Synthesis and Properties of Ti-Rich TS-1
A R T I C L E S
The catalytic property of TS-1 depends on the lattice Ti content,
which is, however, usually less than 2 wt %.8,9 The effective
way to increase the Ti content in the framework of TS-1 is still
a huge challenge. Thangaraj and Sivasanker reported that eight
Ti ions could be incorporated in the lattice sites per unit cell
by an improved method (method B) in which titanium tetra-n-
butoxide was first dissolved in isopropyl alcohol before addition
to the hydrolyzed tetraethyl orthosilicate aqueous solution for
the purpose of avoiding formation of TiO2 precipitate by
reducing the hydrolysis rate of the alkoxide,10 but Schuchardt
et al. could not reproduce it, and found that there was no
difference in the framework Ti content between the samples
synthesized by methods A and B.11 This is also proven by our
data shown in the following section.
To synthesize Ti-rich TS-1, it is necessary and helpful to make
its crystallization mechanism clear. However, very few reports
have been devoted to the study of this subject.12 This might be
due to the complexity of the zeolite crystallization mechanism.
Indeed, it involves both nucleation and crystal growth,12–27 and
both have numerous simultaneous and independent equilibria
and condensation steps while nanoaggregates might evolve into
zeolite crystals.17–27 In addition, the synthesis materials, i.e.,
silica source, solvent, and templating/structure-directing agent
as well as the crystallization promoter or inhibitor, also
significantly affect the crystallization mechanism. As a result,
with respect to nucleation, there are several models, including
homogeneous nucleation, heterogeneous nucleation, autocata-
lytic nucleation, and secondary nucleation, and different nucle-
ation processes have been tried to be accounted for by different
models.12,13,17–27 In the case of crystallization growth, a
transition period involving a slow growth of a crystalline phase
and a relatively rapid crystal growth period are included.
Crystallization in hydrothermal systems may occur through the
liquid-phase transformation mechanisms,13–27 whereas in non-
aqueous systems the crystallization mechanism via solid-phase
transformation seems to predominate.13,14 In contrast, in the case
of synthesis of ZSM-5 in the system free of solvent by using
TPABr and NH4F as cotemplates, a vapor phase transport
mechanism has been suggested.28 Therefore, up to date, although
more than 170 types of zeolitic materials with various uniform
topologies have been synthesized and some of them have been
widely used in petrochemical and fine chemical industries as
active and selective catalysts and/or effective adsorbents, their
crystallization mechanisms have not been clear yet. A full
understanding of the crystallization mechanisms is desirable for
the synthesis of conventional zeolites with significantly im-
proved performance and new zeolites.
The crystallization process of titanosilicates is much more
complex than that of aluminosilicates because Ti4+ has a weak
structure-directing role and is much more difficult to be
incorporated into the framework than Al3+. As we know,
isomorphous substitution of metal atoms for Si in zeolites is
not only related to zeolite structures/framework composition
flexibility and the chemical nature of metals29–33 but is also
strongly influenced by the crystallization mechanism. The
framework composition flexibility of zeolites is chemically
important. Isomorphous substitution of transition metal ions for
Si requires a flexibility of the framework to bring together a
cluster of oxygen atoms around metal cations.32 This could be
made easier by hosting many defect sites in the framework.9,34,35
Therefore, the more flexible the framework of zeolites is, the
higher the substitution degree is.33 Thus, more Ti species could
be incorporated into the framework of Ti-Beta and Ti-MWW
than into that of TS-1.36,37 Nevertheless, Ti K edge extended
X-ray absorption fine structure (EXAFS) studies have shown
that the Ti-O bond length of tetrahedral Ti(OSi)4 species is
about 1.80 Å in contrast to the 1.61 Å of the Si-O bond
length.35,38–43 The Ti-O bond is much longer than the Si-O
bond, probably making the local structure around Ti seriously
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