IntroductionThe Aldol condensation reaction, as a pivotal organic reaction in forming C-C bonds, plays a crucial role in various fields such as pharmaceutical synthesis, total synthesis of natural products, and the industrial production of perfumes and dyes. Conventional Aldol condensation reactions often rely on liquid alkaline catalysts, which suffer from some issues such as difficulty in recovery, complex waste liquid treatment, strong corrosion to equipment, and severe environmental pollution, thereby restricting their application in industrial production. Therefore, the development of efficient, recyclable, and environmentally friendly solid alkaline catalysts becomes a key to promoting the greening and efficiency of the Aldol condensation reactions. Layered double hydroxides (LDHs), with their unique layered structure and adjustable composition have a great potential in catalysis. Recent research indicates that functional modifications, such as ion exchange, intercalation, and doping, can further enhance their catalytic performance. Among them, sulfate-intercalated magnesium aluminum LDH (MgAl-SO₄²⁻-LDH) as a special LDH material, through the introduction of sulfate ions, can enhance the structural stability of LDH and introduce additional acidic sites. This makes MgAl-SO₄²⁻-LDH effectively activate aldehyde or ketone molecules during the catalysis of Aldol condensation reactions, thereby promoting the progress of the condensation reaction. In addition, the morphology of the catalyst has a significant impact on its catalytic performance. The flower-like spherical structure, as a special nanostructure, has attracted much attention in the field of catalysis due to its high specific surface area, abundant pore structure, and superior mass transfer performance. Combining the flower-like spherical structure with MgAl- SO₄²⁻-LDH can be expected to further improve the catalytic efficiency of the catalyst.MethodsThe flower-like MgAl-SO₄²⁻-LDH catalyst was prepared in the molar ratios of n(Mg) : n(Al)=2 : 1 and n(urea) : n(NO₃^–)=1 : 1. In the preparation, magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) and urea were mixed and dissolved in deionized water under stirring. The mixed solution was then transferred to a hydrothermal reaction vessel and reacted at 120 ℃ for 6 h. The resulting product was washed with deionized water for several times and then dried thoroughly in an oven. The dried material was calcined in a muffle furnace at 500 ℃ for 8 h to obtain the magnesium-aluminum composite oxide (LDO). Subsequently, a series of ammonium persulfate solutions ((NH₄)₂S₂O₈) with different concentrations (i.e., 0, 0.05, 0.10, 0.15 mol/L, and 0.20 mol/L) were prepared. The reaction was proceeded in a nitrogen atmosphere at room temperature for a certain period, and the solvent was removed through filtration and drying, yielding the S₂O₈^2– intercalated magnesium-aluminum hydrotalcite compounds (i.e., S₂O₈^2–/LDH). Finally, the prepared S₂O₈^2–/LDH was calcined in a muffle furnace at 550 ℃ for 8 h to obtain the modified magnesium-aluminum composite oxide. The modified magnesium-aluminum composite oxide was uniformly dispersed in deionized water, and the hydration-reduction process was repeated when sulfate ions were introduced to replace persulfate ions. A final catalyst MgAl-SO₄²⁻-LDH was obtained after completing the hydration-reduction and subsequent treatment.Results and DiscussionMgAl- SO₄²⁻-LDH catalyst prepared has a flower-like spherical structure assembled from two-dimensional MgAl-SO₄²⁻-LDH sheets with a uniform morphology and a diameter of 2 μm. The SEM images reveal that, in contrast to the irregular shape of traditional MgAl-LDH catalysts, the MgAl-SO₄²⁻-LDH sample after sulfate ion intercalation modification exhibits a flower-like spherical structure. This interconnected sheet structure divides the catalyst surface into numerous small chambers, which provides abundant reaction sites for the catalytic reaction and promotes the mass transfer process between reactants and products, thereby contributing to an enhanced catalytic performance. The results of EDS analysis confirm that sulfate ions are introduced into the catalyst structure and distributed uniformly.The XRD patterns indicate that the crystallinity of MgAl-SO₄²⁻-LDH catalyst is inferior to that of the MgAl-LDH, exposing more active sites due to the intercalation of sulfate ions. The FTIR spectra reveal that MgAl-SO₄²⁻-LDH catalyst contains a rich number of Brönsted acid sites, enhancing the activation of acetone and p-nitrobenzaldehyde, thus effectively improving catalytic performance. The optimized MgAl-SO₄²⁻-LDH catalyst has a p-nitrobenzaldehyde conversion rate of 70.5%, and after one regeneration the catalytic conversion rate remains 64.03%, demonstrating a good reusability. The FTIR spectra of the reaction products indicate a mixture of 4-(4-nitrophenyl)-3-hydroxy-2-butanone and 4-(4-nitrophenyl)-3-penten-2-one.ConclusionsThe study prepared a flower-like sulfate-intercalated MgAl-SO₄²⁻-LDH catalyst with a uniform microstructure and a diameter of 2 μm, assembled from two-dimensional MgAl-SO₄²⁻-LDH sheet structures. The catalyst showed an improved surface activity due to the intercalation of sulfate ions, thus increasing the number of active sites. The FTIR spectra confirmed the abundance of the Brönsted acid sites in MgAl-SO₄²⁻-LDH catalyst, which significantly enhanced the activation of acetone and p-nitrobenzaldehyde, effectively improving catalytic performance. The optimized catalyst showed a high conversion rate of 70.5% for the aldol condensation of acetone with p-nitrobenzaldehyde, and after regeneration it maintained a conversion rate of 64.03%, indicating a good reusability. The reaction products consisted of a mixture of 4-(4-nitrophenyl)-3-hydroxy-2-butanone and 4-(4-nitrophenyl)-3-penten-2-one, indicating a possible mechanism involving the Brönsted acid sites on MgAl-SO₄²⁻-LDH catalyst facilitating the nucleophilic addition reaction between the activated enol of acetone and the protonated aldehyde group of p-nitrobenzaldehyde.