A PHYSICOCHEMICAL STUDY OF THE AGING OF COLLOIDAL SILICA-GELS USED IN ZEOLITE-Y SYNTHESIS

An investigation was carried out to determine the effects of room-temperature aging on the structure and composition of NaY zeolite synthesis gels prepared from colloidal silica. Alkaline solutions containing varying amounts of sodium aluminate were added to a colloidal silica sol to produce gels of composition 4 Na2O, x Al2O3, 10 SiO2, 180 H2O (x = 0.0 - 1.5). Each of the gels was homogenized and then allowed to age at room temperature. Changes in the composition and structure of the solid and liquid phases of zeolite synthesis gels were followed as a function of time using elemental analysis; n.m.r., i.r. and Raman spectroscopies; and X-ray diffraction. Initial mixing of the colloidal silica, caustic, and aluminate has no noticeable chemical or structural effects beyond gelation via flocculation of the silica particles and adsorption of Al and Na onto the surface of the particles. The colloidal silica dissolves during aging as a consequence of successive cleavage of siloxane bonds. Adsorption of Al reduces the rate of silica dissolution. The slow dissolution of the silica releases monomeric silicate anions that rapidly react with aluminate anions to form an aluminosilicate precipitate, estimated to have a Si/Al ratio of 1. Once the dissolved aluminum is depleted from the liquid phase, the remaining colloidal silica dissolves away, leaving only an amorphous aluminosilicate in the solid phase, which can then rearrange through interaction with the silicates anions in solution. As discussed in a companion study, the aluminosilicate precipitate formed during aging serves as the precursor for nuclei which induce the cyrstallization of NaY zeolite.