Synthesis, characterization and acidic properties of nanopowder ZSM-5 type ferrisilicates in the Na +/K + alkali system

Abstract

In this study Aluminium-free ZSM-5 type nanopowder ferrisilicates (as K–FZ or Na–FZ) were synthesized successfully from silicic acid, potassium (or sodium) carbonate and iron (III) nitrate using hydrothermal procedure (in relatively low crystallization time) in the presence of tetrapropylammonium bromide (TPABr) as a template. Prepared samples have been characterized by XRD, TEM, EDAX, NH 3-TPD, BET surface area, FTIR, TGA/DTA, SEM and ICP analysis. FTIR spectra confirm the production of ZSM-5 type ferrisilicates by the zeolite pentacycle peak at 547 cm − 1 and the symmetric stretching peak of Si–O–Fe at 656 cm − 1. XRD diffractograms confirmed ZSM-5 crystal phase. The results clearly show that K + cation (like Na +) plays a role of structure directing as well as a role of charge balancing agent. This could be assigned from NH 3-TPD analysis that ferrisilicate pretreatment procedure and iron-exchanging can significantly affect the strength of acidic sites. NH 3-TPD indicates that using potassium instead of sodium, leads to an increasing of the global acidity. Furthermore Na–FZ exchanging with iron ions (as Fe–NaFZ) leads to the same results. Thermal analysis revealed 14.7% weight loss due to the decomposition of TPABr template. Scanning electron microscope revealed that there is clustering of spherical particles in dry state. The TEM images confirm the particle size of the ferrisilicates in the range of 15–30 nm and show that iron-exchanged ferrisilicate has well-dispersed nano-sized iron oxide particles with various sizes in the range of 5–15 nm.