Preparação e caracterização de aerogel de nanofibras de celulose com grafite oxidado sulfonado e sua aplicação como catalisador em reações de esterificação na produção de biodiesel

Abstract

The biodiesel production routes through the esterification and transesterification processes are similar due to the common use of a catalyst to accelerate the reaction. Acid heterogeneous catalysis has the advantages of greater ease of product purification, biodegradability, ease of separation and reuse. Some requirements, however, affect the efficiency of heterogeneous catalysis, namely, the surface area and hydrophobic character. In this sense, the airgel derived from cellulose nanofibers (NFC) is interesting due to its high porosity, low cost and biodegradability, as well such as, the use of graphite is relevant due to its hydrophobicity. Therefore, this study aims to produce, in an innovative way, a heterogeneous organic acid catalyst in the form of cellulose nanofiber airgel with sulfonated oxidized graphite, to produce biodiesel through the oleic acid esterification process. The graphite was oxidized in low proportions by 2,2,6,6-Tetramethylpiperidine 1-oxyl free radical (TEMPO). The graphite sulfonation process was carried out using the reagents sulfanilic acid, sulfuric acid and sodium dodecylbenzenesulfonate (SDBS). The functionalized graphite was added to the NFC in a proportion of 50% and the mixture was subjected to the freeze-drying process, resulting in the catalyst in the form of an airgel. To test the efficiency of the catalysts, their use in an esterification reaction was analyzed, together with the reagents oleic acid and methanol, the latter being in excess. The results pointed to low oxidation and sulfonation of graphite, with the exception of oxidized graphite functionalized with sulfanilic acid, which presented lower electrostatic stability and, on the other hand, greater thermal stability, greater incorporation of sulfonic groups, essential for better catalytic performance, as well as, high surface area, with greater measurement of pore volume and diameter, highlighting that these findings show promise for better accessibility of reagents to active sites. The crystallinity of the graphite structure did not suffer significant interference in the functionalization processes. The aerogels produced were used in the synthesis of biodiesel by esterification processes, using conventional heating and the methyl route. Among the variation in the conditions for esterification to occur, temperature was the most significant, due to the kinetic and thermodynamic favoring of the occurrence. The occurrence esterified for 10 min and 100° C, without manifestation (white), declared the conversion of 33% to methyl oleate, report the autocatalysis of the occurrence mediated by carboxylic acid. The highest conversion involved the conditions of molar ratio methanol: oleic acid 10:1, body occurrence of 5% (m/m) in relation to the mass of oleic acid, time of 2 h and temperature of 150°C, reaching 70% for the sample esterified by NFC/GPS airgel – acid. sulfanilic. Therefore, to obtain greater process efficiency with its use, the need to optimize the reaction conditions is evident.