# Carbon-Based Catalysts for Biodiesel Production--A Review
## 2 Carbohydrate Precursors  
A novel branch of carbonaceous catalysts, formed by the incomplete carbonisation of carbohydrate sources such as D-glucose, sucrose, starch and cellulose followed by subsequent sulphonation has been shown to exhibit excellent catalytic activity for the esterification and transesterification reactions in biodiesel production [10]. [PERSON] et al. [46] utilised D-glucose to prepare a CBAS to catalyse the esterification of oleic acid and subsequently waste cooking oil. D-glucose was incompletely carbonised at 400 \({}^{\circ}\)C for 15 h under N\({}_{2}\) flow, following the carbonisation the material was ground to a powder before being heated to 150  
Figure 3: Mechanism of sulphonated carbon-based acid solid (CBAS) synthesis. \({}^{\circ}\)C in concentrated H\({}_{2}\)SO\({}_{4}\) for a further 15 h. The catalytic ability of this sulphonated sugar catalyst was compared to three typical solid acid catalysts, namely sulphated zirconia, niobic acid and amberlyst 15 resin. The reaction was carried out at 80 \({}^{\circ}\)C, with an alcohol to oil molar ratio of 10:1. The sugar catalyst demonstrated a higher catalytic activity than sulphated zirconia and Amberlyst 15. It was reflected in the yields of methyl oleate achieved by the various catalysts, presented in Table 1.  
Niobic acid demonstrated little to no catalytic activity. Despite the larger BET surface area of the sulphated zirconia the increased number of acid sites in the sugar catalyst, lead to an improved catalytic ability. As part of their study, [PERSON] and his team utilised the D-glucose catalyst to catalyse the conversion of waste oil with a high FFA content to biodiesel. Use of this type of oil is not possible applying traditional homogeneous base methods. The D-glucose catalyst achieved a yield of over 90 wt.%, which was far greater than any of the typical commercial catalysts tested, compared in Figure 4. The catalyst retained 93% of its catalytic activity following fifty successive uses demonstrating excellent reusability properties. [PERSON] et al. [39] demonstrated a one-step hydrothermal carbonisation of a glucose catalyst at 180 \({}^{\circ}\)C for 4 h. The resulting catalyst had an acidity of 1.99 mmol g\({}^{-1}\) similar to the work of [PERSON] et al. [46] Waste cooking oil (WCO), high in FFAs, was used to demonstrate the catalytic ability of the glucose catalyst. The reaction was carried out at 60 \({}^{\circ}\)C and a methanol:oil molar ratio of 20:1. A 95% FFA conversion was achieved. The catalyst was reused and after five repeated uses, a 7% increase in FFAs was observed indicating good reusability. [PERSON] [41] demonstrated the catalytic ability of sulphonated incompletely carbonised cellulose. The catalytic material was prepared in a similar way to the D-glucose catalyst by [PERSON] et al. [46] The esterification of oleic acid to methyl oleate was used to demonstrate the catalytic ability of the sulphonated cellulose. The esterification reaction was carried out at 95 \({}^{\circ}\)C. A yield of 99.9% was achieved after 4 h, in six sequential runs the catalyst demonstrated excellent reusability for the esterification reaction, as presented in Figure 5. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Catalyst** & **Catalytic Activity \(\upmu\)mol min\({}^{-1}\)** & **Yield of Methyl Oleate wt.\%** \\ \hline Sugar Catalyst & 67 & 95 \\ Sulfubated Zirconia & 21 & 85 \\ Amberlyst 15 & 7 & 15 \\ Niobic Acid & 1.5 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Catalytic activity and methyl oleate yield (esterification) for various catalysts. Figure 4: Esterification yield for carbohydrate-derived catalysts. [PERSON] [41] also investigated the ability of the catalyst to catalyse the transesterification reaction; the cellulose catalyst was used to catalyse the transesterification of triolein to methyl oleate in the presence of methanol. A conversion of > 98% was achieved; the catalyst demonstrated good reusability over five runs for the transesterification reaction, presented in Figure 5.  
[PERSON] et al.