Solid State Nuclear Magnetic Resonance Study Of Al-Based Hydrotalcite Materials

Layered double hydroxide（LDH）has attracted the attention of many researchers due to its wide applications in catalysis,medicine and adsorption,and etc.Since these excellent properties are directly related to the arrangement order of metal cations in the LDH laminates,the local structure of the materials should be studied in depth first to achieve effective regulation of LDH materials.However,the characterization of LDH is challenging due to their low crystallinity,small particle size and easy stacking between layers.Solid state nuclear magnetic resonance（Solid state NMR）is an effective tool to detect local structures at the atomic level.Therefore,the work of this thesis mainly uses NMR as the core analysis technology,and combines with other traditional characterization techniques to conduct in-depth exploration of the local structure of LDH,and obtains some innovative results.It lays a theoretical foundation for the design and preparation of new functional materials based on LDH.The work of this thesis consists of two aspects:（1）The types of interlayer anions(CO₃^2-,Cl^-,NO₃^-)and synthesis methods（hydrothermal method,constant p H co-precipitation method）in Mg₂Al-LDH are changed,²⁷Al NMR and ¹H NMR are used to explore their effects on the local environment of Mg₂Al-LDH.The results show that two Al sites with different local environments are present in the laminate,and no Al-O-Al environment is detected.The Al-1 site represents a locally symmetrical and ordered Al environment,while the Al-2 site represents a locally disorder Al environment,which are related to the type of interlayer anion,crystallization temperature and Mg defect,and etc.The difference of isotropic chemical shift is about 2 ppm,and the difference of quadrupole coupling constant is about 2 MHz due to the difference of local environment between the two sites;By designing and adjusting the layer element types（M^Ⅱ:Mg,Zn,Cu）and composition ratios（M^Ⅱ/Al=0.5,1,2,3,4,5）of binary MAl-LDH,a series of Mg Al-LDH,Zn Al-LDH and Cu Al-LDH with carbonate intercalation were synthesized by co-precipitation method.The results of ²⁷Al NMR study indicate that the local Al-O-Al environment exists in the laminate when the composition ratio M^Ⅱ/Al<2（M^Ⅱ=Mg,Zn）.When M^Ⅱ/Al≥2（M^Ⅱ=Mg,Zn）and the elements in the laminate are the same,the change of M^Ⅱ/Al has little effect on the quadrupole parameters,however,the²⁷Al isotropic chemical shift is related to the electronegativity of M^Ⅱatom with different elements.The ²⁷Al NMR spectra of Cu Al-LDH are very special.Only when Cu^Ⅱ/Al≤2,the resonance peak can be observed in the spectra,and no Al-O-Al environment is detected in the laminate.Due to the Jahn-Teller effect of Cu²⁺,the isotropic chemical shift decreases gradually with the increase of Cu^Ⅱ/Al ratios.（2）By adjusting the element types（M=Zn,Cu,Ga）and composition ratios（Mg/Zn ratios,Mg/Cu ratios,Ga/Al ratios are 1,2,3,4,5;M^Ⅱ/M^Ⅲ=2）in ternary Mg MAl-LDH laminate,a series of Mg Zn Al-LDH,Mg Cu Al-LDH,Mg Ga Al-LDH materials were successfully prepared by constant p H co-precipitation method.The effect on LDH local environment is studied by NMR,and the results are analyzed in detail.²⁷Al NMR results show that all LDH laminates contain two Al sites with different local environments,and the variation of Mg/M^Ⅱratios（M^Ⅱ:Mg,Zn）has an effect on the local environment around the Al nuclear,however,both ²⁷Al NMR and ⁷¹Ga NMR show that the local structure changes are not caused by different Ga/Al ratios,and there is a correlation between the quadrupole parameters of ⁷¹Ga NMR and ²⁷Al NMR.In addition,¹H NMR shows that Mg,Zn and Al metal cations are distributed in order in Mg Zn Al-LDH laminate,and Mg Zn Al-OH environment is dominant,no Al-O-Al is detected.