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51
2.9. Current Understanding of MTO Mechanism
Figure 2.3 summarizes the MTO mechanism as currently understood
for HSAPO-34. Methylbenzenes are formed and trapped in the chabazite
cage and are originally formed upon oligomerization of olefins, specifically
cyclotrimerization of propene to from a trimethylbenzene. The
methylbenzenes provide an organic scaffold that can be methylated,
primarily through a side-chain alkylation mechanism, and olefins are
generated through cracking of the side-chain from the methylbenzene.
Hexamethylebenzene is the most active organic reaction center in HSAPO-
34, but a range of methylbenzenes and methylnaphthalenes constitute the
hydrocarbon pool organics. The acid site proton is transferred to an organic
molecule to form a cation, facilitating the methylation reaction, and the
anionic sites of zeolite framework stabilize these organic cations.
Methylating agents in this process are methanol, dimethyl ether, and
trimethyl oxonium (2). Polycyclic aromatic hydrocarbons accumulate in the
cages over time and deactivation occurs once these coke materials block a
majority of the cages, thus the acid sites. Regeneration can be
accomplished by combusting the entrained organics at higher temperature
(873 K) in air.
In general, the MTO mechanism operating on HZSM-5 is similar to
that on HSAPO-34 but there are several important differences. The same
methylating agents are present, however, the organic reaction centers in
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 62 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 51 2.9. Current Understanding of MTO Mechanism Figure 2.3 summarizes the MTO mechanism as currently understood for HSAPO-34. Methylbenzenes are formed and trapped in the chabazite cage and are originally formed upon oligomerization of olefins, specifically cyclotrimerization of propene to from a trimethylbenzene. The methylbenzenes provide an organic scaffold that can be methylated, primarily through a side-chain alkylation mechanism, and olefins are generated through cracking of the side-chain from the methylbenzene. Hexamethylebenzene is the most active organic reaction center in HSAPO- 34, but a range of methylbenzenes and methylnaphthalenes constitute the hydrocarbon pool organics. The acid site proton is transferred to an organic molecule to form a cation, facilitating the methylation reaction, and the anionic sites of zeolite framework stabilize these organic cations. Methylating agents in this process are methanol, dimethyl ether, and trimethyl oxonium (2). Polycyclic aromatic hydrocarbons accumulate in the cages over time and deactivation occurs once these coke materials block a majority of the cages, thus the acid sites. Regeneration can be accomplished by combusting the entrained organics at higher temperature (873 K) in air. In general, the MTO mechanism operating on HZSM-5 is similar to that on HSAPO-34 but there are several important differences. The same methylating agents are present, however, the organic reaction centers in |
