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32
direct observation is unlikely. Most experimental evidence in favor of these
species was from carbon labeling studies, extracting mechanistic details from
the isotopic compositions of products.
Scheme 2.2 summarizes the details of the carbene mechanism. As
mentioned previously, the most likely source of a carbene species from
methanol is a framework-bound methoxy species. The carbene (a) could
react with methanol to produce ethanol (b), which would dehydrate in the
acidic zeolite environment to produce ethylene (c).
Scheme 2.2. Proposed Carbene Mechanism
2.5.2. Carbonium Mechanism
Ono and Mori proposed the carbonium route in 1980.10 As with the
carbene mechamism, Ono and Mori suggested framework-bound methoxy
groups were a key intermediate, generating a carbonium species from
dissociation of the methoxy from the anionic site. This step is represented by
a in Scheme 2.3. The next step described by Ono and Mori involved reaction
of the carbonium species with dimethyl ether (b) to a pentavalent carbon
Object Description
| Title | Modification of methanol-to-olefin hydrocarbon pool species by oxygenates on acidic zeolites |
| Author | Hayman, Miranda Jeanette |
| Author email | mirandah@usc.edu; mirandahayman@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2011-02-11 |
| Date submitted | 2011 |
| Restricted until | Unrestricted |
| Date published | 2011-04-26 |
| Advisor (committee chair) | Haw, James F. |
| Advisor (committee member) |
Flood, Thomas C. Jessen, Kristian |
| Abstract | The mechanism of methanol-to-olefin (MTO) catalysis employs organic reaction centers, both aromatic and olefinic, to generate olefins on acid zeolites. Generally, propene is the favored MTO olefin on most zeolite catalysts, but ethylene is a more desirable olefin due to its prevalence in consumer plastics. Much research has been conducted to alter the MTO product selectivities to favor ethylene. This focus of this dissertation is selective modification of the olefinic reaction centers, converting them into aromatic reaction centers known to be responsible for the majority of ethylene production.; Formaldehyde reactivity was studied on HSAPO-34, and found to react with propene through a Prins reaction to form butadiene, which readily cyclized to aromatic species. Evidence of formaldehyde formation was observed from methanol oxidation on the stainless-steel surface of the reactor tubing. This reaction was then studied in HZSM-5 where olefinic reaction centers dominate the hydrocarbon pool. The olefinic reaction centers were converted to aromatic species, and a significant increase in ethylene selectivity was observed. Other oxygenated species, such as acetaldehyde, were also studied in conjunction with methanol on HZSM-5 and an improvement in ethylene selectivity was noted. The consequence of the increased ethylene selectivity however was an increase in the rate of deactivation due to the accelerated formation of aromatic species. |
| Keyword | MTO; methanol-to-olefins; zeolite; heterogeneous catalysis; hydrocarbon pool; HZSM-5 |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3780 |
| Contributing entity | University of Southern California |
| Rights | Hayman, Miranda Jeanette |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Hayman-4358 |
| Archival file | uscthesesreloadpub_Volume23/etd-Hayman-4358.pdf |
Description
| Title | Page 43 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 32 direct observation is unlikely. Most experimental evidence in favor of these species was from carbon labeling studies, extracting mechanistic details from the isotopic compositions of products. Scheme 2.2 summarizes the details of the carbene mechanism. As mentioned previously, the most likely source of a carbene species from methanol is a framework-bound methoxy species. The carbene (a) could react with methanol to produce ethanol (b), which would dehydrate in the acidic zeolite environment to produce ethylene (c). Scheme 2.2. Proposed Carbene Mechanism 2.5.2. Carbonium Mechanism Ono and Mori proposed the carbonium route in 1980.10 As with the carbene mechamism, Ono and Mori suggested framework-bound methoxy groups were a key intermediate, generating a carbonium species from dissociation of the methoxy from the anionic site. This step is represented by a in Scheme 2.3. The next step described by Ono and Mori involved reaction of the carbonium species with dimethyl ether (b) to a pentavalent carbon |
