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C. C. BALLARD, E. C. BROGE,R. K. ILER, D. S. ST.JOHNAND J, R. MCWHORTER
Vol. 65
ESTERIFICATION OF THE SURFACE OF AMORPHOUS SILICA BY C. C. BALLARD, E. C. BROGE,R. K. ILER, D. S. ST.JOHNAND J. R. MCWHORTER Contributionfrom the Industrid and Biochemicals Department, E. I. du Pont de Nemours & Co., Inc., Wilmin.gtm,Delaware Received A w w l 10, lQS0
Primary and secondary alcohols react with the silanol groups on the surface of particles of amorphous silica until a monomolecular, oriented layer of chemically-bound organic groups is formed. Numerous types of amorphous silica are shown to take part in this type of topochemical reaction. This chemisorption, with formation of silicic ester groups, proceeds only under essentiallyanhydrous conditions. The reaction proceeds rapidly to completion only a t elevated temperature and p r e s sure. The surface-esterified amorphous silica powders have an organic content which increws with specific surface area and molecular weight of the alcohol used. The products are highly hydrophobic and organophilicin nature. Packing of the alkoxy grou 8 on the silica surface is correlated with molecular size. The equilibrium constant for the reaction S i O H ROH Si8R HlO was measured in the case of n-butyl alcohol. The energy of activation for the esterification reaction is 21.9kcal. per mole of butoxy groups and for hydrolysis is 9.0 kcal. mole-’. The esterification reaction is endothermic and requires 12.1 kcal. mole-’.
+
+
Introduction Although the physical association of alcohols with silica gels and powders has been studied by numerous investigators1-‘ the possibility of a topochemical reaction between alcohol and the surface of amorphok silica has only recently been appreciated. As described by Iler,6s6 Braendle,’ and Stevenson,8 esterification of the surface of finely divided silica renders the powder organophilic and hydrophobic. Recently, Stober, Bauer and ThomasQ studied the chemisorption of aliphatic alcohols on the surface of pure fumed silica and showed that the alkyl groups must be oriented normal to the surface and are essentially closepacked. There are about 8 hydroxyl (silanol) groups per square millimicron on the original silica surfacea6 Only about three alkoxy groups such as n-butoxy, are required to cover this area, so that two-thirds of the silanol groups are thus covered over and remain inaccessible to further reaction. Esterification of the surface to produce hydrophobic products occurs readily with primary or secondary alcohols containing 2 or more carbon atoms. In the case of methano1,lO highly anhydrous conditions and elevated temperature and presdure are required. Tertiary alcohols do not react readily and, furthermore, tend to be decomposed by dehydration at elevated temperature. Substituted alcohols will esterify the surface ; if the substituent groups are polar, the resulting surface assumes the polar character of the substituent group. 11-16 (1) D. C. Jones and L. Outridge, J . Cham. Soc., 1574 (1930). (2) L Robert, Compl. Rend.. 284, 2066 (1952). (3) F. E. Bartell and D. J. Donahue, J . PA#& Chem., 66, 665-670 (1952). (4) F. E. Bartell and R. M. Suggitt, ibid., 68, 39 (19543. (5) R. K.Iler. U. 9. Patent 2,657,149 (E. I. du Pont de Nemours & Co., Inc., Oct. 27 (1953). (6) R. K. Iler, “The Colloid Chemistry of Silica and Silicates.” Cornell University Preaa, Ithaca, N. Y., 1955. (7) G. C. Meyer and R. 0. Braendle, “A New Class of Siliceoua Thickening Agents,” National Lubricating Grease Institute, San Francisco, Cal., October 25-27, 1954. (8) A. C. Stevenson, “Effects of Surface Modification on Silica fill^^," Elaetomer and Plastics Group, Northeastern Eection, Amer. Chem. SOC.,Cambridge, Mass., Nov. 16, 1954. (9) W. Stober, G. Bauer and K. Thomas, Ann. Chsm., 604,104-110 (1957). ( 1 0 ) E. C. Broge, U. S. Patent 2,736,668 (E. I. du Pont de Nemoura and Co.. Inc., 1956). (11) R. K Iler, U. S. Patent 2,739,074 (E. 1. du Pont de Nemours & Co.. 1956). (12) R. K. Iler, U. S. Patent 2,739,076 (E. I. du Pont de Nemours L Co..1956).
The present paper relates primarily to the kinetics of the esterifkation of the silica surface with some of the lower saturated aliphatic alcohols, principally 1-butanol.
Experimental Silica Substrates.-Finely divided amorphoua ailica o different ultimate particle siaea and specifio surface m a s were repared from sodium silicate by methods which have gee, described previously and are categoriged as tabulated. Type of eilica
Speciijc surface, m.Va.
Method of preparation
A B
25-100 -100 180-200 300 300-800
Drying sol” Sodium silicate and acids & l h g 801, drying Sodium silicate and acid’s Special process19
C D E
Sols were purified by ion exchange; gels were washed with acid and water. Unless noted otherwiee, the mlim was employed in the wet condition for further reaction with dcohols. Methods of EsterScation.-In preliminary studies, the silica was suspended in alcohol and free water removed from the system, usually by azeotropic distilhtion. With lower boiling alcohols, the anhydrous mixture waa heated either in stainless steel autoclaves or in sealed g h tubes. The alcohol vapor was then either vented from the pressure vessel, leaving a dry product, or the mixture waa cooled rapidly and the silica recovered by filtration and dried under vacuum a t 100-120°. With high boiling alcohols the mixture was heated a t atmospheric pressure, cool+, filtered and the product wm wsshed with a suitable volatde solvent such aa acetone, to remove adsorbed alcohol, then dried under vacuum a t 100-125’. The kinetics and equilibria of the surface esterification of alica with 1-butanol were studied by determining the rate of change of carbon content at a series of temperatures ranging from 118 to 320’. Data at 118” were obtained from an extended azeotropic dehydration of silica gel (300 m.*/g. specific surface area) a t atmospheric pressure. Samples of slurry (averaging around 8% silica) were re moved a t intervals and analyzed to determine the water content and the degree of esterification. Data for higher temperatures were obtained by pumping slurry, which had been partially esterified at 118”, through an oil-heated heat exchanger operated under pressures chosen to maintain (13) E. C. Broge, 0.8. Patent 2,739,078 (E. I. du Pont de Nemours & Co., 1956).
(14) M. T.Goebel. U. 8.Patent 2,739,077 (E. I. du Pont de Nemours & Co., Inc.. 1956). (16) R. K. Iler, U. 8. Patent 2,739,076 (E. I. du Pont de Nemours L Co., 1956). (16) K. L. Berry, R. M. Joyce and J. E. Kirby, U. 8. Patent 2,757,098 (E.I. du Pont de Nemourr & Co., bo.. 1956). (17) G. B. Alexander and R. R. Iler, J . Phyr. Chsm., 67, 932 (1953). (18) See reference 5, Example 2. (19) G. B. Alexander, E. C. Broge, R. K. Her, U. 8. Patent 2,765.242 (E. I. du Pont de Nemoura & Co., Inc., 1956).
ESTERIFICATION OF THE SURFACE OF AMORPHOUS SILICA
Jan., 1961
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TABLEI DIFFERENT DEGREES OF ESTERIFICATION OP THE SILICA SURFACE WITH B BUTANOL Expt. no.
Type of subatrate
D D D D D
r-Reaction Temp., "C.
Silica
conditions-
Time, hr.
?z: esterif.
sub-
strate Sp. surf., m.*/s.
