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. This sintering leads to the coexistence of porous AlPO4-5 and dense tridymite. As established by cristallinity and pore volume measurements. the presence of vanadium decreases by 100 to 300°C the threshold temperature at 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 conditions 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 within the porosity. Since only very small amounts of intraporosity VIV species (VIV/VT<1%) are detected by ESR, whatever the extent of the Sintering which generates tridymite, it is concluded that the thermal stability of V-loaded AlPO4-5 is determined by the dispersion of Vv species within the porosity. This conclusion is in line with earlier propositions made to explain the thermal behaviour of V-loaded zeolites and V-contamined FCC catalysts.
Research on Chemical Intermediates – Springer Journals
Published: Apr 15, 2009
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