“…In this context, mixed oxides, other mixed compounds (Jindapon et al, 2016), sulfated oxides (Istadi et al, 2015), zincates (Rubio-Caballero et al, 2009), and magnetic catalysts (Dantas et al, 2017) have been reported for the processing of vegetable oils and animal fats with low acid contents (Dias et al, 2012;Pasupulety et al, 2015). Other authors employed solid catalysts for the processing of raw materials with high FFA contents, such as sulfonated solids, used in reactions with materials containing FFA contents of 10 % (Fadhil et al, 2016), 15 % (Dawodu et al, 2014) and up to 43.73 % (Konwar et al, 2014), and bifunctional catalysts with both Brönsted and Lewis acid sites, used for the processing of materials with FFA contents of 1.78 % (Pan et al, 2018) and from 8.6 % to 60 % (Wang et al, 2017).…”