| Zeolite is one of the most important solid catalysts and it is widely used in heterogeneous catalysis, such as petroleumchemical, pharmaceuticals and fine chemicals industry.Lower alkanes (such as methane and ethane) have abundant reserves in nature. As a result of oil crisis, they may be used as chemical feed stocks instead of petroleum and coal to satisfy the increasing energy demands of modern society. Metal modified zeolite catalysts exhibit high reactivity and selectivity in lower alkane’s activation and conversion. Among them, Zn modified zeolites are widely investigated. Although it was found that lower alkane aromatization and the activation and conversion of methane can be realized over Zn modified zeolites, the structure of active centers and the reaction mechanisms are still poorly unstood, which might restrict to some extent the development of highly active catalysts for lower alkane’s activation.Herein, we studied the active centers on Zn modified ZSM-5catalysts, and the structure-property relationship by using in situ solid-state NMR spectroscopy in combination with IR nd ESR spectroscopy. The main content of this thesis consists of three parts:(1) The low-temperature reactivity of Zn+ions confined in ZSM-5zeolite towards CO oxidation was studied. In this work, we prepared a Zn+containing ZnZSM-5catalyst by reacting of Zn vapor with H-ZSM-5zeolite. The ZnZSM-5catalyst can catalyze CO oxidation at room temperature. The CO conversion yield was65%at298K, and it grew up to98%at310K. The reaction mechanism was identified by in situ ESR spectroscopy:molecular O2is readily activated by Zn+ions at room temperature, leading to formation of two types of reactive O2species and Zn2+ions. The active O2-species oxidizes CO into CO2, and the Zn+ions are simultaneously recovered from the Zn2+ions. The reversible electron transfer between molecular O2and Zn+ion indicates a complete catalytic cycle for CO oxidation. The present findings provide valuable insights into the mechanism of CO oxidation at low temperature, also be helpful for rational design of new non-precious metal-based zeolite catalysts for O2activation and related oxidation reactions via modification of their electronic property.(2) Room temperature stable carbonyl zinc complex formed in zeolite ZSM-5 and its hydrogenation reactivity were studied. We reported room temperature observation of stable carbonyl complexes by adsorption of CO on the ZnZSM-5zeolite catalyst. In addition to monocarbonyl zinc complex, the dicarbonyl zinc complex was identified. By measuring the C-C bond distance, the geometric structure of the dicarbonyl zinc complex was determined. The reactivity of the carbonyls was manifested by a stepwise hydrogenation reaction with the formation of different hydrogenated products (such as methanol, dimethyl ether and methane). The results presented here might be helpful for a better understanding of the nature of zinc carbonyls related to heterogeneous catalysis and enrich our knowledge about the metal carbonyls chemistry.(3) The catalytic active centers for methane H/D exchange reaction on Zn modified ZSM-5zeolite were studied. We prepared three kinds of67Zn modified ZSM-5zeolites, one of them is G2-ZSM-5which was prepared by grinding method. The other two are I2-ZSM-5and I6-ZSM-5prepared by impregnated method. For comparison, the Zn natural abundance Ex-ZSM-5sample was prepared by ion exchange method. The reaction rates of methane H/D exchange on these Zn modified ZSM-5zeolites were studied by in-situ1H MAS NMR spectroscopy. We found that the Ex-ZSM-5sample exhibits the highest reactivity; I2-ZSM-5and I6-ZSM-5samples have similar reactivity which is higher than that of G2-ZSM-5sample. The the spatial relationships and interactions between Br(?)nsted acid sites and Zn species inside ZSM-5pores were investigated by1H/67Zn TRAPDOR NMR experiments. It was demonstrated that the methane H/D exchange reaction was catalyzed by the Br(?)nsted acid sites in close proximity to Zn2+species on ZSM-5framework. |
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