Esterification ring opening reaction
Twenty (20) g of epoxy methyl esters (oxirane) sample
(EMPKOo) was placed in a round bottom flask. 10 ml of
ethyl acetate was added to the flask. Each of the mixtures was stirred
continuously and followed by the addition of 4 g of acid anhydride. The
flask were purged with nitrogen (to exclude air) and followed by the
addition of 1ml boron trifluoro diethyletherate to the flask. The epoxy
methyl esters (oxirane) sample in the flask was heated to 70 °C. The
mixtures were left to react at this temperature for 7 hrs in fume
cupboard with continuous stirring. The organic phase (oily layer) of
sample was purified three times with 5 % NaHCO3 (3 x 15
ml) to neutralize the unreacted acid present. Thereafter, organic phase
of sample was also washed three times with saturated solutions of sodium
chloride NaCl (3 x 15 ml) to obtain the branched methyl ester sample.
The sample was dried over anhydrous magnesium sulphate. The solvent
(ethyl acetate) was then removed using a rotary evaporator. The
unreacted anhydrides were removed at atmospheric pressure using
distillation at 80 °C. The distillation unit can only process about 20
ml of sample each time, limiting the amount of sample that can be
processed.
The obtained branched methyl ester sample was then used as precursors
for the synthesis of modified triester-derivatives by acetylation with
octanoyl chloride to obtain the palm kernel biolubticant
(PKBLE). Fig. 2 shows the reaction scheme for the
esterification of EMPKOo (epoxide).
Fig. 2: Reaction Scheme for Esterification of
EMPKOo (epoxide).
- Results and Discussion
- Transesterification of PKME with TMP for palm kernel
biolubricant production
Figs. 3 and 4 show the progress of the transesterification reactions at
different times for 3.0 and 4.0 molar-ratios, respectively. From the
plots, it could be seen that transesterification advanced stepwise. At
first, monoester (ME) formation reached a maximum value. This was
immediately followed by diester (DE) formation. At maximum DE formation
stage, triester (TE) formation increased at a fast rate. This was
attributed to the fact that during the transesterification of FAME with
TMP which occurs stepwise, intermediate products are produced, prior to
the final preferred product formation, which is the triester (TE)
[25]. Firstly, during the reaction, the ME, that is a single branch
polyol ester was formed. However, increasing the ME quantity, would
result to immediate conversion to DE with another FAME molecule.
Finally, the TE was formed by the reaction of DE and FAME (PKME). It is
worth noting from the plots that the TE concentration increased with
decrease in concentrations of DE and ME, as evident in the Figs. 3 and
4. This reaction mechanism has been reported by a number of other
researchers [4,26-27].
[CHART]
Fig 3: Transesterification between TMP and PKME of 140 °C and
3:1 molar-ratio, using 1 wt % Ca(OH)2 catalyst
[CHART]
Fig 4: Transesterification between TMP and PKME of 140 °C and
4:1 molar-ratio, using 1 wt % Ca(OH)2 catalyst.
Temperature Effects
For the determination of temperature effect on synthesis of biolubricant
from PKME, experiment was conducted at FAME to TMP molar ratio of 4:1,
with catalyst concentration of 1.0% wt/wt of reaction mixture. At these
conditions, the reaction was carried out at 80, 100, 120, 140 and 160
°C, in other study the effect of temperature on biolubricant synthesis.
Figs. 5 and 6 show the temperature effects results for the synthesis PKO
biolubricant. It could be seen from the Fig. 5 that an increase in the
temperature, resulted to the increase in TE composition, until at about
temperature of 140 °C, when the TE composition increase became
negligible. This was attributed to the fact that at higher temperature
(see Fig. 6); the quantity of FAME in the reactor was low, due to
vaporization, favoring the reverse reaction. In other words, the
re-condensation of the FAME vapor back into the reactor. Hence, the
reverse reaction would be contained; thereby leading to more
esterification of the DE to TE. Therefore, there is need for the
condenser water to be cold enough, in other to ensure condensation of
the vaporized FAME back into the reactor [4,28].
[CHART]
Fig. 5: Effects of temperature on PKME composition at 4:1
PKME-TMP mole ratio and 1% catalyst loading for 5 hrs.
[CHART]
Fig. 6: Temporal yield of PKTE at various temperatures, 4:1
PKME-TMP mole ratio and 1% catalyst loading.
Mole Ratio Effects
In other to ensure improve yield of the triester during the
transesterification of FAME with TMP, excess quantity of FAME or TMP is
used. This is because transesterification reaction is a reversible
reaction. However, in this study, excess FAME was chosen over TMP due to
economic reasons of its lower cost. The molar ratio of FAME to TMP was
varied at 3:1, 4:1, 5:1, 6:1 and 7:1, at temperature of 100oC, time of 5hr and Ca(OH)2/wt
catalyst weight of 1.0% wt. Fig. 7 shows the effect of PKME: TMP mole
ratio on % composition of TMP ester at 100 °C and catalyst loading of
1% each. Then again, Fig. 8 shows the temporal yield of palm kernel
triester (PKTE) at various mole ratios for temperature of 100 °C and 1
% catalyst loading. It could be seen from Fig. 7 that as the molar
ratio of FAME: TMP was increased, the yield of TE increased as well. In
other to obtain better and more product yield, the reactants molar
ratios were kept above the stoichiometric values. This is because the
reaction was driven more toward completion. However, in Fig. 8, it was
evident that increasing the molar ratio beyond 4:1 gave negligible
improvement in TE yield for palm kernel biolubricant. This phenomenon
could be attributed to the low rate of conversion of DE to TE, as well
as occurrence of reverse reaction that caused the breaking of DE to TE
[4,29]. Noureddini and Zhu [29], reported that for the chemical
and enzymatic transesterification of rapeseed methyl ester using TMP,
maximum conversion was attained at mole ratios of 3.3:1 and 3.5:1,
respectively. In this work, it could be seen that the conversion to TE
increased from 71.34 to 77.10 % as the mole ratio increased from 3:1 to
4:1 (see Fig. 7). However, a small amount of DE was an added value to
the properties of the lubricant. Meanwhile, the large excess of FAME
remaining in the final product would affect the physical properties of
lube which would require additional energy to remove [26].
[CHART]
Fig. 7: Effects of PKME: TMP mole ratio on % composition of
TMP ester at 100 °C and 1% catalyst loading
[CHART]
Fig. 8: Temporal yield of PKTE at various mole ratios,
100oC and 1% catalyst loading