On the role of balancing cations in the entrapment of CO in ZSM-5 zeolite at room temperature: FTIR study

Abstract

In continuation of an earlier FTIR study (Phys. Chem. Chem. Phys., 1999, 1, 191), the effect of cation exchange on encapsulation of CO in NaZSM-5 zeolite is investigated. The exchange of Na⁺ with a proton/deuteron or a group 2 cation gave rise to a new set of frequency-shifted ν(CO) and ν(CO₂) bands on adsorption of CO. The intensity of the new bands was related approximately to the extent of cation exchange. The value of Δν was positive in the case of samples exchanged with group 2 cations and negative for those exchanged with a proton or deuteron. The frequency shift of above-mentioned bands showed a correlation with the change occurring in pore volume due to cation exchange and also with the electrostatic potential (e/r, i.e. charge/radius) associated with the balancing cation. The results suggest that pores of two different kinds may exist simultaneously in a zeolite, the original ones and the others modified by exchanged cations. These pores exhibit their independent adsorption behavior, the adsorption contribution of a particular kind of pores being proportional to their concentration. Also, the pores of larger volume are filled preferentially at lower pressures. The influence of pore size on IR bands is in conformity with the formation of weakly bonded (CO)_n or (CO₂)_n clusters, occluded in the zeolitic cages and stabilized under the cationic field.