Rapid analysis of occluded Pr2NH in the AlPO4-11 and VPI-5 molecular sieves by direct mass spectrometry

Abstract

The aluminophosphate molecular sieves AlPO₄-11 and VPI-5 have been prepared in the presence of dipropylamine (Pr₂NH), a structure-directing organic incorporated into their synthesis gels. The AlPO₄-11 molecular sieve's unidimensional channel system contains occluded Pr₂NH, detected by TG, in its straight channel structural voids; VPI-5 contains no significant amount of Pr₂NH (also determined via TG). The as-synthesized Pr₂NH-AlPO₄-11 was placed in a mass spectrometer probe (ca. 0.1 mg), which was evacuated to ca. 10^–6 mbar and heated at 2 °C s^–1, and its cracking pattern analysed with time. The Pr₂NH cracking pattern was observed from ca. 275 °C and above, peaking at ca. 350 °C. This is assigned as Pr₂NH removed from the AlPO₄-11 channel system during the direct mass-spectrometric analysis. Trace amounts of Pr₂NH were also observed from the as-synthesized VPI-5. For each material, the above results also indicated that the Pr₂NH was unprotonated. Thus, temperatureprogrammed mass spectrometry proves to be a rapid method for the qualitative analysis of guest molecules in molecular-sieve hosts.