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Determination of kinetic parameters for the reaction between phenol and formaldehyde by differential scanning calorimetry
European Polymer Journal. Vol. 10, pp. 273 Io 278. Pergamon Press 1974. Printed in Great Britain
DETERMINATION OF KINETIC PARAMETERS FOR THE REACTION BETWEEN P H E N O L A N D FORMALDEHYDE BY DIFFERENTIAL S C A N N I N G CALORIMETRY A. SEBENIK, I. VIZOVI~;EKa n d S. LAPANJE* Department of Chemistry, University of Ljubljana, and Chemical Institute B. Kidri~, Ljubljana, Yugoslavia (Received 6 June 1973) Abstract--The kinetic parameters of the complex reaction between phenol and formaldehyde in the presence of sodium hydroxide (NaOH) have been obtained by differential scanning calorimetry (DSC). The two dominant reactions appear to be addition of formaldehyde to phenol with formation of o-hydroxymethyl-phenol and subsequent condensation of the latter. For both reactions, the activation energy (E,), reaction order and rate constants at different temperatures have been determined. E° for addition changes from 23.7 to 193 kcal mole -~ and for condensation from 22.9 to 19.1 kcal mole -1 when the amount of NaOH is increased from 0-25 to 1-00 per cent. The reaction order for addition is 2 and for condensation 1. Thus DSC appears useful for studying the kinetics of more complex polymerization reactions.
Methods. From phenol, paraformaldehyde and solid NaOH, mixtures (with a constant 1 : 1 phenol-formaldehyde ratio) were prepared so that the amount of NaOH was changed from zero to 1 per cent. A differential scanning calorimeter DSC-1B (Perkin-Elmer) was used. Each mixture was weighed directly into the stainless steel pan and closed with a lid. The lower part of the lid was lined with a Teflon gasket. The pan design was similar to that described by Freeberg and Alleman [8]. Each pan was checked for tightness under experimental condftions; the reference pan was always empty, the amount of mixture (121-4-62. 13 mg) in the reaction cell being less than 1 per cent of the We report here the results of studies by D S C aimed pan weight. Using individual cell pairs, straight baselines at obtaining kinetic parameters for the reaction were obtained. Thermograms, i.e. curves giving heat evolubetween phenol a n d formaldehyde which eventually tion as a function of temperature, were recorded at a range leads to insoluble a n d infusible resins, i.e. cross-linked setting of 8 mcal/sec and a heating rate of 4°/min starting at networks. For this type of reaction, D S C a p p e a r s espe305 K and ending when the reaction was over. i.e. when the cially appropriate, considering the paucity of useful recorded curve turned into a straight line representing conmethods for studying such reactions. T h o u g h the exact tinuation of the baseline at the beginning of the experiment. mechanisms of the reaction between phenol a n d forEach experiment was repeated at least twice. The mean S.D. maldehyde are n o t fully established yet, it appears that isestimated at + 3 per cent. The heating rate of 4°/min proved most suitable in preliminary experiments. At the heating rate two reactions are dominating: addition and condensaof 2°/min thermograms were spread out, and peaks were tion. Thus data obtained in calorimetric m e a s u r e m e n t s poorly expressed and also shifted a few degrees towards can be subjected to a theoretical analysis noting that lower temperatures. However, the DSC results for both simplified chemistry a n d a s s u m p t i o n s underlying the heating rates were the same within the limits of experanalysis will contribute to uncertainties in the results imental error. At heating rates of 8°/min and higher, therobtained. mograms became more and more compressed and thus their evaluation more difficult. Moreover, peaks were shifting towards higher temperatures which apparently reflected the EXPERIMENTAL thermal lag due to high heating rates. Calibration of the calorimeter was performed by deterMaterials. Phenol and paraformaldehyde were pro analysi mining the heats of fusion for benzoic acid and tin. The products from Merck AG and Riedel-de-Hat"n, respectively. values found were within 1 per cent of the literature values. All other reagents were of analytical grade, o-HydroxyThin-layer chromatography was used to identify intermemethyl-phenol was prepared by the method of Seto and diates in the reaction. The ascending method with Silica Gel Horiuchi [7]; the product was repeatedly recrystallized G as adsorbent was used. Samples obtained by stopping from benzene (melting point was 85°). reaction in the calorimeter at a given temperature were dissolved in a 4:1 benzene-dioxan mixture and then applied on * To whom correspondence should be addressed. Silica Gel G. The plates were developed in well sealed glass 273
With the advent of commercial differential scanning calorimeters, the determination of b o t h t h e r m o d y n a mic a n d kinetic parameters for chemical reactions by measuring accompanying heat effects is likely to become quite popular [ 1-3]. Differential scanning calorimetry (DSC) lends itself very well also to the study of various polymerization reactions as can be inferred from the increasing n u m b e r of publications dealing with applications.
A. ~EBENIK.I. VIZOVI~EKand S. LAPANJE
274
320
340
360
380
400
420
44.0
Temperoture,
460
4R0
500
K
Fig. I. Thermograms of the reaction between phenol and formaldehyde. 1 (
) no catalyst added; 2 ( - - - - ) , 0.25 per cent NaOH; 3 ( - - - ) , 0-50 per cent NaOH; 4 (. . . . ), 0.75 per cent NaOH; 5 (. . . . . . ), 1 per cent NaOH ; 6 ( ), condensation of o-hydroxymethyl-phenol.
tanks saturated with solvent vapour of the mobile phase (benzene-dioxan 4: 1). Chromatograms were then exposed to iodine vapours (detection). Infra-red spectra were recorded with a Perkin-Elmer M 21 spectrophotometer. Samples obtained by stopping reaction at different temperatures, cf. below, were applied in a thin layer on a sodium chloride plate, except the one at the end of reaction which was ground and mixed with KBr. RESULTS AND DISCUSSION
Chemistry of the reaction. Before an evaluation of the recorded thermograms (Fig. 1) is attempted in order to obtain kinetic parameters, it is necessary to ascertain which reactions actually occur. The first clue in this connection is provided by the form of thermograms. With catalyst added, all curves have two more or less clearly expressed peaks with a dip between them. Thus it appears a plausible assumption that only two major or dominating reactions are involved, and that each curve is composed of two individual ones with overlapping final and initial parts, To identify the main product of the first reaction, thin-layer chromatography was used. In addition to two samples obtained by stopping reaction at 360 K, which is the estimated temperature of completion for the first reaction (cf. Fig. !) o-hydroxymethyl-phenol and phenol were applied as well. Figure 2 shows a typical chromatogram; it can be inferred that o-hydroxymethyl-phenol appears in the samples also and apparently is the main monomeric compound. This can be considered as proof that o°hydroxymethyl-phenol is the main intermediate and the first reaction is essentially an aldol condensation, i.e. addition of formaldehyde to phenol [9]. This finding is in essential agreement with observations of Freeman and Lewis [10] and Zavitsas and Beaulieu [1 I], that, in the reaction between phenol and formaldehyde, prevailingly hydroxymethyl-phenois are being formed and that the ratio between the ortho- and
para-forms exceeds one. Yeddanapalli and Gopalakrishna [12], on the other hand, found for the above ratio values below one. However, none of the abovementioned authors performed their studies under our experimental conditions, i.e. with pure reactants in 1 : 1 ratio. Therefore little additional information could be obtained by detailed analysis of their results. The second reaction was followed by means of infrared (i.r.) spectroscopy. Reaction in the presence of I per cent N a O H was stopped at 360 K, 400 K, 430 K and at completion. The i.r. spectra of these samples are
I
2
3
4
5
6
7
8
9
I0
0000000000
O,O0ooo
( s0 0@
Fig. 2. Reproduction of the thin-|ayer chromatogram of phenol, 1.3 ; o-hydroxymethyl-phenol 2,4; and of reaction mixture phenol-formaldehyde (reaction stopped at 360 K): 5,6,7, 1 per cent NaOH ; 8,9,10. 0"5 per cent NaOH.
Reaction between phenol and formaldehyde Frequency, 66 7
15
800 I
I000 I
cm
-I
1200 I
2000 I
5000 [ .
I
1
I
I
I
I
I
I
i
I
i
[
14
13
12
II
I0
9
8
F
6
..%
4
3
Wovelength,
#m
Fig. 3. Infra-red spectra of reaction mixture phenol-formaldehyde in the presence of 1 per cent NaOH. Reaction stopped at 360 K. 1; at 400 K. 2 ; at 430 K. 3 ; at completion, 4.
275
Calculation o f kinetic parameters. After establishing the two dominating reactions, it is possible to separate the respective heat contributions. For this purpose, pure o-hydroxymethyl-phenol, prepared as described .above, with 1 per cent N a O H added was allowed to react in the calorimeter. Since this is the main monomeric compound, the heat obtained from the area under the thermogram can be considered equal to the heat of condensation in catalyzed reactions. Therefore the initial and final parts of the thermograms for catalyzed reactions were drawn in such a way as to make the areas pertaining to condensation equal to that of o-hydroxymethyl-phenol. By subtracting this area from the total area, the area pertaining to addition was found and from it, the heat of addition. Since the total areas in catalyzed reactions were equal within a few per cent, this procedure appears meaningful. In Fig. 1, the thermograms recorded in a series of experiments without catalyst and with various amounts of catalyst are shown. For comparison, the thermogram ofo-hydroxymethyl-phenol is included. The thermogram for the reaction without catalyst is different from those with added catalyst. It is shifted towards higher temperatures; addition and condensation run simultaneously most of the time and therefore one peak only appears. The curves for catalyzed reactions are similar but, with increasing amount of catalyst, they are shifted towards lower temperatures. Having obtained the heats for addition and condensation reactions, we can now apply the analysis of Borchardt and Daniels [.15], which was validated by Reed et al. [16]. The analysis leads to an expression, c f below, which gives x and the activation energy from a single graph. The relevant equation is k = (VA/n°)~-I dH/dt
(1)
( A - a)"
shown in Fig. 3. The spectrum lor 360 K shows relatively high absorption at 690cm -1 which can be assigned to the bending vibration of monosubstituted benzenes [13], in our case to unreacted phenol. At 400 K, this peak is considerably lower. Vibration at 1010 cm-~ is attributed to the stretching of - - C - - O H bonds in o-hydroxymethyl-phenol [13]. At higher temperatures, the intensity of this vibration diminishes. The third characteristic vibration, at 7 8 0 c m - L is assigned to out-of-plane C - - H bending in 1,2,4 substituted benzene rings i-14]. This absorption first appears in the second spectrum, 400 K, and shows considerable increase with temperature. The absorption band at 752cm -~ can be attributed to out-of-plane C - - H bending in 1,2 substituted benzene rings [14] : its intensity decreases with increasing temperature. Finally. the absorption band around 820-830cm-~ can be assigned to 1,2 and 1,4 substituted benzene rings and accordingly its intensity is seen to decrease with increasing temperature [14]. From these observations, one can conclude that the curve appertaining to the second peak reflects mainly condensation of hydroxymethyl groups and formation of methylene groups.
where k is the rate constant, A the total area under the curve (mcal)corresponding to the total heat evolved, and a (mcal) to the heat evolved up to any time t, d H / d t is the rate of heat evolution, no the number of moles of reactant present initially, Vthe volume and x the reaction order. We can now proceed with a straightforward evaluation of the activation energy and reaction order for the two reactions. In Eqn. (1) we replace d H / d t at individual temperatures by d, the distance (mcal/sec) between the reaction curve and the baseline [ 17]. k
-
(VA/n°~
ld
(A - aF
(la)
Then by application of the Arrhenius equation In k = In Z - Eal/RT,
(2)
where Z is the frequency factor, E,, the activation energy and Tthe temperature in K, and Eqn. (la), the following equation is obtained In
d2/dl A - a2/A - a I
Ea ( 1 / T 2 - 1/7"1) +x, R ln(A -- a2/A - a l )
(3)
276
A. ~EBENXK, I. V I Z O W ~ K and S. LAPANJE
Eqn. (la), the corresponding rate constants. For condensation we obtain d k = - A -a'
x= -
±
-
. .\i.-~
"%.N
-2
and the computation of k at different temperatures is straightforward. In Fig.6 the rate constant for condensation at 400 K is plotted as a function of added catalyst. For addition, x is 2 and the computation is more circumstan tial. Equation (1a) now becomes
,,
..\
-I
--
x
(4)
tO-*
*'-'--7
(VA/no) d k -
3
Fig. 4. Typical plots from which the activation energy, Ej, and reaction order, x, for addition and condensation reactions were obtained. Slope is - E,/R, ordinate intercept is x. Catalyst 0"5 per cent NaOH. t-l, Addition; O, condensation; A, condensation of o-hydroxymethyl-phenol (1 per cent NaOH).
-
-
(.4 -
(5)
a ) 2"
Since Vis equal to m,,,/p,, where rnm is the mass of the reaction mixture and ,o,, its density, determined pyknometrically, and no, the initial number of moles of phenol, is m l / M l (i.e. its mass divided by its molecular weight) we obtain from Eqn. (5) k = (nlMl + n2M2)A d nlP,,(A _ a) 2 ,
(6)
24
where TI and 7"2 are any two temperatures within the temperature range of reaction, al and a 2 the corresponding areas, and dl and d2 the corresponding distances curve-baseline. By plotting the left-hand side of Eqn. (3) vs (1/T2 - I/TI)/In(A - a : / A - al), the activation energy is obtained from the slope of the resulting straight line; its intersection with the axis of ordinate gives the reaction order. The plots for both reactions in the presence of 0'5 per cent NaOH are presented in Fig. 4. Numerical values of activation energy calculated from these and other plots not given here are assembled in Table 1 (Fig. 5). The plot for condensation ofo-hydroxymethyl-phenol in the presence of I per cent N a O H is also included. The reaction order read off the plots is 2 for addition, and 1 for condensation. A few calculations were also performed by using a relation derived by Rogers and Smith [18] which also gives x and Eo from a single plot, and essentially identical results were obtained. Knowing (he reaction order, we can compute, using
22 T o E
20
o u UJ
18
16
~4
I
I
I
0.25
0.50
0.75
I 1.00
C o n c e n t r a t i o n , % No OH Fig. 5,. Activation energy of addition and condensation reactions as a function of added catalyst. Upper curve, addition; lower curve, condensation.
Table 1. Activation energies of addition and condensation reactions determined directly (from thermograms) and indirectly (from rate constants) NaOH conc (wt %) 0.25 0.50 0.75 1.00
Addition E, (kcal mole- 2) Direct Indirect 23.7 20.6 19.7 19.3
22.9 20-3 20-0 19-1
Condensation E, (kcal mole- 1) Direct Indirect 17.6 16"6 16.3 15.7"
18.0 17.0 16.2 15.5"
* The directly and indirectly determined values of E° for condensation of o-hydroxymethyl-phenol are 22.6 and 22.5 kcal/mole, respectively.
Reaction between phenol and formaldehyde where the subscript 2 refers to formaldehyde. In our case n 1 is equal to n2 and k - (MI + M2) A d p,,,(A -- a) 2
(7)
'
from which k is computed. In Fig. 6, the rate constant for addition at 350 K is given as a function of added catalyst. 4
2
I
I
I
I
0"25
0"50
0-75
I'0
Concen't'rot"ion, %NoOH
Fig. 6. Rate constants of addition and condensation reactions as a function of added catalyst. Upper curve, addition at 350 K ; lower curve, condensation at 400 K. As a check on the calculations, from the rate constants at individual temperatures for constant amount of catalyst, the activation energies for the two reactions were calculated. Agreement of these values with those obtained from Eqn. (3) is within the limits of experimental error, cf. Table 1. From inspection of plots in Fig. 5 and the data in Table 1, the following picture emerges. The activation energy for both reactions, as expected, decreases with increasing amount of catalyst. The slope of the curves in Fig. 5, however, decreases with increasing amount of catalyst which shows that a small amount of catalyst causes a substantial decrease of activation energy. At 0.5 per cent added NaOH, the activation energy for addition is 20.5 kcal/mole and that for condensation 16.6kcal/mole. Zavitsas and Beaulieu [I1] found by using a titration method for the activation energy of addition of o- and p-hydroxymethyl-phenols values between 18.5 and 21.1 kcal/mole, depending on concentration, and Yeddanapalli and Gopalakrishna [12] by applying paper chromatographic analysis 19.8 kcal/mole. Considering different reaction conditions, agreement with our values is quite satisfactory. There are also several comparable data for the activation energy of condensation; Katovi6 [13] found for the activation energy of condensation values between 18.3 and 21.3 kcal/mole by applying differential thermal analysis, thermogravimetric analysis and i.r. spectroscopy. The values of Sprung and Gladstone [19] for condensation of m-cresol and o-hydroxymethyl-phenol, obtained by a titration
277
method, are 15.8 kcal/mole and 18-5 kcal/mole, respectively. The latter value compares favourably with our value 22"6 kcal/mole, Table I. Agreement of our values for condensation with published values can thus be deemed satisfactory. The reaction order 2 for addition appears to be reasonable; it has also been observed by other authors [10-12]. Whatever reactions, in addition to the dominant one, run in parallel, they apparently are either also of second order or their extent is negligible. The value 1 determined for condensation agrees with that found by Katovi6 [13], as well as with the value of Sprung and Gladstone [19] for o-hydroxymethylphenol condensation, which, incidentally, equals our value for the same reaction, cf. Fig. 4. Also, as can be inferred from plots similar to those in Fig. 4, the amount of added catalyst does not influence the reaction order. It is necessary to comment on the relatively large difference in activation energy for condensation in the presence of 1 per cent NaOH, 15.7 (15.5) kcal/mole, and for the condensation of o-hydroxymethyl-phenol also in the presence of 1 per cent NaOH (22.6 and 22.5 kcal/mole, respectively) since both values were obtained by the same method. Does the difference therefore reflect the fact that in the first case one has a mixture of different compounds, though one prevailing. and in the second a pure compound? It is well known [10,20] that the relative reactivities of methyl and methylol substituted phenols depend on the number and positions of substituents. N Osa.tisfactory answer to this question can be given without knowing the composition of the mixture obtained in addition reaction. However, the fact that the heating curve for o-hydroxymethyl-phenol as compared with others, cf. Fig. 1, is shifted towards higher temperatures suggests such an interpretation. In conclusion, it can be said that DSC has proved useful for studying the kinetics of the quite complex reaction between phenol and formaldehyde. The values of kinetic parameters obtained can therefore be considered as fairly reliable. Uncertainties which undoubtedly exist arise not only from the shortcomings of the method or the simplifying assumptions on which the equations have been derived but also from the simplified treatment of chemistry of the reaction. Acknowledgements--We thank Dr. Z. Katovi¢5 for many helpful discussions. We also thank (Mrsj T. Malava~i~ for useful advice regarding the calorimetric measurements and F. Cvek and J. Dolinar for running the i.r. spectra. The study was supported by the Boris Kidri~ Fund.
REFERENCES I. K. E. J. Barrett, J. appl. Polym. Sci. 11, 1617 (1967). 2. J. M. Thomas and T. A. Clarke, J. chem. Soc. A, 457 (1968). 3. G. Beech, J. chem. Soc. A, 1903 (t969).
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A.
~EBENIK,I. VIZOV1~EKand
4. K. E. J. Barrett and H. R. Thomas, J. Polym. Sci. AI, 7, 2621 (1969; K. E. J. Barrett and H. R. Thomas, Br. Polym. J. 2, 45 (1970). 5. K. Horie, J. Mita and H. Kambe, J. Polym. Sci. Al, 6, 2663 0968); 7, 2561 (1969). 6. F. Delben and V. Creseenzi, Annali Chirt 60, 782 (1970). 7. S. Seto and H. Horiuchi, J. chem. Soc. Japan, Ind. chem. Sect. 57, 687 (1954). 8. F. E. Freeberg and T. G. Alleman, Analyt. Chem. 38, 1806(1966). 9. K. Hultzsch, Chemie der Phenolharze. Springer-Vedag, Berlin (1950). 10. J. H. Freeman and C. W. Lewis, J. Am. chert Soc. 76, 2080 (1954). 11. A. A. Zavitsas and R. D. Beaulieu, Art chem. Soc., Div. Org. Coatings Plast, Chem., Preprints_27, I00 (1967).
S. LAPANJE
12. L. M. Yeddanapalli and V. V. Gopalakrishna, Makromolek. Chem. 32, 112, 124, 130 (1959). 13. Z. Katovi6, J. appl. Polym. Sci. 11, 85, 95 (1967). 14. P. J. SecresL J. Paint Technol. 37, 187 (1965). 15. H. J. Borchardt and F. Daniels, J. Am. chem. Soc. 79, 41 (1957). 16. R. L. Reed, L. Weber and B. S. Gottfried, Ind. E n g ~ Chem. Fundamentals 4, 38 (1965). 17. R. N. Rogers and E. D. Morris, Jr., Analyt. Chem. 38, 412 (1966). 18. R. N. Rogers and L. C. Smith, Thermochim. Acta 1, 1 (1970). 19. M. M. Sprung and M. T. Gladstone, J. Art chem. Soc. 71, 2907 (1949). 20. G. R. Sprengling and C. W. Lewis, J. Art chem. Soc. 75, 5709 (1953).
R6sam6--On a obtenu par calorimdtrie diffdrentielle (DSC), les param6tres cindtiques de la rdaction complexe entre le phdnol et la formalddhyde en prdsence d'hydroxyde de sodium (NaOH). Les deux rdactions dominantes semblent &re raddition du formald6hyde sur le ph6nol avec formation d'o-hydroxym6thylphdnol et la condensation ultdrieure de ce dernier. On a ddtermind/~ diffdrentes tem~ratures pour les deux rdactions, l'dnergie d'activation (E,), rordre de la rdaction et les constantes de vitesse. E, pour raddition passe de 23,7-19,3 kcal/mole et pour la condensation de 22.9-19,1 kcal/mole quand la teneur en NaOH augmente de 0,25-1,00 pour cent. L'addition est une rdaction d'ordre 2 et la condensation, d'ordre 1. La DSC apparait utile pour I'dtude cindtique de rdactions de polymdrisation plus complexes. sommario---Mediante calorimetria differenziale a scansione (DSC), si sono ricavati i parametri cinetici della eomplessa reazione tra fenolo e formaldeide in presenza di idrossido di sodio (NaOH). Sembra che le due reazioni predominanti siano quella dell'aggiunta di formaldeide al fenolo con formazione di o-idrossimetil-fenolo e raltre della successiva condensazione di quest'ultima sostanza. Per entrambe le reazioni si sono determinate, a differenti temperature, le constanti di velocitY, ordine di reazione ed energia di attivazione (E,). E, per cambiamenti per aggiunta da 23,7 a 19,3 kcal mole -~ e per condensazione da 22,9 a 19,1 kcal mole - ~ quando la quantit~ di NaOH viene aumentata da 0,25 a 1,00 per cento. L'ordine di reazione per l'aggiunta 6 di 2 e pa la condensazione di 1. Ecco perch6 la DSC sembra utile per 1o studio della cinetica di reazioni di polimerizzazione piti complesse.
Zusammenfassung--Die kinetischen Parameter for die komplexe Reaktion yon Phenol mit Formaldehyd in Gegenwart yon Natriumhydroxid (NaOH) wurden durch Differentialscanningkalorimetrie erhalten (DSC). Danach scheinen die beiden dominierenden Reaktionen zu sein einmal die Anlagerung yon Formaldehyd an Phenol unter Bildung von o-Hydroximethylphenol und zum zweiten die anschleiBende Kondensation der letzteren Verbindung. F/ir beide Reaktionen wurden die Aktivierungsenergien (E,), die Reaktionsordnungen und die Geschwindigkeitskonstanten bei verschiedonen Temperaturen bestimmt. Fiir steigende Konzentration von NaOH von 0,25 bis 1,00 Prozent ~indert sich Eo fiir die Anlagerung yon 23,7 nach 19,3 kcal/Mol und ffir die Kondensation yon 22,9 nach 19,1 kcal/Mol. Die Additionsreaktion verl~iuft nach der 2, die Kondensationsreaktion nach der 1 0 r d n u n g . Damit wird gezeigt, dab DSC eine wertvolle Methode zur Untersuchung der Kinetik yon komplexen Reaktionen ist.
DETERMINATION OF KINETIC PARAMETERS FOR THE REACTION BETWEEN P H E N O L A N D FORMALDEHYDE BY DIFFERENTIAL S C A N N I N G CALORIMETRY A. SEBENIK, I. VIZOVI~;EKa n d S. LAPANJE* Department of Chemistry, University of Ljubljana, and Chemical Institute B. Kidri~, Ljubljana, Yugoslavia (Received 6 June 1973) Abstract--The kinetic parameters of the complex reaction between phenol and formaldehyde in the presence of sodium hydroxide (NaOH) have been obtained by differential scanning calorimetry (DSC). The two dominant reactions appear to be addition of formaldehyde to phenol with formation of o-hydroxymethyl-phenol and subsequent condensation of the latter. For both reactions, the activation energy (E,), reaction order and rate constants at different temperatures have been determined. E° for addition changes from 23.7 to 193 kcal mole -~ and for condensation from 22.9 to 19.1 kcal mole -1 when the amount of NaOH is increased from 0-25 to 1-00 per cent. The reaction order for addition is 2 and for condensation 1. Thus DSC appears useful for studying the kinetics of more complex polymerization reactions.
Methods. From phenol, paraformaldehyde and solid NaOH, mixtures (with a constant 1 : 1 phenol-formaldehyde ratio) were prepared so that the amount of NaOH was changed from zero to 1 per cent. A differential scanning calorimeter DSC-1B (Perkin-Elmer) was used. Each mixture was weighed directly into the stainless steel pan and closed with a lid. The lower part of the lid was lined with a Teflon gasket. The pan design was similar to that described by Freeberg and Alleman [8]. Each pan was checked for tightness under experimental condftions; the reference pan was always empty, the amount of mixture (121-4-62. 13 mg) in the reaction cell being less than 1 per cent of the We report here the results of studies by D S C aimed pan weight. Using individual cell pairs, straight baselines at obtaining kinetic parameters for the reaction were obtained. Thermograms, i.e. curves giving heat evolubetween phenol a n d formaldehyde which eventually tion as a function of temperature, were recorded at a range leads to insoluble a n d infusible resins, i.e. cross-linked setting of 8 mcal/sec and a heating rate of 4°/min starting at networks. For this type of reaction, D S C a p p e a r s espe305 K and ending when the reaction was over. i.e. when the cially appropriate, considering the paucity of useful recorded curve turned into a straight line representing conmethods for studying such reactions. T h o u g h the exact tinuation of the baseline at the beginning of the experiment. mechanisms of the reaction between phenol a n d forEach experiment was repeated at least twice. The mean S.D. maldehyde are n o t fully established yet, it appears that isestimated at + 3 per cent. The heating rate of 4°/min proved most suitable in preliminary experiments. At the heating rate two reactions are dominating: addition and condensaof 2°/min thermograms were spread out, and peaks were tion. Thus data obtained in calorimetric m e a s u r e m e n t s poorly expressed and also shifted a few degrees towards can be subjected to a theoretical analysis noting that lower temperatures. However, the DSC results for both simplified chemistry a n d a s s u m p t i o n s underlying the heating rates were the same within the limits of experanalysis will contribute to uncertainties in the results imental error. At heating rates of 8°/min and higher, therobtained. mograms became more and more compressed and thus their evaluation more difficult. Moreover, peaks were shifting towards higher temperatures which apparently reflected the EXPERIMENTAL thermal lag due to high heating rates. Calibration of the calorimeter was performed by deterMaterials. Phenol and paraformaldehyde were pro analysi mining the heats of fusion for benzoic acid and tin. The products from Merck AG and Riedel-de-Hat"n, respectively. values found were within 1 per cent of the literature values. All other reagents were of analytical grade, o-HydroxyThin-layer chromatography was used to identify intermemethyl-phenol was prepared by the method of Seto and diates in the reaction. The ascending method with Silica Gel Horiuchi [7]; the product was repeatedly recrystallized G as adsorbent was used. Samples obtained by stopping from benzene (melting point was 85°). reaction in the calorimeter at a given temperature were dissolved in a 4:1 benzene-dioxan mixture and then applied on * To whom correspondence should be addressed. Silica Gel G. The plates were developed in well sealed glass 273
With the advent of commercial differential scanning calorimeters, the determination of b o t h t h e r m o d y n a mic a n d kinetic parameters for chemical reactions by measuring accompanying heat effects is likely to become quite popular [ 1-3]. Differential scanning calorimetry (DSC) lends itself very well also to the study of various polymerization reactions as can be inferred from the increasing n u m b e r of publications dealing with applications.
A. ~EBENIK.I. VIZOVI~EKand S. LAPANJE
274
320
340
360
380
400
420
44.0
Temperoture,
460
4R0
500
K
Fig. I. Thermograms of the reaction between phenol and formaldehyde. 1 (
) no catalyst added; 2 ( - - - - ) , 0.25 per cent NaOH; 3 ( - - - ) , 0-50 per cent NaOH; 4 (. . . . ), 0.75 per cent NaOH; 5 (. . . . . . ), 1 per cent NaOH ; 6 ( ), condensation of o-hydroxymethyl-phenol.
tanks saturated with solvent vapour of the mobile phase (benzene-dioxan 4: 1). Chromatograms were then exposed to iodine vapours (detection). Infra-red spectra were recorded with a Perkin-Elmer M 21 spectrophotometer. Samples obtained by stopping reaction at different temperatures, cf. below, were applied in a thin layer on a sodium chloride plate, except the one at the end of reaction which was ground and mixed with KBr. RESULTS AND DISCUSSION
Chemistry of the reaction. Before an evaluation of the recorded thermograms (Fig. 1) is attempted in order to obtain kinetic parameters, it is necessary to ascertain which reactions actually occur. The first clue in this connection is provided by the form of thermograms. With catalyst added, all curves have two more or less clearly expressed peaks with a dip between them. Thus it appears a plausible assumption that only two major or dominating reactions are involved, and that each curve is composed of two individual ones with overlapping final and initial parts, To identify the main product of the first reaction, thin-layer chromatography was used. In addition to two samples obtained by stopping reaction at 360 K, which is the estimated temperature of completion for the first reaction (cf. Fig. !) o-hydroxymethyl-phenol and phenol were applied as well. Figure 2 shows a typical chromatogram; it can be inferred that o-hydroxymethyl-phenol appears in the samples also and apparently is the main monomeric compound. This can be considered as proof that o°hydroxymethyl-phenol is the main intermediate and the first reaction is essentially an aldol condensation, i.e. addition of formaldehyde to phenol [9]. This finding is in essential agreement with observations of Freeman and Lewis [10] and Zavitsas and Beaulieu [1 I], that, in the reaction between phenol and formaldehyde, prevailingly hydroxymethyl-phenois are being formed and that the ratio between the ortho- and
para-forms exceeds one. Yeddanapalli and Gopalakrishna [12], on the other hand, found for the above ratio values below one. However, none of the abovementioned authors performed their studies under our experimental conditions, i.e. with pure reactants in 1 : 1 ratio. Therefore little additional information could be obtained by detailed analysis of their results. The second reaction was followed by means of infrared (i.r.) spectroscopy. Reaction in the presence of I per cent N a O H was stopped at 360 K, 400 K, 430 K and at completion. The i.r. spectra of these samples are
I
2
3
4
5
6
7
8
9
I0
0000000000
O,O0ooo
( s0 0@
Fig. 2. Reproduction of the thin-|ayer chromatogram of phenol, 1.3 ; o-hydroxymethyl-phenol 2,4; and of reaction mixture phenol-formaldehyde (reaction stopped at 360 K): 5,6,7, 1 per cent NaOH ; 8,9,10. 0"5 per cent NaOH.
Reaction between phenol and formaldehyde Frequency, 66 7
15
800 I
I000 I
cm
-I
1200 I
2000 I
5000 [ .
I
1
I
I
I
I
I
I
i
I
i
[
14
13
12
II
I0
9
8
F
6
..%
4
3
Wovelength,
#m
Fig. 3. Infra-red spectra of reaction mixture phenol-formaldehyde in the presence of 1 per cent NaOH. Reaction stopped at 360 K. 1; at 400 K. 2 ; at 430 K. 3 ; at completion, 4.
275
Calculation o f kinetic parameters. After establishing the two dominating reactions, it is possible to separate the respective heat contributions. For this purpose, pure o-hydroxymethyl-phenol, prepared as described .above, with 1 per cent N a O H added was allowed to react in the calorimeter. Since this is the main monomeric compound, the heat obtained from the area under the thermogram can be considered equal to the heat of condensation in catalyzed reactions. Therefore the initial and final parts of the thermograms for catalyzed reactions were drawn in such a way as to make the areas pertaining to condensation equal to that of o-hydroxymethyl-phenol. By subtracting this area from the total area, the area pertaining to addition was found and from it, the heat of addition. Since the total areas in catalyzed reactions were equal within a few per cent, this procedure appears meaningful. In Fig. 1, the thermograms recorded in a series of experiments without catalyst and with various amounts of catalyst are shown. For comparison, the thermogram ofo-hydroxymethyl-phenol is included. The thermogram for the reaction without catalyst is different from those with added catalyst. It is shifted towards higher temperatures; addition and condensation run simultaneously most of the time and therefore one peak only appears. The curves for catalyzed reactions are similar but, with increasing amount of catalyst, they are shifted towards lower temperatures. Having obtained the heats for addition and condensation reactions, we can now apply the analysis of Borchardt and Daniels [.15], which was validated by Reed et al. [16]. The analysis leads to an expression, c f below, which gives x and the activation energy from a single graph. The relevant equation is k = (VA/n°)~-I dH/dt
(1)
( A - a)"
shown in Fig. 3. The spectrum lor 360 K shows relatively high absorption at 690cm -1 which can be assigned to the bending vibration of monosubstituted benzenes [13], in our case to unreacted phenol. At 400 K, this peak is considerably lower. Vibration at 1010 cm-~ is attributed to the stretching of - - C - - O H bonds in o-hydroxymethyl-phenol [13]. At higher temperatures, the intensity of this vibration diminishes. The third characteristic vibration, at 7 8 0 c m - L is assigned to out-of-plane C - - H bending in 1,2,4 substituted benzene rings i-14]. This absorption first appears in the second spectrum, 400 K, and shows considerable increase with temperature. The absorption band at 752cm -~ can be attributed to out-of-plane C - - H bending in 1,2 substituted benzene rings [14] : its intensity decreases with increasing temperature. Finally. the absorption band around 820-830cm-~ can be assigned to 1,2 and 1,4 substituted benzene rings and accordingly its intensity is seen to decrease with increasing temperature [14]. From these observations, one can conclude that the curve appertaining to the second peak reflects mainly condensation of hydroxymethyl groups and formation of methylene groups.
where k is the rate constant, A the total area under the curve (mcal)corresponding to the total heat evolved, and a (mcal) to the heat evolved up to any time t, d H / d t is the rate of heat evolution, no the number of moles of reactant present initially, Vthe volume and x the reaction order. We can now proceed with a straightforward evaluation of the activation energy and reaction order for the two reactions. In Eqn. (1) we replace d H / d t at individual temperatures by d, the distance (mcal/sec) between the reaction curve and the baseline [ 17]. k
-
(VA/n°~
ld
(A - aF
(la)
Then by application of the Arrhenius equation In k = In Z - Eal/RT,
(2)
where Z is the frequency factor, E,, the activation energy and Tthe temperature in K, and Eqn. (la), the following equation is obtained In
d2/dl A - a2/A - a I
Ea ( 1 / T 2 - 1/7"1) +x, R ln(A -- a2/A - a l )
(3)
276
A. ~EBENXK, I. V I Z O W ~ K and S. LAPANJE
Eqn. (la), the corresponding rate constants. For condensation we obtain d k = - A -a'
x= -
±
-
. .\i.-~
"%.N
-2
and the computation of k at different temperatures is straightforward. In Fig.6 the rate constant for condensation at 400 K is plotted as a function of added catalyst. For addition, x is 2 and the computation is more circumstan tial. Equation (1a) now becomes
,,
..\
-I
--
x
(4)
tO-*
*'-'--7
(VA/no) d k -
3
Fig. 4. Typical plots from which the activation energy, Ej, and reaction order, x, for addition and condensation reactions were obtained. Slope is - E,/R, ordinate intercept is x. Catalyst 0"5 per cent NaOH. t-l, Addition; O, condensation; A, condensation of o-hydroxymethyl-phenol (1 per cent NaOH).
-
-
(.4 -
(5)
a ) 2"
Since Vis equal to m,,,/p,, where rnm is the mass of the reaction mixture and ,o,, its density, determined pyknometrically, and no, the initial number of moles of phenol, is m l / M l (i.e. its mass divided by its molecular weight) we obtain from Eqn. (5) k = (nlMl + n2M2)A d nlP,,(A _ a) 2 ,
(6)
24
where TI and 7"2 are any two temperatures within the temperature range of reaction, al and a 2 the corresponding areas, and dl and d2 the corresponding distances curve-baseline. By plotting the left-hand side of Eqn. (3) vs (1/T2 - I/TI)/In(A - a : / A - al), the activation energy is obtained from the slope of the resulting straight line; its intersection with the axis of ordinate gives the reaction order. The plots for both reactions in the presence of 0'5 per cent NaOH are presented in Fig. 4. Numerical values of activation energy calculated from these and other plots not given here are assembled in Table 1 (Fig. 5). The plot for condensation ofo-hydroxymethyl-phenol in the presence of I per cent N a O H is also included. The reaction order read off the plots is 2 for addition, and 1 for condensation. A few calculations were also performed by using a relation derived by Rogers and Smith [18] which also gives x and Eo from a single plot, and essentially identical results were obtained. Knowing (he reaction order, we can compute, using
22 T o E
20
o u UJ
18
16
~4
I
I
I
0.25
0.50
0.75
I 1.00
C o n c e n t r a t i o n , % No OH Fig. 5,. Activation energy of addition and condensation reactions as a function of added catalyst. Upper curve, addition; lower curve, condensation.
Table 1. Activation energies of addition and condensation reactions determined directly (from thermograms) and indirectly (from rate constants) NaOH conc (wt %) 0.25 0.50 0.75 1.00
Addition E, (kcal mole- 2) Direct Indirect 23.7 20.6 19.7 19.3
22.9 20-3 20-0 19-1
Condensation E, (kcal mole- 1) Direct Indirect 17.6 16"6 16.3 15.7"
18.0 17.0 16.2 15.5"
* The directly and indirectly determined values of E° for condensation of o-hydroxymethyl-phenol are 22.6 and 22.5 kcal/mole, respectively.
Reaction between phenol and formaldehyde where the subscript 2 refers to formaldehyde. In our case n 1 is equal to n2 and k - (MI + M2) A d p,,,(A -- a) 2
(7)
'
from which k is computed. In Fig. 6, the rate constant for addition at 350 K is given as a function of added catalyst. 4
2
I
I
I
I
0"25
0"50
0-75
I'0
Concen't'rot"ion, %NoOH
Fig. 6. Rate constants of addition and condensation reactions as a function of added catalyst. Upper curve, addition at 350 K ; lower curve, condensation at 400 K. As a check on the calculations, from the rate constants at individual temperatures for constant amount of catalyst, the activation energies for the two reactions were calculated. Agreement of these values with those obtained from Eqn. (3) is within the limits of experimental error, cf. Table 1. From inspection of plots in Fig. 5 and the data in Table 1, the following picture emerges. The activation energy for both reactions, as expected, decreases with increasing amount of catalyst. The slope of the curves in Fig. 5, however, decreases with increasing amount of catalyst which shows that a small amount of catalyst causes a substantial decrease of activation energy. At 0.5 per cent added NaOH, the activation energy for addition is 20.5 kcal/mole and that for condensation 16.6kcal/mole. Zavitsas and Beaulieu [I1] found by using a titration method for the activation energy of addition of o- and p-hydroxymethyl-phenols values between 18.5 and 21.1 kcal/mole, depending on concentration, and Yeddanapalli and Gopalakrishna [12] by applying paper chromatographic analysis 19.8 kcal/mole. Considering different reaction conditions, agreement with our values is quite satisfactory. There are also several comparable data for the activation energy of condensation; Katovi6 [13] found for the activation energy of condensation values between 18.3 and 21.3 kcal/mole by applying differential thermal analysis, thermogravimetric analysis and i.r. spectroscopy. The values of Sprung and Gladstone [19] for condensation of m-cresol and o-hydroxymethyl-phenol, obtained by a titration
277
method, are 15.8 kcal/mole and 18-5 kcal/mole, respectively. The latter value compares favourably with our value 22"6 kcal/mole, Table I. Agreement of our values for condensation with published values can thus be deemed satisfactory. The reaction order 2 for addition appears to be reasonable; it has also been observed by other authors [10-12]. Whatever reactions, in addition to the dominant one, run in parallel, they apparently are either also of second order or their extent is negligible. The value 1 determined for condensation agrees with that found by Katovi6 [13], as well as with the value of Sprung and Gladstone [19] for o-hydroxymethylphenol condensation, which, incidentally, equals our value for the same reaction, cf. Fig. 4. Also, as can be inferred from plots similar to those in Fig. 4, the amount of added catalyst does not influence the reaction order. It is necessary to comment on the relatively large difference in activation energy for condensation in the presence of 1 per cent NaOH, 15.7 (15.5) kcal/mole, and for the condensation of o-hydroxymethyl-phenol also in the presence of 1 per cent NaOH (22.6 and 22.5 kcal/mole, respectively) since both values were obtained by the same method. Does the difference therefore reflect the fact that in the first case one has a mixture of different compounds, though one prevailing. and in the second a pure compound? It is well known [10,20] that the relative reactivities of methyl and methylol substituted phenols depend on the number and positions of substituents. N Osa.tisfactory answer to this question can be given without knowing the composition of the mixture obtained in addition reaction. However, the fact that the heating curve for o-hydroxymethyl-phenol as compared with others, cf. Fig. 1, is shifted towards higher temperatures suggests such an interpretation. In conclusion, it can be said that DSC has proved useful for studying the kinetics of the quite complex reaction between phenol and formaldehyde. The values of kinetic parameters obtained can therefore be considered as fairly reliable. Uncertainties which undoubtedly exist arise not only from the shortcomings of the method or the simplifying assumptions on which the equations have been derived but also from the simplified treatment of chemistry of the reaction. Acknowledgements--We thank Dr. Z. Katovi¢5 for many helpful discussions. We also thank (Mrsj T. Malava~i~ for useful advice regarding the calorimetric measurements and F. Cvek and J. Dolinar for running the i.r. spectra. The study was supported by the Boris Kidri~ Fund.
REFERENCES I. K. E. J. Barrett, J. appl. Polym. Sci. 11, 1617 (1967). 2. J. M. Thomas and T. A. Clarke, J. chem. Soc. A, 457 (1968). 3. G. Beech, J. chem. Soc. A, 1903 (t969).
278
A.
~EBENIK,I. VIZOV1~EKand
4. K. E. J. Barrett and H. R. Thomas, J. Polym. Sci. AI, 7, 2621 (1969; K. E. J. Barrett and H. R. Thomas, Br. Polym. J. 2, 45 (1970). 5. K. Horie, J. Mita and H. Kambe, J. Polym. Sci. Al, 6, 2663 0968); 7, 2561 (1969). 6. F. Delben and V. Creseenzi, Annali Chirt 60, 782 (1970). 7. S. Seto and H. Horiuchi, J. chem. Soc. Japan, Ind. chem. Sect. 57, 687 (1954). 8. F. E. Freeberg and T. G. Alleman, Analyt. Chem. 38, 1806(1966). 9. K. Hultzsch, Chemie der Phenolharze. Springer-Vedag, Berlin (1950). 10. J. H. Freeman and C. W. Lewis, J. Am. chert Soc. 76, 2080 (1954). 11. A. A. Zavitsas and R. D. Beaulieu, Art chem. Soc., Div. Org. Coatings Plast, Chem., Preprints_27, I00 (1967).
S. LAPANJE
12. L. M. Yeddanapalli and V. V. Gopalakrishna, Makromolek. Chem. 32, 112, 124, 130 (1959). 13. Z. Katovi6, J. appl. Polym. Sci. 11, 85, 95 (1967). 14. P. J. SecresL J. Paint Technol. 37, 187 (1965). 15. H. J. Borchardt and F. Daniels, J. Am. chem. Soc. 79, 41 (1957). 16. R. L. Reed, L. Weber and B. S. Gottfried, Ind. E n g ~ Chem. Fundamentals 4, 38 (1965). 17. R. N. Rogers and E. D. Morris, Jr., Analyt. Chem. 38, 412 (1966). 18. R. N. Rogers and L. C. Smith, Thermochim. Acta 1, 1 (1970). 19. M. M. Sprung and M. T. Gladstone, J. Art chem. Soc. 71, 2907 (1949). 20. G. R. Sprengling and C. W. Lewis, J. Art chem. Soc. 75, 5709 (1953).
R6sam6--On a obtenu par calorimdtrie diffdrentielle (DSC), les param6tres cindtiques de la rdaction complexe entre le phdnol et la formalddhyde en prdsence d'hydroxyde de sodium (NaOH). Les deux rdactions dominantes semblent &re raddition du formald6hyde sur le ph6nol avec formation d'o-hydroxym6thylphdnol et la condensation ultdrieure de ce dernier. On a ddtermind/~ diffdrentes tem~ratures pour les deux rdactions, l'dnergie d'activation (E,), rordre de la rdaction et les constantes de vitesse. E, pour raddition passe de 23,7-19,3 kcal/mole et pour la condensation de 22.9-19,1 kcal/mole quand la teneur en NaOH augmente de 0,25-1,00 pour cent. L'addition est une rdaction d'ordre 2 et la condensation, d'ordre 1. La DSC apparait utile pour I'dtude cindtique de rdactions de polymdrisation plus complexes. sommario---Mediante calorimetria differenziale a scansione (DSC), si sono ricavati i parametri cinetici della eomplessa reazione tra fenolo e formaldeide in presenza di idrossido di sodio (NaOH). Sembra che le due reazioni predominanti siano quella dell'aggiunta di formaldeide al fenolo con formazione di o-idrossimetil-fenolo e raltre della successiva condensazione di quest'ultima sostanza. Per entrambe le reazioni si sono determinate, a differenti temperature, le constanti di velocitY, ordine di reazione ed energia di attivazione (E,). E, per cambiamenti per aggiunta da 23,7 a 19,3 kcal mole -~ e per condensazione da 22,9 a 19,1 kcal mole - ~ quando la quantit~ di NaOH viene aumentata da 0,25 a 1,00 per cento. L'ordine di reazione per l'aggiunta 6 di 2 e pa la condensazione di 1. Ecco perch6 la DSC sembra utile per 1o studio della cinetica di reazioni di polimerizzazione piti complesse.
Zusammenfassung--Die kinetischen Parameter for die komplexe Reaktion yon Phenol mit Formaldehyd in Gegenwart yon Natriumhydroxid (NaOH) wurden durch Differentialscanningkalorimetrie erhalten (DSC). Danach scheinen die beiden dominierenden Reaktionen zu sein einmal die Anlagerung yon Formaldehyd an Phenol unter Bildung von o-Hydroximethylphenol und zum zweiten die anschleiBende Kondensation der letzteren Verbindung. F/ir beide Reaktionen wurden die Aktivierungsenergien (E,), die Reaktionsordnungen und die Geschwindigkeitskonstanten bei verschiedonen Temperaturen bestimmt. Fiir steigende Konzentration von NaOH von 0,25 bis 1,00 Prozent ~indert sich Eo fiir die Anlagerung yon 23,7 nach 19,3 kcal/Mol und ffir die Kondensation yon 22,9 nach 19,1 kcal/Mol. Die Additionsreaktion verl~iuft nach der 2, die Kondensationsreaktion nach der 1 0 r d n u n g . Damit wird gezeigt, dab DSC eine wertvolle Methode zur Untersuchung der Kinetik yon komplexen Reaktionen ist.