ELSEVIER Microporous Materials 4 (1995) 31 42 MICROPOROUS MATERIALS Critical assessment of reported procedures for the synthesis of thermally stable VPI-5 Arne Karlsson*, Duncan Akporiaye, Michael St6cker SINTEF, P.O. Box 124, Blindern. N-0314 Oslo, Norway Received 20 June 1994; accepted 29 September 1994 Abstract Three procedures from the literature for the synthesis of a VPI-5 phase that is stable at high temperatures in air have been tried. However, reproducible results were not obtained. Extensive washing or boiling of the products in water overnight did not improve the thermal stability of the products. The thermal stabilities of the samples were assessed in unsealed sample holders at 120°C in static air and heated in quartz trays to 400~C at a heating rate of Y=C/min in a flow of dry air. Additionally, the samples were heated under thermal conditions specified in the relevant literature (in unsealed vessels), but all samples transformed to AIPO4-8. Details of the XRD patterns, earlier reported to be significant for the stable and unstable VPI-5 phases, are believed to be ambiguous. Kevwor&'. VPI-5; Synthesis; Thermal stability; AIPO4-8; Transformation 1. Introduction In the development of novel catalyst systems, designed for the cracking of heavy fractions in crude oils, the focus has been on microporous aluminosilicate and aluminophosphate materials with very large pore openings. Until recently, VPI-5 was the material with the largest known pore openings (ca. 12 A) [ 1 ]. Unfortunately, VPI-5 is often reported with poor thermal stability (see for example Refs. [2]-[12]) and is known to undergo a phase transition under mild thermal treatments. In the hydrated material, part of the water molecules filling the pores possess a triple helix configuration, anchored to the framework via the A104(H20)2 water molecules bonded to octahedrally coordinated aluminium atoms [13] (Fig. 1). Through careful dehydration, the * Corresponding author. 0927-6513.95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 1~927-6513 (94)0008 1-6 Fig. l. Structure of (hydrated) VPI-5 in a view along the [001] direction obtained using coordinates from McCusker et al. [13]. Aluminium and phosphorus atoms alternate in positions at the intersecting lines and are linked via oxygen atoms. The aluminium atoms positioned in the centres of the double 4-rings are octahedrally coordinated to two water molecules (open circles) in addition to four framework oxygens.