On the formation of COS from H2S and CO2 in the presence of zeolite/salt compounds

A gaseous mixture of hydrogen sulphide and carbon dioxide (20% H2S, 80% CO2) was brought into contact at 25 degrees C with NaY and NaX zeolites in an as-synthesized form as well as after modification by the inclusion of salts (NaCl, NaBr) in the small cages of the aluminosilicate framework and ion exchange with aqueous CaCl2 solution. At small contact times (5 h), the degree of conversion of H2S according to the reaction H2S + CO2 --> COS + H2O by the various samples was found to follow the sequence NaY/NaCl approximate to NaY/NaBr approximate to NaX/NaCl(CaCl2) < NaY << NaX/NaCl approximate to NaX. Long-term runs with NaY and NaY/NaBr revealed that the latter zeolite retained a very low level of H2S conversion for contact times as long as 250 h. It is concluded that such low H2S conversion requires the absence of low-coordinated sodium cations in the supercages or their replacement by calcium ions, and blocking of the beta-cages with salt anions.