The Theoretical Studies On The Mechanism Of Syngas Conversion

Converting syngas feedstock into alcohols,aldehyde with low carbon number is an effective way to improve resource utilization and reduce pollution,and has attracted much interest in the research of homogeneous and heterogeneous catalysis.During the process of heterogeneous catalysis,MoP is a kind of noble-metal-like catalyst,and has the traits on low cost,sulfur-poisoning resistance and high hydrogenation activity.In past decades,MoP catalysts have been found to possess high activity and selectivity in the direct conversion of syngas into higher alcohols.Especially when K is employed as a promoter,the catalytic performance of MoP catalyst is significantly improved.So far,the systematic understanding about the detailed mechanism was still yet to be elucidated.In homogeneous catalysis,hydroformylation can convert the alkenes and syngas to aldehydes.Co-based catalyst is one of the most important catalysts,which has the benefits of low cost,high earth abundance,and anti-poisoning properties.Until now,the in silico mechanistic studies of Co-catalysed hydroformylation are mainly focused on phosphine-free cobalt carbonyl catalysts.There are few theoretical studies of the effect from phosphine ligands on the activity and selectivity of hydroformylation.Thus,the tool of density functional theory has been utilized in this thesis to systematically investigate the mechanism of syngas conversion catalysed by MoP heterogeneous catalyst and Co-based homogeneous catalyst,including the promoting effect of K on the conversion of syngas to higher alcohols proceeding on the MoP and the effect of phosphine ligands on the activity and selectivity of Co-catalysed olefin hydroformylation.The main conclusions are as follows:（1）The adsorption and dissociation of H₂ on the selected（101）,（112）,（100）,（111）,（110）,（102）and（001）representative facets of MoP catalyst were systematically studied at first.The results suggest that the dissociative adsorption of H₂ on MoP surfaces is very preferable.However,each surface has quite different saturation coverage,i.e.,the P-terminated（001）surface has 1 ML,Mo-terminated（001）surface has 16/9 ML,（100）and（101）surfaces have 2 ML,（102）has 3 ML,while（110）,（111）and（112）surfaces have 4 ML.Using the ab initio atomic thermodynamic method,the equilibrium phase diagrams of hydrogen adsorptions were established,and the saturation coverage of H at different temperatures and pressures was explored.Computation results also pointed out that the hydrogen adsorptions could modify the relative stability of different MoP surfaces,which led to the obvious changes on the morphology of MoP nanoparticle.The（101）surface is identified as the most stable and exposed surface of MoP nanoparticles at a wide range of temperatures and H₂ partial pressures.Therefore,subsequent mechanism studies are mainly focused on the MoP（101）surface.（2）The mechanism of syngas conversion to CH₄ and CH₃OH on the K/MoP（101）surface has been studied.The optimal route for CO hydrogenation form CH₄ has been obtained as follow:CO→CHO→CH₂O→CH₂→CH₃→CH₄.The optimal route for of CH₃OH formation is through CO→CHO→CH₂O→CH₃O→CH₃OH.The formation of CH₄ is more favorable on the MoP（101）and K/MoP（101）surfaces.Compared with the clean surface,the modification with K promoter lowers the total barriers for the formations of CH₄and CH₃OH.Therefore,K promoter suppresses the formations of CH₄and CH₃OH.（3）The mechanistic investigation of transforming syngas to C₂H₅OH on the K/MoP（101）surface revealed that the best path to C₂H₅OH is via CO→CHO→CH₂O→CH₂→CH₂CHO→CH₂CH₂O→CH₃CH₂O→C₂H₅OH.Compared with the MoP（101）surface,K promoter not only lowers the total barrier of C₂H₅OH formation,but also reduces the energy barrier of the C-C coupling step,which favors C₂H₅OH and C₂₊formation.（4）The effect of phosphine ligands on the activity and selectivity of Co-catalysed olefin hydroformylation has been studied.For the unmodified cobalt carbonyl catalyst,only terminal alkene with two bulky substituents exhibited good regioselectivity towards linear products;For phosphine ligand modified Co-based catalyst,the introduction of phosphine ligand with large Tolman cone angle improved the regioselectivity towards the linear product.The presence of more electron-donating phosphine ligand made the splitting of H₂ both kinetically and thermodynamically more favored,meanwhile it has negative impact on the subsequent reductive elimination.Moreover,these results also disclosed the origin of good regioselectivity towards linear product for PBu₃-modified cobalt carbonyl catalyst in the alkene hydroformylation.The modification with PBu₃ can also alter the chemoselectivity to alkane,despite that the selectivity towards the alkane is still significantly lower than that towards aldehyde.The obtained aldehyde product can further be consumed via the hydrogenation or the reductive hydrocarbonylation to produce such as alcohol and formic ester in the same catalytic system.