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

摘要

Catalytic properties of superparamagnetic gamma-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 alpha-ferric oxide.Analysis of the Mossbauer 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 gamma-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 Mossbauer spectra and magnetic properties suggests that during the catalytic reaction the Fe~(III)ions in the gamma-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 gamma-Fe2O3 nanoparticles.