Effect of thermal treatment on surface and bulk properties of Fe/ZSM-5 zeolites prepared by different methods

Fe/ZSM-5 samples (5 wt.% Fe) prepared by in situ incorporation using TPABr template under hydrothermal conditions (Fe-ZSM-5in), chemical liquid deposition (Fe-ZSM-5imp) and solid-solid (Fe-ZSM-5ss) interaction were characterized by N2 physisorption, TG/DSC, X-ray diffraction, FTIR spectroscopy, UV-Vis diffuse reflectance spectroscopy and 57Fe Mössbauer spectroscopy techniques. Calcination at 550 °C leads to almost complete removal of template that was associated with dislodgment of significant fraction of Fe to external positions as recognized for the in situ prepared sample (Fe-ZSM-5in). This sample showed an increase in lattice volume suggesting the presence of the majority of Fe ions in tetrahedral positions inside zeolite channels and offered as well the lowest crystallites size (75 nm) and maximum SBET (453 m2/g) between all samples. On the other hand, Fe-ZSM-5ssbef, resulting from solid-solid interaction and subjected to heat treatment in vacuum at 200 °C, measured the lowest mean pore radius (r−; 23 Å), and pore volume (Vp; 0.3887 cm3/g), giving a hint about the probability of finding neutral iron oxide nanoparticles (α-Fe2O3) as a separate phase that has been validated by Mössbauer (IS = 0.3 mm/s, QS = −0.2 mm/s, Heff = 520 kOe) and UV-Vis (400 nm) investigations. This sample also demonstrated that the majority of Fe occupied framework positions beside a fraction identified as small oligonuclear oxo-iron ions (Fex3+-O; 290 nm). Interestingly, Fe-ZSM-5ssaft, resulting from solid-solid interaction and subjected to air calcination at 550 °C, measured maximum Vp (0.6380 cm3/g) and r− (39 Å) values, reflecting the enforced location of Fe in this sample leading to an effective pore widening and thus a pronounced mesoporosity is attained. IR bands due to νasT-O in ZSM-5 (1105 cm−1) showed a shift to lower wave numbers (1059 cm−1) following Fe incorporation reflecting the extent of exchanging Fe in this sample (Fe-ZSM-5ssaft), unlike the rest of the samples which showed splitting, which accounted for the presence of residual Al3+ beside Fe3+ ions in the same site. Mössbauer data of this sample confirmed the latter result and indicated the maximum lattice imperfection and showed as well the lowest degree of crystallinity. The Fe-ZSM-5impaft sample, subjected to heat treatment at 550 °C, showed α-Fe2O3 species where that heated at 110 °C presented the lowest SBET (361 m2/g). More correlations were evaluated and discussed on the effect of thermal treatment on the existence of various Fe species (either framework or non-framework), their electronic states and local structures.