Chemistry Reference
In-Depth Information
pre-catalyst
product C
starting
material A
catalyst
intermediate
another
interm
e
diate
starting
material B
by-product X
Figure 1.21
A catalytic cycle.
gas (generated by the reaction between zinc and sulfuric acid) played upon a piece of platinum foil in air, the
gas immediately ignited even though no flame was present. This discovery not only provided the world's first
lighter, but also introduced the concept of catalysis, a term coined by the great Swedish chemist, Berzelius.
37
Catalysis has become increasingly important. Many organometallic processes - though not all - are
catalytic. Catalysis reduces waste and reduces cost. Ideally, the loading of the catalyst should be as low as
possible. Many reactions employed in academic labs are run at 5 mol%. It seems likely that these could work
at much lower loadings. While low catalyst loading is obviously desirable, it must be remembered that there is
nothing in the definition of a catalyst about the loading. Thus, a material used in excess can still be a catalyst.
Whatever the loading, the test as to whether something is acting as a catalyst is to draw the mechanism as
a catalytic cycle (Figure 1.21). If the cycle brings the species back to where it started, then the reaction is
catalytic.
The term catalyst is widely used. It is often applied to molecules that are not the catalyst, but a precursor
for the catalyst. These should be termed “pre-catalysts”. This is particularly the case is industrial processes. A
combination of metal salts, reagents, ligands and supports are combined in a reactor. Somehow, they combine
in situ
to generate the catalyst. Due to this combination process, some reactions may have an induction
period.
The activity of the catalyst is an important issue. While the reactivity can be judged empirically by looking
at the reaction conditions, temperature, concentration, pressure if a gas is involved, for catalysts, the loading
is an important factor. In academic laboratories, it is common practice to employ 5 or 10 mol% as a standard
loading. In academic research, cost of chemicals and waste disposal is often less of an issue, and the cost
of a catalyst is of lesser importance when the investigator is fifteen steps into a thirty-step sequence! An
additional factor is that many academic reactions are run on milligrammes of substrate, and measuring
anything less than 5 mol% of catalyst is very difficult. The situation is quite different in industry. Catalyst
loading must be reduced to reduce the cost of the process, to reduce the cost of waste disposal and to
minimize the amount of residual metal that may contaminate the final product. In this age, it is time for all
academic labs to address the issue of catalyst loading to train students to think in this way for green and cost-
effective chemistry.
So how do we measure the activity of the catalyst? This is usually discussed as the turnover number (TON),
a concept adapted from enzymology. For mechanistic studies in organometallic chemistry, the turnover
number is the number of times the catalyst can go around the catalytic cycle before becoming deactivated.
38
