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Title: The oxidative dehydrogenation of 1-butene to 1,3-butadiene over a series of metal ferrites
Author: Black, Cory
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2018
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1,3-Butadiene is traditionally produced as a by-product of ethylene production, by the steam cracking of naphtha and higher crude oil fractions and is used mainly as a monomer in the production of synthetic rubbers and elastomers in a wide variety of commercial applications. However, as a result of the rising cost of 1,3-butadiene coupled with increasing demand for 1,3-butadiene products, there has been significant interest in alternative routes of producing 1,3-butadiene. One of the most popular alternative methods for 1,3-butadiene production is the oxidative dehydrogenation (ODH) of C4 alkanes and alkenes, metal ferrite catalysts have been shown to be active catalysts for this reaction. An investigation was conducted looking at the oxidative dehydrogenation of 1-butene to 1,3-butadiene over ferrite catalysts. A significant proportion of the investigation looked specifically at ZnFe2O4, however a comparison between ZnFe2O4, MnFe2O4, MgFe2O4, CuFe2O4, NiFe2O4 and Fe3O4 was also carried out. All of the metal ferrite catalysts were produced by a coprecipitation method and were characterized by XPS, atomic absorption, Raman spectroscopy, XRD, EPR, SEM/EDX and TGA. The ODH reaction was carried out in a continuous flow tube reactor with a feed of 1-butene, oxygen and steam directed over the catalyst for 80 hours. The first stages of the project concerned investigating the effect on a ZnFe2O4 catalyst of an increasing the Fe:Zn ratio. Results highlighted the negative effect of excess Fe on the catalytic activity of ZnFe2O4. It was shown that as the Fe:Zn ratio was increased the Fe began to segregate out as α-Fe2O3. α-Fe2O3 was shown in an independent run to be an extremely poor 1-butene ODH catalyst with a significant selectivity towards combustion and trans-hydrogenation products. It was therefore proposed that the production of α-Fe2O3 as a separate phase negatively effects the catalytic activity by severely depleting the oxygen available in the reaction system though by catalysing a competing combustion reaction. The following section of the project aimed to optimise the ZnFe2O4 preparation procedure in order to improve catalytic activity. Two modifications to the catalyst preparation procedure were identified as having a beneficial effect on catalytic activity. The implementation of a premixing step before addition of the metal precursor salt solutions to the NaOH resulted in an enhanced 1-butene ODH catalyst. Catalytic activity was then further enhanced through the implementation of a pH probe in place of litmus paper for the determination of the end of the washing procedure, once pH 7 has been achieved. The combination of these two modifications resulted in a ZnFe2O4 catalyst with a significantly increased 1-butene conversion and 1,3-butadiene selectivity, up 25 % and 10 % respectively after 40 hours on stream. The catalyst produced with these changes implemented allowed a steady state 1-butene conversion of 72 % and 1,3-butadiene selectivity of 97 % to be achieved. This enhanced catalytic activity was attributed to an increased surface area and pore size and the improved removal of any residual Na+ from the catalyst. The modified preparation method was used for all further catalysts preparations. An in depth investigation into the ODH of 1-butene over ZnFe2O4 was conducted as the penultimate stage of the project. This looked at parameters including calcination temperature, variation of butene isomer as reactant, the effect of the presence of steam in the reactant feed and the effect of altering the gas hourly space velocity (GHSV). Results showed that there was very little effect on catalytic activity with varying the GHSV, indicating that, in agreement with previous work, the 1-butene ODH over ZnFe2O4 was occurring in a diffusion controlled regime. Only minor differences in the catalytic activity of the ZnFe2O4 catalyst were observed for the three butene isomer reactant gases, with cis-2-butene showing the highest 1-butene conversion and trans-2-butene showing the highest 1,3-butadiene selectivity. Although due to the evidence pointing towards a diffusion controlled system, it was difficult to determine whether these were real differences in catalytic activity or were a result of slight variations in the catalyst packing. Variations in calcination temperature highlighted the negative effect of an increased calcination temperature, possibly due to increased crystallinity of the ZnFe2O4. An experiment investigating the effect of steam in the reactant feed showed that the ODH reaction proceeded in the absence of steam; however, upon the addition of steam into the reactant feed a promotional effect was observed, this was believed to be a result of hydroxylation of the catalyst surface by the H2O. The final stage of the investigation was to compare the properties of a variety of different metal ferrites and determine the effect of varying the A2+ cation on the catalytic activity of the ferrites for 1-butene ODH. For the A2+ cations tested 1-butene conversion was shown to decrease in the order Zn > Ni > Mn > Mg > Cu > Fe and 1,3-butadiene selectivity was shown to decrease in the order Mn > Mg > Fe > Zn > Ni > Cu. MnFe2O4 was identified as a promising alternative to ZnFe2O4 at extended time on stream due to its high 1,3-butadiene selectivity and its slowly increasing 1-butene conversion, compared to the slow deactivation observed over ZnFe2O4. CuFe2O4 was found to be the worst 1-butene ODH catalyst, however it showed moderate selectivities towards isomerization products, which were produced with differing selectivities, indicating that they may be being produced on separate sites.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry