Studies On Dynamic Synthesis Of ZSM-12 Zeolites

ZSM-12 molecular sieve is a MTW high silica zeolite,which has a unidimensional 12-membered ring channel system with pore openings of 0.57×0.61 nm.ZSM-12 molecular sieve is a kind of very useful catalyst in cracking,isomerization,aromatization and various other acid catalysis reactions due to its particularly framework structure,adjustable higher acid site density and hydrogen transfer capability.Therefore,the synthesis of ZSM-12 molecular sieve has achieved more and more attention.But so far,research has mainly focused on the static crystallization of ZSM-12 molecular sieves,yet in industrial production,the production of molecular sieves usually adopts dynamic crystallization style due to the needs of the mass and heat transfer,this urge researches on study of synthesis of ZSM-12 under dynamic crystallization style.Adopting the dynamic hydrothermal synthesis method,we systematically investigated the influence of silicon source,aluminum source,alkalinity,water content,template content and crystallization temperature on the low Si/Al ratio ZSM-12 molecular sieve.Further more,we have comparative studied the crystallization kinetics of two kind of crystallization style,namely dynamic crystallization and static crystallization.The main achievements are as follows:1)ZSM-12 molecular sieve is a kind of high silica zeolite.Under the conditions of this experiment,the results show that lower Si/Al ratio of ZSM-12 molecular sieve is more difficult to synthesis,when the content of aluminium is high,it tends to generate MFI and BEA mixed crystal.When the ratio of Si/Al<40,the product is mainly the BEA crystal;When the ratio of Si/Al≥40,pure ZSM-12 molecular sieve is accessibility,and the relative crystallinity of products increases with the increase of Si/Al ratio.2)Silicon source plays a particularly important role in the synthesis processes of ZSM-12 molecular sieve.The results show that silica sol are the best silicon source in the synthesis conditions.When the Si/Al=40,only the silica sol can synthesize pure ZSM-12 molecular sieve.When the Si/Al = 50,all three kinds of silicon source can synthesize ZSM-12 molecular sieve.3)Aluminum source plays a particularly important role in the synthesis processes of ZSM-12 molecular sieve.When the Si/Al = 40,only NaA102 acts as aluminum source can get pure ZSM-12 molecular sieve.Al₂（SO₄）3·18H₂0 and pseudo-boehmite act as aluminum source,the products are the amorphous.When the Si/Al=50,NaAlO₂ and Al₂（SO₄）3·18H₂O act as aluminum source the product can get ZSM-12 molecular sieve.But the former is pure phase and the relative crystallinity is highest.4)Alkalinity plays a particularly important role in the synthesis processes of ZSM-12 molecular sieve.When the Si/Al = 40,when alkalinity is lower,the ability of dissolving silicon and aluminum species is insufficient,thus leading to the slow crystallization speed and amorphous products.When alkalinity is too high,it will easily generates MFI type molecular.Thus the best OH^-/SiO₂ ratio is 0.16 under the experiment conditions.When the Si/Al=50,alkalinity has a wide synthetic range,the OH^-/SiO₂ ratio during 0.08～0.22 all can get pure ZSM-12 molecular sieve.The SEM results show that with the increasing of alkalinity,ZSM-12 molecular sieves’s particle size decreases and the shape changes from cube to spherical.5)Water content have a great influence on the crystal growth process and products crystal phase.When the Si/Al = 40,the higher content of water will leads to the lower concentration of reaction species,thus leads to amorphous products.When water content is lower,ZSM-12 molecular sieve can be successfully synthesized,as a result of the higher concentration of reaction species.And the best H20/SiO₂ ratio is 0.13.when the Si/Al = 50,water content has a wide synthetic range,but when the water content is rather lower,ZSM-12 molecular sieve is synthesized with MFI structure mixed crystal.When the water content is rather higher,ZSM-12 molecular sieve is synthesized with BE A structure mixed crystal.6)Template content plays a particularly important role in the synthesis processes of ZSM-12 molecular sieve.The results show that When Si/Al =40,TEA+/SiO₂= 0.12,the product is amorphous;When TEA+/SiO₂= 0.14,the product is pure ZSM-12 molecular sieve;When TEA+/SiO₂= 0.16,BEA type molecular sieve appears with three-dimensional porous.When Si/Al =50,TEA+/SiO₂ = 0.14,the product is ZSM-12 molecular sieve with MFI structure crystal;When TEA+/SiO₂= 0.16～0.18,the product is pure ZSM-12 molecular sieve;When TEA+/SiO₂≥0.20,the product is ZSM-12 molecular sieve with BEA structure crystal;7)Crystallization temperature plays a particularly important role in the synthesis processes of ZSM-12 molecular sieve.The results show that when crystallization temperature is 140℃,the product is amorphous;When crystallization temperature is 150℃～160℃,the product is pure ZSM-12 molecular sieve,with the increase of crystallization temperature,the products’ relative crystallinity increases.When crystallization temperature is 170℃,the product is MFI structure crystal.8)Crystallization method plays a particularly important role in the synthesis processes of the ZSM-12 molecular sieve.Adopting the dynamic and static hydrothermal synthesis method,we systematically investigated the synthesis of two kinds of crystallization method.The results show that the Si/Al ratio of synthetic product is smaller than that of initial gel.Under the dynamic hydrothermal conditions,with the increasing of the Si/Al ratio,the difference between the Si/Al ratio of product and initial gel is very small.However,the difference becomes obvious in the static hydrothermal conditions.The crystallization kinetics curve shows that compared with the static hydrothermal method,dynamic hydrothermal method can promote forming the molecular sieve and speed up crystallization processes,thus the dynamic hydrothermal method is more appropriate method in the synthesis of ZSM-12 molecular sieve.