Structure and Reaction Mechanism of Alkylation of Phenol with

Dec 22, 2007 - This work was supported in part by Grants from the Thailand Research Fund (to J.L.) and the Kasetsart University Research and Developme...
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J. Phys. Chem. C 2008, 112, 540-547

Structure and Reaction Mechanism of Alkylation of Phenol with Methanol over H-FAU Zeolite: An ONIOM Study Bavornpon Jansang,†,‡ Tanin Nanok,†,‡ and Jumras Limtrakul*,†,‡,§ Laboratory for Computational and Applied Chemistry, Department of Chemistry, Faculty of Science, Kasetsart UniVersity, Center of Nanotechnology, Kasetsart UniVersity Research and DeVelopment Institute, Kasetsart UniVersity, and NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart UniVersity, Bangkok 10900, Thailand ReceiVed: September 10, 2007; In Final Form: October 17, 2007

The competitive methylation of phenol with methanol over nanoporous faujasite zeolite was studied within the framework of our-own-N-layered-integrated molecular orbital + molecular mechanics (ONIOM) approach with two proposed reaction pathways: stepwise and concerted mechanisms. The results obtained by the twolayer ONIOM(B3LYP/6-31G(d,p):UFF) scheme elucidate that the methylation preferentially takes place via a single-step concerted mechanism rather than by the stepwise mechanism. The reactivity difference between the O- and C-methylation became more obvious in the concerted mechanism where the O-methylation proceeded faster than the C-methylation. The activation energies of the O- and C-methylation were estimated to be 14.9 and 19.2 kcal/mol, respectively, comparing well with the available experimental data of phenol methylation on acidic zeolites. In the stepwise reaction mechanism, the formation of the surface methoxide species (activation energy of 39.6 kcal/mol) was the rate-determining step, while the phenol methylation was the fast step of the reaction with the activation energies of 11.1 and 12.0 kcal/mol for the O- and C-methylation, respectively. In this reaction, it was found that anisole was a kinetically primary product, whereas o-cresol was the most thermodynamically stable product.

1. Introduction Porous materials have gained recognition as an important source of catalysts for industrial applications. Among these, zeolites, with their shape-selective properties, are the preferred choice for the realization of the desired end products. With the escalating concern and attention being paid to pollution control and global warming, it is, therefore, industrially relevant that zeolites have been found to be environmentally friendly. Faujasite zeolite, in particular, is considered to be one of the major catalysts used in petrochemical industries. Its protonic form has been found to be a significant active constituent for the catalytic cracking of hydrocarbons.1-4 In addition, its supercage with a diameter of approximately 13 Å enables it to operate in the same manner as a nanoreactor for the reactions of bulky molecules, including the alkylation of phenol and its family members.5-10 The latter process is currently being pursued by environmentally concerned chemical industries. The obtained products are used as important raw intermediates for the synthesis of drugs, resins, pharmaceuticals, and dyestuffs. Alkylation of phenol with methanol on zeolites involves two competitive reactions, i.e., the O- and C-methylation. The product of O-methylation is anisole, whereas the product of C-methylation is cresol. Experimental results revealed that the acid-base properties of the catalysts play an important role in the product selectivity in addition to the experimental factors.6,10-12 In literature, there are several references which report that anisole formation requires a site of lower acid strengths * Corresponding author. E-mail: [email protected]. † Department of Chemistry. ‡ Kasetsart University Research and Development Institute. § NANOTEC Center of Excellence, National Nanotechnology Center.

Figure 1. 178T nanocluster of faujasite zeolite subdivided into two layers of calculation methods according to the two-layer ONIOM (ONIOM2) scheme: the 14T cluster (balls) was computed with the B3LYP/6-31G(d,p) level of theory, and the remainder (lines) was computed with the universal force field (UFF).

compared to that for cresol formation.5,7,11,13,14 In addition, recent in situ CF (continuous flow) MAS (magic-angle spinning) NMR

10.1021/jp077246b CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007

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SCHEME 1

SCHEME 2

spectroscopic results on the methylation of phenol by methanol over acidic H-Y zeolite indicated that anisole was an unstable primary product and that it could be converted to cresol directly.9 Nevertheless, the real mechanism of the alkylation of phenol with methanol over solid acid catalysts has never been elucidated, even though it is still comprehensively discussed in various literatures.5,7,9-13,15-22 Until the present time, it has been understood that the reaction occurs through the concerted route or via the step-by-step route. If it takes place through the concerted route, it will process via one step by the coadsorption of phenol with methanol and conversion to the product directly. However, many references in literature have reported the alkoxideintermediateformationduringalkylatingprocesses.8-10,18-21 Furthermore, theoretical calculations have confirmed that alkoxide intermediates are stable on the zeolitic framework and are found to be reactive intermediates.23-26 Theoretical calculations based on quantum chemistry are a useful tool to analyze the reaction mechanism in deep detail. Consequently, several theoretical approaches have been proposed to study the reaction mechanism on zeolite catalysts, such as the bare cluster model,23-27 the periodic density functional theory (DFT) calculation,28,29 and the combined quantum mechanics/molecular mechanics (QM/MM).30-32 However, drawbacks or limitations exist: the bare cluster model calculation, for example, represents only a small active region of the zeolite catalyst, but despite its ease of use and its more economical computational demand, it does not take into consideration the environmental effect of the zeolitic framework such as short- and long-range interactions. Therefore, it cannot adequately describe the behavior of the adsorbate molecule in the real system of the nanocavity of a zeolite. The periodic DFT calculation, although it is able to represent the zeolitic framework, is impractical to calculate very the large unit cells of the zeolite because of its high computational demand; hence, it is feasible only for investigating small unit cell zeolites. However, the van der Waals (vdW) contribution has been found to be an important effect of the extended zeolitic framework on the energetics of the adsorbing molecule in the nanocavity of a zeolite.33 In our recent studies, our-own-N-layered-integrated molecular orbital + molecular mechanics (ONIOM) approach has been successfully employed to study the adsorption properties and the reaction mechanisms of organic molecules over different types of zeolite catalysts.34-40 This approach employs minimal computational demand and is practical for calculating a large system.41,42 We are not aware of any theoretical reports on the mechanism of alkylation of phenol with methanol over zeolites and other porous materials. The elucidation of the reaction mechanisms does not only provide insights into the fundamental steps of the reaction but also helps to optimize the reaction conditions and design catalysts for industrial production. The logical progression, which is the aim of our present work, is to propose the mechanism of alkylation of phenol with methanol on acidic

faujasite zeolite by employing the ONIOM method. The structures and energetic properties along the reaction coordinates of both the stepwise and concerted routes are presented. Finally, the reactivities for the O- and C-methylations as well as its product stability are discussed. 2. Theoretical Methods The faujasite zeolite was modeled by the 178T nanocluster taken from the lattice structure of faujasite zeolite.43 The dangling bonds resulting from cutting the Si-O bonds, which were terminated by the H atoms with the Si-H bond distances of 1.47 Å, pointed in the same direction as the crystallographic Si-O bonds. This nanocluster covers an area of two supercages connected to each other through the 12T-membered ring window. To increase the computational efficiency, the 178T cluster was subdivided into two layers of calculation methods according to the two-layer ONIOM (ONIOM2) scheme (Figure 1). The 14T cluster, including the 12T-membered ring window opening to two supercages and two additional basal T building units, was treated as the inner layer and computed with the hybrid DFT approach (B3LYP) combined with the 6-31G(d,p) basis set for describing the local active site of faujasite zeolite. The remainders of the 178T extended framework connected to the 14T active site were treated as the outer layer and computed with the universal force field (UFF). This force field has been found to provide a good description of the long-range vdW interactions.33-40 All calculations were performed using the Gaussian 03 code.44 During optimization, only the basal 5T of the 14T inner layer, tSiO(Hz)Al(OSi≡)2OSit, was permitted to relax while the remainders were fixed along the position of the crystallographic lattice. Frequency calculations were performed at the same level of theory to ensure that the obtained transition state structure has only one imaginary frequency that corresponds to a saddle-point of the required reaction coordinates. 3. Results and Discussion Two possible reaction pathways of alkylation of phenol with methanol have been proposed: the stepwise and concerted mechanisms as shown in Schemes 1 and 2, respectively. Following the experimental formation of the reactive surface methoxide species in the H-zeolites,9,19-21 the stepwise mechanism was proposed to take place via the creation of an active methoxide intermediate in the first step. This active species can, then, immediately methylate phenol to give anisole or cresol as the products depending on whether the O- or C-methylation is more favored. Under normal experimental conditions, where many molecular species are present in the reaction, the formation of the surface methoxide species may be less important. The reaction is able to take place through an alternate concerted route where methanol and phenol can directly collide with each other to form the products. Concerning the C-methylation of phenol,

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Figure 2. Relative energies (kcal/mol) of all species involved in the stepwise mechanism of phenol methylation with methanol over H-FAU zeolite occurring through the O-methylation (solid line) and the C-methylation (dot line).

Figure 3. Optimized structures of the methanol adsorption (a), Ads_Met, the transition state for the surface methoxide formation (b), TS1_Met, and the surface methoxide intermediate (c), [Int_Met‚‚‚H2O].

TABLE 1: Optimized Geometric Parameters of All Species Involved in the Methoxide Formation Step of the Stepwise Mechanisma methoxide formation parameters Al-Hz Al-O1 Al-O2 Si-O1 Si-O2 O-Hz Hz-Omet Omet-Cmet Cmet-O2 Al-O1-Si Al-O2-Si O1-Hz-Omet O2-Cmet-Omet a

zeolite 2.44 1.89 1.69 1.68 1.62 0.97 1.42

126.5 130.3

Ads_Met Distances 2.30 1.85 1.69 1.66 1.61 1.06 1.45 1.45 3.44 Angles 125.7 133.7 161.0 72.2

TS1_Met

Int_Met‚‚‚H2O

1.74 1.76 1.60 1.63 3.06 0.98 1.91 2.13 126.8 133.1 111.2 164.8

1.70 1.84 1.60 1.70 0.96 3.14 1.52 126.9 129.2

Distances are in angstroms, and angles are in degrees.

the attention was mostly focused on an aromatic nucleophilic substitution reaction at an ortho position. This is because o-cresol is the main product found experimentally in the phenol methylation process.10,12,13

3.1. Stepwise Reaction Pathway. 3.1.1. Surface Methoxide Intermediate Formation. In the step of the surface methoxide formation, the reaction was initialized by the adsorption of a methanol molecule on the active site of H-FAU and subsequently by the activation of the methanol C-O bond cleavage to generate the surface methoxy intermediate. The calculated relative energies of all species involved in this step are summarized in Figure 2. The optimized structure of the adsorption complex, Ads_Met, was stabilized by two neutral hydrogen bonds between the OH group of methanol and the Brønsted acid site (O1-Hz) of the zeolite: one between the Brønsted proton (Hz) and the methanol oxygen atom (Omet), and the other between the methanol hydrogen atom (Hmet) and the basic oxygen atom of the zeolite active site (O3). A similar result has been reported by Haase and Sauer for the 1:1 coverage adsorption of methanol on the H-zeolites.28 The calculated adsorption energy was predicted to be -21.6 kcal/mol. This value compares well with the experimental estimates of the methanol heat of adsorption in acidic H-ZSM-5 zeolite, ranging from 15 to 27 kcal/mol.45,46 In the adsorption complex (Figure 3a), the O1-Hz bond length of zeolite was lengthened from 0.97 to 1.06 Å and the interatomic distance between O3 and Hmet was estimated to be 2.11 Å. The estimated O1‚‚‚Omet distance of 2.48 Å compares well with an experimental report for the O1‚‚‚Omet distance in the methanol adsorption on

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Figure 4. Optimized transition state structures of the O-methylation (a), TS2_anisole, and the C-methylation (b), TS2_o-cresol, in the stepwise mechanism.

TABLE 2: Optimized Geometric Parameters of All Species Involved in the O- and C-Methylation Steps of the Stepwise Mechanisma O-methylation parameters

Int_Met‚‚‚Phe

TS2_anisole

C-methylation

Prod_anisole

TS2_o-cresol

Prod_cresol

Distances Al-O1 Al-O2 Si-O1 Si-O2 Cmet-O2 Cmet-Ophe Cmet-Cphe Ophe-Hphe Cphe-Hphe

1.71 1.85 1.60 1.69 1.50 3.53 3.79 0.97 1.09

1.74 1.76 1.60 1.62 2.16 1.88

1.85 1.69 1.67 1.61 1.44

0.97

1.50

1.74 1.76 1.60 1.63 2.14

1.85 1.71 1.66 1.62

2.20

1.51

1.09

3.40

Angles Al-O1-Si Al-O2-Si O2-Cmet-Ophe O2-Cmet-Cphe a

125.8 130.5 102.8 109.0

126.5 133.6 174.2

125.6 132.9

127.7 132.7

126.2 131.3

172.1

Distances are in angstroms, and angles are in degrees.

H-ZSM-5 using multinuclear solid-state NMR spectroscopy,47 which is in the range of 2.5-2.6 Å. The optimized geometric parameters of the methanol adsorption complex, transition state, and methoxide intermediate are tabulated in Table 1. In the second step, the methanol C-O bond cleavage was activated by the attack of the Brønsted proton Hz on the methanol oxygen atom Omet. As a result, a water molecule was produced and the surface methoxide species was created at the zeolite framework. Numerous theoretical calculations and experimental studies have intensively focused on whether this species can be generated and is stable.8,18-21 Blaszkowski and van Santen reported that the methoxide species was obtained with the reaction of methanol on the Brønsted acid site of zeolite.23 The in situ MAS NMR spectroscopic investigations proved that adsorption of methanol on the H-Y zeolite can lead to the formation of the surface methoxide species at room and higher temperatures.8,19-21 These observations coincide with

the result obtained by Govid et al. in the periodic DFT study on the H-FER zeolite.26 In the transition state, TS1_Met, methanol was protonated, which resulted in the weakening of its C-O bond. As shown in Figure 3b, the protonated methanol OH group is leaving as the methyl group is attacked by the active zeolite O atom. As a consequence of this, we observe the dissociation of the CmetOmet bond and the association of the Cmet-O2 bond, respectively. The Cmet-Omet bond was lengthened from 1.45 to 1.91 Å, whereas the Cmet‚‚‚O2 distance was contracted to be 2.13 Å and the corresponding Omet-Cmet-O2 angle was estimated to be 164.8°. In this configuration, the geometry of the methyl group was altered from the tetrahedral to the trigonal planar structure. The active site structure of zeolite was slightly affected by the progression of the reaction. The bond distances were changed by at most 0.1 Å from the adsorption complex. With respect to the adsorption complex, the activation energy for this

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Figure 5. Relative energies (kcal/mol) of all species involved in the concerted mechanism of phenol methylation with methanol over H-FAU zeolite occurring through the O-methylation (solid line) and the C-methylation (dot line).

transition state was predicted to be 39.6 kcal/mol, lower than that obtained by the previous DFT and ab initio calculations of 44-46 kcal/mol.23-26 The surface methoxide species, Int_Met, was created by the covalent bond formation between the methyl Cmet and the O2 atom of the zeolite active site with the Cmet-O2 bond distance of 1.52 Å (Figure 3c). The reaction was endothermic by 13.3 kcal/mol ([Int_Met‚‚‚H2O] in Figure 2), indicating that the intermediate is thermodynamically unstable and that it can react with a generated water molecule or other species in the system without difficulty.3,8,22 3.1.2. Alkylation Reaction Step: O- and C-Methylation. Once the methoxide intermediate, Int_Met, was generated and water was eliminated from the system, a phenol molecule was inserted into the zeolite supercage and was coadsorbed nearby the active methoxide species. The calculated relative energies of all species involved in this step are included in Figure 2. The corresponding adsorption energy was calculated to be -11.1 kcal/mol ([Int_Met‚‚‚Phe] in Figure 2). The closest Cmet‚‚‚CPhe and Cmet‚‚‚OPhe interatomic distances between the methoxy intermediate and phenol were estimated to be 3.79 and 3.53 Å, respectively (cf., Table 2). The shorter Cmet‚‚‚OPhe interatomic distance implies that the O-methylation occurs more readily than

Jansang et al. the C-methylation. For the O-methylation, the transition state involved the concerted action of the O2-Cmet bond breaking and the Cmet-OPhe bond forming, whereas it was associated with the simultaneous action of the O2-Cmet bond cleavage and the Cmet-CPhe bond formation for the C-methylation. The O2-Cmet bond distance in the transition state structure was estimated to be 2.16 and 2.14 Å for the O- and C-methylation, respectively, whereas the Cmet-OPhe and Cmet-CPhe bond distances were evaluated to be 1.88 and 2.20 Å for the O- and C-methylation, respectively (Figure 4, parts a and b). With respect to the coadsorption complex, the activation energies of the O- and C-methylation were estimated to be 22.2 and 23.1 kcal/mol, respectively (TS2_anisole and TS2_o-cresol in Figure 2). This result clearly indicates that the methylation of phenol is a competitive reaction between the O- and C-methylation. The O- and C-methylation reactions were completed with the proton back-donation from the methylated phenol to the zeolitic framework. The reaction energy for the production of anisole was exothermic by 16.7 kcal/mol (Prod_anisole in Figure 2), whereas it was exothermic by 30.4 kcal/mol for the production of cresol (Prod_o-cresol in Figure 2). The more exothermic formation of o-cresol indicated that it is more thermodynamically stable than anisole. 3.2. Concerted Reaction Pathway. Alternatively, for the concerted mechanism, the reaction was initialized by the coadsorption complex between methanol and phenol over the Brønsted acid site and followed by the direct methylation of phenol with methanol without the formation of the methoxide intermediate. The calculated relative energies of all species involved in this mechanism are summarized in Figure 5. Methanol was adsorbed on the acidic proton via the hydrogen bond complex in a similar way to that in the step-by-step mechanism with the coadsorption of the phenol molecule, Co_Ads. The O1-Hz bond length of zeolite was increased to 1.05 Å with the estimated O1‚‚‚Omet distance of 2.48 Å. The closest Cmet‚‚‚OPhe and Cmet‚‚‚CPhe interatomic distances between the methanol and phenol molecules were evaluated to be 4.10

Figure 6. Optimized transition state structures of the O-methylation (a), TS_anisole, and the C-methylation (b), TS_o-cresol, in the concerted mechanism.

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Figure 7. Relative energies (kcal/mol) with respect to methanol adsorption of important species involved in the stepwise (blue) and concerted (red) mechanisms of phenol methylation with methanol over H-FAU zeolite occurring through the O-methylation (solid line) and the C-methylation (dot line).

TABLE 3: Optimized Geometric Parameters of All Species Involved in the O- and C-Methylation of the Concerted Mechanisma O-methylation parameters

Co_ads

TS_anisole

C-methylation

Prod_anisole

TS_o-cresol

Prod_cresol

Distances Al-Hz Al-O1 Al-O2 Si-O1 Si-O2 O-Hz Hz-Omet Cmet-Omet Cmet-Ophe Cmet-Cphe Ophe-Hphe Cphe-Hphe

2.32 1.84 1.70 1.66 1.61 1.05 1.46 1.44 4.10 3.99 1.09 1.09

1.74 1.73 1.61 1.61

1.71 1.81 1.60 1.67

0.97 1.98 1.97

1.45

0.99

1.44

1.74 1.71 1.60 1.60

1.70 1.82 1.60 1.67

0.97 2.09 2.21

1.51

1.09

4.67

126.44 137.47

124.51 135.61

Angles Al-O1-Si Al-O2-Si O1-Hz-Omet Omet-Cmet-Ophe Omet-Cmet-Cphe a

126.63 132.24 160.41

126.6 134.97

125.41 133.08

168.35 169.82

Distances are in angstroms, and angles are in degrees.

and 3.99 Å, respectively (cf., Table 3). The adsorption energy corresponding with this complex was estimated to be -35.0 kcal/mol. In the transition state, methanol was protonated by the zeolite acid site. Unlike in the stepwise mechanism where the protonated methanol was used to prepare an active methoxy intermediate, a methoxonium cations (a protonated form of methanol) reacted directly with phenol through the O- and C-methylation (Figure 6, parts a and b, respectively). The methanol Cmet-Omet bond was activated and lengthened to 1.98 and 2.09 Å for the O- and C-methylation, respectively, whereas the Cmet‚‚‚OPhe and Cmet‚‚‚CPhe distances were contracted to 1.97 and 2.21 Å for O- and C-methylation, respectively. The OmetCmet-OPhe and Omet-Cmet-CPhe angles were estimated to be 168.4° and 169.8° for the O- and C-methylation methyl, respectively. In both cases, the hybridization of the methyl Cmet resembled the trigonal planar geometry. With respect to the coadsorption complex, the activation energies for the O- and C-methylation (TS_anisole and TS_o-cresol in Figure 5) were calculated to be 28.3 and 32.6 kcal/mol, respectively. These values are in between the activation energies for the methoxide

formation (39.6 kcal/mol) and methylation (22.2 and 23.1 kcal/ mol for O- and C-methylation, respectively) in the stepwise mechanism. The O-methylation was found to more readily occur over the C-methylation. However, the reaction energy for the O-methylation was less exothermic than that for the Cmethylation. In the coadsorption complex, the formation of anisole, Prod_anisol, was endothermic by 7.3 kcal/mol, whereas the formation of o-cresol, Prod_o-cresol, was exothermic by 13.1 kcal/mol. These results are consistent with that obtained from the stepwise reaction pathway and are also in good agreement with previous experimental reports in which anisole is an unstable primary product of the phenol methylation.5,6,9,12 3.3. Comparison of Stepwise and Concerted Reaction Pathways. From the methylation of phenol with methanol through the stepwise mechanism, it is found that the formation of the surface methoxide intermediate is the limiting step of the overall reaction (Figure 2). This result agrees well with the previously studied aniline methylation on the H-Y zeolite using in situ MAS NMR, where the zeolite samples were methoxylated by methanol or methyl iodide prior to aniline adsorption.8 Once the methoxide is generated, it can react rapidly with phenol,

546 J. Phys. Chem. C, Vol. 112, No. 2, 2008 and therefore, the reactivity difference between the O- and C-methylation on the basis of the activation energy difference becomes less obvious (only 0.9 kcal/mol). On the other hand, when the methylation undergoes the concerted mechanism without the formation of the methoxide species (Figure 5), the reactivity difference between the O- and C-methylation becomes more observable (4.3 kcal/mol). Furthermore, it is apparent that the direct methylation of phenol through the concerted mechanism requires less activation energy (28.3 and 32.6 kcal/mol for the O- and C-methylation, respectively) than that for the methoxide formation in the stepwise mechanism (39.6 kcal/ mol). This infers that the activated surface methoxide intermediate is not easily observed under normal experimental conditions. Instead, the protonated methanol or the methoxonium cation, that can be straightforwardly formed, would be the dominant species in the phenol methylation process. Thus, it is worthwhile to consider the methanol adsorption on the Brønsted acid site as being the key step of the overall methylation reaction. When the adsorbing methanol is protonated, it can react with the phenol molecule and other molecular species adsorbed nearby immediately. This explanation is rationalized by the observation that the dimethyl ether (DME) or coke species are also detected in a system that contained a high molar ratio of methanol.3,10,19 Figure 7 summarizes the calculated relative energies of some important species involved in both the stepwise and concerted mechanisms. In the case of the methanol adsorption complex, the concerted activation energies for O- and C-methylation were estimated to be 14.9 and 19.2 kcal/mol, respectively. These values lie between the activation energies of the methoxide formation (39.6 kcal/mol) and the methylation steps (11.1 and 12.0 kcal/mol for the O- and C-methylation, respectively) in the stepwise mechanism and compare well with the available experimental apparent activation energies of phenol methylation with methanol over H-ZSM-517 and NaX4 zeolites of 12-17 and 13.7 kcal/mol, respectively. From these results, it could be concluded that the phenol methylation with methanol over the H-FAU zeolite favors the concerted mechanism and is a competitive reaction between the O- and C-methylation in which the O-methylation proceeds more rapidly than the C-methylation. Although anisole is a kinetically favorable product of phenol methylation, under some circumstances, at higher temperatures and with a longer reaction time, it can be transformed to cresol directly.6,9,12,13 Previous experimental studies have shown that o-cresol is the most populated product of phenol methylation.10,12,13 This suggests that o-cresol is a more thermodynamically stable product of the reaction, consistent with our finding that the formation energy of o-cresol (-15.3 kcal/mol) is more exothermic than that of anisole (-1.6 kcal/mol). 4. Conclusions By using the two-layered ONIOM(B3/6-31G(d,p):UFF) method, a mechanistic investigation has demonstrated that the methylation of phenol with methanol over the proton faujasite zeolite undergoes a competitive reaction between the O- and C-methylation. The reactivity for the O-methylation is virtually the same as for the C-methylation when the reaction occurs through the surface methoxide species. However, when the reaction takes place through the concerted mechanism, the O-methylation becomes obviously faster than the C-methylation. The methylation reaction occurs more readily through the concerted mechanism with the activation energies of 14.9 and 19.2 kcal/mol for the O- and C-methylation, respectively, which compares well with the apparent activation energies of 12-17

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