Catalytic efficiency in zeolite-based reactions critically depends on molecular diffusion kinetics within the confined channels.Nanosized ZSM-5 zeolites with a reduced b-axis thickness shorten the diffusion path and m...Catalytic efficiency in zeolite-based reactions critically depends on molecular diffusion kinetics within the confined channels.Nanosized ZSM-5 zeolites with a reduced b-axis thickness shorten the diffusion path and maximize the pore-opening configuration of the straight channels,enhancing mass transfer to active sites.However,achieving precise control over such nanostructures remains challenging.Herein,we report a urea-nanoseed-assisted synthesis of ZSM-5 nanoplates with low b-axis thickness in a sustainable,fluoride-free system.Comprehensive optimization of the synthesis yielded uniform crystals with a short b-axis(30-80 nm),a-axis(75-115 nm),and c-axis(130-210 nm).Mechanistic studies revealed a reversed crystal growth pathway:crystallization initiates at amorphous aggregate surfaces and propagates inward,critically directed by the organic template in the presence of urea.In methanol-to-olefin(MTO)reactions,these nanoplates outperformed commercial ZSM-5,delivering a 30%higher propylene/ethylene ratio and doubling the catalyst’s lifespan.This work provides a rational strategy for synthesizing diffusion-optimized zeolites for applications where diffusion plays a critical role.展开更多
This study investigates the impact of selenium in different oxidation states,specifically Se^(6+)and Se^(4+),on the structural features of MFI-type zeolites.Se^(4+)is characterized by trigonal pyramidal coordination,f...This study investigates the impact of selenium in different oxidation states,specifically Se^(6+)and Se^(4+),on the structural features of MFI-type zeolites.Se^(4+)is characterized by trigonal pyramidal coordination,forming a[Se^(4+)O_(3)E]^(2-)entity(E being an active lone pair of electrons).The three-coordinated[Se^(4+)O_(3)E]^(2-)entity represents an“empty-corner tetrahedron”,analogous to four-coordinated[Se^(6+)O_(4)]^(2-)and[SiO_(4)]4−tetrahedra.Aiming to understand the impact of lone pair bearing complexes on MFI,Se^(4+)and Se^(6+)were introduced through a one-pot synthesis approach giving rise to Se(Ⅳ)-MFI and Se(Ⅵ)-MFI samples.Both the Se(Ⅳ)-MFI and Se(Ⅵ)-MFI samples exhibit monoclinic symmetry at room temperature,transitioning into orthorhombic symmetry at temperatures above 100℃ and reverting to monoclinic symmetry upon cooling.A significant difference was observed in terms of the silanol content,i.e.Se(Ⅳ)-MFI shows a lower concentration of SiOH compared to Se(Ⅵ)-MFI.These observations are coherently supported by ^(29)Si and ^(77)Se NMR,Se K-edge XAS,FT-IR,and Raman spectroscopy,suggesting that Se^(4+)oxycomplexes with a lone pair of electrons are more favorable for incorporation into the MFI framework compared to Se^(6+).^(77)Se NMR analyses revealed similar Se tetrahedral coordination in both Se(Ⅵ)-MFI and Se(Ⅳ)-MFI,indicating the stabilizing role i.e.oxidation of Se^(4+)to Se^(6+)directly during crystallization of MFI.Considering the differences in the silanol content and the similarities in Se coordination states in Se(Ⅳ)-MFI and Se(Ⅵ)-MFI,the transformation of“empty-corner tetrahedra”into regular tetrahedra i.e.selenite to selenate favors the introduction of Se in the MFI framework.These results provide valuable insights into the tunability of zeolite frameworks using selenium in different oxidation states.展开更多
基金support from National Natural Science Foundation of China(NSFC No.22408285)support from PetroChina(No.KYWX-21-021,KYWX-22-012)+2 种基金support from NSFC No.22178389 and 22472202co-funded by the European Union(ERC,ZEOLIghT,101054004)the Centre for Zeolites and Nanoporous Materials,Label of Excellence,Normandy Region(CLEARRN,00172938).
文摘Catalytic efficiency in zeolite-based reactions critically depends on molecular diffusion kinetics within the confined channels.Nanosized ZSM-5 zeolites with a reduced b-axis thickness shorten the diffusion path and maximize the pore-opening configuration of the straight channels,enhancing mass transfer to active sites.However,achieving precise control over such nanostructures remains challenging.Herein,we report a urea-nanoseed-assisted synthesis of ZSM-5 nanoplates with low b-axis thickness in a sustainable,fluoride-free system.Comprehensive optimization of the synthesis yielded uniform crystals with a short b-axis(30-80 nm),a-axis(75-115 nm),and c-axis(130-210 nm).Mechanistic studies revealed a reversed crystal growth pathway:crystallization initiates at amorphous aggregate surfaces and propagates inward,critically directed by the organic template in the presence of urea.In methanol-to-olefin(MTO)reactions,these nanoplates outperformed commercial ZSM-5,delivering a 30%higher propylene/ethylene ratio and doubling the catalyst’s lifespan.This work provides a rational strategy for synthesizing diffusion-optimized zeolites for applications where diffusion plays a critical role.
基金the European Union(ERC,ZEOLIghT,101054004)is acknowledgedCofunding from the Region of Normandy(CLEAR)for the Centre for Zeolites and Nanoporous Materials is acknowledged.
文摘This study investigates the impact of selenium in different oxidation states,specifically Se^(6+)and Se^(4+),on the structural features of MFI-type zeolites.Se^(4+)is characterized by trigonal pyramidal coordination,forming a[Se^(4+)O_(3)E]^(2-)entity(E being an active lone pair of electrons).The three-coordinated[Se^(4+)O_(3)E]^(2-)entity represents an“empty-corner tetrahedron”,analogous to four-coordinated[Se^(6+)O_(4)]^(2-)and[SiO_(4)]4−tetrahedra.Aiming to understand the impact of lone pair bearing complexes on MFI,Se^(4+)and Se^(6+)were introduced through a one-pot synthesis approach giving rise to Se(Ⅳ)-MFI and Se(Ⅵ)-MFI samples.Both the Se(Ⅳ)-MFI and Se(Ⅵ)-MFI samples exhibit monoclinic symmetry at room temperature,transitioning into orthorhombic symmetry at temperatures above 100℃ and reverting to monoclinic symmetry upon cooling.A significant difference was observed in terms of the silanol content,i.e.Se(Ⅳ)-MFI shows a lower concentration of SiOH compared to Se(Ⅵ)-MFI.These observations are coherently supported by ^(29)Si and ^(77)Se NMR,Se K-edge XAS,FT-IR,and Raman spectroscopy,suggesting that Se^(4+)oxycomplexes with a lone pair of electrons are more favorable for incorporation into the MFI framework compared to Se^(6+).^(77)Se NMR analyses revealed similar Se tetrahedral coordination in both Se(Ⅵ)-MFI and Se(Ⅳ)-MFI,indicating the stabilizing role i.e.oxidation of Se^(4+)to Se^(6+)directly during crystallization of MFI.Considering the differences in the silanol content and the similarities in Se coordination states in Se(Ⅳ)-MFI and Se(Ⅵ)-MFI,the transformation of“empty-corner tetrahedra”into regular tetrahedra i.e.selenite to selenate favors the introduction of Se in the MFI framework.These results provide valuable insights into the tunability of zeolite frameworks using selenium in different oxidation states.
