“Structure-activity” Relationship Of Copper-based Catalysts In Reaction Of Propene Selective Oxidation To Acrolein

Acrolein is an important organic chemical synthetic intermediate,which is mainly used in the production of resin and methionine.Copper-based catalysts have been considered as one of the most promising candidates and widely used in selective oxidation of propene to produce acrolein.However,to date,the major problems towards the reported copper-based catalysts are the low copper efficiency,controversial identification of active sites(Cu²⁺or Cu⁺),as well as the seldom exploration of reaction mechanism for acrolein generation from selective oxidation of propene.In addition,how to significantly increase the selectivity of acrolein and simultaneously reduce the generation of by-products,especially carbon dioxide,greatly limits the application of copper-based catalysts in industry.Therefore,it is our goal to design and synthesize new copper-based catalysts with low cost and high performance on propene selective oxidation to produce acrolein and elucidate the related“structure-activity relationship”.Based on the above research targets,we prepared a series of copper-based catalysts by mainly using deposition-precipitation method in this thesis.By regulating the supports,copper loadings and thermal treatment temperatures of such copper-based catalysts,copper species?clusters or nanoparticles?with different sizes,surface structures,and dispersion conditions were constructed,and their catalytic performance were systematically studied and optimized in selective oxidation of propene to produce acrolein.Combined with a variety of characterization techniques?in situ and ex situ?,the“structure-activity relationships”between the structural evolution of active center?site?and catalytic performance of the traditional silica and the new silicon nitride-supported copper-based catalysts were established.The main research findings are summarized as following:1.Determining the appropriate reaction conditions and catalyst compositions are the research basis for the reaction of selective oxidation of propene to acrolein.Therefore,in this thesis,the pretreatment conditions,reaction gas compositions,other metal doping and different supports of the catalysts were initially explored,and it was found that the copper-based catalysts exhibited excellent catalytic activity after reduction treatment at 300°C.Moreover,the composition of reaction gas was closely related to the distribution of catalytic products.The propene conversion and acrolein selectivity were optimized when the molar ratio of propene to oxygen was 1:1.However,it remains to be explored by doping other non-noble metals to improve the catalytic activity of such catalysts in this reaction.Finally,traditional silica and novel silicon nitride materials supported copper-based catalysts showed excellent catalytic activities in this reaction.2.The copper-based catalysts supported on high surface area silica were synthesized by deposition precipitation method,and the small-size 10wt.%Cu catalyst reacted at 300°C showed remarkable catalytic reactivity for selective oxidation of propene to acrolein,and the acrolein formation rate reached 127 mmol·h^-1·g_Cu^-1.In this section,multiple characterization techniques were applied to reveal the structural evolution and the formation of active sites of Cu/SiO₂ catalysts during catalytic reaction process.X-ray absorption fine structure?XAFS?combined with aberration-corrected high-angle annular dark-field scanning transmission electron microscopy?HAADF-STEM?techniques confirmed the small-size copper species transformed from amorphous copper oxide clusters to cuprous oxide clusters before and after the reaction.By using in situ X-ray diffraction?XRD?and in situ dual beam Fourier transform infrared spectroscopy?DB-FTIR?,we have convincingly identified that C₃H₆ can be effectively adsorbed on the surface of small-size copper species,and the allyl intermediate?CH₂=CHCH₂*?was clearly observed together with the as-formed Cu₂O species during catalytic test,the intermediate may react with oxygen species in the adjacent Cu2O to form acrolein during catalytic test,and the small-size Cu₂O clusters play key roles in this reaction.In addition,a combination of N₂O chemisorption,temperature-programmed surface reaction?TPSR?and CO temperature-programmed reduction?CO-TPR?measurements substantiated that the highly dispersed small-size Cu₂O clusters boost the catalytic reactivity on selective oxidation of propene at low temperatures?³00?C?,while large-size Cu₂O particles favor relatively higher temperatures?>330?C?.3.For the purpose of synthesizing highly thermally stable copper-based catalysts to significantly enhance their catalytic activities in propene selective oxidation reaction,nanosized amorphous silicon nitride?Si₃N₄?was selected to stabilize a series of copper oxides using a deposition-precipitation approach and followed by air-calcination treatment at various temperatures.It was found that by adjusting the thermal treatment temperature,the activity of the catalyst in selective oxidation of propene to acrolein can be effectively modified,especially the selectivity of acrolein,and the 800°C-calcined10wt.%Cu/Si₃N₄ catalyst showed superior catalytic activity with 24.0%C₃H₆conversion and 86.2%acrolein selectivity at 325?C,featuring an acrolein formation rate of 137 mmol.h^-1g_Cu^-1 or 12.7 mmol.h^-1g^-1_cat.By using transmission electron microscope?TEM?,X-ray absorption fine structure?XAFS?,and in situ X-ray diffraction?in situ XRD?,we have identified that the Cu₂O species of large-size were beneficial to the formation of acrolein at higher temperature?325°C?.Furthermore,with the help of CO-temperature programmed reduction?CO-TPR?and temperature-programmed surface reaction?TPSR?,we have confirmed that the presence of surface copper hydroxyl groups?Cu-OH?was closely related to the selectivity of acrolein,which favors the formation of CO₂ at the initial stage of the reaction.The surface copper hydroxyl groups could be effectively tuned by adjusting the calcination temperature of the catalyst,and thus show the optimized selectivity of acrolein together with a reduced CO₂ release,as well as the enhanced catalytic activity.The above results have provided a significant guidance for the rational construction of high activity and thermal stability copper-based catalysts,as well as the elucidation of the“structure-activity relationship”in the selective oxidation of propene to produce acrolein.