Self-Assembly Ultrathin Fe-Terephthalic Acid as Synergistic Catalytic Platforms for Selective Hydrogenation

Abstract

Open AccessCCS ChemistryCOMMUNICATION3 Oct 2022Self-Assembly Ultrathin Fe-Terephthalic Acid as Synergistic Catalytic Platforms for Selective Hydrogenation Qinglin Liu, Jiahui Xian, Yinle Li, Quan Zhang, Hiroshi Kitagawa and Guangqin Li Qinglin Liu 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 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 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 , Quan Zhang Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502 Google Scholar More articles by this author , Hiroshi Kitagawa Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502 Google Scholar More articles by this author and Guangqin Li *Corresponding author: E-mail Address: [email protected] 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.022.202201801 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Alloy nanoparticles (NPs) with numerous exposed catalytic active sites have been extensively studied as efficient heterogeneous catalysts. However, it is challenging to synthesize alloy NP catalysts with high activity while avoiding aggregation. Herein, we report a facile method to encapsulate alloy NPs loaded metal–organic framework (MOF) catalysts (alloy NPs/MOFs) within an ultrathin metal–organic layer using a terephthalic acid (BDC) assisted method. A series of metal-BDC encapsulated PtM/MOFs (M = Fe, Co, and Ni) catalysts were synthesized successfully and showed significantly enhanced catalytic performance toward selective hydrogenation reaction. Especially, the Fe-BDC encapsulated PtFe2/UiO-66-NH2 demonstrated an impressive conversion efficiency of cinnamaldehyde (98.9%) and a high selectivity toward cinnamyl alcohol (95.4%) in 12 h reaction under mild conditions with remarkable stability, much better than Pt/UiO-66-NH2 and PtFe2/UiO-66-NH2 synthesized without BDC, indicating that the Fe-BDC layer could prevent the aggregation of the alloy catalysts effectively, thereby improving the stability of the catalysts. This work provides a valuable strategy to develop active and robust catalysts for heterogeneous reactions. Download figure Download PowerPoint Introduction Nanocatalysts often encounter problems with aggregation- and leaching-induced degradation of catalytic activity. Thus, encapsulating the nanocatalysts with protective layers such as mesoporous polymers, porous carbon, and oxide is mainly employed to improve the recyclability of the nanocatalysts.1–3 However, these methods are always accompanied by sacrificing the reaction activity of the catalysts.1,4 Therefore, exploring an effective way of creating nanocatalysts with good stability and desired catalytic activity is a crucial but challenging task.5–7 The selective hydrogenation products of α,β-unsaturated aldehydes, unsaturated alcohol (UOL), and saturated aldehyde (HUAL) are widely applied in industrial manufacture.8–11 Nevertheless, due to the complex conjugated system of α,β-unsaturated aldehydes, it remains a considerable challenge to obtain a high yield of desired hydrogenation product, especially for UOL.12,13 Hence, designing highly effective catalysts to produce selective UOL has been an arduous task for researchers. Pt is one of the best catalysts for selective hydrogenation of α,β-unsaturated aldehydes; however, the degree of selectivity is still unsatisfactory.4,14–17 Although alloying a 3d-transition metal with Pt has shown remarkable performance for selective hydrogenation reaction, the synthetic methods of Pt-based alloy catalysts are always harsh, in particular, PtFe alloy, which restricts their large-scale preparation.18–23 Therefore, establishing a gentle approach to prepare PtFe alloy-based catalysts is of paramount importance. Herein, we demonstrate a facile one-step synthesis of PtFe alloy nanoparticles (NPs) supported on metal–organic frameworks (MOFs) UiO-66-NH2 covered with ultrathin Fe-BDC (BDC = terephthalic acid or benzene-1,4-dicarboxylic acid) layer (Fe-BDC encapsulated PtFe2/UiO-66-NH2) under mild conditions. The structure of the Fe-BDC encapsulated PtFe2/UiO-66-NH2 obtained was confirmed by powder X-ray diffraction (XRD) and scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX) mappings. Compared with the pure Pt/UiO-66-NH2 and PtFe/UiO-66-NH2, the Fe-BDC encapsulated PtFe2/UiO-66-NH2 showed superior reaction activity and UOL selectivity. More importantly, it exhibited superior recycling stability. We also demonstrated the versatility of our new method by synthesizing different types of metal-BDC encapsulated alloy/MOFs nanocatalysts, which yielded almost the same functionality. Results and Discussion Structural and morphological characterizations on catalysts UiO-66-NH2 was initially prepared by a hydrothermal method (see Supporting Information Figure S1). Since the amine groups in MOF could strongly couple with the loading of metal NP,16 UiO-66-NH2 was chosen to be the ideal support. The pure Pt NPs located on the surface of UiO-66-NH2 were synthesized through a wet-chemical process as illustrated in Scheme 1, and the function of BDC was deeply researched. Without BDC, only a small amount of Pt NPs was reduced onto the UiO-66-NH2, while the distribution of Pt NPs became evenly and intensively after adding a moderate amount of BDC, with particles size ∼5.17 nm, which was confirmed by transmission electron microscopy (TEM; see Supporting Information Figures S2 and S3). The high-resolution TEM (HRTEM) image showed the lattice points, confirming that the Pt element was distributed homogeneously in the entire UiO-66-NH2. Thus, the significance of BDC was recognized as reducing metals on MOFs. The powder XRD patterns (see Supporting Information Figure S4) of the Pt/UiO-66-NH2 samples synthesized with varying amounts of BDC, all matched those of the pristine UiO-66-NH2, indicating that the structure of MOF was well sustained using our developed preparation protocols. Scheme 1 | Fabrication of PtFe alloy/UiO-66-NH2 covered by ultrathin Fe-BDC in one step. Download figure Download PowerPoint An analogous method was applied to synthesize PtFe2/UiO-66-NH2 using 18 mg BDC. As shown in Figure 1a, the NPs were uniformly dispersed on UiO-66-NH2 with a mean size of 4.29 nm, slightly smaller than the one of Pt/UiO-66-NH2 (see Supporting Information Figure S5). The HRTEM image in Figure 1b showed that the interplanar spacing of 0.217 nm was assigned to the (111) facet of PtFe alloy. The smaller lattice spacing implied the successful incorporation of Fe atoms into the Pt nanostructure.24,25 To ensure the existence and distribution of Fe, EDX element mappings were tested and presented homogeneous element distribution of Pt and Fe in the entire UiO-66-NH2 (Figure 1c and Supporting Information Figure S6). Moreover, the compositional line-scanning profile in Figures 1d and 1e further certified that PtFe alloy was synthesized successfully by this developed method under mild reaction conditions. Similarly, to assess the function of BDC in the synthesized PtFe2/UiO-66-NH2, we utilized varying amounts of BDC in a parallel experiment and characterized by TEM. As shown in Supporting Information Figure S7, when synthesized without BDC, large particles were found on the surface of UiO-66-NH2, while the XRD patterns were preserved when BDC was added to the preparation (see Supporting Information Figure S8). PtFe2/UiO-66-NH2 synthesized with different BDC were all used as catalysts for selective hydrogenation of cinnamaldehyde. Experiment results were exhibited in Supporting Information Figure S9 and Table S1, PtFe2/UiO-66-NH2 synthesized with 18 mg BDC performed best, and 18 mg was considered as the optimized amount of BDC in the following experiments. Figure 1 | (a) TEM image of PtFe2/UiO-66-NH2 synthesized with 18 mg BDC. (b) HRTEM image of PtFe alloy NPs. (c) High-angle annular dark-field (HAADF)-STEM-EDX element mappings of PtFe2/UiO-66-NH2. (d) HAADF-STEM image. and (e) Elemental line-scanning profiles. Download figure Download PowerPoint Encouraged by these promising results, we further explored the universality of this newly developed method. First, we prepared PtFe alloy located on MOF by adding different molar ratios of metal precursor, and the morphology of as-synthesis PtFe1/UiO-66-NH2 and PtFe3/UiO-66-NH2 was also characterized by TEM. As shown in Supporting Information Figures S10 and S11, the metal NPs were dispersed homogeneously on UiO-66-NH2 and obtained the lattice fringe of 0.221 and 0.213 nm, respectively, and both were smaller than the (111) plane of Pt. The average sizes of alloy NPs of PtFe1/UiO-66-NH2 and PtFe3/UiO-66-NH2 were 4.01 and 4.21 nm, respectively. Generally, noble metal alloying with transition metal usually forms larger-sized NPs if prepared by a coreduction method, mainly because of the slow reduction rate of the transition metal precursor, possibly leading to inferior catalytic performance.26–28 However, the smaller particles size of PtFe alloy compared with the pure Pt on UiO-66-NH2 indicated that our developed recipe would not negatively influence the alloy NPs size, promising intrinsic catalytic activity.29–31 The actual metal content was detected by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and the results are listed in Supporting Information Table S2. The element ratio of Pt∶Fe was close to the feeding amount, meaning this method could readily control the element ratio only by changing the feeding ratio of the metal precursors. To explore the function of –NH2 group, another type of MOFs UiO-66 was used as support for metal NPs. Employing a similar method, Pt/UiO-66 and PtFe2/UiO-66 were readily obtained and presented a good structure as other samples (see Supporting Information Figures S12 and S13). To understand the crystalline structure of different samples, powder XRD was performed (see Supporting Information Figure S14). The Pt/UiO-66-NH2 and PtFe/UiO-66-NH2 patterns were well-matched with that of the pristine support; once again, this certified that the structure of UiO-66-NH2 was perfectly maintained by applying this developed preparation recipe. No prominent diffraction peak was observed corresponding to the Pt or PtFe alloy, probably because of the low metal content and small size of the NPs.32–34 Besides, the relative strong intensity diffraction peak of UiO-66-NH2 concealed the peak of NPs.35,36 To eliminate the influence of the peak belonging to UiO-66-NH2, we synthesized PtFe2 supported on carbon black (CB) by the same recipe, changing the support. As shown in Supporting Information Figure S15, the as-synthesis PtFe2/CB showed a larger particle size than the metal supported on UiO-66-NH2, which might be another proof for a faster reduction rate of metal precursor when interaction with the amino group on MOF. The shark peak observed in the XRD pattern was located between the standard pattern of Pt and Fe, further confirming the successful preparation of PtFe alloy by our developed method. To deeply explore the structure of as-prepared materials, Fourier transform infrared (FT-IR) and N2 adsorption–desorption isotherm was examined. From the FT-IR spectrum in Supporting Information Figure S16, we can find out that the peak at 1250 cm−1, which was related to the vibration modes induced by the −NH2 group, had an intensity decrease after reducing metal precursor onto UiO-66-NH2, implying the strong interaction of metal precursors with the −NH2 groups.36,37 According to the N2 adsorption–desorption isotherm profiles in Supporting Information Figure S17, the Brunauer–Emmett–Teller (BET) surface area and total pore volume of UiO-66-NH2 were calculated to be 876.3 m2 g−1 and 0.26 cm3 g−1, respectively (see Supporting Information Table S3). After loading Pt NPs, the BET surface area and total pore volume were decreased accordingly to 721.3 m2 g−1 and 0.20 cm3 g−1. A similar phenomenon was found with the result of PtFe2/UiO-66-NH2; the BET surface area and pore volume were further dropped to 658.5 m2 g−1 and 0.21 cm3 g−1, attributed to the blockage of cavities of UiO-66-NH2.38,39 These results indicated that more metal NPs were dispersed on the surface of UiO-66-NH2 owing to the reduction of both Pt and Fe precursor, consistent with the TEM images of PtFe2/UiO-66-NH2. To better understand the exact structure of PtFe2 alloy/UiO-66-NH2, different control samples were prepared by modifying the preparation condition and further characterized by XRD, TEM, and EDX. The XRD data in Supporting Information Figure S18 confirmed that the Fe(acac)2 was able to react with BDC to form Fe-BDC. The TEM images in Supporting Information Figure S19 figured out that the Fe-BDC would cover the metal particle and MOF substrates, further proven by the EDS test in Supporting Information Figure S20. In a word, a thick Fe-BDC layer covered the PtFe2/UiO-66-NH2 structure when the amount of metal precursor and BDC was significantly increased, implying that ultrathin Fe-BDC could also form in the standard preparation process of PtFe2/UiO-66-NH2. To further realize the external element composition of PtFe2 alloy/UiO-66-NH2, an Ar ion etching experiment was conducted, and then characterized by X-ray photoelectron spectroscopy (XPS), as shown in Supporting Information Figure S21 and Table S4. When the etching was more profound, the atomic ratio of Fe decreased explicitly, while the content of Pt was sustained, which yielded another evidence for the existence of ultrathin Fe-BDC on the surface of the entire structure. The intrinsic catalysis behavior greatly depends on the electronic properties of the surface atom of catalysts.40,41 Therefore, the chemical state of Pt in Pt/UiO-66-NH2 and PtFe2/UiO-66-NH2 was characterized by XPS (see Supporting Information Figure S22) and X-ray absorption spectra (see Supporting Information Figure S23). In Figure 2a, the binding energy of Pt 4f in PtFe2/UiO-66-NH2 was slightly lower than that of Pt/UiO-66-NH2, meaning that very few electron transfers to Pt after alloy with Fe, which might boost the activity of hydrogenation reaction.14 As for the Fe element in PtFe2/UiO-66-NH2, a 2p spectrum was further fitted. The peaks at 709.5 and 722.0 eV were corresponding to Fe0 2p5/2 and 2p3/2, respectively (Figure 2b). The other peaks were assigned to three groups, the peak at 711.8 and 717.7 eV belonged to Fe3+ 2p5/2 and its satellite, and the peak located at 714.3 eV was related to the hydroxide groups.42 Supporting Information Figure S22 displayed normalized X-ray absorption near-edge structure (XANES) spectra; Figures 2c and 2d showed enlarged XANES spectra, as well as the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra in R-space of Pt L3-edge. The valence of Pt in PtFe2/UiO-66-NH2 was slightly lower than that of Pt/UiO-66-NH2. After alloying with Fe, the EXAFS of Pt L3-edge recorded slightly weakened Pt–Pt scattering intensity and shortened Pt–Pt bond length, which was another evidence of alloy formation.43–45 Figure 2 | XPS spectra of (a) Pt 4f in Pt/UiO-66-NH2 and PtFe2/UiO-66-NH2, (b) Fe 2p in PtFe2/UiO-66-NH2 and Fe-BDC. (c) The enlarge XANES spectra and (d) the Fourier-transformed EXAFS spectra in R-space of Pt L3-edge. Download figure Download PowerPoint To expand the practical application of our developed method, different transition metal precursors were applied in the synthesis. PtCo2/UiO-66-NH2 and PtNi2/UiO-66-NH2 were synthesized with the same method as PtFe2/UiO-66-NH2, except for using Co(acac)2 and Ni(acac)2 instead of Fe(acac)2, respectively. As shown in Supporting Information Figure S24, the morphology of PtCo2/UiO-66-NH2 was quite similar to that of PtFe2/UiO-66-NH2. The NPs dispersed homogeneously on the UiO-66-NH2 with an average size of ∼4.63 nm. Contrast samples Co-BDC and PtCo2/UiO-66-NH2(h) were also prepared (detailed experimental procedure in Supporting Information). As shown in Supporting Information Figure S25, a thick layer of Co-BDC protected the whole PtCo2/UiO-66-NH2, further verifying the versatility of our developed method. Interestingly, the particle size of PtNi alloy was a little larger, ∼7.67 nm, which might be attributed to the slowly reduced speed of the Ni precursor (see Supporting Information Figures S26 and S27).26,46 Pure Fe2/UiO-66-NH2 was prepared without Pt salt, but no particle was found on MOFs, indicating that the single Fe precursor was hardly reduced to form particle without Pt salt (see Supporting Information Figure S28). Nonetheless, according to the result of ICP-AES, the mass loading of Fe was 2.59 wt %, once again certifying the conclusion that Fe(acac)2 was able to react with BDC to form Fe-BDC. Further, another type of noble metal was induced using this method; PdFe2/UiO-66-NH2 was also prepared successfully and characterized, as displayed in Supporting Information Figure S29. Other supports like MIL-101 Cr, MOF-74, and Al2O3 were used to verify the universality of our developed approach. As shown in Supporting Information Figure S30, PtFe alloy NPs with an average size of 4.04 nm were stuck on the surface of MIL-101 Cr. Also, two other types of catalysts, PtFe2/Al2O3 and PtFe2/MOF-74, were obtained successfully using a similar recipe; detailed information is exhibited in Supporting Information Figures S31 and S32. Catalytic performance of selective hydrogenation Since the hydrogenation products of α,β-unsaturated aldehydes, HUAL and UOL, are widely applied in reaction intermediate, obtaining a high yield of selective hydrogenation product under mild conditions, especially for UOL, is always a considerable challenge.47–51 We chose cinnamaldehyde (CAL) as a model molecule to evaluate the catalysis performance due to its essential role in industrial manufacture.52–54 Figure 3a demonstrates the possible hydrogenation product of CAL, hydrocinnamyl alcohol (HCOL) was produced by further hydrogenation of cinnamyl alcohol (COL) or hydrocinnamaldehyde (HCAL). First, the catalytic performance of Pt/UiO-66-NH2 was evaluated at room temperature under 1 bar H2. As shown in Figure 3b, after a 12 h reaction, the selectivity of COL was 63.5% at the CAL conversion of 48.5%. For comparison, PtFe alloy/UiO-66-NH2 catalyzed hydrogenation reaction under the same reaction conditions, and excellent catalytic performance was exhibited, as shown in Figure 3c and Supporting Information Figure S33. With the addition of Fe, the reaction activity increased sharply, with high selectivity for COL during the entire reaction process. The catalysis results listed in Table 1 (entries 1–4) pointed out that PtFe2/UiO-66-NH2 gave the optimized performance, with the highest conversion of CAL (98.9%) and selectivity for COL (95.4%). To verify the intrinsic catalytic activity of the Fe element, a monometallic Fe2/UiO-66-NH2 catalyst was also applied in a hydrogenation reaction. The Fe2/UiO-66-NH2 did not possess catalytic ability for hydrogenation of CAL under the same reaction conditions. We then performed the catalytic reaction with a physical mixture of pure Pt and Fe catalysts. The physical mixture showed an unsatisfactory performance compared to the PtFe2/UiO-66-NH2 (Table 1, entry 15), meaning that the impressive catalytic activity and product selectivity were contributed to the synergistic effect of PtFe alloy. Figure 3 | (a) Schematic of CAL hydrogenation. Activity and selectivity changes of (b) Pt/UiO-66-NH2 and (c) PtFe2/UiO-66-NH2 for hydrogenation of CAL with different time. (d) Reusability test of PtFe2/UiO-66-NH2 for selective hydrogenation of CAL. (Reaction condition: 0.3 mmol CAL + 5 mg catalyst+ 2.5 mL isopropanol + 2.5 mL water, RT, H2 balloon, 12 h.) Download figure Download PowerPoint Table 1 | CAL Hydrogenation Activities and Selectivity over Different Catalystsa Entry Catalysts Conv. (%) SelHCAL (%) SelHCOL (%) SelCOL (%) TOFd (h−1) 1. Pt/UiO-66 21.5 22.0 7.5 70.4 7.7 2. Pt/UiO-66-NH2 48.5 25.4 11.1 63.5 8.5 3. PtFe1/UiO-66-NH2 98.5 — 10.4 89.6 18.2 4. PtFe2/UiO-66-NH2 98.9 0.4 4.1 95.4 19.5 5. PtFe3/UiO-66-NH2 98.8 — 6.8 93.1 20.7 6. PtCo2/UiO-66-NH2 92.6 11.8 7.2 80.9 19.8 7. PtNi2/UiO-66-NH2 98.0 3.7 70.8 25.5 18.1 8. PdFe2/UiO-66-NH2 15.0 81.9 14.8 3.2 3.2 9. PtFe2/MIL-101 Cr 99.9 0.49 29.0 70.5 19.2 10. PtFe2/MOF-74 67.9 51.2 6.8 41.9 10.8 11. PtFe2/Al2O3 77.2 2.3 1.7 96.1 12.1 12. PtFe2/UiO-66 74.7 2.2 1.6 96.2 13.2 13. PtFe2/CB 1.6 12.5 4.4 82.5 0.3 14. Fe2/UiO-66-NH2 N.D — — — 15. Physical mixture Pt + Fe catalystsb 31.2 10.3 — 89.7 5.6 16. Commercial Pt/Cc 93.6 31.3 57.7 11.0 22.8 17. UiO-66-NH2 N.D. — — — — 18. Pt(acac)2 N.D. — — — — aReaction condition: 0.3 mmol CAL + 5 mg catalyst + 2.5 mL isopropanol + 2.5 mL water RT, H2 balloon, 12 h. b2.5 mg Pt/UiO-66-NH2 + 2.5 mg Fe2/UiO-66-NH2. c0.5 mg 20 wt % Pt/C. dTOF is calculated according to the amount of Pt and the conversion after reaction for 12 h. Other catalysts synthesized by different metal precursors or support were also employed in CAL hydrogenation under the same reaction conditions. The pure Pt supported on UiO-66 provided a slightly lower turnover frequency (TOF) value than the catalyst Pt/UiO-66-NH2, but the selectivity toward different products was close to that of Pt/UiO-66-NH2. Interestingly, in a parallel experiment, using PtFe2/UiO-66 in the hydrogenation reaction presented inferior conversion of CAL (74.7%) but high selectivity for COL (96.2%), compared with PtFe2/UiO-66-NH2. We inferred that the lone pair electrons on the amine groups could attract the electropositive C=O group of CAL, leading to an improvement of the reaction rate.16,55–58 PtFe2 support on CB exhibited trace conversion of CAL (1.6%), further confirming the significance of UiO-66-NH2 lone pair electrons on –NH2 boosting the reaction speed.59,60 To verify the superiority of the synergistic effect between Pt and Fe in the alloy for selective hydrogenation, other Pt-based alloys were used for CAL hydrogenation. High conversion but poor selectivity for COL (80.9% and 25.5%) were achieved over PtCo2/UiO-66-NH2 and PtNi2/UiO-66-NH2, respectively. When the Pt was replaced by Pd, the generated PdFe2/UiO-66-NH2 showed inferior reaction performance than the standard PtFe2/UiO-66-NH2 sample. Considering the catalysis results mentioned above, we concluded that alloying Pt with transition metal improved the reaction rate, in contrast to the pure Pt catalysts, but the improvement of selectivity for COL might be attributed to the special synergistic effect of the PtFe alloy. Recycle ability is considered to be another significant factor in evaluating the practicability of catalysts. Principally, metal NPs immobile on the surface of MOFs decay the catalytic ability readily due to the loss of metal during the successive reaction cycles.61,62 Favorably, PtFe2/UiO-66-NH2 still retained an impressive catalytic activity and selectivity for COL after eight successive cycles of CAL hydrogenation reaction (Figure 3d). The morphology and structure of the reused catalyst were well-preserved, as shown in Supporting Information Figure S34, indicating that the metal NPs were hardly loose or aggregate, owing to the protection of ultrathin Fe-BDC, which ensured the durable catalytic ability of PtFe2/UiO-66-NH2. With this excellent catalytic performance in hand, we used the optimized PtFe2/UiO-66-NH2 to hydrogenate some other α,β-unsaturated aldehydes (see Supporting Information Table S5). All of α,β-unsaturated aldehydes converted to the corresponding UOL via selective hydrogenation under mild conditions. However, due to the steric hindrance from the –CH3 and –N(CH3)2 groups, the α-methyl-cinnamaldehyde and 4-dimethyl-amino-cinnamaldehyde achieved slightly lower conversion and even prolonged the reaction time, but fortunately, remained highly selective for the corresponding UOL. Other substrates like 4-methoxycinnamaldehyde and 3-(2-furyl)acrolein could reach over 90% conversion, accompanied by nearly 95% selectivity toward UOL products catalyzed by PtFe2/UiO-66-NH2. Briefly, PtFe alloy supported on UiO-66-NH2 with the coverage of ultrathin Fe-BDC, synthesized by our developed method, could be widely applied in selective hydrogenation of different α,β-unsaturated aldehydes, achieving excellent catalytic performance. Being aware of the versatility of PtFe2/UiO-66-NH2, the reason why a high yield of UOL was obtained via hydrogenation in mild conditions should be further explored. Thus, the intermediate product COL and HCAL were hydrogenated with the catalysis of Pt/UiO-66-NH2 and PtFe2/UiO-66-NH2 under the same reaction conditions. Surprisingly, the catalytic performances of the two catalysts were different ( Supporting Information Figure S35). The COL gradually turned into HCOL with increasing reaction time using pure Pt/UiO-66-NH2 as the catalyst. In contrast, with PtFe2/UiO-66-NH2 as a catalyst, more than 22% of HCAL was hydrogenated to HCOL after the reaction proceeded for 12 h, while COL hardly converted to HCOL in the entire reaction, implying that after alloying Pt with Fe, the hydrogenation rate of C=O bond was sharply accelerated, but the hydrogenation of C=C bond was inhibited to a great extent. This must be the reason for the high yield of COL when using PtFe2/UiO-66-NH2 as catalyst.63,64 In general, the selectivity enhancement of UOL was possibly based on the electronegative Pt in PtFe2/UiO-66-NH2 compared with Pt/UiO-66-NH2, capable of activating the C=O bond in the CAL leading to the high yield of COL, superior to most of the hydrogenation catalysts reported before (see Supporting Information Table S6). Besides, the electrophilic C=C bond hardly interacted with the electropositive Fe site, which inhibited further hydrogenation of COL to sustain the high yield of the desired product even after prolonged reaction time.24,49,65,66 Conclusion We initially developed an approach for one-step synthesis of PtFe alloy/UiO-66-NH2 encapsulated with ultrathin Fe-BDC. What is more, our method could be widely extended using different metal precursors and MOF supports. The optimized catalyst PtFe2/UiO-66-NH2 achieved 95.4% selectivity toward COL, with 98.8% conversion under mild reaction conditions. The ultrathin Fe-BDC not only protects the metal NPs from losing during the recycle test but also activates to C=O bond, leading to satisfied selectivity of COL. This work opens a new avenue for synthesizing the hydride structure of metal NPs and MOFs for selective hydrogenation. Supporting Information Supporting Information is available and includes experimental materials and details, TEM images, element mappings, XRD, IR, nitrogen adsorption isotherms, XPS, XANES, and catalytic performance tests. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work has been supported by funding from 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 100 Talents Plan Foundation of Sun Yat-sen University, the Program for Guangdong Introducing Innovative and Entrepreneurial Teams (no. 2017ZT07C069), the NSFC Projects (nos. 22075321, 21821003, and 21890380), and the China Postdoctoral Science Foundation (nos. 2019M653141 and 2020M682042).