Evolution of Aromatic Species in Supercages and Its Effect on the

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

Evolution of Aromatic Species in the Supercages and Its Effect on the Conversion of Methanol to Olefins over HMCM-22 Zeolite: A Density Functional Theory Study Sen Wang, Yan-Yan Chen, Zhihong Wei, Zhangfeng Qin, Tingyu Liang, Mei Dong, Junfen Li, Weibin Fan, and Jianguo Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08154 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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 64

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

Evolution of Aromatic Species in the Supercages and Its Effect on the Conversion of Methanol to Olefins over H-MCM-22 Zeolite: A Density Functional Theory Study Sen Wang,a,b Yanyan Chen,a Zhihong Wei,a Zhangfeng Qin,*,a Tingyu Liang,a,b Mei Dong,a Junfen Li,a Weibin Fan,a Jianguo Wang*,a a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China b

University of Chinese Academy of Sciences, Beijing 100049, PR China

*

Corresponding authors. Tel.: +86-351-4046092; Fax: +86-351-4041153. E-mail address:

[email protected] (Z. Qin); [email protected] (J. Wang)

ABSTRACT: H-MCM-22 zeolite is a potential catalyst for the conversion of methanol to olefins (MTO). Previous studies indicated that three types of pores in H-MCM-22, viz., the supercages, sinusoidal channels, and pockets, are different in their catalytic action; however, the evolution of aromatic species in the supercages and its effect on MTO are still highly controversial. In this work, density functional theory considering dispersive interactions (DFT-D) was used to investigate the evolution of aromatic species including their formation, reactivity and deactivation behavior in the supercages; the active role of the supercages in catalyzing MTO was elucidated. The results demonstrated that benzene can be generated in the supercages through aromatization of light olefins; after that, polymethylbenzenes (polyMBs) are formed through methylations, in competition with the construction of naphthalenic species. Both polyMBs (e.g. hexamethylbenzene) and polymethylnaphthalenes 1

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

(polyMNs, e.g. dimethylnaphthalene) exhibit high reactivity as the hydrocarbon pool species in forming light olefins. Owing to the appropriate electrostatic stabilization and space confinement effects, naphthalenic species in the supercages are inclined to serve as the active intermediates to produce light olefins rather than act as the coke precursors in the initial period of MTO; as a result, the supercages contribute actively to the initial activity of H-MCM-22 in MTO, though they may be prone to deactivation in the later reaction stage in comparison with the sinusoidal channels. The insights shown in this work help to clarify the evolution of aromatic species and the active role of the supercages in MTO over H-MCM-22, which is of benefit to the development of better MTO catalysts and reaction process.

1. INTRODUCTION

The conversion of methanol to olefins (MTO) over acidic zeolite catalysts is now considered as an important non-petroleum route to get light olefins.1–8 Great progress has been made both in the industrial application of MTO and in the fundamental research on the catalytic mechanism.9–14 The hydrocarbon pool (HCP) mechanism proposed by Dahl and Kolboe has received wide recognition,6–8 which assumes that the organic molecules trapped in the zeolite pores, namely the HCP species, interplay with the inorganic framework and serve as a co-catalyst; the olefin products are eliminated from the HCP. According to the sort of the HCP species, the HCP mechanism can be further divided into aromatic cycle (polymethylbenzenes or polymethylnaphthalenes as the HCP species) and alkene cycle (higher alkenes as the HCP species).15–20 A variety of zeolites have been employed as the catalysts for MTO.21–24 Among them, H-MCM-22 zeolite of MWW-type exhibits excellent catalytic performance in MTO, with high ratio of propene to ethene in the effluent products and relatively long lifetime.25–27

2


Page 2 of 64

Page 3 of 64

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


H-MCM-22 contains three types of pores, as shown in Figure 1(a): the two-dimensional sinusoidal channels with an elliptical ring cross section of 4.1×5.1 Å, the pockets on the external surface (7.1 Å in diameter and 7.0 Å in height), and the cylindrical supercages (7.1 Å in diameter and 18.2 Å in height) that are accessible through 10-membered ring (4.0×5.5 Å) windows.28 Previous experimental and theoretical investigations demonstrated that these three types of pores are different in their catalytic action on MTO, because of the large differences in pore size and shape.25,27, 29 – 32 It is well accepted that the pockets are detrimental to MTO due to the facile formation of coke, as certain large intermediates are easily formed in the pockets but difficult to decompose for lacking of an electrostatic stabilization effect from the zeolite framework.29 The active acid sites in the sinusoidal channels may dominate the MTO reaction in the steady stage, which favors the alkene cycle and the formation of higher olefins (mainly C3–5 olefins).25,30

{Figure 1}

However, the evolution of aromatic species in the supercages of H-MCM-22 and its effects on MTO are still highly controversial. Through theoretical calculation, it was found that the supercages present in H-MCM-22 played a very important role in catalyzing the MTO reactions;29 owing to the efficient reaction space and proper electrostatic stabilization effect, low olefins were readily produced simultaneously via both the polymethylbenzene (polyMB) and alkene cycles. Through the regulation of framework aluminum siting in H-MCM-22, however, Chen and co-workers illustrated that the acid sites located in the surface pockets and supercages were prone to carbonaceous deposition, whereas those acid sites in the sinusoidal channels were crucial for the methanol to hydrocarbons (MTH) in the steady reaction stage.30 Min and co-workers investigated the catalytic performance of H-MCM-22 and H-ITQ-2 in MTO;25 they found that although H-ITQ-2, which could be

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

considered as delaminated H-MCM-22 without supercages, exhibited higher catalytic stability and selectivity to propene, it gave a much lower initial methanol conversion than H-MCM-22. Bjørgen, Wang and co-workers also proved that the supercages were crucial for MTO in the initial period;33,34 aromatic species such as higher polyMBs were important active intermediates for the formation of light olefins, which could accelerate the reaction process and give a high methanol conversion.

The catalytic performance of H-MCM-22 in MTO was strongly related to the evolution of aromatic species (benzenic or naphthalenic) in the supercages, which could dramatically influence on the catalytic stability and product selectivity.29,33– 35 Benzenic species (dominantly polyMBs) encapsulated in the zeolite pores were generally taken for the HCP species, which were highly active for the formation of light olefins.36–38 On the other hand, naphthalenic species may play two different roles in MTO; they can either serve as the active intermediates to form olefins or turn into the precursors of the deactivated coke species. Song and co-workers investigated the catalytic roles of polymethylnaphthalenes (polyMNs) by selectively synthesizing naphthalenic species at 873 K over H-SAPO-34; 39 they demonstrated that polyMNs could also serve as the reaction centers for the elimination of olefins, just like their benzenic counterparts. In general, the generation of larger aromatic species such as anthracene, phenanthrene and pyrene represents the commencement of zeolite catalyst deactivation, because these polycyclic aromatics are less active as the reaction centers; they may either block the pores or cover the acid sites, which then induce diffusion limitations and/or deactivate the zeolite catalyst.40 By using the first principle computation, Hemelsoet and co-workers illustrated that the bicyclic naphthalenic compounds exhibited certain activity for the formation of olefins, but they were easily converted into coke species due to the high energy barriers for the cracking steps.41,42 Therefore, it is

4


Page 4 of 64

Page 5 of 64

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


essential to investigate the evolution of aromatic species in the supercages and its effect on MTO, to have a better understanding on the reaction mechanism of MTO over H-MCM-22.

In this work, a systematic assessment on the evolution and catalytic behavior of aromatic species in the supercages for MTO over H-MCM-22 was carried out by using the density functional theory considering dispersive interactions (DFT-D). The formation, reactivity and deactivation behavior of aromatic species in the supercages for MTO, including benzenic, naphthalenic, anthracene and phenanthrene ones, were evaluated through the intrinsic and apparent kinetics at real reaction temperature; the active role of the supercages in catalyzing MTO was then elucidated.

2. COMPUTATIONAL MODELING AND METHODS

The 54T cluster model covers 12-membered ring cylindrical supercages (7.1×7.1×18.2 Å) and 10-membered ring crossing windows (4.0×5.5 Å), as shown in Figure 1(b); a silicon atom is substituted with an aluminum atom at T4 site and the charge-balancing proton is bonded with O3.43,44 It has been proved that such a cluster model can rationally describe the space confinement and electrostatic stabilization effects of the zeolite frameworks,29,45,46 similar to the periodic model does.47 Such a 54T cluster contains 53 Si and 1 Al atoms, corresponding to a Si/Al ratio of 53, which can well match the typical Si/Al ratios of H-MCM-22 for MTO in practice.30,48 Moreover, the cluster approach is able to lower the computational cost, whilst gives a more straightforward description on the localization of transition states and the vibrational analysis.45,46 Terminal hydrogen atoms were utilized to saturate the peripheral silicon atom for the cluster model; the distances between the hydrogen atoms and the corresponding silicon atoms are 1.47 Å and the direction of Si–H bonds is along the pre-existing Si–O bonds.

5



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 6 of 64

All density functional theory (DFT) calculations were performed with the Gaussian 09 package.49 The standard B3LYP functional and the 6-31G (d, p) basis set were used in all geometry

optimizations

and

frequency

calculations.

The

active

region

of

“SiOHAl(OSi)2OSi” and the reacting molecules were allowed to relax, whereas the rest of the framework kept fixed at the crystallographic coordinates. The convergence criteria used during geometry optimization were: Max Force = 0.00045 a.u., RMS Force = 0.0003 a.u., Max Displacement = 0.0018 a.u., and RMS Displacement = 0.0012 a.u. Transition states (TS) were guessed by the OPT=TS method and confirmed by the quasi-internal reaction coordinate (quasi-IRC) approach, to verify that each transition state was connected with the corresponding reactants and products. Furthermore, the transition state is a first-order saddle point of potential energy surface, with only a single imaginary frequency. It was verified that the obtained reactants and products were situated in the energy minima points of potential energy surface, with only real frequencies. To obtain accurate interaction energies, single-point calculations with the 6-311+G (2df, 2p) basis set were refined by the ωB97X-D functional including dispersion interactions, which was a promising method for the main group thermochemistry, kinetics, and non-covalent interactions.50,51

Kinetics. To directly compare the competing elementary reactions within a similar reaction network and conveniently evaluate the reaction rates between the unimolecular and bimolecular reactions, it was assumed that in each step all species were first adsorbed on the active sites in the zeolites as a single molecule.46,52,53 Hence, the intrinsic free energy barrier and unimolecular rate constant were determined. The free energies (∆G), enthalpies (∆H), and entropies (∆S) were obtained from the ωB97X-D/6-311+G (2df, 2p) total electronic energy and the thermal correction from the B3LYP/6-31G (d, p) frequency calculation by using the partial Hessian vibrational analysis (PHVA) method at 673 K, which included the

6


Page 7 of 64

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


atoms that were relaxed during the geometry optimization. The intrinsic rate constant (k) at 673 K was calculated by using the classical transition-state theory (TST), following the equation:54

k=

k BT kT ≠ ≠ ≠ exp – ∆Gint RT = B exp ∆Sint R exp – ∆H int RT h h

(

)

(

) (

)

where kB is the Boltzmann’s constant, h is the Planck’s constant, and ∆Gint≠, ∆Hint≠ and ∆Sint≠ are the changes of standard molar Gibbs free energy, enthalpy, and entropy at 673 K between the reactants and the transition state (TS), respectively. The intrinsic rate constants are used for a mutual comparison among various competing elementary reactions within the same reaction network, which are superior to the separate kinetic parameters of activation energies (Ea) and pre-exponential factors (A) for such a comparison.52 In this approach, all of the reactants are considered to be adsorbed on the active sites of the zeolite and have formed a preactivated complex; this complex can henceforth be treated as a single molecule undergoing internal rearrangements.

Thermodynamics. The reaction free energies (∆GR), reaction enthalpies (∆HR), and reaction entropies (−T∆SR), defined as the energy difference between the reactants and products in each reaction step, were calculated to express the thermodynamic feasibility of different reaction pathways.

A larger cluster model (81T) and ONIOM method (ωB97X-D/6-311+G(2df, 2p)//B3LYP/6-31G(d, p): PM6) were also utilized for calculating a few selected reaction steps to check the validity of the methodologies and model systems used in this work; the results for comparison are provided in the Supporting Information (Table S1, and Figure S8).

7



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

To visualize the noncovalent interactions in zeolite systems, the isosurface plots of the reduced density gradient (RDG) for some intermediates confined in H-MCM-22 zeolite were obtained by calculating the RDG functions (RDG(r) = 1/(2(3π2)(1/3))|∆ρ(r)|/ρ(r)(4/3)), ρ represents the electron density) and quantity sign (λ2)ρ (sign (λ2)ρ < 0, H-bonding interaction; sign (λ2)ρ ≈ 0, weak van der Waals interaction; sign (λ2)ρ > 0, strong repulsive interaction) with the Multiwfn software.55

The proton affinity (PA) is obtained as the energy difference between the protonated zeolite and the deprotonated one, i.e. PA = E(Z−) − E(HZ). The ammonia adsorption energy (∆Eads) in the active sites is calculated by the equation ∆Eads= ENH3-HZ − (EHZ + ENH3), where ENH3-HZ is the total energy of the zeolite system after the adsorption of ammonia, EHZ is the energy of the zeolite system before the adsorption and ENH3 is the energy of isolated ammonia. Both the proton affinity (PA) and ammonia adsorption energy (∆Eads) are obtained from the ωB97X-D/6-311+G (2df, 2p) total electronic energy with the thermal correction from the B3LYP/6-31G (d, p) frequency calculation.

3. RESULTS AND DISCUSSION

3.1. Formation of Benzenic and Naphthalenic Species. The reaction networks for the formation of benzenic and naphthalenic species in the supercages of H-MCM-22 are proposed, as depicted in Scheme 1. The aromatization process involves protonation, deprotonation, methylation, cyclization, oligomerization, and hydride transfer reactions. Small amounts of light olefins generated in the induction period for MTO are taken for the precursors of aromatization; however, a detailed investigation about the induction period and the formation of first C–C bonds in MTO is beyond the scope of this work.56

8


Page 8 of 64

Page 9 of 64

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


{Scheme 1}

3.1.1. Benzene Formation. As illustrated in Scheme 1, propene is protonated (P1(a)) by the acidic proton of zeolite, forming 1-propyl carbenium ion, which can react with another propene by oligomerization (O1(a)) to generate 2-hexyl carbenium ion. 1-Hexene is then formed by deprotonation (D1(a)) of 2-hexyl carbenium ion. After that, the hydride transfer (HT1(a)) between alkoxy (hydride acceptor) and 1-hexene (hydride donor) results in the formation of alkane and 1-hexene carbenium ion; methoxy is here chosen as the representative of alkoxy, because both methanol and dimethyl ether can be easily dehydrated to the adsorbed methoxy.57 1-Hexene carbenium ion is further converted to cyclohexene by 1,6-cyclization (C1(a)) and deprotonation. 58 Finally, benzene is produced by repeated deprotonation and hydride transfer reactions (HT2(a) and HT3(a)).

The free energy profiles of benzene formation are depicted in Figure 2 and the detailed kinetic and thermodynamic results at 673 K are summarized in Table 1. The optimized transition states for benzene formation are presented in Figure S1(a1–a4) and Figure S2(a1–a3) of the Supporting Information. The protonation (P1(a)) and oligomerization (O1(a)) need to overcome free energy barriers of 125 and 107 kJ mol−1, with rate constants of 2.87×103 and 7.53×104 s−1, respectively; the reaction free energies of P1(a) and O1(a) are 26 and 25 kJ mol−1, respectively. The deprotonation (D1(a)) and 1,6-cyclization (C1(a)), exothermic by 10 and 12 kJ mol−1, respectively, are very fast with free energy barriers of 66 and 87 kJ mol−1, respectively, which are similar to the results obtained by Vandichel and co-workers from DFT calculations using the ONIOM method.59

Before the hydride transfer reactions, some extra steps (A2, A3, and A4) highlight the formation of methoxy species that serve as the hydride transfer acceptor. Methoxy with

9



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

methane formation is regarded as the dominant hydride transfer acceptor, as the selectivity of methane is higher than that of other alkanes in MTO at 673 K over H-MCM-22.25 Besides the alkoxy, hydrion as the hydride acceptor to produce hydrogen is also investigated in our previous theoretical work;60 with hydrion it needs to overcome much higher free energy barrier than that with methoxy as the hydride acceptor. As a result, methoxy may play a vital role in the hydride transfer reactions. Moreover, the formation of methoxy and their coverage in the zeolite framework may highly depend on the reaction temperature. Li, Yamazaki and co-workers have verified that the methoxy species could be easily generated and were thermally stable over the acid sites in zeolites at 673 K.56,61 Three hydride transfer steps of HT1(a), HT2(a) and HT3(a) require free energy barriers of 174, 148 and 143 kJ mol−1, with rate constants of 4.53×10−1, 4.20×101 and 1.05×102 s−1, respectively; their reaction free energies are −38, −30 and −78 kJ mol−1, respectively. There may be some other conceivable reaction pathways; however, current one should be one of the most potential pathways. These results suggest that the hydride transfer reaction HT1(a) is the rate-determining step for benzene formation, which is in line with the observation of Wang and co-workers;62 for the aromatization over H-SAPO-34, they found that hydride transfer is the predominant obstacle for the formation of alkylbenzene.

Due to the high free energy barriers of the hydride transfer reactions for the formation of aromatics, an increase in the reaction temperature should be of great benefit to promoting the aromatization process.63 However, the increase in temperature can also markedly enhance the alkene cycle and the activity of aromatics as the HCP species in the aromatic cycle forming olefins. As shown in Figure S3 of the Supporting Information, the rate constants of the methylation steps (M1–M3) in the alkene cycle are larger than that of the rate-determining step (HT1(a)) in aromatization at 298–773 K. Moreover, as depicted in

10


Page 10 of 64

Page 11 of 64

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


Figure S4(a,b), the lower energy barriers demanded in the alkene cycle suggest that the alkene cycle in MTO is intrinsically more active than the aromatization, leading to higher selectivity to light olefins in the effluent products. Moreover, it should be noted that only the active role of the supercages in catalyzing MTO are considered in this work. Besides the supercages, the sinusoidal channels, in which the alkene cycle dominates the MTO reactions, also play a crucial role in the overall MTO behavior over H-MCM-22;12,29,30 an increase in the reaction temperature surely also promotes the alkene cycle MTO reactions in the sinusoidal channels besides the reactions in the supercages. All these are in accord with the experimental observation of Bhan and co-workers that the aromatic cycle is probably more preferential at lower temperatures, whereas the alkene cycle is more prominent at higher temperature.64

{Figure 2 & Table 1}

3.1.2. Naphthalene Formation. Recently, Bjørgen and co-workers proposed a similar reaction pathway for the formation of naphthalene from methylbenzene and light olefins.33 In this work, benzene and butene are regarded as the precursors for the formation of naphthalene so that the evolution of naphthalenic species can be directly compared to that of benzenic species in MTO. It should be mentioned that benzene and butene are not abundant species in the effluent products for MTO over the zeolite catalysts. So far as we know, there is an induction period for MTO over the zeolite catalyst, which is in general ascribed to the time requested to build the HCP species. For MTO over H-MCM-22 in the supercages, in this work, the polyMBs and polyMNs are considered as the active HCP species in MTO; the relatively small quantity of benzene and butene in the effluent products may also explain the induction period needed to build the HCP species for MTO, though we admit that these factors ought to be considered for a detailed investigation on the aromatization mechanism,

11



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

especially for the processes like methanol to aromatics (MTA) with aromatics as the targeted products.

As shown in Scheme 1, benzene reacts with 1-butyl carbenium ion, which is generated by butene protonation (P1(b)), forming butylbenzene (O1(b)). Hydride transfer (HT1(b)) between methoxy and butylbenzene leads to the formation of methane and 1-butylbenzene carbenium ion, which can be further converted to phenylcyclohexane by 1,6-cyclization (C1(b)) and deprotonation. After the repeated deprotonation and hydride transfer reactions (HT2(b) and HT3(b)), naphthalene is obtained.

The formation of benzene or naphthalene originates from the adsorption of propene or butene on the zeolite acid sites, which builds the relevant π-complex system and produces the propyl or butyl carbenium ions by the subsequent protonations. The adsorbed alkene π-complexes are almost as stable as the relevant carbenium ions, due to the low free energy barriers and reaction free energies of the protonations; both of them are simultaneously present in zeolites, in agreement with the theoretical results of Vandichel and co-workers.59 Dai and co-workers also found that the carbenium ions such as the butyl carbenium ion existed in the zeolite and were important intermediates in MTO. 65 Meanwhile, the co-existence of adsorbed alkene π-complex and alkoxide species has been also verified during the oligomerization of ethene and propene by means of FT-IR spectroscopy.66

The free energy profiles of naphthalene formation are depicted in Figure 2 and the detailed kinetic and thermodynamic results at 673 K are given in Table 1. The detailed optimized transition states for naphthalene formation are described in Figure S1(b1–b3) and Figure S2(b1–b3) of the Supporting Information. The hydride transfer steps of HT1(b), HT2(b) and HT3(b) are exothermic by 33, 44 and 64 kJ mol−1, respectively; their free energy

12


Page 12 of 64

Page 13 of 64

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


barriers are in the range of 155–165 kJ mol−1, with rate constants of 2.32×100–1.16×101 s−1. Similar to that for benzene formation, the rate-determining step for naphthalene formation also belongs to the hydride transfer reaction. Although the hydride transfer reactions (HT1–HT3, 143–174 kJ mol−1) are the main obstacle for the aromatization process, they are all thermodynamically favorable (exothermic by 30–78 kJ mol−1); meanwhile, their reaction enthalpies and reaction entropies are also quite low. The intrinsic free energy barrier and rate constant of the rate-determining step for benzene formation (HT1(a), 174 kJ mol−1 and 4.53×10−1 s−1) are similar to those of naphthalene formation (HT1(b), 165 kJ mol−1 and 2.32×100 s−1), implying that the formation of naphthalene is also kinetically feasible. As shown in Figure 2, the apparent overall free energy height (energy difference between the highest point and the zero point of the free energy profile) for naphthalene formation (216 kJ mol−1) is a little lower than that of benzene formation (236 kJ mol−1). Meanwhile, the overall reaction free energy (energy difference between the final product and the zero point of free energy profile) for naphthalene formation (−4 kJ mol−1) is also more negative than that for benzene formation (11 kJ mol−1), suggesting that thermodynamically, naphthalene is more attainable than benzene in the supercages. As a result, once benzene is generated, it is quite feasible to convert part of benzene to naphthalene through further aromatization in the supercages of H-MCM-22. This is in agreement with the recent experimental observations that both benzene and naphthalene can be clearly detected by GC-MS in the initial period for MTO over H-MCM-22.48 It should be emphasized that the apparent and intrinsic kinetics for MTO could be determined by various methods.62,67 Although the intrinsic free energy barrier and intrinsic reaction rate constants obtained here cannot be directly compared to the experimental results, they are very useful for evaluating the difference between various competitive reactions within a

13



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

similar reaction network.

To analyze the electrostatic stabilization and space confinement effects, the contributions of enthalpy barrier (∆Hint≠) and entropy loss (−T∆Sint≠) to the total free energy barrier at 673 K are also analyzed. As summarized in Table 1, the free energy barriers of aromatization are mainly contributed by the enthalpy barriers, whereas the entropy losses are less important. The rate-determining steps for benzene formation (HT1 (a)) and naphthalene formation (HT1(b)) show similar entropy losses (15 and 23 kJ mol−1), whereas the former has a higher enthalpy barrier (159 vs. 142 kJ mol−1), suggesting that the supercages in H-MCM-22 can provide evident electrostatic stabilization effect on the aromatic species, especially on the larger naphthalene molecules with two conjugated rings.

It should be noticed that the transition states in this work were searched by OPT=TS method and the relevant reactants and products were then attained by the quasi-IRC approach; as a result, the optimized structure of the reactants was then related to the transition states attained. Therefore, the products and reactants of two consecutive reaction steps may take a different optimized structure and have a different energy level, as these two steps are quite different in the attained transition states.60

3.1.3. PolyMB and PolyMN Formation. The substituted counterparts of benzene and naphthalene, namely polyMBs and polyMNs, can be constructed by repeated methylations (M1(a)–M6(a) and M1(b)–M5(b)), as shown in Scheme 1. The detailed kinetic and thermodynamic results for methylations at 673 K are given in Table 2. The free energy profiles for the formation of polyMBs and polyMNs are presented in Figure S5 and the detailed optimized transition states of various methylation steps are depicted in Figure S6(a–f) and Figure S7(a–e) of the Supporting Information.

14


Page 14 of 64

Page 15 of 64

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} The free energy barriers for the formation of pentamethylbenzene (PMB, 130 kJ mol−1) and hexamethylbenzene (HMB, 127 kJ mol−1) are about 30–40 kJ mol−1 lower than those of lower polyMBs (toluene to tetramethylbenzene (TeMB), 161–168 kJ mol−1), illustrating that higher polyMBs are kinetically attainable; moreover, the formation of higher polyMBs is also thermodynamically favorable due to the low reaction free energies and reaction enthalpies. The large supercages of MCM-22 may match the higher polyMBs with suitable sizes better and can provide a proper electrostatic stabilization effect, which can effectively reduce the free energy barriers and enthalpy barriers of methylations. Therefore, higher polyMBs can be easily constructed from lower polyMBs through repeated methylations, which is consistent with the conclusion of Van Speybroeck and co-workers over H-SAPO-34 and H-SSZ-13.68 Similarly, Van Speybroeck and co-workers also found that the methylation of benzene with methanol was energetically favored over H-ZSM-5 in comparison with that over H-Beta, owing to the efficient stabilization effect of the H-ZSM-5 zeolite on benzene with relatively small molecular size.52

For the formation of polyMNs, the differences among various methylation steps (M1(b)–M4(b)) in the free energy barrier are very small (< 17 kJ mol−1), as given in Table 2. The methylation of tetramethylnaphthalene (TeMN) to get pentamethylnaphthalene (PMN) requires a free energy barrier of 149 kJ mol−1 (M5(b)), similar to that of trimethylnaphthalene (TMN) methylation to form TeMN (M4(b), 143 kJ mol−1); however, the free energy surface of the transition state for producing PMN (150 kJ mol−1) is higher than that for getting TeMN (125 kJ mol−1), as shown in Figure S5 of the Supporting Information, implying that further methylation of TeMN to PMN is some difficult. As reported by Haw and Marcus, it was hard to observe naphthalenic species with more than

15



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 16 of 64

four methyl groups in the initial period of MTO over SAPO-34 and SAPO-18.69 Although there is no relevant experimental result available for the successive methylations of naphthalenic species during MTO over H-MCM-22, it is reasonable to assume that the polyMNs higher than PMN is insignificant in investigating MTO over H-MCM-22.

The formation of alkylbenzene by the oligomerization (O1(b)) between benzene and butene is relatively easy, with a low free energy barrier of 108 kJ mol−1. However, the further growth of alkylbenzene into the naphthalenic species is some difficult; the free energy barrier of the rate-determining step in the formation of naphthalene (HT1(b), 165 kJ mol−1) is similar to that of the benzene methylation (M1(a), 167 kJ mol−1). These results demonstrate that the formation of polyMBs through benzene methylation is in competition with the construction of naphthalene through benzene aromatization; both reactions may take place simultaneously in the supercages of H-MCM-22.

3.2. Reactivity of PolyMB and PolyMN for Propene Formation. To evaluate the reactivity of polyMB and polyMN acting as the hydrocarbon pool (HCP) species for MTO in the supercages of H-MCM-22 to form light olefins, four polyMBs (namely p-xylene (PX), 1,2,4-trimethylbenzene (TMB), 1,2,4,6-tetramethylbenzene (TeMB) and hexamethylbenzene (HMB))

and

three

polyMNs

(including

1,4-dimethylnaphthalene

(DMN),

1,4,5-trimethylnaphthalene (TMN) and 1,4,5,8-tetramethylnaphthalene (TeMN)) are considered.

3.2.1. Reactivity of PolyMB. The detailed polyMB cycle represented by the HMB pathway is depicted in Scheme 2. The whole cycle starts from the adsorption of methanol and polyMB at the acid sites. The gem-methylation of HMB with methanol (M1) forms 1,1,2,3,4,5,6-heptmethylbenzenium cation, which loses one proton in its side chain methyl

16


Page 17 of 64

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


group to give hexamethyl-methylene-cyclohexadiene containing exocyclic double bond (D1). The

second

methanol

molecule

attacks

the

exocyclic

double

bond

to

get

1,1,2,3,5,6-hexamethyl-4-ethylbenzenium cation (M2), which is then deprotonated to form hexamethyl-ethylene-cyclohexadiene (D2). After that, hexamethyl-ethylene-cyclohexadiene is further methylated to 1,1,2,3,5,6-hexamethyl-4-isopropylbenzenium cation with the third methanol molecule (M3). Finally, propene is eliminated through the internal hydrogen-shift (E1).11,70,71 Although the internal H-shift demands relatively high energy, such a one-step intramolecular reaction can give a clear description on the relation between the free energy barriers of olefin elimination and the number of methyl groups in the aromatic ring.

{Scheme 2, Figure 3, Table 3 & Figure 4}

The free energy profiles of MTO via the polyMB cycle at 673 K are depicted in Figure 3; the detailed kinetic and thermodynamic results for all steps are given in Table 3 and Table S2 of the Supporting Information. The detailed optimized transition states of various reaction steps for the polyMB (represented by HMB) cycle are presented in Figure 4(a–f). The framework of supercages as well as methanol and various polyMBs in gaseous phase are taken as the reference state to get the energies of other species. The calculated adsorption free energies of methanol, PX, TMB, TeMB and HMB are 12, 28, 21, 16 and 2 kJ mol−1, respectively, as listed in Table S3 of the Supporting Information. The gradual decrease of the adsorption free energy of polyMBs with the addition of methyl groups in the aromatic ring is mainly ascribed to the decrease of the adsorption enthalpy (more negative), which is directly related to the proton affinity (PA) of aromatic species.72 The adsorption of higher polyMB (HMB) on acid sites is more stable than the lower counterparts (PX to TeMB), because HMB with higher basicity (higher PA, 880 kJ mol−1) has a stronger interaction with the acid sites in zeolite than the lower polyMBs (PX to TeMB, PA = 797–851 kJ mol−1).

17



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

For the PX pathway, three methylation steps of M1, M2 and M3 need to overcome free energy barriers of 171, 125 and 141 kJ mol−1, with rate constants of 7.03×10−1, 2.73×103 and 1.66×102 s−1, respectively, as given in Table S2 of the Supporting Information; their reaction free energies are 70, −73 and −7 kJ mol−1, respectively. Two deprotonation steps of D1 and D2 are very fast, with free energy barriers below 76 kJ mol−1. The elimination of propene (E1) is endothermic by 71 kJ mol−1; it requires a free energy barrier of 181 kJ mol−1, with the rate constant of 1.25×10−1 s−1. With the increase of methyl groups in polyMB (from PX to HMB, as the active HCP species), the free energy barrier of methylation steps are decreased from 171 to 134 kJ mol−1 for M1 step, and from 125 to 99 kJ mol−1 for M2 step, whilst the rate constant is increased from 7.03×10−1 to 5.04×102 s−1 for M1, and from 2.73×103 to 2.86×105 s−1 for M2. However, PX, TMB, TeMB and HMB as the HCP species show similar free energy barriers for the M3 methylation step (viz., 141, 129, 132 and 138 kJ mol−1, respectively). Meanwhile, the free energy barrier for propene elimination (E1) is gradually decreased from 181 to 157 kJ mol−1 with the increase of methyl groups in polyMBs from PX to HMB, in line with that for the methylation steps. As a whole, it seems that higher polyMBs are more active as the HCP species in the supercages for the formation of propene in MTO over H-MCM-22.

In current work, propene is taken as the representative of light olefins for MTO in the supercages of H-MCM-22, because higher polyMBs, which show higher activity than lower polyMBs, favored the production of propene in MTO over large pore zeolites.25,36 Nevertheless, the formation of ethene has been also considered (but not included in this work), which illustrates that the elimination of ethene needs to overcome much higher free energy barriers (193 and 167 kJ mol−1, for the HMB and DMN pathways, respectively) than the elimination of propene (157 and 149 kJ mol−1, for the HMB and DMN pathways,

18


Page 18 of 64

Page 19 of 64

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


respectively).

To get further insights into the electrostatic stabilization and space confinement effects, the contribution of enthalpy and entropy to the total free energy of different pathways is acquired. As depicted in Figure S9 of the Supporting Information, the energy surface of enthalpy is gradually decreased from the PX pathway to HMB pathway, whereas the changes of various pathways in entropy are similar, indicating that the electrostatic stabilization effect plays a vital role for MTO in the supercages and higher polyMB has a stronger interaction with the framework. Via the polyMB cycle, the addition of methyl groups in benzenic ring of active HCP species can decrease the enthalpy barriers of methylation and elimination reactions, whereas it has minor influence on the entropy losses, as summarized in Table 3 and Table S2. These results clearly illustrate that the relatively lower free energy barriers of higher polyMB (e.g. HMB) cycle is mainly contributed by the elevation of the electrostatic stabilization effect on the reaction intermediates/transition states from the supercages. This can be further demonstrated by visualizing the isosurfaces of the reduced density gradient in real space.29,73,74 As depicted in Figure 5(a–d), the isosurfaces of the transition states of olefin elimination reactions (E1) exhibit a marked increase in the green region with the addition of methyl groups in polyMB, confirming that the supercages can provide stronger van der Waals interaction on higher polyMB (e.g. HMB).

{Figure 5}

As shown in Figure 3, the HMB pathway shows lower free energy barrier for the rate-determining step (157 kJ mol−1) and overall free energy height (209 kJ mol−1) than those of the lower polyMB (PX to TeMB) pathways (176–186 kJ mol−1 and 223–258 kJ mol−1, respectively). Meanwhile, the HMB pathway, with a lower reaction free energy and reaction

19



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 64

enthalpy for the rate-determining step, is also thermodynamically more favorable than the lower polyMB pathways; the overall reaction free energy of HMB pathway (−78 kJ mol−1) is more negative than that of PX, TMB and TeMB pathways (32, 7 and −14 kJ mol−1, respectively). As a result, HMB exhibits higher reactivity for the formation of light olefins for MTO in the surpercages of H-MCM-22 than other polyMBs; it should be the dominant reaction intermediate. Such a result is in agreement with the experimental observation of Bjørgen and co-workers;33 through co-feeding 13C methanol with 12C benzene, they quickly detected HMB at a short time on stream in the supercages and verified that HMB play a vital role in the formation of light olefins. Moreover, Hereijgers and co-workers investigated the product shape selectivity and deactivation behavior of H-SAPO-34 with different particle sizes by isotopic switch experiments; 75 they also demonstrated that the

13

C fraction

increased with an increase in the number of methyl groups on the aromatic ring and HMB is the most active polyMB intermediate.

3.2.2. Reactivity of PolyMN. The reaction scheme of polyMNs to form light olefins is similar to that of polyMBs, as shown in Scheme 2. The free energy profiles of polyMN cycle in the supercages of H-MCM-22 at 673 K are described in Figure 6; the detailed kinetic and thermodynamic results for all steps are given in Table 3 and Table S4 of the Supporting Information. The optimized transition states of various reaction steps for the polyMN (represented by DMN) cycle are presented in Figure 7(a–f).

{Figure 6 & Figure 7}

The free energy barriers of methylation steps in the polyMN cycle are in the range of 121–161 kJ mol−1, which are close to that of the lower polyMB cycle (PX to TeMB, 112–171 kJ mol−1); however, they are about 20 kJ mol−1 higher than that of the HMB cycle (99–138

20


Page 21 of 64

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


kJ mol−1). On the other hand, the elimination step for the polyMN cycles exhibits lower free energy barriers (144–149 kJ mol−1) than those of the polyMB cycles (181, 176, 186, and 157 kJ mol−1 for the PX, TMB, TeMB, and HMB pathways, respectively), suggesting that propene can be easily eliminated via the polyMN cycle once the alkyl side-chain is generated. As shown in Figures 3 and 6, the free energy barrier of the rate-determining step (147–161 kJ mol−1) in the polyMN cycle is similar to that of the HMB cycle (157 kJ mol−1) and the overall free energy height of the polyMN cycle (174–199 kJ mol−1) is slightly lower than that of the HMB cycle (209 kJ mol−1). Similar to the HMB pathway, meanwhile, the polyMN (DMN to TeMN) pathways, with a lower reaction free energy and reaction enthalpy, also exhibit higher thermodynamic feasibility than the lower polyMB pathways. These results illustrate that polyMNs with two to four methyl groups are also highly active as the HCP species in the supercages of H-MCM-22 for the formation of light olefins in MTO; they are as active as HMB.

Moreover, as shown in Figure S10 of the Supporting Information, the polyMN and HMB pathways are nearly in the same level in the enthalpy and entropy surfaces, demonstrating that polyMNs and HMB have similar interaction with the supercages, which can provide appropriate electrostatic stabilization and space confinement effects to lower the energy barriers for MTO. The isosurface plots of the reduced density depicted in Figure 8(a–c) also illustrate that the olefin elimination reactions in the polyMN pathways display large green regions in the isosurface, similar to that of HMB pathway. Such results are also supported by the recent experimental studies.76,77 Borodina and co-workers analyzed the influence of reaction temperature on the nature of active species over H-SSZ-13 with small pore-mouth and large elliptic cages; they illustrated that the methylated naphthalene carbocations were important active intermediates for the formation

21



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

of light olefins at high temperature (623–723 K).76 Qi and co-workers investigated the role of naphthalene during the induction period in MTO over H-ZSM-5 and also found that methylnaphthalenes were able to act as the initial active HCP species; introducing naphthalene could enhance the aromatic cycle and promote the formation of light olefins.77

The apparent free energy barriers of the gem-methylation (M1) as well as the overall free energy height of different aromatic-based reaction cycles are depicted in Figure 9. The apparent free energy barriers of the polyMN pathways (DMN to TeMN, 120–149 kJ mol−1) are similar to that of the HMB pathway (137 kJ mol−1), but much lower than those of the lower polyMB pathways (PX to TeMB, 152–199 kJ mol−1). The apparent free energy barriers are dominantly determined by the apparent enthalpy barriers which are sensitive to the number of methyl groups in polyMBs or polyMNs, whereas the apparent entropy losses are less sensitive to the number of methyl groups. Meanwhile, as stated above, the HMB and polyMN (DMN to TeMN) pathways, as expressed by their lower overall free energy height and overall reaction free energy, are kinetically and thermodynamically more feasible than the lower polyMB pathways. As a result, HMB and polyMNs with two to four methyl groups are probably the dominant active HCP species for MTO in the supercages of H-MCM-22.

{Figure 9}

3.3. Deactivation Behavior of Aromatic Species in the Supercages. Along with the MTO reaction process, the naphthalenic species in the supercages of H-MCM-22 may age into larger polycyclic aromatics such as anthracene and phenanthrene and finally develop into coke species. The anthracenic and phenanthrenic species have been detected in MTO over H-MCM-22 with different Si/Al ratios by GC-MS.48 The formation of anthracene and phenanthrene is outlined in Scheme 3. Naphthalene reacts with 1-butyl carbenium ion to

22


Page 22 of 64

Page 23 of 64

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


form butylnaphthalene (O1). Hydride transfer reaction (HT1) between methoxy and butylnaphthalene leads to the formation of methane and 1-butylnaphthalene carbenium ion, which forms the precursors of anthracene and phenanthrene by 1,6-cyclization (C1(a) and C1(b)). After the subsequent deprotonation and hydride transfer reactions (HT2(a, b) and HT3(a, b)), polycyclic aromatic species are generated. The free energy profiles for the formation of anthracene and phenanthrene at 673 K are depicted in Figure 10 and the detailed kinetic and thermodynamic results of various reaction steps are given in Table 4. The optimized transition states of various reaction steps for the formation of anthracene and phenanthrene are presented in Figure S11(a–h) of the Supporting Information.

{Scheme 3, Figure 10 & Table 4}

The oligomerization and cyclization steps of O1, C1(a) and C1(b) are endothermic by 49, 6 and 3 kJ mol−1, respectively; they need to overcome free energy barriers of 89, 98 and 70 kJ mol−1, respectively. The hydride transfer steps of HT2(a) and HT2(b) are exothermic by 46 and 59 kJ mol−1, respectively; they need to step over high free energy barriers of 216 and 205 kJ mol−1, with low rate constants of 2.52×10−4 and 1.72×10−3 s−1, respectively. The polycyclic aromatics generated can then be adsorbed on the acid site of H-MCM-22, with the adsorption enthalpies of −100 and −103 kJ mol−1 (free energies of −50 and −54 kJ mol−1) for anthracene and phenanthrene, respectively.

The formation of anthracene and phenanthrene exhibits higher free energy barrier for the rate-determining step (205–216 kJ mol−1) and overall free energy height (241–242 kJ mol−1) than those of benzene formation (HT1(a), 174 and 236 kJ mol−1, respectively) and naphthalene formation (HT1(b), 165 and 216 kJ mol−1, respectively), indicating that the formation of anthracene and phenanthrene in the supercages is kinetically more difficult in

23



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 24 of 64

comparison with the formation of benzene and naphthalene. Moreover, the free energy barriers of methylation (121–161 kJ mol−1) and olefin elimination steps (144–149 kJ mol−1) in the polyMN pathways are also much lower than those of the hydride transfer reactions (216 kJ mol−1 for HT2(a) and 205 kJ mol−1 for HT2(b)) in the formation of anthracene and phenanthrene. Meanwhile, the overall reaction free energies of the polyMN pathways for producing olefins are more negative than those of anthracene and phenanthrene formation, suggesting that the formation of olefins via the polyMN pathways is also thermodynamically more feasible than the formation of polycyclic aromatics.

These

kinetic

and

thermodynamic

results

suggest

that

naphthalene

or

polymethylnaphthalene are inclined to serve as the active HCP species to produce light olefins rather than to act as the precursors for further aromatization to form the large anthracene and phenanthrene molecules in the initial period of MTO. As a result, H-MCM-22 with the supercages shows relatively high catalytic activity in MTO. The electrostatic stabilization effect from the supercages on the reaction intermediates and transition states can effectively promote the elimination of light olefins via the polyMB and polyMN pathways; moreover, the space confinement effect can also restrict the rapid formation of large anthracene and phenanthrene molecules.29,33

The intrinsic reactivity of the polycyclic aromatics, represented by dimethylanthracene (DMA), is also evaluated by the side-chain route (similar to that shown in Scheme 2). The free energy profiles of MTO via the DMA pathway at 673 K are depicted in Figure S12 of the Supporting Information and the detailed kinetic results for all steps are given in Table S5 of the Supporting Information. The detailed optimized transition states of various reaction steps for the DMA pathway are presented in Figure S13 of the Supporting Information; a distortion of the aromatic rings is observed for the polycyclic species in the supercages of

24


Page 25 of 64

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


H-MCM-22. The free energy barriers of methylation and elimination steps as well as the free energy height of the DMA pathway is lower than those of the HMB and DMN pathways, as shown in Figure S12 of the Supporting Information. These results strongly imply that the aromatic species, even the polycyclic ones, exhibit quite high intrinsic reactivity for the formation of light olefins in the supercages. Various aromatic species in the supercages are able to serve as the active intermediates, which can accelerate the MTO reaction process and promote the formation of light olefins; such an observation may well explain the fact that H-MCM-22 shows high methanol conversion and high selectivity to light olefins in the initial period of MTO.

Although the polycyclic aromatics such as DMA show relatively high intrinsic reactivity in MTO, the generation of large polycyclic aromatics such as anthracene and phenanthrene always represents the commencement of catalyst deactivation. With the progress of MTO reaction, the large polycyclic aromatics are accumulated in the supercages and gradually developed to coke species, which finally causes the catalyst deactivation.25 Therefore, it is still essential to take appropriate measures to restrict the formation of large polycyclic aromatics.

The above results clearly illustrate that polyMBs and polyMNs exhibit high reactivity in forming light olefins; they are important active HCP species in the supercages for MTO over H-MCM-22. Owing to the appropriate electrostatic stabilization and space confinement effects, naphthalenic species in the supercages are inclined to serve as the active intermediates to produce light olefins rather than to act as the coke precursors to form anthracene and phenanthrene in the initial period of MTO. As a result, the supercages contribute actively to the initial activity of H-MCM-22 in MTO, though they may be prone to deactivation in the later reaction stage in comparison with the sinusoidal channels.

25



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

3.4. Influence of Acid Strength on the Evolution of Aromatic Species. The strength of acid sites plays an important influence on the catalytic performance of zeolites in MTO.78–80 To evaluate the effect of acid strength on the evolution of aromatic species, the rate-determining steps in the processes of benzene, naphthalene, and anthracene formation are considered over various trivalent-heteroatom substituted MCM-22 zeolites, viz., Al-MCM-22, Ga-MCM-22 and B-MCM-22, whose acid strength is decreased in succession on the basis of the calculated proton affinity (PA) and ammonia adsorption energy (∆Eads) at the active sites, as shown in Table S6 of the Supporting Information.81,82 The detailed free energy barriers of the hydride transfer reactions for the formation of benzene, naphthalene and anthracene over H-MCM-22 with different acid strengths are listed in Table S6 of the Supporting Information, whereas the relationship between the free energy barrier and the ammonia adsorption energy representing the acid strength is depicted in Figure 11.

{Figure 11}

It can be found that the free energy barrier of the hydride transfer reactions decreases linearly with the acid strength (R2 ≥ 0.94), similar to the previous observation.60,83 Over B-MCM-22 with the weakest acid strength, the free energy barriers of the rate-determining step for the formation of benzene, naphthalene and anthracene are 210, 205 and 254 kJ mol−1, respectively, which are markedly higher than those over Ga-MCM-22 and Al-MCM-22 with stronger acid strength; such a result demonstrates that the formation of aromatic species can be dramatically suppressed over the zeolite catalyst with weaker acid sites. Moreover, the hydride transfer reaction HT2 for the formation of anthracene is more sensitive to the acid strength than HT1 for the formation of benzene and naphthalene, as the former displays a steeper energy line with the acid strength. As a result, a decrease in the acid strength is of benefit to suppressing the accumulation of aromatics, especially the larger polycyclic ones,

26


Page 26 of 64

Page 27 of 64

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 then improving the catalytic stability of H-MCM-22 in MTO. This is also supported by the recent experimental observation of Mihalyi and co-workers;84 for the transformation of ethylbenzene and m-xylene over H-MCM-22 zeolites with different acidities, they found that the catalytic stability of H-MCM-22 can be enhanced by decreasing its acidity through isomorphous substitution of framework Al with B.

4. CONCLUSIONS

The evolution of aromatic species, including their formation, reactivity and deactivation behavior, for MTO in the supercages of H-MCM-22 zeolite were investigated by DFT-D and the active role of the supercages in catalyzing MTO was elucidated.

The results demonstrated that benzene can be generated in the supercages through aromatization of light olefins; after that, polymethylbenzenes (polyMBs) are formed through methylations,

in

competition

with

the

construction

of

naphthalenic

species.

Hexamethylbenzene (HMB) and polymethylnaphthalenes (polyMNs) with two to four methyl groups exhibit high reactivity in forming light olefins.

PolyMBs and polyMNs exhibit high reactivity in forming light olefins; they are important active hydrocarbon pool species in the supercages for MTO over H-MCM-22. Owing to the appropriate electrostatic stabilization and space confinement effects, naphthalenic species in the supercages are inclined to serve as the active intermediates to produce light olefins rather than to act as the coke precursors to form anthracene and phenanthrene in the initial period of MTO. As a result, the supercages contribute actively to the initial activity of H-MCM-22 in MTO, though they may be prone to deactivation in the later reaction stage in comparison with the sinusoidal channels.

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

The acid strength has an important influence on the evolution of aromatic species; the formation of polycyclic aromatics like anthracene is more sensitive to the acid strength than the formation of benzene and naphthalene. A decrease in the acid strength is of benefit to suppressing the accumulation of aromatics, especially the larger polycyclic ones, and then improving the stability of the zeolite catalyst in MTO.

The insights shown in this work help to clarify the evolution of aromatic species and the active role of the supercages for MTO over H-MCM-22, which is of benefit to the development of better MTO catalysts and reaction process.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx. Free energy barriers of a few selected steps calculated with different models and by different methodologies; calculated adsorption energies of various species; calculated kinetic and thermodynamic results of various aromatic pathways; calculated results for the formation of benzene, naphthalene and anthracene over heteroatom-substituted H-MCM-22; rate constants in the aromatization and in the alkene cycle forming olefins as a function of the reaction temperature; free energy profiles for the methylation of benzene and naphthalene as well as for the aromatic cycle (HMB, DMN, DMA pathways) and the alkene cycle; enthalpy and entropy energy profiles of the polyMB and polyMN cycles; detailed optimized transition states of various steps which are not included in the main manuscript, for MTO in the supercages of H-MCM-22 (PDF). 28


Page 28 of 64

Page 29 of 64

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


AUTHOR INFORMATION

Corresponding Authors *Z. Qin: tel, +86-351-4046092; fax, +86-351-4041153; e-mail, [email protected]. *J. Wang: e-mail, [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The calculations were performed on the Computer Network Information Center of Chinese Academy of Sciences, National Supercomputer Centers in Shenzhen and Lüliang of China. The authors are grateful for the financial supports of the National Natural Science Foundation of China (21273264, 21273263, 21227002, 21573270, U1510104), Natural Science Foundation of Shanxi Province of China (2013021007-3, 2015021003), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

REFERENCES

(1)

Hickman, D. A.; Schmidt, L. D. Production of Syngas by Direct Catalytic-Oxidation of Methane. Science 1993, 259, 343–346.

(2)

Laurendeau, N. M. Heterogeneous Kinetics of Coal Char Gasification and Combustion. Energy Combust. Sci. 1978, 4, 221–270.

(3)

Wen, W. Y. Mechanisms of Alkali-Metal Catalysis in the Gasification of Coal, Char, or

29



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

Graphite. Catal. Rev. Sci. Eng. 1980, 22, 1–28.

(4)

Asadullah, M.; Ito, S.; Kunimori, K.; Yamada, M.; Tomishige, K. Biomass Gasification to Hydrogen and Syngas at Low Temperature: Novel Catalytic System Using Fluidized-Bed Reactor. J. Catal. 2002, 208, 255–259.

(5)

Sutton, D.; Kelleher, B.; Ross, J. R. H. Review of Literature on Catalysts for Biomass Gasification. Fuel Process. Technol. 2001, 73, 155–173.

(6)

Dahl, I. M.; Kolboe, S. On The Reaction-Mechanism for Propene Formation in the MTO Reaction over SAPO-34. Catal. Lett. 1993, 20, 329–336.

(7)

Dahl, I. M.; Kolboe, S. On the Reaction-Mechanism for Propene Formation in the MTO Reaction over SAPO-34. 1. Isotopic Labeling Studies of the Co-Reaction of Ethene and Methanol. J. Catal. 1994, 149, 458–464.

(8)

Dahl, I. M.; Kolboe, S. On the Reaction Mechanism for Hydrocarbon Formation from Methanol over SAPO-34. 2. Isotopic Labeling Studies of the Co-Reaction of Propene and Methanol. J. Catal. 1996, 161, 304–309.

(9)

Kolboe, S. Methanol Reactions on ZSM-5 and Other Zeolite Catalysts-Autocatalysis and Reaction Mechanism. Acta Chem. Scand. Ser. 1986, 40, 711–713.

(10) Wang, M.; Buchholz, A.; Seiler, M.; Hunger, M. Evidence for An Initiation of the Methanol-to-Olefin Process by Reactive Surface Methoxy Groups on Acidic Zeolite Catalysts. J. Am. Chem. Soc. 2003, 125, 15260–15267.

(11) Wang, S.; Chen, Y. Y.; Wei, Z. H; Qin, Z. F.; Chen, J. L.; Ma, H.; Dong, M.; Li, J. F.;

30


Page 30 of 64

Page 31 of 64

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


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.

(12) Wang, S.; Wei, Z. H.; Chen, Y. Y.; Qin, Z. F.; Li, J. F.; Dong M.; Fan W. B.; Wang J. G. Recent Research Progresses in the Effect of Framework Structure and Acidity of Zeolites on Their Catalytic Performance in Methanol to Olefins (MTO). J. Fuel Chem. Techno., 2015, 43, 1202–1214.

(13) Lesthaeghe, D.; Van Speybroeck, 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.

(14) Lesthaeghe, D.; Van Speybroeck, V.; Marin, G. B.; Waroquier, M. What Role Do Oxonium Ions and Oxonium Ylides Play in the ZSM-5 Catalysed Methanol-to-Olefin Process? Chem. Phys. Lett. 2006, 417, 309–315.

(15) 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.

(16) Olsbye, U.; Svelle, S.; Bjørgen, M.; Beato, P.; Janssens, T. V. W.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity. Angew. Chem. Int. Ed. 2012, 51, 5810–5831.

(17) Hemelsoet, K.; Van der Mynsbrugge, J.; De Wispelaere, K.; Waroquier, M.; Van Speybroeck, V. Unraveling the Reaction Mechanisms Governing Methanol-to-Olefins

31



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

Catalysis by Theroy and Experiment. ChemPhysChem 2013, 14, 1526–1545.

(18) Liang, T. Y.; Chen, J. L.; Qin, Z. F.; Li, J. F.; Wang, P. F.; Wang, S.; Wang, G. F.; Dong, M.; Fan, W. B.; Wang, J. G. Conversion of Methanol to Olefins over H-ZSM-5 Zeolite: Reaction Pathway Is Related to the Framework Aluminum Siting. ACS Catal., 2016, 6, 7311–7325.

(19) Xu, S. T.; Zheng, A. M.; Wei, Y. X.; Chen, J. R.; Li, J. Z.; Chu, Y. Y.; Zhang, M. Z.; Wang, Q. Y.; Zhou, Y.; Wang, J. B., et al. Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol-to-Olefin Reaction over Chabazite Zeolites. Angew. Chem. Int. Ed. 2013, 52, 11564–11568.

(20) Sun, X. Y.; Mueller, S.; Shi, H.; Haller, G. L.; Sanchez-Sanchez, M.; Van Veen, A. C.; Lercher, J. A. On the Impact of Co-feeding Aromatics and Olefins for the Methanol-to-Olefins Reaction on HZSM-5. J. Catal. 2014, 314, 21–31.

(21) Stöcker, M. Methanol-to-Hydrocarbons: Catalytic Materials and Their Behavior. Micropor. Mesopor. Mater. 1999, 29, 3–48.

(22) Djieugoue, M. A.; Prakash, A. M.; Kevan, L. Catalytic Study of Methanol-to-Olefins Conversion in Four Small-Pore Silicoaluminophosphate Molecular Sieves: Influence of the Structure Type, Nickel Incorporation, Nickel Location, and Nickel Concentration. J. Phys. Chem. B 2000, 104, 6452–6461.

(23) Lee, J. H.; Park, M. B.; Lee, J. K.; Min, H. K.; Song, M. K.; Hong, S. B. Synthesis and Characterization of ERI-Type UZM-12 Zeolite and Their Methanol-to-Olefin Performance. J. Am. Chem. Soc. 2010, 132, 12971–12982.

32


Page 32 of 64

Page 33 of 64

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


(24) Dai, W. L.; Wang, X.; Wu, G.; Guan, N.; Hunger, M.; Li, L. D. Methanol-to-Olefin Conversion on Silicoaluminophosphate Catalysts: Effect of Bronsted Acid Sites and Framework Structures. ACS Catal. 2011, 1, 292–299.

(25) Min, H. K.; Park, M. B.; Hong, S. B. Methanol-to-Olefin Conversion over H-MCM-22 and H-ITQ-2 Zeolites. J. Catal. 2010, 271, 186–194.

(26) Lacarriere, A.; Luck, F.; Swierczynski, D.; Fajula, F.; Hulea, V. Methanol to Hydrocarbons over Zeolites with MWW Topology: Effect of Zeolite Texture and Acidity. Appl. Catal. A 2011, 402, 208–217.

(27) Huang, L. Z.; Wang, P. F.; Li, J. F.; Wang, J. G.; Fan, W. B. Effect of Acid Leaching and Catalytic Properities of Zeolite [Al,B]-MWW Utilized as Ethene and Methylation Catalyst. Micropor. Mesopor. Mater. 2016, 223, 230–240.

(28) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. MCM-22-A Molecular-Sieve with 2 Independent Multidimensional Channel Systems. Science 1994, 264, 1910–1913.

(29) 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.

(30) Chen, J. L.; Liang, T. Y.; Li, J. F.; Wang, S.; Qin, Z. F.; Wang, P. F.; Huang, L. Z.; Fan, W. B.; Wang, J. G. Regulation of Framework Aluminum Siting and Acid Distribution in H-MCM-22 by Boron Incorporation and Its Effect on the Catalytic Performance in Methanol to Hydrocarbons. ACS Catal. 2016, 6, 2299−2313.

33



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 64

(31) Laforge, S.; Martin, D.; Guisnet, M. m-Xylene Transformation over H-MCM-22 Zeolite. 2. Method for Determining the Catalytic Role of the Three Different Pore Systems. Micropor. Mesopor. Mater. 2004, 67, 235–244.

(32) Rigoreau, J.; Laforge, S.; Gnep, N. S.; Guisnet, M. Alkylation of Toluene with Propene over H-MCM-22 Zeolite. Location of the Main and Secondary Reactions. J. Catal. 2005, 236, 45–54.

(33) Bjørgen, M.; Akyalcin, S.; Olsbye, U.; Benard, S.; Kolboe, S.; Svelle, S. Methanol to Hydrocarbons over Large Cavity Zeolites: Toward a Unified Description of Catalyst Deactivation and the Reaction Mechanism. J. Catal. 2010, 375, 170–180.

(34) Wang, X.; Dai, W. L.; Wu, G. J.; Li, L. D.; Guan, N. J.; Hunger, M. Phosphorus Modified

HMCM-22:

Characterization

and

Catalytic

Application

in

Methanol-to-Hydrocarbons Conversion. Micropor. Mesopor. Mater. 2012, 151, 99–106.

(35) Matias, P.; Lopes, J. M.; Laforge, S.; Magnoux, P.; Guisnet, M.; Ramoa Ribeiro, F. n-Heptane Transformation over a HMCM-22 Zeolite: Catalytic Role of the Pore Systems. Appl. Catal. A 2008, 351, 174–183.

(36) Bjørgen, M.; Joensen, F.; Lillerud, K. P.; Olsbye, U.; Svelle, S. The Mechanisms of Ethene and Propene Formation from Methanol over High Silica H-ZSM-5 and H-beta. Catal. Today 2009, 142, 90–97.

(37) Ilias,

S.;

Bhan,

A.

The

Mechanism

of

Aromatic

Dealkylation

in

Methanol-to-Hydrocarbons Conversion on H-ZSM-5: What are the Aromatic

34


Page 35 of 64

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


Precursors to Light Olefins? J. Catal. 2014, 311, 6–16.

(38) Wang, C.; Xu, J.; Qi, G. D.; Gong, Y. J.; Wang, W. Y.; Gao, P.; Wang, Q.; Feng, N. D.; Liu, X. L.; Deng, F. Methylbenzene Hydrocarbon Pool in Methanol-to-Olefins Conversion over Zeolite H-ZSM-5. J. Catal. 2015, 332, 127–137.

(39) Song, W. G.; Fu, H.; Haw, J. F. Selective Synthesis of Methylnaphthalenes in HSAPO-34 Cages and Their Function as Reaction Centers in Methanol-to-Olefin Catalysis. J. Phys. Chem. B 2001, 105, 12839−12843.

(40) Fu, H.; Song, W. G.; Haw, J. F. Polycyclic Aromatics Formation in HSAPO-34 during Methanol-to-Olefin Catalysis: ex situ Characterization after Cryogenic Grinding. Catal. Lett. 2001, 76, 89–94.

(41) Hemelsoet, K.; Nollet, A.; Vandichel, M.; Lesthaeghe, D.; Van Speybroeck, V.; Waroquier, M. The Effect of Confined Space on the Growth of Naphthalenic Species in a Chabazite-Type Catalyst: A Molecular Modeling Study. ChemCatChem 2009, 1, 373–378.

(42) Hemelsoet, K.; Nollet, A.; Van Speybroeck, V.; Waroquier, M. Theoretical Simulations Elucidate the Role of Naphthalenic Species during Methanol Conversion within H-SAPO-34. Chem. Eur. J. 2011, 17, 9083–9093.

(43) Zhou, D. H.; Bao, Y.; Yang, M. M.; He, N.; Yang, G. DFT Studies on the Location and Acid Strength of Bronsted Acid Sites in MCM-22 Zeolite. J. Mol. Catal. A 2006, 244, 11–19.

(44) Zheng, A. M.; Lei, C.; Yang, J.; Zhang, M. J.; Su, Y. C.; Yue, Y.; Ye, C. H.; Deng, F.

35



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

Combined DFT Theoretical Calculation and Solid-State NMR Studies of Al Substitution and Acid Sites in Zeolite MCM-22. J. Phys. Chem. B 2005, 109, 24273–24279.

(45) Van der Mynsbrugge, J.; De Ridder, J.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V. Enthalpy and Entropy Barriers Explain the Effects of Topology on the Kinetics of Zeolite-Catalyzed Reactions. Chem. Eur. J. 2013, 19, 11568–11576.

(46) Van Speybroeck, V.; Van der Mynsbrugge, J.; 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.

(47) 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.

(48) Wang, P. F.; Huang, L. Z.; Li, J. F.; Dong, M.; Wang, J. G.; Tatsumi, T.; Fan, W. B. Catalytic Properties and Deactivation Behavior of H-MCM-22 in the Conversion of Methanol to Hydrocarbons. RSC Adv. 2015, 5, 28794–28802.

(49) 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.02; Gaussian, Inc., Wallingford CT, 2009.

(50) Chai, J. D.; Head-Gordon, M. Long-Range Corrected Hybrid Density Functionals with Damped Atom-Atom Dispersion Corrections. Phys. Chem. Chem. Phys. 2008, 10,

36


Page 36 of 64

Page 37 of 64

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


6615–6620.

(51) Goerigk, L.; Grimme, S. A Thorough Benchmark of Density Functional Methods for General Main Group Thermochemistry, Kinetics, and Noncovalent Interactions. Phys. Chem. Chem. Phys. 2011, 13, 6670–6688.

(52) Van der Mynsbrugge, J.; Visur, M.; Olsbye, U.; Beato, P.; Bjørgen, M.; Van Speybroeck, 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.

(53) De Wispelaere, K.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V. Complete Low-barrier Side-chain Route for Olefin Formation during Methanol Conversion in H-SAPO-34. J. Catal. 2013, 305, 76–80.

(54) Eyring, H. The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935, 3, 107–115.

(55) Lu, T.; Chen, F. W. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580–592.

(56) Li, J. F.; Wei, Z. H.; Chen, Y. Y.; Jing, B. Q.; He, Y.; Dong, M.; Jiao, H. J.; Li, X. K.; Qin, Z. F.; Wang, J. G., et al. A Route to Form Initial Hydrocarbon Pool Species in Methanol Conversion to Olefins over Zeolites. J. Catal. 2014, 317, 277–283.

(57) Ilias, S.; Bhan, A. Mechanism of the Catalytic Conversion of Methanol to Hydrocarbons. ACS Catal. 2013, 3, 18−31.

(58) Joshi, Y. V.; Thomson, K. T. Brønsted Acid Catalyzed Cyclization of C7 and C8

37



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

Dienes in HZSM-5: A Hybrid QM/MM Study and Comparison with C6 Diene Cyclization. J. Phys. Chem. C 2008, 112, 12825–12833.

(59) Vandichel, M.; Lesthaeghe, D.; Van der Mynsbrugge, J.; Waroquier, M.; Van Speybroeck, V. Assembly of Cyclic Hydrocarbons from Ethene and Propene in Acid Zeolite Catalysis to Produce Active Catalytic Sites for MTO Conversion. J. Catal. 2010, 271, 67–78.

(60) 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.

(61) Yamazaki, H.; Shima, H.; Imai, H.; Yokoi, T.; Tatsumi, T.; Kondo, J. N. Evidence for a “Carbene-like” Intermediate during the Reaction of Methoxy Species with Light Alkenes on H-ZSM-5. Angew. Chem. Int. Ed. 2011, 50, 1853–1856.

(62) Wang, C. M.; Wang, Y. D.; Xie, Z. K. Insights into the Reaction Mechanism of Methanol-to-Olefins Conversion in HSAPO-34 from First Principles: Are Olefin Themselves the Dominating Hydrocarbon Pool Species? J. Catal. 2013, 301, 8–19.

(63) Wu, X. C.; Abraha, M. G.; Anthony, R. G. Methanol Conversion on SAPO-34: Reaction Condition for Fixed-bed Reactor. Appl. Catal. A 2004, 260, 63–69.

(64) Ilias, S.; Khare, R.; Malek, A.; Bhan, A. A Descriptor for the Relative Propagation of the Aromatic- and Olefin-based Cycles in Methanol-to-Hydrocarbons Conversion on H-ZSM-5. J. Catal. 2013, 303, 135–140.

38


Page 38 of 64

Page 39 of 64

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) Dai, W. L.; Wang, C. M.; Yi, X. F.; Zheng, A. M.; Li, L. D.; Wu, G. J.; Guan, N. J.; Xie, Z. K.; Dyballa, M.; Hunger, M. Identification of tert-Butyl Cations in Zeolite H-ZSM-5: Evidence from NMR Spectroscopy and DFT Calculations. Angew. Chem. Int. Ed. 2015, 54, 1–5.

(66) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Borello, E. IR Study of Ethene and Propene Oligomerization on H-ZSM-5: Hydrogen-bonded Precursor Formation, Initiation and Propagation Mechanisms and Structure of the Entrapped Oligomers. J. Chem. Soc., Faraday Trans. 1994, 90, 2827–2835.

(67) Hill, I.; Malek, A.; Bhan, A. Kinetics and Mechanism of Benzene, Toluene, and Xylene Methylation over H-MFI. ACS Catal. 2013, 3, 1992−2001.

(68) Van Speybroeck, V.; Hemelsoet, K.; De Wispelaere, K.; Qian, Q. Y.; Van der Mynsbrugge, J.; De Sterck, B.; Weckhuysen, B. M.; Waroquier, M. Mechanistic Studies

on

Chabazite-Type

Methanol-to-Olefin

Catalysts:

Insights

from

Time-Resolved UV/Vis Microspectroscopy Combined with Theoretical Simulations. ChemCatChem 2013, 5, 173–184.

(69) Haw, J. F.; Marcus, D. M. Well-defined (supra)molecular Structures in Zeolite Methanol-to-Olefin Catalysis. Top Catal. 2005, 34, 41–48.

(70) 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.

39



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 40 of 64

(71) Lesthaeghe, D.; Horre, A.; Waroquier, M.; Marin, G. B.; Van Speybroeck, V. Theoretical Insights on Methylbenzene Side-Chain Growth in ZSM-5 Zeolites for Methanol-to-Olefin Conversion. Chem. Eur. J. 2009, 15, 10803–10808.

(72) Nicholas, J. B.; Haw, J. F. The Prediction of Persistent Carbenium Ions in Zeolites. J. Am. Chem. Soc. 1998, 120, 11084–11085.

(73) Johnson, E. R.; Keinan, S.; Mori-Sanchez, P.; Contreras-Garcia, J.; Cohen, A.; Yang, W. T. Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498–6506.

(74) Wang, C.; Chu, Y. Y.; Zheng, A. M.; Xu, J.; Wang, Q.; Gao, P.; Qi, G. D.; Gong, Y. J.; Deng,

F.

New

Insight

into

the

Hydrocarbon-Pool

Chemistry

of

the

Methanol-to-Olefins Conversion over Zeolite H-ZSM-5 from GC-MS, Solid-State NMR Spectroscopy, and DFT Calculations. Chem. Eur. J. 2014, 20, 1–13.

(75) Hereijgers, B. P. C.; Bleken, F.; Nilsen, M. H.; Svelle, S.; Lillerud, K. P.; Bjørgen, M.; Weckhuysen, B. M.; Olsbye, U. Product Shape Selectivity Dominates the Methanol-to-Olefins (MTO) Reaction over H-SAPO-34 Catalysts. J. Catal. 2009, 264, 77–87.

(76) Borodina, E.; Meirer, F.; Lezcano-Gonzalez, I.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Basahel, S. N.; Ruiz-Martinez, J.; Weckhuysen, B. M. Influence of the Reaction Temperature on the Nature of the Active and Deactivating Species during Methanol to Olefins Conversion over H-SSZ-13. ACS Catal. 2015, 5, 992–1003.

(77) Qi, L.; Li, J. Z.; Wei, Y. X.; Xu, L.; Liu, Z. M. Role of Naphthalene during the Induction Period of Methanol Conversion on HZSM-5 Zeolite. Catal. Sci. Technol.,

40


Page 41 of 64

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


2016, 6, 3737−3744.

(78) Chu, Y. Y.; Han, B.; Zheng, A. M.; Deng, F. Influence of Acid Strength and Confienment Effect on the Ethylene Dimerization Reaction over Solid Acid Catalysts: A Theoretical Calculation Study. J. Phys. Chem. C 2012, 116, 12687−12695.

(79) Erichsen, M. W.; Svelle, S.; Olsbye, U. The Influence of Catalyst Acid Strength on the Methanol to Hydrocarbons (MTH) Reaction. Catal. Today 2013, 215, 216–223.

(80) Erichsen, M. W.; Svelle, S.; Olsbye, U. H-SAPO-5 as Methanol-to-Olefins (MTO) Model Catalyst: Towards Elucidating the Effects of Acid Strength. J. Catal. 2013, 298, 94–101.

(81) Chu, C.; Chang, C. Isomorphous Substitution in Zeolite Frameworks. 1. Acidity of Surface Hydroxyls in [B]-ZSM-5, [Fe]-ZSM-5, [Ga]-ZSM-5, and [Al]-ZSM-5. J. Phys. Chem. 1985, 89, 1569–1571.

(82) Wang, Y.; Zhou, D. H.; Yang, G.; Miao, S. J.; Liu, X. C.; Bao, X. H. A DFT Study on Isomorphously Substituted MCM-22 Zeolite. J. Phys. Chem. A 2004, 108, 6730−6734.

(83) Moses, P. G.; Nørskov, J. K. Methanol to Dimethyl Ether over ZSM-22: A Periodic Density Functional Theory Study. ACS Catal. 2013, 3, 735−745.

(84) Mihalyi, M. R.; Kollar, M.; Klebert, S.; Mavrodinova, V. Transformation of Ethylbenzene-m-xylene Feed over MCM-22 Zeolites with Different Acidities. Appl. Catal. A 2014, 476, 19–25.

41



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 64

Table 1. Calculated Kinetic Results of Free Energy Barriers (∆Gint≠), Relative Rate Constants (k),

Enthalpy

Barriers (∆Hint≠) and

Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation of Benzene and Naphthalene during MTO in the Supercages of H-MCM-22.

step

kinetics ∆Gint≠ (kJ mol−1)

thermodynamics

∆Hint≠ (kJ mol−1)

k (s−1)

−T∆Sint≠ (kJ mol−1)

∆GR (kJ mol−1)

∆HR (kJ mol−1)

−T∆SR (kJ mol−1)

Benzene Formation P1(a)

125

2.87×103

95

30

26

−9

35

O1(a)

107

7.53×104

82

25

25

−2

27

D1(a)

66

9.99×107

74

−8

−10

18

−28

174

4.53×10−1

159

15

−38

−12

−26

87

2.53×106

79

8

−12

−21

9

HT2(a)

148

4.20×101

109

39

−30

−14

−16

HT3(a)

143

1.05×102

111

32

−78

−44

−34

HT1(a) C1(a)

Naphthalene Formation P1(b)

124

3.34×103

93

31

6

−15

21

O1(b)

108

5.45×104

105

3

42

31

11

HT1(b)

165

2.32×100

142

23

−33

−16

−17

86

2.74×106

87

−1

6

5

1

HT2(b)

155

1.16×101

124

31

−44

−2

−42

HT3(b)

156

1.01×101

136

20

−64

−30

−34

C1(b)

42


Page 43 of 64

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



Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation PolyMBs and PolyMNs during MTO in the Supercages of H-MCM-22.

step

kinetics ∆Gint≠ (kJ mol−1)

thermodynamics


k (s−1)


∆GR (kJ mol−1)

∆HR (kJ mol−1)


PolyMBs Formation M1(a)

167

1.41×100

154

13

73

70

3

M2(a)

168

1.29×100

158

10

53

52

1

M3(a)

163

3.34×100

153

10

66

61

5

M4(a)

161

4.69×100

149

12

58

59

−1

M5(a)

130

1.15×103

128

2

16

16

0

M6(a)

127

2.04×103

122

5

19

24

−5

PolyMNs Formation M1(b)

160

5.14×100

150

10

54

42

12

M2(b)

155

1.30×101

144

11

24

28

−4

M3(b)

149

3.83×101

146

3

57

60

−3

M4(b)

143

1.15×102

133

10

9

36

−27

M5(b)

149

3.95×101

132

17

30

54

−24

43



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 44 of 64



Enthalpy

Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for MTO in the Supercages of H-MCM-22 via the PolyMB Cycle (HMB Pathway) and PolyMN Cycle (DMN Pathway).

step

kinetic ∆Gint≠ (kJ mol−1)

k (s−1)

thermodynamic ∆Hint≠ (kJ mol−1)


∆GR (kJ mol−1)

∆HR (kJ mol−1)


PolyMB Cycle: HMB Pathway M1

134

5.04×102

123

11

15

21

−6

M2

99

2.86×105

90

9

−119

−102

−17

M3

138

2.33×102

122

16

−64

−60

−4

D1

74

2.72×107

43

31

57

31

26

D2

91

1.13×106

58

33

24

16

8

E1

157

9.19×100

132

25

14

48

−34

PolyMN Cycle: DMN Pathway a M1

146 (150)

6.34×101

137

9

61

72

−11

M2

139

2.37×102

118

21

−46

−37

−9

M3

160

5.74×100

132

28

−4

−1

−3

D1

39

1.28×1010

10

29

13

−13

26

D2

56

6.49×108

23

33

14

−3

17

3.55×101

124

25

57

81

−24

E1 a

149 (142)

The values in the parentheses are obtained with the ωB97X-D function during the

optimization and frequency calculation.

44


Page 45 of 64

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



Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation of Anthracene and Phenanthrene during MTO in the Supercages of H-MCM-22.

kinetic

step ∆Gint≠ (kJ mol−1)

thermodynamic


k (s−1)


∆GR (kJ mol−1)

∆HR (kJ mol−1)


89

1.59×106

91

−2

49

54

−5

169

1.04×100

137

32

−39

−16

−23

98

3.45×105

92

6

6

24

−18

HT2(a)

216

2.52×10−4

184

32

−46

−4

−42

HT3(a)

153

2.01×101

132

21

−81

−38

−43

70

5.05×107

73

−3

3

27

−24

HT2(b)

205

1.72×10−3

169

36

−59

−8

−51

HT3(b)

148

4.26×101

121

27

−86

−49

−37

O1 HT1

Anthracene Formation C1(a)

Phenanthrene Formation C1(b)

45



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 64

Captions Table 1. Calculated Kinetic Results of Free Energy Barriers (∆Gint≠), Relative Rate Constants (k),

Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation of Benzene and Naphthalene during MTO in the Supercages of H-MCM-22. Table 2. Calculated Kinetic Results of Free Energy Barriers (∆Gint≠), Relative Rate Constants (k),

Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation PolyMBs and PolyMNs during MTO in the Supercages of H-MCM-22. Table 3. Calculated Kinetic Results of Free Energy Barriers (∆Gint≠), Relative Rate Constants (k),

Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for MTO in the Supercages of H-MCM-22 via the PolyMB Cycle (HMB Pathway) and PolyMN Cycle (DMN Pathway). Table 4. Calculated Kinetic Results of Free Energy Barriers (∆Gint≠), Relative Rate Constants (k),

Enthalpy


Entropy

Losses

(−T∆Sint≠),

and

Thermodynamic Results of Reaction Free Energies (∆GR), Reaction Enthalpies (∆HR) and Reaction Entropies (−T∆SR) at 673 K of Each Reaction Step for the Formation of Anthracene and Phenanthrene during MTO in the Supercages of H-MCM-22.

46


Page 47 of 64

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


Figure 1. (a) Framework of H-MCM-22 zeolite with three types of pores: supercages, pockets, and sinusoidal channels; (b) 54T cluster model used to represent the supercages of H-MCM-22 zeolite, where a silicon atom is substituted with an aluminum atom at T4 site and the charge-balancing proton is bonded with O3. Atom coloring: yellow (Si), red (O), white (H) and pink (Al).

Figure 2. Free energy profiles for the formation of benzene and naphthalene at 673 K during MTO in the supercages of H-MCM-22 zeolite. Besides the zeolite framework, the adsorbed propene and benzene + butene (A1) are taken as the reference states for the formation of benzene and naphthalene, respectively. A2, A3 and A4 represent the formation of methoxyl at the active site before the hydride transfer reactions.

Figure 3. Free energy profiles of the polyMB cycle at 673 K for MTO in the supercages of H-MCM-22 zeolite. The framework of supercages as well as methanol and different polyMBs in gaseous phase are taken as the reference state. A1, A3 and A4 are the adsorption of methanol; A2 is the additional adsorption of polyMB.

Figure 4. Optimized transition states of various reaction steps in the HMB pathway for MTO in the supercages of H-MCM-22 zeolite. (a), (c) and (e) are for the methylation steps of M1, M2 and M3, respectively; (b) and (d) are for the deprotonation steps of D1 and D2, respectively; (f) is for the elimination step of E1. Atom coloring: yellow (Si), red (O), white (H), pink (Al) and blue (C). The unit of bonding distances is Å.

Figure 5. Isosurface plots of the reduced density gradient (s = 0.500 a.u.) for the transition states of the olefin elimination reactions in the polyMB cycle for MTO in the supercages of H-MCM-22 zeolite: (a) PX pathway; (b) TMB pathway; (c) TeMB pathway; and (d) HMB pathway. The isosurfaces of the reduced density gradient are colored according to the values 47



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

of the quantity sign (λ2)ρ with the indicated RGB scale. VDW represents the van der Waals interactions.

Figure 6. Free energy profiles of the polyMN cycle at 673 K for MTO in the supercages of H-MCM-22 zeolite. The framework of supercages as well as the methanol and different polyMNs in gaseous phase are taken as the reference state. A1, A3 and A4 are the adsorption of methanol; A2 is the additional adsorption of polyMN.

Figure 7. Optimized transition states of various reaction steps in the DMN pathway for MTO in the supercages of H-MCM-22 zeolite. (a), (c), and (e) are for the methylation steps of M1, M2 and M3, respectively; (b) and (d) are for the deprotonation steps of D1 and D2, respectively; (f) is for the elimination step of E1. Atom colorings: (Si), red (O), white (H), pink (Al) and blue (C). The unit of bonding distances is Å.

Figure 8. Isosurface plots of the reduced density gradient (s = 0.500 a.u.) for the transition states of the olefin elimination reactions in the polyMN cycle for MTO in the supercages of H-MCM-22 zeolite: (a) DMN pathway; (b) TMN pathway; and (c) TeMN pathway. The isosurfaces of the reduced density gradient are colored according to the values of the quantity sign (λ2)ρ with the indicated RGB scale. VDW represents the van der Waals interactions.

Figure 9. A comparison of calculated apparent free energy barriers (∆Gapp), apparent enthalpy barriers (∆Happ) and apparent entropy losses (−T∆Sapp) of the gem-methylation step (M1) as well as the overall free energy height (∆GH) and overall reaction free energy (∆GR) among various polyMB and polyMN cycles at 673 K for MTO in the supercages of H-MCM-22 zeolite.

48


Page 48 of 64

Page 49 of 64

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


Figure 10. Free energy profiles for the formation of anthracene and phenanthrene at 673 K for MTO in the supercages of H-MCM-22 zeolite. Besides the zeolite framework and naphthalene, the adsorbed butene (A1) is taken as the reference state for the formation of anthracene and phenanthrene. A2, A3 and A4 represent the formation of methoxyl at the active site before the hydride transfer reactions.

Figure 11. Free energy barriers of the hydride transfer reactions for the formation of benzene, naphthalene and anthraceneas a function of the ammonium adsorption energy for MTO over the Al, Ga and B substituted H-MCM-22 zeolites in the supercages. (a) HT1 for benzene formation: y = 0.78x + 277.6, R2 = 0.94; (b) HT1 for naphthalene formation: y = 0.87x + 280.2, R2 = 0.95; (c) HT2 for anthracene formation: y = 0.91x + 331.7, R2 = 0.99.

Scheme 1. Reaction network for the formation of benzenic and naphthalenic species during MTO in the supercages of H-MCM-22 zeolite.

Scheme 2. Detailed polyMB (represented by HMB) cycle and polyMN (represented by DMN) cycle for MTO in the supercages of H-MCM-22 zeolite.

Scheme 3. Reaction network for the formation of anthracene and phenanthrene during MTO in the supercages of H-MCM-22 zeolite.

49



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 64

Figure 1.

T4 O3

(a)

(b)

Figure 1. (a) Framework of H-MCM-22 zeolite with three types of pores: supercages, pockets, and sinusoidal channels; (b) 54T cluster model used to represent the supercages of H-MCM-22 zeolite, where a silicon atom is substituted with an aluminum atom at T4 site and the charge-balancing proton is bonded with O3. Atom coloring: yellow (Si), red (O), white (H) and pink (Al).

50


Page 51 of 64

Figure 2.

Benzene Formation Naphthalene Formation HT1

300 -1

Free Energies (kJ mol )

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


250

HT3 HT2

200

C1

O1

150

P1 D1

100 50 0

A2

A1

A3

A4

Reaction Coordinate

Figure 2. Free energy profiles for the formation of benzene and naphthalene at 673 K during MTO in the supercages of H-MCM-22 zeolite. Besides the zeolite framework, the adsorbed propene and benzene + butene (A1) are taken as the reference states for the formation of benzene and naphthalene, respectively. A2, A3 and A4 represent the formation of methoxyl at the active site before the hydride transfer reactions.

51



Figure 3.

M2

250

M1

-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 52 of 64

200

M3 D1

150 100 50

A3 A2 A1

A4

0 -50

E1

D2

PX TeMB

TMB HMB

-100 Reaction Coordinate

Figure 3. Free energy profiles of the polyMB cycle at 673 K for MTO in the supercages of H-MCM-22 zeolite. The framework of supercages as well as methanol and different polyMBs in gaseous phase are taken as the reference state. A1, A3 and A4 are the adsorption of methanol; A2 is the additional adsorption of polyMB.

52


Page 53 of 64

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


Figure 4.

(a) M1

(b) D1

(c) M2

(d) D2

(e) M3

(f) E1

Figure 4. Optimized transition states of various reaction steps in the HMB pathway for MTO in the supercages of H-MCM-22 zeolite. (a), (c) and (e) are for the methylation steps of M1, M2 and M3, respectively; (b) and (d) are for the deprotonation steps of D1 and D2, respectively; (f) is for the elimination step of E1. Atom coloring: yellow (Si), red (O), white (H), pink (Al) and blue (C). The unit of bonding distances is Å.

53



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

Figure 5.

(a)

(b)

(c)

(d)

Figure 5. Isosurface plots of the reduced density gradient (s = 0.500 a.u.) for the transition states of the olefin elimination reactions in the polyMB cycle for MTO in the supercages of H-MCM-22 zeolite: (a) PX pathway; (b) TMB pathway; (c) TeMB pathway; and (d) HMB pathway. The isosurfaces of the reduced density gradient are colored according to the values of the quantity sign (λ2)ρ with the indicated RGB scale. VDW represents the van der Waals interactions.

54


Page 54 of 64

Page 55 of 64

Figure 6.

M2

200 -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


M1

M3

150 D1

100 50

E1

D2 A1

A3

0 -50 -100

A2

DMN TMN TeMN

A4


Figure 6. Free energy profiles of the polyMN cycle at 673 K for MTO in the supercages of H-MCM-22 zeolite. The framework of supercages as well as the methanol and different polyMNs in gaseous phase are taken as the reference state. A1, A3 and A4 are the adsorption of methanol; A2 is the additional adsorption of polyMN.

55



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 56 of 64

Figure 7.

(a) M1

(b) D1

(c) M2

(d) D2

(e) M3

(f) E1

Figure 7. Optimized transition states of various reaction steps in the DMN pathway for MTO in the supercages of H-MCM-22 zeolite. (a), (c), and (e) are for the methylation steps of M1, M2 and M3, respectively; (b) and (d) are for the deprotonation steps of D1 and D2, respectively; (f) is for the elimination step of E1. Atom colorings: (Si), red (O), white (H), pink (Al) and blue (C). The unit of bonding distances is Å.

56


Page 57 of 64

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


Figure 8.

(a)

(b)

(c)

Figure 8. Isosurface plots of the reduced density gradient (s = 0.500 a.u.) for the transition states of the olefin elimination reactions in the polyMN cycle for MTO in the supercages of H-MCM-22 zeolite: (a) DMN pathway; (b) TMN pathway; and (c) TeMN pathway. The isosurfaces of the reduced density gradient are colored according to the values of the quantity sign (λ2)ρ with the indicated RGB scale. VDW represents the van der Waals interactions.

57



Figure 9.

∆Happ ∆GH

-T∆Sapp ∆GR

40 20 0

200

-20 -40

150

-60

100

-80 -100

50 0

-120 PX TMB TeMB HMB DMN TMN TeMN

-1

-1

250

∆Gapp

Reaction Energies (kJ mol )

300 Energy Barriers (kJ mol )

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 58 of 64

-140

Figure 9. A comparison of calculated apparent free energy barriers (∆Gapp), apparent enthalpy barriers (∆Happ) and apparent entropy losses (−T∆Sapp) of the gem-methylation step (M1) as well as the overall free energy height (∆GH) and overall reaction free energy (∆GR) among various polyMB and polyMN cycles at 673 K for MTO in the supercages of H-MCM-22 zeolite.

58


Page 59 of 64

Figure 10.

300

Anthracene Formation Phenanthrene Formation

250

HT2

HT1

-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


HT3

200 150

C1

O1

100 50 0

A4

A2 A3

A1


Figure 10. Free energy profiles for the formation of anthracene and phenanthrene at 673 K for MTO in the supercages of H-MCM-22 zeolite. Besides the zeolite framework and naphthalene, the adsorbed butene (A1) is taken as the reference state for the formation of anthracene and phenanthrene. A2, A3 and A4 represent the formation of methoxyl at the active site before the hydride transfer reactions.

59



Figure 11.

-1

Free Energy Barriers (kJ mol )

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 60 of 64

260

Al Ga

B cene nthra A , T2 (c ) H

240 220 200

( a) H

180 160

T1

ation Form e n e z , Ben

T1, N (b) H

-130

-120

ation F orm

ion rmat o F e alen aphth

-110

-100

-90

-80 -1

Ammonium Adsroption Energy (kJ mol )

Figure 11. Free energy barriers of the hydride transfer reactions for the formation of benzene, naphthalene and anthraceneas a function of the ammonium adsorption energy for MTO over the Al, Ga and B substituted H-MCM-22 zeolites in the supercages. (a) HT1 for benzene formation: y = 0.78x + 277.6, R2 = 0.94; (b) HT1 for naphthalene formation: y = 0.87x + 280.2, R2 = 0.95; (c) HT2 for anthracene formation: y = 0.91x + 331.7, R2 = 0.99.

60


Page 61 of 64

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


Scheme 1.

Scheme 1. Reaction network for the formation of benzenic and naphthalenic species during MTO in the supercages of H-MCM-22 zeolite.

61



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

Scheme 2.

Scheme 2. Detailed polyMB (represented by HMB) cycle and polyMN (represented by DMN) cycle for MTO in the supercages of H-MCM-22 zeolite.

62


Page 62 of 64

Page 63 of 64

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


Scheme 3.

Scheme 3. Reaction network for the formation of anthracene and phenanthrene during MTO in the supercages of H-MCM-22 zeolite.

63



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 64 of 64

Table of Contents Graphic

Active MTO HCP Species (CH3)n

MTO HCP Species

64


(CH3)n

Evolution of Aromatic Species in Supercages and Its Effect on the

Recommend Documents