Catalytic effects in propene ammoxidation achieved through substitutions in the M2 phase of the Mo-V-Nb-Te-oxide system

摘要

M2-type phases M_3TeO_(10)(M = Mo and V),belonging to the Mo-V-Nb-Te-oxide system,were prepared by substituting to various degrees the M-metal with W,Ti,Nb or Fe,and Te with Ce.The compositions were characterized by BET,XRD and XPS and were studied for propene ammoxidation.The variations of the unit cell parameters with substitution reveal that the orthorhombic,pseudo-hexagonal M2 structure is preserved to different degrees depending on the substituent and its amount.It is noted that partial substitution with W,Ti,Ce and Fe all increase the activity of the base composition and that Ce,and especially W,improve the selectivity to acrylonitrile at the expense of acrolein.The results of the activity measurements and the XPS investigation indicate that the site,activating propene and ammonia on the unsubstituted M2 phase is a surface ensemble of Mo and Te.W and Ce were found to be able to mimic the role of Mo and Te,respectively;W mainly via site isolation and electronic effects,Ce via redox effects.V-and Ti-sites are shown to be inactive,having merely the role of structure adjusting elements,and serving as templates to complex the excess Te on the surface.