Theoretical Study Of Conversion Of CH₄ With CO₂ Or Other Oxygenates Through Direct C-C Coupling On Metal Oxide Surface

Catalytic conversion of CO₂ to high energy density chemicals could address both the greenhouse climate issues and alleviate the shortage of fossil fuels.CH₄,as the main component of shale gas,provides both the energy and hydrogen sources for CO₂conversion.Compared to the reforming of CO₂ with CH₄ to syngas,conversion of CO₂with CH₄ through direct C-C coupling is more energy-efficient,yet present formidable challenges:selective C-H bond activation of CH₄,possibility of C-C coupling in heterogeneous field,and the C-C coupling based on simultaneous activation of CO₂ and CH₄.In the present work,we first studied the selective activation of CH₄ on single Zn atom doped ceria surface by using density functional theory calculations.The results show that Zn site could stabilize the CH₃*species from CH₄ activation.CO₂ coming from the gas phase could insert into Zn-CH₃*bond to achieve the C-C coupling process.The insertion follows an S_E2 mechanism.The facile insertion barrier makes the C-C coupling in heterogeneous field possible,however,the generated acetate species was too stable on the surface to desorb as a product.We designed Zn site ensemble on ceria surface to explore the possibility of further C-C coupling of acetate species with methyl species.Our calculations demonstrate that CH₄ could be selectively activated on Zn site and CO₂ inserted into the Zn-CH₃*bond to form the acetate species.In the following process,another CH₄was activated on the second Zn site.The C-C coupling of CH₃*with acetate species generates acetone as product.Compared to the CO₂ insertion reaction through Eley-Rideal mechanism,the C-C coupling process via Langmuir-Hinshelwood mechanism is expected to be more efficient.To achieve the simultaneous activation of CO₂ and CH₄,we designed the Zn O cluster supported on In₂O₃ surface,considering the high activity of In₂O₃ for CO₂activation.The results show that CO₂ was activated at the oxygen vacancy site and CH₄dissociated at Zn-O pair.There is a synergy between the activation of CO₂ and CH₄.In the transition state of C-C coupling,a four centered Zn-C-C-O bond is formed.Microkinetics analysis shows that CH₄ plays a dominant role in C-C coupling reaction.In order to explore the possibility of C-C coupling of CH₄ with its derivate,formaldehyde,we designed the single Pd supported on In₂O₃ catalyst.The calculations suggest that CH₄ activated on Pd site could couple directly with formaldehyde to form ethanol finally.The comparison of the key steps in ethanol formation,CH₄ activation,C-C coupling,and ethoxyl hydrogenation,on different single atoms supported In₂O₃surface indicates that Pd/In₂O₃ shows the best activity.In addition to the C-C coupling between CO₂ and CH₄,the derivates of them could also convert through direct C-C coupling reaction.We took formaldehyde and acetone as model reactants and studied the effect of aqueous phase on aldol condensation reaction.AIMD simulation and DFT calculations indicate that?-H abstraction in aqueous phase could only be achieved through proton transfer process.The existence of aqueous phase pronouncedly lowered the Gibbs energy of activation for dehydration step in hydrogenation pathway,thus affecting the reaction pathway.In vapor phase,the reaction goes through a mixed reaction route and in aqueous phase,the hydrogenation pathway is dominant.