The adsorption of CH3Cl on ZSM-5, Mo2C/ZSM-5 and Mo2C/SiO2 was investigated
by temperature-programmed desorption (TPD) and Fourier transform infrared
spectroscopy (FTIR). The formation of methoxy species was clearly identifie
d on all the three catalysts. Three elementary steps were considered to exp
lain the absorption bands observed in the IR spectra and to describe the re
action products found at higher temperatures: (i) the interaction of CH3Cl
with the OH groups of ZSM-5, (ii) dissociation of CH3Cl yielding CH3 and (i
ii) the elimination of HCl from the adsorbed CH3Cl to give CH2 species. A n
ew finding was the identification of the alkenyl carbocation formed on the
Lewis acidic sites of ZSM-5. The decomposition of CH3Cl on ZSM-5 proceeded
at 673 K with a high conversion yielding propylene, ethylene, butane, metha
ne and benzene in decreasing selectivities. Deposition of Mo2C on ZSM only
slightly modified the catalytic behavior of ZSM-5. Its promoter effect came
into prominence on silica surface. (C) 2000 Published by Elsevier Science
B.V.