Locations And Adsorption Mutagenesis Mechanism Of Acid Sites Inside The Confined Channels Of Zeolites

Zeolite as one of highly important heterogeneous catalysts,have been widely applied in many industry reactions like petrochemical engineering,biomedicine and environmental protection.In order to further improve the catalytic activity,in-depth studies of zeolites including their initial synthesis and post treatments have been conducted by the researchers from all over the world during the past decades.Although there have been a large number of fundamental researches on the overall framework types,pore structures and element composition of zeolites,and many insightful results have been obtained,the comprehensive understanding of some key points in zeolites such as the local structure of acid center,the distribution of acid sites and the interaction between reactants and active site,are still under debate.And these problems play a significant impact on how to improve the catalytic activity and how to understand the catalytic reaction mechanism of zeolites.Therefore,such important scientific problems as mentioned above are needed to be solved urgently in the basic research of zeolites.In this thesis,solid-state nuclear magnetic resonance(SSNMR)technology combined with synchrotron X-ray diffraction(SXRD),neutron powder diffraction(NPD)and other advanced characterization techniques and Density Function Theory(DFT)calculations were adopted to characterize the acidity properties of commonly used zeolites like H-ZSM-5 and H-SAPO-34.The main content consists the following three parts:(1)Due to the limitation of current characterization techniques,direct three-dimensional visualization and quantitative investigation of the distribution of Bronsted acid site(BAS)in zeolites has long been a challenging task.In the first part of this thesis,TMPO as the probe molecule was adopted to determine the stable and preferential locations of BAS in the H-ZSM-5 zeolites.By using SXRD and NPD techniques coupled with structural refinements,the accurate adsorption configurations of TMPO including the bond length and bond angle in H-ZSM-5 were visually confirmed.Up to three TMPO molecules adsorbed on different BAS were observed,one adsorbed at the T8(T8-O(H)-T7)site in the straight channel and other two adsorbed on the T12(T12-O(H)-T3)and T6(T6-O(H)-T9)sites in the intersection void,respectively.In addition,similar results were also observed in other H-ZSM-5 zeolites with different Si/Al ratio.It should be noted that our experimental analysis methods are not only applicable to H-ZSM-5 zeolite,but can also be extended to other types of zeolites,which provides an intensive understanding of the relationship between catalytic behaviors and the location of BASs in zeolites.(2)Given that the accurate positions of BAS in H-ZSM-5 zeolites were confirmed,in the second part,the solvent effect originated from the interaction between the adsorbed species and the acid sites inside the nanocage of zeolite was investigated.Two types of solvent effect namely polar and nonpolar solvent effect were both demonstrated by using SSNMR experiments and DFT calculations.As for the polar solvent effect,acetone and nitromethane were adopted as NMR probe and the polar solvent molecule,respectively.13C SSNMR results of acetone before and after coadsorbed with nitromethane on H-ZSM-5 show that the apparent acid strength of H-ZSM-5 was significantly enhanced with the coadsorption of nitromethane(the greater the 13C chemical shift,the stronger the apparent acidity of zeolite),and such effect of can be further enhanced with the increasing of nitromethane concentration.Theoretical calculations suggest that the electrostatic interaction between the nitro group in nitromethane and the methyl group of acetone affects the charge distribution of the acetone molecule,resulting in the increasing of 13C chemical shift.In addition,the non-polar solvation effect on the H-ZSM-5 was also studied.Based on the SSNMR data,similar results of apparent acidity enhancement in H-ZSM-5 was observed upon the coadsorption of acetone and non-polar solvent molecule such as naphthalene and benzene,in which there exist large nucleophilic ? bond moiety.Theoretical calculations show that the electrostatic interaction between the large ? bond in the naphthalene/benzene molecule and the acetone molecule(CH/? interaction)promotes the basicity of carbonyl group,resulting the departure of acidic proton from the zeolite framework.(3)In the third part,the interaction between the adsorbed species/reacting molecules and active site of zeolites was studied to reveal the dynamic changes of the active site structure under the reaction conditions.We found that BAS in SAPO zeolites can be induced into a Frustrated Lewis pair(FLP)active site under the adsorption of guest molecules like acetone and methanol.By using SSNMR,SXRD,NPD,In-situ DRIFT and DFT calculations,it is confirmed that there are two types of adsorption states,apart from the BAS adsorption,guest molecules can also directly coordinate to the framework Al atom of BAS to induce the cleavage of Al-O bond,reaching a FLP state(the skeleton Al atom serves as a Lewis acid,and the Si-OH part serves as a Lewis base).It is the new adsorption induced FLP site that promotes the dehydration of methanol to the surface methoxy on SAPO zeolites under room temperature.In-situ DRIFT results of 18O/16O-labeled methanol demonstrate that the C-O bond of methanol is not broken,and the O of produced water comes from the framework Si-OH group instead of methanol molecules under the traditional view.Theoretical calculations also suggest that the BAS in SAPO zeolites can be induced into FLP by adsorption of small polar molecules.Our unprecedented finding opens up a new avenue to understanding of the dynamic establishment of active sites for adsorption or chemical reactions under molecular bombardments to zeolitic structures.