Solid state NMR study of methanol conversion reaction

Dr. Zhimin Yan (1999-2003)	Directed by Prof. Xinhe Bao and Prof. Xiuwen Han
Abstract	Present address: Department of Chemistry, University of Western Ontario, Canada

Solid state NMR can provide unique and important information on the structure of solid catalysts, and the configuration and motion of adsorbates as well as the sequence of reaction steps. Owning to the wealth of structural and dynamical information available via NMR, particularly NMR atomic specificity, high resolution, and quantitative capabilities, solid state NMR method has played a crucial role in the study of the heterogeneous catalysis. In this thesis, the reaction of methanol conversion into hydrocarbons on HZSM-5 and SAPO molecular sieves, which is of great importance to

industry, is selected as probe reaction to get deeper insights into the structure of solid catalysts and the interaction of reactants with catalysts during adsorption and catalytic reaction by solid state NMR.

1. The results taken from in situ MAS NMR support strongly the existence of the so-called carbon-pool in the conversion of methanol, which serves as the reaction precursor not only for the coupling of the species to form hydrocarbons, but also for uncontrolled polymerization to cause coke on the surface. The organic components of carbon-pool vary from saturated hydrocarbons to both saturated and unsaturated hydrocarbons with the progressing of MTH reaction. Meanwhile, the organic components of carbon-pool are found to be different on HZSM-5 and SAPO molecular sieves. The dynamic processes of Brönsted acid sites involved in the formation of intermediates are observed directly by 1H MAS NMR. The activation of the adsorbed methanol species on HZSM-5 results in the formation of various surface alkyoxy species with different rigid characters.
2. ¹H MAS NMR shows that the nanosized HZSM-5 has higher affinity to methanol than microsized HZSM-5. A large cluster of methanol adsorption complexes can be deduced. The acid strength of Brönsted sites in HZSM-5 is found to be stronger than that in SAPO-34 molecular sieve according to the ¹H NMR chemical shift of hydoxyl group of adsorbed methanol.
3. The lattice of SAPO molecular sieves is not rigid after template removal and are very sensitive to adsorbates even at room temperature. The calcination and presence of adsorbed phases could lead to rearrangement of Si species in framework, and reaction of MTH aggravates desilication of the SAPO molecular sieves framework. 2D ²⁷Al 3Q MAS NMR spectra is used to distinguish the various Al species and the Al environments with or without Si neighbors. A distorted 4-coordinated Al species is identified on the SAPO-34 sample after MTH reaction.
4. The dealumination of USY zeolites by nitric acid and oxalic acid treatment was systematically investigated by multinuclear solid-state NMR and MQ MAS NMR experiments.     There are five aluminum species clearly resolved in the parent USY zeolite: octahedral aluminum species within and outside the zeolite framework(Al^octF,Al^octNF) tetrahedral framework Al (Al^tetraF), distorted tetrahedral Al (Al^dis-tetra.F) and 5-coordinated non-framework Al (Al^pentNF).
       Under a much rigorous dealumination condition, even the tetrahedral framework aluminum is removed from the lattice, accompanying with the formation of tetrahedral non-framework aluminum (Al^tetraF). The different distribution of Al species in these samples accounts for the different catalytic performance of n-dodecane cracking.
5. The SAPO-34 with ultra-small size is synthesized by fast crystallization with proper Al source choice. It is found that the crystal size of SAPO-34 does not play significantly role on the selectivity of the light olefins, but on the product distribution of alkanes and higher olefins. Small-sized SAPO-34 shows high catalytic life due to its strong coke resistance.

	Investigation of the catalyst SAPO-34 upon exposure to 13C-methanol by 1H MAS NMR.