Fly ash as a new versatile acid-base catalyst for biodiesel production Robinson Mu ~ noz a, b , Aixa Gonz  alez c , Fabiola Valdebenito d, e , Gustavo Ciudad f, g , Rodrigo Navia g, h , Gina Pecchi e, i , Laura Az  ocar d, e, * a Scientific and Technological Bio-resource Nucleus, Universidad de La Frontera, Temuco, Chile b Doctorado en Ciencias de Recursos Naturales, Universidad de La Frontera, Temuco, Chile c Departamento de Procesos Industriales, Facultad de Ingeniería, Universidad Cat olica de Temuco, Temuco, Chile d Departamento de Química Ambiental, Facultad de Ciencias, Universidad Cat olica de la Santísima Concepci on, Chile e ANID e Millennium Science Initiative Program- Millennium Nuclei on Catalytic Process towards Sustainable Chemistry (CSC), Chile f Instituto del Medio Ambiente (IMA), Universidad de La Frontera, Temuco, Chile g Departamento de Ingeniería Química, Universidad de La Frontera, Temuco, Chile h Centre for Biotechnology & Bioengineering (CeBiB), Universidad de La Frontera, Temuco, Chile i Universidad de Concepci on, Facultad de Ciencias Químicas, Concepci on, Chile article info Article history: Received 16 June 2020 Received in revised form 18 August 2020 Accepted 21 September 2020 Available online 7 October 2020 Keywords: Biodiesel Fatty acid methyl ester Fly ash Waste frying oils Response surface methodology Transesterification abstract The production of fatty acid methyl esters (FAME) from waste frying oil (WFO) was studied using fly ash as received as a heterogeneous catalyst. The fly ash used in this research had a high content of both CaO and SO 3 , two compounds that have been previously proposed as catalysts in FAME production. The study was carried out on the basis of a response surface methodology (RSM). The model generated by RSM predicted as optimal conditions to obtain a 100% FAME yield at a methanol-to-oil molar ratio of 3.1:1,11.2 (wt.% based on oil weight) fly ash and a temperature of 59  C with agitation at 245 rpm and 6 h of reaction time. Additional experiments comparing anhydrous with aqueous medium showed that fly ash presented a high catalytic capacity to transform free fatty acids (FFA) into FAME through consecutive hydrolysis and esterification processes (hydroesterification) compared with that associated with the transesterification mechanism. According to the results, the fly ash used in this study would act as a multipurpose or “versatile” catalyst due to its chemical composition with constituents that act as acidic and basic catalysts, therefore, catalyzing the transesterification and hydroesterification reactions simultaneously and increasing the conversion yields of FAME. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Biodiesel is a mixture of long-chain mono-alkyl esters derived from fatty acids commonly obtained from edible vegetable oils. The high prices of these raw materials are the main reason for the biodiesel production cost. Therefore, in order to make the invest- ment in this industry more attractive, the use of waste frying oil (WFO) has been investigated [1e4]. Conventionally, biodiesel production is carried out by the transesterification of triglycerides (TG) with methanol in the presence of a homogeneous basic catalyst, such as sodium or po- tassium hydroxide. In the transesterification, 1 mol of TG reacts with 1 mol of alcohol producing a mixture of fatty acids methyl esters (FAME) and glycerol, where the conversion of one ester to another occurs by exchange of the alkoxy group, without obtaining the free acid [5]. The use of the homogenous basic catalyst presents some drawbacks when WFO is used as a raw material because its high level of free fatty acids (FFA), causing undesirable saponifica- tion reactions. The production of soap consumes the catalyst, decreasing FAME production yield and preventing the effective separation of the produced FAME from glycerin [6]. Additionally, the homogeneous characteristic of basic catalysts complicates their removal after the reaction, favoring the production of abundant wastewater due to the necessity of a wash stage to remove the remainder of the catalyst. Catalysts such as lipases and acids have been proposed to solve these drawbacks since the FFA can be directly esterified to FAME by * Corresponding author. Departamento de Química Ambiental, Facultad de Ciencias, Universidad Cat olica de la Santísima Concepci on, Chile. E-mail addresses: r.munoz07@ufromail.cl (R. Mu~ noz), agonzalez@uct.cl (A. Gonzalez), fvaldebenito@ucsc.cl (F. Valdebenito), gciudad@ufro.cl (G. Ciudad), rnavia@ufro.cl (R. Navia), gpecchi@udec.cl (G. Pecchi), lazocar@ucsc.cl, lhazocar@ gmail.com (L. Az ocar). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2020.09.099 0960-1481/© 2020 Elsevier Ltd. All rights reserved. Renewable Energy 162 (2020) 1931e1939