Ind. Eng. Chem. Res. 2002, 41, 2611-2616
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Improved Method for Dehydrating Secondary Alcohols Using Inorganic Sulfates Supported on Silica in Refluxing Octane Edmund J. Eisenbraun,* Kirk W. Payne, Jeremy S. Bymaster, Asfaha Iob, and Allen Apblett† Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
The dehydration of cyclododecanol with copper(II) sulfate (anhydrous, monohydrate, and pentahydrate) unsupported and supported on silica (prepared in situ and as a preformed catalyst) was studied to develop an improved method for dehydrating secondary alcohols. The dehydration progress, synergism of the catalyst(s), and purity of the products were monitored by gas chromatography. Reactions were studied in refluxing octane, a series of aromatic hydrocarbons, and tetrachloroethylene. A comparison is made of dehydration methods contrasting the use of a reflux condenser with no provision for removing water, a conventional Dean-Stark apparatus, and a modified Dean-Stark apparatus incorporating a molecular sieve (AW-500). Aluminum potassium sulfate, supported on silica, was also shown to be an effective dehydration catalyst. The supported catalysts did not cause formation of side products and were found to retain activity in multiple runs. Characterization of the catalysts was performed using Brunauer-EmmettTeller surface area measurements, Fourier transform infrared spectroscopy, thermogravimetric analysis/differential thermal analysis, and Hammett indicators. The use of acidic inorganic catalysts, including unsupported sulfates and those supported on high-surfacearea materials, is receiving attention in current organic chemistry1-3 and has potential for industrial applications. These supported catalysts display a range of Brønsted vs Lewis acid properties.2,3 In this paper, we present an extension of the procedure used to produce high-purity R-cedrene from cedrol4 that included the use of a modified Dean-Stark/molecular sieve apparatus to maintain anhydrous reaction conditions. This former study4 demonstrated that a broad variety of inorganic sulfates were effective dehydration catalysts for tertiary alcohols in the presence of refluxing benzene. The current study extends this dehydration procedure to secondary alcohols using cyclododecanol as a model compound and is intended to expand the concept of filterable, solid inorganic sulfates as useful acidic catalysts in organic chemistry. This is made possible by enhancing the catalytic activity through support of CuSO4 or AlK(SO4)2 on silica and by increasing the reaction temperature through replacement of benzene (bp 80 °C) with octane (bp 126 °C). References to the industrial-scale dehydration of alcohols to alkenes are dominated by preparation of ethylene and terminal alkenes from primary alcohols with infrequent mention of the dehydration of secondary alcohols. While this reflects current industrial trends, there remains a need for effective and environmentally acceptable procedures to prepare cyclic and internal alkenes from secondary alcohols regardless of scale. The catalysts described in this paper are effective for the dehydration of cyclododecanol, as a model secondary alcohol, on a preparative scale and may provide the means to achieve large-scale dehydrations of secondary alcohols. All reactions in this paper were carried out as described in the typical * Corresponding author. Phone: (405) 744-6673. Fax: (405) 744-6007. E-mail:
[email protected]. † Corresponding author regarding catalyst characterization. Phone: (405) 744-5943. E-mail:
[email protected].
Figure 1. Dehydration rate as a function of the reaction temperature.
method in the Experimental Section unless otherwise noted. Following exploratory studies to optimize the reaction and isolation of the products, all variables (i.e., apparatus, stirring rates, heating rates, reaction volumes, catalyst-to-reactant ratios, and sampling times) were carefully duplicated so that only the alteration of the catalyst would influence the reaction. No side products were observed in the current study. Selection of Solvent and Reaction Conditions The reaction conditions used for dehydrating the tertiary alcohol cedrol4 (i.e., CuSO4 in refluxing benzene) were ineffective in dehydrating the secondary alcohol cyclododecanol. Accordingly, increasing the boiling point of the solvent to increase the reaction temperature and supporting CuSO4 on silica to increase the reactivity of the catalyst provided the needed changes for successful dehydration. As shown in Figure 1, the reaction rate, using the same type of catalyst (see catalyst A in the Experimental Section for details), of the dehydration increases with the boiling point of the recycling solvents.
10.1021/ie010549m CCC: $22.00 © 2002 American Chemical Society Published on Web 04/30/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002
Figure 2. Comparison of dehydration methods using the same batch of preformed activated catalyst (G).
Because aromatic hydrocarbons can become involved in side product formation under acidic conditions, they were not selected for solvents in this study. However, the aromatic hydrocarbons shown in Figure 1 served to establish the needed time-temperature profile of the reaction but were not further studied because octane provided the conditions needed to study the reaction. In addition, the close proximity of the boiling ranges of octane and tetrachloroethylene permitted comparison of these two solvents. This comparison was important because tetrachloroethylene had previously been reported to be an effective solvent in the dehydration of secondary and tertiary alcohols using a preformed catalyst prepared from CuSO4 supported on silica.5,6 However, in these earlier tetrachloroethylene studies, there were no provisions for removal of the water formed during the dehydration.5,6 It may be argued that the insolubility of water in tetrachloroethylene and the density of the latter can block the return of water to the reaction site on the catalyst, thus making it more effective than a hydrocarbon solvent in water-forming reactions and obviating the need for water removal.7 Nevertheless, as shown in Figure 1, tetrachloroethylene is less effective than octane. The use of octane represents a challenge to the continued use of tetrachloroethylene in dehydrating alcohols. Therefore, it became essential to thoroughly compare all experimental parameters of these two solvents. Cyclododecanol was dehydrated, with octane and tetrachloroethylene in competition, using the same batch of preformed catalyst (see catalyst G). Reaction procedures using a reflux condenser, a conventional Dean-Stark apparatus, and the Dean-Stark/molecular sieve apparatus8 were compared. The results from these dehydration studies, presented in Figure 2, clearly show that octane is superior (ca. 2 times more effective) to tetrachloroethylene. Further, tetrachloroethylene is unreliable as a reaction medium because upon distillation it has a recognized propensity to form trichloroacetyl chloride and/or phosgene that can result in acidity,9 lead to formation of side products, and therefore interfere with recovery, purification, and reuse of the solvent. In addition, the future permitted use of tetrachloroethylene is questionable because of human toxicity and photochemical activity in ozone depletion.9,10 For these reasons, it was excluded from further study. The data in Figure 2 also show that the removal of water through the use of a Dean-Stark apparatus is beneficial. A comparison of the performance of a DeanStark apparatus with or without molecular sieves was inconclusive.
Figure 3. Synergism of supported catalyst vs individual catalyst components.
Figure 4. Comparison of different in situ mixed catalysts (copper(II) sulfate supported on silica).
Synergism of the Supported CuSO4/Silica Catalysts The previously described use of CuSO4 supported on silica involved a separate procedure for the preparation of the catalyst.5,6 We repeated the preparation (catalysts F and G) and use of these catalysts but also sought to simplify the overall procedure by preparing the catalyst in situ. Our in situ mixed catalyst refers to a catalyst prepared through addition and mixing of all reaction components at the beginning of the reaction. The remarkable synergism, i.e., a 10-fold increase in the reactivity of the supported catalyst, is illustrated by comparing the reaction curves in Figure 3. The upper curve, obtained from the use of the in situ mixed catalyst (catalyst A), is in dramatic contrast to those obtained through the use of the silica support alone (middle curve) or CuSO4‚5H2O alone (lower curve). To further probe synergism, the catalyst components were combined in a variety of ways to determine the most effective combination. Curves for similar but slower reactions were obtained for the in situ mixed catalysts (catalysts B and C) and their individual components when predried at the reaction temperature (CuSO4‚H2O stage)11-14 or when predried to the anhydrous stage.11-14 All forms of the supported catalyst displayed synergism. Reaction curves resulting from the use of the different catalysts formed via in situ mixing can be seen in Figure 4. These curves clearly show the superiority of the catalyst prepared from the pentahydrate salt (catalyst A) over those prepared from the anhydrous salt (catalyst C was 1/3 as fast) or the monohydrate salt (catalyst B was 1/5 as fast). These data support the concept that the catalyst surface (in situ mixed) develops as the reaction
Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2613 Table 1. Comparison of Reaction End Times during Catalyst Reuse reaction end time (min) and use no.
Figure 5. Comparison of catalysts formed by different processes.
proceeds. The higher reactivity of the in situ mixed catalyst prepared from the pentahydrate may be due to the available water allowing better interaction of the copper salt with the silica surface. This could occur by localized concentrations of water on the silica surface allowing partial dissolution of the copper salt. Formation of this liquid phase would facilitate distribution of CuSO4 on the silica surface. Because octane is hydrophobic, free water will collect on the silica surface if it is being produced at a rate faster than its removal by evaporation. The increase in the catalytic activity with time of the in situ mixed catalysts prepared from the anhydrous or monohydrate salt also supports this hypothesis. In this case, water released by dehydration of the alcohol may also play a role in optimizing the distribution of CuSO4 on the silica surface. These data suggest that the most active catalyst has a degree of hydration greater than 1 equiv/copper ion, a supposition that is borne out by the catalyst characterization discussed below. Efficacy of Catalysts Formed by Different Processes The efficacy of the catalysts resulting from different methods of preparation was evaluated. The reaction curves resulting from the use of the catalysts prepared as the preformed catalyst (catalysts F and G, unactivated and activated, respectively), in situ mixed catalyst (catalyst A), and in situ azeotroped catalysts (catalysts D and E) can be seen in Figure 5. All preparation procedures are described in the Experimental Section. The preformed preparation results in a Nishiguchitype5,6 catalyst. The in situ mixed catalyst is as described in the previous section. Except where noted, the in situ azeotroped catalyst was formed by dissolving CuSO4‚5H2O in a small amount of water, adding silica, removing the excess water by azeotropic distillation, cooling the flask contents to room temperature, adding cyclododecanol, adjusting the volume with octane, and then beginning the reaction. Of these various catalysts, the in situ azeotrope method produced on average a catalyst 1.3-2 times more effective than the preformed catalyst and ca. 2.8 times more effective than the in situ mixed catalyst. The different reactivities of the catalysts shown in Figure 5 may partially reflect variability in the distribution of CuSO4 on the silica surface. This phenomenon is important because, by itself, CuSO4 has a low surface area (i.e., CuSO4 prepared by drying in vacuo at 285 °C had a surface area of 4.1 m2/g) and the reactivity should increase with the surface area. As seen with the in situ mixed catalysts, the presence of water
catalyst type
catalyst
1st
2nd
3rd
4th
in situ mixed in situ azeotroped preformed unactivated preformed activated preformed AlK(SO4)2
A D F G H
330 46 75 71 75
310 74 84 71 120
200 84 85 120 120
na 87 70 69 71
during catalyst formation is beneficial for generation of catalytic activity. Thus, the in situ azeotroped catalyst (catalyst D) prepared with CuSO4‚5H2O dissolved in water was much more reactive than the in situ azeotroped catalyst (catalyst E) prepared without the addition of water. This in situ azeotroped catalyst (catalyst E) performed better than the in situ mixed catalyst (catalyst A). Thus, removal of the water by azeotropic distillation yields a more reactive catalyst. This activity difference may be due to the longer maturation of the catalyst (see the former discussion of the in situ mixed catalysts) or the presence of the alcohol (cyclododecanol) in the in situ mixed catalyst competing with water for the surface sites on the silica. This azeotropic effect is also seen in the higher activity of the in situ azeotroped catalyst (catalyst D) versus that of the preformed activated catalyst (catalyst G). Surprisingly, activation of the preformed catalyst (catalyst F to catalyst G) at 200 °C under a stream of nitrogen had a negligible effect on the reactivity. Reusability of the Different Catalysts The reusability of these different catalysts was evaluated by adding a fresh charge of cyclododecanol and distilling out octane to maintain a constant volume during each subsequent run. All forms of supported catalyst were effective in multiple batch runs. The reaction end times for the multiple runs are shown in Table 1. The catalysts did not exhibit side-product formation even during multiple uses. A preformed catalyst (catalyst H) made from AlK(SO4)2‚12H2O and silica is included, and it demonstrated comparable effectiveness. The end times for the fourth use of the catalyst became unreliable because cyclododecenes accumulating from prior runs caused elevation of the reaction temperature, resulting in an increase in the reaction rate. A set of reaction curves, normalized because of nonisolation of products in subsequent runs, resulting from the multiple use of an in situ azeotroped catalyst (catalyst D), are shown in Figure 6. These reaction curves are typical for the reuse of all of the evaluated catalysts. Reproducibility of the Catalysts The batch-to-batch reproducibility of the in situ catalysts is inherently greater than the preformed type catalyst. This results from their preparations requiring less handling steps, a less complicated procedure, and most importantly, all of the material used in the production of the catalyst remains in the flask in which the reaction subsequently takes place. The in situ azeotroped catalyst was found to exhibit a reproducibility of greater than 98%, based on catalyst performance. The in situ mixed catalysts displayed similar
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Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 Table 3. Surface Areas and pK’s for the Different Catalysts
Figure 6. Multiple use of an in situ azeotroped catalyst (D) without catalyst isolation. Table 2. TGA Results for the Different Catalysts and the Support catalyst type
catalyst
silica (Davisil 922,
