Estudos de pré-tratamentos do bagaço de caju utilizando líquido iônico prótico visando a obtenção de produtos de valor agregado

Abstract

Lignocellulosic materials represent a promising renewable feedstock for the production of fermentable sugars and biofuels. Cashew apple bagasse (CAB) waste of the cashew apple agribusiness is an example of lignocellulosic material, mainly composed of cellulose, hemicellulose and lignin, which form a complex and intricate structure, and these features one of the greatest difficulties for the use of lignocellulosic residues by biotransformation, both microbial and enzyme, thereby limiting the use and prevents the conversion economically feasible value-added products. Different pretreatments of CAB were evaluated using diluted sulfuric acid, sodium hydroxide, alkaline hydrogen peroxide, to modify its structure, and among alternative technologies to pretreatment, which is gaining focus is on ionic liquids application. In this context, the present study aims to evaluate different procedures of pretreatment of CAB using the protic ionic liquid 2-(hydroxy) ethylammonium acetate (2-HEAA). Initially, the ionic liquid was synthesized for neutralization acid-base reaction using acetic acid and monoethanolamine in the 1:1 ratio at 35 °C for 24 h. Five methodologies of pretreatment from CAB were evaluated: A method - the pretreatment was conducted in a oven at 120 °C for 3 h using 5% (w/w) solid; B method - the pretreatment was conducted in a oven at 130 °C for 24 h, using 8.68% (w/w) solid, after complete dissolution ethanol 96% was added; C method - the pretreatment was conducted in a autoclave at 120 °C for 3 h, using 15.23% (w/w) solid; D and E methods - the pretreatment was conducted for heated at 80 °C for 6 h or 24 (D and E methods, respectively) using 10% (w/w) solid under continuous stirring in a reactor, after complete dissolution, was treated with a 3% (w/w) NaOH. The analysis of 13C NMR CP/MAS indicate the solids formation with crystalline cellulose structure after pre-treatment, each method has a different mechanism of action on the structure of bagasse, but there was no relationship of crystallinity in the digestibility of cellulose. All studied methods showed an increase in the digestibility of cellulose and high yield of glucose. The digestibility of the cellulose CAB after pretreatment for the methods A, B, C, D and E was 60.65%, 98%, 73.10%, 88.59% and 94.88%, respectively. Glucose yield 270 mgglucose/gCAB-IL (CAB-IL-A), 747.72 mgglucose/gCAB-IL (CAB-IL-B), 541.84 mgglucose/gCAB-IL (CAB-IL-C), 626.43 mgglucose/gCAB-IL (CAB-IL-D), 720.92 mgglucose/gCAB-IL(CAB-IL-E), while the CAB reported 48 mgglucose/gCAB at 96 h of enzymatic hydrolysis. The results demonstrated that increasing crystallinity is a factor of the second order in enzymatic hydrolysis and decreasing the lignin percentage in that the samples positively affects the hydrolysis, taking the 2-HEAA the percent delignification of up to 91.54% (CAB-IL-E), also shown in the FTIR spectra. The pretreatment that was more satisfactory was the method B, that showed a higher level of remaining solids, high digestibility (97.80%), high yield glucose 747.72 mgglucose/gCAB-IL and 8.68% (w/w) of CAB/IL. The potential of reuse of the IL was examined using the method B, the IL was used in three pre-treatment processes (once pure and two times reused), which resulted glucose yield of 463.09 mgglucose/gCAB-IL (CA-IL-R-1) and 505.04 mgglucose/gCAB-IL (CAB-IL-R-2) at 72 h of enzymatic hydrolysis. This work appears as innovative and important model for advancing the use of ILs protic in the pretreatment of CAB, presenting a promising alternative that results in high extraction of lignin from CAB and in high yields in the enzymatic hydrolysis, which may then serve as substrates for the production of value added products.