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 large cavity of Y zeolite seemed to permit that thiophene and furan monomer radicals reacted further to form higher oligomerization products.
It seemed that channel structure of zeolites studied affects the formation of monomer or dimeric radical species.
EPR 방법을 이용하여 티오펜 및 퓨란화합물과 Cu-ZSM-5 제올라이트와의 상호 작용을 Cu-Y 제올라이트에서의 상호작용과 비교하여 연구하였다. Cu-ZSM-5와 Cu-Y에 이러한 헤테로고리 화합물을 흡착시키면 Cu(II) 가 Cu(I) 으로 환원되면서 헤테로고리 화합물의 양이온 라디칼이 관찰되었다.
Cu-ZSM-5에서 2,5-디메틸티오펜과 2,5-디메틸퓨란의 단위체 라디칼이 관찰 되었으나 Cu-Y에서는 이들의 이량체 라디칼만이 관찰되었다. 2-메틸티오펜과 2-메틸퓨란의 경우, Cu-ZSM-5와 Cu-Y에서 모두 이량체 라디칼이 관찰되었다. 티오펜과 퓨란은 Cu-Y에서 초미세구조를 갖지 않고 폭이 좁아진 EPR신호를 보였으며 Cu-ZSM-5에서는 복잡한 초미세 구조가 나타났다. ZSM-5 제올라이트의 세공입구와 세공 교차점의 크기는 Y에서보다 작기 때문에 Cu-ZSM에서 2,5-디메틸티오펜과 2,5-디메틸퓨란의 이합체 라디칼이 생성되기에는 공간이 부족하여 단위체 라디칼이 관찰된 것으로 해석되었으며, 2-메틸티오펜과 2-메틸퓨란의 경우 에는 ZSM-5와 Y에서 모두 이합체 라디칼이 관찰되었다. 메틸기가 치환 되지 않은 티오펜과 퓨란은 이합체로 부터 반응이 더욱 진행된 소중합체 상태인 것으로 해석되었다. 벤젠을 이용하여 흡착되는 분자간의 상호 작용을 제한시킨 경우에는 Cu-ZSM-5에서 티오펜, 2-메틸티오펜, 2-메틸퓨란의 단위체 라디칼이 관찰되기도 하였다.
즉 라디칼의 생성과정에서도 제올라이트의 세공구조에 의한 형상 선택성이 나타난 것으로 해석되었다.
