Role of Si/Al Ratio on Immobilization and Stability of Rhodium Complexes on ZSM-5

Abstract

Rhodium complexes within the pores of zeolite ZSM-5 with varying Si/Al ratios (Si/Al 23, Si/Al 50, and Si/Al 280) were prepared from Rh1+(CO)2(C5H7O2), Rh22+(CO2CH3)4, and Rh3+(C5H7O2)3 followed by thermal treatment in He. IR results indicate that changing the Si/Al ratio stabilizes different Rh species, with the formation of Rh(I), Rh(II), or Rh(III) complexes along with the ability to coordinate two to four ligands. Through X-ray absorption spectroscopy, changes in oxidation state are followed and the final structure determined. Both the Si/Al ratio and rhodium precursor structure play a role, with Rh clusters of two atoms, four atoms, or six atoms immobilized on ZSM-5. Differences in bond lengths are affected by Si/Al ratio, creating tighter or more expanded clusters. This variation in structure caused by the Si/Al ratio shows potential for affecting the amount of hydrogen produced from the partial oxidation of methanol.