Adsorption calorimetric measurements and 27Al DOR NMR studies on the molecular sieve AlPO4-18

Highly cryst. samples of the chabazite-related aluminophosphate AlPO4 18 (AEI) were prepd. and characterized by adsorption measurements as well as by 27Al double rotation (DOR) NMR studies. While the heat curves and isotherms of n-paraffins on AEI show the common feature for adsorption on nonpolar mol. sieves with a given pore dimension, MeOH gives an unusual heat curve with a deep min. for a loading > 4 mols. per cavity. This corresponds to the extended low-pressure hysteresis loop of the isotherm which is absent in SAPO 34. In accord with the structure detn. and Rietveld refinement of the as-synthesized AEI, 3 crystallog. inequivalent Al positions including one pentacoordinated Al can be detected and assigned by 27Al DOR NMR. Calcination as well as the adsorption of polar mols. results in a structure change. While H2O and NH3 generate octahedrally coordinated Al, MeOH gives only AlV as was found for the template-contg. sample. The MeOH adsorption was studied in more detail. The formation of the AlV requires > 1 MeOH mol. per Al1 site. This process occurs for a loading between 1 and 4 mols. per cavity; it is isobaric and compares to a hydration process. Two addnl. mols. can be accommodated in the AEI cavity which seems to be related to the 2nd step of the MeOH isotherm and the extended low-pressure hysteresis loop which is absent in the isotherm of H2O. This is accompanied by another reversible structure change resulting in 4 NMR lines for the 6 different Al positions in the double 6-ring, the secondary binding unit of the AEI. [on SciFinder (R)]