Comparative preparation of microporous VPI-5 using conventional and microwave heating techniques

The crystallization of the microporous AlPO4 phase VPI-5 using conventional boiling and refluxing systems (CR), conventional hidrothermal reactors (CH), microwave-activated refluxing systems (MR), and microwave hydrothermal reactors (MH) have been explored. The MR synthesis yielded amorphous materials or pure VPI-5, depending of the synthesis times. The MH and CH synthesis yielded pure VPI-5 and mixtures (VPI-5+AlPO4-H3 and VPI-5+AlPO4-11), respectively. The CR synthesis yielded mixtures of VPI-5+AlPO4-H2. A reduction in the minimum time necessary to crystallize VPI-5 was observed when microwaves activation was used. The performances of the microwave activation techniques are explained in terms of the influence of microwaves in the change of coordination of Al in the gel, from octahedral AI(O) to tetrahedral, Al(T) to yield the specific Al(O)/Al(T) ratio in the gel precursor necessary to direct the crystallization of VPI-5, and in terms of the rapid heating obtained using microwaves, necessary to reduce the probability of formation of intermediate metastable phases. (C) Elsevier Science Inc. 1997.