Esterification of oleic acid to produce biodiesel catalyzed by sulfonated activated carbon from bamboo
Introduction
Due to gradual depletion of the traditional fossil residues and increased deterioration of the environmental pollutions, alternative renewable fuels were paid more and more attentions recent years. Biodiesel [1], usually produced through transesterification of triglycerides from vegetable oils or animal fats, emerges with conspicuous features. Biodiesel possess the similar physicochemical properties with fossil diesel, such as cetane value, lubricity, kinematic viscosity and flash point, etc, and could be directly applied in engine without mechanical modification [2], [3]. Catalytic capabilities of the homogeneous alkali hydroxides or alcoholates would be crippled if free fatty acids (FFAs) exist in the crude oil due to saponification [4]. Thus, conversion of FFAs into esters through esterification becomes a necessary pre-treatment process for resource with FFAs [5]. For esterification, application of the homogeneous acid catalysts, such as H2SO4, HCl or HF, etc, are restricted due to the equipment corrosion and environment pollution, which however can be avoided by the heterogeneous acid catalysts, such as zeolite [6], heteropolyacids [7], cation exchange resin [8] and niobic acid [9]. Unfortunately, they are subjected to more or less drawbacks of low acid sites density, poor mass transfer and expensive preparation cost.
The carbon based heterogeneous acid catalysts were validated to be proper for esterification owing to the advantages of chemical inertness, mechanical stability, structural diversity and surface hydrophobicity. Carbon-containing materials of multiwall carbon nanotube [10], glycerol [11], oilseed cake [12], starch [13] and bagasse [14], etc, have been used as carbon precursors for heterogeneous acid catalyst preparation. Activated carbon is widely used as catalyst support in chemical and environmental application. Besides hydrophobicity of the catalyst surface to guarantee efficient absorption of long chain organic molecule of fatty acids and beneficial elimination of water byproduct to deactivate the catalyst [15], [16], microstructure of the carbon material could be regularly adjusted to be of profound significance for the catalytic capability through physical or chemical activation [17], [18]. Arylation was used to synthesize the 4-sulfophenyl activated carbon based heterogeneous acid catalyst, where 4-sulfobenzenediazonium supplies active sites and H3PO2 presented as reductant. Under low synthesis temperature (<5 °C), the catalyst gained big surface area and exhibited acceptable catalytic capability [19]. Recently, Malins at al [20] conducted arylation of activated carbon with diazonium salts from 20 °C to 85 °C to synthesize the heterogeneous acid catalyst without H3PO2, where the catalyst showed the similar catalytic performance with commercial Amberlyst-15 for biodiesel production. However, reports about the promising approach of arylation performed higher than 5 °C for the carbon based heterogeneous acid catalyst were relatively rare till now.
Our previous researches [21], [22] synthesized a series of carbon based heterogeneous acids catalysts from bamboo through partial carbonization and sulfonation. However, due to the incorporation of high densities of hydrophilic molecules into the carbon bulk by binding with the flexible carbon sheet, direct carbonization led to the sulfonated catalyst with poor porosity [23]. It could be speculated that application of the bamboo activated carbon for heterogeneous acid catalyst synthesis through arylation could simultaneously manifest advantages of the developed microstructure to strengthen mass transfer and plenitudinous active sites to accelerate chemical reaction for biodiesel production, which however has not been reported till now.
In this study, a series of heterogeneous acid catalysts are synthesized from bamboo activated carbon through arylation with sulfanilic acid, where the synthesis conditions are optimized from molar ratio of benzylsulfonic acid group (PhSO3H) to bamboo activated carbon, sulfonation temperature and sulfonation duration. To reveal catalytic mechanism, characterizations of XRD, FTIR, N2 absorption-desorption, EDX and acid-base neutralization titration are conducted. Meanwhile, influences of catalyst added mass percentage (relative to the oleic acid mass), molar ratio of ethanol to oleic acid, esterification temperature and esterification duration on catalytic capability in esterification of oleic acid with ethanol are investigated. Esterification of oleic acid with methanol is also carried out to validate compatibility of the heterogeneous acid with different alcohols. To acquaint stability of the heterogeneous acid in catalyzing esterification, homogeneous catalytic effect, reusability and regenerability are profoundly estimated under the optimized esterification condition.
Section snippets
Catalysts synthesis
Synthesis of heterogeneous acid catalyst from the bamboo activated carbon is familiar with the procedure in the reported literatures, but some adjustments are conducted [20], [24]. After dried at 105 °C in vacuum oven overnight, the bamboo activated carbon is mixed with sulfanilic acid (analytical grade, Kermel Chemical Reagent Co., Ltd, Tianjin, China) under vigorous stirring to load active sites of PhSO3H group and pH value of the mixture emulsion is controlled to be 7–8 by NaOH throughout
Preparation optimization of the heterogeneous acid
Fig. 1a presents influence of molar ratio of PhSO3H group to activated carbon on catalytic capability of the heterogeneous acid. Esterification is obviously depressed at small molar ratios, where efficiencies are only 39% and 53% at the molar ratio of 0.1 and 0.2, respectively. Heightening the molar ratio till to 1 could aggrandize the catalytic capability and efficiency is continuously increased from 66% to 90% with the molar ratio from 0.3 to 1. As the molar ratio exceeding 1, further
Stability of the heterogeneous acid
As reported in previous studies, the supported heterogeneous catalyst would be deactivated due to active sites leaching. In this study, the undesirable homogeneous catalytic effect is determined from the active sites leaching into both ethanol and oleic acid. After mixed and stirred at 85 °C for 180 min, the heterogeneous acid and ethanol are separated through filtration, and oleic acid is esterified by the filtrated ethanol. Under the optimized condition depicted in Fig. 4, esterification
Conclusions
At the molar ratio of PhSO3H group to activated carbon of 1 and sulfonated at 50 °C for 10 min, the heterogeneous acid, which is synthesized through arylation with sulfanilic acid from bamboo activated carbon, gains the maximum total acid density of 1.69 mmol g−1 and mesoporous microstructure with the surface area and pore volume of 225.71 m2 g−1 and 0.12 cm−3 g−1. Esterification efficiency of oleic acid with ethanol catalyzed by the heterogeneous acid is 96% with the catalyst added mass
Acknowledgements
This work is financially supported by the Shenzhen Science and Technology Research and Development Funds (JCYJ20160331184515666), the Shandong Provincial Natural Science Foundation, China (ZR2017QEE004) and the Young Scholars Program of Shandong University (YSPSDU, 2015WLJH33).
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