Effect Of Zn And Cu Modification On The Reaction Mechanism Of Dimethyl Ether Carbonylation Catalyzed By Pyridine Adsorbed HMOR

As an important raw material for the production of high value-added chemicals,the efficient preparation of ethanol has become one of focus all over the world.The synthesis of dimethyl ether（DME）from syngas,DME carbonylation to methyl acetate（MA）and further hydrogenation to ethanol is a relatively mature industrial route of coal-based ethanol at present,which has a bright application prospect.The route is of great significance for alleviating the world energy shortage and protecting our environment.H-mordenite（HMOR）,as heterogeneous carbonylation catalyst,has attracted much attention in the industry since development.Many studies have revealed that metal modification can remarkably improve the catalytic activity of HMOR,especially the nobal metals Cu and Zn,but the reasons are not clear.Based on density functional theory,the reaction mechanism of DME and CH₃OH carbonylation were carried out on HMOR,pyridine adsorbed HMOR（Py-HMOR）,zinc modified Py-HMOR(Zn-Py-HMOR,Zn²⁺-Py-HMOR and（Zn O）₂-Py-HMOR),copper modified Py-HMOR(Cu-Py-HMOR and Cu²⁺-Py-HMOR)and copper-zinc co-modified Py-HMOR(Cu Zn²⁺-Py-HMOR and Cu²⁺Zn²⁺-Py-HMOR)systems,aiming to uncover the mechanism of the modification of Cu and Zn.In the study of HMOR,pyridine adsorbed HMOR and Zn-modified Py-HMOR systems,we found that DME dissociationin Zn-Py-HMOR and Zn²⁺-Py-HMOR systems show higher catalytic activity,and the reaction energy barriers are reduced to 1.32 and 1.18 e V,respectively.The reaction of acetyl group with DME to MA is a spontaneous process in Zn-Py-HMOR system and exothermic by 2.29 e V.Correspondingly,only 0.22 e V energy barrier needs to conquer,and the reaction energy is-2.90 e V in Zn²⁺-Py-HMOR system.The reaction of acetyl group and CH₃OH to form MA also turns into spontaneous reaction in Zn²⁺-py-HMOR system,and the reaction energy is-0.70 e V.During the whole carbonylation process,the energy barriers between DME dissociation and CO insertion into methyl group in the Zn²⁺-Py-HMOR system is close,which are 1.18 e V and 0.93 e V,respectively.Bader charge and charge density differences analyses results show that more oxygen sites participate in charge transfer in the 8-MR channel of Zn²⁺-Py-HMOR system,which leads to stronger electrostatic interaction and further improves the catalytic activity of HMOR.In addition,the appropriate 8-MR confinement effect of Zn²⁺-Py-HMOR system is favorable for the acetyl group reacting with DME to MA.To sum up,the Zn²⁺modified HMOR system shows the best catalytic activity for the DME carbonylation to MA.By exploring the mechanism of the DME and CH₃OH carbonylation over Cu-modified Py-HMOR systems,we found that the modification of Cu atoms and ions could promote the elementary reaction of DME dissociation to form methoxy group,and the activation energies are 1.35 and 1.77 e V,respectively.In the reaction of acetyl with CH₃OH or H₂O to generate acetic acid（AA）,Cu-Py-HMOR system needs to overcome the corresponding activation energies of 1.18 and 0.34 e V,respectively.It can be seen that the Cu-Py-HMOR system possesses high catalytic activity for the AA formation.Compared with Cu²⁺-Py-HMOR,the electrostatic interaction with the CH3O part of DME promotes further dissociation of DME.The Cu-Py-HMOR system has more advantages in catalytic activity for the dissociation of DME,but both are not conducive to the formation of MA.The reaction mechanism of DME or CH₃OH carbonylation in Cu and Zn-modified Py-HMOR systems were studied.We found that Cu Zn²⁺-Py-HMOR system obviously improves the catalytic reactivity of DME carbonylation.Comparing with isolated Zn²⁺or Cu-modified Py-HMOR systems,DME dissociation in Cu Zn²⁺-Py-HMOR system is easier to occur,and the relative reaction energy barrier is 1.04 e V.It can be seen that the synergistic effect of Zn²⁺ion and Cu atom significantly improves the catalytic performance of Py-HMOR for DME dissociation.For the reaction of acetyl group with DME or CH₃OH to form MA,the highest energy barrier that need to overcome is only 0.25 e V in the Cu Zn²⁺-Py-HMOR system,and exothermic by at least 0.97 e V.From the perspective of kinetics,because Cu-modified system is unfavorable for the MA formation,and Zn²⁺-modified system is beneficial to the MA formation,the catalytic activity of Cu Zn²⁺-Py-HMOR system for the MA formation is comparable to that of Zn²⁺-Py-HMOR system.Clearly,the synergistic effect of Zn²⁺and Cu or Cu²⁺greatly improves the catalytic reactivity of Py-HMOR in the carbonylation reaction process,and Zn²⁺plays a critical role.Bader charge and charge density differences analyses show that the coexistence of Cu and Zn²⁺leads to increase polarization of DME,thereby promoting its dissociation;the increase in the reactivity of acetyl group with DME is mainly due to the double electrostatic interaction and the matching confinement effect;Cu and Zn²⁺in the Cu Zn²⁺-Py-HMOR system 8-MR the unique positioning significantly improves the orientation of CH₃OH（parallel to the longitudinal 8-MR side pockets）,making it more conducive to attacking acetyl group,thereby reducing the activation energy of the reaction.Above results offer an essential theoretical basis for insight into the catalytic reactivity of Py-HMOR modified by non-rich metals Zn and Cu,and provide necessary theoretical clues for further designing and developing new DME carbonylation catalysts with high stability,activity and selectivity in the future.