Various ways of preparing tin oxide sols are well known in the art. For example, while not intended to be but a cursory survey of the state of the patent literature, the following patents are nevertheless believed to be representative.
SnO.sub.2 sols have been prepared by electrodialysis (U.S. Pat. Nos. 3,723,273; 4,147,605; and 4,203,822); by treating alkali metal stannate with an acid to precipitate hydrated stannic oxide, washing to remove electrolytes, and peptizing the hydrated stannic oxide floc with potassium hydroxide or potassium stannate (U.S. Pat. Nos. 3,346,468 and 3,462,373); and treating alkali metal stannate with ion exchange resins (U.S. Pat. Nos. 3,455,794 and 3,888,788).
Nyacol Products Inc., assignee of the instant application, manufactures and sells a tin oxide sol under the trade name SN-20 prepared by ion exchange and which finds particular use as an additive to crude oil as a scavenger for metals that can adversely affect the petroleum cracking process.
In the preparation of this commercial product, a weak acid ion exchange resin is placed in a reactor, water added to form a slurry and thereafter adding to the slurry an aqueous solution of an alkali metal stannate. The stannate is deionized by the resin to precipitate out small particle size tin oxide. However, the particles grow to a larger particle size, e.g. on the order of 100 nm. The desired small particle size is then obtained by adding a proprietary additive and then heating and evaporating to the final desired concentration, e.g. 25.0.+-.0.2@25.degree. C., pH on the order of about 10.5, viscosity &lt;10. While as heretofore alluded to, the foregoing commercial synthesis is entirely satisfactory for the preparation of a tin sol for use in the petroleum cracking process, the presence of the additive precludes its use in many other projected commercial applications. Thus, while the presence of such additives in the relatively "dirty" crude oil mix is of no consequence, in other applications envisioned for use it is.
Stated simply, the task of the present invention is to provide a method of preparing aqueous tin oxide sols having negatively charged SnO.sub.2 particles which do not require the presence of any additive to provide a mean particle size no greater than about 50 nm and most preferably on the order of 5-20 nm.