Applying solid-state nuclear magnetic resonance techniques to characterize heterogenous catalysts or related complex materials

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Wang, Zhuoran
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Marek . Pruski
Aaron D. Sadow
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In general, solid-state nuclear magnetic resonance (SSNMR) spectroscopy can provide much more detailed information about atomic-level structures and dynamics compared to solution-state NMR spectroscopy, since various anisotropic spin interactions present in the solid state offer invaluable insights into the structural and dynamic properties of materials. However, these orientation-dependent spin interactions also result in the broadening of resonance lines, which obfuscates the interpretation of SSNMR spectra. In order to obtain high-resolution spectra, modern SSNMR experiments are often performed under magic-angle spinning (MAS). This mechanical rotation makes the anisotropic spin interactions time-dependent. Because the appearance of SSNMR spectra is determined by these interactions, understanding how these interactions change with time under MAS becomes a key step for developing and applying SSNMR techniques. Therefore, in Chapter 1, the general methods to treat rotations of spin interaction Hamiltonians and the important results are reviewed.

The spin dynamics can be also manipulated by radio-frequency (RF) pulse sequences. Therefore, analyzing the spin dynamics under different pulse sequences becomes another key step to understanding SSNMR spectroscopy. There are two general approaches for this task, relying on analytical approximation theories and numerical simulations. In Chapter 2, we introduce the average Hamiltonian theory (AHT), which is the most useful analytical theory, and SIMPSON, which is the most widely used simulation package. Then, the specific pulse sequence for selective homonuclear dipolar recoupling is analyzed, using both AHT and SIMPSON simulations as an example.

As shown in Chapter 2, we show how distances between nuclei of the same type can be accurately measured by recoupling of homonuclear dipolar interactions. Similarly, the distance information can be obtained for heteronuclear spin pairs by recoupling heteronuclear dipolar interactions. Such distance measurements can provide valuable constraints for deriving atomic- or molecular-level structures. In Chapter 3, the cross polarization with variable contact (CPVC) experiment under fast or ultrafast MAS, which has been recently proposed as another approach for heteronuclear distance measurement, is analyzed both theoretically and experimentally. From the results, the experiment set-up procedure is recommended and the estimates of measurement accuracy limitations are given. Lastly, this technique is further extended to enable determination of the distribution of internuclear distances resulting from structural disorder.

Intrinsically, NMR spectroscopy suffers from low sensitivity, because the magnitudes of spin magnetic moments are very small. The sensitivity becomes further diminished when detecting nuclei with long spin-lattice relaxation time, low gyromagnetic ratio, low natural abundance and/or low concentration. To overcome this problem, several general methods have been developed, including paramagnetic doping, isotope enrichment, indirect detection and hyperpolarization, e.g., by dynamic nuclear polarization (DNP). With the advent of low-temperature fast MAS technology, it becomes possible, for the first time, to perform indirect detection experiment under DNP condition. In Chapter 4, the sensitivity gain resulting from the combined use of indirect detection and DNP is examined for 13C, 15N and 89Y nuclei in functionalized mesoporous silica nanoparticles (MSNs) and Y2O3 nanoparticles. It is found that for nuclei with very low gyromagnetic ratio, this combined method provides sensitivity that exceeds all existing SSNMR approaches, allowing the acquisition of high-quality two-dimensional 1H-89Y correlation spectra of the surface of Y2O3 nanoparticles in less than 2 h.

In Chapter 5, the application of SSNMR spectroscopy to characterizing a potential supporting material for heterogeneous catalysts, ordered mesoporous carbon (OMC), is presented. In this study, 13C cross polarization magic angle spinning (CPMAS) and quantitative direct polarization magic angle spinning (DPMAS) experiments were firstly performed on natural abundance and isotope enriched OMCs. From the results, the key structure components of OMCs calcined at different temperatures were identified. Meanwhile, the evolution trends of these components at different calcination stages were also deduced. Then, the dipolar dephasing experiments, based on differences in spin-spin relaxation time constants (T_2^'), were performed to determine the sizes and structures of aromatic clusters that comprise the OMC structures at different stages of calcination. Based on the spectroscopic data, we were able to determine the temperature-induced structural evolution of the OMC framework and provide basis for further tailoring of these materials for applications in heterogenous catalysts.

In Chapter 6, multinuclear and multidimensional SSNMR spectroscopy was applied to thoroughly characterize an organometallic heterogenous catalyst, La{C(SiHMe2)3}n@MSN, which is synthesized by grafting homoleptic tris(alkyl)lanthanum La{C(SiHMe2)3}3 onto the surface of MSN and used for catalyzing the ring-opening hydroboration reactions of aliphatic and styrenic epoxides with pinacolborane (HBpin). The SSNMR experiments included 13C CPMAS, 29Si DPMAS and CPMAS, 1H{29Si} indirectly detected heteronuclear correlation (idHETCOR) spectroscopy, 29Si J-resolved spectroscopy, 11B DPMAS, 11B multiple-quantum magic angle spinning (MQMAS) and 1H{11B} HETCOR spectroscopy. Based on the results, firstly, we determined the podality of the surface bonded organometallic complex and helped to determine the optimal synthesis conditions. Then, the basic structures of the surface species and the detailed nature of the secondary La ↼ H–Si interaction were established. Lastly, the structure of the active catalytic center was identified, which suggested that the catalytic mechanism is the same as the one previously proposed for the corresponding homoleptic precursor. Notably, the La{C(SiHMe2)3}n@MSN catalyst exceeded its soluble precursor in terms of per-site catalytic activity, selectivity and recyclability.

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