Deep eutectic solvent boosted ruthenium catalysts for acetylene hydrochlorination

Abstract

Despite the potential of Ru-based catalysts to achieve green sustainability in acetylene hydrochlorination, they are plagued by a lack of persistent active sites. Deep eutectic solvents (DESs), considered a novel type of ionic liquid (IL) analogue, can coordinate with metals and adsorb HCl. Hence, to investigate the role of DES in modifying Ru-based catalysts for acetylene hydrochlorination, a range of Ru-DES/AC catalysts were prepared and evaluated for their catalytic performance. The experimental results showed that the formation of DES from a hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) resulted in a more negative electrostatic potential (ESP) minima and stronger electron-donating ability. The interaction of DES with Ru precursors can effectively modulate the microchemical environment around the Ru active site and improve the dispersion of the active components, thereby boosting the activity of Ru-DES/AC catalysts. The addition of DES not only makes the Ru species more stable but also reduces the formation of coke deposition, thus enhancing the stability of the catalyst. Meanwhile, we found that the synergistic effect between HBD and HBA in DES on the performance enhancement of Ru-based catalysts is universal. Therefore, to scientifically design more efficient catalysts, we evaluated the potential descriptors of DES.

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Design, System, Application

In the development of mercury-free catalysts for acetylene hydrochlorination, Ru-based catalysts have certain development potential, but they also suffer from the problem of being unstable and easy deactivation of its active species. In this work, we report a facile and novel strategy to precisely modulate the microchemical environment around the active centre in Ru-based catalysts using deep eutectic solvents (DESs), thereby enhancing their catalytic efficacy. Specifically, DESs facilitate the formation of local structural domains with Ru species through coordination, modulating the electronic structure of the active centre Ru. In addition, the synergistic effect between HBD and HBA in DES is crucial for enhancing the performance of Ru-DES/AC catalysts. By employing a wide range of HBDs and HBAs, we have designed DESs with different hydrogen bond lengths (HBLs) and observed a volcanic relationship between the HBL and conversion of acetylene for Ru-DES/AC catalysts, thus the HBL can be used as a potential descriptor for the design of high-performance Ru-DES/AC catalysts.

1. Introduction

Polyvinyl chloride (PVC) is a versatile resin utilized across various industries. In coal-rich regions, vinyl chloride monomer (VCM) is primarily synthesized through acetylene hydrochlorination. However, this industrial process necessitates the employment of highly toxic and volatile HgCl₂/AC catalysts, which pose a serious threat to the environment and human health.¹ In recent years, numerous researchers have endeavoured to develop highly efficient and alternative catalysts to address this issue. These include precious metal catalysts such as Au,^2–6 Ru,^7–10 Pt,^11–13 and Pd,^14–17 as well as non-precious metal catalysts like Cu,^18–22 Sn,^23–25 and so on. While precious metal catalysts exhibit high activity, limited resources and exorbitant costs hinder their industrial application. Conversely, non-precious metal catalysts are more affordable but exhibit inferior activity, making it challenging to meet industrial demands. It has been demonstrated that RuCl₃ possesses a significantly lower activation barrier compared to AuCl₃ and HgCl₂via density functional theory (DFT) calculations, with a price that is merely one-fourth that of Au.²⁶ Consequently, Ru-based catalysts hold the potential to supplant HgCl₂ catalysts, facilitating a green and sustainable advancement in the acetylene hydrochlorination process.

To address the problems of poor dispersion, serious coke deposition and lack of persistent active sites in traditional Ru/AC catalysts,^5,27 Ru-based catalysts have been modified by adding co-metals,⁷ ligands,²⁸ and ionic liquids (ILs), and carrier modification.^10,13 Among these, researchers constructed local structural domains by ligand coordination with Ru precursors, such as choline chloride,²⁹ thiourea,³⁰N,N′-dimethylpropyleneurea,³¹ N-heterocyclic carbene,³² azole,³³ and amide.²⁸ These localized structural domains enhance the dispersion of active species, facilitate electron transfer between ligands and Ru precursors, and stabilize Ru species, which in turn improves the performance of the catalyst.³⁴ Metals activate acetylene (C₂H₂) by interacting with the C≡C of C₂H₂ in the hydrochlorination of acetylene.³⁵ However, the activated C₂H₂ is prone to undesired side reactions such as polymerization or dehydrogenation without HCl. These by-products accumulate on the catalyst surface, leading to clogging of the catalyst pores and coverage of the active sites, which is an important cause of catalyst deactivation.^13,36 Although the ligand can form local structural domains with the metal precursor by coordination, it cannot adsorb the reactant HCl independently. On the other hand, ILs have a strong adsorption capacity for HCl gases, and they can also be used with metal precursors to modulate the microchemical environment of the active sites through coordination, thereby enhancing the performance of Ru-IL/AC catalysts, such as Ru-15TBAH/AC,¹⁰ Ru-1IL5/AC,³⁷ Ru-15TPPB/AC³⁸ and Ru-10[BMIM]BF₄/AC catalysts.³⁹ ILs are liquid compounds composed of cations and anions linked by electrostatic interaction forces. ILs have shown promise in the development of mercury-free catalysts for the hydrochlorination of acetylene. However, the ‘greenness’ of ILs is often questioned due to their poor biodegradability, biocompatibility and sustainability.⁴⁰

DESs are mainly low eutectic mixtures of HBAs (quaternary ammonium salts or nonionic compounds) and HBDs (compounds such as amides, carboxylic acids, and polyols) assembled through a complex network of hydrogen bonds. DESs are regarded as a new type of IL analogue, possessing many properties of traditional ionic liquids, such as low vapour pressure, high polarity and tunable chemistry. Compared with ILs, DESs are more facile to synthesize, cheaper and greener, demonstrating promising applications across diverse fields.^40,41 In terms of gas capture, DESs have a unique network of powerful hydrogen bonds to trap acidic gases, such as H₂S, CO₂, SO₂, NH₃, and HCl. For example, Hu et al.⁴² selected 1,3-dimethyl-2-imidazolidinone (DMI) as an HBA to form DESs with amide derivatives as HBDs. These DESs have multiple electronegative atoms (e.g., O and N), which can efficiently accommodate HCl and achieve an HCl capacity of up to 2.16 mol mol⁻¹ at 30 °C and 1.0 bar. Meanwhile, the HBDs in this type of DES contain amide groups, which form localised structural domains through coordination with metal precursors. This type of ligand can substantially enhance the performance of metal catalysts in acetylene hydrochlorination.^28,43

Herein, we first selected the DES synthesised by DMI and dimethylurea (DMU) as HBA and HBD, respectively, as an additive to prepare a Ru-[2DMI-DMU]/AC catalyst, aiming to improve the catalytic performance of catalysts in acetylene hydrochlorination. We investigated the synergistic interaction between DMI and DMU in 2DMI-DMU and the regulatory mechanism of 2DMI-DMU on the active centre Ru species. We selected tetramethylurea (TMU), which has a similar molecular structure to DMU, as the HBA and methylurea (MU), urea (U) and caprolactam (CLAA) as HBDs to form seven additional DESs, which were used to investigate the effect of the synergistic effect of HBD and HBA in the DES on the Ru-DES/AC catalyst performance (Scheme 1). Potential descriptors were evaluated to better guide the selection of DES. This approach opens a new avenue for the development of mercury-free catalysts for acetylene hydrochlorination. This approach opens up a novel route for the development of mercury-free catalysts for acetylene hydrochlorination.

Scheme 1 Schematic diagram of DES modulation strategy on Ru-DES/AC catalysts.

2. Experiment

2.1. Materials

Activated carbon (AC, 20–40 mesh) was purchased from Fujian Sensen Carbon Technology Co, Ltd. Ruthenium trichloride hydrate (RuCl₃·H₂O, Ru content: 37%) was purchased from Yurui (Shanghai) Chemical Co, Ltd. 1,3-Dimethyl-2-imidazolidinone (DMI, 98%), tetramethylurea (TMU, 98%), dimethylurea (DMU, 98%), methylurea (MU, 98%), urea (U, 99.7%), and caprolactam (CLAA, 99%) were purchased from Tianjin Hynsopod Technology Co, Ltd. Hydrochloric acid and ethanol (EtOH, 99.7%) were purchased in Tianjin Jiangtian Chemical Co, Ltd. Nitrogen (N₂, 99.999%) and acetylene (C₂H₂, 99.99%) were purchased from Dongrun Gas Sales (Tianjin) Co, Ltd. Gaseous hydrogen chloride (HCl, 99.9%) was purchased from Tianjin Shengtang Gas Co, Ltd. All materials and reagents were used directly in the experiment without further purification.

2.2. DES preparation

In a round-bottom flask, 5.2 g (45.0 mol) of DMI and 2.0 g (22.5 mmol) of DMU were added sequentially and stirred vigorously at 50 °C until a homogeneous clear liquid was formed, followed by drying treatment in a vacuum at 70 °C for 24 h. The synthesised DES was named 2DMI-DMU, with a molar ratio of DMI–DMU of 2 : 1. As shown in Fig. S1,† the HBAs used for the synthesis of DESs were DMI and TMU, and the HBDs were DMU, MU, U, and CLAA. 2DMI-MU, 2DMI-U, 2DMI-CLAA, 2TMU-DMU, 2TMU-MU, 2TMU-U, and 2TMU-CLAA were sequentially prepared in the above procedure with the HBA–HBD molar ratio of 2 : 1. In addition, 0.5DMI–DMU, DMI–DMU and 4DMI–DMU were sequentially prepared by changing the molar ratio of DMI and DMU to 0.5 : 1, 1 : 1 and 4 : 1.

2.3. Ru-DES/AC catalysts preparation

In a beaker, AC (100 g) was immersed in 500 mL of hydrochloric acid solution (HCl = 1 mol L⁻¹) for 6 h at 60 °C. Then, HCl was removed from AC by rinsing with deionised water and then dried at 150 °C for 24 h to obtain the AC to be used. In a beaker, DES (0.2 g) was added to EtOH (8 mL) and stirred thoroughly, then an ethanol solution of Ru prepared by mixing 0.054 g of RuCl₃·H₂O and 1 mL of ethanol was added to the above solution and stirred for 1 hour. After the addition of 2.0 g AC, the beaker was sealed and immersed in a water bath at 60 °C for 6 h. The solvent was slowly evaporated by zapping the holes until no liquid was visible and then dried at 120 °C for 12 h. The catalyst obtained was named Ru-10[DES]/AC, where “10” indicates that the DES was added at 10 wt% of AC. Following the above procedure, Ru-10[DMI]/AC, Ru-10[DMU]/AC and Ru/AC catalysts were prepared. The Ru content in all prepared catalysts was 1% by mass of AC.

3. Results and discussion

3.1. Preparation and characterisation of 2DMI-DMU

DESs are mainly formed by hydrogen bonds, while 2DMI-DMU is prepared by mixing DMI and DMU by simple heating. To ascertain the successful preparation of 2DMI-DMU, the FT-IR spectra of DMI, DMU and 2DMI-DMU were compared (Fig. 1a). In the FT-IR spectrum of DMI, C–H stretching vibration exhibited discernible absorbance at 2939.27 cm⁻¹ and 2862.98 cm⁻¹. C=O carbonyl stretching vibration was observed at 1701.09 cm⁻¹. The characteristic peaks of C=O and N–H sec-amine stretching vibrations in the FT-IR spectrum of DMU were at 1624.88 cm⁻¹ and 3342.82 cm⁻¹, respectively. In comparison to DMI and DMU, C=O (1694.16 cm⁻¹) and N–H (3361.30 cm⁻¹) stretching vibrations appeared shifted as well as broadened in 2DMI-DMU, which can be attributed to the formation of hydrogen bonds between N–H on DMU and C=O on DMI (NH⋯O, Fig. S2†).^44,45 This indicates that 2DMI-DMU was successfully prepared. The FT-IR spectra of yDMI-DMU with varying molar ratios of DMI/DMU were also analyzed, revealing similar changes in both the N–H and C=O characteristic peaks of yDMI-DMU post-DES formation (Fig. S3†), suggesting successful preparation. In addition, the ¹H NMR spectrum of DMU showed the presence of an N–H signal at δ4.964 (Fig. 1b).^46,47 However, for 2DMI-DMU, the N–H signal shifted to δ4.788, further indicating the formation of hydrogen bonds in this DES. In summary, based on the aforementioned analyses, it can be concluded that DESs are formed between DMI and DMU through hydrogen bonds.

Fig. 1 FT-IR spectra (a) and ¹H-NMR spectra (b) of DMI, DMU and 2DMI-DMU.

3.2. Preparation and characterisation of Ru-based catalysts

The Ru-10[2DMI-DMU]/AC catalyst was prepared by combining 2DMI-DMU with the Ru precursor in a specific proportion and dissolving it in an ethanol solution. Subsequently, the Ru-[2DMI-DMU] complex was loaded onto AC using the impregnation technique. To investigate whether there is an interaction between the Ru precursor and DESs, we carried out UV-vis spectroscopy on ethanol solutions of RuCl₃, Ru-[2DMI-DMU] and [2DMI-DMU]. As shown in Fig. 2a, the solution of RuCl₃ showed the absorption peak of Ru³⁺ at 349 nm, whereas that of Ru-[2DMI-DMU] exhibited a blue-shift of the absorption peak of Ru³⁺ near 340 nm, indicating a direct interaction between the Ru precursor and DESs. In addition, another absorption peak of Ru-[2DMI-DMU] appeared at 397 nm, which might be due to the charge transfer, i.e., n–π* electron transitions, between Cl⁻ and DESs.⁴⁸ Also, there was an interaction between the Ru precursor and DMI or DMU, and the characteristic peak was observed only near 206 nm in the UV-vis spectra of DMI, DMU and [2DMI-DMU] solutions (Fig. S4†). As shown in Fig. 2(b), compared with [2DMI-DMU] and DMU, there is no obvious change in the individual characteristic bands of [2DMI-DMU] in the FT-IR spectrum of Ru-[2DMI-DMU], indicating that the hydrogen bond of Ru-[2DMI-DMU] are still present and stable after the interaction of [2DMI-DMU] with the Ru precursor. The catalytic performance of the modified Ru-based catalysts was evaluated for acetylene hydrochlorination (Fig. 2c). The C₂H₂ conversion of the Ru-10[2DMI-DMU]/AC catalyst was consistently maintained at more than 98.5% during 20 h of continuous reaction. In stark contrast, the initial C₂H₂ conversion of the benchmark Ru/AC catalyst was only 70.0%, which decreased to 63.9% after 20 h. The addition of 2DMI-DMU increased the activity of the Ru-10[2DMI-DMU]/AC catalyst by 34.6% and effectively prevented its deactivation. It should be noted that the DMI and DMU monomers also interact with the Ru precursor, resulting in a higher C₂H₂ conversion for Ru-10[DMU]/AC (94.6%) compared to Ru-10[DMI]/AC (84.5%) after 20 h. While the Ru-10[DMU]/AC and Ru-10[DMI]/AC catalysts showed an increase in acetylene conversion of 30.7% and 20.6%, respectively, compared to that of Ru/AC, this improvement is not as significant as the enhancement achieved through the modification process involving 2DMU-DMI. Furthermore, the modified Ru-based catalysts exhibited improved selectivity to VCM (>99%) in acetylene hydrochlorination (Fig. S5†). In summary, [2DMI-DMU] demonstrates the most favourable effect in improving the performance of Ru-based catalysts.

Fig. 2 (a) UV-vis spectra of Ru, Ru-[2DMI-DMU], Ru-[DMI] and Ru-[DMU]; (b) FT-IR spectra of DMU, 2DMI-DMU and Ru-[2DMI-DMU]; (c) conversion of C₂H₂ for Ru-10[2DMI-DMU]/AC, Ru-10[DMU]/AC, Ru-10[DMI]/AC and Ru/AC catalysts. Reaction conditions: T = 170 °C, GHSV(C₂H₂) = 360 h⁻¹, and V_HCl/V_C2H2 = 1.1; (d) conversion of C₂H₂ for Ru-based catalysts after 20 h.

We further refined the DMI–DMU molar ratio and dosage in the Ru-x[yDMI-DMU]/AC catalysts to optimize its performance (Fig. 3a and b). The DMI–DMU molar ratio in the Ru-10[yDMI-DMU]/AC catalysts had little effect and the optimal ratio was 2 : 1. With the increase of DES dosage, the Ru-x[2DMI-DMU]/AC catalysts showed a volcanic trend in acetylene conversion, and the optimum DES dosage was 10%. The He adsorption–desorption isotherms of the samples all exhibited type I isothermal microporous adsorption behaviour, as shown in Fig. S6,† indicating that the addition of 2DMI-DMU did not alter the nature of the pore texture of the catalysts. In Table S1,† different amounts of 2DMI-DMU affected the structural parameters of the Ru-x[2DMI-DMU]/AC catalysts, including the reduction of the specific surface area (S_BET) and pore volume (V_P), which mainly occurred in the micropores. It was shown that 2DMI-DMU was successfully incorporated into the interior of the pores as a liquid-phase layer to protect the Ru species, but the additional amount should not be too large to lead to clogging of the pores and reduce the mass transfer efficiency. The optimized Ru-10[2DMI-DMU]/AC catalyst also exhibited a good VCM space–time yield compared to the ligand- or IL-modified Ru-based catalysts (Table S2†). Therefore, Ru-DES/AC catalysts are of great research value and industrial application in the hydrochlorination of acetylene, providing new ideas for green and sustainable development.

Fig. 3 (a and b) Optimisation of DMI–DMU molar ratio and DES dosage for Ru-x[yDMI-DMU]/AC catalysts. (c) Lifetime test of Ru-10[2DMI-DMU]/AC catalysts. Reaction conditions: T = 170 °C, (a and b) GHSV(C₂H₂) = 720 h⁻¹, (c) GHSV(C₂H₂) = 180 h⁻¹, and V_HCl/V_C2H2 = 1.1.

3.3. Regulation mechanisms of Ru species by [2DMI-DMU]

Considering that the catalytic activity of the Ru-10[2DMI-DMU]/AC catalyst is higher than that of monomeric Ru-10[DMU] and Ru-10[DMI] catalysts, we speculate that there is some kind of synergistic effect between HBD and HBA in DES, which further enhances the activity of the Ru-10[DES]/AC catalyst. According to the literature,^49,50 it is well-established that the electron transfer between HBD and HBA can be carried out through hydrogen bonds, resulting in a redistribution of electrons within the DESs. We then used Multiwfn to perform molecular ESP analysis of 2DMI-DMU, DMI and DMU, and the ESP near the oxygen atom of DMU in [2DMI-DMU] appeared to be extremely small at −57.90 kcal mol⁻¹, smaller than that of DMI (−47.32 kcal mol⁻¹) and DMU (−45.03 kcal mol⁻¹), as shown in Fig. 4a. After the formation of DES by DMI and DMU, the ESP minima of [2DMI-DMU] were more negative, and thus the electron supply capability was enhanced.²⁸ We carried out DFT calculations on the Mulliken charge distribution of the Ru complexes and showed that DMI, DMU and 2DMI-DMU transferred charges to RuCl₃, and also confirmed the change of the microchemical environment around Ru (Table S3†), with the highest amount of charge transfer to RuCl₃ by 2DMI-DMU. The interaction of [2DMI-DMU] with the Ru precursor modulates the microchemical environment of the active centre, which may be the important reason for the improved activity of the Ru-based catalysts. Consequently, we performed X-ray photoelectron spectroscopy (XPS) tests on the unreacted Ru-based catalysts and mainly analysed the electronic structure information of the different valences of the Ru species. The Ru 3p XPS spectra of the Ru/AC catalyst corresponded to the characteristic peak of Ru⁰, Ru/RuO_y, RuCl₃, RuO₂, and RuO_x species with binding energies of 461.6 eV, 462.2 eV, 463.3 eV, 464.4 eV, and 465.7 eV in Fig. 4b, respectively.^28,38 The relative content of high valence Ru^m+ (m > 0) in the Ru-10[DMI]/AC, Ru-10[DMU]/AC, and Ru-10[2DMI-DMU]/AC catalysts was 87.8%, 88.6%, and 89.8%, respectively, and the binding energy of the Ru³⁺ species was sequentially increased by 463.3 eV, 463.6 eV, and 463.9 eV, suggesting a change in the electronic nature of the active centre (Table S4†). As can be seen from the H₂-TPR curves of the Ru-based catalysts in Fig. 4c, the reduction peaks of the Ru species were at 164 °C and 243 °C in the Ru/AC catalyst. However, the reduction temperatures of high-valence state Ru in the Ru-10[DMI]/AC, Ru-10[DMU]/AC and Ru-10[2DMI-DMU]/AC catalysts increased sequentially to 340 °C, 430 °C and 441 °C, which further suggests that the 2DMI-DMU interacted strongly with the Ru species, stabilised their valence states and prevented their reduction. In conclusion, DMI and DMU in 2DMI-DMU synergistically modulated the microchemical environment of the active centre Ru, which should be the key to the increased activity of the Ru-10[2DMI-DMU]/AC catalyst.

Fig. 4 (a) ESP on the van der Waals (vdW) surfaces of DMI, DMU and 2DMI-DMU; (b) Ru 3p XPS spectra of the unreacted Ru-based catalysts and relative changes in Ru species content; (c) TPR profiles of the Ru-based catalysts and 10[2DMI-DMU]/AC.

To better understand the mechanism of the influence of 2DMI-DMU on the Ru-based catalysts during the reaction process, further investigation was conducted on both unreacted and reacted catalysts, considering that the local structural domains formed by the coordination interaction between 2DMI-DMU and Ru precursors could improve the dispersion of Ru species on the carrier surface. The Ru nanoparticles (NPs) in the Ru/AC catalysts exhibited an average size of ca. 1.82 nm. In contrast, after 2DMI-DMU modification, the Ru NPs in the Ru-10[2DMI-DMU]/AC catalyst exhibited a smaller size (ca. 1.20 nm) and more uniform distribution (Fig. 5a and c). Through the comparison of mapping images for the Ru element in both catalysts, it is evident that the Ru-10[2DMI-DMU]/AC catalyst exhibits a higher degree of Ru species dispersion. The sintering of the active components is unavoidable during the reaction process. The addition of 2DMI-DMU resulted in a milder aggregation effect of Ru NPs in the reacted Ru-10[2DMI-DMU]/AC catalyst (2.08 nm) than that in the Ru/AC catalysts (2.66 nm). In addition, as shown in Fig. 5e and S7,† the Ru⁰ content of the Ru/AC catalyst was 12.3%, which increased to 17.4% after the reaction. Conversely, the Ru⁰ content of the Ru-10[2DMI-DMU]/AC catalyst was 10.2% and only increased to 13.6% (Fig. 5f). This further suggests that the interaction between 2DMI-DMU and Ru precursors inhibits the transfer of the highly valent Ru^m+ (m > 0) active component to the inert Ru species, which is more favourable for the Ru-10[2DMI-DMU]/AC catalyst to maintain high activity (Table S4†). Since 2DMI-DMU can donate electrons to RuCl₃, the adsorption of acetylene can be weakened,⁵¹ and 2DMI-DMU has a high adsorption capacity for HCl, effectively inhibiting the occurrence of polymerization or dehydrogenation of acetylene (Table S5†). Analysis of the unreacted and reacted catalysts via thermogravimetric analysis (TGA) revealed that the coke deposition on the Ru-10[2DMI-DMU]/AC catalyst was only 1.19%, while that on the Ru/AC catalyst was 6.20%. A comparison of the structural parameters of both sets of catalysts showed a decrease in S_BET and V_P values for the reacted catalysts, primarily due to carbon deposition on the catalyst surface during the reaction (Table S6, Fig. S8†). Notably, the S_BET and V_P values of the Ru/AC catalysts declined more obviously after the reaction. This finding is largely consistent with the results of TGA, and the Ru-10[2DMI-DMU]/AC catalyst was able to effectively inhibit the formation of the polymer, thereby enhancing the stability of the catalyst. In summary, from the above analysis, it can be concluded that the addition of 2DMI-DMU increases catalytic active sites, stabilizes Ru species, inhibits coke deposition, and consequently improves the activity of Ru-based catalysts.

Fig. 5 HAADF-TEM images, statistics of the particle size and elemental mapping images of C and Ru of (a and b) Ru/AC and (c and d) Ru-10[2DMI-DMU]/AC catalysts; (e and f) relative changes of Ru species content and (g and h) thermogravimetric curves in Ru/AC and Ru-10[2DMI-DMU]/AC catalysts before and after the reaction.

3.4. The generality and descriptors of DES

To verify the generality of synergistic interactions between HBD and HBA in DES for the performance enhancement of Ru-10[DES]/AC catalysts, we selected different HBDs and HBAs to synthesise seven more DESs by a similar method. Firstly, we performed the FT-IR spectra analysis of DESs with the corresponding HBDs and HBAs (Fig. S9†). The results showed that the N–H and C=O characteristic peaks appeared differently after the formation of DESs, which indicated that a hydrogen bond was formed between HBDs and HBAs, and thus DESs were successfully synthesised. Subsequently, we prepared Ru-10[DES]/AC, Ru-10[HBD]/AC and Ru-10[HBA]/AC catalysts with DESs and monomers and evaluated the catalytic performance. As shown in Fig. 6a and b and S10,† the C₂H₂ conversion of both Ru-10[DES]/AC, Ru-10[HBD]/AC and Ru-10[HBA]/AC catalysts were somewhat improved compared with the Ru/AC catalyst. The C₂H₂ conversion of the Ru-10[DES]/AC catalyst was higher than that of both Ru-10[HBD]/AC and Ru-10[HBA]/AC catalysts (Fig. 6c), implying that synergistic effects also exist between HBDs and HBAs in other DESs. ESP analysis of HBAs, HBDs, and DESs was also conducted using Multiwfn (Fig. 6d and S11†). Compared with the corresponding monomers, the ESP minima of DES exhibited more negative values, except for 2DMI-CLAA. The Mulliken charge distribution of the Ru complex was calculated by DFT, and DES transferred more charges to the Ru precursor compared to HBD (Table S7†). This suggests that DES can better regulate the microchemical environment of the active centre and further improve the catalytic activity of Ru-10[DES]/AC catalysts. Overall, the synergistic effect between HBD and HBA in DES is universal for the enhancement of Ru-based catalyst performance, and DESs are a better choice compared to monomers in metal catalyst modification.

Fig. 6 Conversion of C₂H₂ for (a) Ru-10[DES]/AC and (b) Ru-10[HBD/HBA]/AC catalysts. Reaction conditions: T = 170 °C, GHSV(C₂H₂) = 360 h⁻¹, and V_HCl/V_C2H2 = 1.1; (c) conversion of C₂H₂ for Ru-10[HBD]/AC, Ru-10[DES]/AC and Ru-10[HBA]/AC catalysts after 20 h; (d) ESP on the vdW surfaces of HBD, DES and HBA.

To further optimise the development of high-performance Ru-DES/AC catalysts, the DES screening process was accelerated and a descriptor capable of describing the relationship between DES and catalyst performance was established. As shown in Fig. S12 and S13 and Table S8,† it can be seen that the relative value of the hydrogen bond energy (E_HB) decreases with the increase of the hydrogen bond length (HBL) of DES, i.e. the hydrogen bond strength is weakened. The C₂H₂ conversion of the Ru-10[DES]/AC catalyst showed a volcanic relationship with the HBL and E_HB of DES, respectively (Fig. 7 and S14†). From this, we can introduce that the HBL influences the hydrogen bond strength; too strong or too weak is not favourable to catalyse the reaction. The HBL in DES affects the electron transfer ability between HBD and HBA, which changes the ESP distribution on DES. From the Mulliken charge distribution of the Ru complex (Table S7†), 2DMI-DMU transferred the highest number of charges to the Ru precursor among the eight DESs, and thus the Ru-10[2DMI-DMU]/AC catalyst had the best catalytic performance. However, 2TMU-CLAA with shorter HBL showed low catalytic performance, which may be due to the enhanced interaction between acceptor and donor in DES with too strong hydrogen bonding strength, resulting in too strong interaction between DES and Ru, which is unfavourable for the activation of Ru to the reactants, leading to lower catalytic performance. Conversely, the longer HBL results in a low charge provided by the donor to the acceptor in DES and limited electron transfer from DES to the Ru precursor. 2DMI-CLAA with weak hydrogen bond strength has low catalytic activity. Therefore, we considered the HBL of DES as a descriptor to describe the relationship between DES and catalyst performance. In the future studies, we will thoroughly investigate the mechanism of the effect of DES on the performance of Ru-DES/AC catalysts, mainly from various aspects, such as the electronic effect and the spatial site resistance effect. The local environment of the metal active centre can be effectively altered with scientifically designed DES, leading to the design of more efficient catalysts.

Fig. 7 Correlation of HBL of DESs with conversion of C₂H₂ over Ru-10[DES]/AC catalysts.

4. Conclusions

In summary, DESs were successfully prepared using different HBDs and HBAs, and a range of Ru-10[DES]/AC catalysts were prepared for catalytic performance evaluation. Under the reaction conditions of 170 °C, GHSV(C₂H₂) of 360 h⁻¹ and V_HCl/V_C2H2 = 1.1, the Ru-10[2DMI-DMU]/AC catalyst consistently exhibited C₂H₂ conversion rates exceeding 98.5%, representing a remarkable 34.6% increase over the Ru/AC catalyst. The activity of the Ru-10[DES]/AC catalysts was further improved compared with the Ru-10[HBA]/AC and Ru-10[HBD]/AC catalysts. Through comprehensive analysis involving XPS, TPR, DFT calculations, and other characterization techniques, it can be seen that the electron transfer between HBD and HBA in DESs through hydrogen bonds makes the ESP minima of DES more negative and the electron donating ability is enhanced. The activity of Ru-10[DES]/AC catalysts can be effectively enhanced by the increase in the content of Ru^m+ (m > 0) species and the active component dispersion after the interaction of DESs with the Ru precursor. Additionally, the use of DESs in Ru-DES/AC catalysts can enhance the stability of the catalysts by protecting the active Ru species and inhibiting the occurrence of side reactions during the reaction. Furthermore, it was verified that the synergistic effect between HBD and HBA in DES was prevalent in enhancing the performance of Ru-DES/AC catalysts. The HBL of DESs showed a volcano-type relationship with the C₂H₂ conversion of Ru-10[DES]/AC catalysts, which indicated that the HBL could be used as a descriptor for screening Ru-DES/AC catalysts. It provides a new idea for the development of mercury-free catalysts and advances the industrialisation of green and sustainable production processes for vinyl chloride.

Author contributions

L. Li performed the experimental and characterisation test analysis, data compilation and wrote the manuscript. B. Wang and T. Zhang performed the DFT calculations and data analysis. H. Zhang and W. Li contributed to the experimental design and drafted the results section. J. Wu and J. Zhang secured the project funding and contributed to the experimental design, data interpretation and revision of the manuscript. All authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22068031, 22104054, and 22378308) and the special fund of the Yunnan Precious Metal Laboratory (YPML-2022050237 and YPML-2023050202).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4me00045e

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