In situ solid-state nuclear magnetic resonance studies of reactions on basic heterogeneous catalysts /
The reactions of alkyl halides, alcohols, and unsaturated
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1996.
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| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=739363421&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | The reactions of alkyl halides, alcohols, and unsaturated hydrocarbons on solid bases have been studied using in situ solid state NMR spectroscopy. Halide conversion to oligomeric hydrocarbon products in basic and bifunctional zeolites occurs at low temperatures, in contrast to solid acid catalytic processes. Base sites in zeolites were identified as the initial dehalogenation sites leading to framework-bound methoxyl formation. Methoxyls were subsequently methylated by reaction with methyl halides to form ethoxyls. Metal cations were proposed to stabilize the transient framework carbanion formed in this rate determining step. Complete reaction occurred below room temperature on zinc zeolites, in contrast to the methanol-to-gasoline reaction on acidic zeolites at 550 K. Framework- and surface- bound alkoxyl species are generated by chemisorption of alkyl halides in basic and bifunctional zeolites and of alcohols on cesium oxide. These species were identified in situ through NMR characterization techniques. The structure and reactivity of alkoxyls are proposed based on their chemical shift and chemical shift anisotropy characteristics. Magnetization transfer and dipolar dephasing experiments were used to aid in studying structure and dynamics in the catalytic system. Commercially-prepared cesium oxide was characterized using solid state NMR, Xray diffraction, EPR and elemental oxygen analyses. The "oxide" was composed of cesium cations plus peroxide and superoxide anions in an amorphous matrix. Only a small percentage of the 133CS (<10 %) was NMR observable at 298 K due to the intimate dispersion of the paramagnetic superoxide anion. This percentage improved dramatically either by heating the oxide in situ or by ex situ hydrogen or cesium reduction. Paramagnetic species became mobile above 400 K, as evident in both increased 133CS observation percentages (> 50 %) and rapid oxidation of hydrocarbon adsorbates. Hydrogen gas or cesium metal reduction also improved 113CS observation by reducing the superoxide anions to hydroxides or peroxides respectively. 133Cs and "C NMR were used to study interactions with weakly acidic adsorbates. Reaction with water and carbon dioxide produced hydroxides and carbonates consistent with acid/base chemistry. Methanol, acetylene, and isopropanol were deprotonated upon room temperature adsorption, demonstrating base site strength at low temperatures. The products of this interaction were surface-stabilized carbanion intermediates which were rapidly oxidized at 400 K. |
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| Item Description: | Vita. "Major Subject: Chemistry". |
| Physical Description: | xiv, 186 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilms Inc. |
| Bibliography: | Includes bibliographical references: pages 170-181. |