Abstract
Organic xerogels were functionalized by incorporating sugarcane bagasse lignin from soda pulping black liquor, not used so far in this materials, with the aim of introducing new functional groups on traditional gels that could improve its adsorptive capacity. Two mixing designs were applied to identify the reactive combinations that allow a well gel formation and to adjust models that predict physical properties. The designs study five components: resorcinol (R, 0.04–0.3), lignin (L, 0.004–0.14), formaldehyde (F, 0.08–0.17), water (W, 0.45–0.8), and NaOH (C, 0.0003–0.0035). The first experimental design was an extreme vertices design and its results showed shrinkage between 4.3 and 59.7 and a bulk density from 0.54 to 1.3; a mass ratioLR/Fnear 1.5 was required for gel formation. In the second design a D-Optimal was used to achieve better adjusted coefficients and incorporate the largest possible amount of lignin in the gels. Bulk density varies from 0.42 to 0.9, shrinkage varies from 3.42 to 25.35, and specific surface area reaches values of 451.86 m2/g with 13% lignin and 270 m2/g with 27% lignin. High catalyst content improves lignin dissolution and increase shrinkage and bulk density of xerogels and bulk density. Lignin contributes to reducing shrinkage and specific surface area due to his compact and rigid structure.
Highlights
The first organic gel was obtained by Pekala in a solgel polymerization of resorcinol and formaldehyde under alkaline conditions and supercritical drying [1]; this was called aerogel
Lignin molecular weight distribution was determined by size exclusion chromatography (SEC) in a Sephadex G-75 column (57 cm × 1.8 cm) eluted at 1 mL/min with 0.5 M sodium hydroxide using an AKTA 10 GE chromatograph set with a UV detector fixed at 280 nm
Two peaks are observed: one near 4000 g/mol and another smaller in 27000 g/mol; it is similar to molecular weight distribution of wheat straw lignin [42]
Summary
The first organic gel was obtained by Pekala in a solgel polymerization of resorcinol and formaldehyde under alkaline conditions and supercritical drying [1]; this was called aerogel. If the method of drying the gel is at subcritical conditions, the the gel is called xerogel; under freezing conditions, is called cryogel. Organic gel production costs are high; new process and raw materials have been studied to obtain a product commercially attractive [5]. Other phenolic compounds have been used: phenol [6, 7], tannins [8, 9], 2,4-dihydroxybenzoic acid [10], pyrocatechol [11], technical lignin [12,13,14,15,16], and so forth. Furfural has been used as crosslink [17, 18]
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