Small crystal ZSM-5, as a catalyst

DETAILED DESCRIPTION OF THE INVENTION In one embodiment of the invention, a component of the crystallization reaction mixture is a silica precipitate, the particle size of which is in the range of 1 to 500 microns.
Preferably, a silica precursor is formed under conditions which insure certain minimal particle size of the precipitated silica precursor and subsequent reaction of that silica precursor, in crystallization reaction mixtures of controlled composition, to form the porous crystalline silicate.
Such a procedure has the advantage of increased throughput and yield.
The silica precursor is formed from a solution of a silica source.
Conveniently, the solution is an aqueous solution of a pH ranging from 9 to 12.
The source of silica can be any soluble silicate and is preferably sodium silicate.
The silica precursor can be formed by its continuous precipitation from the solution phase.
Accordingly, precipitation can comprise initiating precipitation and maintaining said precipitation.
More particularly, the precipitation step can be continuous.
Alteration of the composition of the solution of silica source can be undertaken by introducing a precipitating reagent.
In one embodiment, the precipitating reagent can be an acid solution.
The acid of the solution may be any mineral acid, such as H.
sub.
2 SO.
sub.
4, HCl, HNO.
sub.
3, etc.
The acid solution can have a pH ranging from essentially 0 to about 6.
Thus in one embodiment of the invention, precipitation can be effected by acid neutralization of a basic solution of a silicate.
In one of two alternative embodiments, the silica can be precipitated alone, i.
e.
, in the absence of sources of other crystalline silicate framework elements.
In this embodiment, both the precipitating reagent and the solution of silica source can be free of intentionally added alumina or alumina source.
However, silicate precipitation can also occur as coprecipitation in the presence of soluble sources of other porous crystalline silicate framework elements, e.
g.
, gallium, indium, boron, iron and chromium