Preparation Of Palladium-based Composite Membrane And Catalytic Performance Of Palladium Membrane Reactor

Palladium-based composite membranes have good hydrogen permeability and selectivity,and are widely used for hydrogen separation and hydrogen-related reactions such as hydrogenation and dehydrogenation.The one-step oxidation of benzene in H₂-O₂system using palladium membrane reactor has achieved excellent catalytic reaction effect,but there are still problems such as poor stability of membrane reactor and low yield of phenol.Propylene preparation of propylene oxide reaction is expected to be a new research hotspot for palladium membrane reactor,the reaction mostly adopts Au-Ti catalytic system,using hydrogen and oxygen for direct epoxidation of propylene,which has problems such as low conversion of propylene,easy deactivation of catalyst,and safety hazards caused by mixed hydrogen and oxygen feed.Based on the above questions,the relevant experimental investigations and the main conclusions were conducted as follows:（1）Alumina（Al₂O₃）composite hollow fibers doped with yttrium oxide stabilized zirconia（YSZ）were prepared by phase transition spinning and sintering technique,and dense palladium membranes（VCFELP-Pd）were prepared on the inner surface of YSZ-Al₂O₃ composite hollow fibers by continuous flow vacuum chemical plating（VCFELP）method,and the related properties were tested.The results showed that the VCFELP-Pd membrane was tightly bonded to the support,the deposited palladium particles were uniform in size with an average size of about 2.5μm,and no nitrogen permeation was detected at room temperature.The hydrogen permeance of the VCFELP-Pd membrane was 0.075 mol·m^-2·s^-1 at 100 KPa,773 K,and no nitrogen permeation was detected after 240 h of long-term operation.The membrane performance remained stable after 24 H₂/N₂ gas exchange cycles,40 high and low temperature thermal cycles,and 20 pressure cycles.（2）The above-prepared inner surface palladium-based composite membranes were assembled into a palladium composite membrane reactor to investigate the catalytic performance of one-step oxidation of benzene to prepare phenol under different reaction conditions.The findings showed that the benzene conversion and phenol selectivity reached 23.5%and 91.6%when the hydrogen,oxygen and benzene flow rates were 7ml·min^-1,2.5 ml·min^-1 and 1 ml·min^-1,respectively.The benzene conversion and phenol selectivity were basically unchanged during 120 h.The reactor has excellent stability and catalytic performance.The reactor has a large specific surface area(4.0 mm^-1)and a small reaction channel（1.0 mm）,which effectively improved the catalytic performance.（3）Various types of palladium membrane reactors were constructed by loading palladium membranes on the outer surface of ceramic tubes through the chemical plating（ELP）method to investigate the catalytic performance of propylene epoxidation for the preparation of propylene oxide under different conditions.The results showed that the hydrogen permeance of the Pd/α-Al₂O₃ composite membrane was 0.197 mol·m^-2·s^-1 at 100KPa and 773 K,and the ideal selectivity of H₂/N₂ was 980.The reaction temperature was473 K,and the flow rates of oxygen,hydrogen,propylene and nitrogen were 18 ml·min^-1,3 ml·min^-1,0.5 ml·min^-1 and 14 ml·min^-1,and the conversion of propylene was 7.02%and the selectivity of propylene oxide was 1.52%.To further improve the catalytic performance,TS-1/Pd/α-Al₂O₃ composite membrane was constructed by loading the TS-1 molecular sieve on the surface of palladium membrane through coupling agent method,and the propylene conversion was able to reach 8.21%and the selectivity of propylene oxide was1.77%.