Investigation of potential of a bifunctional catalyst for the simultaneous esterification and transesterification of high free fatty acid feedstock

Abstract

There has been growing concern around the depletion of the world’s oil reserves as well as the negative environmental impacts fossil fuels pose. Therefore, there has been a crucial demand in the exploration of clean, sustainable, and alternative fuels/ fuel sources. Biodiesel, which can be derived from edible plant oils, waste oils as well as animal fats has been regarded as a clean, renewable and feasible substitute for diesel compression ignition engines without the need for modification. This study aimed to investigate the simultaneous esterification and transesterification of various feedstocks using bi-functional catalysts. Four catalysts consisting of varying ratios of CaO and Al2O3 were synthesised through co-precipitation in order to ensure their bifunctionality where CaO provided basic sites and Al2O3 provided acidic sites. The four catalysts (CaO: Al2O3 ratios of 80:20, 70:30, 60:40, and 50:50) were calcined at a temperature of 600 °C. Catalysts were characterised using Brunauer Emmet Teller (BET), Scanning Electron Microscope (SEM) and X-Ray diffraction (XRD). They exhibited adequate morphological and catalytic characteristics where pore sizes were ≥209 Å, surface areas ≥11 m2/g and pore volumes ≥0.072 cm3/g. Seven feedstock samples were used: 3 virgin oils (canola (VCO), sunflower (VSO) and palm (VPO)), 4 non-edible oils (waste canola (WCO), waste sunflower (WSO), waste palm (WPO) and neem oil (NO)). Free fatty acid contents ranged from 0.22 to 3.25%. Feedstocks were pretreated through filtration, to eliminate solid particles and dehydrated to reduce moisture. Thereafter, each feedstock underwent simultaneous esterification and transesterification using the four catalysts. These experiments were carried out at temperature of 65 °C, agitation rate of 1200 rpm, 2.5 wt% catalyst loading and a methanol to oil molar ratio of 12:1 at a reaction time of 4 hours under reflux as the optimised conditions stipulated by (Zabeti et al., 2009). The results showed that feedstocks with low FFA content required catalysts with less acidic sites to produce the highest yield of biodiesel. In contrast, feedstock with a high FFA content (>1 wt%) favoured a catalyst with a higher quantity of acidic sites. This was evident as VPO, VCO and VSO, which had FFA contents of 0.67%, 0.33% and 0.22% respectively, performed optimally through the use of 80% CaO:20%Al2O3 with yields of 97.85%, 98.95% and 95.6% respectively. Neem oil, which had the highest FFA content of 3.25% achieved the highest yield (97.63%) with the use of a catalyst with more acidic sites (60% CaO: 40%Al2O3). Therefore, the relationship between FFA and acid site quantity was evident. The effect of feedstock saturation on catalyst performance was also investigated. The highly saturated feedstocks (WPO and NO) had a saturated fatty acid (SFA) content of 46.79 and 44.06%, respectively, where the highly unsaturated feedstocks (SO and WSO) had a polyunsaturated fatty acid (PUFA) content of 67.8 and 64.0%, respectively. Monounsaturated fatty acids (MUFA) were dominant in WCO and CO at values of 66.6 and 71.2%, respectively. There were negligible differences in yields between PUFA, MUFA and SFA dominant feedstocks. It was evident that the degree of unsaturation of the feedstock had no significant effect on catalyst performance. Transportation fuel characteristics of the biodiesel synthesised were also determined. These included oxidation stability, density, kinematic viscosity, flash point, sulphur content, total acid number (TAN) and water content in accordance with the ASTM reference test methods (D4502, D 664, D 445, D 93, D 5453 and D 2709). Biodiesel density values ranged from 0.88 to 0.889 g/m3, viscosity from 0.4 to 30.4 cSt, flash point from 130 to 170 °C, sulphur content from 0 to 334 ppm, TAN from 0.17 to 0.88 mgKOH/g and water content from 0.02 to 0.19 wt% The RANCIMAT procedure was also utilised for the measurement of induction time/ oxidation stability. The fuels adhered to the ASTM and EU restrictions with the exception to neem oil biodiesel (NB100). This was attributed to the physio-chemical nature of the corresponding feedstock used. Catalyst robustness was also investigated. It was found that all four catalysts maintained excellent catalytic activity for up to 8 runs. Loss of catalytic activity thereafter was attributed to loss of mass during the cleaning and drying process as well as exposure to moisture and air. The current study provided evidence for the need to synthesise tailor-made bifunctional catalysts with robust nature for the E and TE of low-cost feedstocks.