Page 98 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN002C-86 May 17, 2001 20:36

496 Catalysis, Industrial

support, as well as other promoters. Stereoselectivity (for A mechanism for catalyst oxidation-reduction (others can
polypropyleneandhigherolefins)iscontrolledbyaddition be written) is the following:
of aromatic esters such as ethyl benzoate.
8. Polyethylene and polypropylene (metallocenes). V 2 O 5 SO 3 + SO 2 ↔ (VOSO 4 ) 2
1
Metallocenes of different types are being used in a variety (VOSO 4 ) 2 ↔ V 2 O 4 + SO 2 + SO 3 + O 2
2
of commercial processes to make polymers with differ-
1
ent properties than traditional Ziegler-Natta catalysts. The V 2V O 4 SO 2 + O 2 ↔ V 2 O 5 SO 3
2
metallocene catalysts can be optimized for chain length 14. Polyphenylene oxide. Oxidative polymerization of
and sterochemical control of the product polymer. 2,6-xylenol to the engineering resin polyphenylene ox-
9. Polyvinyl chloride. Organic peroxides are used to ide (PPO) is catalyzed by copper and manganese amines.
catalyze the free radical polymerization of vinyl chloride Pyridine is a typical amine used in the polymerization.
monomer in water. The organic peroxide is selected to The active copper catalyst is Cu(Cl)(OH)(NR 3 ) 2 . The
generate free radicals thermally at the temperature of poly- first step in the reaction is the following, where ArOH
merization. stands for 2,6-xylenol:
10. Polystyrene. The polymerization of styrene is most
commonly done under free radical conditions. Peroxides Cu OH HOAr Cu OAr H 2 O
are used to initiate the reaction at low temperatures. At
∼100 C styrene acts as its own initiator. Below 80 C the
◦
◦
Electron transfer from oxygen to copper gives a phe-
termination mechanism primarily involves combination of
noxyl radical, which couples with another copper-bound
◦
radicals. Above 80 C both disproportionation and chain
radical to form the C O C dimer and Cu(I). The reaction
transfer with the Diels-Alder dimer are important.
behaves as a step reaction rather than a chain reaction. A
11. Polybutadiene. Most polybutadiene is made by an
quinol ether rearrangement occurs to equilibrate polymer
emulsion process with a free radical initiator. If stereo-
and monomer. High-molecular-weight polymer is formed
regular cis-1,4-polybutadiene is desired, a titanium-based
only in the late stages of reaction. Indeed, other phenols
Ziegler-Natta catalyst is used. The catalyst is similar to
are incorporated into the polymer if they are added at the
those used for polyethylene and polypropylene in type
end of the reaction because of the quinol ether rearrange-
and mechanism.
ment.
12. Polyformaldehyde. Polyformaldehyde or polyac-
15. Polycarbonates. Phenol and phosgene react under
etal is made by two different processes. Delrin is made
basic (sodium hydroxide) conditions to form diphenyl car-
from formaldehyde by anionic polymerization catalyzed
bonate. Bisphenol A and diphenyl carbonate are melted
by a tertiary amine. The homopolymer is end-capped
together with a small quantity of basic catalyst (Na, K,
with acetic anhydride. Celcon is made from trioxane
Li) 2 CO 3 . The temperature is slowly raised to 250 C, and
◦
cationic copolymerization using boron trifluoride catalyst
phenol is removed in the polymerization step.
and ethylene oxide (2–3%) as the comonomer. Boron tri-
16. Acetylene hydrogenation. Selective hydrogenation
fluoride is a Lewis acid that associates with trioxane and
of acetylene to ethylene is performed at ∼200 C over sul-
◦
opens up the six-membered ring. Ethylene oxide provides
fided nickel catalysts or carbon-monoxide-poisoned pal-
the end capping. Without an end cap, polyformaldehyde
ladium on alumina catalyst. Without the correct amount
is thermally unstable and loses formaldehyde units.
of poisoning, ethane would be the product. Continuous
13. Sulfuric acid. The oxidation of SO 2 to SO 3 is the
feed of sulfur or carbon monoxide must occur or too
step in sulfuric acid manufacture that requires catalysis.
much hydrogen is chemisorbed on the catalyst surface.
The oxidation is exothermic, and the equilibrium becomes
Complex control systems analyze the amount of acety-
more unfavorable for SO 3 at higher temperatures.
lene in an ethylene cracker effluent and automatically ad-
1 just the poisoning level to prepare the catalyst surface for
SO 2 + O 2 ↔ SO 3
2
removing various quantities of acetylene with maximum
The rate law can be expressed as selectivity.
x
z
y
R f = kp (SO 2 )p (O 2 )p (SO 3 ) 17. Alcohols from esters. The major problem is reac-
tion selectivity. Paraffin by-product in alcohol results if
Where x and y are between 0.5 and 1.0, and z is usually the catalyst activity is too high. Yet the reduction of es-
0.0 to −1.0. ters to alcohols is a difficult reaction. Copper chromite
Normal operating temperature for the vanadium cata- catalyst, 3000–5000 psig hydrogen, and a temperature of
◦
lyst is 450–550 C. 270–300 C are required for the reduction. An alternate
◦
Multiple stages are used for heat transfer and to prevent catalyst is CuO/ZnO, which is used for methyl ester re-
◦
temperaturesabove600 C,whichcandamagethecatalyst. duction only. Hydrogen solubility in alcohol is limiting.