SO3H-Functionalized Ionic Liquid Catalyzed Alkylation of Catechol

Jul 26, 2010 - Several SO3H-functionalized ionic liquids (FILs) were synthesized and their catalytic performances for catechol (CAT) alkylation with t...
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Ind. Eng. Chem. Res. 2010, 49, 8157–8163

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SO3H-Functionalized Ionic Liquid Catalyzed Alkylation of Catechol with tert-Butyl Alcohol Xiaowa Nie,†,‡ Xin Liu,† Lei Gao,† Min Liu,† Chunshan Song,*,†,‡ and Xinwen Guo*,† State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian UniVersity of Technology, Dalian 116012, China, and EMS Energy Institute and Department of Energy & Mineral Engineering, PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802

Several SO3H-functionalized ionic liquids (FILs) were synthesized and their catalytic performances for catechol (CAT) alkylation with tert-butyl alcohol (TBA) were studied theoretically as well as experimentally. Under optimized reaction conditions, the conversion of CAT was 41.5%, and the selectivity for 4-tert-butyl catechol (4-TBC) could reach 97.1%. This electrophilic reaction exhibits a kinetics-dependent character. The higher frontier electron density at the C4 site, comparatively lower activation energy barrier (Ea), and higher thermodynamic stability all act in concert to make the reaction proceed preferentially toward 4-TBC. The FILs can lead to a much higher CAT conversion and a similar selectivity to 4-TBC compared to a Beta zeolite catalyst. Because the frontier electron density [fr(E)] of CAT was not significantly affected by the solvent, the product selectivity rule would not vary, whereas the addition of a solvent with a higher dielectric constant would slow the reaction by increasing Ea, thus resulting in a lower CAT conversion. 1. Introduction Catechol (CAT) tert-butylation is an industrially important reaction because the product, 4-tert-butyl catechol (4-TBC), can be used as an antioxidant, stabilizer, and polymerization inhibitor, as well as an important intermediate for the production of dye developers, heat stabilizers for agricultural chemicals, as well as many other products.1,2 4-TBC can be produced by acid-catalyzed alkylation of CAT with tert-butyl alcohol (TBA), which is a typical Friedel-Crafts reaction.3,4 For this conversion, the conventional Brønsted acid or Lewis acid catalysts widely used are liquid acids, solid acids, and metal halides, which have many disadvantages. Liquid acid catalysts cause equipment corrosion and environmental pollution, and solid acids can deactivate rapidly as a result of coke formation.5 Recently, ionic liquids (ILs) have been applied and shown to be effective in some catalytic conversions.6-12 They have important and unique properties such as negligible vapor pressure, excellent chemical and thermal stability, potential recoverability, and convenience for the separation of products from reactants.13-16 In particular, Brønsted acidic functionalized ionic liquids (FILs) have been reported as novel ecologically benign catalysts for some acid-catalyzed reactions.17-22 In this work, tert-butylation of CAT was studied using laboratorysynthesized SO3H-functionalized ionic liquids (SO3H-FILs) as acidic catalysts. These FILs exhibit superior catalytic activity. Experimental results show that the final products from the SO3HFIL-catalyzed tert-butylation of CAT are a mixture of 3-tertbutyl catechol (3-TBC), 4-tert-butyl catechol (4-TBC), and 3,5di-tert-butyl catechol (3,5-DTBC), with 4-TBC as the dominant product. However, tert-butyl catechol ether (TBCE) was not detected in the experiments, as reported in some other related studies.1,2 The acidic character of the SO3H-FIL catalyst is one of the key factors affecting this catalytic conversion. Solvents * To whom correspondence should be addressed. Tel.: 814-863-4466 (C.S.), +86- 411-39893990 (X.G.). Fax: 814-865-3573 (C.S.), +86411-39893991 (X.G.). E-mail: [email protected] (C.S.), [email protected] (X.G.). † Dalian University of Technology. ‡ Pennsylvania State University.

with different dielectric constants were tested to investigate their influence on product selectivity and CAT conversion. By combining the experimental results and theoretical calculations, the detailed mechanism of this SO3H-FIL acid-catalyzed tertbutylation of CAT was elucidated at the molecular level, and the factors affecting the reactivity are highlighted in the discussion herein. 2. Experimental Section 2.1. Preparation of Ionic Liquids. Four SO3H-functionalized ionic liquids (IL1, IL2, IL3, and IL4, as shown in Table 1) were synthesized according to the procedure in our previous work.23 Trimethylamine or triethylamine was mixed with 1,4butane sultone and stirred at 333 K for 24 h. After solidification of the mass, the product (zwitterion) was washed three times with methanol and ethyl acetate and then dried under a vacuum (353 K, 0.01 Torr). A stoichiometric amount of sulfuric acid or p-toluenesulfonic acid was added to the precursor zwitterion. The mixture was stirred at 353 K for 8 h to form the ionic liquid. The product phase was washed with solvent and dried in a vacuum. 2.2. Characterization of Ionic Liquids. The synthesized ionic liquids were identified by 1H nuclear magnetic resonance (NMR) spectrometry, 13C NMR spectrometry (Varian INOVA 400 MHz), and electrospray ionization mass spectrometry (ESIMS; Q-TOF Micro Mecromous, VK). Data on the thermal stabilities of the ionic liquids were obtained by thermogravimetric analysis (TGA; SDT851e) at a heating rate of 10 °C/ min under nitrogen, with a flow rate of 20 mL/min.23 2.3. Catalytic Tests. The tert-butylation of CAT with TBA was carried out in a round-bottom flask. The typical reaction conditions were as follows: CAT, 20 mmol; TBA, 10 mmol; ionic liquid, 1 mmol; reaction temperature, 423 K. The mixture was stirred for 3 h under condensing reflux. After the catalytic reaction, methanol was used to dissolve the reaction solution. A qualitative analysis of the product was conducted on an HP6890/5973 gas chromatography/mass spectrometry (GC/MS) system; quantitative analysis was carried out on a HP6890 GC instrument equipped with a HP-5 column.23

10.1021/ie100800c  2010 American Chemical Society Published on Web 07/26/2010

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Table 1. Structures of the SO3H-Functionalized Ionic Liquids (FILs) Synthesized and Examined in This Work

2.4. Computational Methods. Quantum chemical calculations for full geometry optimization of all species involved were conducted within the framework of density functional theory (DFT) with the Gaussian 03 code.31 Energy estimations and frequency analysis were carried out with the B3LYP24-26 hybridized functional at the 6-31+G(d,p)27 and 6-311+G(d,p)28,29 levels. Calculations with the B3PW9130 functional at the same levels were also conducted for comparison. The stability of the wave function was tested for each case. The zero-point-energy (ZPE) corrections were applied, and the Gibbs free energies corrected for ZPE were computed from the harmonic frequencies at 423 K and 1 atm, in order to make results comparable with the experiments. Local atomic charges and frontier electron densities [fr(E)] were obtained by ChelpG population analysis at the same level. The transition state (TS) of each elementary reaction was located by the QST method.31 The obtained transition state was confirmed by the single imaginary frequency along the reaction direction first and then reverified by an intrinsic reaction coordinate (IRC) scan.32,33 The solvent interaction was studied on the basis of the gas-phase-optimized geometries with the integral equation formalism of the polarizable continuum model (IEFPCM)34-37 as implemented in Gaussian 03 package.31 3. Results and Discussion 3.1. Experimental Results. 3.1.1. Characterization of the Ionic Liquids. The four presynthesized SO3H-functionalized ionic liquids were characterized by 1H and 13C NMR spectroscopies and ESI-MS.23 The structural characterization data are provided in Tables S1 and S2 of the Supporting Information. The structures are in good agreement with those previously reported in the literature.18,38,39 Thermogravimetry-differential thermogravimetry (TG-DTG) tests of the four FILs were carried out, and the decomposition temperatures were obtained from the DTG curves. The results show that the decomposition temperatures of IL1-IL4 were 618, 606, 586, and 598 K, respectively, indicating the excellent thermal stabilities of these FILs. 3.1.2. Catalytic Performance of the Ionic Liquids. All four FILs listed in Table 1 showed catalytic activity for CAT tertbutylation, and they all exhibited higher selectivity to 4-TBC than to 3-TBC. However, it appears that the anion in the FILs (as shown in Table 1) was more influential than the cation on the catalytic activity. FILs with sulfate anion but different cations (IL1, IL3) were found to be more active than those with the p-tolylsulfonic anion and different cations (IL2, IL4). Among the four FILs, IL1 showed the highest activity for CAT conversion, as well as the highest selectivity to 4-TBC.23

Table 2. tert-Butylation of CAT with TBA Using Different Catalystsa selectivity (%) catalyst

conv X(CAT) (%)

3-TBC

4-TBC

3,5-DTBC

H2SO4 p-CH3 · C6H4SO3H Hβ IL1

49.6 43.2 21.1 41.5

1.5 1.8 0.7 2.2

98.1 98.0 97.6 97.1

0.4 0.4 0.5 0.7

a Reaction conditions: n(CAT)/n(TBA)/n(IL1) ) 2:1:0.1, T ) 423 K, t ) 3 h, m(Hβ)/m(CAT + TBA) ) 0.1.

Therefore, IL1 was used as a representative FIL for the subsequent study. Under the optimized reaction conditions [n(CAT)/n(TBA)/ n(IL1) ) 2:1:0.1, t ) 3 h, T ) 423 K], the conversion of CAT was 41.5%, and the selectivity for 4-TBC could reach 97.1%.23 Therefore, our laboratory-synthesized SO3H-FIL catalysts can exhibit high activity for CAT tert-butylation with high selectivity for 4-TBC. The conversion and selectivity as functions of reaction time for CAT tert-butylation with IL1 are shown in the Supporting Information (Figure S1). The results show that the conversion of CAT and the selectivity for 4-TBC increased rapidly and reached the highest level after 3 h.23 The best reaction time was 3 h because the catalyst activity reached a stable higher active state. A recycle test was also performed with the most promising IL1. The results given in Supporting Information (Table S3) show that there were no apparent changes in the selectivity to 4-TBC, which remained at around 96%, in five repeated cycles. However, the conversion of catechol decreased from 50% to 42% in the first recycle and then remained steady at around 40% to the fifth cycle. The recycle test indicates that the synthesized ionic liquids could be recycled. The results of tert-butylation of CAT with TBA using different catalysts under comparable reaction conditions at 423 K are presented in Table 2. From these results, the FIL exhibited good catalytic performance for this tert-butylation reaction. The catalytic activity of the FIL appears to be lower than that of the strong liquid acid (H2SO4), but similar to that of p-CH3 · C6H4SO3H. It is worth noting that the FIL (IL1) exhibited a better catalytic activity than Hβ catalyst, which is a well-known zeolitic solid acid catalyst. From these results, SO3H-FIL has a potential to become a favorable acid catalyst for this tert-butylation reaction, because of its excellent activity with high selectivity, much lower corrosion tendency compared to conventional liquid acids, excellent thermal stability, ex-

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010 Table 3. Influence of Solvent on tert-Butylation of CAT with IL1 under Mild Conditionsa solvent

dielectric constant

none toluene chlorobenzene dichloromethane

2.38 5.62 8.93

conv X (CAT) (%) 3-TBC 32.8 32.6 31.2 17.5

25.3 29.5 29.9 27.6

selectivity (%) 4-TBC

3,5-DTBC

61.4 56.6 54.6 52.9

13.3 13.9 15.5 19.5

a Reaction conditions: n(CAT)/n(TBA)/n(IL1) ) 2:1:0.1, T ) 343 K, t ) 3 h. The solvent amount was 3 mL.

tremely low vapor pressure, good recoverability, and convenience for separation of products from reactants. The influence of solvents tested under mild conditions is shown in Table 3. The use of a neutral solvent such as toluene or a slightly polar solvent such as chlorobenzene did not show any significant impact on the catalytic activity but decreased the selectivity for 4-TBC. It is apparent from Table 3 that addition of a solvent with a high dielectric constant such as dichloromethane can lead to a major reduction in the catalytic activity of IL1 for CAT conversion, together with a further decrease in the selectivity for 4-TBC. These results suggest that, for selecting the reaction medium, one should avoid solvents with high dielectric constants. 3.2. Computational Results. 3.2.1. Proposed Reaction Mechanism. Density functional theory (DFT)40-44 calculations based on quantum chemistry were conducted to understand the reaction mechanism at the molecular level. There are two possible reaction pathways, including one with t-C4H9OH as the t-butyl donor directly and the other with p-tolylsulfonate as the t-butyl donor after dehydration of t-C4H9OH. In the tertbutylation process of CAT, a proton was donated from the SO3H-FIL. Before examining the two proposed mechanisms, we first examined the interaction between the anion of FIL (HSO4- or p-CH3 · C6H4SO3-) (or substitutional sulfonic acid anion derived from the FIL) and the protonated tert-butylation intermediate. The crucial issue here was to verify which part of the FIL played the role of taking the proton from the protonated intermediate for both reaction mechanisms, that is, the anion of the FIL (HSO4- or p-CH3 · C6H4SO3-) or the substitutional sulfonic acid anion derived from the FIL. To provide reasonable evidence for this investigation, the E(HA) E(A-) values, which represent the proton affinities (PAs) of the corresponding acid anions, were calculated. The corrected electronic energy of each species was obtained using the B3LYP and B3PW91 functionals at the 6-31+G(d,p) and 6-311+G(d,p) levels for comparison. Calculation results presented in Table 4 show that the functional and basis set had small influences on the estimations of PAs. B3LYP is the most widely used functional, and the B3LYP/6-311+G(d,p) method adopted in this study is more appropriate, for the reason that it provided more reasonable electronic energies than the other methods in Table 4. Based on this consideration, B3LYP/6-311+G(d,p) was employed for the subsequent calculations. In general, the more negative the PA, the stronger the tendency of the corresponding acid anion to take the proton. The PA values calculated for HSO4-, p-CH3 · C6H4SO3-, and the substitutional sulfonic acid anion using the B3LYP/6-311+G(d,p) method were 1293.9, 1329.5, and 1109.2 kJ/mol, respectively. From these results, the anion of the FIL (HSO4- or p-CH3 · C6H4SO3-) tends to function as the carrier for cropping the proton from the protonated intermediate, because of its stronger PA, as compared with that of the substitutional sulfonic acid anion derived from the FIL. Therefore, after the deprotonation of the protonated intermediate, there might be a proton transfer from the H2SO4 or

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p-CH3 · C6H4SO3H to the substitutional sulfonic acid anion to keep the persistent SO3H-FIL acidic catalyst. For further investigation of the reaction mechanism, a computational model based on the following approximations was used: (1) Because this study aimed at elucidating the reaction pathway at the molecular level and clarifying the factors affecting the reactivity, a comparatively simple acid model was introduced to represent the structure of SO3H-FIL acidic catalyst, based on the fact that this SO3H-FIL-catalyzed reaction is a Brønsted-acid-catalyzed reaction as well. (2) Because the cation group (quaternary ammonium part) of the SO3H-FIL played only a minor role in catalyzing this tert-butylation reaction (as expected from our experimental results) and because the acidity of CH3SO3H is similar to that of quaternary ammonium ion substituted sulfonic acid, a CH3SO3H group was selected as the acid model to represent the SO3H-FIL-based Brønsted-acid catalyst for the sake of computational efficiency. According to the above approximations, the scheme of the two proposed reaction pathways with our computational model selected is depicted in Figure 1. In the mechanism shown in Figure 1a, t-C4H9OH works as the t-butyl donor; the protonated intermediate is formed through the interaction between t-C4H9OH and CAT under CH3SO3H acidic medium, simultaneously releasing a water molecule. In the mechanism shown in Figure 1b, t-C4H9OH first interacts with CH3SO3H to form CH3SO3-t-C4H9 and water. Then, CH3SO3-t-C4H9 will act as the t-butyl donor to attack CAT. In the tert-butylation process of CAT for the two pathways, because of the resonance effect of the nonbonding pz orbitals on the oxygen atoms and the π bonding orbital on the benzene ring, the local charge density and frontier electron density [fr(E)] on oxygen atoms are relatively larger than those on carbon atoms in a CAT molecule. This factor would facilitate the formation of an O intermediate (Int_O, as shown in Figure 1) instead of C intermediates (Int_3 and Int_4 in Figure 1). Int_O is likely to convert to Int_3 and Int_4 in this acid-catalyzed medium.16 With the progress of tert-butylation, the primary tert-butylation products 3-TBC and 4-TBC can be further converted to 3,5DTBC, which is a sequential reaction and is omitted from Figure 1 for clarity. 3.2.2. Comparison of the Two Mechanisms. The reactivity difference between these two proposed mechanisms lies in the different t-butyl donors as the electrophilic agents for the tertbutylation of CAT. To verify which mechanism is preferred, extensive DFT calculations were carried out to delineate the reaction paths by clarifying the transition states (TSs) and exploring the potential energy surface (PES). The activation energy barriers (Ea) for tert-butylation of CAT with TBA to form Int_3, Int_4 and Int_O in the mechanism with t-C4H9OH working as the t-butyl donor, as well as the dehydration step of t-C4H9OH to form CH3SO3-t-C4H9 and water in the other mechanism were predicted to be 125.8, 110.7, 98.9, and 138.8 kJ/mol, respectively, from the corrected electronic energy (E) calculation results. On the other hand, the free energy barriers from Gibbs free energy (G) calculations at 423 K were estimated to be 120.3, 102.5, 93.1, and 130.4 kJ/mol, respectively for those corresponding steps mentioned above. The Gibbs free energies produce much lower barriers than those predicted from the electronic energies. By comparison of Ea of the direct tertbutylations and the dehydration of t-C4H9OH, it can be concluded that the pathway with t-C4H9OH as the direct t-butyl donor is kinetically more preferred than that with the dehydration of t-C4H9OH as the first step.

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Table 4. Corrected Electronic Energies (E, Ha) and Proton Affinities (PAs, kJ/mol) Calculated Using the B3LYP and B3PW91 Functionals at the 6-31+G(d,p) and 6-311+G(d,p) Levels

E(H2SO4) (Ha) E(HSO4-) (Ha) PA (kJ/mol) E(CH3 · C6H4SO3H) (Ha) E(CH3 · C6H4SO3-) (Ha) PA (kJ/mol) E(substitutional sulfonic acid) (Ha) E(substitutional sulfonic acid anion) (Ha) PA (kJ/mol)

B3LYP/6-31+G(d,p)

B3LYP/6-311+G(d,p)

B3PW91/6-31+G(d,p)

B3PW91/6-311+G(d,p)

-700.187 -699.696 -1294.8 -895.247 -894.743 -1328.4 -955.669 -995.247 -1112.7

-700.195 -699.704 -1293.9 -895.255 -894.751 -1329.5 -955.676 -995.255 -1109.2

-700.171 -699.681 -1292.5 -895.232 -894.727 -1330.5 -955.652 -955.230 -1112.2

-700.179 -699.688 -1293.8 -895.240 -894.734 -1333.0 -955.660 -955.239 -1110.4

3.2.3. Reactivity of tert-Butylation of CAT with TBA. To clarify the factors influencing the conversion of CAT and the product selectivity, further DFT calculations were performed to study the mechanism with t-C4H9OH as the t-butyl agent, which is more favored kinetically for this tert-butylation reaction. Electronic energies (E) for tert-butylation of CAT were calculated, and ZPE corrections were made. The results are illustrated in Figure 2. It shows that 4-TBC is the thermodynamically most stable product, with 3-TBC ranked second, but TBCE is energetically not favored because it has the highest E value. Further insight was gained from the kinetics analysis. For the initial electrophilic attack of t-C4H9OH on CAT, Ea for the formation of Int_O was lowest, at 98.9 kJ/mol, indicating

that formation of Int_O would be faster than formation of Int_3 and Int_4. On the other hand, Ea values for the formation of Int_3 and Int_4 were estimated to be 125.8 and 110.7 kJ/mol, respectively; therefore, these two tert-butylation reactions compete with each other. Moreover, Ea for the subsequent TBCE formation step was predicted to be 63.8 kJ/mol, which was 34.5 and 42.3 kJ/mol higher than the values for 3-TBC and 4-TBC formation via deprotonation. The Ea values of these deprotonation steps indicate that formation of TBCE would be much slower and need more demanding reaction conditions. Ea for 4-TBC formation was lowest, at only 21.5 kJ/mol. This means that, apart from its relatively higher thermodynamic stability, 4-TBC is also a kinetically preferred product.

Figure 1. Proposed reaction pathways for tert-butylation of CAT with TBA: (a) mechanism with t-C4H9OH as the direct t-butyl donor; (b) mechanism with the dehydration of t-C4H9OH as the first step.

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reported the formation of TBCE. They observed that the TBCE selectivity decreased whereas the 4-TBC selectivity increased as the reaction temperature was increased from 120 to 200 °C. This phenomenon might result from the consumption of Int_O for the formation of Int_3 and Int_4, which, in turn, lead preferentially to 3-TBC and 4-TBC, rather than the formation of TBCE. During this process, conversion to 4-TBC is favored kinetically. In this context, the present computational results and analysis are consistent with Anand et al.’s results.

Figure 2. Schematic view of the corrected electronic energies (E) for tertbutylation of CAT in the mechanism with t-C4H9OH as the direct t-butyl donor. The relative E values were obtained from B3LYP/6-311+G(d,p) calculations.

However, TBCE was not detected in the final product mixture according to the experimental results. One could expect some secondary reactions that consumed Int_O, instead of TBCE production from Int_O. These secondary reactions might occur from Int_O to Int_3 and Int_4. Because of the structural similarities between Int_3 and Int_4, Ea for the conversion of Int_O to Int_3 was predicted to be 33.9 kJ/mol, which was only 5.9 kJ/mol higher than for the conversion of Int_O to Int_4. Therefore, Int_O would more likely be converted to Int_3 and Int_4 owing to the lower Ea, rather than forming TBCE. Although Int_O could be formed faster, the subsequent reactions that consumed Int_O were more preferred, which would inhibit the generation of TBCE from Int_O. These results highlight the kinetics-dependent behavior of this SO3H-FIL-catalyzed tertbutylation reaction. Anand et al.45 conducted the tert-butylation of CAT on HY and dealuminated HY zeolite catalysts and

The enthalpy change (∆H) and Gibbs free energy change (∆G) for the overall reaction of each tert-butylation process were calculated at 423 K. The computational ∆H results show that the overall reaction for 4-TBC formation was exothermic by 3.4 kJ/mol, but the overall reaction was endothermic by 1.2 and 30.3 kJ/mol for 3-TBC and TBCE formation. These values indicate that 4-TBC is more thermodynamically stable than the reactant, which is inverse for 3-TBC and TBCE. On the other hand, the calculated ∆G values of the overall reactions for the formation of 4-TBC, 3-TBC, and TBCE were estimated to be 5.2, 13.4, and 40.1 kJ/mol, respectively, which mean that, under the same conditions, reaction for TBCE formation might be much more difficult and need more additional energy. Figure 3 shows the transition state (TS) geometries for the tert-butylation of CAT to form Int_O, Int_3, and Int_4, together with the deprotonation of these three intermediates to produce TBCE, 3-TBC, and 4-TBC, respectively. The important structural parameters of these transition states are listed in Table 5. The TS geometries for the tert-butylation steps (Figure 3a-c) show that a planar t-butyl carbenium transition state is created through the electrophilic interaction of t-C4H9OH with CAT, followed by release of a water molecule. The TS configurations of the subsequent deprotonation steps, as shown in Figure 3d-f, confirm that the anion of the FIL (CH3SO3-) obtained the proton from the protonated intermediate to produce the final monoalkylation product.

Figure 3. Transition state (TS) geometries for tert-butylation of CAT: (a) TS_O_1st step, (b) TS_3_1st step, (c) TS_4_1st step, (d) TS_O_2nd step, (e) TS_3_2nd step, (f) TS_4_2nd step.

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Table 5. Important Structural Parameters for the Transition State Geometries of tert-Butylation of CAT TS_O_1st step O1-H1 (nm) O2-H1 (nm) C1-O2 (nm) C1-O3 (nm) O1-H1-O2 (deg) H1-O2-C1 (deg)

TS_3_1st step

0.179 0.098 0.315 0.352 169.6 104.9

TS_O_2nd step O1-H1 (nm) O2-H1 (nm) C1-O2 (nm) O1-H1-O2 (deg) H1-O2-C1 (deg)

O1-H1 (nm) O2-H1 (nm) C1-O2 (nm) C1-C2 (nm) O1-H1-O2 (deg) H1-O2-C1 (deg)

TS_4_1st step

0.182 0.099 0.307 0.346 172.6 89.7

O1-H1 (nm) O2-H1 (nm) C1-O2 (nm) C1-C2 (nm) O1-H1-O2 (deg) H1-O2-C1 (deg)

TS_3_2nd step

0.123 0.120 0.154 175.1 108.9

O1-H1 (nm) C2-H1 (nm) C1-C2 (nm) O1-H1-C2 (deg) H1-C2-C1 (deg)

conversion. This conclusion is supported by the experimental results shown in Table 3.

0.176 0.098 0.311 0.342 173.9 106.9

TS_4_2nd step

0.121 0.149 0.157 170.0 106.6

O1-H1 (nm) C2-H1 (nm) C1-C2 (nm) O1-H1-C2 (deg) H1-C2-C1 (deg)

0.121 0.150 0.156 167.5 105.4

Table 6. Frontier Electron Density fr(E) of CAT in Vacuum and Solvent Environmentsa) fr(E) solvent

dielectic constant

C3

C4

O

vacuum chlorobenzene dichloromethane ethanol nitromethane DMSO water

1.0 5.62 8.93 24.55 38.20 46.70 78.39

0.048 0.036 0.033 0.031 0.030 0.030 0.030

0.139 0.151 0.156 0.159 0.160 0.160 0.161

0.299 0.311 0.306 0.302 0.301 0.300 0.300

a fr(E) on O is an average value for the two oxygen atoms because they show small difference.

Table 7. Activation Energies of the Two Elementary Steps for 4-TBC Formation in the Presence of Solvents Ea (kJ/mol) for formation reaction medium

dielectric constant

Int_4

4-TBC

cyclohexane toluene chlorobenzene dichloromethane

2.02 2.38 5.62 8.93

110.7 130.2 141.3 199.6 309.6

21.5 28.8 35.2 44.1 68.5

3.2.4. Influence of Solvent. Further computations were performed on the electronic structure of CAT. Frontier electron densities [fr(E)] derived from the Fukui function can provide a more accurate description of the soft-soft interactions.46 fr(E) for an electrophilic substitution reaction represents the density of electrons in the highest occupied molecular orbital (HOMO).47 It is known that the most reactive position where electrophilic attack occurs most likely has the highest frontier electron density based on the frontier molecule orbital theory. The calculated fr(E) values in vacuum and solvent environments are listed in Table 6. It is apparent from a comparison of the two carbon positions on the ring that fr(E) at the C4 site is much larger than that of the C3 site. Therefore, formation of 4-TBC is also electronically preferred over 3-TBC formation. This superiority of the C4 site over the C3 site in terms of fr(E) is not altered in the presence of several different solvents, as can be seen from Table 6. The reactivity of 4-TBC formation in solvent environments was further studied, and the results are presented in Table 7. In the presence of solvent, the Ea values for the two elementary steps both become higher as compared with those in gas phase. Therefore, solvent will hinder the ionic interaction between cation and anion, which is more important for this electrophilic reaction. Because increasing the dielectric constant of the solvent increased the Ea value monotonically, high-dielectric-constant solvent will strongly inhibit the ionic interaction, which will decrease the reaction rate and indirectly decrease the CAT

4. Conclusions Several SO3H-functionalized ionic liquids (SO3H-FILs) have been synthesized, and their catalytic performances for catechol alkylation with tert-butyl alcohol have been studied by a combination of experimental and computational techniques. The mechanism involved and the reactivity have been studied at the molecular level. Between the two proposed reaction pathways, the mechanism with t-C4H9OH as the t-butyl donor directly (without dehydration) is kinetically more preferred than the one with the dehydration of t-C4H9OH as the first step. This electrophilic tert-butylation reaction shows a kinetics-dependent character. The higher frontier electron density at the C4 site of catechol, lower Ea for 4-TBC formation, and higher stability of 4-TBC all together make this reaction proceed with a high selectivity toward 4-TBC. However, the formation reaction of TBCE from Int_O is much slower because of the higher Ea in the deprotonation step, and TBCE has the lowest thermodynamic stability among the three potential monoalkylation products (4TBC, 3-TBC, and TBCE). As a result of its unstable character, Int_O would more likely be converted to Int_3 and Int_4, rather than forming TBCE. The FILs can lead to a much higher CAT conversion and a similar selectivity to 4-TBC compared to a Beta zeolite catalyst. As the relative frontier electron density values of reactive positions of catechol are not significantly influenced by the solvent, the product selectivity rule is not changed. However, the addition of solvent with high dielectric constant slows the reaction by increasing Ea and results in a decrease of the CAT conversion. Acknowledgment This work was financially supported by the program for New Century Excellent Talent in University (NECT-04-0268) and the Plan 111 Project of the Ministry of Education of China. The authors also thank the Pennsylvania State University for partial financial support for X.W.N. as a part of the collaborative research in the ongoing PSU-DUT Clean Energy Research Center efforts. Supporting Information Available: Experimental results on conversions and selectivities as a function of reaction time for CAT tert-butylation with IL1, catalytic performance of ILl recycled and used in the tert-butylation of CAT, and characterizations of the ionic liquids by 1H and 13C NMR spectroscopies and ESI-MS. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Yoo, J. W.; Lee, C. W.; Park, S. E.; Ko, J. J. Alkylation of Catechol with t-Butyl Alcohol over Acidic Zeolites. Appl. Catal. A 1999, 187, 225. (2) Zhou, C. H.; Ge, Z. H.; Li, X. N.; Tong, D. S.; Li, Q. W.; Guo, H. Q. Alkylation of Catechol with tert-Butyl Alcohol Catalyzed by Mesoporous Acidic Montmorillonite Heterostructure Catalysts. Chin. J. Chem. Eng. 2004, 12, 388. (3) Malloy, T. P.; Engel, D. J. (UOP Inc.). Alkylation of hydroxysubstituted aromatic compounds. U.S. Patent 4,323,714, 1982. (4) Aoki, T.; Sawa, K. Manufacture of 4-tert-butyl catechol. Japanese Patent 04273838, 1992. (5) Burk, R. E. Twelfth Report of the Committee on Catalysis; Wiley: New York, 1940.

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ReceiVed for reView April 2, 2010 ReVised manuscript receiVed July 8, 2010 Accepted July 11, 2010 IE100800C