The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 237
Effects of Buffers on Micellar Catalysis
(9) I
,
(10) (11) (12) (13) (14)
(15)
kelones, S.K. Pollack and W. J. Hehre, bid., 99, 4845 (1977); (d) methyleneimine, D. J. DeFrees and W. ,I. Hehre, J . Phys. Chem., 82,391 (1978); (e) a similar approach was employed by t3eauchamp in the determination of the heat of formation of: CF,, ,I. Vogt and J. I.. Beauchamp, ibid., 97, 6682 (1975). G. 6.Goode. A. J. Fsrrer-Correia. and K. R. Jenninas. " Int. J . Mass Spectrum Ion Pbys., 5, 229 (1970). J. F. Wolf, R. H. Staley, I. Koppei, M. Taagepera, R. T. rulcIver, Jr., J. I.. Beauchamp, and R. W. Taft, J. Am. Cbem. S a c , 99, 5417 (1977). (a) R. T. McIver, Jr., Rev. Sci. Instrum., 4Q, 111 (1978); (b) R. L. Hunter and R. T. McIver, Jr., Cbem. Phys. Lett., 49, 577 (1977). Chwpka has dete+rmined a threshold of about 11 eV for vibrational excitation of NH, . W. A. Chupka and M. E. Russell, J. Chem. Pbys., 48, 1527 (1968). D. K. Bohme, R. S.Hemsworth, and H. W. Rundle, J. Chem. Phys., 59, 77 (1973). A referee has pointed out that Yuan Lee has recently determined PA(",) = 205 f 2 kcal mol-' at 298 K and that Houle and Beauchamp have revised reference data leading to A/-lf(NH4+)to yield PA(",) = 207 f 2 kcal mol-'. In addition, Ausloos and Lias have reported PA(",) = 207.3 f 2 kcal mol-' (J. Am. Chem. Soc., 100, 4594 (1978)). The results presented in this paper suggest that the value of 202.3 is a better estimate than these since the calculated values for AH:(NH~.) and EA(",-) are in good agreement with independent experimental determinations. Reaction 16 has been investigated in a flowing afterglow apparatus where it is reported that AGOm = -1.9 f 0.2 kcal mol: G. I. Mackey,
(16) (17) (18) (19) (20) (21)
(22)
(23) (24) (25) (26)
R. S. Hemsworth, and D. K. Bohme, Can. J. Cbem., 54, 1624 (1976), see also ref 13. S.W. Benson, "Thermochemical Kinetics", 3rd ed, Wiley, New York, 1976. (a) J. L. Beauchamp and R. C. Dunbar, J . Am. Chem. Soc., 92, 1477 (1970); (b) W. T. Huntress, Jr., M. M. Mosesman, and 13. D. Elleman, J. Chem. Phys., 54, 843 (1971). R. P. Clow and J. H. Futrell, Int. J. Mass Spectrom. Ion Pbys., 8, 119 (1972). D. K. Bohme in "Interactions between Ions and Molecules", P. Ausloos, Ed., Plenum Press, New York, 1974, p 489. R. L. Hunter, unpublished. The ability of CF:,H+ derived from electron impact on CF,H to donate its proton to a base changed by 10 kcal mol-' upon the addition of 2 X mol of an inert buffer gas, B. A. Levi and W. J. Huhre, unpublished observation. (a) J. D. Baldeschwieler and S. S. Woodgate, Acc. Chem. Fles., 4, 114 (1971); (b) J. L. Beauchamp, Annu. Rev. Pbys. Cbem., 22, 527 (1971); (c) T. A. Lehman and M. M. Bursey, "Ion Cyclotron Resonance Spectrometry", Wiley, New York, 1976. S.W. Benson and H. E. ONeal, "Kinetic Data on Gas Phase Unimolecular Reaction", US. Government Printing Office, Washington, D.C., 1970. D. Feldman, Z. Naturforscb. A , 26, 1100 (1971). K. C. Smyth and J. I. Brauman, J . Cbem. Phys., 56, 4620 (1972). (a) J. L. Beauch,ampand S. E. Buttrill, Jr., J. Chem. Pbys., 48, '1783 (1968); (b) J. I. Brauman and L. K. Blair, J . Am. Cbem. Sac. 92, 5986 (1970).
-
Effects of Buffers on Micellar Catalysis in the Basic Hydrolysis of p-Nitrophenyl Carboxylates Norlakl Funasakl Kyoto Co//egeof Pharmacy, Yamasbina-ku, Kyoto 607, Japan (Received May 90, 15178; Revised Manuscript Received September 1 I, 1978) Publication costs assisted by Kyoto Co//egeof Pharmacy
The effect of the kind and concentration of buffers on the basic hydrolysis of p-nitrophenyl carboxylates catalyzed by hexadecyltrimethylammonium bromide (CTAB) was measured at 25 "C. The basic component of buffers (carbonate ions and triethylamine) catalyzed this reaction and was suggested to be incorporated on CTAB micelle. (AMP), however, was not incorporated because of its hydrophilicity, Since 2-amino-2-methyl-1,3-propanediol an AMP-HBr buffer is suitable for the present reaction system. Using this buffer, the effect of CTAB concentration on the pK value for thymol blue and on rate constants was investigated and analyzed in terms of the surface potential +o of the micelle. An increase in pK values was explicable on the basis of a decrease of q0 with CTAB concentration. Tlhe shape of the rate-surfactant concentration profile markedly varied with the kind and concentration of bufifers. The profile obtained in the AMP buffer was explicable, taking into account the surface hydroxide ion concentration estimated from pK shifts. The ratio of rate constants on the micelle and in bulk solution did not significantly depend on the chain length of the acyl group.
Introduction Micellar catalysis of bimolecular reactions in water depends a t least in part upon the ability of the micelle to concentrate reagents into the small volume of the Stern Several theories5-10 of micellar catalysis which quantitatively take into account this concentration effect have been proposed and compared with experimental data. Most micelle-catalyzed reactions involve hydrophilic ions and buffers, and studies on their incorporation have been reportedl.lbl3 Incorporated ions and buffers complicate the analysis of micelle-catalyzed reactions,10J2 since they may act as catalysts1°J4 and moreover produce changes in the micellar properties. The present study concerns the buffer effect on the basic hydrolysis of p-nitrophenyl carboxylates catalyzed by hexadecyltrimethylammonium bromide (CTAB), since there is considerable information on this reaction availA 2-amino-2-methyl-1,3-propanediol able.10,1!j-18 (AMP)-ammonium bromide buffer was suitable for the 0022-3654/79/2083-0237$0 1.OO/O
present reaction system, since hardly any AMP and its ammonium ion are incorporated onto the CTAB micelle. Furthermore, the ratesurfactant concentration profile was obtained with this buffer and analyzed theoretically in terms of the surface potential.
Experimental Slection Materials. p-Nitrophenyl acetate (PNPA) from Tohyo Kasei Organic Chemicals Co. was recrystallized from 1propanol before use. p-Nitrophenyl propionate (PNFP) was synthesized from p-nitrophenol and propionic acid by the dicyclohexylcarbodiimide methodlg and recrystallized three times from 1-propanol. p-Nitrophenyl butyrate (PNPB) from Sigma Chemical Co. was used without further purification. CTAB from Nakarai Chemicals Co. was extracted with diethyl ether and recrystallized three times from a mixture of ethanol and acetone (1:l). Impurities in sodiurn bromide were removed with active charcoal after extraction with diethyl ether. Thymol blue 0 1979 American
Chemical Society
238
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
G
Noriaki Funasaki
I
i
1
Ok'
5' 10 15 ' ' ' ' ' total carbonate ion concentration (mM)
OO
0.01 0.02 TEA*HBr concentration (M)
20
Figure 1. First-order rate constants plotted against total concentrations of carbonate and hydrogen carbonate ions in the carbonate buffer (pH 9.46) for p-nitrophenyl butyrate at 25 OC. The number attached to the curves indicates concentration (mM) of CTAB added.
(TB), a sulfonphthalein p H indicator, from Tokyo Kasei Organic Chemicals Co. and other reagents were used without further purification. The ion-exchanged water was twice distilled from the all-glass apparatus and used throughout. Kinetics. Kinetic measurements were carried out spectrophotometrically using a Shimazu UV-210 double-beam spectrophotometer equipped with a cell holder through which water from a thermostated bath was continuously circulated. All hydrolyses were followed by observing the appearance of the p-nitrophenoxide ion at 400 nm. Each reaction mixture was made up of 3 mL of a buffer and 3 ,uL of an acetonitrile solution of 0.01 M ester. First-order rate constants were obtained from plots of log ( D , - D ) against time, in the usual manner. Absorbance at equilibrium, D,, was read 24 h later. Absorbance D was digitally read at 1-or 2-min intervals. Buffers of AMPHBr (pH 9-59),triethylamine (TEA)-HBr (pH 9.53), and carbonate-hydrogen carbonate (pH 9.46) were employed. The p H value of the buffers was determined using an Orion digital pH/mV meter 801A equipped with a Beckman glass electrode A-5U. p K Values. pK values were determined from absorbance measurements at an absorption maximum of T B in a strongly alkaline solution of CTAB. The pK was calculated from where K is the acid dissociation constant, DA and DB are absorbance in acidic and strongly alkaline solutions, and D is absorbance in the same buffer as used in the kinetic measurement. Critical Micellizution Concentration (cmc) The cmc in sodium bromide solutions was determined by the surface tension method. The surface tension was measured by the Wilhelmy method as already reported.21 All measurements of kinetics, pK values, and surface tension were carried out at 25 "C. Results As Figures 1-3 show, the first-order rate constant hl for PNPB varied with buffer concentrations, while the ratio of the basic and acidic components of the buffers was kept constant. These figures show a linear relationship between the rate constant and buffer concentration in the absence of CTAB and similar linear relationships were established for PNPA and P N P P in the AMP-HBr buffer. The basic hydrolysis of PNPA i s knownik17to be catalyzed by general bases as well as hydroxide ions: Klb bbaee[ba~elb f k2b[OH]b (2)
0.03
Figure 2. First-order rate constants plotted against TEA-HBr concentrations in the TEA buffer (pH 9.53) for p-nitrophenyl butyrate at 25 OC. The number attached to the curves indicates concentration (mM) of CTAB added.
-
0.08-
C
'E
v
J
0.04.
01 0
0.01
0.02
0.03
1
AMP.HBr concentration ( M )
Figure 3. First-order rate constants plotted against AMP-HBr concentrations in the AMP buffer (pH 9.59) for p-nitrophenyl butyrate at 25 OC. The number attached to the curves indicates concentrations (mM) of CTAB added. The dashed line is calculated from eq 11 employing data shown in Tables I11 and IV.
TABLE I: pK Values and Catalytic Coefficients for p-Nitrophenyl Carboxylates in the Absence of Surfactants kzbase. L/mol.min
base
pK
PNPA
PNPP
PNPB
AMP TEA CO,3OH-
8.97 0.26 0.24 0.14 10.87' 13 10.4 1.00, 1.06 5.6 15.7b 306: 890b Fb 3.1 0.0010 H,Ob -1.7 6 X lo-' a Taken from ref 22. Taken from ref 16 and references cited therein. ' Taken from ref 15.
In the absence of CTAB, values of j z z b are calculated from the slope of linear relationships shown in Figures 1-3 and collected in Table I together with values reported in the literature.15-17 The symbols used in this paper are summarized in Table 11. The Bronsted equation23holds for basic hydrolysis of p-nitrophenyl carboxylates as is well known:15-17 (3) log k2base = a(pW + b The values of a and b were 0.8 and -5.5 for PNPA16 and 0.8 and -7.3 for PNPB, respectively. Thus, bases as well as hydroxide ions also catalyze this reaction. This factor must be taken into accountlobefore attempting to interpret the data in Figures 1-3. In the presence of CTAB, Figures 1-3 show that the relation of rate constants to buffer concentrations depends markedly on the kind of buffer. The rate constant can be increased with increasing buffer concentration by the following factors: (i) increase in micellar concentration
Effects 'of Buffers on Micellar Catalysis
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979 239
TABLE 11: Nomenclature a
b b k m C
cc
cs ct
F
K
K, Ki POH'
R T rY
P i o
coefficient defined in eq 3 constant defined in eq 3 subscript denoting bulk phase rate constant subscript denoting micellar phase total surfactant concentration critical micellization concentration salt concentration total concentration of free counterions Faraday constant acid dissociation constant in the presence or absence of a surfactant association constant for a micelle and ester acid dissociation constant at zero surface potential distribution coefficient of hydroxide ions at zero surface potential gas constant absolute temperature coefficients defined in eq 7 degree of counterion binding to micelles surface potential of micelles
resulting from a decrease in crnc (cf. 'Fable IV) (This factor is negligible, since the cmc is much lower than CTAB concentration used and moreover the cmc change is slight.); (ii) incorporation of the basic component of the buffer, into micelles, which acts as a catalyst (Since carbonate ions, bivalent counterions, catalyze this reaction as Bhown in Table I[, the data in Figure 1 may be explained by this factor.); (iii) increase in the bulk rate constant. On the other hand, the factors decreasing the rate constant are (i) a decrease in surface hydroxide ion concentration with increasing ionic strength. (The data on the AMP buffer are explicable by this factor alone as described below in detail. This factor also affects, in part, the data on the TEA buffer.); (ii) a decrease in surface hydroxide ion concentration resulting from a decrease in the surface charge density, which is brought about by penetration of the basic component of buffers onto micelles. This factor needs to be taken into account in the case of the 'TEA buffer. Since AMP is hydrophilic, however, this factor is negligible. Thus, for the present reaction system, the AMP buffer was most suitable of the three buffers. In the following experiment the 0.01 M AMP-HBr buffer was employed throughout (Figures 4 and 5 ) . Figure 4 shows the effect of CTAB concentration C on the rate constant for PNPA, PNPP, and PNI'B. For comparison, the dashed line illustrates the rate-surfactant concentration profile for PNPB in a 0.01 M carbonate buffer (pH 9.87).1° Similar profiles have been reported for other p-nitrophenyl carboxylates1°J8 and Thus, in general, the kind and concentration of buffers must be taken into consideration in order to analyze these profiles. Under the experimental conditions employed in the present study, the hydroxide ion is the most important catalyst for the micellar reaction. Hydroxide ion concentratlion a t the micellar surface, [OH],, may be correlated5 with that in bulk solution, [OH],
[OH], = po~O[oHIbexp(Wo/RT) (4) where the PoHo is the distribution coefficient of hydroxide ions a t the surface potential $, = 0. A value of IJoHo =1 has so far been adopted.8J0 Martinek et aL5 derive
C (mM)
Figure 5. pKvalues (of thymol blue plotted against CTAB concentrations in the AMP buffer (pl-l 9.59) at 25 OC. The dashed and dot-dash lines are calculated from eq 9 employing, respectively, values of a = 0.862 and 1.
where K, is the constant of association between a micelle and a p-nitrophenyl ester, C, is the cmc, and kz, is the second-order rate constant at the micellar surface. In order to apply this equation to rate data, the surface potential and cmc have to be evaluated by some theory or experiment. Recent s t ~ d i e s showed ~ ~ - ~ that ~ the surface potential of micelles can be estimated from the pK value of pH indicators fully bound30 onto micelles pK = pKi - F+o/2.30RT (6) where pKi is the pK a t $o = 0 and close to the value for the indicator bound in nonionic surfactant micelle^:^^-^^ such as Brij 35 and Triton X 100. Figure 5 shows pK values of T B as a function of CTAB concentration in the presence of the 0.01 M AMP-HBr buffer. In general, the surface potential of micelles may be correlated with total concentration of free univalent counterions, Ct: 2.30RT (-a log c, 4- log y) $0 = (7) According to the Gouy-Chapman theory for a plane double layer,31a equals one and y depends on the surface charge density, dielectric permittivity, and temperature. Experimentally,26 the a value is reported to be 0.862 for CTAB micelles in sodium bromide solutions. In AMP-HBr buffer solutions, the total concentration of free counterions is given by c, = c, + c, (C - CJ(1- 0) (8)
+
240
The Journal of Physical Chemistry, Vol. 83, No. 2, 1979
TABLE 111: Kinetic and Equilibrium Data on p-Nitrophenyl Carboxylates with and without CTAB Micelles
PNPA 54a 0.0490 2.31 X 0.0310 0.0745 PNPP 170' 0.0420 1.35 X l o w 3 0.0247 0.0547 PNPB 530' 0.0289 1.02 X 0.0186 0.0548 ' Taken from ref 10.
where C, is the AMP-HBr concentration and p is the degree of bromide ion binding to the CTAB micelles. In this equation the contribution of the hydroxide ion is neglected since its concentration is relatively low in this experiment. Combination of eq 6-8 yields pK = pK, - log y + O( log [C, + C, + (C - CJ(1 - p)] (9) The surface potential of CTAB micelles in solutions containing 2 mM CTAB and 10 mM NaBr is reportedz6 to be 125 mV. Assuming that $o in solution containing 2 mM CTAB and 10 mM AMPSHBr equals this value, a value of pKi = 10.97 is obtained from eq 6 and Figure 5. In the absence of CTAB, the pK value for T B measured in the 10 mM AMPVHBr buffer was 8.97. A similar difference between bulk p K and micellar pKi has been r e p ~ r t e d ~for~ some ~ * ~ sulfonphthalein ,~~ dyes analogous to T B and this difference is ascribed to the lower dielectric permittivity of the micellar s u r f a ~ e . ~ ~ - ~ ~ In Figure 5 the solid line is drawn smoothly through the experimental points. Moreover, the dashed and dot-dash lines are calculated from eq 9 employing values of N = 0.862 and y = 2.57 and N = 1 and y = 1.37, respectively. Here a value of /3 = 0.802 is employed as already determinedlo by a bromide ion selective electrode32and the y values were determined so that the three lines are crossed a t 2 mM CTAB concentration. Taking into account the experimental error (h0.01 as relative pK), the two theoretical lines conform with the observed values to a similar extent. The cmc of CTAB in sodium bromide solutions was measured by the surface tension method and could be expressed as a function of sodium bromide concentration by log C, = --0.60 log (C, i- C,) - 4.84 (10)
c,
Combination of ey 5 and 6 yields
The use of this equation tacitly assumes that T B and p-nitrophenyl carboxylates are solubilized a t the same location with respect to CTAB micelles. The association constant Ka for the CTAB micelle and p-nitrophenyl carboxylates has already been determinedlo as shown in Table 111. The value of the acid dissociation constant K was read from the solid line shown in Figure 5. The solid lines shown in Figure 4 are calculated from eq 1 1 employing the values of h,,Po~'[oH]b shown in Table 111. The calculated values agree well with the observed data. Furthermore, as Table I11 shows, the ratio of the catalytic coefficients a t the micellar surface and in bulk solution, k2m/h2b,does not almost depend on the chain length of the carboxylates as already seen for a carbonate buffer.1° Table IV shows that the pK value for T B increases with AMP-HBr concentration in the presence of 3.2 mM CTAB.
Noriaki Funasaki
TABLE IV: Changes of pK (in 3.2 mM CTAB), cmc, and A M P Buffer Concentrations
k l b with
[AMP.HBr], mM
PK
cmc, mM
5 10 20
8.71, 8.86; 9.14,
0.34 0.23 0.15
k l b , min-'
-_
0.0236 0.0289 0.0395
The dashed line in Figure 3 is calculated from eq 11 employing values of K, cmc, and h,,, shown in Table IV and k2mI.'o~o[OH]b for PNPB given in Table 111, agreeing well with the experimental results. Thus, it is concluded that hardly any AMP and its ammonium ion are incorporated into or onto the CTAB micelle.
Discussion As the ratio of rate constants at the micellar surface and in bulk solution, kzm/kzbis physically more significant than kzm[OH]b/kl~According to Bender,33at least three factors influence this ratio when the reactant or reactants are incorporated into or onto a micelle: proximity, electrostatic, and medium effects. The value of kzm/h2breported for some other r e a c t i o n ~ranges ~ ~ ~ from ~ 0.033 to about 10, being independent of the chain length for acylation of m-bromobenzaldoximej but dependent for acylation of benzimidazol anionsa5 As yet this ratio for the present reaction system cannot be predicted theoretically. According to Stigter, in the Stern layer, the Po value for hydrophilic counterions is smaller than 1, although in a A similar situation diffuse double layer it equals is encountered in inorganic salt solution^.^^ It has been experimentally observed that hydroxide ions do not strongly bind to CTAB micelles."J3 Thus although PoHo is definitely smaller than 1, the value cannot be estimated a t present. Therefore, absolute hydroxide ion concentration at the micellar surface cannot be evaluated by the pK shift method, and instead, the relative concentration, which is proportional to K/Ki, viz., exp(F$o/RT), was used to analyze the kinetic data. In a carbonate buffer compared with in an AMP buffer, as Figure 4 shows, a steeper increase in rate constants above the cmc is brought about by catalyses of carbonate as well as hydroxide ions bound to CTAB micelles and then a more abrupt decrease is attributed to a decrease in carbonate and hydroxide ion concentrations, as already suggested.1° Romsted6 has derived an useful equation applicable to the condition where the total hydroxide ion concentration is kept constant and he applied this equation to hydroxide ion reactions in both buffered and unbuffered systems without taking into account the complications in the buffered systems. In the present experiment, however, hydroxide ion concentration in bulk solution was kept constant by buffers and the total hydroxide ion concentration increases with CTAB concentration. Therefore, unless the Romsted equation is modified, it is not applicable to the present experiment. More details on the theory for micellar effects on chemical reactions will be published elsewhere.37 In summary, an adequate buffer must be selected to facilitate the analysis of micellar catalyses; an AMP-HBr buffer was suitable for the present reaction system. Using this buffer, surfactant concentration-rate profiles for basic hydrolysis of homologous esters were obtained and analyzed on the basis of eq 11. This equation takes into consideration surface hydroxide ion concentration, whose relative value was estimated from pK values of a p H indicator fully bound to the micelle. The ratio of the rate
Hydrogenation of Hydrocarbons on Platinum Catalysts
constants a t the micellar surface and in bulk solution was almost, independent, of the chain length of carl)oxylates. Achmowledgment. Thanks are due to M. ()hara for assistance with the manuscript.
References and Notes
The Journal of Physical Chemktry, Vol. 83, No. 2, 1979 241
(17) M. L. Bender, Chem. Rev., 60,53 (1960). (18) L. R. Romsted i3nd E. H. Cordes, J. Am. c k m . Soc., 90,4404(1968). (19) H. Zahn and F. Schade, Chem. Ber., 98, 1747 (1963). (20) The term "crlical micellization concentration" instead of "critical micelle concentration" is recommended by IUPAC Commission 1.6 in "Physical Chemistry: Enriching Topics From Colloid and Interface Science", H. van Olphen and K. J. Mysels, Ed., Theorex, La Jolla,
- ., 1975 . - . - , nn I 4 5
Calif -...
E. H. Cordes, "Reaction Kinetics in Micelles", Plenum Press, New York, 1973. C. A. Bunton, Prog. Solid Safe Chem., 8, 239 (1973). I. V. Berezin, K. Martinek, and A. K. Yatsimirski, Russ. Chem. Rev.,
42,787 (1973). J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975. K. Martinek, A. K. Yatsimirski, A. V. Levashov, and I. V. Berezin In "Micellization, Solubilization, and Microemulsions", Vol. 2 K. L. Mlttal, Ed., Plenum Press, New York, 1977,p 489. L. S.Romsted in "Micellization, Solubilization, and Microemulsions", Vol. 2, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 509. F. M. Menger and C. E. Portnoy, J. Am. Chem. Soc., 89,4698 (1967). K. Shirahama, Bull Chem. SOC.Jpn., 48, 2673 (1975). D. Piszklewlcz, J . Am. Chem. Soc., 99, 7695 (1977). N. Funasaki, J . Colloid Interface Sci., 64,461 (1978). J. W. Larsen and L. J. Magid, J. Am. Chem. Soc., 96,5774 (1974). C. A. Bunton and M. J. Minch, J . Phys. Chem., 78, 1490 (1974). C. A. Bunton, K. Ohmenzetter, and L. Sepulveda, J . Phys. Chem.,
81,2000 (1977). J. C. Jagt and J. B. F. N. Engberts, J. Am. Chem. SOC.,99,916
(1!377). M. L. Bender and B. W. Turnquest, J . Am. Chem. Soc., 79, 1656
(1957). W P. Jencks and J. Carriuolo, J. Am. Chem. SOC.,82,1778 (1960).
rr
18R
' ' - 9
(21) N. Funasaki arid S.Hada, Bull. Chem. SOC.Jpn., 49,2899 (l976). (22) A. Albert and E. P. Serjeant, "Ionization Constants of Acids and Bases", Methuen, London, 1962,Chapter 8. (23) J. N. Bronsted and K. J. Pederson, 2.Phys. Chem., 108,185 ('1923); ref 33,chapter 4. (24)J. V. Moller and U. Kragh-Hansen, Biochemistry, 14,2317 (1975). 125) P. Mukeriee and K. Baneriee. J . fhys. Chem., 68,3567 (1964). (26) M. S.Ferkndoz and P. Fromherz, J. phys. Chem., 81,1755 (1977). (27)N. Funasaki, Nippon Kagaku Kaishi, 722 (1976). (28)N. Funasaki, J . Colloid Interface Sci., 60,54 (1977). (29)N. Funasaki, J . Colloid Interface Sci., 62,336 (1977). (30) Some evidence and conditions for the full binding of dyes to micelles are given in reif 10 and 24-29. (31) E. J. W. Verwey and J. Th. 0 . Overbeek, "Theory of the Stisbility of Lyophobic Colloids", Elsevier, Amsterdam, 1948,Chapter 2. (32) By the same method and different analysis (ref 1 l),p is deterrnined to be 0.77and 0.78. (33)M. L. Bender, "Mechanisms of Homogeneous Catalysis from Protons to Proteins", VViley-Interscience, New York, 1971,p 73. (34) K. Martinek, PI. K. Yatsimirski, A. P. Osipov, and I. V. Berezin, Tetrahedron, :29,963 (1973). (35) D. Stigter, J. Phys. Chem., 68,3603 (1964). (36)R. A. Robinson and R. H. Stokes, "Electrolyte Solutions", Butterworths, London, 1965,p 80. (37) N. Funasaki, iri manuscript preparation.
Role of Carbonaceous Deposits in the Hydrogenation of Hydrocarbons on Platinum Catalysts Tadashi Hattori' and Robert L. Burwell, Jr." Ipatieff Laboratory, Department of Chemistry, Northwestern University, Evanston, Illlnois 6020 1 (Received August 11, 1978) Publication costs assisted by the Ipatieff Fund, Northwestern University
+
Pulses of cyclopropane hydrogen and of ethylene + hydrogen have been injected into flowing hydrogen which passed over Pt/SiOz at 0 and -31 O C , respectively. Conversions were independent of pulse number including pulse 1,and turnover frequencies per surface atom of platinum (Pt,) were the same as in a flow reactor. Small concentrations of slowly reacting "carbonaceous residue" accumulate during the 11-pulsesequence, with C3/Pt, and Cz/Pt, about 0.04, but less than 0.02, after pulse 1. The residues appear to be catalytically inactive and they are completely removed by flowing hydrogen at temperatures 50-100 "C above the temperature of a run and appear exclusively as propane (in the hydrogenolysis of cyclopropane) and as ethane (in the hydrogenation of ethylene). These data indicate that the reactant hydrocarbon adsorbs on the surface of platinum to form about a monolayer and that the adsorbed material reacts sequentially with two H-Pt, and desorbs as alkane. A mechanism in which hydrogen is transferred to reactant via carbonaceous residues is unlikely. Catalysts cooled from 450 "C in helium to reaction temperature were initially more active than those cooled in hydrogen. Traces of oxygen in the hydrogen led to declines in activity. When ethylene + hydrogen was injected into carrier helium, hydrogen ran ahead of t8heethylene in the catalyst bed and a surface layer of adsorbed hydrocarbon remained. This adsorbed material was removed as ethane by one pulse of hydrogen at -31 "C.
Introduction The lloriuti-Polanyi mechanism ( 1934)2was one of the first mechanisms in heterogeneous catalysis to propose specific structures for surface intermediates in heterogeneous catalytic reactions. As amplified and extended, it is still widely employed for the hydrogenation of olefins and related reactions. In the simplest form, as applied to ethylene, the mechanism is given in eq 1-4. Here, * H2 + 2* 2H* (1) *CH2CH2* (2) CH,=CH2 + 2" or -+
+
0022-3654/79/2083-0241$01 .OO/O
CH,=CH,(ads) *C2H5
-
+ H*
+ H*
+
*CCzHS -t 1 or 2"
CZH,
+ 2*
(3) (4)
represents a site on the surface of platinum although it is not clear as to wh.ether the site is atop one platinum atom or centered between two, three, or four atoms. C2Hs*is a surface alkyl and (CH2=CH2)* is a surface T complex. In recent years, many molecular, homogeneous catalysts for hydrogenation which consist of organometallic complexes have been developed, rather similar mechanisms have been accepted for hydrogenations in the presence of such catalysts, and considerable interest has developed in intercomparing catalytic reactions on transition metals ,and 0 1979 American Chemical Society
