Phase-selective synthesis of a silicoaluminophosphate molecular sieve from dry gels

Silicoaluminophosphate (SAPO) molecular sieves with AFI and CHA structures have been synthesized from a reaction mixture containing triethylamine (TEA) as a template or structure-directing agent. The syntheses were carried out at 180–200 °C for several days both from dry gels (dry gel conversion) and homogeneous gels (conventional electric heating). From similar reaction mixtures, SAPO-5 (AFI structure) and SAPO-34 (CHA structure) molecular sieves are phase-selectively obtained from dry gel conversion and conventional electric heating, respectively. The phase-selectivity may be explained by the slow crystallization of the dry gel conversion due to low water or TEA concentration under the reaction conditions and the difference of the relative stability of SAPOs (AFI < CHA) under hydrothermal conditions. This phase-selective synthesis with dry gel conversion may lead to a new method for the synthesis of relatively unstable molecular sieves (having large pores) such as VFI or VPI-5.