Ethoxylation and propoxylation reactions are often performed
together or in alternation to obtain
surfactants with particular properties or random and block copolymers.
Both reactions are
normally performed in the same reactor, in the presence of an alkaline
catalyst, at relatively
low temperature, 120−130 °C, to avoid the intervention of the side
reactions that are typical of
the propoxylation. The propoxylation of a primary fatty alcohol is
slower than the corresponding
ethoxylation and gives place to a secondary hydroxyl terminal group
that is still less reactive.
On the contrary, ethoxylation restores more reactive primary
hydroxyl terminal groups.
Therefore, it is important for optimizing the described industrial
operations to know the reactivity
of ethylene and propylene oxide with, respectively, primary and
secondary hydroxyls, in the
presence of the most used KOH catalyst. In this paper, the
kinetics of both the ethoxylation
and propoxylation of 1- and 2-octanol catalyzed by KOH have been
studied for this purpose.
We will show, first of all, that propylene oxide ring opening
occurs selectively giving only
secondary hydroxyls as terminal groups. The ratio of addition rate
of ethylene oxide to primary
and secondary alcohol with respect to that of propylene oxide is always
greater than 1. Kinetic
data collected have been interpreted by using a kinetic model able to
simulate during the time
the consumption of both the octanol and the alkoxide and the evolution
of the oligomer
distributions. The kinetic model and related parameters can be
easily extrapolated to different
industrial situations in which ethoxylation and propoxylation occur
together or in alternation.
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