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Figure 2.5 Acetylacetonate (anionic form of pentane-2,4-dione, Hacac).
d
n
9
r
4
n
g
|
7
LM(acac) + 2HO(O)Al
zeolite
LM{O
2
Al}
+
Hacac-HO(O)Al
→
Scheme 2.1 Schematic representation of reaction of an acetylacetonate metal com-
plex, LM(acac), with an acid site of a zeolite. The brackets indicate the
surface of a support.
Gates and his co-workers used a series of metal acac complexes to syn-
thesize zeolite-supported metal complex catalysts. Examples of the metal
complex precursors include Au(CH
3
)
2
(acac),
74
Rh(CO)
2
(acac),
19,55
Rh(Z
2
-
C
2
H
4
)
2
(acac),
23,46,47,75,76
Ir(Z
2
-C
2
H
4
)
2
(acac)
71,77-80
and Ru(Z
2
-C
2
H
4
)
2
(acac)
2
.
53-63
Metal atoms in these complexes are cationic and have formal charges of
þ
1 (Au, Rh, Ir) or
þ
2 (Ru). Syntheses of these samples are typically con-
ducted in Schlenk flask using a standard air-exclusion technique. A calcined
zeolite in the proton form is reacted with a metal precursor complex in a dry
n-pentane solvent under an inert atmosphere. Upon coming into contact
with zeolites, acac ligands in metal acac complexes undergo ligand exchange
with oxygen atoms of the zeolite. After the ligand exchange completes, the
solvent is removed in dynamic vacuum. Supported catalysts synthesized by
this method are readily used for catalytic reactions without pretreatments at
elevated temperatures. Using metal complexes having other ligands such as
halides complicates structural characterization
19,76
and may cause un-
.
desired catalytic consequences.
Ligand exchange of acac ligands with bidentate ligands of a zeolite is
triggered presumably by the protonation of an acac ligand by a proton of a
zeolite and dissociated Hacac (protonated form of acac) becomes adsorbed
on the surface of a zeolite support (Scheme 2.1).
82,83
Protonated acac ligands
are removed from the metal center and become adsorbed on a zeolite as
characterized by IR spectroscopy.
23,77,82,83
Interaction energy of protonated
acac (Hacac) with a silanol group via hydrogen bonding was estimated as
6kcalmol
1
by DFT calculations.
83
Furthermore, this value could easily be 10
kcal mol
1
depending on the exact form of the hydroxyl group. Thus, the
interaction of Hacac with a zeolite surface provides an additional driving force
for the facile ligand exchange. The ligand exchange reaction appears to be
stoichiometric if a zeolite has enough number of accessible acid sites as
compared with the amount of the metal precursors
81,84
and pore aperture of a
zeolite is large enough to allow the access of metal complexes to the acid sites
of the zeolite.
56
Goellner et al. synthesized a zeolite-supported rhodium complex by the
reaction of Rh(CO)
2
(acac) with dealuminated zeolite Y (denoted as HY).
19
Rh(CO)
2
(acac)
is
a well-known metal
complex precursor used in
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