Olefination And Coupling Of N-Alkanes And Derivatives On Single Crystalline Metal Surfaces

Alkanes,especially light alkanes,are one of the most inexpensive and abundant carbon-based feedstocks on earth.However,the molecule has no directing and activating groups and contains only low-polarity C(sp3)-H bonds and high-energy C(sp3)-C bonds,so that alkanes after petroleum cracking are usually used as energy sources for heat,propulsion or power generation,which greatly limits their utilization in synthetic chemistry.This is a fascinating and challenging task because the conversion of greenhouse gases into chemical feedstocks or high value-added chemicals would be of great value to the chemical industry and the environment.In addition,the almost indistinguishable bond dissociation energy of C(sp3)-1 bonds at all levels in alkanes leads to the dilemma of functionalizing specific C-H bonds.The current research ideas focus on two aspects:first,to find more efficient metal or metal complex catalysts for direct functionalization of alkanes(olefins,alkynes,aromatic hydrocarbons);second,to prefunctionalize alkanes for further conversion and reuse of alkane derivatives.The scanning probe microscope(SPM)technique,which was introduced 40 years ago,has made it possible to study chemical reactions at submolecular scale.Different metal surfaces provide suitable adsorption sites and catalytic sites for precursor molecules,which in turn enable the construction of rich low-dimensional nanostructures.More importantly,advanced microscope techniques enable the identification of reactants and reaction products,which can be combined with spectroscopy and density flooding theory calculations to reveal reaction pathways.In this thesis,the metal surfaceinduced hydrocarbon functionalization reactions are systematically investigated using characterization tools such as ultra-high vacuum low-temperature scanning tunneling microscope(STM),non-contact atomic force microscope(nc-AFM),and synchrotron radiation photoelectron spectroscopy(SRPES),which can be divided into four parts as follows:(1)The transformation of n-alkanes to poly acetylene on Cu(110)substrate and the regulation of linkage during chain growth.n-alkanes annealed on Cu(110)surface to about 450 K undergo dehydrogenation into conjugated polyene monomers.After further annealing to 470 K,intermolecular coupling occurs between polyene monomers to form polyacetylene.non-contact atomic force microscope(nc-AFM)was used to analyze the linkages between monomers during chain growth.Three different linkages were found:1)Perfect all-trans polyacetylene chain;2)Cis-trans isomerized polyacetylene chain;3)Polyacetylene chains coupled across grooves on a Cu(110)substrate.More interestingly,we found that three different linkages could be regulated by adjusting the molecular precoverage strategy,and a reasonable regulation mechanism was proposed by density functional theory calculation.(2)The direct dehydrogenation of octadecyl alcohol to polyenal under mild conditions.Specifically,we used octacosan-l-of(the most abundant chemical component in wheat wax)to realize the C(sp3)-H to C(spF)-H transformation via dehydrogenation on Cu(110)surface at a temperature of approximately 350 K,leading to the formation of conjugated polyenal.The combined scanning tunneling microscope(STM),non-contact atomic force microscope(nc-AFM),synchrotron radiation photoemission spectroscopy(SRPES)measurements,reveal that the hydroxyl group in octacosan-l-ol(C28H57OH)is oxidized to the aldehyde group after the first dehydrogenation caused by annealing,generating a corresponding alkyl-aldehyde product,i.e.octacosanal(C27H55CHO).The subsequent sequential dehydrogenation steps result in the formation of conjugated polyenal(octacosa-tridecaenal).Density Functional theory(DFT)calculation results clearly confirm that the strong electron-absorbing induction effect of the generated C=O and the hyperconjugation effect of the hydrogen bonding of α-C(C2)to C=O enhance the activation reactivity of C(sp3)-H bond,resulting in the continuous dehydrogenation of alkyl aldehydes to polyenal at annealing as low as 350 K.(3)The dehydrogenation reaction of n-alkanes before and after annealing on(2×1)O-Cu(110)substrate was studied.Scanning tunneling microscope(STM)combined with synchrotron photoelectron emission spectroscopy(SRPES)showed that n-alkanes on(2×1)-O-Cu(110)substrates could undergo dehydrogenation to α-alkenes at room temperature.Further studies on the adsorption of polyene,dotriaconta-1,31-diene,tetradeca-1,13-diene and icos-1-ene on(2×1)-O-Cu(110)substrates confirmed that the alkane turned at room temperature was the result of terminal position functionalization(alkenization).In addition,dotriaconta-1,31-diene molecules show different C-H activation temperatures at different adsorption sites on Cu(110)substrate,which proves that the optimal adsorption site of metal substrate is not necessarily the optimal activation site.(4)The adsorption activity and reactivity of light alkanes(isobutane)on Au(110)substrates.Scanning tunneling microscope(STM)shows that the rational design of the chemical structure of molecules,the surface molecular coverage and the annealing temperature could achieve the construction of Au(110)substrates with different degrees of reconfiguration.The isobutane can still be stably adsorbed on the Au(110)substrate at room temperature,indicating that the(1×6)reconstituted Au(110)substrate has higher adsorption energy for light alkanes.And the limited number of alkane molecules adsorbed in the reconstituted tank provides the possibility of controllability of the polymerization products in the later stage.