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Polymerization and Heterogeneous Catalysis

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Alkene Metathesis. Schrock catalyst. Grubbs catalyst. Mechanism. ROMP. Polymerization. Free radical ... Draw the structures of the products A-E. Give the metal ... – PowerPoint PPT presentation

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Title: Polymerization and Heterogeneous Catalysis


1
Polymerization and Heterogeneous Catalysis
  • ROMP
  • Ziegler-Natta polymerization
  • Free radical polymerization
  • Condensation polymerization
  • Surface characterization
  • Heterogeneous catalysts

2
Alkene Metathesis
Grubbs catalyst
Schrock catalyst
3
Mechanism
4
ROMP
5
Polymerization
  • Free radical
  • Carbocation
  • Metal alkene
  • Metal alkyl
  • Metal alkylidene

6
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
7
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
8
Polyolefin Production
9
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10
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11
Cossee Mechanism
12
(No Transcript)
13
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14
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15
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

16
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

17
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.

18
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
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