دانلود رایگان مقاله وقوع واکنش کانیزارو بر روی هیدروتالکیت های منیزیم و آلومنیوم

Abstract

Mg-Al mixed oxides and reconstructed hydrotalcites prepared from mixed oxides by rehydration attract much attention as solid basic catalysts for organic reactions, such as aldol condensation. Therefore, the understanding of parameters that govern their catalytic performance is of great importance. Cannizzaro reaction in the presence of mixed oxide and reconstructed HTC as basic catalysts was used as a model reaction to characterize both catalytic systems and to understand their behavior in aldol condensation. The performance of the samples was investigated in the conversion of aqueous furfural mixture using stirred batch reactor operating at T = 50 °C. The properties of both initial samples and catalysts after reaction were studied by different physico-chemical methods, such as XRD, DRIFT and TGA. It was shown that both mixed oxide and reconstructed HTC possessed poor activity in the conversion of dried furfural proving the importance of water presence for the reaction to take place. Addition of water to the reaction mixture resulted in growth of the yield of both furfuryl alcohol and furoyl furoate as reaction products. Furoic acid was not identified by GC analysis, but the interaction of the acid with the basic sites of catalysts was confirmed by DRIFT and TGA analyses indicating that at optimum reaction conditions, most of the basic sites were involved in the formation of furoate species. The formation of the surface furoate species influenced the behavior of the basic catalysts in aldol condensation of furfural and acetone. It allows concluding that the occurrence of Cannizzaro reaction should be taken into account when considering the behavior of basic catalysts in organic reactions with furfural as a reactant.

4. Conclusions

The rehydration of Mg-Al mixed oxide (calcined as-prepared hydrotalcite) using pure water results in the formation of reconstructed hydrotalcite that possesses OH− groups rather than carbonates in interlayer as proved by TGA-MS results. The comparison of DRIFT spectra of the as-prepared and the reconstructed materials allowed concluding that these newly formed hydroxyls are characterized by the band at 1370 cm−1 which is usually considered as a characteristic band of carbonate groups. The results also show that Mg-Al mixed oxide can be successively rehydrated with aqueous furfural mixture resulting in the formation of reconstructed hydrotalcite structure. The amount of water in the mixture influences the rehydration degree and the total reconstruction of the hydrotalcite structure is observed only when using biphasic (water-organic) reaction conditions. The catalytic data allow concluding unequivocally that Brønsted basic sites ofthe reconstructed HTC catalyze furfural transformation by Cannizzaro reaction. Both furfuryl alcohol and furfuryl furoate formed during the reaction are evidenced by GC analysis. On the contrary, furoic acid is not detected among reaction products by routine GC analysis because it remains adsorbed onthe basic sites ofthe catalystforming surface furoates. These species can be detected by both TGA (a peak in the range of T = 320 ◦C) and DRIFT (characteristic band at 1481 cm−1) methods. The formation of the surface furoates leads to a gradual catalyst deactivation, as evidenced by the time dependency of furfural conversion and yield of furfuryl alcohol