Infrared studies of reactions on oxide surfaces. 5. Lewis acid sites on

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1995

Lewis Acid Sites on Dehydroxylated Silica

Infrared Studies of Reactions on Oxide Surfaces. 5. Lewis Acid Sites on Dehydroxylated Silica B. A. Morrow* and 1. A. Cody Department of Chemistry, University of Ottawa, Ottawa, Ontario, KIN 6N5, Canada (Received March 5, 1976) Publications costs assisted by the National Research Council of Canada

When silica is subjected to a vacuum degassing a t about 600 "C two new infrared bands are observed at 908 and 888 cm-l and the intensity of these bands increases with higher degassing temperature reaching a maximum a t about 1200 "C. These bands are associated with the formation of a new active site on to which pyridine and trimethylamine can reversibly coordinate without dissociation of the site or of the adsorbate. It is suggested that a surface silicon atom of strained siloxane bridge is electron deficient thereby acting as a Lewis acid center. The nature and unusual chemical reactivity of this site are discussed in the following paper We have previously shown1 that when silica is subjected to a high temperature degassing under vacuum (400-1200 "C), new active sites are formed which are capable of acting as centers for the subsequent room temperature chemisorption of ammonia, methanol, or water (no chemisorption occurs a t 20 "C without the vacuum degassing). The new site was associated with the appearance of two infrared bands at 888 and 908 cm-l in the spectrum of degassed silica? and the site was thought to be a strained siloxane bridge (Si-0-Si). Both the 908- and 888-cm-l bands disappeared in unison when titrated with small doses of reactant (or with the boron halides2). In the present paper we report that these active sites are also capable of displaying typical Lewis acidic behavior following the adsorption of pyridine or trimethylamine whereby only one of the aforementioned infrared bands disappears. This is the first evidence for the presence of such sites on silica and as such may have considerable importance with respect to the mechanism of the rehydration of hydrophobic silica^.^,^ Preliminary details of the latter are discussed in the following paper which is concerned with the chemisorption of NH3 H2O on such silicas, and further mechanistic details will follow in part 7.

Experimental Section (This section applies to this and the following paper.) The silica used in this work was Cab-0-Si1 HS-5 and had a BET (N2) surface area of 320 m28-l. The bulk powder was generally heated in a muffle furnace a t 700 "C.prior to disk pressing in order to oxidize any hydrocarbon impurities, although no differences were observed if this procedure was omitted. One inch diameter disks were pressed in a stainless steel die at a pressure of 1000 lb in.-2 for 2 s and the optical thickness was 10 or 40 mg cm-2. The quartz cell has been described previously5 and NaCl windows were sealed with either Glyptal or with Tec Kits E-7 epoxy resin. Heating temperatures