Study of small-port and large-port mordenite modifications. Part 2.—Ion-exchange properties of thermally treated ammonium forms

Abstract

The ion-exchange reactions 2H⁺ Pt(NH₃)²⁺ ₄ and H⁺ Na⁺ have been studied for H-mordenite obtained from its ammonium forms by a deep-bed-like calcination. Two kinds of procedures have been considered. The first corresponds to equilibrated acidic solutions starting from NaCl or Pt(NH₃)₄Cl₂; the second corresponds to equilibrated alkaline solutions starting from NaOH. Under acidic pH conditions the exchange capacities of the HM forms are drastically reduced compared with that of the initial NH₄-mordenite. This can be interpreted as being due to (i) a reduction in the number of exchange sites brought about by framework dealumination and (ii) occupancy of some of the remaining exchange sites by unexchangeable cationic Al^VI species formed during framework dealumination. The exact nature of the Al^VI species is related to the calcination temperature, high temperatures favouring the formation of neutral hydroxyaluminium complexes. At high pH the decrease in apparent ion-exchange capacity is very limited. Reactions other than pure ion exchange, for instance the formation of sodium aluminates from the Al^VI species present in the channels after the calcination step, are involved.