A Study On Structural Reconstruction Mechanism For Germanosilicates

Based upon the formation of siliceous D4R units is kinetically and thermodynamically unfeasible in hydrothermal system and surrounding two main drawbacks for the practical use of Ge containing zeolites,the hydrothermal instability and the expensive price of germanium,this paper designed firstly a versatile crystallization-disassembly-dissolution-recrystallization(CDDR)route for germanosilicates and obtained three different kinds of high-silica zeolites with high hydrothermal stabilities and unexpected crystalline topologies,ECNU-16-RC,IM-20-RC and IM-12-RC.Meanwhile,the heteroatom could be incorporated readily into the framework during CDDR-recrystallization,and formed the active center of catalytic reaction.As follows:In the first part,starting from typical Ge-rich germanosilicates(ECNU-16(UOS),IM-20(UWY),IM-12(UTL)),we successfully removed and recycled the noble germanium sources by utilizing the hydrolysis of T-O-Ge(T = Si,Ge)bonds in acid or water solution.The structures of zeolites obtained through hydrothermal recrystallization process were transferred the low-silica instability into high-silica stability.Various available techniques including 19F NMR,TGVV UV-Raman and ICP were used to clarify the mechanism of synthesizing high-silica zeolite in recrystallization process.The results showed the presence of F-,SBUs,OSDA and less Ge species played an important role in achieving highly crystalline high-silica zeolites.The explored CDDR technique is believed to open a general route for synthesizing practically useful zeolite catalysts and meanwhile resolve favorably the two main drawbacks.In the second part,based upon the above mentioned CDDR mechanism,by choosing IM-20 zeolite as a representative example,we incorporated successfully readily the aluminium into the framework by adding Al source,and Al-IM-20-RC zeolite was obtained,which involving different Si/Al ratios and possessing high crystallinity and stability as well as well-defined pore systems.In comparison to Beta and ZSM-5 zeolite catalysts,Al-containing IM-20-RC,with an open pore system,exhibited higher catalytic activities for the alcohols with different molecular lengths in the reaction of protecting hydroxyl groups,i.e.tetrahydropyranylation of alcohols.