Direct production of biodiesel from high-acid value Jatropha oil with solid acid catalyst derived from lignin
© Pua et al; licensee BioMed Central Ltd. 2011
Received: 6 May 2011
Accepted: 7 December 2011
Published: 7 December 2011
Solid acid catalyst was prepared from Kraft lignin by chemical activation with phosphoric acid, pyrolysis and sulfuric acid. This catalyst had high acid density as characterized by scanning electron microscope (SEM), energy-dispersive x-ray spectrometry (EDX) and Brunauer, Emmett, and Teller (BET) method analyses. It was further used to catalyze the esterification of oleic acid and one-step conversion of non-pretreated Jatropha oil to biodiesel. The effects of catalyst loading, reaction temperature and oil-to-methanol molar ratio, on the catalytic activity of the esterification were investigated.
The highest catalytic activity was achieved with a 96.1% esterification rate, and the catalyst can be reused three times with little deactivation under optimized conditions. Biodiesel production from Jatropha oil was studied under such conditions. It was found that 96.3% biodiesel yield from non-pretreated Jatropha oil with high-acid value (12.7 mg KOH/g) could be achieved.
The catalyst can be easily separated for reuse. This single-step process could be a potential route for biodiesel production from high-acid value oil by simplifying the procedure and reducing costs.
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.
Results and discussion
Characterization of solid acid catalyst
Brunauer, Emmett, and Teller (BET) method analysis on surface area and pore volume
Kraft lignin char (only pyrolysis)
Pretreated Kraft lignin char (phosphoric acid pretreatment + pyrolysis)
Sulfonated Kraft lignin char (catalyst)
Esterification of oleic acid
Orthogonal experimental design of optimization study for oleic acid esterification
Catalyst loading (percentage based on weight of oleic acid)
Molar ratio oleic acid:methanol
Reaction time (h)
Oleic acid conversion rate (%)
Esterification of oleic acid under optimized conditions
Optimized experimental variables
Catalyst loading (based on weight of oleic acid)
Molar ratio oleic acid:methanol
Oleic acid conversion rate
This result suggests that the solid acid catalyst was highly active and stable. The higher conversion rate of oleic acid is possibly due to the high density of the acid (SO3-H) sites from sulfonation in the pores of activated carbon by treatment with phosphoric acid and pyrolysis , as confirmed by EDX spectra in Figure 5.
One-step production of biodiesel from Jatropha oil
The solid acid catalyst derived from lignin achieved a higher biodiesel yield as compared with the catalyst derived from carbohydrate (starch) in a previous study, which achieved a 93% yield from waste oils with high FFAs . There was an investigation reporting on preparation of solid acid catalyst from a glucose-starch mixture that obtained 90% biodiesel yield from high FFA content waste cottonseed oil at 80°C . Another advantage of our work is that we used waste lignin to produce the catalyst that is much more environmentally friendly and inexpensive than in other work in the literature.
In this study, a solid acid catalyst produced from waste Kraft lignin via treatment by phosphoric acid, pyrolysis and sulfuric acid was shown to be useful for esterification and one-step biodiesel production from low-qualified oils due to its high acid density. Assessment for its catalytic activity via esterification proved that it was highly effective in converting oleic acid to ester. This catalyst was further successfully used for biodiesel production, with high yield (96.3%) from non-pretreated Jatropha oil. Besides use in biodiesel production, a lignin derived solid acid catalyst may find other applications as a heterogeneous green catalyst.
Materials and catalyst preparation
The analytical grade concentrated sulfuric acid (98%) and phosphoric acid (85%) used for the catalyst preparation was purchased from Chongqing Chuandong Chemical (Group) Co. Ltd., Chongqing, China. Kraft lignin powders were purchased from Sigma-Aldrich, Shanghai, China (produced in St. Louis, Missouri, US). Industrial oleic acid (186 mg KOH/g) (Kermel Corp., Tianjin, China) and Jatropha oil (acid value of 12.7 mg KOH/g) from Xishuangbanna Tropical Botanical Gardens  were used as model materials for FFAs and crude oil in all experiments, respectively. Dehydrated methanol was from Xilong Chemical Corp., Shantou, China. A 350-ml high-pressure autoclave (FCFD05-30, Yantai Jianbang Chemical Mechanical Co., Ltd., Yantai, China; temperature and pressure can be used up to 320°C and 40 MPa) was used for the esterification and transesterification experiments. A temperature of 60 to 120°C was used in this work.
For the preparation of catalyst, lignin powders were pretreated with concentrated phosphoric acid (85%) . The mixed slurry was left for 1 h at room temperature in air. It was then dried at 105°C for 24 h to allow free vaporization of water, and subsequently pyrolyzed at 400°C for 1 h under nitrogen gas flow [26, 29]. The pyrolyzed char was washed several times with hot and cold distilled water to remove residual chemicals, mineral matter and impurities, and oven-dried overnight at 105°C. Sulfonation was carried out at 200°C for 120 min with 1-g char immersed and stirred in 10-ml concentrated sulfuric acid (98%). The sulfonated sample was rigorously washed with hot and cold distilled water to remove any physically adsorbed species until free of sulfate ions. The resulting sample was dried in an oven at 105°C for 48 h and used as the catalyst.
Esterification of oleic acid
Oleic acid, methanol and the produced solid acid catalyst were loaded and mixed together in an autoclave for the esterification reaction. Experiments were carried out at 60 to 90°C for 5 h. The molar ratio of oil to methanol was 1/6, 1/9, and 1/12. The percentages of catalyst loading were 1 to 5 wt% of oleic acid (Table 2). After reaction, the catalyst was separated from the produced mixture by filtration. Phase separation of filtrate resulted in the isolation of methyl oleate and water. Methanol was removed by distillation of the mixture using a vacuum rotating evaporator.
One-step conversion of Jatropha oil to biodiesel
Similar to the above, non-pretreated crude Jatropha oil with methanol and the catalyst were loaded into an autoclave (reaction temperature: 120°C) for esterification and transesterification to biodiesel directly. The reaction parameters were selected based on the results obtained in the esterification reaction of oleic acid. Phase separation of filtrate resulted in the isolation of FAMEs and glycerol. Methanol was removed by distillation of the mixture using a vacuum rotating evaporator. The upper layer of the resulting mixture was recovered as biodiesel (FAMEs).
Characterizations of solid acid catalyst
The morphology of the solid acid catalyst was examined using SEM (Leo 1450VP). The pore size and pore volume of the as-synthesized product was examined using a BET (Micromeritics ASAP-2020) analyzer operated at -196°C; automatic degas and equilibration interval was 10 s. The elemental composition of solid acid catalyst was examined using EDX. The acid sites on the surface of solid acid catalyst were also determined by the titration method . Solid acid catalyst was added into 0.01 M NaOH aqueous solution and stirred for 2 h at room temperature. Supernatant solution from the centrifugal separation was titrated with 0.01 M HCl.
Characterization of the products
Where AV0 is the initial acid value of oleic acid and AVN is the instant acid value for oleic acid.
Biodiesel obtained from Jatropha oil was analyzed by GC (GC-2014, Shimadzu, Japan) with capillary column of Rtx-wax (30 m × diameter 0.25 mm × 0.25 μm). The prepared biodiesel (5 ml) was dissolved in 20-ml dichloromethane and 1-ml internal standard solutions for GC analysis. Heptadecanoic acid methyl ester was used as the internal standard to quantify the yield of esters. The column temperature was 220°C, while the temperatures of the injector and detector were 260°C and 280°C, respectively. Identification of methyl ester peaks was performed by comparing the retention times between the samples and the standard compounds. Methyl esters were quantified by comparing the peak area between the samples and the standard compounds [2, 12].
The authors wish to acknowledge the financial support from Chinese Academy of Sciences (Bairen Jihua and Knowledge innovation key project (KSCX2-YW-G-075)), Yunnan Provincial Government (Baiming Haiwai Gaocengci Rencai Jihua and Provincial Natural Science Foundation), and China National Natural Science Foundation (No: 21076220). The Ministry of Higher Education of Malaysia (MOHE) for Fundamental Research Grant (UKM-ST-07-FGRS-0233-2010), Universiti Kebangsaan Malaysia for sponsorship from Student Mobility Programme (Outbound).
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