P-Doped V-Cr composite oxide catalysts for the ammoxidation of 2,6-dichlorotoluene.

This study presents a novel P-doped V-Cr composite oxide catalyst for the ammoxidation of 2,6-dichlorotoluene. P-doped V-Cr composite oxides, which crystallized in the hexagonal CrVO3 structure, were synthesized by the solvothermal method with H3PO4 addition. The V-Cr composite oxide particles were composed of numerous nanorods (∼10 nm wide, 100-200 nm long), which assembled into compact microspheres (0.5-1 µm in diameter). The addition of H3PO4 led to the formation of loosely packed irregular and porous nanoparticles with increased specific surface area and pore sizes. Elemental mapping revealed a homogeneous surface distribution of V, Cr, and P. As the molar ratio of V, Cr, and P was set as 1 : 1 : 0.1 in the solvothermal reaction system, the resulting VCrP0.10-N600 product exhibited a maximum specific surface area of 38.7 m2 g-1 and an average pore size of 7.8 nm. In contrast to the P-free VCr-N600, medium-strong acid sites predominated on the VCrP0.10-N600 product, with their concentration exceeding those of weak and strong sites by over an order of magnitude. The catalytic performance of P-doped V-Cr composite oxides was evaluated for the ammoxidation of 2,6-dichlorotoluene to 2,6-dichlorobenzonitrile. The VCrP0.10-N600 catalyst, featuring the optimal P content, achieved a conversion of 92.7%, a molar yield of 78.7% and a selectivity of 82.7%. These values represented improvements of 5.5%, 7.5% and 4.8%, respectively, compared with the P-free VCr-N600 catalyst. First principles methods calculations confirmed that P doping broadened the density of states (DOS) near the Fermi level, thereby increasing the yield, while the introduction of highly localized empty states enhanced selectivity. These theoretical findings were consistent with our experimental results.