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Heterogeneous acid catalysts in the etherification of glycerol with tert-butyl alcohol and isoamyl alcohol

Abstract : Different heterogeneous acid solids based on zeolites and sulfonated graphene oxides have been evaluated as catalysts in the etherification of glycerol with tert-butyl alcohol. The zeolites used in this study, such as mordenite, faujasite, beta and H-ZSM5, were compared to study the effect of pore structure and size, as well as the size of the crystal in the conversion of glycerol and the selectivity towards the di and tri-tert-butyl ether of glycerol. The results showed that, in a liquid phase reaction sensitive to diffusion limitations, the porous zeolite network is not sufficient to predict activity, selectivity and stability. The accessibility to the pore must be taken into account according to its internal volume. The zeolites *BEA and MFI have similar pore volumes, but the difference in the opening of the pores (more than 1 Å) is sufficient to generate strong diffusion limitations in the case of the medium pore zeolite. The activity of the zeolite * H-BEA is proportional to the concentration of the Brønsted acid site as long as the size of the crystals is less than 100 nm (without limitation of internal diffusion). In the case of faujasite with supercages, successive etherification is preferred.In the second series of catalysts, the graphene oxide was obtained by the modified Hummers method and its subsequent reduction by three different routes: i - hydrazine, ii - Zn/HCl and iii - ascorbic acid. Graphene oxide has a different C/O ratio at the surface. Finally, these last solids were functionalized using sulfanilic acid, generating new Brønsted acid sites. Catalytic tests have shown that, regardless of the graphene oxide, the conversion of glycerol is greater than that obtained with Amberlyst® (A-15), which is the reference catalyst for this reaction. The selectivity after 10 hours of reaction is also comparable to that of A-15. This study has shown that the control of the hydrophilic/hydrophobic balance is very important because it depends on the amount of oxygenated groups on the surface.Regarding the mechanism of reaction, it turned out to be different in both cases, favoring Eley-Rideal (ER) in the reaction with zeolites, whereas, in the case of graphene oxides, a mechanism of type Langmuir-Hinshelwood (LH) predominates. Finally, zeolite Beta, Amberlyst® 15 and sulfonated graphene oxide have been evaluated as catalysts in the etherification of glycerol with isoamyl alcohol. The results show that the activation of the primary alcohol is more difficult in the "moderate" conditions of the reaction studied than with that of tert-butyl alcohol. On the other hand, it has been found that the self-etherification of isoamyl alcohol (to form isoamyl ether) favors the etherification of glycerol.
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Cristian David Miranda Munoz. Heterogeneous acid catalysts in the etherification of glycerol with tert-butyl alcohol and isoamyl alcohol. Catalysis. Université de Poitiers; Universidad del Valle (Cali, Colombie), 2019. English. ⟨NNT : 2019POIT2272⟩. ⟨tel-02500703⟩

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