Defects And Active Oxygen Species Regulation Of Cerium-Based Catalysts For Low-Temperature Oxidative Dehydrogenation Of Cyclohexane

As an important organic chemical material,cyclohexene is widely used in agrochemicals,pharmaceuticals,feed,polyester,and other fields.Oxidative Dehydrogenation（ODH）of cyclohexene to produce cyclohexene is a promising green process that can realize the resource utilization of low-value-added cyclohexane and the recycling of "benzene-cyclohexanecyclohexene".During this process,factors such as the activation of the C-H bonds,the dissociation of oxygen,and the desorption of cyclohexene on the catalyst surface will directly affect the conversion of cyclohexane and the formation of cyclohexene.Thus,well-designed catalysts with high activity and selectivity play a significant role in balancing the contradiction between cyclohexane conversion and cyclohexene selectivity.This thesis is committed to the improvement of cerium-based catalysts,mainly by means of crystal facet control,bulk phase doping,and surface modification.Through these methods,the morphology and size of particles were controlled at the nanoscale,the reactivity and redox properties of Ce³⁺/Ce⁴⁺ ion pairs were improved with the surface defect concentration and active oxygen species distribution adjusted,and their performance in catalyzing the low-temperature ODH of cyclohexane was investigated.The main contents are as follows:1.Facet Control of CeO₂.CeO₂ nanocrystals with cube,rod,and octahedral morphology mainly exposed to {100},{110} and {111} crystal surfaces were synthesized by hydrothermal method,respectively,and the catalytic activities of different crystal facets of CeO₂ were investigated.The results show that the catalytic activity is related to the content of Ce³⁺on the surface.The {110} crystal plane is most favorable for cyclohexane conversion,while the {100}crystal plane is conducive to the formation of cyclohexene.Specifically;the exposed crystal facet can affect the redox properties,the concentration of oxygen vacancies of the catalyst,and the desorption ability of the target product cyclohexene.Therefore,CeO₂ nanocubes with moderate oxygen vacancy/Ce³⁺concentration and cyclohexene adsorption energy can effectively activate cyclohexane,while also having a good ability to desorb cyclohexene,resulting in a better cyclohexene yield（7.8%）.Further research shows that the cyclohexane ODH to produce cyclohexene on ceria follows the Langmuir-Hinshelwood mechanism.Oxygen vacancies function as the active sites for competitive adsorption of cyclohexane and oxygen,and the lower reaction temperature makes the main active agent electrophilic oxygen species.2.Bulk Phase Doping of Zr.To further improve the defects concentration of cerium-based catalysts to promote the activation of cyclohexane,a series of ceria-zirconia mixed oxides catalysts with different Ce/Zr molar ratios were synthesized by hydrothermal method.It is found that the appropriate amount of Zr doping can effectively improve the cyclohexane conversion and cyclohexene selectivity,among which Ce_0.3Zr_0.7O₂ has the best catalytic performance with a cyclohexene yield of 11.0%and exhibit excellent activity stability at 350℃.The characterization results show that the activity of cyclohexane ODH is not directly related to the basicity of the catalyst,but positively correlated with the concentration of Ce³⁺;The distribution of C6 products is affected by the acidity of the catalyst,i.e.,the weak acid site is beneficial to the improvement of the selectivity of cyclohexene,while the medium and strong acid sites will lead to the formation of more deep dehydrogenation products,benzene.By changing the feed material and conducting three independent hydrocarbon oxidation reactions of cyclohexane,cyclohexene,and benzene,the results show that COx is largely generated by the direct combustion of cyclohexane.It is speculated that oxygen vacancies promote the formation of highly reactive oxygen species,making it easier to break the C-C bonds,which provides a new perspective for further improving the catalytic performance.3.Surface Modification of Cl.To inhibit the direct combustion of cyclohexane,the surface of CeO₂ nanocubes was impregnated with different concentrations of NH₄Cl solution to improve the nucleophilicity of active oxygen species.The results show that NH₄Cl treatment can significantly decrease the degree of cyclohexane over-oxidation,and 47.1%cyclohexene selectivity can be achieved on 4Cl-c-CeO₂ at 350℃ with a cyclohexene yield of 11.8%.Specifically,chlorine modification can not only regulate the content of peroxides(O₂^2-)on the ceria surface,but also occupy the surface oxygen vacancies and interact with Ce³⁺ to form CeOCl,thus realizing a synergistic effect of both CeOC;and O₂^2-sites to promote the formation of cyclohexene.According to the characterization results and theoretical calculation analysis,chlorine modification has no significant impact on the energy barrier of initial C-H bond activation in cyclohexane molecules,mainly changing the electronic properties and acidity on the surface of CeO₂ nanocubes.Its contribution to the O-terminal CeO₂{100} surface lies in the reduction of surface defects,resulting in the decrease of electrophilic peroxides(O₂^2-),thereby inhibiting the complete oxidation of cyclohexane;At the same time,the increase of nucleophilic CeOCl species and weak acid sites promotes the rapid desorption of cyclohexene from the catalyst surface.4.Surface Modification of Halogen Elements.Based on the successful application of chlorine-modified cubic ceria in promoting the selective conversion of cyclohexane to cyclohexene,CeO₂ nanorods with more abundant surface oxygen species and more defects were selected as the research object,and NH₄F,NH₄Cl and NH₄I solutions of the same concentration were used for impregnation treatment to further clarify the action mechanism of halogen modification on surface active oxygen species.The results show that the surface modification of fluorine,chlorine,and iodine could inhibit the over-oxidation of cyclohexane in varying degrees and effectively improve the selectivity of cyclohexene.Halogen surface modification,on the one hand,regulates the surface acidity of the catalyst,on the other hand,affects the distribution of surface active oxygen species(O_<sup>2-,O₂^2-,O-and O_<sup>2-),producing more active lattice oxygen species and increasing the nucleophilicity of surface oxygen species.Normalizing the acidity of each catalyst by using surface area further clarifies the gain effect of weak acid density on cyclohexene selectivity.Through the in-depth study of pulse oxygen isotope exchange experiments,it has been found that halogen modification weakens the kinetics of oxygen exchange on the CeO₂ surface,and inhibits the total oxygen exchange activity by limiting the rate of oxygen incorporation,resulting in a decrease in the reactivity of surface oxygen,which in turn has an impact on catalytic performance.By correlating the catalytic performanc,O_<sup>2-and O_<sup>2-species are found to be conducive to the formation of cyclohexene,while O₂^2-and O-species are prone to produce COx.