Vapor phase nitration of aromatic compounds

In a process for the vapor phase nitration of aromatic compounds where the aromatic compound is contacted with a nitrating agent in the vapor phase to yield the corresponding nitroaromatic compound, the improvement comprising conducting the nitration in the presence of a nitration promoting catalyst which comprises the adduct of:

• (a) an alumina-silica-metal oxide combination represented by the formula:
• (Al.sub.2 O.sub.3).sub.a (SiO.sub.2).sub.b (M.sub.2/n O).sub.c
• wherein M is a metal cation selected from the group consisting of the lanthanides of rare earths, Groups 1b, 2b, 5b, 6b, 7b, and 8 of the Periodic Table of the Elements, and mixtures thereof, and a, b, and c represent weight percent of the Al.sub.2 O.sub.3, SiO.sub.2, and M.sub.2/n O components, respectively, in the alumina-silica-metal oxide combination, with a being 0 to 100, b being 0 to 100, and c being 0 to 50, and n represents an integer from 1 to 7 of the valence of the metal cation, with the proviso that the sum of (a+b) must be greater than 0, and
• (b) a catalytically effective amount of sulfur trioxide.