Upgrading of Ethanol to Value-Added Oxygen-Containing Chemicals
Author:Wang Qing Zuo
Supervisor:lu an hui
Ethanol is a widely available,versatile platform molecule,which is readily converted catalytically to oxygen-containing value-added chemicals.However,in ethanol upgrading reactions,achieving selectivity for specific products remains challenging due to the complexity of competing reactions,such as dehydration,dehydrogenation,esterification,ketonization,coupling,etc.Therefore,the key is to design efficient active catalysts based on the reaction mechanisms.In this thesis,we fabricated C/SBA-15 composite-supported Cu catalysts,which efficiently catalyze dehydrogenation of ethanol to acetaldehyde.Moreover,hydroxyapatite nanowire rich in[Ca-O-P]was synthesized.High density of the active sites accelerates C-C coupling rates,thus boosting high carbon number species production,such as C6-12 alcohols.In the last part,Co-hydroxyapatite directly catalyzes ethanol to aromatic alcohols,which is an alternative route to the petroleum-based technology.The detailed research contents are as follow:(1)Dehydrogenation of ethanol to acetaldehyde.Carbon-coated SBA-15 composite(C/SBA-15)was prepared by using carbon-coating method.Under H2 atmosphere,the Cu nanoparticle catalyzes the gasification of the carbon in contact with it,thus etches the carbon layer and then,interacts with the SiO2.The obtained Cl/C/SiO2 catalysts include C-SiO2,Cu-C,and Cu-SiO2 interfaces.Carbon covers the[Si-OH]groups on SBA-15 surface and promotes the reduction of Cu+ species to Cu0,which reduces the concentration of active sites for side reactions of CH3CHO,leading to a high product selectivity(up to 98%).The chemical interaction between Cu and SBA-15 of the C/SBA-15 support contributes to the prevention of agglomeration of Cu particles.The designed Cu/C/SBA-15 catalyst showed a high acetaldehyde selectivity(up to 98%)and good stability for a 60 h test at 260 ℃ and weight hourly space velocity of 2.4 gC2H5OH·gcat.-1·h-1 in ethanol dehydrogenation.(2)Upgrading of Ethanol to higher aliphatic alcohols.A nanowire-like hydroxyapatite rich in[Ca-O-P]sites was prepared.The[Ca-O-P]site is responsible for the C-C coupling and its high amount increases the surface coverage of reactants,thus enhancing the C-C coupling reaction rate,which boosts C6-12 alcohols production.This novel catalyst exhibited 30.4%selectivity to n-butanol and 63.9%selectivity to C6-12OH in a conversion of 45.7%at 325 0C and weight hourly space velocity of 0.8 gC2H5OH·gCat.-1·h-1.Ethanol mainly undergoes a "direct-coupling" pathway to form higher aliphatic alcohols over nanowire-like hydroxyapatite.Studies on spent catalysts indicated that oxygenated poly-aromatic species derived from acetaldehyde,which was slighty produced from ethanol dehydrogenation,poison and cover the active sites,thus causing deactivation.The activity was completely recovered after recycling at 500 ℃ under air.(3)Upgrading of ethanol to aromatic alcohols.A cobalt-hydroxyapatite catalyst enabled to direct conversion of ethanol to methylbenzyl alcohols.[Co-O-P]site catalyzes ethanol dehydrogenation to acetaldehyde,which undergoes rapid hydroxyapatite-catalyzed condensation,dehydrocyclization,and hydrogenation reactions to form methylbenzyl alcohols.The rate-limiting step is the dehydrogenation of ethanol to acetaldehyde.The key intermediate is 2-butenal,which undergoes self-condensation and hydrogenation processes to yield aromatic alcohols,that is,2-butenal → 2,4,6-octatrienal →2-methylbenzaldehydes→ 2-methylbenzyl alcohols;2-butenal → 4-methylbenzaldehydes→4-methyibenzyi alcohols.In the presence of Co2+,2-butenal can be partially hydrogenated to 2-butenol;interestingly,this selective hydrogenation does not hinder aromatization because of the cross-coupling between 2-butenal and 2-butenol leading directly to MB-OH without passing through MB=Ol that is,2-butenal + 2-butenol →2/4-methylbenzyl alcohols.Both high ethanol partial pressures and high reaction temperatures promote the hydrogenation of carbonyl groups in aromatic aldehydes and 2-butenal,using H2(from ethanol dehydrogenation)and/or ethanol itself as H-donor,thereby favoring aromatic alcohols formation.This cobalt-hydroxyapatite catalyst showed a 54%selectivity of 2-methylbenzyl alcohols in a conversion of 35%at 325 ℃ and weight hourly space velocity of 1.0 gC2H5OH·gCat.-1·h-1.