Study on the catalytic properties of Cu(Ⅱ)ion exchanged ZSM-5 zeolite

Abstract

Interaction of furan, thiophene, and their methyl derivatives with Cu-ZSM-5 is studied by the EPR technique and compared with that of Cu-Y zeolite. With the adsorption of these 5-membered heteroaromatic compounds on Cu-ZSM-5 as well as on Cu-Y, new high field bands with g=2.003 appeared while the original signal of Cu(II) decreased. This phenomenon was interpreted as the reduction of Cu(II) into Cu(I) forming radical cations of heteroaromatics. Monomer radicals of 2,5-dimethylthiophene and 2,5-dimethylfuran was observed on Cu-ZSM-5 whereas only the dimeric radicals were observed on Cu-Y zeolite. Dimeric radicals of 2-methylthiophene and 2-methylfuran were detected on Cu-ZSM-5 as well as on Cu-Y. Thiophene and furan on Cu-Y gave single exchange narrowed EPR signals without hyperfine structure. The pore openings and intersection space available in ZSM-5, are smaller than in zeolite Y. It may that in the case of ZSM-5, the formation of dimeric radical of 2,5-dimethylfuran and 2,5-dimethylthiophene was hindered due to the restricted space available in the channel structure. When mono methyl substituted furan and thiophene were adsorbed both on Cu-ZSM-5 and on Cu-Y, EPR spectra due to monomer radicals were not observed. When 2-methylthiophene and benzene mixture was adsorbed on Cu-ZSM-5 it was possible to observe the spectra arising from monomer radical. It seemed to be that dimeric radical of 2-methylthiophene and 2-methylfuran was formed through the reaction between a radical and a neutral molecule on Cu-ZSM-5. In the case Cu-Y, only the dimeric radicals of 2-methylthiophene and 2-methylfuran were observed. When furan was adsorbed on Cu-ZSM-5 and on Cu-Y no hyperfine structure was observed. When thiophene and benzene mixture was adsorbed on Cu-ZSM-5, monomer radical of thiophene was observed with HFS constants of 13.0G and 3.7G. Both furan and thiophene gave no hyperfine structure when they were adsorbed on Cu-Y yielding single exchange narrowed EPR signals. The l...