Density Functional Theory Investigation on the Synthesis

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Kinetics, Catalysis, and Reaction Engineering

DFT investigation on the synthesis mechanism of vinyl acetate from acetylene and acetic acid catalyzed by ordered mesoporous carbon supported zinc acetate Xiuqin Dong, Yuchun Wang, Yingzhe Yu, and Minhua Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00596 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Industrial & Engineering Chemistry Research

DFT investigation on the synthesis mechanism of vinyl acetate from acetylene and acetic acid catalyzed by ordered mesoporous carbon supported zinc acetate Xiuqin Dong a,b, Yuchun Wang a,b, Yingzhe Yu a,b Minhua Zhang a,b,* a

Key Laboratory for Green Chemical Technology of Ministry of Education, R&D

Center for Petrochemical Technology, Tianjin University, Tianjin 300072, P. R. China b

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin 300072, P. R. China *

Corresponding author

Tel: +86-22-27405972 Fax: +86-22-27406119 E-mail address: [email protected] Abstract The formation mechanism of vinyl acetate in the reaction of acetylene and acetic acid which was catalyzed by ordered mesoporous carbon (OMC) supported zinc acetate was investigated using density functional theory (DFT). Since the surface functional groups of carbon support influence the reaction significantly and play an important role in catalysis designing, we calculate the elementary steps on pristine ordered mesoporous carbon and carbon modified with carboxyl, carbonyl and hydroxyl respectively. After calculating, we find that carbonyl shifts to epoxy group, which indicates the instability of carbonyl on the catalyst support. We propose the possible reaction mechanism and find that the reaction mechanism is not exactly same with different functional group. A remarkably acetate-shift will take place in the existence of carboxyl and the activation barrier of rate-limiting steps in this case is also reduced, while hydroxyl and epoxy group will increase the barrier to some extent. Therefore, when designing the industrial catalysis for this reaction, we can modify the surface of catalysis support directionally to enhance the reaction efficiency.

Introduction Vinyl acetate(VAc) is served as one of the biggest organic chemistry intermediates in the world and it has huge amount of downstream productions such as Polyvinyl Alcohol(PVA), polyacrylonitrile (PAN), vinyl acetate-ethylene copolymer emulsion(VAE), etc. [1]Vinyl acetate can be produce through two ways, from ethylene or acetylene respectively.[2] The former synthesis

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method has been investigated throughout but as for latter method, studies mainly focus on the unit operation while the understanding for the chemical reaction is far more from enough although at present at least 1/3 of vinyl acetate is produced from acetylene method.[3] The synthesis process is a heterogeneous catalytic reaction, acetylene and acetic acid are catalyzed by zinc acetate supported by porous carbon.[4, 5] In order to optimize this production process, researchers try to find other active components to replace zinc acetate. Composite catalyst V2O5-ZnO and Fe2O3-ZnO have been proposed but the results were not satisfied.[6, 7] The properties of catalysis support have also been considered to influence the reaction, especially the surface functional groups on the porous carbon.[8] However, even till now, the accurate influences of these functional groups remain controversial. Some researchers point out that carboxyl (-COOH) and carbonyl (-CO) exert positive influence on the synthesis reaction[9], while hydroxyl(-OH) would have negative effect,[10] while some researchers consider that carbonyl will not favor the reaction. Some researchers[11] even consider that only the Zn(OAc)2 which is adsorbed on the oxygenic functional groups has catalytic activity. The accurate effect of functional groups modification on catalyst support still need to be drilled down into. As for the synthesis mechanism, three hypothesizes have been proposed. Furugawa proposed that the reaction might include three step: firstly, acetylene adsorbs on the catalyst surface and forms a π-complex. Secondly, the л-complex rearranges to form a σ-complex. Thirdly, the σ-complex reacts with acetic acid and produces vinyl acetate.[12] The reaction is illustrated as follow: (1) CH≡CH+Zn(OAc)2 →HC=CH-Zn-( OAc)2 (2) HC+=CH-Zn-( OAc)2→(OAc)-HC=CH-Zn(OAc) (3)(OAc)-HC=CH-Zn(OAc)+CH3COOH→CH2CH-OCOCH3+Zn(OAc)2 Peter Kripylo proposed another mechanism that the first step is the chemical adsorption of acetic acid on Zn(OAc)2, then acetylene begins to adsorb and produce vinyl acetate.[13] There is also an acid catalytic mechanism, which consider that the mechanism has four steps: the first step is the dehydrogenation of acetic acid. The second step is the reaction between dissociative H+ and acetylene which produces vinyl cationic (CH2=C+). The third step is the addition reaction between vinyl cationic and acetate species which produces vinyl acetate.[14] Although understanding the synthesis mechanism is rather important, but the work on demonstrating these hypothesizes or proposed new mechanism is still blank. Theoretical calculation has proved to be an effective method in mechanism investigation, since some intermediate species are difficult to detect. This method has been applied to investigate the synthesis mechanism of vinyl acetate from ethylene and provide meaningful information.[15-18] Since the study of vinyl acetate synthesis mechanism from acetylene is insufficient, using calculating method to investigate it is of great importance. Based on the above discussion, we conduct a comprehensive investigation on the synthesis mechanism of vinyl acetate through acetylene using density functional theory (DFT). By discussing the absorbing conformation we can give a preliminary analysis of the mechanism. We calculate the activation barriers and the reaction energies of the elementary steps of possible reaction mechanism on pristine carbon and carbon modified with carboxyl (-COOH), carbonyl (-CO) and hydroxyl (-OH) to take the influence of functional groups into account. What we have done will present both theoretical and practical significance. The content and structure of this paper are as follows: In section 2, detailed model and


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computational methods are described. In section 3, the optimized catalysis model, adsorption configurations of all the intermediate species and the possible reaction pathways are calculated on catalysis with different surface functional groups. In section 4, conclusions are drawn.

Model and computational methods In the present work, the synthesis mechanism of vinyl acetate on zinc acetate supported on ordered mesoporous carbon was studied based on the program package DMol3 in Materials Studio of Accelrys.[19, 20] The exchange-correlation function we used is the generalized-gradient approximation (GGA) in the form of the Perdew- Burke-Ernzerh (PBE).[21] The basis set was set as double numerical plus polarization (DNP). The electron density was converged within 1.0× 10-5 eV, and the geometry optimizations were performed with DFT until the force on each atom was less than 0.004 Ha/A by using smearing at 0.005Ha. The real-space global cutoff radius was set to 4.4Å. DFT-D correction was considered to improve the accuracy.[22] As the geometric construction of ordered mesoporous carbon will not influence the reaction mechanism during calculating, researchers usually simplify it as a single graphene layer which was saturated by hydrogen atoms to reach electric charge balanced state.[23] Pristine graphene was modeled using a layer consisting of 62 carbon atoms within an orthorhombic supercell with dimensions amounting to 9.88×8.65×14 Å. Based on the pristine carbon model, we use carboxyl (COOH), carbonyl (-CO) and hydroxyl (-OH) to modify the support surface respectively. On each condition we adsorbed one zinc acetate molecule to simplify the reaction. The minimum-energy reaction path for an elementary step and the transition state (TS) associated with it were determined using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method[24]. Adsorption energy are calculated as:

Eads = E adsorbates / slab − E slab − E adsorbates , Where Eslab, Eadsorbates, Eadsorbates/slab are the energies of the clean surface without any adsorbate, the adsorbate molecule isolated in the gas phase, and the surface with the adsorbate molecule, respectively. The activation barrier Ea and reaction energy △E are defined as:

Ea = ETS − E IS

∆E = E FS − E IS Where E IS , ETS and E FS refer to the total energies of the initial, transition and final states, respectively. Positive value of ∆E represents endothermic reaction.



3 Results and discussion

3.1Catalyst model construction The optimized structure of active component zinc acetate (Zn(OAc)2) molecular is illustrated in Fig. 1. Zn atom bonds with four O atoms. The torsion angle between bonds to reach the energy-minimum state, with a value of 83.30°.The detailed structural parameters can be seen in Table S1.

Fig. 1 Optimized structure of zinc acetate molecular. Blue ball: Zn atom; Red ball: O atom and white ball: H atom.

The energy-lowest structures of OMC and which modified by carbonyl (OMC-CO), carboxyl (OMC-COOH) and hydroxyl (OMC-OH) respectively are shown in Fig. 2. Carboxyl connects to the surface through a C-C bond with a length of 2.08 Å. After optimizing, carbonyl shifts to epoxy group since the O atom moves to the bridge site and connects to the surface through two C-O bonds. A C-O bond also forms between hydroxyl and the surface with a length of 1.49 Å.

(a)

(b)

(c) (d) Fig. 2 Optimized structure of zinc acetate molecular. Blue ball: Zn atom; Red ball: O atom and white ball: H atom. Zinc acetate on OMC In this condition, the structure of zinc acetate changes slightly after adsorbing on carbon support, as illustrated in Fig. 3(a). The lengths of four Zn-O bonds change within 1 Å after adsorbing. The torsion angle T(O1,O2,O3,O4) enlarges from 84.30° to 76.41°. The detailed structural parameters are listed in Table. S1. The adsorption energy of Zn(OAc)2 on OMC is calculated as -0.0113 eV, indicating that the interaction between Zn(OAc)2 and OMC is weak. Zinc acetate on OMC-CO The geometric structure and parameters of zinc acetate on OMC-CO are shown in Fig. 3(b) and Table. S1. After adsorbing, the lengths of four Zn-O bonds increase


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from 2.05 Å, 2.05 Å, 2.05 Å and 2.05 Å to 2.09 Å, 2.06 Å, 2.12 Å and 2.08 Å respectively. The configuration of zinc acetate changes and the torsion angle reduces to 55.55°. The adsorption energy in this condition is -0.117 eV, larger than that on OMC while it is still physical adsorption. Zinc acetate on OMC-COOH In this case, the structure of zinc acetate changes a lot after adsorbing. One Zn-O bond breaks and the distance between Zn and this O atom reaches 2.33 Å, see Fig.3 (c) and Table. S1. Zn-O2 and Zn-O3 also increase to 2.07 Å and 2.09 Å. The torsion angel decreases greatly, which is only 24.28°. A new bond forms between the carboxylate oxygen and zinc atom, with a length of 2.32 Å. The adsorption strength is quite intensive, as the adsorption energy is -0.614 eV. Zinc acetate on OMC-OH After adsorbing on OMC-OH, the length of Zn-O2 increases a lot to 2.20 Å but it still not break. The torsion angle of zinc acetate on OMC-OH is 49.86 °. A bond forms between the O atom of H and Zn atom and the length of it is 2.17 Å. The adsorption energy obtained is -0.358 eV.

(b)

(a)

(c)

(d)

Fig. 3 Optimized structure of zinc acetate adsorbed on (a) OMC; (b) OMC-CO; (c) OMC-COOH; (d) OMC-OH. Blue ball: Zn atom; Red ball: O atom and white ball: H atom.

3.2 Adsorption configurations of acetylene and acetic acid Discussing the adsorption properties of acetylene and acetic acid in this reaction exerts crucial influence since it is the first step when investigating the synthesis mechanism. However, even till now, this problem is still under debate. Additionally, since the surface functional groups will influence the synthesis reaction, studying the adsorbing structures of reactants on different functional groups may provide us with valuable insights for understanding the mechanism. Based on the above discussion, in this section, we will calculate the adsorption configurations of acetylene and acetic acid on OMC-Zn(OAc)2, OMC-CO-Zn(OAc)2, OMC-COOH-Zn(OAc)2 and OMC-OH-Zn(OAc)2. CH≡ ≡CH (1) OMC-Zn(OAc)2



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The energy-lowest structure of adsorbed CH≡CH on OMC-Zn(OAc)2 is shown in Fig. 4(a). After adsorbing, the distance between the centroid of CH≡CH and the Zn atom of zinc acetate is rather far, with a value of 2.58 Å. The configuration of Zn(OAc)2 changes a little, the torsion angle T(O1,O2,O3,O4) of zinc acetate on OMC is 76.41° and after CH≡CH adsorbing，T(O1,O2,O3,O4) enlarges to 148.20°. The detailed structural parameters can be seen in Table S2. The adsorption energy is calculated as -0.100 eV, indicating that the adsorption strength is weak. There is no complex forming in this system obviously. (2) OMC-CO-Zn(OAc)2 In this condition, the distance between centroid of CH≡CH and the Zn atom of zinc acetate is calculated as 3.08 Å. The torsion angle T(O1,O2,O3,O4) of zinc acetate in after adsorbing is 169.1°, see Fig. 4(b) and Table. S2. The adsorption energy is calculated as -0.127 eV, indicating that it is physical adsorption. There is no complex forming in this case. (3) OMC-COOH-Zn(OAc)2 The optimized structure of CH≡CH adsorbed on OMC-COOH-Zn(OAc)2 is shown in Fig. 4(c) and the detailed parameters can be seen in Table. S2. After adsorbing, the distance between centroid of CH≡CH and the Zn atom of zinc acetate is calculated as 3.08 Å. The length of C-H bonds in CH≡CH changes after adsorbing, from 1.07 Å, 1.07 Å to 1.00 Å and 1.00 Å. The adsorption energy of CH≡CH on OMC-COOH-Zn(OAc)2 is calculated as -0.0944 eV, which is rather low. No complex forms during adsorption. (4) OMC-OH-Zn(OAc)2 The energy-lowest structure of CH≡CH adsorbed on OMC-OH-Zn(OAc)2 is shown in Fig. 4(d). After CH≡CH absorbing, one of Zn-O bonds in zinc acetate cracks. The distance between CH≡CH and Zn atom is still far as 4.48 Å. The detailed structural parameters is listed in Table S2.

(a)

(b)

(c) (d) Fig. 4 Optimized structure of acetylene adsorbed on (a) OMC-Zn(OAc)2; (b) OMC-CO-Zn(OAc)2; (c) OMC-COOH-Zn(OAc)2; (d) OMC-OH-Zn(OAc)2. Blue ball: Zn atom; Red ball: O atom

and white ball: H atom. ACS Paragon Plus Environment

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The adsorption energy is -0.148 eV, indicating the adsorption is weak. Still we can find no complex is formed. CH3COOH (1) OMC-Zn(OAc)2 The optimized structure of acetic acid adsorbed on catalyst is illustrated in Fig. 5(a). Acetic acid adsorbs chemically with an adsorption energy of -0.672 eV, through a Zn-O bond between the carboxylate oxygen of acetic acid and zinc atom of Zn(OAc)2, with a length of 2.11 Å. After adsorbing, the length of two C-O bonds in acetic acid change a little, from 1.22 Å and 1.37 Å to 1.25 Å and 1.32 Å respectively. Remarkably, after acetic acid adsorbing, a Zn-O bond breaks, producing CH3COO* similar with the adsorbed acetic acid. The detailed structural parameters are listed in Table. S3. (2) OMC-CO-Zn(OAc)2 The adsorption structure and detailed parameters are shown is Fig. 5(b) and Table. S3. There is a Zn-O bond between acetic acid and zinc acetate with length of 2.10 Å. The adsorption energy of acetic acid on OMC-CO-Zn(OAc)2 is -0.585 eV, indicating it is chemical adsorption. One Zn-O bond in zinc acetate breaks after adsorbing, producing a CH3COO* which is similar with the adsorbed acetic acid. (3) OMC-COOH-Zn(OAc)2 Acetic acid adsorbs on OMC-COOH-Zn(OAc)2 through a Zn-O bond. After adsorbing, the configuration of catalyst changes to some extent. Two Zn-O bonds of zinc acetate break. The O-H bond of carboxyl on activated carbon breaks and the H atom of carboxyl shifts and bonds to one of the O atoms of zinc acetate, generating an CH3COOH* and a CH3COO* both of which are similar with the adsorbed acetic acid, see Fig. 5(c) and the insertion picture. The adsorption energy is calculated as -0.746 eV. The detailed structural parameters are listed in Table. S3. (4) OMC-OH-Zn(OAc)2 After acetic acid adsorbing on OMC-OH-Zn(OAc)2, a new Zn-O bond forms between the ketonic O atom of acetic acid and zinc acetate. Two Zn-O bonds in zinc acetic break, generating two acetic radicals, which, to some extent, having similarities with adsorbed acetic acid, see Fig. 5(d) and the insertion picture. The adsorption energy of acetic acid on OMC-OH-Zn(OAc)2 is calculated as -0.729 eV. The detailed structural parameters are shown in Table S3. After calculating the adsorption configurations of acetylene and acetic acid on OMC-Zn(OAc)2 OMC-COOH-Zn(OAc)2, OMC-CO-Zn(OAc)2 and OMC-OH-Zn(OAc)2, some noticeable phenomena can be discovered. On all of the four catalysis, acetylene adsorbs weakly with adsorption energies of less than -0.128 eV and no complex forms after adsorbing. As for acetic acid, after it adsorbing, on all of the four catalysis, Zn-O bond of zinc acetate will break, generating an acetate species which is similar with the adsorbed reactant acetic acid. Remarkably, when it comes to the catalysis modified carboxyl, one CH3COO* and one CH3COOH* will be generated. These phenomena play an important role in discussing the synthesis mechanism of vinyl acetate which will be expounded as follow.



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(b)

(a)

(c)

(d)

Fig. 5 Optimized structure of acetic acid adsorbed on (a) OMC-Zn(OAc)2; (b) OMC-CO-Zn(OAc)2; (c) OMC-COOH-Zn(OAc)2; (d) OMC-OH-Zn(OAc)2. Blue ball: Zn atom; Red ball: O atom and white ball: H atom.

3.3 Analysis of the possible reaction pathways In order to understand the synthesis mechanism of vinyl acetate, we firstly refer to the former hypothesizes as mentioned above. As be mentioned above, Furukawa considers that the first step of this reaction is the formation of л-complex between acetylene and zinc acetate and then the л-complex will shift to σ-complex. According to our calculation on the adsorption of acetylene, on all the four conditions, OMC-Zn(OAc)2, OMC-CO-Zn(OAc)2, OMC-COOH-Zn(OAc)2 and OMC-OH-Zn(OAc)2, acetylene adsorbs physically and the distance between acetylene and Zn(OAc)2 is rather far. Obviously, there is no complex forming during adsorption. Therefore, we consider that this hypothesis on mechanism has some defects to some extent. Peter Kripylo holds the view that acetic acid adsorbs on Zn(OAc)2 chemically, then acetylene diffuses to acetic acid and reacts with it to produce adsorbed CH2CH-OCOCH3. Finally, CH2CH-OCOCH3 desorbs from the catalyst to produce VAM. The reaction mechanism is shown as follow: (1). CH3COOH+Zn(OAc)2→Zn(OAc)2-CH3COOH(CH≡CH+CH3COOH*) (2). CH≡CH+CH3COOH*→CH2=CH-OCOCH3* (3). CH2=CH-OCOCH3*→CH2=CH-OCOCH3+* Another mechanism named acid catalyzed mechanism, involves four steps: first, acetic acid dehydrogenizes, producing H atom and acetate species. The dissociated H atom reacts with acetylene and generates CH2=C+H, which later reacts with acetate species to yield adsorbed CH2=CHOCOCH3. The last step is also VAM desorption from Zn(OAc)2. This reaction pathway can be seen as follow: 1. CH3COOH*→CH3COO-*+H+


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2. CH≡CH+H+→CH2C+H 3. CH2=C+H+CH3COO-→CH2=CHOCOCH3* 4. CH2=CH-OCOCH3*→CH2=CH-OCOCH3+* Based on the above discussion, we conduct transition state search on Peter Kripylo’s mechanism and acid catalyzed mechanisms to gain a deep insight into VAM formation. Since the surface functional groups on carbon support exert vital influence on the reaction as discussed above, we perform our calculation on OMC-Zn(OAc)2, OMC-CO-Zn(OAc)2, OMC-COOH-Zn(OAc)2 and OMC-OH-Zn(OAc)2 respectively.

3.1 Peter Kripylo’s mechanism on catalysts with different functional groups 3.1.1Peter Kripylo’s mechanism on OMC-Zn(OAc)2 CH≡CH+CH3COOH*→CH2=CH-OCOCH3* In the initial state, acetylene and acetic acid co-adsorb on OMC-Zn(OAc)2, of which there is a bonding interaction between acetic acid and zinc acetic, see Fig.S1(a). We have tested many adsorption sites for acetylene in this system and choose the energy-lowest one, the detailed co-adsorption structure and parameters are described in Fig. S1 and Table S4, in which the distance between acetylene and zinc acetic is rather far. When the reaction begins, The H atom of hydroxyl in acetic acid will shift to O4 in zinc acetic and a new O-H bond will form tentatively, with a distance of 1.09 Å. H2 atom of acetylene will move towards C1 and bonds with it, generating a CH2 group, see Fig. 6. The remaining C2 atom moves closely to O3 in zinc acetic, as has been discussed in supporting information, there is an acetate species replacement between acetic acid and the acetate species of zinc acetate during the reaction, which means, acetylene reacts with the acetate species of zinc acetate rather than acetic acid. In TS, the distance between C2 and O3 is 2.59 Å and in FS, a new bond forms between C2 and O3. The H atom bonds to O3 in TS shifts again to C1 to generate a C1-H bond. When the reaction finishes, an adsorbed vinyl acetate is produced. Apart from this, we can find that the Ob atom of acetic acid will bond to Zn atom, producing a new zinc acetic which is similar to the state before reaction, as can be seem in Fig. S4(b). The activation barrier of this reaction is calculated as 2.59 eV the reaction energy is -1.08 eV. CH2=CH-OCOCH3*→CH2=CH-OCOCH3+*



In the initial state of this reaction, vinyl acetylene bonds to zinc acetate through a Zn-O bond, with a length of 2.18 Å. The detailed structural parameters of adsorbed vinyl acetate are listed in Table S7. When the reaction begins, the Zn-O bond begins to prolong and then breaks, with a distance of 3.00 Å in TS. Then the vinyl acetate continue to move away from zinc acetate and stop at a stable site. The tiptilted zinc acetate begins to move closely to the carbon slab and in FS, it almost return to original state which is before the reaction. The reaction process can be seen in Fig.6. This process needs to overcome an activation barrier of 0.639 eV and adsorb an energy of 0.316 eV.

3.1.2 Peter Kripylo’s mechanism on OMC-CO-Zn(OAc)2

CH ≡ CH+CH3COOH* →CH2=CH-OCOCH3* CH2=CH-OCOCH3*→ CH2=CH-OCOCH3+*

Fig. 6 Potential energy schemes of Peter Kripylo’s mechanism on OMC-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

CH≡CH+CH3COOH*→CH2=CH-OCOCH3* In this case. there is a Zn-O bond between Oa and Zn atom, while the distance between acetylene and zinc acetic is rather far. When the reaction begins, the H2 atom of acetylene shifts to C1 and the hydroxylic hydrogen of acetic acid moves to C2. In TS, the distance between the hydroxylic hydrogen of acetic acid and C2 is 2.08 Å, see Table S9. C2 continues to move to O1 of zinc acetate, as has been discussed, there is still an acetate species replacement between acetic acid and the acetate species of zinc acetate during the reaction. In FS, an adsorbed vinyl acetate is generated, with a Zn-O bond connecting with Zn. The Ob atom of acetic acid bonds to Zn atom in FS, generating a structure similar with original zinc acetate. This process needs to overcome an activation barrier of 2.64 eV and release an energy of 1.02 eV.


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CH2=CH-OCOCH3*→CH2=CH-OCOCH3+* In the initial state, vinyl acetate adsorbs on zinc acetate through a Zn-O bond. This Zn-O bond then extends and breaks, producing vinyl acetate. Vinyl acetate moves and adsorbs on an energy-lowest site, which is far from zinc acetate. The remaining part, the tiptilted zinc acetate begins to move closely to OMC-CO-Zn(OAc)2. In FS, we can find that zinc acetate returns to the original state. The reaction progress can be seen in Fig. 7. The activation barrier of this step is calculated as 1.037 eV and the reaction energy is 0.212 eV.

CH ≡ CH+CH3COOH* →CH2=CH-OCOCH3* CH2=CH-OCOCH3*→ CH2=CH-OCOCH3+*

Fig. 7 Potential energy schemes of Peter Kripylo’s mechanism on OMC-CO-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

3.1.3 Peter Kripylo’s mechanism on OMC-COOH-Zn(OAc)2 CH≡CH+CH3COOH*→CH2=CH-OCOCH3* Acetic acid adsorbs on zinc acetate through a Zn-O bond. The H atom of surface carboxylic group moves to O2 of zinc acetate after adsorbing, producing an acetate species. The distances between acetylene and other molecules are all rather far. When the reaction begins, H2 atom of acetylene shifts to C1 and the remaining C1 atom moves closely to O2 atom, as has been discussed, there is still an acetate species replacement between acetic acid and the acetate species of zinc acetate during the reaction. In TS, a new CH2 group forms. The distance of C2 and O2 reaches 2.55 Å. The H atom bonded on O2 moves towards C2, in FS, a C-H bond forms and an adsorbed vinyl acetate is generated, see Fig. S4(f) and Table S9. This reaction needs to overcome an activation barrier of 2.43 eV and the reaction energy is calculated as -0.996 eV. The reaction progress can be seen in Fig. 8 CH2=CH-OCOCH3*→CH2=CH-OCOCH3+*



As has been discussed in the supporting information, the structure involved replacement is more stable. The initial state of this step on OMC-COOH-Zn(OAc)2 is shown in Fig. S2(f) and the detailed parameters can be seen in Fig.S7. A vinyl acetate adsorbs on vinyl acetate through a Zn-O bond. When the reaction begins, the Zn-O bond begins to prolong and then cracks. The vinyl acetate begins to move away from zinc acetate to a stable adsorption site. In FS, we can find that zinc acetate return to the original state which has not participated in the reaction. The detailed structural parameters are shown in Table S9. The activation barrier of this step is calculated as 1.211 eV and the reaction energy is 0.188 eV.

CH ≡ CH+CH3COOH* →CH2=CH-OCOCH3*

CH2=CH-OCOCH3*→ CH2=CH-OCOCH3+*

Fig. 8 Potential energy schemes of Peter Kripylo’s mechanism on OMC-COOH-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

3.1.4 Peter Kripylo’s mechanism on OMC-OH-Zn(OAc)2 CH≡CH+CH3COOH*→CH2=CH-OCOCH3* As we has discussed in supporting information, the structure of adsorbed vinyl acetate in which CH3COO* coming from acetic acid rather than zinc acetate is more stable. Thus, we choose this structure as the initial state of this step on OMC-OH-Zn(OAc)2 When the reaction begins, the H2 of acetylene moves to C1 to produce a CH2 group. C1 moves towards Ob of acetic acid. In TS, the distance between C2 and Ob is 2.68 Å. In FS, a bond forms between C2 and Ob, with a length of 1.40 Å. The hydroxyl hydrogen of acetic acid bonds to C2, producing a C-H bond in length of 1.09 Å. An adsorbed vinyl acetate is generated in FS. The detailed parameters of IS, TS and FS can be seen in Table S9 and the reaction progress is shown in Fig. 9. The activation energy of this process is calculated as 2.61 eV and the reaction energy is -1.02 eV CH2=CH-OCOCH3*→CH2=CH-OCOCH3+* In the initial state of this step on OMC-OH-Zn(OAc)2, vinyl acetate adsorbs on zinc acetate


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through a Zn-O bond. Compared with zinc acetate before reaction, in this case, O1 and O3 do not bond to Zn atom. The distance between Zn and O1, Zn and O3 are 2.26 Å and 3.18 Å respectively. When the reaction begins, the Zn-O bond between vinyl acetate and zinc acetate starts to prolong and then break, the vinyl acetate moves away from zinc acetate to an energy-lowest site. At the same time, O1 and O3 begin to get close to Zn atom, in TS, O1 bonds to Zn with a bond length of 2.01 Å. The distance between O3 and Zn also becomes shorter in TS, as 2.89 Å. Finally in FS, the distance between O3 and Zn reaches 2.62 Å. This step needs to overcome an activation barrier of 0.640 eV and the calculated reaction energy is 0.347 eV.

3.2 Acid catalyzed mechanism on catalysts with different functional groups 3.2.1 Acid catalyzed mechanism on OMC-Zn(OAc)2 CH3COOH*→CH3COO-*+H+ In the initial state, acetate adsorbs on zinc acetate through a Zn-Oa bond, with a length of

0

CH ≡ CH+CH3COOH* →CH2=CH-OCOCH3*

CH2=CH-OCOCH3*→ CH2=CH-OCOCH3+*

Fig. 9 Potential energy schemes of Peter Kripylo’s mechanism on OMC-OH-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

2.09 Å, see Fig. 5(d) and Table S2. After acetic acid adsorbing on OMC-Zn(OAc)2, the Zn-O4 bond breaks. The length of O-H bond of acetic acid is 1.03 Å. The reaction begins with the extension of O-H bond in acetic acid. In TS, the O-H bond breaks and the distance between O atom and H atom is 2.79 Å. The distance between Ob of acetic acid and Zn atom is 2.90 Å. In FS, the Ob of acetate acid acetic acid bonds to Zn atom, with a length of 2.16 Å, see Fig. S4(d) and Table S5. The detailed parameters of IS, TS and FS are listed in Table S10. The activation barrier of this step on OMC-Zn(OAc)2 is rather high, as 5.12 eV . This step also needs to adsorb lots of heat, with a value of 4.83 eV. CH≡CH+H+→CH2C+H Acetylene tends to adsorbs far away from zinc acetic and acetic acid and the distance



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between the dissociative H atom and C1 atom of acetylene is 4.32 Å. Then the H atom and C1 gets closer and in TS, the distance between them reaches 2.84 Å. Finally a CH2 group is produced. The detailed parameters of IS, TS and FS can be seen in Table S10. This step needs to overcome an activation barrier of 0.241 eV and the reaction energy is calculated as -2.45 eV. +

-

CH2=C H+CH3COO →CH2=CHOCOCH3* In the initial state, the distance between C2 of acetylene and O3 of zinc acetate is 6.43 Å. Then C2 and O3 begin to move close to each other. In TS, the Zn-O3 bond breaks and the distance between O3 and C2 is 2.90 Å. In FS, an adsorbed vinyl acetate is generated, bonding to zinc acetate through Zn-O bond. The detailed structural parameters of IS, TS and FS are shown in Table S10. The reaction progress is illustrated in Fig. 10. The activation barrier of this step is calculated as 0.247 eV and the reaction energy is rather high as -3.14 eV.

CH3COOH* → CH3COO-*+H+

CH≡CH+H+→ CH2C+H

+

-

CH2=C H+CH3COO →CH2=CHOCOCH3*

CH2=CH-OCOCH3* →CH2=CH-OCOCH3+*

Fig. 10 Potential energy schemes of acid catalyzed mechanism on OMC-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

3.2.2 Acid catalyzed mechanism on OMC-CO-Zn(OAc)2 *

-*

CH3COOH →CH3COO +H

+


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The IS, TS and FS of this step is shown in Fig. 10 and the detailed parameters are listed in Table S10. In this initial state of this step, the length of O-H bond is 1.04 Å. when the reaction

CH3COOH* → CH3COO-*+H+ CH≡CH+H+→ CH2C+H +

-



Fig. 11 Potential energy schemes of acid catalyzed mechanism on OMC-CO-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.

begins, the length of this bond becomes to prolong and break. In TS, the distance Ob atom and the dissociated H atom is 2.51 Å. A bond forms again betweem O1 of zinc acetate and Zn atom, with a length of 2.17 Å. In FS, the distance between Ob atom and the dissociated H atom reaches 3.80 Å. the activation barrier of this step is very high, as 4.81 eV and the reaction energy is 4.67 eV, indicating this step is difficult to take place. The reaction progress can be seen in Fig. 11. +

+

CH≡CH+H →CH2C H In the reaction, the dissociated H atom will continue move towards acetylene, in IS, the distance between them is 6.76 Å, in TS it is 2.81 Å and finally, a chemical bond forms between them, with a length of 1.10 Å, see Table S10. This step will overcome an activation barrier of 0.674 eV and the reaction energy is 2.36 eV, indicating this step is exothermatic. +

-

CH2=C H+CH3COO →CH2=CHOCOCH3* In the above discussion, we can infer that the structure of vinyl acetate which the acetate species comes from zinc acetate rather than acetic acid is more energy-lower. Thus, during the reaction, C2 of acetylene and O1 of zinc acetate continue moving close to each other, see Fig. 12. In IS, the distance between is 4.55 Å and then a bond forms between them. Apart from the production of adsorbed vinyl acetate, in the remaining parts of this system, we can find a renewed structure of zinc acetate which is similar to the one that has not participated in the reaction. The activation barrier of this step is low as 0.286 eV and the reaction energy is -3.15 eV, indicating this step is exothermatic.



3.2.3 Acid catalyzed mechanism on OMC-COOH-Zn(OAc)2 CH3COOH*→CH3COO-*+H+ The initial state is the adsorption of acetic acid on OMC-COOH-Zn(OAc)2, see Fig.5(c) We can find that after acetic acid adsorbing, the H atom from surface carboxyl will shift and bond to O2, producing two acetate species. Thus, there are two possibilities for the dehydrogenation reaction. One is that the H of reactant acetic acid dissociates, another is the H bonded to O2 dissociates. We calculates the energies of the two co-adsorption structures of CH3COO-* and H+ and we can find that if the dissociated H origins from O2-H, the structure will has lower energy. Therefore, the reaction begins with the extension of O2-H. In TS, the distance between O2 and H reaches 1.79 Å and in FS, this distance will be 9.58 Å. After the process, the surface carboxyl will break away from the carbon slab and producing CO2. This step needs to overcome a rather high activation barrier, as 3.32 eV and adsorbs an energy of 1.91 eV. +

+

CH≡CH+H →CH2C H The distance between C1 of acetylene and dissociated H atom is 2.67 Å and other parameters of IS, TS and FS are illustrated in Table S10. During the reaction, C1 and dissociated H atom moves towards each other, in TS, the distance between them is 2.26 Å and in FS, a CH2 group is generated. The activation barrier of this step is rather low, with a value of 0.0395 and this step will release a heat of -2.25 eV.

CH3COOH* → CH3COO-*+H+

CH≡CH+H+ →CH2C+H +

-



Fig. 12 Potential energy schemes of acid catalyzed mechanism on OMC-COOH-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided. +

-

CH2=C H+CH3COO →CH2=CHOCOCH3* The initial state of this step is shown in Fig.S5(c). The distance between C2 of CH2=C+H and O2 of zinc acetate is rather far, as 7.86 Å, and as the reaction processes, C2 and O1 move towards each other and in the transition state, the distance between them is 3.00 Å .The distance between Zn and O2 also becomes larger, as in IS, there is a bond between Zn and O2 and in TS, this bond breaks. Then in FS, an adsorbed vinyl acetate is generated. The activation barrier of this step is


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rather low, as 0.389 eV and the reaction energy is 0.993 eV. The reaction progress can be seen in Fig. 12. Remarkably, if vinyl acetate is produced through this acid catalysis mechanism, CO2 will also be generated as carboxyl has broken away from the carbon surface in the reaction of acetic acid dehydrogenation.

3.2.4 Acid catalyzed mechanism on OMC-OH-Zn(OAc)2 CH3COOH*→CH3COO-*+H+ The initial state of this step on on OMC-OH-Zn(OAc)2 is illustrated in Fig.5(d). After acetic acid adsorbing on the catalysis, Zn-O1 bond and Zn-O2 bond break and the distances between Zn and O1 and between Zn and O2 is 2.99 Å and 2.00 Å respectively. The length of O-H bond in acetic acid is 1.07 Å. When the reaction begins, the hydroxyl H atom of acetic acid begins to move away and the length of O-H in TS is 3.57 Å. This H atom keeps moving away and in FS, the distance of Ob and H atom is 6.16 Å. The structure of zinc acetate also changes during the reaction, as O1 bonds to Zn again. This step needs to overcome a rather high activation barrier as 5.37 eV and adsorb a lot of energy as 4.79 eV, indicating that this step is hard to take place. +

+

CH≡CH+H →CH2C H In the initial state, the distance between C1 of acetylene and dissociated H is 5.23 Å and this distance keeps shortening and a chemical bond in length of 1.10 Å forms between them. The activation barrier of this step is not high as 0.45 eV and the reaction will release a lot of heat with a value of 2.75 eV. The other structure parameters are listed in Table S10. +

-

CH2=C H+CH3COO →CH2=CHOCOCH3* Different from this step on zinc acetate supported on pure activated and activated carbon

CH3COOH* → CH3COO-*+H+ CH≡CH+H+ →CH2C+H +

-

CH2=C H+CH3COO CH2=CH-OCOCH3* →CH2=CHOCOCH3* →CH2=CH-OCOCH3+*

Fig. 13 Potential energy schemes of acid catalyzed mechanism on OMC-OH-Zn(OAc)2.Blue ball: Zn atom; Red ball: O atom and white ball: H atom. In order to improve the clarity, carbon support is avoided.



modified by –COOH and -CO, the structure of adsorbed vinyl acetate in which the acetate species comes from acetic acid rather than zinc acetic is more stable. Thus, during the reaction, CH2=C+H keeps moving towards Ob of acetic acid. In IS, the distance between Ob and C2 of acetylene is 2.79 Å, in TS is 2.33 Å and in FS a bond forms between them, with a length of 1.40 Å. The activation barrier of this step is 0.463 eV and the reaction energy is -2,91 eV, indicating this step is easy to take place thermodynamically and kinetically. The reaction progress can be seen in Fig. 13. Noticeable, the dissociation process of vinyl acetate in acid catalyzed mechanism is the same with that in Peter Kripylo’s mechanism both on four catalysis with different surface structures. Therefore, this step is not discussed.

3.4 Comparison of the two mechanisms on catalysis with different surface structures After analyzing the activation barriers and reaction energies of all the elementary steps involved in the two mechanisms, we can find that CH≡CH+CH3COOH*→CH2=CH-OCOCH3* and CH3COOH*→CH3COO-*+H+ are rate-limiting steps of Peter Kripylo’s mechanism and acid catalysis mechanism respectively as the activation barriers of these two step are much more higher than other steps in respective mechanism. In order to expound concisely, we compare the activation barriers of rate-limiting steps of two mechanisms on four catalysts, see Fig. 14. On all the four conditions, the activation barriers of the rate-limiting step of acid catalysis mechanism are much higher than those of the rate-limiting step of Peter Kripylo’s mechanism, with values of about 5 eV, see Table S11. This reaction also needs to adsorb a lot of heat in spite of the modification of functional groups, indicating that it is rather hard to occur thermodynamically and kinetically. The activation barriers of the rate-limiting step of Peter Kripylo mechanism are all around 2.5 eV, see Table S12 and heat will be released during the reaction, which means that the reaction is preferable both thermodynamically and kinetically. Therefore, we may preliminarily consider that vinyl acetate is produced through Peter Kripyo’s mechanism. When it comes to the influence of functional groups on the surface of activated carbon on the synthesis reaction of vinyl acetate, we can find that from Fig 14, compared with OMC- Zn(OAc)2, the modification of carboxyl on the carbon support will decrease the activation barriers of the rate-limiting steps of both mechanisms. Thus we can conclude that carboxyl has positive effect on the synthesis reaction. On the opposite, if the carbon surface is modified with hydroxyl, the activation barriers of the two steps will be increased to some extent. In another word, the existence of hydroxyl on activated carbon has negative effect on the reaction. When it comes to carbonyl, it may increase the activation barrier of the rate-limiting step of Peter Kripylo’s mechanism so that it is negative to the synthesis reaction. Although it can decrease the barrier of the rate-limiting step in the acid catalysis mechanism, the activation barrier of this step is still as high as 4.81 eV which indicates this step is rather difficult to occur. Therefore, the modification of carbonyl on the catalysis support is negative to the synthesis of vinyl acetate. Based on the above discussion, in the design and production process of catalysis, we can increase the content of carboxyl suitably and reduce hydroxyl and carbonyl on activated carbon through surface modification to improve the


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production efficiency of vinyl acetate. It is worth noting that a replacement of acetate species is involved in Peter Kripylo’s mechanism, which means that the acetylene react with acetate species coming from zinc acetate rather than acetic acid. In previous research, many researchers have tried to discover other catalysis to take the place of zinc, all of these trials have been proved to be failed acetate as mentioned above. Our research may proposed an explanation to these failures that acetate radical of the catalysis is necessary in this reaction.

Fig 14 Activation energy of the rate-limiting step of Peter Kripylo’s mechanism and acid catalyzed mechanism on catalysis of different surface structures.

Conclusion In this paper we conduct a density functional theory (DFT) calculation on the synthesis mechanism of vinyl acetate catalyzed in the reaction of acetylene and acetic acid by zinc acetate supported on ordered mesoporous carbon modified with different functional groups. We made comprehensive analysis on the complex structural features of all intermediates and their transition states, the activation barriers and the reaction energies of all elementary steps. The adsorption of acetylene is physical adsorption on the catalysis with different surface structures, indicating Furugawa’s mechanism has some defects. The activation barrier of the rate-limiting step of acid catalysis mechanism is almost two times of that of Peter Kripylo’s mechanism and the activation barriers and reaction energies of the latter mechanism all suggest that Peter Kripylo’s mechanism is preferable both thermodynamically and kinetically. After modification of functional groups on the carbon support, we can find that carboxyl can reduce the activation barrier of rate-limiting steps of both mechanisms dramatically while hydroxyl increases them. Carbonyl will shift into epoxy group and it will reduce the barrier of rate-limiting step of acid catalysis mechanism slightly, but it is still rather high as 4.81 eV, while in Peter Kripylo’s mechanism, it will increase the barrier. Thus, it is negative for the synthesis reaction. When producing the catalysis support, we may



directionally increase the content of carboxyl and reduce hydroxyl and epoxy group by surface modification to optimize the production. We also find that there is a replacement of acetate species existing in the synthesis process of vinyl acetate, which means that acetylene reacts with acetate species originating from zinc acetate rather than acetic acid. This discovery may provide an explanation of the relative poor performance of non-zinc acetate catalysts and inspire us when designing active component of the catalysis.


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Table of Contents graphic


Density Functional Theory Investigation on the Synthesis

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