Page 97 - 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

Catalysis, Industrial 495

intelligently, thereby increasing the rate of the desired re- acid strength is too low, olefin polymerization becomes a
action and minimizing the side reactions and their prod- serious side reaction. Considerable effort is in progress to
ucts. Surface area, pore volume, pore volume distribu- use zeolites and solid acid catalysts to replace liquid acids
tion, acidity or basicity, the number of active sites, and the in alkylation.
chemical surroundings of these active sites must be fixed 4. Hydrotreating and hydrodesurfurization. The major
for a catalyst. Reactants must be adsorbed on the active reactions are desulfurization, denitrogenation, and olefin
site, chemical interactions must occur that are neither too saturation of petroleum feedstocks. Aromatic saturation
strong or too weak, the transition state intermediate must is generally undesirable, so hydrogenation activity must
react with other molecules or rearrange, and finally the re- be moderate. The nickel/molybdenum on alumina catalyst
action products must diffuse away from the reaction site. must be sulfided to achieve desired activity and selectivity.
Catalysts are often designed with optimal support geom- Sulfurisremovedashydrogensulfide;nitrogenisremoved
etry with active metals deposited at an optimum distance as ammonia. Substrate acidity should be moderate to low
from the center for maximum selectivity. or excessive coke is formed. Diffusion is an important
Analytical instrumentation is making studies of mech- factor, especially for heavy crudes.
anisms of industrial reactions under operating conditions 5. Automobile and Hydrocarbon Emissions. The oxida-
more feasible. A number of mechanisms of important tion of carbon monoxide and hydrocarbons is catalyzed by
commercial reactions are still not known conclusively. platinum/palladium/rhodium on alumina. If catalyst poi-
Sometimes it is almost impossible to distinguish among sonssuchasleadandphosphorusarenotpresent,themajor
several alternatives. Refinements or sometimes even com- problems become initiation of oxidation at low tempera-
plete changes in mechanisms have been made as analytical ture, thermal stability at high temperature, resistance to
capabilities have improved. thermal schock, and a high external surface area catalyst
Often model systems are used in mechanistic studies. configuration.
Use of a single-component feed rather than a broad boiling If nitrogen oxide control is one of the catalytic require-
range greatly simplifies analysis schemes. However, part- ments, the stoichiometry of air-to-fuel ratio must be kept
per-million quantities of impurities in real feedstocks can nearly stoichiometric to reduce NO; then air must be added
sometimes severely complicate catalyst usage, selectivity, and CO and hydrocarbons oxidized in a second part of the
and life. catalyst bed.
ExampleswillbetakenfromthereactionslistedinTable The vapor-phase, high-space-velocity oxidation to the
I to illustrate mechanisms and their effect on commercial thermodynamic reaction products should be contrasted
catalyst development. with kinetically controlled oxidation of chemical feed-
stocks when the active metal is purposely poisoned or the
1. Catalytic cracking. Catalytic cracking of hydrocar- surface area reduced.
bons to produce lower molecular weight hydrocarbons oc- 6. Polyethylene (chromium catalyst). The chromium on
curs by the heterolytic leavage of a C C bond. A carbo- silica catalyst is quickly reduced from Cr(VI) to Cr(II).
nium ion mechanism is involved. Strong acids such as The active site consists of a single chromium ion present
silica-alumina or zeolites are used as commercial cata- as silyl chromate before reduction with ethylene. Ethylene
lysts. Hydrogen transfer activity relative to -carbon scis- adds to the chromium as indicated.
sion is important in selectivity. Alkane and aromatic prod-
CH 2 CH 3
uctsarepreferabletoolefins.Highlyolefinicproductscoke CH 2 CH 3
Cr
a catalyst faster and neutalize acid sites. Catalysts can be Cr CH 2
CH 2 CH 2 CH 2
regenerated by controlled thermal oxidation in air.
2. Reforming. Reforming is used to upgrade the oc-
C 2 H 4
tane number of gasoline. In the presence of hydrogen
a desirable catalyst should promote isomerization, cy-
clization, and arenes. Such a catalyst has a dual func- C 2 H 4
tion; it is acidic to promote skeletal rearrangement by Cr
carbonium ion mechanism, and it has a hydrogenation- (CH 2 ) 3 CH 3
dehydrogenation component to promote arene and cy-
clization reactions. Termination of the growing chain can occur by hydride
3. Alkylation. Reactions of olefins with isoparaffins re- transfer to the active site instead of to the monomer.
quire a highly acidic catalyst. Both Friedel-Crafts and 7. Polyethylene (Ziegler-Natta catalyst). Most com-
protonic acids are used. The protonic acids, sulfuric (96– mercial catalysts start with titanium tetrachloride, di-
100%) and hydrofluoric acids, are commonly used. If the ethylaluminum chloride, and magnesium chloride as a