ORIGINAL ARTICLE Potassium compounds-Al 2 O 3 catalyst synthesized by using the sol-gel urea combustion method for transesterification of sunflower and waste cooking oils Kavan Ghavami 1 & Faranak Akhlaghian 1 & Farhad Rahmani 1 Received: 26 January 2020 /Revised: 28 March 2020 /Accepted: 15 April 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The nanostructured composite catalyst of potassium compounds-Al 2 O 3 , or briefly K/Al 2 O 3 , was prepared by using the sol-gel urea combustion method using potassium and aluminum nitrates as precursors, which was applied in the transesterification reaction of sunflower and waste cooking oils to produce biodiesel. The molar ratio of potassium nitrate to aluminum nitrate of 0.5 in the synthesizing solution of the catalyst and calcination temperature of 650 °C were obtained from the optimization experi- ments of the catalyst. The optimized K/Al 2 O 3 catalyst was characterized by ICP, XRD, SEM, and TGA. Gas chromatography- mass spectrometry analysis of the produced biodiesel confirmed the formation of methyl esters. The effects of the operating conditions like methanol to oil molar ratio, catalyst dose, reaction temperature, and time on the biodiesel production were investigated, modeled, and optimized using the response surface methodology. The results of statistical analysis of the experi- mental model showed that linear term of temperature and interaction term of temperature and time had the strongest effect on the biodiesel yield. The optimized operating conditions were methanol to oil molar ratio of 17:1, catalyst dose of 8.25 wt.%, temperature of 70 °C, and time of 7 h. Under these conditions, the experimental yield of biodiesel production was 88.74% from sunflower oil and 82.01% from waste cooking oil. The properties of the produced biodiesels like density, pour point, cloud point, cetane number, and acid value were measured, which were comparable with the standards. Keywords Biodiesel . Transesterification . Sunflower oil . Waste cooking oil . Potassium compounds . Urea combustion method 1 Introduction Energy demand is growing due to the increasing population, industrialization, and technological progress in developing countries [1–5]. The transportation division is one of the energy consumers, which conventionally meets its demand from fossil fuels [6]. Declining fossil fuel supply, high costs, and their harmful impacts on environment have motivated researchers to look for renewable biofuels [4, 6, 7]. Biodiesel as an alterna- tive of petrol-diesel has some advantages like biodegradability, non-toxicity, renewability, low carbon and sulfur content, high lubricity, and flash point. Therefore, it has attracted the attention of researchers [1, 4, 6, 8–11]. Biodiesel can be produced from vegetable oils, animal fats, and algae [10, 12]. Biodiesel is the mono alkyl esters of long-chain fatty acids [12–14]. One of the methods to produce biodiesel is the transesterification of natural oils with alcohols in the presence of catalyst [12, 13, 15]. Catalysts are used to improve transesterification reaction rate and yield to produce biodiesel [16]. Transesterification reaction of oils is reversible, and for the complete conversion of oils, alcohols are used in the excess amount. The transesterification reaction includes three consecutive reversible reactions. Triglyceride is extracted from vegetable oils. In the first reac- tion, triglyceride to diglyceride; in the second reaction, diglyc- eride to monoglyceride; and finally in the third reaction, mono- glyceride to methyl esters and glycerol are converted [17]. The effective parameters on biodiesel production are the molar ratio of alcohol to oil, catalyst dose, reaction temperature and time, and kind of oil [17]. Homogeneous, heterogeneous, enzyme catalysts are used to produce biodiesel [18]. Homogeneous cat- alysts react with final product and are separated from reaction * Faranak Akhlaghian akhlaghianfk@gmail.com 1 Department of Chemical Engineering, Faculty of Engineering, University of Kurdistan, Sanandaj, Iran Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-020-00731-z