Biodiesel is considered a promising energy substitute for conventional diesel fuel. There are various methods for biodiesel production (Abbaszaadeh et al., 2012). However, the transesterification reaction is the most widely used due to its simplicity, technical advantages and cost. This reaction for biofuels production can be carried out by mixing vegetable oils, animal fats or other lipid materials with a short-chain alcohol in the presence of a catalyst. Recent studies have focused on the synthesis of various heterogeneous catalysts, since they are easy to separate and can be reused (Devi et al., 2017). In particular, the hydrothermal carbonization (HTC) offers several advantages for preparing carbon-based catalysts in terms of its process performance, energy and economic factors (Kambo and Dutta, 2015). Therefore, this study analyzes the synthesis of heterogeneous biomass-derived catalysts prepared via HTC and their application in biodiesel production. 
Heterogeneous catalysts were prepared using the coconut endocarp as a precursor of carbon-based supports. A Taguchi experimental design L9 was used to prepare the heterogeneous catalysts as shown in Table 1. The variables used in this experimental design were: the biomass/H2O ratio, KOH concentration, carbonization time and temperature. HTC of coconut endocarp particles was carried out with a Hydrion Scientific reactor to obtain a char that was used as a catalytic support. Subsequently, this support was modified with a KOH solution at different concentrations as proposed in the experimental design, see Table 1. Specifically, 6 g of char were mixed with 60 mL of KOH at room temperature under constant stirring for 24 h. This treated solid was separated from the solution by filtration, washed with deionized water to remove excess KOH and dried in an oven at 100 °C for 24 h. The final product was used as heterogeneous catalysts for biodiesel production. The performance of catalysts was evaluated in the transesterification reaction of the commercial safflower oil using a methanol/oil ratio of 15:1, a catalyst load with respect to the oil of 20 % where the reaction conditions were 60 °C and 8 h of reaction time. The biodiesel production was quantified as Fatty Acid Methyl Esters (FAMEs) formation with a Thermo Scientific gas chromatograph. A signal-to-noise analysis was performed to identify the best conditions for the catalyst preparation. Studies for the regeneration and reuse of the best catalyst were carried out using two routes: a) the catalyst recovered from reaction was washed with 40 mL of concentrated methanol at 25 °C under continuous stirring for 2 h and the catalyst was dried in an oven at 100 °C for 24 h; b) the catalyst obtained from route a) was submitted to a physical activation in a tubular oven at 700 °C for 2 h with N2 flow of 170 mL/min. These recycled catalysts were used in the transesterification reaction and the regeneration was also performed several times. Characterization of precursor, supports and catalysts were performed with FTIR spectroscopy and X-ray fluoresce analysis to determine the functional groups, crystalline structures and their elemental composition.

Results of catalysts performance are shown in Table 1. Carbonization yields ranged from 58.2 to 65.18 %, which were in accordance with the HTC of other lignocellulosic biomasses. Catalysts showed attractive percentages of biodiesel formation with values from 59.81 to 98.68 %. Catalyst sample No. 8 achieved the highest FAMEs formation. Note that the char used as catalytic support achieved only 1.63 % of FAMEs formation and this performance justified the need of modifying this support to tailor its catalytic properties. On the other hand, the signal-to-noise (S/N) analysis of experimental design showed that the impact of variables analyzed in the catalyst preparation followed the trend: KOH concentration > HTC time > biomass/H2O ratio > HTC temperature. The best conditions to prepare the catalyst were 1/3 of biomass/H2O, 200 °C, 12 h of HTC time and 4 M KOH. XRF analysis indicated the presence of hydrated potassium carbonate in the materials modified with KOH. For illustration, Figure 2 b) shows the FTIR spectra of the chars and catalysts. FTIR spectrum of char contained the main functional groups of lignocellulosic biomasses: hydroxyl, aliphatic, carboxylic, quinones, aldehydes, ketones and esters.

Synthesis of biodiesel using heterogeneous catalysts derived from the hydrothermal carbonization of the coconut endocarp (cocos nucifera)

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