MCM-48 Supported Tungstophosphoric Acid: An Efficient Catalyst for

Tungstophosphoric acid (HPW) and MCM-48-supported HPW catalysts are potential catalysts for the esterification of long-chain fatty acids and alcohols ...
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Ind. Eng. Chem. Res. 2008, 47, 2538-2544

MCM-48 Supported Tungstophosphoric Acid: An Efficient Catalyst for the Esterification of Long-Chain Fatty Acids and Alcohols in Supercritical Carbon Dioxide Ayyamperumal Sakthivel, Kenichi Komura, and Yoshihiro Sugi* Department of Materials Science and Technology, Faculty of Engineering, Gifu UniVersity, Gifu 501-1193, Japan

Tungstophosphoric acid (HPW) and MCM-48-supported HPW catalysts are potential catalysts for the esterification of long-chain fatty acids and alcohols in supercritical CO2 medium even at low temperature (100 °C) and a short reaction period (6 h). The high performances of HPW in sc-CO2 medium are due to the effective mass transfer of reactants and products for the catalysis under the conditions. The yields of esters were enhanced with the increase in the chain length of acids and alcohols in the esterification in sc-CO2 medium. The MCM-48-supported HPW catalysts were active even after several recycle experiments. 1. Introduction Fatty acid esters are used as raw materials for emulsifiers, oiling agents for foods, spin finish, and textiles; lubricants for plastics; paint and ink additives and for mechanical processing; personal care emollients; surfactants and base materials for perfumes, and so forth.1,2 They are also used as solvents, cosolvents, and oil carriers in the agricultural industries.1,2 Conventionally, they have been prepared by the esterification of fatty acids and alcohols using mineral acids3 and Lewis acids.4-6 These catalysts suffer from inherent problems of corrosiveness, high susceptibility to water, difficulties in catalyst recovery, environmental hazards, waste control, and so forth. Further, the above methods need large amounts of catalyst and a large excess of either fatty acids or alcohols to achieve high yields of esters. It is an important green chemical technology to find efficient catalysts for the synthesis of fatty acid esters, particularly of long fatty acids and alcohols. Heteropolyacids, particularly tungstophosphoric acid (H3PW12O40‚xH2O; HPW), were found to be efficient catalysts for several acid-catalyzed organic transformations and industrial applications.7-13 Further, their cesium salts were also employed for the esterification of several long-chain fatty acids with methanol.12-14 Moreover, HPW supported on solid materials with high surface area is expected to be an excellent acid catalyst with an eco-friendly nature and good reusability.14-22 Among the various supports, the ordered mesoporous materials were found to be an efficient support to enhance their catalytic performances.20-23 Several mesoporous materials functionalized with sulfonic acid groups and with ZrOCl2‚8H2O have been reported for the esterification of fatty acids with glycerine or fatty alcohols.23-30 One of key tasks for the heterogeneous catalytic reaction is how to remove strongly chemisorbed reactants and products on the catalytic active species for the prevention of the deactivation.21,22,30 In these regards, supercritical carbon dioxide (scCO2) media have several advantages such as high reaction rates due to increased solubility and diffusivity of reactants and products in them, thus eliminating mass transfer resistance, enhancement of heat transfer from catalyst to dense gases, easy * To whom correspondence should be addressed. Tel.: +81-58-2932597. Fax: +81-58-293-2653, Email: [email protected].

separation of solid catalysts and products, and reduction of coke formation on solid acid catalysts.31-33 There have been few reports34 on the esterification of longchain fatty acids with short-chain alcohols using HPW in scCO2 medium. We have been encouraged the attempt to apply for HPW catalysts for the esterification of esters of fatty acids and alcohols in sc-CO2 medium to establish an efficient catalytic system. In the present study, HPW catalysts, which are unsupported and supported on 3D mesoporous MCM-48, are applied for the esterification of some long-chain fatty acids and alcohols. 2. Experimental Section 2.1. Reagents. 1-Butyric acid, 1-hexanoic acid, 1-octanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, isostearic acid, 1-butanol, 1-hexanol, 1-octanol, 1-nonanol, 1-decanol, lauryl alcohol, myristic alcohol, cetyl alcohol, 2-decanol, 2-dodecanol, and 2-hexadecanol were purchased commercially (Nacalai Tesque, TCI, and Kanto Chemical, Japan). All of the chemicals for catalyst synthesis including, HPW, LUDOX HS-40 colloidal silica (40 wt % SiO2, Aldrich), cetyltrimethylammonium bromide (CTMABr), polyoxyethylen(4)-lauryl ether (Brij-30), and NaOH solution (8 M solution) were obtained commercially and used without purification. 2.2. Catalysts. MCM-48 was prepared35 by the following procedure with a molar gel composition of 5.0 SiO2/2.5 NaOH/ 0.86 CTMABr/0.14 Brij30/400 H2O. First, a surfactant mixture solution was prepared by dissolving both CTMABr (7.74 g) and Brij30 (1.35 g) simultaneously in distilled water (60 mL). Then, the NaOH solution (9.65 g of 8 M solution) was added to the surfactant solution and stirred for 0.5 h. The silica solution (18.77 g) was then added to the above-described solution, and the mixture was shaken vigorously for 0.5 h. The resulting gel was kept at 100 °C for crystallization. After 2 d, the mixture was cooled to room temperature, and the pH of the solution was adjusted to 10 with acetic acid. After pH adjustment, the crystallization was continued for another 2 days at 100 °C. The resulting final product was filtered and washed with an ethanol/ water mixture, and dried in an oven at 100 °C, followed by calcination in air at 550 °C for 6 h. A typical procedure for the preparation of MCM-48-supported HPW is as follows: the required amount of MCM-48 (dried in

10.1021/ie071314z CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008

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vacuo) was added to a solution of HPW in methanol (15 mL) and stirred for 12 h at room temperature. The HPW-supported catalyst was dried in vacuo, and the resulting solid was dried at 200 °C for 8 h. MCM-48-supported HPW catalysts are abbreviated as MCM-48-xHPW, where x denotes the percentage of HPW loading (wt %). 2.3. The Esterification. HPW and MCM-48-supported HPW were applied for the esterification of fatty acid (3 mmol) and fatty alcohol (3 mmol) in sc-CO2 medium in a 100 mL SUS316 autoclave at the appropriate pressure, temperature, and period. Acid, alcohol, catalyst, and dry ice were placed in the autoclave and it was heated to the desired temperature, and the reaction was started with agitation. The pressure of CO2 necessary for the reaction conditions was calculated from the empty volume of the reactor and the reaction temperature. The dry ice used in the present study was used immediately after purchase, which may contain water only at the ppm level. After the reaction finished, the autoclave was cooled to room temperature, and CO2 was slowly depressurized. The contents were collected by washing repeatedly with ethanol and chloroform. The products were analyzed using a Shimadzu Gas Chromatograph 14A equipped with a capillary column: Ultra-1 (25 m × 0.3 mm and 0.32 µm thick layer; Agilent Technologies Inc.). 2.4. Recycling of Catalysts. After the reaction, the catalyst was separated by filtration, and washed well with ethanol, chloroform, and acetone. It was subjected to a recycling experiment after heat treatment at 200 °C for 6 h in air. The esterification was carried out in the same procedures as described above. Tungsten amounts in the catalyst before and after use were measured using inductive coupled plasma atomic emission spectroscopy on a JICP-PS-1000 UV spectrophotometer (Leeman Labs Inc.). The samples were prepared using (0.02 g) of catalyst fused with potassium carbonate (0.1 g), and the resultant potassium salt was dissolved using water (100 mL) containing concentrated HNO3 (6.3 mL). The used catalyst was first calcined at 550 °C in air to remove absorbed organic and then the above procedure was followed. The analysis of HPW leached in the solution during the reaction was measured after the recovery of HPW by extraction with water. 2.5. Characterization of Catalysts. Powder X-ray diffraction was measured by a Shimadzu XRD-6000 diffractometer with KR radiation (R ) 1.5418 Å). FTIR spectra were measured by the KBr method using a Nexus 470 Spectrometer (Thermo Electron Corp.). N2 adsorption measurements were carried out on a Belsorp 28SA apparatus (Bel Japan Co. Ltd.). TG and DTA analyses were carried out using a Shimadzu DTG-50 analyzer with a temperature-programmed rate of 10 °C/min in air stream. 3. Results and Discussion 3.1. Characterization of MCM-48-Supported HPW Catalysts. X-ray diffraction patterns of MCM-48-supported HPW (MCM-48-HPW) are shown in Figure 1. The XRD pattern of MCM-48 exhibits a main characteristic peak corresponding to the (211) plane along with a broad shoulder peak from the (220) plane at a 2θ angle of about 2.3 and 3.0°, respectively. These peaks, together with the sextet pattern observed between 2θ angles of 3-6°, indicate the typical MCM-48 characters.35 The broadening and decrease in the intensity of the main peaks were observed by increasing loadings of HPW on MCM-48. A similar observation has also described in previous papers.20-23 No XRD peaks corresponding to crystalline HPW were observed above a 2θ angle of 10°. In the case of highly HPW-loaded MCM-

Figure 1. XRD patterns of MCM-48-supported HPW catalysts. (a) MCM48, (b) MCM-48-5HPW, (c) MCM-48-10HPW, (d) MCM-48-15HPW, (e) MCM-48-20HPW, (f) MCM-48-30HPW, and (g) MCM-48-50HPW.

Figure 2. FTIR spectra of MCM-48-supported HPW catalysts. (a) HPW, (b) MCM-48, (c) MCM-48-5HPW, (d) MCM-48-10HPW, (e) MCM-4815HPW, (f) MCM-48-20HPW, (g) MCM-48-30HPW, and (h) MCM-4850HPW.

48, that is, MCM-48-50HPW showed a broad hump in the range of 2θ angle of 15-40°, but no crystalline peak strongly indicates that the most of the HPW were well dispersed in the MCM-48 channels. FTIR spectra of HPW and MCM-48-supported HPW are shown in Figure 2. The absorption bands observed in the range of 800-1100 cm-1 in both HPW and MCM-48-supported HPW catalysts are ascribed to the Keggin ion.9 A broad absorption band around 1200 cm-1 appeared by overlapping the stretching vibration of P-O bands of HPW (1080 cm-1) in MCM-48supported HPW. However, the absorption bands at 982 (νassym WdO) corresponding to (WdO) vibration and 895 cm-1 (νassym W-O-W) due to stretching vibrations of (W-O-W) can be seen in MCM-48-supported HPW. The N2 adsorption/desorption isotherms of MCM-48-supported HPW were of type (IV) according to the IUPAC,36 which are characteristics for ordered mesoporous solids. A well-defined sharp inflection was observed between the relative pressures (p/p0) of 0.3-0.4 due to capillary condensation of nitrogen inside the primary mesopores (Figure 3). The decrease in surface area and pore volume with the increase in HPW loading (Table 1) supports the high dispersion of HPW in the MCM-48 channels.

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Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008 Table 2. Effect of Loading of HPW on the Esterification of Palmitic Acid and Cetyl Alcohola catalyst

HPW on MCM-48 (wt %)b

yield of cetyl palmitate (%)

MCM-48 MCM-48-5HPW MCM-48-10HPW MCM-48-15HPW MCM-48-20HPW MCM-48-30HPW MCM-48-50HPW HPW

5 10 15 20 30 50 100

20.9 18.1 27.4 60.8 96.6 82.8 86.0 80.9

a Reaction conditions: palmitic acid, 0.7693 g (3 mmol); cetyl alcohol, 0.7273 g (3 mmol); catalyst, 0.006 mmol of HPW (substrate/ HPW ) 500); CO2 pressure, 11 MPa; period, 6 h. b Loading amount of HPW against support (wt %).

Figure 3. N2 adsorption/desorption analysis of MCM-48-supported HPW catalysts. (a) MCM-48, (b) MCM-48-10HPW, (c) MCM-48-20HPW, (d) MCM-48-30HPW, and (e) MCM-48-50HPW. Legends: Adsorption: (a) 9, (b) b, (c) 2, (d) 1, (e) [. Desorption: (a) 0, (b) O, (c) 4, (d) 3, (e) ].

Figure 4. Influence of reaction time on the esterification of palmitic acid and cetyl alcohol over HPW and MCM-48-supported HPW catalysts. Reaction conditions in sc-CO2: palmitic acid, 769.3 mg (3 mmol); cetyl alcohol, 727.3 mg (3 mmol); catalyst, 86.5 mg (0.006 mmol as HPW); 100 °C; CO2 pressure, 11 MPa. Reaction conditions in mesitylene medium: temperature, 100 °C. Legend: In sc-CO2 medium 9, HPW; 1, MCM-48-5HPW; b, MCM-48-10HPW; 3, MCM-48-15HPW; 0, MCM48-20HPW; 2, MCM-48-30HPW; and 4, MCM-48-50HPW. In mesitylene medium: O, MCM-48-20HPW. Table 1. Physicochemical Properties of MCM-48-Supported HPW catalyst

BET surface area (m2/g)

BJH pore diameter (nm)

pore volume (mm3/g)

MCM-48 MCM-48-5HPW MCM-48-10HPW MCM-48-15HPW MCM-48-20HPW MCM-48-30HPW MCM-48-50HPW

790 709 703 629 627 581 220

2.52 2.32 2.32 2.32 2.32 2.32 2.12

888 781 727 704 674 586 321

3.2. The Esterification. Figure 4 shows the influence of the reaction period on the esterification of palmitic acid and cetyl alcohol at 100 °C under 11 MPa of CO2. The yield of cetyl palmitate was gradually enhanced with an increase in the reaction period and reached a maximum at 6 h. However, there was no significant change in the yield with further prolongation

of the reaction period. The higher-loading MCM-48-supported HPW catalysts (MCM-48-20HPW, MCM-48-30HPW, and MCM-48-50HPW) had relatively high initial catalytic activities than unsupported HPW; however, all of the catalysts reached maximum yield after around 6 h. These results show that the esterification proceeds effectively over HPW in sc-CO2 medium and that the mass transfers of reactants and products under our conditions are rapid enough for the catalysis because of their high solubility in the sc-CO2 medium. On the other hand, low HPW-containing catalysts (MCM-48-5HPW, MCM-48-10HPW, and MCM-48-15HPW) showed lower yields than the higher HPW-supported catalysts. It is considered that the acidity of HPW on MCM-48 and -41 is in the same level of unsupported acids because the acidity of HPW appears in a molecular level. However, the support on mesoporous silicas enhances the dispersion of HPW, thus resulting in the increase in acid amounts of HPW. The improvement of the acidity of HPW by support over the silica surface was already confirmed by Kim et al.37 using 31P CP MAS NMR with trimethylphosphine as a probe molecule adsorbed on the acid sites of HPW. The low loading catalysts, such as MCM-48-5HPW and MCM-48-10HPW, can expect high dispersion; however, they only have low catalytic activities. These low activities of low loading catalysts possibly due to the trapping of the acidic proton of HPW with the surface of silica result in decrease of acid amounts; particularly, it is highly influenced at lower loading.37-39 The 20% loading is considered to be the most appropriate for the esterification because moderately dispersed HPW works for the catalysis. Table 2 summarizes the influence of HPW loading on MCM48 in the esterification of palmitic acid and cetyl alcohol at the constant amount of HPW. MCM-48-5HPW and MCM-4810HPW showed a relatively low yield as discussed above. The highest yield of cetyl palmitate was obtained for MCM-4820HPW; however, MCM-48-15HPW, MCM-48-30HPW, MCM48-50HPW, and unsupported HPW gave lower yields than MCM-48-20HPW, although the latter three catalysts gave finally a similar yield. These results show that 20% loading is the most appropriate for the esterification: this may be due to the highly uniform distribution of HPW in MCM-48 channels at the loading. Table 3 summarizes the influence of HPW weight on MCM48-supported HPW and unsupported catalysts for the esterification of palmitic acid and cetyl alcohol. It was clearly seen from the Table 3 that HPW supported on MCM-48 enhanced the catalytic activities, resulting in improvement of the yield of cetyl palmitate. MCM-48-20HPW gave relatively higher yield than other catalysts. This may be due to uniform distribution of HPW on the surface of MCM-48 as discussed above. On

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catalyst

substrate/ catalyst yield cetyl molar ratiob palmitate (%)

MCM-48- 1000 (86.4) 10HPW MCM-48- 500 (86.4) 20HPW MCM-48- 333 (86.4) 30HPW MCM-48- 200 (86.4) 50HPW MCM-41- 500 (86.4) 20HPW MCM-48

catalyst

substrate/ catalyst yield cetyl molar ratiob palmitate (%)

25.7

HPW

1000 (8.6)

26.6

96.6

HPW

500 (17.2)

80.9

88.4

HPW

333 (25.8)

81.0

79.4

HPW

200 (43.2)

82.0

43.6 20.9

no catalyst

16.3

Reaction conditions: temperature, 100 °C; pressure, 11 MPa; period, 6 h. b Molar ratio based on HPW loading (in parenthesis: MCM-48supported HPW weight (mg)). a

Figure 6. Influence of reaction temperature on the esterification of palmitic acid and cetyl alcohol over HPW and MCM-48-supported HPW catalysts. (a) MCM-48-20HPW and (b) HPW. Reaction conditions: palmitic acid, 769.3 mg (3 mmol); cetyl alcohol, 727.3 mg (3 mmol); catalyst, 86.5 mg for MCM-48-20HPW and 17.4 mg for HPW (0.006 mmol as HPW); CO2 pressure, 11 MPa; period, 6 h.

Figure 5. XRD pattern of MCM-48-supported HPW used in the esterification of palmitic acid and cetyl alcohol. (a) MCM-48-20HPW, (b) MCM48-20HPW after the reaction, (c) MCM-41-20HPW, and (d) MCM-4120HPW after the reaction.

the other hand, 20% HPW supported on MCM-41 (MCM-4120HPW) gave the ester only in 44% yield. The observed higher activity in the case of MCM-48-20HPW is mainly due to differences of structure between MCM-48 and MCM-41: 3D pore opening for MCM-48 and 1D pore opening for MCM-41. These differences result in easy diffusion of reactant molecules for MCM-48-20HPW compared to MCM-41-20HPW. Further, as it can be seen from Figure 5, a comparison of the XRD pattern of the catalyst after the reaction shows that MCM-48-20HPW retained its structure although the crystallinity decreased; however, MCM-41-20HPW collapsed completely. The mesoporous 1D MCM-41 have relatively less hydrothermal stability than 3D MCM-48,40 and thus the collapse of MCM-41-20HPW occurs during the esterification due to water formed in the course of the reaction. Thus, MCM-48-20HPW was used for further investigation. Figure 6 shows the influence of reaction temperature on the esterification of palmitic acid and cetyl alcohol over HPW and MCM-48-20HPW catalysts. The yield of cetyl palmitate increased with the increase in reaction temperature over both catalysts. The maximum yield of the ester was obtained at 100 °C on both catalysts. However, the yield of the ester saturated or slightly decreased with further increase in the temperature: this may be due to reverse reaction at higher temperatures by water formed during the esterification.30 From these results, the optimum reaction temperature is considered as 100 °C for the present catalyst system. Figure 7 shows the influence of CO2 pressure on the esterification of palmitic acid and cetyl alcohol over MCM-48-

Figure 7. Influence of supercritical CO2 pressure on the esterification of palmitic acid and cetyl alcohol over HPW and MCM-48-supported HPW catalysts. (a) MCM-48-20HPW and (b) HPW. Reaction conditions: palmitic acid, 769.3 mg (3 mmol); cetyl alcohol, 727.3 mg (3 mmol); catalyst, 86.5 mg for MCM-48-20HPW and 17.4 mg for HPW (0.006 mmol as HPW); temperature, 100 °C; period, 6 h.

20HPW at 100 °C. The yield of cetyl palmitate linearly increased with the increase in CO2 pressure, and saturated gradually at higher pressure than 11 MPa. These results mean that CO2 around 11 MPa is enough for the acceleration of product transfer between the catalyst surface and the bulk phase during the esterification. The catalytic activities under high CO2 pressure (13 MPa) in heterogeneous catalysis showed a slight decrease in conversion compared to the CO2 pressure of 11 MPa, which is a similar phenomena observed in the previous works.30 From the data in the study, the optimum reaction conditions, temperature, 100 °C; period, 6 h; and pressure, 11 MPa, were chosen for further studies. Table 4 summarizes the influence of the chain length of fatty acids and alcohols on the esterification over HPW and MCM48-20HPW. The increase in the chain length of the fatty acid resulted in increasing the yield of the corresponding ester. Similarly, the increase in the yield of corresponding esters was

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Table 4. Esterification of Long Chain Fatty Acids and Alcohols over HPW and MCM-48-20HPW

Table 5. Esterification of Branched Long Chain Fatty Acids and Alcohols over HPW and MCM-48-20HPW

Yield of ester (%)

Yield of ester (%)

acid

alcohol

MCM-48-20HPW

HPW

acid

alcohol

MCM-48-20HPW

HPW

1-Butyric acid 1-Hexanoic acid 1-Octanoic acid 1-Decanoic acid Lauric acid Myristic acid Palmitic acid Stearic acid Palmitic acid Palmitic acid Palmitic acid Palmitic acid Palmitic acid Palmitic acid Palmitic acid

Cetyl alcohol Cetyl alcohol Cetyl alcohol Cetyl alcohol Cetyl alcohol Cetyl alcohol Cetyl alcohol Cetyl alcohol 1-Butanol 1-Hexanol 1-Octanol 1-Nonanol 1-Decanol Lauryl alcohol Myristyl alcohol

48.2 59.6 80.9 91.4 87.9 93.2 97.0 86.3 52.1 69.7 70.5 70.1 74.8 90.1 95.7

46.4 51.7 71.1 85.8 77.6 88.5 80.9 91.6 42.1 56.9 62.5 69.4 53.5 86.5 88.3

palmitic acid palmitic acid palmitic acid oleic acidc isostearic acidc isostearic acidc isostearic acidc isostearic acid

2-decanol 2-decanol 2-hexadecanol cetyl alcohol cetyl alcohol cetyl alcohol cetyl alcohol cetyl alcohol

0 9.0b 2.0 58.3 26.8 32.5d 42.5e 50.4f

2.3 61.1b 2.5 89.3 35.5a 41.4d 60.6e 76.83

a Reaction conditions: substrate/HPW ratio, 500; temperature, 100 °C; CO2 pressure, 11 MPa, period, 6 h. b Period ) 12 h. c Trace amount of unidentified products. d Substrate/catalyst ratio, 150. e Substrate/catalyst ratio, 100. f Substrate/catalyst ratio, 50.

Reaction conditions: substrate/HPW ratio, 500; temperature, 100 °C; CO2 pressure 11 MPa, period 6 h.

observed with increasing the chain length of the fatty alcohol with palmitic acid. Major merits of the sc-CO2 medium are the elimination of mass transfer resistance of the substrate and the products resulting in the prevention of strong adsorbed species on the catalyst as indicated in the Introduction. These merits of sc-CO2 effectively enhance the esterification of long-chain acids and esters. However, they do not effectively work if the shortchain acids or alcohols are involved in the reaction. It is difficult to fully understand the phenomena; however, the following is one possible explanation. The short-chain acids or alcohols strongly adsorb on the acid sites through hydroxyl moieties and occupy acid sites, thus resulting in retardation of the esterification involving short-chain acids and alcohols. The reasons for the strong adsorption of short-chain acids and alcohols are probably due to the less hydrophobic effects of their short alkyl moieties, which give them more ionic nature, thus resulting in their strong interaction with acid sites on the surface. Water formed by the esterification is another reason of the low yield for short-chain acids and alcohols: short-chain acids and alcohols may be significantly influenced compared to those with long-chains because they are much more hydrophilic than the latter, resulting in retardation of the esterification of short-chain acids and alcohols. These interactions may further result in lower activities of short-chain acids and alcohols for the esterification. The solubility properties of these acids and alcohols in sc-CO2 may also be important factors for these phenomena; however, they have not been found fully in the literature. Further research is necessary for clarification of the details of the phenomena. The esterification of palmitic acid and secondary alcohols, namely 2-decanol and 2-hexadecanol, gave the corresponding esters in very low yields (Table 5). The yield of esters still remained low over MCM-48-20HPW, although there were increases in the HPW amount and the reaction period. This is mainly due to low reactivities and steric hindrance of secondary alcohol and the corresponding ester in the channel of MCM48. Oleic acid gave cetyl oleate in moderate yield in the esterification with cetyl alcohol over MCM-48-20HPW; however, HPW gave the ester in high yield, where no isomerization to cetyl elaidate was observed over both catalysts. The esterification of isostearic acid showed relatively low yields over MCM-48-20HPW (Table 4). However, the yields were improved by an increase in the catalyst amount for isostearic acid. The esterification of palmitic acid and cetyl alcohol was examined over HPW and MCM-48-20HPW in mesitylene medium at 100 °C; however, the cetyl palmitate was obtained

Figure 8. TGA profile of MCM-48-20HPW after the catalyzes. Reaction conditions: palmitic acid, 769.3 mg (3 mmol); cetyl alcohol, 727.3 mg (3 mmol); catalyst, 86.5 mg for MCM-48-20HPW (0.006 mmol as HPW); temperature, 100 °C; period, 6 h. (a) Before reaction, (b) after first run in sc-CO2 medium, (c) after fourth run sc-CO2 medium, and (d) after first run in mesitylene medium. Table 6. Recyclability of MCM-48-20HPW for the Esterification of Palmitic Acid and Cetyl Alcohol Yield of cetyl palmitate (%) run

MCM-48-20HPW

MCM-41-20HPW

1st 2nd 3rd 4th

96.6 60.2 38.9 37.3

43.6 30.5

a Reaction conditions: substrate/HPW, 500; temperature, 100 °C; CO 2 pressure, 11 MPa; period, 6 h.

only in low yield (Figure 4). These results indicate that higher yield in sc-CO2 medium is due to rapid diffusion of the reactants and products in the channels of MCM-48-20HPW. Further, scCO2 medium carries reactants and product on the catalyst surface; however, it does not allow long stays on the active catalytic surface. Thus, active sites remain clean and hence enhances the catalytic activity. TG analysis of MCM-48-20HPW (Figure 8) after the reaction in sc-CO2 medium showed a roughly 22% weight loss due to absorbed organic compounds inside the channel of MCM-48; however, about 50% weight loss was observed from the catalyst used in mesitylene medium. MCM-48-20HPW catalysts were recyclable for several runs, as summarized in Table 6. The yield of cetyl palmitate gradually decreased during recycles of the catalyst. The initial decrease in the yield during the first recycle may be due to the leaching of a small amount of HPW: the amount of HPW on the catalyst

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after the first recycles was about 17% HPW (by ICP analysis), and no detectable tungsten was observed by ICP after the second recycle. The observed small leaching of HPW mostly in the first recycle may be due to some of the HPW present on the external surface of the MCM-48 support with weak interaction; this may come out during the first run of catalytic reaction. On the other hand, the accumulation of organic compounds is clearly seen from TGA analysis of the recycled catalysts (parts b and c of Figure 8). These results show that the decrease in catalytic activity during second to fourth recycles is mainly due to adsorbed organic molecules in MCM-48 channels, although scCO2 medium is a good solvent for prevention of the accumulation of heavy molecules. The catalytic activity of MCM-4820HPW was also lower than MCM-41-20HPW during the recycles. The differences between MCM-48 and MCM-41 as the support are due to their differences in pore dimensionalities and in hydrothermal stabilities under the reaction conditions. It is very interesting that MCM-48-supported HPW was active and stable under the reaction conditions; however, the mesoporous structure of MCM-41 was collapsed under the esterification conditions. MCM-41 with 1D structure has less hydrothermal stability than MCM-48 with 3D interconnecting structure.40 The water formed during the esterification reaction collapses MCM-41 easily compared to MCM-48. The catalysis in sc-CO2 medium works well in the esterification of long-chain fatty acids and alcohols. These results mean that reactants and products are rapidly transported to the bulk phase by sc-CO2 medium and that the heavy compounds formed on the acid sites are removed from the catalyst surface by the dissolution, although some of them still remain on the catalyst. These effects of sc-CO2 medium are the most promising properties for green catalytic processes. 4. Conclusion HPW and MCM-48-supported HPW catalysts were highly active in the esterification of long-chain fatty acids and alcohols in sc-CO2 medium. The catalytic activities in sc-CO2 medium were significantly higher than as in mesitylene medium. The high yield in sc-CO2 medium is due to rapid diffusion of the reactants and products in the MCM-48 channels and to the high contact of reactants with the catalyst. HPW gave higher activity for the esterification by loading on MCM-48. The yields of esters were enhanced with the increase in chain length of acid and alcohols in the esterification in sc-CO2 medium. MCM48-supported HPW was recyclable for the esterification although catalytic activities were considerably decreased. The HPW catalysts were found to be potentially useful for the esterification of fatty acids with primary fatty alcohols. However, relatively very low yields were obtained in the esterification involving some short acid and alcohols, secondary alcohols, and functional acids. Acknowledgment A part of this work was financially supported by grants (B) 16310056 and 1931006, from the Japan Society for the Promotion of Science (JSPS). A.S. is grateful to JSPS for a postdoctoral fellowship. Literature Cited (1) Lauridsen, J. B. Food emulsifiers: Surface activity, edibility, manufacture, composition, and application. J. Am. Oil Chem. Soc. 1976, 53, 400-407. (2) Fatty Acids in Industry: Processes, Properties, DeriVatiVes; Johnson, R. W., Fritz, E., Eds.; Marcel Dekker: New York and Basel, 1988.

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ReceiVed for reView September 28, 2007 ReVised manuscript receiVed January 8, 2008 Accepted February 8, 2008 IE071314Z