Highly uniform Rh nanoparticles supported on boron doped g-C₃N₄ as a highly efficient and recyclable catalyst for heterogeneous hydroformylation of alkenes

Abstract

A series of boron doped g-C₃N₄ supported rhodium nanoparticle catalysts has been synthesized for the first time and exhibited excellent catalytic activity and easy recycling properties in the hydroformylation of styrene due to the boron doping, which modified the delocalized conjugated π structure of g-C₃N₄ and increased the adsorption and dispersion of Rh nanoparticles.

---

Hydroformylation of alkenes, discovered by Roelen as early as 1938, represents one of the most significant and largest catalytic industrial processes for the production of aldehydes and alcohols.^1,2 As a 100% atom-economic reaction, nowadays, the production capacity is beyond 12 million tons every year.^3–5 More importantly, these aldehydes and alcohols are considered as important versatile intermediates, which can be further transformed into a wide variety of high-performance chemicals, such as detergents, surfactants and plasticizers.^6–9

In traditional technology, homogeneous Rh-based complexes are commonly used and widely accepted as catalysts for hydroformylation of alkenes.^10–12 However, the application of homogeneous catalytic systems confronts problems of high cost of noble metals and difficulty in the separation of thermally sensitive Rh-based complexes from the low volatilized products.^13–15 With the increasing development of green chemistry and sustainable chemistry,¹⁶ transition metal nanocatalysts lead the organic synthesis towards a more economical and more sustainable development direction due to their novel electronic, magnetic, optical and catalytic properties.^17,18 To date, great progress in the hydroformylation of alkenes catalyzed by rhodium nanoparticle catalysts has been made; nevertheless, Rh nanoparticles alone usually suffer from serious agglomeration, leading to the rapid decay of catalytic activity and the poor durability of the catalyst. To stabilize the nanocatalyst system, a common colloidal method to prepare metal particles involves the reduction of metal ions in the presence of stabilizing or capping materials such as surfactants and polymers.^19–22 The important benefit of colloidal nanoparticle synthesis for catalytic applications is the ability to systematically tune particle size and shape, as well as the composition of multielement particles. The stability of Rh nanoparticles can be controlled by the type of stabilizers; however, the strong encapsulation of metal particles within the stabilizing agents results in a loss of catalytic activity due to a capping agent effect.^23–26

To stabilize and expose the catalytically “clean” metal nanoparticles, an additional approach is immobilizing metal nanoparticles onto various solid supports, such as dendritic scaffolds, polymers, metal oxides, mesoporous materials and various kinds of carbon.^27–30 Among these support materials, graphitic carbon nitride (g-C₃N₄) has been widely attractive for heterogeneous catalysis due to its unique 2-dimensional (2D) structure, high surface area, facile modification, desirable thermal and chemical stability and facile preparation from abundantly available precursors such as urea and melamine.^31–34 In particular, the framework topology of g-C₃N₄ is in fact a defect-rich, N-bridged “poly (tri-s-triazine ring)”,^35,36 which would tend to form π-conjugated planar layers like that of graphite.³⁷ This electronic structure of the π-network would effectively adsorb metal nanoparticles and prevent them from agglomeration on the g-C₃N₄.³⁸ Furthermore, g-C₃N₄ contains so-called “nitrogen pots” with abundant melon moieties, which are potential ideal sites for heteroatom doping.³⁷ Among chemical modifications with foreign atoms, boron-doped g-C₃N₄ has been broadly covered to amplify activity for the targeted reaction.^39–41 In particular, the selection of boron source may result in different doping sites and further affect Lewis acidity on the surface of g-C₃N₄, which can modify the electronic structure and also probably act as specific reaction sites for reactant molecules.^42,43 Inspired by these findings in hydroformylation, it is expected that introducing boron in g-C₃N₄ may be an effective strategy to optimize the catalytic performance of Rh nanoparticles.

In the present study, a boron doped g-C₃N₄ supported Rh nanoparticle catalyst has been synthesized (cf. ESI,† Scheme S1) and tested in styrene hydroformylation. The g-C₃N₄, with rich pyridine-like nitrogen, can trap Rh nanoparticles to form potential active sites, while the loading boron can modulate the textural, electronic, and structural properties of g-C₃N₄ to promote the electron transfer and form a more stable π conjugation system, which strengthens the adsorption and deposition of Rh nanoparticles and alkenes onto the g-C₃N₄. By considering the importance of boron for deposition of Rh, the deposited Rh nanoparticles on boron doped g-C₃N₄ could exhibit excellent catalytic activity in the hydroformylation of styrene, as well as easy separation and recyclability.

The structures of the as-prepared pure g-C₃N₄, 3%B-g-C₃N₄ and Rh/X%B-g-C₃N₄ were evaluated by X-ray diffraction (XRD), as shown in the Fig. S1 (cf. ESI†). The XRD patterns confirmed the formation of graphitic stacking C₃N₄ layers. Two peaks are observed in the XRD patterns for all the samples. The high-intensity peak at around 27.7° is a characteristic of an interlayer stacking of conjugated aromatic systems, which is indexed to the (002) plane of graphitic stacking g-C₃N₄ corresponding to the average interlayer distance of d = 0.326 nm. The small peak observed at 13.1°, indexed as the (100) plane, is relevant to the in-plane structural packing motif of tri-s-triazine units, with an average distance of d = 0.675 nm. This suggests that the support materials are intact and maintained their graphitic structure after the doping process. For the boron doped samples, the (100) peak shifts slightly to higher angles with increasing content of H₃BO₃, which should be caused by the lattice contraction from the doping behavior.⁴⁰ After the anchoring of Rh nanoparticles, there is no obvious diffraction peak related to Rh due to the low loading amount and fine dispersion of Rh nanoparticles on the g-C₃N₄ support.

The morphologies of pure g-C₃N₄, 3%B-g-C₃N₄ and Rh/3%B-g-C₃N₄ were examined by scanning electron microscopy (cf. ESI,† Fig. S3). The SEM picture of pure g-C₃N₄ clearly reflects a two-dimensional lamellar structure consisting of small flat sheets with wrinkles and rolling edges. The thickness of the layer is measured to be about 20 nm. Further observation of the SEM picture indicates that there are some pores on the layers’ surface, which is due to the release of a large number of gases such as NH₃ and CO₂ during thermal condensation of urea. After the doping of boron and deposition of Rh nanoparticles on the g-C₃N₄ (cf. ESI,† Fig. S3b–d), the platelet-like morphology of g-C₃N₄ remains intact, which is consistent with the XRD and FTIR results (cf. ESI,† Fig. S1 and S2).

In order to study the promoting effect of boron on the catalytic performance of the Rh/g-C₃N₄ catalyst, the distributions of the Rh nanoparticle size in Rh/g-C₃N₄ and Rh/3%B-g-C₃N₄ were investigated by TEM. As shown in Fig. 1, both Rh/g-C₃N₄ and Rh/3%B-g-C₃N₄ samples exhibit a clear lamellar structure of g-C₃N₄, which means the boron doping does not affect the morphology of g-C₃N₄. After the introduction of Rh nanoparticles, numerous dark Rh nanoparticles appear on the lamellar g-C₃N₄. All the Rh nanoparticles are attached to the surface of g-C₃N₄ strongly.

Fig. 1 TEM images of Rh/g-C₃N₄ (a and b) and Rh/3%B-g-C₃N₄ (c and d).

The TEM micrograph of Rh/g-C₃N₄ (Fig. 1b) reveals that Rh nanoparticles with an average diameter of 6–7 nm are well dispersed on the surface of g-C₃N₄. As for Rh/3%B-g-C₃N₄ (Fig. 1d), the Rh nanoparticles homogeneously disperse on the surface of g-C₃N₄ and the particle size of the deposited Rh nanoparticles becomes small and evenly-distributed (ca. 1 nm). Thus, these results reveal that highly uniform Rh nanoparticles can be supported successfully on the boron doped g-C₃N₄. Moreover, the strong interaction between boron and the π bonded planar-layers could induce the formation of small-sized Rh nanoparticles and then stabilize the resulting nanoparticles.

To further probe the composition of samples, X-ray photoelectron spectroscopy (XPS) was carried out to explore the presence of C, N, B and Rh peaks in the sample of Rh/3%B-g-C₃N₄, as shown in Fig. 2. Fig. 2a shows the high-resolution spectrum of C 1s. The C 1s XPS spectrum reveals two different signals at 284.6 and 287.9 eV, respectively. The peak at 284.8 eV is ascribed to graphitic carbon. The weaker C peak located at 288.7 eV can be assigned to N–C=N groups of triazine rings. The N 1s peak in the relevant XPS spectrum can be deconvoluted into three peaks centered at 398.1, 399.8, and 404.2 eV (Fig. 2b), assigned to sp²-bonded N in triazine rings (C=N–C), the bridging N atoms in N(–C)₃ and terminal NH₂, respectively. The B 1s XPS spectrum of Rh/3%B-g-C₃N₄ in Fig. 2c displays one peak at 191.6 eV, which is higher than 190.1 eV for h-BN and lower than 194 eV for B₂O₃.⁴⁴ This indicates that boron inserts in the BCN frame in the form of N–B–N coordination, which implies boron is a substitute for some carbon atoms in the CN framework. XPS analysis of the Rh 3d core level shows the presence of two bands at 307.1 and 311.6 eV which may be assigned to metallic Rh 3d_5/2 and Rh 3d_3/2, respectively. Another two peaks at 309.1 eV and 314.4 eV are attributed to the Rh³⁺ 3d_5/2 and Rh³⁺ 3d_3/2, respectively. This confirms that the deposition–precipitation method effectively loaded nanosized particles of Rh on the boron doped g-C₃N₄.

Fig. 2 XPS spectra of (a) C 1 s, (b) N 1 s, (c) B 1s and (d) Rh3d of the Rh/3%B-g-C₃N₄ sample.

In order to further understand the promotion effect of boron addition, the catalytic performances of styrene hydroformylation over the Rh/g-C₃N₄ and Rh/X%B-g-C₃N₄ catalysts were evaluated in detail, as shown in Table 1. It can be seen that the Rh/g-C₃N₄ catalyst exhibits an excellent catalytic activity for the hydroformylation of styrene (entry 1, TOF = 7100 h⁻¹). For the Rh/1%B-g-C₃N₄ catalyst, the conversion of styrene increases to 79.1% with a higher TOF (entry 2). Encouraged by these results, a series of boron doped g-C₃N₄ supported Rh catalysts were prepared with increasing boron loading. As shown in entries 2–7, the modification of boron results in higher activity of the catalysts. Doping boron atoms on the surface of g-C₃N₄ can act as the anchoring sites to support Rh nanoparticles, resulting in the uniform deposition of Rh nanoparticles on the surface of g-C₃N₄. In addition, the styrene with the delocalized π bonds, as an electron donor, can be adsorbed easily on the boron Lewis acid sites with the formation of electron-deficient aromatic intermediates, resulting in an increase in the catalytic performance of the Rh/X%B-g-C₃N₄ catalysts. The effect of boron loading in the hydroformylation of styrene catalyzed by Rh/X%B-g-C₃N₄ catalysts was also evaluated in detail, the conversion of styrene gradually increases until a loading of boron up to 3%, suggesting that the introduction of boron into the catalyst made the catalyst expose many more active sites and that a synergetic effect could be occurring between the Rh species and the boron species. On further increasing the boron loading, the styrene TOF kept unchanged, which may be due to the much higher amount of boron on the surface which cannot disperse and expose more active Rh sites. Thus, the optimum loading of boron in the Rh/X%B-g-C₃N₄ catalysts is found to be 3% in the tested range. This loading was chosen for further studies.

Table 1 Hydroformylation of styrene catalyzed by Rh/g-C₃N₄ and Rh/X%B-g-C₃N₄ with various boron contents^a

Entry	Catalyst	Conversion (%)	TOF^b (h^−1)	Selectivity (%)
				Aldehydes	B : L^c
a Reaction conditions: catalyst: 0.01 g, toluene: 20 mL, styrene: 1.0 mL, reaction time: 3 h, temp.: 100 °C, syngas (CO/H2 = 1): 6.0 MPa. b TOF = number of moles of product formed/(number of moles of Rh × time). c B : L = 2-phenylpropanal : 3-phenylpropanal.
1	Rh/g-C3N4	58.9	7100	100	57 : 43
2	Rh/1%B-g-C3N4	79.1	9600	100	58 : 42
3	Rh/2%B-g-C3N4	85.2	10 300	100	57 : 43
4	Rh/3%B-g-C3N4	99.0	12 000	100	57 : 43
5	Rh/4%B-g-C3N4	99.0	12 000	100	56 : 44
6	Rh/5%B-g-C3N4	99.0	12 000	100	56 : 44
7	Rh/6%B-g-C3N4	99.0	12 000	100	55 : 45

---

The lifetime is an important aspect concerning the use of catalysts, and therefore the reusability of the Rh/3%B-g-C₃N₄ was examined for 7 cycles under the optimum reaction conditions and the results are displayed in Fig. 3. After the reaction, the catalyst was easily separated from the reaction solution and directly reused in the next catalytic cycles. As shown in Fig. 3, the Rh/3%B-g-C₃N₄ catalyst can be reused at least 7 times with little change in the catalytic performance and selectivity, indicating that the Rh/3%B-g-C₃N₄ is an efficient and stable catalyst for hydroformylation of alkenes. Namely, the strong interaction between the doped boron and π bonded planar-layers of g-C₃N₄ may be acting to stabilize the Rh nanoparticles, which can strengthen the adsorption and dispersion of Rh nanoparticles on the surface of g-C₃N₄.

Fig. 3 Reusability of the Rh/3%B-g-C₃N₄ catalyst for the hydroformylation of styrene. Reaction conditions: Rh/3%B-g-C₃N₄: 0.01 g, toluene: 20 mL, styrene: 1.0 mL, temp.: 100 °C, syngas (CO/H₂ = 1): 6.0 MPa.

In this work, a series of boron doped g-C₃N₄ supported rhodiun nanoparticle catalysts has been prepared and exhibit significantly enhanced activity in the hydroformylation of styrene (the highest TOF = 12 000 h⁻¹). The excellent catalytic performances of the Rh/B-g-C₃N₄ catalysts should be attributed to its unique two-dimensional lamellar structure, which determined its largest exposed Rh surface area, highest nanoparticle dispersion and smallest nanoparticle size. More importantly, the Rh nanoparticles of Rh/B-g-C₃N₄ dispersed more homogeneously and the Rh particle size (about 1 nm) was more uniform than those of the Rh/g-C₃N₄. The strong interactions between boron and the π bonded planar-layers of g-C₃N₄ could induce the formation small-sized Rh nanoparticles and then stabilize the resulting nanoparticles. In addition, Rh/B-g-C₃N₄ also exhibited excellent catalytic activities for the hydroformylation of various alkenes with high selectivity to aldehydes. More importantly, the Rh/B-g-C₃N₄ can be easily separated from the products and directly reused in the next cycle without an evident loss in the catalytic activity and selectivity after 7 cycles. In summary, we envision that the research results will constitute a new insight for further applications of boron-doped or boron dominant materials towards heterogeneous hydroformylation catalysis.

Conflicts of interest

There are no conflicts to declare.

References

1. F. Hebrard and P. Kalck, Chem. Rev., 2009, 109, 4272–4282.
2. R. Franke, D. Selent and A. Borner, Chem. Rev., 2012, 112, 5675–5732.
3. S. A. Jagtap and B. M. Bhanage, Appl. Organomet. Chem., 2018, 32, 4478–4486.
4. I. Piras, R. Jennerjahn, R. Jackstell, A. Spannenberg, R. Franke and M. Beller, Angew. Chem., 2011, 123, 294–298.
5. M. Beller, B. Cornils, C. D. Frohning and C. W. Kohlpaintner, J. Mol. Catal. A: Chem., 1995, 104, 17–85.
6. S. Fuchs, D. Lichte, T. Jolmes, T. Rösler, G. Meier, H. Strutz, A. Behr and A. J. Vorholt, ChemCatChem, 2018, 10, 4126–4133.
7. G. M. Torres, R. Frauenlob, R. Franke and A. Borner, Catal.: Sci. Technol., 2015, 5, 34–54.
8. J. Wildt, A. C. Brezny and C. R. Landis, Organometallics, 2017, 36, 3142–3151.
9. C. Y. Li, K. J. Sun, W. L. Wang, L. Yan, X. P. Sun, Y. Q. Wang, K. Xiong, Z. P. Zhan, Z. Jiang and Y. J. Ding, J. Catal., 2017, 353, 123–132.
10. M. Haumann and A. Riisager, Chem. Rev., 2008, 108, 1474–1497.
11. M. Ahmeda and A. Sakthivel, J. Mol. Catal. A: Chem., 2016, 424, 85–90.
12. J. Liu, L. Yan, Y. J. Ding, M. Jiang, W. D. Dong, X. E. Song, T. Liu and H. J. Zhu, Appl. Catal., A, 2015, 492, 127–132.
13. Y. P. Zhao, X. M. Zhang, J. Sanjeevi and Q. H. Yang, J. Catal., 2016, 334, 52–59.
14. N. C. C. Breckwoldt, N. J. Goosen, H. C. M. Vosloo and P. V. Gryp, React. Chem. Eng., 2019, 4, 695–704.
15. B. C. E. Makhubela, A. Jardine and G. S. Smith, Green Chem., 2012, 14, 338–347.
16. L. B. Wang, W. B. Zhang, S. P. Wang, Z. H. Gao, Z. H. Luo, X. Wang, R. Zeng, A. W. Li, H. L. Li, M. L. Wang, X. S. Zheng, J. F. Zhu, W. H. Zhang, C. Ma, R. Si and J. Zeng, Nat. Commun., 2016, 7, 14036–14043.
17. F. Y. Xiao, Y. W. Qin, N. Wang and D. W. Pan, Appl. Clay Sci., 2018, 166, 74–79.
18. F. Huang, Y. C. Deng, Y. L. Chen, X. B. Cai, M. Peng, Z. M. Jia, P. J. Ren, D. Q. Xiao, X. D. Wen, N. Wang, H. Y. Liu and D. Ma, J. Am. Chem. Soc., 2018, 140(41), 13142–13146.
19. I. Nakamula, Y. Yamanoi, T. Imaoka, K. Yamamoto and H. Nishihara, Angew. Chem., Int. Ed., 2011, 50, 5830–5833.
20. X. R. Xue, Y. Song, Y. C. Xu and Y. H. Wang, New J. Chem., 2018, 42, 6640–6643.
21. W. Alsalahi, W. Tylus and A. M. Trzeciak, ChemCatChem, 2018, 10, 2051–2058.
22. Z. Sun, Y. H. Wang, M. M. Niu, H. Q. Yi, J. Y. Jiang and Z. L. Jin, Catal. Commun., 2012, 27, 78–82.
23. Y. K. Shi, X. J. Hu, B. L. Zhu, S. R. Wang, S. M. Zhang and W. P. Huang, RSC Adv., 2014, 4, 62215–62222.
24. M. E. Grass, S. H. Joo, Y. W. Zhang and G. A. Somorjai, J. Phys. Chem. C, 2009, 113, 8616–8623.
25. Y. Borodko, S. E. Habas, M. Koebel, P. D. Yang, H. Frei and G. A. Somorjai, J. Phys. Chem. B, 2006, 110, 23052–23059.
26. Y. Borodko, S. M. Humphrey, T. D. Tilley, H. Frei and G. A. Somorjai, J. Phys. Chem. C, 2007, 111, 6288–6295.
27. Z. M. Jia, F. Huang, J. Y. Diao, J. Y. Zhang, J. Wang, D. S. Su and H. Y. Liu, Chem. Commun., 2018, 54, 11168–11171.
28. L. X. Xia, D. Li, J. Long, F. Huang, L. N. Yang, Y. S. Guo, Z. M. Jia, J. P. Xiao and H. Y. Liu, Carbon, 2019, 145, 47–52.
29. C. Hou, G. F. Zhao, Y. J. Ji, Z. Q. Niu, D. S. Wang and Y. D. Li, Nano Res., 2014, 79, 1364–1369.
30. X. J. Hu, Y. K. Shi, Y. J. Zhang, B. L. Zhu, S. M. Zhang and W. P. Huang, Catal. Commun., 2015, 59, 45–49.
31. W. Tian, Q. H. Shen, N. X. Li and J. C. Zhou, RSC Adv., 2016, 6, 25568–25576.
32. W. Liu, M. L. Wang, C. X. Xu and S. F. Chen, Chem. Eng. J., 2012, 209, 386–393.
33. Y. J. Fu, C. A. Liu, C. Zhu, H. B. Wang, Y. J. Dou, W. L. Shi, M. W. Shao, H. Huang, Y. Liu and Z. H. Kang, Inorg. Chem. Front., 2018, 5, 1646–1652.
34. Y. S. Fu, T. Huang, B. Q. Jia, J. W. Zhu and X. Wang, Appl. Catal., B, 2017, 202, 430–437.
35. Y. Wang, X. C. Wang and M. Antonietti, Angew. Chem., Int. Ed., 2012, 51, 68–89.
36. J. D. Hong, X. Y. Xia, Y. S. Wang and R. Xu, J. Mater. Chem., 2012, 22, 15006–15012.
37. Q. Liu and J. Y. Zhang, Langmuir, 2013, 29, 3821–3828.
38. W. Y. Zhang, H. J. Huang, F. Li, K. M. Deng and X. Wang, J. Mater. Chem. A, 2014, 2, 19084–19094.
39. J. R. Wei, W. L. Shen, J. Zhao, C. W. Zhang, Y. H. Zhou and H. Y. Liu, Catal. Today, 2018, 316, 199–205.
40. C. H. Lu, R. Y. Chen, X. Wu, M. F. Fan, Y. H. Liu, Z. G. Le, S. J. Jiang and S. Q. Song, Appl. Surf. Sci., 2016, 360, 1016–1022.
41. W. Q. Kong, X. F. Zhang, B. B. Chang, Y. N. Zhou, S. R. Zhang, G. L. He, B. C. Yang and J. J. Li, Electrochim. Acta, 2018, 282, 767–774.
42. G. B. Zhou, Y. Pei, Z. Jiang, K. N. Fan, M. H. Qiao, B. Sun and B. N. Zong, J. Catal., 2014, 311, 393–403.
43. Y. K. Shi, X. J. Hu, L. Chen, Y. Lu, B. L. Zhu, S. M. Zhang and W. P. Huang, New J. Chem., 2017, 41, 6120–6126.
44. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2010, 26, 3894–3901.

---

Footnote

† Electronic supplementary information (ESI) available: Characterization, and supporting figures and tables. See DOI: 10.1039/c9nj05385a

---