Synthesis Of MFI Zeolite From Industrial Hexafluorosilicic Acid And The Corresponding Transalkylation Catalytic Performance Evaluation

In this paper,SiO₂ and NH₄F were prepared by using phosphate fertilizer by-product fluorosilicic acid,then the recovered SiO₂ as silicon source and ammonium florid as mineralizer were used to synthesize the molecular sieves which has MFI structure.The transalkylation reaction between toluene and 1,2,4-trimethylbenzene over the molecular sieves catalyst was studied.The main contents of this thesis are as follows:Firstly,the synthesis process of producing SiO₂ and NH₄F over the two-step aumoniation method which used phosphate fertilizer by-product fluosilicic acid as raw material was optimized,and the optimal reaction conditions of the first to second step of ammoniated were investigated as:in the first step,with 42%total amount of ammonia,the addition of ammonia of 2 mL/min,the reaction temperature 40?and the reaction time was 20 min;in the second step,with 58%total amount of ammonia,addition of ammonia of 2 mL/min,the reaction temperature 50?and the reaction time 70 min,the recovery of fluorine is up to 85%and that of SiO₂ is up to 55%,and the production SiO₂ contains fewer impurities.Secondly,the hydrothermal synthesis method was investigated to prepare the MFI structure molecular sieves?ZSM-5?.The results indicate that silica,aluminium and fluorine sources,Si/Al molar ratio,crystallization time and temperature,the amount of TPABr and NH₄F will affect the properties of synthesized ZSM-5.To characterize the molecular sieves,XRD,SEM,TG,BET,and EDS method were used.The results show that when the recovered SiO₂ and NH₄F as silica sources and mineralizer respectively,Al?OH?₃as aluminium source,high-quality ZSM-5 can be synthesized.Using SiO₂ which was prepared by fluosilicic acid as silica source,ZSM-5 can be obtained under both static and dynamic conditions.Moreover,comparing with static condition,the crystallization time is shorten and the crystallization is higher under dynamic condition.With the increase of hydrothermal condition and Si/Al molar ratio,ZSM-5's crystal form changes from orthorhombic system to monoclinic system.When the molar ratio of SiO₂ and TPABr is lower than 8,the incorporated amount of TPABr into the framework almost reach the maximum of about 12 wt%.When silica source is fumed SiO₂,ZSM-5 cannot be obtained without NH₄F,with the extra addition of NH₄F as mining agent during hydrothermal condition.But for the recovered SiO₂,even without NH₄F,ZSM-5 can be synthesized and the amount of NH₄F can control the crystal size of ZSM-5.Thirdly,the borosilicate MFI zeolite?B-ZSM-5?was successfully synthesized which use the recovered SiO₂ as raw material.The effect of Si/B molar ratio,crystallization time and temperature,the amount of template and mineralizer to the properties of B-ZSM-5were studied.The results show that B-ZSM-5 can be obtained when Si/B molar ratio is in the range of 1-70 and Al and Mg as impurities in the recovered SiO₂ can not be introduced in B-ZSM-5.When Si/B molar ratio is 12,B-ZSM-5 has the best crystallization after the dynamic response for 5 days,and with the increase of Si/B molar ratio,the crystal form of B-ZSM-5 transfer from orthorhombic system to monoclinic system.When the molar ratio of Si and templates is in the range of 4-20,B-ZSM-5 can be synthesized.And the more templates,the larger specific surface area of B-ZSM-5.The crystal size of B-ZSM-5 will increase with the increase of fluorine ion concentration.Finally,the effection of different molecular sieves as catalysts for the transalkylation reaction of toluene and 1,2,4-trimethylbenzene was investigated.The results show that with the SiO₂/Al₂O₃ molar ratio decreases,the conversion rate of raw materials and the selectivity of benzene and dimethylbenzene increase.When SiO₂/Al₂O₃ molar ratios are similar,comparing with the condition of adding NH₄F,the size of HZSM-5 with b axis preffered growth is smaller without NH₄F,and the average conversion rate of such transalkylation reaction is up to 55%for toluene and 63%for 1,2,4-trimethylbenzene,and the selectiveties of dimethylbenzene and benzene are 62.5%and 28%respectively.The larger size HZSM-5 molecular sieve which was synthesized with NH₄F is not a good choice for the transalkylation reaction for the selectivity of benzene is law but of 1,2,3-trimethylbenzene is high.Although the catalytic performance of borosilicate B-ZSM-5zeolite for the toluene and 1,2,4-trimethylbenzene disproportionation reaction is lower than aluminosilicate ZSM-5 zeolite,it can efficiently catalyze the toluene and 1,2,4-trimethylbenzene disproportionation reaction that indicating the boron has been incorporated to the zeolite crystaland,and it has the shape-selective catalysis.After removing the boron atoms from the B-ZSM-5 zeolite and introduction of Al atoms to the Al-?B?-ZSM-5 zeolite,which catalytic activity for the toluene and 1,2,4-trimethylbenzene disproportionation reaction is sharply increased.B-ZSM-5 zeolite as a precursor can be modified or isomorphous substitued by other heteroatom to enhance activity.This method has effective to produce high value-added of microporous molecular sieves.The new method for recovered silica from the H₂SiF₆ as silicon source to synthesize MFI structure of molecular sieve has been developed.Not only it solved the environmental problem and improve the economic value-added of hexafluorosilicic acid,but also reduced the cost of production the MFI structure of molecular sieve that is widely used in petrochemical industry.This method can expand and link the chain of phosphorous chemical industry and fusion phosphorus chemical industry and petrochemical industry.