Synthesis and Characterization of AlPO4-36 and MAPO-36 with Different Magnesium Content

The objective of this work was to perform a systematic study regarding the effect of different synthesis parameters on the crystallization of AlPO4-36 and MAPO-36 (aluminophosphate molecular sieve type 36, ATS, structure). MAPO-36 samples were synthesized from gels containing different magnesium contents in order to obtain catalysts with different degrees of acidity. Under the synthesis conditions employed in this work, the formation of AlPO4-36 and MAPO-36 increased when the reaction mixture was aged at room temperature; however, small amounts of AlPO4-5 always crystallize parallel to the ATS materials. The presence of magnesium in the reaction mixture can also favor the formation of the ATS structure; nevertheless, the synthesis of pure MAPO-36 from reaction mixtures having very low magnesium content was not possible under several conditions. Pseudoboehmite was a better aluminum source than aluminum isopropoxide for the formation of MAPO-36. A practically pure ATS structure was obtained from reaction mixtures containing molar fractions of Mg between 0.033 and 0.133. For higher magnesium contents, traces of another unidentified phase were detected. Energy-dispersive spectrometry analysis indicates a uniform chemical composition of the MAPO-36 particles, and Scanning electron microscopy shows that the ATS samples have a needlelike morphology. Thermogravimetric analysis of ATS samples reveals that the weight loss corresponding to the decomposition of protonated amine is linearly correlated with magnesium content in the solid up to 0.8 mmol of magnesium per gram of solid.