Ind. Eng. Chem. Res. 1992,31, 2419-2421
2419
Kinetics and Mechanisms of Fatty Alcohol Polyethoxylation. 2. Narrow-Range Ethoxylation Obtained with Barium Catalysts E. Santacesaria,* M. Di Serio, R. Garaffa, and G. Addino Cattedra di Chimica Industriale, Dipartimento di Chimica dell'llniversitd, Via Mezzocannone, 4, 80134 Napoli, Italy
Dodecanol ethoxylation has been studied in the presence of a barium catalyst. An induction period
has been observed in all the runs performed, probably because of the formation of barium alcoholate as the true catalyst. Barium catalysts give rise to a narrow-range molecular weight distribution of ethoxylated oligomers, compared with the potassium catalysts. This behavior can be well described with about the same kinetic model applied in a previous paper to describe the reaction catalyzed by KOH and with about the same kinetic parameters. The narrow range distribution can be described, however, only by assuming different values for the proton-exchange equilibrium between the alcohol and the ethoxylated oligomers in the two ca~es.For KOH, a unique equilibrium constant of 4.8 can be assumed for all the proton-exchange reactions, while for barium different decreasing equilibrium constants from 5 to 2.05 for the first four adducts must be taken to reproduce the narrow range distribution. On the basis of these findings, suggestions are given on the reaction mechanism.
Introduction Many catalysts useful for promoting narrow-range ethoxylation in the production of nonionic surfactants have been described recently (seefor example Yang, 1980; Yang et al., 1980a,b; Brace et al., 1988, 1989). Narrow-range ethoxylation is industrially interesting as it strongly reduces the amount of unreadsd substrate and of oligomers with a small number of adduds not useful as surfactants because they are insoluble in water. Narrow range distributions can easily be achieved with acidic catalyst, but this has not become common practice because acid catalysis gives rise to undesired and toxic products. Alternatively, alkaline earth metals can be used as catalysts with good results. In spite of the large number of patents devoted to the argument, very few papers discuss the mechanietic aspects of the narrow-range ethoxylation (see Matheson et al., 1986) while no paper considers the kinetic aspects. In a previous paper (Santacesaria et al., 1990) we described a general kinetic model able to represent ethoxylation in many of its aspects, that is, ethylene oxide consumption and oligomer distributions for different pressures, temperatures, substrate acidity, catalyst type, and concentration. In another paper (Santacesaria et al., 1992) we applied an extended kinetic model to the ethoxylation of 1-dodecanol in the presence of KOH as catalyst. In this paper the same kinetic model will be used for the ethoxylation of 1-dodecanol in the presence of barium catdysta. These runs showed an induction period which was probably necessary for the formation "in situ" of the catalyst. This phenomenon has been investigated in this paper and qualitatively interpreted. Experimental Section Apparatus, Techniques, and Reagents. All the techniques used have been described in details in the preceding paper (Santacesaria et al., 1992). Barium catalyst was usually introduced as barium dodecanoate (CllH23C00)2Ba. In this form, barium is readily and completely soluble in the reaction mixture and gives reproducibility of the kinetic results.
* To whom correspondence should be addressed.
Table I. List of Kinetic Runs Performed and of Corresponding Owrating Conditions
total dodecanol (CllH,COO)zBa P ~ o T carboxylic run (mol) (mol) (atm) ("C) acid (mol) 1 1.796 0.00653 4 180.0 0.01306 2 1.796 0.00685 4 150.0 0.01370 3 1.796 0.0091 4 128.0 0.01810 4 1.796 0.00685 2 150.0 0.01370 5 1.796 0.0175 4 150.0 0.03504 0.00685 6 1.796 4 150.0 0.03467 Table 11. Equilibrium Constants and Kinetic Parameters Applied in the Model 5.00 3.95 K;; 2.05 In A (preexponential factor) (cms equiv-l 8-l) 24 1 activation energy (cal/equiv) 16456 438 1"
Oligomers have been analyzed with the HPLC technique
as described elsewhere (Santacesaria et al., 1992). The density of reaction mixtures and the solubility of ethylene oxide in these mixtures have been determined, also as described by Santacesaria et al. (1992). Kinetic Runs and Discussion. The operating conditions of the kinetic runs performed are summarized in Table I. As can be seen, runs have been made at different temperatures, pressures, and catalyst concentrations. In Figure 1ethylene oxide consumptions are reported, as a function of time, for the different cases considered in Table I. An induction period can be observed in all the cases, in particular at low temperatures. If we do not consider the induction period, it is possible to reproduce the described runs with the same model employed for KOH Catalyst, in a previous paper (see Santaceearia et al., 1992) and with about the same kinetic parameters provided that we take different values for the proton-transfer equilibrium constants related to different oligomers. The optimized values for these constants are reported in Table 11. These constants are practically independent of temperature. The same table also reports the kinetic parameters applied in the simulations of the kinetic runs. Barium concentration has been expressed as equivalents/ cm3. Figure 2 shows experimental and calculated values of ethylene oxide consumption related to the runs 1,2, and
0888-6885/92/2631-2419$03.00/00 1992 American Chemical Society
2420 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992
I l h 120'
-
E
40
st!%-
00
0
4 0
4
8 0 4 8 Number of adducts
4
8
2
Figure 4. Simulation of the oligomer distribution at different reaction times in the case of run 2. 0
80
160
240
Time (minl
Figure 1. Ethylene oxide consumption as a function of time for different operating conditions (see Table I).
Number of adducts
Figure 5. Simulation of the oligomer distribution at different reaction times in the case of run 3.
teraction of the alcohol anion. For potassium we can write the equilibria RO-K+ + RO(E0)iH e ROH + RO(EO);K+ (1) less stable more stable Similarly in the case of barium we have RO-BaX+ + RO(E0)iH F? ROH + [RO(EO)i]-BaX+ less stable more stable (2) Time ( m i n )
Figure 2. Simulation of ethylene oxide consumption as function of time for runa 1,2,and 3, by considering only the pseudo-eteady-state conditions. Dotted lines correspond to the induction period that has been not simulated.
::EL, 60
-E s
30'
20
0
0
4
LLLJ
0
4
0 4 8 12 1L 0 Number of adducts
4
8.~ 1 2 0
4 8 12 Number of -adducts adducts
8
1
8
2
12
16
Figure 3. Simulation of the oligomer distribution at different reaction times in the case of run 1.
3. Figures 3-5 report experimentaland calculated oligomer distributions, at different times, for the same kinetic runs. Narrower distributions obtained in this case, with respect to KOH catalyst, are the consequence of decreasing the proton-transfer equilibrium constanta from KO,to Koi(with i 2 4). In fact, KO,has about the same value in both cases, while Koiis smaller in the case of barium (2.05instead of 4.8). The interaction of the ethoxylated chain anion with the cation is reasonably greater, in all cases, than the in-
where X can be RO- or RO(E0); with j = 1, n. The equilibrium constant for i = 1is about the same in both cases; then, it remains at the same level, independently of the chain length for potassium, while it decreases for barium until i = 4. An attempt to explain these behaviors will be discussed later. As for the induction period, it must be pointed out that a rough proportionality exists between the length of the period and the amount of the acid introduced (see curves 2 and 6 of Figure 1). That is, to dissolve barium easily and completely in the organic phase, it is necessary to prepare a carboxylated salt, because preparation of an alcoholate needs time and is hardly complete. Induction time is mainly the time necessary to ethoxylate the carboxylic anion coupled with barium. Consequently, reaction rates gradually increase with the progressive ethoxylation of the carboxylic anion. When different amounts of free carboxylic acids greater than the stoichiometric one are added to the reaction mixture, induction times increase proportionately. The reaction starts only when the whole acid has reacted, because ethoxylated acid is readily displaced by free acid. The presence of water can create induction delay by a similar mechanism as can oxygen which gives hydroperoxides by autoxidation. As the effect of the acid concentration is very strong in determining the induction time, a quantitative description of the phenomenon requires knowing exactly the kinetics of the acid anion ethoxylation for both the species Ba(RC00)2and Ba(RO)(RCOO)as well as the interference that the carboxylic species could have with the protontransfer equilibria. More investigations are necessary to describe quantitatively this phenomenon which is crucial for the optimization of the catalyst formulation. Conclusions First of all we confirmed that narrow-range ethoxylation of fatty alcohols promoted by alkaline earth metal catalpta
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2421
0
2
4
6 8 10 12 Number of adducts
14
16
In the case of barium, we must use at least four different equilibrium constants for a satisfactory simulation of the oligomer distribution. The first constant is 5, which is about the same value as for potassium, then we have decreasing values until Ko4= Koi = 2.05. This low value of the proton-transfer equilibrium constant is responsible for the narrow range distribution compared to potassium. The different behavior of barium compared to potassium can be explained by assuming a tight structure for the potassium ion pairs. As a consequence, potassium can interact only with the last uncharged oxygen before the negative charge of the anion, giving rise to complexes of the same stability as the following:
Figure 6. Comparison of the oligomer distributions for KOH and barium dodecanoate at 180 OC and 4 atm for a ratio EO/ROH = 7. Points are experimental while lines are simulated. Data are related tom 1. 12
0
2
4
6
8 10 12 14 16 Number of adducts
18
20
22
Figure 7. Comparison of the oligomer distributions for KOH and barium dodecanoate at 180 OC and 4 atm for a ratio EO/ROH = 10.9. The Poisson distribution and distribution obtained with nonylphenol and KOH are also reported in the plot for comparison purposes. Points are experimental while lines are simulated. Data are related in run 1.
has not a kinetic origin but is related to the stability of the ethoxylate ion pair with respect to the alcoholate ion pair. Ale0 the different behavior of alkali and alkaline earth metal catalysta can mainly be explained on the basis of ion pair stabilities. In fact, in the case of fatty alcohol ethoxylation in the presence of potassium and barium, respectively, we observed about the same kinetic parameters but different proton-transfer equilibrium constants and consequently different oligomer distributions (see, for example, Figures 6 and 7). In Figure 6 a comparison of the oligomer distributions for respectively KOH and barium dodecanoate at 180 OC and 4 atm is reported for a ratio EO/ROH equal to 7, while in Figure 7 the same comparison is reported for a ratio EO/ROH equal to 10.9. In the same figurea are also shown the Poisson distribution and the distribution obtained, under the same conditions, by employing nonylphenol as substrate and KOH as catalyst. As can be seen, the acidity of the substrate produces narrow range distribution. In the other casea we have large differences between the behavior of potassium and barium catalpta. In the case of potassium we observed that a unique value, independent of the temperature, of about 4.8 for the proton-transfer equilibrium between the alcoholate anion and all the oligomers is sufficient to well reproduce the observed oligomer distribution.
However, the second charge of barium may interact with more than one oxygen, giving rise to complexes different from (11); that is, barium cation ionic pairs shown in equilibria 2 may be more solvated than the corresponding potassium ionic couples. The observation that all the proton-transfer equilibrium constants are poorly dependent on the temperature suggests that differences are not energetic but conformational. When a carboxylic acid is present in the reaction mixture, the proton-transfer equilibrium RCOO- Ba2+ROEO-+ RCOOH e (RC00-)2Ba2++ ROEOH is completely shifted to the right. Therefore, in this case, the ethoxylation occurs only when the whole acid has been ethoxylated. This behavior is similar to the one already described in a previous paper (Santacesaria et al., 1990) for the ethoxylation of nonylphenol and explains the presence of an induction time in the ethoxylation of fatty alcohol with (CllH,C00)2Ba as catalyst. Registry No. CH3(CH2)110H, 112-53-8;(CllH29C00)2Ba, 4696-57-5;KOH,1310-58-3;ethylene oxide, 75-21-8.
Literature Cited Brace, E. L.; Shannon, M. L.; Wharry, D. L. U S . Patent (to Vista Chem. Co.), 4,775,653,1988. Brace, E. L.,; Shannon, M. L.; Wharry, D. L. US. Patent (to Vista Chem. Co.), 4,835,321,1989. Matheson, K. L.; Matson, T. P.; Yang, K. JAOCS, J . Am. Oil Chem. SOC. 1986,63, 3. Santacesaria, E.; Di Serio,. M.;. Lisi,. L.;. Gelosa, D. Znd. Eng. - Chem. Res. 1990,.29,-719. Santaceaaria. E.: Di Serio. M.: Garaffa. R.:Addino. G. Ind. E M . Chem. Res. 1992, preceding'paper in'thi issue. ' Yang, K. US. Patent (to Conoco Inc.), 4,239,917,1980. Yang, K.; Nield, G. L.; Washceheck, P. H. US.Patent (to Conoco Inc.), 4,210,764,1980a. Yang, K.;Nield, G. L.; Washceheck, P. H. U.S.Patent (to Conoco Inc.), 4,223,164,1980b.
-
Receiued for reuiew June 30, 1992 Accepted July 9,1992
