Synthesis and properties of MgAPO-5
Magnesium aluminophosphate molecular sieves with the AlPO4-5 structure have been synthesized with varying Mg : (Mg + Al + P) mole ratios using triethylamine as an organic template. The results indicated that highly crystalline MgAPO-5 material can be obtained using starting reaction gel compositions containing up to 50 mol% of Mg. The MgAPO-5 product was associated with small amounts of MgAPO-47 below 20 mol% Mg addition and in the range 30–50 mol% Mg it was associated with an unknown phase designated as phase (I). In either case, the contribution due to the impurity phase was <5%.
Substitution of Mg into the AlPO4-5 framework was demonstrated by the variation in the unit-cell volume with Mg mole fraction, thermogravimetric study of the amount of occluded amine, chemical analysis and 27Al and 31P NMR data. The 27Al NMR spectra of MgAPO-5 samples containing up to 50 mol% Mg showed a single peak at 39 ppm which is assigned to Al(4P). The 31P NMR spectra showed a major peak at –29 ppm followed by low intensity peaks with decrease in intensity order at –23, –17 and –11 ppm corresponding to P(4Al), P(3Al, 1Mg), P(2Al, 2Mg)P and (1Al, 3Mg) units, respectively. The magnesium phosphate sample prepared in the absence of aluminium gave a symmetrical single 31P NMR peak near 0 ppm due to P(4Mg) units. Temperature-programmed desorption of pyridine indicated that the MgAPO-5 samples contain structural Brønsted acid sites.