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 Bronsted 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.