Identifying the catalytic active site of durable Ru-based liquid-phase catalyst for acetylene hydrochlorination
Linfeng Li , Bao Wang , Tiantong Zhang , Xinyuan Wang , Dingqiang Feng , Wei Li , Jiangjiexing Wu , Jinli Zhang
To meet the compliance requirements of the Minamata Convention, the development of environmentally friendly mercury-free catalyst systems has become essential for achieving sustainable development in the polyvinyl chloride (PVC) industry [1]. As a core step in PVC production, the green and efficient catalysis of the acetylene hydrochlorination reaction is fundamental to ensuring both the economic viability and environmental safety of the industrial process [2]. In recent years, researchers have developed various solid-phase mercury-free catalysts, such as Au, Pd, Ru, Pt, and Cu, and have improved their performance by adding ligands or ionic liquids [3–7]. Characterisation analysis combined with theoretical calculations has demonstrated that these modifications can enhance the dispersion of active components, regulate the electronic structure and steric hindrance of active sites, and thereby improve the activity and stability of catalysts [8–11]. However, acetylene hydrochlorination is a highly exothermic reaction (ΔH = −124.8 kJ/mol), and the gas-solid phase system suffers from low heat transfer efficiency. This often leads to local heat accumulation in the catalyst bed during the reaction, which can damage active species and promote carbon deposition [12,13].
In contrast, liquid-phase catalytic systems offer significant technical advantages due to their super heat transfer properties. In recent years, progress has been made in liquid-phase catalytic systems based on transition metals such as Pd, Au, Ru, and Cu [12,14,15]. Researchers have constructed composite media systems by dissolving active components in ionic liquids (such as 1‑butyl‑3-methylimidazolium chloride and tetrabutylphosphonium stearate) or organic solvents (such as tetradecane and tetramethylene sulfone (TMS)), achieving initial C2H2 conversion rates exceeding 95% with Pd/Ru-based catalysts achieved [16–18]. From a cost perspective, Ru, being relatively cheaper than Pd and Au, has great potential for industrial applications. Currently, research on liquid-phase catalysts primarily focuses on the screening of metals and liquid media, with a lack of in-depth understanding of the active sites of the catalysts. In solid-phase catalysis, active sites are typically located on the surface or interface of the catalyst and can be easily observed using techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy [19,20]. However, in liquid-phase systems, active components are uniformly dispersed in the solution, and their activities originate from molecular or ionic species in the liquid, making it challenging to directly identify them using the above characterisation techniques as solid catalysts [21]. Furthermore, in liquid-phase reactions, active species, solvents, substrates, and products coexist in the same phase, and highly efficient liquid-phase catalysts often exhibit excellent performance at extremely low concentrations, further complicating the resolution of specific structural information [22]. Therefore, a thorough understanding of the structure of active sites in liquid-phase catalysts is crucial for advancing the study of liquid-phase acetylene hydrochlorination.
In this study, a Ru-based liquid-phase catalyst (Ru-DIPEA/TMS) was developed and optimised for liquid-phase acetylene hydrochlorination. The organic solvent TMS was used as the liquid medium and the specific ligand diisopropyl formamide (DIPEA) was introduced to optimise the catalyst composition and reaction conditions to improve both the activity and stability of the catalyst. Through a series of characterisation techniques, including UV–vis spectroscopy, FT-IR spectroscopy, XPS, single-crystal XRD and X-ray absorption spectroscopy (XAS), combined with density functional theory (DFT) calculations, we investigated the actual structure of the active sites and their transformation mechanisms under the synergistic effects of ligands and HCl. This study provides an in-depth analysis of the structural characteristics of catalytic active sites through precise characterisation, offering robust theoretical support for the design and optimisation of liquid-phase catalytic systems.
The optimal ratios of active components for the xRu-yDIPEA/TMS catalysts were systematically investigated by precisely adjusting the molar concentration ratio of RuCl3·H2O to the DIPEA ligand in the TMS solvent under a nitrogen-protected atmosphere (Fig. S2 in Supporting information). When the fixed Ru concentration was 0.008 mol/L, the C2H2 conversion showed an increasing and then decreasing trend (92.3% → 92.5% → 93.8% → 91.3% → 89.3%) with the increase of DIPEA concentration from 0.2 mol/L to 1.6 mol/L, and the optimum DIPEA concentration was determined to be 0.6 mol/L. Further investigation of the effect of Ru content revealed that the catalyst activity increased in a gradient (89.1% → 93.8% → 94.7%→ 95.5% → 95.8%) when the Ru concentration was increased from 0.006 mol/L to 0.014 mol/L, but the increase in activity tended to slow down (Δη < 0.3%) after > 0.012 mol/L. Based on this, the optimal composition of the Ru-DIPEA/TMS catalyst was finally determined to be 0.012 mol/L Ru with 0.6 mol/L DIPEA. Comparative experiments (Fig. 1a and Fig. S3 in Supporting information) showed that the initial activity of the unmodified Ru/TMS catalyst was only 40.8%, and the deactivation rate after 24 h was 20.1% (32.6%). After the introduction of the amide ligand, the activity and stability of the Ru-Amide/TMS series catalysts were significantly enhanced and the C2H2 conversions were, in order, 49.3% (DMBA), 52.8% (DMPA), 66.3% (DMAC), 69.5% (DMF), 85.1% (DEF), 86.6% (DPF), 95.5% (DIPEA), 81.6% (DBF). Notably, the catalytic activity of DIPEA-modified catalysts was enhanced by 63.2% compared to Ru/TMS, and DIPEA/TMS alone had no catalytic activity (Fig. S4 in Supporting information), confirming that the co-action of Ru species with DIPEA ligands is the key to achieving high performance of Ru-DIPEA/TMS catalysts.
Subsequently, the effect of reaction conditions on the performance of the Ru-DIPEA/TMS catalysts was systematically investigated. As shown in Fig. S5 (Supporting information), the C2H2 conversion increased linearly from 91.9% to 96.1% when V(HCl):V(C2H2) was increased from 1.05 to 1.20, but the increase tended to slow down (Δη < 1.1%) in the interval 1.10–1.20. The reaction temperature and GHSV(C2H2) experiments (Figs. 1b and c) showed that the C2H2 conversion increased exponentially with increasing temperature in the range of 120–160 ℃ (50.2% → 95.8%). When GHSV(C2H2) was decreased from 90 h−1 to 30 h−1, the C2H2 conversion increased significantly from 90.8% to 99.0%. T = 150 ℃, GHSV(C2H2) = 50 h−1, and V(HCl):V(C2H2) = 1.15 were chosen as the optimum reaction conditions for energy consumption and efficiency considerations. In a continuous 900-h long test under the optimised conditions (Fig. 1d), the Ru-DIPEA/TMS catalyst showed excellent stability: the C2H2 conversion was stable at 95.5% after 24 h of reaction and remained above 91.1% at the end of the test, and the VCM selectivity was always > 99.9%. As shown in Table S1 (Supporting information) [14–16,18,23], the present catalysts exhibited excellent catalytic activity and stability at lower reaction temperatures (150 ℃ vs. 160–180 ℃) compared to the liquid-phase catalytic systems reported in the literature, with significant potential for industrial applications.
Encouraged by the above results, the synergistic mechanism of DIPEA and HCl activation on the active site structure and catalytic performance of Ru-DIPEA/TMS catalysts was systematically revealed through a series of experiments combined with multiscale characterisation. As shown in Fig. 2a, UV–vis spectra analysis revealed that TMS and DIPEA exhibited characteristic absorption peaks in the 200–250 nm range, but there was no obvious signal in the > 300 nm interval. Compared to the typical Ru3+ characteristic peak at 418 nm for Ru/TMS [24,25], the introduction of the DIPEA ligand resulted in a significant blue shift of the Ru3+ characteristic peaks of Ru-DIPEA/TMS (Δλ = 31 nm, 387 nm), which was attributed to the ligand effect between the Ru precursor and DIPEA. FT-IR spectra analysis further revealed that the sharp peak of DIPEA at 1661 cm−1 corresponded to the stretching vibration of the carbonyl group (νC=O) (Fig. 2c). In Ru-DIPEA/TMS, νC=O splits into two characteristic peaks (1661 and 1611 cm−1), and the latter shows a blue shift of 50 cm−1 [26]. This may be due to the blue shift of the characteristic νC=O peak caused by the interaction of some carbonyl groups of DIPEA with the ruthenium precursor. Taken together, the above spectra evidence confirms that DIPEA may in turn optimise its catalytic performance through the coordination of the carbonyl group with Ru3+.
It has been reported that the introduction of ligands not only forms coordination interactions with metal precursors, but also significantly enhances the adsorption of HCl, thus effectively improving the activity and stability of catalysts. For example, ligands containing the heteroatom N can react with HCl to form the corresponding hydrochloride salts (e.g., N-methyl pyrrolidone, NMP) [23,27]. Inspired by these findings, we systematically investigated the effect of HCl activation on the catalytic performance of Ru-DIPEA/TMS. As shown in Fig. 2b and Fig. S6 (Supporting information), the initial C2H2 conversion of Ru-DIPEA/TMS exhibited a monotonically increasing trend when the HCl activation time was increased from 0 to 12 h. When the HCl activation time was 12 h, the initial C2H2 conversion of the catalyst reached a maximum value of 96.2% and the induction period of the catalyst completely disappeared. Notably, when the catalyst was activated with C2H2 for 1 h, the induction period was not shortened, and the catalytic activity decreased to 89.5%. This phenomenon suggests that the HCl activation process triggered the reconfiguration of the active site. To elucidate the intrinsic mechanism, Ru-DIPEA/TMS-HCl was characterised in detail after HCl activation. The FT-IR spectra (Fig. 2c) showed a significant attenuation of the original νC=O signal intensity at 1611 cm−1, while new characteristic peaks attributed to νRu–CO appeared at 2045 cm−1 and 1969 cm−1 in Ru-DIPEA/TMS-HCl [28], indicating a shift in the coordination environment of the Ru active centre from Ru–O to Ru–CO configuration. XPS analysis further revealed (Fig. 2d) that the Ru 3p3/2 binding energy of Ru-DIPEA-HCl was negatively shifted by 0.7 eV (463.5 → 462.8 eV) compared to the Ru-DIPEA, suggesting that HCl activation contributed to an increase in the electron cloud density of the Ru centre [29]. The above characterisation analyses indicate that the charge distribution around the Ru active site and the coordination environment in the Ru-DIPEA/TMS catalysts were significantly altered in the presence of HCl.
To analyze the real structure of the active site Ru in the Ru-DIPEA/TMS-HCl catalyst after HCl activation, two characteristic crystals of the Ru-DIPEA/TMS-HCl catalyst were successfully obtained using the solvent slow evaporation method (white crystals: ligand derivatives; red crystals: active centre complexes) [30]. Fig. 3a shows the crystal structure of the white crystals, which were analysed using single-crystal XRD to determine the structural parameters detailed in Table S2 (Supporting information). Diisopropylamine hydrochloride (C6H15N·HCl) was produced by the protonation reaction of DIPEA during the activation of HCl (Table S3 in Supporting information). This transformation was further confirmed by FT-IR spectroscopy and NMR spectroscopy (Figs. S7 and S8 in Supporting information). In the FT-IR spectra of C6H15N·HCl, the characteristic νC=O peak disappeared, and a sharp absorption peak appeared at 2718.17 cm−1, corresponding to the characteristic peak of the secondary amine salt (NH2+) [31]. On the other hand, Fig. 3b shows the crystal structure of the red crystal, in which the Ru ion is located in the centre of the crystal and shows a six-coordinated configuration. The Ru ion was bound to two carbonyl (CO) carbon atoms with a Ru–C bond length of approximately 1.855 Å and a C≡O bond length of approximately 1.130 Å, and the Ru ion was also bonded to two independent chloride ions (Cl−) with a Ru–Cl bond length of approximately 2.406 Å. Additionally, the Ru ion was also bonded to two Cl− ions in C6H15N·HCl with a Ru–Cl bond length of approximately 2.449 Å (Table S4 in Supporting information). Based on these structural features, the valence state of Ru is inferred to be +2, and its molecular structure is Ru(CO)2Cl2(C6H15N·HCl)2, which conforms to the typical octahedral coordination configuration based on its bond angle data (Table S5 in Supporting information).
Synchrotron XAS analysis was further carried out to verify the valence and coordination environment of the Ru active centre in the Ru-DIPEA/TMS catalyst. As shown in Fig. 3c, the Ru K-edge XANES spectra reveal the absorption threshold of the catalyst, which is situated between the Ru foil and RuCl3. This observation indicates that the Ru in the catalyst possesses a positive charge and an average valence state between 0 and +3 [32–34]. This finding agrees with the results obtained from the single-crystal XRD analysis. The EXAFS oscillations on the Ru K side were analysed by Fourier transform (FT), and Fig. 3d shows the FT-EXAFS function of the catalysts concerning the sample. The peak at 2.33 Å for Ru foil corresponds to the Ru–Ru bond, the peak at 1.84 Å for RuCl3 corresponds to the Ru–Cl bond, and the peak at 1.50 Å for RuO2 corresponds to the Ru–O bond in the first shell layer. The R-space and k-space fitting curves obtained after fitting the EXAFS data for Ru foil, RuCl3, RuO2, and Ru-DIPEA/TMS-HCl are shown in Figs. 3e and f, and Fig. S9 (Supporting information). The peaks of Ru-DIPEA/TMS-HCl at 1.59 and 2.02 Å correspond to the Ru–C and Ru–Cl coordination in the first shell layer, respectively. The peak at 2.52 Å is 0.19 Å longer than the Ru-Ru (2.33 Å) peak of Ru foil, which indicates that the Ru-Ru bond does not exist in the catalyst. Combined with the above single-crystal XRD results, the 2.52 Å peak can be attributed to the Ru-O path in the second shell layer Ru–C≡O. The fitting results indicate (Table S6 in Supporting information) that in Ru-DIPEA/TMS-HCl, the Ru–C and Ru–Cl bond lengths are 1.895 ± 0.041 Å and 2.421 ± 0.025 Å, respectively, with coordination numbers of 2.4 and 3.6. Thus, the first coordination shell of the Ru atom is determined to be 6, considering the error margins. The local coordination structure of Ru in the catalyst was ultimately determined to be [Ru(CO)2Cl4]2− core stabilized by two [C6H15N]+ units through a hydrogen bond network, perfectly corroborating the single-crystal XRD results.
The Ru-DIPEA/TMS catalysts were sequentially tested by UV–vis spectroscopy at the initial stage (Initial) and at HCl activation times of 0.5–12 h (HCl, x h) to monitor the dynamic evolution of the Ru-DIPEA/TMS catalysts during the HCl activation process (Fig. 4a). At the initial stage, the catalyst showed a characteristic Ru3+ absorption peak at 387 nm. After 0.5 h of HCl activation, the peak shifted blue to 294 nm (Δλ = 93 nm). As the activation time extended to 12 h, the characteristic peak gradually shifted red to 371 nm, and a distinct shoulder peak appeared at 499 nm, corresponding to metal-Ru-to-ligand charge transfer [5]. The above UV–vis spectroscopy analysis revealed that the coordination environment of the active Ru centre underwent significant changes upon HCl activation.
To reveal the energy evolution of the process, DFT calculations were performed to determine the formation energies of key intermediates. As shown in Fig. 4b, the initial coordination process of RuCl3(DIPEA)3 releases −98.59 kcal/mol energy. However, when the fourth ligand is introduced to form RuCl2(DIPEA)4, the formation energy increases sharply to +38.83 kcal/mol due to the spatial resistance effect. HCl activation triggers ligand dissociation, protonation of DIPEA to form C6H15N·HCl, CO release, and the formation of RuCl2(C6H15N·HCl)4, which releases −49.40 kcal/mol of energy. Finally, CO competes with Cl− for coordination, forming the more stable Ru(CO)2Cl2(C6H15N·HCl)2 (−30.84 kcal/mol) [35], which is in agreement with the UV–vis spectroscopic observations. The electron transfer mechanism was further revealed by Mulliken charge analysis, which showed that the charge of the Ru active centre decreased from 0.857 eV to 0.376 eV under the synergistic effect of the ligand and HCl. These results indicated that the ligand transfers electrons to the central Ru (Table S7 in Supporting information), thereby altering the electronic structure around the active Ru centre. Using UV–vis spectroscopy and DFT calculations, we confirm that the Ru active centre gains electrons and tends to form a more stable coordination structure during HCl activation. This process is a key factor in enhancing the stability and activity of the catalyst.
In summary, the composition and reaction conditions of the Ru-DIPEA/TMS catalysts were systematically optimised, with the optimal ratios determined to be 0.012 mol/L for Ru and 0.6 mol/L for DIPEA. The optimal reaction conditions were determined as T = 150 ℃, GHSV(C2H2) = 50 h−1, and V(HCl)/V(C2H2) = 1.15. Under the above conditions, the catalyst exhibited excellent C2H2 conversion (95.5%), stability, and VCM selectivity (99.9%) in tests of up to 900 h, showing significant potential for industrial applications. Characterisation techniques such as UV–vis and FT-IR spectra revealed substantial coordination between the Ru precursor and the DIPEA ligand in the Ru-DIPEA/TMS catalyst. After HCl activation, the catalyst’s induction period was completely eliminated, and the characteristic peak of νRu–CO appeared in the FT-IR spectra, indicating significant changes in the active sites of the catalyst. To further investigate the actual structure of the Ru active site and its evolution after HCl activation, single-crystal XRD analysis was employed. The results showed that under HCl activation, DIPEA decomposed to form C6H15N·HCl, which ultimately gave rise to a six-coordinated Ru(CO)2Cl2(C6H15N·HCl)2 structure. XPS, XAS, and UV–vis spectra analyses showed that the electron cloud density of the Ru species increased, its valence state decreased, and its coordination environment of the Ru species was significantly altered during the HCl activation process. Additionally, DFT calculations of the formation energies of ruthenium complexes at different stages provided strong theoretical support for the active site evolution mechanism. This study provides valuable insights into the structural evolution of active sites during acetylene hydrochlorination, enhancing our understanding and offering practical guidance for industrial applications, as well as laying a solid foundation for the advancement of liquid-phase catalysis.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Linfeng Li: Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Data curation, Conceptualization. Bao Wang: Validation, Data curation, Conceptualization. Tiantong Zhang: Software, Methodology, Data curation. Xinyuan Wang: Investigation, Conceptualization. Dingqiang Feng: Writing – review & editing, Writing – original draft, Data curation. Wei Li: Supervision, Resources, Methodology. Jiangjiexing Wu: Writing – review & editing, Supervision, Methodology, Funding acquisition, Data curation. Jinli Zhang: Supervision, Resources, Project administration, Funding acquisition.
This work was supported by the National Natural Science Foundation of China (No. 22378308), Jing-Jin-Ji Regional Integrated Environmental Improvement-National Science and Technology Major Project (No. 2024ZD1200301–2) and the Scientific and Technological Project of Yunnan Precious Metal Laboratory (No. YPML-2023050202). Computing resources and the Gaussian 09 software package were provided by Shenzhen Supercomputing Centre. The authors thank Dr. Chenliang Ye for his help with data analysis.
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) Catalytic performance of Ru-Amide/TMS catalysts with different amide ligands. (b) Catalytic performance of Ru-DIPEA/TMS catalysts at different reaction temperatures. (c) Different GHSV(C2H2). (d) Stability test of Ru-DIPEA/TMS catalysts and C2H2 conversion of liquid-phase catalysts from literature.
Figure 2 (a) UV–vis spectroscopy of Ru-DIPEA/TMS, Ru/TMS, DIPEA/TMS, and TMS. (b) C2H2 conversion of Ru-DIPEA/TMS catalysts with different HCl/C2H2 activation times. (c) FT-IR spectra and (d) Ru 3p XPS of Ru-DIPEA/TMS before and after HCl activation and other samples.
Figure 3 Single-crystal XRD structures of C6H15N·HCl (a) and Ru complexes (b) in Ru-DIPEA/TMS-HCl. (c) Ru K-edge X-ray absorption near-edge structure (XANES) spectra and (d) R-space extended X-ray absorption fine structure (EXAFS) spectra of the Ru K-edge of Ru foil, Ru-DIPEA/TMS-HCl, RuCl3, and RuO2 sample. Fitted curves of the Ru K-edge of Ru-DIPEA/TMS-HCl samples in (e) R-space and (f) k-space.
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