# Carbon-Based Catalysts for Biodiesel Production--A Review
## 2 Carbohydrate Precursors
[10] prepared and compared four carbohydrate-derived catalysts from D-glucose, sucrose, cellulose and starch. The preparation method was as the same as that by [PERSON] et al. [46] except carbonisation was carried out at 400 \({}^{\circ}\)C rather than 450 \({}^{\circ}\)C. The esterification of oleic acid was used to evaluate the catalytic activity of the four catalysts. Each catalyst was added to a methanol oleic acid mixture, with a 10:1 molar ratio, and reacted at 80 \({}^{\circ}\)C. After 3 h, the starch, cellulose, sucrose and D-glucose catalysts had achieved a conversion of 95%, 88%, 80% and 76%, respectively. Figure 4 clearly indicates that the catalyst derived from starch was much more active for the esterification reaction than the other three catalysts. A yield of 95% was achieved in under 3 h whereas the cellulose, D-glucose and sucrose catalysts required 4-5 h to achieve a comparable yield. For all four catalysts, the XRD pattern was typical of amorphous carbon. The starch-derived catalyst had the highest sulphur content, the highest acid site density and the highest contribution for SO3H to the acid site density. In terms of textural properties, the starch catalyst also possesses the most favourable properties, the largest surface area of the four carbohydrate-derived catalysts the largest average pore volume and average pore size (Table 2). The larger pore size, pore volume and surface area allowed reactants easier accessibility to the SO3H sites leading to the higher efficiency of the starch catalyst. Despite the inferior surface area of the carbohydrate-derived catalysts in comparison with the sulphated zirconia and niobic acid, the four prepared catalysts exhibited much higher esterification and transesterification activity. The starch catalyst in particular had an esterification and transesterification activity comparable to that of the concentrated H2 SO4 (Figure 6). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Catalyst Derived** & **S Content** & **Total Acid Site** & **SO3H Acid Site** & **Surface** & **Average Pore** & **Average Pore** \\
**From** & **wt.\%** & **Density mmol/G** & **Density mmol/G** & **Area m\({}^{2}\)/g** & **Volume cm\({}^{3}\)/g** & **Size nm** \\ \hline D-Glucose & 4.7 & 1.6 & 1.47 & 4.1 & 0.44 & 4.0 \\ Sucrose & 5.1 & 1.71 & 1.59 & 5.0 & 0.52 & 5.1 \\ Cellulose & 5.4 & 1.82 & 1.68 & 5.7 & 0.65 & 6.4 \\ Starch & 5.9 & 1.97 & 1.83 & 7.2 & 0.81 & 8.2 \\ \hline \end{tabular}
\end{table} 
Table 2: Textural properties of various solid acid catalysts. Figure 5: Reusability of cellulose-derived catalyst for esterification and transesterification reaction. Following the initial experimentation that showed the effectiveness of the four carbohydrate-derived catalysts, [PERSON] et al. [10] conducted a further experiment. The goal of which was to assess the effectiveness of the catalyst for the simultaneous esterification and transesterification of waste cooking oils (27.8 wt.% FFA). The reaction was carried out at 80 \({}^{\circ}\)C with a methanol: oil molar ratio of 20:1. As presented in Figure 7 the starch achieved the highest FAME yield in the shortest time of any of the catalyst. Through experimentation and assessing the return vs. the cost, it was determined that the optimal oil:methanol ratio was 30:1, and the catalyst loading was optimised at 10 wt.%. The optimal reaction temperature and time was 80 \({}^{\circ}\)C for 8 h. Under these conditions, a yield of 92% was achieved. It was also determined that, the starch-derived catalyst retained 93% of its catalytic activity after fifty successive reuses. [PERSON] et al. [40] completed a study on the effect of carbonisation temperature on the catalytic ability of a D-glucose catalyst. The D-glucose was carbonised at 300, 400, 500 and 600 \({}^{\circ}\)C. The acid content due to SO3H (which is representative of the catalytic performance) of the four D-glucose was compared, and the results are shown in Figure 8.  
Figure 6: Activity of various catalysts for the esterification and transesterification reaction.