A PHYSICOCHEMICAL STUDY OF THE ALUMINATION OF SILICALITE AND THE DEALUMINATION OF ZEOLITE ZSM-5

The nature of occurrence of acidic sites generated by treatment of silicalite with aluminum trichloride in the vapor phase have been assessed by measurements of ion-exchange capacity, powder X-ray diffraction, X-ray photoelectron spectroscopy, e.s.r. of exchanged cooper (II), n-hexane cracking, and methylene blue adsorption. The results point to the formation of a range of intracrystalline acidic sites, while aluminum ions bound to the external surface during the vapor treatment are removed by subsequent dilute acid washing. The inability of the acid sites to persist on the external surface of silicalite exposed to aluminum trichloride vapor is shared by similarly treated dodecasil-1H and dodecasil-3C. Similar measurements show that effective dealumination of ZSM-5 occurs by deposition of a silica layer from aqueous solution but is accompanied by a significant presence of amorphous silica. The formation of a silica layer on ZSM-5 by gas-phase silylation with tetramethoxysilane results in effective surface coverage, though a minor component of the material (about 1%) is unaffected by this treatment. High-temperature treatment with steam or silicon tetrachloride of ZSM-5 does not remove aluminum selectivity from the crystalline surface. © 1990.