Adsorption calorimetric measurements and aluminum-27 DOR NMR studies on the molecular sieve aluminum phosphate-18
| Autor: | M.P.J. Peeters, J.H.C. van Hooff, J.W. de Haan, L.J.M. van de Ven, U. Lohse, J.J. Janchen, I. Girnus |
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| Rok vydání: | 1993 |
| Předmět: | |
| Zdroj: | The Journal of Physical Chemistry. 97:12042-12046 |
| ISSN: | 1541-5740 0022-3654 |
| DOI: | 10.1021/j100148a033 |
| Popis: | 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)] |
| Databáze: | OpenAIRE |
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