In Situ NMR of SAPO-34 Crystallization

SAPO-34 (SAPO: silicoaluminophosphate) has been synthesized and examined by in situ multinuclear high-resolution NMR under hydrothermal conditions (170 °C) in the presence of HF, using morpholine as the structure-directing agent. Four routes have been examined: (a) from a SAPO gel with HF (triclinic SAPO-34), (b) from a SAPO gel without HF (trigonal SAPO-34), (c) from an AlPO 4 gel with HF (triclinic AlPO 4 -34), and (d) from a slurry containing the layered AlPO 4 F prephase (triclinic AlPO 4 -34). In situ 13 C, 19 F, 27 Al, and 31 P NMR have uncovered the main steps of the reaction. The gel dissolves into small units condensed to no more than 4-rings (4R). The next steps are formation of a layered intermediate (the prephase), redissolution of the prephase into 4R building units of different types depending on the fluoride contents and aluminum coordination, and the subsequent nucleation and crystallization of triclinic AlPO 4 /SAPO-34. The initial fast process leads to the formation of the prephase made of an alternating stacking of 4R type-II units. When approaching 170 °C, this phase redissolves in 4R type-III units with aluminum in five-coordination. With additional defluorination, 4R type-IV units transform into 4R type-V units and both types connect in a chabazite topology. One 4R type-IV unit clips two fluorine atoms into the bridging position in the 4-ring and subsequently shapes the network topology into its crystalline order. The mechanism provided can now be subjected to further chemical testing for purposes of syntheses optimization.