The selective oxidation of lower alkane components of natural gas, such as ethane, to partial oxygenates remains a major challenge for both industry and academia. At present 60% of industry’s 7.8 Mt annual acetic acid demand is met through the carbonylation of methane, the operation of which is highly energy intensive, leads to formation of corrosive iodide by-products and requires high pressures of CO. Meanwhile ethene, a feedstock of great industrial importance, is produced via steam cracking of alkanes such as ethane, a process which is typically operated at > 800 oC and accounts for ca. 40% of the petrochemical industry’s annual energy consumption. The development of an atom efficient, low temperature, environmentally benign process for the direct conversion of ethane to either of these molecules would circumvent the need for current practices and thereby represent an important milestone in the valorisation of natural gas. The heterogeneous catalyst system explored in this thesis is based upon zeolite (ZSM-5) catalysts, which may be modified via post synthesis deposition of either or both of iron and copper and are shown to selectively transform ethane to a variety of higher value products including but not limited to ethanol, acetaldehyde, acetic acid and ethene under mild, environmentally benign conditions (H2O2 as oxidant, water as solvent, temperatures of < 90oC). C-C scission of C2 products led to formation of carbon centred radicals and yielded C1 products including methylhydroperoxide, methanol and formic acid, whilst deep oxidation yielded CO2, typically at selectivities of < 5%. The method by which catalysts were prepared was shown to impact significantly upon catalyst performance, with chemical vapour impregnation, a novel vapour deposition technique being shown to yield highly active catalysts. Investigations of reaction conditions such as ethane pressure, ethane partial pressure, temperature, oxidant concentration and catalyst mass [Chapter 1] III were conducted in a batch reaction system. All parameters were found to impact significantly upon both catalytic activity and product distributions allowing for directed selectivity to either ethanol or acetic acid as major product. Through extensive mechanistic studies, it was shown that a complex reaction scheme operates with these catalysts, which results in the primary C2 products ethanol, ethylhydroperoxide and ethene. Of these the former two were shown to undergo consecutive oxidation to acetaldehyde and acetic acid, whilst ethene was shown to react under test conditions to yield acetic acid. Additionally, upon deposition of Cu2+ onto ZSM-5 catalysts ethene was shown to become the major reaction product, with selectivities of 45.5% at 1.15% conversion. Following development under batch reaction conditions, the ZSM-5 catalysts were then applied under a continuous flow regime through co-feeding of an aqueous hydrogen peroxide solution and mixed ethane/ argon feed through a custom built fixed bed trickle bed reactor system. Through optimisation of reaction conditions 23% ethane conversion to acetic acid (73% selectivity) was observed. Through varying the catalyst bed makeup, turnover frequencies equal to those observed at comparable conditions under batch reaction conditions were observed, thereby showing the viability for translating this catalyst system to a more industrially viable continuous flow system. High ethene and acetic acid selectivities of 37.8% and 43.0% were observed, respectively at 3.3% ethane conversion upon testing of 1.25% Fe 1.25% Cu/ZSM-5 (30) within this regime at 50 oC. Subsequent studies focused upon elucidating the role played by aluminium sites within the zeolite framework in catalyst activity and determining product selectivities. This has led to the development of Cu/ZSM-5 and FeCu/ZSM-5 catalysts which are not only highly active for the activation of ethane, but also highly selective for the formation of ethene, with productivity to ethene alone shown to reach 25.6 mol (ethene) kg-1 (catalyst) [Chapter 1] IV h-1 (at 50% ethene selectivity). Through studies of ZSM-5 catalysts of varying support composition, headway has been made in decoupling the effects which increased exchange capacity (aluminium content) has upon catalyst performance, thereby paving the way for future development of more active, selective catalysts for the transformation of ethane to higher value products, specifically ethene. An interesting aspect of this work was the discovery that copper oxide particle size and size distribution may be controlled through varying of the ZSM-5 support’s SiO2/Al2O3 ratio due to a metal support interaction.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:610986 |
| Date | January 2013 |
| Creators | Armstrong, Robert |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/59701/ |
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