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113
increased as methanol conversion decreased. The deactivation study was
halted after 8 hours when the methanol conversion dropped below 50%
Typically, HZSM-5 is fairly resistant to deactivation because the
coking materials, specifically higher methylated methylbenzenes like
pentamethylbenzene, have difficulty forming in the channels.14 Under the
MTO conditions of 648 K and 600 sccm of helium, HZSM-5 can convert over
64 times its weight of methanol and still have full methanol conversion. With
the addition of formaldehyde, in the form of 1,3,5-trioxane, HZSM-5 only
converts four times its weight in methanol before deactivating. The
increased rate of deactivation in the presence of an aldehyde is due to the
increased rate of aromatic species, as discussed in Chapter 3. While the
aromatics have the benefit of increasing ethylene selectivity, these same
species age into other aromatics such as naphthalenes and larger polycyclic
aromatic hydrocarbons. In fact, after only five minutes time on stream, 3% of
the aromatic product selectivity is due to naphthalene species. While this
percentage is fairly small, it is still significant given the short time on stream.
Deactivation studies were conducted on several other zeolites,
including Ferrierite (FER), Beta (*BEA), and Mordenite (MOR), to examine
the effect of an aldehyde on the rate of deactivation. The topologies of these
zeolites are discussed extensively in Chapter 1. The effect of the various
topologies on MTO with an aldehyde is illustrated Figure 4.8, with the
corresponding product selectivities provided in Table 4.8.
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 124 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 113 increased as methanol conversion decreased. The deactivation study was halted after 8 hours when the methanol conversion dropped below 50% Typically, HZSM-5 is fairly resistant to deactivation because the coking materials, specifically higher methylated methylbenzenes like pentamethylbenzene, have difficulty forming in the channels.14 Under the MTO conditions of 648 K and 600 sccm of helium, HZSM-5 can convert over 64 times its weight of methanol and still have full methanol conversion. With the addition of formaldehyde, in the form of 1,3,5-trioxane, HZSM-5 only converts four times its weight in methanol before deactivating. The increased rate of deactivation in the presence of an aldehyde is due to the increased rate of aromatic species, as discussed in Chapter 3. While the aromatics have the benefit of increasing ethylene selectivity, these same species age into other aromatics such as naphthalenes and larger polycyclic aromatic hydrocarbons. In fact, after only five minutes time on stream, 3% of the aromatic product selectivity is due to naphthalene species. While this percentage is fairly small, it is still significant given the short time on stream. Deactivation studies were conducted on several other zeolites, including Ferrierite (FER), Beta (*BEA), and Mordenite (MOR), to examine the effect of an aldehyde on the rate of deactivation. The topologies of these zeolites are discussed extensively in Chapter 1. The effect of the various topologies on MTO with an aldehyde is illustrated Figure 4.8, with the corresponding product selectivities provided in Table 4.8. |
