Selectivity of Aldehyde CH Activation Reactions in Nanovessels

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Selectivity of Aldehyde C-H Activation Reactions
in Nanovessels
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Not encapsulated
Encapsulated

• Challenges in nanovessel recognition and
  chemistry
• Understanding physical factors that control size
  and shape selectivity in host-guest systems
• Installation and reactivity of functional groups
  inside nanovessels
• Understanding intravessel stereochemical
  relationships and isomerization reactions
• Developing efficient enantiorecognition in
  host-guest systems
• Developing efficient nanovessel-catalyzed
  reactions

2
Membranes Potentially, an efficient method for
improved selectivity 1) Ionically conductive
membranes for achieving selectivity for
hydrogen production potentially also for
electrochemical production of other
chemicals. 2) Membranes for fine
chemicals 3) Development of membranes
with well-defined pore structures (zeolites,
microporous materials) 4) Strong Pd membranes
(supported in or on another structure to allow
thin, cost-effective use of Pd.)
Figure from report by Air Products Inc.
3
Spin control of selectivity
Future challenges

• Establish whether spin changes can indeed
  significantly effect reactivity - need more
  unambiguous examples of two state reactivity
  (notion is still controversial)
• Emphasize study of reactions featuring open shell
  compounds as reagents, intermediates, or products
• Identify chemical system and circumstances under
  which spin blocking is expected to be important
  (e. g. reaction types, metals etc.)
• Demonstrate that spin change can be used to
  control selectivity and offers a dimension of
  catalyst design
• Develop better computational methods for
  characterizing reactions involving multiple spin
  surfaces (i. e. location of minimum energy
  crossing points and calculation of crossing
  probabilities)

Potential energy surface of an oxygen atom
transfer reaction involving crossing from a
doublet to a quartet surface
4
Microwave Control of Catalysis an example
Auto-exhaust Catalyst lightoff

• Microwave Energy
• Suppresses SO2 poisoning (selective desorption)
• Lowers Lightoff Temp. (selective heating of
  sites)
• Changes Selectivity (selective desoprtion)
• CAN Also
• Enhance Catalyst Synthesis (zeolites other
  more uniform morphology)

SO2
µwave
Conner et al., Catalysis Letters 71, 3-4, 133-8
(2001)
5
Origins of Surface Chirality

• Natural chirality
• Originates from the surface structure of the
  substrate.

• 2. Chirally modified surface
• Isolated chiral modifiers induce 11
  enantiospecific interactions with chiral
  adsorbate.

• 3. Chirally templated surface
• Chiral modifiers form structures with long-range
  chiral order.

• Challenges
• Preparation of high surface area, naturally
  chiral materials.
• Predictive control of enantio-specific
  modifier-adsorbate interactions.

6
Rationale Heterogeneous photoelectrochemistry
provides paths to important chemical syntheses
Example CO2 Fixation promoted by MnS
CO2

• Future Challenges
• Understand how to design photocatalysts for
  specific applications
• Control of selectivity promoted by understanding
  of mechanism

X.V. Zhang, Scot T. Martin, C.M. Friend,
Harvard University Funded by NASA
7
How Do Coordination and Redox Properties of
Active Surface Sites Affect Selectivity?

• Rationale
• Selectivity Controlled by Relative Rates of
• Competing Pathways
• Relative Rates Influenced by Coordination and
• Redox Properties
• Optimizing Selectivity Requires Controlling
• Coordination and Redox Properties

• Future Work
• Establish Relationship Between Coordination and
• Redox Properties
• Establish Relationship Between
  Coordination/Redox
• Properties and Kinetics of Competing Pathways

• More Active Surface Redox Sites are More
  Selective

8
How Does Synthesis Method Affect
Structure/Function Properties?

• Future Work
• Establish Relationship Between
  Structure/Function Properties and
• Synthesis Method
• Establish Relationship Between
  Structure/Function Properties and
• Kinetics of Competing Pathways

• Rationale
• Selectivity Controlled by Relative Rates of
  Competing
• Pathways
• Relative Rates Influenced by Structure/Function
• Properties
• Optimizing Selectivity Requires Controlling the
  Specific
• Structures/Functions of Active Surface Sites

Structure of Active Surface Site

• Synthesis Method Controls Distribution of Active
  Surface Sites

9
Tandem Catalysis in Alkene Polymerization
Tandem Catalysis. Below is an example of tandem
catalysis whereby three independent catalytic
centers work together to provide a single product
(branched polyethylene) from a single starting
material (ethylene). The optimum reaction
conditionswere obtained by using high throughput
technologies developed by Symyx.
Non-statistical oligomerization reactions. There
are no catalysts that allow for the selective
formation of pentamers or higher condensation
products for any substrate available at the
present moment. For example, in the
production of decene from ethylene, one
oligomerizes ethylene with a suitable catalysts
to generate a Schultz-Flory distribution of
products. Distillation into size specific
products is then required, which results in the
generation of waste and undesirable fractions. A
dream set of reactions is shown below, in which
Cat1, Cat2, etc. correspond to size-specific
oligomerization catalysts.
10
Supported catalysts may alter reactivity by
suppressing certain bimolecular reactions and
favoring others
Example Single-site metal catalysts that
form polyesters by ring-opening polymerization
also enter into bimolecular trans-esterification.
Supported single-site catalysts grow chains
that are released into the solution as
exclusively cyclic polymers/oligomers by
intrachain trans-esterification
Potential New classes of cyclic polyesters and
polycarbonates