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107
decrease in the amount of aldehyde molecules in relationship to the amount
of organic reaction centers. At an elevated temperature the ability for
acetaldehyde to react with those olefinic reaction centers remains unchanged
– the aldehyde is supplied at a constant concentration and rate, but the
higher temperatures allow for more methanol to react and for that reaction to
occur at a faster rate. Consequently, the ability of the zeolite to produce
those reaction centers is enhanced and this counters the effects of the co-fed
acetaldehyde as the native HZSM-5 pool is predominantly olefinic. In other
words, higher ethylene selectivity would be expected at lower temperatures
because any olefinic pool that does develop should be converted to the
aromatic pool by the acetaldehyde and methanol will react on the organic
scaffold provided: the aromatic hydrocarbon pool. As temperature increases,
more olefinic reaction centers are produced and the product selectivity will
shift correspondingly.
Finally, the effect of altering the concentration of the aldehyde was
examined using various molar ratios of methanol and acetaldehyde on
HZSM-5 (SI/Al = 40) at 623 K. The results are shown in Figure 4.6 and the
relative product selectivities are provided in Table 4.6. The highest aldehyde
concentration also has the highest ethylene to propene ratio at 1.5. Aromatic
species comprised 36% of the product selectivity, followed by the C4 to C6
hydrocarbons at 24%. Both ethylene and propene has a selectivity of 18%.
As the concentration of acetaldehyde was decreased, the ethylene to
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 118 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 107 decrease in the amount of aldehyde molecules in relationship to the amount of organic reaction centers. At an elevated temperature the ability for acetaldehyde to react with those olefinic reaction centers remains unchanged – the aldehyde is supplied at a constant concentration and rate, but the higher temperatures allow for more methanol to react and for that reaction to occur at a faster rate. Consequently, the ability of the zeolite to produce those reaction centers is enhanced and this counters the effects of the co-fed acetaldehyde as the native HZSM-5 pool is predominantly olefinic. In other words, higher ethylene selectivity would be expected at lower temperatures because any olefinic pool that does develop should be converted to the aromatic pool by the acetaldehyde and methanol will react on the organic scaffold provided: the aromatic hydrocarbon pool. As temperature increases, more olefinic reaction centers are produced and the product selectivity will shift correspondingly. Finally, the effect of altering the concentration of the aldehyde was examined using various molar ratios of methanol and acetaldehyde on HZSM-5 (SI/Al = 40) at 623 K. The results are shown in Figure 4.6 and the relative product selectivities are provided in Table 4.6. The highest aldehyde concentration also has the highest ethylene to propene ratio at 1.5. Aromatic species comprised 36% of the product selectivity, followed by the C4 to C6 hydrocarbons at 24%. Both ethylene and propene has a selectivity of 18%. As the concentration of acetaldehyde was decreased, the ethylene to |
