ADSORPTION CALORIMETRIC MEASUREMENTS AND AL-27 DOR NMR-STUDIES ON THEMOLECULAR-SIEVE ALPO4-18

Highly crystalline samples of the chabazite-related aluminophosphate A lPO4-18 (AEI) have been prepared and characterized by adsorption measu rements as well as by Al-27 double rotation (DOR) NMR studies. Whereas the heat curves and isotherms of n-paraffins on AEI show the common f eature for adsorption on nonpolar molecular sieves with a given pore d imension, methanol gives an unusual heat curve with a deep minimum for a loading >4 molecules per cavity. This corresponds to the extended l ow-pressure hysteresis loop of the isotherm which is absent in SAPO-34 . In accordance with the structure determination and Rietveld refineme nt of the as-synthesized AEI, three crystallographically inequivalent Al positions including one pentacoordinated Al could be detected and a ssigned by Al-27 DOR NMR. Calcination as well as the adsorption of pol ar molecules results in a structure change. While water and ammonia ge nerate octahedrally coordinated Al, methanol gives only Al-v as has be en found for the template-containing sample. The methanol adsorption h as been studied in more detail. It has been found that the formation o f the Al-v requires more than one methanol molecule per Al-1 site. Thi s process occurs for a loading between one and four molecules per cavi ty; it is isobaric and compares to a hydration process. Two additional molecules can be accommodated in the AEI cavity which seems to be rel ated to the second step of the methanol isotherm and the extended low- pressure hysteresis loop which is absent in the isotherm of water. Thi s is accompanied by another reversible structure change resulting in f our NMR lines for the six different Al positions in the double 6-ring, the secondary building unit of the AEI.