Preparation Of Au/ZSM-5 Catalysts And Their Catalytic Cracking Performance For Increasing Propylene Production

Propylene is widely used as raw materials in the preparation of various chemicals.The production of propylene can be achieved by catalytic cracking of macromolecular hydrocarbons.The Au/ZSM-5 catalyst can reduce the temperature of the catalytic cracking reaction,and it can improve the reactant conversion,propylene selectivity and micro-reactivity index at the same time.Therefore,Au nanoparticles supported on ZSM-5zeolites had been prepared for catalytic cracking reaction of n-octane and light diesel oil in this paper.The reactant conversion,propylene selectivity and micro-reactivity index of different series of catalysts was investigated at different reaction temperatures.The relationship between the stability of Au nanoparticles,the size of Au nanoparticles,the Au species and catalytic performance was revealed under high temperature reaction system.The main research contents and results of the paper are as follows:?1?0.5%Au/x%La-ZSM-5 catalysts with different La loadings were prepared by an improved deposition precipitation method.0.3 wt.%La could effectively improve the dispersibility of Au nanoparticles.During the reaction,it showed the effect of stabilizing Au nanoparticles and preventing their agglomeration.Comparing with 0.5%Au/ZSM-5catalyst,0.5%Au/0.3%La-ZSM-5 catalyst showed better catalytic cracking performance.While achieving full conversion of n-octane,the selectivity of propylene increased from51.7%to 52.9%,and the reaction stability was also improved significantly.The experimental results of catalytic cracking of light diesel oil showed that the conversion,micro-reactivity index and propylene selectivity of 0.5%Au/0.3%La-ZSM-5 catalyst increased from 71.6%,72.2%and 48.4%to 76.6%,77.1%and 49.2%at 360oC,respectively.The conversion,micro-reactivity index and propylene selectivity increased from 69.4%,70.0%and 55.2%to 72.1%,72.7%and 57.1%at 460oC,respectively.In addition,the 0.5%Au/0.3%La-ZSM-5 catalyst showed less carbon deposits,which also indicated that the catalyst had good stability.?2?Using the urea precipitation method,0.5%Au/ZSM-5-S series catalysts with different Au nanoparticle sizes were obtained by changing the preparation time,temperature and amount of urea.Catalysts with different Au nanoparticle sizes showed different catalytic activities for the catalytic cracking reactions of n-octane and light diesel oil.Catalysts with small Au nanoparticle size showed more excellent catalytic performance.The 0.5%Au/ZSM-5-S₁ catalyst with Au nanoparticle size of 6.9 nm showed the highest n-octane conversion and propylene selectivity.Its n-octane conversion was as high as 98.6%at 410oC.The propylene selectivity was 47.8%,and the selectivity of propylene was as high as 58.6%while the full conversion of n-octane was achieved at 460oC.In addition,the size of Au nanoparticles had a greater impact on propylene production from catalytic cracking of light diesel oil.When the size of Au nanoparticles was reduced from 8.6 nm to 6.9 nm,the conversion and micro-reactivity index of the catalyst increased from 59.0%,59.7%to 77.9%,78.4%at 260oC,respectively.The propylene selectivity increased from 51.6%to 61.8%at 460oC.?3?Using the urea precipitation method,a series of 0.5%Au/ZSM-5-T catalysts with different Au species were prepared by changing the calcination temperature.0.5%Au/ZSM-5-uncalcined and 0.5%Au/ZSM-5-200°C catalysts with more positively-charged Au showed that the n-octane conversion was only 12.0%and 18.0%,and propylene selectivity was only 4.3%and 11.6%at 260°C,respectively.In comparison,0.5%Au/ZSM-5-400°C and 0.5%Au/ZSM-5-600°C catalysts which mainly contained zero-valence Au.They showed the n-octane conversion was 70.8%and 89.6%,the propylene selectivity was 33.6%and 32.7%,respectively.In addition,zero-valence Au was not conducive to formation of by-products?C₄ alkanes and aromatic compounds?.Compared with 0.5%Au/ZSM-5-200°C catalyst containing more positively-charged Au,the experimental results of catalytic cracking of light diesel oil showed that the conversion,micro-reactivity index and propylene selectivity of 0.5%Au/ZSM-5-600°C catalyst with more zero-valence Au was increased from 63.1%,63.8%,25.8%to 89.3%,89.5%,28.4%at 260°C.This indicated that the positively-charged Au was not conducive to the progress of this reaction,while zero-valence Au had a positive effect on the improvement of catalytic performance.