Hydrogen mordenite and hydronium mordenite. Ion exchange and thermal stability

Abstract

Isotherms for the Na⁺⇌ H₃O⁺ exchange in mordenite showed irreversibility. Hydronium mordenite prepared from the Na-form could be fully re-converted to the Na-form with aqueous NaCl, NaOH or CH₃COONa, but hydronium mordenite made by heating the NH₄-form to 450°C and subsequent hydration underwent only ∼70 % exchange to the Na-form. This may be understood if loss of NH₃ and H₂O at temperatures upward of 400°C is accompanied by removal of some framework Al with formation of such species as AlOOH, AlO⁺, AlOH²⁺ and Al³⁺ with corresponding loss of exchange capacity. To explore these possibilities samples of NH₄-mordenite were heated to different constant temperatures in the range 455 to 970°C and separate portions were then treated respectively with NaOH and with NaCl solutions. Release to solution of soluble Al-bearing cations was inappreciable in all cases, but NaOH was absorbed. This result was interpreted as a consequence of reactions such as AlO⁺+ NaOH → Na⁺+ AlOOH, AlOH²⁺+ 2NaOH → 2Na⁺+ AlOOH + H₂O, Al³⁺+ 3NaOH → 3Na⁺+ AlOOH + H₂O and of reaction with the intracrystalline hydronium ions formed by exposing the zeolite still containing residual [graphic omitted] to water. NaCl solutions liberated acid from samples previously heated below about 700°C. Samples heated above this temperature did not liberate appreciable acid with NaCl. This behaviour suggested that NaCl reacted primarily or only with the residual hydronium while NaOH reacted additionally with Al-bearing cations.