Removal of tetramethylammonium cations from zeolites

Zeolite α (high-silica LTA), a potential shape-selective catalyst, is synthesized in the presence of tetramethylammonium (TMA) ions. Since TMA+ ions are incapable of forming olefins at low temperature, temperatures in excess of 500 °C are required to thermally decompose them and burn off the carbonaceous deposits, frequently causing damage to the structure. In this paper, the thermal decomposition of zeolitic TMA+ ions is investigated. This work led to a less severe method for removing TMA+ ions by stepwise reaction with ammonia at low temperatures. TMA+ ions located in the supercage can easily be removed at a temperature as low as 250 °C, generating mono- and dimethylamine. Sodalite cage TMA+ ions require a temperature of not more than 400 °C to be degraded. Although this treatment raises the Si/Al ratio somewhat, damage to the structure is minimal. Since the size of the zeolitic pores defines the type of molecules capable of escaping from the zeolite cavities, decomposition of TMA+ ions in NaTMA-Y and NaTMA-high-silica sodalite have been included for comparison.