Article pubs.acs.org/JPCC
Confinement Effect of Zeolite Cavities on Methanol-to-Olefin Conversion: A Density Functional Theory Study Xu Li, Qiming Sun, Yi Li,* Ning Wang, Junran Lu, and Jihong Yu* State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, People’s Republic of China
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S Supporting Information *
ABSTRACT: The confinement effect of zeolite cavities on the methanol-toolefin (MTO) conversion is investigated through density functional theory calculations. According to the side-chain mechanism, we select several hydrocarbon pool (HP) intermediates that may exist during the MTO conversion process and optimize their structures within the cluster models of zeolite cavities cha, lev, and lta, respectively. The transition states during methylation, deprotonation, methyl shift, and olefin production are also located within these cavities. According to our results, all of the HP intermediates are stabilized in zeolite cavities, especially in cha and lta. Moreover, the cha cavity displays the lowest intrinsic free-energy barriers for all of the methylation and olefin-production steps, indicating its high MTO catalytic activity. We find that the differences in reaction barriers and reaction energies are highly related to the different confinement effects of zeolite cavities. In comparison with lev and lta, the cha cavity matches the dimensions of HP species very well, so it is able to provide the most suitable confinement to HP species. Our discovery will provide further understanding of the side-chain mechanism, which is important for finding new catalysts for MTO conversion.
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INTRODUCTION Zeolites are the most important solid catalysts in chemical industry because of their unique shape selectivity.1−4 Methanolto-olefin (MTO) conversion over acidic zeolite catalysts has been an important non-petrochemical process to produce industrially demanding light olefins via natural gas, coal, or even biomass.5−7 The small-pore silicoaluminophosphate zeolite SAPO-34 has been used in industry as a highly selective catalyst for MTO conversion.8,9 The actual mechanism for MTO conversion over acidic zeolite has been considerably debated for a long time. So far, the hydrocarbon pool (HP) mechanism has been generally accepted, which was proposed by Dahl and Kolboe on the basis of their experimental studies.10 However, the HP mechanism is very complex;11,12 it depends on many factors, such as zeolite framework topology, framework composition, strength of acid site, pore openings, the shape of the pores, and the orientation of extraframework species, etc.13−17 Many studies have been conducted to investigate the mechanism of MTO conversion over zeolite catalysts. Through in situ solid state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, some important HP intermediates, such as the heptamethylbenzenium and hexamethylmethylenecyclohexadiene have been found in DNL-6, ZSM-5, and β during MTO conversion.18−20 According to the side-chain route, the formation of light olefins undergoes multiple steps of methylation, deprotonation, methyl shift, and side-chain elimination, producing a series of HP species (Figure 1).21,22 At the beginning, hexamethylben© 2014 American Chemical Society
Figure 1. Side-chain route of HP mechanism for MTO conversion.
Received: June 8, 2014 Revised: October 2, 2014 Published: October 2, 2014 24935
dx.doi.org/10.1021/jp505696m | J. Phys. Chem. C 2014, 118, 24935−24940
The Journal of Physical Chemistry C
Article
instance, SAPO-35 can be deactivated more easily during MTO conversion.30 The distinct MTO catalytic performances of these zeolites might result from the different confinement effects of different zeolite cavities.31 Among previous studies, theoretical calculations have provided an important insight into the mechanism of MTO conversion over acidic zeolite catalysts.32 Many theoretical studies have demonstrated that the framework topology decides the reactivity and selectivity of zeolites; their cavities provide the spatial room for the HP intermediates and transition states generated during MTO conversion.33 Through the quantum mechanics/molecular mechanics (QM/MM) method, van Speybroeck and co-workers found that the activation energy for the methylation of polymethylbenzene was extremely low in the cha cavity (60.8 kJ/mol) compared to that in the channel intersections in zeolite MFI (161.7 kJ/mol).34 They also elucidated the effect of confined space on the growth of naphthalenic species in zeolites SAPO-34 and SSZ-13.35 Similarly, Iglesia and co-workers investigated the confinement effect on the selectivity of alkane cracking and dehydrogenation in the pores of zeolite MOR.36 In this work, we employ the density functional theory (DFT) calculations to investigate the MTO catalytic cycle according to the HP mechanism. The confinement effect of different zeolite cavities, including the cha, lev, and lta cavities are evaluated assuming the same side-chain mechanism. Our theoretical calculations may provide a further insight into the confinement effect of zeolite cavities during the MTO conversion.
zene (intermediate A) reacts with methanol in SN2 fashion to form heptamethylbenzenium (intermediate B) and a water molecule; this is the first methylation step (Me1). Heptamethylbenzenium is then deprotonated (Dep1) to form hexamethylmethylenecyclohexadiene (intermediate C), which may transform into Intermediate D through the second methylation step (Me2). Intermediate D may transform into Intermediates E and F through additional steps of deprotonation (Dep2) and methylation (Me3). The side chains on intermediates D and F may undergo several 1,2-methyl shift steps (Shift1−3 and Shift4−6) and turn into isomers D′ and F′, respectively. Ethylene and propylene are produced from Intermediates D′ and F′ through elimination steps (Elim1 and Elim2), respectively, closing the catalytic cycle. Water molecules play an important role during these catalytic cycles by passing protons between intermediates and zeolite frameworks.23 Several types of zeolites have been tested as catalysts for MTO conversion. SAPO-34 containing cha cavities (11.6 Å × 10.6 Å; Figure 2a) has shown high selectivity in the production
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Figure 2. Cluster models of (a) the cha cavity consisting of 36 T atoms, (b) the lev cavity consisting of 30 T atoms, and (c) the lta cavity consisting of 48 T atoms. Si atoms are shown in yellow, Al in purple, O in red, and H in white. Atoms calculated at the high ONIOM layer are shown as spheres.
COMPUTATIONAL DETAILS
The cluster models of lev, cha, and lta (Figure 2) were cut from the ideal silica structures of zeolites LEV, CHA, and RHO, which contain 30, 36, and 48 T atoms, respectively.37 One Brönsted acid site was considered for each cavity by replacing one Si atom by one Al. The locations of acid sites were chosen at the 8-ring window where methanol and light olefin molecules can diffuse easily. We tried several possible O atoms in the 8ring windows and selected the ones with the best proton affinity as the acid sites for the following calculations. More details about the selection of the acid sites are shown in Figure S1 and Table S1 in the Supporting Information (SI). The dangling bonds in all of these cluster models were saturated by H atoms.
of light olefins.24,25 The 8-ring windows of the cha cavity allow the free diffusion of light olefins, and the space inside cha favors the formation of HP intermediates of specific size and shape.26,27 Other zeolites, such as SAPO-35 with lev cavities (10.1 Å × 9.7 Å; Figure 2b) and DNL-6 with lta cavities (11.9 Å × 11.9 Å; Figure 2c), have also been tested for MTO conversion.28,29 As compared to SAPO-34, SAPO-35 and DNL6 have displayed different MTO catalytic performances. For
Table 1. Intrinsic Reaction Barriers (kcal/mol) for MTO Steps within Different Zeolite Cavities at 673 K Assuming the Same Side-Chain Mechanism cha
lev
lta
reacn
ΔE
ΔH
ΔG
−TΔS
ΔE
ΔH
ΔG
−TΔS
ΔE
ΔH
ΔG
−TΔS
Me1 Dep1 Me2 Shift1 Shift2 Shift3 Elim1 Dep2 Me3 Shift4 Shift5 Shift6 Elim2
14.6 21.5 11.2 27.1 19.2 21.9 16.0 15.1 10.6 23.3 22.0 20.7 14.0
14.5 18.9 10.8 26.1 19.1 21.3 15.6 18.6 10.5 23.3 21.9 21.8 13.0
18.5 30.2 15.0 30.8 17.7 23.2 19.2 21.7 16.1 17.3 21.1 17.7 17.2
4.0 11.3 4.2 4.7 −1.3 1.9 3.6 3.1 5.6 −6.0 −0.8 −4.1 4.2
28.9 14.2 13.2 14.0 15.4 21.9 19.6 28.1 23.7 21.2 15.1 16.0 16.2
28.4 10.9 14.0 24.6 15.3 21.3 18.6 36.5 24.7 19.4 14.4 15.7 14.0
33.9 26.4 19.7 23.4 14.0 24.8 23.7 47.3 30.9 24.2 17.4 17.4 21.5
5.5 15.5 5.7 −1.1 −1.3 3.5 5.1 10.8 6.2 4.8 3.0 1.8 7.5
20.2 16.1 17.2 17.3 19.5 26.2 17.6 9.3 17.4 25.6 23.6 15.2 13.3
19.7 14.0 17.8 16.7 18.9 25.6 18.2 7.3 17.7 24.4 23.3 15.1 12.7
27.1 21.6 20.6 15.9 21.4 29.4 26.5 14.2 24.3 26.3 22.7 13.8 20.3
7.4 7.6 2.8 −0.9 2.5 3.8 8.3 6.9 6.6 1.9 −0.6 −1.3 7.6
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dx.doi.org/10.1021/jp505696m | J. Phys. Chem. C 2014, 118, 24935−24940
The Journal of Physical Chemistry C
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Table 2. Intrinsic Reaction Energies (kcal/mol) for MTO Steps within Different Zeolite Cavities at 673 K Assuming the Same Side-Chain Mechanism cha
lev
lta
reacn
ΔE
ΔH
ΔG
−TΔS
ΔE
ΔH
ΔG
−TΔS
ΔE
ΔH
ΔG
−TΔS
Me1 Dep1 Me2 Shift1 Shift2 Shift3 Elim1 Dep2 Me3 Shift4 Shift5 Shift6 Elim2
−2.5 16.7 −31.7 −6.9 2.8 −1.8 12.8 12.7 −34.3 2.5 3.7 −7.0 2.1
−1.5 15.6 −30.9 −7.4 3.2 −1.4 14.1 15.0 −33.1 2.4 2.5 −5.4 2.8
0.4 21.0 −33.0 −3.1 0.2 −2.5 7.2 15.2 −33.3 2.9 6.2 −11.8 0.1
1.9 5.4 −2.1 4.2 −3.0 −1.1 −6.9 0.2 −0.2 0.5 3.7 −6.4 −2.7
4.9 11.6 −14.1 −15.3 13.4 −19.5 15.8 21.0 −17.4 0.8 −6.3 4.5 −8.5
6.3 9.5 −23.3 −4.3 13.5 −18.6 14.9 31.0 −15.3 0.6 −6.3 4.1 −6.9
3.2 20.1 −19.1 −7.2 12.3 −24.0 19.5 34.8 −13.9 1.6 −6.7 1.5 −12.0
−3.1 10.6 4.2 −2.9 −1.2 −5.4 4.6 3.8 1.4 1.0 −0.5 −2.7 −5.1
−5.2 14.6 −13.9 −3.4 −7.3 7.5 13.1 9.7 −26.0 −0.4 5.9 −8.1 −1.0
−4.5 12.8 −12.3 −4.5 −6.4 7.0 16.0 5.1 −24.3 −1.0 5.6 −7.4 0.4
−0.2 15.6 −12.4 −3.5 −5.6 4.2 10.8 12.1 −19.9 0.1 6.3 −13.9 −2.8
4.3 2.8 −0.1 1.0 0.8 −2.8 −5.2 7.0 4.4 0.9 0.7 −6.5 −3.2
mol).34 The free-energy barriers of Me2 follow the sequence of cha < lev < lta, and those for Me3 follow the sequence of cha < lta < lev. The free energies of these methylation reactions show a slightly different trend (Table 2). For the first methylation reaction, the free energies in cha and lta are generally the same, both of which are slightly lower than that of lev. The second and the third methylation steps in three cavities are all exothermic; the reaction free energies of cha are much lower than those of lev and lta. All of these results indicate that the methylation steps might take place more easily in cha than in the other two cavities. Dep1 occurs between intermediates B and C. As those in the first methylation step, the bond distances (Table S3 in the Supporting Information) involving a H-shift in the first deprotonation step are similar in different cavities. However, different from the trend in the methylation steps, the intrinsic free-energy barriers for the first deprotonation step at 673 K follow the sequence of lta (21.6 kcal/mol) < lev (26.4 kcal/ mol) < cha (30.2 kcal/mol). Dep2 occurs between intermediates D and E. The intrinsic free-energy barriers for this step follow the sequence of lta (14.2 kcal/mol) < cha (21.7 kcal/mol) < lev (47.3 kcal/mol). The free energies of the two deprotonation reactions show the same trend as their freeenergy barriers. Both of the deprotonation steps are endothermic. Olefin Production. In comparison with the direct-cracking mechanism, the concerted mechanism proposed by Van Speybroeck and co-workers is a lower-barrier pathway to yield olefins.22 According to this mechanism, intermediates D and F turn into isomers D′ and F′ through methyl shift reactions (Shift1−3 and Shift4−6); D′ and F′ transform back into A through side-chain eliminations, producing ethylene (Elim1) and propylene (Elim2), respectively. SI Figure S2 shows the geometries of transition states of the elimination step within different zeolite cavities. The geometries of these transition states are similar among different cavities. To generate ethylene, the H1 atom in the methyl group is attracted by the water molecule, elongating the C1−H1 bond. With the assistance of the water molecule, the H1 atom shifts from C1 to Ow. The C2−CHP bond is then broken, and the ethylene molecule is eventually generated from intermediate D′. Propylene is generated in a similar way (Table S4 in the Supporting Information).
All of the DFT calculations were carried out using the exchange-correlation functional B3LYP as implemented in the Gaussian 09 package.38 The B3LYP functional has been widely applied to investigate the catalytic reactions over zeolite clusters.39 Because the cavity models were too large for regular DFT calculations, we employed the two-layer ONIOM method for all of our calculations.40 The SiO3−O−AlO2−OH−SiO3 cluster and HP species were in the high level, and the remaining framework atoms were in the low level. The cavity models and HP species were optimized together at the ONIOM (B3LYP/ 6-31g(d,p):AM1) level. Transition states were confirmed through the intrinsic reaction coordinate (IRC) approach.41 The achievement of true energy minima and saddle points was checked by frequency calculations at the same level. Enthalpy, entropy, and free-energy calculations were performed at 673 K. The single-point energy calculations were performed at the ONIOM (B3LYP/6-31g(d,p):HF/6-31g(d)) level. Dispersion corrections were performed to the single-point energies according to the D3 method proposed by Grimme and coworkers.42
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RESULTS AND DISCUSSION Methylation and Deprotonation. Assuming the sidechain mechanism (Figure 1), generating ethylene requires two methylations (Me1 and Me2) and one deprotonation (Dep1) steps, and generating propylene requires one additional methylation (Me3) and one additional deprotonation (Dep2) steps. The calculated reaction barriers and reaction energies of these reaction steps within protonated cha, lev, and lta cavities are listed in Table 1 and Table 2, respectively. The optimized geometries of the transition states for the first methylation and the first deprotonation in different cavities are shown in Figure S2 in the Supporting Information. For the transition states of the first methylation step (Me1), the Cm−Om breaking distances in methanol (cha, 2.14 Å; lev, 2.13 Å; lta, 2.14 Å) and the Cm−CHP forming distances between methanol and hexamethylbenzene (cha, 2.21 Å; lev, 2.24 Å; lta, 2.17 Å) are quite similar within different zeolite cavities (Table S2 in the Supporting Information). However, as shown in Table 1, the intrinsic free-energy barriers at 673 K are quite different (cha, 18.5 kcal/mol; lev, 33.9 kcal/mol; lta, 27.1 kcal/mol). For the cha cavity, the zero-point-corrected energy barrier of 14.6 kcal/mol for the first methylation step is close to that reported by Van Speybroeck and co-workers (15.2 kcal/ 24937
dx.doi.org/10.1021/jp505696m | J. Phys. Chem. C 2014, 118, 24935−24940
The Journal of Physical Chemistry C
Article
Figure 3. H···O distances (
