Base-Catalyzed Silica Gels: Structure and Chemistry
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BASE-CATALYZED SILICA GELS: STRUCTURE AND CHEMISTRY GARRY J. GARVEY and BRUCE E. SMITH AKZO Chemicals Inc., Livingstone Ave.,
Dobbs Ferry,
NY
10522.
ABSTRACT The chemical and physical properties of two base-catalyzed silica gels systems were characterized. The sol-gel transition Pyrolysis behavior was studied by titration, rheometry, and NMR. was monitored by TGA and mass spectroscopy. Many characteristics were strikingly similar for the two base-catalyzed systems. However ammonium hydroxide additions resulted in heterogeneities Ammonium at lower concentrations than ammonium carbonate. carbonate enhanced oxidation on pyrolysis. INTRODUCTION Gelation of hydrolyzed tetraethyl orthosilicate (TEOS) sols can be induced by addition of aqueous bases, which under some circumstances results in heterogeneous gelation due to localized regions of high pH. This commonly occurs for highly concentrated ammonium hydroxide additions. The problem can be avoided by normality of a weaker base such as ammonium using an equal carbonate. The current study is a comparison between silica gels prepared using these two bases. The two-step process of gel formation is an excellent method of rapidly producing a silica gel. First, aqueous acid catalysis Second, of TEOS produces rapid and nearly complete hydrolysis. base catalysis of the hydrolyzed sol promotes rapid This process has been well polymerization and gel formation. characterized [1] and chemical mechanisms have been proposed (2,3]. Ion specific effects have been noted [4]. The conditions under which this two step process occurs The mechanical properties of the solaffect the gel structure. The gel transition have been studied by shear rheometry [5]. evolution of gel structure over long time periods has been Gel investigated by transverse loading of cast gel bars [6,7]. The surface structure has also been characterized by NMR [8,9]. area, residue hydroxyl, and carbon contents of TEOS-derived dry gels have been investigated [10]. The effects of chemistry on the pyrolysis and densification of dry gels were addressed [11]. The current study seeks to apply several of these approaches to define differences generated by two bases, ammonium hydroxide and ammonium carbonate. This study follows the evolution of some chemical and physical properties from solution to calcined silica gel in an attempt to identify these differences. PROCEDURE A clear sol was prepared from TEOS, [AKZO, Silbond Pure ®] using 1N nitric acid (Fisher, ACS reagent), ethanol (Quantum, U.
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
224
S. P.) and deionized water (18 MC) and refluxed for ten minutes. The sol composition was 1 mole TEOS, 2.7 moles water, 0.05 moles ethanol at 0.0125 N nitric acid concentration. The sol sample was analyzed by capillary GC (HP-5890 with a 30m x 0.25mm DB5 column). Hydrolysis was more than 80 % complete as determined by ethanol peak intensity. No unreacted monomer was 0 detected. The hydrolyzed TEOS sols, thermostated to 25 C, were titrated using each base catal
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