Ionothermal synthesis of crystalline microporous aluminophosphates: Systematic study on the conditions affecting the framework type

Abstract

In a systematic study on the synthesis of aluminophosphates (AlPOs) under ionothermal conditions, initially using 1-butyl-3-methylimidazolium bromide ([C4mim]Br) as ionic liquid solvent and structure-directing agent, the effect of the reaction conditions (i.e. molar P/Al, F/Al and ionic liquid/Al ratios, alternative fluoride sources, influence of the ionic liquid's cation or anion, temperature, reaction time) on the framework type was studied in detail. In [C4mim]Br, the formation of the more thermodynamically stable AEL framework type proceeds via AFI. The framework type can be changed by choosing another anion or cation of the ionic liquid. Hence, the successful ionothermal synthesis of the AFI framework AlPO is reported by using either N-ethylpyridinium bromide ([C2py]Br) or 1-butyl-3-methylimidazolium chloride ([C4mim]Cl). The mineraliser [Me4N]F, rather than HF, has been used for the first time as an alternative fluoride source in ionothermal synthesis, which can also affect the framework type. Hence, a very efficient synthesis of the LTA framework type is reported in [C4mim]Br using [Me4N]F. Ab initio molecular dynamics (AIMD) studies showed that the anion bridges between the aluminium atoms of the framework and the cation. The interaction is more favoured in the presence of the bromide than the chloride, which may be a clue to the question why the AEL framework is not formed in the chloride-based ionic liquid. This study opens several routes to pursue in the future as numerous ionic liquids are available which can be used in ionothermal synthesis.