Synthesis Of Beta Zeolite Induced By Seeds With Low Structure Directing Agent Content

Naphtha catalytic cracking process has a wide developmental foreground in the production of light olefins due to its low cracking temperature,low energy consumption,and flexibility to adjust the ratio of propylene to ethylene,etc.Considering that the processing technique has been increasingly improved continuously,the focus of research is transferred to its catalyst.Beta zeolite is often applied to catalytic cracking catalyst because of its high thermal and hydrothermal stability and suitable surface acidity.It is mainly used as the additive in the catalytic cracking catalyst due to the limitation of its high synthetic cost.Therefore,this research used industrial silica sol as silicon source and synthesizing Beta zeolite by single template method and adding seeds method respectively in order to reduce the synthetic cost.Then optimizing the synthesis conditions,and selecting several appropriate products for n-heptane catalytic cracking reaction.Beta zeolite was first hydrothermally synthesized by single template method.The results displayed that the system which had higher template content had a wider synthetic phase region.For example,Beta zeolites with good crystallinities were obtained under the crystallization temperature of 140°C¹70°C in the system where TEAOH/SiO₂=0.4,while Beta zeolites only can be obtained under the crystallization temperature of 150°C and 160°C in the system where TEAOH/SiO₂=0.3.The product synthesized under 160°C for 3 days with a gel ratio of SiO₂:0.033Al₂O₃:0.3TEAOH:0.05NaOH:17.6H₂O had the highest relative crystallinity.The minimum ratio of template to silica at which could obtain Beta zeolite was 0.2.Secondly,Beta zeolite was synthesized by adding homemade seeds extra.The seed was the product synthesized under the best condition in the system above.The results showed that the addition of seeds can shorten the crystallization time and reduce the dosage of template.Pure Beta zeolites with good crystallinity were obtained when OH^-/SiO₂=0.2⁰.38 in the system where the ratio of template to silica was 0.2,and the shortest crystallization time was decreased to 24 hours in this system.The product synthesized under 160°C for 48 hours with a gel ratio of SiO₂:0.033Al₂O₃:0.2TEAOH:0.18NaOH:12.7H₂O had the highest relative crystallinity.The minimum ratio of template to silica at which could obtain Beta zeolite was decreased to 0.1.Finally,acid properties and n-heptane catalytic cracking performances of several Beta zeolite samples were investigated.Py-FTIR results showed that all the samples had both B acid sites and L acid sites.S7 and S8 samples synthesized with seeds had higher B/L values compared with other samples.The corresponding B/L values were 0.930 and 1.099respectively,which were much higher than that of commercial Beta zeolite.NH₃-TPD results showed that the acid content distributions of all samples were similar to each other and weak acid content took about 75%of total acid amount,but the total acid amount of each sample was different.S4,S7 and S8 samples had higher total acid amounts compared with other synthetic samples.The results of catalytic cracking reaction manifested that the synthesized Beta zeolite sample had a higher yield of light olefins and a higher relative selectivity of propylene than those of commercial Beta zeolite.