Kinetics of Alkaline-Catalyzed Phenol-Forrnaldehvde Reaction J
AMMONIA L. M. DEBING, G. E. MURRAY, AND R. J. SCHATZ Monsanto Chemical Co., Springfield, Mass.
A
LTHOUGH the reaction between phenols and formaldehyde is of great importance in the manufacture of synthetic resins, there have been very few quantitative studies of the reaction kinetics published. There have, of course, been numerous empirical approaches to the problem. Among them may be mentioned the work of Novak and Cech (14), Stager (167, Granger (6),and particularly that of Holmes, Megson, and Paisley (5,9-12). Attempts to follow the course of the reaction have been by such methods as following the decrease in bromine value ( 1 4 ) ; the time required to yield ‘‘a compact, granular or powdery form of precipitate on neutralization” ( 5 ) ; and the time required for the development of a permanent cloudiness in mixtures of various phenols and paraformaldehyde (6, 9-12). These studies were largely concerned with the determination of resinification end points and thus were not adaptable to the classification of reaction rate.
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direct measurement of the uncombined formaldehyde content of the reaction mixture vs. time. EXPERIMENTAL
Although rate studies are usually made in dilute solutions, it was decided to use concentrations that would more closely a p proximate those of industrial importance. Consequently, unless otherwise noted, the experiments discussed below used a concentration of 5 moles of phenol and 5 moles of formaldehyde per kilogram of reaction mixture. Catalyst concentrations are also expressed in moles per kilogram of reaction mixture. The reactions were carried out in an ordinary 2-liter, 3-necked flask, equipped with a condenser, stirrer, thermometer, and a vacuum sampling device. The flask was heated by a Glas-Col mantle controlled by a Variac and the reaction temperature was maintained within 4 ~ 0 . 5C. ~ by heating or, if necessary during the initial stages, by cooling. The phenol, formaldehyde, and part of the water were weighed into the flask and the mixture was brought to the desired reaction temperature. The catalyst was dissolved in the rest of the water and also heated to the reaction temperature. The catalyst solution was then added to the reaction mixture and zero time taken as the time of addition. Samples were removed a t intervals by the vacuum sampling device and analyzed for free formaldehyde by the hydroxylamine hydrochloride method (16). The reaction mixture generally se arated into two phases before the reaction was completed. d w e v e r measurements were continued until the reaction had proceeded to the extent that representative samples could no longer be obtained.
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t
/N M I N U T E S
Figure 1.
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First-Order Rate Plots Using 0.05 Mole Hexa at Various Temperatures
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Standard charge, 5 moles of phenol and 5 moles of formaldehyde per kilogram of aqueous reaction mixture
On the qdantitative side can be mentioned the work of Sprung
(16),who made an accurate study of the reaction kinetics of various phenols with paraformaldehyde in the absence of water, using
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triethanolamine as the catalyst. Under these conditions the reaction was found to be fist order. Euler and Kispeczy (3) studied the reaction of xylenols and formaldehyde and found it to be approximately second order. Nordlander (IS)studied the ammonia-catalyzed reaction and, although this work was never published, the abstract indicates the reaction to be of &st order. Jones (7)made a thorough study of the acid-catalyzed reaction which he found to be second order. He also did some work on the base-catalyzed reaction which he believed to be first order. He did not state what catalyst was used on the alkaline si2e and this will be shown to be a controlling factor. Inasmuch rn the current literature deals primarily with resinification studies-i.e., condensation stage-f the alkaline-catalyzed reactions between phenol and formaldehyde, it was decided to initiate research on the kinetics of the early stages of this reaction-i.e., addition of formaldehyde to phenol-as followed by
Figure 2.
/eo
240
300
M i N u r c s Ar 80°C.
Fir2t-Order Plots Using Various Concentrations of Hexa
The catalyst used in these ex eriments was hexamethylenetetramine rather than ammonia, Eecause of the greater ease and accuracy of handling (1,2). The percentage of free formaldehyde was plotted against time and the best smooth curve drawn through the measured Doints. For the rate calculations, formaldehyde-values were read from the smooth curve rather than using the actual values obtained. The variables studied kinetically are temperature, Figure 1; catalyst concentration, Figure 2; and the ratio of phenol to formaldehyde, Figure 3. REAGENTS.The raw materials used in the experimental work were Baker’s C.P. Analyzed or the Eastman Kodak Co. White Label chemicals, except for the phenol and formaldehyde. The formalin was the 37% methanol-free type, meaning less than 354
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1952
3% methanol, and the phenol was Monsanto's commercial grade, distilling from 180' to 182" C. DISCUSSION
In this work, a series of experiments were run using hexamethylenetetramine concentrations of from 0.025 to 0.15 mole per kilogram of aqueous reaction mixture and temperatures from 60" to 100"C. The rgsults obtained by testing the formaldehyde consumption rate data for conformity to the fist-order reaction rate are shown in Figures 1, 2, and 3. The reaction is first order in agreement with the abstract of Nordlander's work ( l a ) and Yanagita's work (17).
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TABLEI. FIRST-ORDER CONSTANTSFOR HEXA-CATALYZED REACTION First-Order Constantsd Temp., O C. 0.026b 0.0375b 0.0501, 0.1506 100 2.89 3.89 00 2.18 80 0.82 1.28 3.09 0 First-order constants are given as k (in seo. -1) X lo'. 1, Catalyst concentration in moles of hexa per kilogram of reaotion mixture
calculated E as 17,000 calories per mole. When IC is plotted versus the catalyst concentration and the data from this plot are combined with the temperature coefficient] the following equation is obtained relating the rate constant to the catalyst concentration and the temperature T - 100 (3) k = (0.00688C - 0.00004)(1.98) -10 where C is the catalyst concentration in moles per kilogram of reaction mixture and T is the temperature in o C. PH EFFECTS.The pH of the hexa-catalyzed reaction increases slightly, at least up to the point a t which two-phase separation occurs at room temperature.
Figure 3. First-Order Plots of Two Phenol-Formaldehyde Ratios with 0.05 Mole Hexa The hexa-catalyzed reaction was found to exhibit an incubation period, the length of which depends on the catalyst concentration and reaction temperature, as is shown in Figure 4.
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Figure 5.
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Various coneentrations of hexa (moles per kilogram) used at
Cloud Point us. Moles of Formaldehyde Reacted 1000 c.
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eo
0
LO
Figure 4.
Incubation Period a t Low Temperatures and Catalyst Concentrations
Table I gives a few typical fist-order constants obtained by inserting the slope values from the first-order plots in the equation
k =
log t 2.303
a
2 . z
2.303 X slope
(8
+
or
E = -2.303B X slope and E is fou.id to be 15,400 calories per mole.
PO
80'C.
o
0.1 0.2 0.3 0.4 0.1 0 6 0.7 M O L C . 5 fOffMULDCHYOC R C A C T C D
Figure 6.
From the above data, the temperature coefficient can be found to be approximately 1.98 per 10' C. By plotting log k versus the reciprocal of the absolute temperature a straight line is obtained. From the slope of this line, the activation energy, E , can be calculated from the Arrhenius equations log IC =
40
P
-
Yanagita (17)
a8 0.9 MOLEh
€ N O A
Cloud Point vs. Moles of Formaldehyde Re'acted
0.05 mole of hexa per kilogram used at various temperatures
CLOUD POINT. When samples of reaction mixtures containing hexa as the catalyst were withdrawn from the main batch, they separated into two phases on cooling. As the reaction continued the temperature a t separation became steadily higher. This phenomenon seemed t o be analogous to the eritical solution temperature and, since this temperature is a valuable analytical tool in many cases, this phenomenon] which was designated as the cloud point, was investigated more thoroughly. While this work was in progress] an article appeared describing this phenomenon (4). The article by Finn and Rogers ( 4 )describing cloud point is an
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excellent study. The major part of their report has been confirmed in this laboratory but little has been added. A small contribution has been made, however, by plotting mole ratios of formaldehyde consumed per mole of phenol a t various catalyst concentrations (Figure 5) and a t various temperatures (Figure 6). These curves also illustrate the influence of a higher temperature-up to 100" C.-a greater catalyst concentration, and a lower methanol content for the reaction mixture. The sharp change in rate of cloud point temperature with reaction temperature and catalyst concentration is better illustrated by this work, and in Figure 5 it can be seen that when catalyst content and temperature are both high, a straight line is encountered a short way from the origin-0.25 mole hexa a t 100' C .
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The cloud point of resin may be considered as a measure of the degree of condensation occurring when standard conditions, such as temperature, concentration of reactants, catalyst concentration, etc., are maintained. SUMMARY
Studies of reaction rates of hem-catalyzed phenol-formaldehyde reactions have been made and found to follow first-order behavior. Reaction rate constants have been determined a t different catalyst concentrations and temperatures. An independent discovery of a previously reported phenomenon called cloud point has been described along with additional contributions made by this investigation.
(Kinetics of Alkaline-Catalyzed Phenol-Formaldehyde Reactidn)
SODIUM HYDROXIDE L. bl. DEBING, G. E. MURRAY, AND R. J. SCHATZ
T
HIS investigation was originally intended to encompays a wider range of qhenol-formaldehyde ratios, temperature, and catalyst concentrations. Unfortunately, this project has been temporarily terminated before all of the original objectives could be achieved. It was decided, however, to publish the somewhat incomplete results obtained to date rather than to await completion a t an undetermined time.
reaction mixture and reaction temperatures of 60' to 100" C. Figures 7 and 8 show some of the results obtained when part of these data were tested for conformity to the first- and second-order equations by plotting, respectively, log a / a -x us. time and x / a - x us. time. It is at once apparent that the reaction does not follow eit,her order but appears to be somewhere between them, Upon consideration, it was realized that while the reaction is gendally written as
EXPERIMENTAL
The details of the experimental work are the same as those usen with the hexamethylenetetramine catalyst, described in the preceding section, with the exception that sodium hydroxide is the catalyst. I n this work, there were no phase separations during the rate studies and it was observed that the p H of the reaction mixtures dropped rapidly and finally became essentially constant --e.g., from a p H of 8.1 to 7 . 5 (0.05 mole of sodium hydroxide at 100" C . and a 1 t o 1 mole ratio).
OH I
OH
I
and in previous work the molar concentratlions of phenol and formaldehyde have been used for rate calculations, this is an oversimplification. Phenol has three positions-namely, two ortho and one para-that can react with formaldehyde, and since formaldehyde exists in solution as methylene glycol-CH~( 0H)z(8)it has two active positions that can react with phenol. The reaction can, therefore, be more truly represented as
--b OH
+ CHz(OH), ---+ :&H20H
-
Figure 7.
_ _ -_-- _ - "
-
0.0/,?5
+ HzO
-
I
I
Illustration of Nonconformity to First-Order Reaction
Various concentrations (moles per kilogram) of sodium hydroxide
The variables studied kinetically are temperature, catalyst concentration, the influence of amines as catalysts, and various mole ratios of phenol-formaldehyde, In all expressions of phenol-formalaehyde mole ratios, phenol is expressed first as unity, and formaldehyde seeondly as the prescribed mole ratio.
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DISCUSSION
A seriea of experiments was made using catalyst concentrations of 0.0125 to 0.05 mole of sodium hydroxide per kilogram of
60
/PO
/EO 240 200 /NMINUT.5S
7iHE
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Figure 8. Illustration of Nonconformity to Second-Order Rate a t Various Temperatures 0.050 mole of s o d i u m hydroxide per kilogram
(5)
