A Density Functional Theory Study On The Structure And Acidity Of The Interlayer-expanded Zeolite COE-4

The interlayer-expanded zeolite COE-4possesses a unique two-dimensional10-MR pore structure with the Si(OH)2hydroxyl groups attached to the linker position between the FER-type layers, showing an excellent physicochemical properties and functions. Isomorphous substitution of the Si atom with a heteroatom would open up a new method to the structural and functional modifications for the-created acidity and oxidation of T-COE-4zeolites. In this thesis, the structre stability and acidity of T-COE-4zeolites were investigated in details by DFT calculations. The main findings are summarized as follows:1. The density functional theory (DFT) method was used to study the framework stability and Bransted acidity of Fe-, B-, Ga-or Al-doped T-COE-4zeolites, in which the tetravalent Si was isomorphously substituted by the trivalent heteroatom at the linker position. It is found that the range of the T-O-Si angle was widened to maintain the structure stability of isomorphously substituted zeolites T-COE-4. The smaller the O1-T-O2bond angle was, the more difficult was to form the regular tetrahedral unit. Thus the substitution energies at the linker positions increased in the following sequence:Al-COE-4<Ga-COE-4<Fe-COE-4<B-COE-4. The adsorption of NH3as a probe molecule indicates that the Bransted acid strength could affect the hydrogen-bonding interaction between (N-H…O2) and (N…H-O2), which could play an important part in the stability of the NH3adsorption complexes. The relative Bransted acid strength of the interlayer-expanded T-COE-4zeolites decreased in the order:Al-COE-4> Ga-COE-4> Fe-COE-4> B-COE-4. Moreover, the interlayer distance upon incorporation of a boron center instead of a silicon center might shrink significantly or even lead to the collapse of the framework structure with the compensating proton.2. DFT method was used to study the structure stability and Brensted acidity of Ti-, Sn-or Zr-doped interlayer-expanded zeolites T-COE-4. It is found that the substitution energies of Ti-, Sn-or Zr-inserted at the linker site could increase thermodynamically in the sequence of Ti-COE-4<Sn-COE-4<Zr-COE-4. The range of the T-O-Si angle was widened to maintain the framework stability of Ti-, Sn-or Zr-substituted zeolites T-COE-4. The interlayer distance Si2-…Si3between the layers was not increased remarkably for Ti-, Sn-or Zr-dopeed zeolites T-COE-4. The incorporation of Ti-, Sn-, or Zr-hetroatoms into T-COE-4zeolites could produce the Bransted acidity, which could decrease in the order of Sn-COE-4> Zr-COE-4> Ti-COE-4. Moreover, NH3molecule was stabilized on the surface of T-COE-4zeolites by the physical-adsorption manner, to form the (N…H-O2) hydrogen bond for the NH3adsorption complexes over the Bronsted acid site.3. DFT and the dispersion corrected DFT-D3methods were used to investigate the relative Lewis acid strength of Ti(Ⅳ), Sn(Ⅳ), or Zr(Ⅳ)-doped interlayer-expanded T-COE-4zeolites. The computed Fukui function values based on the Mulliken population analysis as well as NH3, C5H5N, H2O and (CH3)3PO adsorptions were used to predict the Lewis acid strength in the increasing sequence:Ti-COE-4<Zr-COE-4<Sn-COE-4. In general, the interaction energies demonstrated that the sequence of stability of four different T-COE-4-L complexes were (CH3)3PO> C5H5N> NH3> H2O. In addition, the six O-T-O bond angles were divided into different extent to form the analogous trigonal bipyramid structures for the optimized ligand adsorbed-complexes. The bond length variations of T-COE-4-L fell in limited range, whereas bond angles could occur to different disturbed deviations.