Incorporation of manganese and iron into the zirconia lattice in promoted sulfated zirconia catalysts

Two series of Mn- or Fe-promoted zirconia samples were prepared: (i) a series of sulfate-free reference compounds via co-precipitation of aqueous solutions containing zirconium and the promoter cation and (ii) a series of catalysts via incipient wetness impregnation of a sulfated zirconium hydroxide. The promoter content was varied between 0 and 5 wt% metal. All promoter-containing materials were calcined at 923 K. The reference materials contained mainly isolated Mn or Fe species incorporated into the zirconia lattice as evidenced by stabilization of the tetragonal zirconia phase, EPR (isolated ions in highly symmetric environment), and a shrinking unit cell volume (XRD) of the tetragonal zirconia phase with increasing promoter content. Only the Mn-promoted catalysts showed such shrinkage in unit cell volume with increasing promoter content. At 2 wt% promoter content, Fe could and Mn could not be detected by ion scattering spectroscopy on the surface of the catalysts. The Fe-promoted catalysts contained Fe2O3-like surface species (EPR, XANES), which could at least in part be removed by washing with oxalic acid. Catalysts were tested for isomerization at 338 K using 1 kPa n-butane in balance of N2. At 0.5 wt% promoter content the maximum rates produced by the 0.5 wt% Mn- and Fe-promoted sulfated zirconia were about 80 and 20 μmol g−1 h−1, respectively. Mn was thus more effective as a promoter for n-butane isomerization than Fe, despite the more extensive incorporation into the zirconia lattice.