Synthesis of large single crystals of the large-pore aluminophosphate molecular sieve VPI-5

The synthesis of VPI-5 has been optimized for the production of large singl e crystals with dimensions up to 400 x 35 x 35 mu m(3). All syntheses produ ced aggregates of needle-like crystallites. In most cases these were highly intergrown, but under the optimized conditions, individual hexagonal needl es radiating from a central point were formed. The synthesis procedure of J ahn et al., which was known to produce a highly crystalline material and wh ich is unusual in that polyphosphoric acid rather than orthophosphoric acid is used as the P source, was taken as a starting point. Optimization was d one in three stages: (1) using polyphosphoric acid with an average chain le ngth of 8 (n(av)=8) and di-n-propylamine as the organic additive; (2) using a shorter, commercially available polyphosphoric acid (n(av)= 4) and a var iety of organic additives; and (3) using polyphosphoric acid with n(av)= 8 and di-n-butylamine (DBA), which was found to be the most suitable organic additive in the second stage. Key parameters were found to be the use of th e longer chain polyphosphoric acid, the water content of the synthesis mixt ure, the ultrasonic treatment of the gel, the selection of the organic addi tive and the reaction temperature. With a gel composition of Al2O3:P2O5:95. 4H(2)O:DBA and a reaction temperature 123 degrees C, large single crystals could be grown. (C) 1999 Elsevier Science B.V. All rights reserved.