Self-aldol condensation of acetaldehyde to crotonaldehyde over Lewis acidic metal-incorporated *BEA zeolites

Abstract

Metal-incorporated *BEA zeolites have shown remarkable catalytic performance in the self-aldol condensation of various aldehydes. The substitution of heteroatom sites can modulate the self-aldol condensation performance of aldehydes, while the underlying mechanisms have been insufficiently explored, with even less attention given to acetaldehyde-related processes. Herein, seven transition metal-incorporated *BEA zeolites were post-synthesized and applied in the self-aldol condensation of acetaldehyde. All the M-BEA zeolites exhibited excellent crotonaldehyde selectivity, exceeding 80 % and most surpassing 85 %, with various acetaldehyde conversions. SEM, XRD, N₂ physisorption, FT-IR, UV–Vis, XPS, Py-IR, and NH₃-TPD were employed to characterize the physicochemical properties. The M-BEA zeolites predominantly exhibit Lewis acidity, and the amount of Lewis acid with medium strength dominated the acetaldehyde conversion variations. The probe-pulse and TPSR experiments were further employed to investigate the reaction pathways to explain the product distribution on the Zr-BEA zeolite, which demonstrated the highest crotonaldehyde yield. The main side products were long-chain unsaturated aldehydes and cyclic compounds from excessive aldol condensation and cyclization. A reaction network for acetaldehyde conversion on Zr-BEA zeolite was ultimately proposed.