Synthesis Of ZSM-48 Molecular Sieves With Low Silicon-to-Aluminum Ratio

ZSM-48 molecular sieve has a^*MRE-type framework structure with one-dimensional ten-membered ring pore structure and pore diameter of 0.53×0.56 nm.By virtue of its unique pore structure,ZSM-48 possesses excellent catalytic performance in a variety of reactions including methanol to hydrocarbon,xylene isomerization,C₄-olefin cracking,hydroisomerization and other reactions.Due to the low acid content of ZSM-48 with high silicon-to-aluminum（SiO₂/Al₂O₃）ratio,studying the synthesis of ZSM-48 with low SiO₂/Al₂O₃ratio is more practical.However,most of the current researches focus on synthesizing high SiO₂/Al₂O₃ratio ZSM-48 of 200～400.There are few studies on the synthesis of low SiO₂/Al₂O₃ratio ZSM-48,and the current research has some disadvantages:（1）The preparation of low SiO₂/Al₂O₃ratio ZSM-48 requires to use a large amount of hexamethonium bromide as template agent,which makes its fabrication cost too high.（2）At present,there are few literature on the pilot scale-up synthesis of ZSM-48.Therefore,this article adopts hydrothermal synthesis method to systematically explore the influence of a series of synthesis parameters on the preparation of ZSM-48.And preliminary pilot scale-up synthesis study in 1 L reactor.The main conclusions are as follows:（1）The effects of silicon sources and aluminum sources on the synthesis of low SiO₂/Al₂O₃ratio ZSM-48 of 100 are investigated.The results show that silica sol and white carbon black are used as silicon sources to synthesize rod-shaped and strip-shaped morphologies of ZSM-48 with high relative crystallinity respectively.Three different manufacturers of silica gel are used as silicon sources to synthesize ZSM-48 with low relative crystallinity and severe agglomeration phenomenon.In the system using silica sol as silicon source,the effects of four aluminum sources of liquid sodium partial,pseudo boehmite,sodium metaaluminate and aluminum sulfate octadecahydrate are investigated.Experiments find that the relative crystallinity of ZSM-48 synthesized with liquid sodium partial as aluminum source is the highest（101.1%）,while the relative crystallinity of ZSM-48 synthesized with pseudo boehmite,sodium metaaluminate,and aluminum sulfate octadecahydrate as the aluminum source is low at 86.6%,89.8%,and 54.0%respectively.（2）The effect of a series of synthetic parameters on the synthesis of low SiO₂/Al₂O₃ratio ZSM-48 is investigated by using silica sol as silicon source.The results are shown as follows.1)The alkalinity has a significant effect on the relative crystallinity and morphology of the ZSM-48.When Na₂O/SiO₂is 0.024,the morphology of ZSM-48 is strip-shaped,with low relative crystallinity（87.2%）.When Na₂O/SiO₂is in the range of0.029～0.053,the morphology of the sample changes from strip shape to rod shape,and the relative crystallinity increases continuously.When Na₂O/SiO₂is 0.053,the relative crystallinity of ZSM-48 is the highest（103.7%）.When Na₂O/SiO₂is in the range of0.057～0.077,the morphology of ZSM-48 changes from petal shape to monodisperse spherical particles,and the relative crystallinity decreases continuously.2)The template content has a great influence on the crystal phase and relative crystallinity of ZSM-48.When HMBr₂/SiO₂is less than 0.0203,the synthesized samples have quartz phase.When HMBr₂/SiO₂is in the range of 0.0203～0.0268,the pure phase ZSM-48 can be obtained.When HMBr₂/SiO₂is 0.0203,the relative crystallinity of ZSM-48 is the highest（103.7%）.3)The water content mainly affects the relative crystallinity of ZSM-48.ZSM-48 with the pure phase can be synthesized within the range of H₂O/SiO₂from 13 to 26.With the increasing of H₂O/SiO₂,the relative crystallinity firstly increases and then decreases.When H₂O/SiO₂is 20,the relative crystallinity of ZSM-48 is the highest（103.0%）.4)The seed amount mainly affects the relative crystallinity of ZSM-48.When the seed amount is less than 3%,the synthesized ZSM-48 is amorphous.When the seed amount is 5%,the relative crystallinity of the synthesized sample is up to 95.5%.Finally,we select the scheme with higher relative crystallinity for pilot scale-up synthesis in 1 L reactor.（3）The effect of a series of synthetic parameters on the synthesis of low SiO₂/Al₂O₃ratio ZSM-48 is investigated by using white carbon black as silicon source.The results are shown as follows.1)The alkalinity has a great effect on the relative crystallinity and particle size of ZSM-48.When Na₂O/SiO₂is less than 0.034,the relative crystallinity of the synthesized sample is low.When Na₂O/SiO₂is in the range of 0.034～0.053,the particle size gradually decreases.When Na₂O/SiO₂is 0.053,the relative crystallinity of ZSM-48 is the highest（109.0%）.When Na₂O/SiO₂is more than 0.053,mordenite is produced.2)The template content has an effect on the crystal phase of ZSM-48.When HMBr₂/SiO₂is less than 0.0102,the synthesized sample has quartz phase.When HMBr₂/SiO₂is 0.0102,the pure phase ZSM-48 can be synthesized.Compared with silica sol system,HMBr₂/SiO₂is reduced by about one time.3)The water content mainly affects the crystal phase of the synthesized ZSM-48.When H₂O/SiO₂is less than 18,it is easy to produce mordenite and quartz phase.When H₂O/SiO₂is in the range of 18～22,the pure phase ZSM-48 can be synthesized.When H₂O/SiO₂is more than 22,amorphous is produced in the synthesized sample.4)The seed amount mainly affects the crystal phase of the synthesized ZSM-48.When the seed amount is less than 4%,mordenite and quartz phase are produced.When the seed amount is 8%,the relative crystallinity of ZSM-48 is the highest（100.7%）.Three different morphologies of ZSM-48 of petal shape,strip shape and rod shape are selected for the reaction performance of O-cresol isomerization to produce M-cresol.The optimum ratio is selected to conduct the pilot scale-up synthesis in1 L reactor,and the influence of stirring speed is investigated.