Langmuir 1989, 5 , 1061-1071 structural difference between these two supports is in the aluminum content, which would affect the attainable local concentration of Cd2+and hence of CdS. Hydrogen generation was also observed from CdS on the small pore zeolite 4A, Table 111. The activity of CdS on SiOz and zeolite A decreased markedly upon repeated washing, however, suggesting that unlike the CdS-loaded X and Y zeolites, the active photocatalyst was attached to the surface of the supports and was thus more susceptible to mechanical removal.
Conclusions Zeolites are effective supports for photoactive CdS particles. Hydrogen evolution ensues when solutions containing sacrifical donors are brought into contact with the photoactivated semiconductor. The rate of hydrogen evolution was drastically increased by surface attachment of ZnS or Pt as hydrogen evolution cocatalysts. The photoactive CdS particle is held within the zeolite cavities and channels, as is indicated by the ineffectiveness of photoplatinization with PtCle2- as gauged by the absence of Pt in the atomic absorption spectrum after attempted photoplatinization with an anionic platinizing agent and
1061
by poor hydrogen evolution from aqueous ferricyanide. The absorption spectra of the CdS-loaded zeolites suspended in water is consistent with semiconductor particles of diameters greater than 100-200 A, although much smaller cluster sizes are attainable by ion exchange in acetonitrile. Since the zeolite cavities are much smaller than the observed particle size, agglomeration of the smaller cage-contained particles of CdS must have occurred, producing materials that “snake” through the pores of the zeolite and which have grown beyond the limited diameter of the a-cage. The increased hydrogen production rate observed with samples in which the CdS particle is generated in situ in the photochemical reaction medium is consistent with initial production of many dispersed small particles (large surface area) which aggregate further upon aging. The encapsulated CdS powders retain nearly the same activity as large particles adsorbed onto an inert silica support. Acknowledgment. This work was supported by the Army Research Office. Our program on organic-inorganic composites is supported by the Texas Advanced Research Program. We also thank John McNally for performing the zinc analysis.
Autoxidation of Tetralin Catalyzed by Cobalt-Pyridine Complexes in Polymer Latexes Rama S. Chandran, Sanjay Srinivasan, and Warren T. Ford* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 Received December 19, 1988. I n Final Form: April 13, 1989 Cobalt(I1) has been bound quantitatively to copolymer latexes prepared from acrylic acid, styrene, and divinylbenzene; methacrylic acid, styrene, and divinylbenzene; sodium 4-styrenesulfonate (NaSS),styrene, and divinylbenzene; and NaSS, 4-vinylpyridine, styrene, and divinylbenzene. With added pyridine, the cobalt acrylate and methacrylate latexes catalyzed the autoxidation of 1,2,3,4-tetrahydronaphthalene(tetralin) in aqueous suspension up to 3.3 times faster than Co(I1) and pyridine in aqueous solution at 50 “C. The cobalt NaSS/Cvinylpyridine latexes were only slightly more active than Co(I1) and pyridine in aqueous solution. The autoxidation proceeds by a free radical mechanism. Introduction Aqueous polymer colloids are a new environment for catalysis reported only recently. Examples include sulfonic acid catalysts for ester hydrolysis1*2and inversion of su~ r o s ea, ~primary amine for ester hydr~lysis,~ imidazoles for ester hydrolysis,6 an immobilized enzyme,6 a histamine-Cu(I1) complex for oxidation of ascorbic acid,’ and phase-transfer catalysts.8 We have reported cobalt catalysts boynd to synthetic latex particles for autoxidation of tetralin,gJO2,6-di-tert-b~tylphenol,~~J~ and l-decane(1)Fitch, R. M. In Macromolecules; Benoit, H., Rempp, P., Eds.; Pergamon Press: Oxford, 1982;pp 39-63. (2)(a) Arai, K.; Sugita, J.; Ogiwara, Y. Makromol. Chem., Rapid Commun. 1986,7,427.(b) Arai, K.; Maseki, Y.; Ogiwara, Y. Makromol. Chem., Rapid Commun. 1986,7,655. (3) Kim, J. H.; E l - h e r , M. S.; Klein, A.; Vanderhoff, J. W. J. Polym. Sci., Polym.Chem. Ed., in press. (4)Hopkins, A,; Williams,A. J. Chem. SOC.,Perkin Trans. 2 1983,891. ( 5 ) Kitano, H.; Sun, Z.-H.; Ise, N. Macromolecules 1983,16, 1306. (6)Kitano, H.; Nakamura, K.; Ise, N. J. Appl. Biochem. 1982,4,34. (7)Sun,Z.;Yan, C.; Kitano, H. Macromolecules 1986,19, 984. (8)Bernard, M.; Ford, W. T.; Taylor, T. W. Macromolecules 1984,17, 1812. (9)Chandran, R. S.;Ford, W. T. J. Chem. SOC.,Chem. Commun. 1988, 104.
thiol.12 The idea of using colloidal polymers as catalyst supports came from studies of the effect of particle size on the activities of polymer-supported phase-transfer ~ata1ysts.l~The activities of heterogeneous catalysts depend on their surface areas. The smaller the catalyst particles, the greater the surface area per unit mass, and the higher the activity when the reaction rate is limited either by mass transfer of reactant to active sites on the particle surface or by intraparticle diffusion of a rea~tant.’~ Catalysts supported on particles 10-1000 nm in diameter should have activities much greater than conventional catalysts supported on 0.1-5.0-mm particles. Transition-metal catalysts are often classified as either homogeneous or heterogeneous. Homogeneous catalysts (10)Ford, W. T.; Chandran, R.; Turk, H. Pure Appl. Chem. 1988,60, 395. (11)Turk, H.; Ford, W. T. J. Org. Chem. 1988,53, 460. (12)Hassanein, M.; Ford, W. T. Macromolecules 1988,21,525. (13)(a) Tomoi, M.;Ford, W. T. J . Am. Chem. SOC.1980,102,7140. (b) Tomoi, M.; Ford, W. T. J. Am. Chem. SOC.1981,103,3821.( c ) Ford, W.T.; Tomoi, M. Adu. Polym. Sci. 1984,55,49. (14)Satterfield, C. N.Mass Transfer in Heterogeneous Catalysis; MIT Press: Cambridge, MA, 1970.
0743-7463/89/2405-1061$01.50/00 1989 American Chemical Society
1062 Langmuir,Vol. 5, No. 4 , 1989 in solution are generally considered to be more selective than catalysts supported on mineral surfaces. Their selectivity is probably due to the lower temperatures at which they are used. The main advantage of heterogeneous catalysts is ease of separation of catalyst from reactants and products, which enables reuse of the catalyst. Many analogues of homogeneous catalysts, specific transition-metal complexes, have been supported on insoluble polymers and on silica to try to achieve the advantages of both homogeneous and heterogeneous ~ata1ysts.l~Selectivities as high as and sometimes different from those of soluble catalysts have been attained. The aim of using latex catalysts is to attain the selectivity of homogeneous catalysts along with the activity and recoverability of high surface area heterogeneous catalysts. Catalysts also often are classified as chemical or biological. Chemists are devoting great effort to mimic the activity and specificity of enzymes. The kinetics of enzyme catalysis are usually analyzed from the standpoint of homogeneous catalysis, but in living organisms the enzymes are located in the cytoplasm of the cell, which is more of a gel than a solution, inside cellular organelles, or in the cell or organelle lipid bilayer membranes. Biological catalysis proceeds naturally in heterogeneous aqueous environments. The surface of a latex particle resembles a membrane surface, a region of high charge density separating its lipophilic interior from an aqueous solution, but the interior of a latex particle is an amorphous polymer that is much less ordered than the aliphatic lipid chains in a bilayer. This paper describes the autoxidation of tetralin catalyzed by latex-bound cobalt-pyridine (Copy) complexes. Cobalt ions in acetic acid activated by HBr or by methyl ethyl ketone are the preferred catalysts for autoxidation at the benzylic positions of alkylaromatic hydrocarbons.16 p-Xylene is converted to terephthalic acid on a large scale by this meth0d.l' We chose tetralin to study autoxidation of a hydrocarbon using latex catalysts because it is more reactive than methylbenzenes,ls and the product mixtures were expected to be relatively easy to analyze. Many years of effort in other laboratories have been devoted to autoxidations in homogeneous solutions catalyzed by cobalt ions and complexes, but the mechanisms of the reactions still are not thoroughly understood. In general, cobaltcatalyzed autoxidations have been shown to function by electron-transfer reactions involving Co(I1) and Co(II1) species. In most reports, there is no balanced set of microscopic reaction steps that is consistent with all of the products and the kinetics. However, there is a consensus that most autoxidations at the benzylic position of alkyaromatic hydrocarbons proceed by a free radical mechanism.1622 The cobalt ions appear to catalyze reactions of alkyl hydroperoxide intermediates and may also be involved in less well understood reaction steps. ~
(15) (a) Chauvin, Y.; Commereuc, D.; Dawans, F. Progr. Polym. Sci. 1977, 5 , 95. (b) Pittman, C. U., Jr. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1983, Vol. 8, pp 553-611. (16) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oddations of Organic Compounds; Academic Press: New York, 1981; Chapter 3, pp 38-42; Chapter 5, pp 120-127. (17) (a) Brill, W. F. 2nd. Eng. Chem. 1960,52,837. (b) Chester, A. W.; Landis, P. S.; Scott, E. J. Y. CHEMTECH 1978, 366. (18) Kamiya, Y. J. Catal. 1974, 33, 480. (19) Woodward, A E.; Mesrobian, R. B. J . Am. Chem. SOC.1953, 75, 6189. (20) Kamiya, Y.; Beaton, S.; Lafortune, A.; Ingold, K. U. Can. J . Chem. 1963,41, 2020. (21) Hay, A. S.; Blanchard, H. S. Can. J. Chem. 1965, 43, 1306. (22) Lunak, S.; Vaskova, M.; Lederer, P.; Vepreksiska, J. J. Mol Catal. 1986, 34, 321.
Chandran et al. Table I. Autoxidation of Tetralin Catalyzed by Cobalt-Amine Complexes i n WaterO GLC analysis, mol % of initial tetralin mol of ligand (L) L/Co tetralin tetrol tetralone THPf NH,b excess 54 19 20 6 pyridinec 4 76 5 13 6 pyridined 6 37 21 28 14 acetic acid' 68 11 19 2 pyridine-2-carboxylic 3 72 97 2,2'-bipyridine 3 95 1 4 2,2'- bipyridine 1 14 4 10 12 "Except as noted, reactions were carried out with 0.13 M tetralin dispersed in 30 mL of water containing 1.3 mM cobalt(I1) chloride at 50 "C for 48 h a t 740-750 mmHg pressure of dioxygen. The pH was adjusted to 8.5 with KOH a t the start. 1.0 mM Co(II), 65 h, >lo0 mols ammonia per Co(I1). c2.1 mM Co(II), 0.14 M tetralin, 6 h. d1.8mM Co(II), 0.10 M tetralin. 'Anhydrous cobalt(I1) acetate in acetic acid. f 1-Hydroperoxytetralin.
Scheme I
OOH I
THP
I
d temlone
Experimental Section Complete experimental details are provided in the supplementary material available in microform.
Results Homogeneous Cobalt Catalysts. Oxidation of 110 mM tetralin in the presence of 1.9 mM cobalt(I1) acetate in acetic acid proceeded to 25% conversion at 50 OC in 24 h. Cobalt@) acetate in water w a ~ completely inactive, but that is not surprising because the solubility of tetralin in water is estimated to be less than lo4 M (naphthalene, 1X M a t 25 OC; tetralin, 1.4 X 10" M a t 150 0C).23 The cycling of cobalt between oxidation states I1 and I11 is likely vital for its activity in autoxidation of hydrocarbons. The oxidation potential for conversion of Co(I1) to Co(II1) is 1.84 V in water but is lower when amine ligands are present. For example, the oxidation potential for conversion of [ C O ( N H ~ ) ~to] ~[ C + O ( N H ~ ) ~is]only ~ + 0.1 V.24 Cobalt(I1) chloride in aqueous ammonia catalyzed (23) Solubilities of Inorganic and Organic Compounds; Stephen, H., Stephen, T., Eds.; Pergamon Press: New York, 1963; pp 533,616. (24) (a) Cotton, F. A.; Wilkmaon, G. Advanced Inorganic Chemistry, 4th ed.; Wiley-Interscience: New York, 1466; pp 863-876. (b) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueocls Solution, English ed.; Pergamon Press: New York, 1966, p 323. (c) Laitinen, H. A.; Bailar, J. C., Jr.; Holtzclaw, H. F., Jr.; Quagliano, J. V. J.Am. Chem. SOC.1948, 70, 1948.
Cobalt-Pyridine Complexes os Catalysts
Longmuir, Vol. 5, No. 4, 1989 1063
Table 11. Colloidal Acrylic Acid and Methylacrylic Acid Copolymers and Cobalt Catalysts copolymer composition' mgatom of wt% styrene acid DVB COOH/Co(II)' Co/g of polymer solids c . nm' 79 20MA 1 18.6 0.11 2.1 5ir f 2.0 79 20MA 1 8.0 0.25 1.3 58 f 2.0
cntalyst LC-I LC-2 LC-3 LC-4 LC-5
79 23 79
20AA 76AA 2QAA
1 1 1
17.4 2.3 42.6
0.12 4.2
0.05
2.0 1.0 4.4
+
62 2.2 140 + 7.0 60 f 2.5
d./d.d 1.14 1.14 1.20 2.1 1.20
OMol 9b styrene and ethylvinylbenzenes (from the DVB), acrylic acid (AA) or methacrylic acid (MA), and divinylbenzenes (from 55% technical DVB) in the monomer mixture. bMol of COOH in the original latex/mol of Co in the catalyst. 'Number-average diameter of particles with standard deviation. d d . = weightaverage particle diameter.
Scheme I1 R f
M*-kw *
KaS@@WO, CH2.CH
H,O.SDS
**.R-H
&"a* "I
' P
the oxidation of tetraliin at a higher rate than did cobalt(II) in acetic acid (see Table I). Pyridine, pyridine-3-carboxylic acid, and 1-methylimidazoleligands produced even higher autoxidation rates. The bidentate ligands 2,2'-bipyridine, 1.10-phenanthroline, and pyridine-&carboxylic acid were less active. More ligands were tested with soluble Co(I1) catalysts for tetralin autoxidation to determine which ligand bound to cobalt in a latex catalyst should lead to the most active catalyst. No ligand much more active than pyridine was found (Table I), so pyridine was used for all latex catalysts. We shall use Copy to designate a cobalt-pyridine complex, either soluble or polymer bound. Autoxidations of tetralin with soluble cobalt catalysts produced major amounts of 1-tetrol and 1-tetralone and a minor amount of 1-hydroperoxytetralin (THP, Scheme I). All three products were identified by GLC analysis and by COmpariSOh with authentic samples. Pure T H P partly reacted to form tetrol and tetralone under the conditions used for the GLC analyses of the product mixtures, and the analyses are corrected for this decomposition. Preparation of Acrylate and Methacrylate Latex Catalysts. Latex copolymenr of styrene, acrylic acid (AA) or methacrylic acid (MA), and divinylbenzene were prepared by emulsion polymerization to complete conversion by using sodium dodecyl sulfate (SDS)as an anionic surfactant and potassium persulfate/sodium bisulfite as a redox initiator (Scheme II).% Latexes containing 20 and 76 mol % acid groups were prepared (Table 11). They have been colloidally stable for more than 24 months. The latexes were purified by ultrafiltration through cellulose acetate/nitrate membranes stated to have 0.1-am pores. The membranes retained even samples with average diameters of 0.06 pm without significant loss of latex during ultrafiltration. The precursor to catalyst LC-1 shown in the transmission electron micrograph of Figure 1A has a number-average diameter of 58 nm. Conversion of the acid sites in a 20% acrylic acid copolymer to the Co(I1) salt was achieved by firstconverting (25) (a) Campbell,G. k;Upsan, D. k Macmmol.Synth.. in pres. 6) Blackley, D. C.; Sebaatim. S. A. R. D.Br. Polym. J. 1987. 19, 25.
Figurc 1. ( A ) I.atex 1,-1. micrograph t n k m at 480(KIX. Sample was prepared hy the drop methrd and s t n i n d with 5% uranyl acetate. IR) Cocatalyst 1,C-I. 29(KXlX; drop method. 1 % uranyl acetate. ( C )Cocatalyst IS:-1 recovered from reaction mixture, 1OOooOx; drop methid. 1% uranyl acetate
50% of the available sites with KOH to the potassium salt followed by adding an equivalent amount of cobalt(1I) acetate or chloride solution during sonication. Such "amples were colloidally stable, but addition of Co(I1) in e 'cess of the available potassium ions resulted in slow coagulation of the latex over a period of 2 months. Addition of Co(I1) directly to the latex in carhoxylir arid form also caused
1064 Langmuir, VoE. 5, No. 4, 1989
Chandran et al.
Table 111. Autoxidation of Tetralin Using Latex Catalystsa
catalystb LC-1 LC-le LC-1 LC-2 LC-2f LC-3 LC-4 LC-5 LC-58 L-5, Kt COPY' Co(OAc)d
latex, mg 182 182 510 79 79 365 14 400 400 400
initial composition Co(II), mmol Py, mmol tetralin, mmol 0.020 0.020 0.057 0.020 0.020 0.042 0.042 0.020 0.020 0.057 0.057
0.120 0.120 0.360 0.120 0.120 0.252 0.252 0.120 0.125 0.125 0.360
1.83 1.83 3.00 1.83 1.83 3.90 3.90 1.83 1.85 1.83 3.00 3.30
OzC consumed
tetralin
69 74 67 62 52 68 48 82 105 58 45 25
35 23 38 42 53 37 55 21 20 25 57 76
GLC analysis mol % of initial tetralind tetrol tetralone 21 22 20 20 15 19 16 25 18 11 14 2.5
41 43 39 36 29 42 26 49 50 18 27 22
THP 6 8 6 4 4 4 2 12 12 46 4 2
Reactions were carried out at 50.0 f 0.1 OC for 24 h a t dioxygen pressure of 720 mmHg (20 mmHg less than atmospheric pressure) with magnetic stirring of 30.0 mL of reaction mixture. bTable 11. cDioxygen consumed from the gas buret as mol % of initial tetralin. "Corrected for 34-50% decomposition of THP in the injection port of the gas chromatograph. The sums of components found by GLC account for 96-107% of the initial tetralin in these experiments, which indicates the range of experimental error in the product isolations and analyses. OThe mixture was agitated by using a platform shaker having an amplitude of 2.5 cm and a frequency of 1 s-l. foxidation was carried out in air at atmospheric pressure (150 psi of dioxygen). S A fresh batch of LC-5 was used. h50% of the available COIH sites was neutralized with 0.40 mmol of KOH. No Co(I1) was added. 'Catalyst was soluble Co(II)/pyridine in water at pH 8.0. jCobalt(I1) acetate in acetic acid.
coagulation, as expected for the treatment of an anionic colloid with a divalent metal saltsz6 Catalyst LC-4 was prepared from a 76% acrylic acid latex by addition of 0.95 equiv of KOH based on acrylic acid units, followed by addition of 0.95 equiv of Co(I1) acetate. Thus more than 90% of the acrylic acid units was in Co salt form. The Co(I1)-bound latexes were purified by ultrafiltration until the conductivity of the filtrate had decreased 30-fold and reached a constant value at about 40 X lo4 R-' cm-'. An upper limit of 3 X lo4 M Co(I1) in the ultrafiltrate was established by visible spectrophotometric analysis using l-nitroso-2-naphthol-3,6-disulfonic acid disodium salt (nitroso-R-salt),2' which forms a red complex with Co(I1). This corresponds with an upper limit of 0.15% of the added Co(I1) in all of the ultrafiltrate. Within error of measurement, all of the Co(I1) was bound to the latexes. Properties of the stable latex catalysts are in Table 11. A transmission electron micrograph of the catalyst LC-1 is shown in Figure 1B. Autoxidations with Latex Catalysts. The latexes containing Co(I1) and pyridine catalyzed the autoxidation of tetralin. Addition of 20 mol of pyridine/mol of Co(I1) did not cause either coagulation of the latex or extraction of cobalt ions from the bound latex. Ultrafiltration of the latex catalysts gave less than 10 mM of Co(I1) in the filtrate by the nitroso-R-salt test. During initial studies, the pH of the reaction mixture was adjusted to 8.5 with 0.1 N KOH. The catalysts were preequilibrated under dioxygen for 30 min at 50 "C prior to addition of tetralin. A decrease of as much as 1pH unit was observed after 50% conversion of tetralin. Autoxidation of tetralin was carried out with these latex catalysts at 50 "C for 24 h. The catalyst LC-1, recovered from one reaction mixture and analyzed by transmission electron microscopy, still contains discrete colloidal particles of the original size as shown in Figure 1c. Relative activities of the colloidal catalysts and of soluble Copy were determined by measuring the dioxygen uptake with a gas buret for 6-18 h (Table 111). In our preliminary communication: T H P was not identified, and the product distributions of tetrol and tetralone were not corrected for (26) Ottewill, R. H. In Emulsion Polymerization; Piirma, I., Ed.; Academic Press: New York, 1982; Chapter 1. (27) Vogel, A. I. A Textbook of Quantitatioe Inorganic Analysis, 3rd ed.; Longman: London, 1961; pp 795-796.
T H P decomposition during GLC anelysis. Those corrections have been made in Table 111, which contains both experiments reported earlier and new ones. All experiments in Table I11 had an induction period of 30-70 min before consumption of oxygen was observed. Then the volume of oxygen consumed was a linear function of time from 10% to at least 55% conversion, indicating zero-order rate dependence on tetralin concentration. From Table I11 we note the following: (1)The relative zero-order rate constants of catalyzed oxidations are LC-1, 9.0; soluble Copy in water, 4.5; and cobalt(I1) acetate in acetic acid, 1.0. (2) The catalysts LC-1, -2, -3, and -5, prepared from 20% acrylic acid or methacrylic acid copolymers, are more active than LC-4, prepared from a 76% acrylic acid copolymer. (3) Catalyst LC-5 showed the highest activity (Figure 2). LC-5 contains less Co per unit weight of particles than the other catalysts. When the rate comparisons of Table VI1 are made on the basis of equal amounts of cobalt, there is much more polymer present in the LC-5 experiments. During experiments with LC-1 to LC-4, part of the tetralin was present as droplets of a separate organic phase. This phase separation was observed after turning off the stirrer. On the other hand, no tetralin droplets were observed in the presence of LC-5. In a separate experiment, using the typical amounts of polymer and substrate present in the latex-catalyzed reaction mixtures, 0.4 g of dry LC-5 imbibed completely 0.25 g of tetralin. (4) Latex catalyst LC-5 showed slightly higher activity when the reaction mixture was maintained at pH 8.5 with 3.3 mM borate buffer than it did when the pH was adjusted to 8.5 with KOH a t the start (Figure 2), but the difference is within limits of experimental error in the rate constant. The Copy in water showed higher activity when the mixture was buffered a t pH 8.5. The pH did not change during the buffered reactions, but it decreased when only KOH was used, possibly due to the formation of small amounts of 3-(2-hydroxyphenyl)butyricacid and 2- (2-carboxypheny1)propionicacid from the Baeyer-Villiger oxidation of tetralone.28 Table IV reports the precision of rate data with LC-5. As expected for such complex heterogeneous mixtures, the precision of rate constants is not high. The induction times (28) Robertson, A,; Waters, W. A. J. Chem. SOC.1948, 1578.
Langmuir, Vol. 5, No. 4, 1989 1065
Cobalt-Pyridine Complexes as Catalysts 30
30
20
20
2
ci
E
v
E
8
Y
x w
%
3 0
a
.-n 0
10
10
0 0
400
200
600
800
0
Time (min)
Figure 2. Autoxidation of tetralin at 50 "C with (a) LC-1 with initial pH adjusted to 8.5 with KOH, (b) LC-5 with initial pH adjusted to 8.5 with KOH, and (c) LC-5 buffered at pH 8.5 with sodium borate and HCl. Conditions were 1.83 mmol of tetralin, 0.120 mmol of pyridine, and 510 mg of LC-1 or 400 mg of LC-5 diluted to 30 mL with water.
0
100
200
300
400
Time (min)
Figure 3. Autoxidation of tetralin at 50 "C in buffered pH 8.5 mixture with (a) LC-5 and (b) soluble Copy. For conditions, see Figure 2.
Table IV. Rates and Induction Periods of Tetralin Oxidation Catalyzed by LC-5" LC-5 date of k, mL of induction expt sampleb expt 0, min-1 time, min 1 A 9-30-87 0.074 20 2 A 6 10-2-87 0.100 3 A' 10-8-87 0.125 9 4 B 5-19-88 0.085 128 5 B 11-24-88 0.094 46 6 B' 11-25-88 0.095 46 7 B 11-26-88 0.096 59 avg k = 0.096 f 0.022 (99% confidence limit) avg k = 0.091 0.016 excluding expt 3 avg k = 0.094 f 0.008 excluding expt 1 and 3
*
"0.020 mmol of CO(OAC)~, 0.120 mol of pyridine, 1.80 mmol of tetralin in 30.0 mL of aqueous mixture adjusted to pH 8.5 with 1.0 mL of 0.1 M sodium borate/HCl; 50.0 f 0.1 "C; 730 mmHg of dioxygen. bCo(OAc)z was added to L-5 to prepare A on 6-12-87 and to prepare B on 12-4-87. 'LC-5 was not ultrafiltered after addition of CO(OAC)~, as it was in other experiments.
are unpredictable and not correlated with the rate constants. Since most other rate constants were measured only once, we shall consider differences in rate constants significant only if the difference exceeds 25% of the mean value. Additional parameters that affect the rate of tetralin autoxidation were studied with LC-5 in mixtures buffered a t pH 8.5. All of the experiments exhibited a zero-order dependence of dioxygen uptake on the tetralin concentration to at least 55% conversion. The oxidation catalyzed by LC-5 was 4 times faster than the homogeneous Copy-catalyzed reaction (Figure 3). Using LC-5 as a catalyst, more than 85% of the tetralin was converted to products in 10 h. The rate increased at higher temperature, as shown in Figure 4. The molar amount of dioxygen consumed was slightly more than the molar amount of products formed in all cases where the conversions were greater than 30%, probably due to the secondary oxidation of tetrol to tetralone. The dependence of product composition on time during the LC-5 latex catalyzed autoxidation at 50 "C is
0
200
400
600
800
Time (min) Figure 4. Autoxidation of tetralin catalyzed by LC-5 at 30, 50, and 75 "C. For conditions, see Figure 2c. shown in Figure 5. The tetralone to tetrol ratio increased from about 1 at 2-10% conversions to about 3 a t 80% conversion. In an independent experiment, 1-tetrol (1.9 mmol) in the absence of tetralin and THP, but otherwise under the same conditions as the experiments in Figure 5, was oxidized to tetralone in 20% conversion in 20 h. The induction periods suggest a free radical chain mechanism for autoxidation. Attempted autoxidation of 2,6-di-tert-butylphenol (DTBP) failed using the colloidal catalyst LC-5 and the same reaction conditions as for tetralin autoxidation. The autoxidation of tetralin was completely inhibited by addition at the beginning of the reaction of 2 mol % DTBP. Also, 1mol % of either DTBP or 4-methyl-2,6-di-tert-butylphenol inhibited any further oxidation of tetralin when introduced after 40 mol % of dioxygen (based on initial tetralin) had been consumed.
1066 Langmuir, Vol. 5, No. 4, 1989
Chandran et al. U"
50
-
h
i
40-
v
E
x
0
r \
30-
w
0
10
20
30
40
Time (min) 100 1
80
-
20
I
\
-
B
tetrdin
I in 4 100
0
200
Time (min)
Figure 7. Autoxidation of THP catalyzed by LC-5 at 50 "C. For other conditions, see Figure 2c.
04 0
I 2000
1000
Time (min)
Figure 5. Composition of reaction mixture during autoxidation of tetralin catalyzed by LC-5 at 50 O C : (A) at low conversion with time = 0 taken as the time of first observable consumption of
dioxygen; (B) a separate experiment at higher conversions. For conditions, see Figure 2c.
0
200
100
300
Time (min)
Figure 8. Autoxidation of tetralin catalyzed by LC-5 at 50 O C : (a) without added THP; (b) with 2.1 mol % added THP; (c) with 11 mol % added THP. For conditions, see Figure 2c.
0
10
20
30
40
Time (min)
Figure 6. Autoxidation of tetralin catalyzed by LC-5 at I 5 O C : (a)with 0.5 mol % AIBN; (b) without AIBN. For other conditions, see Figure 2c.
The induction period for LC-5-catalyzed autoxidation of tetralin at 75 "C was reduced to less than 1 min when the free radical initiator AIBN [azobis(isobutyronitrile), 0.5 mol '701 was added to the reaction mixture (Figure 6). 1-Hydroperoxytetralin (THP) is an intermediate that produces both tetrol and tetralone. Pure T H P was converted to a mixture of tetrol and tetralone as shown in Figure 7 by using LC-5 at 50 "C under 1-atm pressure of dioxygen. We expected that THP might act as an initiator. T H P (2.2 and 10 mol '70 based on tetralin) shortened but
did not eliminate the induction periods of the LC-5-catalyzed reaction at 50 "C, and the initial rate of autoxidation decreased slightly as shown in Figure 8. The zero-order rate constant of oxidation of tetralin with LC-5 is 3.3 times greater under dioxygen than under air at 1 atm of pressure as shown in Figure 9. This suggests a less than first-order dependence on partial pressure of dioxygen, but we did not obtain data at other partial pressures. The colloidal catalyst was active over four successive additions of tetralin, leading to a total turnover of 300 mol of tetralin consumed/mol of cobalt. The oxidation rate recovered to its initial maximum rate after the second and third additions of tetralin (Figure 10) but not after the fourth addition (not shown). There was an induction period after each addition of tetralin. The pH of the reaction mixture had dropped to 6.7 after the third cycle (32~5h and 5.5 mmol tetralin) and was returned to 8.5 with fresh addition of borate buffer before starting the fourth cycle. A separate organic phase appeared in the reaction
Langmuir, Vol. 5, No. 4, 1989 1067
Cobalt-Pyridine Complexes as Catalysts
Scheme 111
M
SO,N* 4.w
20
lca
-
3
1.1
3.2
N.SS
1.3
mnol
E
E
%
2
6
10
0 100
200
Time (.m i d. Figure 9. Autoxidation of tetralin catalyzed by LC-5 at 50 " C (a)under 1atm of dioxygen; (b) under 1atm of air. For conditions, see Figure 2c.
1000
A
2
Time (min) Figure 10. Autoxidation of tetralin injected into the reaction mixture at three different times with LC-5at 50 "C and 1.8 mmol of tetralin at (a) t = 0, (b) t = 587 min,and (c) t = 1244 min. Inset shows induction periods after each injection. For other conditions, see Figure 2c. mixture after the fourth addition of tetrnlii. The contents of the reaction mixture after each cycle are reported in the Experimental Section. 4-Styrenesulfonate a n d 4-Vinylpyridine Latex Catalysts. Polymer latexes which provide both the ionic binding sites for the Co(I1) and the pyridine ligands for catalytic activity were prepared by emulsion copolymerization of sodium styrene-4-sulfonate (NaSS) and 4-vinylpyridine (4-VP) with styrene and DVB. We used the two stage "shot-growth" emulsion polymerization process in order to incorporate >3 mol % of the ionic monomer.28 This process provides latexes from styrene and NaSS with high charge density and minimal concur(29) Kim. J. H.;Chainey, M.;E l - h e r , M.s.;Vanderhoff,J. W.J. Polym. Sei.. Polym. Chem. Ed., in presa.
Figure 11. (A) Cocatalyst LGT36OOOX, spray method; (B)Co catalyst LC-7, lOOOOX, drop method, 1% uranyl acetate. rent formation of water-soluble polyelectrolyte. The latexes contained up to 20 mol % 4-VP and 3.5 mol % NaSS. The synthetic route for LC-8is shown in Scheme
1068 Langmuir, Vol. 5, No. 4 , 1989
catalyst LC-6 LC-7 LC-8 LC-9
Chandran et al.
Table V. Colloidal CoDolymers from Anionic Monomers a n d Cobalt(I1)-Bound Colloidal Catalysts copolymer composition, mol % mole ratio mgatom Co/g wt% styrenea anionic 4-VP DVB anion/Co of polymer Py/Co(II) solids d,, nmb 96 3.1 NaSS 0.0 0.9 2.6 0.10 6 4.2 170 f 1 80 2.5 NaSS 17.8 0.6 4.5 0.06 40 3.5 202 f 1 86 9.8 3.3 NaSS 0.7 5.9 0.05 17 4.0 95 f 7 98.6 1.4' 0.0 0.0 2.0 0.068 6 0.17 92 f 41d
Includes ethylvinylbenzene from the technical DVB. acid. Irregular clusters of smaller primary particles.
Number-average particle diameter and standard deviation. 2-Dodecylpropenoic
Table VI. Oxidation of Tetralin Using N a S S Colloidal Catalssts" GLC analysis, mol % of initial tetralin 0 2 catalyst consumedb tetralin tetrol tetralone T H P LC-6' 67 36 15 25 24 LC-6c'd 70 24 19 24 32 LC-7 49 54 10 23 12 LC-8 62 41 13 35 10 LC-9e 76 30 22 44 4 "Reactions were carried out at 50.0 f 0.1 "C for 14 h a t 730mmHg pressure of dioxygen with 0.020 mmol of Co catalyst and 1.8 mmol of tetralin with magnetic stirring of 30.0 mL of reaction mixture. The pH was maintained a t 8.5 with 1 mL of 0.1 M borate buffer. bAs mol % of initial tetralin. With 0.13 mmol of added pyridine. d25-h reaction time. eReaction was carried out a t 50.0 f 0.1 "C for 25 h a t 731-mmHg pressure of dioxygen with 0.002 mmol of Co catalyst, 0.97 mmol of tetralin, and 0.013 mmol of pyridine with magnetic stirring of 30.0 mL of reaction mixture. The pH was maintained a t 8.5 with 0.5 mL of 0.1 M borate buffer.
111. The latexes prepared by this process consisted of larger and more uniform particles than the acrylic acid latexes (Figure 11). They were purified by ultrafiltration until a 40-50-fold decrease in conductivity of the filtrate had been achieved (initial conductivity 900 x lo4 f2-I cm-l, final (18-20) x IO4 V1cm-I). Co(II) ions were incorporated by adding 0.025 M aqueous cobalt(I1) chloride during ultrasonic agitation. Up to 40% of the ionic sites could be substituted with Co(11) ions without coagulation (Table VI. The 4-VP/NaSS latex catalysts were used for autoxidation of tetralin and compared with a similar catalyst prepared from styrene, NaSS, and DVB, with pyridine added to the reaction mixture but no 4-VP in the copolymer (Table VI). The catalysts containing polymerbound pyridine ligands were less active than the Co latex catalyst with pyridine added externally (Figure 12). The catalysts LC-6, LC-7, and LC-8 prepared from the NaSS latexes were less active than the acrylic acid latex catalyst LC-5 (Table V). Stable latexes were also obtained by emulsion polymerization of methyl acrylate, 4-VP, styrene, and DVB. However, they coagulated partly or completely during attempts to hydrolyze the ester groups with acid or base. Even in partially hydrolyzed latexes containing carboxylate and pyridine units, addition of small amounts (
