Structure and shape-selective properties of MFI type ferrisilicates. A comparison with aluminosilicate analogues
A comparison of H-forms of Fe- and Al-substituted silicates of MFI structure and of a comparable crystal size is given from the point of view of the acidity of SiOHMe groups, activity as well as para -selectivity in toluene disproportionation and toluene alkylation with ethylene. It has been found that SiOHFe groups exhibit weaker acidity than SiOHAl groups which is reflected in ammonia desorption at lower temperatures and lower toluene conversion for ferrisilicates. The ferrisilicates exhibit lower para -selectivity compared to aluminosilicates; moreover, higher coking is observed. While the aluminosilicates contain mostly aluminum in the framework sites forming SiOHAl groups, the ferrisilicates contain, beside Fe linked in SiOHFe groups, also tricoordinated Fe 3+ cations or even extraframework Fe species. The latter two species can be responsible for a considerably higher coking of ferrisilicates compared to aluminosilicates. Simultaneously, such Fe species can contribute to observed lower para -selectivity of ferrisilicates in comparison with aluminosilicates, when they are located on the outer surface of zeolite crystals.