Surface Properties of Hydrotalcite-Based Zn(Mg)Al Oxides and Their Catalytic Activity in Aldol Condensation of Furfural with Acetone

Abstract

Basic mixed oxides MgAl, ZnMgAl, and ZnAl were successfully prepared from hydrotalcite precursors synthesized by urea method. Materials with the same molar ratio (M2+/Al3+) = 2 were studied to describe the influence of Mg/Zn ratio on their physicochemical properties. Materials were tested as catalysts of the aldol condensation of furfural with acetone. For samples with similar particle sizes and surface BET areas, the varying catalytic activity was related to the different acidobasic properties. Higher furfural conversion and selectivity to longer carbon chain F2Ac product was observed for samples with higher total amount of basic sites. More specifically, it correlated with the population of Me2+–O2– pairs that represented dominant type of basic sites in all studied catalysts. At the same Al loading, Mg2Al mixed oxide exhibited higher specific surface area, higher total amount of basic sites and higher amount of acid sites than Zn2Al oxide.

Pyroaurite‐based MgFe mixed oxides and their activity in aldol condensation of furfural with acetone: effect of oxide composition

“…The thickness of the anion layer is influenced by the number, orientation and strength of the bonds between anions and hydroxyls groups of the cation layer. The values of parameter a in the present MgFe series ranged from 0.310 to 0.312 nm and were typical for layered double hydroxides . The cell parameters a were very similar across the samples in this MgFe series.…”

“…Nevertheless, a contribution of other basic sites is not fully excluded. As we showed previously on MgAl and Zn‐Mg‐Al oxides, medium strong basic sites are highly heterogeneous Me n+ ‐O 2− sites covering a wide range of CO 2 interaction energies . Based on large energetic heterogeneity of medium strong Me n+ ‐O 2− basic sites, the portion of medium strong basic sites can essentially contribute to the aldol condensation reaction.…”

“…The concentration of acid sites varied from 32 to 91 μmol g −1 . Compared to hydrotalcite‐derived MgAl oxides, the studied MgFe oxides are significantly less acidic . Actually, as shown by Ordóñez et al ., the substitution of Al 3+ by Fe 3+ in the structure of the resulting mixed oxide leads to a slight decrease in the concentration of basic sites and a more marked decrease in the concentration of acid sites.…”

“…The least stable complexes, which are desorbed on the temperature ramp first, are usually assigned to bicarbonates being formed on weak basic sites represented by OH − groups. Medium stable complexes are bidentate carbonates (chelating and bridged), formed on highly heterogeneous Me n+ ‐O 2− pairs . Bidentate carbonates are often observed as dominant species within hydrotalcite‐derived mixed oxides .…”

“…The comparison of FAc‐OH and FAc selectivity indicated that with higher reaction time, the ability of the catalyst to dehydrate the intermediate alcohol is higher. It was concluded before that the increased selectivity to FAc (and also F 2 Ac) could be due to the (partial) reconstruction of HTC structure as a result of the interaction of the catalyst with water released during the reaction …”