Brazilian açaí berry seeds: an abundant waste applied in the synthesis of carbon-based acid catalysts for transesterification of low free fatty acid waste cooking oil.

Residues of açaí seeds (Euterpe oleracea Mart.) were a novel source for the synthesis of the acid heterogeneous catalyst applied in the conversion of low free fatty acid waste cooking oil (WCO) to biodiesel. Yield of activated carbon (AC) and catalyst (CAT), as well as density of SOH groups and total acidity, was analyzed in an entirely random designed experiment using multiple linear regression, one-way ANOVA, and Tukey's post hoc test. Time, temperature, dosage of KOH, and ratio of HSO/AC were the predictor variables with 3 levels each, at a significance level of α = .05. A significant yield variation portion of AC was explained by the experimental factors (R = .891, F (3, 23) = 62.9, p < .0001), as did the yield of CAT (R = .960, F (3, 23) = 185.7, p < .0001), density of SOH (R = .969, F (3, 23) = 242.2, p < .0001), and total acidity (R = .973, F (3, 23) = 280.6, p < .0001). Levels of time (p = .001) and KOH dosage (p = .006) were significant to the yield of AC, and temperature levels were not influent on density of SOH (p = .731) or total acidity (p = .762). CAT showed a S of 249 m g, V of 0.104 cm g, low crystallinity, high thermal stability, and a mesoporous amorphous structure. Optimized catalytic tests resulted in 89% conversion of WCO and 11 cycles of reuse, better than pure HSO or pure KOH (p < .0001) and also better than many biomass-derived catalysts reported in the literature.