Page 56 - Modeling of Chemical Kinetics and Reactor Design

26 Modeling of Chemical Kinetics and Reactor Design

ACID-BASE CATALYSIS: HOMOGENEOUS LIQUID PHASE

A catalyst is defined as a substance that influences the rate or the
direction of a chemical reaction without being consumed. Homogeneous
catalytic processes are where the catalyst is dissolved in a liquid
reaction medium. The varieties of chemical species that may act as
homogeneous catalysts include anions, cations, neutral species, enzymes,
and association complexes. In acid-base catalysis, one step in the
reaction mechanism consists of a proton transfer between the catalyst
and the substrate. The protonated reactant species or intermediate
further reacts with either another species in the solution or by a
decomposition process. Table 1-1 shows typical reactions of an acid-
base catalysis. An example of an acid-base catalysis in solution is
hydrolysis of esters by acids.

Acid
R COOR + H O [ R COOH + R OH (1-113)
1 2 2 1 2

Mechanism
1 (
+
+
R COOR + H [ R COOR H ) * (1-114)
1 2 2
1 (
*
+
R COOR H ) + H O [ R COOH + R OH + H + (1-115)
2 2 1 2
AUTOCATALYTIC REACTIONS

There are many reactions in which the products formed often act
as catalysts for the reaction. The reaction rate accelerates as the
reaction continues, and this process is referred to as autocatalysis. The
reaction rate is proportional to a product concentration raised to a
positive exponent for an autocatalytic reaction. Examples of this type
of reaction are the hydrolysis of several esters. This is because the
acids formed by the reaction give rise to hydrogen ions that act as
catalysts for subsequent reactions. The fermentation reaction that
involves the action of a micro-organism on an organic feedstock is a
significant autocatalytic reaction.
Normally, when a material reacts, its initial rate of disappearance
is high and the rate decreases continuously as the reactant is consumed.
However, in autocatalytic reaction, the initial rate is relatively slow