Synthesis Of Germanosilicates With Well-designed Quaternary Ammonium As Structural Directing Agent

Zeolite materials have shown excellent performance in the petrochemical industry and fine chemical synthesis,which simulates the exploration and development of zeolites with novel structure.In the present study,the design synthesis of novel germanosilicates was carried out using new ammonium salts as structural directing agent(SDA)with the help of germanium atoms as the co-SDA.The structure of ammonium salts,including diquaternary ammonium cations and polycyclic ammonium cations,the composition of synthetic gels,and the existence of germanium and F-ions were considered for the final structure of zeolites.1.Using diquaternary ammonium cations as SDA in germanosilicate system,four types of zeolites were obtained,including NES zeolite with shortened crystallization time,ECNU-16 zeolite with definitely novel structure,STW zeolite with chiral channel,and ITH zeolite with odd-ring pore channels.The length of the chain and the head structure of diquaternary ammonium cations were tuned to investigated the influence of SDA structure on zeolite synthesis.It was found that the SDA molecules with longer chain favored the large pore zeolite structures.Moreover,the unique directing effect of Ge atoms to the formation of double-4-ring(D4R)subunits was confirmed,which then favored the construction of zeolite structures different from those in all-silica or aluminosilica synthetic system.2.The structure of novel ECNU-16 germanosilicate was determined by combining the Rotation Electron Diffraction(RED)and high resolution powder X-ray diffraction(PXRD)technique.The ECNU-16 zeolite,constructed by 6-ring subunits,possesses three-dimsional pore channels,including two sets of 8-ring pore channel along [001] and [100] direction and 10-ring pore channel along [010] direction.ECNU-16 zeolite,with the space group of C2/m1,has a monoclinic symmetry.The unit cell parameters of ECNU-16 zeolite are a=17.834(5)?,b= 15.127(4)?,c=10.671(3)?,?=108.552(6)°,?=?=90° The specific surface area and micropore volume is 367 m2·g-1 and 0.111 cm3·g-1.3.ECNU-16 zeolite was found to have the same composite building units(CBUs)and similar layer structure as IM-16 zeolite with the UOS topology.However,the symmetry element leads to the final two different structures.The SDA 1,1'-(1,4-butanediyl)bis(3-methyl-1H-imidazolium)(4BI)used for synthesizing ECNU-16 zeolite is the dimer style of the SDA 1-ethyl-3-methylimidazolium(1E3MI)used in the preparation of IM-16 zeolite.The comprehensive computational study reveals that the zeolite phase selectivity is determined by the unique configuration and behavior of the “dimer” and “monomer” organics,which is closely correlated to their rotation freedom as well as the included volume of host zeolite channels.1E3 MI could rotate in the channel while the 4BI could not because of its liner and plate configuration.Thus,the ECNU-16 framework was constructed through inversion centers,while the IM-16 framework was build up through a mirror plane to provide a larger accessible pore.The fundamental viewpoint will provide new guidelines for the rational design and synthesis of new zeolites in the future.4.A novel ammonium of 1,8-diazabicyclo[5,4,0]undec-7-ene(DBU)and its quaternary ammonium were firstly used in the synthesis of zeolites.In the germanosilicate system,UTL zeolite with shortened crystallization time of 28 h and IWR zeolite with unique nanosheets aggregated hollow sphere morphology was obtained.In contrast to the NON zeolite obtained in all-silica system using DBU as SDA,the introduction of Ge atoms changed the directing role of DBU from cage-based style to channel-based aggregation style,which resulted in large or extra-large pore zeolite structures.5.The ECNU-20(IWR)germanosilicate,with a nanosheet aggregated hollow sphere morphology,was obtained using DBU as SDA.Based on the experimental data form TEM images and the simulation using Diffax method,the nanosheets was determined to be ~10-20 nm along c axis.Through theoretical calculation,DBU molecules preferred to interacted with the ab face with more silanol groups forming H-bonding,which largely decrease the surface free energy and then prohibit the crystal growth of ab face.On the other hand,the SEM images showed that the high-concertation SDA favored the formation of sphere morphology,which induced the reserved crystal growth mechanism and then the unique hollow spehere morphology.Compared to traditional Ti-containing IWR zeolites,Ti-ECNU-20 zeolite with nanosheets morphology showed higher activity in the epoxidation reactions.