Theoretical Simulation And Solid-State Nuclear Magnetic Resonance Study Of The Framework Aluminum Distribution In Zeolites

Zeolites have been widely used in petroleum catalysis and other industrial applications due to their unique confined pores,accessible acid sites,good hydrothermal stability,and environmental friendliness.The location of framework Al atoms in zeolites as well as the confined pores of zeolites are key factors influencing its catalytic activity and selectivity.Therefore,the systematic investigation of framework Al distribution in zeolites and the microscopic revealing of the catalytic mechanism in the confined environment are helpful to deepen the understanding of the properties of zeolites,which is significant for the targeted regulation of acid site distribution of zeolites.In this thesis,the distribution pattern of framework Al atoms in zeolites affected by synthesis conditions such as organic structure-directing agent(OSDA)and the existing form of 2-norbrnyl carbocation(2NB+)in the confined zeolite pores have been investigated mainly by using periodic density functional theory,ab initio molecular dynamics simulations and solid-state NMR experiments,and some innovative results have been achieved,mainly as follows:(1)The influence mechanism of different OSD As on the framework Al location in ZSM-5 zeolite was investigated by periodic density functional theory.It was found that when tetrapropylammonium(TPA+)and tetraethylammonium(TEA+)cations with similar structures were used as OSDAs,their directed framework Al distributions were also similar,mainly at T1,T10,T11 and T12 sites.When 1,2,4-trimethylpyrazolium(124TMP+),which has a different structure from TPA+and TEA+,was used as OSD A to synthesize ZSM-5 zeolite,the T4 site was the most stable location.Further energy analysis showed that OSDA directed Al atoms to specific T sites mainly through electrostatic interaction.This result indicates that OSDA can effectively regulate the distribution of framework Al atoms in zeolites,which provides the theoretical guidance for the precise design of zeolites.It also demonstrates methodologically that it is necessary to take account of the synthesis conditions(OSDA)for the accurate prediction of Al location in the calculation of the framework Al distribution in zeolites.(2)The impacts of different synthesis conditions(e.g.,OSDA,Na+and H2O)on the framework Al distribution in SSZ-13 zeolite was systematically investigated by periodic density functional theory(DFT).It was found that when the synthesis conditions were not considered,the framework Al distribution in zeolites violated the Lowenstein's rule,i.e.,the conformation containing-Al-O-Al-was more stable;however,when the effects of OSDA,inorganic Na+ and solvent H2O molecules were taken into account,the structure that obeys Lowenstein's rule is more stable.The above results indicate that OSDA can direct the location of Al atoms into zeolite frameworks.Additionally,by combining the ab initio molecular dynamics(AIMD)simulation as well as the directing probability,the directing effect of OSDA on framework Al atoms was able to be quantified and visualized,which expands the research method of framework Al distribution and enriches the knowledge of the distribution pattern of OSDA-guided framework Al atoms.(3)In addition to the location of individual Al atoms,the spatial relationship between framework Al atoms in zeolites is of great significance for the study of the synergistic catalysis mechanism of multiple acid sites.Firstly,1H-1H DQ NMR experiments were used to investigate the correlation between Br(?)nsted acid(-Al-O(H)-Si-)in zeolites to illustrate the proximity of Al atoms.The Br(?)nsted acidic protons were observed in the ZSM-5 zeolite(Si/Al=150)with low Al content,indicating the spatial proximity between acid sites,which indirectly illustrated the existence of enrichment of framework Al atoms in zeolites.Subsequently,the high-throughput theoretical calculation results also demonstrate that the configuration of framework Al atoms closer to each other is more stable,which also proves the enrichment behavior of framework Al atoms in zeolites from the thermodynamic point of view.Finally,the stability of zeolite trapped with the coexistence of OSDA and H2O was investigated by the AIMD simulation,which further demonstrated the possibility of enrichment of framework Al atoms in zeolites.(4)Based on the determination of the framework Al atom and Bronsted acid proton locations in zeolites,the formation and transformation mechanism of carbocation at the Br(?)nsted acid sites inside confined zeolite pores were further investigated.Carbocations are important intermediates in the catalytic transformation of hydrocarbons,its characterization has always been a challenge by theoretical and experimental investigations.In this thesis,we first determined the bond distance criteria for distinguishing the classical and non-classical 2-norbornyl cation(2NB+)by calculating the electronic structure of 2NB+.Subsequently,the dynamic changes of 2NB+in the confined ZSM-5 channels were traced by AIMD simulations at room temperature,and the statistical results show that 2NB+mainly exists in non-classical form.Next,the reaction process of 2-norbornene with H-ZSM-5 zeolite was further investigated by solid-state NMR method,and confirmed 2NB+exists as the non-classical form in ZSM-5 zeolite.Finally,the host-guest interaction analysis showed that the confined pores of ZSM-5 zeolite stabilize non-classical 2NB+ by electrostatic interaction and van der Waals interaction.The findings of this work provide experimental and theoretical guidance for the capture and characterization of other highly active species.