Synthesis, characterisation and catalytic performance of the solid acid DAF-1

Abstract

The novel aluminophosphate-based microporous solid DAF-1 may be prepared in aqueous or non-aqueous media. Its framework composition being determined by the need to charge balance organic cations taken up into the structure. In aqueous conditions the framework charge is 0.22^– per AIPO₄ unit which may result from magnesium (for aluminium) or magnesium plus silicon (for phosphorus) substitution. Computer simulation of the framework–template interaction for the successful octa- and deca-methonium templates suggests templating action occurs in the narrower channels, with the organic cation acting only as a void filler in the larger cavities. DAF-1 is capable of assimilating heteroatoms into the framework in varying concentrations : Co substituting for Mg, Ga and Al and Si for P. Characterisation of the framework structure by advanced energy-minimisation methods and taking (Mg, Al)/P ordering into account predicts the true space group to be P6/mcc. Calcined Mg-DAF-1 is shown by IR spectroscopy of adsorbed pyridine to possess mild Brønsted and Lewis acidity of strenght similar to MgAPO-5 but much weaker than MgAPO-36. This is reflected in the results of isobutane conversion, where DAF-1 and MgAPO-5 give a high selectivity to butenes and propene with few of the saturated hydrocarbons typical from strong acids such as MgAPO-36 and H-ZSM-5. The higher conversion rate of isobutane over DAF-1 than over MgAPO-5 is due to the higher concentration of acid sites within the former.