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

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 zeol ite is investigated. The exchange of Na+ with a proton/deuteron or a group 2 cation gave rise to a new set of frequency-shifted nu(CO) and nu(CO2) ban ds on adsorption of CO. The intensity of the new bands was related approxim ately to the extent of cation exchange. The value of Delta nu was positive in the case of samples exchanged with group 2 cations and negative for thos e exchanged with a proton or deuteron. The frequency shift of above-mention ed 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. cha rge/radius) associated with the balancing cation. The results suggest that pores of two different kinds may exist simultaneously in a zeolite, the ori ginal ones and the others modified by exchanged cations. These pores exhibi t their independent adsorption behavior, the adsorption contribution of a p articular kind of pores being proportional to their concentration. Also, th e 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 w eakly bonded (CO)(n) or (CO2)(n) clusters, occluded in the zeolitic cages a nd stabilized under the cationic field.