CO interacts with exchangeable cations M+ (gray spheres in the picture) of zeolites to form M+⋅⋅⋅CO and M+⋅⋅⋅OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M+=Na+) the difference in interaction energy amounts to 3.8 kJ mol−1, as determined by means of variable-temperature FT-IR spectroscopy.