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46
In 2006, Michel Waroquier and co-workers published several
theoretical studies on MTO mechanisms.33,34 Prior to this work, theoretical
studies on the MTO direct mechanism were widely scattered and difficult to
directly compare due to the variety of zeolite cluster models used (typically
1T or 3T size, where T refers to tetrahedral sites as described in Section 1.1)
as well as the levels of theory. Waroquier and co-workers studied many
direct mechanism pathways to ethylene on HZSM-5 at the DFT level and
used an extensive 46T cluster in their calculations. A two-layered ONIOM
method was employed wherein a small 5T cluster was studied at a high level
of theory (B3YLP) while the remaining cluster was considered at a lower
theory (6-31g(d)-).
Feasibility of various reaction pathways was determined based on the
calculated absorption energies, reaction barriers, and rate coefficients; and
the researchers concluded direct mechanisms were unlikely in MTO
mechanisms based on these criteria. Specifically related to the ylide
(Section 2.4.3) pathway, they deduced the oxygen bridge was not sufficiently
basic to facilitate the proton abstraction. In addition, ylides were found to be
very unstable and the zeolite framework did not provide any additional
stabilization. However, oxonium ions such as trimethyl oxonium (2 in
Scheme 2.4) were found to be stable species due to stabilization by the
anionic zeolite framework and proved to be good methylating agents. These
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 57 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 46 In 2006, Michel Waroquier and co-workers published several theoretical studies on MTO mechanisms.33,34 Prior to this work, theoretical studies on the MTO direct mechanism were widely scattered and difficult to directly compare due to the variety of zeolite cluster models used (typically 1T or 3T size, where T refers to tetrahedral sites as described in Section 1.1) as well as the levels of theory. Waroquier and co-workers studied many direct mechanism pathways to ethylene on HZSM-5 at the DFT level and used an extensive 46T cluster in their calculations. A two-layered ONIOM method was employed wherein a small 5T cluster was studied at a high level of theory (B3YLP) while the remaining cluster was considered at a lower theory (6-31g(d)-). Feasibility of various reaction pathways was determined based on the calculated absorption energies, reaction barriers, and rate coefficients; and the researchers concluded direct mechanisms were unlikely in MTO mechanisms based on these criteria. Specifically related to the ylide (Section 2.4.3) pathway, they deduced the oxygen bridge was not sufficiently basic to facilitate the proton abstraction. In addition, ylides were found to be very unstable and the zeolite framework did not provide any additional stabilization. However, oxonium ions such as trimethyl oxonium (2 in Scheme 2.4) were found to be stable species due to stabilization by the anionic zeolite framework and proved to be good methylating agents. These |
