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d
n
9
r
4
n
g
|
7
Figure 2.4 HAADF-STEM (Z-contrast) images characterizing the catalyst prepared by
the reaction of Ir(Z
2
-C
2
H
4
)
2
(acac) with zeolite HSSZ-53: (A) the initially
prepared catalyst and (B) the catalyst after it had been used for ethene
hydrogenation. The images show individual Ir atoms (some of them
indicated by white circles) well dispersed inside the one-dimensional
channels of the zeolite and in the absence of detectable iridium clusters.
Reproduced from ref. 53.
Control of the distribution of acidic sites of zeolites provides another
benefit to the synthesis of structurally uniform supported metal complexes.
Synthesis of a zeolite with specific tetrahedral aluminium sites is chal-
lenging particularly when zeolites consist of various crystallographically
different T atoms. However, a recent report shows that the synthesis of
zeolites by a particular combination of SDAs induces a specific distribution
of aluminium sites.
73
.
2.4.2 Precursor Metal Complexes
A variety of organometallic precursors can be used in the synthesis of
oxide-supported metal complexes.
33-35
Although various organometallic
precursors (see examples in ref. 33-35) can be also used in the synthesis of
zeolite-supported metal complexes, metal complex precursors are preferred
to have ligands that mimic the surface of zeolite ligands. Acetylacetonate
(C
5
H
7
O
2
, abbreviated as acac hereinafter) ligands schematically shown in
Figure 2.5 act as anionic bidentate ligands and coordinate to a cationic
metal ion via two oxygen atoms. Because acac ligands mimic oxygen atoms
at an acidic site of a zeolite (Figures 2.1 and 2.5), the ligand exchange of
acac ligands with those from a zeolite is expected to cause a minimal
structural exchange (slight changes in the metal-oxygen bond distances
are often observed by ligand exchange because of the differences in the
electron density of oxygen atoms in acac and those in a zeolite, see
Section 2.4.1).
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