# Carbon-Based Catalysts for Biodiesel Production--A Review
## 3 Biomass Precursors
Figure 9: Reusability of polonga seed cake catalyst and peanut shell catalyst. The eco-friendly vegetable oil asphalt gave a significantly higher conversion of FFAs than either of the petroleum asphalt catalysts used. Similarly, under the stated reaction conditions the vegetable oil asphalt outperformed the petroleum oil asphalt in the transesterification reaction. Biochar is obtained as a solid residue in the pyrolysis of biomass [50, 51]. Biochar is of interest as a catalytic material for a wide variety of reasons such as; large surface area, high pore volume, long term stability, the potential for surface functional groups and low cost of production [51, 52, 53, 54]. Depending on the heating rate during pyrolysis the biochar can be produced as the major component or as a by-product. Fast pyrolysis is used to produce bio-oil, and biochar is generated as a by-product of the reaction (about 15 wt%), whereas slow pyrolysis with a lower heating rate favours the production of biochar giving a yield of 30-50 wt% [55, 56]. [PERSON] et al. [21] prepared two sulphonated biochar catalysts from woody biomass and examined their esterification and transesterification ability. The catalysts were prepared using two different methods. Method A: conc. H\({}_{2}\)SO\({}_{4}\) was added to the biochar and heated; method B: the biochar was pre-treated with 7 M KOH to improve the porosity and surface area, and fuming H\({}_{2}\)SO\({}_{4}\) was added to the treated biochar and heated. Despite the lower surface area, catalyst B displayed a much higher sulphonic group density and total acidity than catalyst A (Table 4). Catalyst A showed little to no catalytic activity for the transesterification of canola oil in the presence of ethanol. However, the catalyst demonstrated good esterification ability when tested with waste vegetable oil. Catalyst B showed significantly better activity for the transesterification reaction due to the increased acid site density and in particular, the increased sulphonic group density as this is the only acid group strong enough to catalyse the transesterification process. Despite the increased activity of catalyst B for the transesterification reaction, the maximum yield obtained was in the range of 10%. A further study was completed by [PERSON] et al. [57] in which an improved sulphonated biochar catalyst was produced from woody biomass using a method similar to method B with the carbonisation of the biochar being carried out at a range of temperatures. As expected, the surface area decreased following sulphonation but as depicted in Table 5 the percentage decrease in surface area was drastically different \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Catalyst** & **Surface Area m2/g** & **Sulphonic Group Density mmol/g** & **Total Acidity \(\mu\)mol/g** \\ \hline Method A & 14.4 & 0.59 & 83 \\ Method B & 2.6 & 1.04 & 3200 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Catalyst characterisation for method A and B.  
Figure 10: Esterification conversion for petroleum and vegetable asphalt catalysts. for the catalysts carbonised at different temperatures. The catalyst carbonised at 450 \({}^{\circ}\)C showed the largest decrease in surface area, pore size, pore volume, and for the catalysts carbonised at 675 and 875 \({}^{\circ}\)C, the decrease in surface area, pore volume, and pore size was substantially lower. The reason for this is believed to be the elevated number of C-C bonds when the biochar was carbonised at higher temperatures, the C-C bond is more resistant to collapse leading to a lesser reduction in surface area, pore size and pore volume. In contrast, the total acid density showed the opposite trend, with increasing carbonisation temperature and increasing degree of carbonisation, an increasing number of C-H bonds are ruptured which in turn reduces the active acid sites available on the catalyst surface. The transesterification of canola oil with methanol was used to evaluate the catalytic activity of the prepared catalyst.