Process for preparing silica-alumina

DETAILED DESCRIPTION OF THE INVENTION The term "silica-alumina hydrogel" as used herein is defined as a coagulated colloid of coprecipitated, hydrated silica-alumina containing an embedded aqueous phase.
The terms "acidic sites" and "acidity" of silica-alumina compositions as used herein refer to the presence of any sites (on the surface of the silica-alumina composition) which can chemically interact with any Bronsted or Lewis base.
The acidity of silica-alumina can be determined by any of the methods described in the literature such as in "Solid Acids and Bases--Their Catalytic Properties" by Kozo Tanabe, 1970, Academic Press, New York, NY, pages 5-23, and "Surface Acidity of Solid Catalysts" by H.
A.
Benesi et al, Advances in Catalysis, Volume 27, 1978, Academic Press, pages 99-112, both literature citations being herein incorporated by reference.
The process for making silica-alumina in accordance with this invention comprises the step of mixing a first aqueous solution, comprising an aluminum salt plus an ammonium salt of an acid having a pKa of less than 2, with a second aqueous solution comprising an alkali metal silicate, wherein the amount of the ammonium salt in said first solution is such that a silica-alumina hydrogel is coprecipitated upon said mixing.
Suitable aluminum salts are those that are substantially water-soluble such as Al(NO.
sub.
3).
sub.
3, Al.
sub.
2 (SO.
sub.
4).
sub.
3, KAl(SO.
sub.
4).
sub.
2, NH.
sub.
4 Al(SO.
sub.
4).
sub.
2, aluminum acetate and the like.
Suitable ammonium salts include NH.
sub.
4 Cl, NH.
sub.
4 Br, NH.
sub.
4 NO.
sub.
3, (NH.
sub.
4).
sub.
2 SO.
sub.
4, NH.
sub.
4 ClO.
sub.
4, ammonium trichloroacetate and the like.
Suitable water-soluble alkali metal silicates include Na.
sub.
2 SiO.
sub.
3, K.
sub.
2 SiO.
sub.
3, Cs.
sub.
2 SiO.
sub.
3, Na.
sub.
2 Si.
sub.
2 O.
sub.
5, K.
sub.
2 Si.
sub.
2 O.
sub.
5, Na.
sub.
2 Si.
sub.
2 O.
sub.
7, Na.
sub.
4 SiO.
sub.
4 and the like.
Presently preferred process ingredients are KAl(SO.
sub.
4).
sub.
2.
16H.
sub.
2 O, (NH.
sub.
4).
sub.
2 SO.
sub.
4 and Na.
sub.
2 SiO.
sub.
3.
Preferably, the mixing of the two solutions is carried out with agitation, e