Why Are Support Surfaces of Heterogeneous Catalysts Important Ligands for Selectivity Control

1

• DOE Basic Energy Sciences Contractors Meeting
• Selectivity Breakout Session May 25, 2004
• Report
• Examples of recent advances and future prospects,
  for specific topics selected from those below,
  are given in the
• appendix.
• The role of mechanistic understanding in
  controlling selectivity at catalyst active sites
• a) Using mechanism-based understanding
  (especially good kinetic information) to
  understand and predict
• selectivity
• b) Better methods for using kinetics and
  steric and electronic effects to understand the
  relationship between structure and selectivity in
  well-defined heterogeneous catalysts
• c) How to apply what is known about
  selectivity control in homogeneous systems to
  heterogeneous systems. Example how to transfer
  information from enantioselective homogeneous
  systems to enantioselective heterogeneous
  systems. Other types of selectivity should be
  explored as well.
• d) Start looking at surfaces as ligands.
  Most supported catalysts are oxidesneed to look
  at other supports e.g. MgCl2, TiCl3, understand
  how structure of the catalyst influences the
  selectivitye.g., chiral crystals

2
2) Understanding and utilizing novel methods for
controlling selectivity at active sites of
catalysts (e.g., spatial and temporal control by
compartmentalization, constrained volume,
reaction sequence, shape control, orientation
control, phase separation). a) How to
employ the mechanisms used by enzymes in
synthetic catalytic systems (e.g., hydrogen
bonding, electrostatic effects, shape
selectivity, etc.) b) Examining size
effects on selectivity issues at the nanoscale
i. e., synthesis of different sized particles
and examination of how their size affects
catalytic reactivity and selectivity (e.g.,
Pt-catalyzed alkane hydrogenation)
c) How to stabilize nanoparticles on
heterogeneous supports new anchors to avoid
sintering during catalytic reactions
d) Use of membranes (for reactant
selection/product separation, or as a catalyst
support) to control catalyst
selectivity e) Immobilized enzymes how to
anchor them to surfaces and control and
understand their behavior f) Spin control
of reactivity g) Understanding selectivity
of naturally anchored enzymes (e.g., those bound
to membranes, inorganic surfaces)
3
h) Controlling the timing of sequential
reactions product activation of catalysts to
initiate subsequent reactions i)
How to control selectivity between one-electron
and multi-electron reactivity j)
Cascade reactions chemoselective consecutive
reactions how to control multiple catalysts in
the same solution improving
selectivity by coordination of tandem reactions
kinetics and transport issues k) Methods
are needed to develop new supramolecular and
nanovessel catalysts, as well as heterogeneous
catalysts with a wider range of pore and
binding site sizes with the goal of moving from
binding studies to the development of
well-defined materials with specifically tailored
active sites. These should not just recognize
molecules, but also accelerate their
reactions. 3) New non-thermal methods of
catalyst activation (e.g., photochemical,
microwave, plasmas, phase transition)
a) Use of alternate activation methods to change
selectivity e.g., microwave irradiation,
high-energy plasmas, photochemistry,
etc., and understanding the physical basis of
these phenomena controlling selectivity in
highly energetic species by specific
bond activation b) Channeling of energy
released in exothermic steps into the next
endothermic steps to lower the energy required
for the overall reaction.
4
4) Determining the structure of the active site
at the molecular level a) How to
define the target size in both homogeneous and
heterogeneous catalysis? b) Better methods
for depositing nanoclusters on solid supports and
understanding their structure and function
c) How to design active sites of
heterogeneous catalysts to control selectivity.
Need more structure-activity
relationships for heterogeneous systems d)
Developing surface chemistry e.g., cutting
crystals at certain angles to develop
enantioselective chemistry e) Need
to control redox properties and coordination
properties to control reactivity f) How to
deal with the problem of how many sites on a
surface are actually active, and how many are
inactive? Need ways to focus on the
actually active sites use of isotope labeling,
poisoning experiments to sort this out g)
Ways to tie structure/function to
synthesis/characterization 5) Polymerization and
oligomerization selectivity (stereochemical
control, molecular weight control) a)
Modification of polymers, e.g. with chiral
ligands b) Use of constrained binding to
affect non-statistical oligomerization of
alkenes, or multiple activation of substrates
c) Selectivity in sequencing polymers
d) Controlling molecular weight and
measurement of kprop vs kct, the rate constants
involved in determining this,
especially with regard to structure-activity
relationships for different ligands, metals, etc.
Also for copolymerization, there is a
need for fundamental kprop vs kcts for different
co-monomers. e) What is required to
efficiently incorporate polar co-monomers? New
strategies are needed for instilling metal
preference for CC vs heteroatom lone electron
pairs, as has been accomplished for Ru metathesis
catalysts.
5
Selectivity Breakout SessionAppendix Examples
of Recent Accomplishments and Future Prospects
6
How Does Understanding Mechanism in Homogeneous
Catalysis Serve as a Guide to Selectivity?
Example Tacticity control in olefin
polymerization can be achieved by understanding
the mechanism and structural characteristics of
the metal-olefin complexes Ansa
linkage permits control of side-to-side and
top-to-bottom coordination.
Example Alkane s-complexes lead to selective
activation at the thermodynamically preferred
position on an alkane Metal can
migrate up and down alkyl chain until it finds
the best position for insertion, leading to
selective activation of terminal methyl groups
with many metal complexes.
What is needed Further studies of mechanism are
likely to lead to new advances in alkane
functionalizations, polymerizations, reductions,
hydrogenations, and green chemical reactions.
How can reactions in alternative media/geometries
be used to control selectivity? Calculations can
provide insight into relative barriers for
processes that cannot be directly observed.
7
Catalyst Support Surfaces Ligands for
Selectivity Control

• Future Work
• Characterize bonding between Catalysts and
• Supports Experimentally Theoretically
• Establish Relationship Between Local Molecular
• Structures/Electronic Interactions and Support
• Ligands
• Establish Relationship Between Local Molecular
• Structures/Electronic Interactions and
  Kinetics of
• Competing Pathways

• Rationale
• Selectivity Controlled by Relative Rates of
  Competing
• Pathways
• Relative Rates Influenced by Local Molecular
• Structures and Electronic Interactions
• Support Ligands Affect Local Molecular
  Structures
• and/or Electronic Interactions
• Optimizing Selectivity Requires Controlling the
  Local
• Molecular Structures and/or Electronic
  Interactions

Example
Redox
Acid
Redox
Redox
Acid
Acid

• Support Ligands Control Electron Density/Cluster
  Size of Active Surface Site

8
Known Single-Site Catalysis on Super Brønsted
Acids
97 ? 2 Sites Active!
87 ? 2 Sites Active!
Possible Tandem Processes
Ziegler Site Oligomerization Site ROMP Site Chain
Transfer Site
Cationic Site Anionic Site Second Ziegler
Site Hydrogenation Site
Close Proximity ? Multiple Coupled
Transformations ?
9
Stereocontrol in Alkene Polymerization Reactions
10
How to take advantage of enzymes and their
sophisticated control mechanisms in synthetic
catalytic systems?

• Why enzymes?
• Enzymes embed and use all what we know as smart
  features in catalysis (self-regulation, on-off
  switches, etc.)
• Understanding these features at the molecular
  and atomic levels is critical for successful
  integration in synthetic catalytic systems.
• The structure and dynamics of enzymes control
  very tightly nonproductive competing pathways and
  therefore selectivity.
• Emerging concepts in catalysis point to a
  critical role of dynamics -even for heterogeneous
  systems traditionally thought as rigid- in the
  control of both selectivity and activity at the
  atomic level.
• Possibilities for immobilization of enzymes -or
  bioengineered functionally-active subdomains- are
  expanding and will open invaluable opportunities
  at the frontiers of biomimetics and heterogeneous
  catalysis.

Examples Control features in NO catalytic
biosynthesis by NOS enzymes
Dynamic active site of NOS enzymes
Fe
Shell/protein-assisted control of electronic
properties of the active site by long range
H-bonding/p-stacking
Static H-bonding/p-stacking of the shell ensures
the right electronic properties of the core
iron-heme

• Areas to be targeted
• Determine paradigm mechanisms used by enzymes,
  coupling selectivity to high activity at the
  molecular and atomic levels.
• Investigate the role of soft tuning of
  prosthetic catalytic sites in enzymes allowing
  the same core catalyst (e.g. heme) perform very
  different reactions (e.g. reductions, oxidations,
  mono- or di-oxygenation..).
• Integrate smart biocatalytic features in
  heterogeneous and homogeneous model systems.
• Develop efficient immobilization procedures for
  bioengineered catalytic components in synthetic
  catalytic systems.

Cofactor/substrate-driven dynamic redox
modulation of the active site
Dynamic binding of cofactor and/or substrate
assists in fine-tuning redox properties of iron
to facilitate electron transfers required for
oxygen activation.