a) Effect of Temperature
To study the temperature effect, experiments were performed at fixed conditions of molar ratio of reactants (OA: TMP of 3:1), reaction time of 20 h and catalysts dosage of 3 wt.% . Figure 4 depicts the variation of catalytic behavior over the temperature range from 100oC to 140oC. For the two catalysts, the activity (total conversion) increased with temperature, reaching a maximum of 87.2% and 71.73% at 140oC for IL1/SBA-15 and IL2/SBA-15, respectively. Meanwhile, the selectivity of the two catalysts, it is increased with the temperature towards the objective product (TMPTO) up to 130oC (95.6% and 92.9% for IL1/SBA-15 and IL2/SBA-15, respectively). Above this temperature, the selectivity begins to decrease with increasing of di-ester that may have been the result of the backward reaction, viz ., breakdown of tri-ester to di-ester (Xm, 2011). Through the studied temperature range, IL1/SBA-15 exhibits higher activity than that of IL2/SBA-15. This is due to stronger acidity of IL1/SBA- 15 than that of IL2/SBA-15, induced by the variance of substituents on the imidazolium ring.
Beside the GC analysis, the obtained product was investigated by kinematic viscosity analysis. Of all the methods used for the study of used oil, none have better repeatability or accuracy of methods than viscosity. The lubricants with high viscosity require more energy to flow, and in the same time, low viscosity may not reduce friction (Wu, 2013). Hence, the optimum viscosity value can be defined as the least viscosity reducing friction between surfaces in mutual contact. From Table 2, the viscosity of the obtained TMPTO is lower than that of a commercial oil at 40oC; indicating that the produced bio-lubricant is appropriate for low load conditions (Qiao, 2017). The TMPTO viscosity index is obviously higher than that of the commercial oil, which means that TMPTO has improved viscosity-temperature properties, viz., TMPTO will retain good viscosity on condition that the temperature obviously changes. The pour point of TMPTO is low enough to satisfy the requirements of lubrication oil.