Figure 1. (a) XRD patterns of different ZnFe2O4 catalysts; (b) fresh and spent ZnFe2O4@H-ZSM-5, (c)XRD patterns of H-ZSM-5*, K-ZSM-5, La-ZSM-5, Ce-ZSM-5, H-ZSM-5.
The tailor-made capsule catalyst consists of two components: ZnFe2O4 as core catalyst and ZSM-5 as shell catalyst. XRD patterns of ZnFe2O4and capsule catalysts were shown in Figure 1. As seen, the main phase of core catalyst is mainly ZnFe2O4 apart from a small amount of Fe2O3, when NaOH as an alkaline thermal-solvent is used. It has been reported that the crystallinity of spinel species increases as the concentration of Na decreases.28 By contrary, the main Fe phase is ZnFe2O4 without Fe2O3 formation for K-ZnFe2O4, which indicates that KOH is more conducive to the formation of spinel structure. These phenomena are also consistent with our previous findings about ZnFe2O4.24 As drawn in Figure 1b, the ZnFe2O4@ZSM-5 capsule catalyst and H-ZSM-5 exhibit same diffraction peaks that 2θ value of 8.0, 8.9, 23.3, 23.9, and 24.4o (JCPDS 44-0003), suggesting that the MFI topology structure of ZSM-5 zeolite is not destroyed during the encapsulation process of ZnFe2O4 particles. In addition, XRD spectra of the capsule catalyst have no obvious peaks of Fe phase, indicating that the surface of the nuclear catalyst is completely wrapped by ZSM-5 shell. To verify the structure of the capsule catalyst, the surface morphology of the K-ZnFe2O4@K-ZSM-5 capsule catalyst was analyzed by SEM in Figure 2d-i. An obvious boundary between the two different components can be observed in the cut section of the capsule catalyst where having different elements distribution profiles in mapping images that Zn, Fe belong to K-ZnFe2O4 and Si, Al belong to K-ZSM-5 presented. Zn and Fe elemental signals are obviously surrounded by Al and Si elemental from mapping images of the K-ZnFe2O4@K-ZSM-5 capsule catalyst, which indicates that the ZnFe2O4 core catalyst is wrapped inside by K-ZSM-5 shell catalyst (Figure 2e). These above results certify that the capsule structure catalyst with the ZnFe2O4 as core componets uniformly encapsulated by K-ZSM-5 has been successfully synthesized.