Influence of Mg–Al Mixed Oxide Compositions on Their Properties and Performance in Aldol Condensation

Abstract

The influence of chemical composition of Mg−Al mixed oxides on their properties and catalytic performance in aldol condensation of furfural and acetone was studied. Pure alumina, Mg−Al layered double hydroxides with a wide range of Mg/Al molar ratios from 0.5 to 15, and pure magnesia were prepared by the same synthesis method, and corresponding oxides were produced by calcination of the as-prepared samples at T = 450 °C. The physicochemical properties of the samples were investigated by XRD, N 2 physisorption, SEM, CO 2 -TPD, and NH 3 -TPD, and their catalytic performance was evaluated in aldol condensation of furfural and acetone. SEM images revealed that the Al content in the samples influenced the size and the morphology of both large agglomerates and plate-like crystals. The CO 2 -TPD measurements proved that the concentration of basic sites and the distribution of their strengths was determined by the Mg/Al molar ratio in the Mg−Al mixed oxides. However, over the whole range of Mg/Al compositions studied here, no clear correlation between the chemical composition and the number and strength of basic sites was found. Nonetheless, the change in the strength and in the density of basic sites affected the catalytic performance of calcined Mg−Al mixed oxides. Additionally, morphology and crystal size also influenced the performance of the catalysts. Finally, the composition of reaction products was affected by acid−base and textural properties of the prepared Mg−Al-mixed oxides.

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

“…For high synthetic Mg:Fe ratios, it was obviously more difficult to incorporate Mg into the structure of catalyst precursors within the synthesis, the remaining amount of Mg being in the precipitation solution. Such behaviour already has been observed by our co‐workers for a Mg/Al hydrotalcite series, where at higher Mg:Al ratios (>3) the samples lacked magnesium, suggesting that the precipitation of this element was not completed during the synthesis …”

“…It is obvious that the increasing Mg:Fe ratio is connected with the abundance of strong basic sites (O 2− ), whereas the concentration of medium (Me n+ ‐O 2− pairs) and weak (OH − ) basic sites increases only slightly. This observation is in line with the fact that the presence of separate MgO is probable for high content of magnesium in the sample (see section entitled Physicochemical characteristics) and the basicity of pure MgO oxide is relatively high . In addition, the presence of highly basic O 2− ions can be related to the abundance of surface defects in such samples.…”

“…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.…”

“…The concentration of weak basic sites, OH − ions, only slightly increased with magnesium content in the sample. In general, compared to the often studied hydrotalcite‐based MgAl oxides, MgFe oxides have lower total concentration of basic sites than MgAl oxides (for similar Mg:Me 3+ ratios approximately two times), but the population of strong basic sites is higher for MgFe oxides, namely those with high ratios of Mg:Fe >5. For MgAl oxides, the dominant species were actually medium strong Me n+ ‐O 2− sites in the whole series of Mg:Al ratios in the range of 0.5–10 …”

Our Contributions in Synthesis of Diverse Heterocyclic Scaffolds by Using Mixed Oxides as Heterogeneous Catalysts

“…Thus mixed oxides and their composites provide a wide range of applications in heterogeneous catalysis, as those are easy to handle, less corrosive than the traditional homogeneous catalysts plus cheaper due to recyclability and reusability . MMOs are used in variety of oxidation, reductions and condensation reactions . Mixed metal oxides (MMOs) are known to play a key role as effective catalysts in producing biologically active scaffolds and fine chemicals …”