3820
Ind. Eng. Chem. Res. 1998, 37, 3820-3829
Mechanism and Kinetics of the Acid-Catalyzed Dehydration of 1and 2-Propanol in Hot Compressed Liquid Water Michael Jerry Antal, Jr.,* Magnus Carlsson, and Xiaodong Xu Hawaii Natural Energy Institute, 2540 Dole Street, University of Hawaii at Manoa, Honolulu, Hawaii 96822
Donald G. M. Anderson Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138
Only propene and 1-propanol are observed as products of the acid-catalyzed dehydration of 2-propanol in compressed liquid water after 100 s or less at 320 °C. Likewise, only propene, 2-propanol, and traces of n-propyl ether are observed as products of the acid-catalyzed dehydration of 1-propanol under the same conditions. Kinetic analyses of the experimental data indicate that propene is formed from 1-propanol by an acid-catalyzed E2 mechanism that involves only protonated 1-propanol as an intermediate. Byproduct n-propyl ether is formed by an AdE3 mechanism involving propene, 1-propanol, and acid. The formation of propene from 2-propanol is kinetically consistent with both an acid-catalyzed E2 mechanism and an acid-catalyzed E1 mechanism (that involves the bare i-propyl cation in addition to protonated 2-propanol as intermediates in the dehydration chemistry). Thus, our kinetic analysis does not enable us to discriminate between the two mechanisms for 2-propanol dehydration. On the other hand, results of the kinetic analysis point to the important role of the undetected product i-propyl ether in the formation of propene from both 2-propanol and 1-propanol. This ether (which is not stable in hot liquid water) is formed by an AdE3 mechanism and decomposes via an uncatalyzed, unimolecular reaction. Experimental studies of the reactions of n-propyl ether and i-propyl ether are consistent with model predictions. Introduction Due to problems with environmental pollution, expanding reliance on imported sources of petrochemical feedstocks and undervalued agricultural commodities, increasing attention is being given to possibilities for using fermentation products as alternative feedstocks for the synthesis of higher value chemicals and oxygenated fuels.1 These fermentation products are born in water, and the cost of distillation is high. Thus, it is natural to inquire: What utility may water enjoy as a chemical reaction medium? For example, earlier work has shown that, in the presence of a low concentration of acid, ethanol rapidly equilibrates with ethene in supercritical water.2,3 After the products leave the reactor and are cooled, phase separation of ethene from the water-ethanol product mixture occurs. Thus, a pure ethene stream is made available at high pressure. Economic prognostications4,5 indicate that this process could be a cost-competitive step in the production of poly(vinyl chloride) (PVC) from grain alcohol. 1-Propanol and 2-propanol are also fermentation products; consequently, there is similar interest in their use as feedstocks for propene production in near- and supercritical water. To enable these alcohols to become economical sources of propene and thereby compete with established petrochemical technologies for manufacturing olefins, a deep knowledge of the dehydration chemistry of 1- and 2-propanol is needed. There is agreement that primary alcohols undergo dehydration via a concerted E2-type * Phone: (808)956-7267. Fax: (808)956-2336. E-mail: antal@ wiliki.eng.hawaii.edu.
elimination mechanism,6-10 but the mechanism of dehydration of secondary alcohols is not well-understood.7,9,11,12 In particular, scant attention has been paid to the role of ethers as intermediates in the dehydration chemistry at temperatures of industrial concern. Recent research points to their formation as byproducts of the acid-catalyzed dehydration of other alcohols.2,3,13,14 Furthermore, ethers are known to be susceptible to solvolysis in hot liquid water;15,16 consequently, they may easily open new pathways by assuming the role of unstable intermediates in the reaction chemistry. The goal of this work is to employ chemical kinetics to develop a better understanding of the mechanism of the acid-catalyzed dehydration of 1- and 2-propanol in water. We seek to determine if kinetic models can fit a broad range of data acquired from several flow reactors at 320 °C with reactant concentrations between 0.05 and 2.0 M and sulfuric acid concentrations below 0.025 M. If the data can be fit, we seek to use measures of the quality of the fit to discriminate between models and thereby gain insight into the mechanism of dehydration. Finally, we explore the influence of different data sets on the estimated values of the rate constants. Although all the data reported here was acquired at 34.5 MPa, the fact that liquid water is virtually incompressible leads to the conclusion that our findings are meaningful over a range of pressures above the saturation pressure of water that are of potential interest to industry. Likewise, experience teaches that a reaction mechanism prevails over a range of temperatures.13 For this reason, we expect that our mechanistic work depicts the important acid-catalyzed reaction chemistry of propanol in hot liquid water over a broad range of temperatures.
S0888-5885(98)00204-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/18/1998
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3821
Apparatus and Experimental Procedures Five annular and two capillary tube flow reactors were used during the course of this work to detail the effects of reactant and catalyst concentrations and residence time on product formation. As discussed in earlier publications,14,17,18 each reactor was designed to satisfy criteria which ensure the legitimate use of the plug flow idealization of reactant and product flow within the reactor. When our work was initiated, only one product sampling system was available, but by the end of the research three sampling systems were in use. These three systems employed different principles to capture samples of the reactor’s effluent. Earlier papers2,3,13,14,19,20-23 and theses24-27 provide a complete description of this equipment. Because many students learned how to operate flow reactors during the course of this work, we found it necessary to institute a strict program of quality assurance for the experimental data used in modeling. Typically, for each reaction condition three samples of the reactor’s effluent were taken in each of two different sampling systems. Two injections of each sample were analyzed, and the mean values of the gaseous and liquid product yields were calculated. These values were used to determine the carbon balance for each sample. A typical mean value of the carbon balance for all six samples was 1.024 ( 0.029, where the ( value indicates the sample standard deviation over the six samples. For this example, the mean value of the carbon balance for the three samples taken by the first sampling system was 1.010, and the mean value obtained from the second sampling system was 1.038. Usually, samples with a mean carbon balance outside the range 1.00 ( 0.05 were rejected and the experimental condition was re-run. Likewise, a significant disagreement between the results obtained from the two different sampling systems was cause for rejection of the experiment. Conditions that favored the formation of ethers (which were usually not detected by our analytical procedures) occasionally resulted in carbon balances as low as 0.92. These experimental results were not rejected. Prior to Dec 30, 1991, liquid products were analyzed by high-performance liquid chromatography (HPLC) according to procedures described earlier.21,22 Over a 4-year period we conducted numerous analyses of external standards by HPLC and discovered that our HPLC analyses of the two propanols in water were less reliable than those obtained by gas chromatography (GC). Consequently, we abandoned HPLC in favor of analysis by a Hewlett-Packard model 5890 GC equipped with a 30 m × 0.53 mm (5-µm film thickness) J&W Scientific DB-1 column. The column was operated at 40 °C with a helium flow of 15 mL/min. Quantitative analysis of the products was based on external standards, analyzed together with each set of samples. From the standard injections straight line calibration curves were determined by linear regression. Usually, two calibration lines were calculated: one for the lower concentration region and one for the higher. We employed this procedure because the flame ionization detector’s response was manifestly nonlinear over the range of propanol concentrations examined in this work. Hewlett-Packard model 5840 and 5890 gas chromatographs, each equipped with a flame ionization detector and a Poropak Q column, were used for gas analysis. The column was held at 35 °C for 4.2 min, followed by a 15 °C/min ramp to 227 °C, and a 70 °C/min ramp to
350 °C with a 11.24-min hold at 350 °C. The carrier gas was 8.5% hydrogen in helium. Some qualitative analyses of the gas and liquid samples were accomplished using a Hewlett-Packard model 5790 GC equipped with a Hewlett-Packard model 5970 mass selective detector and a J&W FSOT capillary column. All reactant solutions were prepared with degassed, distilled water or HPLC grade, deionized water. The water was degassed by drawing it through a Whatman Micron Sep membrane filter and into an evacuated vessel. Fisher certified-grade 1-propanol and Fisher HPLC-grade 2-propanol were used as reactants. No impurities were detected in these reagents by HPLC or GC analyses. Fisher certified-grade 10 N sulfuric acid was diluted and used as a catalyst. Propene (Aldrich Chemical Co.) and air standards were used to calibrate the GC. Kinetic Models and Parameter Estimation To represent a particular kinetic model, we employ notation like “E2/AdE3;E2/AdE3/Uni”. In this notation, nomenclature prior to the semicolon describes the reaction chemistry of 1-propanol, whereas that following the semicolon represents the reactions of 2-propanol. A “/” separates mechanisms (composed of several elementary chemical reactions) that are included in the model. In this example (i.e., E2/AdE3;E2/AdE3/Uni), 1-propanol decomposes by an acid-catalyzed E2 elimination to produce propene, which is subsequently transformed to di-n-propyl ether by an AdE3 reaction (i.e., a third-order electrophilic addition). Likewise, propene is produced from 2-propanol via an E2 elimination. The product propene reacts to form di-isopropyl ether via an AdE3 reaction, which subsequently undergoes a unimolecular decomposition to produce propene and 2-propanol. More exactly, the E2/AdE3;E2/AdE3/Uni model is displayed as eqs 1-8 in Figure 1. This model is representative of those we considered, and it realizes a good fit to all of our experimental data (see later, especially eq 10, for a discussion of fits). Consequently, the mathematics of its formulation are summarized here to serve as an illustrative example of model development. Equations 1-8 define a set of seven ordinary differential equations (ODEs) which govern the timedependent behavior of the concentrations of 1-propanol (1-PrOH), 2-propanol (2-PrOH), protonated 1-propanol (1-PrOH2+), protonated 2-propanol (2-PrOH2+), protonated di-n-propyl ether [(1-Pr)OH+(1-Pr)], di-isopropyl ether [(2-Pr)O(2-Pr)], and protonated di-isopropyl ether [(2-Pr)OH+(2-Pr)]. For example, the ODE governing the concentration of (2-Pr)O(2-Pr) is
d[(2-Pr)O(2-Pr)]/dt ) k7[(2-Pr)OH+(2-Pr)][H2O] k-7[(2-Pr)O(2-Pr)] [H3O+] - k8[(2-Pr)O(2-Pr)] (1) where [H3O+] is the concentration of hydronium ions and [H2O] is the concentration of water. To avoid unnecessary integrations, three algebraic equations are employed to represent the concentrations of propene, hydronium ion, and water. Carbon conservation results in the following expression for propene:
[C3H6] ) [PrOH]0 - [1-PrOH] - [2-PrOH] [1-PrOH2+] - [2-PrOH2+] - 2[(1-Pr)OH+(1-Pr)] 2[(2-Pr)O(2-Pr)] - 2[(2-Pr)OH+(2-Pr)] (2)
3822 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998
we find
1 [H3O+] ) (-S + xS2 + 4Kw) 2
(4)
where
S ) [1-PrOH2+] + [2-PrOH2+] + [(1-Pr)OH+(1-Pr)] + [(2-Pr)OH+(2-Pr)] - [H2SO4]0 (5) Finally, the concentration of water is assumed to remain constant throughout the reaction with a value given by
[H2O] ) (1 - initial mole fraction of reactant FH2O,RTP (6) propanol) × 55.55 × FH2O,NTP where FH2O,NTP is the density of water at 0.1 MPa and 25 °C and FH2O,RTP is the density of water at reaction temperature and pressure (RTP). The initial conditions (t ) 0 at the entrance of the reactor) are as follows:
[PrOH]0 ) (NTP concentration of reactant FH2O,RTP (7) propanol) × FH2O,NTP [C3H6]0 ) [1-PrOH2+] ) [2-PrOH2+] ) [(1-Pr)OH+(1-Pr)]0 ) [(2-Pr)O(2-Pr)]0 ) [(2-Pr)OH+(2-Pr)]0 ) 0 (8) and
1 [H3O+]0 ) ([H2SO4]0 + x4Kw + [H2SO4]02) (9) 2
Figure 1. Elementary reactions governing the acid-catalyzed dehydration of 1-propanol and 2-propanol in hot water. (a) E2/ AdE3 model for 1-propanol. (b) E2/AdE3/Uni model for 2-propanol. (c) E1/AdE3/Uni model for 2-propanol.
where [PrOH]0 is the initial concentration of propanol that enters the reactor. When acid is used as a catalyst, a charge balance offers the following equation:
Since the intermediate species included in the model are undetected by our analytic instrumentation, some assumptions must be made concerning their disposition at the exit of the reactor. We elaborate these assumptions in the context of each model discussed below. Since model values of the concentrations of these undetected intermediates are always low, these assumptions do not strongly influence the fit of the model to the data. Finally, to calculate the residence time of the reactant at RTP, the density of the 1-propanol or 2-propanol solution is assumed to be that of pure water at the same conditions. The rate constants ki, i ) 1,2, ..., n, are parameters whose values are chosen to give a best fit (in the least squares sense) of the model to the experimental data. The quality of this fit is measured by the value of the objective function χν2:
[H3O+] + [1-PrOH2+] + [2-PrOH2+] +
m
[(1-Pr)OH+(1-Pr)] + [(2-Pr)OH+(2-Pr)] ) [HSO4-] (3) Since H2SO4 fully dissociates into H3O+ at the conditions discussed in this paper,27
-
and HSO4 and HSO427 does not further dissociate, we have [HSO4-] ) [H2SO4]0. Employing the relationship Kw ) [H3O+][OH-],
χν2 )
2
∑ ∑ (ej,k/σk)2/ν j)1k)1
(10)
where the residual ej,k ) (yj,kexp - yj,kmod) and yj,k is the yield of each of the two alcohols (1-PrOH or 2-PrOH corresponding to k ) 1 or 2) included in the model at reaction conditions j ) 1, ..., m, as measured experimentally (exp) or calculated by the model (mod). Also,
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3823 Table 1. Experimental and Calculated Fractional Yields of 1-Propanol and 2-Propanol at 320 °C, 34.5 MPa yields 1-PrOH
2-PrOH
no.
date
reac. IDa
res. time (s)
[PrOH] (mol/dm3)
[H2SO4] (mol/dm3)
exp
model 1b
model 2c
exp
model 1b
model 2c
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
3/4/87 1/21/87 1/14/87 1/21/87 1/14/87 d 12/23/86 12/23/86 d d d 6/3/87 4/19/93 d e d 10/26/89 e 10/26/89 5/18/90 8/29/90 5/18/90 8/29/90 11/3/92 3/13/90 5/15/90 12/12/91 5/15/90 4/19/93 12/30/91 4/1/93 12/30/91 1/6/92 1/6/92 1/6/92 2/3/93 2/3/93 2/18/93 4/1/93 4/1/93 2/3/93 2/3/93 4/1/93 1/6/94 1/6/94 1/6/94 1/6/94
1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 2 2 2 1 1 1 1 2 2 2 1 1 1 2 1 1 1 1
7.50 × 101 3.90 × 101 3.80 × 101 3.90 × 101 3.90 × 101 1.00 × 102 2.20 × 101 4.30 × 101 2.90 × 101 3.40 × 101 3.50 × 101 3.10 × 101 2.60 × 101 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.30 × 100 1.00 × 102 2.50 × 101 2.60 × 101 2.90 × 101 2.60 × 101 2.50 × 101 1.10 × 100 1.30 × 100 1.30 × 100 4.00 × 101 3.00 × 101 3.00 × 101 1.10 × 102 1.00 × 102 2.60 × 100 2.60 × 100 2.70 × 100 3.60 × 101 3.80 × 101 1.30 × 100 2.80 × 100 2.70 × 100 1.90 × 100 1.80 × 100
5.00 × 10-1 5.00 × 10-1 5.00 × 10-1 5.00 × 10-1 5.00 × 10-1 1.10 × 100 5.00 × 10-1 5.00 × 10-1 5.00 × 10-2 1.00 × 10-1 2.00 × 10-1 1.00 × 100 1.00 × 100 1.00 × 10-1 1.00 × 10-1 1.00 × 100 1.00 × 100 1.00 × 100 2.00 × 10-1 2.00 × 100 1.00 × 10-1 1.00 × 10-1 5.00 × 10-1 1.00 × 100 1.00 × 100 1.00 × 100 1.00 × 100 1.00 × 100 1.90 × 100 1.00 × 10-1 9.80 × 10-1 5.00 × 10-1 1.00 × 10-1 2.00 × 10-1 1.00 × 10-1 9.90 × 10-1 1.00 × 10-1 9.70 × 10-2 9.70 × 10-1 1.10 × 100 1.00 × 10-1 1.00 × 10-1 9.80 × 10-1 1.00 × 100 2.00 × 100 2.00 × 100 1.00 × 100
3.00 × 10-3 9.00 × 10-3 1.00 × 10-2 1.60 × 10-2 2.50 × 10-2 1.00 × 10-2 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 1.00 × 10-3 5.00 × 10-3 5.00 × 10-3 2.00 × 10-3 1.00 × 10-2 5.00 × 10-3 1.00 × 10-2 2.00 × 10-3 5.00 × 10-3 5.00 × 10-3 1.00 × 10-2 3.00 × 10-3 3.00 × 10-3 1.00 × 10-2 1.00 × 10-2 5.00 × 10-3 5.00 × 10-3 5.00 × 10-3 2.00 × 10-3 1.00 × 10-2 1.00 × 10-3 1.00 × 10-3 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-3 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2 1.00 × 10-2
7.40 × 10-1 6.40 × 10-1 6.40 × 10-1 4.70 × 10-1 3.30 × 10-1 4.10 × 10-1 8.80 × 10-1 7.90 × 10-1 8.50 × 10-1 8.30 × 10-1 8.20 × 10-1 8.30 × 10-1 8.40 × 10-1 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 0.00 × 100 3.10 × 10-2 9.10 × 10-1 8.80 × 10-1 7.20 × 10-1 7.90 × 10-1 8.70 × 10-1 0.00 × 100 0.00 × 100 0.00 × 100 6.40 × 10-1 9.80 × 10-1 9.90 × 10-1 4.50 × 10-1 5.10 × 10-2 0.00 × 100 0.00 × 100 9.40 × 10-1 9.70 × 10-1 6.20 × 10-1 0.00 × 100 9.50 × 10-1 9.90 × 10-1 9.90 × 10-1 9.90 × 10-1
7.60 × 10-1 6.60 × 10-1 6.40 × 10-1 4.80 × 10-1 3.30 × 10-1 4.30 × 10-1 8.70 × 10-1 7.70 × 10-1 8.30 × 10-1 8.00 × 10-1 7.90 × 10-1 8.50 × 10-1 8.70 × 10-1 1.50 × 10-5 2.60 × 10-4 1.00 × 10-4 2.70 × 10-5 2.70 × 10-4 2.30 × 10-4 1.30 × 10-4 5.40 × 10-5 2.60 × 10-4 1.60 × 10-4 2.90 × 10-2 9.20 × 10-1 9.10 × 10-1 7.50 × 10-1 7.60 × 10-1 9.00 × 10-1 1.90 × 10-4 1.10 × 10-4 4.20 × 10-5 6.00 × 10-1 9.60 × 10-1 9.60 × 10-1 4.10 × 10-1 5.40 × 10-2 1.90 × 10-3 8.30 × 10-4 9.60 × 10-1 9.50 × 10-1 6.20 × 10-1 2.90 × 10-4 9.60 × 10-1 9.70 × 10-1 9.80 × 10-1 9.80 × 10-1
7.60 × 10-1 6.50 × 10-1 6.40 × 10-1 4.80 × 10-1 3.30 × 10-1 4.30 × 10-1 8.70 × 10-1 7.70 × 10-1 8.20 × 10-1 8.00 × 10-1 7.90 × 10-1 8.50 × 10-1 8.70 × 10-1 1.40 × 10-5 2.70 × 10-4 1.90 × 10-4 3.80 × 10-5 5.80 × 10-4 2.60 × 10-4 3.80 × 10-4 5.30 × 10-5 2.70 × 10-4 2.40 × 10-4 2.90 × 10-2 9.10 × 10-1 9.10 × 10-1 7.50 × 10-1 7.60 × 10-1 9.00 × 10-1 1.90 × 10-4 2.00 × 10-4 4.90 × 10-5 5.90 × 10-1 9.60 × 10-1 9.60 × 10-1 4.10 × 10-1 5.40 × 10-2 2.20 × 10-3 1.50 × 10-3 9.60 × 10-1 9.50 × 10-1 6.10 × 10-1 6.10 × 10-4 9.60 × 10-1 9.70 × 10-1 9.80 × 10-1 9.80 × 10-1
n.m.f n.m.f n.m.f n.m.f n.m.f 2.80 × 10-1 n.m.f n.m.f 6.50 × 10-2 1.10 × 10-1 9.20 × 10-2 n.m.f 4.70 × 10-2 9.20 × 10-1 6.80 × 10-1 6.60 × 10-1 8.50 × 10-1 5.70 × 10-1 6.50 × 10-1 5.80 × 10-1 8.30 × 10-1 6.80 × 10-1 6.40 × 10-1 4.30 × 10-1 3.30 × 10-2 4.90 × 10-2 9.90 × 10-2 9.80 × 10-2 3.90 × 10-2 6.90 × 10-1 7.00 × 10-1 8.10 × 10-1 1.70 × 10-1 1.60 × 10-2 1.40 × 10-2 2.70 × 10-1 4.90 × 10-1 5.10 × 10-1 4.90 × 10-1 1.30 × 10-2 0.00 × 100 1.60 × 10-1 5.90 × 10-1 1.20 × 10-2 1.00 × 10-2 6.60 × 10-3 7.60 × 10-3
1.10 × 10-1 1.50 × 10-1 1.60 × 10-1 2.40 × 10-1 3.10 × 10-1 2.60 × 10-1 5.50 × 10-2 1.00 × 10-1 7.60 × 10-2 8.90 × 10-2 9.10 × 10-2 6.40 × 10-2 5.50 × 10-2 9.00 × 10-1 6.60 × 10-1 6.90 × 10-1 8.40 × 10-1 5.60 × 10-1 6.60 × 10-1 5.90 × 10-1 8.20 × 10-1 6.60 × 10-1 6.70 × 10-1 4.60 × 10-1 3.40 × 10-2 3.50 × 10-2 1.10 × 10-1 1.00 × 10-1 3.80 × 10-2 6.90 × 10-1 6.80 × 10-1 8.20 × 10-1 1.80 × 10-1 1.40 × 10-2 1.40 × 10-2 2.70 × 10-1 4.40 × 10-1 4.80 × 10-1 4.90 × 10-1 1.20 × 10-2 1.80 × 10-2 1.80 × 10-1 5.50 × 10-1 1.20 × 10-2 1.00 × 10-2 6.30 × 10-3 7.00 × 10-3
1.10 × 10-1 1.50 × 10-1 1.60 × 10-1 2.40 × 10-1 3.10 × 10-1 2.60 × 10-1 5.50 × 10-2 1.00 × 10-1 7.60 × 10-2 9.00 × 10-2 9.20 × 10-2 6.50 × 10-2 5.50 × 10-2 9.00 × 10-1 6.60 × 10-1 6.90 × 10-1 8.40 × 10-1 5.60 × 10-1 6.60 × 10-1 5.90 × 10-1 8.20 × 10-1 6.60 × 10-1 6.70 × 10-1 4.60 × 10-1 3.40 × 10-2 3.50 × 10-2 1.10 × 10-1 1.00 × 10-1 3.80 × 10-2 6.90 × 10-1 6.80 × 10-1 8.20 × 10-1 1.80 × 10-1 1.40 × 10-2 1.40 × 10-2 2.70 × 10-1 4.40 × 10-1 4.70 × 10-1 4.90 × 10-1 1.20 × 10-2 1.80 × 10-2 1.80 × 10-1 5.50 × 10-1 1.20 × 10-2 1.00 × 10-2 6.40 × 10-3 7.10 × 10-3
a 1 ) 1-PrOH, 2 ) 2-PrOH. b Model 1 ) E2/Ad 3;E2/Ad 3/Uni mechanism. c Model 2 ) E2/Ad 3;E1/Ad 3/Uni mechanism. E E E E of two experiments. e Average of three experiments. f n.m. ) not measured.
σk is the sample standard deviation associated with the measurement of yj,kexp, and ν is the number of degrees of freedom.28 As is evident from eq 10, the value of σk plays a pivotal role in the formulation of χν2. During the past decade many statistical studies3 of the accuracy of both HPLC and GC instruments with automatic injectors have shown σk = µyj,kexp, where µ ) 0.04 for experiments with inlet reactant concentration [PrOH]0 g 0.1 mol/dm3, µ ) 0.08 for 0.1 > [PrOH]0 g 0.01 mol/ dm3 and µ ) 0.2 when [PrOH]0 < 0.01 mol/dm3. These values of µ reflect our recent experience, which indicates that there is a significant loss of accuracy when low concentrations of alcohols in water are measured. We remark that our sampling procedure involves several washes of the sample loop to recover all of the products. These washes result in a significant dilution of the sample, which necessarily causes an increase in the value of σi associated with the concentration measurement. Also, we remark that if the nonlinear responses
d
Average
of the HPLC’s RI detector and the GC’s FID detector are overlooked, systematic errors can invalidate the experimental results. Results Table 1 lists the results of 47 experiments involving the acid-catalyzed decomposition of 1-propanol and 2-propanol. Because our initial goal was to describe the reaction chemistry of 1-propanol, the formation of 2-propanol (from propene) was not at first monitored (see runs 1-5, 7, 8, and 12). In the case of this early data, missing experimental measurements were accounted for by omitting the corresponding term in the calculation of χν2 and adjusting ν accordingly. Later in the experimental campaign, we realized the necessity of studying the joint reaction chemistry of both 1-propanol and 2-propanol, and a complete analysis of the reactor’s effluent was conducted thereafter.
3824 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 2. Elementary Rate Constants and the Associated η˜ a for the E2;E2 Type Models
a
model
E2;E2
E2/AdE3;E2
E2/AdE3;E2/AdE3/Uni
E2/AdE3;E2/AdE3/Uni
no. of param. no. exp. data χν2 k1/ (dm3/(mol‚s)) η˜ 1 k-1/ (dm3/(mol‚s)) η˜ -1 k2/ (dm3/(mol‚s)) η˜ 2 k-2/ (dm6/(mol2‚s)) η˜ -2 k3/ (dm6/(mol2‚s)) η˜ 3 k-3/ (dm3/(mol‚s)) η˜ -3 k4/ (dm3/(mol‚s)) η˜ 4 k-4/ (dm3/(mol‚s)) η˜ -4 k5/ (dm3/(mol‚s)) η˜ 5 k-5/ (dm6/(mol2‚s)) η˜ -5 k6/ (dm6/(mol2‚s)) η˜ 6 k7/ (dm3/(mol‚s)) η˜ 7 k8/ (1/s) η˜ 8
8 86 1.02 5.92 × 101 1.61 × 10-2 5.80 × 100 1.66 × 10-2 1.98 × 10-1 1.52 × 10-2 6.16 × 10-3 1.29 × 10-1 N/A N/A N/A N/A 1.41 × 102 1.96 × 10-2 1.01 × 100 2.32 × 10-2 2.09 × 102 2.37 × 10-2 6.17 × 102 2.38 × 10-2 N/A N/A N/A N/A N/A N/A
10 86 0.782 1.88 × 100 1.24 × 10-2 1.60 × 100 1.05 × 10-1 4.96 × 101 1.06 × 10-1 1.50 × 10-1 1.14 × 10-1 6.52 × 100 6.22 × 10-2 1.93 × 10-2 6.47 × 10-2 7.42 × 102 1.64 × 10-2 5.00 × 101 1.70 × 10-2 9.47 × 100 1.64 × 10-2 3.20 × 100 1.90 × 10-2 N/A N/A N/A N/A N/A N/A
13 86 0.702 1.90 × 100 1.14 × 10-2 2.75 × 100 9.07 × 10-2 8.51 × 101 9.18 × 10-2 2.52 × 10-1 9.79 × 10-2 4.64 × 100 4.92 × 10-2 9.81 × 10-3 5.23 × 10-2 5.44 × 102 1.94 × 10-2 1.25 × 102 2.06 × 10-2 3.55 × 101 2.04 × 10-2 3.31 × 100 2.27 × 10-2 3.47 × 103 1.73 × 10-1 2.47 × 101 3.16 × 10-1 3.43 × 100 1.54 × 10-1
13 92b 1.49 1.75 × 100 1.66 × 10-2 1.85 × 100 1.18 × 10-1 4.94 × 101 1.19 × 10-1 1.64 × 10-1 1.29 × 10-1 3.58 × 100 1.22 × 10-1 1.59 × 10-2 1.28 × 10-1 4.78 × 102 2.24 × 10-2 1.26 × 102 2.36 × 10-2 4.04 × 101 2.34 × 10-2 3.48 × 100 2.62 × 10-2 2.47 × 100 3.33 × 10-1 1.84 × 10-2 5.70 × 10-1 3.16 × 100 2.11 × 102
See Xu et al. (1997) for the definition of η˜ . b Includes six outliers.
Experience teaches that experimental data which lie at extremes of reaction conditions are most useful for model development and discrimination. Consequently, the core of the experimental plan used in this work includes measurements of product yields at high and low values of the reactant and catalyst concentrations and conversions for both propanols (a total of 2 × 23 conditions). On the basis of our experience and practical considerations, reactant concentrations above 0.5 M, catalyst concentrations above 5 mM, and conversions above 0.3 were considered “high”, and other values were “low”. But in some cases, a particular combination could not be achieved in our equipment. For example, we could not realize residence times long enough to achieve a high conversion of a high concentration of 1-propanol with a low concentration of acid catalyst. The data set given in Table 1 also includes some studies of trends (for example, see runs 2-5) and reproductions of problematic experiments (such as runs 25 and 26, 33 and 42, and 15 and 30). In some cases, multiple runs on different days at a single condition were accumulated, and the mean value of the measured yields was employed (for example, run 18 represents the mean value of data obtained on Oct 26, 1989, Jan 4, 1980, and May 18, 1990). Experiments 44-47 were chosen to improve our ability to discriminate between different mechanistic models of the reaction chemistry (see below). In Table 1 reactant concentrations range from 0.05 to 2 M, catalyst concentrations from 1 to 25 mM, and residence times from 1.1 to 142 s. These conditions evoked 1-propanol conversions as large as 59% and 2-propanol conversions as large as 57%. 2-Propanol was usually a major byproduct of the dehydration of 1-propanol, and 1-propanol was observed to be a minor byproduct of 2-propanol dehydration. In keeping with the chemist’s rule of thumb that secondary alcohols are far more reactive than primary alcohols, a cursory
examination of Table 1 reveals 2-propanol to be far more prone to dehydration than 1-propanol. We also remark that neither alcohol evidenced decomposition in our reactors in the absence of an acid catalyst. Discussion An acid-catalyzed E2 elimination (see Figure 1a,b) is the least complex model for the formation of propene from both alcohols, and the interconversion of the two alcohols. As discussed above, this E2;E2 model has eight free parameters (rate constants k(1, k(2, k(4, and k(5) and involves two undetected intermediates (i.e., the protonated alcohols). This simple model realizes an agreeable fit χν2 ) 1.02 to the data (see Table 2). But some evidence suggests that the secondary alcohol may undergo dehydration via an acid-catalyzed E1 mechanism; consequently, we examined the ability of a 10 parameter (see Figure 1c) E2;E1 model (involving rate constants k(1, k(2, k(9, k(10, and k(11) to fit the data. This somewhat more complicated model also achieves a good fit to the data with a value χν2 ) 1.06. Earlier work in our laboratory3,14,13 has clearly revealed the importance of ethers as byproducts and intermediates of acid-catalyzed dehydrations of alcohols. Consequently, we employed GC-MS to search for evidence of ethers in the effluent of the reactor. n-Propyl ether was detected as a byproduct of 1-propanol dehydration, but no ethers were found in the products of 2-propanol dehydration. These findings led us to hypothesize that n-propyl ether plays a role in the reaction chemistry. On the other hand, either isopropyl ether is not a significant intermediate or it is not stable at reaction conditions. To explore the matter further, we executed semiquantitative studies of the reactions of n-propyl ether and isopropyl ether in water at 320 °C. Without acid the n-propyl ether evidenced no decomposition after 47 s, but the addition of 1 mM sulfuric
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3825
Figure 2. (a) Inconsequential reactions involving 1-propanol or n-propyl ether that were considered in addition to those listed in Figure 1a for the E2/AdE3 model. (b) Inconsequential reactions involving 2-propanol or i-propyl ether that were considered in addition to those listed in Figure 1b for the E2/AdE3/Uni model. (c) Inconsequential reactions involving 2-propanol or i-propyl ether that were considered in addition to those listed in Figure 2b (above) and 1c for the E1/AdE3/Uni model. Also, many elementary reactions involving the crossed ether (1-Pr)O(2-Pr) were considered, but none improved the fit of the model to the data.
acid caused it to suffer a 20% decomposition to propene and 1-propanol. On the other hand, isopropyl ether decomposed to propene and 2-propanol both in the presence and absence of acid. Thus, isopropyl ether is not stable under any of the reaction conditions explored in this work. These results were consistent with the hypothesis that both ethers play a role in the dehydration chemistry of the two alcohols and caused us to examine more complicated models of the reaction chemistry. To begin, we added two pathways to the eight parameter E2;E2 model to account for the reversible formation of n-propyl ether from propene, 1-propanol, and acid via an AdE3 mechanism (see pathway 3, Figure 1a). This 10-parameter E2AdE3;E2 model achieved a fit χν2 ) 0.782 to the data (see Table 2). To secure further improvements in the fit, we included many other reaction pathways (see Figure 2a) in the kinetic model. No other pathways involving 1-propanol or n-propyl
ether resulted in an improvement of fit. But as expected, some improvement was obtained when pathways governing the formation of isopropyl ether from propene, 2-propanol, and acid, and its unimolecular decomposition were included in the model. Thus, the inclusion of pathways 6-8 (see Figure 1b, but note that a best fit was obtained with values k-6 ) k-7 ) 0) in the model reduced the value of χν2 to 0.702. On the other hand, the addition of other pathways (see Figure 2a,b) to the model offered no improvement to the fit. The accord of this 13-parameter E2/AdE3;E2/AdE3/Uni model with the 86 experimental data points is further illustrated in Figure 3a,b, which display parity plots of the simulated values and experimental measurements of the yields of 1-propanol and 2-propanol. Table 2 lists the best fitting rate constants for the E2/AdE3;E2/AdE3/ Uni model. We remark that, in addition to providing a good fit to the data, this model is also fully consistent with experimental findings concerning ether formation and decomposition detailed above. To learn more about our ability to discriminate between various models, we also examined the ability of the related E2/AdE3;E1/AdE3/Uni mechanism (see pathways 9-14 in Figure 1b) to fit the data. Note that this model differs from the E2/AdE3;E2/AdE3/Uni model in the involvement of the carbocation C3H7+ as a key intermediate in the formation of both propene and isopropyl ether from 2-propanol. To our surprise, this 15-parameter model achieved a comparable fit of χν2 ) 0.734 (see Table 3 and Figure 3a,b). The addition of other pathways (see Figure 2a-c) to the model offered no further improvement to the fit. Evidently, it is difficult to discriminate between E2 and E1 mechanisms in kinetic descriptions of the acid-catalyzed dehydration of 2-propanol. Often, it is possible to gain insight into the reaction chemistry by a careful examination of the magnitudes of the simulated yields of intermediate species exiting the reactor. As expected, the simulated yields of charged intermediates (including protonated n-propyl ether) are small (,1%) in all the experiments irrespective of the numerical model used to effect the simulation. This finding is consistent with the fact that n-propyl ether was not detected as a product by our usual methods of chemical analysis. On the other hand, the yields of isopropyl ether reach 28% (experiment 20) in the E2/AdE3;E2/AdE3/Uni model and 23% (experiment 24) in the E2/AdE3;E1/AdE3/Uni model. Recall that isopropyl ether is not a stable product of the reaction chemistry; hence, it may decompose at the exit of the reactor and thereby avoid detection. Alternatively, its remnants may phase-separate over time from the aqueous products and thereby escape detection, but a second phase in the liquid product sample was never actually observed. Also, carbon balances close to 1.0, realized in most experiments, imply that isopropyl ether cannot be a significant, stable product. In our modeling work, we obtained a best fit to the experimental data when we assumed that protonated n-propyl ether does not decompose at the outlet of the reactor. Also, we realized a best fit with the assumptions that both protonated and unprotonated isopropyl ether decompose to 2-propanol and propene at the reactor’s outlet. These assumptions are consistent with our experimental observations (see above). Clearly, future work should give fastidious attention to the potential role of isopropyl ether as a key intermediate in the acid-catalyzed,
3826 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 Table 3. Elementary Rate Constants and the Associated η˜ a for the E2;E1 Type Models
a
model
E2;E1
E2/AdE3;E1
E2/AdE3;E1/AdE3/Uni
E2/AdE3;E1/AdE3/Uni
no. of param. no. exp. data χν2 k1/(dm3/(mol‚s)) η˜ 1 k-1/ (dm3/(mol‚s)) η˜ -1 k2/(dm3/(mol‚s)) η˜ 2 k-2/ (dm6/(mol2‚s)) η˜ -2 k3/ (dm6/(mol2‚s)) η˜ 3 k-3/ (dm3/(mol‚s)) η˜ -3 k9/ (dm3/(mol‚s)) η˜ 9 k-9/ (dm3/(mol‚s)) η˜ -9 k10/ (1/s) η˜ 10 k-10/ (dm3/(mol‚s)) η˜ -10 k11/ (dm3/(mol‚s)) η˜ 11 k-11/ (dm3/(mol‚s)) η˜ -11 k12/ (dm6/(mol2‚s)) η˜ 12 k13/ (dm3/(mol‚s)) η˜ 13 k14/ (1/s) η˜ 14
10 86 1.06 3.24 × 101 1.61 × 10-2 3.04 × 100 1.71 × 10-2 1.95 × 10-1 1.56 × 10-2 6.33 × 10-3 1.29 × 10-1 N/A N/A N/A N/A 1.49 × 102 1.98 × 10-2 1.18 × 100 2.31 × 10-2 9.26 × 103 2.36 × 10-2 3.06 × 103 2.37 × 10-2 4.62 × 102 2.37 × 10-2 3.76 × 103 2.39 × 10-2 N/A N/A N/A N/A N/A N/A
12 86 0.812 1.48 × 101 1.27 × 10-2 1.61 × 101 1.45 × 10-2 2.31 × 100 1.43 × 10-2 5.64 × 10-3 1.14 × 10-1 5.34 × 100 7.00 × 10-2 1.74 × 10-2 7.33 × 10-2 1.26 × 102 1.64 × 10-2 4.03 × 100 1.90 × 10-2 1.07 × 104 1.90 × 10-2 3.51 × 103 1.90 × 10-2 5.52 × 102 1.90 × 10-2 1.19 × 103 1.92 × 10-2 N/A N/A N/A N/A N/A N/A
15 86 0.734 5.85 × 100 1.15 × 10-2 5.11 × 101 1.67 × 10-2 2.43 × 101 1.67 × 10-2 1.11 × 10-2 9.86 × 10-2 4.60 × 100 4.93 × 10-2 9.94 × 10-3 5.22 × 10-2 1.94 × 102 1.83 × 10-2 2.00 × 101 2.21 × 10-2 1.88 × 104 2.19 × 10-2 4.15 × 102 2.21 × 10-2 3.05 × 101 1.98 × 10-2 2.78 × 102 2.10 × 10-2 7.64 × 100 1.46 × 10-1 3.55 × 100 1.12 × 100 1.07 × 10-1 2.57 × 10-1
15 92b 1.32 3.57 × 100 1.47 × 10-2 8.57 × 101 2.70 × 10-2 8.23 × 101 2.72 × 10-2 2.37 × 10-2 1.05 × 10-1 2.87 × 100 7.31 × 10-2 5.44 × 10-3 8.05 × 10-2 6.53 × 103 2.09 × 10-2 6.68 × 102 2.05 × 10-2 5.33 × 104 3.05 × 10-2 2.05 × 103 3.04 × 10-2 2.04 × 101 2.62 × 10-2 1.10 × 102 3.23 × 10-2 1.79 × 102 1.18 × 10-1 1.40 × 101 6.80 × 10-1 4.53 × 10-1 1.71 × 10-1
See Xu et al. (1997) for the definition of η˜ . b Includes six outliers.
aqueous-phase dehydration chemistry of both 2-propanol and 1-propanol. Toward the end of the experimental campaign, we were anxious to identify experimental conditions which could enable us to determine if the bare carbocation plays a significant role in the dehydration chemistry of 2-propanol. Assuming that an E2-type mechanism governed the evolution of propene from 2-propanol, we employed the E2/AdE3;E2/AdE3/Uni model to predict the products that would result from various experimental conditions, and we subsequently examined the ability of the E2/AdE3;E1/AdE3/Uni model to fit such synthetic data. In this model, the carbocation was assumed to revert to propene at the reactor’s outlet. After many simulations, we concluded that models involving the carbocation intermediate could not realize a good fit to experimental data which emphasized the formation of 2-propanol from propene derived from 1-propanol at very short residence times. The simulations suggested that the inclusion of the additional intermediate (the bare carbocation) would delay the appearance of 2-propanol as a byproduct of 1-propanol dehydration. This rationale lay behind the execution of experiments 4447. Unfortunately, these additional data were fit easily by models involving either E2 or E1 chemistry. Thus, the value of χν2 actually increases by 10.3% when these four data points are omitted and a best fit is obtained using the E2/AdE3;E2/AdE3/Uni model. Likewise, χν2 increases by 10.5% when the E2/AdE3;E1/AdE3/Uni model is used to fit the remaining 43 experimental conditions. The addition of these “hard-to-fit” data actually improved the fits of both models to the entire data set. These improvements in the values of χν2 occur because both models realize extraordinary fits (see Table 1) to the measured 2-propanol yields in experiments 44-
47; consequently, the numerator in χν2 does not increase, whereas the denominator increases due to the increase in the number of experimental data points. This exercise in frustration caused us to conclude that measurements of the yields of the posited ether intermediates are needed to elucidate the mechanism of 2-propanol dehydration. Figure 4a,b display the E2/AdE3;E2/AdE3/Uni mechanism and the E2/AdE3; E1/AdE3/Uni mechanism (respectively). We immediately acknowledge the complexity of the reaction networks displayed in these figures. We also profess sympathy towards skeptical readers who react to these pictures with the thought that such work as ours is merely a demonstration of mathematical curve fitting and has nothing to do with the actual, governing elementary chemistry. The following observations offer evidence that disputes this skeptical pointof-view. (1) At 34.5 MPa and 385 °C, the primary alcohol ethanol undergoes an acid-catalyzed dehydration in water to form ethene. This reaction chemistry is governed by an E2-elimination mechanism.3 Our finding that 1-propanol (also a primary alcohol) undergoes dehydration by an E2 mechanism is consistent with the earlier findings concerning ethanol. (2) Ethanol also reacts in supercritical water to form diethyl ether via an AdE3 mechanism involving ethene, ethanol, and acid.3 This result supports our finding that n-propyl ether is formed by an analogous AdE3 mechanism as shown in Figure 4a. (3) Our experimental examination of n-propyl ether decomposition revealed its stability in hot water (even in the presence of acid), which is consistent with the model results that cause us to omit the Uni and Bi pathways given in Figure 2a.
Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3827
Figure 4. Acid-catalyzed reactions of 1-propanol and 2-propanol in hot liquid water. (a) E2/AdE3;E2/AdE3/Uni mechanism. (b) E2/ AdE3;E1/AdE3/Uni mechanism.
Figure 3. (a) Parity plot of 1-propanol yields. The error bars represent one sample standard deviation associated with the experimental measurement of yield. (b) Parity plot of 2-propanol yields. The error bars represent one sample standard deviation associated with the experimental measurement of yield.
(4) In light of item 2 above, it is not surprising to find that the formation of isopropyl ether is also governed by an AdE3 mechanism involving propene, 2-propanol, and acid as shown in Figure 4a. (5) On the other hand, earlier work with tert-butyl alcohol13,14 indicated the formation of di-tert-butyl ether from the tert-butyl cation and the alcohol via an AdE2 mechanism. It is a curious fact that the analogous mechanism does not seem to play a role in the formation of isopropyl ether from 2-propanol. Instead, we find in Figure 4b an AdE3 mechanism involving carbocation, propene, and water.
(6) The earlier work with ethanol3 also demonstrated the unimolecular decomposition of diethyl ether to ethanol and ethene at 385 °C in supercritical water. This mechanism is analogous to the unimolecular decomposition of isopropyl ether displayed in Figure 4a,b. In agreement with the modeling results, our experimental examination of isopropyl ether decomposition also revealed the role of an uncatalyzed, unimolecular decomposition of the ether to propene and 2-propanol. (7) Likewise, earlier work with tert-butyl alcohol13,14 revealed the role of an analogous unimolecular decomposition of di-tert-butyl ether to isobutylene and tertbutyl alcohol in hot water at 225, 250, and 320 °C. (8) On the other hand, both the earlier ethanol and tert-butyl alcohol research indicated the role of an elementary bimolecular reaction of the relevant ether with water (see Figure 2a), which was not observed in this work with isopropyl ether. In an effort to learn more about the equilibrium composition of products derived from our reactants, we executed several long residence time experiments with 0.1 mol/dm3 of 1-propanol reactant and 10 mM sulfuric acid catalyst (see experiments 48-50 in Table 4). Because the results of these experiments could not be well-fit by any of the models we studied, we were eventually led to do a GC-MS analysis of the products. In addition to 1-propanol and 2-propanol, the GC-MS detected an unknown product which it tentatively identified as methyl pentanol after a search of library spectra. We did not attempt to confirm the identity of this unknown species. Experiments that employed shorter reaction times (
