Biodiesel production catalyzed by highly acidic carbonaceous catalysts synthesized via carbonizing lignin in sub- and super-critical ethanol
Graphical abstract
Introduction
Compared with currently used fossil fuels, biodiesel derived from edible and non-edible oils is renewable, biodegradable, less polluted, non-toxic and environmental friendly, and gains significant attentions [1], [2], [3]. Non-edible oils such as Jatropha curcas L. oil are important raw materials for biodiesel production, because they do not compete with land for food production, and are cheap and sustainable [4], [5], [6]. Solid catalysts including solid acids [7], [8], [9], [10] and solid bases [3], [11] are widely studied for biodiesel production because they are less corrosive than liquid acids or liquid bases, produce less liquid pollutants after catalytic reactions and can be easily recycled. For the transformation of Jatropha oil, the critical challenge is its high content of free fatty acids (FFAs) that greatly limits the activity and reuse of base catalysts, and brings serious saponification in base-catalyzed reactions [6]. But, solid acid catalysts can solve this problem, as they can catalyze both esterification and transesterification and produce biodiesel directly from high acid value (AV) oils in one step [7].
Lignin is the second-most abundant organic material in the nature after cellulose. It is largely obtained from industrial pulping process and can be used as feedstock for producing energy, chemicals and fuels [12], [13], [14]. Lignin has higher carbon content than carbohydrates that benefits the economics in biochar production. Recently, carbonaceous catalysts from lignin have been studied (Table 1) [4], [15], [16], [17]. Compared with char from pyrolysis, hydrothermal biochar prepared under relatively mild condition of hot water provides rich oxygen-containing groups [18], [19], [20]. Generally, hydrothermal char from carbohydrates (but not from lignin) has regular morphology with well-distributed functional groups [21]. The main reason is that sugars dissolved in hot-compressed water undergo homogeneous carbonization, however lignin decomposes under both heterogeneous and homogeneous conditions with different routes towards polyaromatic hydrochar [15], [22], [23]. A proper solvent is required for lignin reaction system such as small molecule alcohols that can help to dissolve lignin to limit heterogeneous routes for the production of lignin biochar with better surface properties to load active sites [24], [25].
This work aims to produce carbonaceous acids with high acid content from lignin for biodiesel production. Ethanol as solvent was first screened from different alcohols for lignin char production, and further optimized under sub- and super-critical conditions. The yielded char was sulfonated to carbonaceous acids for the production of biodiesel from oleic acid, Jatropha oil and blended soybean oils with high AV according to single-factor and orthogonal experimental designs. The carbonaceous acid prepared in this work with novel alcohothermal carbonization possessed much higher acid content (5.05–5.35 mmol [H+]/g) than those achieved by conventional methods with pyrolysis in previous studies (1.3–3.5 mmol [H+]/g) (Table 1) [4], [16], [17].
Section snippets
Materials
De-alkaline lignin (CAS 9005-53-2) was purchased from J&K scientific Ltd. (Beijing). It was repeatedly washed by distilled water until filtrate’s pH reached 4.8 before use. Element composition of washed lignin was C56.7H6.6O33.7S2.9N0.2 analyzed by an elemental analyzer (Vario EL III, Elementar Analysensysteme GmbH, Hanau, Germany). Ash content was 3.4% measured by calcination of lignin in air at 600 °C for 2 h in a muffle furnace (4–10, Ever light medical equipment Co., Ltd., Beijing). Reagents
Char yield and element analysis
Carbonization of lignin in different alcohols was studied and results were listed in Table 2. At carbonization temperature of 260 °C and 7.3–9.0 MPa, the order of char yield in different alcohols was methanol ≈ ethanol > iso-propanol >> 1, 2-propanediol ≈ glycerol (No. 1–5, Table 2) with the highest char yield (>66%) for methanol and ethanol. It demonstrated that alcohols with shorter carbon chain benefited char formation. The highest C (75.7 wt%) and lowest O (16.0 wt%) contents in ethanol, whereas the
Conclusions
Biochars were synthesized from dealkaline lignin in sub- and super-critical alcohols for the production of carbonaceous acids by sulfonation. Two biochars produced by carbonization in subcritical ethanol (180 °C) and pyrolysis (400 °C) or carbonization in supercritical ethanol (260 °C) were screened with abundant surface oxygen-containing functional groups (2.81 and 1.35 mmol [H+]/g) for sulfonation, resulted in carbonaceous acids with high acid content (5.35 and 5.05 mmol [H+]/g). Both catalysts
Acknowledgements
The authors wish to acknowledge the financial support from Nanjing Agricultural University, Chinese Academy of Sciences [CAS 135 program (XTBG-T02) and equipment R&D grant (No.YZ201260)], the Yunnan Provincial Government (Baiming Haiwai Gaocengci Rencai Jihua), the National Natural Science Foundation of China (No. 31400507 and 31400518).
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