Open AccessCCS ChemistryCOMMUNICATION14 Jul 2022Ultrafine PdRu Nanoparticles Immobilized in Metal–Organic Frameworks for Efficient Fluorophenol Hydrodefluorination under Mild Aqueous Conditions Wenqian Yang†, Qinglin Liu†, Jun Yang, Jiahui Xian, Yinle Li, Guangqin Li and Cheng-Yong Su Wenqian Yang† State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 †W. Yang and Q. Liu contributed equally to this work.Google Scholar More articles by this author , Qinglin Liu† State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 †W. Yang and Q. Liu contributed equally to this work.Google Scholar More articles by this author , Jun Yang State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 Google Scholar More articles by this author , Jiahui Xian State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 Google Scholar More articles by this author , Yinle Li State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 Google Scholar More articles by this author , Guangqin Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 Google Scholar More articles by this author and Cheng-Yong Su *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Optoelectronic Materials and Technologies, MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry Sun Yat-Sen University, Guangzhou 510006 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101230 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Fluorinated organic compounds are of great importance to modern industries, while their release to the environment is inevitable, causing extreme environmental pollution and the subsequent hazardous effect on ecosystems. This is because the degradation of fluorinated compounds under mild conditions remains a challenging task due to the strong C–F bond strength. In this study, we report preparation of [email protected] through immobilizing ultrafine PdRu alloy nanoparticles with a mean diameter of ∼2 nm into the metal–organic framework (MOF), MIL-101(Cr), which was highly active and stable in the hydrodefluorination of 4-fluorophenol (4-FP) under mild aqueous conditions. The optimized catalyst Pd0.5Ru0.5@MIL-101 achieved impressive hydrogenation performance with a 98.5% conversion of 4-FP and a 97.7% selectivity of cyclohexanol, much better than the single metal-doped [email protected] and [email protected] catalysts. The excellent catalytic behavior contributed to the synergistic effect of combining the PdRu alloying effect and the MOF nanospace confinement effect, providing a promising strategy to develop highly efficient hydrodefluorination catalysts to assist environmental restoration and green ecology. Download figure Download PowerPoint Introduction Fluorinated organic compounds are now widely applied in several formulations, including pesticides, medicines, and functional materials, but the use of some fluoroaromatics leads to severe pollution on account of their release into the environment, which include perfluoroalkylated compounds.1–6 Because the binding energy of C–F is the strongest among the carbon–halogen bonds, the activation of the C–F bond is believed to be one of the most inactive organic processes.7–9 Most defluorination reactions can only be carried out under harsh conditions such as high temperature and pressure, so under mild conditions, the degradation of aryl fluorides remains a challenging task.10–12 To date, the single metal-center catalysts such as Pt,6 Ni,13 and Mg14 have been reported to have the ability of C–F activation, but their catalytic performance is unsatisfactory. Rh is able to express a good defluorination effect, albeit rare and expensive.15–17 Alternatively, the Kitagawa group18–20 developed a solid-solution-alloy of PdRu via high-temperature heating in the presence of polyvinylpyrrolidone (PVP), and obtained ∼10 nm PdRu nanoparticles with excellent Rh-like performance for CO-oxidizing. Recently, PdRu alloy nanoparticles were also applied in the hydrogenation of benzoic acid.21 Hence, we suspected that PdRu alloy nanoparticles might have a great power to break chemical bonds, and therefore, be a good candidate for hydrodefluorination via C–F breakage. Metal–organic frameworks (MOFs) are a class of porous materials assembled through coordination bonding between metal ions and organic linkers.22–40 Due to their finely adjustable and uniform pore structure, MOFs represent a kind of ideal support for encapsulating metal nanoparticles and preventing the aggregation and loss of metals during reactions.41–43 Furthermore, some MOFs possess unsaturated sites, beneficial for substrate adsorption, leading to improved catalytic performance.44–47 Therefore, we became interested in encapsulating ultrafine PdRu alloy nanoparticles in the confined nanospace of MOFs as a promising catalyst for hydrodefluorination. We successfully synthesized the ultrafine PdRu alloy nanoparticles with the mean diameter of ∼2 nm and confined in the MOF, MIL-101, to afford [email protected] catalysts. Then we employed 4-fluorophenol (4-FP) as a model fluoroaromatic compound to evaluate the hydrodefluorination ability of our developed catalysts. Remarkably, optimized Pd0.5Ru0.5@MIL-101(Cr) exhibited an excellent hydrodefluorination rate of 4-FP with outstanding reusability and versatility, significantly surpassing [email protected](Cr) and [email protected](Cr) under mild conditions of room temperature, standard atmospheric pressure, and water environment. Moreover, an alloying effect and MOF nanospace confinement effect were proposed to contribute to the excellent catalytic performance. Results and Discussion Structural and morphological characterizations on catalysts A typical hydrothermal method was adopted to prepare the MOF MIL-101(Cr) precursor (see Supporting Information Figures S1 and S2).48 To precisely immobilize PdxRu1−x (x = 0, 0.2, 0.5, 0.8, and 1.0) nanoparticles with size and location under control in MIL-101, we used a double solvents method (DSM) and the overwhelming reduction approach (Figure 1a). A variety of metal contents were tested to obtain better control of PdRu alloy nanostructure in the MOFs. We found that 3 wt % is the suitable amount of metal content for the formation of well-distributed PdRu nanoparticles (see Supporting Information Figure S3). There was no apparent nanoparticle aggregation, and the quantity of metal alloy was sufficient. The actual contents and composition of Pd and Ru were determined by inductively coupled plasma mass spectrometry (ICP-MS), summarized in Supporting Information Figure S4 and Tables S1 and S2. Figure 1 | (a) Schematic representation of immobilization of the PdRu nanoparticles in MIL-101(Cr). (b) TEM image. (c) Particle size distribution. (d) HAADF-STEM image of Pd0.5Ru0.5@MIL-101. (e) Compositional line profiles of Pd (red) and Ru (green). (f) HAADF-STEM-image and its EDX map of Pd-L and Ru-L and overlay image over Pd0.5Ru0.5@MIL-101. Download figure Download PowerPoint As shown in the transmission electron microscopy (TEM) image (Figure 1b), Pd0.5Ru0.5 nanoparticles were uniformly and densely dispersed in MIL-101(Cr) with a mean size of 2.01 nm (Figure 1c). We further investigated Pd0.5Ru0.5 nanoparticles by line-scan analysis (Figures 1d and 1e). The energy dispersive X-ray (EDX) line-scan position of the nanoparticle is denoted by the yellow arrow (Figure 1d). The composition of line profiles with Pd and Ru in Pd0.5Ru0.5@MIL-101 indicates the formation of atomic-level PdRu alloy.18,20 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and corresponding STEM−EDX maps (Pd-L, Ru-L, overlay) demonstrated that Pd and Ru were homogeneously distributed throughout the whole MOF structure (Figure 1f). Other synthesized MOF catalysts, PdxRu1−x@MIL-101(Cr), displayed similar morphologies (see Supporting Information Figure S5), and the actual molar ratios of Pd/Ru were almost in line with the feedstock used in the preparation, indicating a successful immobilization of the ultrafine PdRu alloy in the confined nanospace of MIL-101(Cr). As far as we know, the size of the encapsulated PdRu alloy (∼2 nm) is the smallest reported to date, which provides a unique opportunity to investigate their catalytic performance at such an ultrafine nanoscale level. The solid-state structure of the prepared catalysts was investigated by powder X-ray diffraction (PXRD). The diffraction patterns of PdxRu1−x@MIL-101(Cr) were well comparable with that of pure MIL-101(Cr), and no diffraction peaks of Pd or Ru were found, indicative of the formation of much smaller PdxRu1−x nanoparticles (see Supporting Information Figure S6). From the Fourier transform infrared (FT-IR) spectrum (see Supporting Information Figure S7), we observed that PdxRu1−x@MIL-101(Cr) nanoparticles retained the spectroscopic features of MIL-101(Cr), implying that the structure of MIL-101(Cr) was maintained after encapsulating the PdRu nanoparticles. The Brunauer–Emmett–Teller (BET) surface area and total pore volume of PdxRu1−x@MIL-101 decrease remarkably when compared with pure MIL-101, attributable to the incorporation of PdxRu1−x nanoparticles into the pores of MIL-101 (see Supporting Information Figure S8). Catalytic evaluation toward defluorination reaction We chose hydrodefluorination reaction, one of the most passive organic catalytic reactions, to explore the performances of the prepared catalysts. The reactions were carried out with a 4-FP substrate under mild conditions (room temperature, standard atmospheric pressure, and water environment). Briefly, 4-FP was initially hydrodefluorinated to form phenol and then further hydrotreated to cyclohexanone and eventually to cyclohexanol (Figure 2a). The hydrodefluorination performance with different catalysts is shown in Figures 2b and 2c. In the presence of bare MIL-101(Cr), no 4-FP conversion was detectable even after 12 h reaction; the same observation was made when commercial Pd and Ru nanoparticles were utilized in the catalysis (see Supporting Information Table S3). Interestingly, when these metal nanoparticles were confined in MIL-101(Cr), they displayed hydrodefluorination activity. However, the defluorination ability of [email protected] and [email protected] was relatively poor, and only 3.6% and 5.5% conversions of 4-FP were achieved, respectively. Figure 2 | (a) Hydrodefluorination reaction of 4- FP process under mild condition. (b) Defluorination conversion and (c) cyclohexanol yield catalyzed by different catalysts after 12 h reaction. (d) The activity and selectivity changes and (e) the stability test for hydrodefluorination of fluorophenol with Pd0.5Ru0.5@MIL-101. Download figure Download PowerPoint Surprisingly, when PdRu alloy catalysts were applied, the reaction activity was sharply enhanced. The detailed reaction performance is summarized in Supporting Information Table S3, in which the optimized Pd0.5Ru0.5@MIL-101(Cr) exhibited the best performance. The time-course of PdxRu1−x @MIL-101(Cr) (x = 0.2, 0.5, and 0.8) is shown in Figure 2d and Supporting Information Figure S9–S11. For Pd0.5Ru0.5@MIL-101(Cr), the conversion of 4-FP reached 91.1%. Whereas the conversions of 4-FP decreased by 63.0% and 81.7% when the molar ratio of Pd/Ru was adjusted to 4:1 and 1:4, respectively. In addition, cyclohexanol yield reached the maximum at a Pd/Ru ratio of 1:1. When the reaction time was extended for more hours (4 h), Pd0.5Ru0.5@MIL-101(Cr) gave 4-FP conversion as high as 98.5% and selectivity of 97.7% toward cyclohexanol (Figure 2d and Supporting Information Table S4). In contrast, [email protected](Cr) and [email protected](Cr) exhibited 4-FP conversion of 5.3% and 7.9%, respectively, under similar reaction conditions (see Supporting Information Table S5). Since recyclability is another significant factor in evaluating the practicability of catalysts, we assessed the reusability of Pd0.5Ru0.5@MIL-101(Cr) in consecutive reactions. From Figure 2e, we can see that a superior 4-FP conversion level is still maintained above 96% with no apparent reduction in cyclohexanol yield for five-time runs. The PXRD (see Supporting Information Figure S12) and TEM (see Supporting Information Figure S13) were carried out to characterize Pd0.5Ru0.5 @MIL-101(Cr) after recycling experiments. Compared with the fresh one, the crystallinity of used Pd0.5Ru0.5 @MIL-101(Cr) was maintained; there was neither distinct aggregation of PdRu nanoparticles nor significant change of the morphologies of the supporting MOFs. For comparison, Pd0.5Ru0.5 alloy nanoparticles were synthesized without encapsulation in MOF and applied in a defluorination reaction under the same conditions (see Supporting Information Figures S14–S17). Notably, only about one-third of 4-FP conversion was obtained for the free Pd0.5Ru0.5 alloy, compared with Pd0.5Ru0.5@MIL-101(Cr) (see Supporting Information Table S3). Therefore, we inferred that the MOF nanospace played a crucial role in confining and stabilizing the ultrafine PdRu nanoparticles and facilitated spillover of H2,49–51 thereby enhancing the catalytic activity. Since the Pd and Ru nanoparticles showed no catalytic activity while the bare Pd0.5Ru0.5 alloy nanoparticles were inefficient in defluorination catalysis and difficult to recycle for further utilization, we envisioned that the superior defluorination activity of Pd0.5Ru0.5 @MIL-101(Cr) was attributed to the synergistic action of the MOF nanospace confinement effect and the RdRu alloying effect, whereby: (1) encapsulating the Pd0.5Ru0.5 alloy nanoparticles with ultrafine size exposed more active sites and significantly increase the atomic utilization, (2) preventing the nanoparticles from agglomeration led to an excellent defluorination activity and stability. To investigate the universality of Pd0.5Ru0.5@MIL-101(Cr), we tested several different aryl fluorides as substrates to undergo hydrodefluorination reactions with the detailed results (Table 1). For difluorophenol, Pd0.5Ru0.5@MIL-101 removed the fluorine atoms successfully while maintaining high selectivity of 99.9%. Interestingly, when fluorine atom and unsaturated functional groups were both present on the benzene ring, as found in structures like 4-fluorobenzoic acid and 4-fluorobenzamide, the C=O group was maintained in the process of hydrodefluorination. Usually, benzene rings have higher resonance energy, so the hydrogenation of the benzene ring required more stringent conditions than the hydrogenation of unsaturated functional groups. In our case, the benzene rings were selectively hydrogenated. This finding opens up a promising application toward the selective reduction of aryl fluorides with unsaturated functional groups (e.g., carboxyl, amide). Hence, the Pd0.5Ru0.5@MIL-101 could be regarded as a high-performance catalyst for hydrodefluorination, favorable toward sustainable development of the environment by degrading excess fluorochemicals. Table 1 | Hydrodefluorination of Different Aryl Fluorides with Pd0.5Ru0.5@MIL-101(Cr) Entry Substrate Product Time (h) Conv. (%) Sel. (%) 1a 16 98.5 97.7 2a 16 92.5 90.0 3a 12 <99.9 <99.9 4a 12 97.0 82.8 5b 12 <99.9 <99.9 6b 12 <99.9 <99.9 7b 12 91.9 <99.9 8b 12 86.2 <99.9 9b 12 <99.9 <99.9 aReaction condition: 10 mg catalyst, 0.05 mmol substrate, 2.5 mL water, 25 °C, RT, H2 balloon. bReaction condition: 10 mg catalyst, 0.05 mmol substrate, 2.5 mL water, 70 °C, 1 MPa H2. Structure-selectivity relationship consideration To understand why Pd0.5Ru0.5@MIL-101(Cr) exhibited excellent defluorination performance, X-ray photoelectron spectroscopy (XPS) investigations were conducted. When metal nanoparticles were localized in MIL-101(Cr), the binding energy of the Cr 2p in MIL-101 moved to lower energy, verifying the electron transfer between the MOF and the small metal nanoparticles (Figure 3a). The strong interaction between metal nanoparticles and the MOF framework might prevent the loss of an active site and guarantee the stability of the catalyst during the recycling tests. The changes in the electronic structure after forming atomic-level PdRu alloy were also revealed by XPS. Compared with [email protected], the binding energies of Pd 3d 3/2 and 3d 5/2 in PdxRu1−x@MIL-101 (x = 0.2, 0.5, and 0.8) exhibited consecutive positive shifts with increasing Ru content, which identified with Pd oxidization (Figure 3b). On the contrary, the binding energies of Ru 3p 3/2 and 3p 1/2 in PdxRu1−x@MIL-101 (x = 0.2, 0.5, and 0.8) exhibited consecutive negative shifts with decreasing Pd content, compared with [email protected], which identified with Ru reduction (Figure 3c). Such shifts suggested that the electrons of Pd were slightly transferred to Ru; at the same time, the precise surface control of PdxRu1−x@MIL-101 offered the potential for tuning the catalytic behavior in the defluorination reaction. Figure 3 | (a) XPS spectra of Cr 2p of MIL-101(Cr). XPS spectra of Pd 3d (b) and Ru 3p (c) of different PdxRu1-x @MIL-101. Download figure Download PowerPoint According to the reported theoretical calculations of the d-band center of the near-Fermi-level, the electronic structure of Pd0.5Ru0.5 is similar to that of Rh.52 Moreover, Rh is located between Pd and Ru in the Periodic Table, known to activate the C–F bond. Therefore, the outstanding performance of Pd0.5Ru0.5@MIL-101 for C–F bond activation in hydrodefluorination might be due to its Rh-like electronic structure. The adsorption mechanism of substrates Moreover, the adsorption of substrates is a critical initial step in heterogeneous catalysis, and the different active sites on bimetallic alloys for adsorption have a great effect on the reaction rate.53–55 To further clarify the defluorination process, we speculated a plausible mechanism of adsorption based on reported theoretical calculations studies and our experimental results (Figure 4 and Supporting Information Figure S18). Pd and Ru atoms are two potential active adsorption sites toward –F on the surfaces of Pd0.5Ru0.5@MIL-101. According to the related density of states reported, the estimated d-band center of Ru relative to Fermi energy is closer to that of Pd0.5Ru0.5 when compared with that of Pd, and the d-band center of Ru goes down after the introduction of Pd in the alloy.52 Furthermore, we found that the [email protected] could convert fluorophenol to the end product of hydrogenation (cyclohexanol) under mild conditions, but [email protected] could only convert fluorophenol to cyclohexanone under the same reaction conditions. Therefore, the Ru atom is more likely to be the active site inclined to adsorb –F in Pd0.5Ru0.5@MIL-101 to promote the progress of defluorination. Figure 4 | Proposed adsorption mechanism of substrates in the presence of Pd0.5Ru0.5@MIL-101. Download figure Download PowerPoint Conclusion We have successfully synthesized uniform PdRu alloy ultrafine nanoparticles immobilized in MIL-101(Cr). The optimized Pd0.5Ru0.5@MIL-101 catalyst showed an excellent hydrodefluorination effect with a 4-FP conversion of 98.5% and a selectivity of 97.7% toward cyclohexanol, maintaining good stability and recyclability under mild conditions. The outstanding hydrodefluorination catalysis of Pd0.5Ru0.5@MIL-101 could be readily applied to other aryl fluoride substrates, leading to impressive chemoselectivity that leaves the unsaturated functional groups (e.g., carboxyl, amide) intact. The enhancement of the catalytic activity is attributable to a synergistic effect that combines the alloying effect between Pd and Ru and the nanospace confinement effect through encapsulating ultrafine PdRu nanoparticles into the pores of MIL-101(Cr). The adsorption tendency of Ru atoms toward the F group of substrates might accelerate the departure of fluorine atoms. This work provides a compelling clue to design catalysts for hydrodefluorination of fluorophenol, which has great promise in preventing groundwater from being polluted and benefits sustainable development of the environment. Supporting Information Supporting Information is available and includes detailed experimental procedures and additional figures. Conflict of Interest The authors declare no conflict of interest. Acknowledgments This research was supported by the National Key R&D Program of China (no. 2018YFA0108300), the Overseas High-level Talents Plan of China and Guangdong Province, the Fundamental Research Funds for the Central Universities, the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (no. 2017ZT07C069), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (no. 2017BT01C161), and the NSFC Projects (nos. 22075321, 21821003, 21890380, and 21905315).