An EPR and NMR study on Mo/HZSM-5 catalysts for the aromatization of methane: Investigation of the location of the pentavalent molybdenum

EPR and ²⁷Al NMR were used to probe the nature of the phases that formed during the conversion of methane at high temperature on the Mo/HZSM-5 catalyst. It was shown that ammonium 6-molybdoaluminate III supported over silica gave rise, when reduced, to an EPR signal due to Mo⁵⁺ ions with the odd electron interacting with the nonzero nuclear spin of aluminum in much the same way as in HZSM-5. Therefore, the occurrence of this particular spin interaction should not be taken as evidence of the location of the Mo ⁵⁺ ions at the exchange sites of the zeolites. Rather, combined ²⁷Al NMR and EPR investigation of the reduction of a pure polycrystalline aluminum molybdate suggested that the observed signal in the case of the zeolite composite catalyst was due to Mo⁵⁺ ions formed within the Al₂(MoO₄)₃ extra phase that formed upon dealumination of the zeolite in the presence of molybdenum.