Using phosphorus doping of MoO3/ZSM-5 to modify performance in methane dehydroaromatisation

The performance of methane dehydroaromatisation catalysts prepared by the impregnation of H-ZSM-5 with ammonium heptamolybdate and phosphomolybdic acid precursors has been found to be different. Particular attention has been paid to hydrogen production which, although the major gas-phase product of dehydroaromatisation, is generally not reported. The use of phosphomolybdate leads to lower activity catalysts. When 5wt.% of phosphorus is added to the catalyst prepared from ammonium heptamolybdate, the activity is suppressed and H2 is seen as the sole gas-phase product. The rate of hydrogen formation increases with time on stream passing through a maximum, which is associated with the loss of an inorganic phosphite phase from the catalyst. Bulk MoO3 and P doped MoO3 samples are observed to produce H2 as a result of carburisation. Although the general pattern of hydrogen formation is similar to the 5wt.% doped zeolite catalyst, in these cases gas-phase carbon oxides are also associated with the hydrogen maximum.