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9
would have no charge and would be uninteresting catalytically. However,
when T-sites are silicon as well as aluminum (3+ oxidation state), an anionic
site is created for each aluminum atom substituted in place of a silicon atom.
The negative charge resulting from this substitution localizes on an oxygen
atom bridging between both the silicon and aluminum atoms. Any cation of
appropriate charge can be used to counterbalance the negative charge, such
as sodium, ammonium, or a proton. When a proton is used as the balancing
cation, an acidic site is produced, which opens the door to a wide variety of
catalytic applications taking advantage of the numerous topologies for shape-selective
catalysis. Figures 1.1 and 1.2 depict the Brønsted acid sites of
Figure 1.2. The acid site of the silico-aluminophosphate HSAPO-34. Similar
to aluminosilicates, an acid site in silico-aluminophosphates is created when
a proton counters negative charge generated by substitution. The
substitution in this case is silicon substituting aluminum in the aluminum-phosphorus-
oxygen framework.
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 20 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 9 would have no charge and would be uninteresting catalytically. However, when T-sites are silicon as well as aluminum (3+ oxidation state), an anionic site is created for each aluminum atom substituted in place of a silicon atom. The negative charge resulting from this substitution localizes on an oxygen atom bridging between both the silicon and aluminum atoms. Any cation of appropriate charge can be used to counterbalance the negative charge, such as sodium, ammonium, or a proton. When a proton is used as the balancing cation, an acidic site is produced, which opens the door to a wide variety of catalytic applications taking advantage of the numerous topologies for shape-selective catalysis. Figures 1.1 and 1.2 depict the Brønsted acid sites of Figure 1.2. The acid site of the silico-aluminophosphate HSAPO-34. Similar to aluminosilicates, an acid site in silico-aluminophosphates is created when a proton counters negative charge generated by substitution. The substitution in this case is silicon substituting aluminum in the aluminum-phosphorus- oxygen framework. |
