Study On The Reaction Mechanism Of Acetic Acid And Ethanol Synthesis From Methane-Syngas Via Two-Step Reaction On Rh And Co Catalysts

The synthesis of acetic acid and ethanol from methane-syngas via two-stepreaction have attracted growing interest due to the extensive sources of methaneand syngas, the efficient and cleaning utilization of energy, etc.. However, fewstudies have been carried out to fully understand the internal mechanism ofacetic acid and ethanol synthesis from methane-syngas via two-step reaction onRh and Co catalysts due to the complexity of the involved reactions. Thus,clarifying the mechanism of acetic acid and ethanol synthesis frommethane-syngas via two-step reaction on Rh and Co catalysts and understandingthe key steps that involved in the synthesis process will be significant to proposethe methods to improve the selectivity and productivity of acetic acid andethanol from methane-syngas.In this paper, the reaction mechanisms of acetic acid and ethanol synthesisfrom methane-syngas via two-step reaction on Rh and Co catalysts have beensystematically investigated using density functional theory (DFT) methodtogether with the periodic slab models. The adsorption, dissociation anddesorption of CO and H2with different pressure over Rh and Co surfaces are firstly investigated by using various coverage models of CO and H2; then, thesource and major species of CHx(x=1-3) intermediates, the formation of C-Cchain that leading to the formation of C2oxygenates and the formation of aceticacid and ethanol have been systematically investigated to explore the detailedreaction mechanisms involved in the synthesis of acetic acid and ethanol frommethane-syngas via two-step reaction on Rh and Co catalysts. The mainconclusions are as follows:(1) For the adsorption of CO on Rh and Co catalysts with different pressure,on Rh(100) surface, only molecule adsorption CO exist, and the saturatedadsorption coverage is1ML. On Co(0001) surface, CO only adsorbed withmolecule state, and the saturated adsorption coverage of CO is7/9ML. OnCo(100) surface, the saturated coverage of molecule adsorption CO is7/9ML,when the coverage is1/9ML, the dissociative adsorption of CO is morefavorable; when the coverage is2/9to5/9ML, both the dissociative andmolecule adsorption of CO co-exist; when the coverage is greater than5/9ML,CO only adsorbed with the molecule state.(2) For the adsorption of H2on Rh and Co catalysts with different pressure,on Rh(100) surface, H2mainly adsorbed with the dissociative state, few H2prefers to be the molecule adsorption, and the saturated adsorption coverage is6/12ML. On Co(0001) surface, H2mainly adsorbed with the dissociative state,few H2prefers to be the molecule adsorption, and the saturated adsorptioncoverage is4/9ML. On Co(100) surface, H2mainly adsorbed with the dissociative state, few H2prefers to be the molecule adsorption, and thesaturated adsorption coverage is7/9ML. Thus, the dissociative adsorption ismore preferred for H2on Rh and Co catalysts.(3) The formation mechanism of CHx(x=1-3) intermediates frommethane-syngas via two-step reaction on Rh and Co catalysts have beenobtained. For CHx(x=1-3) formation on Rh(111) surface, CH species is thedominant product of methane dissociation, while for CHxformation from syngas,CH and CH3species are the dominant products rather than CH2and CH3OH,which are formed through the pathway of CO+Hâ†'CHO+Hâ†'CHOHâ†'CH+OHand CO+Hâ†'CHO+Hâ†'CHOH+Hâ†'CH2OH+Hâ†'CH3+OH, respectively. OnCo(0001) surface, CH is the dominant product of methane dissociation, whilefor CHxformation from syngas, CH is the main products rather than CH2, CH3and CH3OH, which is produced through two parallel routes of CO+Hâ†'CHOâ†'CH+O and CO+Hâ†'CHO+Hâ†'CH+OH. Meanwhile, the activity ofmethane dissociation on Rh catalyst is higher than that on Co catalyst, while theformation of CH from syngas on Co catalyst is easier than the formation of CHand CH3from syngas on Rh catalyst.(4) The source and major species of CHx(x=1-3), as well as the C-C chainformation of C2oxygenates on Rh and Co catalysts are clarified. On Rh(111)surface, CH species from methane are favorable both kinetically andthermodynamically compared to CH and CH3formed from syngas; On Co(0001)surface, CH species from methane are favorable both kinetically and thermodynamically compared to CH formed from syngas. Thus, the majorCHx(x=1-3) species on Rh and Co catalysts is CH that derived from methane,while CO in syngas mainly participates into the insertion reactions with CH toproduce CHCO, which is the C2oxygenate precursor that leading to theformation of acetic acid and ethanol. Meanwhile, the formation of C-C chain onCo catalyst is easier than that on Rh catalyst.(5) The formation mechanisms of acetic acid from methane-syngas viatwo-step reaction on Rh and Co catalysts are obtained. On Rh(111) surface,acetic acid is mainly produced via the pathway of CHCO+OHâ†'CHCOOH+Hâ†'CH2COOH+Hâ†'CH3COOH. On Co(0001) surface, acetic acidis mainly produced via the route of CHCO+Hâ†'CH2CO+Hâ†'CH3CO+OHâ†'CH3COOH. Meanwhile, the activity and selectivity of acetic acid on Rh catslystis higher that on Co catalyst, suggesting that Rh catalyst is more favorable foracetic acid synthesis than Co catalyst.(6) The formation mechanisms of ethanol from methane-syngas viatwo-step reaction on Rh and Co catalysts are obtained. On Rh(111) surface,ethanol is mainly produced via the pathway of CHCO+Hâ†'CH2CO+Hâ†'CH3CO+Hâ†'CH3CHO+Hâ†'CH3CH2O+Hâ†'CH3CH2OH. On Co(0001) surface,ethanol is mainly produced via two parallel routes of CHCO+Hâ†'CH2CO+Hâ†'CH3CO+Hâ†'CH3CHO+Hâ†'CH3CH2O+Hâ†'C2H5OH and CHCO+Hâ†'CH2CO+Hâ†'CH2CHO+Hâ†'CH3CHO+Hâ†'CH3CH2O+Hâ†'C2H5OH. Meanwhile, theactivity and selectivity of ethanol on Rh catslyst is higher that on Co catalyst, suggesting that Rh catalyst is more favorable for ethanol synthesis than Cocatalyst.