Synergistic Catalysis by MgO Induced Acid-Base Bifunctional Sites for Cross Condensation of HCHO and Acetaldehyde

Graphical The HCHO-acetaldehyde condensation is a typic aldol reaction and a critical step to produce 1,3-propanediol from methanol and ethanol. The cross condensation of HCHO and acetaldehyde to acrolein has been conducted over MgO/SiO 2 catalyst, which shows the synergistic catalysis by the acidic sites (Mg-O-Si structure) and basic sites (Mg-O-Mg structure) for activation of HCHO and acetaldehyde, respectively. Acid-base synergistic catalysis greatly enhances the performance of MgO-based catalysts in the Aldol condensation. In this paper, the effect of acid-base strength/concentration/proportion and the synergistic catalysis on the performance of cross condensation between HCHO and acetaldehyde were studied. The NH 3 /CO 2 -TPD results indicated that the weak acidic sites combined with the medium basic sites were beneficial for the cross condensation of formaldehyde and acetaldehyde. The loading of MgO nanocrystals on SiO 2 induced the generation of new acidic sites (Mg-O-Si). Combined with the inherent basic sites (Mg-O-Mg) and Mg-O-Si, the acid-base bifunctions sites could catalyze the cross condensation of HCHO and acetaldehyde synergistically. The appropriate proportion of acidic/basic sites (10/1) was critical for the cross condensation in the presence of vast water in formalin. The effect of reaction parameters, including molar ratio of substrates, reaction temperature, linear velocity and weight hourly space velocity, were investigated for the cross condensation. The results showed that 10 wt% MgO/SiO 2 catalysts exhibited the highest catalytic activity (57.9% conversion of acetaldehyde and 84.3% selectivity of acrolein) under the optimal reaction parameters among the investigated catalysts. The catalytic performance of MgO/SiO 2 could be recovered after in situ regeneration in air to remove the carbon deposition.