Thermal stability of V-loaded AlPO4-5 materials

A progressive sintering of the porosity of both V-free and V-loaded AlPO4-5 samples is observed upon calcination in air at increasing temperatures. Th is sintering leads to the coexistence of porous AlPO4-5 and dense tridymite . As established by cristallinity and pore volume measurements, the presenc e of vanadium decreases by 100 to 300 degrees C the threshold temperature a t which this sintering begins, in spite of the low V amount in the samples (V/(V+Al+P) atomic ratio of 0.5-1%). This threshold temperature depends on the method used to introduce V (by hydrothermal synthesis, under static con ditions or agitation, and/or by grinding vanadia with a V-free AlPO4-5) and on the morphology of the AlPO4-5 crystals as observed by scanning electron microscopy. These factors are related to the dispersion of the V species w ithin the porosity. Since only very small amounts of intraporosity V-IV spe cies (V-IV/V-T < 1%) are detected by ESR, whatever the extent of the sinter ing which generates tridymite, it is concluded that the thermal stability o f V-loaded AlPO4-5 is determined by the dispersion of V-V species within th e porosity. This conclusion is in line with earlier propositions made to ex plain the thermal behaviour of V-loaded zeolites and V-contamined FCC catal ysts.