Catalytic conversions of chloroolefines over iron oxide nanoparticles 3. Electronic and magnetic properties of γ-Fe₂O₃ nanoparticles immobilized on different silicas

Abstract

Catalytic properties of superparamagnetic γ-ferric oxide nanoclusters, which are uniform in terms of size and magnetic properties were studied. The catalysts were supported on the activated silica gel matrix (AGM) prepared from the KSK-2 silica gel of globular structure and on the activated silica matrix (ASM) prepared from layered natural vermiculite. The clusters are active in some reactions of chloroolefin conversions: isomerization of dichlorobutenes and alkylation of benzene with allyl chloride. Their activity in these reactions is many times higher that of usual supported catalysts based on α-ferric oxide. Analysis of the Mössbauer spectra of the 2.5 wt.% Fe/AGM and 2.5 wt.%Fe/ASM samples before and after the reaction at T = 3–300 K shows that during the reaction some Fe^III ions arranged in ∼2–3-nm γ-Fe₂O₃ nanoclusters magnetically ordered at 6 K are reduced to form a high-spin Fe^II complex in the paramagnetic state. According to the macroscopic magnetization data (SQUID) of the initial clusters, curves with hysteresis are observed at 2 K in the plots of forward and backward magnetization, while the 2.5 wt.%Fe/ASM catalyst after the reaction at T = 2 K demonstrates a linear field dependence of the magnetization passing through the coordinate origin. Analysis of the Mössbauer spectra and magnetic properties suggests that during the catalytic reaction the Fe^III ions in the γ-Fe₂O₃ nanoclusters interact with chloroolefin with the allylic structure to be partially reduced to the Fe^II ions that are bound in a complex containing chloride ions and O^II ion(s) of the silicate matrix as ligands. This is a reason, probably, for the high catalytic activity of γ-Fe₂O₃ nanoparticles.