Incorporation of iron in sodalite structures and their transformation into other iron containing zeolites: Synthesis of Fe-NaA (LTA)

Abstract

In the low temperature digestion of gibbsitic aluminium ores an iron content impurity phase, ‘desilication product’ (DSP), will be separated from the sodium aluminate liquor. Due to its main component: (hydroxy- or carbonate-) sodalite (SOD) and amount, reaching 7–10% of the feedstock, the DSP is an important raw material for synthesising various zeolites. Applying the sol–gel technique the authors developed synthesis methods to prepare iron content (chloride-) sodalites, in order to model how they can be transformed to commercially interesting types of zeolites (e.g. to NaA (LTA)). Using diluted mineral acids (e.g. 5 wt.% sulfuric acid) in amounts equivalent (or rather in 10% excess) to the ion exchange capacity, a heat-treatment at 100°C, lasting 3 h leads to an amorphous product which can easily be recrystallised to various zeolites. Outgoing from iron containing SOD, a new kind of NaA (LTA) could be prepared possessing various amounts of iron. Mössbauer, XP, ESR and UV–VIS spectroscopic investigations revealed that up to 3 wt.% total iron content, as an average, about 60–62% iron is sited in framework (FW) positions, in T h co-ordination. The other part is located in the voids, in extra-framework (EFW) positions, in O h co-ordination, as highly dispersed iron oxide. This component exhibits strong magnetic interaction which manifests itself in large linewidths when taking the ESR spectra. Mössbauer spectroscopy revealed that the FW/EFW iron ratio decreased with the total iron content. Due to the ‘chemistry’ of sol–gel technique, in alkaline environments complete iron incorporation into zeolitic frameworks can never be attained. Applying complexing agents or other methods EFW iron can be dissolved during the acid treatment permitting the synthesis of low module zeolites with uniform Fe(III) siting. The various kinds of iron content LTA zeolites may catalyse (biomimetic) selective oxidation reactions between reactants capable to enter the pore structure.