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31
sparked an extensive discussion in literature with over 20 different
mechanisms proposed.2 The only point on which all the mechanisms agreed
was that the reaction was autocatalytic, with the initial products acting as co-catalysts.
Initially, zeolites were believed to be superacids (species with a proton
strength greater than that of concentrated sulfuric acid) and early
mechanisms proposed for MTO reflected this presumed acidity with high-energy
intermediates in the direct production of olefins from methanol.2 A
few of these direct mechanisms are examined in further detail below.
2.5.1. Carbene Mechanism
In a 1977 publication,9 Salvador and Kladnig proposed carbenes
(:CH2) could be formed by decomposition of a framework-bound methoxy
species 1, which were chemisorption products of methanol. In their early
work on MTG catalysis, Chang and Silvestri, who discovered MTG at Mobil,
favored a carbene route.6 Carbenes are short-lived transient species thus
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 42 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 31 sparked an extensive discussion in literature with over 20 different mechanisms proposed.2 The only point on which all the mechanisms agreed was that the reaction was autocatalytic, with the initial products acting as co-catalysts. Initially, zeolites were believed to be superacids (species with a proton strength greater than that of concentrated sulfuric acid) and early mechanisms proposed for MTO reflected this presumed acidity with high-energy intermediates in the direct production of olefins from methanol.2 A few of these direct mechanisms are examined in further detail below. 2.5.1. Carbene Mechanism In a 1977 publication,9 Salvador and Kladnig proposed carbenes (:CH2) could be formed by decomposition of a framework-bound methoxy species 1, which were chemisorption products of methanol. In their early work on MTG catalysis, Chang and Silvestri, who discovered MTG at Mobil, favored a carbene route.6 Carbenes are short-lived transient species thus |
