Effect of interlayer anions on the catalytic activity of Mg-Al layered double hydroxides for furfural and acetone aldol condensation reaction
Said Arhzaf, Jamal Houssaini, Mohammed Naciri Bennani, Marwa Alaqarbeh, Mohammed Bouachrıne
Abstract
The aldol condensation reaction between furfural (F) and acetone (A), which typically produces 4-(2-Furyl)-3-buten-2-one (FA), is a critical reaction due to the application of the final product as a flavoring agent in various food items. Traditionally, this reaction is catalyzed by liquid-based catalysts. However, homogeneous liquid-base catalysis often leads to environmental damage. Solid-base catalysis is highly desirable to address the environmental issue due to liquid-base catalysis as it reduces the need for excessive solvents and reagents, thus preserving the environment. In this study, a series of Mg-Al hydrotalcites (HT) intercalated with nitrate ion (HT-NO3), carbonate ion (HT-CO3), or acetate ion (HT-CH3COO) were prepared using three different coprecipitation procedures. Upon calcination at 450°C, the solids were transformed into mixed metal oxides. Among the calcined hydrotalcites, the catalysts with acetate anion (cHT-CH3COO) or carbonate anion (cHT-CO3) exhibited the highest basicity and thus showed superior catalytic activity for the aldol condensation reaction. Optimal conversion and selectivity were achieved at 90°C for 2 hours using the most basic catalysts (cHT-CO3 and cHT-CH3COO). These catalysts yielded over 98% conversion with FA selectivity of 76% for cHT-CO3 and 65% for cHT-CH3COO, respectively. Notably, the catalyst cHT-CH3COO exhibited higher selectivity towards F2A (32%) than the cHT-CO3 catalyst (18%). The effect of interlayer anions on the structural properties of the Mg-Al hydrotalcites was analyzed using X-ray diffraction. Fourier-transform infrared spectroscopy (FT-IR) was employed to investigate the interactions corresponding to different types of anions. The specific surface area and pore volume of the calcined Mg-Al hydrotalcites were determined using nitrogen adsorption (BET), while their total basicity was evaluated through acid-base titration. The reaction kinetics were monitored using gas chromatography.