Study on aldol condensation of acetic acid with formaldehyde to acrylic acid over TiO₂ modified VPO

A class of TiO₂ modified VPO catalysts were prepared by organic solvent-heating method in the present work. The catalysts were characterized by TEM, XRD, XPS, NH₃-TPD and CO₂-TPD techniques. The catalytic performances were evaluated in the aldol condensation of acetate acid and formaldehyde to acrylic acid in a fixed-bed reactor. The results show that the addition of TiO₂ significantly changes the proportion of V content forming V⁵⁺ and V⁴⁺ ion pair to total V with the unmodified VPO catalyst. As the precursor of TiO₂ is rutile phase and Ti/V molar ratio is 2.0, the proportion of V content forming V⁵⁺ and V⁴⁺ ion pair to total V reaches the maximum, resulting in the best acrylic acid yield (18.0%) and acrylic acid space-time yield (6.61 mmol/(g·h)). It indicates that for the VPO catalyst modified by TiO₂, the redox cycle of V⁵⁺ and V⁴⁺ ion pair plays a major role in the catalytic reaction of formaldehyde and acetate acid to acrylic acid.