First-principles Studies On Selective Dehydrogenations Of Aliphatic Derivatives

Aliphatic compounds,also known as aliphatic hydrocarbons,are hydrocarbons that do-not contain benzene or other aromatic rings in their molecular structures,which are a group of substances that occur widely in nature and an important class of chemical intermediates.Saturated aliphatic compounds and their derivatives are limited by the chemical stability of C-H bonds,making them less active in many chemical reactions.On-surface synthesis,as an extension of heterogeneous catalysis,has shown obvious advantages in achieving selective dehydrogenation of aliphatic derivatives.Although numerous scholars and researchers have successfully implemented experiments on single crystal metal surfaces for catalytic alkane conversion,the mechanism of activating the carbon-hydrogen(C-H)bond and the reaction pathway during the reaction of aliphatic hydrocarbons on the surface of metal single crystals are still lacking of in-depth studies due to the limitation of experimental characterization and other factors.In recent years,with the increase in arithmetic power brought by the localization of high-performance supercomputers and the development of Density Functional Theory(DFT),we are now able to use computer simulations to study experimental intermediate states and reaction paths that cannot be directly detected the pathway that reactions occur by modern highresolution characterization instruments such as atomic force microscopy(AFM)and scanning tunneling microscopy(STM)Based on density flooding theory calculations,the aim of this thesis is to investigate the reaction mechanism of aliphatic derivatives dehydrogenation on metal surfaces.By analyzing the reaction paths,studying the adsorption behavior of aliphatic molecules on metal surfaces and their diffusion migration,and analyzing the electronic structures of different states on the reaction paths,in this thesis,DFT calculations have been performed to investigate reaction pathways of selective dehydrogenations of aliphatic derivatives on single crystal metal surfaces.The underlying reaction mechanisms are studied by identifying the most favored reaction pathways,optimizing adsorption configurations of molecular precursors,identifying the diffusion behaviors of precursors and reaction intermediates,and analyzing the electronic structures of key intermediates.Detailed results of the presented thesis are described below:(1)Introducing terminal functional groups into alkane molecules to facilitate C-H activations.The selective dehydrogenation reaction of hexanol molecule(n-C6H13OH)on the Cu(110)surface at different sites of carbon-hydrogen bond activation was used as a model to study the potential barriers for the dehydrogenation of hexanol molecule at different sites,and the order of selective dehydrogenation reaction of this molecule on the Cu surface was found by comparing the potential barriers for the reaction to occur.Theoretical calculations have shown that the terminal alcohol hydroxyl group(-OH)in the hexanol molecule(n-C6H13OH)and the methylene(-CH2)carbon-hydrogen bond adjacent to the alcohol hydroxyl group will be dehydrogenated first,resulting in the oxidation of the terminal hydroxyl group and the methylene(-CH2OH)adjacent to the hydroxyl group to an aldehyde group(-CHO),followed by a cascade of dehydrogenation reactions on the carbon chain starting from the aldehyde group end,eventually producing Conjugated polyene.Subsequent experimental observation has demonstrated that straight-chain alkanes that introduce terminal hydroxyl groups have lower dehydrogenation temperatures compared to straight-chain alkanes,agreeing well with our theoretical predictions.2)Effect of different substituted functional groups on the activation of neighboring hydrocarbons.In this work,the influence of the functional groups in aliphatic derivatives in C-H activations has been investigated from first-principles calculations.Herein,the reaction pathways of C-H activations in aliphatic derivatives containing hydroxyl,aldehyde,and carboxylic acid are studied.In addition,the charge transfer on carbon atoms adjacent to functional groups in aliphatic derivatives is employed as a descriptor to probe the C-H activation of aliphatic derivatives.We found the Calculations shown that the CH activations can be facilitated when the methylene group gains charges from terminal functional groups,while the charge transfer from methylene group to terminal functional groups will prevent the dehydrogenation on Cu(110).(3)Reaction mechanism of acrolein formation by dehydrogenation of n-propanol on copper low index crystal planes.In this work,the reaction pathways of n-propanol(C3H7OH)molecules on three surfaces at Cu(110),Cu(111),and the step of Cu(111)have been investigated to generate n-acrylaldehyde by carbon-hydrogen activation.The conversion of n-propanol to propyl alcohol exhibits the best catalytic performance on the step of Cu(111).In addition,the adsorption energy of n-propanol can be employed as a descriptor to predict the catalytic activity of different surfaces.Calculations have shown that the better catalytic performance occurs on the surface with stronger adsorption of npropanol.Such descriptor provides good predictions so that tedious transition state calculations can be avoided.