Aunque Te Vieras Así Siempre

oTopics in Catalysis Vol. 27, Nos. 1–4, February 2004 (# 2004)

83

Esterification and transesterification of renewable chemicals
Hans E. Hoydonckx, Dirk E. De VosÃ, Suhas A. Chavan, and Pierre A. Jacobs
Centre for Surface Chemistry and Catalysis, Kasteelpark Arenberg 23, 3001 Leuven, Belgium

An overview is given of newly developed Lewis or Brønsted acid and base catalysts foresterification, transesterification and
ester interchange reactions. The most relevant applications of these catalysts and reactions in the domain of renewable resources, in
particular, oils and fats, are discussed.
KEY WORDS: esterification; ester exchange; acid-catalyzed esterification.

1. Introduction
Direct esterification starting from an acid and an
alcohol, transesterification starting from anester and an
alcohol, and the ester interchange reaction between two
esters are of major economic importance in the
processing of renewable chemicals [1]. Direct esterification can be catalyzed or can even proceed in a
spontaneous fashion, depending on the miscibility of
the reactants, the temperature and the acidity of the
carboxylic acid reaction partner. High temperature
tends toincrease the mutual solubility of, for instance,
glycerol and fatty acids, and if the reagents are stable
enough, spontaneous reaction at high temperature may
lead to useful process conditions, e.g., for production of
dimethyl terephthalate. In the esterification of lauric
acid with glycerol, addition of a small amount of methyl
lactate provides sufficient miscibility for the reaction to
proceedat 130  C without catalyst [2].
Esterification and transesterification are equilibrium
reactions, and they often produce volatile molecules
such as water or lower alcohols. In practice, an excess of
one of the reagents can be used to displace the
equilibrium, or water is continuously removed by
adsorption on drying agents or by codistillation with
‘‘entrainers’’ such as benzene, or by useof a waterselective membrane. The reactivity of alcohols and
carboxylic acids toward esterification primarily depends
on the steric hindrance of both the alcohol and the
carboxylic acid. Thus, esterifications with tertiary acids
or alcohols are slow; rates decrease in the following
order:
CH3 OH > primary alcohols > secondary alcohols
> tertiary alcohols
HCO2 H > CH3 CO2 H > RCH2 CO2 H >R2 CHCO2 H
> R3 CCO2 H:
à To whom correspondence should be addressed.
E-mail: dirk.devos@agr.kuleuven.ac.be

The acid strength of the carboxylic acid has just a minor
effect on the rate of the esterification reaction.
Transesterification is often a technical alternative to
direct esterification in case the carboxylic acids are
poorly soluble or have a very high viscosity, or simply if
theactual feedstock consists of esters.
Besides the direct esterification, the transesterification and the ester interchange reaction, there exist
several atom-efficient, waste-free routes to esters. A
prominent example is the addition of carboxylic acids to
olefins like isobutene, leading to tert-butyl esters. Not
only H2 SO4 but also acid zeolites are suitable catalysts
[3]. Another route isthe addition of carboxylic acids to
epoxides, catalyzed by multimetal cyanides [4].

2. Catalysts
This overview is restricted to catalysts that can use
either acids or esters as acyl source; hence, methods
using anhydrides or acyl chlorides are not considered.
Moreover, focus is especially on recent developments in
catalyst design.

2.1. Catalysts for direct esterification
2.1.1.Homogeneous Brønsted acids
Simple Brønsted acid catalysts are the most frequently used catalysts for the direct esterification. For
easy reactions, e.g., with methanol or ethanol, an
organic resin with sulfonic acid groups is industrially
applied [1]. However, esterification is generally slower
when either alcohol or acid is more sterically hindered,
as in the case of long-chain fatty alcohols,...