Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 33

crystalline Sn-b-zeolite. 48 This zeolite has tin atoms within the zeolite
framework. Their work elucidated the structure at tin sites as well as their
distribution in the zeolite framework. Their work also suggests that a
similar technique may be applicable to characterization of zeolite-supported
molecular metal complex catalysts if they have high structural uniformity as
well as uniform distribution within a zeolite.
Some of the spectroscopic techniques listed in Table 2.1 can be carried out
under reactive atmosphere to investigate catalysts under working con-
ditions 49-54 and sometimes multiple techniques are used in concert with
transient method to characterize their formation of active catalytic species
on supports and to show how the structures evolve during catalysis. 55 Re-
sults are correlated with catalysis data obtained in separate experiments
using a flow reactor. In those cases, flow patterns in sample cells used in
spectroscopic measurements should match those used in the catalytic re-
action experiments so that mass transfer does not disguise results and direct
comparisons can be made.
Structural uniformity of zeolite-supported metal complexes has been
investigated by IR spectroscopy using CO as a probe molecule. Solid state
13 C MAS NMR spectroscopy was used to investigate dynamic structural
uniformity of zeolite-supported rhodium diethene complex. These examples
are described in detail in Section 2.4.3.

d n 9 r 4 n g | 7

2.4 Guide for the Synthesis and Characterization
of Supported Molecular Metal Complexes with
a High Degree of Structural Uniformity

.

2.4.1 Types of Zeolite for Support Materials
There are several criteria for choosing the type of a zeolite as a support
material. The sites where cationic metal complexes are anchored are acid
sites (near framework aluminium sites) of zeolites (Figure 2.1). Therefore, to
ensure site isolation of supported metal complexes, a high silica-alumina
ratio is preferred. At the same time, acidic sites should be accessible to the
metal precursors. Ideally, these anchoring sites are crystallographically
equivalent so that these sites function as identical ligands.
Choice of the type of a zeolite is limited by the relative pore aperture of a
zeolite. 56 FAU zeolite is often used because of the largest pore aperture
among commercially available zeolites. 18-26,32 Ogino et al. investigated mo-
lecular sieving effects in the synthesis of zeolite-supported rhodium and
ruthenium complexes. 56 Zeolites tested in these investigations were HSSZ-
42, Hb, HMOR, and HZSM-5. Critical molecular dimensions of the pre-
cursors (Rh(Z 2 -C 2 H 4 ) 2 (acac) (acac ¼ acetylacetonate, C 5 H 7 O 2 , see Section
2.4.2 for more details) (I), Ir(Z 2 -C 2 H 4 ) 2 (acac) (II), Ru(Z 2 -C 2 H 4 ) 2 (acac) 2 (III),
and Rh(CO) 2 (acac) (IV)) and the narrowest pore apertures of the zeolites are
shown in Figure 2.2.