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.