NaY Catalyst

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Synthesis of High-Performance and High-Stability Pd(II)/NaY Catalyst for CO Direct Selective Conversion to Dimethyl Carbonate by Rational Design Hong-Zi Tan, Zhe-Ning Chen, Zhong-Ning Xu, Jing Sun, Zhi-Qiao Wang, Rui Si, Wei Zhuang, and Guo-Cong Guo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00286 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019

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ACS Catalysis

Synthesis of High-Performance and High-Stability Pd(II)/NaY

Catalyst

for

CO

Direct

Selective

Conversion to Dimethyl Carbonate by Rational Design Hong-Zi Tan,ac# Zhe-Ning Chen,a# Zhong-Ning Xu,a* Jing Sun, ab Zhi-Qiao Wang,ab Rui Si,d Wei Zhuanga* and Guo-Cong Guoa*

a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure

of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. b

Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Chinese Academy of

Sciences, Fuzhou, Fujian 350002, P. R. China. c

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

d

Shanghai Institute Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R.

China. #

These authors contributed equally.

*To

whom

correspondence

should

be

addressed:

E-mail:

[email protected];

[email protected]; [email protected] Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday.

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ABSTRACT Direct selective catalytic conversion of CO to C2+ oxygenates with high-performance and high-stability is challenging. Generating dimethyl carbonate (DMC), a high added-value oxygenate, by CO direct catalytic conversion with Pd-based catalyst is of particular interest as an economic and environmentally friendly potential alternative of the traditional transesterification route. The Pd(II)-based catalysts were often used in this route to selectively generate DMC. However, Pd(II) easily reduces to Pd(0) in the presence of CO, which leads to the byproduct of dimethyl oxalate (DMO). A high-performance and high-stability catalytic system selective towards DMC is thus hard to achieve. We herein reveal, by density functional theory calculations, that the mononuclear-isolated nature of Pd(II) centers is critical for the selectivity towards DMC while the aggregation of Pd(0) centers leads to the selective generation of DMO. Inspired by this picture, heterogeneous catalyst with mononuclear-isolated Pd(II) centers anchored on Y-type silico-alumina zeolite was produced experimentally using an ultrasonic-assisted ammonia evaporation approach. This catalyst demonstrates a high selectivity (>99.5%) to target DMC over 100 hours, a stability beyond any other reported results. Characterization of the catalyst structure as well as the reaction intermediates and kinetics further verified the computational mechanism. Current work provides a high-performance catalytic system to selectively produce DMC with high-stability through rational design and controlled synthesis, and sheds the light on how to synthesize high-performance and long-lived catalysts for the future industrialization of this process route. KEYWORDS: CO, Pd(II), Pd(0), selective, dimethyl carbonate, DMC, dimethyl oxalate, DMO


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INTRODUCTION Carbon monoxide (CO) is one of the most versatile and cost-efficient chemical building block obtainable from natural gas, coal and biomass. Direct catalytic conversion of CO into the high added-value chemicals becomes progressively more important due to the rapid decline of petroleum and coal resources.1-3 The challenge of these conversions lies in the realization of high-selectivity and high-stability during the reaction routes. For the CO direct catalytic conversion to hydrocarbons (C2=–C4= light olefins), the selectivity and stability of catalytic system has been significantly improved during the years.4, 5 On the other hand, to achieve satisfying selectivity and stability in the direct catalytic conversion of CO to C2+ oxygenates remains more challenging.6-8 Dimethyl carbonate (DMC) is an important oxygenate needed for the polycarbonate production,9,

10

lithium batteries,10-14 fuel oil additives15-17 as well as

replacing the toxic chemicals (e.g., dimethyl sulfate, phosgene) in the reactions.18-21 Industrially, over 90% of the DMC production adopts a transesterification route, in which the raw materials of propylene oxide first reacts with CO2 to form cyclic carbonate intermediate, then the transesterification process occurs between the cyclic carbonate intermediate and methanol to generate DMC. However, propylene oxide is derived from petroleum via a chlorohydrination process, which leads to a high cost of production as well as a serious pollution issue.22 Producing DMC through CO direct catalytic conversion is considerably more economic and environmentally friendly. This route involves two separated reactions. The first reaction is the catalytic synthesis of DMC from CO and methyl nitrite (MN, CH3ONO); and the second reaction is the non-catalytic synthesis of MN from methanol, O2 and recyclable NO, which was produced in the first reaction.23 Using Pd(II)-based catalysts in the first reaction lead to the production of DMC (Scheme 1). However, the reduction of Pd(II) to Pd(0) is inevitable in the presence of CO, which leads to the production of dimethyl oxalate (DMO).23-26 Industrial application of this CO direct catalytic conversion route is therefore impeded by the stability of the catalyst used. Furthermore, molecular mechanism behind this selectivity remains uncertain, making it more difficult to improve the catalyst used herein. To prevent the catalyst deactivation, chloride was usually introduced to suppress the reduction of Pd(II) and the further aggregation of Pd(0).27-29 However, chloride often quickly changes into the methylchloroformate byproduct and lead to a rapid


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catalyst deactivation.23, 29 Moreover, chloride can contaminate DMC and corrode the reaction apparatus.30 On the other hand, the performance of existing chloride-free catalysts are highly influenced by the local environments of active species30 and the type of acidic sites on carrier17. Their stabilities are not satisfying due to the unavoidable slow deactivation.30 We herein combined rational catalyst design and state-of-art synthesis method to obtain a high-performance and high-stability catalytic system for CO direct selective conversion to DMC. Density functional theory (DFT) calculations were carried out to understand the molecular mechanism for producing DMC and DMO in details. It was suggested that production of DMC occurs between an adsorbed *COOCH3 intermediate and a MN in atmosphere on a mononuclear-isolated Pd(II) center with an Eley-Rideal mechanism (E-R, in which the reactant in gas phase directly reacts with the intermediate adsorbed on the catalyst surface). The creation of DMO, on the other hand, happens between two adsorbed *COOCH3 intermediates on the neighboring palladium centers with a Langmuir-Hinshelwood mechanism (L-H, in which the reactants are adsorbed onto the catalyst surface before the reaction occurs). Further calculations proposed a heterogeneous catalyst model with mononuclear-isolated Pd(II) centers anchored on Y-type silico-alumina zeolite to achieve high-selectivity and high-stability for generation of DMC, where the six-member rings (6-MRs) of sodalite cage in Y-type silico-alumina zeolite provides a desirable coordination surrounding to isolate and stabilize the Pd(II) centers. The structure and performance of catalysts highly depend on the preparation method, and can be of great disparity even with the same composition. Therefore, we further developed an ultrasonic-assisted ammonia evaporation method to synthesize the Pd(II)/NaY catalyst under the guidance of rational catalyst design, in which the Pd(II) species are coordinated by zeolite framework oxygen atoms with an average coordination number of 3.8. This catalyst exhibits the superior catalytic performance and stability: a high DMC selectivity (>99.5%) is observed during the entire 100 hour evaluation, with the average weight-time yield (WTY) of 1600 g·kgcat.-1·h-1. This is beyond the performance of other reported state-of-the-art catalysts for this reaction. Characterization of the catalyst structure as well as the reaction intermediates and kinetics further verified the theoretical prediction that DMC is generated through an E-R mechanism.

RESULTS AND DISCUSSIONS


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Theoretical Model Indicates that the Selectivity between DMC and DMO Depends on the Isolation of Pd Species. The redox reaction producing DMC or DMO from MN and CO includes an oxidative addition process followed by a reductive elimination (Scheme 1). It is known that using the Pd(II)-based catalysts lead to the generation of DMC, whereas the Pd(0)-based catalysts lead to DMO.23-26 The palladium methoxy intermediate was generally considered to form first after the absorption of MN. Subsequently, palladium methoxy carbonyl intermediate can be generated via the insertion of the adsorbed CO molecule.17, 23, 29, 31 It was suggested that DMC is produced by the reaction between palladium methoxy carbonyl intermediates and MN from atmosphere, while DMO is obtained by the coupling between two palladium methoxy carbonyl intermediates.1,

17, 23, 31

However,

outstanding question remains of why the Pd(II)-based catalysts favor the creation of DMC while Pd(0)-based catalysts favor DMO. We next conducted the DFT calculations to understand the molecular mechanism behind this selectivity.

Scheme 1. The selectivity toward DMC or DMO over Pd-based catalysts.

For the DMC generation, the Pd(acac)2 species with a known selectivity to DMC was used as a model system of Pd(II)-based catalysts.26 Figure S2 presents the calculated thermodynamics of the oxidative addition and the CO approaching to the palladium center, respectively. Unexpectedly, the oxidative addition of MN, which was considered as the first step to form the palladium methoxy intermediate, appears to be unfeasible energetically (strong endothermic of 20.1 kcal/mol). On the other


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hand, the approaching of CO to palladium center is much more favorable (slight endothermic of 5.7 kcal/mol), which leads to the formation of palladium carbonyl species (Figure 1a). The palladium carbonyl species is then oxidized to the tetravalent palladium methoxy carbonyl intermediate (LM2) in an oxidative addition process by MN. The energy barrier of this step is 22.0 kcal/mol (TS1) with respect to the initial reactants. Subsequently, the tetravalent palladium intermediate is reduced back to the divalent Pd(acac)2 species, in a reductive elimination process, by an additional MN molecule in the reacting atmosphere. This step generates the target DMC product, and the energy barrier is 26.2 kcal/mol (TS2) with respect to the tetravalent palladium intermediate LM2. This route is therefore an E-R process along a Pd(II)-Pd(IV)-Pd(II) pathway. It should be noted that the (NO)2 dimer is generated in conjunction with DMC according to our calculated pathway. Under the experimental conditions, (NO)2 dimer will rapidly dissociate into NO monomer.32, 33 The Pd13 cluster is used to mimic the Pd(0)-based catalysts.34, 35 The Pd13CO12 model, in which all the surface palladium atoms covered by CO molecule, is used as the initial catalytic species (Figure 1b), since CO is abundant (even excessive) in the reacting atmosphere. Again, the intuitively expected formation of palladium methoxy intermediate, through the direct oxidative addition reaction between MN and the palladium surface, is energetically unfeasible (strong endothermic of 23.9 kcal/mol) under this circumstance (Figure S3). Instead, MN molecule directly contacts with the surface carbonyl species other than the bare metal with a feasible energy barrier of 16.6 kcal/mol, forming the surface methoxy carbonyl intermediate (Pd13COOCH3). Therefore the surface methoxy carbonyl should be the most significant intermediate in reaction instead of the palladium methoxy species. As shown in Figure 1b, a typical L-H process between two neighboring palladium methoxy carbonyls experiences an energy barrier of 18.1 kcal/mol. On the other hand, reaction between surface methoxy carbonyl and MN in the reacting atmosphere leads to producing DMC with an E-R process, of which the energy barrier is 27.2 kcal/mol, while another reaction channel leading to the formation of DMO through the palladium methoxy dicarbonyl intermediate (Pd13COCOOCH3) has a very high energy barrier (41.4 kcal/mol) for the insertion of one CO molecule to surface methoxy carbonyl. The selectivity for reaction on Pd(0) catalysts thus favors the production of DMO through the reaction between two neighboring palladium methoxy carbonyls.


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Figure 1. Computed molecular mechanism for generation of (a) DMC on Pd(II)-based catalyst [Pd(acac)2], and (b) DMC and DMO on Pd(0)-based catalyst [Pd13 cluster] from reaction of CO and MN (unit in kcal/mol).


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Selectivity between the pathways leading to DMC or DMO therefore depends on whether the palladium methoxy carbonyl intermediates are separated from each other. Pd(II)-based catalysts are often the coordination compounds, the palladium atoms in which are isolated from each other naturally, and there is no neighboring palladium methoxy carbonyl pairs. The reductive elimination reaction occurs between the palladium methoxy carbonyl intermediate and a MN in the reacting atmosphere (E-R mechanism), which leads to the generation of DMC. On the other hand, abundant neighboring palladium methoxy carbonyl pairs exist in the Pd(0)-based catalysts, their reactions (L-H mechanism) lead to the production of DMO. However, if the Pd(II) is reduced to Pd(0) species, the aggregation of palladium atoms becomes unavoidable, which prevents the selectivity to DMC.17, 23, 24, 36

Computational Heterogeneous Catalyst Model of Stability for Selective Generation of DMC. A catalytic system for DMC production therefore requires the stable isolated Pd(II) centers. The organometallic system, such as Pd(acac)2 and Pd(OAc)2, provides the desired selectivity toward DMC.26 However, the stability of these catalysts is not satisfying. The supported catalysts based on the inorganic carriers can act as the more stable alternatives of the organic ligands if the similar coordination surrounding can be provided. The metal oxides (e.g., MgO37, Al2O316, 29, 38)

and zeolites25, 30, 39 are often used as the carriers to prepare the supported palladium

catalytic systems for this reaction. However, the Pd(II) centers on the surface of these metal oxide carriers are largely unsaturated (Figure 2a), implying the instability of this supported catalytic system. Instead, placing the Pd(II) centers into the 6-MRs of sodalite cage in Y-type zeolites (Figure 2a) provides the desired coordination surrounding, where four coplanar oxygen atoms are coordinated with the Pd(II) center to form a saturated 16e reactive center. Using the charged silico-alumina zeolites instead of the silico zeolites can further enhance the stability of Pd(II) center since an additional electrostatic interaction between metal center and carrier skeleton is introduced. The calculated electron affinity and HOMO-LUMO gap for these three different supported Pd(II) catalysts are used to evaluate their relative stabilities (Figure 2b). Our calculations suggested that the electron affinity of supported Pd(II) catalysts increases from the MgO carrier, to the silico zeolite carrier then to the silico-alumina


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zeolite carrier. Reduction of the Pd(II) center in these catalysts therefore becomes more difficult in this sequence. A similar trend can be found for the HOMO-LUMO gap, suggesting a better stability for the silico-alumina zeolite carrier supported catalyst. Reaction mechanism on Pd(II)/SiAl-Y catalyst was also calculated, which show that reaction on this zeolite carrier supported catalyst follows the similar reaction pathway on complex Pd(acac)2 (see Figure S4 and S5). These findings suggest the selectivity and stability towards DMC could be achieved on the Pd(II)/SiAl-Y catalyst.

Figure 2. (a) Optimized geometries of model supported Pd(II) catalysts and (b) their calculated electron affinity and HOMO-LUMO gap.

Preparation and Characterization of Pd(II)/NaY Catalytic System with mononuclear-isolated Pd(II) centers. A computational heterogeneous catalyst model of Y-type silico-alumina zeolite supported Pd(II) species has been proposed to achieve high-selectivity and high-stability for generation of DMC, where the 6-MRs of sodalite cage in Y-type silico-alumina zeolite provides a desirable coordination surrounding to isolate and stabilize the Pd(II) centers. According to such


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computational proposal, we synthesized a Y-type silico-alumina zeolites supported Pd(II) catalyst, with mononuclear-isolated Pd(II) centers, to achieve high-performance and high-stability for the aforementioned CO direct selective conversion to DMC. We developed an ultrasonic-assisted ammonia evaporation method for this purpose (Scheme 2).40 Pd(acac)2 first reacts with NH3·H2O to form a coordination compound, which then diffuse to the suitable channels of NaY zeolite by ultrasonic treatment. The acac ligands in these coordination compounds react with the Na cation to leave off. Subsequently, all ammonia ligands in the palladium ammonia coordination ions leave off during ammonia-evaporation process, and the original coordination structure of Pd(II) species are preserved to form stable mononuclear-isolated Pd(II) centers anchored on the NaY zeolite framework. It was reported that only the paired Al substitution in 6-MR with two Na cations located could charge balance a divalent counter ion,41 in which the Al-O(Si-O)2-Al linkages may be the most stable arrangement to anchor a divalent counter ion due to the symmetric geometrical configuration. Considering the sizes of 6-MR windows and palladium ammonia coordination compound, we speculated that the Pd(II) species most likely exchanged to the 6-MR windows with Al-O(Si-O)2-Al linkages in supercages.

Scheme 2. Diagram of the formation process of Pd(II)/NaY catalyst.


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Figure 3. (a) FT-IR spectroscopy of NaY zeolite, Pd(II)/NaY catalyst and the centrifugal supernatant during preparation process; (b) XRD pattern of NaY zeolite, Pd(II)/NaY catalyst and Pd(acac)2 precursor.

Fourier Transform infrared (FT-IR) spectroscopy of the Pd(II)/NaY catalyst (Figure 3a) has an additional peak at 1405 cm-1 compared with that of NaY zeolite, which is attributed to the Pd-O bond stretch. On the other hand, the characteristic IR signals of acac ligand from Pd(acac)2 precursor (1654 cm-1 for νCOring, 1560 cm-1 for νC-C-C and 1412 cm-1 for δC-H, respectively42) are absent in the signal of Pd(II)/NaY catalyst, and observed in the signal of centrifugal supernatant. These observations indicate the removal of the acac ligand in Pd(II)/NaY. Furthermore, the X-ray diffraction (XRD) angle of Pd(II)/NaY catalyst (Figure 3b) is the same as that of NaY zeolite without obvious diffraction signals of Pd(acac)2, indicating that the NaY zeolite framework remains intact and the Pd species are highly dispersed on the carrier. It was remarkable that the pattern of Pd(II)/NaY lost intensity at around 6 degrees, reflecting the ion exchanged degree, which was caused by the exchanged Pd species in channels with changed local electric field.43 Combined with the presence of Na signal (17.38 mg·L-1) in the centrifugal supernatant by inductively coupled plasma (ICP) measurement, it is clear that the acac ligands react with the framework Na cation of NaY zeolite and then leave the catalytic system during the preparation process.


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Figure 4. (a) TEM, (b) HAADF-STEM, (c) aberration-corrected HAADF-STEM and (d) HAADF-STEM-EDX mapping of Pd(II)/NaY catalyst.

Consistent with the Pd loading of 0.935% by ICP, the Energy Dispersive X-ray (EDX) spectroscopy (Figure S7) also reveals the presence of Pd species in the Pd(II)/NaY catalyst. No Pd granule is observed in the Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM) of Pd(II)/NaY catalyst (Figure 4a, 4b). On the other hand, mononuclear-isolated Pd centers could be clearly identified in the high-resolution STEM image (Figure 4c), and the Pd component were homogeneously dispersed in the mapping results (Figure 4d). The result of X-ray absorption near-edge structure (XANES) spectroscopy on the Pd(II)/NaY catalyst (Figure 5a) coincides with that of the PdO powder, indicating a Pd(II) valence state of Pd species in Pd(II)/NaY. The absence of Pd-Pd contribution in the extended X-ray absorption fine structure (EXAFS) spectroscopy of Pd(II)/NaY catalyst (Figure 5b) further indicates that the Pd species remain mononuclear-isolated in nature,42,

44-47

which are in accord with the result of high-resolution STEM.

Meanwhile, the metal-support interaction is further verified by the presence of the Pd-O bond with an average coordination number of 3.8 at an average bond distance of


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2.01 Å (Table S1). Since an average Pd-O coordination number of approximately 4 was observed for Pd(II)/NaY catalyst, we conclude that four framework oxygen atoms at Al sites in NaY zeolite act as two bidentate ligands to chelate Pd moieties.42, 48 It was remarkable that there was only Pd-O but no Pd-O-Pd signal in the Pd(II)/NaY catalyst, which further revealed that the Pd-O signal were derived from the interaction between mononuclear-isolated Pd(II) centers and zeolite framework oxygen atoms but not form the PdO bulk phase. The NaY zeolite anchored Pd(II) species by coordination were therefore highly uniform and essentially mononuclear-isolated in character, consistent with the higher degree of crystallinity of NaY zeolite and the existence of well-defined binding sites on NaY zeolite surface.49

Figure 5. (a) The normalized XANES spectroscopy at the Pd K-edge of Pd foil, PdO powder and Pd(II)/NaY catalyst; (b) Fourier transforms of the experimental EXAFS spectroscopy of Pd foil, PdO powder and Pd(II)/NaY catalyst.

High-Performance and High-Stability for CO Direct Selective Conversion to DMC on Pd(II)/NaY Catalyst Undergoes an E-R Mechanism. We next performed the catalytic tests of CO direct catalytic conversion on the Pd(II)/NaY catalyst prepared. The catalyst was conducted in a continuous flow of CO and MN mixture feed at 120 °C for 100 h. The results (Figure 6) shows a very high DMC selectivity (>99.5%) and a good CO conversion ratio (>80%) with no obvious decay during the whole test. The average WTY of DMC is up to 1600 g·kgcat.-1·h-1, largely exceeds that of the reported catalysts.


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Figure 6. CO conversion, product selectivity and WTY of DMC in long-term CO

direct catalytic conversion reaction on Pd(II)/NaY catalyst. Reaction conditions: 0.2 g catalyst, 120 °C, 0.1 MPa, weight hour space velocity (WHSV) = 2500 L·kgcat.-1·h-1, CO: MN: Ar: N2 = 19% : 45% : 3% : 33%.

In addition, the catalytic performance of Pd(0)/MgO catalyst was also evaluated at the same reaction condition displayed in Figure S8, which shows nearly 90% DMO selectivity with 93% CO conversion. The Pd species exist on the MgO support as nanoparticles about (4.75±2.25) nm with 0.982% loading (Figure S9). It was known that the Lewis acidity and Lewis alkalinity of supports would benefit the selectivity towards DMC and DMO, respectively.22 To further analyze the effect of Pd species status on product selectivity excluding supports roles, as shown in Table S2, we compared the catalysts with different Pd species status on the same support (Pd(II)/NaY and Pd(0)/NaY). The Pd(0)/NaY catalyst shows 29.4% DMC selectivity and 70.6% DMO selectivity even with Lewis acidic support of NaY zeolite, which demonstrated that the superior DMC selectivity in Pd(II)/NaY catalyst was mainly derived from the mononuclear-isolated nature of Pd(II) centers rather than the Lewis acidic NaY zeolite support (Figure S10). Moreover, the impact of confinement effect on product selectivity was also excluded (Figure S11), in which both DMC and DMO molecules could penetrate the channels in NaY zeolite without limitation.


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In order to verify the key difference between the reaction mechanism of DMC (E-R) and DMO (L-H), kinetics experiments were performed (Figure 7). In terms of the mass action law, the rate of elementary reaction is proportional to the product of the power of the reactants concentration. With constant CO concentration, the MN concentration dependence of DMC formation rates on Pd(acac)2 and Pd(II)/NaY catalyst are very different from that of DMO on the Pd(0)/MgO catalyst (see kinetic experiments section in Supplementary Information). Before the adsorbed *COOCH3 intermediates reach saturation on catalyst surface, approximate quadratic relations were found between the product formation rates and the MN concentration on all three catalyst. After the saturation, the DMC formation rate was linear with MN concentration on Pd(acac)2 and Pd(II)/NaY catalyst, while the DMO formation rate becomes constant. The DMO formation rate therefore has no correlation with the gaseous MN concentration. The kinetics reveals that DMC was produced via E-R mechanism depending on the gaseous MN concentration, while DMO was produced via L-H mechanism with a constant formation rate.

Figure 7. Kinetics experiments of (a) DMC formation rate on Pd(acac)2 catalyst; (b) DMC formation rate on Pd(II)/NaY catalyst; (c) DMO formation rate on Pd(0)/MgO catalyst with increasing MN concentration. Reaction conditions: 0.02 g Pd(acac)2, 0.05 g Pd(II)/NaY and Pd(0)/MgO catalyst, 120 °C, 0.1 MPa, CO space velocity = 85.48 mmol·g-1·h-1.


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The in situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS) of the reaction between CO and MN on Pd(II)/NaY catalyst (Figure 8) verifies the existence of palladium methoxy carbonyl intermediates. The peak at 1720 and 1191 cm-1 are ascribed to the C=O and C-O stretching vibrations of adsorbed *COOCH3 intermediates.29 Moreover, the peak at 1450 cm-1 is assigned to the C-H deformation of adsorbed *COOCH3 intermediates on catalyst surface. Strong new peaks appear at 1770 and 1303 cm-1 with the passing of time, which are ascribed to the C=O and C-O stretching vibrations of gaseous DMC product, respectively.38 Meanwhile, the peaks appeared at 1909 and 1844 cm-1 are assigned to the bimodal peaks of gaseous NO,50 indicating that NO was also produced during the reaction.

Figure 8. in situ DRIFTS of the reaction between CO and MN on Pd(II)/NaY catalyst.

In accord with the theoretical calculations, the Pd 3d X-ray photoelectron spectroscopy (XPS) of fresh and used Pd(acac)2 catalyst (Figure 9) reveal the presence of high valence palladium intermediates during reaction. Because the elusive photoelectron signal of high valence palladium intermediates could often be covered by background signal in the Pd(II)/NaY catalyst, we used Pd(acac)2 catalyst to substitute. The details of XPS measurement were elaborated clearly in SI. The binding energy (B.E.) of Pd 3d3/2 and 3d5/2 peaks at 343.1 and 337.7 eV were ascribed to the Pd(II) species in fresh Pd(acac)2 catalyst. By rapid freeze to cease reaction with liquid


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nitrogen, two new strong peaks at 344.4 eV and 339.2 eV appear in the used Pd(acac)2 catalyst except for the Pd(II) characteristic signal peaks, which were attributed to the 3d3/2 and 3d5/2 of higher oxidation state Pd species than Pd(II) species, maybe Pd(IV) species. The existence of high valence palladium intermediates in the used Pd(acac)2 catalyst demonstrated that the initial Pd(II) species would undergo a oxidative addition to higher valence palladium intermediates, and then a reductive elimination to recycle during DMC formation, in agreement with the Pd(II)-Pd(IV)-Pd(II) pathway proposed in DFT calculations.

Figure 9. Pd 3d XPS of (a) fresh and (b) engaged Pd(acac)2 catalyst.

CONCLUSIONS Direct selective catalytic conversion of CO to DMC with high-performance and high-stability is challenging. Through theoretical model, we revealed that DMC is selectively produced on the mononuclear-isolated Pd(II) centers via an E-R mechanism. However, the Pd(II) would be easily reduced to Pd(0) in the presence of CO, leading to the production of DMO. Calculations further proposed a heterogeneous catalyst model of Y-type silico-alumina zeolite supported Pd(II)


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species to obtain a high-performance catalytic system with high-selectivity and high-stability for generation of DMC, where the 6-MRs of sodalite cage in Y-type silico-alumina zeolite provides a desirable coordination surrounding to isolate and stabilize the Pd(II) centers. Benefited from the computational model, we developed an ultrasonic-assisted ammonia evaporation method to successfully synthesize the Pd(II)/NaY catalyst, in which the Pd(II) species are coordinated by zeolite framework oxygen atoms with an average coordination number of 3.8 and remains mononuclear-isolated in nature. This catalyst not only shows a high selectivity (>99.5%) towards DMC with an average WTY reaching 1600 g·kgcat.-1·h-1, but also has an excellent long-term stability with no obvious deactivation in 100 h evaluation, which exceeds that of previously reported state-of-the-art catalysts for this reaction. Further characterizations of the catalyst structure as well as the reaction intermediates and kinetics verified that DMC is generated through an E-R mechanism. Current work provides a high-performance and high-stability catalytic system for CO direct selective conversion to DMC, and further sheds the light on how to prepare more efficient catalysts for the future industrialization of this process route.

ASSOCIATED CONTENT Supporting Information The Supporting Information includes catalyst synthesis, catalytic performance, computational methods, computed thermodynamics data, computed molecular mechanism, kinetics experiments, XPS measurement details, EDX spectroscopy, EXAFS parameters, TEM image, NH3-TPD, CO2-TPD, in situ DRIFTS, XPS spectroscopy.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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This work was supported by the National Key R&D Program of China (2017YFA0206800, 2017YFA0700103), the National Natural Science Foundation of China (91545201, 91645116, 21433014), the Programs of the Chinese Academy of Sciences (ZDRW-CN-2016-1, QYZDJ-SSW-SLH028), and the Natural Science Foundation of Fujian Province (2018J06005) and Youth Innovation Promotion Association CAS.

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NaY Catalyst

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