ACRYLONITRILE FROM PROPANE ON (VO)2P2O7 WITH PREADSORBED AMMONIA .2. STUDY OF THE MECHANISM OF AMMONIA ADSORPTION AND REACTION WITH C3 HYDROCARBONS

Infrared data on the time-on-stream changes of the adsorbed species on vanadyl pyrophosphate with preadsorbed ammonia during interaction with propane and oxygen are reported, together with an infrared and thermogravimetric analysis of the ammonia interaction with this catalyst and of the adsorbed species formed in ammonia and propylene coadsorption. Results indicate that (i) ammonia forms mainly ammonium ions at room temperature, but increasing the temperature of interaction up to 400°C results in a progressive increase in the amount of ammonia coordinated to surface Lewis acid sites: (ii) the amount of adsorbed ammonia at 400°C corresponds to about a 1: 1 NH3-to-P surface ratio, but is lower for higher reaction temperatures; (iii) in the presence of gaseous oxygen, the amount of coordinated ammonia decreases, with the probable formation of an amido (NH2) species; (iv) propylene is coordinated as a π-bonded complex with vanadium Lewis acid sites and, in the absence of gaseous oxygen, is slowly transformed to acrylonitrile through the intermediate formation of propylamine; and v) propane does not form adsorbed intermediates in the absence of gaseous oxygen, but in the presence of gaseous oxygen, acrylate species are detected, It is suggested that two pathways of formation of acrylonitrile from propane are present on vanadyl pyrophosphate, the first in anaerobic conditions involving the intermediate formation of propylamine and the second, when O2 is present, involving the reaction between surface acrylate species and chemisorbed ammonia species. The second pathway is much more rapid than the first. © 1993 Academic Press, Inc.