The Role Of Crystal Structures Of Copper Chloride Hydroxides In The Oxidative Carbonylation Of Methanol By DFT

As an important green chemical intermediate, dimethyl carbonate (DMC) is an environmentally benign compound with versatile chemical reactivity and excellent prospects of application. It was reported that different crystal structures of copper chloride hydroxide was found to be the active component on the catalytic synthesis of DMC by the vapor phase oxidative carbonylation of methanol. The focus of this dissertation is to investigate the catalytic activities of different crystal structures of copper chloride hydroxide (Cu(OH)Cl and atacamite) toward vapor phase DMC synthesis using first-principles calculations based on the density functional theory(DFT) and periodical slab model.In this dissertation we suggest that the oxidative carbonylation of methanol can be separated into four separate steps: the substitution of methoxide, the insertion of CO to the methoxide, the surface reaction to form DMC and the reoxidation of the Cu(Ι). The adsorption energies, geometries and charges for reactants and the various intermediates were computed, and the transition state and the activation barrier for every step were also calculated.The optimized results indicate that the methanol molecule is inclined to adsorb with either of the geometries (vertical adsorption or parallel adsorption) on the top site of Cl atoms of Cu(OH)Cl(001) surface or atacamite(011) surface. The parallel geometry of methanol is more stable in thermodynamics, while the vertical geometry is more active in kinetics. The activation barrier of the substitution reaction of methoxide declines greatly by the presence of Cl atom in active site. The role of Cl atom in the substitute reaction of methoxide is as follow: Firstly, the adsorption of methanol in active site is active adsorption, and Cl atom plays a role of pre-reaction for HM-OM bond activating. Secondly, in the transition state system of TS1, the energy released by the formation of HM-Cl1,1 bond backdonates the energy needed by breaking the HM-OM bond. Moreover, the intermediate HCl molecular weekly adsorbs on the surface of catalyst. This can explain the phenomenon of the leaching of Cl species which was found in the lab researches.The activation barrier calculation indicates that the surface reaction is the rate-controlling step in the process of oxidative carbonylation of methanol. And the activation barrier of every step on atacamite(011) surface is lower than that on Cu(OH)Cl(001) surface, mainly because of the week bonding of M-Cu1,1 (M stands for Cl1,1, OM,1 and Cc) on atacamite(011) surface which facilitates the bond-breaking in chemical reaction. Hence, the atacamite crystal structure has better catalytic activity.