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 single crystals with dimensions up to 400×35×35 μm3. All syntheses produced aggregates of needle-like crystallites. In most cases these were highly intergrown, but under the optimised conditions, individual hexagonal needles radiating from a central point were formed. The synthesis procedure of Jahn et al., which was known to produce a highly crystalline material and which is unusual in that polyphosphoric acid rather than orthophosphoric acid is used as the P source, was taken as a starting point. Optimization was done in three stages: (1) using polyphosphoric acid with an average chain length of 8 (nav=8) and di-n-propylamine as the organic additive; (2) using a shorter, commercially available polyphosphoric acid (nav=4) and a variety of organic additives; and (3) using polyphosphoric acid with nav=8 and di-n-butylamine (DBH), which was found to be the most suitable organic additive in the second stage. Key parameters were found to be the use of the longer chain polyphosphoric acid, the water content of the synthesis mixture, the ultrasonic treatment of the gel, the selection of the organic additive and the reaction temperature. With a gel composition of Al2O3:P2O5:95.4H2O:DBA and a reaction temperature 123°C, large single crystals could be grown.