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.