Preparation And Reaction Mechanism Of Supported Gold Catalysts For Selective Hydrogenation Of Alkadienes

Supported gold nanocatalysts have been demonstrated to be active in a number of oxidation-reduction reactions.Although the catalytic activity of gold catalysts was found to be lower than that of Pt catalysts,a striking feature in the selective hydrogenation of alkadienes is that it can achieve a 100%selectivity to alkene,which offers new opportunities to significantly improve the economics.However,it will take a long time before the use of gold in large industrial processes.Mechanistic aspects regarding the activity of supported gold nanoclusters are still under debate.At present,the origins of catalytic activity of Au have been proposed to stem from three contributions:?i?quantum size effects,which causes electron density increased;?ii?presence of low coordination Au sites,such as edge and corner atoms;?iii?charge transfer between reducible oxides and gold nanoclusters,which causes the part of particles became either electron-rich or electron-poor atoms at the interface and periphery.Because it is possible that the real catalyst systems include all of them,the relative contributions of each of these factors to the special catalytic properties are worth deep exploration.Therefore,this fundamental research tries to cast a little further light on the design of Au catalysts with high activity in the futher.The emphasis in this dissertation is organized by the factors related to the activity,and corresponding reaction mechanism of alkadienes hydrogenation was also understood.Four supported Au catalysts were prepared by co-deposition-precipitation with urea?DPU?and chemical reduction?CR?.After characterized by H₂-TPR,XRD,HRTEM and in situ FT-IR spectroscopy of CO adsorption,four categories of active site?anionic Au on ZrO₂,cationic Au on SiO₂,metallic Au on functionalized SiO₂?N–SiO₂?and anionic Au with OH groups on CeO₂?were observed,according to the reducibility of as-prepared gold samples,crystallinity of supports,size distribution of Au nanoparticles and charged species populated at support-particle interface,which were formed by the SMSI effect.It is the first time to test these four catalysts in the reaction of selective hydrogenation of isoprene.The different performance showed that the charge states have significant effect on the intrinsic activity.Thus the?charge sensitive reaction?was observed rather than the?size-dependence reaction?.The role of anionic gold should not be underestimated.Furthermore 3,1-isomer is the major product at low isoprene conversion??80%?.After that,the most stable 2,2-isomer predominated the product distribution.Isopentane was not observed for the whole range of isoprene conversion,therefore the selectivity to isopentenes is still excellent.The small Au nanoparticles?NPs?with narrow size distribution in solution were prepared according to the Brust-Schiffin method.Then these Au NPs were directly attached onto the NH₂-SiO₂ support via ultrasonic treatment.The Pt/N-SiO₂ was prepared as a reference catalyst.After six Au and Pt/N-SiO₂ catalysts evaluated in the reaction of 1,3-butadiene hydrogenation between 213-223 K,it was found that supported Au NPs have the ability to catalyze the hydrogenation reaction,while the Pt catalyst is inactive,which is contradictory with the common observations.The hydrogenation reaction is size-dependent in this condition.The highest TOF stops at 2-3 nm rather than 1-2 nm and the selectivity to butene is still as high as 100%.In addition,the?-bonded species on Au particles have been characterized by in situ FT-IR spectroscopy,which are the result of double bonds hybridization from butadiene to chemically adsorb on the most active surface.This observation is not primary but important evidence that Au NPs have high catalytic activity at ultralow temperature.Again the selectivity to 1-butene in this condition is also different,because the isomerization reaction to2-butene is almost negligible in the mixture of H₂ and 1-butene.At last,more detailed experimental and theoretical studies are needed to establish the role of low temperature on the electron affinity of Au and the reason why the Pt NPs are inactive.The plausible scenario in this reaction is that the first H atom was added to a terminal carbon of?-bonded species to produce the radical,then another H atom was added to an internal carbon to produce 1-butene,which is less easily to be adsorbed.A kind of ionic liquid,octyl triphenylphosphonium bromide?OTPPBr?was prepared as the capping agent to stabilize Au NPs.The most stable ZrO₂ on thermodynamics and Al₂O₃used in many hydrogenation applications were used as supports.At the beginning,the thermal treatment condition for OTPPBr@Au/ZrO₂ was determined by combined thermogravimetry and differential scanning calorimetry?TG-DSC?results.The Au NPs were oxidized by OTPPBr at 425?in N₂.However,almost all the Au NPs sintered to large particles simultaneously,which lost their activities.Then,the OTPPBr-DPU@Au/ZrO₂ catalysts were prepared by DPU method in order to control the Au NPs in small size after reduced at 200?in H₂.Among them,the 0.01 gOTPPBr-DPU@Au/ZrO₂ has the stronger ability to dissociate H₂ according to the H₂-TPD results.Unfortunately,1,3-butadiene hydrogenation reaction could not occur over this catalyst.It is possible that a lot of OTPPBr with large molecule size have blocked some pore canals,which are strong diffusional resistances inside the catalyst for C₄ molecules.Thus the ability for hydrogen dissociation is a necessary but not sufficient condition for the real hydrogenation catalyst.When the corresponding support was replaced by Al₂O₃ which has larger pore size and volume,the catalytic activity was improved.This evidence means it is the suitable textural properties that have a significant effect on the interaction between active sites and reactant.Compared 0.01 gOTPPBr-DPU@Au/Al₂O₃ with reference catalyst Au/Al₂O₃ with negatively charged Au atoms,only neutral Au atoms were observed when the OTPPBr was added.Furthermore,the Au NPs dispersion increased as well as 1,3-butadiene conversion.However,the TOF did not increase by at least one order of magnitude,so the role of OTPPBr is only limited at optimizing the size distribution.The positive Au atoms were not produced and the negative Au atoms have less effect on catalytic activity.The product distribution,1-butene?cis-2-butene?trans-2-butene,did not change whatever the active site.