Recently, biodiesel has gained significant attention as it is a renewable, biodegradable, less pollutant emitting, non-toxic and more environmentally friendly fuel source as compared with the fossil diesel fuel available at present. It is a renewable and biodegradable fuel that consists of fatty acid methyl esters (FAMEs). It is carbon neutral because the carbon content in the exhaust is equal to the amount initially fixed from the atmosphere [1–5]. According to previous reports, the raw materials for biodiesel production account for almost 75% of the total biodiesel cost [3, 6]. Therefore, a number of research projects have been carried out using non-edible oils such as Jatropha oil or fats, and other waste oils, to reduce the raw material cost. Nevertheless, such oils usually contain a high percentage of free fatty acids (FFAs) that severely affect the biodiesel production process. The high FFA content (>1 wt%) will form soap when a homogenous base catalyst (for example, NaOH) is used, resulting in difficulty in separating products and causing a low biodiesel yield [3, 7, 8]. Therefore, a two-step process of acid esterification and base transesterification is normally used to convert such oils to biodiesel [9–13]. Production of FAMEs is usually catalyzed by homogenous basic or acidic catalysts such as NaOH, KOH and NaOCH3 or sulfuric acid and phosphoric acid [13–16]. However, these homogeneous catalysts create several problems at the end of the reactions, including difficulty in separation of the catalysts, production of pollutants, corrosion of the reactor, sulfur contamination in the biodiesel, and formation of soap [3, 17]. In contrast, solid acid catalysts possess advantages over conventional homogeneous acid and base catalysts by being easier to separate from the end products, having comparable catalyst activity and giving a lower amount of pollutants . Recently, wide attention has been given to producing a solid acid catalyst for replacing homogeneous acid catalysts. Previous work has produced carbon-based solid acid catalysts by sulfonating carbonized polymer for hydrolysis . Many studies have been performed using solid acid catalysts for biodiesel production. A good solid acid catalyst should simultaneously catalyze esterification of fatty acids in the oil and transesterification of triglycerides [17, 19–22]. Therefore, the use of solid acid catalysts has gained more and more attention in recent years.
Lignin is the second-most abundant natural organic material after cellulose, and the richest aromatic organic biopolymer. It has high carbon content and should be usable as a precursor for activated carbon. Lignin is generally collected from the major waste material from paper mills: black liquor. Waste black liquor lignin can be a low-cost material for the preparation of solid acid catalysts [23, 24]. However, there has been little research performed with regard to its application in biodiesel production. Only high-cost carbohydrate-based biomass (for example, starch, glucose) has been used as a raw material to make solid acid catalysts, showing high catalytic activity for biodiesel production from low-qualified oils with high FFAs [10, 25].
In this study, we prepared a solid acid catalyst from Kraft lignin by treatment with phosphoric acid, pyrolysis and sulfuric acid, and subsequently it was used as catalyst to synthesize biodiesel from high-acid value Jatropha oil. In the biodiesel production process with the catalyst, first, the esterification of oleic acid was studied with an orthogonal experimental design to optimize reaction variables. Various reaction parameters, such as catalyst loading, reaction temperature and oil-to-methanol ratio on the esterification rate were optimized. Under these optimized conditions, crude Jatropha oil with high FFAs was directly converted to biodiesel with the solid acid catalyst.