Abstract
Kraft lignin was catalytically graphitized to graphene-based nanostructures at high temperature under non-oxidative atmospheres. To obtain the best catalytic performance, a uniform catalyst–lignin mixture must be made by bonding transitional metal (M) ions to oxygen (O), sulfur (S) or nitrogen (N)-containing functional groups in kraft lignin. One of the strategies is to dissolve or disperse kraft lignin in a suitable solvent, whereby the polymer chains in the condensed lignin molecules will be detangled and stretched out while the functional groups are solvated, and when mixing lignin solution with catalyst metal solution, the solvated metal ions in an aqueous solution can diffuse and migrate onto lignin chains to form M-O, M-S, or M-N bonds during the mixing process. Therefore, solvent effects are important in preparing M–lignin mixture for production of graphene-based nanostructures. Fe–lignin precursors were prepared by dissolving lignin with different solvents, including water, methanol, acetone, and tetrahydrofuran (THF). Solvent effects on the catalytic performance, size and morphology of graphene-based nanostructures were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM), and nitrogen sorption measurements. The sizes, morphologies, and catalytic properties of the products obtained from Fe–lignin precursors are greatly influenced by the solvents used. It was found that Fe–lignin (THF) had the highest iron dispersion and the smallest iron particle size. Furthermore, Fe–lignin (THF) exhibited the best catalytic performance for graphitization of kraft lignin while the graphitization degree decreased in the order: Fe–lignin(THF) > Fe–lignin(Acetone) > Fe–lignin(methanol) > Fe–lignin(water).
Highlights
To massively manufacture low-priced graphene materials, a molecular cracking and welding (MCW) method has been developed for producing few-layer graphene from solid carbon resources, especially biomass sources [1,2]: graphene-encapsulated transitional metal nanostructures are first produced by graphitization of mixtures of metal catalyst and solid carbon materials
10 mg of samples were loaded with argon (99.99% purity, 50 mL/min) gas flowing through the Thermogravimetric Analyses (TGA) at 50 mL/min as temperature was ramped at 10 ◦ C/min
The third mass loss (Figure 4), corresponding to the catalytic decomposition of solid residue yielded from the second stage, indicated that the remaining functional groups of kraft lignin continued to decompose as the temperature increased, which led to the formation of char matrix
Summary
To massively manufacture low-priced graphene materials, a molecular cracking and welding (MCW) method has been developed for producing few-layer graphene from solid carbon resources, especially biomass sources [1,2]: graphene-encapsulated transitional metal nanostructures are first produced by graphitization of mixtures of metal catalyst and solid carbon materials. Kraft lignin has relatively high hydrophobicity since it is condensed to huge molecules when recovered from black liquors through the acidity precipitation process; the polymer chains are entangled and curled, and a metal salt aqueous solution is repelled due to its hydrophobic property It is very difficult for metal ions to penetrate kraft lignin particles if an impregnation method is used, which yields a poor contact degree between metal and kraft lignin [2,4]. To prepare a uniform M–lignin mixture, it is very important to select a suitable solvent to dissolve or disperse kraft lignin, and the selected solvent should be miscible with water since the catalyst metal salt is usually dissolved in an aqueous solution. Materials investigate effects of different solvents on the uniformity of Fe ion dispersion in lignin
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