Mesoporous MAl2O4 (M = Cu, Ni, Fe or Mg) spinels: Characterisation and application in the catalytic dehydrogenation of ethylbenzene in the presence of CO2 Original Research Article

The catalytic properties of nanostructured MAl2O4 (M = Cu, Ni, Fe or Mg) were investigated in the dehydrogenation of ethylbenzene with CO2. The catalysts were characterised by XRD, Raman spectroscopy, textural properties, acidity (TPD-NH3) and basicity (CO2-TPD) measurements, SEM and TEM analyses and TPR techniques. XRD revealed the spinel structure of MAl2O4, except for the NiO, CuO, Fe2O3 and MgO phases. All solids exhibited the nanostructured features of the SBA-15 template, including high values of textural properties and morphologies characteristic of the mesoporous silica. The acidic strength follows the electronegativity trends of the cations present on the spinel aluminates: CuAl > NiAl > FeAl and MgAl, which is the inverse order of the basicities. The selectivities in the dehydrogenation of ethylbenzene with CO2 showed that styrene was the predominant product for all catalysts except for those based on Ni, which was highly selective for the production of toluene. The FeAl2O4 catalyst provided the best catalytic performance among the solids studied due to the continuous oxidation of Fe3+ sites by the CO2 from the reaction. This solid has the additional advantage of being stable over the timescale of the reaction, as compared to the traditional Fe-based catalysts, due to its nanostructured features.