Secondary Growth Of ZSM-5 Membrane And Preliminary Study On The Catalytic Cracking Of N-Dodecane Under Supercritical Conditions

As flight speed increases to the high supersonic and hypersonic regime, the temperature of the ram air taken on the aircraft becomes too high to cool the structure. Therefore it is necessary to utilize the fuel as the primary coolant. In this case, sensible heating of the fuel is not enough, and endothermic reactions like catalytic cracking are needed. ZSM-5 is a fuel cracking catalyst which has a superior performance. In this work, ZSM-5 membranes were synthesized on nonporous stainless steel supports by combining one-step in situ seeding with one secondary membrane growth step.The suitable technological conditions of synthesis of ZSM-5 membranes were investigated in this paper. The effects of different synthesis parameters on crystal morphology of the membranes were characterized by DLS, XRD, SEM and TEM. The optimized thermal treatment temperature, crystallization time and the molar composition for silicalite-1 seed were 98℃, 48h and 9TPAOH:25TEOS:1450H2O:100EtOH respectively, while 185℃, 48h and 4TPAOH:25TEOS:2750H2O:0.2Al(NO3)3 for secondary growth. The results of SEM observation may provide an explanation for the suitable parameter of Silicalite-1 seeds which coating on the inner suffer of stainless steel tube. And the difference of synthesis zeolite membranes in crystallization reactor and stainless steel tube (ID=2mm) has been discovered. The thickness of zeolite membranes will not increase continuously with prolonging the time of growth, because of the exhaustion of raw materials.The activity of HZSM-5 membrane which synthesized on the surface of triangular spiral stainless steel packing by secondary growth method has been evaluated by cracking n-dodecane under supercritical condition using the fixed-bed reactor. The results clearly demonstrate that the conversion of catalytic cracking is far more than that of pyrolysis at the same condition and that catalytic cracking can reduce the temperature required for a given conversion rate by 80℃at least. Meanwhile, with the increasing of the catalytic reactions, H2 molar ratio in gas production will increase, resulting from catalytic coking. The optimized treatment for ZSM-5 membranes to HZSM-5 catalystic is accomplished by combining the results of catalytic reaction with thermal analysis (TGA and DSC). Firstly, in order to remove the TPA template molecules from the zeolite channels, all samples with films were heated in air at 450 for 3h using a heating and cooling rate of 1.0℃min-1. After calcination, the samples were rinsed in stronger ammonia water under ultrasound treatment. Finally, repeat the calcination in step 1.The temperature of burning coke deposition from zeolite membranes was investigated. The aim is to estimate the influence of temperature on the activity of zeolite. After Calcination at 700℃, the activity of zeolite has decreased, but 600℃has not. The suitable temperature is 550℃.