The compact ZSM-5 membranes was synthesized on porous alumina substrates by pre-coating narrosized ZSM-5 seeds and then employing the twice hydrothermal synthesis with adding the promoter NaCl. Effect of Na+ concentration on the formation of membrane was investigated in this work, indicating that adding a certain mount of NaCl can boost the growth of zeolite crystals on the substrates, thus forming the compact membrane consisted of highly intergrown crystals. The separating performance of zeolite membranes indicated that using a mixture with a composition of Al2O3: 84SiO(2): 10Na(2)O: 100NaCl: 15TPABr: 3500H(2)O as synthesis gel, the ideal selectivity of H-2/C3H8 could reach a maximum value of 23.7 at room temperature and pressure difference of 0.1 MPa, and the permselectivity decreased to 9.4 at testing temperature of 200 degreesC, which is still higher than the corresponding Knudsen diffusion value (4.69), suggesting that the membrane synthesized by this method was defect-free
