228 The Journal of Supercritical Fluids, 1990,3, 228-232

Mechanism and Kinetics of the Acid-Catalyzed Dehydration of Ethanol in Supercritical Water?

Xiaodong Xu, Carlos De Almeida, and Michael J. Antal, Jr.*

Department of Mechanical Engineering and the Hawaii Natural Energy Institute, University of Hawaii at Manoa, 2.540 Dole Street, Holn,: TS 305, Honolulu, HI 96822

Received April 4, 1990; accepted in revised form October 4, 1990

In the presence of a low concentration (<O.Ol mol dm-3) of sulfuric acid, ethanol undergoes rapid and selective dehydration to ethene in supercritical water. The kinetics of this reaction are consistent with an acid-catalyzed E2 mechanism.

Keywords: supercritical water, ethanol, acid-catalysis, dehydration, mechanism, kinetics, parameter estimation, E2 mechanism

INTRODUCTION Everyone is familiar with the aqueous phase proper-

ties of liquid water and the gas-like properties of steam. For example, salts like NaCl dissolve in water, but not in dry steam. From a more fundamental point of view, aqueous (liquid phase) water stabilizes the formation of ions; whereas steam does not. Consequently, heterolytic chemistry (involving the formation of charged transition states and intermediates) is important in aqueous phase water; whereas homolytic chemistry (involving the forma- tion of free radical intermediates) is important in steam. The ability of water to stabilize the formation of charged species can be judged by the value of its ion constant, K,. When K, 2 10-14, aqueous phase chemistry can be ob- served in water; whereas when K, << lo-l4 free radical chemistry prevails. Typically, K, exceeds lo-l4 when the density of water p is > 0.3 g cmd3.1 Thus, if aqueous phase chemistry is desired, the pressure P and temperature T of the reaction environment should be selected to pro- vide p > 0.3 g cm-3. This conclusion is important be- cause acid catalyzed, aqueous phase (heterolytic) chemistry is often highly specific; whereas free radical chemistry is noteworthy for its non-specificity.

Fermentation products are usually born in water and rarely achieve concentrations exceeding 2 mol dm-3. Separation of the desired fermentation products from water by distillation or semi-permeable membranes is costly. In addition, fermentation products do not command a high

?Paper presented at the Symposium on Supercritical Fluids, 1990 AIChE Spring National Meeting, March 18-22, 1990, Orlando, FL, USA.

value as a fuel supplement. Both problems would be solved if an aqueous phase chemical reaction could be used to transform the fermentation product into a water insoluble, higher value chemical. In the case of ethanol, this higher value product is ethene and the dehydration re- action proceeds to a significant extent when T 2 T, (374 “C) with P > P, (22 MPa). These conditions ensure K, > lo-14. To properly design a supercritical flow reactor intended to achieve an optimal conversion of ethanol to ethene, a detailed knowledge of the mechanism and related kinetics of the dehydration reaction is required. The goal of this paper is to detail the mechanism and kinetics of the acid-catalyzed ethanol dehydration reaction in supercrit- ical water at 385 “C and 34.5 MPa.

In spite of the fact that a well known undergraduate text casually asserts the governing role of an El mecha- nism in thk acid-catalyzed dehydration of ethanol,2 authori- tative reviews of the literature3,4 conclude that a concerted E2 type mechanism more probably governs the dehydra- tion of primary alcohols. Nevertheless, no studies of the mechanism of ethanol dehydration in. supercritical water have been reported in the literature. Considerable work on the acid-catalyzed hydration of alkenes at lower tempera- tures (typically less than 200 “C) has been reported.5,6 In a key study Baliga and Whalley found the acid-catalyzed hydration of ethene to be first order in ethene and H+ con- centrations, with an activation volume of -15.5f1.5 cm3 mol-’ at 180 “C between 10 and 300 MPa. This value led them to conclude that the activated complex for the hydration of ethene includes at least one firmly bound molecule of water and can be represented as (ethene*H+*H20)‘.

0896-8446/90/0304-0228$4.00/O 0 PRA Press

The Journal of Supercritical Fluids, Vol. 3, No. 4, 1990 Dehydration of Ethanol 229

----L 2 (4 H,o+ + H,c-CH,OH

7 H,O I- H,C--CH,O;, 7 H,O+ + H,O + H2C=CH2

-2

1 2 @I H,O+ + H,C-CH,OH -

T H,O + H,C-CH,0+H2 -7 H,O + H,O + H,C-CH;

-2 -3 3

Ti H,O + H,O+ + H,C=CH,

Figure 1. Acid-catalyzed (a) E2 and (b) El mechanism of ethanol dehydration.

Early work in this laboratory7s9 established the role of a heterolytic reaction mechanism in the acid catalyzed de- hydration of ethanol in supercritical water with K, > lo-14. High yields of ethene were obtained in the presence of 0.001 to 0.03 mol dm-3 H2S04 after about 70 s at 38.5 “C and 34.5 MPa. Trace (2 to 5%) amounts of diethyl ether were also detected.8 Closely related work9 concerning the acid-catalyzed dehydration of 1 -propanol in water at 375 “C and 34.5 MPa established kinetic agreement between the experimental data and an acid catalyzed E2 mechanism. The related El mechanism, and a simpler steady state E2 mechanism, did not fit the experimental data. A study of the effect of pressure on the acid-catalyzed rate of dehydration of 1-propanol in water near its critical point also supported the role of a concerted E2 dehydration mechanism.‘O In this paper, we employ kinetics to examine the mechanism of ethanol dehydration in supercritical water at 385 “C and 34.5 MPa. To avoid complications, this study is restricted to low ethanol concentrations (0.5 mol dm-3 or less). In this concentration regime diethyl ether is not detected as a reaction product. A subsequent paper will deal with the role of diethyl ether in the reaction mechanism at higher ethanol concentrations.

APPARATUS AND EXPERIMENTAL PRO- CEDURES

The two flow reactors used in this work have been described in detail in earlier publications.7-10 *Although these reactors operate in a laminar flow regime, consider- able research11*12 has shown that no significant error is in- troduced into the kinetic evaluation when the plug flow idealization is used to calculate the residence time of the reactants within the reactor.

All reactant solutions were prepared using degassed, distilled water. Punctilious ethanol (U.S. Industrial Chemicals Co., 190 proof: the balance being water) was used as the reactant. No impurities were detected in this reagent by HPLC or GC analyses. The sulfuric acid used was a Fisher certified grade 10 N solution.

At each operating condition, triplicate samples of the reactor effluent were collected for analysis. Quantification of liquid products was accomplished by triplicate analysis of each of these samples using a Waters High Perfor-

mance Liquid Chromatograph (Model 6000A solvent de- livery system, Perkin Elmer LC 600 autosampler and a differential refractometer) and a Hewlett-Packard Model 3388A integrator. An Alltech C 18 column was employed with degassed, distilled water as the solvent at a flow rate of 2 cm3/min. Gaseous products were analyzed using a Hewlett-Packard Model 5840 Gas Chromatograph equipped with a flame ionization detector. A Poropak Q column operating at 200 “C with a 8.5% hydrogen in he- lium as the carrier gas was used to separate the gaseous products. Ethanol, ethene, (99.5% pure Matheson C.P. grade), and air standards were used for calibration.

KINETIC MODEL AND PARAMETER ESTI- MATION

Figures la and lb display the acid-catalyzed E2 and El mechanisms for the dehydration of ethanol. Note that the El mechanism involves two more rate constants (kinetic parameters) than the related E2 mechanism. We also examine the ability of the steady state idealization of the E2 mechanism (E2SS) with four rate constants, and a simple equilibrium model (EQ) with two rate constants to simulate our data. Mass action kinetics define sets of three and four coupled, non-linear, ordinary differential equations (ODE’s) associated with the E2 and El mechanism (respectively). The E2SS model and the EQ model involve only two ODE’s. The solutions to these ODE’s govern the time-dependent behavior of the concentrations of the reactant, the reaction intermediate(s), and the products(s) exiting the reactor. This behavior depends upon the values of the rate constants, as well as the initial reactant and acid concentrations. If ci represents the concentration of ethanol exiting the reactor in the ith experiment, and G represents the calculated value obtained by numerical integration of the governing ODE’s with the appropriate initial conditions and residence time, then the weighted residual ei = (ci - c<)/Oi is a measure of the disparity between the model and the experiment. We choose 0, to be the sample standard deviation of the experiment, which has been determined by studies of experimental reproducibjlity to be given by Oi = 0.08 ci. With this definition of ii,’ the familiar chi-squared statistic is given by

230 Xu et al. The Journal of Supercritical Fluids, Vol. 3, No. 4, 1990

TABLE I Experimental and Calculated Fk dctional Yields of Ethanol at 385 “C, 34.5 MPa,

Using El, E2, EQ, and E2SS Models

Reactant Sulfuric Residence Cont. Acid Cont. Time

(mol/dm3) (mol/dm3) 6) Exp.

Ethanol Yield

Calc. Calc. El E2

Calc. EQ

Calc. E2SS

0.2 0.03 75 0.43izo.035 0.434 0.440 0.479 0.423 0.2 0.03 39 0.45+0.037 0.470 0.473 0.499 0.470 0.2 0.03 23 0.52i~O.042 0.532 0.535 0.556 0.554 0.5 0.02 78 0.54f0.044 0.521 0.504 0.481 0.470 0.1 0.03 18 0.56f0.046 0.522 0.541 0.596 0.576 0.2 0.03 17 0.58f0.047 0.577 0.583 0.605 0.611 0.2 0.03 15 0.59f0.048 0.597 0.604 0.628 0.637 0.5 0.01 72 0.62kO.05 1 0.626 0.606 0.546 0.597 0.5 0.01 74 0.64kO.052 0.622 0.602 0.543 0.593 0.5 0.01 37 0.70f0.057 0.718 0.703 0.664 0.720 0.5 0.01 24 0.73f0.059 0.773 0.763 0.745 0.791 0.5 0.005 78 0.75f0.061 0.713 0.698 0.654 0.711 0.5 0.015 14 0.76f0.062 0.786 0.779 0.768 0.809 0.5 0.01 14 0.80f0.065 0.834 0.829 0.831 0.862 0.5 0.01 18 0.81+0.066 0.807 0.800 0.794 0.832 0.5 0.005 14 0.85kO.069 0.896 0.896 0.908 0.924

where m is the number of experimental measurements and v is the number of degrees of freedom.

The inverse chemical kinetic problem is to minimize xv2 by an appropriate choice of rate constants. If the val- ues of the lei I are less than the confidence interval associ- ated with the experimental measurement, or if the value of x?’ is small (typically less than one), then the mechanisti- cally based kinetic model is said to be consistent with the experimental data. If these conditions are not satisfied, then the mechanism is not consistent with the experimen- tal data. We employ the IMSL13 subroutine BCLSF to search for optimal values of the rate constants which min- imize x,2. Values of the ei required by BCLSF are calcu- lated by integrating the coupled set of non-linear, first order ODE’s associated with each reaction mechanism us- ing the IMSL subroutine IVPAG. Further details con- cerning the ordinary differential equations and the numeri- cal methods employed in this work are given by Narayan and Antal

RESULTS AND DISCUSSION Table I lists 16 experimental measurements of the

fractional yield (yield = final ethanol concentration/initial ethanol concentration) of ethanol in supercritical water at 34.5 MPa and 385 “C for a variety of residence times, ini- tial acid and reactant ethanol concentrations. Error bars associated with these measurements represent the 95 % confidence interval based on variations in the experimen- tally determined ethanol yield during multiple experimen-

1.0

0.9 I 0.91 “A 3 I ‘7”/ -I s 0.e 2 k

0.5

0.4

0.3 LIIf!cJ 0.3 0.5 0.7 0.9

0.3 0.3 0.4 0.5 0.6 0.7 0.6 0.9 1.0

Experimental fractional yield

Figure 2. Calculated vs. experimental yields of ethanol us- ing the E2 model (central line), the El model (left inset) and the E2SS model (right inset).

tal runs using Students t distribution. The calculated, best-fit ethanol yields for the E2, El, E2SS, and EQ models are also displayed in Table I.

Figure 2 displays calculated vs. experimental yields using the E2 model. In Figure 2, agreement of the calcu- lated value with the experimental measurement is indi- cated by an intersection of the error bar displayed in the figure with the diagonal line. The E2 model agrees with

The Journal of Supercritical Fluids, Vol. 3, No. 4, 1990 Dehydration of Ethanol 231

0.8

0.6

0.4

0.2

0.0 0.0 1 0.02 0.03

initial acid concentration at NTP (Ml

Figure 3. Comparison of El and E2 models using the best-fit kinetic parameters (0.5 M ethanol reactant, residence time = 14 s at 385 “C 34.5 MPa).

all the experimental data. The insets in the upper and lower corners of Figure 2 display the agreements of the El model and the E2SS model with the experimental data. Plainly, the El model is also consistent with the experimental data, but the E2SS model is not consistent with the data. Values of x$ for the E2, El, E2SS, and EQ models are as follows: 0.52, 0.40, 1.04, and 1.79 (respectively). These values also indicate that only the E2 and El models enjoy a good agreement with the experimental data.

Figure 3 displays the calculated fractional yields of ethanol together with the protonated ethanol (El and E2 models) and carbocation (El model only) intermediates as a function of initial NTP acid concentration at 14 s resi- dence time, 385 “C and 34.5 MPa using the optimal ki- netic parameters for each model. The curves in Figure 3 reveal that the El model achieves a good fit to the exper- imental data by a choice of rate constants that predict car- bocation concentrations which greatly exceed the proto- nat&d alc-oX61 concentrations. Thus the El model is al- tempting to assert that the bare carbocation is more stable than the protonated alcohol. This result seems highly improbable. Although many attempts were made to find rate constants which would enable the El model to fit the experimental data arid maintain a low carbocation concentration, none succeeded. Effectively the El model succeeds in fitting the experimental data by mimicking the EL? model, with the carbocation assuming the role of the protonated ethanol intermediate. These results cause us to conclude that the El mechanism is not kinetically consistent with the experimental data.

TABLE II Elementary Rate Constants for the

El and E2 Models

El model E2 model

k, = 7.26 dm3/mol-s k, = 3.81 dm3/mol-s k-, = 1.16 dm3/mol-s k_, = 0.006 dm3/mol-s k, = 57.83 l/S k:! = 7.48 dm3/mol-s k-2 = 0.012 dm3/mol-s k-, = 134.6 dm6/mo12-s k, = 0.48 dm’/mol-s k-, = 428.9 dm3/mol-s

Table II displays values of the elementary rate con- stants which were found to optimize the model and mini- mize the value of 2,‘. We examined the role of rapid equilibria in the model by testing the sensitivity of x,2 to variations in the rate constants ki which maintained the re- lationship ki/k_i = constant. Because the value of x$ ex- hibited some sensitivity to individual values of the ki, we have listed the best-fit values in Table II. More work is required to determine the confidence interval associated with each value of the k,.

CONCLUSIONS The results of this study lead us to the following

conclusions:

1) The acid-cialyzed E2 mechanism for ethanol dehydration in supercritical water is

232 Xu et al.

consistent with all of our experimental data at 385 “C and 34.5 MPa.

2) The El mechanism achieves a good fit to the data by mathematically mimicking the E2 mechanism in a manner which violates conven- tional chemical precepts.

3) The steady state idealization of the E2 mechanism and a simple equilibrium model are not consistent with the experimental data.

4) The experimental data apparently contains sufficient information to enable a calculation of the individual rate constants associated with each elementary step of the E2 reaction mecha- nism.

ACKNOWLEDGMENTS This work was supported by the National Science

Foundation under grant CBT85-14867 and CBT88-12954. The authors thank Professor Donald G.M. Anderson (Harvard University) of his suggestion concerning the format of Figure 2, and Professor Maitland Jones, Jr. (Princeton University) for a critique of our analysis of the reaction mechanisms discussed in this paper. We also thank Dr. Duane Bruley and Dr. Maria Burka (NSF) for their contmuing interest in this work.

The Journal of Supercritical Fluids, Vol. 3, No. 4, 1990

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