Dimethyl maleate (DMM) is an important chemical intermediate^[1]. At present, the synthesis route of dimethyl maleate is mainly maleic anhydride route through esterification with methanol. The industrial production methods of maleic anhydride are benzene oxidation method, butene oxidation method and C4 olefin method^[2−3], but the reaction temperature is above 350 ℃. Meanwhile, the energy crisis motivated us to develop a potential route direct synthesis of dicarboxylic acid ester to replace the route of petroleum. Acetylene alkoxy- carbonylation is a well-known reaction to synthesize α,β-unsaturated carboxylic acids and their derivatives^[4]. Thus, acetylene dicarbonylation has gained increasing attention in academic research.

In 1964, Tsuji et al. ^[5] reported a palladium salt-catalyzed carbonylation system. By using C₂H₂ and CO and catalyzed by PdCl₂, muconyl chloride was a major product, accompanied by a considerable amount of fumaryl and maleyl chloride. As a result, much effort has been devoted to developing an efficient homogeneous Pd based catalytic system such as PdCl₂-thiourea^[6], PdCl₂/HgCl₂^[7], PdCl₂/SnCl₂^[8], Pd(xantphos)Cl₂^[9], PdCl₂/CuCl₂/HCl/O₂^[10], PdCl₂/FeCl₃/ H₂SO₄/O₂^[11], PdBr₂-LiBr^[12], PdI₂/KI^[13], PdCl₂/KI^[14]. However, in homogeneous catalysis system, there are still problems such as difficult catalyst separation, inability to recycle and reuse, and complex product purification and separation process ^[15]. In 2020, Wei et al. ^[16−17] used Pd(nano- sheet)/AC to catalyze the acetylene dicarbonylation reaction, and obtained the yield of 43.8% and the DMF selectivity of 99%. They also used Pd/α-Fe₂O₃ catalyst with highly dispersed Pd clusters to catalyze the acetylene dicarbonylation reaction, obtained about 75% dimethyl fumarate conversion in 80 ℃. The main reason for the increase in reactivity is the SMSI effect of Pd/α-Fe₂O₃ catalyst, where electrons are transferred from Pd nanoclusters to α-Fe₂O₃ support to generate electron-deficient Pd centers, which promotes Pd^δ+↔Pd⁰ cycling and thus improves acetylene dicarbonylation reactivity. In 2021, Zhao et al. ^[18] prepared a series of bimetallic Co/Pd alloy nano-catalysts by one-step reduction method. With acetonitrile as solvent, the total yield was 77.23%. However, the alloy catalyst still has the disadvantages of small specific surface area, active component waste and poor thermal stability.

Herein, we constructed CoO_x-Pd/HAC heterogenous acetylene dicarbonylation catalytic system. As expected, acetylene conversion reached a maximum of 75.4% at 80 ℃, which is three times higher than that of Pd/AC catalyst and the catalyst can be stably recycled 7 times. Moreover, the catalysts surface structure was characterized by TEM, XRD. CO-TPD and H₂-TPR were evaluated the catalysts adsorption and reduction performance. XPS were characterized to the catalysts surface chemical composition.

1 Experimental section 1.1 Chemicals

All chemicals used in the experiments were of analytical grade, purchased from Shanghai Aladdin Industrial Corporation in China, and directly used without further purification. In all reactions, the purity of all gases used in the experiment was 99.999%, and the resistivity of deionized water was 18.25 MΩ∙cm.

1.2 Activated carbon treatment

Firstly, 1.0 g activated carbon (AC) was added into 15 mL HNO₃ (65%~68%), and then stirred at room temperature for 1 h, washed with water and ethanol for three times, dried in an air oven at 70 ℃ for 12 h, the obtained sample was donated as HAC.

1.3 Catalyst preparation

Co/HAC sample were prepared by an impregnation method employing the synthesized HAC as support. Typically, 0.093 g Co(NO₃)₂ was dissolved into 10 mL deionized water, added 1 g activated carbon impregnating for 24 h, dried and put it in muffle furnace, reduced at 400 ℃ under H₂ atmosphere for 8 h, the obtained sample was donated as 3%Co/HAC. As the above method, 1%Co/HAC, 5%Co/HAC, 3%Fe/HAC, 3%Mn/ HAC and 3%Cu/HAC samples were obtained.

Co-Pd/HAC catalysts were prepared by an in-situ growth method employing the synthesized HAC as support. Typically, 1 g 3%Co/HAC was dispersed in a mixture of ethanol (20 mL) and distilled water (3 mL), the suspension was first subjected to sonication treatment for 15 min. Then the [Pd₂(μ-CO)₂Cl₄]²⁻ precursor, synthesized as literature method^[19], was added dropwise into the above slurry gradually under vigorous stirring, and maintained for another 1 h. After centrifuged, the obtained powder was washed with deionized water thoroughly, and then dried at 80 ℃ in air overnight. The samples were named as 3%Co-Pd/HAC, 1%Co-Pd/HAC, 5%Co-Pd/HAC, 3%Fe-Pd/HAC, 3%Mn-Pd/HAC, and 3%Cu-Pd/HAC, respec- tively. The nominal Pd mass content is 3% on the basis of support weight.

1.4 Catalyst activity evaluation

The acetylene dicarbonylation reaction was carried out in a 50 mL stainless steel high-pressure reactor, placed in an oil bath and magnetically stirred at a speed of 900 r∙min⁻¹. In a typical experiment, 50 mg catalyst, 10 mg KI and 20 mL methanol were put into the reactor, then 0.27 g of acetylene gas, 0.6 MPa O₂, and 4.6 MPa CO was passed into the reactor, stirring at 60 ℃ for 8 h. The quantitative analyze was performed on a Gas Chromatography (GC) (Shimadzu GC-2014C) with a FFAP capillary column (50 m × 0.32 mm × 0.50 μm), using methyl benzoate as an internal standard. Gas Chromatography-Mass Spectrometer (GC-MS) was conducted on an Agilent 7890A-5975C instrument equipped with a HP-5 MS capillary column (30 m × 0.25 mm × 0.25 μm) to analysis the product selectivity.

1.5 Catalyst characterization

Smart lab-SE type X-ray polycrystalline powder diffracto-meter was used to determine the XRD spectra of the samples. The instrument is characterized by Cu target Kα (λ=0.154 056 nm) ray, tube voltage 40 kV, tube current 40 mA, scanning range of 5°~80°, scanning speed 0.2(°)·s^–1. H₂-TPR on Tianjin Xianquan Instrument Company TP-5080 automatic multi-purpose adsorption. The sample of 50 mg palladium carbon catalyst was loaded into the quartz tube, and the temperature was raised to 100 ℃ and kept constant temperature for He gas purging and dehydration, and then lowered to room temperature. TPR experiment was carried out under 10%H₂-Ar gas flow rate of 30 mL∙min⁻¹, and the temperature was raised to 800 ℃ at 10 ℃∙min⁻¹. The hydrogen consumption signal was detected by thermal conductivity. NH₃-TPD is also carried out on the aforementioned automatic multi-purpose adsorption. The sample of 50 mg palladium carbon catalyst was also loaded into the quartz tube, which was heated to 100 ℃ and kept at constant temperature for 1 h for He gas purging and dehydrating. After falling to room temperature, the TPD experiment was carried out under 10%NH₃-Ar air flow with a flow rate of 40 mL∙min⁻¹, and the temperature was heated to 800 ℃ at a rate of 10 ℃∙min⁻¹. The hydrogen consumption signal was detected by thermal conductivity. X-ray photo-electron spectra were measured by ESCALABXI+ multi-functional photoelectron spectrometer.

2 Results and discussion

Fig. 1(a) shows the XRD patterns of AC, HAC, and Co-Pd/HAC samples, it can be noted that several derivative peaks in HAC were disappeared after treatment with nitric acid, which indicated that crystalline carbons into amorphous carbons or impurities were dissolved. After loading Pd, weak diffraction peaks at 40.1°, 46.1° and 68.1° appeared, which were consistent with the (111), (200) and (220) crystal faces of Pd fcc structure (JCPDS No.46-1043). Moreover, after loading Co, the new diffraction peaks at 36.5°, 42.4°, 61.5° and 20.8°, 42.5° were corresponded to the CoO (JCPDS No.43-1004) and CoO₂ (JCPDS No.89-8399). NaBH₄ solution was used to reduce the CoO_x, XRD patterns showed that the two new diffraction peaks of 23.7° and 50.8° generated Co₂B, so that the catalyst 3%Co-Pd/HAC lost the catalytic activity.

Fig. 1(b)−(c) show the TEM and HRTEM of 3%Co-Pd/ HAC. The TEM image showed that the Pd nanoparticles and CoO_x particles were evenly dispersed on the surface of HAC support. The morphology of catalyst in dark field can be further observed by STEM-HAADF (Fig. 2(a)). Fig. 2(b)−(f)shows the distribution of each element in the catalyst clearly.

The X-ray photoelectron spectroscopy characterization shown in Fig. 3, and it can be noted that the binding energies of Pd 3d of 3%Co-Pd/HAC and 5%Co-Pd/HAC catalysts were shifted toward 0.55 eV higher binding energies than Pd/HAC. As shown in Fig. 3(a), the peaks with binding energies at 780.0 and 795.5 eV belong to Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, respectively, and the peaks at 779.5 and 794.9 eV correspond to Co³⁺ 2p_3/2 and Co³⁺ 2p_1/2, the peaks at 782.5 and 798.0 eV correspond to Co⁴⁺ 2p_3/2 and Co⁴⁺ 2p_1/2. The peaks at around 786.5 and 803.0 eV could be ascribed to the satellite peaks^[20−21]. The Pd 3d XPS spectra of the Co-Pd/HAC is shown in Fig. 3(b), the peaks with binding energies around 334.9 and 340.2 eV belong to Pd⁰ 3d_5/2 and Pd⁰ 3d_3/2, respectively, and the peaks with binding energies around 336.3 and 341.8 eV belong to Pd²⁺ 3d_5/2 and Pd²⁺ 3d_3/2, the peaks with binding energies around 337.6 and 343.5 eV belong to Pd⁴⁺ 3d_5/2 and Pd⁴⁺ 3d_3/2^[22]. The incorporation of CoO_x generated more Pd⁴⁺ and Pd²⁺ species, and the electron deficient Pd species could increase the CO adsorption capacity and improve the reaction activity of the catalyst.

Fig. 4 shows the H₂-TPR spectra of palladium-carbon catalyst and palladium-carbon- cobalt catalyst. After nitric acid treatment, the increase of hydrogen consumption from 400 ℃ is attributed to the increase of oxygen active groups in activated carbon. It can be seen from the spectrum that at around 100 ℃, containing Pd catalysts appear to peaks attributes to the reduction of PdO. After introducing of the additive Co, the oxygen reduction temperature of the in-situ supported palladium nanoparticles on the surface of the palladium carbon catalyst decreases to different degrees at about 300 ℃, which may be because H₂ dissociates into hydrogen on the surface of the Pd nanoparticles^[23] and migrates from the surface of the Pd nanoparticles to the surface of the metal Co element. The oxygen active species on the surface of activated carbon were further reduced to form the hydrogen spillover effect, so that the catalyst showed high activity. 400 and 550 ℃ are the reduction peaks of acidic groups and oxygen-containing groups of activated carbon, respectively, while the new peak formed at about 640 ℃ may be the reduction of CoO_x under the action of hydrogen flow. From previous study^[24], we indicated CoO_x → Co₃O₄ (450 ℃), Co₃O₄ → CoO (600 ℃), CoO → Co (730 ℃).

According to the Fig. 5 CO-TPD test results, CoO_x reduces the desorption temperature of carbon monoxide clearly, and the absolute area of the image is integrated from 400 to 700 ℃ , it can be seen that the surface of the CoO_x catalysts have a higher adsorption capacity of carbon monoxide than the Pd/HAC, indicating that the additive variable valence metals were also the active center to adsorb CO. Moreover, the adsorption of CO could be contributed to the acetylene di- carbonylation.

Acetylene dicarbonylation was selected as a model reaction, which can be obtained the product dimethyl maleate (DMM) and dimethyl fumarate (DMF). The reaction was conducted under the following conditions: 80 ℃, 4.6 MPa CO and 0.6 MPa O₂, 20 mL methanol and using KI as a promoter, methanol as both a solvent and a nucleophile reagent. Table 1 shows the catalytic performance of different catalysts for acetylene dicarbonylation. It can be seen that Pd/HAC with acetylene conversion 48.6%, which is two times higher than that of Pd/AC (24.0%). After introducing other metal oxides (Co, Fe, Mn, and Cu) into Pd/HAC catalyst, the acetylene conversion furtherly increased. Among them, Co-Pd/HAC catalyst has better catalytic activity and selectivity of dimethyl maleate. Moreover, the effect of the Co amount on the acetylene dicarbonylation activity was investigated. It can be noted 3%Co presented the best catalytic activity and selectivity of dimethyl maleate.

The influence of temperature, time, and gas partial pressure on Co-Pd/HAC catalyst was investigated. As shown in Fig. 6(a), the acetylene conversion increased from 46.5% to 75.4% as the temperature increased from 40 to 80 ℃ and the selectivity of dimethyl maleate is always about 85%. In Fig. 6(b), the acetylene dicarbonylation reaction was complete at about 5 h, and the acetylene conversion rate increased with the progress of the reaction, which may be related to the active site saturation of palladium nanoparticles. In Fig. 6(c), adjusting CO pressure and fixing oxygen pressure, when the total reaction pressure is lower than 5.2 MPa, the conversion increases with the increase of pressure, and then the reaction activity decreases. This may be caused by the reduction of O₂ pressure leading to the weakening of the reaction cycle, because acetylene dicarbonylation is a double cycle process that consumes oxygen, and the reduction of oxygen partial pressure leads to the slowing of the Pd^δ+↔Pd⁰ cycle ^[25] and the oxidation cycle of the promoter KI, resulting in the decrease of catalytic activity. Then to evaluate the catalyst recycle stability, the catalytic conditions of 50 mg Co-Pd/HAC,10 mg KI catalyst, 20 mL methanol, 4.6 MPa CO, 0.6 MPa O₂, 6 h under 80 ℃ reaction temperature were adopted. The catalyst after use is washed by deionized water and anhydrous ethanol, dried overnight in an air atmosphere, weighed, and the lost catalyst is supplemented by the side reactor, and then the dicarbonylation reaction continues. The reaction results are shown in Fig. 7(a). It can be found that after several cycles of use of palladium carbon catalyst, acetylene conversion and selectivity decreased to varying degrees, which may be due to the loss of catalyst activity. Some Pd species could no longer complete the reaction cycle, and the particle size of palladium nanoparticles did not change significantly before and after the reaction. However, partial agglomeration of palladium nanoparticles compared with that before the reaction may also be the cause of partial deactivation. Fig. 7(b)−(c) shows the TEM and HRTEM images of the catalyst after the reaction.

3 Conclusion

A series of CoO_x-Pd/HAC catalysts were prepared by [Pd₂(μ-CO)₂Cl₄]²⁻ in situ formation of Pd nanoparticles by equivalent impregnation method. It was found that the 3%Co-Pd/HAC catalyst has the best activity in the acetylene dicarbonylation (acetylene conversion 75.4%, dimethyl maleate selectivity 86.2%). Moreover, it was found that the addition of an appropriate amount of metal cobalt oxide could drive electron migration on the surface of palladium nanoparticles, generate electron-deficient Pd species active centers, and drive the cycle of dicarbonylation reaction to step Pd^δ+↔Pd⁰. CoO_x catalysts have a higher adsorption capacity of carbon monoxide than the Pd/HAC, indicating that the additive variable valence metal oxide were also the active center to adsorption CO for the acetylene dicarbonylation. It has certain research significance to produce cis-product dimethyl maleate and trans-product dimethyl fumarate directly.