P.J. SCHOFIELD, B. J. RALPH,A N D J. H. GREEN
472
Mechanisms of Hydroxylation of Aromatics on Silica Surfaces
by P. J. Schofield, B. J. Ralph, and J. H. Green School of Biological Sciences and Department of Nuclear and Radiation Chemistry, Uninersity of New South Wales, Sydney, Australia (Received August 26, 1963)
Finely divided silica shows unusual surface active qualities. Mixing of silica with polar aromatic compounds both in a dry and a moist state results in hydroxylation of the benzene nucleus. In the case of monohydroxybenzoic acids, hydroxylation results in the formation of dihydroxybenzoic acids, but the hydroxylating power of the silica system is a function of such factors as the concentration of water and metal ion impurities and the effect of pretreatment of the solid. On the basis of these results a possible series of reaction mechanisms is suggested.
It has been known for some time that finely divided silica possesses surface active properties. In 1960 Marasas and Harington112showed that the shaking of siliceous dust with various organic compounds in air effected their oxidation, and in the case of certain aromatic compounds, hydroxylation of the compounds. They suggested that, in these cases, the reaction was possibly initiated by free radicals of a peroxidic nature formed during the shaking of the reaction mixture, because Hauserj had shown that atomic oxygen was liberated during the formation of a fresh silica surface. The results of Marasas and Harington are not isolated cases as it has been recently shown that silica, pretreated in various ways, exhibits a variety of unusual surface active properties. Several of these results, and the diverse reaction mechanisms proposed may be summarized as follows: (a) Benson and Castle4 found that reactions could be effected by grinding silica in the presence of organic compounds or by mixing the organic compounds with silica, which had been maintained in an active state by storage in an inert atmosphere. They suggested a reaction mechanism based on a dual thermal-ionic process which operates during the grinding of the silica. A free-radical process was found to be unlikely. (b) Kohn5 demonstrated that, during irradiation in an inert atmosphere, a reaction occurred between aromatic compounds and dehydrated silica gel, with the formation of highly colored compounds of varying stability. The reaction mechanism was attributed principally to carbonium ion formation and the active intermediate was presumed to be of ionic character. The Journal of Physical Chemistry
(c) I n the radiolysis of pentane, Calffrey and Allen6 postulated that the increased decomposition of the pentane, when it is adsorbed on the silica gel, is due to a transfer of energy from the adsorbent to the adsorbate. The earlier work of Marasas and Harington has been extended as part of a more general study of hydroxylation in this laboratory. The work to be reported in this paper demonstrates that oxidations and hydroxylations of aromatic compounds occur on the surface of finely divided silica, both in a dry and a moist state, and may be attributed to a concurrent series of reactions. In the moist state the predominant reaction is a complexing of the iron impurities in the silica. Thus, for the surface active mechanism, free-radical, ionic, thermal-ionic, energy transfer, and metal complex formation sequences have now been suggested for the diverse conditions used.
Experimental Two experimental procedures under aerobic conditions were adopted: (a) the “dry” reaction, in which the organic compound, the “substrate,” was ground with the silica, and (b) the “wet” reaction, in which the substrate was ground with the silica, a fixed volume of water was added, and the reactions were then followed (1) L. W. Marasas and J. S. Harington, Xature. 188, 1173 (1960). (2) L. UT.Marasas and J . S. Harington, private rommunication. (3) E. A. Hauser, “Silicic Science,” D. van Nostrand Co., Inc., Princeton, K , J., 1955. (4) It. E. Benson and J. E. Castle, J . P h y s . Chem., 62, 840 (1958). (5) H. W. Kohn, i b i d . . 66, 1185 (1962). (6) J. M . Caffrey and A. 0 . Allen, ibid., 62,33 (1958).
HYDROXYLATION O F AROMATICSON SILICA SURFACES
by spectrophotometric analysis of the compounds formed. In the experiments involving quantitative analysis the hydroxylating power of the system was determined by the conversion of salicylic acid (2hydroxybenzoic acid) to gentisic acid (2,5-dihydroxybenzoic acid), which was analyzed by the method of Benati.? Superfine crystalline silica, particle size approximately 10 p , was obtained from Quality Earths Pty. Ltd., Australia, and used in all the experiments. It was made from washed beach sand by further grinding in a pebble mill using quartz pebbles. Spectrographic analysis of the silica by Mr. L. Rannit of the Analytical Chemistry Department indicates the following impurities, reported in % : All 0.3; Fe, 0.15 ; Ti, 0.1; Ca, 0.2; Mg, 0.01; Mn, 0.03; Zr, 0.02; Cu, 0.004. ( a ) D r y Reaction. Salicylic acid (0.2 g.) and 20 g. of silica were ground together manually and 2-g. samples of the mixture were taken and allowed to react for a known time. A known volume of water was then added to the reaction mixture and after removal of the solids by centrifugation, an aliquot was taken. The optical density of the colored compound was determined directly in aqueous solution a t 495 mp, while the gentisic acid was determined by the method of Benati. ( b ) Wet Reaction. The same method as above was used except that after grinding, 1 ml. of water was added to the mixture and the mixture then allowed to react for a known time. (c) Reaction of Salicylic Acid with Moist Ferrous Carbonate. Moist freshly prepared ferrous carbonate was mixed with salicylic acid. Water was added to the reaction mixture and the solids then removed by centrifugation. Traces of ferrous ion or cupric ion were added to aliquots of the clear supernatant layer, and the increase in the optical density of the solution was determined with respect to control samples. ( d ) Irradiation of Silica-Aqueous Benzoic Acid System. Samples containing 0.5 g. of silica and 1ml. of M aqueous benzoic acid were diluted with known volumes of water. The samples were then irradiated for a known time a t a constant dose rate. The irradiated samples were then diluted to a known volume, the solids removed by centrifugation, and the salicylic acid determined in an aliquot of the supernatant layer by the ferric chloride method. Standard calibration curves for the salicylic acid were prepared in each case. ( e ) Pretreated Silica-Aqueous Benzoic Acid System. The silica samples were pretreated by: (1) pre-irradiation, ( 2 ) acid washing followed by pre-irradiation, (3) preheating a t 750’ for 24 hr., (4) pre-irradiation, followed by preheating. The procedure for the irradiation and determination of salicylic acid was identical
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with that for the nontreated silica-aqueous benzoic acid system, and in both cases control samples of aqueous benzoic acid alone were run concurrently.
Discussion and Results Even dry grinding of silica with salicylic acid effected hydroxylation of the aromatic nucleus. Marasas and Harington suggested that this reaction mechanism might involve peroxidic free radicals formed during grinding. However, such a mechanism seems unlikely in view of our results. An e.s.r. spectrometer was used to study the reaction mixture during, and subsequent to, grinding and, although the sensitivity was adequate, no free radicals were found. A second experiment supports these results. Samples of the dry reaction mixture were irradiated in a 200-c. Cs13’ yradiation source, for, if peroxide-like free radicals are involved, then irradiation should increase the possibility of the formation of such free radicals. However, the extent of hydroxylation of the irradiated samples showed no increase with respect to the control samples. It thus seems that free-radical mechanisms are not significant in the reaction. The mechanism of the reaction is known t o be dependent upon several factors, such as the structure of the adsorbent and the concentration of water (Fig. 1 and 2). Bentonite, alumina, and montmorillonite show no surface activity under these conditions,
2.0
%
200 Time, min.
400
Figure 1. Formation of products from the “wet” silica reaction: 0, gentisic acid; 0 , “red complex” ( a t 495 mr).
(7) 0 . Benati, Farm. sei. e tec. (Pavia), 5, 43 (1950).
Volume 68,Number 3 March, 1964
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P. J. SCHOFIELD, B. J. RALPH,A N D J, H . GI~EEN
100
200
300
400
Time, min.
Figure 2. Formation of products from the “dry” silica reaction: 0, gentisic acid; 0 , “red complex” (at 495 mp).
indicating that the activity may be a characteristic property of silica alone. If the silica is pretreated by acid washing to remove metal ion impurities, the hydroxylating power of the solid is decreased. If water is present, the hydroxylating power is also decreased (Fig. 1). These facts suggest that for the dry reaction, an energy transfer process is involved. It has been suggested by Kohn that stable carbonium ions are formed by the transfer of charge of intermediate ions formed in silica gel during irradiation, and it was found that water also inhibited this process. It does seem, then, that in the silica reaction, a similar transfer of energy may be the principal mechanism. This is supported by the fact that removal of the metal ion impurities, which are principally ions of metals of the transition series, significantly decreases the catalytic properties of the silica. An analogous result has been found in the radiolysis of pentane adsorbed on zeolite in that the presence of cobalt ions significantly increases the catalytic power of the system. Allen8 has suggested that the mechanism may be one of transfer of positive charge from the adsorbent to the adsorbate, involving the metal ions as the active intermediate in the transfer process. It thus seems that the mechanism of the silica reaction may be an analogous reaction of energy transfer via a metal ion as the active intermediate. The active sites formed in the silica have been shown to exist for an appreciable Thz Journal of Physical Chemistry
time, and as the time intervals of the experiments described here are relatively short, then it can be assumed that even after several hours, active sites remain and subsequent transfer of energy continues. During the grinding of silica, silicon-to-oxygen bonds are broken and it has been suggested that fragments of an ionic type are f ~ r m e d . It ~ is also known that high temperatures of short duration occur during friction of silica surfaces,1aand for the dry silica reaction it was found that the temperature of the mixture increased a little, while for the wet reaction the temperature of the mixture increased appreciably on the addition of water. As Benson and Castle4 have already suggested, the initiating step in such grinding is probably a dual thermal-ionic process. The surface of the silica is of a polar nature. For polar compounds such as the hydroxybenzoic acids, physisorption of the compounds may occur due to a preferred orientation a t the surface of the silica and the compound, resulting in the formation of an intermediate complex and subsequent hydroxylation of the compound. In the case of 4-hydroxybenzoic acid, only the 3,4-dihydroxybenzoic acid was found, whereas in free-radical hydroxylations both the 3,4- and the 2,4dihydroxybenzoic acids were found. The formation of the 3,4-dihydroxybenzoic acid alone is consistent with hydroxylation by a form of ionic mechanism, while further indirect evidence for intermediate polar interaction was found in the case of the nonpolar compounds, benzene and naphthalene, in which no hydroxylation of the aromatic nucleus occurred. For the wet reaction, the principal reaction is the complexing of the metal ion impurities by the adsorbed organic acid; in the case of salicylic acid, a ferric salicylate complex is formed. In the silica used, the iron impurity may be present either as the ferric oxide, or more particularly as the ferrous carbonate (siderite). Hence, whichever of these impurities is present should react with the salicylic acid in a similar manner to that in the silica reaction. Mixing of the salicylic acid with moist ferric oxide produced only insignificant amounts of the complex. However, mixing of the salicylic acid with moist ferrous carbonate rapidly produced a red-colored complex, with maximum absorption a t approximately 490 mw ; with 3,4-dihydroxybenzoic acid instead of salicylic acid, a blue complex was immediately produced, similar to that obtained for the compound in the wet conditions. (8) A. 0. Allen, Radiation Res. S u p p l . , 2 , 471 (1960). (9) W. A. Weyl, Research (London), 3, 230 (1950). (10) F. P. Bowden, Nature, 166, 330 (1950).
HYDROXYLATION OF AROMATICS OK SILICA SURFACES
475
The mechanism for the formation of the iron-organic acid complex may be established by reference to the following analogous conditions. The violet ferric salicylate complex formed by the addition of aqueous ferric chloride to aqueous salicylic acid exhibits maximum absorption a t 530 mk. This solution will be acidic due to the partial hydrolysis of the ferric chloride. Addition of traces of carbonate ion to this system results in the formation of a red complex with maximum absorption a t approximately 490 mp, the extent of the shift depending upon the concentration of carbonate ion present. Addition of traces of dilute acid to this system again results in the formation of the violet complex. This equilibrium may thus be represented as Hf J
(Fe3+/SA) complex red,
Amax
C
(Fe3+/SA) complex
?
490 mp
violet, Amax
=
530 mp
The red complex formed in the silica reaction exhibits identical shifts under the same conditions, and as carbonate was shown to be present in the silica, the mechanism of the formation of the complex is as follows
+ SA +Fe2+.SA+ C02Fe2+,SA + COS2- + + FeC03
(1)
0 2
-
(Fe3+.SA) complex (2) A, 490 mp
H+
(Fe3+.SA) complex A,
N
490 mp
J
COa2 -
(Fe3+.SA)complex (3) A,
= 530 mp
Evidence was obtained for the fact that the iron is initially present as ferrous ion and is then oxidized by oxygen to the ferric state. The formation of the complex occurs only a t the surface of the silica, and not in the bulk of the reaction mixture where the oxygen concentration is relatively low. After removal of the solid material in the reaction, the optical density of the red supernatant solution increased on standing, indicating aerial oxidation of ferrous ion to ferric ion, with subsequent formation of the ferric salicylate. Addition of traces of free ferrous ion or cupric ion to a synthetic mixture of ferrous carbonate and salicylic acid resulted in similar changes (Fig. 3). Preheating of the silica to elevated temperatures and oxidizing the ferrous carbonate to ferric oxide reduced the subsequent formation of the iron complex. Such pretreatment also reduced the hydroxylating power of the silica, thus verifying that the active form of the iron in the silica is the ferrousstate.
10
20 Time, min.
30
Figure 3. Catalyzed formation of “red complex” from synthetic mixture of ferrous carbonate and salicylic acid: A, no ions added; B, plus free Cu*+; C, plus free Fez+; a, complex from actual silica reaction.
Other mono- and dihydroxybenzoic acids form colored complexes with the iron impurities, and in each case the complex formed in the presence of the carbonate ions is different from that obtained by the addition of aqueous ferric chloride to the acid (Table I). However, addition of traces of acid to the silica reaction iron complex again results in the formation of the complex produced in acid conditions. It thus appears that the probable reaction mechanisms for both the hydroxylation and complexing reactions are a series of distinct steps: (I) A dual thermal-ionic process during grinding, forming active sites which, in this particular case, are probably associated with the surface hydroxyl groups of the silica. (2) Physisorption of the polar adsorbate by a form of polar interaction resulting in the formation of an intermediate complex and resultant hydroxylation by a possible positive ion intermediate and associated with the metal ion impurities. (3) If water is present, possible complexing of the metal ion impurities, with resultant inhibition of the hydroxylating power of the system. The effects of y-radiation on these systems were also studied and it may be appropriate to discuss them here. Irradiation of aqueous aromatic compounds also results in hydroxylation of the benzene nucleus. In the presence of a solid material such as silica, the Volume 68,Number 3 March, 1964
P. J. SCHOFIELD, B. J. RALPH,AND J. €1. GREEN
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250
200
200
d M
5 s
a 150
2
:
-5
0 .e
0
2
.*
4
100
v1
100
50
10
16
10
5
Volume of solution, ml.
Figure 4. Increased formation of salicylic acid in the presence of silica; ?-radiation dose, 18 hr. a t 160 rads/min. (dose rate for 10-ml. sample): 0, silica present; 0 , silica absent.
Table I : Colors of Aqueous Solutions Adsorbate
o-Hydroxybenzoic acid m-Hydroxybenzoic acid p-Hydroxybenzoic acid 2,4-Dihydroxybenzoic acid 3,4-Dihydroxybenzoic acid 3,5-Dihydroxybenzoic acid a
Silica"
Red Blue Green Red-green Blue Olive-green
FeClaa
Lilac Pale yellow Yellow Lilac Green Brown
Color of the aqueous solution from the silica reaction.
* Color of the aqueous solution from the reaction of ferric chloride with the adsorbate.
extent of the radiation-induced hydroxylation may either be reduced or increased. Competition by the solid decreases the concentration of solvent radicals available for the hydroxylation reaction, and the yield of hydroxylated product might consequently decrease. However, it has been shown for the radia-
The J O U Tof ~Physical Chemistry
20
Volume of solution, ml.
Figure 5. Effect of preconditioning on the catalytic activity of the silica; ?-radiation dose, 18 hr. a t 160 rads/min. (for 10-ml. sample): 0, untreated; 0 , pre-irradiated ( 5 days a t 160 rads/min.); VI preheated (at 750" for 24 hr.); 0, pre-irradiated and preheated.
tion-induced hydroxylation of aromatic compounds that the initial step is the formation of a hydroxycyclohexadienyl radical, which then reacts with any oxygen present to form the corresponding peroxy radical. l 1 Spontaneous decomposition of this radical results in the formation of the phenolic compound. Hence, in the presence of an added solid, which is capable of adsorbing both the solute and oxygen in close contact, the possibility of forming the intermediate peroxy compound is increased, and hence the yield of hydroxylated product is correspondingly increased. The presence of the silica enhanced the radiationinduced hydroxylation of aqueous benzoic acid (Fig. 4) and of aqueous phenylalanine. Pretreatment of the silica by irradiation or heating significantly decreases the catalytic property of the solid (Fig. 5). This decrease is presumably due to the destruction of the active sites on the silica. (11) L. M. Dorfman, R. E. Suehler, and I. A. Taub, J . Chem. Phys., 3 6 , 649 (1962).
