Optimisation of Heterogeneous Catalysed Esterification Reaction for
n-Hexyl Acetate Synthesis part 1
The kinetics of heterogeneously catalysed esterification reaction of acetic acid and
n-hexanol to synthesise n-hexyl acetate were undertaken in a jacketed stirred batch reactor in the presence of ion exchange resins in order to ascertain the optimal conditions required for high yield of ester and fastest initial reaction rate.
The optimal parameters were attained with Purolite CT-124 catalyst at temperature 368 K, catalyst loading 5% (w/w), feed molar ratio of 4 : 1 (n-hexanol : acetic acid) and stirrer speed 500 rpm.
However, agitation of the reacting mixture had virtually no effect on the rate and extent of acetic acid conversion. Furthermore it was discovered that the catalyst which yielded more ester product at the fastest rate was the gelular CT-124 resin. This study
infers the success of the CT-124 resin catalyst due to it’s suitability for interaction with aqueous reacting mixtures as opposed to the macroporous species resin, which couldn’t surpass the conversion achieved by the gelular resin even with the macroporous
resin’s superior specific surface area value. Additionally, this body of work successfully confirmed the re-usability property of the CT-124 resin.
Acetic acid features heavily as a feedstock and effluent stream stemming from industrial processes such as petrochemical processing, wood treatment and production of acetic anhydride and cellulose esters (an example of fine chemical industries) as reported
by Saha et al. (2000), Singh et al. (2006) and Chuang and Xu (2009). One crucial step in these processes is to recover the acetic acid from the aqueous streams prior to discharge into the environment (Chuang and Xu, 2009).
The solution can be found in acid catalysed esterification, to generate revenue by creating a valued ester,
n-hexyl acetate in this case, from waste acetic acid streams. This is in stark contrast to a generic treatment of distilling the aqueous stream to separate water from acetic acid which would prove to be economically unviable (Chuang and Xu, 2009; Huang
et al., 2005).
Esters are highly valued in the production of perfumes, foodstuffs, pharmaceuticals and polymers. Most notably,
n-hexyl acetate is renowned for its aromatic-fruit scent and is used accordingly in flavouring manufacture (Goodwin
et al., 2006).
Ion exchange resins (IERs) have been observed ever since the turn of the 20th century primarily due to environmental, economic and reaction selectivity concerns surrounding the conventional use of strong acid catalysis. Strong acid catalysts tend to taint
the product ester and hinder the productivity of associated down-stream processes.
Counter-measures used to neutralise such acids will then precipitate contaminants (sludge) (Harmer and Sun, 2001). The principal advantages that IERs hold over acid catalysts are they exist in solid state and so are easily filtered and retained; insoluble,
so eliminates leaching into waterways and the key issue of vessel corrosion due to caustic acidity of strong acid catalysts is negated. (Alexandratos, 2009 and Sharma and Chakrabarti, 1993).