Removal of tetramethylammonium cations from zeolites

Zeolite alpha (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 degreesC 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 degreesC, generating mono- and dimethylamine. Sodalite cage TMA(+) ions require a temperature of not more than 400 degreesC 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. (C) 2002 Published by Elsevier Science B.V.