Sb. Derouane-abd Hamid et al., Site isolation and cooperation effects in the ammoxidation of propane withVSbO and Ga/H-ZSM-5 catalysts, TOP CATAL, 15(2-4), 2001, pp. 161-168
The performances of a conventional mixed oxide catalyst, VSbyOx, and of a n
ovel zeotite-based catalyst, Ga-modified H-ZSM-5 (Ga2Z27), are compared for
the ammoxidation of propane. In both cases, the effects of reaction temper
ature and ammonia to propane ratio were investigated. Increasing productivi
ty to acrylonitrile is observed when the temperature is raised and the ammo
nia to propane ratio increased. The maximum yields in acrylonitrile are 8.5
and 11.4% for the unpromoted VSbyOx and Ga2Z27 catalysts, respectively, at
comparable propane conversions (ca. 30%). The activity and productivity to
acrylonitrile of Ga2Z27 are about 50% of those of VSbyOx, at 500 degreesC,
in the absence of any additional promoters. As reported previously, propen
e and carbon oxides, CO and CO2, were observed to be major by-products when
using VSbyOx catalysts, 10-20 and 20-40%, respectively. By contrast, the G
a2Z27 catalyst shows a very low selectivity to carbon oxides (1% or less) a
nd a rather high and interesting selectivity to C-4 hydrocarbons, consistin
g mainly of isobutane and isobutene. These observations can be explained by
differences in the reaction mechanisms describing the ammoxidation of prop
ane on these two systems. For VSbyOx, the classical mechanism applies. Prop
ene is the primary reaction product which is easily oxidised to carbon oxid
es. For Ga2Z27, propane is initially activated to a protonated pseudo-cyclo
propane (PPCP) transition state on bifunctional sites involving dispersed g
allium species and Bronsted acid sites of the zeolite. The PPCP transition
state can evolve in several ways leading to propane, C-2-C-4 hydrocarbons,
and N-containing compounds. Isolation of gallium species is necessary for G
a/H-ZSM-5 in order to, active sites, whereas vanadium sites isolation is re
quired to avoid overoxidation for VSbyOx. Phase generate the bifunctional,
Ga3+-H+ cooperation is necessary for the VSbyOx catalysts, as reported prev
iously, whereas site cooperation is required for Ga-modified H-ZSM-5.