Acidity Of H-ZSM-5 Zeolite As Studied By Two-dimensional Solid-state NMR Spectroscopy

Zeolites have been widely used in the petrochemical industry because of their high surface area,high hydrothermal stability,tunable acidic properties,and well-defined pore structure.In numerious acid-catalyzed reactions,the catalytic performance(reactivity and selectivity)of zeolites is known to be dictated by its acidic properties(viz.type,strength,distribution,and concentration).Understanding the acidic properties of zeolites is crucial to explore the catalytic mechanism and the structure-activity correlations.Acidity properties of solid acid catalysts including zeolites,metal oxides and heteropoly acid etc.have been extensively investigated by various analytical techniques,such as titration,thermal desorption,and FT-IR.In addition to the aforementioned techniques,one-dimensional(1D)solid-state NMR spectroscopy has been shown to be a powerful tool for acidity characterization with a suitable probe molecule containing the desirable NMR-sensitive nucleus,such as 1H,13C,15N,and 31P.Futhermore,high-resolution two-dimensional(2D)experiments were carried out to provide more detailed informations about interactions between different species,such as the interactions between zeolite and adsorbate,between two adsorbed species,and between different acid sites,which provide technical support for enabling the unambiguous discrimination of acid type,local structure and synergy effect of acid sites.In this dissertation,the detailed acid property of H-ZSM-5 zeolite was studied in depth by 2D ssNMR experiment,deepening the understanding of acid features of zeolites,the main results are listed below:(1)2D heteronuclear correlation ssNMR experiments of quadrupolar nuclei.We demonstrated a low power heteronuclear dipolar recoupling schemes to establish polarization transfers between the central transitions of two quadrupolar nuclei,such as 23Na-27A1 and 11B-27Al.Quantitative comparisons of sensitivity of different recoupling routes in D-HMQC and D-RINEPT experiments,demonstrated that the polarization transfer efficiency was strongly influenced by the quadrupole coupling constant and relaxation time.Moreover,concerning the low-power recoupling schemes,R211 has a higher efficiency than SR4 and R3(yy)during on-resonance,however,SR4 and R3(yy)both exhibit much broader recoupling bandwidths than R211,which is useful for structure determinations in applied systems containing inequivalent sites with different chemical shifts.(2)Tri-coordinated framework aluminum species and Br(?)nsted/Lewis acid synergy in H-ZSM-5 zeolite.The tri-coordinated framework aluminum(FAL)is usually considered as "NMR-invisible" species due to its extremely distorted local environment.We demonstrated that tri-coordinated FAL species can interact with the basic probe TMPO molecules to form distorted tetrahedral FAL site,which becomes NMR-visible.Three A1 species were well-resolved by 2D 27Al MQMAS NMR experiment on the TMPO/H-ZSM-5 sample,and 2D 31P{1H} CP-HETCOR NMR experiment was used for discriminating the acidic features.With the help of 2D 31P-27Al HMQC NMR experiment,two types of tri-coordinated FAL sites acting as Lewis acid sites have been unambiguously identified,which amount to 11.6%of the total acid sites.Moreover,the 31P-27A1 distances in the adsorption complex TMPO/H-ZSM-5 were measured by fitting the dephasing curve in 31P{27Al}RESPDOR NMR experiments,which are in good agreement with DFT calculation results.Finally,the Bronsted/Lewis acid synergistic effects arising from the close spatial proximity between the tri-coordinated FAL site and the Br(?)nsted acid site,leading to the generation of superacidity in the zeolite,was confirmed by 27A1-27A1 DQ-SQ NMR experiment and DFT calculations.(3)The adsorption behavior of TMPO probe molecules on H-ZSM-5 zeolites for acidity characterization.2D heteronuclear correlation NMR experiments demonstrated that the residual CH2Cl2 solven did not have any influence on acidity characterization.Moreover,the variations of 31P and 1H chemical shifts predicted by DFT calculations agree well with the upward parabolic characteristic as observed by 2D 31P{1H} CP-HETCOR NMR experiment,i.e.the 1H chemical shift decreased after increased with increasing acidic strength(the 31P chemical shift increases).The saturation adsorption behavior of TMPO was also explored by 2D 31P-31P DQ-SQ NMR experiments,indicating that two TMPO probe molecules adsorbed in 10-ring channel with close spatial proximity.To verify the adsorbate structure,DFT calculation was used to indentify the trend of 31P chemical shift in different models.The calcilation results indicated that two probe molecules are co-adsorbed on the same acid site of H-ZSM-5 zeolite.