Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 78

Glycerol can be transformed into valuable chemicals using catalytic techniques such as hydrogenolysis, etherification, steam reforming, esterification, oxidation, dehydration, and cyclization. Various catalytic methods adopted for the conversion of crude glycerol as feedstock are outlines in Figure 4.5. A large variety of valuable derivatives, such as fine chemicals, basic units for polymers, fuels, esters, synthesis gas, hydrogen, and fuel additives can be produced using these techniques.

Several solid acid catalysts such as metal oxide (A1₂O₃), zeolite H-ZSM-5, metal sulfide (CdS), immobilized liquid acid (e.g. HF/AlCl₃), heteropoly acid (e.g. H₃PW₁₂O₄₀), solid superacid (SO₄^2–/ZrO₂), natural clay, etc., have been tested for different catalytic processes [20]. None of the above catalysts have shown full potential for glycerol valorization on large scale. Carbon-based materials have a large potential to be used as supports for many active metals as well as catalysts after modification owing to their large surface area, stability in both acidic and basic solutions, functional properties, and desirable acidic or basic sites. The carbon has been used as a support for various metals such as Ru, Pt, Re, Cu, etc. for the glycerol conversion into useful products [21]. In some processes, the carbon-based catalyst with acidic sites has received tremendous interest compared to homogeneous catalysts. This is attributed to its stability, efficiency, viability, and sustainability. Furthermore, carbon catalysts can be recycled numerous times without losing their activity. In particular, carbon-based sulfonated catalysts (CBSCs) are a rapidly growing field for glycerol valorization due to their easy recovery, recyclability, long-term activity, and stability.

Figure 4.5 Roadmap of selected glycerol valorization reactions.

Biomass-derived CBSC is an excellent catalyst for various applications owing to its low cost and abundance. The CBSCs contain a stable and insoluble carbon skeleton with –SO₃H functionalized groups. It is amorphous having –SO₃H and –COOH groups. In addition, the structure is aromatic in nature with the presence of high density –OH groups. The probable structure of the biomass-derived CBSC is given in Figure 4.6. The –SO₃H groups attached to CBSC are the main acidic sites for catalysis whereas the –OH and –COOH group behave as hydrophilic reactant which favor the catalytic performance by providing access to –SO₃H sites.

The basic principles, mechanisms and role of different carbon-based catalysts for different catalytic routes have been explained in the next section.