Polymerization and Heterogeneous Catalysis

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Polymerization and Heterogeneous Catalysis

• ROMP
• Ziegler-Natta polymerization
• Free radical polymerization
• Condensation polymerization
• Surface characterization
• Heterogeneous catalysts

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Alkene Metathesis
Grubbs catalyst
Schrock catalyst
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Mechanism
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ROMP
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Polymerization

• Free radical
• Carbocation
• Metal alkene
• Metal alkyl
• Metal alkylidene

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Quiz
Draw the structures of the products A-E. Give the
metal oxidation state and total electron count
of each complex.
A Fe(II), 18e
B Fe(II), 18e
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Quiz
Draw the structures of the products A-E. Give the
metal oxidation state and total electron count
of each complex.
Ta(V), 18e
Ta(V), 18e
Ta(V), 10e
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Polyolefin Production
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Cossee Mechanism
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Heterogeneous Catalysis History

• 1875 Pt catalyzed oxidation of SO2 to SO3 (then
  sulfuric acid)
• 1903 Oswald process, NH3 oxidation on Pt gauze
  to form nitrogen oxides (then nitric acid)
• 1908 Haber process for the synthesis of NH3 from
  N2 and H2 over iron oxides
• 1923 Fischer-Tropsch synthesis of hydrocarbons
  from CO and H2
• Various hydrocarbon modification processes
  (cracking, reforming) WWII
• More recent are hydrodesulfurization and other
  hydro-treating processes
• Most heterogeneous catalysts are finely divided
  metal on a metal oxide support. The metal can be
  an oxide or a sulfide as well as elemental metal.
  Modifiers and activators are used

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Terms

• 1. Catalyst
• Increases the rate of a chemical reaction
  without being consumed. It may be radically
  changed due to the reaction. It can't change the
  ultimate equilibrium of a chemical reaction.
• 2. Selectivity
• The extent to which a catalyst accelerates the
  reaction to form one or more desired products
  instead of all of those that could be formed.
  This varies with temperature, pressure and extent
  of conversion.
• of reactant used which forms the desired
  product
• 3. Yield
• Reported on either a weight of product/weight of
  reactant or volume of product/volume of reactant.
  If the products have a lower density than the
  reactants, the yield may be more than 100

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Terms

• 4. Site or Active Site
• The reaction takes place only at particular
  sites on the solid material. It may be a group or
  cluster of atoms on the surface. It may be a
  defect in the crystal lattice.
• Structure sensitive reactions are those whose
  rates vary with the detailed structure of the
  surface.
• Structure insensitve reactions are those whose
  rates depend only on the total number of exposed
  metal atoms on the surface.
• 5. Turnover number
• Number of molecules that react per site per unit
  time. It is difficult to determine the true
  number of active sites.

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Catalyst Deactivation

• Poisoning
• Compound that reacts at the surface is a poison,
  reduces activity, sulfur
• Fouling
• Physical blockage, coking
• Reduction in surface area
• Sintering, growth of crystallites
• Loss of active species