Browsing by Author "Brosius, Roald"

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    Open Access
    Carbon monoxide clean-up of reformate gas by preferential oxidation
    (2015) Muziki, Sibongile; Fletcher, Jack; Brosius, Roald
    The preferential oxidation (PrOx) activity of two Ru/Al2O3 catalysts prepared using different methods was tested. The first catalyst was prepared by wetness impregnation at a high pH and the second was prepared by incipient wetness impregnation. Catalytic activity was measured at varying temperatures, space velocities as well as O2/CO ratio. The Ru catalyst catalyst prepared using wetness impregnation at high pH was found to exhibit higher CO conversion despite having a lower Ru dispersion compared to the Ru catalyst prepared using incipient wetness impregnation at the tested temperature range. For both Ru catalysts the trends observed with varying temperature, space velocity as well as O2/CO ratio were similar. Increasing temperature increased CO conversion up to a maximum after which a further increase in temperature led to a decrease in CO conversion. At low temperatures, increasing space velocity resulted in a decrease in CO conversion. An increase in CO conversion was observed with increasing space velocity at higher temperatures. Increasing space velocity led to a decrease in CH4 formation at high temperatures. Furthermore it was determined that mass transfer limitations played a role during the catalytic process. The effects of mass transfer limitations could be reduced by increase the linear space velocity. A Pt-Fe/mordenite catalyst was prepared in this study using solid state ion exchange to deposit Fe and competitive ion exchange to deposit Pt. This method was proposed in order to try and improve the preparation method reported in literature. The synthesised catalyst did not perform as well as the Pt-Fe/Mordenite reported in literature. A maximum CO conversion of 99 % with 47 % CO2 selectivity at 180 °C, 120 000 ml/(h gcat) and O2/CO ratio of 1 was achieved.
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    Open Access
    Hydrocracking of n-C16 over MFI Zeolite Nano-sheets - Effect of the Si/Al Ratio
    (2018) Parker, Mohamed Habeeb; Kooyman, Patricia J; Brosius, Roald
    The combination of MFI zeolite nano-sheets with competitive adsorption of water (H2O) in hydrocracking of long-chain paraffins presents a promising opportunity to produce diesel with high yield and with high cetane number. Thus, in wet hydrocracking of a long-chain paraffin (n-hexadecane (n-C16)) over MFI nano-sheets, it was investigated whether catalytic activity increased with increasing number of Brønsted acid (H + ) sites (decreasing silicon-to-aluminium (Si/Al) ratio), while secondary cracking remained completely suppressed. Also, it was investigated whether more Al atoms could be incorporated into the framework of MFI nano-sheets by modifying the new synthesis method. It was demonstrated that the new synthesis method, which utilizes C22H45–N + (CH3)2–C6H12–N + (CH3)2–C6H13 (C22-6-6) as structure-directing agent (SDA), could be extended to various Si/Al ratios in the range 25 – 100. The nano-sheets exhibited extra-framework Al (EFAl) species. Nano-sheets with Si/Al = 75 exhibited an oddly large amount of EFAl species compared to the other nano-sheets. For nano-sheets with Si/Al = 75, a high fraction of the EFAl species may have formed during calcination of the ammonium form and may encompass flexible Al species with predominantly Al in octahedral coordination (AlVI). Nano-sheets were loaded with 0.9 wt% platinum (Pt) via incipient wetness impregnation (IWI). Pt/nano-sheets with Si/Al = 25, 50 and 100 exhibited similar and high Pt dispersion (γPt). In contrast, Pt/nano-sheets with Si/Al = 75 exhibited a very low γPt, which was probably a result of the abundance, nature and/or location of EFAl species present in the support. In dry hydrocracking of n-C16, the catalytic activity increased with decreasing Si/Al ratio, strongly suggesting that the number of H+ sites increased with decreasing Si/Al ratio. Nano-sheets with Si/Al = 75 most likely contained AlVI species associated with Brønsted acidity, supporting the presence of flexible AlVI species. In wet hydrocracking of n-C16, at a constant and sufficiently high γPt, the activity increased with increasing number of H+ sites (decreasing Si/Al ratio), while secondary cracking remained completely suppressed. Pt/nano-sheets with Si/Al = 75 displayed a lower activity than 2 Pt/nano-sheets with Si/Al = 100, which may be a result of the very low γPt of Pt/nano-sheets with Si/Al = 75, underlining the importance of high γPt. For Pt/nano-sheets with Si/Al = 25, 50 and 100, H2O favoured linear cracking products at low cracking yields. In contrast, for Pt/nano-sheets with Si/Al = 75, H2O favoured branched cracking products, which may be a result of Pt sites on the external surface of the support being too far from the H+ sites inside the micropores. The new synthesis method could be extended to a modified SDA, namely C22H45–N + (CH3)2–C6H12–N + (CH3)2–C3H7 (C22-6-3), at various Si/Al ratios in the range 25 – 100. Replacing the terminal –C6H13 group in C22-6-6 with –C3H7 resulted in an increase in the framework Al (FAl) content of MFI nano-sheets with Si/Al ≥ 50, with the increase being the most pronounced for nano-sheets with Si/Al = 50. This was due to the increased occupancy of the zeolite framework by the hydrophilic region of C22-6-3 in comparison to the hydrophilic region of C22-6-6 under the given set of synthesis conditions, since –C3H7 was less bulky than –C6H13. Calcined nano-sheets were loaded with 1 wt% Pt via competitive ion exchange (CIE). In dry and wet hydrocracking of n-C16, the activity increased with decreasing Si/Al ratio and in wet hydrocracking, secondary cracking was not completely suppressed up to high conversions. This was probably due to the presence of additional H+ sites generated after SDA removal. H2O favoured linear cracking products at low cracking yields. Sodium (Na+ ) ion-exchanged nano-sheets were loaded with 1 wt% Pt via CIE. The average Pt size (dPt) of the Pt/Na+ nano-sheets were larger than the dPt of the Pt/calcined nano-sheets, which may be a result of the nature and/or location of EFAl species present in the Na+ supports. In dry and wet hydrocracking of n-C16, differences in activity were observed and in wet hydrocracking, secondary cracking was not completely suppressed up to high conversions. This was probably due to insufficient intimacy between H+ sites and Pt sites such that the rate was controlled by diffusion of olefinic intermediates from H+ sites to Pt sites and vice versa. H2O favoured linear cracking products at low cracking yields.
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    Platinum Nanoparticles to Study Metal Location in Shape-Selective Hydrocracking
    (2018) Nel, Dayle; Brosius, Roald; Kooyman, Patricia; Fletcher, Jack
    In the petrochemical industry, bifunctional hydrocracking is used to further refine long chain hydrocarbon product molecules into smaller, more valuable fragments (such as diesel and aviation fuel). The current study is focused on using hydrocracking to increase diesel yield, as the preferred automotive fuel in terms of efficiency and environmental impact. However, even though conventional hydrocracking can significantly increase diesel yield, it is not selective to improving diesel quality. In a recent study by Brosius et al., 2016, the use of a Pt/MFI bifunctional hydrocracking catalyst in the presence of H2O has shown, for the first time, selective hydrocracking to produce not only a high yield but also high cetane number diesel product. The current study is a continuation of the work by Brosius et al., 2016, with a focus on improving the catalyst design and, in particular, the metal function, as in this study the Pt metal was supported using wet impregnation which does not allow for control of metal size and location. The hydrocracking mechanism consists of a (de)hydrogenation function at the metal sites and cracking/isomerisation steps at the acid sites. The reason a high yield of high cetane diesel is produced from hydrocracking over Pt/MFI in the presence of H2O is twofold: firstly, hydrocracking inside the MFI micropores is selective to C-type β-scission which results in linear cracking products, and secondly, H2O competes at the acid sites which allows for the desorption of the linear primary cracking products. From previous literature it is evident that metal location affects both activity and selectivity during hydrocracking over various catalyst types due to diffusion limitations. Furthermore, with hydrocracking over MFI, metal location is also expected to influence the amount of reactant molecules which crack inside the micropores. Therefore, the aim of this study was to quantify the extent that metal located inside or outside the micropores of MFI influences the cracking and isomerisation steps in the hydrocracking of n-C16 in the presence of H2O. Pt metal was selectively placed inside and outside the MFI micropores using different preparation techniques. To place the metal exclusively outside the micropores, monodisperse Pt NPs, which were bigger than the micropore diameter of 0.55 nm, were pre-synthesised and then contacted with the MFI support. To place the metal exclusively inside the micropores, a competitive ion exchange (CIE) method was used where Pt ions were chemically bonded within the MFI structure. Besides bulk MFI, the Pt NPs were also supported on MFI nanosheets (NS), which have been used in previous research to decrease diffusion limitations between metal and acid sites, to assess the effects of metal location. The prepared catalysts were characterised using TEM, ICP-OES, CO chemisorption, and NH3-TPD to ensure metal location was the only varied parameter. The catalysts were then analysed by dry and wet hydrocracking of n-C16 in a fixed-bed, trickle reactor, where the product composition was determined using an online GC. The performance of PtNPs/MFI and Pt-CIE/MFI were compared directly with Pt/MFI from Brosius et al., 2016. Catalyst activity was first analysed using conversion versus temperature plots and Arrhenius plots, primary hydrocracking was then assessed using C4/C12 ratio and once achieved, branched versus linear product selectivity could be compared. Looking at activity, H2O decreased the activity by ~30 °C, which is expected as H2O blocks acid sites. Comparing the individual samples showed relatively similar results in wet and dry hydrocracking, showing metal location does not play a major role in overall activity. From the Arrhenius plots, the activation energies could be compared and showed that the difference between wet and dry hydrocracking was relatable to the isosteric heat of adsorption of H2O on MFI. Primary hydrocracking was achieved for all three catalyst samples in the presence of H2O, again showing that metal location does not largely influence the yield of primary cracking products. However, when comparing branched versus linear primary cracking products, a significant difference was observed based on metal location. At low conversion, the greatest differences are seen, with Pt/MFI and Pt- CIE/MFI achieving ~100 % n-alkanes in C1-C15 cracked products, whereas PtNPs/MFI achieved only ~40 %. The reason for the vast differences is because having the metal inside the micropores, firstly, provides a non-diffusion hindered steady supply of reactant molecules to encourage desorption of the primary cracked products and, secondly, the cracking/isomerisation steps need to occur inside the MFI micropores which is more likely with internal metal. Furthermore, placing Pt NPs on NSs showed that even with thin NSs of 2 nm thickness, if the metal is placed outside, the hydrocracking products are just as branched as those from PtNPs/MFI. Post-mortem TEM characterisation of the catalyst samples revealed that no significant changes to the Pt metal sites were seen after ~10 days of reaction under varying hydrocracking conditions, ensuring that metal location was the only parameter varied in the study. In conclusion, regardless of metal location, primary hydrocracking can be achieved in the presence of H2O. Furthermore, placing metal exclusively inside the MFI micropores results in a significant increase in linear primary cracking products in comparison to placing metal completely outside the micropores, which results in mostly branched primary cracking products.