Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2021Ionic Liquid-Stabilized Single-Atom Rh Catalyst Against Leaching Shipeng Ding, Max J. Hülsey, Hua An, Qian He, Hiroyuki Asakura, Min Gao, Jun-ya Hasegawa, Tsunehiro Tanaka and Ning Yan Shipeng Ding Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 , Max J. Hülsey Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 , Hua An Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 Joint School of the National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207 , Qian He Department of Materials Science and Engineering, National University of Singapore, Singapore 117575 , Hiroyuki Asakura *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Molecular Engineering, Graduate School of Engineering; Japan and Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8510; 615-8245 , Min Gao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Institute for Catalysis, Hokkaido University, Sapporo 001-0021 , Jun-ya Hasegawa Institute for Catalysis, Hokkaido University, Sapporo 001-0021 , Tsunehiro Tanaka Department of Molecular Engineering, Graduate School of Engineering; Japan and Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Kyoto 615-8510; 615-8245 and Ning Yan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585 https://doi.org/10.31635/ccschem.021.202101063 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Single-atom catalysts (SACs) are promising in some reactions typically promoted by homogeneous catalysts. However, the leaching of active species from the support in liquid-phase reactions hinders their potential applications. Herein, taking the hydroformylation reaction as an example, we report ionic liquid (IL)-induced stabilization of single Rh atoms against leaching. Among the ILs examined, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([OHEmim][Tf2N])-stabilized single-atom Rh1/TiO2 showed a turnover frequency (TOF) of around 800 h−1 in styrene hydroformylation and was able to be recycled for five runs without significant deactivation. At the same time, the activity for unprotected Rh1/TiO2 decreased over two orders of magnitude under similar conditions. Density-functional theory (DFT) simulations revealed that the IL acts as a linker to connect both Rh and TiO2, thereby enhancing the binding energy of Rh species on TiO2 from 0.69 to 1.19 eV. This work offers a new approach to enhance the stability of atomically dispersed metal catalysts against leaching. Download figure Download PowerPoint Introduction Due to the distinct electronic structure and coordination environment of the isolated metal atoms on the support compared with metal clusters and nanoparticles, single-atom catalysts (SACs) have shown exceptional catalytic performance in diverse applications.1–16 A survey of the single-atom catalysis research showed that around two-thirds of published articles in 2019 focused on liquid-phase reactions. Indeed, organic transformations widely used to synthesize fine chemicals are commonly conducted in solvents. Liquid-phase hydroformylation, for example, is regarded as one of the most important homogeneously catalyzed reactions in industry.17 The formed aldehydes are essential intermediates for fine chemicals such as alcohols, carboxylic acids, and amines. Rh SACs are promising in hydroformylation,18–21 benefiting from the homogeneity of the active sites, convenient separation of products from catalysts, and the avoidance of phosphine ligands. Single-atom Rh1/CoO showed much higher activity than nanoparticle counterparts in propene hydroformylation.22 Likewise, Qiao et al.23 developed a single-atom Rh catalyst on ZnO nanowires that displayed excellent activity comparable to that of Wilkinson's catalyst. Later, by coupling hydroformylation with a water-gas shift reaction, a high selectivity toward linear aldehyde from styrene was achieved over Rh1/CeO2.18 However, the styrene conversion decreased dramatically from the initial 80% to around 40% after two cycles due to Rh leaching. The leaching of metal atoms into the solvent is a major challenge for the practical applications of SACs in liquid-phase reactions.19,24–26 Ionic liquids (ILs) have been successfully applied as reaction solvents and stabilizers to protect metal nanoparticles and homogeneous/single-site catalysts on the support.27–29 Stable Ir, Rh, Pt, and other nanoparticles were prepared in ILs for the hydrogenation of arenes and alkenes,30,31 as well as catalytic transformation of biomass.32 The size of IL-stabilized nanoparticles could be easily tuned by varying the anions or changing the alkyl chain length of the cations.33 In the domain of homogeneous catalysts, Cole-Hamilton et al. reported the hydroformylation of alkenes in a supercritical CO2-IL biphasic system containing a Rh complex, which could be operated continuously for a few weeks without obvious deactivation.34 ILs were also used to tune the selectivity of the Ir complex in selective hydrogenations by modulating the electronic environment of Ir atoms.35,36 The coating of n-ethyl-3-methyl-imidazolium acetate on SiO2-supported Ir complex enhanced the electron density of Ir and improved the stability in H2 reduction.37 CO2 hydrogenation to formic acid was achieved using Ru complex as the catalyst and basic IL 1-butyl-2,3-dimethylimidazolium acetate as the buffer.38,39 IL-Modified porous Re organometallic polymers were reported to be highly active for the photocatalytic CO2 reduction to CO.40 Supported ILs containing homogeneous Rh complex effectively catalyzed hexene to heptanal in the hydroformylation reaction.41 IL was further proven efficient to stabilize SACs.42 In our previous study, an electrostatic double layer of ILs surrounding the positively charged Pt1 and Pd1 metal active sites was constructed to prevent metal atom aggregation in gas-phase hydrogenation.42 To the best of our knowledge, however, there has been little study on the stabilization of SACs against leaching in liquid-phase reactions using ILs. We propose taking advantage of the ability of ILs to simultaneously bind both the metal and the metal oxide43 to enhance the adhesiveness of single-atom species on the support. In this work, ILs containing common anions and cations were used to study the stabilization. Specifically, ILs bearing anion with weak binding affinity were selected because overly strong binding of ILs on metal would deactivate the SACs. We also deliberately selected cations with functional groups, which could render the ILs improved chemical and physical characteristics in metal catalysis.44 Proper choice of ILs with suitable interaction with Rh atoms and the support is the key to enhance the stability without compromising activity. Density-functional theory (DFT) simulation demonstrates that IL increases the binding strength of single-atom Rh species on the support by acting as the linker to connect Rh atoms and the support, thus improving its antileaching property. Results and Discussion The Rh1/TiO2 catalyst with a Rh loading of 0.1 wt % was prepared via wet impregnation using Rh(acac)(COD) as the precursor and rutile TiO2 as the support (Scheme 1). After drying and calcination in air at 200 °C for 1 h, the catalyst was obtained, denoted as 0.1Rh1/TiO2. The isolated Rh species likely occupied the defect sites on TiO2 such as the edge-sites, or substituted the Ti sites.13,45 The Fourier transform infrared spectroscopy (FTIR) spectrum of 0.1Rh1/TiO2 did not show C–H vibration, hinting at complete removal of ligands during calcination ( Supporting Information Figure S1). Three ILs ( Supporting Information Figure S2) bearing the same [Tf2N−] anion, with either [OHEmim+], [N1444+], or [Omim+] as cation were employed to modify the Rh catalyst, and the ratio of Rh to IL was fixed at 1∶10. The weight percentage of the ILs was in the range of 2.7%–4.7%, depending on the molecular weight. The obtained catalysts were labeled as OHEmimTf2N-0.1Rh1/TiO2, N1444Tf2N-0.1Rh1/TiO2, and OmimTf2N-0.1Rh1/TiO2, respectively. The Brunauer-Emmett-Teller (BET) surface area of 0.1Rh1/TiO2 was 40.7 m2/g ( Supporting Information Figure S3), and the surface area of the anion [Tf2N] was determined to be 190 Å2 using the van der Waals surface of the IL pair in the condensed phase.46 Thus around 2.7 monolayers of ILs were coated on the 0.1Rh1/TiO2. Scheme 1 | Schematic illustration of ionic liquid stabilized single-atom Rh against leaching in hydroformylation. Download figure Download PowerPoint As evident from the circled bright dots in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, Rh was atomically dispersed on both 0.1Rh1/TiO2 and OHEmimTf2N-0.1Rh1/TiO2 catalysts (Figures 1a–1b). The white line intensity at the K-edge X-ray absorption near-edge structure (XANES) spectra for all Rh samples closely resembled that of the Rh2O3 reference (Figure 1c), suggesting the average oxidation state of Rh was close to 3+ before reaction. The k3-weighted Fourier transform spectra of extended X-ray absorption fine structure (EXAFS) for Rh displayed a dominant peak at around 1.6 Å, which was assigned to the Rh–O first shell (Figure 1d). There was no obvious peak at 2.3 Å attributable to Rh–Rh scattering, confirming the atomic dispersion of Rh on IL-free and IL-stabilized 0.1Rh1/TiO2 samples.47–49 Based on EXAFS fitting results ( Supporting Information Figures S4 and S5 and Table S1), the Rh–O coordination number (CN) for Rh SAC catalysts was in the range of 6–7, similar to that of single Rh atoms supported on CeO2,50 CoO,22 phosphotungstic acid,47 and TiO2.51 The CN was also consistent with the theoretical prediction, since the Rh atom substituted at the six-fold Ti sites forming Rh1O6 structure is identified as the most favorable configuration for Rh on rutile TiO2 under air calcination.51 Figure 1 | HAADF-STEM images of (a) 0.1 Rh1/TiO2 and (b) OHEmimTf2N-0.1Rh1/TiO2. (c) The normalized XANES spectra at K-edge for SAC Rh catalysts, Rh foil and Rh2O3. (d) The k3-weighted Fourier transform spectra of EXAFS for SAC Rh catalysts, Rh foil and Rh2O3. Wavelet transforms of the k3-weighted EXAFS for (e) Rh2O3, (f) Rh foil, and (g) 0.1Rh1/TiO2. (h) CO adsorption on SAC Rh catalysts. Download figure Download PowerPoint EXAFS wavelet transform analysis was performed to discriminate the relative weight of the backscattering atoms (Figures 1e–1g). Rh foil and Rh2O3 showed an intensity maximum at around 9 Å−1 attributed to Rh–Rh and 6 Å−1 assigned to Rh–O scattering contributions, respectively. Besides, the Rh2O3 also showed a peak at around 11.7 Å−1, which was assigned to the second shell Rh–Rh. This peak was not visible, and only the intensity maximum at 9 Å−1 was observed for SAC Rh catalysts (Figure 1g and Supporting Information Figure S6), indicating the absence of Rh2O3 clusters/particles consistent with the HAADF-STEM and EXAFS results that Rh species were atomically dispersed. The catalysts were further probed by CO adsorption. 0.1Rh1/TiO2 showed a characteristic pair of peaks centered at 2088 and 2019 cm−1 attributable to symmetric and asymmetric CO vibrations on gem-dicarbonyl Rh species with the oxidation state of 1+ (Figure 1h).47,51,52 The inconsistency of the valent state compared with the XANES analysis was due to the reduction of Rh from 3+ to 1+ during CO treatment.47 Besides, the simulated XANES spectra for Rh foil, Rh2O3 and Rh+, and the structure of Rh+ for the simulation were also shown in Supporting Information Figure S7. The simulated spectra were consistent with the experimental. The CO adsorption on IL-modified Rh catalysts showed a red shift relative to 0.1Rh1/TiO2, with a sequence of OHEmimTf2N-0.1Rh1/TiO2 > OmimTf2N-0.1Rh1/TiO2 > N1444Tf2N-0.1Rh1/TiO2, suggesting electron transfer from ILs to positively charged Rh atoms. From these, both anion and cation of the ILs played a part in the electron transfer, hinting that the electronic properties of SACs could be finely tuned by choosing different cation and anion combinations. For the Rh(CO)2 gem-dicarbonyl species, the angle between two carbonyl groups was associated with the ratio of integrated absorbance of the symmetric and asymmetric vibrations.53 The adsorption peaks (1300–1000 cm−1) of ILs showed a blue shift after coating on 0.1Rh1/TiO2 or TiO2 support ( Supporting Information Figure S8), potentially as a result of the interaction between ILs and the TiO2 support. The catalytic performance of the SAC Rh catalysts in styrene hydroformylation was examined in a batch reactor. The conversion was kept below 25% to compare the activity under the kinetic control region ( Supporting Information Table S2). The IL-stabilization resulted in the activity decrease of single Rh atoms, particularly [Bmim][BF4]- and [NH2Emin][Tf2N]-coated ones ( Supporting Information Table S2). The activity drop was likely due to the relatively strong binding of [BF4−] and –NH2 to Rh atoms.54 In the literature, the binding energies between Pd clusters and anions of ILs followed the order of [BF4−] > [CF3SO3−] > [PF4−] ≈ [Tf2N−],54,55 nicely matching the hydroformylation activity of single-atom Rh modified by ILs. Plausibly, ILs induced steric hindrance and restricted the access of substrates to Rh atoms. The decrease of the Rh oxidation state could also be a factor, given that the electron-donating ligands lowered the reactivity of homogeneous Rh complex in hydroformylation.56,57 We further performed Bader charge analysis to examine the oxidation state of Rh.58 The charge localized on Rh was +0.48 |e| for HRh(CO)3/TiO2 and dropped slightly to +0.47 |e| for HRh(CO)3-IL/TiO2. Apart from ILs, inorganic salts were also tested, but the activity decreased significantly, probably due to the mass transfer limitation of organic substrate to inorganic salt doped Rh sites.59–61 Nevertheless, neither the ILs nor the inorganic salts modification affected product selectivity. We further examined the stabilization effect of ILs on single-atom Rh species against leaching, using [OHEmim][Tf2N], [Omim][Tf2N], and [N1444][Tf2N] as examples. 0.1Rh1/TiO2 exhibited a turnover frequency (TOF) of 1250 h−1 in the first run (Figure 2a). However, the TOF decreased to only 10 h−1 after recycling five times, due to the leaching of Rh as demonstrated by inductively coupled plasma atomic emission spectroscopy (ICP-OES) analysis ( Supporting Information Table S3). No obvious Rh leaching into the solution was detected when the reaction was performed in the absence of CO, indicating that the leached Rh species might be in the form of carbonyl complexes. For IL-stabilized catalysts, the initial TOF was 878 h−1 for OHEmimTf2N-0.1Rh1/TiO2, 722 h−1 for N1444Tf2N-0.1Rh1/TiO2, and 416 h−1 for OmimTf2N-0.1Rh1/TiO2, slightly lower than that of the unprotected catalyst.62 Significantly improved recyclability, however, was observed on IL-stabilized Rh catalysts, particularly on OHEmimTf2N-0.1Rh1/TiO2, which maintained a TOF of around 800 h−1 after five cycles. The OHEmimTf2N contained a hydroxyl functional group, which is known to be an excellent stabilizer for metal nanoparticles due to its weak interaction with the metal.63 Figure 2 | Hydroformylation of (a) styrene and (b) 4-methylstyrene over IL-free and IL-stabilized 0.1Rh1/TiO2. (c) Hydroformylation of styrene on single-atom 0.1Rh1/ZnO and OHEmimTf2N-0.1Rh1/ZnO. Reaction condition: 10 bar H2, 10 bar CO, 100 °C, 2 h. Download figure Download PowerPoint We further performed the reaction using 4-methylstyrene as a substrate (Figure 2b). OHEmimTf2N-0.1Rh1/TiO2 was able to recycle at least six times with a TOF kept at around 300 h−1, while rapid deactivation was again observed on 0.1Rh1/TiO2. The Rh catalysts showed a lower TOF in 4-methylstyrene hydroformylation than in styrene hydroformylation, in agreement with previous reports that the p-substitution of the phenyl ring with electron-donating methyl groups led to decreased reaction rate.64 We then prepared Rh1/ZnO and [OHEmim][Tf2N]-Rh1/ZnO catalysts and confirmed the atomic dispersion of Rh on ZnO by CO adsorption ( Supporting Information Figure S9). Not unexpectedly, [OHEmim][Tf2N] suppressed the leaching of Rh atoms from the ZnO host (Figure 2c). The Rh dispersion of the 0.1Rh1/TiO2 catalyst after styrene hydroformylation was examined. As evidenced by HAADF-STEM images, the Rh species remained atomically dispersed on the spent OHEmimTf2N-0.1Rh1/TiO2 without identifiable nanoparticles (Figure 3a). The XAS results showed that all spent Rh catalysts showed a dominant Rh–O peak (Figure 3b), and Rh stayed in a high oxidation state ( Supporting Information Figure S10), indicating that Rh atoms maintained their electronic and geometric characteristics. After two runs, the loading of Rh in the IL-free 0.1Rh1/TiO2 was decreased by about 50%, while the majority of Rh atoms were preserved on IL-stabilized 0.1Rh1/TiO2 catalyst ( Supporting Information Table S3). CO adsorption on the spent OHEmimTf2N-0.1Rh1/TiO2 provided the same result as that collected over the fresh one. In contrast, no identifiable absorption peak existed for the spent 0.1Rh1/TiO2, possibly due to the low density of Rh sites on TiO2 after recycling (Figure 3c). The absorption peaks that were assigned to va(CF3), va(SO2), and vs(SO2) vibrations from ILs65 were still identifiable after 5 catalytic runs, confirming the strong interaction between ILs and Rh1/TiO2 (Figure 3d). Figure 3 | (a) HAADF-STEM images of OHEmimTf2N-0.1Rh1/TiO2 and (b) Fourier transform of EXAFS for Rh catalysts after styrene hydroformylation for 2 runs. (c) CO adsorption on Rh catalysts and (d) FTIR of ILs coated on 0.1Rh1/TiO2 after styrene hydroformylation for 5 runs. Download figure Download PowerPoint DFT calculations were performed to clarify the role of ILs in the stabilization of 0.1Rh1/TiO2. H-Rh(CO)2-styrene (HRh(CO)2Ph) was modeled as the leaching species, based on a previous report.66 Rutile TiO2(110), consisting of two types of coordinated Ti (five and six coordinated Ti atoms, Ti5c and Ti6c) and two types of coordinated O atoms (bridged two and three coordinated O atoms, O2c and O3c), was used to represent the TiO2 surface67 (Figure 4a). The interaction between HRh(CO)2Ph/ILs and TiO2 was investigated over these four sites. Two adsorption geometries of HRh(CO)2Ph on TiO2 were located (Figures 4b–4c): HRh(CO)2Ph either interacted with the O2c sites coordinating to the Rh atom or the Ti5c sites with coordination of O atoms of the CO. The low coordinated sites Ti5c and O2c on the TiO2 surface usually serve as the electron acceptor and donor, respectively. The adsorption energy of HRh(CO)2Ph on Ti5c site was −0.69 eV, stronger than the one at O2c sites (−0.58 eV). The difference comes from the electronic repulsion between the TiO2 surface and adsorbed HRh(CO)2Ph. In the case of O2c coordinating to the Rh atom, the short distance (∼2.9 Å) between the Rh–CO plane and the TiO2 surface induced more electronic repulsion, making the system less stable. The HRh(CO)4 adsorption on TiO2 was also considered, since it is not impossible that CO occupies all the valence orbitals of Rh(I) to form HRh(CO)4 ( Supporting Information Figure S11). Similar to the case of HRh(CO)2Ph, HRh(CO)4 coordinated to Ti5c sites but with a longer distance of Ti5c–O and a weaker binding energy of −0.33 eV. Therefore, HRh(CO)2Ph coordinated to the Ti5c site was identified as the most favorable configuration. Figure 4 | (a) Optimized geometries of TiO2(110) surface; HRh(CO)2Ph complex coordinated to (b) Ti5c and (c) O2c on TiO2 (110) sur face; (d) The most stable geometry of HRh(CO)2Ph with [OHEmim][Tf2N]; (e-f) [OHEmim][Tf2N] stabilized HRh(CO)2 on TiO2 surface. Download figure Download PowerPoint Next, we conducted conformation searching between isolated IL and HRh (CO)2Ph. Totally, 233 conformers of IL-HRh(CO)2Ph were located. The most stable conformer had an interaction between the Rh center and O atom of [Tf2N−] (Figure 4d). [OHEmim+] did not interact directly with HRh(CO)2Ph, but stayed around [Tf2N−] to balance the charge of the system. Thus, the interaction between [Tf2N−] and HRh(CO)2Ph was mainly focused to investigate the role of ILs. Figures 4e–4f show the geometry of HRh(CO)2Ph with ILs on TiO2. Both [OHEmim+] and [Tf2N−] species interacted with the TiO2 surface with a large energy of −3.02 eV. Energy decomposition analysis was done to exclude interaction between the TiO2 surface and ILs. The binding energy of HRh(CO)2Ph increased to 1.19 eV, which was much larger than the one without ILs (0.69 eV). [Tf2N−] bound on both the TiO2 surface and HRh(CO)2Ph with two O atoms of [Tf2N−] coordinating to low coordinated Ti5c sites and one O atom coordinate to Rh atom. The O atom of HRh(CO)2Ph still bound to the Ti5c sites, which was similar to the one without ILs. As the TiO2 surface usually contains oxygen vacancies, which could affect the interaction between the leaching species HRh(CO)2Ph, the binding energy of the leaching species on TiO2 with oxygen vacancies was calculated. Acting as Lewis acid sites, the presence of oxygen vacancies further enhanced the binding energy of leaching species on TiO2 from 0.69 to 0.75 eV for the unprotected single-atom Rh, and from 1.19 to 1.67 eV for the IL-stabilized Rh atoms ( Supporting Information Figure S12). Again, the presence of ILs significantly enhanced the stability of Rh. Conclusion In summary, we report that IL doping remarkably enhances the leaching resistance of the SAC Rh/TiO2 catalyst prepared by wet impregnation without compromising catalyst activity and selectivity. Acting as a linker to bind to both the support and single metal atoms, ILs stabilize single Rh species in hydroformylation by increasing the binding energy of Rh on the support. This work offers a simple approach to stabilize single-atom metals against leaching in liquid-phase reactions. Supporting Information Supporting Information is available and includes the characterization data, experimental procedures, and computational details. Conflict of Interest There is no conflict of interest to report. Funding Information We thank the ANR-NRF project from the Singapore National Research Foundation (ANR-17-CE06-001701 and WBS: R-279-000-530-281) for their finiancial support. The XAS were collected at the beamline BL01B1 at the SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan) under the approval of JASRI (Proposal No. 2019A1398). Q.H. thanks the NRF Fellowship (NRF-NRFF11-2019-0002) from the National Research Foundation of Singapore. M. G. and J. H. acknowledge the MEXT Project of Integrated Research Consortium on Chemical Science (IRCCS), the Photoexcitonix Project at Hokkaido University, and JSPS KAKENHI(JP20K05217). Some of the computations were performed at RCCS (Okazaki, Japan). References 1. Flytzani-Stephanopoulos M.; Gates B. 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