Application Of Paramagnetic Relaxation Enhancement Solid-state NMR To The Heterogeneous Catalysis Of Zeolites

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy can provide a unique insight into the structure of heterogeneous catalysts and a detailed description of the related catalytic reaction mechanism. Paramagnetic relaxation enhancement (PRE) refers to the acceleration of relaxation rates resulting from the dipole-dipole interactions between unpaired electrons and the surrounding nuclei. PRE is a well-established tool for investigating the structure and dynamics of biological proteins and functional materials. The introduce of paramagnetic metal ions into heterogeneous solid catalysts not only can act as active reaction center, but also serve as a probe to study the structure-property relationship of heterogeneous catalysts from a new view.In this dissertation, solid-state NMR in combination with PRE was introduced to explore the detailed structrual information about the active centers and elucidate the catalytic reaction mechnism in heterogeneous catalytic systems. Moreover, versatile solid-state NMR techniques were employed to determine the interaction between histidine and Zn(II) over a wide pH range. These work might provide basis and new insights into the structure-property relationship in complicated systems containing paramagnetic metal ions.i) We proposed a new technique via PRE MAS NMR to monitor catalytic reaction processes using natural abundance reactants. After introducing paramagnetic Cu(II) ions into the HY zeolite, the relaxation rates for both the framework 29Si sites and the adsorbed reactants were significantly enhanced, enabling the fast detection of NMR signals with short recycle delay for signals accumulation. The application of in-situ PRE MAS NMR techniques to elucidate the heterogeneous catalytic reaction pathway using nature abundance reactants not only can save cost for purchasing the 13C isotropic enriched samples, but also is promising to shorten the NMR spectrometer time.ii) PRE solid-state NMR was further used to monitor the valence state alternation of copper species doped in HY zeolite during catalytic reaction process. The valence state variation of copper species was strongly correlated to the change of longitudinal relaxation rates of framework 29Si nuclei as well as the reactants loaded onto the catalysts during the oxidation of trimethylphosphine (TMP) and CO over Cu-HY zeolite. The combination of PRE and in-situ NMR spectroscopy facilitates the detection of copper species serving as active center as well as monitoring of evolution from reactants, intermediates to products in heterogeneously catalyzed process, which is of great importance for elucidating the detailed catalytic reaction mechanism,iii) Solid state NMR was employed to investigate the carbonylation of methanol with CO over Cu-H-MOR in comparison with H-MOR zeolite. It was found that the carbonylation performance over Cu-H-MOR zeolite was much better than that of Cu-free H-MOR zeolite. The high reactivity on Cu-H-MOR zeolite was ascribed to the presence of Cu(I), which can efficiently stabilize the intermediate product DME and suppress the unwanted coke formation. The tightly adsorbed CO on Cu(I)ions was not the active species in carbonylation reaction, while it suppressed the adsorption of DME on Cu(I) ions, consequently resulting in the formation of hydrocarbon. Our works might provide new insights into the catalytic reaction pathway for carbonylation of methanol with CO over Cu-H-MOR.iv) Multi-nuclear and multi-dimensional solid-state NMR techniques were utilized to provide a detailed description about the interaction between histidine and Zn(Ⅱ) from pH 3.5 to 14. The interactions between histidine and metal species play essential roles in a wide range of important biological processes including enzymes catalysis and signal transduction.2D homo-and hetero-nuclear correlation NMR experiments were used to extract the 1H,13C,15N chemical shifts in various histidine-Zn(II) binding complexes. Several histidine-Zn(II) binding models were proposed on the basis of experimental results as well as DFT theoretical calculations. All these findings give a comprehensive set of benchmark values for NMR parameters and structural geometries in variable histidine-Zn(II) binding complexes over a wide pH range and might provide insights into the structure-property relationship of histidine-metal complexes in biological metalloproteins.