# Carbon-Based Catalysts for Biodiesel Production--A Review
###### Abstract  
In recent years, a new class of superior heterogeneous acid catalyst for biodiesel production has emerged. These catalysts offer advantages over their predecessors such as high surface area, elevated acid site density, enhanced catalyst activity, good operation stability and relevant economic affordability in an environmentally friendly frame. This review was concerned with carbon-based solid acid (CBAS) catalysts derived from both carbohydrate and pyrolysis products. A series of CBASs with various origins such as D-glucose, sucrose, starch, cellulose and vegetable oil asphalt, converted to char and sulphonated, have been explored as potential heterogeneous catalysts. Catalyst preparation and synthesis methods were briefly summarized. Catalyst characterization and performance for biofuels related reactions were elucidated, identifying potential research applications. Three catalysts in particular were identified as having potential for industrial application and requiring further research. heterogeneous acid catalyst; biofuel; transesterification; carbohydrate; biomass; biochar 29 December 201923 January 20  
The esterification of FFAs in the presence of a short-chain alcohol and a suitable catalyst is the second route for FAME production, presented in Figure 2.  
Traditional biodiesel production methods utilise homogeneous base catalysis for the transesterification reaction, the reaction is carried out at atmospheric pressure and mild temperatures (50-80 \({}^{\circ}\)C) and an oil to alcohol ratio in the region of 5:1 [13, 14]. While homogeneous base catalysis boasts a high yield of biodiesel, mild operating conditions and a fast reaction rate, there are several obstacles that prevent the process from being economically viable [10, 15, 16]. Firstly, the homogeneous nature of the process means that the separation of the homogeneous base is difficult and requires an energy and cost intensive neutralisation and water-washing step [6, 15]. Secondly, while liquid base catalysts catalyse the transesterification very well, the presence of FFAs presents a problem, as a saponification reaction will occur when FFAs are present in the feedstock for a liquid base catalysed reaction. The formation of soap inhibits the separation of the FAMEs, the water and the glycerol [6, 17, 18, 19, 20]. As a result, the oil feedstock for homogeneous base catalysis must be a virgin vegetable oil, with a very low FFA content (< 0.5 wt.%), or waste oils (such as cooking oil) must be pre-treated to lower the FFA content [12, 17, 18]. It is estimated that 70%-90% of the cost of biodiesel production is due to the cost of raw materials [12, 21, 22]. First-generation biodiesel production uses edible feedstock to produce FAMEs; while the FFA content is low, this process is uneconomical due to the competition for these oils with the food market [11, 23, 24]. Second-generation biodiesel production utilises lower-cost inedible feedstock or waste cooking oils, and these lower-grade plant oils and waste oils often have elevated FFA and water content, making homogeneous base catalysis an economically unfavourable process [11, 19]. Homogenous acid catalysis is insensitive to FFA content and has the potential to carry out simultaneous esterification and transesterification. Despite this distinct advantage, homogeneous acid catalysis presents the same separation issues as homogeneous base catalysis. As well as this, the rate of reaction for the transesterification process is much slower than homogeneous base catalysis, making the process economically challenging due to the increased energy requirements [2, 25, 26, 27, 28, 29, 30, 31]. Heterogeneous catalysis using a solid base catalyst for the transesterification of TGs shares many of the same properties as the homogeneous base catalysis such as mild operating conditions, a fast rate of reaction and a high conversion [28, 29, 32, 33, 34]. However, heterogeneous base catalysis has a distinct  
Figure 1: Transesterification of triglycerides. Figure 2: Esterification of free fatty acids.