Active Site Ensembles Enabled CâC Coupling of CO2 and CH4 for

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Active Site Ensembles Enabled C-C Coupling of CO and CH for Acetone Production 2

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Yuntao Zhao, Hua Wang, Jinyu Han, Xinli Zhu, and Qingfeng Ge J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02359 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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The Journal of Physical Chemistry

Active Site Ensembles Enabled C−C Coupling of CO2 and CH4 for Acetone Production Yuntao Zhao, † Hua Wang, † Jinyu Han, † Xinli Zhu*, † and Qingfeng Ge*, †, ‡

†

Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China ‡

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois

62901, United States

Corresponding Authors *Tel.:86-22-27890859 Email: [email protected] (X.Z.) *Tel.:1-618-453-6406 Email: [email protected] (Q.G.)

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ABSTRACT The production of acetone through ketonization of acetic acid is both energy-intensive and environmentally-detrimental as it emits equimolecular CO2. Herein, a novel approach for acetone production from CO2 and CH4 was proposed through consecutive C−C coupling based on an extensive density functional theory computational study. To realize the consecutive C−C coupling, CeO2-based catalyst doped with Zn site ensemble was constructed. Direct coupling of CH3* with the acetate species, formed from C-C coupling of CO2 and stabilized CH3*, could be achieved to produce acetone. The results show that carbon chain growth is possible on the active site ensembles constructed on a CeO2-based catalyst. The coupling of the acetate species with the methyl moiety follows a nucleophilic addition mechanism and has apparent activation energies of 0.25 and 0.14 eV on Ovac (oxygen vacancy) and neutral surfaces, respectively. This process could potentially replace the traditional acetone production technology based on ketonization of acetic acid.

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INTRODUCTION Acetone is widely used in many applications, including the production of pesticide and herbicide as well as other important industrial chemicals.1-3 Ketonization of acetic acid is the major route to produce acetone in industry.4-11 Ketonization reaction has been reported widely on metal oxides, such as TiO2, ZrO2, and CeO2.5,6 Although considerable studies have been carried out in aqueous,12 gas,5,13 and organic phases,8 there is little consensus on the mechanism of ketonization.5,6,8,14,15 Among the proposals, the pathway involving β-ketoacid intermediate is believed to be the most likely one, albeit β-ketoacid has never been detected. In the β-ketoacid route, dissociative adsorption of one acetic acid molecule resulted in carboxylate species. Subsequent cleavage of α-C−H bond, either via direct α-hydrogen abstraction or through enolization, makes the carboxylate species nucleophilic. Reaction of the nucleophile with the neighboring carboxylate will produce a β-ketoacid intermediate. Decarboxylation of β-ketoacid results in ketone, CO2 and H2O. Since the production of acetone in this process is accompanied by the stoichiometric formation of CO2, a high yield of acetone corresponds to a high CO2 emission. The studies on ketonization reaction demonstrated that the acetate species originated from acetic acid dissociation on the M−O site were the important intermediates4,5,15,16 and the C−C bond formation was the kinetically limiting step. We showed that C−C coupling of Zn stabilized CH3 and CO2 could be realized on Zn-doped ceria, resulting in a strongly adsorbed acetate species.17 The recent report of the binuclear CuII complex formed from the mobile CuI species for NOx reduction demonstrated the importance of the multinuclear ensembles in catalysis.18 These results inspired us to construct a Zn site ensemble in CeO2 to further C-C coupling, i.e. adding additional Zn stabilized CH3 to acetate intermediates. We demonstrate that this Zn

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ensemble enables acetone production from CO2 and CH4 and overcomes the CO2 emission issue of the ketonization reaction of acetic acids. Conversion of CO2 and CH4 to chemicals with high commercial value and energy density is gaining increasing interest due to the availability of CH4 from shale gas and the potential of mitigating the negative environmental effects of these greenhouse gases.19-22 CH4 and CO2 are both C1 sources, which could be employed to build longer chain hydrocarbons. However, the difficulty in selectively breaking the C−H bond in CH4 and the inertness of CO2 present a formidable challenge to utilize them in the construction of C−C bonds under moderate conditions.19,23,24 Therefore, most studies have focused on reforming CH4 with CO2 at high temperatures to produce syngas.25-32 Recently, conversion of CH4 and CO2 to acetic acid by C−C coupling has made significant progresses. For example, we demonstrated that doped Zn in CeO2 stabilizes CH3 from CH4 activation and enables facile insertion of CO2 into the Zn−CH3 bond through the SE2 mechanism.17 Very recently, Shavi et al.33 reported a CeO2-ZnO/MMT catalyst exhibited high activity and selectivity towards acetic acid production experimentally. Density functional theory (DFT) studies of CH4 and CO2 to acetic acid conversion on M-exchanged MFI catalyst (M = Be, Co, Cu, Mg, Mn, Zn) revealed the detailed properties related to CH4 dissociation and confirmed the SE2 insertion mechanism.34 Tu et al. achieved a single-step conversion of CH4 and CO2 to chemicals including acetic acid, methanol, ethanol and formaldehyde at room temperature and atmospheric pressure through plasma-driven catalysis.35 However, only one-step C−C coupling was involved in those reported processes. To the best of our knowledge, no consecutive C−C coupling towards multicarbon (C≥3) products from CH4 and CO2 has been reported. Acetone production from CO2 and CH4 through a consecutive C−C coupling is therefore of great

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significance. Herein, we demonstrate that a CeO2-based catalyst with an active Zn site ensemble enables carbon chain growth by coupling a second Zn-stabilized CH3 with the acetate species that is formed from direct C−C coupling of CO2 and the first Zn-stabilized CH3. We mapped out a detailed reaction mechanism based on results of DFT calculations. This process provides not only an alternative for acetone production using CO2 and CH4 instead of acetic acid but also a new approach for CO2 and CH4 utilization.

COMPUTATIONAL METHODS All spin polarized DFT calculations were carried out using the Vienna ab initio simulation package (VASP).36,37 Projector-augmented wave (PAW) potentials were used to represent the core-valence interaction.38,39 The wavefunctions of valence electrons are expanded in a basis set of plane waves with a cutoff energy of 400 eV. The Perdew-Burke-Ernzerhof (PBE) functional was used to determine the exchange and correlation energies.40 DFT+U approach with U = 5 eV to localize the f orbitals of cerium was adopted, in the same fashion as our previous study on Zndoped ceria.17 Previous studies have demonstrated that the U values depend on the valence states and chemical environment,41,42 and may affect the initial, transition and product states differently.43 For CeO2(111), U in the range of 3 - 5.5 eV was shown to provide a balanced results of reaction energetics and surface reducibility.43 The slab geometry and parameters were also kept the same. Herein, the Zn ensemble were built by introducing a pair of Zn to one side of the slab consisting of 72 O atoms and 36 Ce atoms in three O-Ce-O trilayers. The slab was separated by a vacuum space of 15 Å. The surface Brillouin zone was sampled by a (2×2×1) kpoint grid. During structure optimization, the bottom trilayer was fixed to their bulk positions and the top two trilayers were allowed to relax by using either the conjugate gradient algorithm

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or the quasi-Newton method. The convergence criterion was set to 0.02 eV/Å for all unconstrained atoms. All optimized structures were verified by normal mode frequency analysis and only one imaginary mode was found in transition states. The Zn site ensemble were built on the basis of the previous Zn-doped CeO2(111) surface by substituting another Ce atom with a Zn atom in the supercell. At this level of doping, the doped Zn atoms may exist either as isolated Zn sites or an ensemble of Zn pair. We compared the relative stability of the two possibilities and found that the Zn pair is 0.87 eV more stable than the isolated Zn sites. The results clearly indicate that the doped Zn atoms tend to form Zn ensembles on CeO2(111). We note that doping two Zn atoms in the unit cell requires removal of two oxygen atoms to maintain formal charge neutrality. The slab with stoichiometric metal/oxygen ratio is denoted as the neutral surface and the stab with one additional surface oxygen atom being removed is denoted as Ovac. The oxygen vacancy concentration is 8.3% on the neutral surface and 12.5% on the Ovac surface. Among three possible arrangements, the two O atoms in the ortho-position (Figure S1) is the most stable one and was chosen as the model for subsequent studies. The oxygen vacancy formation energy (Ef) was calculated according to the following formula: 1 = + −

2 where , , and represent the energy of the Ovac slab, gas phase O2, and the neutral slab, respectively. In CeO2(111) doped with one Zn atom, the surface oxygen vacancy was calculated to be 0.56 eV more stable than the subsurface vacancy. Therefore, we only considered surface oxygen vacancy in the present study. The same adsorption energy definition17 was adopted in this study, where a positive value indicates an endothermic process and a 6


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negative value demonstrates exothermic adsorption energy.

RESULTS AND DISCUSSION

Figure 1. (a) Optimized structures of Zn ensembles in neutral and Ovac surfaces. (b) Structures of transition (upper panel) and final (lower panel) states of CH4 dissociation on neutral and Ovac surfaces. Ce is shown in ivory, O in red, Zn in purple, H in white, and C in grey. Distances are in Å. Dissociative Adsorption of CH4 The dissociation of CH4 was carried out on both neutral and Ovac surfaces. Among different configurations of oxygen vacancies on the neutral surface (Figure S1), the removal of two 7



adjacent O atoms is the most stable one, as shown in Figure 1a. On the basis of the most stable neutral surface, the formation energy of the Ovac surface is 1.27 eV, comparable to the value of 1.21 eV on the surface with one doped Zn atom. This result indicates that doped Zn atoms in CeO2 stabilizes the surface oxygen vacancy as the vacancy formation energy is reduced from 2.13-2.43 eV on pure CeO2 surface.44 The optimized adsorption structures as well as corresponding transition states of dissociative adsorption of CH4 are given in Figure 1b. Heterolytic dissociative adsorption of CH4 proceeds through breaking the C−H bond, with the methyl group occupying the Zn site and the H atom on the O site. The dissociation barrier on the Ovac surface is 0.74 eV, which is lower than 1.22 eV on the neutral surface. As shown in Figure 1b, in the transition state on Ovac surface, the H was balanced with attraction from both surface O and C of CH4, with a H−O distance at 1.33 Å and H−C distance at 1.35 Å, respectively. Compared to H−C bond length of 1.09 Å in pure CH4 molecule, H−C distance in transition state was elongated by 0.26 Å. Meanwhile, methyl group moved toward surface Zn site, with a Zn−C distance at 2.20 Å. The three C−H bonds stayed the same way as they were in the pure CH4 molecule. However, on the neutral surface, the dissociation is different as a similar adsorption mode could not be stabilized. In such a case, the adsorbed methyl species underwent a configuration inversion during the activation process and was stabilized on the Zn site away from the OH species. Zn-doped CeO2 surfaces have been suggested to promote activation of CH4.17,45 We reported that the dissociation barrier of CH4 on the single Zn doped CeO2 surface with one oxygen vacancy was 0.36 eV.17 This value is lower than 0.74 and 1.22 eV on Ovac and neutral surfaces doped with two Zn atoms, respectively. These results indicate that the formation of the Zn ensemble on the surface and the existence of oxygen vacancies play critical roles in activating

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the first C−H bond of CH4. The existence of oxygen vacancies clearly enhances the dissociation process, consistent with the fact that CH4 dissociation was enhanced by the existence of oxygen vacancies on the iron oxide surfaces.46 Coupling CO2 with CH3* on the Zn Site Ensemble We first examined the C−C bond formation between adsorbed CH3 species and CO2 on the

Figure 2. Configurations of CO2 insertion (top row) and corresponding acetate species (bottom row) on neutral (left) and Ovac surfaces (right). Distances are in Å.

Zn ensembles as shown in Figure 2. As shown in Figure 2, on CeO2 surfaces doped with Zn ensemble, formation of the C−C bond from CH3* and CO2 follows the same SE2 mechanism through CO2 insertion into the Zn−CH3 σ-bond as we reported previously.17 The calculated C−C coupling barriers are 0.57 and 0.49 eV on Ovac and neutral surfaces, respectively. The corresponding reaction pathways are displayed in Figure 3. In the transition state, CO2 was activated to a bent conformation.47 Density of states analysis indicated that the C atom in activated CO2 has gone through a transformation of hybridization from sp to sp2 (Figure S2). CeO2(111) doped with isolated Zn sites shows the possibility of CO2 insertion into the Zn −CH3 bond with an insertion barrier of 0.51 eV.17 As shown above, the CO2 insertion barriers on 9



the surfaces with the two-Zn atom ensemble with and without Ovac have similar values. These results demonstrate that both the number of doped Zn atoms and the existence of oxygen vacancy do not significantly affect the formation of the first C−C bond. Although the first C−C bond formation on the ensemble is no superior to the single Zn site, the Zn site ensembles facilitate further C−C coupling with the additional Zn site of the ensemble stabilizing CH3 from CH4 dissociation. This additional C-C coupling reaction helps to convert the strongly bound acetate species on the active site and promotes the regeneration of the active sites. We also examine the possibility of the Zn ensemble catalyzed coupling of the methyl species

Figure 3. Potential energy diagrams for CH4 adsorption and C−C coupling reaction on (a) neutral and (b) Ovac surfaces. A* signifies the adsorption state of species A. CH3COO* and CH3COO*_2 represent the monodentate and bidentate adsorption states, respectively. on the two Zn sites as this reaction is likely to compete with CO2 insertion. Our results show that a second methyl species could not be stabilized on the next Zn site on neutral surface. Even on Ovac surface, the dissociative adsorption of the second CH4 becomes 0.52 eV endothermic, which 10


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is comparable to the C−C coupling activation barrier (Figure 3). Therefore, we conclude that the coupling of adsorbed CH3 with CO2 would be the dominant reaction pathway.

Coadsorption of CH3COO* and CH3* Coupling of the methyl species and CO2 leads to the acetate species. The acetate species formed is also the intermediate for acetone production through ketonization. In order to form the second C−C bond, another CH3* from CH4 activation needs to be stabilized in the presence of the acetate species on the Zn ensembles. Before the second CH3* stabilization, we tested desorption of the acetate species in the form of acetic acid and found it energetically unfavorable, similar to the acetate species on the singly doped CeO2. The acetate species formed from insertion of CO2 into the Zn−CH3 σ-bond strongly binds to the surface, making desorption of the product in the form of acetic acid highly energetic. The alternative route of transferring the proton through the Zn site is even more endothermic. In addition to forming acetic acid, the acetate species may undergo conversion to acetaldehyde or ethanol, which has been reported in a number of studies.48-50 Previous studies suggested that acetaldehyde is the key intermediate in the interconversion among the three species. The formation of acetaldehyde from acetic acid requires breaking the C−O bond. However, both direct C−O bond rupture and hydrogenation of the acetate species followed by deoxygenation are strongly inhibited on the Zn-doped ceria surface.17 In this work, two Zn sites stabilize both the acetate species from the CO2 insertion reaction and the methyl species from CH4 activation. The dissociative adsorption of the second CH4

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molecule on the next Zn site following acetate formation is shown in Figure 4.

Figure 4. The second CH4 dissociation on neutral (left) and Ovac (right) surfaces, respectively. Transition states are shown in upper row and the stabilized states of CH3* and H* are in the lower row. Distances are in Å.

The dissociation barrier of CH4 on Ovac and neutral surfaces are calculated to be 1.25 and 1.04 eV, respectively, while the dissociation energies are 0.49 and 0.63 eV, respectively. As shown above, the presence of an oxygen vacancy on the surface enhances the activation of the first CH4. However, the dissociation barrier of the second CH4 on the Ovac surface is slightly higher than that on the neutral surface. These barriers indicate that the activity toward CH4 dissociation is reduced from that of the fresh Zn sites. Nevertheless, the second CH4 dissociation presents apparent activation energies of -0.19 and -0.41 eV on Ovac and neutral surfaces, respectively, following the highly exothermic acetate formation process.

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Coupling CH3* with CH3COO* Coadsorption of stabilized CH3* and the acetate species on the Zn site ensemble establishes the initial configuration for further C−C coupling. The structures along the coupling process are shown in Figure 5 and the parameters are summarized in Table 1. Before the coupling proceeds, the adsorbed monodentate acetate species undergoes a rotation to form a bidentate structure (CH3COO*_2 in Figure 3) with one O of CH3COO* attached to surface Zn and the other O bonded to surface Ce, which is more stable on both Ovac and neutral surfaces. The rotation energies on Ovac and neutral surfaces are calculated to be −0.56 and −0.49 eV exothermic,

Figure 5. Initial (IS), transition (TS), and final states (FS) of C−C coupling from CH3COO* and CH3* to CH3CO(O)CH3* on both surfaces. Insets are top views.

respectively.

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Table 1. Geometric Parameters of Initial (IS), Transition (TS), and Final States (FS) of C-C Coupling from CH3COO* and CH3* to CH3CO(O)CH3* over Neutral and Ovac Surfaces. Distances Are in Å and Angles Are in Degree. Neutral Zn−C C−C

Ovac

C−O

∠C-CO-O

Zn−C C−C

C−O

∠C-CO-O

IS

2.01

3.14

1.26/1.29

180.0

2.04

3.57

1.26/1.29

180.0

TS

2.37

2.09

1.31/1.35

136.1

2.53

2.13

1.31/1.36

136.7

FS

-

1.53

1.41/1.44

121.0

-

1.53

1.41/1.44

121.3

As shown in Figure 5 and Table 1, in the transition state, the methyl species was displaced from the Zn site, and the coplanar structure consisting of two C and two O atoms of the acetate species were destroyed. On Ovac surface, Zn−C distance was elongated to 2.53 Å, from 2.04 Å in coadsorbed state. Away from the surface, the methyl group tilted toward the acetate species and the C of CH3 still remains sp3 hybridization, unlike the case in CO2 insertion step. For acetate species, the structural distortion occurred at a ∠ C-CO-O angle of 136.7°, and the bulged C, connected to two O atoms, tilted toward methyl group. After the distortion, two C−O bonds are elongated to 1.31 and 1.36 Å from 1.26 and 1.29 Å in stabilized acetate species respectively. The results reflect that C−O bonds in the acetate species has been further activated, close to be C−O single bonds. The C−C coupling process on neutral surface shows analogy with that on Ovac surface. The C−C distance is measured to be 2.13 and 2.09 Å on Ovac and neutral surfaces, respectively. Bader charge analysis showed that the carbon atom of the methyl group on the neutral and Ovac surfaces has a net charge of −0.34 |e| and −0.38 |e|, respectively, relative to the value of the carbon atom in the CH4 molecule. These results indicate that the methyl group served as nucleophile in the coupling reaction, consistent with its role in the formation of the acetate species. On the other hand, the carboxylic center of the acetate species acted as an 14


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electrophile. Results from electron charge density difference confirmed that there are corresponding electron accumulation and depletion centered on the two carbon atoms when the C−C bond is formed, shown in Figure 6. The loss of electron on carbon center of acetate species makes it an electrophilic center. We note that the carboxylic center of acetate species stems from the linear CO2, with sp hybridization in C. During the process of the first C−C coupling, C of CO2 went through a transformation from sp to sp2 hybridization. When the carbon center was attacked by the methyl group, a distortion occurred, making the two C−O bonds deviated from the original C-CO-O plane. Consequently, the hybridization of carbon center changed from sp2 to sp3 in the second coupling process. The electron accumulated on the methyl group fills the new empty sp3 orbital. Meanwhile, surface Zn and Ce atoms are involved in the coupling reaction,

Figure 6. Isosurface of the electron charge density difference at ±0.001 e bohr-3 for C−C coupling on Ovac surface. Charge accumulation and depletion are represented by the yellow and blue regions.

and hence, stabilize the generated CH3CO(O)CH3*. This coupling step shares several mechanistic features with the ketonization reaction from carboxylic acids. In the ketonization reaction, the carboxylic carbon atom serves as an electrophilic center and the α-carbon atom of the other carboxylate servers as a nucleophilic center after abstraction of the α-hydrogen or enolization.15,16 In the present work, the stabilized methyl group on the Zn site functions similarly to the enolate or the anion after α-hydrogen 15



abstraction in the ketonization mechanism, i.e. acting as a nucleophilic center to achieve C−C coupling. The formation of the C−C bond leads to dioxo-isopropylidene (CH3CO(O)CH3*), with two O atoms anchored on the surface through the O−Zn and O−Ce bonds. When the C−C bond is formed, the C−O bonds are further stretched to longer distances of 1.44 and 1.41 Å, respectively. These results demonstrate that the second C−C coupling reaction follows a nucleophilic addition mechanism. The reaction pathway and energy for conversion to acetone is

Figure 7. Potential energy profiles for acetone production from co-adsorbed CH3COO*_2 and CH3* through a C−C coupling reaction on (a) neutral and (b) Ovac surfaces. A* represents the adsorption state of species A.

depicted in Figure 7.

The barrier for the coupling step was determined to be 1.69 and 1.52 eV on Ovac and neutral surfaces, respectively. The apparent activation energy on both surfaces was calculated to be only 16


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0.25 and 0.14 eV. These results are generally in agreement with those reported for ketonization of acetic acid. Experimentally, the intrinsic activation energy of C−C bond formation was obtained from fitting the kinetics data with the Arrhenius and Van’t Hoff equations. A barrier of 1.67 eV was reported on a Ru/TiO2 catalyst.4 Recently, ketonization of acetic acid was performed over Zr/Mn mixed oxides and an apparent activation barrier of 1.67 eV was reported12. The ketonization activity has been linked with the acidity or basicity of sites on the catalyst surface, as well as the surface vacancies.5,6,12,16 However, the results in this study showed similar bond lengths, bond angles, dihedral angles, as well as the similar activation barriers in the transition state with and without oxygen vacancies. Therefore, the presence of oxygen vacancy should not have a significant effect on the coupling reaction. In fact, on Zn-doped CeO2, the oxygen vacancy does not change the redox property of the Zn site ensemble. The coupling of CH3* and the acetate species depends strongly on the synergy of the Zn sites in the ensemble. Once dioxo-isopropylidene is formed, further transformation to acetone can be achieved either via direct C−O rupture or hydrogenation of the C−O bond, as shown in Figure 7. After breaking the C−O or C−OH bond, the acetone species adsorbs on the surface through an O−Ce bond. To complete the catalytic cycle, the O atom can react with H atoms from dissociation of CH4 and form a water molecule. The corresponding structures of intermediates are provided in Table S1. The overall reaction energy from gaseous CO2 and CH4 to gaseous acetone is 0.84 eV.

CONCLUSIONS Active Zn site ensemble was created in CeO2 to achieve consecutive C−C coupling of CO2 and CH4 to produce acetone. The process utilizes the Zn-stabilized CH3* as a nucleophile to couple with the acetate species, making the process energy-efficient and free from CO2 emission.

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The formation of acetone follows a nucleophilic addition mechanism, with apparent activation energies of 0.25 and 0.14 eV on Ovac and neutral surfaces, respectively. The presence of oxygen vacancies facilitates CH4 activation but has no significant effect of the C−C coupling reaction. These findings demonstrate that active ensembles on a catalyst are important to tune the selectivity and provide a new approach to produce C3 oxygenates from CO2 and CH4 directly.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website. Supporting results with associated figures and tables (PDF)

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Key Research and Development Program of China (2016YFB0600900) and National Natural Science Foundation of China (Grants 21373148, 21576204, and 21676194). Q.G. acknowledges the support of NSFCBET program (Award CBET-1438440).

REFERENCES

(1) Zhou, J.; Zhang, H.; Zhang, Y.; Li, Y.; Ma, Y. Designing and creating a modularized synthetic pathway in cyanobacterium synechocystis enables production of acetone from carbon dioxide. Metab. Eng. 2012, 14, 394-400. (2) Lozano, A.; Yip, B.; Hanson, R. K. Acetone: A tracer for concentration measurements in gaseous flows by planar laser-induced fluorescence. Exp. Fluids 1992, 13, 369-376. (3) Mestres, R. A green look at the aldol reaction. Green Chem. 2004, 6, 583-603. (4) Pham, T. N.; Shi, D.; Resasco, D. E. Reaction kinetics and mechanism of ketonization of aliphatic carboxylic acids with different carbon chain lengths over Ru/TiO2 catalyst. J. Catal. 18


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