Aromatization of methane on Mo modified zeolites: Influence of the surface and structural properties of the carriers

摘要

Mo/zeolite type catalysts based on ZSM-5 and MCM-22 type carriers having a comparable amount of Mo (4.67 wt%) were investigated in methane conversion. The effect of the different structure and surface properties of zeolite samples has been studied. XRD, FTIR, TPDA, SEM and model reaction measurement were applied for the catalysts characterization. It has been revealed that deep bed treatment (DBT) of ZSM-5 has a negative effect as regards the catalyst performance in methane conversion. Results obtained on MoMCM-22 showed a lower resistance of this catalyst towards coking compared with MoZSM-5 type catalyst probably due to the channel system of MCM-22 forming supercages. Catalytic results on two MoZSM-5 catalysts which differ only in particle size of the ZSM-5 samples showed that Mo catalyst based on microsized ZSM-5 is significantly more active, selective and stable in methane conversion compared with catalyst based on ZSM-5 sample having particle size in nanosized region. Physico-chemical measurements indicated that Mo is located preferentially on high external surface area in nanosized catalyst, while Mo species in microsized HZSM-5 interact with the internal Bronsted acid sites probably due to its smaller external surface area. It is generally accepted that these species are the precursors for the formation of the most active carburized Mo sites. Mo does not have inorganic salts which are in the state of isolated cations so a wet impregnation with the solution of ammonium heptamolybdate (AHM) is usually used for Mo incorporation into zeolite resulting in the localization of large AHM molecules mainly on the external surface. The results that indicate probably the decrease of external surface supports the migration of Mo oxide species, forming during the activation of the catalyts, into intracrystalline space of zeolite crystals, where they interact with the internal Bronsted acid sites.