TEXTURAL, ACIDIC AND CATALYTIC PROPERTIES OF NIOBIUM PHOSPHATE AND OF NIOBIUM OXIDE - INFLUENCE OF THE PRETREATMENT TEMPERATURE

Niobium pentoxide and niobium phosphate pretreated at a low temperature (e.g. 423 K) present a large surface area (160-170 m2 g-1), a large number of strong acid sites (heat of ammonia adsorption greater than 110 kJ mol-1) and a great activity for 3,3-dimethyl-1-butene isomerization (similar to that of a USHY zeolite). Treatment under vacuum at higher temperatures causes the elimination of water, the decrease of the surface area and of the number and strength of the acid sites. The effect of the pretreatment temperature is more pronounced with niobium oxide than with niobium phosphate. While these compounds can be rehydrated they do not recover their texture and their acid properties. The pretreatment under nitrogen flow prior to the measurement of the activity being less effective for dehydration than the pretreatment under vacuum, niobium oxide and particularly niobium phosphate keep a high isomerization activity even with a pretreatment temperature of 773 K.