Yf. Chang et al., AN O-18(2) TEMPERATURE-PROGRAMMED ISOTOPE-EXCHANGE STUDY OF TRANSITION-METAL-CONTAINING ZSM-5 ZEOLITES USED FOR OXYDEHYDROGENATION OF ETHANE, Journal of catalysis, 154(1), 1995, pp. 24-32
The oxydehydrogenation of ethane was studied over a number of transiti
on-metal containing ZSM-5 catalysts, e.g., Co-H-ZSM-5, Cu-Na-ZSM-5, Nb
-ZSM-5, and V-ZSM-5. It has been shown that the introduction of transi
tion metal cations into ZSM-5 zeolite, either as charge balancing cati
ons (Co2+, Nb5+) or as lattice cation (V5+), enhances the activity and
selectivity of the catalysts for the oxydehydrogenation of ethane. Th
e O-18(2) temperature-programmed isotope exchange technique was used t
o probe the likely active sites on the catalysts. It was demonstrated
that the introduction of transition metal cations results in the forma
tion of active sites responsible for the formation of single-step doub
le exchange between gas phase oxygen and lattice oxygen. The activity
for formation of single-step double exchange O-16(2) is a good indicat
or for the activity for oxydehydrogenation of ethane to form ethylene.
The introduction of transition metal cations (Cu2+, Co2+, etc.) into
ZSM-5 zeolite lowers the temperature required for isotope exchange bet
ween gas phase oxygen and lattice oxygen. The temperature required for
isotope exchange depends on the cations introduced. The activity for
facilitating single-step double exchange between gas phase O-18(2) and
lattice oxygen (O-16) and the activity for oxydehydrogenation of etha
ne to ethylene decrease in the same order: Co-H-ZSM-5 > Cu-Na-ZSM-5 >
[V]-ZSM-5 > Nb-ZSM-5 > Na-ZSM-5 similar to H-ZSM-5 similar to Cu-ZSM-5
. It appears that the structural defects created by introducing transi
tion metal cations are responsible for the enhanced activity and selec
tivity for the oxydehydrogenation of ethane. NH3-TPD results show that
the presence of strong Bronsted acid sites are undesired for oxydehyd
rogenation reactions. However, the presence of relatively weaker acid
sites is not harmful for the reaction. (C) 1995 Academic Press, Inc.