The effect of silica on the reduction of precipitated iron-based fischer-tropsch catalysts
- Authors: Coombes, Matthew
- Date: 2016
- Subjects: Fischer-Tropsch process Reduction (Chemistry) , Catalysts
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/14873 , vital:27888
- Description: Iron Fischer-Tropsch (FT) catalysts are typically prepared as iron oxides which are reduced to FT-active iron metal and iron carbide prior to FT synthesis. The iron oxides contain a variety of different chemical and structural promoters to alter FT-activity. Silica is a common structural promoter which stabilises the formation of small crystallites and provides mechanical integrity to the catalyst. However, silica inhibits the reduction of the oxide precursor to the FT-active phases. This ultimately affects catalyst activity and product selectivity. It has been proposed that the silica interacts with the iron to form encapsulating shells of fayalite (Fe2SiO4), or fayalite rafts between the iron oxide and the silica support. In this study, six silica-promoted iron oxide samples were prepared using a simple co-precipitation technique. Samples contain varying amounts of silica, and the samples are named 100/x Fe/SiO2, where x is the weight of silica for 100 weight iron, with x taking on values of 0, 10, 25, 50, 100 and 200. The resulting iron oxides were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRPD), M¨ossbauer spectroscopy (MS), magnetic susceptibility measurements (MM), Raman spectroscopy, thermal gravimetric analysis (TGA) and nitrogen physisorption. Their reduction in a hydrogen atmosphere was investigated using temperature programmed reduction (TPR), in situ XRPD and TEM. The reduction in hydrogen of 100/0 Fe/SiO2 and 100/10 Fe/SiO2 was also studied using in situ gas flow TEM cells. These cells allow the samples to be studied in the electron microscope at temperature and pressure conditions approaching those experienced in a real reactor environment. In the absence of a silica promoter (100/0 Fe/SiO2), hematite particles are formed with mean particle diameters of 39 ± 12 and 52.7 ± 0.2 nm determined using TEM and XRPD respectively. MM data reveals a magnetic transition (Morin transition) at≈230 K, consistent with a mean particle size of≈50 nm. In a hydrogen atmosphere, the hematite reduces to metallic iron via a two-step process viz. hematite → magnetite → iron. The final iron particles have an average crystallite size of 68.0 ± 0.2 nm. The presence of lower amounts of silica in the samples 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 results in the formation of silicasubstituted 2-line ferrihydrite particles. Bands in the Raman spectra of these samples shift on increasing silica content, which indicates an increasing number of Fe-O-Si bonds within the ferrihydrite framework. MM reveals typical superparamagnetic (SPM) behaviour above a blocking temperature in the range 39 - 68 K which gives mean particle sizes of 4.2, 3.6 and 3.5 nm for 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 respectively, in good agreement with particle sizes determined using TEM (3.1±0.4, 2.4±0.3 and 2.4±0.3 nm respectively). MS data at 300 K and 4.2 K were fitted with distributions of ∆EQ and Bhf respectively. The median values of Bhf decrease with increasing silica content, indicating greater degrees of distortion in the Fe3+ environments induced by increased silica substitution. The reduction to metallic iron occurs via a three-step process viz. hematite → magnetite → wu¨stite → iron, with the silica stabilising the wu¨stite phase. The increasing amount of Fe-O-Si bonds on increasing silica content shifts reduction to higher temperatures broadens each reduction step as a result of local Fe-O-Si concentration variations. Fractions of each sample are not completely reduced even at 1000°C, with the relative proportion increasing with increasing silica content. In situ gas flow TEM studies reveal that the mechanism of reduction involves the liberation of atomic iron atoms from the silica-substituted iron oxides which agglomerate and grow into final iron particles. This leaves a poorly crystalline Fe-O-Si bonded framework behind. STEM-EDS and STEM-EELS reveal low concentrations of silicon at the surface of the resulting iron particles, however they do not form encapsulating shells of fayalite as previously suggested. The majority of the silica remains in the Fe-O-Si material which may crystallise into separate fayalite particles at elevated temperature. The presence of silica in high proportions (100/100 Fe/SiO2 and 100/200 Fe/SiO2) results in the formation of a two-phase system consisting of silicasubstituted 2-line ferrihydrite particles which are encapsulated in an ironinfused amorphous silica network. As with the other silica-bearing samples, there is an increase in Fe-O-Si bonds and an increase in the degree of distortion at Fe3+ sites with increasing silica content. The large amount of silica suppresses the blocking temperature of the SPM crystallites. In a hydrogen atmosphere, the reduction to metallic iron follows the same three step process as the other silica-bearing samples. Reduction temperatures are further shifted to higher values and given reduction steps are considerably broader with increasing silica content. The fraction of iron not fully reduced also increases. Iron particle diameters are very small, since encapsulation by the silica matrix prevents growth of particles.
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- Date Issued: 2016
A Comparative study of two copper(II) based metal-organic frameworks : Cu2¼(OH)½B4C•8H2O and Cu2Na(OH)B4C•7H2O
- Authors: Coombes, Matthew
- Date: 2013
- Subjects: Copper , Organometallic compounds , Supramolecular organometallic chemistry
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4533 , http://hdl.handle.net/10962/d1016245
- Description: This study focussed on two copper(II)-containing metal-organic frameworks (MOFs): Cu2Na(OH)B4C•7H2O and Cu2¼(OH)½B4C•8H2O (B4C = 1,2,4,5- benzenetetracarboxylate). They are both covalent, three-dimensional metalorganic framework polymers containing voids filled with water molecules. Both were characterised by elemental analysis, infrared spectroscopy, X-ray powder diffractometry (both in situ and regular), thermogravimetric analysis, differential scanning calorimetry and X-ray photoelectron spectroscopy. These two MOFs are essentially identical, with the only difference being the substitution of sodium by copper at every 4th site (disordered throughout the crystal). The guest inclusion properties of both MOFs were studied and compared. Although both structures collapse on dehydration, it was observed that Cu2Na(OH)B4C•7H2O is able to take up signifcant amounts of water, methanol and ethanol. All these processes are fully reversible. Car-Parrinello molecular dynamics studies suggest that it is a strong interaction between the oxygen atoms on these molecules with the sodium cation of the MOF that is responsible for this signifcant uptake. In contrast, Cu2¼ (OH)½ B4C•8H2O, the MOF without a sodium cation, did not demonstrate any methanol or ethanol uptake, but was able to take up some water. The uptake of water, however, is not a fully reversible process. The absence of sodium likely results in insuffcient energy to draw methanol and ethanol into the framework, while a subtle rotation of a carboxylate group on dehydration decreases the ability of the framework to form hydrogen bonds, thus reducing the ability to take up water. A series of hydrothermal syntheses were performed in order to develop a method of synthesis superior to the current gel-based synthesis that requires several months and has poor yields. The hydrothermal products were characterized by elemental analysis, infrared spectroscopy, X-ray powder diffractometry, thermogravimetric analysis and differential scanning calorimetry. It was shown that the MOF Cu2Na(OH)B4C•7H2O may be synthesised in almost 100% yield by using a temperature of 120°C over a period of 72 hours. It was not possible to synthesise Cu2¼ (OH)½ B4C•8H2O in a 100% yield - it was only obtained as a minor product.
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- Date Issued: 2013