Ind. Eng. C h e n . Res. 1992,31, 2413-2418
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KINETICS AND CATALYSIS ~~
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Kinetics and Mechanisms of Fatty Alcohol Polyethoxylation. 1. The Reaction Catalyzed by Potassium Hydroxide E. Santacesaria,* M. Di Serio, R. Garaffa, and G. Addino Cattedra di Chimica Zndustriale, Dipartimento di Chimica dell'llniversitd, Via Mezzocannone, 4, 80134 Napoli, Italy
The kinetics of fatty alcohol ethoxylation has been studied by performing kinetic runs in two types of reactors characterized by different dispersed phases. In one case gaseous ethylene oxide was dispersed in the liquid alcohol reagent; in the other case, alcohol was sprayed into the ethylene oxide gaseous mixture. In both cases chemical reaction rates were not affected by diffusion, in the adopted experimental conditions. The reaction occurs through an SN2 mechanism. A kinetic model of general utility for interpreting ethoxylation behavior has been developed. The model describes the ethylene oxide consumption, the alcohol conversion, the molecular weight distribution, and the change with time of the reaction mixture volume and of the ethylene oxide solubility.
Introduction Nonionic surfactants are industrially produced by base-catalyzed reactions of hydrophobic compounds containing active hydrogen, such as alkylphenols, fatty alcohols, fatty acids, mercaptans, and alkylamines, with ethylene and/or propylene oxide (Schick, 1967,1987). In spite of the importance of these products, few papers have been published on the kinetics of polyethoxylation. In particular for fatty alcohols, Gee et al. (1959) have found a firsborder dependence of reaction rates on both ethylene oxide and catalyst concentration. However, Satkowski and Hsu (1957) observed an order of less than 1by using KOH as catalyst at 135-140 "C. Ischii and Ozaki (1960) suggested that below 130 O C , in the absence of solvents, the reaction occurs with a mechanism involving the ionic couple while at higher temperatures the anionic mechanism predominates. Ethoxylation rates may depend also on the fatty alcohol chain length as suggested by Satkowski and Hsu (1957). As can be seen, many conclusions reported in the literature are controversial. In particular, kinetic results may be unreliable because they have normally been obtained by interpreting curve8 of ethylene oxide added during time, without considering diffusion limitation, the change of the reaction mixture density, the change of ethylene oxide solubility, the change of catalyst concentration, and, above all, the change in the oligomer distribution. In fact, it is interesting to observe that the use of CsOH as catalyst, instead of NaOH, gives rise to a narrow range oligomer distribution, as observed by Matheson et al. (1986). This behavior is strongly enhanced in the presence of alkaline earth metal catalyts such as barium, calcium, or strontium compounds, as reported in many patents and papers recently published (Yang et al., 1980a,b; Yang, 1980; Brace et al., 1988, 1989). In the present paper, a kinetic model of general validity and able to reproduce results commonly obtained will be described. This model will be applied here to the inter-
* To whom correspondence should be addressed.
pretation of kinetic runs of 1-dodecanolpolyethoxylation catalyzed by KOH. In a succeeding paper, the same model will be applied successfully to the narrow-range ethoxylation obtained in the presence of a barium catalyst. The previously mentioned kinetic model is an extension of a model already published by Santacesaria et al. (1990) and applied to nonylphenol polyethoxylation in the presence of KOH. In all cases, polyethoxylation was performed in a wellstirred isothermal semibatch reactor. The mathematical model developed is able to reproduce the evolution with time of the ethylene oxide consumption and the molecular weight distribution for different temperatures, ethylene oxide pressures, and catalyst concentrations, by taking into account also the change with time of the volume of the reaction mixture and of the ethylene oxide solubility. For this purpose, solubility and density parameters have been determined by independent experiments.
Experimental Section Apparatus, Techniques, and Reagents. Kinetic runs were performed in a jacketed, 1.5L stainless steel reactor, equipped with a magnetically driven stirrer, consisting of a turbine connected as in Figure 1, to a holed rod, able to develop a great interfacial area. The control of pressure, temperature, and ethylene oxide feeding the reactor was fully automated by a computer. The ethylene oxide consumption was directly measured and recorded at each instant with a weight balance connected to the computer, measuring the loss of weight of the ethylene oxide bottle. The ethylene oxide in the bottle was pressurized with nitrogen at about 10 atm. Also temperature and pressure were controlled and recorded at each instant. Temperature was kept constant through two thermostated fluids, one for heating and the other for cooling the reaction mixture. The heating fluid always circulated in the reactor jacket, while the freezing one was automatically fed, when necessary, to a coil inside the reactor. Ethylene oxide was automatically fed to the reactor to keep a preset value of the pressure constant.
0888-5885192/263I-2413$Q3.O010 0 1992 American Chemical Society
2414 Ind. Eng. Chem. Res,, Vol. 31, No. 11, 1992
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Figure 3. Scheme of the Pressindustria type reactor. Figure 1. Scheme of the automized plant employed in the ethoxylation. A = computer; B = computer interface; C = on-off valve for feeding ethylene oxide; D = ethylene oxide bottle; E = pressure transducer; F = manometer; G = exit for withdrawing; H = jacketed reactor; I = freezing coil; L = holed stirred; M = Magnedrive stirrer; N = thermocouple. Details of the stirrer, L, are shown.
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Figure 4. Kinetic behavior of nonylphenol in the ethoxylation performed at different temperatures. The plot is related to runs performed at 3 atm of EO by using 0.785 mol of nonylphenol and about 0.0146 mol of KOH. Time
Figure 2. Example of chromatogram obtained by HPLC. Numbers are related to the EO adducts in the molecule. The f i t peak is the excess of derivatizing agent.
Kinetic runs were made using 1-dodecanolas fatty alcohol and KOH as catalyst, by changing the ethylene oxide preeaure, the temperature, and the catalyst concentration. Samples of the reaction mixtures were taken at different times, derivatized with 3,6-dinitrobenzoyl chloride as suggested by Desbene et al. (19871, and analyzed by the HPLC technique. The elution gradient technique was used with solvent A (99/1 (v/v) = n-heptane/CHzC12-2propanol (95/6)) and solvent B (60/50 (v/v) = n-heptane/CH2C12-2-propanol (95/5)). The sample was injected on a 26- X 0.4-cm column of Lichrospher 100 Diol furnished by the Merck Co. The solvent feed rate was l cm3/minand the sample was eluted with a gradient of the following type: B was increased from 0 to 100% during the fmt 60 min, then only B was fed for 15 min, and from 65 to 80 min B was decreased from 100 to 0%. The UV detector was kept at 254 nm. The different oligomers were recognized by injecting standard monodispersed samples furnished from the Nikkol Chem. Co. and by observing a proportionality between retention times and number of ethylene oxide adducts. Figure 2 shows an example of the chromatograms obtained. Some kinetic runs,performed in a completely different reactor, a ’Pressindustria type”reactor, 10-L volume, have also been interpreted. In this case the liquid was sprayed into the ethylene oxide atmosphere as shown in Figure 3. Finally, also kinetic runs performed by Satkowski and Hsu (1957) were interpreted for comparison with the kinetic model we suggest. As mentioned, for accurate kinetic evaluation it is important to know how the density of the reaction mixture
and the ethylene oxide solubility change with the extent of reaction. For this purpose, the density of 1-dodecanol polyethoxylate was measured with a pycnometer at different mean molecular weights and temperatures. Solubility runs have been performed in the same reactor of Figure 1using, respectively, pure dodecanol and polyethoxylated mixtures treated several times with ion-exchange resins of the Amberlite IRC 76 type,i.e., resins with moderate acidity, to eliminate any trace of the potassium catalyst. Runs have been performed as already described by Santacesaria et al. (1990). Reagents used were of the maximum purity commercially available: 1-dodecanol was furnished by Aldrich Co., ethylene oxide by the SIAD Co., potassium hydroxide by the Prolabo Co., and Amberlite resins by the Rohm and Haaa Co. The jacketed reactor was built by INOX-Impianti Co. Reaction Pattern and Kinetic Model. Santacesaria et al. (1990) showed that, in the ethoxylation of nonylphenol, the acidity of nonylphenol has a strong influence on both the rate of the initiation step and the equilibrium of proton transfer. As a consequence thereof, reaction ratea are initially slow, until all the nonylphenol has reacted, and then reaction rates increase as shown in Figure 4. In the case of 1-dodecanol, these effects are absent; therefore, it is reasonable to assume the same reaction constant for both the initiation and propagation step. Then, considering the same acidity for 1-dodecanol and the ethoxylatad oligomers, as suggested by Nagase and Sakaguchi, (1961), Stockburger and Brandner, (1963), and Lowe and Weibull, (19541, the proton-transfer equilibrium RO- + RO(E0)iH ROH + RO(EO)[ (1) should have a constant equal to about 1. With this last assumption it is impossible to describe correctly the oligomer distribution. In particular, it is impossible to extend the model to the description of the
Ind. Eng. Chem. Res., Vol. 31, No. 11,1992 2415
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Figure S. Density aa a function of temperature for different average number of EO adducts. Number of adducts: (0) 0, ( 0 )2.13, (A) 3.35,( 0 )4.31,( 0 )= 16.44.
narrow-range ethoxylation when cesium or alkaline earths metals are used as catalysts. It must be concluded that the type of cation in the catalyst influences the oligomer distribution and is, therefore, involved in the reaction mechanism. By considering the ability of polyethoxylated surfactants to give crown ether complexes at low temperature, as described by Chan et al. (1970), Bailey et al. (19761, and Balasubramanian and Chandani (1983), it is reasonable to assume that, at the reaction temperature, interactions between the ether groups and metal are still possible and affect above all the proton-transfer equilibrium. However, thiseffect is too weak, compared with that of anion nucleophilicity, at the reaction temperatures, to influence also kinetic constants. Equilibrium 1becomes RO-M+ + RO(E0)iH s ROH + RO(EO);M+ (2) and the equilibrium constant could be different from 1and different for each oligomer. Therefore, the reaction scheme becomes the following one: RO-M+ + EO
2RO(EOI1-M+
RO(EO)
