Browsing by Author "Fletcher, Jack"
Now showing 1 - 20 of 35
Results Per Page
Sort Options
- ItemOpen AccessAcid catalysed alkylation of diphenyl ether with methanol over shape-selective zeolites(2005) Ntshabele, John Phenyo Pheko; Fletcher, Jack; Böhringer, WalterSelectivity in methylation of phenol over acidic catalysts depends primarily on temperature and the acidity of the catalyst. It is reported that direct methylation of phenol with methanol over weakly acidic catalysts yields predominantly o-cresol, whilst catalysts with medium acidity yield primarily o- and p-cresol, with significant anisole formation at lower temperatures (≈ 200°C). m-Cresol is, however, formed with significant selectivity over strongly acidic catalysts or at higher temperatures (typically 300°C and more). To date, only one industrial process is known which is selective to p-cresol but has the disadvantage of co-formaiton of inorganic salt, which is costly to dispose of.
- ItemOpen AccessCarbon monoxide clean-up of reformate gas by preferential oxidation(2015) Muziki, Sibongile; Fletcher, Jack; Brosius, RoaldThe 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.
- ItemOpen AccessCatalytic conversion of Ethylbenzene over acid zeolites(2000) Mabaso, Evans Itai; Fletcher, Jack; O'Connor, CyrilIn this study, the performance of this test reaction was employed to compare the acidity of various zeolites, viz. lanthanum/sodium-exchanged zeolite Y (LaNaY-72), hygrogen/sodium-exchanged zeolote Y (LZY-52, LZY-62, LZY-82), hydrogen-exchanged zeolite Beta (H-BEA-15, H-BEA-25) and hydrogen-exchanged ZSM-5 (H-MFI-45).
- ItemOpen AccessCharacterisation of acidic/basic properties of alumina supports(2002) Ferreira, Riki; Fletcher, Jack; Visagie, Kobus; Böhringer, WalterPrevious experience with the preparation and testing of Co/alumina catalysts for Fischer-Tropsch synthesis has revealed that, while commercial available aluminas result in materials of significantly different catalytic performance, no correlation between the physical properties of the aluminas and the resulting catalytic performance was evident. Consequently, it was proposed that differences in the chemical (acid/base) nature of the alumina surfaces might be responsible for the observed differences in catalytic behaviour. In this study, isopropanol conversion was evaluated as a possible test reaction for characterisation of the acid/base nature of commercial aluminas - literature indicates acetone to result from isopropanol reaction on basic sites, and DIPE and propene products to result from isopropanol conversion over acid sites of varying strength.
- ItemOpen AccessThe characterization and elimination of the external acidity of ZSM-5(1993) Weber, Reinier Willem; Fletcher, Jack; O'Connor, CyrilThe zeolite ZSM-5 is well-known for its unique intersecting channel system. This channel system has a great bearing on the shape-selective properties and the long life-times of ZSM-5. In this study, ZSM-5 was modified in various ways to eliminate the external acidity of the catalyst to further improve these properties, and the success and effects of these modifications were investigated primarily using temperature programmed desorption techniques. The internal surface of ZSM-5, a medium pore zeolite, plays a major role in the shape selective properties of this catalyst, due to the diffusional restrictions imposed by the channel system on bulky molecules. Even though the number of acid sites on the external surface is small compared to the total number of acid sites, these easily accessible and non-shape selective acid sites may provide a high turnover rate for non-shape selective reactions. Furthermore, the main cause of deactivation of ZSM-5 is thought to be the formation of polyaromatic molecules on the external surface, which block access to the channels of the catalyst.
- ItemOpen AccessCharacterization of a novel laboratory internal recycle reactor for HTFT studies(2008) Vallabh, Hema; Fletcher, JackThis study aims to fully characterise the Stirred from Top Internal Recycle Reactor (STIRR) by means of a residence time distribution (RTD) study to determine its suitability for catalyst testing for High Temperature Fischer-Tropsch studies. It is required to not only ensure that the reactor behaves as a perfectly mixed CSTR, and but also to confirm that there is indeed sufficient flow through the catalyst bed thus ensuring adequate gas-catalyst contact.
- ItemMetadata onlyThe conversion of citronellal over sulphonated polystyrene resin catalysts(2016) Maharaj, Riddhi Anubhav; Böhringer, Walter; Fletcher, JackCitronellal was converted over sulphonated polystyrene resin catalysts of different acid loading. The catalysts had been prepared sulphonating a commercial polystyrene resin with sulfuric acid. Catalyst activities were screened in the liquid phase and in the gas phase in a temperature range of 50 to 110°C, varying space velocity. Catalyst activities (in terms of rates of citronellal conversion) followed the degree of sulphonation and were similar in both the environments, but apparently governed by external mass transfer control in the liquid phase or limited by the very low partial pressure of citronellal in the gas phase. No effect of the degree of sulphonation on selectivity was observed. [Please note: the fulltext of this thesis has been deferred to 9 June 2018]
- ItemOpen AccessDevelopment of bimetallic Pd-Zn catalysts for methanol steam reforming: hydrogen production for fuel cells(2015) Xalabile, Philasande; Fletcher, Jack; Luchters, Niels; Malatji, PeterProton exchange membrane fuel cell (PEMFC) has been reported as clean and efficient energy technology from conversion of H₂. However, one of the main challenges remains the storage and transport of hydrogen. The promising alternative is to produce H₂ on site by a reformer using a H₂-dense liquid as a fuel, a technology known as fuel processing. Methanol is an attractive source of H₂ compared to other fuels as it presents several advantages, i.e. it is obtained sulphur-free, has a high H to C ratio and therefore produces a H₂-rich reformate, can be reformed at low temperatures (200 - 300°C) and is a liquid at ambient conditions so that it can be easily handled. Typically, Cu-based catalysts are used for steam reforming of methanol due to their high activity (i.e. H₂ production) and high selectivity towards CO₂. As CO poisons anodic catalyst of PEMFC, high selectivity towards CO₂ is crucial so as to eliminate or at least minimize CO removal load downstream a fuel processor. However, Cubased catalysts are thermally unstable and suffer deactivation due to sintering at high temperatures (> 250°C). Moreover, Cu-based catalysts are pyrophoric and therefore difficult to handle. Recent studies show that PdZn catalysts are very promising as they exhibit comparable activity and selectivity to Cu-based ones. Furthermore, PdZn catalysts are thermally stable in the typically methanol steam reforming temperature range (200 - 300°C). Most literature attributes high CO₂ selectivity of PdZn catalysts to formation of PdZn alloy. It is generally agreed that PdZn alloy is formed when PdZn catalysts are reduced in H₂ at high temperatures (> 250°C). In this work, a Pd/ZnO catalyst aimed at 2.5 wt% Pd was successfully prepared via incipient wetness impregnation and the duplicate preparation of the catalyst was successful. Both impregnation catalysts were confirmed by ICP-OES to contain similar weight Pd loadings i.e. 2.8 and 2.7 wt%, respectively. The actual Pd loading (ICP-OES) was slightly higher than the target loading (2.5 wt%) due to Pd content of Pd salt underestimated during catalyst preparation. Furthermore, crystallite size distribution, i.e. PdO crystallites on ZnO support, was similar (i.e. 6.7 ± 2.4 nm and 6.3 ± 1.9 nm) for both impregnation catalysts.
- ItemOpen AccessThe effect of metal type and loading on n-paraffin hydrocracking conversion and selectivity(2014) Wynne, Peter DT; Böhringer, Walter; Fletcher, JackWith the continued decline in global oil reserves, there is a growing need to develop alternative sources of conventional fuels to complement the current dependence on crude oil feedstocks. Natural gas, coal and biomass have been identified for this purpose. The distinctive advantage of using natural (stranded) gas is that it is turned into a useful product, thereby increasing its value and reducing the environmental impact of simply flaring it. The value-added work up of natural gas is effected by Gas-to-Liquid conversion via the Fischer-Tropsch Synthesis. Long-chain hydrocarbon waxes are produced and these are subsequently hydrocracked into the required middle distillate fuels, preferably diesel, as diesel engines are more efficient than their petrol counterparts. Hydrocracking may be carried out using a bifunctional catalyst, consisting of metal and acid components. Industrially, hydrocracking is used to crack heavy crude oil fractions into the desired fuel range, however, the catalysts used are sulphided transition metals. These are less suitable for cracking Fischer-Tropsch waxes as they would introduce sulphur into a clean feedstock. Moreover, at reaction temperatures of around 250°C, transition metal sulphide catalysts display little activity. Thus, one may consider noble metals such as palladium or platinum, whilst shape selective zeolites may be used as the acid component.
- ItemOpen AccessThe effect of oxygenates on the oligomerisation of propene over zeolite ZSM-5(1993) Langford, Steven Thomas; Fletcher, JackThe oligomerisation of alkenes such as propene and butene represents an important route to the production of environmentally clean transportation fuels. When these olefins originate from Fischer-Tropsch product streams they are often contaminated with small amounts of oxygenates such as acetic acid, ethanol, butanol, methyl-ethyl-ketone (MEK), acetone and water. Complex feeds can result in competitive reaction between the feed components and may result in observed selectivities that cannot be predicted from pure component data alone. ZSM-5 has been shown to be an excellent catalyst for alkene oligomerisation and is also active for the conversion of oxygenates. When pure oxygenates are fed over ZSM-5 at oligomerisation temperatures, acetone and MEK cause the catalyst to deactivate and the conversions are low whereas ethanol and n-butanol are completely converted and no deactivation occurs. At 250°C acetic acid undergoes a low conversion (4%) and at higher temperatures a decrease in conversion is also observed with time. The decrease in conversion of acetone has been attributed to the formation of a surface cyclic ketonic species. During acetic acid reaction dehydroxylation of the zeolite has been proposed as the cause of decreasing activity with time but this work has shown that the catalyst is regenerable and the acidity of the catalyst, as determined by ammonia TPD, remains unchanged after reaction with acetic acid. The propene oligomerisation and hexane cracking activity of ZSM-5 (Si/Al = 30) is reduced when the catalyst is exposed to oxygenates such as acetic acid, ethanol, n-butanol, MEK, acetone and water (mole fraction in feed < 0.01). In the case of all except acetic acid the activity is almost completely restored when the oxygenate is removed. Acetic acid causes irreversible loss of activity for propene oligomerisation and hexane cracking at 250°C. For MEK and acetone the activity for propene oligomerisation is restored to a greater extent (90% for acetone and 50-60% for MEK) than acetic acid but not fully. The decrease in activity for propene oligomerisation and hexane cracking at 250°C is proposed to be due to site poisoning by preferential strong adsorption of the oxygenates, especially acetic acid, onto the zeolite surface. An adsorption complex for acetic acid and the surface hydroxyls has been proposed in which the bond angles and lengths of the molecule are similar to those observed in the formation of the acetic acid dimer, consistent with the proposed strong adsorption.
- ItemOpen AccessThe effect of zeolite type on the hydrocracking of long n-paraffins(2008) Kukard, Ross S; Fletcher, JackAlthough it is debatable as to the lifetime of the planet’s crude oil reserves, it is indisputable that they are finite and will, eventually, become exhausted. As such it is desired to devise methods whereby currently available non-crude oil derived hydrocarbon feedstocks may be utilised for the production of clean, high quality liquid fuels (particularly middle distillate fuels such as diesel and jet fuel), aiding in alleviating the demand on crude oil reserves. One technique whereby this may be achieved involves the conversion of the non-crude oil derived hydrocarbon feedstock (for example stranded gas, remote natural gas, coal or biomass) to syngas (a mixture of CO and H2). This syngas is subsequently converted to paraffinic wax by Fischer-Tropsch Synthesis, and this wax selectively hydrocracked down to the desired distillate fuels fraction. This hydrocracking may be conducted utilising either monofunctional or bifunctional catalysts. Monofunctional catalysts, such as the supported sulphided base metals or metal oxides utilised in many crude oil refineries, yield a product with minimal additional branching, as desired, though would contaminate the otherwise clean Fischer-Tropsch wax with sulphur. Bifunctional catalysts, utilising a metal on an acidic support, yield a product with significant branching, yet do not contaminate the product. Furthermore, the acid supports utilised in oil refinery applications are limited to either amorphous silica-alumina, or H-USY (a thermally treated large pore zeolite) due to the presence of large, bulky polycyclic and highly branched molecules in the feedstock. Fischer-Tropsch wax exhibits only minimal branching, and it was hence the aim of this investigation to determine whether zeolites with pores smaller than those utilised in the hydrocracking of crude oil derived feedstocks, specifically medium pore zeolites such as H-MFI, may be utilised in the bifunctional hydrocracking of this wax to impart shape selectivity upon the reaction, thereby limiting the extent to which branching may occur. In this regard, four different zeolites (H-MFI, H-BEA, H-USY and H-MOR) were tested under the same, industrially relevant conditions, and the results collated so as to quantify the effects of the each zeolite’s unique properties, in particular their pore geometries (in terms of pore size, shape and channel inter-connectivity), the performance of each catalyst in terms of its activity (the overall conversion of the feedstock) and selectivity (with regards to both carbon number distribution and the degree of branching). Furthermore, it was desired to determine the extent to which the anticipated transition state shape selectivity of some of the zeolites affected the stability (on-stream lifetime) of the catalyst through a reduction in coke formation. The results of this investigation indicated that medium pore zeolites show significant potential for use in the selective hydrocracking of a Fischer-Tropsch wax feedstock. It was found that those zeolites possessing medium sized pores (specifically H-MFI and H-BEA) exhibited a significantly higher activity than did those with only larger pores (H-USY and H-MOR), a phenomenon theorised to be due to the more orderly and efficient configuration of the adsorbed molecules within the medium pores promoting contact with active acid sites. Furthermore, it was found that H-MFI, with a porous network comprised entirely of medium pores, showed an improved selectivity towards the desired linear products, whilst zeolites with only wide pores (H-USY and H-MOR) and those with intersecting wide and medium pores (H-BEA) showed branched product selectivities roughly equivalent to one another, all greatly favouring the production of mono-methyl branched species. Unfortunately, due primarily to the large variations in the observed activity between the zeolites tested, the results of the deactivation analysis were inconclusive.
- ItemOpen AccessFactors influencing the catalytic activity of Fe-ZSM-5 during the catalytic conversion of N₂O(2015) Van der Walt, Franschua Johan; Fletcher, JackZeolites have found widespread applications as acid catalysts for decades. By introducing transition metal ions in the cation position, the zeolite is transformed into a redox catalyst. The nature of the trivalent heteroatom influences the properties of the zeolite. Contrary to Al-zeolites, Fe-containing zeolites show redox properties, since Fe can easily change its oxidation state (Fe²⁺, Fe³⁺, or Fe⁴⁺). Catalytic function of isolated redox sites within zeolite cavities (or channels) may result in a material with specific redox properties (Kiwi-Minsker et al., 2003). The properties of transition metal exchanged zeolites have been studied from the 1960's onwards and the conversion of N₂O over Fe-Y zeolites has been studied by Fu et al. (1981) in late 1970's. In this study, the preparation of iron ZSM-5 zeolite catalysts by mechanochemical means and thermally induced solid-state ion exchange was studied. After grinding the NH4-Zeolite and ferrous chloride, no x-ray reflections characteristic of ferrous chloride are detected. After heating the sample to 120 and 200 °C reflections characteristic of ferrous chloride are visible but disappear upon further heating to 300 °C. No porosity is observed after grinding and heating up to 200 °C as a result of pore mouth blocking. Moreover, upon heating up to 500 °C porosity starts to develop with pore volumes and pore sizes slightly lower than those of the parent zeolite. From the thermogravimetric analysis it is evident that the ion exchange takes place during calcination from 150 and 420 °C in agreement with the literature. In the second part of the study commercial Fe-ZSM-5 catalyst samples with different N₂O conversion activities (in the presence of H₂O and NO at 425 °C), ranging between 70 and 90 % (high, mid and low activity) are studied and characterised. The effect of temperature during calcination of the plant produced and laboratory calcined extrudate catalyst material was investigated. Panov et al. (1996) reported in the literature that the Fe²⁺ is oxidised to Fe³⁺ in the presence of N₂O forming what they called the α-oxygen, a form of active surface oxygen, with the evolution of molecular nitrogen. During the conversion, two surface α-oxygen atoms migrate, combine and desorbs as molecular oxygen from the surface. The α-oxygen forms between 200 and 350 °C and desorbs as molecular oxygen above 350 °C (Taboada et al., 2005). In this study, no correlation to N₂O conversion activity could be found for the α-oxygen content and correspondingly the concentrations of the respective iron oxides and iron hydroxides in the Fe-ZSM-5 samples.
- ItemOpen AccessHigh throughput experimentation: a validation study for use in catalyst development(2016) Luchters, Niels; Fletcher, JackHigh throughput and combinatorial experimentation is becoming more and more used in catalysis research. The benefits of parallel experiments are not only limited to shorten the time - to - market, but also give opportunities to study the process in more depth by performing more experiments. The influence of a parameter, for example the amount of the active metal and/or promoter, to the process is better understood with a broader parameter space investigated. To study the parameter space, multiple experiments need to be performed. It is of paramount importance to understand the variability of the data between these experiments. This is not always defined, specifically when literature gives contradictory results, most often due to the time for duplicate experiments necessary. In this project the reproducibility and variance in high throughput catalyst preparation and testing was determined and the use of parallel experimentation was demonstrated within a catalyst development study. The high throughput equipment was used for catalyst development studies for fuel processing, the production of fuel cell - grade hydrogen from hydrocarbon fuels. Fuel processing consists of three catalytic reactions, namely reforming, water - gas shift and a CO clean - up through either selective methanation or preferential oxidation. Focus has been placed on the first two reactions, steam methane reforming (SMR) and medium temperature water - gas shift (WGS), using platinum group metals (PGM). All catalysts in this study (except for the commercial WGS catalyst) were prepared using automated synthesis robot (Chemspeed ISYNTH) and the activity testing was performed on the Avantium Flowrence. For both reactions two types of studies were performed, one - to - many and many - to - many; referring to one catalyst tested in many reactors or many prepared catalysts (same composition, different batches) tested in many reactors. For the WAGS one - to - many a commercial low temperature shift catalyst was selected and for SMR a single batch of Rh/Al 2 O 3 . The many - to - many experiments comprised of eight batches of prepared catalysts for both reactions. The WGS reaction was performed with 1 wt% Pt/Al 2 O 3 catalysts and for the reforming reaction batches of 0.5 wt% Rh/Al 2 O 3 was used. It was proven that in all these studies the experimental standard deviations in the data is 6%, from preparation to activity measurements. A study on the rhodium metal loading on alumina in the steam methane reforming catalyst was studied between 0.05 and 0.6 wt%. A 0.4 wt% Rh/Al 2 O 3 was found to have the highest activity per amount of rhodium. Lower Rh content would require decreased space velocity, whereas higher metal content does not increase the conversion due to larger crystals sizes. This study has been performed up to a metal loading of 0.6 wt% and it is recommended to follow - up with studying the range of 0.6 to ~2.5 wt% to investigate the optimal metal loading. It was shown that the use of automated experimentation (parallel preparation and evaluation under same condition) for catalyst development results in highly reproducible results with a relative standard deviation of ~6% activity. The high throughput equipment was demonstrate d to be a very powerful tool in catalyst research
- ItemOpen AccessThe hydrocracking of Fischer-Tropsch wax : using n-tetradecane as a model compound(2005) Kotsiopoulos, Athanasios; Fletcher, Jack; Böhringer, WalterIncreasingly stringent legislation has been applied to transportation fuels to minimise or eliminate aromatics and sulphur compounds in diesel fuel. This has led to manufacturers determining alternative production methods to comply to legislation. Part of the current diesel fuel is being produced by hydrocracking heavier fractions derived from crude oil. These hydrocracking processes utilise bi-functional catalysts which have a metal (hydrogenating/dehydrogenating) function and an acid (cracking) function. The most common of these hydrocracking catalysts are combinations of either noble metals and acid zeolites, such as Pt/ HY, or combined sulphides of group VIA and VIIIA metals on amorphous acidic supports, such as CoMo/SiO2-Al2O3. For good quality diesel, the fuel should have a high cetane number and the aromatics and sulphur content should also be kept to a. minimum (e.g. EU legislation: sulphur content must be below 10 ppm (wt) by 2008). Fischer-Tropsch wax is made up predominantly of long-chain linear paraffins with exceptionally low aromatics and heteroatom content (sulphur and nitrogen-containing compounds) and therefore a good source for very 'clean', good quality diesel. The objective of this study was therefore to investigate the suitability of a conventional bi-functional hydrocracking catalyst namely, CoMo/SiO2-Al2O3 in unsulphided form for the hydrocracking of Fischer-Tropsch wax using n-tetradecane as a model compound. The purpose of using the catalyst in unsulphided form was not to introduce any sulphur to the already sulphur-free feedstock.
- ItemOpen AccessHydrocracking of long chain n-Paraffins under Fischer-Tropsch conditions(2014) Koen, Matthew Anthony; Fletcher, Jack; Broisius, RoaldA number of various iron-palladium loaded H-MFI zeolites used for the hydrocracking of n- hexadecane under Fischer-Tropsch conditions were tested to address the inherent low CO tolerance of the pure palladium noble metal hydrocracking catalysts. The hydrocracking mechanism consists of two functions, namely the metal de-/hydrogenation (HD/DHD) and the acidic -scission function. The addition of CO to reactions involving monometallic palladium hydrocracking catalysts has led to an imbalance between these functions due to the migration of the noble metal resulting in significant and undesirable secondary cracking. However, the inclusion of iron to the hydrocracking catalyst may allow for chemical anchoring of the noble metal (Wen et al., 2002) reducing the effect of the migration and thus retaining the bifunctional balance. The consequent palladium-iron alloy (Garten, 1976) also has the potential for an improved rate of de-/hydrogenation (Fukuoka et al., 1990) resulting in a greater rate of intermediary carbenium ions which in turn could lower any undesired secondary cracking reactions already present. The Fe/H-MFI precursor was prepared using a solid-state ion exchange after which incipient wetness impregnation was used to add the palladium. Different loadings of palladium and iron were used to prepare the PdFe/H-MFI catalysts in order to determine an optimum ratio loading. All experiments were conducted at standard low temperature Fischer-Tropsch conditions in a plug-flow fixed trickle-bed reactor equipped with a homogeneously operating evaporator and on-line GC-FID analysis. It was found that none of the bimetallic catalysts produced showed any greater tolerance to carbon monoxide when compared to the monometallic catalyst. The results indicated that the behaviour of the bimetallic catalyst was near identical to that of the monometallic catalyst in the presence of CO. It was thus concluded that the preparation method used, in particular the Fe/H- MFI precursor through solid state ion-exchange, was unsuitable for the production of an alloyed PdFe/H-MFI catalyst. An effect of iron was noted in the low palladium high iron loaded catalyst i.e. PdFe/H-MFI (16,12). In the absence of CO, this catalyst showed a significantly improved selectivity when com- pared to the low palladium low iron catalyst, PdFe/H-MFI (16,24). This effect of iron was attributed to the blockage of the H-MFI pores due to the large amount of iron present. As a consequence of this, access to the internal acid sites is severely limited and therefore are essentially removed from the hydrocracking reaction. As such the PdFe/H-MFI (16,12) has an improved metal:acid site balance. Poisoning by water (a Fisher-Tropsch product) was found to significantly reduce secondary cracking due to deactivation of the acid sites (lowering of total acidity) resulting in improved selectivity through intermediary olefin product promotion. From this, almost pure primary cracking was possible allowing the noble metal catalysts to retain its ideal hydrocracking properties at very high conversions (as evident by the high isomerization selectivity). This indicates that if the total acid strength of the H-MFI zeolite could be reduced (e.g. dealumination), the overall catalyst selectivity could be improved. Testing into whether the effect of water in reducing secondary cracking could be used to offset the effect of an increase in secondary cracking by CO addition, proved ineffective. It is therefore thought that CO not only causes palladium migration and clustering on the external zeolite but also poisons the active metal sites still available. As a result the balance between the metal and acid function could not be restored. It is thus recommended that for future work a zeolite with a lower total acid strength be used in conjunction with a alternate method for iron addition. Furthermore, testing into higher loadings of palladium may prove fruitful in balancing its migratory nature in the presence of carbon monoxide.
- ItemOpen AccessThe hydrocracking of long chain n-paraffins under Fischer-Tropsch conditions(2012) Binneman, Jacqueline; Fletcher, JackInterest in the area of hydrocracking has grown rapidly over the years. In the early 1960's companies such as Chevron and Universal Oil Products (UOP) introduced new hydrocracking processes to manufacture high octane gasoline. The demand for transportation fuels such as diesel and jet fuel has increased significantly which results in the continuous development of hydrocracking techniques and catalysts. The conversion of normal long chain paraffins from the Fischer-Tropsch synthesis to clean distillate fuels is a particular area of interest. The objective of this project is to investigate the hydrocracking of long chain paraffins under F-T conditions. The aim is to achieve in situ, the hydrocracking of low temperature Cobalt-based F-T wax by combining F-T synthesis and hydrocracking in a single reactor. For the purpose of this thesis, it involves subjecting the hydrocracking catalyst to F-T conditions. Synthesis gas (carbon monoxide and hydrogen), the paraffin n-C16 and water were co-currently fed to a fixed-bed reactor containing only the hydrocracking catalyst. Therefore care was taken to match the experimental conditions of the hydrocracking experiments to those that prevail in the Fischer-Tropsch synthesis. Practically this means the hydrocracking of n-hexadecane was studied at the space velocity, the reaction temperature and pressure and under partial pressure of H2, CO and water, at which n-hexadecane is produced in F-T process assuming that n-hexadecane is the only hydrocarbon product and that n-hexadecane is a model compound for the low temperature F-T process. The results of this investigation show that the hydrocracking reaction over a Pd catalyst supported on H-MFI Zeolite under F-T conditions is non-ideal. At low feed (n-C16) conversions, product distributions are strongly dominated by secondary reactions. The ability of the metal site is significantly inhibited by the presence of CO and water. The product distributions show exactly this due to the increase in unsaturated and more branched species. Feed conversion in the presence of water and CO increase with increasing reaction temperature. The absence of methane in the product spectrum is an indication that the hydrogenolysis reaction is an unfavorable pathway for the catalyst used. The data obtained from this investigation suggests that the combination of low temperature Fischer-Tropsch and hydrocracking into a single reaction step is feasible.
- ItemOpen AccessInfluence of preparation techniques on the Fischer-Tropsch performance of supported cobalt catalysts(2003) Chirinos Maruri, Ada Elida; Claeys, Michael; Fletcher, Jack; Van Steen, EricCobalt based catalysts are generally used for the FT synthesis due to their high activity and selectivity for linear hydrocarbons, low activity for the water gas shift reaction and lower price compared to noble metals [22]. There can, however, be a large effective loss of active metal due to strong metal-support interaction forming complexes that are not reduced at temperatures below 400°C.
- ItemOpen AccessInvestigation into the behaviour of a wash-coated PGM-based catalyst layer onto micro-channel reactors for the steam reforming of methane(2017) Van Niekerk, Wesley; Fletcher, Jack; Luchters, NielsA wash-coating method which had originally been used for wash-coating a Rh/Al₂O₃ catalyst onto stainless steel micro-channels (MC) for the reforming of propane [24] was tested in the steam reforming of methane. The robustness of this method was unknown and was therefore tested for its possible application in methane steam reforming, which has far harsher reaction conditions. A 1 wt% Rh/Al₂O₃ catalyst was wash-coated onto heat treated MC reactor plates and tested at 700 °C with steam to carbon ratio of 3 at a number of catalyst mass specific space velocities (scc᛫(gcat᛫h)⁻¹). The MC tests yielded conflicting results with some tests having stable catalysts and the majority have unstable catalysts due to poor wash-coat adhesion. The unsuccessful cases were due to a loss of catalyst. The change in catalyst stability was postulated to be the result of the wash-coating suspension size being reduced too much. In the cases where catalyst instability due to poor adhesion and ultimately loss of the catalyst the suspension batch size was reduced such that the surface tension of the viscous suspension now exceeded the intermolecular forces in the liquid. This resulted bubble formation and due to the high viscosity of the suspension due to the presence of the polyvinyl alcohol (PVA) binder the bubbles remained during the wash-coating process which is thought to have adversely affected the wash-coats adhesion. Another possible cause which is thought to have amplified the poor adhesion of the unstable catalyst runs is the thermal expansion of the stainless-steel reactor plates. The results of this study could not give outright and straightforward conclusions as to why there were 2 stable runs and unstable runs due to a loss of catalyst. As a result, further work is required to confirm the postulations and trends seen in this study. Future work should concentrate on using a larger batch of suspension to mitigate bubble formation, adding an alumina primer layer before wash-coating the catalyst to aid adhesion through additional oxide bond formation and the use of a more thermally stable stainless steel reactor plate to mitigate thermal expansion.
- ItemOpen AccessMethanol conversion to olefins and propene oligomerization over modified SAPO-34 and dealuminated mordenite(1992) Van Niekerk, Miles; Fletcher, Jack; O'Connor, Cyril T; Kojima, MasamiSAP0-34 and mordenite, catalysts with quite different pore structures, are known to be suitable for methanol conversion to light olefins and propene oligomerization to a distillate type product, respectively. In this study, these catalysts were modified in various ways and the effect of these modifications on the activity and selectivity of the above two reactions investigated. SAP0-34, a small-pore silicoaluminophosphate molecular sieve, is highly selective in the formation of ethene and propene from methanol, but deactivates rapidly due to coke formation. This catalyst was synthesized and modified in various ways in an attempt to increase the catalyst lifetime and selectivity to ethene. Mild hydrothermal conditions encountered during deep-bed calcination of SAP0-34 were found to increase the catalyst lifetime. A number of further modifications were made to this deep-bed calcined material in an attempt to increase further the lifetime of this material. These modifications were : (i) Silanization - in order to neutralize the acidity on the external surface of the crystallites and hence reduce pore-mouth blockage by coke species on the crystallite external surface; (ii) Steaming - to investigate the effect of more severe hydrothermal conditions than those encountered under deep-bed-calcination conditions; (iii) Acid site poisoning by ammonia - in an attempt to reduce the rate of coke formation which takes place readily on strong acid sites; (iv) Boron impregnation - in order to reduce the intercrystalline void volume and thereby sterically hinder the formation of bulky coke molecules within the SAP0-34 pores; (v) Acid and caustic treatments - in order to reduce the catalyst acidity and thereby limit the rate of the coke formation reaction.
- ItemOpen AccessPerformance of gold catalysts for low temperature water gas shift(2001) Roberts, Stephen; Fletcher, JackThe ultimate objective of the study was to investigate the performance of suitably prepared gold catalysts for low temperature water gas shift (LTS) - and more specifically to investigate the performance of these catalysts at temperatures below those traditionally utilised. As opposed to the research undertaken to-date on gold catalysed water gas shift, the reaction was performed, as far as possible, under conditions resembling those found industrially, viz. conditions of temperature, pressure, WHSV and dry feed gas composition typical of those applicable to industrial LTS units. Important to this study was therefore the generation of a comprehensive performance benchmark for the commercially available LTS catalyst, a copper-based material, against which to compare the performance of the gold deposited catalysts. The gold catalysts were prepared by deposition-precipitation, a preparation procedure found to yield nano-sized gold particles, suggested in literature as being crucial for activity, on the metal oxide support. Using this procedure, gold promoted low (copper) and high (iron oxide) temperature shift catalysts and commercial zinc oxide supports were prepared and tested. A wide spectrum of Au particle sizes were prepared on the supports, ranging from approximately 3-500 nm. The gold promoted LTS catalyst was found to exhibit slightly higher activity than the commercially available catalyst at temperatures below the conventional LTS range. It would appear as if gold promotion is advantageous to the industrial catalyst and could impact greatly on LTS catalyst life. Even though substantially less active than the commercial copper catalyst was found, the gold promoted commercial zinc oxide catalyst exhibited significantly higher activity than that previously quoted in literature and better performance than the iron oxide supported catalysts of this study. Consequently, the Au/ZnO system exhibits good potential for further developments in terms of water gas shift conversion.