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Identification of a Key Precursor Phase for Synthesis of SAPO-34 and Kinetics of Formation Investigated by in Situ X-ray Diffraction

2001; American Chemical Society; Volume: 105; Issue: 50 Linguagem: Inglês

10.1021/jp0110758

1520-6106

Ørnulv B. Vistad, Duncan Akporiaye, Karl Petter Lillerud,

Polyoxometalates: Synthesis and Applications

The kinetics of formation of SAPO-34 in the presence of HF and with morpholine as a structure-directing agent has been monitored by in situ X-ray powder diffraction. Examination of the gel as a function of various heating profiles and final synthesis temperatures demonstrates the importance of the heating history. At lower temperatures, a layered "prephase" has been obtained with general empirical formula resembling [AlPO4F] × 0.87 C4H9ON−H. On indexing powder X-ray diffraction data, a monoclinic unit cell with dimensions a = 14.473 Å, b = 6.930 Å, c = 9.239 Å, and β = 101.54° was found with high figures of merit. An additional disordered phase, most probably a layered structure, appears as a transient phase during transformation of the prephase to SAPO-34. It has been shown that these two precursors are of major importance for obtaining SAPO-34 under the conditions studied. In addition, use of the prephase as starting material with the addition of water produces triclinic AlPO4-34 (Si free) after hydrothermal treatment at 180 °C for 24 h. The kinetics of formation have been examined in the presence of hydrofluoric acid as a function of the final synthesis temperature using a two-step heating profile. The crystallization curves were fitted to the Avrami−Erofeyev equation, and activation energy of 158 kJ/mol for the formation of triclinic SAPO-34 was determined from an Arrhenius plot.