Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 516

For many of these technical applications, the zeolite particles (typical size of a
few m) have to be partially consolidated or attached to substrates. Usually,

spheres, granules, rods, or tubes with a diameter of millimeters are used in different
types of packed-bed reactors. However, there is still need to optimize these catalytic
systems. For example, for fast reactions, like the partial oxidation of benzene to
phenol, the use of thin catalytically active layers of small zeolite crystals would be
preferable (Hiemer et al. 2004 ). A promising approach to comply with both
requirements is the use of cellular monoliths such as ceramic foams coated with the
desired zeolite type.
In principle, there are three different methods to obtain zeolite-covered cellular
monoliths. The simplest one is dip coating the foam in a slurry that contains the
zeolite particles, a dispersant/solvent, and a binder. However, by using this method,
only foam monoliths with a large cell size are coatable, and the homogeneity of the
layer thickness inside the cells is not very high. A second approach is the synthesis
of a desired zeolite structure directly on the foam surface. The hydrothermal
crystallization of a zeolite-forming reaction mixture in the presence of PDC foams
can be used even for support foams with relatively small cell sizes and leads to a
zeolite layer of very homogenous thickness (Zampieri et al. 2004 ). The layer
thickness can be varied by synthesis parameters such as synthesis time or the ratio
of support/reaction mixture. However, in this case, the zeolite growth reduces the
cell diameter and can cause a blocking of the cells.
A more sophisticated technique is the partial

µ

transformation of the three-

dimensional cellular structure. In case of filler-containing polymer-derived foams,

the type of filler can be chosen with respect to the requirements for the zeolite

synthesis. Silicon powder can be used to obtain Si-rich zeolites such as MFI-type
zeolite (which is an important catalyst for isomerization reactions, for the decom-
position of nitric oxides, and also for partial oxidation reactions) after a hydro-
thermal treatment in the presence of a structure-directing agent (SDA) (Schef

fl er

er 2006 , 2008 ). By 29 Si-MAS NMR spectroscopic investigations, it
could be shown that the silicon

and Schef fl

filler in the starting mixture is transformed into the
zeolite coating during the hydrothermal treatment (Schef

er et al. 2005a ). Al-rich
zeolites, which are important components in hydrocracking catalysts as well as in
separation processes such as pressure or temperature swing air separation, might be
obtained by the partial transformation of PDC foams

fl

filled with Al powder as
shown in Fig. 17 . The properties of the zeolite layer such as crystal morphology can
be in

uenced by varying the reaction parameters. The special feature of these
materials is the very good interdigitation between the support structure of the foam
and the zeolite layer formed by the partial transformation of the

fl

filler component,
even if more than 50 % of the foam is transformed. Al-rich zeolites and their
composites are also good candidates for thermochemical energy storage via the
reversible sorption of water vapor.