|  Girish N, Niju S, Meera Sheriffa Begum K M, et al. Utilization of a cost effective solid catalyst derived from natural white bivalve clam shell for transesterification of waste frying oil [J]. Fuel, 2013, 111: 653–658. Zhang Q Y, Xu C, Zhou K Z, et al. Synthesis of biodiesel with silica gel column chromatography catalyzed with immobilized K2CO3 [J]. Journal of Fuel Chemistry and Technology, 2011, 39(10): 754–758. (In Chinese) Badday A S, Abdullah A Z , Lee K T. Transesterification of crude Jatropha oil by activated carbon-supported heteropolyacid catalyst in an ultrasound-assisted reactor system [J]. Renewable Energy, 2014, 62: 10–17 Karabas H. Biodiesel production from crude acorn (Quercus frainetto L.) kernel oil: an optimisation process using the Taguchi method [J]. Renewable Energy, 2013, 53: 384–388 Jiménez-Morales I, Santamaría-González J, Maireles-Torres P, et al. Zirconium doped MCM-41 supported WO3 solid acid catalysts for the esterification of oleic acid with methanol [J]. Applied Catalysis A: General, 2010, 379: 61–68. Zhang Q Y, Li H, Qin W T, et al. Solid acid used as highly efficient catalyst for esterification of free fatty acids with alcohols [J]. China Petroleum Processing and Petrochemical Technology, 2013, 15(1): 19–24. Zi J F. Synthesis of butyl oleate catalyzed by compound inorganic salts [J]. Industrial Catalysis, 2013, 21(1): 50–52. (In Chinese) Gao D Z, Wang L, Liu L H. Comparison of catalytic activity of inorganic salts in the esterification of high acid value oil [J]. Shandong Chemical Industry, 2009, 38(8): 7–9. (In Chinese) Cheng D Y, Li D S. Preparation of isoamyl butyrate using inorganic ferric salt heterogeneous catalyst [J]. Natural Gas Chemical Industry, 2004, 29(5): 65–67. (In Chinese) Park Y M, Lee D W, Kim D K, et al. The heterogeneous catalyst system for the continuous conversion of free fatty acids in used vegetable oils for the production of biodiesel [J]. Catalysis Today, 2008, 131: 238–243. Corro G, Ba?uelos F, Vidal E, et al. Measurements of surface acidity of solid catalysts for free fatty acids esterification in Jatropha curcas crude oil for biodiesel production [J]. Fuel, 2014, 115: 625–628. Zhang Q Y, Li H, Liu X F, et al. Modified porous Zr-Mo mixed oxides as strong acid catalysts for biodiesel production [J]. Energy Technology, 2013, online. Doi: 10.1002/ente.201300109. He L Q, Qin S J, Chang T, et al. Biodiesel synthesis from the esterification of free fatty acids and alcohol catalyzed by long-chain Br?nsted acid ionic liquid [J]. Catalysis Science & Technology, 2013, 3(4): 1102–1107. Su C H. Kinetic study of free fatty acid esterification reaction catalyzed by recoverable and reusable hydrochloric acid [J]. Bioresource Technology, 2013, 130, 522–528. Salis A, Pinna M, Monduzzi M, et al. Biodiesel production from triolein and short chain alcohols through biocatalysis [J]. Journal of Biotechnology, 2005,119, 291–299. Mao L X. Comparison of catalytic activity of some inorganic salts in esterification of propanoic acid with butanol [J]. Fine Chemical Intermediates, 2001, 31(6): 34–35. (In Chinese) Santos J S, Dias J A, Dias S C L, et al. Acidic characterization and activity of (NH4)xCs2.5-xH0.5PW12O40 catalysts in the esterification reaction of oleic acid with ethanol [J]. Applied Catalysis A: General, 2012, 443–444: 33-39. Tang B H. Synthesis of ethyl acetate catalyzed by inorganic salt [J]. Advanced Materials Research, 2012, 455–456: 1060–1063.