SYNTHESIS AND PROPERTIES OF MGAPO-5
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abstract
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 Brnsted acid sites.
