Fe(III)-modified montmorillonite and bentonite : Synthesis, chemical and UV-Vis spectral characterization, arsenic sorption, and catalysis of oxidative dehydrogenation of propane

Two major species were identified in Fe-treated montmorillonite: monomeric or dimeric hydroxoaqua cations Fe(OH) - (form I), and polymeric structures with edge-shared Fe(O,OH) (form II). These species have different electron spectra (absorption maximum is 29,600 cm in form I, and 26,000 and 28,000 cm in form II), chemical and thermal stability, and electrochemical behavior. Form I behaves as a partly exchangeable cation in interaction with Cu from Cu-trien solution and Ni from Ni-EDTA, that can be used for selective quantitative analysis. On heating above the dehydration temperature (∼100-150°C) montmorrilonite with Fe in form I is converted to a mica-like structure and Fe ions are fixed more strongly in the montmorillonite structure. Form II behaves similarly to hydrous ferric oxides, but its thermal crystallization to hematite is postponed to ∼500-600°C. The Fe cations in the interlayer space are much less thermally stable than Al pillars in pillared interlayered clays (PILCs). Form I is more active in oxidative dehydrogenation of propane, while form II is the active species in sorption of As and the non-specific combustion of propane. To produce only form II by the treatment of montmorillonite with Fe, its load must be kept below ∼20 wt.%; otherwise the usual hydrous ferric oxides are formed