Vapor phase aldol condensation of methyl acetate with formaldehyde over a Ba–La/Al2O3 catalyst: the stabilizing role of La and effect of acid–base properties

Abstract

Vapor phase aldol condensation of methyl acetate with formaldehyde was studied over Ba–La/Al₂O₃ with different amounts of lanthanum catalysts. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, pyridine absorption performed via Fourier transform infrared spectroscope (Py-IR), NH₃ and CO₂ temperature-programmed desorption (NH₃ and CO₂-TPD), thermal analysis (TG-DTA) and scanning electron microscopy (SEM). The catalytic performance was evaluated using a fixed-bed microreactor. The results showed that bare Al₂O₃ was intrinsically active but poorly selective to methyl acrylate. The addition of barium species significantly improved the catalytic activity and selectivity. However, the Ba/Al₂O₃ catalyst was not stable in the continuous reaction due to a large amount of carbon deposition on the catalyst surface. Compared with adding individual components (BaO), the combination of the two promoters (BaO and La₂O₃) showed higher catalytic stability. Although the activity of the Ba–La/Al₂O₃ catalyst was not obviously increased compared with the Ba/Al₂O₃ catalyst, the carbon deposition was obviously suppressed in the target reaction due to the alkaline function of La₂O₃. Combined with the characterization results, we found that the addition of lanthanum species could significantly reduce the number of strong acid sites on the catalyst surface, inhibit the generation of carbon species in the reaction process, and stabilize the catalytic activity of the catalyst. In addition, the lifetime of the optimum 5Ba–0.5La/Al₂O₃ catalyst was evaluated over a continuous period of 300 h, and the initial catalytical activity did not exhibit an obvious decrease.