PRECURSORS OF THE ZEOLITE ZSM-5 IMAGED BY CRYO-TEM AND ANALYZED BY SAXS

The microstructural evolution in the nucleation stage of a synthesis r eaction of ZSM-5 zeolite was studied, with particular emphasis on the role of the organic cation, TPA+ (tetrapropylammonium). Direct observa tion of the microstructure was achieved by cryo-transmission electron microscopy (cryo-TEM). The quanitative evaluation of the structural un its was obtained by small-angle X-ray scattering (SAXS) measurements. In the presence of the organic cation TPA+, we found globular structur al units 5 nm in diameter that aggregate to elongated bodies 44 nm lon g. The globular structural units were found in reaction mixtures with or without the organic template TPA-OH, as long as the pH of the initi al solution was kept above 11.6. Without TPA+, aggregation to cylindri cal particles was not observed after 2 h of heating, and no ZSM-5 crys tallinity was attained at the end of the reaction (after 8 d). These r esults suggest that the zeolite ZSM-5 building blocks are globular str uctural units, 5 nm in diameter, containing silica, alumina, organic c ation, and water, which fuse together to produce elongated aggregates that may compose the final unit cell. It is concluded that the globula r structural unit is a cluster of tetrapods similar to those found in the final ZSM-5 crystals.