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.