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The catalytic oxidative dehydrogenation of n-octane over iron and other metal molybdates

Bugler, Keith 2017. The catalytic oxidative dehydrogenation of n-octane over iron and other metal molybdates. PhD Thesis, Cardiff University.
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This work studies the gas phase oxidative dehydrogenation (ODH) of n-octane to produce octenes, catalysed by metal molybdates of the formula AMoO4, where A equals iron, cobalt or nickel in the +2 oxidation state. An in-situ reduction study from previous work had shown that iron molybdate where iron is in the +2 oxidation state is a superior catalyst to an iron molybdate catalyst with iron in the +3 state. This was tested and found to be the case. Subsequently all iron molybdate catalyst testing was performed with iron molybdate where iron is in the +2 oxidation state. This was achieved through a pre-reduction step in catalyst preparation. The pre-reduced iron molybdate catalyst with a 2.7:1 molar excess of molybdenum to iron was found to be composed of the species FeMoO4 + Mo4O11. This catalyst exhibited high selectivity to octenes from an n-octane feedstock at 400 °C in a plug flow reactor. Changes in gas hourly velocity (GHSV) and temperature (ranging from 350-550 °C) affected catalyst activity and selectivity, as did varying the ratio of carbon to oxygen in the gas feed. Optimum conditions for the production of octene were found to be; 400 °C reactor bed temperature, 4000 h-1 GHSV and an 8:1 carbon to oxygen ratio, no carbon oxides were observed at these conditions. Increasing temperature results in higher conversion of n-octane but lower selectivity to octenes. Aromatic species become the major products at higher temperatures. Carbon oxide selectivity also rises with temperature. Increasing partial pressure of oxygen in the gas feed leads to higher conversion but the major products formed are carbon oxides. Lowering the level of oxygen from an 8:1 carbon to oxygen ratio saw lower conversions with similar selectivity. This suggested oxidative dehydrogenation was occurring. Lowering the GHSV from 4000 h-1 to 1000 h-1 resulted in product selectivity to aromatic species, ethyl benzene, xylene and styrene. Higher conversion as a result of greater contact time between catalyst and product was observed. Styrene and xylene selectivity increased in line with temperature, while selectivity to ethyl benzene fell, suggesting a competing pathway between aromatic formation, or that ethyl benzene underwent further dehydrogenation to styrene. Increasing GHSV to 6000 h-1 resulted in an even greater selectivity to octenes than 4000 h-1. However conversion was lower, likely due to contact time effects. These findings suggested that the product selectivity from n-octane over an iron molybdate catalyst has a strongly kinetic element. Increasing the concentration of n-octane in the gas feed showed a shift in the optimal conditions for the production of octene. A higher GHSV was required to yield octenes as the dominant product, this had the unfortunate effect of lowering conversion percentage. While this was off-set in some way by the increased concertation of noctane it does suggest future difficulties on scaling up the process. Time on line studies showed the catalyst was stable at temperatures of 550 °C for 20 hours or more. In addition anaerobic studies were carried out on the catalyst were tested to elucidate the mechanism of the catalyst. The change in selectivity and activity showed the catalyst most likely operates by a Mars and van-Krevelen type system. After oxygen deprivation for 25 hours catalyst deactivation occurred. Analysis showed both carbon laydown and reduction of the molybdenum lattice from Mo4O11 had occurred. Product selectivity analysis indicated that lattice oxygen from the iron molybdate or the bulk Mo4O11 phase was responsible for the ODH of n-octane to octene. Carbon oxides were formed via oxygen in the gas feed. Stoichiometric nickel molybdates and cobalt molybdates were prepared and compared against nickel and cobalt molybdates with a molybdenum molar excess of 1.5:1 for the catalytic conversion of n-octane to octene. These catalysts were then compared against stoichiometric iron molybdate (FeMoO4) to compare catalytic effectiveness. Iron molybdate outperformed nickel molybdate and cobalt molybdate which have been more heralded ODH catalysts in the literature. Cobalt molybdate was found to exhibit high selectivity to aromatic species while nickel molybdate produced carbon oxides and cracked hydrocarbon products. Nickel molybdate and cobalt molybdate with an excess of molybdenum performed better as catalysts than stoichiometric nickel and cobalt molybdates.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 25 May 2017
Last Modified: 20 Apr 2021 11:05

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