tert-Butylation of Phenol over Ordered Solid Acid Catalysts in

Res. , 2006, 45 (18), pp 6118–6126 ... Cite this:Ind. Eng. Chem. ... The tert-butylation of phenol gave 2,4-di-tert-butylphenol (2,4-DTBP) in 65% yi...
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Ind. Eng. Chem. Res. 2006, 45, 6118-6126

tert-Butylation of Phenol over Ordered Solid Acid Catalysts in Supercritical Carbon Dioxide: Efficient Synthesis of 2,4-Di-tert-butylphenol and 2,4,6-Tri-tert-butylphenol Gunda Kamalakar, Kenichi Komura, and Yoshihiro Sugi* Department of Materials Science and Technology, Faculty of Engineering, Gifu UniVersity, Gifu 501-1193, Japan

The tert-butylation of phenol was carried out over ordered solid acid catalysts (zeolites and MCM-41-supported heteropolyacids and rare-earth-metal triflates) in supercritical CO2. The catalytic performances in supercritical CO2 were superior to those in other reaction media such as liquid-phase and N2 atmosphere conditions. The tert-butylation of phenol gave 2,4-di-tert-butylphenol (2,4-DTBP) in 65% yield over H-Y zeolites and 2,4,6tri-tert-butylphenol (2,4,6-TTBP) in 40% yield over Sc(OTf)3 supported on MCM-41 [Sc(OTf)3/MCM-41]. The coke formation in supercritical CO2 was much lower than that in other reaction media. These high performances are due to the high solubilities of reactants and products and to the minimal coke formation: supercritical CO2 works as the reaction medium; removes coke precursors, which are heavy aromatics from phenol and/or isobutene oligomers, before they convert to coke materials; and keeps the catalytic sites clean during the catalysis. The spatial requirement for catalytic active sites is also important for the selective synthesis of 2,4-DTBP and 2,4,6-TTBP. H-Y zeolites can afford the reaction environment to allow the formation of 2,4-DTBP, but not 2,4,6-TTBP. 2,4-DTBP can be easily formed inside H-Y zeolites and diffuse out from their 12-membered pores. Sc(OTf)3/MCM-41 can allow the formation of 2,4,6-TTBP and subsequent diffusion of the products from the channels. The yields of 2,4-DTBP and 2,4,6-TTBP were significantly influenced by reaction conditions, particularly CO2 pressure. The optimum CO2 pressure was 10 MPa at 130 °C. However, a further increase of CO2 pressure rapidly decreased the formation of 2,4-DTBP and 2,4,6TTBP. The highly dense CO2 might decelerate the catalysis by preventing access of the reactants to the catalytic active sites. The catalysts can be recycled without significant loss of the activity and were stable after three cycles. Introduction Environmentally conscious catalyses using solid acid catalysts are key technologies for “green chemistry”.1-6 Conventionally, Lewis acids such as AlCl3, FeCl3, and SnCl4; Brønsted acids such as H2SO4, H3PO4, and HF; and superacids such as SbF5, HF-SbF5, and SbF5-HSO3F have been used for acid catalysis in organic synthesis. However, they generally suffer from inherent problems of corrosiveness, high susceptibility to water, difficult catalyst recovery, environmental hazards, toxicity, and waste control of spent catalyst. It is important to replace these catalysts with environmentally conscious catalysts that are active under mild conditions, can be easily recovered after the reactions, and can be reused for repeat reactions. Solid acid catalysts are effective alternatives to achieve these goals.5,6 The synthesis of bulky organic molecules for industrial purposes is an important research area for solid acid catalysis.7-9 tert-Butylated phenols, typically bulky molecules, are commercially important chemical intermediates; they have been widely used for the synthesis of chemicals, antioxidants, antiseptics, agrochemicals, resins, UV absorbers, and stabilizers for polymers.10,11 Extensive research has been published on the tert-butylation of phenols over catalysts such as zeolites,12-17 hydrotalcites,17,18 clays,19,20 heteropolyacids,21-23 metal-substituted mesoporous silicates,24-27 modified mesoporous aluminophosphates,23,28 ClSO3H-modified mesoporous zirconias,29-31 mixed oxides,32 and modified zirconias.22,33,34 These catalysts are used in either liquid- or vapor-phase conditions. However, * To whom correspondence should be addressed. Tel.: +81-58-2932597. Fax: +81-58-293-2653. E-mail: [email protected].

the majority of these catalysts yield mainly mono-tert-butylated phenols: 2,4-di-tert-butylphenol (2,4-DTBP) and 2,4,6-tri-tertbutylphenol (2,4,6-TTBP) were obtained in only low yield because of severe deactivation by coke formation during the reaction. Hence, it is an important task to find suitable catalysts for the synthesis of these bulky alkylated phenols as a green chemical technology. A key for this task is how to remove the coke precursors formed on the catalytically active species for the prevention of the deactivation. Supercritical fluids are important reaction media that have been used extensively in recent times for organic transformations.35-52 Supercritical fluids provide advantages such as higher dissolution power to remove coke precursors from catalytically active sites in heterogeneous catalysis. They also exhibit intermediate properties between those of liquid- and vapor-phase reaction conditions that facilitate the better mass transfer of reactants and products, thus maintaining the catalytic activity with considerably lower deactivation even after longer reaction periods.42-48 Supercritical CO2 fluids also have the advantage of tunable dissolving power based on the changing solubilities of reactants and products brought about by variation of pressure and temperature.35-45,49-52 Recently, interest in the area of heterogeneous catalysis in supercritical CO2 has increased because the deactivation caused by coke formation in heterogeneous catalysis in supercritical CO2 can be minimized compared to that occurring in other reaction media.35-41 Other merits of using supercritical CO2 as a reaction medium are recyclability, noncorrosiveness, elimination of hazardous organic solvents, minimal generation of waste, and easy separation of products from

10.1021/ie060440k CCC: $33.50 © 2006 American Chemical Society Published on Web 07/28/2006

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the catalytic system. These advantages of supercritical CO2 offered by heterogeneous catalytic systems have prompted us to study the tert-butylation of phenol in supercritical CO2. In the present article, we describe the tert-butylation of phenol in supercritical CO2 over zeolites and mesoporous MCM-41supported scandium triflate [Sc(OTf)3] and tungstophosphoric acid (HPW) to establish the efficient synthesis of 2,4-DTBP and 2,4,6-TTBP. Experimental Section Catalysts and Reagents. Zeolites H-Y (SiO2/Al2O3 ) 5.2, Tosoh Corporation, Tokyo, Japan; SiO2/Al2O3 ) 12, 30, 60, and 80, Zeolyst CV, Groningen, The Netherlands), and H-beta (SiO2/Al2O3 ) 25, Zeolyst CV) were obtained commercially and calcined before use in an air stream at 500 °C for 5 h. Rareearth-metal triflates, RE(OTf)3 (RE ) Sc, La, Ce, and Yb), and tungstophosphoric acid (H3PW12O40‚nH2O; HPW) were purchased from Nacalai Tesque (Kyoto, Japan) and Aldrich Japan (Tokyo, Japan), respectively. Mesoporous MCM-41 was synthesized by a procedure reported in the literature.53 RE(OTf)3 and HPW supported on MCM-41 were prepared by the impregnation method from solutions of methanol and ethanol, respectively. Phenol and tert-butanol were obtained commercially and used as received without further purification. tert-Butylation of Phenol. A typical reaction was carried out as follows: Phenol (1.0 g, 10.6 mmol), tert-butanol (1.57-2.75 g, 2.12-37.1 mmol), and a catalyst (0.1 g) were placed in a 100-mL SUS-316 autoclave, and dry ice was loaded into the reactor. The amount of CO2 necessary for the reaction conditions was calculated from the empty volume of the reactor. Then, the reactor was heated to the desired reaction temperature while the reaction mixture was stirred. After the specified reaction time, the reactor was cooled to room temperature and slowly depressurized. The contents of the reactor were washed with acetone and analyzed by gas chromatograph (GC-18, Shimadzu Corporation, Kyoto, Japan) with an Ultra-1 capillary column (30 m × 0.1 mm; Agilent Technologies, MA). The products were identified by GC-MS (Shimadzu GC-MS5000 instrument equipped with an Ultra-1 capillary column) and were also compared with authentic samples. Recycling of Catalysts. After completion of the reaction, the catalyst was separated by filtration and washed well with acetone. The catalyst was subjected to repeated reactions after being dried at 100 °C overnight. The reaction procedures are the same as described above. Characterization of Catalysts. XRD analysis of the catalysts was carried out on a Shimadzu XRD-6000 diffractometer with Ni-filtered Cu KR radiation (λ ) 1.5418 Å). Thermogravimetric analysis was performed on a Shimadzu DTG-50A thermobalance at a programmed rate of 10 °C/min. Results and Discussion tert-Butylation of Phenol over H-Y Zeolite. Table 1 summarizes the tert-butylation of phenol in supercritical CO2 over solid acid catalysts at 130 °C and 10 MPa CO2 pressure with a 1:2 molar ratio of phenol to tert-butanol. H-Y and H-beta zeolites and HPW/MCM-41 were employed as the catalysts. The products found in the reaction mixtures were 2and 4-tert-butylphenols (2- and 4-TBP) and 2,4-DTBP. H-Y(5.2) zeolite gave the highest yield of 2,4-DTBP, and the conversion of phenol decreased with increasing SiO2/Al2O3 ratio. The yields of 2- and 4-TBP and 2,4-DTBP decreased with increasing SiO2/ Al2O3 ratio; however, the decrease of 2- and 4-TBP was more

Table 1. Synthesis of 2,4-Di-tert-butylphenol by the tert-Butylation of Phenola yield (%) catalyst

conversion (%)

2-TBP

4-TBP

2,4-DTBP

H-Y(5.2)b H-Y(12)b H-Y(30)b H-Y(30)b,c H-Y(40)b H-Y(60)b H-Y(80)b H-beta(25)b HPW/MCM-41d,e [Al]-MCM-41(30)b H-Y(5.2)f H-Y(5.2)g

100.0 78.0 73.0 89.3 70.5 68.5 60.5 72.1 99.0 41.0 59.1 68.0

21.5 13.4 11.0 12.3 10.4 10.6 8.7 17.2 18.2 19.1 22.1 21.8

13.5 14.3 12.4 8.5 13.6 14.7 13.2 52.3 20.5 21.9 31.2 11.1

65.0 50.3 49.6 68.5 46.5 43.2 38.6 2.6 60.3 5.8 35.1

a Reaction conditions: catalyst, 0.1 g; phenol (substrate), 1.0 g (10.6 mmol); tert-butanol (substrate), 1.57 g (21.2 mmol); temperature, 130 °C; time, 6 h; CO2 pressure, 10 MPa. b Values in parentheses are SiO2/Al2O3 ratios. c Substrates: phenol, 1 g (10.6 mmol) and tert-butanol, 2.75 g (37.1 mmol). d HPW (30 wt %) supported on MCM-41. e 2,4,6-TTBP (1.8% yield) was also found in the products. f Under autogenous pressure in hexane. g Under N pressure (10 MPa). 2

extensive than that of 2,4-DTBP. An excess of tert-butanol in the reaction mixture enhanced the formation of 2,4-DTBP even with a SiO2/Al2O3 ratio of 30, but there was no formation of 2,4,6-TTBP even with a tert-butanol/phenol ratio of 3. On the other hand, H-beta(25) zeolite exhibited only low catalytic activity and principally gave a mixture of 2- and 4-TBP. From these results, we can infer the following points for the successful synthesis of 2,4-DTBP: (1) The reaction is enhanced on strong acid sites with appropriate amounts of acid. (2) 2,4-DTBP can be formed inside H-Y zeolites and diffuse out from the 12membered pores. However, there is no such space to form 2,4DTBP in the channels of H-beta. Estimated molecular diameters of phenol, 2-TBP, 4-TBP, 2,4DTBP, and 2,4,6-TTBP as determined using Chem 3D software (CambridgeSoft Corp., Cambridge, MA) are 0.53, 0.55, 0.53, 0.68, and 0.87 nm, respectively. These values suggest that the formation of mono-tert-butylphenols (2- and 4-TBP) and ditert-butylphenol (2,4-DTBP) is possible in the pores of H-Y zeolites, which have a pore diameter of 0.74 nm, but that the formation of 2,4,6-TTBP is prevented because of the shapeselective limitation of H-Y pores. HPW/MCM-41 also gave 2,4-DTBP in reasonably high yield; however, [Al]-MCM-41(30) gave 2- and 4-TBP in 41% yield. These results show that strong acidity is essential for the formation of 2,4-DTBP. HPW has acidity strong enough for the catalysis, but the acidity of [Al]-MCM-41 is too weak for the catalysis. A small amount of 2,4,6-TTBP was also formed in the tert-butylation over HPW/MCM-41, which is due to the wide channels of mesoporous silicate facilitating the formation of 2,4,6-TTBP. We carried out the tert-butylation of phenol in liquid-phase conditions in hexane as the solvent and in N2 atmosphere without solvent. The reaction over H-Y(5.2) zeolite was carried out in hexane (15 mL) under autogenous pressure; 5.8% yield of 2,4-DTBP was obtained with 59.1% conversion of phenol. The tert-butylation of phenol under a N2 pressure of 10 MPa (without solvent) gave a better yield of 2,4-DTBP (35.1%) than reaction in liquid-phase conditions but a lower yield than the reaction in supercritical CO2. Severe deactivation of the catalysts was observed in liquid-phase conditions, whereas almost no coke formation was observed in supercritical CO2. These results suggest that phenol and tert-butanol (probably converted to isobutene under the reaction conditions) are dissolved in

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Figure 1. Influence of reaction temperature on the tert-butylation of phenol over H-Y zeolite in supercritical CO2. Reaction conditions: H-Y(5.2), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol, 1.57 g (21.2 mmol); CO2 pressure, 10 MPa; time, 4 h.

Figure 2. Influence of CO2 pressure on the tert-butylation of phenol over H-Y zeolite. Reaction conditions: H-Y(5.2), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol, 1.57 g (21.2 mmol); temperature, 130 °C; time, 4 h.

supercritical CO2 and that coke precursors are efficiently removed from the catalyst surface. Figure 1 shows the influence of reaction temperature on the tert-butylation of phenol under 10 MPa of CO2 pressure over H-Y(5.2) zeolite. The conversion of phenol and the yield of 2,4-DTBP increased with increasing reaction temperature, and particularly, they were enhanced at temperatures higher than 110 °C: 65% yield of 2,4-DTBP was achieved at 130 °C and CO2 pressure of 10 MPa. The yield for 2,4-DTBP was considerably higher than those of the 2- and 4-TBP isomers at all temperatures. The yield of 2-TBP was maximized at 110 °C and decreased at higher temperatures, whereas a gradual increase in the yield of 4-TBP with temperature was observed. These results show that 2,4-DTBP was formed consecutively from 2and 4-TBP. Judging from the profiles of 2- and 4-TBP as a function of reaction temperature, the less bulky 4-TBP might yield 2,4-DTBP faster than the bulkier 2-TBP. The formation of 2,4-DTBP is slower for 2-TBP than for 4-TBP because of the presence of the bulkier tert-butyl group in the ortho position (ortho effect), which can be understood from the product profiles of 2- and 4-TBP as a function of the reaction temperature. Figure 2 shows the influence of CO2 pressure on the tertbutylation of phenol over H-Y(5.2) zeolite at 130 °C. The yield of 2,4-DTBP increased considerably as the CO2 pressure increased from 8.5 to 10 MPa, and the highest yield of 2,4-

Figure 3. Influence of reaction time on the tert-butylation of phenol over H-Y zeolite in supercritical CO2. Reaction conditions: H-Y(5.2), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol, 1.57 g (21.2 mmol); temperature, 130 °C; CO2 pressure, 10 MPa.

DTBP was 65% under a CO2 pressure of 10 MPa. However, the conversion of phenol and the yield of 2,4-DTBP decreased dramatically with a further increase of CO2 pressure. These results mean that the reaction rate decreases with increasing density of CO2 at higher pressures. The higher CO2 pressures (>10 MPa) might prevent access of the reactant molecules to the catalytic center through filling of the channels with highly dense CO2. From these results, we can infer that an appropriate reaction temperature and CO2 pressure are essential for the successful synthesis of 2,4-DTBP. However, further studies are necessary to clarify the phenomena occurring in high-pressure supercritical CO2. The influence of reaction time on the tert-butylation of phenol over H-Y(5.2) is shown in Figure 3. The conversion and yield of the product increased with increasing reaction time, and 100% conversion of phenol was achieved within 6 h. The yield of the 2,4-DTBP was 70% after 6 h; this yield of 2,4-DTBP is the highest among all values previously reported in the literature.13-34 These results mean that no significant deactivation of the catalyst occurred during the reaction. The product profile as a function of reaction time shows that the tert-butylation of phenol proceeds principally through a consecutive reaction mechanism. Phenol gives 2- and 4-TBP, and then 2,4-DTBP is generated from 2- and 4-TBP. In these paths, the shape-selective formation of 2- and 4-TBP from phenol is not probable judging from the data shown in Figures 1-3. The formation of 2,4-DTBP occurs through both 2- and 4-TBP, although the less bulky 4-TBP reacts rapidly compared to the bulky 2-TBP under appropriate conditions. It is also possible that some 2,4-DTBP was formed directly from phenol inside the pores of H-Y zeolites. However, the formation of 2,4,6-TTBP was prevented even at higher ratios of tert-butanol to phenol because of shape-selective limitation of H-Y zeolite. tert-Butylation of Phenol over Scandium Triflate Supported on MCM-41. The observation of high yields of 2,4DTBP and the prevention of coke formation on H-Y zeolites in the tert-butylation of phenol prompted us to investigate the synthesis of 2,4,6-TTBP in supercritical CO2. For this purpose, it is important to design a strong acid catalyst with a mesoporous structure. We chose ordered mesoporous silicate MCM-41 as the support, because the bulky 2,4,6-TTBP can be accommodated and formed in its channels. Rare-earth triflates were chosen as strong acids as another important key for the synthesis of 2,4,6-TTBP because they are versatile Lewis acid for organic

Ind. Eng. Chem. Res., Vol. 45, No. 18, 2006 6121 Table 2. Synthesis of 2,4,6-Tri-tert-butylphenol by the tert-Butylation of Phenola catalyst Sc(OTf)3/MCM-41b La(OTf)3/MCM-41b Ce(OTf)3/MCM-41b Yb(OTf)3/MCM-41b Sc(OTf)3 Sc(OTf)3/SiO2c Sc(OTf)3/Al2O3d HPW/MCM-41e,f HPW/MCM-41e Sc(OTf)3/MCM-41b,g Sc(OTf)3/MCM-41b,h

yield (%) conversion (%) 2-TBP 4-TBP 2,4-DTBP 2,4,6-TTBP 86.0 44.0 38.0 10.2 36.5 21.6 20.2 99.0 84.2 59.0 72.2

9.0 16.5 19.0 6.5 11.4 8.5 8.5 18.2 10.8 15.2 12.6

7.0 10.0 9.5 3.7 8.2 7.3 6.3 20.5 7.4 8.6 8.0

34.0 17.5 9.5

36.0

9.7 4.3 4.2 60.3 38.7 25.4 37.4

7.2 1.5 1.2 1.8 27.3 9.8 14.2

a Reaction conditions: catalyst, 0.1 g; phenol (substrate), 1 g (10.6 mmol); tert-butanol (substrate), 2.75 g (37.1 mmol); temperature, 130 °C; time, 4 h; CO2 pressure, 10 MPa. b Sc(OTf)3 (25 wt %) supported on MCM-41. c Fuji Silysia; Caricat, G-10. d Obtained from Catalysts & Chemicals Industries Co., Ltd., Kawasaki, Japan. e Tungstophosphoric acid (30 wt %) supported on MCM-41. f Substrates: phenol, 1.0 g (10.6 mmol) and tertbutanol, 1.57 g (2.12 mmol). g Under autogenous pressure in hexane. h Under N atmosphere of 10 MPa. 2

syntheses.54-56 Supported metal triflates are very efficient catalysts for several acid-catalyzed organic transformations because of the high dispersion of the acidic components over them.57-59 Table 2 summarizes the influence of type of the catalyst on the tert-butylation of phenol in supercritical CO2. The tertbutylation of phenol was examined over the rare-earth-metal trilflates Sc(OTf)3, La(OTf), Ce(OTf)3, and Yb(OTf)3 supported on MCM-41. The principal products were 2- and 4-TBP, 2,4DTBP, and 2,4,6-TTBP. Sc(OTf)3/MCM-41 gave the best conversion of phenol and the highest yield of 2,4,6-TTBP. 2,4,6TTBP was not formed at all on Yb(OTf)3, La(OTf)3, and Ce(OTf)3 because of their weak acidities compared to that of Sc(OTf)3. HPW supported on MCM-41 was also an efficient catalyst for the formation of 2,4,6-TTBP. To confirm the effectiveness of mesoporous silicate, we examined the use of silica gel and alumina as supports. The catalytic activity of Sc(OTf)3 was not enhanced over these supports, which might be due to the lower dispersion of Sc(OTf)3 over these supports. Unsupported Sc(OTf)3 also exhibited a low catalytic activity in the tert-butylation of phenol. The highly efficient formation of 2,4,6-TTBP shows that there is no spatial restriction on the formation of bulky 2,4,6-TTBP in mesoporous MCM-41 because there is little steric limitation for the products to diffuse out from the catalyst. However, H-Y(30) zeolite did not yield any 2,4,6-TTBP even at higher ratios of tert-butanol to phenol; this lack of activity for the formation of 2,4,6-TTBP is due to spatial constraints on diffusion from the zeolite. To confirm the effectiveness of supercritical CO2, we examined the reactions over Sc(OTf)3/MCM-41 in liquid-phase conditions (using hexane as the solvent under autogenous pressure): 9.8% yield of 2,4,6-TTBP was obtained with 59% conversion of phenol. The low yield of 2,4,6-TTBP was mainly due to the rapid deactivation of the catalyst by coke formation, which is similar to that of the H-Y zeolite as discussed above. The tert-butylation was also examined under a N2 atmosphere of 10 MPa without solvent. The yields of 2,4,6-TTBP under N2 atmosphere were higher than those in hexane. Compared to these two reaction conditions, supercritical CO2 exhibited the highest catalytic performance with the least catalyst deactivation because of effective removal coke precursors. The influence of reaction temperature on the tert-butylation of phenol over Sc(OTf)3/MCM-41 under a CO2 pressure of 10

Figure 4. Influence of reaction temperature on the tert-butylation of phenol over Sc(OTf)3/MCM-41 in supercritical CO2. Reaction conditions: Sc(OTf)3/MCM-41 (loading, 25 wt %), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol 2.75 g (37.1 mmol); CO2 pressure, 10 MPa; time, 4 h.

Figure 5. Influence of CO2 pressure on the tert-butylation of phenol over Sc(OTf)3/MCM-41. Reaction conditions: Sc(OTf)3/MCM-41 (loading, 25 wt %), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol 2.75 g (37.1 mmol); temperature, 130 °C; time, 4 h.

MPa is shown in Figure 4. The conversion and the yields of 2,4-DTBP and 2,4,6-TTBP increased spontaneously with increasing temperature. However, the yield of 2,4,6-TTBP reached a maximum at 130 °C and then decreased slightly at higher temperatures. The highest yield of the 2,4,6-TTBP was 36% at 130 °C and a CO2 pressure of 10 MPa. The formation of 2,4DTBP and 2,4,6-TTBP predominates over that of 2- and 4-TBP at all temperatures. A temporary decrease in the yields of 2,4DTBP and 2- and 4-TBP was observed at 130 °C. This corresponds to the maximum yield of 2,4,6-TTBP: the formation of 2,4,6-TTBP might be more rapid than the formation of its precursors, 2,4-DTBP and 2- and 4-TBP. These results show the merits of mesoporous environments for acid catalysis in supercritical CO2. Figure 5 shows the influence of CO2 pressure on the tertbutylation of phenol over Sc(OTf)3/MCM-41 at 130 °C. The conversion increased gradually with increasing CO2 pressure up to 10-11 MPa. The yield of 2,4,6-TTBP increased rapidly from as the pressure increased from 8 to 10 MPa, reached maximum at 10 MPa, and then rapidly decreased with further increase in the pressure. The highest yield of 2,4,6-TTBP is 36% at 10 MPa. The yields of 2,4-DTBP, 2- and 4-TBP were 35%, 10-20%, and 5-10%, respectively. However, the yields of 2- and 4-TBP and 2,4-DTBP exhibited minima at 10-11

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Figure 6. Influence of reaction time on the tert-butylation of phenol over Sc(OTf)3/MCM-41 in supercritical CO2. Reaction conditions: Sc(OTf)3/ MCM-41 (loading, 25 wt %), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol 2.75 g (37.1 mmol); temperature, 130 °C; CO2 pressure, 10 MPa.

Figure 7. Influence of loading amount of Sc(OTf)3 to MCM-41 on the tert-butylation of phenol in supercritical CO 2. Reaction conditions: Sc(OTf)3/MCM-41 (loading, 10-100 wt %), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol 2.75 g (37.1 mmol); CO2 pressure, 10 MPa; temperature, 130 °C; time, 4 h.

MPa. This decrease corresponds to the maximum in the yield of 2,4,6-TTBP. As previously discussed, the decrease in the yield of 2,4,6-TTBP with further increase in CO2 pressure is due to the prevention of access of reactant molecules to catalytic active sites by filling of the channels with highly dense CO2. The influence of reaction time on the tert-butylation of phenol over Sc(OTf)3/MCM-41 at 130 °C is shown in Figure 6. The conversion of phenol increased spontaneously with the reaction time. The formation of 2- and 4-TBP occurred at the initial stages, and then the yield of 2,4-DTBP increased with the progress of the reaction and saturated after 5-6 h of reaction with the complete conversion of phenol. The highest yield of 2,4,6-TTBP was 42.7% after 6 h: this is the highest yield for 2,4,6-TTBP among all values previously reported in the literature.13-34 The change in product profile with reaction time shows that the tert-butylation proceeds through a consecutive reaction mechanism. Figure 7 shows the influence of the Sc(OTf)3 loading on MCM-41 in the tert-butylation of phenol in supercritical CO2. Unsupported Sc(OTf)3 gave a 7% yield of 2,4,6-TTBP with 36% conversion of phenol. The catalytic performance increased rapidly with increasing loading of Sc(OTf)3 to 20%; the yield of 2,4,6-TTBP at this loading was 35%. The highest yield of

Figure 8. Influence of ratio of tert-butanol and phenol on the tert-butylation of phenol in supercritical CO2. Reaction conditions: Sc(OTf)3/MCM-41 (loading, 25 wt %), 0.1 g; phenol, 1.0 g (10.6 mmol); tert-butanol, 1.852.96 g (2.5-4.0 mmol); temperature, 130 °C; CO2 pressure, 10 MPa; time, 4 h.

2,4,6-TTBP was 40% at 70 wt % loading. On the other hand, unsupported Sc(OTf)3 had only a low activity for 2,4,6-TTBP (7.2%). The enhancement of the catalytic activity through use of an MCM-41 support is due to the dispersion of the triflate on the support: the high dispersion of the triflate effectively works as the catalyst for the tert-butylation. We also calculated turn-over number (TON) of Sc(OTf)3 on the catalyst, which correlates with the efficiency of the catalytic species. The TON reached a maximum at 25% loading, and further increases in the loading decreased the TON. These values of TON show that the 25% loading provided the best catalytic performance for the tert-butylation of phenol in supercritical CO2. The influence of the tert-butanol/phenol molar ratio is shown in Figure 8. The increase of the ratio of tert-butanol to phenol to 4.0 significantly enhanced the formation of 2,4,6-TTBP: the highest yield was 40% at a ratio of 4.0. However, the yields of 2,4-DTBP and 2- and 4-TBP were almost constant with increasing the ratio. These results show that a high ratio of tertbutanol to phenol is a key factor for the selective synthesis of 2,4,6-TTBP. Excess water was added to the initial reaction mixture to determine the influence of water on the stability of the catalyst during the reaction. The catalytic performance decreased dramatically upon addition of water (100 mmol): 2,4,6-TTBP was obtained in 13% yield with 30% conversion of phenol. Judging from the XRD patterns of the catalyst used for the reaction (not shown), the decrease of the catalytic activity is mainly due to the destruction of the mesoporous structure of the catalyst by water. However, the water liberated by the dehydration of tert-butanol does not destroy the mesoporous structure of the catalyst. Recyclability of Catalysts. The recyclability of the catalyst is an important key in catalysis. The tert-butylation of phenol over H-Y zeolite was carried out for four cycles to examine its recyclability in supercritical CO2, and typical results are reported in Table 3. The catalyst after reaction was separated from the reaction mixture and washed with acetone. The catalyst was used after being dried at 120 °C overnight. Only a 6-8% decrease was observed in the conversion and the yield of 2,4DTBP after three reaction cycles. Table 4 lists the recyclability results for Sc(OTf)3/MCM-41 in the tert-butylation of phenol. The catalyst was regenerated by the procedure described for H-Y zeolite. The catalyst maintained a high activity even after

Ind. Eng. Chem. Res., Vol. 45, No. 18, 2006 6123 Table 3. Recyclability of H-Y Zeolites for the Synthesis of 2,4-Di-tert-butylphenola yield (%) cycle

conversion (%)

2-TBP

4-TBP

2,4-DTBP

first second third fourth

86.0 85.3 81.2 80.6

16.8 15.3 15.6 16.2

11.6 15.5 14.4 13.8

57.6 54.3 51.2 50.6

a Reaction conditions: H-Y(5.2) zeolite (catalyst), 0.1 g; phenol (substrate), 1.0 g (10.6 mmol); tert-butanol (substrate), 1.57 g (21.2 mmol); temperature, 130 °C; time, 4 h; CO2 pressure, 10 MPa.

Table 4. Recyclability of the Catalyst for the Synthesis of 2,4,6-Tri-tert-butylphenola yield (%) cycle

conversion (%)

2-TBP

4-TBP

2,4-DTBP

2,4,6-TTBP

first second third

86.0 82.3 77.2

9.0 10.8 11.5

7.0 8.5 10.0

34.0 30.7 24.2

36.0 32.3 31.5

a Reaction conditions: Sc(OTf) /MCM-41 (catalyst; loading, 25 wt %), 3 0.1 g; phenol (substrate), 1.0 g (10.6 mmol); tert-butanol (substrate), 2.75 g (37.1 mmol); temperature, 130 °C; time, 4 h; CO2 pressure, 10 MPa.

Figure 9. XRD patterns of fresh Sc(OTf)3/MCM-41 and the catalyst after use for three cycles of the reaction.

four uses: only a 9% loss in the conversion of the phenol and a 6% loss in the yield of 2,4,6-TTBP occurred. These excellent recyclabilities of H-Y zeolite and Sc(OTf)3/MCM-41 in the tert-butylation are mainly due to the minimal deactivation that occurred in supercritical CO2 because the catalytic active sites of these catalysts are maintained free through low coke formation. The XRD patterns of H-Y and Sc(OTf)3/MCM-41 used for the reaction were measured in order to determine the stability of the catalysts during the reaction. The structure of the H-Y zeolite was maintained during the tert-butylation (results not shown). Figure 9 shows the XRD patterns of Sc(OTf)3/MCM41 after three reaction cycles. The representative peaks assigned to the mesoporous structure were found in both fresh and used catalysts at almost the same intensities. These results show that H-Y zeolites and Sc(OTf)3/MCM-41 were stable and active during the tert-butylation. The microporous structure of H-Y zeolite and the mesoporous structure of Sc(OTf)3/MCM-41 catalyst were not destroyed by water liberated from the dehydration of tert-butanol during the reaction. Coke Formation during the tert-Butylation of Phenol. To elucidate the merits of supercritical CO2 as a reaction medium, we performed a thermogravimetric (TG) analysis of the catalysts

Figure 10. TG profiles of H-Y zeolite used for the reaction. Reaction conditions: see Table 1.

used for the reaction. Figure 10 shows profiles of H-Y zeolites used under various experimental conditions for 6 h. Judging from the TG profiles, coke formation in supercritical CO2 was low. On the other hand, coke formation was clearly observed on the catalysts used in liquid-phase reactions in hexane as the solvent and in the reaction under N2 atmosphere without solvent. A peak in the range of 500-600 °C clearly indicates the combustion of coke materials. Their amounts were 3.5% for liquid-phase reaction and 2.6% for the reaction under N2 atmosphere after 6 h. Coke formation took place during the initial stage of the reaction (0.5 h) under liquid-phase conditions. Coke formation under these conditions shows that coke precursors converted easily to coke materials because they were not effectively removed from acidic sites. On the other hand, they are removed easily from acid sites in supercritical CO2 before they convert to coke materials. This is the most important role of supercritical CO2 in the tert-butylation of phenol. Similar TG profiles were observed for Sc(OTf)3/MCM-41 after it had been used for the reaction. The profiles of Sc(OTf)3/ MCM-41 from 250 to 700 °C are shown in Figure 11. No significant peaks were observed for fresh Sc(OTf)3/MCM-41 above 400 °C, although some peaks were found below 350 °C because of the decomposition of Sc(OTf)3. After being used for catalysis, Sc(OTf)3/MCM-41 exhibits apparent peaks due to coke formation. These results mean that very little or no deactivation caused by coke formation occurs during the reaction in supercritical CO2. On the other hand, peaks from 400 to 550 °C appeared for Sc(OTf)3/MCM-41 used in liquid-phase conditions or under N2 atmosphere. These peaks are due to coke formation under these reaction conditions. Coke formation under these conditions shows that coke precursors converted easily to coke materials because they were not effectively removed from the acidic sites. These results are quite similar to the case of H-Y zeolite. Supercritical CO2 for the tert-Butylation of Phenol. Many researchers have studied the synthesis of 2,4-DTBP over many solid acid catalysts under vapor- and liquid-phase conditions.13-34 Many articles have also been published on the tert-butylation of phenol using mesoporous materials as the catalysts in vaporor liquid-phase conditions.24-31 However, the principal products were 2- and 4-TBP, and these catalysts were not efficient for the synthesis of 2,4-DTBP and 2,4,6-TTBP. These low activities are due to severe deactivation of the catalysts because of low mass transfer of coke precursor from the active sites.17 The

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Figure 11. Thermogravimetric profiles of Sc(OTf)3/MCM-41 used for the reaction. Reaction conditions: see Table 2.

weak acidity of metal-substituted MCM-41 is a reason for the low activity. We also confirmed the tert-butylation of phenol to determine the catalytic activities of H-Y zeolites and Sc(OTf)3/MCM-41 under liquid-phase conditions in hexane and without solvent under N2 atmosphere. The yields for 2,4-DTBP with H-Y zeolites and for 2,4,6-TTBP with Sc(OTf)3/MCM41 were much lower than those in supercritical CO2. These lower activities are due to deactivation by coke formation caused in liquid-phase conditions in hexane and without solvent under N2 atmosphere. To clarify the behavior of solid acid catalysts in supercritical CO2, we must consider the role of the supercritical CO2 in the catalytic process. Supercritical CO2 provides a good solvent for the tert-butylation because of its high dissolution power for organic compounds. In particular, the properties of supercritical CO2, such as higher density and diffusivity compared to the vapor phase, low viscosity compared to the liquid phase, etc., facilitate better mass transfer of reactants and products. Supercritical CO2 can also remove coke precursors and prevent the coke formation over acid sites; the activity of the catalyst is retained with lower deactivation after prolonged reaction periods to result in the successful synthesis of 2,4-DTBP and 2,4,6TTBP in supercritical CO2. The catalytic performances of acids are also an important key for the efficient synthesis of target molecules even if supercritical CO2 can work as a good solvent and efficiently remove coke precursors to keep active sites clean for the catalysis. H-Y zeolites and Sc(OTf)3/MCM-41 have sufficient acid strengths for solid acid catalysis and work as efficient catalysts for the tert-butylation of phenol: H-Y(5.2) had the highest activity among the H-Y zeolites, and Sc(OTf)3/MCM41 had the highest activity among the supported triflates. The selective formation of 2,4-DTBP is due to the shapeselective character of H-Y zeolite; however, the formation of 2,4,6-TTBP is not allowed for H-Y zeolites because of the steric constraint on accommodating 2,4,6-TTBP in the pores. On the other hand, Sc(OTf)3/MCM-41 has acid sites in its mesoporous channels to allow the formation of bulky products. The channels of MCM-41 are wide enough for the formation of 2,4,6-TTBP to make contact with acidic sites, and supercritical CO2 keeps the active sites free from coke formation.

The CO2 pressure has a marked influence on the tertbutylation of phenol in supercritical CO2. The yields of 2,4DTBP and 2,4,6-TTBP have an optimal CO2 pressure: 10 MPa at 130 °C. An increase in CO2 pressure enhanced the tertbutylation but resulted in rapid decelerations in the yields of 2,4-DTBP and 2,4,6-TTBP with further increases in pressure. The increase of the catalytic performances from low to optimal pressures means that the increase in density of supercritical CO2 accelerates the catalysis through the increasing solubilities of the reactants, products, and coke precursors. However, highly dense CO2 decelerates the catalysis because of the prevention of access of reactant molecules to catalytic active sites by preferential filling of the channels with CO2. Severe coke formation on acid sites has been described in many solid acid catalyses.60-63 It is well documented that coke precursors are oligomers of alkenes, alkylated polyaromatic hydrocarbons, etc. Coke formation occurs by the dehydrogenation of coke precursors and can be prevented if coke precursors are effectively removed before converting to coke materials. Previously, it has been clearly recognized that supercritical CO2 can effectively remove coke precursors.39,44-46 Under our conditions, the removal of coke precursors by supercritical CO2 supports the effective tert-butylation of phenol. The prevention of deactivation by coke formation has been recognized as a key in the alkylation of isobutane with isobutene over solid acid catalysts.63,64 It is also possible that supercritical fluids can reduce the coke formation occurring during alkylation, because they effectively remove coke precursors.39,45-47 Conclusions The tert-butylation of phenol in supercritical CO2 was carried out over H-Y zeolite and Sc(OTf)3 supported on MCM-41 and compared with the reaction performed in other media. The tertbutylation of phenol gave 2,4-di-tert-butylphenol (2,4-DTBP) in 65% yield over H-Y zeolite and 2,4,6-tri-tert-butylphenol (2,4,6-TTBP) in 40% yield over Sc(OTf)3/MCM-41. The catalysts were stable during the reaction and recyclable without a significant loss of the activity after three or four cycles. The catalytic performances in supercritical CO2 were superior to those in liquid-phase reactions in hexane as the solvent and in vapor-phase reactions under N2 atmosphere without solvent, and the coke formation in supercritical CO2 was much reduced compared to those in other media. These results show that the catalysis proceeds efficiently in supercritical CO2 because of the high solubility of reactants and products and the low viscosity of supercritical CO2, which facilitates the better mass transfer of reactants and products. These properties of supercritical CO2 also enhance the removal of coke precursors to keep catalytic active acid sites clean for the catalysis. The spatial requirement for catalytic active sites is also an important key for the selective synthesis of 2,4-DTBP and 2,4,6TTBP. H-Y zeolites can provide an environment suitable for the formation of 2,4-DTBP and its diffusion from the pores, but not for the formation of 2,4,6-TTBP. Sc(OTf)3/MCM-41 can also allow the formation of 2,4,6-TTBP and its diffusion from channels. The yields of 2,4-DTBP and 2,4,6-TTBP varied with CO2 pressure. The optimum pressure was 10 MPa at 130 °C, and a further increase of the CO2 pressure rapidly decelerated the formation of 2,4-DTBP and 2,4,6-TTBP. These results suggest that the catalytic performances were enhanced at the optimum pressure because of the increasing solubility of reactants and products; however, highly dense CO2 decelerates the access of the reactants to the active catalytic sites by its preferential filling of the catalyst channels.

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ReceiVed for reView April 8, 2006 ReVised manuscript receiVed June 21, 2006 Accepted June 26, 2006 IE060440K