Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 28

d n 9 r 4 n g | 7

Schematic representation of a cationic metal complex (M(L)(L 0 )) an-
chored on a zeolite. M represents a metal. L and L 0

Figure 2.1

represent ligands.

When supported metals are small and consist of a single atom or a few
atoms, a support plays important roles as ligands to control their catalytic
properties. 3,4 Thus, non-uniformity of a support leads to a spectrum of various
catalytic properties of supported metals. Using crystalline supports such as
zeolites is advantageous because the hindrance of surface non-uniformity may
be largely overcome. Zeolites provide well-defined anchoring sites for cationic
metal species because isomorphous substitution of Si 41 with Al 31 forms a
negative charge in the zeolite framework, which is compensated by an ex-
changeable cation. 5 When a metal complex is anchored on a zeolite, the zeolite
often acts as an anionic bidentate ligand as schematically shown in Figure 2.1.
Because the density of anchoring sites in a zeolite can be varied by
changing its Si/Al ratio, the spatial distance between supported metal
complexes can be controlled to some extent. In addition, zeolites offer
unique environments for metals when they are anchored in molecular-sized
and ordered micropores of zeolites, enabling some catalytic reactions that
are dicult to be achieved using other catalysts. 5-7 Unique catalytic per-
formance of zeolite-supported metal complex catalysts has been demon-
strated in various reactions such as the oxidation of methane to
methanol 8-10 and deNO x reaction. 11-13
This chapter describes the synthesis and characterization of zeolite-
supported molecular metal complex catalysts (supported mononuclear
metal complexes that function essentially as molecules). These catalysts are
in a subclass of supported metal catalysts. There are many excellent reviews
that show detailed investigations of the local structures of zeolite-supported
metal complexes and their relationships with catalytic performance. 14-17 The
focus of this chapter is the synthesis method of structurally uniform
supported species, characterization to demonstrate the uniformity of
supported species, and investigations of structure-performance relation-
ships using structurally uniform supported species.
Structural uniformity of the zeolite-supported metal complex is a key to
determine their structures precisely and understand catalytic chemistry at
the molecular level. Spectroscopic techniques such as extended X-ray
absorption fine structure (EXAFS) and X-ray absorption near edge structure
(XANES) spectroscopies provide exact structural information if the sup-
ported species have a high degree of uniformity. Structural uniformity offers
the opportunity for investigations of catalytic cycles in detail as performed
for molecular catalysts in solution; a combination of transient experiments

.