Kinetics and Mechanism of Cyclohexanol ... - ACS Publications

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Ind. Eng. Chem. Res. 2001, 40, 1822-1831

Kinetics and Mechanism of Cyclohexanol Dehydration in High-Temperature Water Naoko Akiya and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

We examined cyclohexanol dehydration in pure water at temperatures of 250, 275, 300, 350, and 380 °C with water densities ranging from 0.08 to 0.81 g/cm3. Under these conditions, cyclohexanol dehydrates readily in the absence of added catalysts to form cyclohexene as the major product. The most abundant minor products are 1- and 3-methyl cyclopentenes. The reaction rate and product distribution at 380 °C show a remarkable sensitivity to the water density. At low densities, the reaction is slow, and cyclohexene is the only product. At high densities, the reaction is nearly complete, and methyl cyclopentenes appear along with cyclohexene. The experimental results implied a reaction mechanism that comprises two pathways: (1) reversible cyclohexanol dehydration to form cyclohexene through an E2 mechanism, and (2) subsequent cyclohexene protonation to form the cyclohexyl cation, which rapidly rearranges to form methyl cyclopentyl cations, which then lose a proton to form methyl cyclopentenes. A kinetics model based on the proposed mechanism was able to predict the striking effect of the water density on the product yields at 380 °C and, thereby, to demonstrate that the proposed mechanism captures the trends in the experimental data. An analysis of mechanistic issues regarding cyclohexanol dehydration in high-temperature water (HTW) revealed three roles for water. Water participates in elementary reaction steps as a reactant and as a product, water is the source of the acid catalyst (H3O+), and water also drives the mechanism toward E2 by favoring, through solvation, the oxonium ion rather than the carbocation as the reaction intermediate. This study provides further evidence that acid-catalyzed reactions can be accomplished readily in HTW in the absence of added acid and that HTW has potential applications in environmentally benign industrial chemistry. Introduction High-temperature water (HTW), including both liquid water (T > ∼200 °C) and supercritical water (T > 374 °C, P > 218 atm), is attracting increased attention as a medium for organic chemistry.1,2 Water near its critical point exhibits many properties that are distinctly different from those of ambient liquid water, such as high solubility for organic and gaseous compounds, high ion product, low dielectric constant, and weak hydrogen bonds. These and other properties of HTW vary over a wide range of values as a function of temperature and pressure. This strong temperature and pressure dependence of the properties provides opportunities to tune the reaction environment to optimal conditions for the chemical transformation of interest. The reactions of cyclohexanol and its products in HTW provide an opportunity for the convenient examination of several different types of chemical transformations (dehydration, dehydrogenation, aromatization, rearrangement) in HTW. As such, it is a good model system. The goal of the study described herein is to obtain kinetics data for cyclohexanol dehydration in HTW and to use them to gain mechanistic insights. This study of cyclohexanol dehydration is the first step toward a better understanding of cyclohexanol chemistry in HTW. Crittendon and Parsons3 reported that cyclohexanol is unreactive in pure HTW at a nominal reaction * Corresponding author. E-mail: [email protected]. Phone: 734 764-3386. Fax: 734 763-0459.

temperature of 375 °C for 20 min, but that the addition of a solid metal catalyst (PtO2), acid (HCl), or base (NH4OH) leads to a variety of products. Kuhlmann et al.,4 on the other hand, reported that the dehydration to cyclohexene occurs readily at 300 °C after 60 min without the deliberate addition of any catalysts. They also reported that cyclohexene is the only product observed in their experiments. They speculated that the dehydration was acid-catalyzed, with H3O+ generated from the dissociation of water being the likely catalyst. This hypothesis is consistent with the work by Antal and co-workers, who showed that tert-butyl alcohol dehydration occurs in pure HTW at 250 °C without the addition of any catalyst.5,6 Antal and co-workers have been very active in the field of alcohol (but not cyclohexanol) dehydration in HTW. They have reported mechanisms and kinetics for the dehydration of ethanol, 1- and 2-propanol, and tertbutyl alcohol to the corresponding alkenes.5-8 The quantitative kinetics data have facilitated the conceptual design and economic evaluation of chemical processes based on the new chemistry in HTW.9,10 In contrast to the kinetics and mechanistic information available for the dehydration of the C2-C4 alcohols in HTW, there are no kinetics or mechanisms available for the dehydration of cyclohexanol in HTW. Moreover, the limited data that are available are contradictory. The present investigation of cyclohexanol dehydration in HTW resolves this conflict and fills this gap in the literature on alcohol dehydration in HTW.

10.1021/ie000964z CCC: $20.00 © 2001 American Chemical Society Published on Web 03/27/2001

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1823

Experimental Section We performed experiments at different batch holding times, temperatures, and water densities to determine the effects of these variables on the product yields and reaction rates. The conditions examined are temperatures of 250, 275, 300, 350, and 380 °C, times ranging from 15 to 180 min, and water densities ranging from 0.08 g/cm3 (4.7 mol/L) to 0.81 g/cm3 (45 mol/L). The cyclohexanol concentration was 0.3 mol/L (at reaction conditions) in all experiments. We used batch reactors fashioned from nominal 1/ -in. stainless steel Swagelok tube fittings (one port 4 connector and two caps). The reactor volume is 0.59 cm3. Preliminary experiments indicated that the use of new reactors (as received) produced results that differ from those obtained with reactors that have been used at least once. New reactors led to higher cyclohexanol conversions and the formation of additional (unidentified) byproducts. To eliminate these effects, we seasoned the new reactors prior to use. The reactors were loaded with water and heated in a sand bath at 300 °C for 30 min, after which they were cooled gradually at ambient conditions. The reactors were then washed with acetone and dried. All of the data reported herein were obtained from seasoned reactors. The reactors were loaded with a carefully measured amount of cyclohexanol and placed in a glovebox filled with purified helium. In the same glovebox, we vigorously bubbled helium through distilled and deionized water to eliminate dissolved oxygen and carbon dioxide, gases that might influence the reactions. The reactors were then loaded with a measured amount of deaerated water and sealed under a stream of helium in the glovebox. In some experiments, cyclohexene was the reactant. Because cyclohexene is very volatile, it was added to the reactor in the glovebox immediately after the reactor was loaded with deaerated water. All chemicals were obtained commercially in high purity and used as received. The amount of water added to the reactor set the volume of the vapor phase at subcritical reaction temperatures and the water density at supercritical reaction temperatures. For experiments at subcritical temperatures (250-350 °C), we chose water loadings that ensured that the reactors would be nearly filled with a single liquid phase. The required water loading for each temperature was determined by taking the density of the reactor contents to be that of pure liquid water at the same temperature.11 Because cyclohexanol and cyclohexene are both insoluble in water at room temperature, we were concerned with the possibility that two immiscible liquid phases might be present in the reactor during experiments at subcritical temperatures. Kuhlmann et al.4 ascertained experimentally the solubility of cyclohexanol (and other organic reagents) in water up to 100 °C and concluded that mixtures that are 0.3-0.5 M in organics at room temperature are homogeneous at 300 °C. We conducted our experiments at temperatures of 250-350 °C and cyclohexanol concentrations of 0.4-0.6 M at room temperature. To probe further the solubility of cyclohexanol and cyclohexene in HTW at the temperatures and concentrations used in our experiments, we performed calculations with the chemical process simulator ASPEN PLUS.12 We used the RK-ASPEN thermophysical properties model, which is suitable for a mixture of polar and nonpolar compounds at high

temperatures and pressures. Simulations of cyclohexanol in water and cyclohexene in water predicted a single liquid phase at the experimental conditions. Judging from these calculations and the experimental results of Kuhlmann et al., we believe that the reactor contents were always homogeneous under the reaction conditions used. The loaded and sealed reactors were placed in a preheated, fluidized sand bath (Techne SBL-2) set at the desired reaction temperature. The sand bath was kept isothermal to within (1 °C with a temperature controller (Techne TC-8D). The reactor heat-up time has been measured to be 2-3 min,13 which is short compared to the typical batch holding times used in this study. Upon reaching the desired holding time, the reactors were removed from the sand bath, and the reaction was quenched immediately by immersing the reactors in a cold water bath. The reactor temperature dropped to room temperature in less than 1 min. The reactors were further cooled in a freezer for up to 1 h to condense any volatile components. The reactors were opened, and the contents were recovered through the addition of acetone. It was essential to complete this last step quickly to minimize the loss of volatile compounds. We used two Hewlett-Packard model 5890 gas chromatographs equipped with either a flame ionization detector or a mass spectrometric detector to analyze the reaction products. The sample constituents were separated with a Hewlett-Packard HP-5 fused silica capillary column. Product identification was accomplished by comparing retention times with those of authentic standards and by inspecting mass spectra. The number of moles of each product present was obtained from the chromatographic analyses with experimentally determined detector response factors for each component. We used methyl cyclohexane as the standard. Product molar yields were then computed by dividing the number of moles of product formed by the number of moles of cyclohexanol initially loaded into the reactor. Experimental Results Table 1 and Figures 1-7 present the experimental product yields for cyclohexanol dehydration in pure HTW. The uncertainties reported are 95% confidence intervals determined from multiple (typically 5-10) runs under nominally identical reaction conditions. Under the conditions employed in this study, cyclohexanol readily dehydrates to form cyclohexene in HTW without any added catalyst. The minor products include 1- and 3-methyl cyclopentene, with yields as high as 22 and 6%, respectively. Our identification of 3-methyl cyclopentene remains tentative because the mass spectrum for this compound is also consistent with the fragmentation pattern for 2-methyl cyclopentene. Nevertheless, we are confident that this product is a methyl cyclopentene. Methylene cyclopentane formed in some experiments but in yields of less than 1%. The combined yields of 1- and 3-methyl cyclopentene and methylene cyclopentane appear as “methyl cyclopentenes” in the figures and in Table 1. Cyclohexanone was also present but in yields that were typically less than 1%. The highest cyclohexanone yields (2-3%) were obtained at 380 °C and water densities of 0.20 g/cm3 and lower. The carbon balance generally exceeded 90%, and the lowest value was 79%. The carbon loss showed no notable trend with changes in temperature or water density. Background experiments revealed that the failure to achieve

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Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001

Table 1. Experimental Molar Yields for Cyclohexanol Dehydration in Pure HTWa T (°C)

[H2O]0 (g/cm3)

time (min)

cyclohexanol

cyclohexene

methyl cyclopentenesb

cyclohexanoneb

250

0.81

275

0.76

300

0.73

350

0.59

380

0.08 0.14

30 60 90 120 180 15 30 45 60 90 15 30 45 60 90 15 30 45 60 90 60 30 45 60 75 90 30 45 60 75 90 30 45 60 75 90 60 15 30 45 60 15 30 45 60 90 15 30 45 60 120 180

0.86 ( 0.15 0.70 ( 0.03 0.68 ( 0.08 0.70 ( 0.07 0.63 ( 0.11 0.71 ( 0.08 0.58 ( 0.09 0.43 ( 0.06 0.43 ( 0.08 0.40 ( 0.07 0.63 ( 0.08 0.32 ( 0.07 0.34 ( 0.08 0.27 ( 0.07 0.18 ( 0.03 0.075 ( 0.015 0.071 ( 0.006 0.063 ( 0.023 0.076 ( 0.013 0.074 ( 0.019 0.86 ( 0.07 0.76 ( 0.08 0.75 ( 0.08 0.69 ( 0.09 0.68 ( 0.06 0.65 ( 0.08 0.64 ( 0.05 0.60 ( 0.10 0.57 ( 0.07 0.58 ( 0.08 0.56 ( 0.07 0.52 ( 0.06 0.49 ( 0.09 0.33 ( 0.09 0.26 ( 0.05 0.21 ( 0.05 0.041 ( 0.055 0.032 ( 0.004 0.025 ( 0.010 0.026 ( 0.004 0.032 ( 0.016 0.045 ( 0.010 0.042 ( 0.006 0.048 ( 0.016 0.046 ( 0.006 0.041 ( 0.009 0.068 ( 0.007 0.068 ( 0.013 0.064 ( 0.010 0.059 ( 0.007 0.055 ( 0.018 0.035 ( 0.013

0.077 ( 0.040 0.085 ( 0.043 0.20 ( 0.07 0.23 ( 0.07 0.24 ( 0.06 0.12 ( 0.04 0.27 ( 0.07 0.38 ( 0.06 0.41 ( 0.08 0.50 ( 0.07 0.33 ( 0.07 0.59 ( 0.07 0.64 ( 0.08 0.67 ( 0.06 0.71 ( 0.05 0.80 ( 0.06 0.80 ( 0.03 0.82 ( 0.07 0.76 ( 0.04 0.78 ( 0.05 0.070 ( 0.073 0.12 ( 0.08 0.14 ( 0.02 0.15 ( 0.04 0.17 ( 0.03 0.28 ( 0.06 0.23 ( 0.09 0.28 ( 0.08 0.34 ( 0.05 0.38 ( 0.07 0.34 ( 0.09 0.35 ( 0.06 0.46 ( 0.08 0.58 ( 0.07 0.68 ( 0.07 0.71 ( 0.07 0.91 ( 0.06 0.87 ( 0.03 0.87 ( 0.09 0.86 ( 0.04 0.87 ( 0.05 0.80 ( 0.04 0.81 ( 0.06 0.84 ( 0.03 0.82 ( 0.05 0.74 ( 0.07 0.82 ( 0.07 0.74 ( 0.09 0.70 ( 0.11 0.60 ( 0.05 0.48 ( 0.09 0.48 ( 0.09

nd nd nd nd nd nd nd nd nd nd nd 0.001 ( 0.001 0.002 ( 0.001 0.003 ( 0.001 0.005 ( 0.001 0.013 ( 0.003 0.013 ( 0.003 0.022 ( 0.012 0.033 ( 0.006 0.033 ( 0.014 nd nd nd nd nd nd nd nd 0.001 ( 0.001 nd nd nd nd nd 0.001 ( 0.002 0.001 ( 0.002 0.010 ( 0.003 0.009 ( 0.001 0.012 ( 0.003 0.013 ( 0.002 0.015 ( 0.013 0.026 ( 0.011 0.035 ( 0.046 0.052 ( 0.024 0.061 ( 0.032 0.12 ( 0.05 0.055 ( 0.014 0.078 ( 0.025 0.12 ( 0.04 0.21 ( 0.01 0.26 ( 0.08 0.28 ( 0.04

nd nd nd nd nd nd nd nd nd nd 0.001 ( 0.001 0.001 ( 0.001 0.001 ( 0.001 0.002 ( 0.002 0.002 ( 0.001 0.004 ( 0.002 0.003 ( 0.003 0.002 ( 0.003 0.004 ( 0.000 0.003 ( 0.003 0.021 ( 0.032 0.004 ( 0.006 0.011 ( 0.010 0.020 ( 0.014 0.007 ( 0.014 0.021 ( 0.009 0.007 ( 0.007 0.010 ( 0.010 0.009 ( 0.019 0.011 ( 0.006 0.012 ( 0.005 0.006 ( 0.006 0.012 ( 0.004 0.028 ( 0.021 0.009 ( 0.003 0.011 ( 0.007 0.010 ( 0.019 0.005 ( 0.006 0.004 ( 0.005 nd 0.013 ( 0.028 0.008 ( 0.010 0.004 ( 0.011 0.008 ( 0.004 0.009 ( 0.004 0.006 ( 0.003 nd nd nd 0.005 ( 0.001 0.006 ( 0.001 0.001 ( 0.002

0.17

0.20

0.25 0.34

0.51

0.68

a

[Cyclohexanol]0 ) 0.3 mol/L. b nd ) not detected.

Table 2. Molar Yields of Individual Methyl Cyclopentenes at 380 °C and 0.68 g/cm3 time (min)

1-methyl cyclopentene

3-methyl cyclopentene

methylene cyclopentane

15 30 45 60 120 180

0.038 ( 0.012 0.058 ( 0.021 0.092 ( 0.027 0.17 ( 0.01 0.21 ( 0.06 0.22 ( 0.04

0.016 ( 0.002 0.020 ( 0.005 0.027 ( 0.009 0.037 ( 0.002 0.045 ( 0.014 0.057 ( 0.008

0.001 ( 0.001 nd 0.001 ( 0.002 0.002 ( 0.001 0.004 ( 0.003 0.003 ( 0.002

100% carbon balance is most likely due to losses of volatile products (cyclohexene and methyl cyclopentenes) prior to analysis. Table 2 shows the product yields for individual methyl cyclopentenes at 380 °C and 0.68 g/cm3, the reaction conditions for which the yields of these minor products were the highest. 1-Methyl cyclopentene always had the highest yield, followed by 3-methyl cyclopentene. The

yields of methylene cyclopentane were always an order of magnitude smaller than the yields of the 1- and 3-methyl cyclopentenes. Figures 1-3 show the temporal variations of the product yields for cyclohexanol dehydration in HTW at 250, 300, and 350 °C, respectively. At the lower temperatures, the cyclohexanol yield decreases continuously with time, and this change is accompanied by a corresponding increase in the cyclohexene yield. At 250 °C, cyclohexene is the only product, whereas at 300 °C, methyl cyclopentenes appear in a low yield that increases with time. At 350 °C, the conversion of cyclohexanol is rapid and appears to reach an equilibrium value after no more than 15 min of reaction time. The cyclohexene yield, on the other hand, reaches 80% after 15 min but then gradually decreases at longer times. This decrease in the cyclohexene yield is accompanied by a corresponding increase in the methyl cyclopentenes


Figure 1. Yields of cyclohexanol and cyclohexene at T ) 250 °C and [H2O]0 ) 0.81 g/cm3.


Figure 2. Yields of cyclohexanol, cyclohexene, and methyl cyclopentenes at T ) 300 °C and [H2O]0 ) 0.73 g/cm3.




yield. Taken collectively, Figures 1-3 show that increasing temperature increases the rate of cyclohexanol disappearance and the selectivity toward methyl cyclopentenes. Figures 4-7 show the temporal variations of the product yields at 380 °C and water densities of 0.14, 0.20, 0.51, and 0.68 g/cm3, respectively. A remarkable influence of the water density on the yields is evident. At water densities of 0.14 and 0.20 g/cm3, cyclohexene is the only product observed. The cyclohexanol yield decreases continuously with time, and this decrease is accompanied by a corresponding increase in the cyclohexene yield. At water densities of 0.51 and 0.68 g/cm3, we observe a low and nearly time-invariant cyclohexanol yield, a maximum in the cyclohexene yield, and the appearance of and a steady increase in the yield of methyl cyclopentenes. These data show that higher water densities facilitate the conversion of cyclohexanol

and the formation of methyl cyclopentenes. At these more liquidlike densities (0.51 and 0.68 g/cm3), the methyl cyclopentenes yields at 380 °C are substantially higher than those observed at lower temperatures. This result is consistent with an increase in temperature shifting the selectivity toward methyl cyclopentenes. We also conducted experiments in the absence of water at 300 and 380 °C for 60 min. The cyclohexanol conversion was 8.2 ( 2.0% at 300 °C and 8.5 ( 1.3% at 380 °C. The low cyclohexanol conversion in the absence of water confirms that water plays an important role in the reaction of cyclohexanol. Cyclohexanone was the major product at both temperatures, but small quantities of cyclohexene and methyl cyclopentenes were also produced. At 300 °C, the product yields were 3.5 ( 0.6% for cyclohexanone and 0.2 ( 0.1% for cycloalkenes. At 380 °C, the product yields were 9.5 ( 1.1% for cyclohexanone and 0.5 ( 0.3% for cycloalkenes. Cyclohex-

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anone likely forms as a result of thermally activated free-radical dehydrogenation of cyclohexanol. The much lower yields of cyclohexanone from experiments in water indicate that this pathway is either suppressed in HTW or supplanted by the faster dehydration pathway. Cyclohexanol dehydration and formation of methyl cyclopentenes most likely occur by means of ionic chemistry. We conducted experiments with cyclohexene in HTW to shed light on the fate of cyclohexene after its formation via cyclohexanol dehydration. All experiments were conducted for the batch holding time of 60 min. The cyclohexene conversion was 21.6 ( 9.6% at 300 °C and 39.3 ( 5.7% at 380 °C. At 300 °C, the product yields were 14.3 ( 6.1% for cyclohexanol and 1.7 ( 1.6% for methyl cyclopentenes. At 380 °C and 0.34 g/cm3 of water, the product yields were 2.7 ( 0.5% for cyclohexanol and 15.3 ( 6.0% for methyl cyclopentenes. These results indicate that two reaction pathways are available to cyclohexene in HTW. One pathway is hydration to generate cyclohexanol, and the other pathway is rearrangement to form methyl cyclopentenes. The observed product distribution indicates that hydration is the preferred pathway at 300 °C whereas rearrangement is the preferred pathway at 380 °C and 0.34 g/cm3. The higher yield of methyl cyclopentenes at higher temperature is consistent with the results obtained with cyclohexanol as the reactant. Effect of Metal Surface. Even though the reactors were treated hydrothermally prior to use, there remains a possibility that the treated metal surfaces impose some catalytic or surface effects on the reaction. We investigated such wall effects by performing cyclohexanol dehydration experiments under nominally identical conditions but in reactors with varying surface-tovolume ratios. This variation was achieved by separately adding 7 and 22 mg of stainless steel filings to the reactors. Reactions were carried out at 380 °C, 0.34 g/cm3 of water, and 60 min of batch holding time. In the absence of stainless steel filings, the product yields were 3.2 ( 1.6% for cyclohexanol, 86.9 ( 5.0% for cyclohexene, and 1.5 ( 1.3% for methyl cyclopentenes. With 7 mg of stainless steel filings, the product yields were 3.2% for cyclohexanol, 85.9% for cyclohexene, and 3.0% for methyl cyclopentenes. With 22 mg of stainless steel filings, the product yields were 4.7% for cyclohexanol, 85.9% for cyclohexene, and 1.0% for methyl cyclopentenes. It is clear that the product yields are insensitive to the presence of the added stainless steel filings. These results are consistent with negligible catalytic effects due to the reactor walls.

Figure 8. Yields of cyclohexanol, cyclohexene, and methyl cyclopentenes at T ) 300 °C and [H2O]0 ) 0.73 g/cm3 with deaerated (solid line) and untreated (dashed line) water.

Effect of Dissolved Air. We also examined the effects of dissolved air on the reaction rates at 300 °C. Dissolved oxygen can oxidize the organic compounds, and dissolved carbon dioxide forms carbonic acid, which might catalyze dehydration. Reactors loaded with cyclohexanol and unsparged (hence not deaerated) water were placed under a stream of purified helium and sealed in the helium-filled glovebox to displace air from the reactor headspace. Figure 8 shows the product yields as functions of time for reactions at 300 °C in deaerated and unsparged water. The uncertainties from the experiments with unsparged water are larger because fewer replicates were done at each batch holding time. The cyclohexanol conversion in Figure 8 is consistently higher in the presence of dissolved air, and this rate-enhancing effect appears to be stronger at shorter reaction times. For example, the cyclohexanol conversions in the presence of dissolved air are 66% at 15 min and 85% at 90 min, whereas in deaerated water, they are 37% and 82%, respectively. The greater reaction rate is probably due to the lower pH produced by the dissolved CO2. Regardless of the cause of this effect, however, it is clear that distilled and deionized water exposed to ambient air contained sufficient quantities of dissolved gases to have measurable effects on the reaction kinetics. Therefore, for kinetics and mechanistic studies, it is important to remove dissolved air from water prior to use, even though this necessity is not universally acknowledged in the studies of acid-catalyzed reactions in HTW found in the literature. From a practical point of view, however, the presence of dissolved air might be desirable precisely because of this rate enhancement. Comparison with Previous Experiments. The experimental results in Table 1 are in apparent conflict with those of Crittendon and Parsons,3 who reported no reaction for cyclohexanol after 20 min in HTW (0.33 g/cm3) at 375 °C. We observed 97% conversion after 15 min for reaction in HTW (0.34 g/cm3) at 380 °C. This difference in the observed cyclohexanol reactivity can be easily explained, however. The reactor that Crittendon and Parsons used had a much larger thermal mass than did the reactors that we used. As a result, the time required to heat their reactor from room temperature to 375 ×bcC was much longer (>1 h). In fact, 20 min after being placed in a tube furnace at 375 ×bcC, the temperature inside their reactor was only 268 °C.14 Of course, the average temperature of the reactor contents during the experiments would be much lower than 268


°C. In contrast, the heat-up time is very short (2-3 min) during our experiments. Another important distinction is that, under the conditions employed by Crittendon and Parsons, a large fraction of the reactor volume was occupied by a vapor phase, so cyclohexanol could partition between the vapor and liquid phases. This phase behavior could influence the reactivity. In contrast, we conducted all of our experiments at subcritical temperatures in such a way that the reactor was nearly filled with a single liquid phase. We speculate that the long heat-up time and the partitioning of cyclohexanol between the vapor and liquid phases contributed to the apparent lack of reactivity of cyclohexanol in pure HTW reported by Crittendon and Parsons. The facile formation of cyclohexene that we observe in the absence of added catalysts is consistent with the results of Kuhlmann et al.,4 who reported that cyclohexanol dehydration occurred readily in HTW at 300 °C. They reported a cyclohexene yield of 33% at 60 min, which is lower, however, than the 67% yield listed in Table 1 for the same conditions. Kuhlmann et al. did not report the formation of any other products. Thus, the present results, which identify 1-methyl cyclopentene and 3-methyl cyclopentene as byproducts at 300 °C, albeit in small quantities, are also not in complete accord with the results of Kuhlmann et al. Recall from the Experimental Section that we cooled the reactors in a freezer after the experiments and completed the product recovery step quickly to minimize the loss of volatile compounds, i.e. cyclohexene and methyl cyclopentenes. Exploratory experiments conducted without cooling the reactors in the freezer and with a longer time (by ∼2 min) taken for the product recovery step resulted in much lower cyclohexene yields (30% after 45 min and 36% after 75 min) and no recovery of methyl cyclopentenes. These results are fully consistent with those reported by Kuhlmann et al. Therefore, the likely source of the difference between the present results and those of Kuhlmann et al. is the possible loss of volatile components during their experiments. The paragraphs above provide a means of reconciling the apparently contradictory results that had been published for cyclohexanol dehydration in HTW. The discrepancies can be traced to the different experimental procedures and apparatuses that were used. Reaction Mechanism Previous studies have shown that HTW has a sufficiently high concentration of H3O+ and OH- ions that some acid- and base-catalyzed organic reactions occur in the absence of any added catalysts.5,6,15-17 Figure 9 shows Kw, the ion product for water, as a function of water density at 380 °C.18 The range of Kw is from 10-24 (mol/kg)2 at 0.08 g/cm3 to 10-11 (mol/kg)2 at 0.68 g/cm3. This monotonic increase in the ion product, and hence in the H3O+ concentration, with increasing water density at 380 °C might account for the strong influence of the water density on the cyclohexanol conversion and product yields at 380 °C. Interestingly, the dehydration reaction occurs at 380 °C even at low water densities, where Kw is many orders of magnitude smaller than its value for ambient liquid water [Kw ) 10-14 (mol/kg)2]. This observation suggests that both thermal activation and acid catalysis are important for the rate enhancement of cyclohexanol dehydration in HTW. At temperatures and water densities used in the liquid-phase

Figure 9. Ion product for water at T ) 380 °C.

experiments, Kw decreases from 10-11 to 10-12 (mol/kg)2 as the temperature increases from 250 to 350 °C,18 yet the data show that the reaction rate increases with temperature. Again, the role of thermal activation is evident. Alcohol dehydration is a classic organic reaction that has been studied extensively. The established mechanisms for alcohol dehydration are E1cB, E1, and E2. These notations stand for elimination (“E”) with a ratelimiting step that is either unimolecular (“1”) or bimolecular (“2”). A given alcohol can undergo dehydration by any one of these mechanisms, depending on the reaction conditions. The dominant mechanism is determined by various factors, including the acid/base properties of the catalyst, the interactions of the reactant and intermediate species with the reaction medium, the reaction temperature, and the structural features of the alcohol.19-22 The balance of the acidity and basicity of the catalyst often determines the dominant mechanism. In the presence of a strong base and a weak acid, alcohol dehydration occurs primarily via a carbanion intermediate, which is formed when the base abstracts a proton from the β carbon (E1cB mechanism). In the presence of a strong acid and a weak base, alcohol dehydration occurs primarily via a carbocation intermediate, which is formed by the loss of the protonated hydroxy group (-OH2+) as water (E1 mechanism). In the E1cB and E1 mechanisms, the (unimolecular) formation of the ionic intermediate is the rate-limiting step, and the subsequent formation of the alkene is facile. In the E2 mechanism, alcohol dehydration occurs by β elimination, in which the proton on the β carbon and the hydroxy group are eliminated in a concerted fashion. The reaction is bimolecular because a nucleophile is required to initiate elimination. Although the E2 mechanism is typically observed in the presence of an acid and a base of balanced strength, the reaction is acid-catalyzed, since the protonated hydroxy group is an excellent leaving group. In pure water, no base is present that is strong enough to abstract a proton from the carbon backbone. Therefore, the E1cB mechanism is highly unlikely. The remaining candidates are acid-catalyzed E1 and E2 mechanisms, with H3O+ from water being the catalyst. Figure 10 shows the reaction network for acid-catalyzed cyclohexanol dehydration. Formation of the oxonium ion is the necessary first step for both the E1 and E2 mechanisms. What distinguishes the two mechanisms is that the carbocation is the key intermediate in the E1 mechanism. There is only one previously reported attempt to

1828


Figure 10. Reaction network for cyclohexanol dehydration in HTW.

elucidate the reaction mechanism of cyclohexanol dehydration. Whittaker and co-workers performed deuterium-labeling studies for the gas-phase dehydration of cyclohexanol over zirconium phosphate.23,24 Dehydration experiments at 350 °C using 2,2′,6,6′-[2H4]cyclohexanol as the starting material generated cyclohexene with significant deuterium scrambling. The authors attributed this outcome to the reaction intermediate being a carbocation with highly mobile positive charge, in which case the dehydration mechanism would be E1. There is some question, however, as to whether the reaction mechanism observed for the gas-phase experiments would also operate in HTW. Solvation can play an important role in reactions in water, modifying the reactivity of the species involved in ionic chemistry.19,25 There are numerous examples of direct observation of carbocations by spectroscopic methods in (anhydrous) superacids,26 but not in weaker acids such as sulfuric acid solutions, because of the interactions of cations with solvating molecules.25 This literature suggests, then, that carbocations (R+) in water are solvated by water molecules and exist largely as oxonium ions (ROH2+),25 in which case the dehydration mechanism should be predominantly E2. This hypothesis is supported by a recent ab initio study that showed that, in the presence of room-temperature water, the oxonium ion is the stable intermediate, even for secondary (i-propyl) and tertiary (tert-butyl) alcohols.19 A recent 13C NMR study also showed that, even in a moderately concentrated solution of sulfuric acid, dehydration of tert-butyl alcohol occurs via the oxonium ion intermediate and not the carbocation intermediate.27 The spectroscopic evidences noted above were obtained at room temperature, and there is always a possibility that a different mechanism operates at higher temperatures. Dabbagh and Mohammad Salehi28 observed a dramatic shift in the product distribution with changes in temperature for the dehydration of 1,2diphenyl-2-propanol over alumina. At 200-260 °C, the Hofmann (least branched) alkene is the major product, whereas at 300 °C, the Saytzeff (most branched) alkene is the major product. The authors attributed this result to a shift in the dehydration mechanism from E2 to E1 as the reaction temperature increased. Carrizosa and Munuera29 observed a similar shift in the product distribution with temperature (at about 100 °C) for t-pentanol dehydration over TiO2. Thomke30,31 found that the mechanism of 2-butanol dehydration over ThO2 shifts from E1cB below 400 °C to E2 and even E1 above 400 °C. Although none of these studies was conducted in aqueous environments, they suggested that the formation of carbocations in water might also be favored at higher temperatures. To summarize, this inspection of the relevant literature suggests that an oxonium ion intermediate (E2 mechanism) would be anticipated for dehydration in HTW, but that the elevated temperature might admit

Figure 11. Proposed reaction mechanism for cyclohexanol dehydration in HTW.

the formation of carbocations. The experimental data in Table 1 support this hypothesis. Cyclohexene is the major product under all reaction conditions examined in this study. At 250 and 275 °C, cyclohexene is the only product, but at higher temperatures, methyl cyclopentenes are also produced, in a yield that increases with increasing temperature. At 380 °C, the methyl cyclopentene yield also increases with increasing water density. If the secondary cyclohexyl carbocation is generated during cyclohexanol dehydration, as required in the E1 mechanism, it should quickly rearrange to the tertiary methyl cyclopentyl carbocation, which is much more stable.26 Thus, the absence of methyl cyclopentenes at 250 and 275 °C suggests that the E2 mechanism operates at these temperatures. The formation of methyl cyclopentenes at higher temperatures suggests that the formation of carbocations becomes increasingly more favorable as the reaction temperature increases. Because cyclohexene is consistently the major product, however, the E2 mechanism probably remains the dominant dehydration mechanism, even at temperatures of 300 °C and above. On the basis of the experimental data reported herein and supporting evidence from the literature as discussed in the preceding paragraphs, we propose the following scenario. Cyclohexanol dehydration to cyclohexene in pure HTW occurs through the E2 mechanism. Some of the cyclohexene molecules are then protonated to form the secondary cyclohexyl carbocation. This carbocation very quickly rearranges to form the tertiary methyl cyclopentyl carbocation, which then loses a proton to form 1-methyl cyclopentene. Charge migration within the methyl cyclopentyl carbocation can yield structures that deprotonate to form 3-methyl cyclopentene and methylene cyclopentane, both of which were observed experimentally. Both the increase in temperature and the isothermal increase in water density favor the carbocation formation. Figure 11 shows the detailed reaction mechanism that we propose to account for cyclohexanol dehydration in HTW. Species appearing above the arrow in a reaction step are co-reactants, and those appearing below the arrow are coproducts. The mechanism described in Figure 11 is similar to the E2/ AdE3;E1/AdE3/Uni mechanism proposed by Antal et al. to account for the reactions of 1- and 2-propanols in HTW.7 This mechanism includes the conversion of 1-propanol to propylene via the E2 mechanism, followed by the conversion of propylene to a secondary carbocation, from which 2-propanol is formed. The consideration of mechanistic issues related to


cyclohexanol dehydration in HTW reveals that the role of water in this reaction is threefold. First, water participates in the elementary reaction steps as a reactant and as a product. Second, water is the source of H3O+, which catalyzes the formation of both cyclohexene and methyl cyclopentenes. Third, water drives the reaction mechanism toward E2, rather than E1, by favoring, through solvation, the oxonium ion (“OXO6” in Figure 11) rather than the carbocation (“CAT6” in Figure 11) as the reaction intermediate. We expect water to make similar contributions to the dehydration of other alcohols in HTW, although the extent to which solvation influences the reaction mechanism should depend on both the nature of the intermediate species and the reaction conditions. Mechanism-Based Model Having proposed a mechanism for cyclohexanol dehydration in HTW, we next sought to demonstrate that the mechanism was consistent with the experimental results. Therefore, we constructed a detailed chemical kinetics model based on the reaction mechanism shown in Figure 11. The model equations, which apply to reactions in a constant-volume batch reactor, are

d[NOL6] ) -k01[H3O+][NOL6] + k10[H2O][OXO6] dt (1)

Table 3. Parameter Estimates for the Model in Equations 7-9 Optimized for Experimental Data at 380 °C and 0.68 g/cm3 parameter

value

units

K1 K2 K3

2.05 × 106 4.39 × 103 4.51 × 101

L mol-1 s-1 L2 mol-2 s-1 L mol-1 s-1

This result suggests that the formation of the cyclohexyl cation from cyclohexene is essentially irreversible. One could rationalize this outcome by recognizing that the formation of a more stable tertiary cation from a secondary cation is highly favorable. It is likely that the cyclohexyl cation rapidly rearranges to the tertiary methyl cyclopentyl cation as soon as it is formed via cyclohexene protonation. Taking one of the steps to be irreversible greatly simplifies the kinetics model. If one assumes that the formation of the cyclohexyl cation is in fact irreversible and, furthermore, employs the quasi-stationary state approximation for the charged, reactive intermediates (OXO6, CAT6, CAT5), one obtains the following system of equations from the reaction mechanism:

(

)

k01k12 d[NOL6] )[NOL6][H2O] + dt k10 + k12 k10k21 [ENE6][H2O][H3O+] (7) k10 + k12

(

)

) -K1[NOL6][H2O] +

d[OXO6] ) k01[H3O+][NOL6] dt

K2[ENE6][H2O][H3O+]

(k10 + k12)[H2O][OXO6] + k21[H2O][H3O+][ENE6] (2) d[ENE6] ) k12[H2O][OXO6] dt

d[ENE5] ) k23[ENE6][H3O+] ) K3[ENE6][H3O+] dt (8) [ENE6] ) [NOL6]0 - [NOL6] - [ENE5]

(k21[H2O] + k23)[H3O+][ENE6] + k32[H2O][CAT6] (3) d[CAT6] ) k23[H3O+][ENE6] dt (k32[H2O] + k34)[CAT6] + k43[CAT5] (4) d[CAT5] ) k34[CAT6] - (k43 + k45[H2O])[CAT5] + dt k54[H3O+][ENE5] (5) d[ENE5] ) k45[H2O][CAT5] - k54[H3O+][ENE5] dt (6) We used a commercial modeling package, Scientist,32 to solve simultaneously the ordinary differential equations above (eqs 1-6) and perform parameter estimations to obtain the rate constants k01, k10, k12, k21, k23, k32, k34, k43, k45, and k54. The minimization algorithm used by Scientist for parameter estimation is a Powell variant of the Levenberg-Marquardt approach.33 We used the experimental concentration profiles acquired at 380 °C and water densities between 0.08 and 0.68 g/cm3 for parameter estimation. We fit the experimental data at each density independently and found that k32 was consistently near zero (∼10-15-10-20 L mol-1sec-1).

(9)

where K1, K2, and K3 are lumped parameters. As one can see, the number of parameters in the model is reduced from 10 to 3, which simplifies the parameter estimation and reduces the likelihood that the data can be fit simply because there are many parameters rather than because the model is correct. We used Scientist to solve simultaneously eqs 7-9 and perform parameter estimation to obtain values for the parameters K1, K2, and K3. We fit the model to a single concentration profile, which was acquired at 380 °C and 0.68 g/cm3. Table 3 lists the optimized values for K1, K2, and K3 under these conditions. If the proposed mechanism is correct, then the model in eqs 7-9 with the parameters in Table 3 should provide reliable predictions of the product yields at 380 °C and all other water densities. To test this expectation, we used the parameter estimates to predict the product yields at other water densities at 380 °C. Figure 12 compares the experimental data and the model predictions for reactions at 380 °C and 60 min as a function of water density. The mechanism-based model is clearly capable of predicting the strong water density effects observed experimentally, even though the rate constants were optimized at only one density (0.68 g/cm3). Note that the model employs no empiricism to account for the water density dependence. The reaction kinetics depend on the water density in two ways. Water is both a reactant and a product in different steps of the mechanism, so the reaction rate is a direct function of

1830


Acknowledgment We thank Michelle Osinski for performing preliminary experiments and Jianli Yu for assistance in the laboratory. Financial support from the National Science Foundation (CTS-9985456), the donors of the ACS Petroleum Research Fund (34644-AC9), and the U.S. Environmental Protection Agency (STAR Fellowship for N.A.) is gratefully acknowledged. Literature Cited

Figure 12. Comparison of experimental data and model prediction for yields at T ) 380 °C and t ) 60 min.

the water concentration. Water is also the source of the acid catalyst, H3O+, the concentration of which is a function of both temperature and water density.18 It is this latter effect that is the more important one in this model. This kinetics modeling exercise clearly demonstrates that the proposed mechanism is consistent with the experimental data. Conclusions We studied cyclohexanol dehydration in HTW at temperatures of 250-380 °C, reaction times of 15-180 min, and water densities of 0.08-0.68 g/cm3. Under these experimental conditions, cyclohexanol dehydrates readily in the absence of added catalysts to form cyclohexene as the major product. The most abundant minor products are 1-methyl cyclopentene and 3-methyl cyclopentene. Increasing temperature and water density increase the rate of cyclohexanol disappearance and the selectivity toward methyl cyclopentenes over cyclohexene. These results provide further evidence that acidcatalyzed reactions can be accomplished readily in HTW in the absence of added acid. The implications for environmentally benign industrial chemistry are clear. The reaction mechanism for cyclohexanol dehydration in HTW comprises the following steps. Cyclohexanol dehydrates reversibly to form cyclohexene by the E2 mechanism. Cyclohexene is then protonated irreversibly to form the cyclohexyl cation. This cation rapidly rearranges to form methyl cyclopentyl cations, which then lose a proton to form methyl cyclopentenes. A detailed chemical kinetics model based on this mechanism captured the trends in the experimental data at 380 °C. The model was able to predict the striking effect of the water density on the product yields, thereby supporting the proposed reaction mechanism. We elucidated three roles for water in cyclohexanol dehydration in HTW. First, water participates in the elementary reaction steps as a reactant and as a product. Second, water is the source of H3O+, which catalyzes the formation of both cyclohexene and methyl cyclopentenes. Third, water drives the reaction mechanism toward E2 by favoring, through solvation, the oxonium ion rather than the carbocation as the reaction intermediate. We expect water to make similar contributions in the dehydration of other alcohols in HTW, although the extent to which solvation influences the reaction mechanism should depend on both the nature of the intermediate species and the reaction conditions.

(1) Savage, P. E. Organic Chemical Reactions in Supercritical Water. Chem. Rev. 1999, 99, 603-621. (2) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical ConditionssApplications and Fundamentals. AIChE J. 1995, 41, 1723-1778. (3) Crittendon, R. C.; Parsons, E. J. Transformations of Cyclohexane Derivatives in Supercritical Water. Organometallics 1994, 13, 2587-2591. (4) Kuhlmann, B.; Arnett, E. M.; Siskin, M. Classical Organic Reactions in Pure Superheated Water. J. Org. Chem. 1994, 59, 3098-3101. (5) Xu, X.; Antal, M. J., Jr.; Anderson, D. G. M. Mechanism and Temperature-Dependent Kinetics of the Dehydration of tertButyl Alcohol in Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1997, 36, 23-41. (6) Xu, X.; Antal, M. J., Jr. Kinetics and Mechanism of Isobutene Formation from t-Butanol in Hot Liquid Water. AIChE J. 1994, 40, 1524-1534. (7) Antal, M. J., Jr.; Carlsson, M.; Xu, X.; Anderson, D. G. M. Mechanism and Kinetics of the Acid-Catalyzed Dehydration of 1and 2-Propanol in Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1998, 37, 3820-3829. (8) Xu, X. D.; DeAlmeida, C. P.; Antal, M. J. Mechanism and Kinetics of the Acid-Catalyzed Formation of Ethene and Diethyl Ether from Ethanol in Supercritical Water. Ind. Eng. Chem. Res. 1991, 30, 1478-1485. (9) Gu, J. Thermodynamics and Conceptual Design of a Supercritical Ethylene from Ethanol Process. M. S. E. Thesis, University of Nebraska, Lincoln, NE, 1991. (10) Halvorsen, J. Process Evaluation for the Production of Ethylene from Ethanol. M. S. E. Thesis, University of Nebraska, Lincoln, NE, 1990. (11) Lemmon, E. W.; McLinden, M. O.; Friend, D. G. Thermophysical Properties of Fluid Systems. In NIST Chemistry Webbook; Mallard, W. G., Linstrom, P. G., Eds.; NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersberg, MD, February 2000 (http://webbook.nist.gov/chemistry/fluid/). (12) ASPEN PLUS, release 9.3-1; Aspen Technology: Cambridge, MA, September 1996. (13) Lawson, J. R.; Klein, M. T. Influence of Water on Guaiacol Pyrolysis. Ind. Eng. Chem. Fundam. 1985, 24, 203-208. (14) Holliday, R. L.; Jong, B. Y. M.; Kolis, J. W. Organic Synthesis in Subcritical Water: Oxidation of Alkyl Aromatics. J. Supercrit. Fluids 1998, 12, 255-260. (15) Lesutis, H. P.; Glaser, R.; Liotta, C. L.; Eckert, C. A. Acid/ Base-Catalyzed Ester Hydrolysis in Near-Critical Water. Chem. Commun. 1999, 20, 2063-2064. (16) Chandler, K.; Deng, F. H.; Dillow, A. K.; Liotta, C. L.; Eckert, C. A. Alkylation Reactions in Near-Critical Water in the Absence of Acid Catalysts. Ind. Eng. Chem. Res. 1997, 36, 51755179. (17) Ikushima, Y.; Hatakeda, K.; Sato, O.; Yokoyama, T.; Arai, M. Acceleration of Synthetic Organic Reactions Using Supercritical Water: Noncatalytic Beckmann and Pinacol Rearrangements. J. Am. Chem. Soc. 2000, 122, 1908-1918. (18) Marshall, W. L.; Franck, E. U. Ion Product of Water Substance, 0-1000 °C, 1-1000 bar: New International Formulation and Its Background. J. Phys. Chem. Ref. Data 1981, 10, 295304. (19) Uggerud, E.; Bache-Andreassen, L. Theoretical Models and Experimental Data for Reactions Between Water and Protonated Alcohols: Substitution and Elimination Mechanisms. Chem. Eur. J. 1999, 5, 1917-1930. (20) Luy, J. C.; Parera, J. M. Acidity Control in Alcohol Dehydration. Appl. Catal. 1986, 26, 295-304.

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1831 (21) Hala´sz, I.; Vinek, H.; Thomke, K.; Noller, H. Rate Determining Step in Alcohol Dehydration on La2O3, ThO2 and MoO3, and Relations to Double Bond Shift in Olefins. Z. Phys. Chem. Neue Folge 1985, 144, 157-163. (22) Winterbottom, J. M. Hydration and Dehydration by Heterogeneous Catalysts. Catalysis 1981, 4, 141-174. (23) Cruz Costa, M. C.; Hodson, L. F.; Johnstone, R. A. W.; Liu, J.-Y.; Whittaker, D. The Mechanism of Gas-Phase Dehydration of Cyclohexanol and the Methylcyclohexanols Catalysed by Zirconium Phosphate and Zirconium Phosphite. J. Mol. Catal. A 1999, 142, 349-360. (24) Johnstone, R. A. W.; Liu, J.-Y.; Whittaker, D. Mechanism of Cyclohexanol Dehydration Catalysed by Zirconium Phosphate. J. Chem. Soc., Perkin Trans. 2 1998, 6, 1287-1288. (25) Kazansky, V. B. Adsorbed Carbocations as Transition States in Heterogeneous Acid-Catalyzed Transformations of Hydrocarbons. Catal. Today 1999, 51, 419-434. (26) Olah, G. A. Carbocations and Electrophilic Reactions. Angew. Chem. 1973, 12, 173-212. (27) Kazansky, V. B.; Figueras, F.; de Me´norval, L. C. In Situ 13C NMR Study of tert-Butanol Interaction with Moderately Concentrated Sulfuric Acid. Catal. Lett. 1994, 29, 311-323. (28) Dabbagh, H. A.; Mohammad Salehi, J. New TransitionState Models and Kinetics of Elimination Reactions of Tertiary

Alcohols over Aluminum Oxide. J. Org. Chem. 1998, 63, 76197627. (29) Carrizosa, I.; Munuera, G. Study of the Interaction of Aliphatic Alcohols with TiO2. II. On the Mechanism of Alcohol Dehydration on Anatase. J. Catal. 1977, 49, 189-200. (30) Thomke, K. Hydrogen-Exchange in Dehydration of Deuterated 2-Butanols on BPO4 (Mechanism E1), Ba3(PO4)2 (Mechanism E2) and ThO2 (Mechanism E1cB). Z. Phys. Chem. Neue Folge 1977, 105, 75-86. (31) Thomke, K. Study on Mechanisms in Dehydration of Deuterated 2-Butanols on ThO2 (with Different Surfaces) and on La2O3. Z. Phys. Chem. Neue Folge 1977, 105, 87-100. (32) Scientist, version 2.01; MicroMath, Inc.: Salt Lake City, UT, 1995. (33) Powell, M. J. D. A FORTRAN Subroutine for Solving System of Nonlinear Algebraic Equations. In Numerical Methods for Nonlinear Algebraic Equations; Robinowitz, P., Ed.; Gordon and Breach Science Publishers: New York, 1970.

Received for review November 13, 2000 Revised manuscript received February 15, 2001 Accepted February 20, 2001 IE000964Z

Kinetics and Mechanism of Cyclohexanol ... - ACS Publications

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