Abstrakt
Transitioning towards green chemistry requires designing sustainable processes to produce both inexpensive and environmentally friendly materials capable of catalyzing complex chemical transformations in fine chemistry. In this context, natural silicate-based catalysts stand out as some of the most promising alternatives, yet vermic-ulites have been mostly overlooked so far, despite their high potential as clay catalysts with high negative layer charge, high number of exchangeable interlayer cations and high Lewis and Br?nsted acidity.
Thus, this work reports a previously undescribed and unusual reaction between benzaldehyde and dioxane in the presence of natural and ion-enriched (Al(3+), Fe(3+), Mg(2+), Ni(2+), Sn(4+)) vermiculite catalysts. The effects of different vermic-ulites on the reaction were assessed both at different activation/calcination temperatures (150 degrees C, 300 degrees C and 450 degrees C) and without activation.
Regardless of temperature activation, all vermiculites catalyzed C-C bond cleavage, decarbonylation/ decarboxylation and radical oxidative coupling of benzaldehyde and dioxane, albeit in different yields. Non-activated, natural vermiculite provided the highest benzaldehyde conversion (92 %) and benzaldehyde-based selectivity to the coupling product, 1,4-dioxan-2-yl benzoate (11 %).
High benzaldehyde conversion rates were observed also in reactions with aluminum-enriched vermiculite (Al-VMT) activated at 150 degrees C (84 %), providing 10% selectivity to the coupling product, and with iron (III)-enriched vermiculite (Fe-VMT) (38 % benzaldehyde conversion, 12% selectivity). Catalyst calcination prevented the formation of the 1,4-dioxan-2-yl benzoate while simultaneously enhancing benzaldehyde degradation to benzene.
Ultimately, this study shows, for the first time, that vermiculites catalyze radical-oxidative coupling, yielding complex structures such as 1,4-dioxan-2-yl benzoate, which can be used as building blocks in fine chemistry.