| Palladium composite membranes have attained more and more application in hydrogen separation,purification and catalytic hydrogen-related reactions due to their unique selectivity of H2, especially in hydroxylation of benzene to phenol. Properties of supports greatly affect preparation and qualities of Pd composite membranes. Palladium composite membranes deposited on outer face of tubular substrates are prone to get polluted and scraped, which easily reduce their gas permeable and catalytic capabilities.In this paper, porous ceramic tubular substrates with large pore size and ceramic hollow fibers were chosen as supports to deposit Pd composite membranes. Two modified electroless plating methods were used to optimize the structure and gas permeability of Pd-Sil-1 composite membranes prepared by "co-seeding" method. Pd-based membranes were deposited on inner face of ceramic hollow fibers by conventional electroless plating. The morphology, structure, gas permeability and selectivity of as-synthesized Pd composite membranes were characterized by means of SEM,TEM and gas permeation tests. A novel Pd membrane reactor with "micro-channel" characteristic was designed and applied in one-step hydroxylation of benzene to phenol. The reaction conditions were optimized and the reaction mechanism was investigated. The main contents and results are as follows:1. Pd-Sil-1 composite membranes were prepared on macroporous ceramic supports with average pore size of 2μm by "co-seeding" method. Different dip-coating time, crystallization temperature were investigated to optimize the growth of Sil-1 membranes. The results showed that the best Sil-1 membranes for subsequent deposition of Pd membranes were achieved under the dip-coating time of less than 20s and the crystallization temperature of 423K. Rotating electroless plating and vacuum-assisted electroless plating were used to improve the performance Pd composite membranes. N2 flux at room temperature of Pd-Sil-1 composite membranes prepared by modified methods were improved. Rotation of supports and vacuum outside the supports could eliminate bubbles generated by redox reaction. Vacuum inside the supports could enhance the adhesion between Sil-1 layer and Pd layer, and encourage the generation of "Pd-legs". All Pd composite membranes prepared by methods mentioned above had poor gas selectivity at high temperature wich indicated that it was hard to prepare good Pd-Sil-1 membranes on macroporous ceramic supports. 2. Palladium-based membranes with a thickness of 2μm were deposited successfully on the inner face of ceramic hollow fibers by conventional electroless plating with 3 times of sensitization/activation at 318K, which had good gas permeability and stability. The hydrogen flux was 0.16 mol·m-2·s-1 at 773K, 0.1MPa and H2/N2 ideal selectivity was 493.10 gas-exchanging cycles between N2 and H2 at 473K and 20 pressure-exchanging cycles between 150kPa and 50kPa at 773K didn't impair its gas permeation flux. After four temperature changing cycles, the H2/N2 ideal selectivity decreased from 493 to 418, which indicated that the thermal stability of the Pd membrane dropped slightly.3. A palladium membrane reactor with "micro-channel" characteristic was constructed and used in direct hydroxylation of benzene to phenol, which was assembled with the palladium membrane deposited on inner face of ceramic hollow fiber. The highest benzene conversion and phenol yield were obtained when H2/O2 feed ratio was 3.5, which were 16.95% and 14.51%, respectively. After 9 days of reactions, the N2 flux of palladium-based membrane slightly increased from 8.14×10-4mol·m-2·s-1 to 8.60×10-4mol·m-2·s-1, and no defect was observed from SEM images, which meant the membrane had good stability in the hydroxylation of benzene to phenol. |
PDF Full Text Request