Page 110 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

458 Catalysis, Homogeneous

are important for a variety of reactions including hy- Kinetics is an important part of catalysis; after all, catal-
drogenation, isomerization of alkenes, oligomerization, ysis is concerned with accelerating reactions. Comparison
polymerization, carbonylation, hydroformylation, hydro- of catalysts and comparison of catalyzed and noncatalyzed
cyanation, metathesis, polyester formation, etc. Many reactions is not a straightforward task. Suppose we have
catalysts display high rates and selectivities for conver- a bimolecular reaction of species A and B with a rate of
sions that would have taken many steps in the absence of product formation:
the catalyst or would not have been possible at all. Many
d[P]/dt = k 1 [A][B]
of the catalytic reactions show a high atom economy, i.e.,
a high percentage of the atoms of the starting materials We don’t know what the rate equation for the catalyzed
ending up in the final product as by-product formation is reaction might look like, but it is reasonable that at least
minimized. the catalyst concentration will occur in it, e.g.:

d[P]/dt = k 2 [Cat][A][B]

I. DEFINITION AND SCOPE OF Hence the dimension of the reaction is different, even in
HOMOGENEOUS CATALYSIS the simplest case, and hence a comparison of the two rate
constants has little meaning. If both reactions occur si-
A. Definition multaneously in a system we may determine what part of
the product is made via the catalytic route and what part
Berzelius coined the term catalysis in 1836 when he had isn’t. In enzyme catalysis and enzyme mimics one often
noticed changes in substances when they were brought in compares the k 1 of the uncatalyzed reaction with k 2 of the
contact with small amounts of certain species called “fer- catalyzed reaction; if the mechanisms of the two reactions
ments.” Many years later in 1895 Ostwald came up with are the same this may be a useful comparison. In prac-
the definition that: A catalyst is a substance that changes tice the rate equation may take a much more complicated
the rate of a chemical reaction without itself appearing form than the ones shown above. The rate equation tells
into the products. This means that according to Ostwald a us something about the mechanism of the reaction.
catalyst can also slow down a reaction. The definition used Before we turn to “mechanisms” let us repeat how a
today reads as follows: A catalyst is a substance that in- catalyst works. We can reflux carboxylic acids and alco-
creases the rate at which a chemical reaction approaches hols and nothing happens until we add traces of mineral
equilibriumwithoutbecomingitselfpermanentlyinvolved. acid that catalyzes esterification. We can store ethene in
The “catalyst” may be added to the reactants in a different cylinders for ages (until the cylinders have rusted away)
form, the catalyst precursor, which has to be brought into without the formation of polyethylene, although the for-
an active form (“activated”). During the catalytic cycle mation of the latter is exothermic by more than 80 kJ/mol.
the catalyst may be present in several intermediate forms We can heat methanol and carbon monoxide at 250 C and
◦
when we look more closely at the molecular level. An ac- 600 bar without acetic acid being formed. After we have
tive catalyst will pass a number of times through this cycle added the catalyst the desired products are obtained at a
of states; in this sense the catalyst remains unaltered. The high rate.
number of times that a catalyst goes through this cycle is A catalyst lowers the barrier of activation of a reaction,
the turnover number. The turnover number (t.o.n.) is the i.e., it lowers the activation energy. When protons or Lewis
totalnumberofsubstratemoleculesthatacatalystconverts acids are the catalysts this description seems fairly accu-
into product molecules. The turnover frequency (t.o.f.) is rate. Take for instance a Diels-Alder reaction catalyzed by
the turnover number in a certain period of time. Substrates a Lewis acid (Fig. 1):
are present in larger amounts than the catalyst; when we The catalyst makes the dienophile more electrophilic. It
report on catalytic reactions the ratio of substrate to cat- lowers the energy level of the LUMO and the interaction
alyst is an important figure. An inhibitor is a substance between the LUMO of the dienophile and the HOMO
that retards a reaction. An inhibitor is also present in “cat-
alytic” or substoichiometric amounts. In a radical chain
reaction an inhibitor may be a radical scavenger that inter-
rupts the chain. In a metal-catalyzed reaction an inhibitor
could be a substance that adsorbs onto the metal making it
less active or blocking the site for substrate coordination.
We also talk about a poison, a substance that stops the
catalytic reaction. We will often see the word co-catalyst,
a substance that forms part of the catalyst or that plays
another role somewhere in the catalytic cycle. FIGURE 1 Lewis acid catalyzed Diels-Alder reaction.