Regulating Electronic Status of Platinum Nanoparticles by Metal–Organic Frameworks for Selective Catalysis

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Regulating Electronic Status of Platinum Nanoparticles by Metal–Organic Frameworks for Selective Catalysis Yu Shen†, Ting Pan†, Peng Wu, Jiawei Huang, Hongfeng Li, Islam E. Khalil, Sheng Li, Bing Zheng, Jiansheng Wu, Qiang Wang, Weina Zhang, Wei David Wei and Fengwei Huo Yu Shen† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 †Y. Shen and T. Pan contributed equally to this work.Google Scholar More articles by this author , Ting Pan† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 †Y. Shen and T. Pan contributed equally to this work.Google Scholar More articles by this author , Peng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Jiawei Huang Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 Google Scholar More articles by this author , Hongfeng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Islam E. Khalil Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Sheng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Bing Zheng Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Jiansheng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Qiang Wang Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Weina Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author , Wei David Wei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 Google Scholar More articles by this author and Fengwei Huo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000278 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Selective hydrogenation of alkynes to alkenes remains challenging in the field of catalysis due to the ease of over-hydrogenated of alkynes to alkanes. Favorably, the incorporation of metal nanoparticles (MNPs) into metal–organic frameworks (MOFs) provides an opportunity to adjust the surface electronic properties of MNPs for selective hydrogenation of alkynes. Herein, we used different metal-O clusters of MOFs to regulate the electronic status of platinum nanoparticles (Pt NPs) toward overhydrogenation, semihydrogenation, and unhydrogenation of phenylacetylene. Specifically, Pt/Fe-O cluster-based MOFs are found to reduce the electronic density on Pt NPs and inhibit the overhydrogenation of styrene, leading to an 80% increase in selectivity toward a semihydrogenation product (styrene). Meanwhile, Cu-O cluster-based MOFs generate high oxidation states of Pt NPs and release Cu2+ ions, which worked together to deactivate Pt NPs in the hydrogenation reaction entirely. Thus, our studies illustrate the critical role of metal-O clusters in governing chemical environments within MOFs for the precise control of selective hydrogenation of alkynes, thereby, offering appealing opportunities for designing MNPs/MOFs catalysts to prompt a variety of reactions. Download figure Download PowerPoint Introduction Selective hydrogenation of alkynes to alkenes is a key transformation reaction in industrial manufacturing of fine chemicals, pharmaceuticals, polymers, and others.1–3 Achieving high selectivity of partial hydrogenation products in an economical, mild, and environmentally benign way is still a challenge because it is easy to overhydrogenate alkynes into alkanes.4,5 Heterogeneous metal catalysts are brought into the spotlight due to their high activity and stability.6 A traditional Lindlar catalyst, composed of palladium nanoparticles (Pd NPs) modified by lead ions and quinolone additives, has been used widely in the semihydrogenation industry for decades.7 However, toxic lead ions in Lindlar catalysts hamper their applications in the green chemical industry. Recently, metal oxides, organic molecule- or metal ion-modified metal nanoparticles (MNPs), multimetallic alloys, and single-atom catalysts have been developed for improving the selectivity of semihydrogenation.8–11 However, all those materials require complicated procedures to tune the electronic status of MNPs to achieve selective hydrogenation. Thus, developing new strategies to regulate the electronic status of MNPs remains a critical issue in the field of selective hydrogenation of alkynes. As an emerging porous material, metal–organic frameworks (MOFs) offer intriguing properties, including diverse organic–inorganic compositions, facile functionalization, and uniform yet tunable cavities, showing promising application prospects in gas separation, sensor platforms, heterogeneous catalysis, and so on.12–14 Besides, MOFs have been used as hosts for MNPs, and those hybrid catalysts combine both the molecular sieving effect of MOFs matrix and the high catalytic activity of MNPs.15–21 Recently, scientists found that MOFs could be used to modulate the electronic status of MNPs.22–24 For instance, Zhao et al.25 demonstrated that controlling the electron transfer in Pt/MOFs improved the catalytic selectivity for hydrogenation of α,β-unsaturated aldehydes. Also, Xiao et al.26 observed that tuning the electron transfer between platinum nanoparticles (Pt NPs) and porphyrinic MOFs allowed an increase in the surface electron densities of Pt NPs and enhanced the alcohol oxidation. Herein, we demonstrate the use of metal-O clusters within MOFs to regulate precisely the interfacial electronic status of Pt NPs for promoting the selective hydrogenation of phenylacetylene (Scheme 1). Specifically, Cr-O, Fe-O, and Cu-O cluster-based MOFs, which had similar ligands and coordination structures, were exploited to create different chemical environments for Pt NPs. We found that Pt/Fe-O cluster-based MOFs catalysts exhibited >99% conversion of phenylacetylene and ∼80% selectivity to styrene. Meanwhile, Pt/Cr-O cluster-based MOFs catalysts showed no influence on the selectivity, and thus, resulted in overhydrogenation, with the formation of ethylbenzene. Surprisingly, Cu-O cluster-based hybrid catalysts were found to lose catalytic activity totally in the hydrogenation reaction. Further, studies using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) confirmed the essential role of metal-O clusters within MOFs in modulating the electronic status of Pt NPs in promoting the activity and selectivity of alkyne hydrogenation. Scheme 1 | Precise control of selective hydrogenation of phenylacetylene using distinct metal-O cluster-based MOFs as modulators to regulate the electronic status of Pt NPs. MOFs, metal–organic frameworks; Pt NPs, platinum nanoparticles. Download figure Download PowerPoint Experimental Methods Preparation of Pt/MOFs catalysts About 30 mL as-synthesized Pt NPs solution (0.6 mM) and 50 mg MOFs powder were stirred under 500 rpm at room temperature for 6 h. Subsequently, Pt/MOFs were collected by centrifugation at 8000 rpm for 5 min and washed twice with ethanol or methanol. Finally, the obtained Pt/MOFs powder was dried at room temperature in a vacuum oven for 12 h. Catalytic hydrogenation of phenylacetylene In a typical procedure, 10 mg of the Pt NPs catalyst was dispersed in 5 mL of methanol solution, and then 100 μL phenylacetylene was added to the above solution. Subsequently, the solution was purged with a H2 balloon. During the catalytic process, the reaction solution was stirred magnetically at room temperature for the desired reaction time. After that, the catalysts were separated by centrifugation, and the solution was analyzed by gas chromatography (GC). Results and Discussion Catalysts preparation To actualize the concept while avoiding the interference of pore size within MOFs and MNPs morphology, the hybrid Pt/MOFs catalysts were rationally designed and synthesized, where presynthesized MNPs were deposited on the MOFs surface. Pt NPs with an average size of 2.8 nm were synthesized by established methods ( Supporting Information Figure S1).27 Several stable MOFs were selected as supports to explore the effect of distinct metal-O clusters on regulating the electronic status of Pt NPs, namely, Cr-based MIL-100(Cr) and MIL-101(Cr); Fe-based MIL-100(Fe), MIL-101(Fe), and MIL-88(Fe); and Cu-based MOF, HKUST-1, and MOF nanosheets, Cu-TCPP.28–30 Pt NPs were dispersed uniformly on the MOFs surface, as revealed by transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) elemental mapping data (Figures 1a–1d). As shown in the large-scale TEM images from Supporting Information Figures S2–S8, no free Pt NPs were observed in the catalysts. Additionally, powder X-ray diffraction (PXRD) patterns of Pt/MOFs composites were identical to those of the corresponding simulated MOFs, indicating that MOFs maintained their original crystal structures after the deposition of Pt NPs ( Supporting Information Figures S9–S15). An inductively coupled plasma mass spectrometry (ICP-MS) measurements confirmed the similarity of Pt concentration in the composites ( Supporting Information Table S1). Figure 1 | TEM, high-angle annular dark-field scanning TEM (HAADF-STEM), and the corresponding EDS elemental mapping images of Pt NPs anchored on different MOFs supports. (a) Pt/MIL-101(Cr), (b) Pt/MIL-101(Fe), (c) Pt/MIL-88(Fe), (d) Pt/HKUST-1. TEM, transmission electron microscopy; EDS, energy-dispersive X-ray spectroscopy, Pt NPs, platinum nanoparticles; MOFs, metal–organic frameworks. Download figure Download PowerPoint Catalytic performance Various Pt/MOFs catalysts were utilized in exploring the chemoselectivity of phenylacetylene hydrogenations (Figure 2a). In a typical catalytic reaction, Pt/MIL-100(Cr) and Pt/MIL-101(Cr) with Cr-O clusters generated mostly overhydrogenation products (ethylbenzene), which were similar to the Pt NPs catalysts (Figure 2b and Supporting Information Table S2). Surprisingly, Pt/MIL-100(Fe), Pt/MIL-101(Fe), and Pt/MIL-88(Fe) with Fe-O clusters showed a high conversion (>99%) and selectivity (∼80%) toward the semihydrogenation product (styrene; Figure 2b), while suppressing the overhydrogenation of styrene to ethylbenzene (Figure 2c). No significant decay in the selectivity was noticed even when the reaction time was prolonged to 24 h ( Supporting Information Figure S16). Pt/HKUST-1 and Pt/Cu-TCPP composite of Cu-O clusters in MOFs showed no activity of phenylacetylene and styrene hydrogenations (Figures 2b and 2c). For comparison, MOFs and the corresponding metal oxide supports were also tested for the phenylacetylene and styrene hydrogenation. As shown in Supporting Information Table S2, all MOFs support exhibited no hydrogenation activity, and Pt/metal oxide catalysts showed no hydrogenation selectivity. The evaluation of Pt/MOFs catalytic performance was based on the similar Pt NPs loading (2%) and full conversion of phenylacetylene ( Supporting Information Figure S17 and Table S1). In short, Pt/MOFs catalysts showed three distinct hydrogenation results: overhydrogenation, semihydrogenation, and unhydrogenation, indicating that metal-O clusters within MOFs functioned as modulators of Pt NPs and altered the catalytic performance of phenylacetylene hydrogenation reaction in Pt NPs. Figure 2 | Performance of various Pt/MOFs catalysts for the hydrogenation reaction yielding phenylacetylene and styrene. (a) Schematic of hydrogenation of phenylacetylene. (b) The yield of phenylacetylene hydrogenation on various catalysts. (c) The yield of styrene to ethylbenzene on various catalysts. MOFs, metal–organic frameworks. Download figure Download PowerPoint Mechanistic studies We sought to gain an understanding of how metal-O clusters within MOFs affected Pt NPs’ activity and selectivity in the hydrogenation of phenylacetylene by exploring the mechanism of the electronic effects and the coordination environments of Pt NPs on MOFs. The electronic properties were experimentally confirmed by XPS and XAS spectra. The Pt 4f spectra of Pt/MIL-101(Cr) and Pt/MIL-101(Fe) showed two main peaks at 71.3 ± 0.1 and 74.6 ± 0.1 eV, corresponding to Pt 4f7/2 and Pt 4f5/2, respectively (Figure 3a).31 Interestingly, an 0.3 eV shift of Pt 4f toward the high-energy side was observed on Pt/HKUST-1, revealing the difference in the electronic status of the Pt NPs supported on HKUST-1, compared with that on MIL-101(Cr) and MIL-101(Fe). Furthermore, the Pt 4f7/2 was fitted with two components, including the predominant metallic Pt0 located in the binding energy of 71.3 eV in the spectra (denoted as red peaks) and Pt2+ at 72.3 eV (denoted as blue peaks).32 Pt/MIL-101(Cr) showed 28% amount of Pt2+ species, which was similar to that of the bare Pt NPs (27%), indicating a weak electronic interaction between Pt NPs and MIL-101(Cr).25 We speculated that this weak electronic interaction was mainly due to the less overlap of the d orbitals between Pt NPs and Cr-O clusters in MIL-101(Cr).23 Notably, the Pt2+ species ratio in Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts increased to 32% and 36%, respectively, revealing that Pt NPs on these two MOFs became electron deficient, compared with the bare Pt NPs. In comparison with the Cr 2p, Fe 2p, and Cu 2p XPS spectra of MIL-101(Cr), MIL-101(Fe), HKUST-1, and Pt/MOFs composites (Figures 3b–3d), the Cr 2p states remained unchanged after the deposition of Pt NPs, whereas noticeable increases in Fe2+ species (from 61.3% to 67.7%) and Cu+ species (from 22% to 41%) were observed. These phenomena suggested that electrons from Pt NPs were transferred to Fe-O and Cu-O clusters-based MOFs across the heterogeneous interface. As shown in the normalized X-ray absorption near-edge structure (XANES) at the Pt L-edge ( Supporting Information Figure S18), Pt/HKUST-1 exhibited a higher white line intensity than Pt/MIL-101(Fe), MIL-101(Cr), and bore Pt NPs, further confirming the electron deficiency of Pt NPs in Pt/HKUST-1.31 The local coordination environment of Pt in Pt/MOFs catalysts was further characterized by extended X-ray absorption fine structure (EXAFS; Supporting Information Figure S19). The stronger peaks at 2.6 and 1.6 Å could be assigned to Pt–Pt and Pt–O coordination, respectively (Figure 3e).33 The intensity of Pt–O peaks exhibited a gradual increase upon the deposition of Pt NPs on MIL-101(Fe), and HKUST-1 supports. The quantitative EXAFS curve fitting analysis revealed that the coordination number of Pt–O bond in Pt/MIL-101(Cr) was 0.4, while in Pt/MIL-101(Fe) and Pt/HKUST-1, it increased to 0.8 and 1.2, respectively ( Supporting Information Table S3). The formation of abundant Pt–O bonds and charge-transfer interactions demonstrated that MOFs matrix shared the same function as inorganic and organic materials to regulate the surface electronic status of Pt NPs. DFT calculations further proved that the exposed metal-O clusters on MOFs could withdraw electrons from Pt atoms in the order of MIL-101(Cr) < MIL-101(Fe) < HKUST-1 and consequently increased the Bader charge of Pt atoms (Figure 3f and Supporting Information Figure S20 and Table S4). Figure 3 | Electronic status of Pt on various Pt/MOFs catalysts for phenylacetylene hydrogenation reaction. (a) XPS profiles of Pt 4f for Pt NPs supported on a series of MOFs. The metallic Pt0 located at the binding energy of 71.3 and 74.6 eV, and oxidation state Pt2+ located at 72.3 and 75.5 eV. The ratio of Pt2+/Pt0 in various catalysts was marked. (b–d) XPS profiles of Cr 2p, Fe 2p, and Cu 2p in pure MOFs and Pt/MOFs catalysts. The ratio of reduction state of Fe2+ and Cu+ within Fe-O and Cu-O cluster-based MOFs was marked. (e) EXAFS spectra of Pt foil, PtO2, and Pt/MOFs catalysts. (f) DFT calculations of Bader charge of Pt atoms on different MOFs. (g) DFT calculations of the binding energy of H atoms on the Pt surface. (h) Photographs of 5 mg Pt/MOFs catalysts mixed with 45 mg of WO3 before and after treatment with H2 gas at 25 °C for 5 min. MOFs, metal–organic frameworks; XPS, X-ray photoelectron spectroscopy; EXAFS, extended X-ray absorption fine structure; DFT, density functional theory; WO3, tungsten oxide. Download figure Download PowerPoint The surface electronic status of Pt NPs directly correlated to the interaction with active H atoms and unsaturated molecules, thus, delivering different catalytic activities and selectivities of alkyne hydrogenation.34,35 The binding energy of active H atoms on Pt(111) (2 × 2) surface with different charges was explored by DFT calculations (Figure 3g and Supporting Information Figure S21 and Table S5). The small binding energy (−0.42 eV) of H atoms on the neutrally charged Pt surface indicated that active H atoms could readily migrate to the adsorbed phenylacetylene and/or styrene, resulting in overhydrogenation on Pt NPs and Pt/MIL-101(Cr) catalysts. When the Pt(111) surface charge increased to a positive value through the interfacial electron transfer from Pt to MOFs, the higher Pt–H binding energy (−1.2 eV) prevented the migration of active H atoms toward the adsorbed phenylacetylene and/or styrene, suppressing the overhydrogenation on Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts. These DFT results were evaluated further by the color change in tungsten oxide (WO3), since active H atoms could react readily with the yellow WO3 to form dark blue HxWO3.36 As depicted in Figure 3h, the Pt/HKUST-1 with WO3 powder mixture exhibited no change in color after the H2 treatment, whereas mixing Pt/MIL-101(Cr) and Pt/MIL-101(Fe) with WO3 powders showed a different extent of color changes, which revealed the distinct binding energies of active H atoms on the Pt surface. These differences in the color change of WO3 were consistent with DFT simulations and the performance of phenylacetylene hydrogenation on distinct Pt/MOFs catalysts. Furthermore, the partially oxidized Pt could adsorb phenylacetylene tightly on its surface, while weakening its interaction with styrene molecules,34 as verified by the lower catalytic activity (5%) of Pt/MOFs(Fe) catalysts for styrene (Figure 2c). The electronic environment of Pt NPs created by Fe-O clusters allowed for the semihydrogenation of phenylacetylene while styrene intermediates preferred desorption from the Pt surface. The results mentioned above demonstrated that the chemical environment of metal-O clusters within MOFs could regulate the catalytic performance of Pt NPs in the hydrogenation of phenylacetylene. It is known that the metal ions also affect the catalytic activity and selectivity of MNPs in the hydrogenation reaction.37 Therefore, we wondered whether the trace of metal ions released during the catalytic process would affect the catalytic performance. As shown in Supporting Information Figure S22, even when Fe3+ ions were increased to 6 ppm, no apparent decay of Pt NPs in the hydrogenation activity was observed. The high conversion (99%) of phenylacetylene to ethylbenzene indicated that the Fe3+ ions could hardly modify Pt NPs to realize alkyne semihydrogenation. In contrast, 6 ppm Cu2+ ions could hinder the hydrogenation activity completely, suggesting that the activity of Pt NPs was sensitive to Cu2+ ions. Considering the potential influence of metal ions on the activity of Pt NPs, we built a core–shell structure [email protected](Cr) as catalysts to better evaluate the influences of metal ions released from MIL-101(Fe) and HKUST-1 on phenylacetylene hydrogenation reaction ( Supporting Information Figure S23). [email protected](Cr) catalysts reached 99% conversion of phenylacetylene to ethylbenzene, and the reduced reaction rate was mainly a result of the diffusion of the reactant through the MOFs channel from the surface to the active sites ( Supporting Information Figure S24). When HKUST-1 powders were introduced to the reaction, the conversion of phenylacetylene only reached 35% within 1 h, after which no further increase occurred ( Supporting Information Figure S24, red dots). This result indicated that the synergistic effect of Cu-O metal clusters and released Cu2+ ions could poison Pt NPs. However, when the MIL-101(Fe) was added to the reaction, the conversion of phenylacetylene reached 100%, and the selectivity was similar to [email protected](Cr) ( Supporting Information Figure S24, blue dots). These results confirmed that the metal-O clusters within MOFs could regulate the catalytic chemoselectivity of Pt NPs in the hydrogenation of alkynes. Furthermore, the catalytic stability of Pt/MIL-101(Fe) catalyst for phenylacetylene hydrogenation was evaluated by PXRD, TEM, and XPS characterizations. There was a slight decrease in Pt NPs content of Pt/MIL-101(Fe) after three reaction cycles (from 2.2% to 2.0%; Supporting Information Figure S25) and negligible influence on catalytic performance ( Supporting Information Figure S26), indicating the catalytic stability of Pt/MIL-101(Fe) catalyst. The crystallinity of MIL-101(Fe) changed slightly ( Supporting Information Figure S27) as the sizes, and electronic status of Pt NPs were retained ( Supporting Information Figures S28 and S29), which confirmed the stability of the Pt/MIL-101(Fe) catalyst. Conclusion We successfully demonstrated that employing different metal-O clusters within MOFs enabled the regulation of interfacial electronic structures of Pt NPs for significant improvement of selective hydrogenation of phenylacetylene. We found that Pt/Fe-O cluster-based MOFs catalysts highly favored the semihydrogenation of phenylacetylene to form styrene while Pt/Cr-O cluster-based MOFs facilitated overhydrogenation to an alkane. More importantly, Pt NPs were deactivated completely when Cu-O metal cluster-based MOFs were used as supports. Our studies affirmed that the electronic status of MNPs modified by metal-O clusters and trace poisonous metal ions released from MOFs is crucial in regulating the hydrogenation of unsaturated molecules, thus, opening up new paths for designing a series of suitable MNP/MOFs catalysts to boost other selective hydrogenation reactions of alkynes. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflict of interest. Funding Information This study was supported by the National Key R&D Program of China (no. 2017YFA0207201), the National Natural Science Foundation (nos. 21727808, 21574065, 21604038, 21971114, 21604040, and 51702155), the National Science Foundation for Distinguished Young Scholars (no. 21625401), and the Jiangsu Provincial Funds for Natural Science Foundation (nos. BK20160975, BK20160981, and BK20170975). Acknowledgments The authors are grateful to the Synchrotron Radiation Research Center (NSRRC) in Taiwan for their help on X-ray absorption spectroscopy measurements.