Designing Of ZrO₂ Based Catalysts And Their Catalytic Performance And Mechanism Studies For Non-oxidative Propane Dehydrogenation

Non-oxidative propane dehydrogenation?PDH?is an important industrial process for the on-purpose production of propene.Nowadays,the commercialized PDH processes mainly include Catofin and Oleflex processes,which apply the K-Cr O_x/Al₂O₃and Pt-Sn/Al₂O₃ based catalysts respectively.Although PDH process has been widely applied in practical production,however,some drawbacks,such as the high toxicity of chromium and highly expensive of platinum based catalysts,limiting their further development and utilization.Therefore,it is of significant practical meaning to develop newly eco-friendly,cost-efficient and superior catalytic performance PDH based catalysts.Recently,Zr O₂ based catalysts have been reported to be a new type of candidate which fulfills the above requirements and have broad research potential and application prospect.However,the active sites and reaction mechanism of Zr O₂ based catalyst in PDH reaction are still not clear.So,the thesis mainly investigates the effects of crystallite phase and crystallite size of bare Zr O₂ catalysts on the activity and selectivity in PDH reaction,then we try to elucidate the structure of active site and reaction mechanism at a molecular level through the combination of complementary characterization techniques and density functional theory?DFT?calculations.On this basis,the obtained catalyst structure-reactivity relationship could be used to guide the catalyst designing and preparation and may give some new enlightenment to the development of more excellent performance catalysts.?1?The first part mainly investigates the effect of crystallite size of bare monoclinic Zr O₂ catalyst on the catalytic performance of PDH and the way of C-H activation.The results reveal that the smaller the crystallite size,the higher the exposed edges,corners and steps defective sites,which should be coordinatively unsaturated Zr cations(Zr_cus),resulting in higher activity and propene selectivity in PDH reaction.DFT calculations also indicate that the active site of defective Zr O₂surface is two Zr_cussites located in one oxygen vacancy,the way of C-H bond activation is homolytic and the rate determining step is H₂ formation.However,the active site of stoichiometric Zr O₂ surface is Zr-O,the way of C-H bond activation is heterolytic and the rate determining step is the activation of C-H bond.The activation barrier of defective surface is higher than that of stoichiometric surface of Zr O₂.?2?The second part mainly compares the catalytic performance of different phases?monoclinic vs.tetragonal?of Zr O₂ catalysts in PDH reaction.The catalytic results show the rate of propene formation of monoclinic Zr O₂ catalyst is higher than that of tetragonal phase,which is explained as the higher reducibility of monoclinic Zr O₂catalysts,resulting in more Zr_cus sites formed.In addition,DFT calculations also indicate the active site of both phases are the same,that is two Zr_cus sites located in one oxygen vacancy.Thus,the intrinsic activity of different phases is very similar and the activity difference comes from the number of active sites.?3?On the basis of the structure-reactivity relationship of Zr O₂ catalysts and the regulation role of surface defective sites in PDH reaction and combined with highly dispersed metal cations and functional topologies of metal-organic frameworks?MOFs?materials.We successfully prepared the highly defects exposed Zr O₂ catalysts by using the Zr cations containing Ui O-66 materials as the precursor of Zr O₂.The obtained catalysts exhibit good catalytic performance in PDH reaction,the propene selectivity could reach 85%at propane conversion of 30%.?4?We prepared different metal nanoparticles?Rh,Ru,Pt and IrNPs?supported Zr O₂ catalysts and then investigate the effects of kinds and loadings of metal NPs on the activity and selectivity in PDH.Catalytic activity results reveal that Rh/Zr O₂ catalysts show the highest activity and the relationship between the rate of propene formation and Rh NPs loadings displays a volcanic curve.The positive effect of Rh NPs,on one hand,could be attributed to the reducibility of Zr O₂ support is largely enhanced with the help of Rh NPs,resulting in formation of more Zr_cus sites and thus increasing the concentration of active sites.On the other hand,the introduction of Rh NPs also enhances the H₂ desorption and thus decreases the energy barrier of H₂ formation.However,the activity will decrease when the loadings of Rh NPs is even higher,which could be explained as the promotion effect of H₂ desorption weakens due to the more difficult desorption of propene result from over-reduction of Zr O₂ support.In addition,we also investigate the effects of other metal NPs,such as Ru,Pt and Ir,on the PDH reactivity,it is found that the rate of propene formation and Ru loadings also displays a volcanic curve,however,increases monotonously with Pt or IrNPs loadings.