-1.74, 2.24-2.28, and 1.92-1.97 Å, respectively. In Ag-FAU, these parameters are about 2.11-2.12, 2.17-2.23, and 1.96-2.04, respectively. These geometrical parameters and energy barriers are close to those in the direct internal H-shift pathway, indicating that the similar geometries generally lead to the similar energy properties. Again, the DCD analysis indicates a clear electron transfer from the aromatic ring to the H3Cδ+ and metal cations (Figures S35 and S36).

3.6. Diffusion Properties of Intermediates from MD Simulations. Generally speaking, if the activation barrier of a reaction step is low, it is probable that the formation of product from the transition state is much faster than the diffusion of reactant and thus the rate of reaction is governed by diffusion process. Therefore, it is important to investigate the diffusion behaviors of intermediates for understanding the reaction mechanisms in detail. In this work, we calculated the MSDs and center of mass (COM) trajectories of several important intermediates in direct internal H-shift (INT-7), indirect spiro (INT-spiro-1) and methyl transfer pathways (INT-ch3trans-3), paring pathway (INT-paring-2), and C6+ pathway of alkene cycle (INT-C6-5). The self-diffusion coefficients of guest molecules were obtained from the slope of MSDs versus time plot. However, all the MSDs of aromatic intermediate molecules exhibit a clear oscillating characteristic after 1.0 ps (Figure S37), which reflects that the guest molecules are captured in the cages of FAU zeolite. Actually, this viewpoint can be validated by the trajectories of COMs of guest molecules. For example, it can be seen from Figure 10(a) that the intermediate molecules can not migrate freely from one cage to another due to their large sizes; for most of the time, the guest molecules move in one single cage. This probably leads to a fast coke phenomenon of catalysts, which is in accord with the experimental deactivation of SAPO-34 due to the formation of alkyl aromatics with large alkyl chains.71 Even if the intermediate is a long chain molecule (INT-C6-5), its COM trajectories indicate that guest molecule is adsorbed around the acidic site of H-FAU (Figure S39(a)). However, for molecules


Page 24 of 52

Page 25 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with smaller sizes such as methanol and propene, the COM trajectories indicate a free migration in H-FAU from one cage to another (Figures S39(b) and 10(b)). The probability density was calculated from diffusion trajectories of studied molecules in 200 stored configurations with a grid of 1.0 Å of FAU unit cell. Normalized density maps were obtained by accumulating a number of guest molecules in grid space, projected to plane and normalized by the total of number of guests. Figure 10(c) shows the calculated COM probability distribution of propene in X-Y plane of FAU zeolite, at 673.15 K and low loading (10 propene/uc). It is seen that

the propene molecules are mainly concentrated in two supercages. The similar observation holds for CH3OH molecules at low loadings (20 CH3OH/uc, Figure S40(b)). The black regions where the probability density is zero imply that the guest molecules can not enter the regions freely. Clearly, the black regions are larger in area for propene than those for methanol, indicating a larger diffusion space for methanol. In summary, the propene as the desirable product in MTP cycles can be released freely from the FAU channels making an on-going MTP process, although the intermediates with large sizes can not migrate freely between two supercages of FAU. However, Dai et al. experimentally pointed out that the picture of deactivation of SAPO-34 materials consisted of a number of different steps and the most important step was the formation of alkyl aromatics with large alkyl chains.71 Thus, the detailed mechanism of deactivation of FAU zeolite in MTP cycles is worthy of an in-depth investigation in our future work.

4. CONCLUSIONS The catalytic abilities of different pathways for the formation of propene in the polyMB and the alkene cycles for MTP over H-FAU zeolite are investigated by a two-layer ONIOM method. The different pathways in polyMB cycle occur in the following order of reactivity: methyl transfer pathway > spiro pathway > direct internal H-shift > paring pathway. Furthermore, the polyMB cycle is more competitive than the alkene cycle in H-FAU, which is different from the result on H-ZSM-5 zeolite. Introduction of Li+ and Ag+ cations into FAU zeolite does not reduce the activation free energy barriers of the methylation steps involved in these two cycles, indicating that the experimental efforts to improve propene selectivity probably should focus on the physical effect of Li+ and Ag+ cations. The MM contributions to the ONIOM total energies are important to elucidate the effect of zeolite framework. In the direct internal H-shift pathway, the stabilizing



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect of zeolite environment on TSs mainly originates from the electrostatic interactions for the methylation steps but VDW interactions for the internal CH3-shift steps. In the spiro and methyl transfer pathways, the zeolite framework stability is mainly attributed to the VDW energies; the electrostatic energies are relatively small. However, in the alkene cycle and aromatization process, the electrostatic interactions are generally more favorable than VDW ones for the formation of TS structures. For the step of formation of propene in polyMB and alkene cycles, the DCDs suggest a clear electron transfer between propene fragment to aromatic ring or propoxy group. The enthalpy barrier and entropy loss are important for analyzing the free energy barrier. Generally speaking, the formation step of propene is an entropy-increased process. The direct internal H-shift and paring pathways are entropy-decreased ones. The entropy effect in the alkene cycle is larger than that in the polyMB cycle due to the larger entropic barriers. The theoretical insights in current study can improve our understanding of the MTP cycles and be useful to experimentally develop high-efficiency zeolite catalysts.

Supporting Information Available: The calculated rate constants, enthalpy barriers, entropy losses, free energy barriers at 673.15 K, QM and MM contributions to free energy barriers, and van der Waals and electrostatic contributions to MM energies of each transition state for C7+ pathway in alkene cycle and aromatization over H-FAU zeolite, the proposed C7+ pathway in the alkene cycle for the formation of propene, free energy profiles calculated for the formation of propene via the C7+ and aromatization pathways, optimized structures of transition states in polyMB and alkene cycles, difference charge densities for several important TSs, the representative MSD plots of INT-7 and INT-C6-5, typical COM trajectories of one INT-7, one INT-spiro-1, one INT-ch3trans-3, one INT-paring-2, one INT-C6-5, one CH3OH, and one propene in X-Y plane in one FAU unit cell at 673.15 K, and plots of COM probability density of 10 propene and 20 CH3OH molecules in X-Y plane in one H-FAU unit cell at 673.15 K. This material is available free of

charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was funded by the National Science Foundation of China (21203118), the Innovation Program of Shanghai Municipal Education Commission (14YZ147), and IIASA Young Scientists Summer Program (21411140044).


Page 26 of 52

Page 27 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Stöcker, M. Methanol-to-hydrocarbons: Catalytic Materials and Their Behavior. Micropor. Mesopor. Mater. 1999, 29, 3−48. (2) Lesthaeghe, D.; Horré, A.; Waroquier, M.; Marin, G. B.; Speybroeck, V. V. Theoretical Insights on Methylbenzene Side-Chain Growth in ZSM-5 Zeolites for Methanol-to-Olefin Conversion. Chem. Eur. J. 2009, 15, 10803−10808. (3) McCann, D. M.; Lesthaeghe, D.; Kletnieks, P. W.; Guenther, D. R.; Hayman, M. J.; Speybroeck, V. V.; Waroquier, M.; Haw, J. F. A Complete Catalytic Cycle for Supramolecular Methanol-to-Olefins Conversion by Linking Theory with Experiment. Angew. Chem. Int. Ed. 2008, 47, 5179 –5182. (4) Svelle, S.; Olsbye, U.; Joensen, F.; Bjørgen, M. Conversion of Methanol to Alkenes over Medium- and Large-Pore Acidic Zeolites: Steric Manipulation of the Reaction Intermediates Governs the Ethene/Propene Product Selectivity. J. Phys. Chem. C 2007, 111, 17981–17984. (5) Lesthaeghe, D.; Speybroeck, V. V.; Marin, G. B.; Waroquier, M. Understanding the Failure of Direct C-C Coupling in the Zeolite-Catalyzed Methanol-to-Olefin Process. Angew. Chem. Int. Ed. 2006, 45, 1714−1719. (6) Song, W. G.; Marcus, D. M.; Fu, H.; Ehresmann, J. O.; Haw, J. F. An Oft-studied Reaction that May Never Have Been: Direct Catalytic Conversion of Methanol or Dimethyl Ether to Hydrocarbons on the Solid Acids HZSM-5 or HSAPO-34. J. Am. Chem. Soc. 2002, 124, 3844−3845. (7) Dahl, I. M.; Kolboe, S. On the Reaction Mechanism for Hydrocarbon Formation from Methanol over SAPO-34: I. Isotopic Labeling Studies of the Co-Reaction of Ethene and Methanol. J. Catal. 1994, 149, 458−464. (8) Wang, C. M.; Wang, Y. D.; Xie, Z. K.; Liu, Z. P. Methanol to Olefin Conversion on HSAPO-34 Zeolite from Periodic Density Functional Theory Calculations: A Complete Cycle of Side Chain Hydrocarbon Pool Mechanism. J. Phys. Chem. C 2009, 113, 4584-4591. (9) Sullivan, R. F.; Sieg, R. P.; Langlois, G. E.; Egan, C. J. A New Reaction that Occurs in Hydrocracking of Certain Aromatic Hydrocarbons. J. Am. Chem. Soc. 1961, 83, 1156−1160. (10) Dessau, R. M. On the H-ZSM-5 Catalyzed Formation of Ethylene from Methanol or Higher Olefins. J. Catal. 1986, 99, 111−116. (11) Svelle, S.; Olsbye, U.; Joensen, F.; Bjørgen, M. Conversion of Methanol to Alkenes over Medium- and Large-Pore Acidic Zeolites: Steric Manipulation of the Reaction Intermediates Governs the Ethene/Propene Product Selectivity. J. Phys. Chem. C 2007, 111, 17981−17984. (12) Bjørgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U. Conversion of Methanol to Hydrocarbons over Zeolite H-ZSM-5: On the Origin of the Olefinic Species. J. Catal. 2007, 249, 195−207. (13) Wang, S.; Chen Y. Y.; Wei, Z. H.; Qin, Z. F.; Ma, H.; Dong, M.; Li, J. F.; Fan, W. B.; Wang, J. G. Polymethylbenzene or Alkene Cycle? Theoretical Study on Their Contribution to the Process of Methanol to Olefins over H-ZSM-5 Zeolite. J. Phys. Chem. C 2015, 119, 28482−28498. (14) Wang, S.; Wei, Z. H.; Chen, Y. Y.; Qin, Z. F.; Ma, H.; Dong, M.; Fan, W. B.; Wang, J. G. Methanol to Olefins over H-MCM-22 Zeolite: Theoretical Study on the Catalytic Roles of Various Pores. ACS Catal. 2015, 5, 1131−1144.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(15) Sassi, A.; Wildman, M. A.; Ahn, H. J.; Prasad, P.; Nicholas, J. B.; Haw, J. F. Methylbenzene Chemistry on Zeolite HBeta: Multiple Insights into Methanol-to-Olefin Catalysis. J. Phys. Chem. B 2002, 106, 2294−2303. (16) Lesthaeghe, D.; Sterck, B. D.; Speybroeck, V. V.; Marin, G. B.; Waroquier, M. Zeolite Shape-Selectivity in the Gem-methylation of Aromatic Hydrocarbons. Angew. Chem. Int. Ed. 2007, 46, 1311–1314. (17) Wang, C. M.; Wang, Y. D.; Liu, H. X.; Xie, Z. K.; Liu, Z. P. Theoretical Insight into the Minor Role of Paring Mechanism in the Methanol-to-olefins Conversion within HSAPO-34 Catalyst. Micropor. Mesopor. Mater. 2012, 158, 264–271. (18) Marchi, A. J.; Froment, G. F. Catalytic Conversion of Methanol into Light Alkenes on Mordenite-like Zeolites. Appl. Catal., A 1993, 94, 91−106. (19) Dehertog, W. J. H.; Froment, G. F. Production of Light Alkenes from Methanol on ZSM-5 Catalysts. Appl. Catal. 1991, 71, 153−165. (20) De Wispelaere, K.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V. Complete Low-barrier Side-chain Route for Olefin Formation During Methanol Conversion in HSAPO-34. J. Catal. 2013, 305, 76−80. (21) Wang, S.; Chen, Y. Y.; Wei, Z. H.; Qin, Z. F.; Chen, J. L.; Ma, H.; Dong, M.; Li, J. F.; Fan, W. B.; Wang, J. G. Theoretical Insights into the Mechanism of Olefin Elimination in the Methanol-to-Olefin Process over HZSM-5, HMOR, HBEA, and HMCM-22 Zeolites. J. Phys. Chem. A 2014, 118, 8901−8910. (22) Galadima, A.; Muraza, O. Recent Developments on Silicoaluminates and Silicoaluminophosphates in the Methanol-to-Propylene Reaction: A Mini Review. Ind. Eng. Chem. Res. 2015, 54, 4891−4905. (23) Clark, L. A.; Sierka, M.; Sauer, J. Computational Elucidation of the Transition State Shape Selectivity Phenomenon. J. Am. Chem. Soc. 2004, 126, 936−947. (24) Sun, Y. X.; Yang, J.; Zhao, L. F.; Dai, J. X.; Sun, H. A Two-Layer ONIOM Study on Initial Reactions of Catalytic Cracking of 1-Butene to Produce Propene and Ethene over HZSM-5 and HFAU Zeolites. J. Phys. Chem. C 2010, 114, 5975–5984. (25) Jansang, B.; Nanok, T.; Limtrakul, J. Structure and Reaction Mechanism of Alkylation of Phenol with Methanol over H-FAU Zeolite: An ONIOM Study. J. Phys. Chem. C 2008, 112, 540–547. (26) Olson, D. H.; Dempsey, E. The Crystal Structure of the Zeolite Hydrogen Faujasite. J. Catal. 1969, 13, 221–231. (27) Maseras, F.; Morokuma, K. IMOMM: A New Integrated Ab Initio + Molecular Mechanics Geometry Optimization Scheme of Equilibrium Structures and Transition States. J. Comput. Chem. 1995, 16, 1170–1179. (28) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215–241. (29) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III; Skiff, W. M. UFF, A Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc. 1992, 114, 10024–10035. (30) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Acc.


Page 28 of 52

Page 29 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem. Res. 2008, 41, 157–167. (31) Zhao, Y.; Truhlar, D. G. A New Local Density Functional for Main-group Thermochemistry, Transition Metal Bonding, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Phys. 2006, 125, 194101–194118. (32) Zhao, Y.; Truhlar, D. G. Density Functional for Spectroscopy: No Long-Range Self-Interaction Error, Good Performance for Rydberg and Charge-Transfer States, and Better Performance on Average than B3LYP for Ground States. J. Phys. Chem. A 2006, 110, 13126–13130. (33) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2006, 2, 364–382. (34) Sun, Y. X.; Mao, X. F.; Pei, S. P. A Two-layer ONIOM Study of Thiophene Cracking Catalyzed by Proton- and Cation-exchanged FAU Zeolite. J. Mol. Model. 2016, 22, 51–66. (35) Gonzalez, C.; Schlegel, H. B. Reaction Path Following in Mass-weighted Internal Coordinates. J. Phys. Chem. 1990, 94, 5523–5527. (36) Bour, P.; Keiderling, T. Partial Optimization of Molecular Geometry in Normal Coordinates and Use as A Tool for Simulation of Vibrational Spectra. J. Chem. Phys. 2002, 117, 4126−4132. (37) Piccini, G.; Sauer, J. Effect of Anharmonicity on Adsorption Thermodynamics. J. Chem. Theory Comput. 2014, 10, 2479−2487. (38) Piccini, G.; Sauer, J. Quantum Chemical Free Energies: Structure Optimization and Vibrational Frequencies in Normal Modes. J. Chem. Theory Comput. 2013, 9, 5038−5045. (39) Mynsbrugge, J. V.; Janda, A.; Sharada, S. M.; Lin, L. –C.; Speybroeck, V. V.; Head-Gordon, M.; Bell, A. T. Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n-Butane in Brønsted Acidic Zeolites. ACS Catal. 2017, 7, 2685−2697. (40) Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553–566. (41) Eyring, H. The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935, 3, 107−115. (42) Mynsbrugge, J. V.; Visur, M.; Olsbye, U.; Beato, P.; Bjørgen, M.; Speybroeck, V. V.; Svelle, S. Methylation of Benzene by Methanol: Single-site Kinetics over H-ZSM-5 and H-beta Zeolite Catalysts. J. Catal. 2012, 292, 201–212. (43) Heinz, H.; Suter, U. W. Atomic Charges for Classical Simulations of Polar Systems. J. Phys. Chem. B 2004, 108, 18341–18352. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009. (45) Chu, Y. Y.; Han, B.; Zheng, A. M.; Yi, X. F.; Deng, F. Pore Selectivity for Olefin Protonation Reactions Confined inside Mordenite Zeolite: A Theoretical Calculation Study. J. Phys. Chem. C 2013, 117, 2194−2202. (46) Liu, J. W.; Liu, Z. F.; Feng, G.; Kong, D. J. Dimerization of Propene Catalyzed by Brønsted Acid Sites Inside the Main Channel of Zeolite SAPO-5: A Computational Study. J. Phys. Chem. C 2014, 118, 18496−18504.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(47) Lu, T.; Chen, F. W. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (48) Han, S. S.; Goddard III, W. A. Metal-Organic Frameworks Provide Large Negative Thermal Expansion Behavior. J. Phys. Chem. C 2007, 111, 15185-15191. (49) Plimpton, S. J. Fast Parallel Algorithms for Short-range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1–19. (50) Frenkel, D.; Smit, B. Understanding Molecular Simulation: From Algorithms to Applications. Boston (MA): Academic Press; 2002. (51) Dubbeldam, D.; Calero, S.; Vlugt, T. J. H.; Krishna, R.; Maesen, T. L. M.; Smit, B. United Atom Force Field for Alkanes in Nanoporous Materials. J. Phys. Chem. B 2004, 108, 12301–12313. (52) Macedonia, M. D.; Maginn, E. J. A Biased Grand Canonical Monte Carlo Method for Simulating Adsorption Using All-atom and Branched United Atom Models. Mol. Phys. 1999, 96, 1375–1390. (53) Granato, M. A.; Lamia, N.; Vlugt, T. J. H.; Rodrigues, A. E. Adsorption Equilibrium of Isobutane and 1-butene in Zeolite 13X by Molecular Simulation. Ind. Eng. Chem. Res. 2008, 47, 6166–6174. (54) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids. J. Am. Chem. Soc. 1996, 118, 11225–11236. (55) Ewald, P. P. Die Berechnung Optischer und Elektrostatischer Gitterpotentiale. Ann. Phys. 1921, 369, 253–287. (56) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids. Oxford: Clarendon Press; 1987. (57) Svelle, S.; Tuma, C.; Rozanska, X.; Kerber, T.; Sauer, J. Quantum Chemical Modeling of Zeolite-Catalyzed Methylation Reactions: Toward Chemical Accuracy for Barriers. J. Am. Chem. Soc. 2009, 131, 816–825. (58) Nguyen, C. M.; Reyniers, M. F.; Marin, G. B. Adsorption Thermodynamics of C1–C4 Alcohols in H-FAU, H-MOR, H-ZSM-5, and H-ZSM-22. J. Catal. 2015, 322, 91-103. (59) Speybroeck, V. V.; der Mynsbrugge, J. V.; Vandichel, M.; Hemelsoet, K.; Lesthaeghe, D.; Ghysels, A.; Marin, G. B.; Waroquier, M. First Principle Kinetic Studies of Zeolite-Catalyzed Methylation Reactions. J. Am. Chem. Soc. 2011, 133, 888–899. (60) Lee, C. C.; Gorte, R. J.; Farneth, W. E. Calorimetric Study of Alcohol and Nitrile Adsorption Complexes in H-ZSM-5. J. Phys. Chem. B 1997, 101, 3811–3817. (61) Li, J. Z.; Wei, Y. X.; Chen, J. R.; Tian, P.; Su, X.; Xu, S.; Qi, Y.; Wang, Q. Y.; Zhou, Y.; He, Y. L.; Liu, Z. M. Observation of Heptamethylbenzenium Cation over SAPO-Type Molecular Sieve DNL-6 under Real MTO Conversion Conditions. J. Am. Chem. Soc. 2012, 134, 836–839. (62) Murdoch, J. R. What Is the Rate-limiting Step of A Multistep Reaction? J. Chem. Educ. 1981, 58, 32–36. (63) Sun, Y. X.; Han, S. Mechanistic Investigation of Methanol to Propene Conversion Catalyzed by H-beta Zeolite: A Two-layer ONIOM Study. J. Mol. Model. 2013, 19, 5407–5422. (64) Bjørgen, M.; Olsbye, U.; Petersen, D.; Kolboe, S. The Methanol-to-hydrocarbons Reaction: Insight into the Reaction Mechanism from [12C]Benzene and [13C]Methanol Coreactions over Zeolite H-beta. J. Catal. 2004, 221, 1–10.


Page 30 of 52

Page 31 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(65) Jentys, A.; Tanaka, H.; Lercher, J. A. Surface Processes during Sorption of Aromatic Molecules on Medium Pore Zeolites. J. Phys. Chem. B 2004, 109, 2254−2261. (66) Pope, C. G. Sorption of Benzene, Toluene and p-xylene on ZSM-5. J. Phys. Chem. 1984, 88, 6312−6313. (67) Zhu, X. X.; Liu, S. L.; Song, Y. Q. ; Xu, L. Y. Catalytic Cracking of C4 Alkenes to Propene and Ethene: Influences of Zeolites Pore Structures and Si/Al2 Ratios. Applied Catalysis A: General 2005, 288, 134–142. (68) Ilias, S.; Bhan, A. Mechanism of the Catalytic Conversion of Methanol to Hydrocarbons. ACS Catal. 2013, 3, 18−31. (69) Boronat, M.; Viruela, P. M.; Corma, A. Reaction Intermediates in Acid Catalysis by Zeolites: Prediction of the Relative Tendency To Form Alkoxides or Carbocations as a Function of Hydrocarbon Nature and Active Site Structure. J. Am. Chem. Soc. 2004, 126, 3300-3309. (70) Boronat, M.; Zicovich-Wilson, C. M.; Viruela, P.; Corma, A. Influence of the Local Geometry of Zeolite Active Sites and Olefin Size on the Stability of Alkoxide Intermediates. J. Phys. Chem. B 2001, 105, 11169-11177. (71) Dai, W. L.; Wu, G. J.; Li, L. D.; Guan, N. J.; Hunger, M. Mechanisms of the Deactivation of SAPO-34 Materials with Different Crystal Sizes Applied as MTO Catalysts. ACS Catal. 2013, 3, 588−596.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 52

Table 1. Calculated Rate Constants (k), Enthalpy Barriers (∆H≠), Entropy Losses (-T∆S≠), Free Energy Barriers (∆G≠) at 673.15 K, QM and MM Contributions to Free Energy Barriers, and van der Waals (VDW) and Electrostatic (Elec) Contributions to MM energies of Each Transition State for Direct Internal H-shift Pathway in PolyMB Cycle over H-FAU Zeolitea species TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 TS9 TS10 TS11 a

k -1

3.46 × 10 4.24 × 105 1.29 × 100 8.13 × 108 9.70 × 103 1.60 × 109 1.51 × 103 1.11 × 10-4 1.35 × 1010 1.20 × 109 4.42 × 108

∆H≠ 175.19 68.05 161.10 49.04 120.63 45.09 109.58 214.67 28.05 39.77 55.17

-T∆S≠ 0.17 28.85 6.90 5.56 -2.59 5.72 18.88 5.70 10.82 12.63 2.84

∆G≠ 175.36 96.90 168.00 54.60 118.04 50.81 128.46 220.37 38.87 52.40 58.01

QM 178.28 80.40 169.75 51.36 121.25 44.45 124.17 215.59 45.21 62.91 79.45

All energy values are reported in kJ mol-1 and rate constants in s-1.


MM -2.92 16.50 -1.75 3.24 -3.21 6.36 4.29 4.78 -6.34 -10.51 -21.44

VDW -0.53 15.29 0.81 -0.43 -0.81 2.50 7.13 4.18 -6.78 -11.23 -20.75

Elec -2.39 1.21 -2.56 3.67 -2.40 3.86 -2.84 0.61 0.44 0.72 -0.69

Page 33 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Calculated Rate Constants (k), Enthalpy Barriers (∆H≠), Entropy Losses (-T∆S≠), Free Energy Barriers (∆G≠) at 673.15 K, QM and MM Contributions to Free Energy Barriers, and van der Waals (VDW) and Electrostatic (Elec) Contributions to MM Energies of Each Transition State for Paring Pathway in PolyMB Cycle over H-FAU Zeolitea species TS-paring-1 TS-paring-2 TS-paring-3 TS-paring-4 TS-paring-5 TS-paring-6 TS-paring-7 TS-paring-8 TS-paring-9 a

k 0

3.33 × 10 5.27 × 10-7 1.80 × 105 6.09 × 1012 6.78 × 10-1 1.99 × 106 7.54 × 1011 5.59 × 101 5.17 × 1011

∆H≠ 154.32 232.66 110.71 -2.47 159.38 88.09 1.01 144.95 9.45

-T∆S≠ 8.36 17.66 -9.00 7.13 12.22 0.17 15.35 1.95 9.02

∆G≠ 162.68 250.32 101.71 4.66 171.60 88.26 16.36 146.90 18.47

QM 155.83 253.11 112.33 2.15 171.16 86.70 13.31 149.52 11.22



MM 6.85 -2.79 -10.62 2.51 0.44 1.56 3.05 -2.62 7.25

VDW 9.79 -2.01 -11.14 0.46 1.96 0.93 1.33 0.44 5.74

Elec -2.94 -0.77 0.52 2.05 -1.52 0.63 1.72 -3.06 1.51


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 52

Table 3. Calculated Rate Constants (k), Enthalpy Barriers (∆H≠), Entropy Losses (-T∆S≠), Free Energy Barriers (∆G≠) at 673.15 K, QM and MM Contributions to Free Energy Barriers, and van der Waals (VDW) and Electrostatic (Elec) Contributions to MM Energies of Each Transition State for Spiro and Methyl Shift Pathways in PolyMB Cycle over H-FAU Zeolitea species TS-spiro-1-CH3OH TS-spiro-1-H2O TS-spiro-2-CH3OH TS-spiro-2-H2O TS-ch3trans-1 TS-ch3trans-2 TS-ch3trans-3 TS-ch3trans-4-H2O TS-ch3trans-4-CH3OH a

k 2

2.51 × 10 1.07 × 103 3.29 × 103 4.08 × 104 4.00 × 108 5.09 × 109 1.71 × 105 1.68 × 1012 4.39 × 1013

∆H≠ 121.12 113.41 117.72 116.76 58.86 37.18 101.60 31.47 12.81

-T∆S≠ 17.38 16.99 6.37 -6.75 -0.29 7.16 0.38 -19.59 -19.19

∆G≠ 138.50 130.40 124.09 110.01 58.57 44.34 101.98 11.88 -6.38

QM 149.14 118.83 114.91 106.12 64.17 44.73 104.93 4.19 -12.92



MM -10.64 11.57 9.18 3.89 -5.60 -0.39 -2.95 7.69 6.54

VDW -13.48 8.57 13.18 7.86 -6.34 -2.15 -3.47 6.69 4.91

Elec 2.84 3.00 -4.00 -3.97 0.74 1.76 0.52 1.00 1.63

Page 35 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Calculated Rate Constants (k), Enthalpy Barriers (∆H≠), Entropy Losses (-T∆S≠), Free Energy Barriers (∆G≠) at 673.15 K, QM and MM Contributions to Free Energy Barriers, and van der Waals (VDW) and Electrostatic (Elec) Contributions to MM Energies of Each Transition State for C6+ Pathway in Alkene Cycle over H-FAU Zeolitea species TS-alkene TS-C6-1 TS-C6-2 TS-C6-3 TS-C6-4 TS-C6-5 TS-C6-6 TS-C6-7 a

k -1

3.96 × 10 4.32 × 106 4.00 × 100 6.72 × 106 8.84 × 101 4.50 × 106 2.59 × 104 6.89 × 107

∆H≠ 150.48 86.33 143.73 88.40 130.58 100.46 155.83 81.43

-T∆S≠ 24.13 -2.42 17.93 -6.97 13.76 -16.77 -43.28 -13.02

∆G≠ 174.61 83.91 161.66 81.43 144.34 83.69 112.55 68.41

QM 171.95 89.82 159.08 84.24 135.00 80.45 117.02 68.83



MM 2.66 -5.91 2.58 -2.81 9.34 3.24 -4.47 -0.42

VDW 5.39 -2.57 5.88 -0.07 12.01 6.90 -0.75 1.85

Elec -2.74 -3.34 -3.30 -2.74 -2.67 -3.66 -3.72 -2.27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 52

internal H-shift pathway paring pathway PolyMB cycle

+

spiro pathway methyl transfer pathway

HMB CH3OH

C6+ pathway

+ Alkene cycle

propene

methylation

+ C7+ pathway

Aromatization

Scheme 1. PolyMB and alkene cycles for the MTO process in H-FAU zeolite.


Page 37 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CH3OH

H-OZ

H OZ

CH3OH

CH3OH adsorbed

H3C

co-adsorbed

H OZ

H

H H3C

C C

C C

C C

H3C

CH3

TS1 H3C

methylation

CH3

C

C

C

CH3

CH3

CH3

INT-1

complex-2

complex-1

CH3 CH3 C

C

C

+ H2O + OZ CH3 OH

CH3

i) H2O desorbed ii) deprotonation TS2

H3C H3C

C

C C

CH3

C C

C

H OZ

CH3OH co-adsorbed

CH3

CH3 H3C

C

C

C

C

C

C

TS4

C

H3C

i) H2O desorbed

C C H3C CH3 C H3C CH3

complex-4

CH3 C

CH3OH

CH3

C

H3C C CH3 H3C CH3

TS6

CH3OH

H3C C

CH

co-adsorbed

C

C

CH3

H3C

C C

C

C

CH3

C C H3C CH3 C H3C CH3

H H3C

H2O desorbed

TS8

INT-7 internal H-shift

C C

C

C

C

CH3

C C H3C C CH3 H3C CH3

INT-8 + CH3-CH=CH2 + OZ H

H H3C

C

CH3

complex-5

INT-7 + H2O + OZ

ii) internal CH3-shift TS9

H3C

H OZ

CH3


i) propene desorbed

HC

INT-6 + H-OZ

INT-5 + H2O + OZ

H3C

C

C

C

ii) deprotonation

H3C C CH3 H3C CH3

methylation

TS5 H3C CH3 C H C C OZ methylation co-adsorbed

CH3

HC

C

TS7

C

CH2

CH3OH

CH2

CH3 C

methylation

CH3

CH3 OH

INT-4 + H-OZ

CH3

TS3

complex-3

CH3 ii) deprotonation H3C

INT-3 + H2O + OZ

H3C

C

C

CH3

CH3

H2C

C

CH3

C

CH3

H3C CH3 H3C CH3 H3C CH3 C i) H2 O desorbed H3C C C C CH3 C C C

C

H3C

INT-2 + H-OZ

H3C

C

C

CH3

C

CH3

CH3 CH3

INT-9 + OZ

TS10 internal CH3-shift

H3C

C

CH3

C CH3 C C H3C C CH3 C

TS11 internal CH3-shift

CH3

INT-10 + OZ

Scheme 2. The proposed direct internal H-shift pathway for the polyMB cycle in H-FAU zeolite.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H3C

H3C CH3 H3C C CH3 C C C

H3C

C

C

H3C

TS-paring-1

CH3

ring contraction

CH3

INT-3 + OZ

CH3

H3C C H3C

C

C

CH2 CH3 C

CH C

C

H3C

CH3

C

CH3

TS-paring-2 H3C

CH3

hydride shift

C C

H3C

C

C

propene removal CH3

CH3

INT-paring-1 + OZ

INT-paring-2 + OZ

CH3 C

C

CH3

H3C

TS-paring-5

C

C

TS-paring-4

INT-paring-3 + OZ

CH3

TS-paring-6

CH2

C C protonation C CH CH3 ii) deprotonation CH2 H3C C H3C C

CH3

TS-paring-3

CH3

i) propene desorbed

C

C

H3C

C

C

H3C

C

Page 38 of 52

ring expansion

CH3

complex-paring-1 + H-OZ

INT-paring-4 + OZ

+ CH3-CH=CH2 CH3 H3C H3C

C C

C C

CH2

TS-paring-7

CH

deprotonation

H3C

CH3

CH3

H3C

C C

C C

C C

C C

CH

CH3OH

CH

co-adsorbed

H OZ

CH3

C CH

CH3

H

CH3 H3C H3C

CH3

C C

C C

TS-paring-8 CH CH

CH3

complex-paring-3

complex-paring-2 + H-OZ

INT-paring-5 + OZ

H3C

CH3OH

CH3 H3C

CH3

i) H2O desorbed ii) deprotonation TS-paring-9

INT-paring-6 + H2O + OZ

H3C H3C

C C

C C

C C

CH3 H

CH3

PMB + H-OZ

Scheme 3. The proposed paring pathway for the polyMB cycle in H-FAU zeolite.


methylation

Page 39 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H3C

H

HC H3C

H3C H3C

C

CH C

CH3 C

CH3

1 iro-sp S T ion nat o t o r dep

C C TS H3C C CH3 -ch H3C CH3 3tr int an ern s

INT-7 + OZ

al

-1

CH

3 -s

hif t

CH2 C

C

CH3

C

C

H3C

TS-spiro-2 propene removal

C

C H3C CH3 H3C CH3

INT-spiro-1 + H-OZ

H3C H3C H3C

CH

C C

H3C

C

CH3

C

CH3

TS-ch3trans-2

H3C

internal CH3-shift

CH3

H3C

C C

internal CH3-shift

H3C H3C

C C

C

C C

CH3 CH3 C CH3 C

CH3

CH3

CH3 C

CH

INT-ch3trans-2 + OZ

CH3

TS-ch3trans-3

CH3

CH3

INT-ch3trans-1 + OZ

H3C CH

C

INT-8 + CH3-CH=CH2 + OZ

CH3

C

C


CH3

C

C

C C

CH3 CH3

H3C

TS-ch3trans-4 propene removal

CH3

INT-ch3trans-3 + OZ

H3C

C C

C C

C C

CH3 CH3

CH3

HMB + CH3-CH=CH2 + H-OZ

Scheme 4. The proposed spiro and methyl shift pathways for the polyMB cycle in H-FAU zeolite.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CH3OH

CH3OH H-OZ

H OZ

co-adsorbed

i) H2O desorbed

TS-alkene

CH3-CH=CH2

Page 40 of 52

CH3-CH=CH2 methylation INT-alkene

ii) deprotonation TS-C6-1

complex-alkene + H2O + OZ

CH3 OH

CH3OH

H OZ

co-adsorbed

TS-C6-2 methylation CH3-CH=CH-CH3 INT-C6-2

INT-C6-1 + H-OZ

complex-C6-1

i) H2O desorbed

CH3OH

ii) deprotonation TS-C6-3

co-adsorbed

+ H2O + OZ

CH3OH H OZ

INT-C6-3 + H-OZ

INT-C6-4

TS-C6-4 CH2 =CH-CH-CH3 CH3 complex-C6-2

i) H2O desorbed

TS-C6-6

ii) hydride shift TS-C6-5

beta scission

INT-C6-5 + OZ

+ H2O + OZ

methylation

+

ZO

INT-C6-6

i) propene desorbed

+ H-OZ ii) deprotonation TS-C6-7

propene +

Scheme 5. The proposed C6 pathway in the alkene cycle for the formation of propene in H-FAU zeolite.


Page 41 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS-arom-1

TS-arom-2

hydride shift

hydride shift

INT-C6-4 + OZ

INT-arom-2 + OZ

INT-arom-1 + OZ

TS-arom-3 internal CH3-shift

TS-arom-4

TS-arom-5

hydride shift

deprotonation INT-arom-4 + OZ

INT-arom-3 + OZ

INT-arom-5 + H-OZ

i) CH3OH adsorbed

TS-arom-6

ii) H2O desorbed iii) CH3 OZ produced

hydride transfer INT-arom-5 + CH3OZ

i) CH4 desorbed

TS-arom-8

ii) hydride shift

cyclization

TS-arom-7 INT-arom-6

INT-arom-8 + OZ

INT-arom-7 + OZ + CH4 + OZ

TS-arom-9 deprotonation

i) CH3OH adsorbed

TS-arom-10

ii) H2O desorbed iii) CH3OZ produced

hydride transfer

INT-arom-10 INT-arom-9 + H-OZ

INT-arom-9 + CH3 OZ

i) CH4 desorbed

i) CH3 OH adsorbed

ii) deprotonation TS-arom-11

ii) H2O desorbed iii) CH3OZ produced

+ CH4 + OZ

TS-arom-12 hydride transfer

INT-arom-12 INT-arom-11 + H-OZ

INT-arom-11 + CH3OZ + CH4 + OZ

i) CH4 desorbed ii) deprotonation TS-arom-13 benzene + H-OZ

Scheme 6. The proposed aromatization pathway for the formation of benzene in H-FAU zeolite.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 52

H supercage

O Al Si

(a)

(b)

Figure 1. A 156T nanocluster model of H-FAU zeolite divided into two regions (a). The inner 18T region (colored balls) is computed with quantum mechanics method and the outer region (lines) computed with UFF. The dashed lines enclose the 18T active region where the MTP reactions take place (b).


Page 43 of 52

TS1 (175.36) 182.94

Energy/kJ mol-1

200

INT-1 +H2O 93.45

100 0

TS3 (168.00)

124.09

136.59

TS4 (54.60) INT-4

53.52

INT-1 27.19

Reactants 8.70 complex-2 0.00

TS2 (96.90)

INT-3 +H2O INT-2 -31.41 -23.16 complex-3

complex-1 7.58

60.67

6.07

49.62

INT-3

-100 -200 (a) TS5 (118.04)

200 Energy/kJ mol-1

55.82

complex-4

14.12

0

172.53

145.55

INT-5 +H2O

100

TS8 (220.37)

TS7 (128.46)

173.86 TS6 (50.81) INT-5

37.04

INT-7 +H2O

INT-6 complex-5

13.21

17.10

INT-8 +propene

-25.03

-13.77

INT-7

-10.98

-47.84

-100 -200 (b) 200

Energy/kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

-100

TS10 (52.40)

TS9 (38.87)

0

TS11 (58.01)

INT-8 -37.53 INT-9 -33.13 INT-10 -37.76

-76.40

-85.83

-95.77

-200 (c) Figure 2. Free energy profiles calculated for the direct internal H-shift pathway illustrated in Scheme 2 over H-FAU zeolite at 673.15 K. The zeolite framework as well as the methanol, PMB, and H2O in gaseous phase at infinite separation are taken as the reference state. For transition states, the activation barriers are given in parentheses.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.410

C3 1.704

C2

C2 2.801 C1

C1

C4 2.391 C3

H 1.688 1.139

O1

O1 O2

O2

Al

Al

(a)

(b)

Figure 3. Optimized structures of (a) TS8 and (b) TS-paring-3.


Page 44 of 52

Page 45 of 52

400 TS-paring-2 (250.32)

300

TS-paring-5

278.80

(171.60)

TS-paring-1 (162.68)

200

TS-paring-3 (101.71) INT-paring-3 165.84 +propene TS-paring-4

Energy/kJ mol-1

168.75

100

123.16

64.13 28.48

0

6.07

141.00

72.04

52.74

33.56

complex-paring-1

INT-paring-1

TS-paring-6 (88.26)

(4.66)

67.38

INT-paring-3

INT-paring-2

205.16

INT-paring-4 -27.09

INT-3

INT-paring-5

-100 -200 -300 -400 (a) 400 300 200

Energy/kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS-paring-8

100 0

-100

(146.90) 72.85

TS-paring-7 (16.36)

INT-paring-6 +H2O 1.54

-10.73 -80.06

complex-paring-2

-74.05

TS-paring-9 -59.19

INT-paring-6

complex-paring-3

(18.47) -40.72 -135.06

PMB

-200 -300 -400 (b) Figure 4. Free energy profiles calculated for the paring pathway illustrated in Scheme 3 over H-FAU zeolite at 673.15 K. The zeolite framework as well as the methanol, PMB, and H2O in gaseous phase at infinite separation are taken as the reference state. For transition states, the activation barriers are given in parentheses. For simplicity, the same steps (from Reactants to INT-3 in Scheme 2) are omitted in Scheme 3.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 52

1.912 C3

1.987 C3

C1 C2 1.134 1.818 H 1.755

C1 1.827 C2 1.129 H 1.749

0.978 1.823 O2

H O1

0.974 H 1.890 O2

O1

Al

Al

(a)

(b)

C4 3.043 C3 C2

C4 2.987 C3 C2 C1 1.251 H 1.540 0.979 O 1.900 H O2

O1

C1

0.988 1.738

H

1.296 1.461

O O2 H

O1

Al

Al

(c)

(d)

Figure 5. Optimized structures of (a) TS-spiro-2-methanol, (b) TS-spiro-2-H2O, (c) TS-ch3trans-4-methanol, and (d) TS-ch3trans-4-H2O.


Page 47 of 52

Energy/kJ mol-1

200

TS-spiro-2-CH 3OH (124.09) 124.14 INT-8-propene-CH3OH 29.73 INT-spiro-1-CH 3OH INT-8-propene-H2O 0.05 TS-spiro-1-H2O TS-spiro-2-H 2O 18.54 (130.40) (110.01) 105.37 109.62

TS-spiro-1-CH3OH (138.50) 121.53

100 INT-7-CH 3OH

-16.97 INT-7

-47.84

-100

INT-spiro-1-H 2O -0.39

-25.03 INT-7-H2O

-200 (a) 200 Energy/kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

-100

TS-ch3trans-1 (58.57) 10.72

TS-ch3trans-3 (101.98) 20.83

TS-ch3trans-2 (44.34) -16.52

INT-7

-47.84

INT-ch3trans-1 -60.86

INT-ch3trans-2 -81.15

TS-ch3trans-4-H 2O (11.88) -31.46

INT-ch3trans-3-H2O -43.33 HFAU-HMB-propene-H2O -125.36

-200 (b) Figure 6. Free energy profiles calculated for the indirect spiro (a) and methyl transfer (b) pathways illustrated in Scheme 4 over H-FAU zeolite at 673.15 K. The solid and dash lines indicate that the pathways are assisted with methanol and H2O, respectively. The zeolite framework as well as the methanol, PMB, and H2O in gaseous phase at infinite separation are taken as the reference state. For transition states, the activation barriers are given in parentheses. For simplicity, the same steps (from Reactants to INT-7 in Scheme 2) are omitted in Scheme 4.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 48 of 52

(b)

Figure 7. Difference charge densities for TS-spiro-2-H2O. (a) positive DCD and (b) negative DCD. The positive DCD represents where charge density increases and the negative DCD represents where charge density decreases.


Page 49 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C5

C4

C3 C6 C3

2.629 C4 C5 C2 2.905 C1 2.429 O1 O2

C2 1.188 H 1.490 C1 C 2.164

O1

O2

Al

Al

(a)

C3 C2

(b)

C4 C6 C5

1.196 H 1.510 C

1.191 H 1.367 C 2.170 O2

Al

C2

C1

C1 C6

O1

C5 C4 C3

O1

2.105

Al

(c)

O2

(d)

Figure 8. Optimized structures of (a) TS-C6-6, (b) TS-arom-6, (c) TS-arom-10, and (d) TS-arom-12.



300

TS-alkene (174.61)

Energy/kJ mol-1

100 0

TS-C6-2 (161.66)

240.05

200

INT-alkene 65.45 +H2O Reactants complex-alkene 62.04

83.00

-0.91

INT-alkene

0.00

170.29

TS-C6-1 (83.91)

8.63 -24.72 complex-C6-1

TS-C6-3 (81.43)

INT-C6-2 +H2O

31.07

9.41

INT-C6-1

-50.37

-58.69

INT-C6-2

-100

INT-C6-3

-200 -300 (a)

300 200 TS-C6-4 (144.34)

Energy/kJ mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 52

100

-100

120.11 TS-C6-6 (112.55)

TS-C6-5 (83.69)

-24.23 complex-C6-2

-47.54 INT-C6-4 +H2O

18.62

-10.55 -94.23 INT-C6-4

-93.92

TS-C6-7 (68.41)

-47.25 INT-C6-6 +propene

INT-C6-5

-32.86 -101.27

-109.51

INT-C6-6

propene +H-OZ

-200 -300 (b) Figure 9. Free energy profiles calculated for the formation of propene via the C6+ pathway illustrated in Scheme 5 over H-FAU zeolite at 673.15 K. The zeolite framework as well as the methanol, propene, and H2O in gaseous phase at infinite separation are taken as the reference state. For transition states, the activation barriers are given in parentheses.


Page 51 of 52

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c) Figure 10. Typical COM trajectories of one INT-spiro-1 molecule (a) and one propene molecule (b), and plot of COM probability density of 10 propene molecules (c) in X-Y plane in one H-FAU unit cell at 673.15 K. The Arabic numbers in the trajectory maps represent the evolution of molecular COM positions with time. The oval dashed lines represent two supercages connected to each other through a 12MR window. The arrow indicates the direction through the 12MR window. The diamond dashed line represents the zeolite framework wall.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 52

TOC Graphic:

0 5.000E-4 1.000E-3 0.001500 0.002000 0.002500 0.003000 0.003500 0.004000

diffuse

The contributions of the polymethylbenzene (polyMB) and alkene cycles to the methanol to propene (MTP) process in FAU zeolite have been investigated by a two-layer ONIOM study. The calculated results demonstrate that the indirect methyl transfer pathway is the most favorable in the polyMB cycle. The polyMB cycle is more competitive than alkene cycle. The TOC graphic above describes the difference charge densities (DCDs) of transition state for the most favorable step in the polyMB cycle.


New Theoretical Insights into the Contributions of Poly(methylbenzene

Recommend Documents