Design And Synthesis Of Novel Porous Materials Based On Structural Modifiability Of Germanosilicates

Germanosilicates have been successfully synthesized in the SiO₂/GeO₂ system benefiting from the longer bond distance and smaller angle of Ge-O-Ge,which facilitate the formation of the secondary building units in the framework such as double four ring?D4R?and/or double three ring?D3R?cages.However,the Ge-zeolites are problematic because structural degradation takes place readily owing to the easy hydrolysis of Ge-O bonds,even by moisture under environmental conditions.Taking the advantage of their structural instability,in this thesis,we employed two kinds of germanosilicates UTL and CIT-13 as the research objects with the purpose to innovatively synthesize novel zeolites with various topologies and catalytic species.Meanwhile,those resultant materials exhibited promising catalytic performance in the particular reaction.The results obtained are as follows:In the first part,the crystalline structure of UTL zeolite experienced an unusual orientated collapse and reconstruction within an extremely narrow time window during the structural stabilization process by nitric acid treatment at elevated temperature.Taking full advantage of this unique structural change,extra large-pore Sn-UTL zeolites,hardly hydrothermally synthesized,were readily post-synthesized with tetrahedrally coordinated Sn ions in the framework.At the same time,the isomophous incroporation of Sn species did not prevent the removal of Ge species.During the whole process,ca.90%of Ge atoms were removed from the UTL precursor,which greatly increased the hydrothermal stability of Sn-UTL zeolite.With intersecting 12-and 14-membered ring pore channels,Sn-UTL showed promising catalytic performance in the Baeyer-Villiger oxidation reactions of ketones as well as in the Meerwein-Pondorf-Verley reaction.In the second part,the calcined UTL sample easily underwent an irreversible structural rearrangement,incapable of recovering the original UTL topology by nitric acid treatment at elevated temperature.However,several structural derivatives with continuously tunable pore sizes were obtained by precisely controlling the acid treatment process.Isomorphous incorporation of Ti active centers and structural reorganization were then simultaneously realized using hexafluorotitanic acid?H₂TiF₆?aqueous solution,giving rise to two titanosilicates of Ti-ECNU-14 and Ti-ECNU-15with different pore sizes varying form medium pores?10-ring?to extra-large pores?14-ring?.Similarly,the extra-large pore Ti-UTL with high Ti content of tertrahedrally coordinated Ti ions in framework also has been post-synthesized using stabilized UTL as precursor via H₂TiF₆ treatment.Compared with traditional titanosilicates,the extra large-pores of Ti-ECNU-15 and Ti-UTL favored the diffusion of bulky substrate molecules and oxidants,improving the accessibility of reacting with Ti active sites inside the pore channels.Especially,they showed a superior conversion in the epoxidation of cyclohexene.In the third part,in view of the failure to hydrolyze the metastable Si-O-Si interlayer bonds vertical to the layers in D4R units when using the traditional acidic-assisted assembly-disassembly-organisation-reassembly route,we developed a new top-down strategy that the hydrolysis in ammonia aqueous solution not only disconnected readily the chemically weak Ge?Si?-O-Ge bonds located within interlayer double four ring units but also cleaved the metastable Si-O-Si bonds therein.The different structural changes of CIT-13 under acidic media and alkaline media have been investigated in detail.Combining the literature description about the structural modification of UTL,we concluded that the germanosilicates encountered a successive structural degradation and reconstruction process in acid solution,whereas zeolites underwent a controllable degradation assembly process using ammonia aqueous solution.Moreover,several novel daughter zeolites?ECNU-n,n=18,21,22,23?were obtained by optimizing alkali treatment conditions.Structure analysis demonstrated individual single four ring?S4R?units in reassembled ECNU-18 replace the Ge-rich D4R units in CIT-13.The corresponding high-resolution transmission electron microscopy?HRTEM?image has been taken for further confirmation of the structure,which allows the observation of well-ordered 10-R channels and disordered8-R channels in the intergrowth sample.Additionally,Al-ECNU-18 was prepared using calcined Al-CIT-13 as the precursor with the same process and the catalytic property in the m-xylene isomerization/disproportionation reaction is in accord with the 12×8-R ideal structure.In the forth part,the random or irregular location of D4R subunits along c direction induced the stacking faults in the germanosilicate CIT-13.Hence,the disordered phenomenon is possibly eliminated after fully removing the interlayer D4R units,realizing the topotactic conversion from intergrowth zeolite into single crystalline one.We continued the work of third part and ECNU-21 was deemed to lose the D4R units extensively.According to the HRTEM images and refined structure,there is no disorder in the projection along b-axis and ECNU-21 comprises only O-linkages between exactly the same layers as the parent CIT-13 zeolite.With Ge-related moderate Lewis acidity confined in ordered 10-R channels,ECNU-21served as a promising selective catalyst in the ethylene oxide?EO?hydration reaction with the EO conversion 97%and ethylene glycol?EG?selectivity 96%under extremely mild conditions,which would provide new idea to solve the problems of low EG selectivity and high energy consumption encounted in the non-catalytic industrial process.