that the presence of a suitable acidic catalyst is necessary for the reaction. Several reports have shown that solid acid catalysts exhibit a higher selectivity towards acrolein; however, catalysts deactivation is the main problem [55]. Deactivation study indicates that polycondensed and cyclic C6+ generated due to reaction of glycerol with acrolein deposited inside the pores and block the active acidic sites. Therefore, the growth of a highly efficient and stable heterogeneous catalyst for the long-term stable transformation of glycerol to acrolein is still required.
Figure 4.8 Glycerol dehydration in the presence of acidic catalyst [55].
Very few studies have been reported on the carbon-based catalyst for glycerol dehydration. Lili et al. have utilized activated carbon-supported silicotungstic acid catalyst for the glycerol dehydration into acrolein. Activated carbon was selected as support due to its high surface area, superior stability over a wide pH range, and strong interaction with acidic silicotungstic material. The activity of the catalyst depends upon the loading of silicotungstic acid, its dispersion, and the relative amount of acidic sites. The catalyst with 10% loading showed the highest activity and selectivity [56].
4.4.2.7 Cyclization
N-heterocyclic carbene–silica nanoparticles have been used for glycerol cyclization into cyclic acetals. The catalyst was prepared by immobilization of 1-Butylimidazole onto rice husk ash by 3-chloropropyltriethoxysilane (CPTES). The chlorine of the catalyst was replaced by the phosphate and sulfate group, and the resulting catalysts were named RHABIm-H2PO4 and RHABIm-HSO4 respectively. The RHABIm-H2PO4 catalyst show superior catalytic efficiency and selectivity as compared to RHABIm-HSO4 owing to the presence of extra free acidic proton. The cyclization reaction followed a pseudo-first-order rate low [57].
Conclusion
The utilization of glycerol to valuable products has attracted the attention of researchers because glycerol is produced in large quantities as waste during biodiesel production. The effective utilization of this waste will be an important factor that can reduce the cost of biodiesel and promote its commercialization. Since glycerol is a biomass-derived chemical feedstock, research efforts are being made globally for its transformation into marketable chemicals which can be employed as substitutes for chemicals derived from fossil resources. A catalytic route is an efficient approach for its transformation into valuable chemicals.
This chapter explained many catalytic processes for the conversion of glycerol into useful chemicals using carbon-based materials as a catalyst. Carbon-based catalysts have shown the potential to exhibit high glycerol conversions with quite a good selectivity. However, only a small number of studies have been conducted using carbon-based catalysts, and reaction conditions have not yet been completely optimized. A very high selectivity towards the desired product is still difficult because of the multifunctional structure of glycerol with the presence of hydroxyl groups of the same reactivity. Therefore, excellent opportunities are available to find new highly active carbon catalysts with high surface area, functional properties, desirable acidic or basic sites, and high hydrothermal stability for the transformation of glycerol to valuable products.
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