As the catalyst concentration increased, the total glycerine content
reached a minimum. Therefore, there is an optimal catalyst
concentration in relation to the acylglycerides conversion. This
behavior has also been observed when using conventional catalysts,
such as sodium methoxide [31]. With high catalyst concentration,
saponification reaction occurs in a higher extent, with
the consequent inhibitor effect in the transesterification reaction.
According to EN-14214 standard, the total glycerine content
must be lower than 0.25 wt%. The results obtained showed that
using tetramethyl ammonium hydroxide as transesterification catalyst,
it was possible to obtain good conversion levels, reaching
total glycerine contents within the range established by the
international standard. The total glycerine varied little for catalyst
concentrations between 1.75 and 2.5 wt%, being 0.14 wt% approximately,
well below the limit. Therefore, in order to decrease the
catalyst consumption, it was adopted for other experiments, a
TMAH concentration of 1.76 wt%. The conversion evolution with
time was studied using this value and also a lower catalyst concentration
(1.09 wt%) which, according to the results presented in
Fig. 1, was not enough to obtain a final total glycerine value lower
than 0.25 wt%. Karavalakis et al. [21] found lower activity for the
TMAH. Using the same methanol:oil molar ratio as in this work,
a high catalyst concentration (approximately 3 wt%) was needed
to reach mono-, di- and tri-glycerides concentration according to
the EN 14214 standard. It is likely that the water content in the
TMAH solution used by Karavalakis et al. [21] was higher that that
used in this work, although it was not reported. In preliminary
experiments, we found that the TMAH as provided, had a low
activity and a poor ester yield, due to a high water concentration,
which was approximately 6 wt%.