STEIN RESEARCH GROUP
POROUS SOLIDS — NANOCOMPOSITES — SELF-ASSEMBLED FRAMEWORKS
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Hierarchical
Templating and Functionalization of Porous
Catalysts and Catalyst Supports
During
the last decade, much progress has been made in the bottom-up design
of porous solids by a wide range
of chemical
techniques,
including self-assembly, surfactant and polymer templating. Materials
exhibiting a uniform arrangement of pores offer a wide variety of applications
that are
based on both the chemical properties of the solid matrix and properties
specific to the pore size and arrangement. Porous materials have been classified
according
to their pore diameters. “Micropores” are those with diameters
less than 2 nm, “mesopores” range from 2 nm to 50 nm, and “macropores” are
greater than 50 nm in diameter. Microporous zeolites have found commercial
applications as adsorbents, molecular sieves, and size or shape selective
catalysts. Mesoporous
silicates, prepared by surfactant templating or block-copolymer templating
can be useful in chemical applications involving larger guest molecules,
such as
biological compounds. These materials possess high surface areas, ordered
channel structures, and narrow pore size distributions. For most applications,
however,
the pore surfaces must be modified. We have developed several synthetic
approaches that permit modification of the mesoporous sieve surface
by direct syntheses.
We have demonstrated that vinyl, thiol, and other organic functions
can be incorporated in these well-ordered materials in-situ, and proved
that
they
are located within
the channels. We have also introduced a method of incorporating organic
functional groups in the walls of the mesoporous support by using precursor
molecules
with two silicate units surrounding the organic group (bissilsesquioxanes).
To
achieve larger pore sizes, we developed novel colloidal crystal templating
techniques, which have been used by us and other researchers
to produce three-dimensionally ordered macroporous (3DOM) solids composed
of insulator, semiconductor, or metal compositions. We showed that
it was possible
to combine colloidal crystal templating with surfactant templating or zeolite
synthesis to produce hierarchical structures: macroporous solids (200-800
nm pores) surrounded by silicate walls with mesopores (2-4 nm pores)
or micropores
(0.5 nm pores). Organic functional groups can also be introduced in these
systems, which combine the selectivity and high surface areas due to
the smaller pores
with easy access through the macropores. Furthermore, it is possible to adjust
other properties of the macroporous solids (e.g., catalytic properties) through
functionalization and structuring of the walls.
We have studied the following systems
in catalytic reactions:
•mesoporous and 3DOM silica functionalized
with polyoxo- metalate clusters
for epoxidation reactions
•mesoporous silica functionalized
with titania clusters for photooxidation and
thermal degradation
of dye molecules
•mesoporous silica loaded with copper
porphyrins for oxidative bleaching of dye
molecules
•mesoporous silica functionalized
with sulfonic acid groups for acid catalysis
•3DOM alpha-alumina modified with
silver for epoxidation of ethylene
•3DOM sulfated zirconia for isomerization
of butane
Fig.
1. Schematic diagram of the preparation and structure of a porous
silicate containing both macropores and mesopores with surface functional
groups. Mesopores are produced by a surfactant templating process and
macropores by colloidal crystal templating. The organic functional
groups are introduced directly during the synthesis and are accessible
for further reaction, such as uptake of heavy metal ions or introduction
of dye molecules within the walls.
Fig. 2. SEM and schematic structure of a macroporous zeolite prepared by combining colloidal crystal templating with the use of a structure directing agent for the zeolite, silicate.The thin silicalite walls contain size-selective, 0.5 nm micropores with short diffusion paths, while the ca. 500 nm diameter macropores permit efficient transport of guest molecules.