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37
2.6.1. Cyclic Olefin Mechanism
Around 1982, Bernd Langner was studying the effect of pore size on
MTH product selectivity.16 Through the course of the study, he co-fed
methanol with several other alcohols, such as ethanol, propanol, and
cyclohexanol, and observed a reduction in the induction period with all the
co-fed alcohols. Cyclohexanol produced the most dramatic effect though, as
only a trace amount of the molecule (0.0036 mol % in gas phase) resulted in
an 18-fold reduction in the duration of the induction period.
Langner’s proposed mechanism, summarized in Scheme 2.6,
centered on cyclic olefins produced via dehydration of a cyclic alcohol like
cyclohexanol. He suggested the cyclic olefins would react with
formaldehyde, produced upon reaction of methanol with an already-present
olefin, to produce higher methylated cyclic olefinic species. In turn, these
species would produce a mixture of olefinic products (propene, isobutene
and isopentene) in addition to the initial cyclic olefin. The already-present
olefin was hypothesized as a potential impurity in the methanol or a result of
some direct mechanism. Langner’s mechanism however, was not able to
explain the increased ethylene selectivity at higher temperatures.
Scheme 2.6. Langner’s Cyclic Olefin Mechanism
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 48 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 37 2.6.1. Cyclic Olefin Mechanism Around 1982, Bernd Langner was studying the effect of pore size on MTH product selectivity.16 Through the course of the study, he co-fed methanol with several other alcohols, such as ethanol, propanol, and cyclohexanol, and observed a reduction in the induction period with all the co-fed alcohols. Cyclohexanol produced the most dramatic effect though, as only a trace amount of the molecule (0.0036 mol % in gas phase) resulted in an 18-fold reduction in the duration of the induction period. Langner’s proposed mechanism, summarized in Scheme 2.6, centered on cyclic olefins produced via dehydration of a cyclic alcohol like cyclohexanol. He suggested the cyclic olefins would react with formaldehyde, produced upon reaction of methanol with an already-present olefin, to produce higher methylated cyclic olefinic species. In turn, these species would produce a mixture of olefinic products (propene, isobutene and isopentene) in addition to the initial cyclic olefin. The already-present olefin was hypothesized as a potential impurity in the methanol or a result of some direct mechanism. Langner’s mechanism however, was not able to explain the increased ethylene selectivity at higher temperatures. Scheme 2.6. Langner’s Cyclic Olefin Mechanism |
