Shape-Selective Methylation of 2-Methylnaphthalene with Methanol

Article pubs.acs.org/JPCC

Shape-Selective Methylation of 2-Methylnaphthalene with Methanol over H-ZSM-5 Zeolite: A Computational Study Xiaowa Nie,†,§ Michael J. Janik,‡ Xinwen Guo,*,† and Chunshan Song*,†,‡,§ †

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China ‡ Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States § EMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Methylation of 2-methylnaphthalene (2-MN) for synthesis of 2,6-dimethylnaphthalene (2,6-DMN) is an industrially important reaction because 2,6-DMN is a key precursor for the advanced polymer material polyethylenenaphthalate. Shapeselective methylation of 2-MN with methanol in an H-ZSM-5 pore was studied using the ONIOM2 model and density functional theory. Two proposed reaction mechanisms, stepwise and concerted, were considered. Computational results reveal that the stepwise path, with methanol dehydration to produce a methoxide intermediate as the rate-limiting step, is kinetically favored. Both the stepwise and concerted path indicated that methylation at the 6-position is favored over methylation at the 7-position; however, the concerted path shows a greater selectivity. 2-MN isomerization and methylation, which may occur on the catalyst external surface and decrease selectivity to the desired 2,6-DMN product were also examined. Isomerization of 2-MN to 1-MN proceeds faster on external surface sites than 2-MN methylation with methanol. Decreasing the external surface acid site concentration will restrict the extent of 2-MN isomerization, therefore increasing the β,β′-DMN selectivity inside the pore.

1. INTRODUCTION Methylation of 2-methylnaphthalene (2-MN) for selective synthesis of 2,6-dimethylnaphthalene (2,6-DMN) is an industrially important reaction. 2,6-DMN is a key precursor of 2,6-naphthalenedicarboxylic acid, which is a monomer of polyethylenenaphthalate (PEN).1,2 PEN is a good gas barrier and has favorable mechanical, thermal, and electrical properties. The demand of monomer for making the advanced PEN polymer material motivates our study of the shape-selective methylation of 2-MN with methanol to produce 2,6-DMN. 2,6-DMN is one of the ten dimethylnaphthalene isomers. For targeted synthesis of 2,6-DMN, the most difficult issue is how to increase the ratio of 2,6/2,7DMN. The boiling points of these two isomers are very similar (only 0.3 °C difference), making their separation difficult. A number of catalytic investigations have attempted to achieve higher selectivity to 2,6-DMN.3−8 Studies in both the open literature1,5,6,9 and patent literature10,11 reveal that medium-pore H-ZSM-5, with its particular pore structure and suitable acid properties, is an optional material for shape-selective synthesis of the β,β′-DMN isomer mixtures. Pu and Inui1 reported a higher selectivity to 2,6-DMN yielded by ZSM-5 compared with other large-pore zeolites including ZSM-12, BEA, and Y. Furthermore, a positive effect on the selectivity to 2,6-DMN and 2,6/2,7-DMN ratio has been found by decreasing or passivating external (nonselective) acid sites on H-ZSM-5.3,5 For this alkylation reaction, the solvent © 2012 American Chemical Society

used (e.g., 1,2,4-trimethylbenzene or mesitylene) may play a complex role and may also take part in the alkylation process,12 but in this article, we are focused on exploring the mechanism of the H-ZSM-5 shape-selectivity for methylation of 2-MN with methanol without an involvement of the solvent molecule. Mechanistic determination may further advance the design of active and selective zeolite-based catalysts for this reaction. In this work, the quantum mechanics/molecular mechanics (QM/MM) embedded “Our own N-layered Integrated molecular Orbital and molecular Mechanics” (ONIOM)13−16 method, with density functional theory (DFT)17−20 used for the QM region, was employed to examine the mechanism of 2-MN methylation with methanol to form 2,6- and 2,7-DMN in an H-ZSM-5 pore. A nonembedded cluster model is used to differentiate the impact of pore structure and to represent the external surface of H-ZSM-5. Reactant isomerization, which leads to a nonselective methylation product, is also considered on the external surface model.

2. COMPUTATIONAL METHODS Previous studies on acid site location within ZSM-5 zeolite reported various results on the Al sitting sites, depending on the Received: September 27, 2011 Revised: December 30, 2011 Published: January 12, 2012 4071

dx.doi.org/10.1021/jp209337m | J. Phys. Chem. C 2012, 116, 4071−4082

The Journal of Physical Chemistry C

Article

Figure 1. 128T cluster represents the pore structure of H-ZSM-5 in (a) straight channel view and (b) zigzag channel view; the 12T cluster represents the external surface acid model of H-ZSM-5 in (c) straight view and (d) side view. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen.)

In an ONIOM scheme, the drawbacks of inaccurate energy evaluation from the UFF molecular force-field and the electron embedded approximation in ONIOM have been demonstrated, and these energetics should not be considered reliable. To improve the energetic properties and take into account the effect of the entire zeolite framework on molecule adsorption, pure DFT single-point energy calculations at a B3LYP/6-31G(d,p) level of the whole 128T cluster were performed on the prior optimized structures from the ONIOM scheme. Recent publications of ONIOM studies on adsorption and reactions over zeolites have reported that a combination of DFT/ONIOM method for energy/optimization estimations is a more accurate representation of dispersion interactions between the adsorbates and zeolite pore.27,30,40,41 Consequently in this article, the use of the 128T B3LYP/6-31G(d,p)//12T ONIOM(B3LYP/ 6-31G(d,p):UFF) approach is chosen. A 12T cluster with an Al atom substituted for Si12 (T12) was used as the external surface acid model, as shown in Figure 1c,d. This 12T cluster was terminated by H atoms bonded to Si atoms and treated at the B3LYP/6-311+G(d,p) level.42−45 For calculations using the 12T cluster, all the H atoms of the acid model were fixed at the initial coordinates. The QST method was applied to isolate transition states, and each transition state was confirmed to have a single imaginary vibrational frequency along the reaction coordinate. Zero-point energy (ZPE) corrections were included. All calculations were performed with the Gaussian03 package.46

computational methodology and structural cluster model used. Lonsinger et al.21 reported a most preferred T12 site for aluminum substitution for silica of ZSM-5; however, the calculated energetics for substitution show that several tetrahedral sites are energetically comparable with regard to aluminum sitting. Chatterjee and Chandra22 found that T6, T9, and T12 are the comparatively stable sites of Al substitution in the crystallographic structure of ZSM-5. Other computational studies have confirmed that T12 is the most favored site of Al within the ZSM-5 framework,23−25 and this site also provides sufficient space to be accessed easily by adsorbates. Thus, the T12 site is a common choice of Al substitution for Si in computational studies of adsorption and reaction with ZSM-5 zeolite.26−31 In this study, we introduced a 128T cluster model27,29,30 taken from the crystallographic structure32−34 of ZSM-5 zeolite, with a single Al atom substituted for Si12 (T12), as shown in Figure 1a,b. The cluster was terminated by H atoms bonded to Si atoms, with the terminal Si−H bond length fixed at 1.47 Å. A two layer ONIOM (ONIOM2) approach27,28,35−38 was applied to examine the reactions taking place in the ZSM-5 pore. In the ONIOM scheme, the higher theory layer included 12T, covering the 10membered ring and two additional basal T units, and was calculated at the B3LYP/6-31G(d,p) level. The remaining extended zeolite framework was treated with the universal force field (UFF).39 During optimization, only the basic 5T cluster within the 12T active region, [((SiO)3Al(OH)Si], was allowed to relax, whereas the rest of the model was fixed to the crystallographic coordinates. 4072



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Scheme 1. Proposed Reaction Mechanisms for Methylation of 2-MN with Methanol to Produce 2,6- and 2,7-DMN Isomers in an H-ZSM-5 Pore, (a) the Stepwise Path and (b) the Concerted Patha

a

Only the basic 5T, [((SiO)3Al(OH)Si], within the 128T H-ZSM-5 model is shown for clarity.

Jansang et al.50 over H-FAU zeolite (39.6 kcal mol−1) and by Maihom et al.27 over H-ZSM-5 (41.2 kcal mol−1). Once the methoxide intermediate is produced, a water molecule may be released from the pore. Coadsorbed water was not considered to further interact with reacting species in the stepwise path; we therefore define the stepwise mechanism to require water desorption prior to methylation. The desorption energy of water is 12.1 kcal mol−1. 3.1.1.2. Methylation of 2-MN. From the adsorbed methoxide state, the methylation of 2-MN proceeds through two primary steps including the addition of the methyl fragment to form a carbenium ion intermediate and deprotonation of the intermediate to produce the adsorbed 2,6- or 2,7-DMN product. Figure 4 illustrates the optimized structures of all species involved in 6- and 7-methylation (meaning methylation at the 6- and 7-position) of 2-MN. The geometric parameters of the coadsorbed intermediate and the transition states for formation of the adsorbed 2,6- and 2,7-DMN in the stepwise mechanism are given in Table 2. Mulliken charges of all the species included both in methanol dehydration and the subsequent 2-MN methylation are presented in Table 3. A 2-MN molecule is coadsorbed in the zeolite pore with the methoxide intermediate, denoted as Int_Met...2-MN, and shown in Figure 4a. The distance between the methoxy carbon (C1) and the naphthalenic carbon to which it will be transferred to is 2.84 Å for C6 and 2.95 Å for C7, indicating a more favorable configuration for reaction at the 6-position. The (C−H) σ covalent chemical bond is formed from the overlap of the s orbital of the H atom and the pz orbital of C atom in the methoxide intermediate. When a 2-MN molecule accesses the methoxide species, electron transfer between the σ bond (C−H) within methoxide and the π orbital (CC) of the benzene ring in 2-MN molecule occurs, and 2-MN can thus be stabilized

3. RESULTS AND DISCUSSION 3.1. Methylation Reactions within the H-ZSM-5 Pore. The shape-selective methylation of 2-MN occurs in the pore channel of the H-ZSM-5 catalyst, with high selectivity to β,β′DMNs.1,5,9,47,48 On the basis of previous studies27,28,31,36,49,50 on reaction path determination for alkylation reactions over zeolites, we proposed two reaction mechanisms for methylation of 2-MN with methanol in the H-ZSM-5 pore: the stepwise and concerted paths, as shown in Scheme 1a and b, respectively. 3.1.1. Stepwise Mechanism. 3.1.1.1. Dehydration of Methanol. In the stepwise mechanism, methanol adsorbs and dehydrates prior to Calcohol−C2‑MN bond formation. A methanol molecule adsorbs at the acid site of H-ZSM-5 through formation of an Oalcohol−Hzeolite hydrogen bond. The methanol C−O bond is then cleaved to generate a methoxide intermediate, releasing a water molecule. The optimized geometries of the adsorbed methanol (Ads_Met), dehydration transition state (TS_Met), and the methoxide intermediate (Int_Met...H2O) are shown in Figure 2. Important geometric parameters are given in Table 1. The configurations of the three molecular structures involved in methanol dehydration are similar to those reported in other ONIOM studies of methanol dehydration over zeolites.27,50 The relative energy of each state involved in methanol dehydration is illustrated in Figure 3. The energy sum of a separate methanol molecule, a separate 2-MN molecule, and the H-ZSM-5 structure is used as the reference energy. The adsorption energy of methanol is −24.4 kcal mol−1, which is within the range reported experimentally for H-ZSM-5 zeolite (−15 to −27 kcal mol−1),51,52 and slightly less exothermic than the calculated value reported by Svelle et al.53 using DFT + D (−27.4 kcal mol−1). The activation energy (Eact) for methanol dehydration to produce the Int_Met...H2O is 41.6 kcal mol−1, which is similar to the computational values reported by 4073



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Figure 2. Optimized geometries of (a) methanol adsorption, Ads_Met; (b) dehydration transition state, TS_Met; (c) methoxide intermediate, Int_Met...H2O. Only the 12T active region within the 128T H-ZSM-5 is shown for clarity. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

Table 1. Geometric Parameters of Equilibrium and Transition States Involved in Methanol Dehydrationa parameter

Ads_Met

TS_Met

Int_Met...H2O

Distances C1−O3 O3−H1 H1−O1 Si−O1 Al−O1 Si−O2 Al−O2

1.47 1.16 1.27 1.59 1.66 1.66 1.80

O3−H1−O1 C1−O3−H1 Si−O1−Al Si−O2−Al

169.9 119.1 131.9 129.2

1.84 0.97 3.96 1.54 1.68 1.69 1.79

3.21 0.97 2.21 1.57 1.69 1.70 1.77

86.5 113.1 131.6 129.9

148.5 71.5 131.3 130.1

Angles

Atom labels are given in Figure 2, distances are in Ångstroms, and angles are in degrees.

Figure 3. Relative energy (kcal mol−1) of each state involved in the stepwise mechanism of 2-MN methylation with methanol over H-ZSM-5 to produce 2,6- and 2,7-DMN. (The energy sum of a separate methanol molecule, a separate 2-MN molecule, and the H-ZSM-5 structure is used as the reference energy.)

around the methoxide by this σ−π interaction. The methoxide species has three σ bonds in this type, and the σ−π interaction may come from the cocontribution of the three σ bonds with the CC bond of the benzene ring. The coadsorption energy of 2-MN with Int_Met is −22.0 kcal mol−1, as noted in Figure 3.

This exothermic coadsorption energy indicates that it is favorable for these two species to colocate in a ZSM-5 pore. The transition states for formation of the carbenium ion 2,6-DMN intermediate, TS_Int_2,6-DMN, and 2,7-DMN intermediate, TS_Int_2,7-DMN, are shown in Figure 4b,c.

a

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Figure 4. Optimized structures of all species involved in 6- and 7-methylations of 2-MN in the stepwise mechanism. The methoxide and 2-MN coadsorbed intermediate is shown in (a) Int_Met...2-MN; transition states for the methylation steps are shown in (b) 6-methylation, TS_Int_2,6DMN, and (c) 7-methylation, TS_Int_2,7-DMN; carbonium ion intermediates are shown in (d) 6-methylation, Int_2,6-DMN, and (e) 7methylation, Int_2,7-DMN; transition states for the deprotonation step are shown in (f) 6-methylation, TS_Ads_2,6-DMN, and (g)7-methylation, TS_Ads_2,7-DMN; adsorbed products are shown in (h) 6-methylation, Ads_2,6-DMN, and (i) 7-methylation, Ads_2,7-DMN. H-ZSM-5 zeolite is shown in part for clarity. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

The methoxide C−O bond dissociates to form a methyl carbenium ion, stabilized between the corresponding 2-MN carbon atom and the negatively charged surface site of ZSM-5. The geometry about the C1 atom of the methyl group changes from tetrahedral in the methoxide to trigonal planar, owing to the C1 hybridization state change from sp3 to sp2. The C1−O2 and C1−C6 distances are 2.44 and 2.53 Å, respectively, in TS_Int_2,6-DMN, whereas they are 2.50 and 2.92 Å for C1−O2 and C1−C7 in TS_Int_2,7-DMN. A hydrogen bond is formed at the transition

state between a hydrogen atom of 2-MN (H6 or H7) and the negatively charged site (O1). The atomic distances of H6−O1 and H7−O1 are 1.74 and 2.49 Å, indicating a stronger hydrogen bond interaction in TS_Int_2,6-DMN than TS_Int_2,7-DMN. The optimized structures of the carbenium ion 2,6- and 2,7DMN intermediates are shown in Figure 4d,e. The delocalized positive charge within the intermediate is stabilized by the negatively charged sites of ZSM-5. The C1−O2 and H6−O1 distances are 3.15 and 2.32 Å in Int_2,6-DMN, while the 4075



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Table 2. Optimized Geometric Parameters of Intermediate and Transition States within the Stepwise Mechanisma 6-methylation of 2-MN Ads_Met...2-MNb

TS_Int_2,6-

C1−CX CX−HX HX−O1 Si−O1 Al−O1 Si−O2 Al−O2

2.84/2.95 1.08/1.09 3.66/5.22 1.58 1.65 1.64 1.79

2.53 1.10 1.74 1.58 1.68 1.57 1.70

C1−CX−HX CX−HX−O1 Si−O1−Al Si−O2−Al

92.0/95.1 83.0/52.4 133.2 133.8

55.9 160.2 134.8 139.9

parameter

7-methylation of 2-MN

TS_Ads_2,6-

TS_Int_2,7-

TS_Ads_2,7-

1.52 1.45 1.62 1.60 1.70 1.54 1.68

2.92 1.08 2.49 1.61 1.71 1.56 1.67

1.54 1.30 1.48 1.59 1.68 1.56 1.69

90.0 169.5 125.9 154.8

60.1 105.0 131.1 140.2

102.7 167.8 128.4 144.0

Distances

Angles

a

Atom labels are given in Figure 4, distances are in Ångstroms, and angles are in degrees. bX represents the 6- or 7-position; for Ads_Met...2-MN; the first value refers to the 6 position, and the second refers to the 7 position.

Table 3. Mulliken Charges of All the Species Included in the Stepwise and Concerted Mechanism of 2-MN Methylation with the 128T H-ZSM-5 Modela stepwise mechanism

Ads_Met TS_Met Int_Met...H2O Int_Met...2MN TS_Int_2,6-DMN TS_Int_2,7-DMN

concerted mechanism

Co_Ads TS_Int_2,6-DMN TS_Int_2,7-DMN Int_2,6-DMN Int_2,7-DMN

a

C1 −0.13 C1 −0.10 C1 −0.09 C1 −0.09 C1 −0.19 C1 −0.15 C6 −0.06 C1 −0.20 C1 −0.20 C6 −0.18 C7 −0.16

O1 −0.71 O2 −0.70 O1 −0.67 C6 −0.11 C6 −0.10 C7 −0.18 C7 −0.11 C6 −0.16 C7 −0.21 O1 −0.74 O1 −0.73

O3 −0.72 O3 −0.54 O2 −0.62 C7 −0.13 O1 −0.68 O1 −0.67 O3 −0.57 O3 −0.53 O3 −0.55 O2 −0.65 O2 −0.68

H1 0.53

Int_2,6-DMN Int_2,7-DMN

H1 0.34 O2 −0.62 O2 −0.65 O2 −0.63 H1 0.45

TS_Ads_2,6-DMN TS_Ads_2,7-DMN Ads_2,6-DMN Ads_2,7-DMN TS_Ads_2,6-DMN TS_Ads_2,7-DMN Ads_2,6-DMN

H6 0.21 H7 0.19

Ads_2,7-DMN

C1 −0.34 C1 −0.34 C6 −0.09 C7 −0.09 C6 0.16 C7 0.14 C6 −0.19 C7 −0.24 C6 0.05 C7 0.07

O1 −0.68 O1 −0.66 O1 −0.70 O1 −0.67 H6 0.38 H7 0.37 O2 −0.66 O2 −0.70 H6 0.36 H7 0.39

O2 −0.66 O2 −0.64 H6 0.29 H7 0.29

H6 0.26 H7 0.25

H6 0.30 H7 0.38

Atom labels are referred to those given in Figures 2, 4, and 6.

C1−O2 and H7−O1 distances are 3.60 and 3.19 Å in Int_2, 7-DMN, indicating a closer association of the positive and negative charges for the intermediate toward 6-methylation, which also can be confirmed by a stronger electrostatic interaction in Int_2,6-DMN than that in Int_2,7-DMN through the Mulliken charge analysis given in Table 3. The activation barriers are 29.9 and 32.5 kcal mol−1 for 6- and 7-methylation, respectively, to produce the 2,6- and 2,7-DMN intermediates from the coadsorbed state. The carbenium ion intermediates go through deprotonation steps to form 2,6- and 2,7-DMN products adsorbed at the zeolite active site. Transition states involved are described in Figure 4f, TS_Ads_2,6-DMN, and Figure 4g, TS_Ads_2,7-DMN. The vibrational animation of the single imaginary frequency for these two transition states shows the proton (H6 or H7) transferring from the carbon atom of the 2,6- or 2,7-intermediate to

the negatively charged site (O1) of the zeolite. The H6−O1 and H7−O1 distances at the transition states are 1.62 Å and 1.48 Å, respectively. The adsorbed 2,6-DMN (Ads_2,6-DMN) and 2,7-DMN (Ads_2,7-DMN) products are shown in Figure 4h,i. The product state is stabilized over the active site in ZSM-5 pore by a π−H bond interaction between the benzene ring in the DMN molecule and the acidic proton of ZSM-5. The average of the two Hzeolite−C distances for the CC bond is 2.73 Å for Ads_2,6-DMN and 2.41 Å for Ads_2,7-DMN, indicating a stronger π−H bond interaction for the adsorbed 2,7-DMN molecule. The activation barriers for the deprotonation step are 3.4 and 8.5 kcal mol−1, respectively, showing a faster proton transfer for 2,6-DMN formation. Figure 3 shows that the formation of the adsorbed 2,6-DMN from the coadsorbed state (Int_Met...2-MN) is exothermic by 36.6 kcal mol−1, whereas it is 45.1 kcal mol−1 exothermic for adsorbed 2,7-DMN formation. 4076



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Calcohol−C2‑MN bond in a single step. The transition states for 6-methylation of 2-MN to form the 2,6-DMN intermediate, TS_Int_2,6-DMN, and 7-methylation of 2-MN to form the 2,7DMN intermediate, TS_Int_2,7-DMN, are illustrated in Figures 6b and c, respectively. At the transition state, the Ozeolite−Hzeolite (O1−H1) bond is broken and the Oalcohol−Hzeolite (O3−H1) bond is formed. Meanwhile, the Calcohol−Oalcohol (C1−O3) bond is broken to produce a water molecule, adsorbed at a framework O atom of ZSM-5 through formation of a hydrogen bond. The Hwater−Ozeolite distance is 2.16 Å for both transition states. The configuration of each transition state involves a trigonal planar methyl carbenium ion with a sp2 hybridization state, which is costabilized by both the delocalized negative charge on the O atom of water and the π electrons on the benzene ring of 2-MN. At the transition state, the proton (H6 or H7) is out of the plane in the benzene ring of 2-MN, indicating a partial transfer of the proton to the negatively charged site of the ZSM-5. A hydrogen bond is formed between the H6 or H7 atom and the O2 atom of the zeolite, and the TS_Int_2,6-DMN has a stronger hydrogen bond interaction as reflected by a shorter H6−O2 (2.71 Å) distance than that of H7−O2 (3.44 Å) in TS_Int_2,7DMN. The carbenium ion intermediates are formed through the above concerted transition states, as shown in Figure 6d, Int_2,6DMN, and Figure 6e, Int_2,7-DMN. Water remains coadsorbed in the concerted path, adsorbed at an Ozeolite atom within the active region of ZSM-5 through hydrogen bonding. The activation barriers for 6- and 7-methylation of 2-MN to form the carbenium ion intermediates are 43.6 and 49.8 kcal mol−1, as shown in Figure 5. The 6.2 kcal mol−1 difference in Eact indicates that 6-methylation of 2-MN is kinetically more favored than 7-methylation in the concerted path, which would lead to a faster formation of the 2,6-DMN intermediate. The carbenium ion intermediate goes through deprotonation to produce the adsorbed 2,6- or 2,7-DMN product through the transition states shown in Figure 6f, TS_Ads_2,6-DMN, and (g), TS_Ads_2,7-DMN. The CDMN−HDMN bond is weakened at the transition state, and the HDMN−Ozeolite bond is incompletely formed. The CDMN−HDMN distance is elongated to 1.34 Å from 1.11 Å in the carbenium ion intermediate for TS_Ads_2,6-DMN, and 1.41 Å from 1.13 Å for TS_Ads_2,7DMN. The HDMN−Ozeolite distance is contracted to 1.42 Å from 3.36 Å for TS_Ads_2,6-DMN, and 1.39 Å from 3.78 Å for TS_Ads_2,7-DMN. Once the proton is donated back to recreate the active site of ZSM-5, the concerted path is completed by the final desorption of 2,6-DMN, 2,7-DMN and water from the active region of the zeolite. The 2,6- and 2,7-DMN adsorbed structures are shown in Figure 6h, Ads_2,6-DMN, and Figure 6i, Ads_2,7-DMN. The product species are adsorbed at the active site of ZSM-5 by π−H bond interaction between the active site and the benzene ring CC bond within the DMN molecule. The activation barriers for the deprotonation step are 1.0 and 2.9 kcal mol−1, respectively, showing a fast proton backdonation. Formation of the adsorbed 2,6-DMN product from the coadsorbed reactant is exothermic by 16.9 kcal mol−1, whereas it is 20.4 kcal mol−1 exothermic for the adsorbed 2,7-DMN formation. 3.1.3. Comparison of the Stepwise and Concerted Mechanisms. Methylation of 2-MN with methanol in an HZSM-5 pore may take place through either the stepwise or concerted mechanism. In the stepwise path, methanol dehydration to form a methoxide intermediate is the rate-limiting step,54 with an apparent activation energy barrier (Eapp‑act) of 41.6 kcal mol−1. The following methylation steps to form the 2,6- and

The final step is product desorption from the zeolite active site. The more exothermic formation of 2,7-DMN results in its more difficult desorption compared to 2,6-DMN desorption. 3.1.2. Concerted Mechanism. The concerted mechanism occurs with coadsorption of 2-MN with methanol, followed by 2-MN methylation directly without formation of a methoxide intermediate. The relative energy of each state in the concerted path is shown in Figure 5, the optimized molecular structures

Figure 5. Relative energy (kcal mol−1) of each equilibrium and transition state in the concerted mechanism of 2-MN methylation with methanol over H-ZSM-5 to produce 2,6- and 2,7-DMN. (The energy sum of a separate methanol molecule, a separate 2-MN molecule, and the H-ZSM-5 structure is used as the reference energy.)

are illustrated in Figure 6, and the geometric parameters of key species are given in Table 4. Mulliken charges of all the species created are included in Table 3. To construct an initial coadsorbed state, the adsorption of methanol and 2-MN were first separately optimized. Adsorption energies are −24.4 and −17.2 kcal mol−1 for methanol and 2-MN, which indicates that methanol adsorption is preferential thermodynamically. Therefore, methanol is adsorbed over the active site of H-ZSM-5 via a hydrogen bond interaction, and the 2-MN molecule is coadsorbed nearby methanol through a π−H bond interaction. This state is denoted as Co_Ads and shown in Figure 6a. The hydroxyl group of methanol interacts with the acid site of the zeolite, meanwhile, the hydrogen atom of hydroxyl group interacts with the 2-MN molecule by π−H bond, with an average H−C bond length being 2.23 Å. The methyl group of methanol shows partial delocalized positive charge because the negative charge on the hydroxyl O atom transfers in part to the proton acid site of the zeolite. The partially delocalized positive charge on the methyl group appears to be stabilized by the π electrons on the benzene ring of 2-MN, with C1−C6 and C1−C7 distances of 3.77 and 3.95 Å, indicating a more optimal position for methylation at the 6-position. The favored C6 site in its configuration can also be demonstrated from the Mulliken charge results given in Table 3, where less negative charge retains on C6 than C7, indicating a stronger electrostatic interaction participation of C6 in the alkylation process. The coadsorption energy of 2-MN with methanol is −30.8 kcal mol−1, as shown in Figure 5. After coadsorption, 2-MN is attacked by methanol directly, leading to the formation of a carbenium ion 2,6- or 2,7-DMN intermediate and releasing a water molecule. The concerted reaction breaks the Ozeolite−Hzeolite bond, forms the Oalcohol− Hzeolite bond, breaks the Calcohol−Oalcohol bond, and forms the 4077



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Figure 6. Optimized molecular structures of all equilibrium and transition state species in 6- and 7-methylations of 2-MN in the concerted mechanism. Coadsorbed methanol and 2-MN reactants are shown in (a) Co_Ads; transition states for the methylation steps are shown in (b) 6methylation, TS_Int_2,6-DMN, and (c) 7-methylation, TS_Int_2,7-DMN; carbonium ion intermediates are shown in (d) 6-methylation, Int_2,6DMN, and (e) 7-methylation, Int_2,7-DMN; transition states for the deprotonation steps are shown in (f) 6-methylation, TS_Ads_2,6-DMN, and (g) 7-methylation, TS_Ads_2,7-DMN; adsorbed products are shown in (h) 6-methylation, Ads_2,6-DMN, and (i) 7-methylation, Ads_2,7-DMN. H-ZSM-5 zeolite is shown in part for clarity. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

2,7-intermediates are competitive (2.6 kcal mol−1 difference in Eact). In the concerted path, the Eapp‑act is 43.6 kcal mol−1 for 6methylation and 49.8 kcal mol−1 for 7-methylation to form the carbenium ion intermediates, where 6-methylation is kinetically favored. Deprotonation reactions are faster, as reflected by the lower activation barriers than the comparable deprotonation steps in the stepwise path. The relative stability of the transition states in 6-methylation are all higher than those in 7-methylation for these two reaction

mechanisms. In the stepwise path, we did not consider the impact of adsorbed water on the subsequent methylation and deprotonation reactions and assumed that if water is retained within the following step, the transition states will be the same as those created in the concerted path. Calculation results on Eapp‑act indicates that the stepwise mechanism is kinetically preferred for 2-MN methylation, but the selectivity difference between 2,6- and 2,7-DMN is less than it would be in the concerted path. 4078



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Table 4. Optimized Geometric Parameters of the Coadsorbed Reactant and the Transition States in the Concerted Mechanisma 6-methylation of 2-MN parameter

Co_Adsb

TS_Int_2,6-

7-methylation of 2-MN

TS_Ads_2,6-

TS_Int_2,7-

TS_Ads_2,7-

1.53 1.34 1.42 0.97 4.52 1.57 1.66 1.62 1.73

2.33 1.09 3.44 0.97 2.16 1.58 1.69 1.56 1.67

1.55 1.41 1.39 0.96 5.39 1.58 1.66 1.63 1.74

100.6 168.9 138.1 129.7

102.1 94.5 128.9 142.7

92.0 159.8 139.4 126.7

Distances C1−CX CX−HX HX−O2 H1−O3 H1−O1 Si−O1 Al−O1 Si−O2 Al−O2

3.77/3.95 1.07/1.09 1.92/3.80 1.55 1.04 1.66 1.77 1.56 1.65

2.43 1.10 2.71 0.97 2.16 1.58 1.69 1.56 1.67

C1−CX−HX CX−HX−O2 Si−O1−Al Si−O2−Al

111.2/115.7 172.5/86.1 128.5 141.8

123.2 126.0 128.9 142.7

Angles

a

Atom labels are given in Figure 6, distances are in Ångstroms, and angles are in degrees. bX represents the 6- or 7-position; for Co_Ads, the first value refers to the 6 position, and the second refers to the 7 position.

The clearly lower activation barrier for the transition state leading to the 2,6-DMN formation compared to that for 2,7DMN formation is consistent with the concept of the restricted electronic transition state selectivity toward 2,6-dialkylnaphthalene inside zeolitic pore proposed in our previous study,55,56 which may be more important for conversion of polycyclic aromatic hydrocarbons compared to one-ring aromatics.57 It should be noted that other selective-effects of ZSM-5 pore may impact its shape-selectivity, as further discussed later. 3.2. Methylation and Isomerization of 2-MN on the External Surface of H-ZSM-5. Though shape-selective methylation occurs in the ZSM-5 pore, nonselective methylation as well as reactant and product isomerization may take place on the catalyst external surface and impact the product selectivity. Experimental studies4−6 have detected 1-MN isomer from the final product mixture and presumed that 1-MN would be stuck in the pore due to its bulky molecular structure. Therefore, 1-MN may be largely produced through 2-MN isomerization on the catalyst external surface. Once 1-MN is produced on the catalyst external surface, it can be further methylated to generate α,α′-DMNs or α,β′-DMNs.9 Experimental studies suggested that decreasing the number of acid sites on the external surface minimizes those competing reactions to produce α,α′DMNs or α,β′-DMNs. Here, we seek to corroborate this by examining the energetics of isomerization and methylation reactions on the external surface, but prior to the start, the important issue is to confirm whether the 1-MN could be produced in the pore. We calculated the energetics for isomerization of 2-MN to 1-MN in the 128T H-ZSM-5 pore. The overall reaction involves three major steps including protonation of the adsorbed 2-MN to form the carbenium ion 2-MN intermediate, methyl group transfer from the 2-MN intermediate to generate the 1-MN intermediate, and deprotonation of the 1-MN intermediate to produce the adsorbed 1-MN. The energy diagram and the key transition state geometry of this isomerization process are shown in Figure 7, with the adsorbed 2-MN molecule as the reference energy. By comparison of the Eapp‑act in 2-MN isomerization (Eapp‑act = 54.2 kcal mol−1) and 2-MN methylation (Eapp‑act = 41.6 kcal mol−1 for the stepwise path; Eapp‑act = 43.6 and 49.8 kcal mol−1 for the concerted path) in the H-ZSM-5 pore, we can conclude that the isomerization of 2-MN is highly restricted in the pore

Figure 7. Energy diagram and the key transition state geometry from different view for 2-MN isomerization to 1-MN in the 128T H-ZSM-5 pore. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

and that the transition state for methyl group transfer from the 2-position to the 1-position is difficult to be stabilized within the pore. Apart from the energetics aspect, we also examined the molecular structure of the formed 1-MN in the pore, as shown in Figure 8. From the optimized configuration, it is apparent that 1-MN molecule is bulky due to a methyl group at the 1-position and is difficult to move through either the zigzag channel or the straight channel of ZSM-5, as illustrated in Figure 8a,b. The above analysis results demonstrate that 1-MN is very difficult to be formed in the pore and is mainly produced from 2-MN isomerization on the catalyst external surface. We use a 12T model to represent the external surface acid site of H-ZSM-5, as shown in Figure 1c,d. The external surface of an H-ZSM-5 zeolite may expose a wide-range of acid site structures. We adopt this 12T cluster because it is directly comparable with our pore model, simply removing the pore constraints and any stabilization offered by extended adsorbate-pore interactions. The acid site strength of the cluster is not significantly different than the original 128T pore structure. The adsorption 4079



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Figure 8. Formed 1-MN molecule through 2-MN isomerization in the H-ZSM-5 pore, (a) straight channel view and (b) zigzag channel view. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

energy of methanol over the 12T is −25.7 kcal mol−1, whereas it is −24.4 kcal mol−1 over the 128T. This 1.3 kcal mol−1 difference in methanol adsorption energy indicates that the 12T cluster is slightly more acidic than the 128T model. 2-MN isomerization to 1-MN as well as its methylation with methanol were examined on the 12T model, and the relative reaction rate ratio for 2-MN isomerization/methylation was calculated. Reaction energy diagrams and key structures for 2-MN isomerization and stepwise and concerted methylation of 2-MN are illustrated in Figures 9a, b, and c, respectively. The Eapp‑act for the isomerization reaction is 34.7 kcal mol−1, representing the energy difference between the methyl transfer transition state and the initial adsorbed 2-MN state. In the stepwise methylation of 2-MN, the Eapp‑act is 40.4 kcal mol−1, and it is 42.1 and 45.2 kcal mol−1 for 6- and 7-methylation in the concerted methylation of 2-MN. Accordingly, we can approximately estimate the reaction rate for 2-MN isomerization and methylation according to the Arrhenius eq 1, and then the relative reaction rate ratio for isomerization/methylation can be determined by eq 2.

k = A exp( − Ea /RT )

(1)

k1/k 2 = (A1/A2) exp(Ea2 − Ea1/RT )

(2)

A=

freq(1) × freq(2) × freq(3) × ··· × freq(n)reactant freq(1) × freq(2) × freq(3) × ··· × freq(n)TS

(3)

In eq 2, the pre-exponential factor A1 and A2 for the corresponding reaction can be predicted by eq 3 based on the transition state theory.44,45 The Eapp‑act (34.7 kcal mol−1) for the isomerization reaction and Eapp‑act (40.4 kcal mol−1) for the kinetically favored stepwise methylation are used as the Ea items in eq 2. The reaction temperature is considered to be 633 K according to experimental studies.3,4 Then, the relative reaction rate ratio for 2-MN isomerization/methylation is close to 650:1 as calculated by eqs 2 and 3. The above results indicate that 2-MN isomerization to 1-MN proceeds much faster than its methylation at an external surface acid site of ZSM-5. Once 1-MN is formed, it can be further methylated to α,α′-DMNs or α,β′-DMNs on the catalyst external surface. Therefore, restricting surface isomerization by decreasing the external surface acid site concentration restricts 2-MN isomerization, which in turn allows for 2-MN diffusion into ZSM-5 to increase the β,β′-DMN selectivity through the shape-selective methylation within the HZSM-5 pore.

Figure 9. Reaction energy diagrams and key structures for (a) isomerization of 2-MN to 1-MN, (b) stepwise methylation, and (c) concerted methylation of 2-MN to form 2,6- and 2,7-DMN on the external surface of H-ZSM-5. The 12T acidic model is shown in part for clarity. (Blue, silicon; red, oxygen; pink, aluminum; white, hydrogen; gray, carbon.)

A question that arises is, does the calculated difference in methylation energy barriers between the transition states for 2,6-DMN and 2,7-DMN reflect interactions with the extended pore structure or the electronic interactions intrinsic to the acid 4080



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the reaction paths with the 128T and 12T H-ZSM-5. This material is available free of charge via the Internet at http://pubs.acs.org.

site? Recall that the methylation of 2-MN inside the pore of H-ZSM-5 was studied by using a 128T cluster model, while the methylation and isomerization of 2-MN on the external surface acid site of H-ZSM-5 by using a 12T cluster model. As seen from comparing the energetics from 12T and 128T cluster models, the energy barriers for TS_Met, TS_Int_2,6-DMN, and TS_ Int_2,7-DMN formation between the 12T and 128T models are almost identical (40.4 vs 41.6 kcal mol−1, 34.6 vs 32.8 kcal mol−1, and 36.6 vs 35.4 kcal mol−1 in stepwise mechanism). Similar results can also be found in the concerted mechanism. These results indicate that the extended pore structure is not necessary to sufficiently distinguish between 2,6-DMN and 2,7-DMN. Preferential formation of 2,6 DMN against 2,7-DMN arises mainly by differences in the intrinsic stability of the transition and intermediate states interacting with the local acid site. Various electronic interactions between the reacting species and surface sites and among the reactant species have been discussed in section 3.1. Such electronic interactions lead to a clearly lower activation barrier, by at least 2 kcal mol−1, for the transition state leading to 2,6-DMN formation compared to that for 2,7-DMN formation. It is essential for the acid site of H-ZSM-5 to show a preference to 2,6-DMN formation, whereas the pore size effect would eliminate the formation of the 1-methylated products that are otherwise favored at external surface sites.56 This is in good agreement with the previously proposed concept called restricted electronic transition state selectivity toward 2,6-dialkylnaphthalene in the alkylation of 2-alkylnaphthalene inside zeolitic pore.55,56 We observed a 2.6 kcal mol −1 difference in energy barrier of 2,6- and 2,7-DMN formation in the stepwise mechanism from calculations on the 128T H-ZSM-5 model, while we obtained a 2.0 kcal mol−1 difference of that with the 12T cluster. Similar results have also been obtained for the concerted mechanism. The value of 2.0 kcal mol−1 for the difference between activation barriers of two reactions is not very large but significant enough to allow for the preferential occurrence of one reaction against the other under mild conditions, judging from both the Arrhenius equation and the experimental results.4−6

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Corresponding Author

*(X.G.) Tel: +86- 411-84986133. Fax: +86- 411-84986134. E-mail: [email protected]. (C.S.) Tel: 814-863-4466. Fax: 814-865-3573. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was financially supported in part by the program for New Century Excellent Talent in University (NCET-04-0268), the Plan 111 Project of Ministry of Education of China, and the US Department of Energy, National Energy Technology Laboratory. We also wish to thank the Pennsylvania State University for partial financial support for X.N. through the PSU-DUT Joint Center for Energy Research.

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REFERENCES

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4. CONCLUSIONS Stepwise and concerted mechanisms for shape-selective methylation of 2-MN with methanol in an H-ZSM-5 pore have been examined using the DFT/ONIOM approach. The stepwise path is kinetically favored, where methanol dehydration is the rate-limiting step with an apparent activation barrier of 41.6 kcal mol−1. Both stepwise and concerted paths show selectivity toward 2,6-DMN formation, though the selectivity difference between 6-methylation and 7-methylation is more significant in the concerted path. At external surface sites, 2-MN isomerization to 1-MN is much faster than its methylation with methanol. Restricting surface isomerization by decreasing the acid site concentration on the catalyst external surface favors 2-MN conversion in the H-ZSM-5 pore for the shapeselective reactions, thus enhancing the β,β′-DMN selectivity.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

* Supporting Information S

Imaginary vibrational frequency for each transition state created in all the reaction paths with the 128T and 12T H-ZSM-5, and the corresponding vibrational mode sketch maps for all the transition states with using the 128T H-ZSM-5 cluster; Cartesian coordinates for all the stationary points and transition states in all 4081



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Shape-Selective Methylation of 2-Methylnaphthalene with Methanol

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