Solid-State NMR Studies Of Layered Double Hydroxides

Layered double hydroxides(LDH)are very important materials with widespread application potential in catalysis,ion exchange,CO2 adsorption,flame retardation and drug delivery,therefore they have attracted much research attention.Rational design of LDH with desired properties requires detailed information on the local structure,for example,the cation ordering in the layer,the interactions between the cations in the layer and anions in the interlayer region,the evolution of LDH to layered double oxides(LDO)during calcination as well as how the structure of LDH can be regenerated from LDO(memory effects).Solid-state NMR(SSNMR)is a powerful tool with which to probe the local structure of materiasl on molecular/atomic scale.In this dissertation,we applied SSNMR techniques,in combination of a variety of other characterization methods,to study LDH and related LDO and extract important structural information.The results provide new insight into the applications of LDH.1.By decreasing 1H-1H homonuclear dipolar coupling with deuteration process,high resolution 1H NMR spectra of Mg-Al LDH were obtaind at low spinning rate for the first time.Three different hydroxyl groups were quantified and it is confirmed that there is no Al-O-Al linkages and non random distribution of cations in the layer.The superlattice structure of LDH was investigated by double resonance NMR techniques for the first time and the results provide a new approach to the cation ordering and interactions in LDH.2.The structure evolution during the calcination process of LDH was investigated by 1H NMR and FTIR on deuterated samples.The results show that three processes occur in the following order:dehydration of interlayer water,dehyroxylation of Mg2Al-OH and dehyroxylation of Mg3-OH.The dehydration temperature is mainly controlled by the interlayer distance,while interaction between the anions and water molecules in the interlayer is also an important factor.The dehyroxylation temperature is directly related to the interactions between the anions and the hydroxyl groups in the layer:the stronger the interaction,the higher the dehydroxylation temperature.3.The structure change during the reconstruction of LDH from LDO was investigated with a variety of characterization methods on Mg-Al-LDH-CO3.Detailed structure information in the reconstruction process was obtained and a mechanism is proposed.4.Owing to the small dipolar interaction between the unpaired electrons and low gyromagnetic ratio nucleus 2H,as well as large Fermi contact shift from paramagnetism,high resolution 2H NMR data were obtained from Ni-Al-LDH for the first time.Spectral assignments were made according to the data obtained with Ni-Al-LDH with different Al molar percentages.These results provide a new method for study the cation ordering,as well as the structure and motion of hydroxyl groups in paramagnetic LDH.