As Catalysts

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					CHBE 551 Lecture 24
      Catalysis I




                      1
        Importance Of Catalysis

   90% of all chemical processes use
    catalysts
   Changes in catalysts have a giant
    influence on rates and selectivity’s of
    reactions.
    More than anything else
   Most real reactor design associated with
    optimizing performance of catalyst



                                               2
         Catalysis Definition

Ostwald defined a catalyst as a substance
 which changed the rate of reaction
 without itself being consumed in the
 process

Not being consumed  catalyst does change




                                            3
Catalytic Reaction Occurs Via A
        Catalytic Cycle:

reactants + catalyst  complex

complex  products + catalyst




                                  4
   Example: Rhodium Catalyzed
      CH3OH+COCH3COOH
                        CH3COOH                            CH3OH
                                            HI


                                            H2O

                           CH3COI                          CH3I
                                       [Rh(CO)2I 2]-



                                  C
                                  O                         CH3
                              I         I              I          I
                        H3C       Rh                        Rh
                              C         I                          I


                                                  C
                                  C                         C
                                                  O
                              O   O                         O
                                                            CO
    Figure 12.1 A schematic of the catalytic cycle for Acetic acid production via the
    Monsanto process.




Printing press analogy

                                                                                        5
 The Rate Enhancement Of A Number Of
 Reactions In The Presence Of A Catalyst

       Reaction         Catalyst       Rate     Temperature
                                    Enhancement
  Ortho H2  Para H2   Pt (solid)      1040       300K
   2NH3N2 + 3H2       Mo (solid)     1020        600K
  C2 H4 + H2  C2 H6   Pt (solid)      1042       300K
   H2 +Br2  2HBr      Pt (solid)     1  108     300K
2NO + 2H2 N2 + 2H2 O Ru (solid)      3  1016    500K
CH3COH  CH4 + CO       I2 (gas)      4  106     500K

 CH3CH3  C2H4 +H2     NO2 (gas)      1  109     750K

   (CH3)3 COH        HBr (gas)      3  108     750K
 (CH3)2CH2CH2+H2O


                                                              6
        Examples Of Effects Of Solvent


Table 13.2 The rate of the SN2 reaction NaCl + CH3I  NaI + CH3Cl
at 350 K
______________________________________________________________
               Rate Constant,                   Rate Constant,
Solvent      liter/(mol-second)  Solvent     liter/(mol-second)
______________________________________________________________
Gas phase         ~10-45          ______            _______

Water           3.5 x 10-6       Methylcyanide          0.13

Methanol       3.1 x 10-6     Dimethylformamide      2.5
______________________________________________________________


                                                                    7
            Types Of Catalysts

•   Homogeneous catalysts:
      Soluble compounds that go in
      solutions

•   Heterogeneous catalysts:
       Solids that sit in reactors




                                     8
   Common Examples Of Catalysts


Homogeneous
  Enzymes (detergents,
  digestion, tears)

Heterogeneous
  Catalytic converter




                                  9
   Rate Laws Different Than With
        Gas Phase Reactions


Rate proportional to the surface area not
 the volume.




                                            10
       Effects Of Surface Area

Consider a platinum catalyzed reaction.
You can run the reaction
1) Run the reaction on the wire
2) Take the wire and smash it with a
   hammer and then run the reaction.

The rate will be higher on the wire you
   smashed with a hammer!


                                          11
      Why Does Smashing A Wire
         Change The Rate?


•   When you squashed the platinum you
    created more surface area.
•   You also changed the shape of the
    surface which can affect the rate.




                                         12
          Turnover Numbers

Rates of catalytic reactions often expressed
 as turnover number


                     RA
                TN =
                     NS
 RA = Rate per unit area (molecules/cm2-sec)
 NS = Number of exposed metal atoms /
      unit area (Atoms/cm2)
                                               13
Turnover Numbers For Some Typical
           Reactions


                                               Dehydrogenation

      -1
                              2    Hydrogenation
                     10
       Turnover Number, sec
                              0                                             Silicon
                     10
                                                                           Deposition
                                                                GaAs
                                                              Deposition
                              -2
                     10


                              -4
                     10               Olefin
                                   Isomerization
                                                Alkane
                              -6
                                             Hydrogenolysis
                     10                       Cyclization
                                   200     400    600     800    1000      1200    1400
                                         Reaction Temperature, K

                                                                                          14
                                         Typical Catalytic Kinetics

                                    13
                               10
     Rate, Molecules/cm -sec


                                                                450 K            450 K
     2




                                    12
                               10                                               440 K
                                                                440 K
                                                                                 425 K
                                                              410 K
                                                                                 415 K
                                                      390 K
                                    11
                               10        -8           -7                   -6        -8            -7               -6
                                    10             10                 10        10               10            10
                                              CO pressure, torr                           O 2 pressure, torr



Figure 2.15 The influence of the CO pressure on the rate of CO
 oxidation on Rh(111). Data of Schwartz, Schmidt, and Fisher



                                                                                                                         15
   Typical Catalyst Kinetics




               k1PCO PO 2
   R CO 
             1  K 2PCO      2


Called a Langmuir-Hinshelwood rate
   law. Also called Monod rate law.

                                      16
                                  Temperature Dependence


                                          PO2=2.5E-8 torr                   PCO=2.E-7 torr
                   -sec


                          1E+13
Rate, Molecules/cm 2




                                                             C
                          1E+12
                                                                                               F
                                                                  B                            E
                                                                                               D
                          1E+11
                                                       A
                                    400        600          800       400        600         800
                                          Temperature, K                    Temperature, K


                                  Figure 2.18 The rate of the reaction CO
                                     + 2 O2  CO2 on Rh(111). Data of
                                    Schwartz, Schmidt and Fisher[1986].

                                                                                                   17
Catalysts Do Not Work Over A Broad Temp
                 Range

                          1000

                              1
    Rate, Moles/lit sec




                                      Catalyst Alone                    Gas Phase -- No
                          0.001                                         Wall Reactions

                           1E-6
                                                                   Approximate
                           1E-9                                    Effect Of Walls


                          1E-12

                          1E-15
                                  0         500        1000     1500        2000     2500
                                                       Temperature, K


 Figure 12.2 The rate of hydrogen oxidation on a platinum coated pore calculated
 with a) only heterogeneous (catalytic) reactions, b) only radical reactions, and c)
 combined radical, homogeneous reactions

                                                                                            18
         Types Of Catalysts:

Homogeneous Catalysts
Heterogeneous Catalysts




                               19
         Homogeneous Catalysts:

   Acids or Bases
   Metal salts
   Enzymes
   Radical initiators




                                  20
       Table 12.2-Some Reactions Commonly
           Catalyzed By Acids And Bases

       Reaction                  Example            Typical Application

      Isomerization          CH2=CHCH2CH3         Octane Enhancement
    (Rearranging the          CH3CH=CHCH3           Monomer Production
structure of a molecule)                           Paraxylene Production
      Alkylation              CH3CH=CHCH3 +          Pharmaceutical
  (Making too little          CH3CH2CH2CH3            Production
molecules into a bigger    (CH3CH2)CH(CH3)(C4H9)   Monomer Production
         one)                                        Fine Chemicals
                                                    Butane + olefin
                                                         octane
        Cracking           C12H24 C7H14 + C5H10   Crude Oil Conversion
(Taking a big molecule                                  Digestion
and making it into two
     littler ones).

                                                                           21
   Acids And Bases As Catalysts



       Benzene  ethylene  ethylbenzene
                     (12.2)



a proton reacts with the ethylene to form an
ethyl ion:

          H   CH 2CH 2   CH 3CH 2 
                     (12.3)



                                               22
 Acids As Catalysts Continued


The ethyl ion reacts with benzene to yield
and ethylbenzene ion:

      CH 3CH 2   C6 H 6   CH 3CH 2 C6 H 6 
                        (12.4)



Then the ethylbenzene ion loses a proton:

    CH 3CH 2C6H 6   CH 3CH 2C6H5  H 
                        (12.5)


                                                     23
          Solid Acids And Bases As
                  Catalysts


Table 12.9 Some common solid acids and bases
     Material            Type           Material        Type
 silica/alumina        solid acid       Mordenite      zeolite
     alumina           solid acid        ZSM-5         zeolite
    Y-zeolite           zeolite           VFI        large pore
    Faugasite                                          zeolite
     Sodalite           zeolite          Offretite     zeolite
    HF-SbF5            superacid         HSO3F       superacid
H2[Ti6O4(SO4)4(        superacid         Sulfated    superacid
     OEt)10]                             Zirconia
      MgO              solid base         Na2O         base



                                                                  24
Very Complex Pore Structure




Figure 12.4 A diagram of the pore structure in Faugasite.
                                                            25
                         Enzymes As Catalysts

 Oxidoreductases (promote                              Transferases
oxidation reduction reactions)            (promote transfer of functional groups)

  NADH         NADH + H2O2         Dimethylallylcis-      Dimethylallyl diphosphate +
peroxidase     NAD(+)+2         transferase (Transfer             isopentenyl
 (Oxidizes        H2O.           dimethylallyl groups)    disphosphatediphosphate +
NADH with                                                        dimethylallylcis-
peroxides                                                    isopentenyldiphosphate
Ferroxidase   4 Fe2+ + 4 H+ +      Glycoaldehyde         Sedoheptulose 7-phosphate + D-
 (oxidizes    O2  4 Fe3+ + 2        transferase         glyceraldehyde 3-phosphate  D-
   Iron)            H2 O              (Transfer’s         ribose 5-phosphate + D-xylulose
                                   Glucoaldeydes)                   5-phosphate
                                     Also called
                                    Transketolase
  Glucose     -D-Glucose +             Alanine            L-Alanine + 2-oxoglutarate 
  oxidase         O2  D-          aminotransferase           pyruvate + L-glutamate
 (oxidizes      glucono-1,5-        (Transfer amino
 Glucose)     lactone + H2O2     groups from alanine)

                                                                                    26
Solvents: As Catalysts

  CH 3I  NaCl  CH 3Cl + NaI
                  (12.17)

  Table.12.6 The rate of reaction (12.17) in
  several solvents. All measurements have
  been extrapolated to 25 C
       Solvent            Rate const,
                          lit/mole sec
     Gas Phase             about 10-45
       Water                3.5  10-5
      Methol                 3  10-6
      Methyl                   0.13
      Cyanide
       DMF                      2.5
                                               27
Next: Heterogeneous Catalysis


Examples of heterogeneous catalysts
  include:

   Supported Metals
   Transition Metal Oxides and Sulfides
   Solid Acids and Bases
   Immobilized Enzymes and Other
    Polymer Bound Species

                                           28
         Supported Metal Catalysts


Use support because platinum
very expensive and only the
surface is active.

Spread platinum out on cheap
support.

Support also provides strength


                                 Figure.12.3 A picture of a
                                 supported metal catalyst.

                                                              29
     Advantage Of Heterogeneous Catalysts
         Compared To Homogeneous:
   Cheaper separation
   More selective
   Generally cheaper

Disadvantage
 Not quite as active or a per metal atom
  basis



                                            30
Typical Mechanism Of Heterogeneous
     Catalysis (H2+C2H4C2H6)

            H 2 + 2S  2 H(ad)
                    (12.18)



         C 2 H 4  S  C 2 H 4 ad 
                    (12.19)



   C2 H 4 ad   H ad   C2 H5ad   S
                    (12.20)



    C2 H5ad   H ad   C2 H 6  2S
                    (12.21)
                                             31
                     Summary

   Catalysts make a tremendous difference to
    rates
       1013 enhancement average 1040 possible
       Kinetics change – rarely linear
       Optimum conditions often at max rate
          Zero order kinetics


   Two types of catalysts
       Homogeneous
       Heterogeneous
   Homogeneous more active
   Heterogeneous less expensive to use/control

                                                  32
                   Query

   What did you learn new in this lecture?




                                              33

				
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