Encapsulation of ferro- and ferricyanide complexes inside ZSM-5 zeolite synthesized from rice straw: Implications for synthesis of Prussian blue pigment

Encapsulation of [Fe(CN)6]4− and [Fe(CN)6]3− complexes in the intracrystalline pores of ZSM-5 zeolite, FeIIL/Z and FeIIIL/Z respectively, by the zeolite synthesis method was reported. The modified zeolites were characterized by powder XRD, FT-IR and UV-vis spectroscopy. The nitrogen adsorption isotherms allow for the evaluation of pore structure of the complex-modified zeolites, whereas the thermal analysis (TGA/DTA) measurements provide insight into the decomposition products of the immobilized complexes. The modified zeolites exhibited smaller pore volumes and surface areas as compared with those of unpromoted ZSM-5, suggesting the inclusion of iron cyanides inside the interconnecting channels of ZSM-5. While the ferricyanide complex enhanced the formation of highly crystalline zeolite, the ferrocyanide one resulted in a lesser effect. The electronic spectra of the colloidal species developed when FeIIIL/Z brought in contact with an aqueous solution of [Fe(CN)6]3− exhibit absorptions attributed to CN− → iron charge-transfer. New bands at 294 and 319 nm due to d-d transitions of FeIII tetrahedral monomeric moieties were emitted concurrently under successive adsorption of [Fe(CN)6]aq3− over FeIIIL/Z, along with a broad band at 555 nm assigned to polymeric [FeII-C-N-FeIII] of Prussian blue (PB). The FT-IR spectra of FeIII/IIL/Z devoted to the adsorption of an aqueous solution of [Fe(CN)6]3− showed a band at 2092 cm−1 assigned to the C-N stretch in the FeII-CN-FeIII linkages. The vibrations attributable to Fe-O-Si bonding along with hydrocarbon and nitroprusside appeared only in the spectrum of FeIIIL/Z, thus was found to be strong evidence for the mutual interaction between [Fe(CN)6]3− and the latter sample.