ABSTRACT
Abstract Self-condensation of acetone is considered a reaction catalyzed typically by bases. In this study a flow microreactor has been used to evaluate the performance of several calcined MgZnAl hydrotalcite-like materials (HTs). The main products were mesityl oxide and aldol compounds. The nature of the catalyst’s composition, i.e., the effect of Zn loading on the reaction intermediates and by-products was reported. In order to give an overall description of the base catalyzed oligomerization of acetone in gas phase, a suitable mechanism was proposed. When increasing the Zn content in the hydrotalcite structure, the activity for the acetone’s oligomerization increased and the selectivity to mesityl oxide reached values about 88% mol. The most active catalyst was the one with the higher Zn content. An interpretation in function of the adsorption-desorption equilibrium of products was proposed, which discloses that the product’s desorption is slow in strong basic sites. Therefore, the activity and selectivity on calcined HTs could be easily regulated by controlling the nature and amount of cations. Introduction The increasing demand for the acetone aldol (dimer) and mesitylene (trimer), employed as important intermediate reagents for a large variety of fine chemicals, renewed the research efforts in these reactions by using heterogeneous catalytic processes (1,2). Indeed, gas-phase acetone condensation reaction has been carried out over basic oxides like MgO (3). Good activity for aldolization reactions has been reported also over doped MgO, and MgAl hydrotalcite (4,5). In fact, basic mixed oxides obtained from hydrotalcite-like materials (HTs) have showed good activities in many organic reactions (6-9). The structure of HTs resembles that of brucite, Mg (OH)2, in which magnesium is octahedrally surrounded by hydroxyls. A HT structure is created replacing some of the M2+ divalent cations by M3+ trivalent cations turning the layered array positively charged. These layers are electrically compensated for by anions which are located in the interlaminar region. A large variety of synthetic HTs can be prepared and are represented by the general formula: [M2+1-x M3+x (OH)2]x+
[An-]x/n• m H2O, where M2+ = Mg2+, Ni2+, Zn2+, etc; M3+ = Al3+, Fe3+, Ga3+, etc; An-= (CO3)2-, Cl-, (NO3)-, etc.
