- Title
- Enhancing allyl alcohol selectivity in the heterogeneous catalytic conversion of glycerol
- Creator
- Sanchez Combita, Gizelle
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The production and commercialisation of liquid fuels from biomass as substitutes for petroleum-based liquid hydrocarbons is currently being heavily researched due to environmental and economic incentives. Biodiesel manufacture is one potential approach, however the development of a by-product waste disposal strategy is required, as surplus glycerol, the major undesirable by-product threatens sustainability of this alternative route. Even though crude glycerol is considered a financial liability for the biodiesel industry, it is also a low cost and readily available reactant. The valorisation of glycerol by means of direct allyl alcohol production has been previously investigated by other authors through processes that are highly selective to allyl alcohol but involve operating at conditions that seem both unsustainable and not economically viable for a potential industrial scale up, both in terms of reactor set up and choice of catalyst. Conversely, continuous catalytic one-step processes, which are much more desirable and potentially economically viable, endure other challenges such as low allyl alcohol selectivity, accelerated catalyst deactivation and unclear reaction mechanism, which create a gap of knowledge which needs to be addressed. This thesis describes the development of a process, at laboratory scale, for the direct and continuous conversion of glycerol into allyl alcohol over different heterogeneous catalysts, where the optimised engineering aspects of the unit operations can be used to scale up the reaction for industrial applications. With this aim, efforts were directed towards gaining a basic understanding of the reaction mechanism. Such information was used for catalyst development and process design. Minimisation of secondary reactions is desired for industrial scale allyl alcohol production, particularly since the presence of by-products such as acrolein is detrimental for further allyl alcohol processing. In previous investigations of the conversion of glycerol, acrolein formation has been linked to catalyst acidity. Thus, the acid-base catalytic properties of alumina-supported iron catalysts were modified by alkali metal deposition through different methods. The resultant catalysts were studied for applicability of glycerol conversion using a vertical packed bed reactor at atmospheric pressure and 340 °C, with nitrogen as carrier gas. Liquid phase products were analysed by GC-FID and GC-MS, while and gaseous phase products were analysed by micro GC and FTIR. It was found that increased catalyst basicity (as determined by TPD) favoured allyl alcohol production (up to a 2.7-fold increase in the allyl alcohol yield was obtained), reduced acrolein formation by up to 75 % and enhanced overall catalyst activity. Product distribution suggested that even though acid sites are required for allyl formation (since the initial dehydration of glycerol must occur), a high number of acid sites results in an increased rate of formation of acrolein. Alkali metals were also found to increase the concentration of hydrogen, carbon monoxide and carbon dioxide in the product gas stream. The increased concentration of the later species provided evidence of the potential involvement of a redox process in allyl alcohol formation from glycerol over alumina supported iron oxide, which is consistent with the stoichiometry of the reaction. The individual effect of reducing agents such as formic acid, oxalic acid, carbon monoxide and hydrogen as well as other organics such as acetic acid and propanoic acid was studied under the same operating conditions. Significant enhancement in the rate of formation and yield of allyl alcohol (which reached levels of 19.5 %) was observed when the reductants were added. This coincided with an increase in the rate of formation of carbon dioxide and a decrease in the yield of hydroxyacetone. However, acetic and propanoic acid had little effect on the rate of formation of allyl alcohol, providing further evidence of the participation of reductive species in the reaction. Further investigation of the reaction of glycerol and formic acid over the alumina-supported iron catalyst involved in situ techniques (temperature programmed desorption-mass spectrometry and in situ Fourier transform infrared spectroscopy). During the adsorption/desorption/reaction of formic acid on the alumina supported-iron catalyst, carbon monoxide was, in addition to water vapour, the lowest temperature decomposition product, while hydrogen and carbon dioxide were formed at higher temperatures. During the reaction of glycerol and formic acid, carbon dioxide formed at much lower temperature than that required for formic acid decomposition over alumina-supported iron oxide catalysts, and with different H₂/CO₂ MS signal ratio to that recorded from formic acid decomposition. IR bands observed in the course of the reaction of glycerol and formic acid over the iron catalyst were absent for heat treatment of neat glycerol and neat formic acid. Thus, they were thought to be a consequence of the catalytic reaction and are most likely attributable to carbon in cyclic compounds and to ring deformation. The introduction of more practical and inexpensive reductants (ammonia and ammonium hydroxide) in glycerol conversion favoured the rate of formation of allyl alcohol by up to 78 % and additionally resulted on the formation of a mixture of polymeric material collected as a solid product. The product was characterised using 1D and 2D NMR spectroscopy, FTIR spectroscopy, qualitative chemical tests and elemental analysis, enabling the identification of building blocks with unsaturated, amido and ester functionalities. The identified chain structures resemble those of polymers currently manufactured from fossil fuels and used in biomedical applications. Long term stability tests (120 hours of time-on-stream) were conducted in order to gain insight into the deactivation mechanism of the alumina-supported iron catalyst. At the end of the test glycerol conversion was approximately 70 % while allyl alcohol yield dropped to 6.1 %. Product distribution was found to be dependent on time-on-stream; poly-aromatic compounds and non-conventional coke (coke containing COO and C=O groups) were identified in used samples by transmission FTIR spectroscopy. The latter species was thought to be formed through poly-condensation of heterocyclic compounds (detected in liquid samples by GC-MS) over iron on alumina catalysts. The presence of carbonaceous deposits decreased the amount of micropores and mesopores as well as resulted in a drop in specific surface area, as determined by nitrogen adsorption. XPS results suggested that coke affected equally Al and active Fe sites, and Fe3+ was reduced to Fe2+ following catalytic measurements. Low and high temperature mechanisms of coke formation were thought to be involved in the formation of strongly absorbed species. Because of their shape selectivity, zeolite iron catalysts were studied in glycerol conversion. The catalyst prepared by alkaline iron exchange afforded moderate allyl alcohol yields. Characterisation of this zeolite revealed partial dissolution of framework silicon and aluminium (by ICP), changes in the micropore volume and increased mesoporosity. Additional enhancement in the rate of formation of allyl alcohol (which reached levels of 12 %) was obtained by rubidium deposition. Despite product distribution suggesting the occurrence of a deoxygenation process, where iron provides redox sites for the reaction to take place, hydroxyacetone was not associated to the formation of allyl alcohol from glycerol over the more acidic iron exchanged zeolites.
- Subject
- allyl alcohol; glycerol; acrolein; catalysis; iron oxide; zeolites; alumina
- Identifier
- http://hdl.handle.net/1959.13/1322454
- Identifier
- uon:24586
- Rights
- Copyright 2016 Gizelle Sanchez Combita
- Language
- eng
- Full Text
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