Transesterification of used cooking sunflower oil catalyzed by hazelnut shell ash Marija R. Miladinovi  c a , Jugoslav B. Krsti  c b , Miodrag V. Zduji  c c , Ljiljana M. Veselinovi  c c , Djordje N. Veljovi  c d , Ivana B. Bankovi  c-Ili  c e , Olivera S. Stamenkovi  c e, * , Vlada B. Veljkovi  c e, f a Faculty of Agriculture, University of Ni  s, Kosan ci ceva 4, 37 000, Kru sevac, Serbia b Institute of Chemistry, Technology and Metallurgy, Center for Catalysis and Chemical Engineering, University of Belgrade, Njego seva 12, 11000, Belgrade, Serbia c Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35,11000, Belgrade, Serbia d Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4,11000, Belgrade, Serbia e Faculty of Technology, University of Ni  s, Bulevar OsloboCenja 124, 16000, Leskovac, Serbia f The Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000, Belgrade, Serbia article info Article history: Received 19 July 2021 Received in revised form 11 October 2021 Accepted 26 October 2021 Available online 1 November 2021 Keywords: Ash Biodiesel Hazelnut shell Kinetics Transesterification abstract Hazelnut shell ash was investigated as a new base catalyst for the transesterification of used cooking sunflower oil to biodiesel. To understand its catalytic properties, the prepared ash was characterized by EDX, XRD, TGA/DTA, Hg porosimetry, N 2 physisorption, FE-SEM, and basic strength measurements. The effects of the catalyst loading in the range of 1e5% of the oil weight and the methanol-to-oil molar ratio of 6:1e18:1 on the kinetics of the fatty acid methyl esters synthesis were established. Moreover, the leaching and reusability of the catalyst were assessed. The obtained results revealed that hazelnut shell ash was mostly composed of K, Ca, and Mg. The highest ester content (98%) was achieved at the catalyst loading of 5%, the methanol-to-oil molar ratio of 12:1, and the reaction time of 10 min. The contribution of homogeneous catalysis because of the catalyst leaching was confirmed but did not determine the overall reaction rate. The catalyst can be reused after the recalcination at 800  C for 2 h achieving the high methyl esters content (>96%) in 30 min after three subsequent runs. The overall reaction followed the pseudo-first-order kinetics with respect to triacylglycerols. A linear relationship between the apparent reaction rate constant and the catalyst loading and the methanol-to-oil molar ratio was determined. The determined value of the reaction rate constant was 0.0576 dm 6 /(min$mol 2 ). © 2021 Published by Elsevier Ltd. 1. Introduction The utilization of biomass as a source of energy has gained great attention through the years because of the possibility for its diverse conversion into thermal and electrical energy, and fuels. Special attention is drawn to biomass-based diesel fuel, i.e., biodiesel because it can be used in diesel engines and as heating fuel. Bio- diesel represents an alternative liquid fuel important for the sus- tainable replacement of fossil diesel in transportation and the reduction of exhaust pollution. To meet the sustainable criteria and reduce the production cost, the production of biodiesel tends to shift towards the utilization of low-cost materials as oily feedstocks and catalysts. Conventional biodiesel production worldwide is mostly based on homogeneous catalysis. Besides the advantages offered by homogeneous catalysts, they have some drawbacks in their utilization such as difficulties associated with the separation from the reaction medium and reusability. The removal of these catalysts from crude biodiesel requires the additional operation of washing, which increases the production costs and generates a large amount of wastewater. This process could be simplified by the utilization of solid catalysts, especially if they are prepared from low-cost materials. Besides the naturally available CaO [1], the waste materials from food processing, such as eggshells [2,3], shells [4,5], and animal bones [6,7] have been investigated as a source of CaO. The utilization of waste materials is important not only economically but environmentally alleviating solid waste disposal * Corresponding author. E-mail address: stamenkovico@tf.ni.ac.rs (O.S. Stamenkovi c). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2021.10.071 0960-1481/© 2021 Published by Elsevier Ltd. Renewable Energy 183 (2022) 103e113