Magnesium- and iron-doped chromium fluoride/hydroxyfluoride: synthesis, characterization and catalytic activity

Abstract

The calcination of α-CrF₃·3H₂O results in the formation of a chromium hydroxyfluoride with the pyrochlore structure. The stepwise replacement of chromium by iron and magnesium leads to considerable alterations in the structure and the surface properties of the calcination products, accompanied by significant changes in the catalytic activity. The dismutation of dichlorodifluoromethane and the dehydrochlorination of 1,1,1 -trichloroethane act as a probe reactions for Lewis acid sites. The syntheses of the catalysts were carried out by coprecipitation of mixed metal fluoride trihydrates and subsequent calcination procedures. The stepwise replacement with iron leads to a rebuilding of the lattice from the cubic pyrochlore structure of CrF_3–x(OH)_x into the pseudo-hexagonal tungsten bronze (HTB) structure of β-FeF₃. The maximum catalytic activity towards CH₃CCl₃ dehydrochlorination was obtained for the 65% iron sample, which is accompanied by a maximum BET surface area and a maximal number of Lewis acid sites. CrF_3–x(OH)_x exhibits a dramatic loss of catalytic activity as well as BET surface area. Possible explanations are given by comparing the pseudo-HTB structure with the cubic pyrochlore structure with regard to the accessibility of the Lewis acid metal cations. Upon substitution of chromium by magnesium we obtained a maximum Lewis acidity for samples with 65–92% magnesium leading to a corresponding maximum in catalytic activity. The Brønsted acidity of both systems is predominantly weak. Bulk and surface hydroxy groups are distinguished.