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Langmuir 2000, 16, 4072-4079
Articles Isomerization of Citronellal to Isopulegol Using Eclectically Engineered Sulfated Zirconia-Carbon Molecular Sieve Composite Catalysts, UDCaT-2 Ganapati D. Yadav* and Jayesh J. Nair Chemical Engineering Division, University Department of Chemical Technology, University of Mumbai, Matunga, Mumbai-400 019, India Received August 18, 1999. In Final Form: February 1, 2000 Sulfated zirconia (S-ZrO2) is a well-known solid superacid used as a catalyst in various reactions of commercial importance such as isomerization, Friedel-Crafts alkylation and acylation, nitration, cracking, esterification, etc. S-ZrO2, per se, is not a shape-selective catalyst. The selectivity toward the formation of the desired product can be greatly enhanced by eclectically designing a shape-selective catalyst by a synergistic combination between S-ZrO2 and carbon molecular sieves (CMS). The paper presents the novelty of the combination of S-ZrO2 and CMS, designated as UDCaT-2, as a shape-selective catalyst in the cyclization of citronellal to isopulegol which has industrial value. UDCaT-2 was found to be the best catalyst, among others, for the selective cyclization of citronellal to isopulegol wherein the shape selectivity can be tailored by proper pretreatment. The conversion and selectivity were found to be the maximum at 95 °C. The formation of isopulegol was found to be dependent on the average pore size of the carbon molecular sieve barrier encompassing S-ZrO2. A detailed kinetic study of the reaction showed that it followed a Langmuir-Hinshelwood-Hougen-Watson type of mechanism whereby citronellal was found to be weakly adsorbed on the catalytic surface sites. The reaction was found to follow first-order kinetics for the disappearance of citronellal.
Introduction Isopulegol is an important intermediate for the manufacture of menthol, which is used extensively in pharmaceuticals, cosmetics, toothpastes, chewing gum, and other toilet goods as well as in cigarettes.1 Isopulegol is manufactured from the cyclization of citronellal. Bogert and Hasselstrom2 have reported the use of UV in the cyclization reaction. Activities and selectivities of acid clinoptilolite, mordenite, and faujasite zeolites in the isomerization of citronellal in n-hexane, chloroform, and dichloromethane, as solvents, have been investigated.3 The activities of the acidic zeolites for the isomerization of citronellal were found to be in the following order: HY(max.) > HCC > (clinoptilolite) > HMCP (clinoptilolite + mordenite) > HX at 84 °C in dichloroethane, and the selectivities to isopulegol were HCC (90%) > HMCP (85%) > HY(80%) > HMP (72%) at 80% conversion level. The activity in these studies was related to the total amount of Bronsted acid sites of the catalysts, and only a fraction of these sites, located mainly on the external surface of the crystal, were accessible to the reactants due to the diffusional resistance. It was observed that the selectivity to the isopulegol ether had increased, with increasing the accessibility to the acidic centers. There are several reports whereby Cu-Cr and * To whom correspondence should be addressed. Fax: 91-22414 5614. Telephone: 91-22-414 5616 (Ext. 291). E-mail:
[email protected]. (1) Leffingwell, J. C.; Shackelford, R. E. Cosmet. Perfum. 1974, 89, 69. (2) Bogert, M. T.; Hasselstrom, T. Synthesis 1930, 53, 4093. (3) Fuentes, M.; Magraner, J.; De Las Pozas, C.; Roque-Malherbe, R.; Pariente, J. P.; Corma, A. Appl. Catal. 1989, 47, 367.
Cu-Cr-Mn4, tris(triphenylphosphine)rhodium chloride,5 and micellar6 catalysts have been employed to catalyze the cyclization reaction. Several Lewis acids have been used7 as catalysts for the preparation of L-isopulegol from D-citronellal. Dean and Whittaker8 have studied this reaction with superacids (e.g., FSO3H/SO2) to observe that the cyclization follows the same path as the normal acids, yielding isopulegol and neoisopulegol. We developed a new catalyst 9-11 based on sulfated zirconia (S-ZrO2) and carbon molecular sieve (CMS), named UDCaT-2, which was found to offer very high selectivity to isopulegol; the details of the catalyst preparation and characterization and kinetics of the reaction are presented here. UDCaT-2 was also found to be highly selective in nitration of aromatic compounds.11,12 Experimental Section Chemicals. Zirconium oxychloride was procured from Loba Chemie. Ammonia, toluene, cyclohexane, acetone, 2-propanol, and concentrated sulfuric acid, all of AR grade, were obtained (4) Kogami, K.; Kumanotani, J. Bull. Chem. Soc. Jpn. 1968, 41, 2530. (5) Sakai, K.; Oda, O. Tetrahedron Lett. 1972, 4375. (6) Clark, B. C.; Theresa, S. C.; Guillermo, A. I. J. Org. Chem. 1984, 49, 4557. (7) Nakatani, Y.; Kawashima, K. Synthesis 1978, 147. (8) Dean, C.; Whittaker, D. J. Chem. Soc., Perkin Trans. 1990, 2, 1275. (9) Yadav, G. D.; Nair, J. J. J. Chem. Soc., Chem. Commun. 1998, 2369. (10) Yadav, G. D.; Nair, J. J. Catal. Lett. 1999, 62, 49. (11) Yadav, G. D.; Nair, J. J.; Narendra V. US Patent Appl. No. 09/ 211,500, 1998. (12) Yadav, G. D.; Nair, J. J. Microporous Mesoporous Mater. 1999, 33, 1. (13) Schimid, J. A. J. Am. Ceram. Soc. 1987, 70, 367.
10.1021/la9911178 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000
Isomerization of Citronellal to Isopulegol
Langmuir, Vol. 16, No. 9, 2000 4073
Figure 1. Pore size distribution plot of (a) S-ZrO2[650], (b) S-ZrO2[230-350], and (c) UDCaT-2. from s.d. Fine Chemicals Ltd. Citronellal was obtained from Arofine Industries Ltd. Preparation of Catalysts. S-ZrO2. The catalyst was prepared by using a conventional precipitation method.9 A 100 g sample of zirconium oxychloride was dissolved in distilled water, and the solution was filtered. The clear solution and 25% aqueous ammonia were added dropwise simultaneously in a beaker with constant stirring, and a white precipitate of zirconium hydroxide was obtained. The precipitation was carried out at a pH of 9-10. After complete precipitation, it was digested as such for 6 h. The precipitate was filtered through a Buchner funnel. It was washed thoroughly with distilled water and made free of ammonia and chloride ions. The filtered precipitate was dried in an oven at 120 °C for 24 h. The dried catalyst was crushed in a mortar and pestle to a fine powder. It was treated with 1 N H2SO4. Here, 15 mL of 1 N H2SO4 was required for 1 g of the catalyst. The sulfated catalyst was dried in an oven at 120 °C for 24 h followed by calcination at 230 to 650 °C. When it was calcined between 230 and 350 °C, it was designated as S-ZrO2[230-350] and that calcined at 650 °C as S-ZrO2[650]. ZrO2[230-350]. The catalyst was prepared by the same method as described above except that no sulfuric acid treatment was given to the dried powder and was calcined directly at 230350 °C for 3 h. UDCaT-2. To 10 g of the above prepared catalyst, an appropriate quantity of poly(vinyl alcohol) (PVA) solution was added dropwise until it was just wet. This was then mixed well to get a uniform coating. It was dried at 100 °C for 1 h and calcined at the desired temperature to yield the UDCaT-2 catalyst. S-ZrO2/Solvent/CMS. S-ZrO2 was initially soaked with different solvents such as benzene, cyclohexane, carbon tetrachloride, and hexane until wetness and then coated with PVA solution by the incipient wetness technique. These were then calcined at 350 °C for 3 h to give S-ZrO2/solvent/CMS catalyst. Characterization of Catalysts. S-ZrO2[650], S-ZrO2[230350] and UDCaT-2 were characterized in detail by using various techniques such as chemical analysis, XRD, FTIR, pore size distribution, pore volume and BET surface area. Characterization of S-ZrO2[650]. Table 1 gives the pore size distribution of S-ZrO2[650]. The nitrogen adsorption isotherm was studied by using a Micromeritics surface area analyzer (model: ASAP 2010) to determine the BET surface area of 100 m2/g. The pore size of the catalyst was found to be in the range of 35-50 Å with maximum pores in the region of 38-42 Å as shown in Figure 1a. The pore volume of the catalyst was found to be 0.108 cm3/g. The FTIR spectra of S-ZrO2[650] (Figure
Table 1. Pore Size Distribution of Various Catalysts catalysts
pore size range, Å
surface area, m2/g
pore volume, cm3/g
S-ZrO2[650] S-ZrO2[230-350] UDCaT-2
35-50 25-35
