Theoretical Insights into the Mechanism of Olefin Elimination in the

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Theoretical Insights into the Mechanism of Olefin Elimination in the Methanol-to-Olefin Process over HZSM-5, HMOR, HBEA, and HMCM22 Zeolites Sen Wang,†,‡ Yanyan Chen,† Zhihong Wei,† Zhangfeng Qin,*,† Jialing Chen,†,‡ Hong Ma,†,‡ Mei Dong,† Junfen Li,† Weibin Fan,† and Jianguo Wang*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China ‡ University of Chinese Academy of Sciences, Beijing 100049, PR China ABSTRACT: The mechanism of olefin elimination in the process of methanol-to-olefins (MTO) over a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by DFT-D calculations, which is a crucial step that controls the MTO product distribution. The results demonstrate that the manners of olefin elimination are related to the pore structure of zeolite catalyst and the interaction between proton transfer reagent (water or methanol) and zeolite acidic framework. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with large pores, as suggested by the energy barrier of rate-determining step and the potential energy surface (PES), but is unfavorable over HZSM-5 with medium-sized pores due to the steric hindrance of spiro intermediates. Over various zeolites, water and methanol perform differently in proton transfer to form the spiro intermediates; over HMOR and HBEA with strong acidity, water is superior to methanol in promoting propene elimination, whereas over HMCM-22 with relatively weaker acidity, methanol is more favorable as a proton transfer reagent.

1. INTRODUCTION As methanol can be expediently produced via syngas from multifarious carbon sources such as coal, natural gas, and biomass,1−5 the conversion of methanol-to-olefins (MTO) over acidic zeolite catalysts has been turning into an increasingly important alternative to naphtha cracking to get light olefins.6−8 A great deal of effort has been devoted in the past decades to elucidate the reaction mechanism of MTO with both experimental and theoretical approaches.9−12 Among them, the hydrocarbon pool mechanism proposed by Dahl and Kolboe has received wide recognition,6−8 which assumed that organic molecules, i.e., the hydrocarbon pool species, trapped in the zeolite pores interplayed with the inorganic framework and served as a cocatalyst; the olefin products were eliminated from the hydrocarbon pool. Moreover, polymethylbenzenes (polyMBs) were considered as the most important active hydrocarbon pool species.13−15 Via the polyMBs-based hydrocarbon pool mechanism, two routes have been proposed for the formation of light olefins, i.e., the side-chain route16−19 and paring route.20,21 By directly pulsing different polyMBs and 13C-methanol onto the large pore HBEA zeolite, Sassi and co-workers found that the sidechain scheme was the predominant route to produce olefins.18 Wang and co-workers also confirmed that the side-chain route for MTO reactions was preferable to the paring one by comparing the kinetics of both routes.22 Through the side-chain © XXXX American Chemical Society

route, olefins were formed mainly through the continuous growth of alkyl side chains of polyMBs by methylation and subsequent dealkylation or olefin elimination.18,19 Extensive experimental and theoretical approaches have been made to investigate the methylation of polyMBs.23−26 Although the elimination of olefins is a crucial step in MTO, the detailed scheme of olefin elimination is still rather elusive and there is a lot of controversy over the reaction schemes.27−29 For example, Wang and co-workers investigated the complete cycle of sidechain hydrocarbon pool mechanism over HSAPO-34 zeolite and demonstrated that the indirect spiro route was energetically more favorable than the direct one, especially for the formation of propene.27 In contrast, Lesthaeghe and co-workers found that both the direct and the indirect spiro mechanisms had the same reaction barriers for the elimination of light olefins over HZSM-5 zeolite.28 Moreover, the relationship between the characteristic of olefin elimination and the zeolite topological structures, which may play an essential role in determining the distribution of olefin products, is still unclear and needs to be considered in detail. Special Issue: International Conference on Theoretical and High Performance Computational Chemistry Symposium Received: January 16, 2014 Revised: February 20, 2014

A

dx.doi.org/10.1021/jp5005055 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Cluster models used to represent the zeolite catalysts: (a) 33T HZSM-5; (b) 38T HMOR; (c) 53T HBEA; (d) 54T HMCM-22.

2. COMPUTATIONAL MODELING AND METHODS

Meanwhile, the presence of other components in the reaction atmosphere also has a significant effect on the olefin elimination. Wang and co-workers observed that water could act as a bridging species over HSAPO-34 zeolite, which was able to facilitate the proton shift between the organic species and the zeolite framework and then promoted the olefin elimination.27 However, other results indicated that the addition of water in the methanol feed for MTO over HZSM-5 zeolite was insignificant in improving the selectivity to light olefins.30 Moreover, Wispelaere and co-workers illustrated that methanol itself might also act as a bridge between the hydrocarbon pool species and the acid sites of the zeolite.31 Therefore, the effect of water and methanol as the proton transfer reagent on the olefin elimination in different zeolites is another important factor worth a thorough consideration. In this work, the mechanism of olefin elimination in a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by the density functional theory calculations with dispersive interactions (DFT-D) employing large cluster models to represent the framework of zeolites. The relationship between the characteristic of olefin elimination and the topological structure of zeolites as well as the effect of water and methanol as the proton transfer reagent on the olefin elimination were thoroughly considered.

33T HZSM-5, 38T HMOR, 53T HBEA, and 54T HMCM-22 cluster models were taken from their lattice structures.32−35 The model of HZSM-5 covers the intersection cavity between the straight channel (5.3 × 5.6 Å) and the zigzag channel (5.1 × 5.5 Å), as shown in Figure 1a; a silicon atom was substituted with an aluminum atom at T12 and a charge-balancing proton was produced at O24.36 For HMOR, the 38T cluster model shown in Figure 1b contains a 12-membered ring channel (6.5 × 7.0 Å) interconnected by 8-membered side pockets (2.6 × 5.7 Å); the aluminum atom prefers the T4 site when replacing the silicon atom and the charge-balancing proton favors bonding with O10 in the MOR framework.37 The 53T cluster model of HBEA consists of large intersected straight channels (5.5 × 6.7 Å); the aluminum defect is located at the T9 position, and the charge-balancing proton is attached to O41 (Figure 1c).38 The 54T HMCM-22 cluster model involves the supercage (7.1 × 7.1 × 18.2 Å), as depicted in Figure 1d; the substituted aluminum atom is located at the T4 site and the charge-balancing proton is at the O3 site.39 The terminal hydrogen atoms of the cluster model were utilized to saturate the peripheral silicon atom. The distances between the hydrogen atoms and the corresponding silicon atoms are 1.47 Å and the Si−H bonds are present along the pre-existing Si−O bonds. B



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spiro intermediate and the breaking of C−C bond to form light olefins. Recently, Wispelaere and co-workers proposed a methyl-shift mechanism over HSAPO-34 zeolite; the methyl shifts toward the ring carbon whereto the isopropyl side chain was concerted, which led to an sp3 hybridized ring carbon atom and simultaneously, the breaking of C−C bond and deprotonation of the terminal methyl group of the isopropyl side chain.31 Although the corresponding free energy barrier of the ratedetermining step in the methyl-shift mechanism over HSAPO34 zeolite is about 97.0 kJ/mol, the transition state for the water-assisted propene elimination step needs very large space, i.e., nearly 7 Å in length. As a result, this mechanism is seriously restricted in most zeolites because of the space limitation. Therefore, in this work, we focused mainly on the spiro mechanism in the indirect routes that require moderate space; the calculations were also limited to the formation of propene as the representative of light olefins, as propene is more expected as the product of MTO than other olefins. 3.1. Direct Mechanism. HZSM-5. The optimized structures of the transition state for propene elimination over HZSM-5 by the direct mechanism are shown in Figure 3a; the selected geometrical parameters are summarized in Table 1. For the proton transfer, the distances C1−H1 and C2−H1 are 1.16 and 1.66 Å, respectively, whereas for the C−C bond breaking, the C1−C2 and C1−C3 bond distances are 1.78 and 1.70 Å, respectively. The related energy diagram is illustrated in Figure 4; the calculated activation energy is 170.57 kJ/mol. HMOR. The optimized structures of the transition state for propene elimination over HMOR by the direct mechanism are shown in Figure 3b, and the related geometrical parameters are also listed in Table 1. The activation energy is 159.81 kJ/mol (Figure 4). HBEA. The optimized structures of the transition state for propene elimination over HBEA by the direct mechanism are shown in Figure 3c. The lengths of C1−H1 and C2−H1 are 1.15 and 1.62 Å, respectively (Table 1). Two key distances of C1−C2 and C1−C3 for C−C bond breaking are 1.82 and 1.64 Å, respectively. This step needs to overcome an energy barrier of 183.76 kJ/mol (Figure 4). HMCM-22. The optimized structures of the transition state for propene elimination over HMCM-22 by the direct mechanism are shown in Figure 3d and Table 1. The activation energy is 165.66 kJ/mol (Figure 4). The above results illustrate that by the direct mechanism, the propene eliminations over various zeolites (HZSM-5, HMOR, HBEA, and HMCM-22) are quite close to each other in their activation energies (170.57, 159.81, 183.76, and 165.66 kJ/mol, respectively) and the structure of transition states, suggesting that the effect of zeolite topological structure on the olefin elimination by the direct mechanism is insignificant, as the whole reaction process involving only internal H-shift, which does not require large interspace by the direct mechanism. 3.2. Indirect Spiro Mechanism with Methanol as the Proton Transfer Reagent. According to the concept of the indirect spiro mechanism, the proton transfer reagent such as water or methanol plays an important role in the proton shift between the reactant species and the zeolite framework. In this section, methanol is chosen as the proton transfer reagent. HZSM-5. The optimized geometries of the transition state for the formation of the spiro intermediate over HZSM-5 are presented in Figure 5(a1) and the selected geometrical parameters are summarized in Table 2. The C2−H1 bond of

All the density functional theory (DFT) calculations were performed with the Gaussian 09 package.40 The standard B3LYP functional and the 6-31G (d) basis set are used in all geometry optimizations and frequency calculations. Only the 5T active region of “SiOHAl(OSi)2OSi” and the reacting molecule are allowed to relax while the rest of the structure is kept fixed at the crystallographic coordinates during geometry optimizations. Transition states were confirmed by the factor that they have only one imaginary frequency. Furthermore, the quasi-internal reaction coordinate (quasi-IRC) approach was used to verify that the transition states were connected with the reactant and product. To obtain high accurate interaction energies, single-point calculations were refined by ωB97XD functional including dispersion interactions, which was a promising method for main group thermochemistry, kinetics, and noncovalent interactions;41,42 the 6-311+G(2df,2p) basis set was then carried out. The activation energy is defined as the energy difference between the reactant and transition state (E(activation barrier) = E(TS) − E(reactant)) in each reaction step and the potential energy surface (PES) height is the energy difference between the starting point (reactant species) and the highest point on the PES; both of them can be used for comparing the feasibility of different reaction routes in the MTO process.43 The proton affinity (PA) was obtained as the energy difference between the protonated zeolite and the deprotonated one, i.e., PA = E(Z−) − E(HZ).

3. RESULTS AND DISCUSSION Olefin elimination may proceed through the direct mechanism or indirect mechanism including spiro and methyl-shift routes, as depicted in Figure 2. In the direct mechanism, one ending

Figure 2. Suggested reaction schemes for propene elimination through (a) the direct mechanism, (b) the indirect spiro mechanism, and (c) the methyl shift mechanism.

hydrogen atom of the side-chain alkyl group has to be shifted directly to the ring carbon and meanwhile the C−C bond breaks between ring carbon and side-chain alkyl carbon; this is a one-step route and as an internal H-shift process, it does not need the assistance of proton transfer reagent and vast space. In the indirect spiro mechanism, the ending hydrogen atom is first taken off by active oxygen atom of the proton transfer reagent to form a spiro intermediate, which is subsequently attacked by a proton from the proton transfer reagent; after that, the C−C bond between ring carbon and side-chain alkyl carbon is then broken to eliminate olefin. The olefin elimination by the spiro mechanism thus consists of two steps, i.e., the formation of C



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Figure 3. Optimized structure of transition states for propene elimination by the direct mechanism over (a) HZSM-5, (b) HMOR, (c) HBEA, and (d) HMCM-22 zeolites. The unit of bonding distances is Å.

Table 1. Optimized Structure Parameters of the Transition State for Propene Elimination over Various Zeolites by the Direct Mechanism bond distance (Å) zeolite

C1−H1

C2−H1

C1−C2

C1−C3

HZSM-5 HMOR HBEA HMCM-22

1.16 1.16 1.15 1.17

1.66 1.63 1.62 1.55

1.78 1.81 1.82 1.82

1.70 1.65 1.64 1.62

the methyl group is elongated to 1.42 Å, whereas the C1−C2 interatomic distance between the isopropyl and ring carbons is 1.71 Å. The distance between active oxygen of methanol and hydrogen of the methyl group (O1−H1) is 1.19 Å. The transition state of C−C bond breaking is shown in Figure 5(a2), and the selected geometrical parameters for this transition state are given in Table 3. Two key C−C distances for C1−C2 and C1−C3 are lengthened to 1.64 and 1.66 Å, respectively. The proton of the methanol will attack the ring carbon; the bond angle of proton transfer (C1−H1−O1) is about 156°, and the distance of C1−H1 is 1.26 Å. By following the indirect spiro mechanism, the elimination of propene requires more space than that by the direct mechanism on the basis of the structure of the transition states. The calculation results indicate that the rate-determining step for the indirect spiro mechanism is the formation of the spiro

Figure 4. Activation energies (Ea) of the rate-determining step and the potential energy surface heights (Eh) of propene elimination by the direct mechanism (DM) and indirect spiro mechanism (SM) over HZSM-5, HMOR, HBEA, and HMCM-22 zeolites.

intermediate with activation energy of 135.52 kJ/mol and the potential energy surface (PES) height is 212.99 kJ/mol (Figure 4). For the elimination of propene over HZSM-5, the activation energy of the rate-determining step via the indirect spiro mechanism is 35.00 kJ/mol lower than that via the direct mechanism. However, the potential energy surface height of the indirect spiro mechanism is about 43.00 kJ/mol higher than that of the direct mechanism (Figure 4). These may suggest that the indirect spiro mechanism energetically does not have a D



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Figure 5. Transition states for the propene elimination by the indirect spiro mechanism over (a) HZSM-5, (b) HMOR, (c) HBEA, and (d) HMCM22 zeolites (methanol as proton transfer reagent): (1) TS1, the step of spiro intermediate formation, and (2) TS2, the subsequent step of C−C bond breaking. The unit of the bonding distances is Å.

HMOR. For the elimination of propene over HMOR, the optimized geometrical parameters of the transition states are

distinct advantage over the direct mechanism for the propene elimination over HZSM-5. E



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intermediate, has an activation energy of 98.62 kJ/mol and potential energy surface height of 137.64 kJ/mol (Figure 4). Due to the large supercage in HMCM-22, the influence of the transition state shape selectivity on the propene elimination by the indirect spiro mechanism is negligible. The activation energy of the rate-determining step and the potential energy surface height for propene elimination over HMCM-22 by the indirect spiro mechanism are 67.04 and 28.02 kJ/mol lower than those by the direct mechanism, respectively. The above results suggest that the actual reaction routes for the olefin elimination are strongly related to the pore structure of zeolite catalysts. Moreover, the confinement of zeolite pore has a greater effect on the indirect spiro mechanism than that on the direct mechanism. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with larger pores, whereas it is not favorable over HZSM-5 with medium-sized pores due to the serious steric hindrance of the spiro intermediates. 3.3. Indirect Spiro Mechanism with Water as the Proton Transfer Reagent. It was reported that the addition of water in the methanol feed could improve the selectivity to light olefins for MTO over HMOR and HSAPO-34 zeolites44−46 and propene could be effectively eliminated through the indirect spiro mechanism when water was served as the proton transfer reagent.27 To consider the effect of water in the feed on the olefin elimination in MTO, the indirect spiro mechanism with water as the proton transfer regent over HZSM-5, HMOR, HBEA, and HMCM-22 zeolites is then considered in detail and compared with that using methanol as the proton transfer regent. HZSM-5. With water as the proton transfer regent, as shown in Figure 6(a1,a2) and Tables 4 and 5, the structures of two transition states for the spiro intermediate formation (TS1) and the C−C bond breaking (TS2) in the indirect spiro mechanism over HZSM-5 are similar to those with methanol as the proton transfer reagent. The rate-determining step is also the formation of the spiro intermediate, with the activation energy of 138.02 kJ/mol and the potential energy surface height of 212.86 kJ/mol (Figure 7), which are also very close to those with methanol as the proton transfer reagent. These suggest that water and methanol are similar in their effects as a proton transfer reagent on the propene elimination over HZSM-5 by the indirect spiro mechanism; i.e., the presence or addition of water in the methanol feed may have a trivial effect on the selectivity to propene for MTO over HZSM-5. Such an observation agrees well with the experimental results of Dehertog and Froment.30 HMOR. With water as the proton transfer reagent on HMOR, the structure of the transition states for the propene elimination shown in Figure 6(b1,b2) and Tables 4 and 5 indicate that the O1−H1 and O2−H2 bond distances in the spiro intermediate formation are about 0.02 and 0.04 Å shorter than those with methanol as the proton transfer reagent, respectively. The activation energy of the spiro intermediate formation as the rate-determining step and the potential energy surface height are reduced by 35.00 and 62.00 kJ/mol, respectively (Figure 7). Such a result suggests that for the propene elimination over HMOR, in conflict with the result observed over HZSM-5, water as the proton transfer reagent is superior to methanol, which can obviously promote the formation of propene in HMOR zeolite. This is consistent with the experimental observation that the addition of water to the feed with

Table 2. Optimized Structure Parameters of the Transition State TS1 of the Spiro Intermediate Formation for Propene Elimination by the Indirect Spiro Mechanism over Various Zeolitesa Bond distance in TS1 (Å)

a

zeolite

C2−H1

C1−C2

O1−H1

O2−H2


1.42 1.41 1.44 1.44

1.71 1.79 1.89 1.83

1.19 1.21 1.19 1.18

1.62 1.69 1.55 1.59

Methanol is regarded as the proton transfer reagent.

Table 3. Optimized Structure Parameters of the Transition State TS2 of C−C Bond Breaking for Propene Elimination by the Indirect Spiro Mechanism over Various Zeolitesa bond distance in TS2 (Å)

a

bond angle in TS2 (deg)

zeolite

C1−C2

C1−C3

C1−H1

C2−H1−O1


1.64 1.69 1.72 1.65

1.66 1.66 1.78 1.93

1.26 1.24 1.19 1.25

156.0 169.8 164.4 173.3

Methanol is regarded as the proton transfer reagent.

shown in Figure 5(b1, b2), and Tables 2 and 3. The ratedetermining step by the indirect spiro mechanism is the formation of spiro intermediate, with the activation energy of 129.20 kJ/mol and the potential energy surface height of 171.75 kJ/mol (Figure 4). The activation energy for the ratedetermining step by the indirect spiro mechanism is 35.00 kJ/ mol lower than that by the direct mechanism, whereas the former has only a slightly higher potential energy surface height (171.75 vs 159.81 kJ/mol). As a result, the indirect spiro mechanism is energetically more favorable than the direct mechanism for the propene elimination over HMOR. This is in opposition to the result obtained over HZSM-5, as HMOR is provided with larger interspace available for the formation of the spiro intermediate than HZSM-5, which may significantly reduce the restriction of the transition state shape selectivity in HMOR via the indirect spiro mechanism. HBEA. Figure 5(c1, c2) and Tables 2 and 3 illustrate the optimized geometrical parameters of the transition states for the propene elimination over HBEA by the indirect spiro mechanism. The rate-determining step is also the formation of spiro intermediate, with the activation energy of 103.40 kJ/mol and the potential energy surface height of 106.68 kJ/mol (Figure 4). Owing to the large pores in HBEA, the transition state shape selectivity exhibits little confinement effect on the propene elimination by the indirect spiro mechanism. Compared with the direct mechanism, the indirect spiro mechanism is energetically much more favorable for propene elimination over HBEA; the activation energy of the rate-determining step and the potential energy surface height by the indirect spiro mechanism are 80.36 and 77.08 kJ/mol lower than those by the direct mechanism, respectively. HMCM-22. For the propene elimination over HMCM-22 by the indirect spiro mechanism, the optimized geometrical parameters are shown in Tables 2 and 3 and Figure 5(d1, d2). The rate-determining step, i.e., the formation of the spiro F



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Figure 6. Transition states for the propene elimination by the indirect spiro mechanism over (a) HZSM-5, (b) HMOR, (c) HBEA, and (d) HMCM22 zeolites (water as proton transfer reagent): (1) TS1, the step of spiro intermediate formation, and (2) TS2, subsequent step of C−C bond breaking. The unit of the bonding distances is Å.

HBEA. For the propene elimination over HBEA with water as the proton transfer reagent, the optimized geometrical

methanol can help to enhance the selectivity to propene over the HMOR zeolite.44 G



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illustrates that in this case water is inferior to methanol as the proton transfer reagent; adding water to the feed is of no use to improve the selectivity to propene for MTO over HMCM-22. The above results suggest that the effect of water as the proton transfer reagent on the propene elimination by the indirect spiro mechanism is strongly related to the special zeolite used as the catalyst. As currently four zeolites all have enough large space for water and methanol molecules, the proton transfer ability or the activity in propene elimination over each zeolite should be determined by the interaction between the proton transfer reagent and the zeolite acidic framework, i.e., the acidic properties of the zeolites and the proton transfer reagents, rather than the zeolite pore confinement. As listed in Table 6, the acid strength, related to the proton affinity (PA) of the zeolite active sites and the O−H bond

Table 4. Optimized Structure Parameters of the Transition State TS1 of Spiro Intermediate Formation for Propene Elimination by the Indirect Spiro Mechanism over Various Zeolitesa bond distance in TS1 (Å)

a

zeolite

C2−H1

C1−C2

O1−H1

O2−H2


1.44 1.46 1.40 1.44

1.72 1.79 1.84 1.84

1.19 1.19 1.22 1.19

1.56 1.65 1.57 1.72

Water is taken as the proton transfer reagent.

Table 5. Optimized Structure Parameters of the Transition State TS2 of C−C Bond Breaking for Propene Elimination by the Indirect Spiro Mechanism over Various Zeolitesa bond distance in TS2 (Å)

a

bond angle in TS2 (deg)

zeolite

C1−C2

C1−C3

C1−H1

C2−H1−O1


1.63 1.65 1.67 1.66

1.69 1.77 1.92 1.87

1.26 1.29 1.23 1.24

164.2 176.7 171.3 171.7

Table 6. Calculated Proton Affinity (PA) and O−H Bond Length (rO−H) of the Active Sites in HZSM-5, HMOR, HBEA, and HMCM-22 Zeolites as Well as the Differences between Water and Methanol as the Proton Transfer Reagents in the Activation Energy (ΔEa) for the Formation of Spiro Intermediate by the Indirect Spiro Mechanism

Water is taken as the proton transfer reagent.

zeolite

PA (kJ/mol)

rO−H (Å)

ΔEa (kJ/mol)


1190.32 1184.05 1188.82 1215.98

0.973 0.978 0.975 0.973

2.50 −56.51 −16.01 39.75

length (rO−H) in these sites, reduces in the order HMOR > HBEA > HZSM-5 > HMCM-22, which is in line with the experimental observations.47,48 Meanwhile, for the propene elimination with water and methanol used as the proton transfer reagents, the difference in the activation energy difference (ΔEa) of the rate-determining step is almost proportionally increased with the proton affinity (related to the acidity) of the zeolite catalyst. This means that water is more conducive to promote the olefin elimination than methanol over the zeolites with stronger acidity. Among the four zeolites considered in this work, HMCM-22 zeolite shows the weakest acidity and the strongest conjugated basicity, indicating that the proton of water or methanol can be easily carried off by the active oxygen atom of zeolite. Meanwhile, the oxygen atom of methanol has stronger nucleophilicity than the oxygen atom of water; thus the ending hydrogen atom of the side-chain alkyl group can be more easily shifted to the oxygen atom of methanol to promote the spiro intermediate formation. As a result, over HMCM-22 zeolite, methanol is superior to water as the proton transfer reagent for the propene elimination. HZSM-5 is provided with stronger acidity than HMCM-22; the ending hydrogen atom of the side-chain alkyl group can also be more easily shifted to the oxygen atom of methanol than to that of water. However, the ability to capture proton of zeolite active sites is reduced gradually with the increase of the zeolite acidity. As the O−H bond of water is less stable than the O−H bond of methanol, the proton shift from water to the active oxygen atom of zeolite will be much easier. Taking into consideration these two aspects, water and methanol as the proton transfer reagents give similar activation energies for the rate-determining step in propene elimination over HZSM-5.

Figure 7. Activation energies (Ea) of the rate-determining step and the potential energy surface heights (Eh) of propene elimination by the indirect spiro mechanism (SM) over various zeolites with methanol and water as the proton transfer reagents.

parameters shown in Figure 6(c1,c2) and Tables 4 and 5 illustrate that the O1−H1 bond distance is about 0.03 Å longer, whereas the distance of C1−C2 is 0.05 Å shorter than those with methanol as the proton transfer reagent in the spiro intermediate formation. Meanwhile, the activation energy of the spiro intermediate formation as the rate-determining step and the potential energy surface height are 16.01 and 19.29 kJ/mol lower than those with methanol as the proton transfer reagent, respectively (Figure 7); these suggest that over HBEA, water as the proton transfer reagent is also superior to methanol, as it can slightly reduce the energy barrier for propene elimination. HMCM-22. As shown in Figure 6(d1,d2) and Tables 4 and 5, for the propene elimination over HMCM-22 with water as the proton transfer reagent, both the distances of C1−C2 and O1− H1 are lengthened by 0.01 Å, compared with that with methanol as the proton transfer reagent in the spiro intermediate formation. The activation energy of the ratedetermining step and the potential energy surface height are 43.0 and 35.0 kJ/mol higher than those with methanol as the proton transfer reagent, respectively (Figure 7). Such a result H



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21273263, 21103216), and Natural Science Foundation of Shanxi Province of China (2012011005-2 and 2013021007-3).

Over HMOR and HBEA zeolites with stronger acidity, the ability of zeolite active sites to capture a proton is further reduced. As a result, water plays a better promoting role than methanol in the indirect spiro mechanism for the olefin elimination over HMOR and HBEA zeolites, because water can make the proton shift to the active oxygen atom of zeolties easier than methanol as the proton transfer reagent. All these results suggest that the determination of appropriate proton transfer reagent is related to acidity of zeolite catalyst over which propene can be effectively eliminated through the indirect spiro mechanism. Water as the proton transfer reagent is able to promote the propene elimination over zeolites like HMOR and HBEA with strong acidity but may be helpless over the zeolites like HMCM-22 with relatively weaker acidity. Figures 4 and 7 illustrate that the indirect spiro mechanism is a low energy and efficient route for the propene elimination over most zeolites; it is energetically more favorable than the direct mechanism on the basis of the lower energy barrier of the rate-determining step and the lower potential energy surface height.

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4. CONCLUSIONS The mechanism of olefin elimination in MTO over a series of zeolites like HZSM-5, HMOR, HBEA, and HMCM-22 was investigated by DFT-D calculations. Two mechanisms (direct and indirect spiro ones) for olefin elimination were compared and the effect of the proton transfer reagent (water and methanol) on olefin elimination was considered. The results demonstrate that the manners of olefin elimination are related to the pore structure of the zeolite catalyst. The indirect spiro mechanism is preferable to the direct mechanism over HMOR, HBEA, and HMCM-22 zeolites with larger pores, on the basis of the energy barrier of the ratedetermining step and the potential energy surface height, whereas it is not favorable over HZSM-5 with medium-sized pores due to the steric hindrance of spiro intermediates. Over various zeolites, water and methanol also perform differently in proton transfer to form the spiro intermediates, which is related to the interaction between the proton transfer reagent and the zeolite acidic framework, i.e., the acidic properties of the zeolites and the proton transfer reagents. Water as the proton transfer reagent is able to promote the propene elimination over zeolites like HMOR and HBEA with strong acidity but may be helpless over the zeolites like HMCM-22 with relatively weaker acidity.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The calculations are performed on the Computer Network Information Center of Chinese Academy of Sciences, Shenzhen Cloud Computing Center and Shanghai Supercomputer Center. The authors are grateful for the financial supports of National Basic Research Program of China (2011CB201400), the National Natural Science Foundation of China (21273264, I



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