Synergistic Effects between Acid and Base Sites and Kinetic for Synthesis of Methylacrolein on the Cs-P/γ-Al<sub>2</sub>O<sub>3</sub> Catalyst
Gang Wang, Guangming Cai
Abstract
Methylacrolein (MAL) is traditionally synthesized from propanal and formaldehyde via the Mannich reaction instead of the aldol reaction in homogeneous catalytic systems due to the undesired self-condensation of propanal. Herein, the direct synthesis of MAL from propanal and trioxane via the aldol reaction was realized on an acid–base bifunctional Cs-P/γ-Al2O3 catalyst, which exhibited good catalytic stability and could be prepared in a large scale with 54.5% yield and 100% selectivity. The influence of P elements on the structure and acid–base properties of Cs-P/γ-Al2O3 and even on the selectivity of MAL, compared with Cs/γ-Al2O3, was elucidated through FT-IR, XRD, XPS, and CO2- and NH3-TPD characterizations in combination with catalytic activity evaluation. The synergistic effects between acid and base sites on MAL production were unveiled where the decomposition of trioxane into a formaldehyde unit followed by condensation between the formaldehyde monomer and propanal proceeded on the acid and base sites. Kinetics studies revealed that the trioxane decomposition into formaldehyde was zero-order-dependent on trioxane concentration and the condensation step conformed to the Langmuir–Hinshelwood mechanism, with activation barriers of 72 ± 0.2 and 65 ± 0.5 kJ/mol, respectively.