Effect On The Structure And Properties Of Mesoporous Molecular Sieves By Hydroxyl Radicals

In 1992,mesoporous pure silicon MCM-41 molecular sieve was synthesized for the first time.It is well known that MCM-41 is of single pore size distribution,regulated aperture from 1.3 to 50 nm,high specific surface area,and regular pore structure.At the same time,the limitation of skeleton atoms is smaller than that of zeolite.These merits make MCM-41 become topic research.However,long crystallization time and poor acidity of MCM-41 seriously restricts its industrial production and application.How to shorten the crystallization time and increase the acidity is a great challenge.Therefore,in this paper,the·OH radical is introduced into the synthesis system of MCM-41,in order to shorten crystallization time and enhance acidity.The specific research results are as follows:（1）Pure silicon mesoporous molecular sieve MCM-41 was synthesized by direct ultraviolet light,H₂O₂ and Na₂S₂O₈ free radical initiator,respectively.The excellent structure and acidity of MCM-41 was synthesized under ultraviolet light for 3 h,pH of 10.5 and ultraviolet light power of 500 w.In addition,the B and L acidic site coexist,and the L acidic site is the major.The acidity of the MCM-41by the H₂O₂ or Na₂S₂O₈ radical initiators was enhanced in one step.The esterification reaction,high cucurbit acid and a series of alcohols（methanol,ethanol,n-butanol,n-octanol）,was used to evaluate the catalysts.When n（high cucurbit acid）:n（alcohol）=0.01:0.3,the reaction temperature 100°C,the reaction pressure 2.5 MPa,the reaction time 4 h,and catalyst 0.05 g,the catalytic activity of MCM-41 induced by hydroxyl radicals is significantly higher than that of traditional hydrothermal MCM-41 and industrial grade ZSM-5.（2）The mechanism of acid-enhancement of MCM-41 synthesized by free radical initiation was studied by NH₃-TPD,Py-IR,ART-FTIR,²⁹Si MAS NMR and ¹H MAS NMR characterization.The results showed that the hydroxyl radical initiates the increase of silicon hydroxy group in MCM-41,which leads to an increase in acidity.（3）The new mesoporous Al@SiO₂@mSiO₂ microspheres was synthesized using CTAB（cetyltrimethylammonium bromide）,TEOS（ethyl orthosilicate）and AlCl₃ as raw materials,ethanol and ammonia as solvents under hydrothermal and ultraviolet light（UV）,respectively.The results show that the Al@SiO₂@mSiO₂microspheres from hydrothermal system have better structure,more uniform dispersion and particle size,stronger acidity than the latter.However,the four-coordinate aluminum in Al@SiO₂@mSiO₂ microspheres from UV is more than that from hydrothermal system.The esterification reaction,high cucurbit acid and a series of alcohols（methanol,ethanol,n-butanol,n-octanol）,was also used to evaluate the catalysts.At the same reaction conditions as the above mentioned,the results showed that the catalytic activity of mesoporous microspheres synthesized under the two systems is significantly higher than that of MCM-41 and industrial grade ZSM-5 synthesized under the traditional hydrothermal system.In conclusion,mesoporous pure silicon MCM-41 molecular sieve,good structure and stability,strong acidity and super catalytic activity,was synthesized in short time through hydroxyl radicals.Meanwhile,Al@SiO₂@mSiO₂microspheres,uniform particle size and dispersion,strong acidity,was synthesized in hydrothermal and UV system.It provides a new way for the synthesis of acidic mesoporous MCM-41 molecular sieves.