Research Of ZSM-5 Modified By Alkali And Gold For Catalytic Cracking Diesel To Produce Propylene

ZSM-5 molecular sieve is the most commonly used catalyst components in fluid catalytic cracking devices to produce propylene, their small hole diameter limits the large hydrocarbon molecules into the channel to contact with the active sites and catalytic activity cannot be fully explored. In order to elimination its diffusion limit, preparation nano-gold catalyst whose gold nano particles are limited within molecular screen channel and illuminate gold's effect on light diesel catalytic cracking yield propylene reaction.Under different conditions we used NaOH solution to treat the ZSM-5, then we used the new gold precursor body Au?en?₂Cl₃ to modify the ZSM-5. Through controlling gold loading, reduction temperature, calcination temperature, we prepared a series of catalyst.Straight-run diesel oil?distillation 235³37 ?? was used as raw materials in the microreaction-activity evaluation unit with different temperatures to investigate selectivity of low carbon olefins such as propylene and micro-activity index of the ZSM-5. The physico-chemical properties of the catalysts were characterized by means of XRD, NH3-TPD, BET, ICP-AES, UV-Vis, TEM, FE-SEM techniques, the following results were obtained:?1? ZSM-5 were treated in NaOH solution with different conditions, in the case of NaOH solution concentrations are not too high we can get the micro-mesoporous composite ZSM-5 with the crystal structure unchanged, pore volume and average pore size of the treated ZSM-5 is increased, but the strong acid sites decreases, further acid treatment reduced the amount of weak acid sites. Compared with the parent zeolite, the ZSM-5 after alkali and alkali-acid treatment showed a higher micro-activity and olefin selectivity. After 800 ? high temperature steam treatment the alkali treatment samples' crystal structure were preserved, the mesoporous volume increased continuously, but the acidic site losed greatly. Compared with the parent zeolite micro-activity index of the ZSM-5 after series treatment did not decline, but the introduction of mesoporous lay a solid foundation for preparing Au/ZSM-5 catalyst of which gold was limited in thechannel of the zeolite.?2? Au?en?₂Cl₃ was used as precursor to prepare a series of Au/ZSM-5 with different gold loading, ICP results showed that the actual load of gold can reach more than theoretical value of 80%. This fact indicate that Au?en?₂Cl₃ can strongly combine with the molecular sieve under the operating conditions, which means it is a effective gold precursor for molecular sieves to load with. But in the loading process it's need to use high concentration NaOH solution to adjust the pH of the solution, so gold-loaded molecular sieve's acid have been impacted, there were two NH3 desorption peak in the parent zeolite, after modification there was only one peak left and its position changed.The higher the load of gold, the lower the acidity of the zeolites. Microreaction-activity results showed that although gold content of the gold-loaded molecular sieve are different, their olefin selectivity and micro-activity index are almost the same, those might be gold has a positive effect on FCC activity and it make up for a catalytic activity decline caused by acid reduced.?3? The Au/ZSM-5 with same gold loading were treated in H₂ atmosphere at different temperatures, BET results showed that the higher the treatment temperature is the smaller the Au/ZSM-5s' surface area and pore volume is, the catalytic activity were get bigger as the temperature rises. We believed that those were due to the highly dispersed gold nano particle aggregate on the surface and they cover the outer surface of the zeolite and blocked pores contribute to the acid of Au/ZSM-5 and eventually affect the catalytic activity.?4? Au/ZSM-5 with the same gold loading were baked in air atmosphere at different temperatures, XRD results showed that with the increase of the treatment temperature the intensity of the characteristic diffraction peaks gradually decreased, but even 1000 ?did not destroy molecular sieves' structure. But molecular sieves' acid amount decreased significantly with the increase of calcination temperature. After baked at 1000 ?molecular sieve's acid sites almost disappeared. Although the molecular sieves had different acid amount, but their activity is almost the same, which can still be attributedto the highly dispersed gold nano particle aggregate on the surface and they cover the outer surface of the zeolite and blocked pores contribute to the acid of Au/ZSM-5 and eventually affect the catalytic activity.