Hydrothermal Synthesis and Growth of Zeolites - Hydrothermal Technology - page 336

metal cations permits the preparation of promising catalysts for both complete
oxidation of alkanes and NO x decomposition.
Guth et al. [88] have developed a new route of the synthesis of microporous
compounds in the presence of fluorine. The addition of fluoride ions in the reaction
medium seems to induce mineralization and allows the crystallization of zeolites at
neutral or acidic pH and the partial substitution of silicon by other tri- or tetravalent
metals, like B 3 1 ,Al 3 1 ,Ga 3 1 ,Fe 3 1 ,Ge 4 1 , and Ti 4 1 . Just like in other phases, fluo-
rine is also incorporated in the open frameworks, and the best example is cloverite,
with 3D 20-membered ring channels, in which fluorine is located at the center of a
D4R cage [168,169] . A similar structural diversity is associated, in part, with the
ability of vanadium-oxygen compounds to form tetrahedral, square-pyramidal, and
octahedral coordination polyhedra, and to build larger aggregates by the condensa-
tion of polyhedra through shared oxygen atoms [170,171] . Such works produce
novel structures/frameworks that encapsulate organic guests to produce large cavi-
ties or tunnels. After the organic template is removed by ion exchange or thermal
methods, the solid may sorb substrate molecules in a size- and shape-selective fash-
ion [172] . Similarly, there are several mixed tetrahedral octahedral framework

structures which are both novel and different from the traditional aluminosilicate
zeolites, where only TO 4 tetrahedra (T

Si, Al, or Ge) are the basic building units.
Mixed TO 6 and TO 4 units are also found in the structures of recently synthesized
microporous

5

176] .
These compounds show good ionic conductivity and humidity-sensing property as
seen in microporous Na-germanates. Leonowicz et al. [177] have obtained a new
boron, MCM-22, zeolite that contains structural features previously unknown
among zeolites. The high-resolution electron micrographs and synchrotron X-ray
diffraction powder data reveal two independent pore systems, both of which are
accessed through rings composed of 10-tetrahedral (T) atoms (such as Si, Al, and
B). One of these pore systems is defined by 2-d, sinusoidal channels. The other
consists of large supercages whose inner free diameter, 7.1 ˚ , is defined by 12 T-O
species (12-rings) and whose inner height is 18.2 ˚ . These coexisting pore systems
may provide opportunities for a wide variety of catalytic applications in the petro-
chemical and refining industries.
Caro et al. [149] have obtained aligned molecular sieve crystals as nonlinear opti-
cal materials. As is well known, the formation of L¨ngmuir

stannosilicates, germanates, and gallophosphates [173,174

Blodgett films and the
crystallization of noncentrosymmetric organic crystals are widely studied as promis-
ing techniques to get ordered molecular arrangements for nonlinear optics and opto-
electronics [147] . The required bulk alignment of molecular dipoles can also be
achieved by their adsorption in the pores of appropriate molecular sieve matrices.
Caro et al. [149] have studied the optical second harmonic generation (SHG) by
para-nitroaniline (PNA) adsorbed in AlPO 4 -5 crystals. No SHG effects could be
observed for the unloaded AlPO 4 -5 matrix. Figure 6.48 shows the experimental
arrangement of the ps-Nd:YAG laser device and the position of the AlPO 4 -5 crystal
in the laser light beam [149] . The relative intensity of the SHG signal as a function
of the angle between the polarization plane of the incident laser light and the c-axis
(length direction [001]) of AlPO 4 -5 crystal is plotted in Figure 6.49 . The vanishing