17O solid-state NMR spectroscopic studies of the involvement of water vapor in molecular sieve formation by dry-gel conversion

Dry-gel conversion is a relatively new approach for molecular sieve synthesis. This method potentially has several advantages over the traditional hydrothermal synthesis and can be used to prepare molecular sieves with certain unique properties. The technique involves treating the predried reactive gel powder with water vapor at elevated temperature and pressure. The role of water vapor in this apparent solid transformation is, however, not clear. In this work, we directly monitored the involvement of 17O-enriched water vapor in crystallization of AlPO4-11 (an aluminophosphate-based molecular sieve) by 17O solid-state NMR spectroscopy. In addition to 17O magic-angle spinning technique, several dipolar-coupling based double-resonance methods including 17O[27Al], 17O[31P] rotational-echo double-resonance, 17O --> 31P and 1H --> 17O cross polarization techniques were used for spectral editing to select different 17O species. The results show that water from the vapor phase slowly exchanges with water molecules strongly bound to the AlPO intermediates first. Then 17O atoms are gradually incorporated in both P-O-H and P-O-Al units in the layered intermediate. There are three different P sites in AlPO4-11. Interestingly, during the transformation from the layered intermediate to AlPO4-11, the 17O atoms prefer to bond to the P2 and P3, but not to P1. The absence of 17O atoms in the first coordination sphere of P1 site suggests that some building units such as joint four- and six-membered rings involving hydrogen bonding with structure-directing agents are common in both layered intermediate and AlPO4-11 and they are not affected by the transformation from the layered phase to the AlPO4-11 framework.