Dimethyl carbonate (DMC) is an environmentally benign chemical product with biodegradability and low toxicity ^[1]. It has attracted more and more attention and widely used as the methylation or carbonylation agent, solvent, and fuel additive ^{[2, 3,4, 5,6, 7,]}.

The synthesis of DMC from CO and methyl nitrite (MN) was developed by Ube Industries ^[8,9]. DMC is synthesized over supported palladium catalysts in the Ube process ^[10,11]. Among all the supported palladium catalysts, activated carbon (AC) supported Wacker-type catalysts (PdCl₂–CuCl₂/AC) exhibited excellent catalytic performance for the synthesis of DMC from CO and MN ^[12,13]. Recently, the potential of various activated carbon supported Wacker-type catalysts promoted by different alkali promoters such as potassium (K) promoters was evaluated in the synthesis of DMC by oxycarbonylation of methanol ^[14,15,16]. It is found that the presence of alkali promoters such as CH₃COOK benefits the vapor-phase DMC synthesis reaction and the well-defined Cu₂Cl(OH)₃ is supposed to be the active phase ^[17]. The essential role of alkali promoters may be as a result of electron-donating properties of K, which has an effect upon the electronic environment of palladium and copper active species in the Wacker-type catalyst ^[14]. Jiang et al. ^[14]. have proved that the catalytic activities of activated carbon supported Wacker-type catalysts promoted by different alkali promoters ranked in the following order: K > Na > Li. And the interaction between CH₃COOK and CuCl₂ or PdCl₂ during the preparation of catalyst resulted in the formation of KCl species which suppressed the chlorine loss of Wacker-type catalysts. However, it has not yet been reported that the effects of potassium promoters of Wacker-type catalysts on the synthesis of DMC from CO and MN.

In this paper, activated carbon supported Wacker-type catalysts promoted by different K promoters were prepared by different methods and the catalytic activities of catalysts with different mass content of K promoter were evaluated. The effect of different K promoters on the catalytic performance of PdCl₂–CuCl₂/AC catalysts for DMC synthesis from CO and MN was investigated to explore an effective K promoted PdCl₂–CuCl₂/AC catalyst for the DMC formation.

Catalyst preparation: The AC supported Wacker-type catalysts were prepared by a two-step impregnation method. 0.057 g KCl was dissolved in 5 mL distilled water and KCl aqueous solution was added drop by drop to activated carbon. After impregnated for 24 h at room temperature, the sample was dried at 373 K for 4 h and the obtained sample is labeled as AC–KCl. PdCl₂–CuCl₂/AC catalyst with KCl promoter prepared by the incipient wetness impregnation method (IWI) is as follows: 7 mL of methanol solution of PdCl2 and CuCl2 was added by drops to AC–KCl, impregnating at room temperature for 24 h, drying at 373 K for 4 h and calcining at 473 K for 4 h. And the obtained catalyst is labeled as IWI-PdCl₂–CuCl₂– KCl/AC. PdCl₂–CuCl₂/AC catalyst with KCl promoter prepared by the excessive impregnation method is as follows: AC–KCl was added into 50 mL of methanol solution of PdCl2 and CuCl2 and stirred for 4 h, followed by removing solvent by vacuum evaporation at 323 K, drying at 373 K for 4 h and calcining at 473 K for 4 h. And the obtained catalyst is labeled as PdCl₂–CuCl₂– KCl/AC. PdCl₂–CuCl₂/AC catalysts with different K promoters (KOH, CH₃COOK and K₂CO₃) were also prepared by the excessive impregnation method and are labeled as PdCl₂–CuCl₂–KOH/AC, PdCl₂–CuCl₂–CH₃COOK/AC and PdCl₂–CuCl₂–K₂CO₃/AC, respectively. The loadings of Cu and Pd for all catalysts were 1.2 and 1.0 wt%, respectively.

Catalyst characterization: Textural properties of different K promoters modified AC supported Wacker-type catalysts were measured with N₂ adsorption and desorption on a Micromeritics ASAP 2020 apparatus at 77 K. H₂-temperature-programmed reduction (H₂-TPR) experiments were carried out on the Micromeritics AutoChem 2920 apparatus. The catalyst surface copper species were analyzed by X-ray photoelectron spectroscopy (XPS), which was carried out on a Perkin-Elmer PHI 1600 ESCA system.

Activity test and product analysis: The DMC synthesis from CO and MNover AC supported Wacker-type catalyst was conducted at 0.2 MPa, 393 K in a continuous flow fixed-bed reactor. 2 mL of catalyst (20–40 mesh) sample were loaded in the middle of fix-bed. The inlet gas was composed of CO, MN and N₂, the volume ratio of CO/MN was 1.0. The gas hourly space velocity (GHSV) of the reaction was 4000 h^-1. The products were analyzed by Agilent 4890A GC equipped with a FID detector.

Fig. 1 shows the catalytic performance of PdCl₂–CuCl₂/AC catalysts with different K promoters, which were CH₃COOK, KCl, KOHand K₂CO₃. FromFig. 1, it is obviously noted that the space time yield (STY) of DMC was around 900 g (L-cat)^-1 h^-1 over PdCl₂– CuCl₂–KCl/AC catalysts,while itwas only around700 g (L-cat)^-1 h^-1 over PdCl₂–CuCl₂/AC without any K promoter. Comparatively, the STY of DMC over PdCl₂–CuCl₂/AC catalysts promoted by KOH, CH₃COOK and K₂CO₃ was much lower, only around 550, 400 and 200 g (L-cat)^-1 h^-1, respectively. The results showed that the STY of DMC and the selectivity of DMC based on MN and CO on PdCl₂–CuCl₂/AC catalysts with different K promoters ranked in the following order: KCl > KOH > CH₃COOK > K₂CO₃. Especially, the addition of KCl promoter significantly improved the catalytic activities of PdCl₂–CuCl₂/AC catalyst for DMC synthesis from CO andMN. Therefore, the additionofKpromoters in the formofKCl can significantly enhance the catalytic activities of DMC synthesis from CO and MN.

Fig. 1

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Fig. 1.Catalytic activities of PdCl2–CuCl2/AC catalysts with different K promoters (K loading 1.0 wt%).

Textural properties of activated carbon and PdCl₂–CuCl₂/AC catalysts with different K promoters are listed in Table 1. It is noted that the specific surface area, pore volume and average pore diameter of all the catalysts were slightly changed, suggesting that the structure and physical properties of PdCl₂–CuCl₂/AC catalysts with different K promoters were similar. Therefore, the different K promoters had negligible effect on the textural properties of K promoters modified PdCl₂–CuCl₂/AC catalysts.

Table 1

Table 1 Textural properties of the catalysts.

Table 1 Textural properties of the catalysts.

H₂-TPR profiles of PdCl₂–CuCl₂/AC catalysts with different K promoters are illustrated in Fig. 2. It can be seen that there is one reduction peak around 200 ^°C for all the catalysts. This reduction peak is due to the co-reduction of Cu²⁺ and Pd²⁺ species. According to previous research, it can be seen that the strong interactions among activated carbon support, PdCl₂ and CuCl₂ species led to the overlap of peaks of Cu²⁺ and Pd²⁺ species into one peak ^[18,19]. It is noted that this reduction peak was shifted to higher temperature, and the reduction temperature of all the catalysts ranked in the following order: PdCl₂–CuCl₂–KCl/AC < PdCl₂–CuCl₂–KOH/AC < PdCl₂–CuCl₂–CH₃COOK/AC < PdCl₂–CuCl₂–K₂CO₃/AC. It is well known that Pd²⁺ species is the main active species of DMC synthesis from CO andMN^[11]. However, Pd₂₊ species is easy to be reduced to Pd⁰ species during the reaction, resulting in the deactivation of the Wacker-type catalysts ^[12]. It is noted that the existence of Cu²⁺ species is responsible for oxidizing Pd⁰ to Pd²⁺. Thus, the higher the reduction temperature of Cu²⁺ species, the more difficult it is to oxidize Pd⁰ to Pd²⁺. Thus, PdCl₂–CuCl₂/AC catalysts with CH₃COOK, KOH and K₂CO₃ promoters are not favorable for DMC synthesis from CO andMNbecause of the higher reduction temperature of Cu²⁺ species.

Fig. 2

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Fig. 2.H2-TPR profiles of PdCl2–CuCl2/AC catalysts with different K promoters.

Fig. 3 shows XPS spectra of PdCl₂–CuCl₂/AC and PdCl₂–CuCl₂– KCl/AC catalysts before and after reaction. It can be seen that there are two peaks associated with copper species with the binding energy around 934.0 eV and 932.0 eV in all the catalysts. The former is ascribed to Cu²⁺ species and the latter is attributed to Cu⁺ species ^[15,20]. The area percentages of Cu 2p for all the catalysts were further analyzed. The proportion of Cu²⁺ is 62.8% and 79.7% for PdCl₂–CuCl₂/AC and PdCl₂–CuCl₂–KCl/AC catalysts before reaction, respectively. Meanwhile, the amount of Cu²⁺ in the PdCl₂–CuCl₂–KCl/AC catalyst after reaction is also higher than that in the PdCl₂–CuCl₂/AC catalyst after reaction. It can be concluded that PdCl₂–CuCl₂/AC catalyst promoted by KCl led to possessing larger amount of Cu²⁺+. It is known that Cu²⁺ species is responsible for oxidizing Pd⁰ to active Pd²⁺+ and more Cu²⁺+ species will benefit the catalytic performance of Wacker-type catalysts for DMC synthesis from CO and MN. Therefore, PdCl₂–CuCl₂–KCl/AC catalyst with more Cu²⁺+ species performed excellent catalytic activities for DMC synthesis.

Fig. 3

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Fig. 3.XPS spectra of PdCl2–CuCl2/AC and PdCl2–CuCl2–KCl/AC catalysts before and after reaction. (a) PdCl2–CuCl2/AC (before reaction); (b) PdCl2–CuCl2/AC (after reaction);(c) PdCl2–CuCl2–KCl/AC (before reaction); (d) PdCl2–CuCl2–KCl/AC (after reaction).

Fig. 4 shows the catalytic performance of PdCl₂–CuCl₂–KCl/AC catalysts with different preparation method for DMC synthesis from CO and MN. For PdCl₂–CuCl₂–KCl/AC catalyst prepared by excessive impregnation method, STY of DMC reached a relative stable value of around 900 g (L-cat)^-1 h^-1 after 10 h reaction, while STY of DMC on IWI-PdCl₂–CuCl₂–KCl/AC catalyst was around 750 g (L-cat)^-1 h^-1. Meanwhile, for the two catalysts, the selectivity of DMC based on MN and CO was basically the same. It is suggested that the catalytic performance of PdCl₂–CuCl₂–KCl/AC catalyst was superior to that of IWI-PdCl₂–CuCl₂–KCl/AC catalyst. Therefore, PdCl₂–CuCl₂–KCl/AC prepared by excessive impregnation method is more favorable for the synthesis of DMC from CO and MN.

Fig. 4

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Fig. 4.Catalytic performance of PdCl2–CuCl2–KCl/AC catalysts with different preparation method.

Fig. 5 shows the catalytic performance of PdCl₂–CuCl₂–KCl/AC catalysts with different K promoter content, which were 0.3 wt%, 1.0 wt% and 2.0 wt%. From Fig. 5, it is noted that both STY of DMC and the selectivity of DMC based on MN and CO were higher over PdCl₂–CuCl₂–KCl/AC catalysts with different K promoter content than that of PdCl₂–CuCl₂/AC catalyst. The reactivity of PdCl₂– CuCl₂–KCl/AC catalysts increased when the mass content of K promoter increased from 0.3 wt% to 1.0 wt%. Meanwhile, when the mass content of K promoter increased from 1.0 wt% to 2.0 wt%, the catalytic activities of PdCl₂–CuCl₂–KCl/AC catalysts remained nearly unchanged. Therefore, 1.0 wt% was chosen as the optimal K promoter content of PdCl₂–CuCl₂–KCl/AC catalysts for DMC synthesis.

Fig. 5

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Fig. 5.Catalytic performance of PdCl2–CuCl2–KCl/AC catalysts with different K promoter content.

The STY of DMC and selectivity of DMC based on MN and CO over PdCl₂–CuCl₂/AC catalysts with different K promoters ranked in the following order: PdCl₂–CuCl₂–KCl/AC > PdCl₂–CuCl₂/ AC > PdCl₂–CuCl₂–KOH/AC > PdCl₂–CuCl₂–CH₃COOK/AC > PdCl₂ –CuCl₂–K₂CO₃/AC. Especially, the addition of KCl significantly improved the catalytic activities of PdCl₂–CuCl₂/AC catalyst for DMC synthesis from CO and MN. PdCl₂–CuCl₂/AC catalysts with different K promoters were characterized by N₂ adsorption,H2-TPR and XPS techniques. The N₂ adsorption data showed that the textural properties of all the catalysts were similar, while the H₂- TPR results proved that the reduction temperature of Cu²⁺ species for all the catalysts ranked in the order: PdCl₂–CuCl₂–KCl/ AC < PdCl₂–CuCl₂–KOH/AC < PdCl₂–CuCl₂–CH₃COOK/AC < PdCl₂ –CuCl₂–K₂CO₃/AC. The amount of Cu²⁺ in the PdCl₂–CuCl₂–KCl/AC catalyst was higher than that in the PdCl₂–CuCl₂/AC catalyst which was evidenced by the XPS results. The existence of KCl promoted the reducibility of Cu²⁺ species and increased the proportion of Cu²⁺ species on PdCl₂–CuCl₂–KCl/AC catalyst, which further benefited the redox cycle between Pd²⁺/Pd⁰ and Cu⁺/Cu²⁺ pairs and resulted in the better catalytic performance of PdCl₂–CuCl₂– KCl/AC catalyst for DMC synthesis from CO and MN.

Financial support from the Natural Science Foundation of China (Nos. 20936003, 21325626, 21176179), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) (No. 20090032110021) is gratefully acknowledged.