Single-atom catalysts for hydroformylation of olefins

Shu Tao; Da Yang; Minmin Wang; Guangxun Sun; Gaoyan Xiong; Wenwen Gao; Youzhi Zhang; Yuan Pan

doi:10.1016/j.isci.2023.106183

. 2023 Feb 19;26(3):106183. doi: 10.1016/j.isci.2023.106183

Single-atom catalysts for hydroformylation of olefins

Shu Tao ^1,², Da Yang ^1,², Minmin Wang ¹, Guangxun Sun ¹, Gaoyan Xiong ¹, Wenwen Gao ¹, Youzhi Zhang ¹, Yuan Pan ^1,^∗

PMCID: PMC10009200 PMID: 36922997

Summary

Hydroformylation is one of the most significant homogeneous reactions. Compared with homogeneous catalysts, heterogeneous catalysts are easy to be separated from the system. However, heterogeneous catalysis faces the problems of low activity and poor chemical/regional selectivity. Therefore, there are theoretical and practical significance to develop efficient heterogeneous catalysts. SACs can be widely applied in hydroformylation in the future, due to the high atom utilization efficiency, stable active sites, easy separation, and recovery. In this review, the recent advances of SACs for hydroformylation are summarized. The regulation of microstructure affected on the reactivity, stability of SACs, and chem/regioselectivity of SACs for hydroformylation are discussed. The support effect, ligand effect, and electron effect on the performance of SACs are proposed, and the catalytic mechanism of SACs is elaborated. Finally, we summarize the current challenges in this field, and propose the design and research ideas of SACs for hydroformylation of olefins.

Subject areas: Single-atom catalysts, Hydroformylation

Graphical abstract

Single-Atom Catalysts; Hydroformylation

Introduction

Hydroformylation, also known as oxo-synthesis reaction, is one of the most important homogeneously catalyzed industrial processes for the production of aldehydes from alkenes and syngas with 100% atom economy, which was found by Otto Roelen as early as 1938.¹^,²^,³^,⁴^,⁵ Up to now, the production of chemical produces through this hydroformylation has exceeded 12 million tons every year, which is one of the most significant commercial uses of soluble homogeneous metal catalyst in the chemical industry.⁶^,⁷ The obtained aldehyde products for hydroformylation can be converted to the valuable and stable products (alcohols, ketones, acetals, amines products, etc.) through oxidation, hydrogenation, or reductive amination, which are extensively utilized in the synthesis of fine compounds such as insecticides, spices, food additives, and plasticizers.⁸^,⁹

The typical catalysts for hydroformylation of olefins are homogeneous complexes of the type [HM(CO)_xL_y], where L can be further CO or an organic ligand. A generally accepted series of the activities of the unmodified metal is as follows:¹⁰ Rh >> Co > Ir > Ru > Pd > Mn > Fe > Ni >> Re. To date, only cobalt and rhodium catalysts can be used in industrial application; other metals only stay in the stage of academic research. Rhodium-phosphine complex catalyst has the advantages of mild reaction conditions, well catalytic performance, and low energy consumption. It has gradually become the mainstream in the industrial hydroformylation instead of cobalt carbonyl catalyst.¹¹ The uniform distribution of active sites, excellent catalytic activity, and superior chem/regioselectivity of homogeneous catalysts are only a few of their many benefits. However, the issue of catalyst separation leads to the loss of active metal and phosphine ligand, which is not conducive to large-scale application in industrial production. In contrast, heterogeneous catalysts can overcome catalyst separate deficiencies. Due to the surface properties of the support, the interaction between metal and support, and the microenvironment of the catalytic sites, heterogeneous catalysts can demonstrate excellent performance. Therefore, the development of heterogeneous catalysts with high activity and high stability for hydroformylation has important theoretical and practical significance.¹² The term “heterogeneous catalyst” describes a catalyst that immobilizes the active metal or metal complex on a solid support. Molecular sieve,¹³^,¹⁴^,¹⁵ carbon materials,⁵^,¹⁶^,¹⁷ inorganic oxides,¹⁸^,¹⁹^,²⁰ magnetic nanoparticles,²¹ and organic polymers²²^,²³^,²⁴ are the examples of supports.

In contrast to traditional heterogeneous catalysts, single-atomic catalysts (SACs) are a recently emerging class of catalytic material featured with unique single-atom dispersion and maximum atomic utilization of active metal.²⁵^,²⁶ The atomically dispersed metal anchored on support brings similar catalytic behavior to homogeneous catalyst. In addition, the heterogenous property of SACs makes them easy to be separated from the liquid-phase reaction mixture and achieve convenient recovery as well as recycling. Combining the advantages of homogeneous catalysts and heterogeneous catalysts, SACs exhibit high catalytic activity and selectivity in hydroformylation.²⁷^,²⁸^,²⁹^,³⁰^,³¹^,³²^,³³

However, up to now, very few reviews of SACs in hydroformylation have been reported. In this paper, we summarize recent advances of SACs for hydroformylation. The effects of microstructure of SACs on the reactivity and chem/regioselectivity of hydroformylation are discussed. The support effect, ligand effect, and electron effect on the performance of SACs in hydroformylation are proposed. The mechanism of SACs in hydroformylation is elaborated. Finally, we summarize the current problems and challenges in this field, and propose the design and research ideas of SACs for hydroformylation (Figure 1).

Overview of the main topics covered herein

In this review, the recent advances, a variety of structural regulation methods, and reaction mechanism of SACs for hydroformylation of olefin are discussed. (Reproduced with permission from Ref.,³⁴ © J. Catal. 2022; Reproduced with permission from Ref.,³⁵ © ACS Catal. 2022; Reproduced with permission from Ref.,³⁶ © ACS Catal. 2021; Reproduced with permission from Ref.,³⁷ © Cell Reports Physical Science 2022; Reproduced with permission from Ref.,³⁸ © Nature 2022; Reproduced with permission from Ref.,³⁹ © CCS Chemistry 2022; Reproduced with permission from Ref.,⁴⁰ © Chem Catalysis 2022; Reproduced with permission from Ref.,⁴¹ © ACS Appl. Mater. Interfaces 2021).

SACs for hydroformylation of olefins

The application of SACs in hydroformylation is still in the early stage. According to the current results, SACs have great potential to achieve high activity and selectivity of hydroformylation (Figure 2) since they have extremely high metal dispersion, low coordination environment in the metal center, and the strong interaction between metal atoms and support. Herein, we focus on the recent development of SACs in the field of hydroformylation.

A brief history of the recent advances of SACs for hydroformylation of olefin

(Reproduced with permission from Ref.,³¹ © Nat. Chem. 2011; Reproduced with permission from Ref.,⁴² © Angew. Chem. Int. Ed. 2016; Reproduced with permission from Ref.,⁴³ © Green Chem. 2016; Reproduced with permission from Ref.,⁴⁴ © J. Catal. 2017; Reproduced with permission from Ref.,⁴⁵ © Angew. Chem. Int. Ed. 2021; Reproduced with permission from Ref.,³⁶ © ACS Catal. 2021; Reproduced with permission from Ref.,⁴⁶ © J. Catal. 2021; Reproduced with permission from Ref.,³⁴ © J. Catal. 2022; Reproduced with permission from Ref.,³⁵ © ACS Catal. 2022; Reproduced with permission from Ref.,⁴⁰ © Chem Catalysis 2022; Reproduced with permission from Ref.,³⁸ © Nature 2022).

Rh SACs

Homogenous phosphines-modified Rh catalysts have shown remarkable performance in the hydroformylation process before the application of SACs. In the 1950s, Union Carbide applied RhCl(PPh₃)₃ to the industry. The “low-pressure oxo-progress” has much higher stability and milder conditions than Co-based catalysts.⁴⁷ Later, Rhone-Poulenc Company and Ruhrchemie Company jointly developed RCH/RP process to achieve a new two-phase (organic/water) catalytic system. In this process, the water-soluble Rh-P complex was dissolved in the water phase; the products were dissolved in the oil phase.⁴⁸ The effective separation of the products and catalyst can be achieved by simple static layering and decanting operation. Compared with Co-based catalysts, Rh-based system possessed higher catalytic activity, selectivity and stability, and the milder conditions.⁴⁹^,⁵⁰ Therefore, Rh SACs are the potential supported catalysts for hydroformylation under mild conditions.

In 2016, Zhang et al.⁴² synthesized Rh SACs by the impregnation method to adsorb Rh³⁺ onto ZnO nanowires (Rh₁/ZnO-nw) for the hydroformylation process (Figure 3A). Compared to the typical Wilkinson’s catalyst RhCl(PPh₃)₃ (Turnover number (TON) = 19000), the Rh₁/ZnO-nw showed excellent activity (TON = 40000), and can be recycled and reused for four times without significant loss of reactivity and selectivity. However, the ratio of linear to branched aldehyde (L/B) was only 1.0. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images clearly distinguished that isolated Rh atoms were distinguishable from the ZnO nanowires (Figure 3B). In the characterization of in situ diffuse reflectance infrared Fourier transform spectroscopy study of CO adsorption (CO-DRIFTS) (Figure 3C), the absence of a Rh⁰-CO bridge adsorption peak implied that Rh was dispersed on ZnO support in the form of single atoms, which was compatible with the characterization results of HADDF-STEM.⁵⁴^,⁵⁵^,⁵⁶

Rh SACs for hydroformylation of olefins

(A) Design configuration of Rh₁/ZnO-nw catalyst for olefin hydroformylation.

(B) HAADF-STEM image of Rh₁/ZnO-nw.

(C) DRIFT spectra of CO adsorption at 25°C on Rh/ZnO-nw catalysts with different Rh loadings⁴² (Reproduced with permission from Ref.,⁴² © Angew. Chem. Int. Ed. 2016).

(D) The Rh K-edge EXAFS spectra in R space for Rh/CoO with different Rh loadings.

(E) Product yields of 0.2% Rh/CoO over the course of five rounds of successive reaction⁵¹ (Reproduced with permission from Ref.,⁵¹ © Nat. Commun. 2016).

(F) HAADF-STEM image of Rh₁/PNP-ND.

(G) Schematic illustration for synthesizing Rh₁/PNP-ND⁵² (Reproduced with permission from Ref.,⁵² © Nat. Commun. 2021).

(H) The Rh K-edge EXAFS spectra in R space for Rh@POL-PTBA-HA-50.

(I) Rh@POL-PTBA-HA-50 catalyzes 1-octene hydroformylation results⁴⁶ (Reproduced with permission from Ref.,⁴⁶ © J. Catal. 2021).

(J) The design configuration of Rh₁/HAP catalyst for “Hydroformylation-reductive amination” tandem reaction of olefins⁵³ (Reproduced with permission from Ref.,⁵³ © Mol. Catal. 2021).

In the same year, Wang et al.⁵¹ reported CoO-supported Rh SACs (Rh/CoO) for the hydroformylation of propylene. The selectivity of butyraldehyde was as high as 94.4% and the turnover frequency (TOF) can reach 2065 h^-1. After five cycles, the activity and selectivity of the catalyst remained at high levels (Figure 3D). Extended X-ray absorption fine structure (EXAFS) revealed that Rh atoms were atomically dispersed for the sample of 0.2% Rh/CoO. As shown in Figure 3E, the peak at ca. 2.0 Å was attributed to Rh-O shell, and no other peaks for Rh-Rh contribution were observed. Additional mechanistic studies revealed that the structural reconstruction of the Rh SACs took place during the catalytic process, which facilitated in the adsorption and activation of the reactants.

Gao et al.⁵² synthesized a phosphorus coordinated Rh SACs (Rh₁/PNP-ND) through the metal-ligand coordination approach (Figures 3F and 3G), and applied it to the hydroformylation of styrene to achieve high conversion (>99%) and high selectivity (>90%) under mild conditions. The abundant carboxyl groups loaded on the surface of nanodiamond (ND) selectively react with the amino groups of pincer ligands (PNP) to obtain PNP-ND, which provided a large number of sites for the highly dispersed anchoring of Rh. The ³¹P solid-state NMR spectra showed that the chemical shift of P atoms migrated to the low-field region after the Rh species were anchored by PNP-ND. This provided direct evidences for the successful anchoring of Rh species in PNP-ND. In order to further demonstrate the adaptability to different substrates, the Rh₁/PNP-ND was applied to the hydroformylation of a series of styrene derivatives, which demonstrated exceptional selectivity and activity, comparable to homogeneous catalysts.

In 2020, Li et al.⁴⁵ synthesized 0.5% Rh/CeO₂ SACs by electrostatic adsorption method innovatively, and coupled hydroformylation with low-temperature water-gas shift reactions. Without using any ligand, the catalytic system not only avoided the hydrogenation of styrene and phenylpropyl aldehyde occurred as the side reaction but also achieved high selectivity to obtain linear aldehyde in the hydroformylation of styrene and its derivatives (L/B = 3). Due to the high Ce vacancy density in CeO₂ support, high loading of active Rh sited can be achieved. HAADF-STEM images and CO-DRIFTS proved that Rh existed in the form of single atom in 0.5% Rh/CeO₂ catalyst. To further comprehend the new reaction route, a number of comparative tests were conducted. In contrast to the conventional reaction with styrene, CO, and H₂ as substrates, the authors found that the higher linear/branched aldehyde ratio obtained was related to the reactants of CO and H₂O. When secondary alcohol dehydrogenation was coupled with hydroformylation reaction, the selectivity of linear products was still higher than that of branched products, although the activity was lower. Based on the above facts, the author proposes a six-membered transition formed by the combination of C=C unsaturated double bond and formic acid. According to Marcovnikov's rule, the active H addition the end of C=C bond. This intermediate not only facilitates the insertion of carbonyl groups into the terminal C=C bond to form the linear aldehydes but also prevents the formation of phenyl Rh species, which ultimately inhibits the formation of linear aldehydes.⁵⁷

Zhao et al.⁴⁶ successfully encapsulated Rh within porous monophosphine polymers (POPs) by one-pot method to prepare Rh@POP-PTBA-HA-50.⁵⁸^,⁵⁹^,⁶⁰ According to the characterization of HAADF-STEM and EXAFS (Figure 3H), it is proved that Rh species were encapsulated as a single-atom in the POPs skeleton. Fourier transform infrared spectroscopy (FT-IR) spectrum of Rh@POP-PTBA-HA-50 showed that a strong C=N stretch at 1623 cm^-1, and the peaks at 1700 and 3345 cm^-1 attributed to aldehyde group were obviously weakened compared to 4,4′,4’’-phosphanetriyltribenzaldehyde (PTBA) and N2H4H2O (HA). In addition, the ¹³C magic angle spinning NMR peak of Rh@POP-PTBA-HA-50 at 162 ppm matched to the carbon atom of the C=N bond. Both of them indicated the formation of imine bonds. Compared to Rh(CO)₂(acac)-PTBA, Rh@POP-PTBA-HA-50 showed a significant improvement in regioselectivity (linear aldehydes) from 62% to 92% in hydroformylation of 1-octene (Figure 3I). Due to the robust coordination of dispersed phosphine ligands with metal active species, the catalyst demonstrated remarkable catalytic activity (TON = 60000) and good thermal stability.

The obtained aldehyde products from hydroformylation can be further converted into high-valuable chemicals like amines, carboxylic acids, and alcohols through additional oxidation, reduction, and hydrogenation.⁶¹^,⁶²^,⁶³^,⁶⁴^,⁶⁵ Hydroformylation followed by other reactions through one-pot method has been extensively explored, such as “hydroformylation-hydrogenation”, “hydroformylation-acetalization”, “hydroformylation-aldol condensation”, “hydroformylation and reductive amination”, and so on. Following the atom economy and low energy consumption in green chemistry, combining SACs with tandem hydroformylation have become a powerful and promising synthetic method. Li et al.⁵³ successfully prepared hydroxyapatite (HAP)-supported single-atom Rh catalyst (Rh₁/HAP) for the tandem hydroaminomethylation of olefins. (Figure 3J). HAADF-STEM and CO-DRIFTS results revealed that Rh atom was atomically dispersed on the HAP support. 1-hexene was almost entirely converted over 0.5Rh₁/HAP under moderate reaction conditions, and the selectivity was 93.2%. The hydroformylation, condensation, and hydrogenation reactions are all parts of the overall hydrocarbamoylation reaction. According to the mechanistic study, the hydrocarbamoylation process is a speed-regulating step. Through separate evaluation of hydroformylation reaction, 0.5Rh₁/HAP guaranteed high activity of hydroformylation reaction, thus ensuring the excellent catalytic activity of the tandem reaction.

Co SACs

Co is another metal catalyst applied for hydroformylation industry. The catalytic activity of Rh is 10³–10⁴ times than that of Co.⁶⁶^,⁶⁷ However, the shortage and the high price of precious Rh limit its development and application in hydroformylation. Co continues to have a long-term role, since its effective antitoxic performance and the weak requirement for olefin’s purity. As early as 1952, the carbonyl cobalt catalysts with HCo(CO)₄ as the active ingredient were first applied in the oxo-synthesis of propylene in 1952. Later, cobalt carbonyl modified by phosphines could decrease pressure to 5–10 MPa in the 1950s and the CO was replaced by PR₃, P(OR), etc. Compared to CO, the phosphines possessed stronger σ-electron-donating ability and weaker π-receptor-accepting ability. The selectivity of linear aldehydes is significantly increased in phosphines modified Co system.⁶⁸^,⁶⁹ However, the hydrogenation of olefins to alkanes occurred, which reduced the activity relatively. In recent years, supported cobalt-based catalysts have attracted a lot of attention. Basic researches have been done in the laboratory, but there is still a significant gap between these efforts and industrial manufacturing.

Recently, Cong et al.³⁴ developed the ultrasound-assisted impregnation method to design Co SACs supported by zirconium phosphate (CoZrP-2.0). The tight coordination of Co atom with phosphate group of ZrP prevented the leaching of Co, and enhanced the activity and stability of catalyst (Figures 4A and 4B). In CoZrP-2.0-catalyzed hydroformylation, the conversion of 1-octence was about 100%, and the selectivity of C₉ aldehyde was 91.3%. After six cycles, the activity and selectivity of the catalyst remained at high levels. Based on the pyridine adsorption FT-IR spectrum and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis of CoZrP-X catalyst (Figure 4C), it was clearly observed that the leaching of Co was closely related to the loss of Brønsted acid. On the CoZrP-2.0, the higher the loss of B-acid sites (CoZrP-2.0: 174.7 mmolg^-1), the lower leaching of Co species. With the increase of P/Zr ratio, more Co atoms combined with phosphate groups by replacing the protons of the B-acid sites, which promoted the formation of ionic Co atoms. The coordination structure and chemical surroundings Co cites were examined in depth using EXAFS structural characterization (Figures 4D and 4E). Similar peaks at 1.5 Å and 2.6 Å corresponded to the initial coordination shells of Co-O and Co-Co, respectively, were presented in samples of CoZrP-0.5, CoZrP-1.0, and CoZrP-1.5. Only the Co-O bond (1.5 Å) was observed on CoZrP-2.0, revealing the single atom characteristic. With the increase of Brønsted acid site and BET surface area with the increase of P/Zr (from 0.5 to 2.0), Co atoms are more evenly dispersed on the support. According to the EXAFS characterization results, when P/Zr = 2.0, the Co atoms are dispersed as a single atom on the support. By comparing the catalytic performance of different P/Zr catalysts for 1-octene hydroformylation, it can be found that with the increase of P/Zr from 0.5 to 2.0, the conversion rate of the catalyst decreased slightly, but the selectivity for aldehydes increased from 59.4% to 89.6%, and the leaching rate of Co decreased sharply from 29.1% to 0.5%. Co SACs mainly take aldehydes as the main product, which has better activity, selectivity, and stability.

Co SACs for hydroformylation of olefins

(A) The detailed preparation process of CoZrP-2.0.

(B) The recyclability of CoZrPx-2.0 catalyst for 1-octene hydroformylation.

(C) Relationships between the loss of Brønsted acid sites and Co leaching for CoZrP-X catalysts with different P/Zr ratios.

(D) EXAFS data of Co-k edge of model compound and CoZrP-X sample.

(E) Experimental 2D WT EXAFS plots for x(k) EXAFS data-collected CoZrP-X samples³⁴(Reproduced with permission from Ref.,³⁴ © J. Catal. 2022).

Ru SACs

Current industrial production of hydroformylation mainly employs Rh-based catalyst. The expensive price of rhodium has promoted the research of other alternative transition metal catalysts in hydroformylation. A pioneering work on ruthenium catalysts for homogeneous hydroformylation in 1965 by Wilkinson et al⁷⁰ is worth noting that Ru can significantly promote the catalytic activity of cobalt catalyst in hydroformylation reaction. Masanobu Hidai et al.⁷¹ studied the synergistic effect of bimetallic catalysts in the hydroformylation of olefin. The initial reaction rate of hydroformylation of cyclohexene catalyzed by Co₂(CO)₈/Ru₃(CO)₁₂ was 19 times higher than that of Co₂(CO)₈, and this bimetallic catalyst has a wide range of substrate applicability. The addition of 1 wt% Ru to 10 wt% Co/AC increased the conversion of 1-hexene by nearly 60%, inhibited the side reaction of alkene isomerism, and improved the selectivity of aldehydes. Zhang et al.¹⁶ believed that the addition of Ru can greatly improve the reducibility, provided more cobalt metal centers for the reaction.

Escobar-Bedia et al.³⁵ developed a novel Ru-based catalyst (Ru@NC) containing isolated single atoms and disordered clusters in nitrogen-doped carbon matrix, that applied to hydroformylation of 1-hexene with good activity and selectivity (Figures 5A and 5B). The strong interaction between Ru and N atoms can improve the dispersion of metal and change the electronic properties of Ru atoms on the surface, thus affecting the stability and activity of catalyst. According to scanning electron microscope energy dispersive spectrometer findings, N and Ru atoms embedded in the carbon substrate were distributed uniformly on the support and their signals overlapped. Moreover, the potential connection between surface N and Ru atoms was confirmed by a prominent Ru-N bond at 460 cm^-1 on the Raman spectra. According to the X-ray absorption near-edge region (XANES, left) spectral and EXAFS spectral (right) analysis (Figure 5C), Ru mainly existed in the form of single atoms in Ru@NC. However, Ru-Ru scattering intensity increased with the increase of the metal loading. Ru-Ru is detected to exist in a highly disordered state in the highly loaded catalyst, indicating the formation of small, dispersed, and highly disordered Ru clusters. In order to further investigate the impact of N atoms on the performance of catalyst, 0.2Ru@NaC and 0.2Ru@NC were conducted as the comparison experiments under the same circumstances (Figure 5D). The results showed that the rate of 0.2Ru@NaC decreased significantly with the prolongation of reaction time; the leaching of Ru in solution increased significantly. Moreover, the regioselectivity of 0.2Ru@NaC in hydroformylation was lower than that of 0.2Ru@NC. The interaction of surface N atom with Ru atom can stabilize and change the electronic properties of Ru atom.

Ru SACs for hydroformylation of olefins

(A) Scheme of Ru@NC-catalyzed olefin hydroformylation.

(B) 0.2Ru@NC catalyst cycling experiment.

(C) XANES (left) and EXAFS (right) spectral of Ru@NC catalyst.

(D) Comparison of activity between 0.2Ru@NC and 0.2Ru@NaC catalysts³⁵ (Reproduced with permission from Ref.,³⁵ © ACS Catal. 2022).

Au SACs

Various transition metals including Rh, Co, Ir, Ru, and Fe have been proven to be efficient catalysts for hydroformylation. Although Au is conventionally considered inactive for hydroformylation, numerous studies have shown that Au exhibits high olefin activity,⁷² H₂ dissociation,⁷³^,⁷⁴ and CO bonding capabilities.⁷⁵ At same time, Au was applied for CO oxidation,⁷⁵ water gas conversion reaction,⁷⁶ and methanol synthesis.⁷⁷^,⁷⁸

Wei et al.⁴⁰ encapsulated dispersed Au into purely siliceous zeolite to prepare Au(0.2%)@S-1 (Figure 6A). The Au SACs showed high activity noticeable stability after 5 cycles in the hydroformylation of propylene, which was one order of magnitude greater than Au nanoparticle catalysts (Au(0.8%)@S-1 and Au(0.2%)/S-1) (Figures 6B and 6C). In addition, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) provided evidence that Au was coated in the molecular sieve and the morphology of the molecular sieve was unaffected by Au species. The utilization of Cs-corrected HAADF-STEM (Figure 6D), XANES spectra (Figure 6E), and EXAFS spectra (Figure 6F) confirmed that the oxygen bridge bond of molecular sieve enclosed the atomically scattered Au to create the Au-O-SiO_x structure, which maximizes the active site’s density and structural stability.

Au SACs for hydroformylation of olefins

(A) Design configuration of catalyst for enhanced hydroformylation of olefin.

(B) Effect of different reaction temperature on the hydroformylation performance of propylene catalyzed by Au(0.2%)@S-1.

(C) Cycling experiments of the hydroformylation of propylene catalyzed by Au(0.2%)@S-1.

(D) Cs-corrected HAADF-STEM images of Au(0.2%)@S-1.

(E) The normalized XANES spectra and of Au(0.2%)@S-1, Au(0.8%)@S-1, Au(0.2%)/S-1, Au foil, and Au₂O₃.

(F) The normalized EXAFS spectra of Au(0.2%)@S-1, Au(0.8%)@S-1, Au(0.2%)/S-1, Au foil, and Au₂O₃⁴⁰ (Reproduced with permission from Ref.,⁴⁰ © Chem Catalysis 2022).

Regulation strategies of SACs for enhanced performance of hydroformylation

Support effect

The supports of heterogeneous catalysts often employed inorganic oxides, POPs, metal-organic frameworks (MOFs), carbon materials, etc. The chem/regioselectivity of reactions increased through the modification of organic phosphine ligands or inorganic materials, and MOF domain limitation. However, the deactivation of catalysts and the loss of active components are still needed to be explored. Table 1 summarizes the catalytic performance of hydroformylation on SACs.

Table 1.

Summary of the application of SACs for hydroformylation for olefin

Entry	Catalyst	Substrate	Conv. (%)^a	Sel. (%)^b	L/B^c	TOF (h^-1)^d	Ref.
1	Rh/ZnO	styrene	/	99.0	1/1	3333	Zhang et al.⁴²
2	Rh/CoO	propylene	/	94.9	/	2065	Wang et al.⁵¹
3	Rh/CeO₂	allylbenzene	79.0	98.0	3/1	/	Li et al.⁴⁵
4	Rh/CeO₂	styrene	96.0	97.0	51/46	130	Amsler et al.⁶⁶
5	Rh/POL-PPh₃	ethylene	96.2	96.1	/	4530	Jiang et al.⁸⁵
6	Xantphos-doped Rh/POPs-PPh₃	1-octene	42.0	87.0	9/1	/	Li et al.⁴⁴
7	Rh@POP-PTBA-HA-50	1-octene	87.2	96.3	92/8	801	Zhao et al.⁴⁶
8	Rh/CPOL-1bp&10P	ethylene	/	93.0	24/1	1209	Li et al.⁴³
9	Rh₁/PNP-ND	styrene	77.0	99.0	1/12	/	Gao et al.⁵²
10	Rh-Co-Pi/ZnO	1-decene	95.0	70.0	9/5	/	Wei et al.⁴¹
11	Rh@Y	1-hexene	91.0	100.0	6/5	6567	Shang et al.³⁹
12	CoZrP-2.0	1-hexene	99.0	92.6	57/35	/	Cong et al.³⁴
13	Co₁/β-Mo₂C	propene	/	95.0	/	749	Wei et al.³⁷
14	Ru@NC	1-hexene	99.9	54.0	93/7	/	Escobar-Bedia et al.³⁵
15	Au(0.04%)@S-1	propene	/	/	/	4096	Wei et al.⁴⁰

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Conversion rate, which indicated the conversion rate of aldehyde.

Selectivity, which indicated the selectivity for the aldehydes.

The molar ratio of linear aldehydes/branched aldehydes.

Turnover frequency.

POPs are a new material composed of C, N, O, and H atoms with high specific surface area, low skeleton density, controllable pore structure, and excellent stability.⁷⁹^,⁸⁰^,⁸¹^,⁸²^,⁸³ It provides a new class of polymer support for the preparation of SACs that possess the advantages of both homogeneous and heterogeneous catalysts. The utilization of POPs was conducive to the diversification of ligand modification, due to the immobilization of phosphines in polymer chains by covalent bonds. Further coordination of Rh atoms with P atoms can realize high loading of active metal. The high concentration of ligand can stabilize the metal atoms and prolong the life of the catalyst. In addition, POPs are typically insoluble in the majority of solvents, which prevents catalyst loss via dissolution.⁸⁴ However, the poor mechanical strength, poor thermal conductivity, complicated synthesis steps, and strict preparation conditions limit the large-scale production of POPs. For the first time, POL-PPh₃ was synthesized via solvothermal polymerization in 2014 by the Xiao team and Ding team.⁵⁸ N₂ adsorption/desorption curves showed high specific surface area of POL-PPh₃ (1086 m²/g), and the concentrated pore size distribution at 0.7, 1.5, and 3–70 nm. The synthesized POL-PPh₃ with graded porosity is conducive to the uniform dispersion of active centers. Jiang et al.⁸⁵ used POL-PPh₃ as a support to synthesize Rh SACs (Rh/POL-PPh₃), which exhibited outstanding activity in the hydroformylation of ethylene in a fixed-bed reactor. After long-term stability test for more than 1000 h, the conversion of ethylene maintained at a stable level, and the loss of Rh catalyst was only 0.0046%. HAADF-STEM image clearly shows that the isolated Rh atoms are uniformly dispersed on the POL-PPh₃ with porous structure. In addition, no sintering or aggregation was seen in the Rh species after long-term stability tests. Only Rh-P and Rh-C bonds were found according to EXAFS spectroscopy, which demonstrated that the strong coordination of Rh atoms with the exposed P atoms in the POL-PPh₃ framework prevented the loss of active metal during the reaction.

Metal oxides (ZnO, CoO, CeO₂, and Al₂O₃) are frequently employed as support due to their outstanding chemical, thermal, and mechanical stability. The interactions between metal and support, the hydrogen overflow effects, and synergistic effects influence the catalytic activity of oxide-supported metal catalysts.⁸⁶^,⁸⁷ However, the interaction between the active metal and the oxide support may lead to the migration of surface particles, and finally the inert oxide will coat the active metal particles, thus deactivating the catalyst.⁸⁸ Amsler et al.⁶⁶ investigated the activity and stability of Rh SACs loaded with different oxides (MgO, CeO₂, and ZnO) in the hydroformylation of olefin by combining theoretical calculation and experimental study. Through calculating the free energy of supported catalysts in comparison to the complex HRh(CO)₄, the atomically dispersed Rh/MgO was determined to be the most stable. HRh(CO)₄ on flat oxide surfaces (CeO₂ (111)) has catalytic activity comparable to that of molecular complexes. However, for the step edge on the MgO (301) surface, the calculation shows that the catalytic activity was significantly reduced. EXAFS characterization showed Rh atoms on MgO in higher coordination environments and higher degrees of confinement. The strong contact between Rh and the support, which interfered with the recovery of active species and the product’s desorption, resulted in the low activity.

Molecular sieves have been applied in numerous catalytic fields because of their special shape selectivity, adjustable acidity, and high water/thermal stability. In addition, molecular sieves can also be used as carriers to support, coat, disperse, and stabilize metal-active species (nanoparticles, clusters, single atoms, and isolated ions), achieving excellent activity and stability in heterogeneous process. The main problem of using molecular sieve as support is that the pore size of the support itself will affect the activity and selectivity of the catalyst. Moreover, there are abundant proton acid sites on the surface of molecular sieve, which may promote side reactions such as aldol condensation.⁸⁹ Shang et al.³⁹ prepared Rh SACs (Rh@Y) utilizing an in situ guiding agent. In hydroformylation of olefins, Rh@Y showed significant catalytic activity, cycle stability, and substrate suitability under relatively mild conditions. Under the same experiment conditions, compared to other kind of catalysts (Rh/S-1, Rh/USY, Rh/ZSM-5, Rh/Beta, Rh/Mor, Rh/Y, and Rh-Y), Rh@Y showed higher catalytic efficiency. The characterization of MAS NMR, XPS, XAS, and HAADF-STEM images revealed that the introduced Rh species had no effect on the structural stability of zeolite. The isolated Rh&+ (& = 2.5) was successfully confined to the molecular sieve structure. The researchers explored the reaction mechanism of hydroformylation on the catalyst through density functional theory (DFT) calculation. The results show that the combination of α-C atom on 1-hexene and C atom on CO is the rate-determining step (RDS) of the reaction. The energy barrier of RDS for straight chain aldehydes is obviously lower than that of RDS for branched chain aldehydes, which is consistent with the experimental results. In addition, the calculation results further revealed the dynamic change information of the active Rh site in the reaction, which confirmed the space limitation of the molecular sieve and the stabilizing effect of the skeleton oxygen atom on the active Rh site in the catalytic system.

Carbon materials have adjustable physical and chemical characteristics, adequate pore size distribution, and appropriate pH, which can improve the high dispersion of active components and accelerate the diffusion of reactants and products. Therefore, carbon materials, of which activated carbon is the most common, offer enormous potential in catalytic reactions.⁹⁰ Ligands play a major decisive role in the reactivity of Rh metal, especially in the selectivity. Due to the limitations of carbon material itself, it is difficult to build a stable bond structure between carbon material and organic phosphine ligand, and it is easy to lose phosphine ligand in the reaction, which will lead to the reduction of selectivity and the loss of Rh atoms, and ultimately affect the life of the catalyst. Feng et al.⁹¹ constructed Rh SACs (Rh₁/AC) on activated carbon for the carbonylation of methanol. The activity of Rh₁/AC was three times than that of homogeneous catalyst ([Rh(CO)₂I₂]^-) and 30 times than that of carbon-supported Rh nanoparticle catalyst. The HAADF-STEM diagram of Rh₁/AC showed that isolated Rh atoms were dispersed on the surface of support before or after the reaction. DFT calculation and differential charge density calculation showed that the carbonyl group with electron donor properties were the optimal anchoring sites of Rh atoms, and the coordination bonds enhanced the electron density of the central Rh atoms and reduced the energy barrier of the speed control step.

MOFs are crystals with adjustable pore structure, large specific surface, high porosity, and stable multi-dimensional network structure generated by the coordination and hybridization of multi-dentate organic ligands with transition metal ions. Selecting an appropriate metal precursor and confining it to the MOF pore through the nanoconfinement effect is the efficient method to achieve atomic-level dispersion of metal. The main problem of using MOFs as support is that the pore size of the support itself will affect the activity and selectivity of the catalyst, just like molecular sieves.⁹²^,⁹³^,⁹⁴ So far, Rh-based nanocluster catalysts combined with MOFs have been applied in the hydroformylation of alkenes, such as Rh@MIL-101,⁹⁵ Rh@IRMOF-3,⁹⁶ Rh@ZIF-8,⁹⁷ and Rh/MnMOF.⁹⁸ This paved the way for the continued development of atomically distributed catalysts based on MOFs.

Ligand effect

Phosphines-modified transition metals are widely employed in the hydroformylation of olefin, due to the high activity and selectivity under mild conditions. When coordinating with transition metals, P atoms can provide lone pair electrons to the empty tracks of transition metal to form σ-bond; at the same time, they also accept the filled d-orbital feedback electronic of metal atoms to form feedback π-bond. The σ-donor and π-acceptor properties of phosphines can regulate the performance of transition metal catalysts in organic reaction.⁹⁹ Compared to ligands composed of other elements of the same main group, phosphine ligands have the highest catalytic activity in hydroformylation (activity sequence: PPh₃ > NPh₃ > NP₃ > AsPh₃, SbPh₃ > BiPh₃).

There are two or more P atoms in bi-/multi-dentate phosphines to coordinate and chelate with the transition metal to form a more stable catalyst active intermediate and avoid the deactivation. A bi-dentate phosphine ligand vinyl-biphephos functionalized by vinyl was copolymerized with vinyl monomer through solvothermal synthesis method reported by Li et al.⁴³ The prepared corresponding Rh catalyst Rh/CPOL-1bp&10P showed higher regioselectivity (L/B > 24) and activity (TOF > 1200 h^-1) in propylene hydroformylation of fixed-bed reactor in comparison to Rh/POL-PPh₃, Rh/POL-dppe, Rh/CPOL-bp&DVB, Rh/CPOL-bp&dppe, and Rh-biphephos/SiO₂ (Figures 7A and 7B). The metal Rh atomically dispersing on the porous polymer support achieved high reactivity of catalysts according to results of EXAFS and HAADF-STEM. Moreover, the same group synthesized Xantphos-doped Rh/POPs-PPh₃ via the copolymerization of vinyl-Xantphos and vinyl-PPh₃ (Figure 7C).⁴⁴ Although the lower conversion of 1-octene was obtained in Xantphos-doped Rh/POPs-PPh₃ system, the selectivity and regioselectivity of the target product were significantly superior to that of Rh/POPs-PPh₃.

Ligand effect on hydroformylation of olefins

(A) The monomers for polymerization (left to right): Vinyl Biphephos, 3vPPh₃, 4vdppe, DVB.

(B) Study on hydroformylation of propylene under different Rh/CPOL-BP&P catalysts⁴³ (Reproduced with permission from Ref.,⁴³ © Green Chem. 2016).

(C) Xantphos-doped Rh/POPs-PPh₃ catalyst for efficient hydroformylation of long-chain alkenes⁴⁴ (Reproduced with permission from Ref.,⁴⁴ © J. Catal. 2017).

(D) HAADF-STEM images of fresh Rh₁/PIPs.

(E) HAADF-STEM images of spent Rh₁/PIPs with H₂S.

(F) Proposed reaction mechanism comparison of hydrocarboxylation of cyclohexene on the Rh₁/PIPs catalyst with H₂S.

(G) *In situ* DRIFTS of Rh₁/PIPs for heterogeneous cyclohexene hydrocarboxylation¹⁰⁰ (Reproduced with permission from Ref.,¹⁰⁰ © ACS Catal. 2022).

In addition to phosphine ligands, some other ligands can also be used to regulate the hydroformylation of olefins in SACs. Yuan et al.¹⁰⁰ synthesized the hydrophilic catalyst (Rh₁/PIPs) through alkalization, polymerization, impregnation, and other steps. When CO feed contained 1000 ppm H₂S, the hydrocarboxylation of olefin was facilitated unexpectedly. The characterization of HAADF-STEM (Figures 7D and 7E) and EXAFS demonstrated that Rh existed as single atom in the Rh₁/PIPs. Ex situ EXAFS and in situ DRIFTS revealed a ternary cycle mechanism of olefin hydrocarboxylation reactions (Figure 7F). The authors used CO and CO-H₂S (1000 ppm H₂S) as probe molecules, and added mixed liquids (cyclohexene, water, iodomethane, and other reactants and auxiliaries) in the form of bubbles to perform in situ DRIFTS (Figure 7G). The findings demonstrated that, in contrast to Rh-H bonds in CO systems, Rh-H bonds in CO- H₂S systems exhibit a red-shift, which is attributable to the coordination of the strong electron ligand S species with Rh atoms. DFT calculation confirmed that the energy barrier of each step can be reduced with the addition of H₂S, including the speed control step. This work offered a sulfur-resistant strategy for the carbonylation reactions, and advanced the theory of SACs in heterogeneous catalysis.

Electron effect

In the catalytic process, the formation and breakage of chemical bonds of substrates, intermediates, or products on the catalyst surface are the results of the interactions between reactant molecules and metal atomic orbitals. Besides the properties of metal elements, the electronic structure of active metals is also influenced by metal size, supports, and the coordination environment of surface atoms. Wei et al.⁴¹ developed a high-performance Rh SACs (Rh-Co-Pi/ZnO) by adding heteroatoms to regulate the microenvironment of active metals (Figure 8A). In addition, the HAADF-STEM image (Figure 8B) of Rh-Co-Pi/ZnO clearly showed that the introduction of Pi greatly significantly increased the dispersion degree of Rh atoms. According to the characterization of CO-DRIFTs (Figure 8C) and XPS, the presence of Co atoms reduced the electron density around Rh and impaired the interaction of Rh-CO. Compared to Rh/ZnO system, the selectivity for linear aldehydes increased from 32.1% to 54.9%, the L/B ratio increased from 0.7 to 2.1, in Rh-Co-Pi/ZnO-catalyzed hydroformylation of 1-decene (Figure 8D). Inductively coupled plasma optical emission spectrometer showed that 94.1% of Rh, 96.4% of Co, and 95.9% of P remained in the recycled Rh-Co-Pi/ZnO. The well thermally stability and recyclability of Rh-Co-Pi/ZnO was reused for five cycles without noticeably decline of catalytic activity.

In previous studies, ionic liquids (ILs) have been shown to be effective in protecting and stabilizing nano- and homogeneous catalysts. Ding et al.¹⁰¹ found that ILs can increase the activation energy of single atoms aggregation and adjust the oxidation valence state of metal atoms, and first proposed a simple and universal strategy of stabilizing SACs. In 2021, the team extended this strategy to Rh SACs to investigate the effect of ILs on the stability of styrene hydroformylation on Rh₁/TiO₂ (Figure 8E). After five cycles of reaction, the TOF of unmodified Rh₁/TiO₂ decreased from 1250 h^-1 to only 10 h^-1, and the loading of Rh decreased from 0.1% to 0.05%. The initial TOF of the IL-stabilized catalyst was lower than that of Rh₁/TiO₂, but its stability was significantly increased. In particular, the TOF value of 1-(2-hydroxyethyl)-3-methylimidazolium bis (trifluoromethanesulfonyl) imide ([OHEmim][Tf₂N])-stabilized Rh₁/TiO₂ only decreased from 878 to 800 h^-1, and the loading of remained at about 0.1%. DFT calculation showed that ILs can increase the binding energy from 0.69 to 1.19 eV by acting as the linker between Rh atoms and TiO₂, thus improving its anti-leaching performance.

Catalytic mechanism of SACs in hydroformylation of olefins

There are few studies on the mechanism of hydroformylation catalyzed by SACs; many models and inferences need to be further explored for verification. Lee et al.³⁶ studied the influence of ReO_x promoter and the mechanism of hydroformylation of ethylene catalyzed by atomically dispersed Rh-ReO_x/γ-Al₂O₃ (Figure 9A). The synergistic effect of atomically dispersed Rh-ReO_x with γ-Al₂O₃ was revealed using DFT calculations and microkinetic modeling. In contrast to the typical Wilkinson’s catalyst, the stable Rh(CO)₂ precursor exhibited a 16-electron square planar structure by coordinating with two oxygen atoms on the surface. The RDS of hydroformylation depended on the local surroundings of Rh atoms. In the absence of ReO_x, the rate was controlled by the CO insertion step; in the presence of ReO_x, the Rh-CO coordination bond weakened, and CO coordination was the rate-controlling step. Meanwhile, ReO_x improved the selectivity of propanal by blocking the main hydrogenation pathway through steric effects.

Rh/POL-PPh₃ synthesized by Jiang et al.⁸⁵ showed excellent performance and ultra-high stability in fixed bed of olefins hydroformylation, which was due to the similar catalytic function to that of homogeneous catalyst HRh(CO)(PPh₃)₃ (Figure 9B). Ma et al.¹⁰² revealed the dual role of the polymer 3V-PPh₃ monomer as both support and ligand in Rh/3V-PPh₃ catalyzing hydroformylation of ethylene through quantum chemistry method. Compared with PPh₃ as support, the adsorption energy of Rh atoms on 3V-PPh₃ increased, indicating that the introduction of vinyl increased the Rh-P bond interaction. Secondly, the high density of P atoms (Rh: P = 1:3) exposed on supports helped to improve the dispersion degree of Rh atoms, to increase the energy barrier formed by Rh-Rh bond, finally to improve the stability of catalyst. In this paper, the dual action mechanism of 3V-PPh₃ as support and ligand in Rh/3V-PPh₃ is studied, and the reason why Rh atoms are not easy to lose is explained from a microscopic perspective, which is helpful to understand the relationship between microstructure and electronic effect, and provides theoretical guidance for the development and design of efficient heterogeneous catalyst.

Gao et al.⁵² used DFT calculation to explore the regioselective mechanism of Rh₁/PNP-ND-catalyzed hydroformylation of styrene (Figure 9C). The Rh₁/PNP-ND model was established based on the coordination of one Rh atom with two P atoms on the similar single-layer graphene. After optimization, a square planar structure was obtained under reaction conditions, with the coordination of one Rh, two P, one H, and one CO. It is generally recognized that the step of olefin insertion is crucial for determining the regioselectivity. The reaction barrier of the formation of branched aldehydes (0.69 eV) was significantly lower than that of linear aldehydes (0.74 eV). After calculation, the ratio of the relative rates of branched aldehydes and linear aldehydes was 5.75. The ratio of two products was predicted to be 85:15, which was consistent with the experimental data. Further thermodynamic analysis revealed that, starting from the same alkene coordination state, the ΔG of the branched alkyl complex and linear alkyl complex is -3.36 and -1.21 kcal/mol, respectively. In conclusion, the coordination environment of Rh atoms in Rh₁PNP-ND was favorable for the formation of branched chain products in both thermodynamics and dynamics.

Wang et al.⁵¹ thoroughly investigated the significant performance of 0.2% Rh/CoO in hydroformylation of propylene combining DFT calculation with characterization data (Figure 9D). The DRIFT spectrum of 0.2% Rh/CoO demonstrated that the adsorption of propylene was greatly enhanced when exposed to H₂ and CO atmosphere. The binding energy of Rh 3d in the mixed atmosphere (H₂, CO, and propylene) was 1.3 eV lower than that without any gas treatment, which was obviously larger than the bias generated by the catalyst exposed to H₂ or CO atmosphere, according to the XPS spectrum of 0.2% Rh/CoO. It can be inferred that Rh atoms in Rh/CoO undergo structural reconstruction during the catalytic process, which promoted the adsorption and activation of reactants. Based on the calculation of DFT, four stable co-adsorption configurations were proposed, designated as configurations Ⅰ, Ⅱ, Ⅲ, and IV. The relative positions of the H atoms and adjacent unsaturated C atoms helped to conclude that configurations Ⅰ and Ⅲ tend to form linear products, whereas configurations Ⅱ and IV tend to form branched products. Further investigation of the paths of configurations Ⅰ and Ⅱ showed that the last step of product formation was the rate-limiting step with the highest energy barrier. The energy barrier of the rate-limiting step in configuration I is 0.063 eV lower than that in configuration II, indicating the favorable formation of linear products.

Wei et al.³⁷ created an effective and stable Co SACs (Co/β-Mo₂C) by utilizing the potent electronic metal–support interaction (EMSI) effects between Co atoms and β-Mo₂C. This catalyst outperformed all previously reported heterogenetic Co-based catalyst in hydroformylation of propylene (TOF up to 749 h^-1). After repeated use for five times, the activity of catalyst did not decline noticeably. Compared to the bulk Co particles, Co/β-Mo₂C with single Co atom has better activity for hydroformylation of olefin, and the TOF value was increased by 8.7 times. Based on the characterization of XPS, Baber charge, charge density, and Co density of states, it can be seen that the EMSI effect between single Co atoms and the support tailored the electronic properties of metal to be positively charged Co²⁺ and reduced the electronic density of Co. According to DFT calculations, the electron deficient property of the Co atoms contributed to CO insertion, thereby increasing the activity of hydroformylation of olefins (Figure 9E). For Co-based hydroformylation, the significant EMSI interaction between single-atom Co and the support in Co/β-Mo₂C was crucial in optimizing the charge density, lowering the reaction potential energy, and stabilizing the active site.

Insoo Ro et al.³⁸ reported heterogeneous Rh-WO_x pair site catalysts for ethylene hydroformylation. Two active sites (Rh atoms and WO_x species deposited on Al₂O₃) worked together to catalyze various stages of the reaction. The structure of catalyst can be altered by varying the loading of WO_x on the support, which regulated the catalytic activity of ethylene hydroformylation in turn (Figure 9F). According to HAADF-STEM (Figure 9G) and CO-Fourier transform infrared spectrometer characterization, Rh and WO_x were located in Rh/0.7W as independent sites and were also forming Rh-W pair sites. Due to the synergistic interaction between the active sites on Rh and WO_x, Rh/0.7W possessed the highest activity (0.1 g_propanal cm^-3h^-1) and selectivity ( > 95%). A bifunctional mechanism was proposed based on the experimental kinetics and First-principles microdynamics simulations (Figure 9H). Rh assisted WO_x reduction, which binded ethylene molecules; ethylene was transferred from WO_x to Rh; H₂ dissociated at the Rh-WO_x interface to form two hydrogen atoms, one of which binded to the Rh-WO_x interface. The bi-functional catalyst also depended on the geometry of the Rh-WO_x interface, the energetics of reconfiguring the coordination of the pair site during the reaction, and the capacity to transfer molecules between the active centers of the pair site.

Methods for probing the real active centers of SACs in the hydroformylation process

In SACs, active metal is loaded on the surface of the support in the form of a single atom, and the requirements for characterization methods of SACs have also reached unprecedented atomic-level accuracy. In recent years, the rapid development of electron microscopy and spectroscopy technology has provided support for analyzing the spatial distribution, electronic structure, and coordination environment of metal centers, and provided reliable evidence for exploring the catalytic performance, structure-activity relationship, and catalytic mechanism. The characterization techniques applicable to SACs include HAADF-STEM, XAS, and CO-DRIFT.

HAADF-STEM

HAADF-STEM improves the measurement accuracy to atomic level, and can clearly observe isolated metal atoms and their spatial distribution on the support, becoming the most direct means to characterize SACs.¹⁰³ It provides strong evidence for further understanding the mechanism of atomic catalytic reaction and identifying the coordination structure of metal center on the support. The brightness of the atoms in the image is proportional to the square of the atomic number, so as to distinguish between heavy atoms (such as Pd, Pt, Ru, Rh, Co, etc.) and light atoms (such as N, O, C, etc.).¹⁰⁴^,¹⁰⁵ In the prepared Rh₁/HAP, Li et al.,⁵³ using HAADF-STEM, can clearly observe that the isolated Rh atoms are evenly distributed on the HAP (Figure 10A). Shang et al.³⁹ observed atomically dispersed Rh species distributed in molecular sieve using Cs-HADDF-STEM. Due to the difference in atomic contrast (Si = 14, O = 8, Al = 13, and Rh = 45), the brightest spot in the image can be identified as the Rh atoms (Figure 10B).

XAS

X-ray absorption spectroscopy (XAS) is used to measure the structure of X-ray absorption coefficient varying with energy. The sample excites its core electrons to transition to the empty orbit by absorbing X-ray (XANES) or transition to continuous state to form wave dry radiation with surrounding atoms (EXAFS). The chemical valence state and electronic structure of elements can be obtained from XANES, and the two-dimensional local structure information of adjacent atoms can be obtained from EXAFS. Therefore, XAS is widely used to study the structural model of active sites and explore the mechanism of monatomic catalysis.²⁷ Ding et al.¹⁰¹ used XAS to characterize the synthesized atomically dispersed Rh₁/TiO₂. The K-edge XANES of Rh was studied with Rh foil and Rh₂O₃ as reference materials. Compared with the standard samples, the energy absorption curve of Rh₁/TiO₂ was very close to that of Rh₂O₃, indicating that the average oxidation state of Rh was close to 3+ (Figure 10C). According to EXAFS, Rh displayed a dominant peak at around 1.6 Å, which was assigned to the Rh-O first shell. There was no obvious peak at 2.3 Å, which was attributable to Rh-Rh scattering (Figure 10D).

CO-DRIFT

The probe molecule infrared spectroscopy shows different vibration frequencies for metal atoms in different chemical environments, which is an effective mean to characterize the dispersion state and electronic state of metal particles in supported catalysts. In the CO-DRIFT spectrum, the adsorption form of CO on metal can be judged according to the position of CO adsorption peak, and then the dispersion state of metal particles can be determined.⁴⁵^,¹⁰⁶^,¹⁰⁷^,¹⁰⁸ Li et al.⁴⁵ used CO-DRIFT technology to identify the existence of Rh loaded on CeO₂. For Rh₁/CeO₂, the positions of infrared absorption peaks of CO were 2010 and 2052 cm^-1, which are attributed to the symmetric and asymmetric vibration of gem-dicarbonyl doublet CO on positively charged Rh atoms (Figure 10E). A peak centered at 2052 cm^-1 was also observed which corresponds to the linear CO adsorption on Rh atoms. For NP-Rh/CeO₂ and 5Rh/CeO₂, the positions of the infrared adsorption peaks of CO were 1860 or 1800 cm^-1, and 2046 and 1960 cm^-1, which respectively correspond to the bridge adsorption between two Pt atoms, the linear adsorption of CO molecules on the surface of Rh atoms and the adsorption at the interface (Figure 10F).

DFT calculation

DFT calculation is often combined with relevant experiments to further explore the reaction mechanism by studying the properties of catalytic materials (such as bond length, adsorption energy, etc.). DFT calculation in catalyst research mainly starts from the following four aspects: structural stability judgment, reaction free energy calculation, electronic structure analysis, and molecular diffusion/adsorption dynamics simulation. It is helpful to predict catalyst structure and stability, evaluate catalyst performance, innovate catalyst design strategies, and finally achieve SACs with high activity, high selectivity, and strong stability.¹⁰⁹^,¹¹⁰^,¹¹¹ Wei et al.⁴⁰ clarified the reaction mechanism of propylene hydroformylation on the catalyst through DFT calculation. Firstly, the most reasonable model of Au(0.2%)@S-1 is determined, that is, a single Au atom replaces the Si atom at the T8 site on the crystal S-1 zeolite. Secondly, the adsorption energy of H, CO, and propylene is calculated on the Au(0.2%)@S-1 model. Among them, the adsorption capacity of H is the strongest (2.19 eV), followed by propylene (1.18 eV) and CO (1.27 eV), indicating that the adsorption capacity of propylene on the Au(0.2%)@S-1 model is moderate, which is conducive to the adsorption and desorption of reactants on the active center. It is also calculated that the adsorption energies of CH₃CH₂CH₂^∗ and CH₃CH₂CH₂CO^∗ are much more negative than those of propylene, indicating that the olefin insertion and CO insertion reactions are thermodynamically favorable. Based on the above analysis, a possible mechanism of propylene hydroformylation on Au(0.2%)@S-1 is proposed (Figure 10G).

Other characterization techniques

It is difficult to separate and detect the free radicals and intermediates in the chemical reaction process, which makes it difficult to speculate the reaction mechanism. In situ Raman, in situ XPS, isotope labeling, and other technologies can monitor the dynamic evolution of catalysts and reaction intermediates in real time under experimental conditions, which helps to accurately understand the structure of catalysts and build theoretical models, making outstanding contributions to the design of various effective catalysts.

Opportunities and challenges of SACs for hydroformylation of olefins

In this paper, the catalytic application and reaction mechanism of SACs in hydroformylation of olefins are summarized. The effects of microstructure regulation on catalytic activity, chemical/regioselectivity, and stability are discussed. The strategies of support effect, ligand effect, and electronic effect are proposed to adjust the performance of SACs. Advanced characterization techniques HADDF-STEM, XAFs, and DFT calculations are used to further study the mechanism. Although the application of SACs in hydroformylation is still in its infancy, SACs have already shown excellent performance. Existing research demonstrates that the SACs, notably the Rh SACs, have distinctive electronic/coordination structure, high atom utilization, unsaturated active center coordination, and tunable central metal electronic structure; the above characteristics make its catalytic activity equal to or even better than that of homogeneous catalyst. More importantly, the strong coordination between active metals and supports can effectively avoid the loss of Rh, which provides a new direction for the development of heterogeneous hydroformylation.

Despite the significant development, there are still a dearth of pertinent studies and numerous pressing issues that need to be resolved.

(1)
In contrast to nanocatalyst and cluster catalyst, the active metal in SACs is atomic dispersion on the support, and the metal surface energy in SACs increases sharply. In the process of preparation and reaction, metal atoms are easy to migrate and agglomerate, which lead to the instability and deactivation of the catalyst. Therefore, the key to the synthesis of catalyst is to select an appropriate support. The coordination between the defect sites on the support surface and the single metal atom to prevent the agglomeration phenomenon can not only stabilize the single metal atom but also expose the active sites of the metal. The atomic dispersion of metal precursors can be achieved by means of space limitation, defect capture, and ligand anchoring, which can effectively limit the migration and aggregation of monodisperse metal atoms on the support. In addition, SACs cannot provide multiple adjacent metal sites, and its metallicity is often regulated by the support. Therefore, when multiple metal active sites are required to be activated cooperatively and active metals are required to have strong metallicity for catalytic reactions, SACs are difficult to achieve efficient catalytic activity. Fully exposed cluster catalysts (FECCs) can not only provide adjacent metal active sites but also partially maintain its metallicity on the basis of 100% metal dispersion. Metals in FECCs are mainly composed of very small clusters, and all atoms in the clusters are in the state of coordination unsaturated. FECCs have been widely used in alkane dehydrogenation, toluene hydrogenation, CO₂ reduction, LGWS, and other reactions, and become an important field in heterogeneous catalysis. FECCs, as an extension of the concept of SACs, can well solve the problem of single active site in SACs, which makes it possible to efficiently carry out multi-step and complex catalytic reaction systems. As a conceptual extension of SACs, FECCs can solve the problem of single active sites in SACs, and provide a new way to design efficient catalysts.
(2)
In order to fill the defect that SACs have a single metal center and low loading, a second metal is introduced to synthesize dual-atom-site catalysts (DASCs) and nano-single-atom-site catalysts (NSASCs). As a further extension of the concept of SACs, DACs/NSASACs achieve low-cost, high selectivity, high stability, and antitoxicity catalysts. They retain the advantages of SACs, and introduce a variety of interactions, such as synergistic effect, geometric effect, and electronic effect. With the diversity of metal atoms in DACs/NSASACs, it is of great significance to save precious metal resources, reduce production costs, and realize industrial applications. So far, the reported DACs/NSASACs have been successfully applied to hydrogen evolution reaction, O₂ reduction/evolution reaction, N₂ reduction reaction, CO oxidation reaction, and other catalytic fields. However, how to control the structure of diatomic sites, improve the density of catalytic sites, and reveal the synergistic effect between atomic sites and the structure-activity relationship of catalysts through accurate structural characterization or theoretical calculation is still a major challenge.
(3)
Most of the reported catalyst supports with remarkable performance are limited to metal oxides and porous polymers. The synthesis of SACs using metal oxide as the support has the advantages of simple synthesis process and straightforward catalyst model, which is conducive to exploring the reaction mechanism of olefin hydroformylation. In addition, the absence of phosphine ligand is extremely valuable for environmental preservation. However, the surface modification of this catalyst is limited, and the poor regioselectivity is difficult to reach the level of homogeneous catalyst. Due to the diversity of synthesis methods of porous polymers, mono/multidentate phosphine ligands can be modified into porous polymer materials. The SACs supported by this method have excellent catalytic activity and selectivity for hydroformylation. The high density of phosphine ligand can avoid the loss of Rh and improve the stability of catalyst. However, the complex synthesis of porous polymer materials, the expensive ligand, and the poor mechanical strength seriously limit the mass production of catalysts.
(4)
As the products of hydroformylation, aldehydes are high value-added intermediates that can be converted into amines, alcohols, or acetals by further reactions. The one-pot tandem hydroformylation-hydrogenation reaction, hydroformylation-adol condensation reaction, hydroformylation-acetalization reaction, and hydroformylation-reductive amination reaction are economical methods to obtain above productions. At present, SACs or even supported catalysts are rarely reported in this area, which calls for more investigation.
(5)
Hydroformylation with synthesis gas as raw material has become the mainstream of modern chemical industry for its mature process, low cost, and suitability. However, due to the high toxicity and explosiveness of syngas, researchers are committed to studying green and efficient alternatives, such as HCHO, CO₂, HCOOH, aldehydes, etc. Among them, the hydroformylation of HCHO as raw material has achieved good progress in reactivity and regioselectivity, and HCHO is cheap and easy to obtain, convenient for storage, transportation, and atmospheric pressure application. Therefore, HCHO is a promising substitute for synthesis of gas. CO₂ is a clean, low-cost, and abundant raw material. However, the inert carbon-oxygen bond in CO₂ makes it difficult to add metal-activated species, resulting in the poor selectivity of target products. The utilization of CO₂ in hydroformylation is still in the laboratory research and development stage. The hydroformylation of olefins using these syngas substitutes often requires the modification of precious metal catalysts with complex phosphine ligands, which leads to high production costs, so it is still a long way for the industrial application.
(6)
The prepared SACs are still in the early stages of fundamental research, with the defects such as poor thermal stability, high metal surface energy, and low active metal loading. Therefore, there are still great challenges in industrial production. The development of high stability and applicability of SACs is crucial for meeting the demands of industrial applications. The macroscopic preparation of SACs is the long-term pursuit and the most challenging ultimate goal in hydroformylation.
(7)
Further research is still required to fully understand the catalytic mechanism of SACs in hydroformylation, including the catalytic active species, reaction mechanism, and inactivation process. The synthesis and regulation of particular catalysts from the atomic scale can be realized by the means of in situ electron microscopy, in situ synchrotron radiation, and other contemporary characterization technologies. Combined with DFT calculations, the catalyst structure and reaction pathway can be simulated. Above methods provide a crucial scientific foundation for explaining the structure-activity relationship of SACs.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos. 22108306, 22102214), Taishan Scholars Program of Shandong Province (No. tsqn201909065), Shandong Provincial Natural Science Foundation (Nos. ZR2021YQ15, ZR2020QB174), and the Fundamental Research Funds for the Central Universities (No. 22CX07009A).

Author contributions

Writing - Original Draft, Conceptualization, S.T. and D.Y.; Review & Editing, M.W., G.X., W.W., and Y.Z.; Writing - Review & Editing, Supervision, Funding acquisition, Y.P.

Declaration of interests

The authors declare no competing interests.

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PERMALINK

Single-atom catalysts for hydroformylation of olefins

Shu Tao

Da Yang

Minmin Wang

Guangxun Sun

Gaoyan Xiong

Wenwen Gao

Youzhi Zhang

Yuan Pan

Summary

Graphical abstract

Introduction

Figure 1.

SACs for hydroformylation of olefins

Figure 2.

Rh SACs

Figure 3.

Co SACs

Figure 4.

Ru SACs

Figure 5.

Au SACs

Figure 6.

Regulation strategies of SACs for enhanced performance of hydroformylation

Support effect

Table 1.

Ligand effect

Figure 7.

Electron effect

Figure 8.

Catalytic mechanism of SACs in hydroformylation of olefins

Figure 9.

Methods for probing the real active centers of SACs in the hydroformylation process

HAADF-STEM

Figure 10.

XAS

CO-DRIFT

DFT calculation

Other characterization techniques

Opportunities and challenges of SACs for hydroformylation of olefins

Acknowledgments

Author contributions

Declaration of interests

References

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