Coking Behavior In The Steam Reforming Of Model Compounds Of Bio-oil:Understanding The Reaction Intermediates And The Properties Of The Coke

This thesis aims to study the correlation between reaction intermediates and properties of the coke in steam reforming of bio-oils for renewable hydrogen production.The nickel-supported mesoporous catalysts（Ni/SBA-15 and Ni/KIT-6）were used.The selection of these catalysts was due to their efficiency for hydrocarbons cracking,their ability for operating at high temperatures,the evidence of the method of synthesis,the availability and affordability of nickel as revealed the literature.Some organic compounds,selected as representing the most abundant organic models constituting the bio-oil such as methanol,ethanol,isopropanol and n-hexanol for alcohol;acetaldehyde for aldehydes;acetone for ketones;formic acid and acetic acid for carboxylic acids;n-hexane for linear aliphatic compounds;and guaiacol and toluene for oxygenated and non-oxygenated aromatic organic compounds,were steam reformed under the normalized conditions.That is a constant temperature between 350 to 750℃,the reaction time of 5h,the catalyst loading of 0.5 g,the steam to carbon ratio（S/C）between 2 to 5（under these conditions,the^*OH supply from the dissociation/adsorption of steam and which help for the gasification of coke precursors is lower,while the coking is favors）,the space velocity（LHSV）between 8 to 12 h^-1.The tests were conducted in atmospheric pressure through the fixed bed continuous flow reactor.While the composition and the yield of the gaseous products were analyzed by the gas chromatograph（GC）,the catalysts were characterized by several techniques such as X-ray diffraction（XRD）,nitrogen adsorption/desorption isotherm,temperature programmed reduction（H₂-TPR）,temperature programmed oxidation（TPO）,element analysis（EA）,thermogravimetric（TG）,Raman,Fourier transformed infrared spectroscopy（FT-IR）,contact angle,and transmission electron microscopy（TEM）.The reaction intermediates were analyzed and identified by the means of in-situ diffuse reflectance infrared Fourier transformed spectroscopy（DRIFTS）.The secondary compounds formed from interconversion of reactant during the reforming reaction were analyzed by the gas chromatograph couple with mass spectrometer（GC-MS）,while the UV-fluorescence analysis helped for identification of aromatic structures formed during the reformation reaction.The results obtained and published in various articles（Dissemination of Research Results）can be resume as presented below.The aliphatic and aromatic structures in bio-oil such as n-hexane and toluene were the greatest contributors to the coking during the steam reforming of bio-oil.Because,as compare to other organics model studied current this thesis,their coke content was the highest while the experiment condition was almost the same.This was mainly due to insufficient oxygen-content intermediates which help for gasifying the precursors of the coke.The main coke precursors were an ethylenic type compounds with the double bond（C=C）.These intermediates tended to rapid polymerization at higher temperature.It was found that these intermediates were formed from various routes depending on hydrocarbon structure.For aliphatic chain hydrocarbon structures such as n-hexane,the intermediate precursor of the coke was mainly formed by dehydrogenation of reactant followed the rapid polymerization,leading to the formation of coke of multiwalled carbon nanotubes with almost smooth surface and hydrophobic,as a main morphology.In contrast,with toluene,the encapsulated coke,the sintering of the nickel particles and the formation of carbon nanotubes with large cavities and rough surfaces with the nickel particles at the top wrap with a carbon layer were identified as the main causes of deactivation of the catalyst.While the polyaromatic hydrocarbons precursors of coke were formed either directly by the polymerization of benzene from demethylation of toluene or by conversion of pyrolytic carbon from the decomposition of the intermediate radical-CH_x^*.These results were totally different from those obtained for oxygen containing organics.Indeed,because of the lack of oxygen-contain intermediates revealed in the steam reforming of n-hexane and toluene,and which the consequence was the abundant formation of the coke,we undertook a study,under similar conditions on oxygenated organic compounds,models of bio-oil,such as ethanol,acetaldehyde,acetone and acetic acid.Lower rates of the coke were formed for total conversion of the reactants compare to those of n-hexane and toluene.This result was attributed to the abundance of CH_xO^*reaction intermediates,which assisted in the gasification of C=C species,precursor of the coke.The main problem encountered in this case was a severe interconversion of reactants which led to the abundant formation of secondary organic compounds.These secondary compounds limited the fully exploitation of hydrogen potential of indicated reactant.On the other hand,the oxygenated reaction intermediates highly affected the properties of the coke formed.For example,in the steam reforming of ethanol,the multi-walled carbon nanotubes with thick wall,large inner diameter and rough surface were formed.Rough surface of the carbon nanotubes was also formed in the steam reforming of acetone with larger outer diameter.Similar properties were obtained in the carbon nanotubes formed in steam reforming of acetic acid with the main difference of the long waist and smooth surface.However,in the contrary,the dehydrogenation of acetaldehyde forms C_xH_yO^*intermediates which undergoes fast polymerization to form amorphous coke of lower thermal stability with higher reactivity to oxidation.This polymeric coke wrapped the catalyst surface leading the rapid deactivation despite the insignificant yield.Following multiples issues encountered during the first’s parts of our investigations,that is the lack of oxygenated organic intermediates in steam reforming of n-hexane and toluene or the sever interconversion of reactant in the steam reforming of ethanol,acetic acid and acetone,we undertook to explore some ways to fight against these issues.Thus,in the first instance,we explored the application of the layered catalysts arrangement in the reactor bed.This strategy,as indicated the literature,have been effective for the steam reforming of hydrocarbons when using the protective catalyst layers as upper layer.Thus,we tested oxygenated models of bio-oil such as ethanol,acetone and acetic acid and non-oxygenated ones such as n-hexane and toluene,always under the normalized condition at 650℃.Almost the similar reaction intermediates as that reported in our previous investigations were obtained.But them,the yield of the coke formed in the steam reforming of ethanol,acetone and acetic acid were relatively lower than that previously obtained,the properties of the coke were almost similar.Nevertheless,the interconversion of reactants was not ameliorated by using this strategy.In contrast,in n-hexane and toluene,the C_aH_b-type reaction intermediates could pass through the upper layer of the catalyst to be reformed on the lower layer,leading to generation of coke with similar yield and properties as that in the upper layer.Furthermore,severe formation of encapsulated coke and sintering were observed in the lower layer of catalyst in steam reforming of toluene.This was the main cause of the drop of conversion and gaseous products yields,revealing the catalyst deactivation,while the carbon nanotubes were speedily formed in steam reforming of n-hexane both in the upper and lower layers of the catalyst.Some other works,exploring the effects of externally oxygen supply on the reaction intermediate and the properties of the coke in the steam reforming of toluene was conducted and the promising results obtained were published in the manuscript 2（Dissemination of Research Results）.While a comparative study of the effects of reaction intermediates generated during steam reforming of oxygenated aromatic compounds such as guaiacol,or alcohol such as n-hexanol,all considered as models of the bio-oil,big contributor to the coking,on the properties of coke were reported in manuscript 5（Dissemination of Research Results）,the effects of ketonization reaction during the steam reforming of acetone and isopropanol on the properties of the coke were reported in manuscript 6（Dissemination of Research Results）.In sum,the results obtained during this thesis,which have been published or are still in process of being published,constitute a greatest source of relevant and exploitable information,mainly in the refining industries for improvement of the maintenance and regeneration policy of the spent catalysts.Moreover,this information can also help in guiding certain decisions when defining the sizing design parameters of certain industrial catalysts for the steam reforming of fuels,bio-fuels or hydrocarbons.