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Article

DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites

by
Yuan Zhou
1,2,
Guoqiang Zhang
1,
Ya Song
1,
Shirui Yu
1,
Jingjing Zhao
1 and
Huayan Zheng
1,*
1
Department of Food Science and Engineering, Moutai Institute, Renhuai 564502, China
2
Experimental Training Teaching Center, Moutai Institute, Renhuai 564502, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(3), 477; https://doi.org/10.3390/catal13030477
Submission received: 30 January 2023 / Revised: 22 February 2023 / Accepted: 24 February 2023 / Published: 26 February 2023
(This article belongs to the Special Issue Recent Trends in Catalysis for Syngas Production and Conversion)
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Abstract

:
In this study, a density functional theory method is employed to investigate the reaction mechanisms of dimethyl carbonate (DMC) formation, through oxidative carbonylation of methanol, on four types of Y zeolites doped with Cu+, Cu2+, Cu2O and CuO, respectively. A common chemical route is found for these zeolites and identified as, first, the adsorbed CH3OH is oxidized to CH3O species; subsequently, CO inserts into CH3O to CH3OCO, which reacts with CH3O to form DMC rapidly; and finally, the adsorbed DMC is released into the gas phase. The rate-limiting step on Cu2+Y zeolite is identified as oxidation of CH3OH to CH3O with activation barrier of 66.73 kJ·mol−1. While for Cu+Y, Cu2O-Y and CuO-Y zeolites, the rate-limiting step is insertion of CO into CH3O, and the corresponding activation barriers are 63.73, 60.01 and 104.64 kJ·mol−1, respectively. For Cu+Y, Cu2+Y and Cu2O-Y zeolites, adsorbed CH3OH is oxidized to CH3O with the presence of oxygen, whereas oxidation of CH3OH on CuO-Y is caused by the lattice oxygen of CuO. The order of catalytic activities of these four types of zeolites with different Cu states follows Cu+Y ≈ Cu2O-Y > Cu2+Y > CuO-Y zeolite. Therefore, CuY catalysts with Cu+ and Cu2O as dominated Cu species are beneficial to the formation of DMC.

1. Introduction

Dimethyl carbonate (DMC), which is considered as one of the environmentally benign chemicals, has been used as a low toxicity solvent and fuel additive. Its production and utilization have recently drawn much attention [1,2,3,4,5,6,7]. Meanwhile, DMC synthesis by oxidative carbonylation of methanol is suggested since phosgene is not produced during the process [7,8,9,10,11]. CuO and Cu2O are p-type semiconductors with a direct band gap of 1.2 and 2.0 eV, respectively, which has been widely used as sensors and active centers in various catalytic reactions due to their unique electronic structure [12,13,14]. Similarly, Cu-exchanged zeolite catalysts, as the chloride-free catalysts, have been considered as one of the most attractive catalysts for DMC synthesis in recent years [15,16,17,18,19], due to the high catalytic activity and selectivity. CuY zeolite catalyst is one of them [17,18,19,20,21,22,23].
The presence of different Cu states in CuY zeolites results in distinct catalytic activities and is achieved using different methods. King [20] reported that Cu+Y zeolite prepared by solid-state ion exchange showed a satisfying catalytic activity in the oxidative carbonylation reaction, while ion-exchanged Cu2+Y zeolite exhibited a poor performance. Cu+ and CuO based Cu-FAU catalysts were prepared by Kieger et al. via ion-exchanged method and incipient-wetness-impregnation, respectively. After characterized by UV-VIS, IR, TPR and NH3-TPD, it was suggested that Cu+ and CuO were formed in Cu-FAU by ion-exchange method and incipient-wetness impregnation, respectively, and Cu+ exhibited a better catalytic activity than CuO [24]. Richter and co-workers showed that CuO was formed in CuY zeolite when the Cu loading was above 10 wt% during incipient-wetness impregnation [25]. They pointed out that, due to the formation of CuOx particles, oxidative carbonylation of methanol proceeded with and without oxygen, meanwhile, CuOx enhanced the formation of DMC [26,27]. Our study showed that with increasing the exchange degree, different Cu states were produced, such as Cu2+, Cu+, Cu2O, CuO and CuY zeolites, leading to different catalytic performance [28].
A number of studies investigated the possible reaction schemes of oxidative carbonylation of methanol to DMC on Cu-exchanged zeolite [16,20,22,25,29,30,31], Pd-exchanged zeolite [9,32], Cu/AC [33], γ-Cu2Cl(OH)3 [34], CuCl [35] and Cu2O [8,36] catalysts. Generally, the molecularly adsorbed methanol is first oxidized by oxygen to methoxide or di-methoxide species. Then the formation of DMC follows two distinct reaction pathways. The first starts with the insertion of CO into methoxide to produce CH3OCO, which subsequently reacts with CH3OH to form DMC. The second involves the CO addition to di-methoxide species. Experimental investigation by Engeldinger et al. [26,27] showed that CuOx aggregates were formed in CuY catalyst when the Cu loading was above 11 wt%, which promoted oxidation and oxocarbonylation reactions of methanol and enhanced the formation of DMC. They suggested that the reaction was closely related to the CH3OCOOH (MMC), which was produced through participation of lattice oxygen from CuOx of the catalyst. Cu was reoxidized by gas phase oxygen according to the Mars–van Krevelen mechanism [37]. Although the role of CuOx has been identified, there is little information on the detailed reaction mechanism which addresses the Cu2O, CuO and Cu2+ species of Y zeolite during oxidative carbonylation of methanol.
In this work, the reaction mechanisms governing oxidative carbonylation of methanol to DMC were studied with Cu+, Cu2+, Cu2O and CuO species in Y zeolites using density functional theory (DFT). An appropriate size of CuY cluster was constructed as the stable configuration, reflecting different Cu states in Y zeolite. Then, the reaction mechanisms for DMC formation on four types of Cu species were investigated, and the order of catalytic activity of different Cu states in Y zeolite was characterized. It is expected that these results will provide a theoretical clue to prepare CuY catalyst with better catalytic activity for the DMC synthesis.

2. Results

A faujasite type structure with various cationic sites and different crystallographic oxygen positions is shown in Figure 1. As reactant molecule, CO is very difficult to diffuse inside the sodalite cages and hexagonal prisms (2.3 Å) [38] of Y zeolite because of the large dynamic diameter (3.76 Å), while they easily enter enter supercages (7.4 Å) [38], suggesting that only copper species located at sites II and III are accessible to CO adsorption and act as the active sites for the oxidative carbonylation of methanol to DMC. Based on the previous studies [30,31,39,40], copper cations in site II are more stable than site III, therefore site II was selected to represent the location of active center Cu species in this study.
According to the literatures [30,41,42], the local conformation and interactions of molecule can be described using the cluster model. The dangling bonds were saturated by H atoms [41,43]. The terminal H atoms were oriented along the bond direction of Y zeolite. The bond length of O-H was set to 1.0 Å, respectively. During numerical optimization, the local structure of Y zeolite was kept unchanged for Yn−, Y, CuY cluster models. The compensating charges, Al atoms and adjacent SiO4 units were relaxed, while other atoms were fixed. For the adsorbate-CuY cluster system, the compensating charges, the absorbed molecules and the 6 MR occupied by the active center Cu+ species were relaxed.
In order to find the appropriate cluster size, five different sized clusters, consisting of 6T, 12T, 24T, 42T, and 60T atoms (T represents an Al or Si atom) (see Figure 2), were constructed.
The binding energies of Cu2+ in these five Y clusters and the adsorption energies of CO on CuY zeolite with these clusters were calculated, as shown in Table 1.
The interaction energy (Eint) between Cu2+ and Y2− zeolite was defined as [44]
Eint = ECu2+ + EY2− − EMY
where ECu2+ is the total energy of Cu2+, EY2− is the total energy of Y2−, and EMY is the total energy of MY, respectively. Here, a larger Eint represents a more stable structure of Cu2+Y system. It can be found from Table 1 that the effect of the cluster size on the adsorption energies of CO is negligible, while the Cu2+ interaction energies are significantly influenced by the cluster size. Therefore, a very small cluster model cannot fully reflect the structure of Y zeolite. Comparison of the Cu2+ interaction energies suggests that the difference between 24T and 60T cluster is within the allowable error range (<80 kJ/mol). The 24T cluster model was selected in this study to reduce the computing cost.
To better represent the structure of Y zeolite in experiments, four Si atoms of the Y zeolite cluster were substituted by four Al atoms according to the Lowenstein–Dempsey rules. The Y cluster with four Si atoms replaced by Al atoms was denoted as Y4−. Based on the 24T cluster model of Y zeolite, the most stable configurations of Y4− cluster was obtained by evaluating the substitution energy [45] and binding energy of Y4−, which are defined as
Esub = EY4− + 4ESi-EY-4EAl
Ebind = 24EH + 60EO + 4EAl + 20ESi-EY4−
where, Esub and Ebind are the substitution energy and the binding energy of Y4−, respectively. EY4− and EY are the total energies of Y4− cluster and the Y cluster without the replacement of Si, respectively. ESi, EAl, EH and EO are the energies of single Si, Al, H and O atoms, respectively. With these definitions, a smaller Esub indicates an easier replacement of Si by Al and larger Ebind means a more stable Y cluster.
Table 2 lists the calculated Esub and Ebind for different distribution of Al atoms.
Due to its least substitution energy and largest binding energy of Y4−, Y4− cluster with the distribution of 4 Al atoms, denoted as 4-11-12-22, is the most stable structure of Y4− cluster (see Figure 3). Negative charges, which are introduced when Si atoms are replaced by Al atoms, are usually compensated by protons associated with crystallographic oxygen atoms adjacent to the Al atoms.
Based on the stable structure of Y4− cluster, the configurations to reflect the different Cu states in Y zeolite were constructed. According to the literatures [44,46], a majority of charge-compensating protons locate at O1 sites, while the others occupy O3 sites to avoid the formation of -OH2 group. In this study, for Y zeolite with five Al atoms, three charge-compensating protons locate at O1 sites, and two protons are at O3 sites (see Figure 3). For Cu2+Y zeolite, Cu2+ is used to balance the negative charge of Al11 and Al12, and charge-compensating protons are located at the O1 site to balance the negative charges of Al4 and Al22, respectively (see Figure 4a). For Cu+Y zeolite, Cu+ balances the negative charge of Al12, when three protons located at the O1 site compensate the negative charges of Al4, Al11 and Al22 (see Figure 4b). For Cu2O-Y and CuO-Y zeolite, all negative charges of Al are compensated by four charge-compensating protons located at O1 and O3 (see Figure 4c,d).

3. Discussion

In this section, the notation (X)* and (X)(Y)* are referred to active center Cu states, such as Cu+, Cu2+, Cu2O and CuO interacting with species X and X and Y, respectively. The optimized geometries of reactants, transition states and products for different reaction pathways of DMC formation were calculated.

3.1. The Desorption and Dissociation of CH3OH

The processes of desorption and dissociation of CH3OH on these four types of zeolites and the corresponding transition states TS1 are shown in Figure 5.
As shown in Figure 5, adsorbed CH3OH on the four types of zeolites are bound to different kinds of active center Cu species through O atom. The adsorption of CH3OH on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites is exothermic with the energy release of 65.59, 85.81, 122.70 and 94.07 kJ·mol−1, respectively. Subsequently, for Cu+Y zeolite, with the presence of oxygen, the O-H bond of CH3OH breaks to form the co-adsorbed (CH3O)*(OH)* configuration (see Figure 5(a2)). Since no TS state has been found, molecularly adsorbed CH3OH is converted rapidly to CH3O species. This demonstrates that the presence of adsorbed O on Cu+Y zeolite exhibits a high surface reactivity toward the formation of CH3O. The results are in good agreement with early reported experimental observations [15,20,22,28].
For Cu2+Y zeolite, adsorbed CH3OH is oxidized by adsorbed O to form CH3O species via a transition state (TS1), as shown in Figure 5(b2). The O-H distance in CH3OH increases from initial 0.975 Å to 1.126 Å of TS1, and finally to 2.275 Å, showing that the O-H bond in CH3OH is destroyed. Meanwhile, the distance between adsorbed O atom and H atom decreases drastically from initial 2.418 Å to 1.417 Å of TS1, then to 0.979 Å of (CH3O)*/(OH)* (see Figure 5(b3)), revealing that a new O-H bond forms on Cu2+Y zeolite. Similar changes are found on Cu2O-Y zeolite. For these two zeolites, the oxidation of adsorbed CH3OH with the presence of oxygen needs to overcome activation barriers of 66.73 and 23.56 kJ·mol−1, respectively (see Table 3).
It is interesting to note that for CuO-Y zeolite, adsorbed CH3OH is oxidized to CH3O species without the presence of O, which is attributed to the presence of lattice oxygen from CuO species. Experimental studies by Engeldinger et al. [26,27] suggested that the formation of methoxy species from the adsorbed CH3OH proceeded with and without oxygen, indicating that lattice oxygen of CuOx was able to participate in the oxidation process. According to the Mars–van Krevelen mechanism [37], gas phase oxygen can re-oxidize Cu to CuO species [26,27]. These structures in Figure 5 further prove that for the oxidation reaction of CH3OH to CH3O, oxygen is needed for Cu2O-Y zeolite but is not essential for CuO-Y zeolite. The oxidation reaction of CH3OH on CuO-Y zeolite is exothermic (85.36 kJ·mol−1) and exhibits an activation barrier of 39.94 kJ·mol−1, as shown in Table 3.

3.2. Insertion of CO into CH3O (Path I)

Figure 6 shows the processes of inserting CO into CH3O to form CH3OCO on these four types of zeolites and the corresponding transition states TS2.
For Cu+Y zeolite, the distance between the C atom of CO and the O atom of CH3O decreases from initially 2.787 Å of (CO)*/(CH3O)* to 1.961 Å of TS2, which suggests the formation of a new C-O bond. It is also seen from Figure 6 that insertion of CO into Cu-OCH3 elongates the Cu-O bond from initially 1.901 Å to 2.455 Å of TS2 and then to 2.840 Å of (CH3OCO)*, indicating that at the final product CH3OCO adsorbs on the Cu+ via the C atom of CO (this C atom is denoted as C’ for the further analysis). The CO insertion reaction exhibits an activation barrier of 63.73 kJ·mol−1 via TS2, which agrees with the results calculated by Zheng et al. [30] (see Table 3). Similar changes from the initial geometries to TS2 states then to the final products happen on the three other types of zeolites. The CO insertion reaction on Cu2+Y, Cu2O-Y and CuO-Y zeolites needs to overcome activation barriers of 64.45, 60.01 and 104.64 kJ·mol−1, respectively (see Table 3).

3.3. CH3O Reacts with CH3OCO to Form DMC (Path I)

(CH3OCO)* adsorbed on these four types of zeolites can react with another (CH3O)* to form DMC, via a transition state TS3 (see Figure 7).
For Cu+Y zeolite, the distance between C′ atom of CH3OC′O and the O atom of the second CH3O (this O atom is denoted as O′ for the later analysis) decreases from initially 2.461 Å of (CH3OC′O)*/(CH3O′)* to 1.863 Å of TS3, and finally the C′-O′ bond in DMC of 1.361 Å. In addition, the bonds of Cu-C′ and Cu-O′ are elongated to 3.433 (not shown in Figure 7) and 2.510 Å, respectively, suggesting the weak (physical) adsorption of DMC on Cu+Y zeolite. Similar changes from the initial geometries to TS3 states then to the final products happen on the three other types of zeolites. The reaction of CH3O with CH3OCO on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites exhibits activation barriers of 28.27, 37.95, 40.90 and 15.95 kJ·mol−1 via TS3, respectively (see Table 3), and the exothermic energies are 164.16, 315.11, 313.18 and 312.58 kJ·mol−1, respectively for these four types of zeolites.

3.4. Formation of (CH3O)2 Species (Path II)

The second pathway to form DMC suggests that (CH3O)*/(OH)* reacts with CH3OH, which results in co-adsorption of (CH3O)2* and H2O. Figure 8 shows these adsorption configurations of (CH3O)*(OH)*/CH3OH and (CH3O)2*/H2O on these four types of zeolites, via a transition state TS4.
For Cu+Y zeolite, the O-H distance in CH3OH increases from initially 1.002 Å to 1.529 Å of TS3, indicating this O-H bond tends to break. Meanwhile, the distance between the H atom of OH in CH3OH and the O atom of (OH)* decreases from initially 1.724 Å of (CH3O)*(OH)*/CH3OH to 1.028 Å of TS4, demonstrating the migration of the H atom away from CH3OH and towards the O atom of OH. This leads to the formation of additional (CH3O)* and H2O. Moreover, the distance between the O atom of CH3OH and Cu+ decreases from 2.855 Å of TS4 to 1.880 Å of (CH3O)2*/H2O, which suggests the formation of (CH3O)2*. This reaction on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites exhibits activation barriers of 93.86, 89.49, 116.38 and 115.29 kJ·mol−1 via TS4, respectively (see Table 3).

3.5. Insertion of CO into (CH3O)2 to Form DMC (Path II)

The processes of inserting CO into (CH3O)2 to form DMC on these four types of zeolites and the corresponding transition states TS5 are shown in Figure 9.
As shown in Figure 9, on Cu+Y zeolite the distance of Cu-CO of (CH3O)2*/CO configuration is 5.477 Å, and the C-O bond (1.142 Å) of CO is similar to that (1.143 Å) of CO in gas phase. This suggests that two CH3O (i.e., (CH3O)2*) molecules adsorbed at the active center Cu effectively inhibit the adsorption of CO, which agrees with the stronger adsorption of CH3O than CO (139.59 kJ·mol−1 vs. 125.25 kJ·mol−1). Starting from the adsorption configuration of (CH3O)2*/CO, the formation of DMC goes through a transition state TS5 (see Figure 9). On Cu+Y zeolite, the distance of the C atom of CO (denoted as C″) and the O atom of the nearest CH3O (denotes as O″) decreases from initially 4.073 Å to 2.015 Å of TS5. The distance between C″ and the O atom of the furthest CH3O (denotes as O‴) decreases from initially 5.477 Å of (CH3O)2*/CO to 2.462 Å of TS5. Furthermore, in TS5, the distance of Cu-O1 is elongated to 2.834 Å from 1.830 Å of (CH3O)2*/CO to accommodate the insertion of CO. Similar calculated results are found on the other three types of zeolites. The reaction step on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites is significantly exothermic by 256.59, 372.06, 232.24 and 323.77 kJ·mol−1, and the corresponding activation barriers are 201.68, 164.95, 253.96 and 210.74 kJ·mol−1 via TS5, respectively (see Table 3).

3.6. Desorption of DMC

Desorption of DMC from Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites is endothermic with the energy input of 24.30, 72.18, 41.31 and 65.69 kJ·mol−1, respectively. These energies needed are compensable by the exothermic reactions of DMC formation on respective zeolites.

3.7. Rate-Limiting Reactions of DMC Formaction

The potential energy curves for two reaction paths are plot in Figure 10.
For the path I of DMC formation on these zeolites, insertion of CO into CH3O is followed by the formation of DMC. On Cu+Y zeolite, the corresponding activation barriers of these two reactions are 63.73 and 28.27 kJ·mol−1, respectively, which suggests the insertion reaction of CO into CH3O is rate-limiting. On the other hand, for path II of DMC formation, insertion of CO into (CH3O)2 is followed by the formation of DMC. On Cu+Y zeolite, the rate-limiting step for path II is insertion of CO into (CH3O)2 with an activation barrier of 201.68 kJ·mol−1. The comparison between the rate-limiting reactions of two paths (63.73 vs. 201.68 kJ·mol−1) suggests the path I is favorable for DMC formation on Cu+Y zeolite. Similar to these processes on Cu+Y zeolite, DFT calculations further confirm that path I is the favorable process of DMC formation over CuO-Y and Cu2O-Y zeolites, with the rate-limiting step of inserting CO into CH3O. For Cu2+Y zeolite, the favorable pathway of DMC formation is also path I, while oxidation of the absorbed CH3OH to CH3O becomes rate-limiting. Zhang et al. [36] found the favorable pathway of DMC formation on Cu2O(111) follows path I, which agrees with the finding in this study. However, they found that path II was favorable for the formation of DMC over CuO(111) and the insertion of CO into (CH3O)2 was considered as rate-limiting step [47]. Comparison of the activation barriers of the rate-limiting steps on Cu2O-Y (this study) and Cu2O(111) (in the literature [36]) (60.01 kJ·mol−1 and 161.9 kJ·mol−1, respectively) suggest the Cu2O species in Y zeolite should exhibit a better catalytic activity than the carrier-free Cu2O crystalline surface (see Table 3). These results indicate that the carrier significantly affects the activation barriers and even the reaction pathways.
Based on the aforementioned analyses, the following reaction route of DMC formation on different Cu states in Y zeolites was proposed. First, the adsorbed CH3OH is oxidized to CH3O species on zeolites. Then CO inserts to CH3O to form CH3OCO, which subsequently reacts with CH3O to form DMC at a relative high reaction rate. Finally, adsorbed DMC is released into the gas phase. A distinction exists for these four types of zeolites investigated in this study. It is found that the rate-limiting step on Cu2+Y zeolite is oxidation of CH3OH to CH3O, while for Cu+Y, Cu2O-Y and CuO-Y zeolites, the rate-limiting step is insertion of CO into CH3O. Moreover, oxidation of CH3OH to form CH3O requires a presence of oxygen on Cu+Y, Cu2+Y and Cu2O-Y zeolites, while on CuO-Y zeolite the adsorbed CH3OH is oxidized by the lattice oxygen of CuO. The latter agrees with experimental findings by Engeldinger et al. [26,27].
The activation barriers of insertion of CO into CH3O over Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites are found to be 63.73, 64.45, 60.01 and 104.64 kJ·mol−1, respectively. Worthwhile to notice, oxidation of CH3OH to CH3O on Cu2+Y zeolite exhibits an activation barrier of 66.73 kJ·mol−1, while oxidation of CH3OH on Cu+Y zeolite is a barrier free reaction, suggesting that Cu+Y zeolite possess a better catalytic activity than Cu2+Y zeolite. As a result, the order of catalytic activities of these four types of zeolites is derived as Cu2O-Y ≈ Cu+Y > Cu2+Y > CuO-Y, which agrees with a previous experimental study [28].

4. Methodology

Density functional theory calculations were performed using the DMol3 program package of Materials Studio 8.0 [48]. The generalized-gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correction functional was used in all calculations [49]. The double numerical plus polarization (DNP) basis set [50], which is equivalently accurate to the commonly used 6-31G** Gaussian basis set, was employed to describe the Si-O-H-Al-Cu system. In this approach, for the non-metal Si, O, H and C atoms were treated with the all-electron basis sets, which considers all valence orbitals, while the inner electrons of the Al and Cu atoms were kept frozen and replaced by an effective core potential (ECP), which is attributed to that the metal atom participated into the reaction mainly occurs by the outer valence electron orbitals. The convergence criteria of DFT calculations were set to 2 × 10−5 Ha for energy, 4 × 10−3 Ha/Å for force, 0.005 Å for displacement. Complete linear synchronous transit (LST) and quadratic synchronous transit (QST) were used to determine the transition states (TS).
For the reaction A + B → AB on CuY zeolite, the reaction enthalpy (ΔH) and activation energy (Ea) were calculated by
ΔH = EAB/CuY − EA+B/CuY
Ea = ETS/CuY − EA+B/CuY
where EAB/CuY is the total energy for the product AB on CuY zeolite, EA+B/CuY is the total energies of the co-adsorbed A and B on CuY zeolite and ETS/CuY is the total energy of the transition state (TS) on CuY zeolite, respectively. The negative ΔH represents an exothermic reaction.
The adsorption energy (Eads) of the adsorbate-cluster system is defined as
Eads = Eadsorbate + ECuY − Eadsorbate/CuY
where Eadsorbate/CuY is the total energy of adsorbate-CuY substrate system in the equilibrium state, ECuY and Eadsorbate are the total energies of CuY substrate and free adsorbate alone, respectively. From this definition, the large adsorption energy indicates a strong interaction between the absorbate and CuY zeolite.

5. Conclusions

In this work, the DFT method was employed to investigate the reaction mechanisms of DMC formation on four types of zeolites doped with Cu+, Cu2+, Cu2O and CuO, respectively, based on two proposed reaction pathways. The calculation results reveal that path I is dominant for the formation of DMC since the activation barriers of rate-limiting steps for path II are much higher than that of path I. Moreover, the calculation results also suggest the following route to describe DMC formation on these zeolites. First, CH3OH is adsorbed and oxidized to CH3O species. Then CO inserts into CH3O to form CH3OCO, which reacts with CH3O to product DMC. Lastly, adsorbed DMC is released into the gas phase. It is found that for Cu+Y, Cu2+Y and Cu2O-Y zeolites, adsorbed CH3OH is oxidized to CH3O with a presence of oxygen, whereas oxidation of CH3OH on CuO-Y utilizes the lattice oxygen of CuO. The rate-limiting step on Cu2+Y zeolite is oxidation of CH3OH to CH3O, while on three other types of zeolites, the rate-limiting step is insertion of CO into CH3O, and the corresponding activation barriers of these rate-limiting steps for Cu2+Y, Cu+Y, Cu2O-Y and CuO-Y zeolites are 66.73, 63.73, 60.01 and 104.64 kJ·mol−1, respectively. Based on above mentioned, the catalytic activities of these four types of zeolites with different Cu states exhibit the order of Cu2O-Y ≈ Cu+Y > Cu2+Y > CuO-Y. These findings are expected to guide the selection and preparation of CuY catalysts with the best catalytic activity for DMC synthesis.

Author Contributions

H.Z. and S.Y. outlined the work plan; Y.Z. and Y.S. conducted the computations; Y.Z. and G.Z. drew the figures and drafted the manuscript. H.Z. and J.Z. revised the drafted manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22262020), Zunyi Technology and Big data Bureau, Moutai institute Joint Science and Technology Research and Development Project ([2021]328) and Research Foundation for Scientific Scholars of Moutai Institute (mygccrc[2022]081 and [2022]080).

Data Availability Statement

All the relevant data used in this study have been provided in the form of figures and tables in the published article, and all data provided in the present manuscript are available to whom they may concern.

Acknowledgments

The authors are grateful to Zhong Li, John Z. Wen and Nilesh Narkhede for their kindly academic discussion and language help.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Faujasite type structure with cationic sites (orange balls) and different crystallographic oxygen positions (red balls). Here, site I is at the middle of hexagonal prism; site I’B is in the sodalite cage adjacent to 6MR which is shared by both hexagonal prism and sodalite cage; site I’A is similar to site I’B, but away from the sodalite cage; site II is in the supercage close to the six-membered ring (6 MR) shared by supercage and sodalite cage; site II* is similar to site II, but located towards the supercage; site III is in the supercage that is next to four-membered rings (4 MR) of sodalite cage.
Figure 1. Faujasite type structure with cationic sites (orange balls) and different crystallographic oxygen positions (red balls). Here, site I is at the middle of hexagonal prism; site I’B is in the sodalite cage adjacent to 6MR which is shared by both hexagonal prism and sodalite cage; site I’A is similar to site I’B, but away from the sodalite cage; site II is in the supercage close to the six-membered ring (6 MR) shared by supercage and sodalite cage; site II* is similar to site II, but located towards the supercage; site III is in the supercage that is next to four-membered rings (4 MR) of sodalite cage.
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Figure 2. The cluster geometries of Y zeolite with different sizes. Red, yellow, pink and white balls stand for O, Si, Al and H atoms, respectively.
Figure 2. The cluster geometries of Y zeolite with different sizes. Red, yellow, pink and white balls stand for O, Si, Al and H atoms, respectively.
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Figure 3. The stable cluster geometry of Y4− zeolite with 24T atoms. See Figure 2 for the color coding.
Figure 3. The stable cluster geometry of Y4− zeolite with 24T atoms. See Figure 2 for the color coding.
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Figure 4. The stable cluster geometries of (a) Cu2+Y, (b) Cu+Y, (c) Cu2O-Y and (d) CuO-Y zeolites. Red, yellow, pink, white and orange balls stand for O, Si, Al, H and Cu atoms, respectively.
Figure 4. The stable cluster geometries of (a) Cu2+Y, (b) Cu+Y, (c) Cu2O-Y and (d) CuO-Y zeolites. Red, yellow, pink, white and orange balls stand for O, Si, Al, H and Cu atoms, respectively.
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Figure 5. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the oxidation of CH3OH to CH3O (unit: Å). (a1) (CH3OH)*/(O)* on Cu+Y, (a2) (CH3O)*/(OH)* on Cu+Y, (b1) (CH3OH)*/(O)* on Cu2+Y, (b2) TS1 on Cu2+Y, (b3) (CH3O)*/(OH)* on Cu2+Y, (c1) (CH3OH)*/(O)* on Cu2O-Y, (c2) TS1 on Cu2O-Y, (c3) (CH3O)*/(OH)* on Cu2O-Y, (d1) (CH3OH)*/(O)* on CuO-Y, (d2) TS1 on CuO-Y and (d3) (CH3O)*/(OH)* on CuO-Y. See Figure 4 for the color coding.
Figure 5. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the oxidation of CH3OH to CH3O (unit: Å). (a1) (CH3OH)*/(O)* on Cu+Y, (a2) (CH3O)*/(OH)* on Cu+Y, (b1) (CH3OH)*/(O)* on Cu2+Y, (b2) TS1 on Cu2+Y, (b3) (CH3O)*/(OH)* on Cu2+Y, (c1) (CH3OH)*/(O)* on Cu2O-Y, (c2) TS1 on Cu2O-Y, (c3) (CH3O)*/(OH)* on Cu2O-Y, (d1) (CH3OH)*/(O)* on CuO-Y, (d2) TS1 on CuO-Y and (d3) (CH3O)*/(OH)* on CuO-Y. See Figure 4 for the color coding.
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Figure 6. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of CH3OCO (unit: Å). (a1) (CH3O)*/(CO)* on Cu+Y, (a2) TS2 on Cu+Y, (a3) (CH3OCO)* on Cu+Y, (b1) (CH3O)*/(CO)* on Cu2+Y, (b2) TS2 on Cu2+Y, (b3) (CH3OCO)* on Cu2+Y, (c1) (CH3O)*/(CO)* on Cu2O-Y, (c2) TS2 on Cu2O-Y, (c3) (CH3OCO)* on Cu2O-Y, (d1) (CH3O)*/(CO)* on CuO-Y, (d2) TS2 on CuO-Y and (d3) (CH3OCO)* on CuO-Y. See Figure 4 for the color coding.
Figure 6. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of CH3OCO (unit: Å). (a1) (CH3O)*/(CO)* on Cu+Y, (a2) TS2 on Cu+Y, (a3) (CH3OCO)* on Cu+Y, (b1) (CH3O)*/(CO)* on Cu2+Y, (b2) TS2 on Cu2+Y, (b3) (CH3OCO)* on Cu2+Y, (c1) (CH3O)*/(CO)* on Cu2O-Y, (c2) TS2 on Cu2O-Y, (c3) (CH3OCO)* on Cu2O-Y, (d1) (CH3O)*/(CO)* on CuO-Y, (d2) TS2 on CuO-Y and (d3) (CH3OCO)* on CuO-Y. See Figure 4 for the color coding.
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Figure 7. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of DMC via path I (unit: Å). (a1) (CH3O)*/(CH3OCO)* on Cu+Y, (a2) TS3 on Cu+Y, (a3) DMC on Cu+Y, (b1) (CH3O)*/(CH3OCO)* on Cu2+Y, (b2) TS3 on Cu2+Y, (b3) DMC on Cu2+Y, (c1) (CH3O)*/(CH3OCO)* on Cu2O-Y, (c2) TS3 on Cu2O-Y, (c3) DMC on Cu2O-Y, (d1) (CH3O)*/(CH3OCO)* on CuO-Y, (d2) TS3 on CuO-Y and (d3) DMC on CuO-Y. See Figure 4 for the color coding.
Figure 7. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of DMC via path I (unit: Å). (a1) (CH3O)*/(CH3OCO)* on Cu+Y, (a2) TS3 on Cu+Y, (a3) DMC on Cu+Y, (b1) (CH3O)*/(CH3OCO)* on Cu2+Y, (b2) TS3 on Cu2+Y, (b3) DMC on Cu2+Y, (c1) (CH3O)*/(CH3OCO)* on Cu2O-Y, (c2) TS3 on Cu2O-Y, (c3) DMC on Cu2O-Y, (d1) (CH3O)*/(CH3OCO)* on CuO-Y, (d2) TS3 on CuO-Y and (d3) DMC on CuO-Y. See Figure 4 for the color coding.
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Figure 8. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of (CH3O)2 (unit: Å). (a1) (CH3O)*/(OH)*/CH3OH on Cu+Y, (a2) TS4 on Cu+Y, (a3) (CH3O)2*/H2O on Cu+Y, (b1) (CH3O)*/(OH)*/CH3OH on Cu2+Y, (b2) TS4 on Cu2+Y, (b3) (CH3O)2*/H2O on Cu2+Y, (c1) (CH3O)*/(OH)*/CH3OH on Cu2O-Y, (c2) TS4 on Cu2O-Y, (c3) (CH3O)2*/H2O on Cu2O-Y, (d1) (CH3O)*/(OH)*/CH3OH on CuO-Y, (d2) TS4 on CuO-Y and (d3) (CH3O)2*/H2O on CuO-Y. See Figure 4 for the color coding.
Figure 8. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of (CH3O)2 (unit: Å). (a1) (CH3O)*/(OH)*/CH3OH on Cu+Y, (a2) TS4 on Cu+Y, (a3) (CH3O)2*/H2O on Cu+Y, (b1) (CH3O)*/(OH)*/CH3OH on Cu2+Y, (b2) TS4 on Cu2+Y, (b3) (CH3O)2*/H2O on Cu2+Y, (c1) (CH3O)*/(OH)*/CH3OH on Cu2O-Y, (c2) TS4 on Cu2O-Y, (c3) (CH3O)2*/H2O on Cu2O-Y, (d1) (CH3O)*/(OH)*/CH3OH on CuO-Y, (d2) TS4 on CuO-Y and (d3) (CH3O)2*/H2O on CuO-Y. See Figure 4 for the color coding.
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Figure 9. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of DMC via path II (unit: Å). (a1) (CH3O)2*/CO on Cu+Y, (a2) TS5 on Cu+Y, (a3) DMC on Cu+Y, (b1) (CH3O)2*/CO on Cu2+Y, (b2) TS5 on Cu2+Y, (b3) DMC on Cu2+Y, (c1) (CH3O)2*/CO on Cu2O-Y, (c2) TS5 on Cu2O-Y, (c3) DMC on Cu2O-Y, (d1) (CH3O)2*/CO on CuO-Y, (d2) TS5 on CuO-Y and (d3) DMC on CuO-Y. See Figure 4 for the color coding.
Figure 9. The structures of reactants, products and transition states on Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolite for the formation of DMC via path II (unit: Å). (a1) (CH3O)2*/CO on Cu+Y, (a2) TS5 on Cu+Y, (a3) DMC on Cu+Y, (b1) (CH3O)2*/CO on Cu2+Y, (b2) TS5 on Cu2+Y, (b3) DMC on Cu2+Y, (c1) (CH3O)2*/CO on Cu2O-Y, (c2) TS5 on Cu2O-Y, (c3) DMC on Cu2O-Y, (d1) (CH3O)2*/CO on CuO-Y, (d2) TS5 on CuO-Y and (d3) DMC on CuO-Y. See Figure 4 for the color coding.
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Figure 10. The reaction mechanism for the formation of DMC over Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites via path I (a) and path II (b).
Figure 10. The reaction mechanism for the formation of DMC over Cu+Y, Cu2+Y, Cu2O-Y and CuO-Y zeolites via path I (a) and path II (b).
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Table
Table 1. The interaction energies (Eint) of Cu2+ and the adsorption energies (Eads) of CO on the clusters of Cu2+Y zeolites with different sizes.
Table 1. The interaction energies (Eint) of Cu2+ and the adsorption energies (Eads) of CO on the clusters of Cu2+Y zeolites with different sizes.
Y Zeolite with Different SizeEint/kJ·mol−1Eads/kJ·mol−1
6T2713.6291.45
12T2588.7683.33
24T2529.5983.08
42T2454.4483.27
60T2442.6182.88
Table
Table 2. The substitution energies (Esub) and binding energies (Ebind) for the Y4− cluster with different distribution of Al atoms.
Table 2. The substitution energies (Esub) and binding energies (Ebind) for the Y4− cluster with different distribution of Al atoms.
The Distribution of Al AtomsEsub/kJ·mol−1Ebind/Ha
1-11-12-2227.3121.0454
2-11-12-2231.1621.0440
3-11-12-2245.1521.0387
4-11-12-220.2821.0557
5-11-12-2237.7221.0415
8-11-12-2231.9621.0436
9-11-12-22122.1121.0093
11-12-14-22107.6021.0148
11-12-17-22210.5220.9756
11-12-20-22129.8121.0064
11-12-22-24125.4921.0080
Table
Table 3. The activation barriers for individual reaction steps based on two proposed reaction mechanisms (/kJ·mol−1).
Table 3. The activation barriers for individual reaction steps based on two proposed reaction mechanisms (/kJ·mol−1).
Catalyst(CH3OH)* + O*→(CH3O)*(OH)*(CH3O)* + CO*→(CH3OCO)*(CH3OCO)* + (CH3O)*→(DMC)*(CH3O)*(OH)* + CH3OH→(CH3O) 2* + H2O(CH3O)2* + CO→(DMC)*Ref.
Cu2O--161.998.868.3308.5[36]
Cu2O-Y23.5660.0140.90116.38253.96This study
Cu+Y--63.7328.2793.86201.68This study
Cu2+Y66.7364.4537.9589.49164.95This study
CuO-Y39.94104.6415.95115.29210.74This study
CuO--114.5200.925.7109.1[47]
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Zhou, Y.; Zhang, G.; Song, Y.; Yu, S.; Zhao, J.; Zheng, H. DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites. Catalysts 2023, 13, 477. https://doi.org/10.3390/catal13030477

AMA Style

Zhou Y, Zhang G, Song Y, Yu S, Zhao J, Zheng H. DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites. Catalysts. 2023; 13(3):477. https://doi.org/10.3390/catal13030477

Chicago/Turabian Style

Zhou, Yuan, Guoqiang Zhang, Ya Song, Shirui Yu, Jingjing Zhao, and Huayan Zheng. 2023. "DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites" Catalysts 13, no. 3: 477. https://doi.org/10.3390/catal13030477

APA Style

Zhou, Y., Zhang, G., Song, Y., Yu, S., Zhao, J., & Zheng, H. (2023). DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites. Catalysts, 13(3), 477. https://doi.org/10.3390/catal13030477

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