Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 31

2.2.3 Synthesis by the Surface Organometallic Chemistry
Approach
The surface organometallic chemistry approach merges disciplines of solu-
tion organometallic chemistry and solid surface chemistry. 33-40 Numerous
examples of syntheses of oxide-supported metal complexes via this route
have been reported. From the perspective of synthesis of zeolite-supported
metal complexes with uniform structure, the surface organometallic chem-
istry approach provides several advantages over other methods. First, syn-
thesis by ligand exchange of an organometallic precursor does not require
high temperature treatments that may result in the formation of multiple
supported species. Second, a wealth of information about organometallic
chemistry provides a basis for investigating the structure and catalytic
chemistry of supported metal complexes. Finally, organometallic precursors
can be designed to have ligands that mimic zeolite surface and those that
function as intermediates in a catalytic cycle.
Organometallic compounds having labile ligands such as Z 3 -allyl and
alkyls have been used as metal complex precursors in the synthesis of oxide-
supported metal complexes. 14,33-40 These organometallic compounds can be
reacted with zeolite in an organic solvent or contacted with zeolite after
being vaporized. Ligands in the initially prepared samples can be post-
synthetically exchanged with other ligands as described later in this chapter.
Gates and his co-workers reported the syntheses of a family of zeolite-
supported mononuclear metal complex catalysts with uniform structure
using the surface organometallic approach. Some criteria of choices of or-
ganometallic precursors and zeolites to synthesize structurally uniform
supported metal species are described in detail in the Section 2.4.

d n 9 r 4 n g | 7

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2.3 Characterization Techniques
Zeolite-supported molecular metal complex catalysts are characterized by
multiple techniques such as EXAFS and XANES spectroscopies, transmis-
sion electron microscopy (TEM), solid state NMR, IR, UV-visible spectro-
scopies and X-ray photoelectron spectroscopy (XPS). 40-42 DFT calculations
guide band assignments in spectroscopic characterization and aid in se-
lecting a candidate structure model. Strengths and limitations for each
characterization techniques have been summarized in several review papers
and some of them are listed in Table 2.1. 16,42 An important point to note is
that these techniques are complementary to each other and thus multiple
techniques are required to characterize supported metal complex catalysts.
In addition, it is important to understand the limitations of each technique.
For example, EXAFS provides direct information about local structures
around a particular atom regardless of the phase of the materials. This
technique determines the inter-atomic distance accurately ( 0.02 Å). 43,44
However, it gives average information of the whole sample. Therefore,
characterization of

zeolite-supported metal complexes consisting of