Efficient Heterogeneous Hydroformylation over Zeolite-Encaged Isolated Rhodium Ions

Open AccessCCS ChemistryRESEARCH ARTICLES20 Jul 2022Efficient Heterogeneous Hydroformylation over Zeolite-Encaged Isolated Rhodium Ions Weixiang Shang†, Bin Qin†, Mingyang Gao, Xuetao Qin, Yuchao Chai, Guangjun Wu, Naijia Guan, Ding Ma and Landong Li Weixiang Shang† School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Bin Qin† School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Mingyang Gao School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Xuetao Qin Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, BIC-ESAT Peking University, Beijing 100871 , Yuchao Chai School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Guangjun Wu School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Naijia Guan School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Ding Ma Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, BIC-ESAT Peking University, Beijing 100871 and Landong Li *Corresponding author: E-mail Address: [email protected] School of Materials Science and Engineering, Nankai University, Tianjin 300350 Haihe Laboratory of Sustainable Chemical Transformations, Tianjin 300192 https://doi.org/10.31635/ccschem.022.202202043 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Alkene hydroformylation is an extremely important industry process currently accomplished via homogeneous catalysis. Heterogeneous hydroformylation is being avidly pursued as a more economical and sustainable process. Herein, we report the construction of zeolite-encaged rhodium catalyst for efficient hydroformylation. Through a facile in situ hydrothermal strategy, isolated Rhδ+ (δ = 2.5) can be encaged in faujasite and efficiently stabilized via interaction with framework oxygen atoms, producing a [email protected] model catalyst with well-defined rhodium sites and coordination environment. [email protected] exhibits high catalytic activity, perfect chemoselectivity, and recyclability in 1-hexene hydroformylation under mild reaction conditions, making it a robust heterogeneous catalyst for potential applications. A state-of-the-art turnover frequency value of 6567 molC=C/molRh/h for [email protected] can be achieved in 1-hexene hydroformylation at 393 K, outperforming all heterogeneous catalysts and most homogeneous catalysts under comparable conditions. With the well-defined structure of [email protected], the detailed mechanism of alkene hydroformylation can be interpreted via theoretical calculations, and the advantages of heterogeneous hydroformylation are well explained. This work provides a promising solution toward efficient heterogeneous noble metal catalysis by encaging stable isolated ions in a zeolite matrix. Download figure Download PowerPoint Introduction With more than one hundred thousand metric tons of oxo chemicals produced annually, alkene hydroformylation is recognized as an extremely important and large-scale industry process for the production of aldehydes via homogeneous catalysis.1,2 The aldehyde products from alkene hydroformylation are widely employed in the synthesis of fine chemicals like alcohols, carboxylic acids, and amines and in further processing for the manufacture of various pharmaceuticals, plasticizers, surfactants, and flavorings.1,3 As early as 1938, Roelen4 disclosed that unmodified carbonyl cobalt, namely Co2(CO)8, could catalyze the hydroformylation of propylene, and the first set of equipment for butyraldehyde production was built in 1947.5,6 The process was operated necessarily under harsh reaction conditions (20–30 MPa syngas pressure, 423–453 K) to obtain a sustainable reaction rate and ensure the stable existence of the catalytically active species HCo(CO)4.7,8 Modifications of carbonyl cobalt by introducing phosphorus ligands, preferentially phosphines, were then investigated to improve the catalyst stability and selectivity for n-aldehyde, and meanwhile, the reaction pressure of hydroformylation could be lowered to 5–10 MPa.9–12 However, such modifications unavoidably decreased the catalytic activity and intensified the hydrogenation side reaction.13 Rhodium was found to exhibit superior intrinsic catalytic properties to cobalt, catalyzing alkene hydroformylation under relatively mild reaction conditions (2–12 MPa, 343–433 K).1,13–16 Since their first industrialization in the 1970s, Rh-complexes employing phosphorus ligands,17–20 particularly phosphine PPh3 and phosphite P(OPh)3, have been employed in commercial hydroformylation processes.13,14,21 Occasionally, homogeneous Ru or Pt-based catalysts were capable of catalyzing the hydroformylation reaction, but more hydrogenation products and/or lower activities were obtained.22–27 Other transition-metal carbonyl complexes of group VIII elements in the periodic table, such as Ir, Os, Pd and Fe, were investigated, but they exhibited significantly inferior activity without exception.28–30 Homogeneous rhodium catalysis is still the most effective and feasible strategy for alkene hydroformylation. However, the process of catalyst separation, purification, and recycling is very complicated, and the loss of rhodium is inevitable, significantly increasing the cost of the hydroformylation process.15,31 To avoid the problem of catalyst separation and recycling, the liquid–liquid biphasic system has been developed, and the hydroformylation catalysis can be achieved by the efficient transfer of organic substrates into the aqueous catalyst system.32–34 The aqueous biphasic Ruhrchemie/Rhône-Poulenc process with water-soluble Rh-tris(m-sulfonatophenyl) phosphine complexes as catalysts has been applied to industrial propylene hydroformylation since 1984.35 However, due to the low reaction rates caused by the low solubility of alkene substrates with increasing carbon number in aqueous media, the biphasic catalysis process is mostly restricted to the hydroformylation of C3–C4 olefins.36 The heterogenization of homogeneous catalyst, anchoring rhodium or its complexes on insoluble supports, is therefore proposed to address these drawbacks.37–45 Zeolite supported rhodium complexes have been investigated as immobilized homogeneous catalysts for decades, which can catalyze the hydroformylation reaction via classic homogeneous catalysis pathway.37–40 Isolated active rhodium sites immobilized onto the phosphine-functionalized polymers46 or oxides47–50 have exhibited remarkable catalytic activity in hydroformylation, even comparable to homogeneous rhodium catalysts, and provide good chemoselectivity. Because of the maximized rhodium utilization efficiency, facile catalyst separation, and strong metal-support interaction to prevent rhodium leaching, supported atomically dispersed or single-atom rhodium catalysts are proposed as promising candidates for hydroformylation with both the advantages of hetero- and homogeneous catalysis.47,49,51 Compared with conventional oxide supports, zeolites are preferable hosts that accommodate highly dispersed or even atomically dispersed active sites. Zeolite confinement effects can modulate the properties of encaged sites and endow more catalytic functionalities, in addition to the simple stabilization of these sites against leaching and aggregation.52–54 Many recent studies have constructed zeolite-encaged isolated sites for various catalytic applications with fruitful output,55–58 inspiring the exploration of zeolite-encaged isolated rhodium sites for heterogeneous hydroformylation. Herein, we report a facile strategy to construct faujasite encaged atomically dispersed rhodium catalyst [email protected] via an in situ hydrothermal route, which demonstrates remarkable catalytic performance in the hydroformylation of a wide variety of alkenes under relatively mild conditions. Due to the well-defined structure of [email protected], the mechanism and advantages of heterogeneous hydroformylation can be well understood. Experimental Methods Rhodium species encaged in faujasite zeolite ([email protected]) were synthesized via an in situ hydrothermal route (experimental details, materials, and methods are shown in the Supporting Information). Various characterization results, including X-ray diffraction (XRD) patterns, Ar adsorption–desorption isotherms, thermogravimetry-differential thermal analysis (TG-DTA) curves, transmission electron microscopy (TEM) images and nuclear magnetic resonance (NMR) spectra, the detailed kinetic measurements, more catalytic data, and all the parameters in the spin-polarized density functional theory (DFT) calculations, are shown in the Supporting Information. Results and Discussion Synthesis and characterization of [email protected] The ligand-protected in situ hydrothermal route has been developed for the encapsulation of transitional metal ions into a zeolite matrix. In this study, 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane was optimized not only to protect rhodium ions against aggregation as ligand, but also to work as the structure-directing agent for the formation of faujasite zeolite. The crystallization of zeolite was accomplished at low temperatures (see Supporting Information for details), completely inhibiting the precipitation of rhodium ions during synthesis. Thus, [email protected] with homogenous rhodium dispersion was successfully prepared via the in situ hydrothermal route. The calcined [email protected] product showed clear diffraction peaks corresponding to pure-phase FAU topology with high crystallinity ( Supporting Information Figure S1), giving type I isotherms with characteristic microporous structure ( Supporting Information Figure S2 and Table S1). The thermogravimetric curve of raw [email protected] revealed that the organic ligand employed in the synthesis could be fully removed at <623 K ( Supporting Information Figure S3). The overview scanning electron microscopy (SEM) image of calcined [email protected] (Figure 1a) showed typical faujasite crystals of cubic shape with uniform size of ∼1.0 μm. Single-particle TEM image and scanning TEM (STEM) element mapping analyses indicated the good dispersion of rhodium species throughout the zeolite crystallite in [email protected] ( Supporting Information Figure S4 and Figure 1b), in significant contrast to Rh/Y and Rh-Y where the enrichment of Rh species near the surface was observed ( Supporting Information Figures S5 and S6). In the representative high-resolution TEM (HR-TEM) image, clear lattice fringes of faujasite zeolite were observed while no signs of rhodium aggregates were detected (Figure 1c). The Cs-corrected high-angle annular dark field (HAADF)-STEM image further indicated the presence of atomically dispersed rhodium species within the zeolite matrix, which were most likely to appear as brighter dots due to the difference in atomic contrast (Si = 14, O = 8, Al = 13, Rh = 45) (Figure 1d,e).59 According to the results from microscopic analyses, atomically dispersed rhodium species were successfully encaged and stabilized by the matrix of faujasite zeolite, creating an opportunity for the maximal utilization of noble metal rhodium in hydroformylation. Figure 1 | Electron microscopy of [email protected] (a) Overview SEM image; (b) selected-area STEM element mapping analyses; (c) HR-TEM image; Cs-corrected HAADF-STEM image along [110] (d) and approximate [111] (e) directions with possible rhodium species highlighted by dashed circles. Download figure Download PowerPoint Next, we focused on the existing states of rhodium sites and their interaction with faujasite zeolite. The structural integrity of zeolite was confirmed by solid-state magic-angle spinning (MAS) NMR. The 27Al MAS NMR spectrum of [email protected] showed only one resonance signal at 62 ppm due to the four-coordinated framework aluminum species, similar to the cases of NaY and Rh/Y ( Supporting Information Figure S7). The 29Si MAS NMR spectra of [email protected], Rh/Y, and NaY all showed five resonance signals at −105, −100, −94, −89, and −85 ppm due to framework Si(OSi)4, Si(OSi)3(OAl), Si(OSi)2(OAl)2, Si(OSi)(OAl)3, and Si(OAl)4 species, respectively ( Supporting Information Figure S8). Thus, the introduction of rhodium species did not affect the structural integrity of zeolite, and the rhodium species should be stabilized via interaction with framework oxygen atoms. In the temperature-programmed reduction profiles (Figure 2a), [email protected] showed a major peak centered at 443 K and a minor peak at 373 K, corresponding to the reduction of rhodium ions with strong and weak interactions with zeolites, respectively.60–62 In significant contrast, Rh/Y and Rh-Y showed a major reduction peak at 353 and 363 K, respectively. The difference in the strength of Rh-zeolite interactions in [email protected] and Rh/Y or Rh-Y were clearly demonstrated. The oxidation states of Rh species in [email protected], Rh/Y, and Rh-Y were investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 2b, two binding energy values at 313.3(4) and 308.5(6) eV due to Rh 3d3/2 and 3d5/2 signals, respectively, were observed for Rh/Y and Rh-Y, suggesting the cationic state of rhodium species.46,49 For [email protected], the Rh 3d binding energy signals slightly shifted to lower values (313.1 and 308.3 eV) compared with those observed for Rh/Y and Rh-Y, indicating the stronger interaction between rhodium ions Rhδ+ (1<δ<3) and framework oxygen and the electron transfer thereof.41,46,47 When treated in flowing H2 at 573 K, the rhodium ions in [email protected] were completely reduced to metallic rhodium, giving binding energy signals at 312.1 and 307.3 eV (Figure 2b). Fourier transform infrared (FT-IR) spectroscopy of CO adsorption was employed to distinguish the highly dispersed rhodium species and aggregates.57,63 As shown in Figure 2c, two clear IR bands at 2105 and 2030 cm−1 ascribed to the symmetric and asymmetric stretching modes of isolated gem-dicarbonyl Rh(CO)2 species, respectively, were observed on [email protected] These observations, together with the absence of linear Rh-CO and bridged-CO species, indicated that the well isolated rhodium sites were encaged in zeolite,49,64 in good accordance with Cs-corrected HAADF-STEM observations (Figure 1d,e). For Rh/Y and Rh-Y, a strong band at 2060–2070 cm−1 due to linear-absorbed CO on rhodium ions, that is, Rh–CO species, was observed, and the weak bands at ∼2110 and ∼2040 cm−1 associated with Rh(CO)2 species were also present.65,66 The presence of Rh–CO species hinted at the formation of small rhodium aggregates, also in accordance with TEM observations ( Supporting Information Figures S5 and S6). The configuration and coordination environment of rhodium sites encaged in zeolite were fully characterized by X-ray absorption spectroscopy (XAS). The Rh K-edge X-ray absorption near-edge structure (XANES) spectra in Figure 2d revealed the existence of positively charged rhodium species Rhδ+ (δ = 2.5), calculated from its white line intensity peak in comparison with reference Rh foil and Rh2O3.47,67 A prominent peak at ∼1.5 Å due to the first shell of the Rh–O path was observed in the FT k2-weighted extended X-ray absorption fine structure (EXAFS) spectrum of [email protected] (Figure 2e), and the average coordination number of Rh–O was determined to be 4.6 from the EXAFS fitting curve and the wavelet-transformed EXAFS oscillations (Figure 2f,g and Supporting Information Table S3). The absence of the Rh–Rh path in the first shell might imply the formation of atomically dispersed Rhδ+ species in [email protected], as confirmed in the HAADF-STEM images (Figure 1d,e). Figure 2 | Characterization of rhodium sites and their interaction with zeolites. (a) H2-TPR profiles of [email protected], Rh/Y, and Rh-Y samples; (b) Rh 3d XPS of Rh/Y, Rh-Y, [email protected], and reduced [email protected] samples; (c) FT-IR spectra of CO adsorption on [email protected], Rh/Y, and Rh-Y samples. Lighter lines indicate the spectra after room-temperature He purging. (d) The Rh K-edge XANES spectra of [email protected], [email protected], Rh2O3, and Rh foil; (e) The Fourier transform (FT) of k2-weighted EXAFS spectra at the K-edge of [email protected], [email protected], Rh2O3, and Rh foil. (f) The Rh K-edge EXAFS spectra in R space and the fitting curve of [email protected] (g) Full-range EXAFS WT 2D plots for [email protected], Rh2O3, and Rh foil. Download figure Download PowerPoint Catalytic behaviors of [email protected] in alkene hydroformylation A variety of Rh-containing zeolites, including Rh/S-1, Rh/USY, Rh/ZSM-5, Rh/Beta, Rh/Mor, Rh/Y, and Rh-Y ( Supporting Information Table S2), were investigated as heterogeneous catalysts for 1-hexene hydroformylation. As shown in Figure 3a, the specific 1-hexene conversion rate of 40–60 molC=C/molRh/h (100% selectivity toward aldehydes) was obtained at a very low temperature of 333 K with most of the Rh-containing zeolites prepared via wet impregnation, while Rh/S-1 showed a much lower specific 1-hexene conversion rate of 10 molC=C/molRh/h. This might be due to the more efficient stabilization of rhodium species by zeolites with the presence of framework aluminum species. [email protected] containing isolated rhodium ions more efficiently stabilized by zeolite matrix (Figure 2a,b) were expected and able to exhibit higher activity, that is, a specific 1-hexene conversion rate of 106 molC=C/molRh/h with good regioselectivity toward heptanal (n/iso = 2.2). A similar activity trend can be achieved with Rh-containing zeolites at high 1-hexene conversions, as shown in Supporting Information Figure S9. Figure 3 | Catalytic performance of [email protected] in 1-hexene hydroformylation. (a) Comparison of various Rh-containing zeolites. Reaction conditions: 0.05 g catalyst, 3 mmol 1-hexene, 1.5 mL toluene, 6 MPa CO/H2 (1/1, mol ratio), 333 K, 1h; (b) Time-dependent substrate conversion and product selectivity in 1-hexene hydroformylation over [email protected] catalyst. Reaction conditions: 0.05 g catalyst, 3 mmol 1-hexene, 1.5 mL toluene, 6 MPa CO/H2 (1/1, mol ratio), 333 K. (c) Arrhenius plots of 1-hexene hydroformylation over [email protected] catalyst. (d) Comparison of Rh-based catalysts in the hydroformylation of LAOs, C5–C10. Solid symbols: heterogeneous catalysts; hollow symbols: homogeneous catalysts. Download figure Download PowerPoint The impacts of reaction parameters like pressure and temperature on the catalytic performance of [email protected] were investigated, and the reaction conditions were optimized for 1-hexene hydroformylation. Typically, the hydroformylation of 1-hexene proceeded smoothly with [email protected] catalyst at different pressures ( Supporting Information Figure S10): the catalytic activity gradually increased while the selectivity toward linear aldehyde gradually decreased with increasing syngas pressure from 2 to 8 MPa. The reaction temperature plays a key role in the liquid-phase hydroformylation reaction.68 As shown in Supporting Information Figure S11, the hydroformylation of 1-hexene occurred at near room temperature (313 K) over [email protected] catalyst, which appeared to be a very low reaction temperature for 1-hexene hydroformylation. The reaction speeded up with increasing temperature, and the full conversion of 1-hexene could be obtained at >353 K within 4 h. Meanwhile, the selectivity to linear enanthal gradually declined (branched enanthal increased instead) with increasing reaction temperature and traces of byproducts (<2%), that is, hexane, heptanol and heptylic acid, could be detected at the reaction temperature of 373 K. It was subsequently revealed that these byproducts only appeared after the full conversion of 1-hexene. Thus, it is possible to achieve 100% chemoselectivity in alkene hydroformylation by controlling the substrate conversion. The time-on-stream behaviors of 1-hexene hydroformylation are shown in Figure 3b. Typically, 1-hexene conversion gradually increased while the selectivity toward linear enanthal decreased (the sum of linear and branched enanthal is 100%) with time-on-stream. A similar trend was observed for Rh/Y at 333 K ( Supporting Information Figure S12) or [email protected] at 353 K ( Supporting Information Figure S13), while the catalytic deactivation seemed to occur on Rh/Y at >50% 1-hexene conversion ( Supporting Information Figure S12). The reaction rates of 1-hexene hydroformylation at 313–333 K over [email protected] catalyst were measured, as shown in Supporting Information Figure S14. Arrhenius plots gave an apparent activation energy value of 60.6 kJ/mol (Figure 3c), which was distinctly lower than that achieved on homogeneous HRh(CO)(PPh3)3 (117 kJ/mol)69 and heterogeneous Rh/TPPTS (87.4 kJ/mol)70 in the hydroformylation of 1-hexene. Of note is the apparent activation energy value was also distinctly lower than that achieved on Rh/Y under similar conditions (73.7 kJ/mol) ( Supporting Information Figures S15 and S16), which might be explained by the electronic confinement of zeolite that reduces the apparent activation energy of reaction in the confined space.57,71 Undoubtedly, the reduced apparent activation energy value and the maximal utilization of atomically dispersed rhodium sites together cause the unprecedented catalytic activity of [email protected] in 1-hexene hydroformylation. A direct comparison of Rh-based catalysts, both homogeneous and heterogeneous, in the hydroformylation of C5–C10 linear α-olefins (LAOs) is shown in Figure 3d (more details shown in Supporting Information Table S4). Typically, the well-known homogeneous phosphite-coordinated rhodium catalyst, HRh(CO)(PPh3)3, gave a TOF value of 2467 molC=C/molRh/h at 373 K,40 while more complicated homobimetallic rhodium complexes chelated by binucleating tetraphosphine ligand gave a higher TOF value of 4380 molC=C/molRh/h at 363 K.72 The highest TOF value of 8200 molC=C/molRh/h was reported for 1-hexene hydroformylation using a rhodium-BIPHEPHOS system at 393 K.73 Heterogeneous catalysts generally exhibited lower TOF values, for example below 1000 molC=C/molRh/h.37,39,41,45,49,74 In this study, [email protected] exhibited a state-of-the-art TOF value of 6567 molC=C/molRh/h at 393 K, outperforming all heterogeneous catalysts and most homogeneous catalysts under comparable conditions. In addition, 100% chemoselectivity toward aldehydes could be obtained under well-controlled reaction conditions, making [email protected] a potential heterogeneous hydroformylation catalyst. Inspired by the great success of [email protected] in 1-hexene hydroformylation, a wide range of alkene substrates was explored and the results are shown in Table 1. Remarkably, the side reaction of hydrogenation and isomerization could be entirely inhibited, and perfect chemoselectivity to aldehydes (>99.9%) could be achieved for all cases. The hydroformylation of C3–C16 LAOs produced a mixture of linear and branched aldehydes, and the catalytic activity decreased slightly with the increase in carbon chain length (entries 1–7), which might be related to the diffusion limitations of the bulky substrate within the zeolite channels. Neohexene underwent hydroformylation to produce the corresponding linear aldehyde in high selectivity, due to the large steric hindrance on the C=C bond (entry 8).49 When terminal aromatic alkenes were employed as substrates, the hydroformylation reaction proceeded smoothly to produce mixtures of aldehydes (entries 9–11), and sometimes the preferential production of branched aldehydes was observed (entries 9 and 10), originating from the superior electronic effect and thermodynamic stability of the intermediate benzyliridium complex.28,29,49 In the case of bulky α-methyl styrene (entry 12), the substrate conversion was much lower (30%) but with perfect regioselectivity to the corresponding linear aldehyde, due to the dominant steric hindrance on the C=C bond. The hydroformylation of cyclic alkenes, namely cyclopentene and cyclohexene, led to the formation of corresponding cycloalkylformaldehydes as single products (entries 13 and 14). For linear non-terminal alkenes (entries 15–18), the hydroformylation reaction proceeded like LAOs, whereas branched aldehydes were obtained as the regioselective products. Isolated dienes, such as 1,5-hexadiene (entry 19) and 4-vinyl-1-cyclohexene (entry 20), underwent hydroformylation to produce aldehydes, and the hydroformylation of 4-vinyl-1-cyclohexene preferably occurred on the vinyl group. In great contrast, the hydroformylation of conjugated dienes like 1,3-butadiene and isoprene (entries 21 and 22) appeared to be unsuccessful, which may be attributed to the stabilization of the C=C bond by the conjugation effect. Overall, [email protected] was verified to be a promising heterogeneous hydroformylation catalyst with high activity, perfect chemoselectivity, and wide applicability. Table 1 | Alkene Hydroformylation over [email protected] Catalysta Entry Substrate Target Product Conversion (%) Selectivity (%; n/iso) 1 100 56/44 2 96 55/45 3 99 58/42 4 99 53/47 5 96 52/45 6 84 62/38 7 82 63/37 8 55 96/4 9 100 16/84 10 100 21/79 11 93 51/49 12 30 100/0 13 100 100/0 14 90 100/0 15 95 15/85 16 99 9/91 17 99 5/95 18 35 1/99 19 43 60/40 20 86 76/24 21 0 — 22 0 — aReaction conditions: 0.05 g catalyst, 3 mmol substrate, 1.5 mL toluene, 6 MPa CO/H2 (1/1, mol ratio), 333 K, 10 h. Recyclability of [email protected] in hydroformylation Recyclability is a key issue for the success of heterogeneous hydroformylation catalysts. Although many heterogeneous catalyst systems have been developed, the leaching of rhodium species during liquid-phase hydroformylation reaction and the poor catalyst recyclability thereof are often unavoidable. In this study, [email protected] catalyst exhibited remarkable recyclability and no activity loss was observed over five cycles (Figure 4a), due to the efficient stabilization of rhodium ions by the zeolite matrix. In contrast, a sharp decline in 1-hexene conversion was observed on Rh/Y catalyst in the first recycle. Meanwhile, server leaching of rhodium species was found in Rh/Y, but no leached rhodium species could be detected in liquid phase with [email protected] catalyst (below the detection limit of 1 ppm). Furthermore, hot filtration tests were conducted to explore the heterogeneous nature of [email protected] catalyzed hydroformylation reaction. As shown in Figure 4b, the reaction was completely terminated upon the removal of [email protected] catalyst by filtration, whereas the hydroformylation reaction proceeded with the filtrate even after removing the Rh/Y catalyst (Figure 4c). Thus, the leached rhodium species in the liquid phase could catalyze the hydroformylation reaction in the manner of homogeneous catalysts, which might also happen with other heterogeneous catalyst systems. To obtain a true sense of heterogeneous catalysis for hydroformylation reaction, the efficient stabilization of rhodium species by solid supports against leaching is indispensable. The existing states of rhodium species in [email protected] after five cycles were investigated by XAS. FT EXAFS fitting spectrum at R space ( Supporting Information Figure S17) and wavelet transform (WT) EXAFS spectrum of [email protected] sample (Figure 4d) demonstrated a major scattering peak at ∼1.5 Å from the first Rh–O shell with a coordination number of 4.5 ( Supporting Information Table S3), confirming that the isolated cationic states of Rh species in [email protected] were well preserved after the hydroformylation reaction. The structural integrity of [email protected] catalyst after five cycles of hydroformylation reaction was directly visualized by HR-TEM and STEM mapping analyses ( Supporting Information Figure S18), and the atomically dispersed rhodium species were observed in the representative Cs-corrected HAADF-STEM image (Figure 4e). According to the above-mentioned results, a true sense of heterogeneous [email protected] catalyst with atomically dispersed rhodium ions has