Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Application of calcined waste cupuaçu (Theobroma grandiflorum) seeds as a low-cost solid catalyst in soybean oil ethanolysis: Statistical optimization Iasmin Maquiné Mendonça a , Flavia Lopes Machado a , Cláudia Cândida Silva a , Sérgio Duvoisin Junior a , Mitsuo Lopes Takeno b , Paulo José de Sousa Maia c , Lizandro Manzato b , Flávio Augusto de Freitas b, ⁎ a Universidade do Estado do Amazonas – UEA, Av. Darcy Vargas, 1.200 - Parque Dez de Novembro, Manaus, AM 69050-020, Brazil b Instituto Federal do Amazonas – IFAM/CMDI, Av. Gov. Danilo de Matos Areosa, 1731-1975 – Distrito Industrial, Manaus, AM 69075-35, Brazil c Universidade Federal do Rio de Janeiro – UFRJ, Av. Aluízio da Silva Gomes, 50 – Novo Cavaleiro, Macaé, RJ 27930-560, Brazil ARTICLE INFO Keywords: Cupuaçu seeds Biodiesel Ethanolysis Statistical optimization Central composite design Response surface methodology ABSTRACT In this work, waste cupuaçu seeds were calcined for 4 h at 800 °C and evaluated as a heterogeneous catalyst for the biodiesel synthesis. The catalyst (CCS) was characterized by X-ray powder diffraction (XRD), wavelength dispersive x-ray fluorescence (WDXRF), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric and differential thermal analysis (TG-DTA) and soluble alkalinity. The catalytic activity was evaluated by CCS-cat- alyzed ethanolysis of soybean oil and the process was optimized using response surface methodology (RSM) and analysis of variance (ANOVA). The significance of the different process parameters and their combined effects were established through a central composite design (CCD) and the optimum process (catalyst loading of 10% (w/w) relative to oil mass, reaction time 8 h, ethanol:oil molar ratio 10:1 and temperature 80 °C) resulted in a conversion of 98.36% with good agreement with predicted conversion, 97%. The catalyst was recycled, main- taining its great catalytic activity and resulting in conversions close to 98% in the first two cycles. The high potential of CCS as a catalyst for biodiesel production was demonstrated. 1. Introduction Biodiesel has attracted a lot of attention as a promising substitute for conventional diesel because of its attractive features. It is renewable, biodegradable, non-toxic, carbon neutral, emits low exhaust emissions, has a higher flash point and excellent lubricity and is also en- vironmentally acceptable in diesel engines without the need for many engine changes [1,2]. The starting materials for the preparation of biodiesel are vegetable oils or animal fats and alcohol for producing alkyl esters of fatty acids. It is worth noting that most of the studies on biodiesel synthesis prefer to use the methanolysis route, since the size of the alcohol chain greatly influences the conversion percentage [3,4]. However, based on cost and performance considerations, the ethanol can be obtained from renew- able agricultural resources, it is non-toxic and it forms stable fatty acid esters [5,6]. Therefore, better suited for the production of this biofuel. Biodiesel synthesis is usually catalyzed by homogeneous, hetero- geneous or enzymatic processes, as well as supercritical technology [7,8]. There are several methods for producing the biofuel, but transesterification is the easiest and most economical approach to produce it [9,10]. The transesterification process can be carried out in the presence of basic catalysts or acids. In transesterification with homogeneous basic catalysis, the process is usually catalyzed by alkali metal hydroxides, such as sodium or potassium, showing high catalytic activity resulting in high conversion rates of oil or fat into biodiesel. However, homogeneous catalysis systems have many disadvantages, such as difficulties in separating the catalyst from the organic phase, the need to use a large amount of water, which consequently results in the production of a considerable amount of wastewater, as well the pro- duction of soaps [11,12]. An alternative method for biodiesel production is the use of solid catalysts. The benefits of heterogeneous catalysis have been empha- sized as a solution for homogeneous catalysis especially for feedstocks containing high free fatty acids, since solid basic catalysts prevent or reduce the amount of soap produced by neutralizing free fatty acids or saponification of triglycerides, which simplifies post-treatment (se- paration and purification) processes and avoids the production of toxic waste water [3,13]. Due to these advantages, the number of studies on https://doi.org/10.1016/j.enconman.2019.112095 Received 9 July 2019; Received in revised form 20 September 2019; Accepted 21 September 2019 ⁎ Corresponding author. E-mail address: freitas.flavio@yahoo.com.br (F.A. de Freitas). Energy Conversion and Management 200 (2019) 112095 0196-8904/ © 2019 Elsevier Ltd. All rights reserved. T