Synthesis of a zeolite membrane as a protective layer on a metallic Pd composite membrane for hydrogen purification
Palladium composite membranes are used widely in the hydrogen purification process owing to their excellent hydrogen permeability and selectivity. However, they can lose hydrogen permeability irreversibly when exposed to a small amount of impurity gases, which severely limits their applications. Here, a seed-free hydrothermal method was developed to synthesize a zeolite membrane on the surface of an ultra-thin metallic Pd composite membrane to offer protection from impurity gases. The impurity gases with a kinetic diameter larger than the zeolite pore size were prevented from coming in direct contact with the Pd composite membrane. The presence of a protective zeolite membrane distinctly suppressed the hydrogen permeance decline in a 5% propylene/hydrogen mixture, which greatly enhanced the stability of the Pd membrane. SEM and XRD showed that the acid pretreatment before the secondary hydrothermal growth process and the rotary state were beneficial for synthesizing thin and uniform zeolite membranes. With increasing synthesis time, the protective effects of the zeolite membrane were enhanced significant due to the reduction of defects.