Supported Nanoscale Catalysts for Oxidative Hydrolysis
Cellulose to Monosaccharide Sugars and their Derivatives
The objective of the research is to improve our fundamental understanding of
composition/structure/function relationships in supported, heterogeneous
catalysts for reactions that oxidatively convert cellulose to monosaccharide
sugars, such as glucose and other chemical intermediates.
In homogeneous catalysis by metals, control of the redox chemistry that
governs the catalysis is accomplished by changing the type of organic ligand
attached to the central metal. Heterogeneous catalysis, while sometimes more
difficult to control, affords us greater latitude to direct the physical and
chemical nature of the metal clusters. By exploiting the ability to size select
specific metal clusters or alloys along with the type of support material, we
can control the redox chemistry of the catalyst for specific reactions. New
cluster synthesis techniques coupled with angstrom level control of the support
material allow us unprecedented ability to tailor the catalytic properties in
this important class of catalysts.
The fundamental understanding that will result from these studies will
provide insight into controlled oxidation of large, biomolecules containing
multiple –OH bonds. The particular focus of this project is on supported metal
clusters with sub-nanometer dimensions. The influence of size and structure on
the catalytic properties of clusters having dimensions <1 nm (<50 atoms) has
been studied to a much lesser degree than particles >1 nm because the small
clusters are difficult to prepare, stabilize, and characterize.
The project will accomplish its objectives through highly integrated efforts
in:
- Novel catalyst synthesis, using advanced techniques, of highly uniform
supported sub-nanometer catalytic metal and alloy clusters,
- Measurements of catalyst atomic and nanoscopic structure and composition
under synthesis, pretreatment and operating catalytic reaction conditions,
and
- Experimental elucidation of kinetics and mechanisms for selected, model
catalytic reactions and surface chemical reactions involving catalytic
intermediates.
The chemical reactions of interest for this project are those that could
oxidatively convert cellulosic materials into glucose, which could then be
converted enzymatically into ethanol for use in transportation fuels. The
overall reaction of interest is: cellulose + O2/H2O
à glucose
The key bond breakage is the glycosidic (C-O-C) bond connecting the two
glucose moieties. In such a reaction, the goal is to minimize the breakage of
non-glycosidic bonds including the C-C and C-OH bonds as well as the C-O-C bond
contained in the glucose ring. The glycosidic bond joins two carbohydrate
(sugar) molecules together to form cellulose. Further oxidation of the –OH side
groups could result in chemical intermediates with, for example, aldehyde and
ketone linkages. The sensitivity of certain reactions to catalyst type will be
tested via the utilization of probe reactants (e.g. acetals) that contain some
of these key bonds but have sufficiently high vapor pressures to allow
investigating their reactivity in the gas phase. We hypothesize that the
activity and selectivity of key reactions in oxidative reforming can be
controlled by supported cluster size and composition via their electronic
properties and their influence on the local concentration of surface
intermediates. We also hypothesize that catalyst stability can be influenced by
properties of the support and by porous oxide overcoats. These hypotheses will
be tested by experimental investigations of the kinetics and mechanisms of key
reactions involving reagents and intermediates along with in-situ and
operando measurements. |
