CO direct esterification to dimethyl oxalate and dimethyl carbonate: the key functional motifs for catalytic selectivity†

Zhi-Qiao Wang ^ab, Jing Sun ^ab, Zhong-Ning Xu *^ab and Guo-Cong Guo *^ab
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China. E-mail: gcguo@fjirsm.ac.cn; znxu@fjirsm.ac.cn
^bFujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108, P. R. China

First published on 20th July 2020

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1. Introduction

Dimethyl oxalate (DMO, C₄H₆O₄) is an important ester chemical raw material in the preparation of various dyes and medicines and is also used as a solvent and extraction agent in industries.^1,2 As an important intermediate in organic synthesis, oxalic acid can be obtained via hydrolysis of DMO at atmospheric pressure. Oxamide, a high quality sustained-release fertilizer, can be synthesized via ammonia hydrolysis of DMO under atmospheric pressure. Notably, ethylene glycol (EG, (CH₂OH)₂) can be prepared via hydrogenation of DMO at low pressure. As an essential and important raw material, EG has been widely applied to synthesize polyester resins and fibers and is also used as solvent, surfactant, antifreeze, etc.^3,4

Many methods have been developed for the synthesis of EG, such as the route of fossil fuels (coal, petroleum, and natural gas) and biomass-based resources.^5–13 Among these synthetic routes, the petroleum route is a prime method to synthesize EG. The corresponding reaction processes are as follows. Firstly, ethylene is obtained from petroleum. Then, ethylene is oxidized to ethylene oxide. Finally, EG is obtained via hydrolysis of ethylene oxide. The petroleum route is popular globally. However, the use of the petroleum route is limited by the lack of oil resources in some countries. Recently, the coal to EG (CTEG) technology, a new technology of “coal replacing oil” has gathered significant attention. Because the price of coal resources is much lower than petroleum, the cost of coal-based EG method could be saved by 39.50% compared to the petroleum-based EG production for the same yield.¹⁴ As one of the early academic institutes for research on direct esterification to DMO in the gaseous phase, the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences built the world's first 200000 ton CTEG plant in 2009, marking it the first industry on CTEG technology achieved in China. This technology includes three main parts: (1) CO dehydrogenation, (2) synthesizing DMO, and (3) synthesizing EG by DMO hydrogenation.^15–26

Notably, synthesizing DMO is regarded as the key step to convert inorganic carbon to organic carbon in CTEG technology. The traditional routes include (1) the conventional esterification method and (2) the one-step esterification dehydrating method. In the conventional esterification method, DMO is synthesized via the esterification of oxalic acid and alcohol, in which inorganic acid or ion exchange resin is used as the catalyst. However, the production cycle of this method is long, and the conventional esterification process generally needs 22–24 hours. Thus, to shorten the production cycle, improve the production efficiency and reduce the power consumption of products, the conventional esterification method has been improved to the one-step esterification dehydrating method. The main reaction of the one-step esterification dehydrating process is similar with the conventional esterification method. The difference is that the industrial oxalic acid, alcohol and dehydrating agent toluene are put into the reactor simultaneously in a certain proportion for esterification, which reduces the step of esterification distillation. Meanwhile, the esterification time is about 6 hours shorter than that for the conventional method.²⁷

Dimethyl carbonate (DMC, C₃H₆O₃) is another important ester chemical raw material. As a green raw material, DMC has abundant reactivity.^28,29 DMC is the main ingredient in the synthesis of polycarbonate.³⁰ DMC also is used as a lithium battery additive due to its high dielectric constant. In recent years, due to its high oxygen content (53%) and high octane number (105), DMC is also recommended as a potential additive in fuel oil. When a suitable amount of DMC is added to fuel oil, the soot particle emission of engines can be reduced, which consequently, further alleviates environmental pollution. The demand for DMC has increased annually because of its various applications. The traditional routes to obtain DMC include: (1) the phosgene route, (2) transesterification route, (3) liquid-phase methanol oxidative carbonylation route, and (4) urea alcoholysis route. However, due to the use of the hypertoxic phosgene as the reaction raw material, the phosgene route has been discontinued presently. Furthermore, although the transesterification route and the liquid-phase methanol oxidative carbonylation route are adopted in the industrial production of DMC currently, they have some shortcomings. On the one hand, the transesterification route produces a large amount of wastewater and residue, which also suffers from high costs. On the other hand, the lifetime of catalysts in the liquid-phase methanol oxidative carbonylation route is severely shortened by the formation of water during the reaction.^31–40

In brief, many ester chemicals (DMO, DMC, methyl formate (MF), etc.) have a wide range of industrial applications. There are many liquid phase and gas phase methods to synthesize ester chemicals. The liquid phase methods include acid and alcohol esterification, transesterification, and liquid phase methanol oxidative carbonylation. However, the liquid phase method usually requires an acid or base as the catalyst, while the by-products are difficult to separate, and the catalyst circulation is difficult. Gas phase methods include the phosgene alcohol method and methanol oxidation carbonylation method. However, phosgene is highly toxic, and the by-product HCl is corrosive in the phosgene alcohol method, while the catalyst may have a limited life in the gas phase methanol oxidation carbonylation method.³⁰ Therefore, it is urgent and important to develop new catalytic techniques with high efficiency, low cost and environmental friendliness to prepare ester chemicals.

The direct esterification of CO involves processes using CO as the starting material and ester chemicals as the products. The ester chemicals include dimethyl oxalate (DMO), dimethyl carbonate (DMC), and methyl formate (MF). The CO direct esterification processes involve at least three reactions as follows:

2CO + 2CH3ONO → (COOCH3)2 (DMO) + 2NO	(1)

CO + 2CH3ONO → CO(OCH3)2 (DMC) + 2NO	(2)

2CO + H2 + 2CH3ONO → 2HCOOCH3 (MF) + 2NO	(3)

The by-product NO can be recycled and reused to produce CH₃ONO with CH₃OH and O₂ in another reactor.

DMO and DMC are two different products of the direct CO esterification reaction, while the selectivity can be regulated by adjusting the microstructure of the catalyst. Compared with the traditional routes to synthesize DMO and DMC, the direct CO esterification reaction is extremely competitive. In a fixed-bed reactor, the separation of product and catalyst with time is beneficial to increase the life of the catalyst. Furthermore, adopting a fixed-bed reactor is beneficial for simplifying the process of product separation and purification. Many institutes, such as Tianjin University, Xiamen University, East China University of Science and Technology, and Shanghai Jiao Tong University, have focused on investigating the active catalyst species, microstructure of the support, additive agent type, reaction mechanism, and reaction kinetics for the direct esterification of CO to DMO.^27,41–49 Direct CO esterification to DMC technology also has good industrial application potential.⁵⁰

This review provides an in-depth analysis of the functional motifs of catalysts¹⁰¹ that affect the selectivity for DMO and DMC in the direct CO esterification reaction. Specifically, the microstructure of the active component of the catalyst and the Lewis acid and basic sites of the support ate the focus. We also provide rational guidance for the direct esterification of CO to DMO and DMC.

2. Effect of valence state of active component Pd

As is known, the structure–activity relationship of catalysts is one of the most important and essential issues in the field of heterogeneous catalysis.⁵¹ According to the literature, Pd-based catalysts are the main catalysts, which have been proven to be good catalytic systems for the direct CO esterification reaction.⁵² Initially, Pd(0) was regarded as the active species for the direct esterification of CO to DMO, while Pd(II) is regarded as the active species for DMC. In this part, we analyze the effect of valence state of the active component Pd on catalytic performance.

2.1 Pd(0) species

In recent years, the research on catalysts with Pd(0) species for the direct CO esterification reaction mainly focus on nanometer Pd-based catalysts. With the rapid development of the controlled synthesis of nanomaterials with different sizes and shapes, new opportunities are provided to study the structure–activity relationship of catalysts.^53–66 Scientists have discovered that novel structures and properties can be obtained by making materials with sizes on the nanoscale.^67–79 The exposed crystal planes, defect structures and surface atomic coordination environment of nanoscale noble-metal catalysts are closely related to their catalytic activity.^80–83 It is worth noting that adopting nanoscale Pd-based catalysts with Pd(0) species is beneficial to develop high performance Pd-based catalysts with a low loading for the direct esterification of CO to DMO, which will reduce catalyst costs by reducing the use of noble metals.

Monodisperse Pd(0) nanoparticles with exposed (111) and (100) facets were synthesized via preferentially oriented facet growth technology to investigate the effect of size and shape of Pd(0) nanoparticles on the direct esterification of CO to DMO (Fig. 1).⁸⁴ Then, the Pd(0) nanoparticles were supported on α-Al₂O₃ to obtain Pd-based catalysts with Pd(0) species. When used for the direct CO esterification reaction, the selectivity of CO Pd(0)/α-Al₂O₃ catalyst with exposed (111) and (100) facets was close to 100%. Meanwhile, the turnover frequency (TOF) value of the Pd(0)/α-Al₂O₃ catalyst with (111) facets was 1.2 s⁻¹, which is much higher than that (0.04 s⁻¹) of the Pd(0)/α-Al₂O₃ catalyst with (100) facets, demonstrating that the (111) facets of Pd(0) nanocrystals are beneficial for the direct esterification of CO to DMO. According to this finding, a high-performance and long-lived supported Pd(0) nanocatalyst with a lower Pd loading for the direct esterification of CO to DMO was successfully obtained via a new wet impregnation-solution chemical reduction method, which well controlled the exposure of (111) facets and size of the Pd nanocrystals.

In addition to studying the effect of palladium morphology on the catalyst, it is of great significance to reduce the palladium loading, which can reduce the amount of palladium in the catalyst so as to reduce its cost. A new synthetic method involving Cu²⁺-assisted in situ reduction at room temperature was developed to obtain a highly active and stable Pd(0)/α-Al₂O₃ nanocatalyst with an ultra-low Pd loading (0.13%, Fig. 2).⁸⁵ Notably, the selectivity of the Pd(0)/α-Al₂O₃ catalyst was 97%, indicating that DMO is the main product in the direct CO esterification reaction. The introduction of Cu²⁺ ions is beneficial to obtain Pd(0) nanoparticles with high dispersion and small size, which can improve the catalytic activity. Considering the mild synthetic conditions, this method may have good potential for industrial application.

2.2 Pd(II) species

According to the literature, Pd-based catalysts have been used in the direct esterification of CO to DMC, including chloride-containing and chloride-free catalysts.^87–91 It was reported that the (Pd-CuCl₂)/γ-Al₂O₃ catalyst with high activity (CO conversion: 70.9%) and high selectivity for DMC (99.8%) was successfully prepared for the direct esterification of CO to DMC, suggesting that Pd(II) species may be the active species for the direct esterification of CO to DMC.⁹² However, chlorine-containing Pd-based catalysts are easily deactivated due to the loss of chloride ions. Thus, the development of high-performance and long-lived chloride-free Pd(II)-based catalysts has been the focus of many researchers in recent years. By choosing different palladium precursors, a series of Pd(II)/NaY catalysts were prepared with high selectivity for DMC (>99%). Subsequently, a chloride-free [Pd(acac)₂]/NaY catalyst presented higher catalytic activity (CO conversion: 60.1%) than that of [Pd(OAc)₂]/NaY (56.1%) and [Pd(NO₃)₂]/NaY (53.9%).⁸⁶ The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image suggested that the Pd species was highly dispersed on the support of NaY in the [Pd(acac)₂]/NaY catalyst (Fig. 3).

Notably, the oxidation states of the Pd species in the catalyst could be tuned by the ability to accept backdonated electrons from the ligands, while there was a positive correlation between catalytic activity and the high oxidation states of the Pd species (Fig. 4). Furthermore, after reducing the corresponding [Pd(acac)₂]/NaY catalysts to Pd(0)/NaY, the main product became DMO, indicating that Pd(II) may be favorable for the formation of DMO and Pd(0) for the formation of DMO.

However, Pd(II) is easily reduced to Pd(0) by CO during the catalytic reaction, which further leads to the formation of DMO as a by-product. This means a highly stable Pd(II)-based catalyst, which produces DMC with high selectivity, is hard to achieve. When the Pd(II) species is reduced to Pd(0) species during the reaction process, the formation of DMC will be prevented because the aggregation of palladium atoms is unavoidable. Therefore, developing catalytic systems with stable isolated Pd(II) for the production of DMC is necessary.

In fact, the stability of organometallic catalysts [Pd(acac)₂ and Pd(OAc)₂] is not good although their selectivity for DMC is desirable (>99%).⁸⁶ Accordingly, inorganic carriers are regarded as stable alternatives for organic ligands to prepare supported catalysts. Metal oxides (MgO, Al₂O₃, etc.) and zeolites are often employed as carriers to obtain supported Pd-based catalysts for this reaction. However, the unsaturated Pd(II) on the surface of metal oxide carriers may lead to instability in the catalytic system. Instead, the desired surrounding coordination can be realized by introducing Pd(II) into the 6-MRs (MR: membered rings) of the sodalite cage in Y-type zeolites, in which a saturated 16e reactive center can be formed by coordinating four coplanar oxygen atoms with the Pd(II) center.⁹³ Then, choosing charged silico–alumina zeolites as the support, the stability of Pd(II) can be further enhanced by improving the electrostatic interaction between the metal and carrier skeleton.

The relative stabilities of three different supported Pd(II) catalysts were evaluated by calculating their corresponding electron affinities and HOMO–LUMO gaps. As shown in Fig. 5, the electron affinity for the supported Pd(II) catalysts followed order of Pd(II)/SiAl–Y > Pd(II)/Si–Y > Pd(II)/MgO, while the reduction of the Pd(II) center in these corresponding catalysts was more difficult in this sequence.⁹³ This trend was also proven by the corresponding HOMO–LUMO gap, indicating the better stability for Pd(II)-supported catalysts with the silico–alumina zeolite carrier. Furthermore, by calculating the reaction mechanism over the Pd(II)/SiAl–Y catalyst, it was found that there is a similar reaction pathway over this silico–alumina zeolite carrier-supported catalyst and the complex Pd(acac)₂, indicating that high selectivity for DMC and stability may be achieved simultaneously.

Based on the theoretical calculations above, we synthesized Y-type silico–alumina zeolite-supported Pd(II) catalysts to achieve high selectivity and stability simultaneously. As shown in Fig. 6, the mononuclear-isolated Pd centers were identified in the high-resolution STEM image clearly, and the Pd species were homogeneously dispersed on the support in the mapping results. The X-ray absorption near-edge structure (XANES) spectroscopy results are shown in Fig. 7, in which the white line of the Pd(II)/NaY catalyst is consistent with that of the PdO powder, suggesting Pd(II) species exists in Pd(II)/NaY. Notably, in the extended X-ray absorption fine structure (EXAFS) spectrum, there was no Pd–Pd contribution in the Pd(II)/NaY catalyst, which further demonstrated that the Pd species is mononuclear-isolated, which is consistent with the high-resolution STEM results. The performance of Pd(II)/NaY in the direct esterification of CO to DMC is presented in Fig. 8. It is worth noting that very high DMC selectivity (>99.5%) and good CO conversion (>80%) with no obvious decay during the whole test were obtained over the Pd(II)/NaY catalyst, suggesting that we obtained a high-performance and long-lived Pd(II)-based catalyst for the direct esterification of CO to DMC. Also, the average WTY (weight time yield) of DMC reached 1600 g kg_cat⁻¹ h⁻¹, which exceeds that of the reported Pd-based catalysts.

3. Effect of aggregate state of active Pd component

It is widely believed that Pd(0) is the active species for the production of DMO from the direct CO esterification reaction, while Pd(II) is beneficial for the generation of DMC. In situ infrared characterization indicates that the carbonyl methoxide (*COOCH₃) intermediate is the active intermediate in the direct esterification of CO to DMO. However, the *COOCH₃ intermediate is also considered as the active intermediate in the direct esterification of CO to DMC. Therefore, understanding the principle of the formation of DMO or DMC from the *COOCH₃ intermediate over Pd-based catalysts is the key to achieving selectivity regulation.

The synthesis of DMC or DMO from MN and CO includes an oxidative addition and a reductive elimination process (Fig. 9). After the absorption of MN, the palladium methoxy intermediate is generally formed first. Then, the palladium methoxy carbonyl intermediate is generated by the insertion of the adsorbed CO molecule.⁹⁴ Subsequently, DMC is formed by the reaction between the palladium methoxy carbonyl intermediates and MN in the atmosphere, while DMO is produced by the coupling of two palladium methoxy carbonyl intermediates. DFT calculations were used to investigate the molecular mechanism to achieve selectivity regulation for DMO and DMC (Fig. 10). The Pd(acac)₂ species was adopted as the model system of Pd(II)-based catalysts for DMC, and the Pd₁₃ cluster was adopted to mimic the Pd(0)-based catalysts.

3.1 Formation process of DMC

Firstly, compared with the oxidative addition of MN (strongly endothermic: 20.1 kcal mol⁻¹), the palladium center is much more favorable to combine with CO (slightly endothermic: 5.7 kcal mol⁻¹), which forms a palladium carbonyl species (Fig. 10). Due to the oxidative addition process of MN, the palladium carbonyl species is further oxidized to form a tetravalent palladium methoxy carbonyl intermediate (LM2). With respect to the initial reactants, the energy barrier is 22.0 kcal mol⁻¹ (TS1) in this step. Then, the divalent Pd(acac)₂ species is formed by reducing the tetravalent palladium intermediate in a reductive elimination process. With respect to the tetravalent palladium intermediate (LM2), the energy barrier is 26.2 kcal mol⁻¹ (TS2) during the formation of the target DMC. This route is regarded as an Eley–Rideal (E–R) process along a Pd(II)–Pd(IV)–Pd(II) pathway.

3.2 Formation process of DMO

Considering that CO is in excess in the reaction system, the Pd₁₃CO₁₂ model is regarded as the initial catalytic species since the surface palladium atoms are covered by CO. Compared with the palladium methoxy intermediate formed by the oxidative addition of MN and the surface palladium (strongly endothermic: 23.9 kcal mol⁻¹), the MN molecule directly contacts with the surface the carbonyl species to form the surface methoxy carbonyl intermediate (Pd₁₃COOCH₃), which has a feasible energy barrier of 16.6 kcal mol⁻¹. Therefore, the methoxy carbonyl on the surface of the catalyst should be a more significant intermediate during the reaction, not the palladium methoxy species. As shown in Fig. 10, the energy barrier of the typical Langmuir–Hinshelwood (L–H) process between two neighboring palladium methoxy carbonyls to form DMO is 18.1 kcal mol⁻¹. However, the energy barrier of the reaction between the surface methoxy carbonyl and MN in the reacting atmosphere to form DMC with an E–R process is 27.2 kcal mol⁻¹. Meanwhile, another reaction channel to obtain DMO, which is formed by the palladium methoxy dicarbonyl intermediate (Pd₁₃COCOOCH₃), also has a very high energy barrier (41.4 kcal mol⁻¹). Based on these results, Pd(0) catalysts indeed favor the production of DMO via the reaction of two neighboring palladium methoxy carbonyls.

Thus, the selective regulation for the reaction of CO and MN depends on whether or not the palladium methoxy carbonyl intermediates are separated from each other. The palladium atoms in Pd(II)-based catalysts are often isolated from each other naturally, in which there is no neighboring palladium methoxy carbonyl pairs during the reaction. The reductive elimination reaction can occur between the palladium methoxy carbonyl intermediate and a gaseous MN molecule (E–R mechanism), which generates a DMC molecule. For Pd(0)-based catalysts, DMO can be formed via the coupling of abundant neighboring palladium methoxy carbonyl intermediates by the L–H mechanism.⁹³

The isolated Pd(0)-based catalysts presented high selectivity to DMC, while aggregated Pd(II)-based catalysts showed high selectivity to DMO. This breaks the dogma that only Pd(0) can lead to the formation of DMO, while Pd(II) benefits the generation of DMC, suggesting the valence state of the active palladium is not the only key to the selectivity of the direct CO esterification reaction. In brief, the isolated Pd-based catalysts will lead to the formation of DMC, while aggregated Pd-based catalysts are beneficial for the production of DMO. In fact, an isolated single-atom Pd₁ catalyst and aggregated Pd(II)-MOF catalyst were successfully synthesized in our lab. The isolated single-atom Pd₁ catalyst is conducive to the formation of DMC, while the aggregated Pd(II)-MOF catalyst is beneficial for the formation of DMO. Interestingly, with a decrease in the incorporated content of active Pd species in the Pd(II)-MOF catalyst, the selectivity was reversed to DMC as the primary product, which should be attributed to the lack of spatially adjacent Pd centers. The corresponding results will be reported soon.

4. Effect of chemical characteristics of supports

As a part of the catalyst, the support also has a great influence on its catalytic performance.^95,96 Lu and co-authors developed Al-fiber@ns-AlOOH composites via the hydrothermal oxidation reaction of Al and H₂O (2Al + 4H₂O → 2AlOOH + 3H₂).^43,44 The Al-fiber@ns-AlOOH composites were used to synthesize Al-fiber@ns-AlOOH@Pd catalysts directly via an incipient wetness impregnation method. When used for the direct esterification of CO to DMO at different reaction temperatures, the CO conversion over the Al-fiber@ns-AlOOH@Pd catalyst was higher than that for the Pd/α-Al₂O₃ reference catalyst, while the selectivity for DMO was slightly higher than that of the Pd/α-Al₂O₃ reference catalyst. The TOF of the Al-fiber@ns-AlOOH@Pd catalyst was 0.39 s⁻¹, which was almost four-fold higher than that (0.10 s⁻¹) of the Pd/α-Al₂O₃ reference catalyst, suggesting that the Al-fiber@ns-AlOOH@Pd catalyst possesses much higher intrinsic activity for the direct esterification of CO to DMO. Considering the almost equivalent Pd loading and dispersion, the activity improvement of the Al-fiber@ns-AlOOH@Pd catalyst is reasonably ascribed to the unique structure of the support, in which the OH-rich surface has been proven to be favorable for the adsorption and activation of CO.

4.1. Supports with Lewis basic sites

Increasing the number of Lewis basic sites is beneficial for the direct esterification of CO to DMO. MgO with Lewis basicity has been proven to be a good carrier material for the direct esterification of CO to DMO. It was reported that the Pd/MgO catalyst prepared via a wet impregnation method exhibited good activity (CO conversion: 63%) and high selectivity for DMO (97%) in the direct esterification of CO to DMO.⁹⁷ Furthermore, we found that hierarchically porous flower-like ZnO nanocrystals as a support exhibited good activity (CO conversion: 67%) for the direct esterification of CO to DMO. Unfortunately, the activity of the Pd/ZnO catalyst was difficult to maintain due to the growth and sintering of the Pd nanoparticles. To overcome the stability issue of the Pd/ZnO catalyst, Mg²⁺ ions were further introduced into the ZnO nanocrystals during their synthesis (Fig. 11). Interestingly, the CO conversion over the Pd/Mg-ZnO catalyst was maintained for at least 100 h, suggesting that the catalytic stability of the Pd/ZnO catalyst can be greatly improved after Mg²⁺ doping (Fig. 12).⁹⁸ After analyzing the structure of the catalyst, it was found that Mg²⁺ was embedded in the crystal lattice of the ZnO support to form a Zn–Mg oxide solid solution.

On one hand, the introduction of Mg²⁺ into the ZnO support further led to a strong metal–support interaction caused by electron transfer from the ZnO to the Pd nanoparticles, effectively restraining the sintering of the active Pd nanoparticles and greatly improving the catalyst stability. On the other hand, the introduction of Mg²⁺ ions into the lattice of the ZnO support improved the number of Lewis basic sites, which is associated with high selectivity for DMO.

4.2. Supports with Lewis acid sites

Ma and co-authors found that the Lewis acid sites in chlorine-free catalysts play an important role in the direct esterification of CO to DMC reaction. As is known, the composition of the Faujasite (FAU) zeolite framework (e.g., SiO₂/Al₂O₃ ratio) has an influence on the Lewis acidic sites. Thus, to investigate the influence of Lewis acid sites on the performance of the direct esterification of CO to DMC, a series of Pd-doped zeolites with different SiO₂/Al₂O₃ molar ratios was obtained via the CTE method.⁹⁹ As the amount of Lewis acidic sites in the Pd/FAU catalysts increased, the TOF (based on the CO conversion per mole Pd per hour) increased linearly. There was a similar trend for the selectivity for DMC (based on MN). Consequently, the Lewis acidic sites in the catalysts promote the direct CO esterification to DMC reaction.

Recently, the effect of Lewis acid sites in supports on the catalytic performance for the direct esterification of CO to DMC was investigated. Metal–organic frameworks (MOFs) materials were firstly introduced as supports due to their well-defined structures, including UiO-66, MIL-101 and MOF-5 (Fig. 13).¹⁰⁰ Remarkably, the catalyst of Pd(II)/UiO-66 presented good catalytic performance with a high WTY of 1653 g kg_cat.⁻¹ h⁻¹ and DMC selectivity of 98.5%. It was proven that the catalytic performance was positively correlated with the amount of Lewis acid sites in the MOF supports. Compared with the Pd(II)/MIL-101 and Pd(II)/MOF-5 catalysts, the abundant Lewis acid sites in UiO-66 were responsible for the high performance of Pd(II)/UiO-66 for the direct esterification of CO to DMC. Furthermore, NH₃-TPD, pyridine-IR, XPS and in situ DRIRS measurements showed that UiO-66 with abundant defects possesses abundant Lewis acid sites. Meanwhile, these Pd(II)/MOFs catalysts with more Lewis acid sites are beneficial for the adsorption of CO, which also facilitates the direct CO esterification to DMC reaction.

The Lewis acid and basic sites of supports will affect the electron density of the active component Pd, which will further change the catalytic selectivity to a certain extent. Thus, increasing the number of Lewis basic sites promotes the formation of DMO, while Lewis acid sites benefit the production of DMC in the direct CO esterification reaction.

5. Conclusion

In this review, we analyzed the functional motifs for the direct esterification of CO to DMO and DMC. Initially, it was believed that the valence state of the active component Pd is responsible for the selectivity for DMO and DMC. However, through the study of isolated and aggregated Pd, it was found that the aggregate state of the active component Pd is deemed as the key functional motif for catalytic selectivity to DMO and DMC. The isolated Pd is conducive for the formation of DMC, while the aggregated Pd is beneficial for the formation of DMO. On the other hand, the Lewis acid and basic sites of the support also have an influence on the selectivity. Increasing the number of Lewis basic sites promotes the formation of DMO, while Lewis acid sites benefit the production of DMC. Currently, the reason why the support microstructure affects selectivity still needs further in-depth study. This review will lay the foundation for the development of a new generation of direct CO esterification to DMO and DMC catalysts and provide useful guidance for the precise control of the selectivity of the direct CO esterification reaction.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We gratefully acknowledge the financial support by the National Key Research and Development Program of China (2017YFA0206802, 2017YFA0700103, 2018YFA0704500), the Natural Science Foundation of China (91545201, 91645116), the Programs of the Chinese Academy of Sciences (XDB20010100, QYZDJ-SSW-SLH028), the Natural Science Foundation of Fujian Province (2018J06005, 2018J05036), and the China Postdoctoral Science Foundation (2016LH0018).

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Footnote

† Dedicated to celebrating 60 years of the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.

This journal is © The Royal Society of Chemistry 2020