FIGURE 6 FESEM images of surface morphologies of Ti membrane
substrate (A1, A2), ZIF-8/Ti membrane (B1, B2) and Cu/ZnO/Ti CMNR (C1,
C2); FESEM images of cross-sectional morphologies of Ti membrane
substrate (A3), ZIF-8/Ti membrane (B3) and Cu/ZnO/Ti CMNR (C3); TEM
images of ZIF-8 nanoparticles in ZIF-8/Ti membrane (D); TEM images (E)
and HRTEM image of Cu/ZnO nanoparticles in Cu/ZnO/Ti CMNR.
The compositions of elements on the cross-section of ZIF-8/Ti membrane
and Cu/ZnO/Ti CMNR were obtained by EDS, as shown in Figure S8. After
ion-exchange and reduction, the atomic percentage of N in the membrane
decreased from 5.15% to 0%, and the atomic percentage of O increased
from 11.85% to 34.69%, which indicated the successful immobilization
of ZnO in the pores of Ti membrane substrate. The element distribution
of the whole cross-section of the Cu/ZnO/Ti CMNR was shown in Figure S9.
It could be obviously found that Cu, Zn, and O elements were uniformly
distributed along the cross-section, which proved that the Cu/ZnO
nanoparticles can be successfully distributed along the thickness
direction of the Ti membrane substrate by flowing synthesis.
3.1.3 Permeability of the CMNRs
The size of the membrane pores would be decreased after the
nanoparticles immobilization in membrane pores, which could reduce the
permeability of the membrane. The permeability of Ti membrane, ZIF-8/Ti
membrane and Cu/ZnO/Ti CMNR were determined before the experiment of
methanol dehydrogenation, in order to explore the effect of the
nanoparticles immobilized in the pores of Ti membrane substrate on the
permeability of the membrane. The results were shown in Figure 7. It can
be seen that the Ti membrane substrate has the best permeability, which
is 105
m3·m-2·h-1·kPa-1.
The permeability of the membrane could be reduced significantly, if
ZIF-8 nanoparticles were immobilized in membrane pores. It can be
calculated that the permeability of the ZIF-8/Ti membrane was only 0.41
m3·m-2·h-1·kPa-1,
which was three orders of magnitude lower than that of Ti membrane
substrate. The decrease of membrane pore size after ZIF-8 immobilization
could lead to the increase of transmembrane resistance for gas
permeating, resulting to the decrease permeability of membrane. Compared
with ZIF-8/Ti membrane, the permeability of the Cu/ZnO/Ti CMNR was
increased significantly and its value was calculated to be 22.2
m3·m-2·h-1·kPa-1.
This implied that the effect of Cu/ZnO nanoparticles immobilization in
Ti membrane substrate on permeability was smaller than ZIF-8, since the
ZIF-8 structure was collapsed during the process of ion-exchange by
Cu2+ and the pores of the membrane can be regenerated.