A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones

Abstract

The selective hydrogenation of chalcone to dihydrochalcone under atmospheric hydrogen pressure is investigated using a thermoregulated phase-transfer ruthenium nanocatalyst, representing the first ruthenium nanocatalyst used for this reaction. Under the optimal conditions, the conversion of chalcone into dihydrochalcone and the selectivity are 98% and >99%, respectively. The ruthenium catalyst can be easily separated from the product and recycled three times without any decrease in the conversion or selectivity. The turnover frequency is higher than that of a reported ruthenium complex catalyst. The thermoregulated phase-transfer ruthenium nanocatalyst is also applied for the atmospheric hydrogenation of other α,β-unsaturated ketones.

Keywords

α,β-unsaturated ketones atmospheric hydrogen pressure Ru nanocatalyst selective hydrogenation thermoregulated phase-transfer catalysis

Introduction

The selective hydrogenation of α,β-unsaturated ketones is an important transformation in both organic synthesis and industrial processes,^1–5 especially in the field of pharmaceuticals, fragrances, and other fine chemicals.^6–9 In the past few decades, the use of transition-metal nanocatalysts for the selective hydrogenation of α,β-unsaturated ketones has attracted much attention. However, most attempts utilize expensive Rh,^10–13 Pd,^14–17 Ir,^18,19 Pt,^20,21 and Pt/Au²² nanocatalysts along with high hydrogen pressures. Until now, reports on this reaction with cheaper Ru nanocatalysts under atmospheric hydrogen pressure are still limited, and the catalyst itself is difficult to reuse.²³ In addition, compared with other hydrogen sources,²⁴ using H₂ leads to higher atom economy and conforms with green chemistry approaches.

In our previous work, a thermoregulated ligand Ph₂P(CH₂CH₂O)₂₂CH₃ (L_P1000) stabilized Ru nanocatalyst was prepared that exhibited thermoregulated phase-transfer ability in an aqueous/1-pentanol biphasic system.²⁵ Specifically, before the reaction, the Ru nanocatalyst was present in the lower aqueous phase due to the formation of intermolecular hydrogen bonds between L_P1000 and H₂O, while the substrate was located in the upper 1-pentanol phase. During heating, the intermolecular hydrogen bonds between L_P1000 and H₂O were broken, and the Ru nanocatalyst is transferred into the upper 1-pentanol phase. Therefore, the reaction proceeds in the same phase. Following completion of the reaction and cooling, the Ru nanocatalyst returns to the aqueous phase due to the regeneration of hydrogen bonds between L_P1000 and H₂O. Thus, by simple phase separation, the Ru nanocatalyst can be separated from the products easily and can be reused directly in the next cycle.

To date, this thermoregulated phase-transfer ruthenium nanocatalyst (TPT-Ru_nano catalyst) has been successfully applied to the selective hydrogenation of 1,5-cyclooctadiene under atmospheric hydrogen pressure.²⁶ In continuation of our ongoing research on thermoregulated phase-transfer catalysis, we herein report a TPT-Ru_nano catalyst for the selective hydrogenation of α,β-unsaturated ketones under atmospheric hydrogen pressure.

Results and discussion

The TPT-Ru_nano catalyst was prepared by reduction of RuCl₃·xH₂O with molecular hydrogen in the presence of L_P1000 as a stabilizer (molar ratio of L_P1000/Ru = 9). The ultraviolet-visible (UV-Vis) absorption spectra of the mixture of RuCl₃·xH₂O and L_P1000 before and after reduction are shown in Figure 1. It is clear that the mixture has a characteristic absorption peak at 382 nm before reduction; however, at the end of the reduction, this characteristic peak disappeared entirely, which indicated that Ru⁰ had formed.²⁷

Figure 1.

UV-Vis absorption spectra of the mixture of L_P1000 and RuCl₃·xH₂O.

In order to further confirm the chemical valence of the Ru element, X-ray photoelectron spectroscopy (XPS) was carried out. Figure 2 shows that there are two peaks at binding energies of 483.7 and 461.5 eV, corresponding to Ru 3p_1/2 and Ru 3p_3/2, respectively, which therefore confirms the existence of Ru⁰.^28,29

Figure 2.

XPS spectra of the TPT-Ru_nano catalyst.

In addition, with the aid of transmission electron microscopy (TEM) analysis, as shown in Figure 3, the average diameter of newly prepared Ru nanoparticles was found to be 1.3 ± 0.3 nm.

Figure 3.

TEM image and the particle size histogram of the newly prepared TPT-Ru_nano catalyst.

With the TPT-Ru_nano catalyst in hand, chalcone, which has wide applications in organic synthesis and medicinal chemistry,^30–35 was selected as a model substrate for the atmospheric hydrogenation, in which an Ru nanocatalyst has not been investigated. First, the influence of the molar ratio of L_P1000 to Ru on the reaction was explored. With the ratio increasing from 2 to 9, the conversion of chalcone increased from 67% to 98% (Table 1, entries 1–4). However, on further increasing the ratio to 10, the conversion of chalcone decreased to 92% (Table 1, entry 5). A possible reason for this is that the excess L_P1000 covered the active sites of the TPT-Ru_nano catalyst, and the effective contact between the TPT-Ru_nano catalyst and chalcone was weakened. Next, the influence of the temperature on the reaction was examined. In the range of 50–80 °C, the conversion of chalcone increased from 21% to 98% (Table 1, entries 4, 6, and 7), but when the temperature was further increased to 90 °C, the conversion of chalcone decreased to 94% (Table 1, entry 8) and the TPT-Ru_nano catalyst exhibited aggregation at this elevated temperature. The influence of reaction time was subsequently investigated. When the reaction time was extended from 30 to 60 min, the conversion of chalcone increased from 53% to 98% (Table 1, entries 4, 9, and 10). On further extending the reaction time to 75 min, the conversion of chalcone remained at 98% (Table 1, entry 11). Finally, we investigated the effect of the chalcone/Ru molar ratio on the reaction. The results indicated that on changing the ratio from 25 to 50, the conversion of chalcone decreased from 98% to 73% (Table 1, entries 4, 12, and 13). The selectivity for dihydrochalcone remained >99% all the time. Therefore, the optimal conditions are shown in Table 1 (entry 4). Under these conditions, the conversion of chalcone was 98%, and the selectivity for dihydrochalcone was >99%. Also, the turnover frequency (TOF) was 24 h⁻¹, which is higher than a reported Ru complex catalyst under atmospheric hydrogen pressure.³⁶

Table 1.

Atmospheric hydrogenation of chalcone catalyzed by the TPT-Ru_nano catalyst.

Entry	L_P1000/Ru (molar ratio)	Temperature (°C)	Time (min)	Chalcone/Ru (molar ratio)	Conversion (%)^a	Selectivity (%)^b	TOF (h⁻¹)^c
1	2	80	60	25	67	>99	17
2	4	80	60	25	71	>99	18
3	8	80	60	25	93	>99	23
4	9	80	60	25	98	>99	24
5	10	80	60	25	92	>99	23
6	9	50	60	25	21	>99	5
7	9	70	60	25	78	>99	19
8	9	90	60	25	94	>99	23
9	9	80	30	25	53	>99	26
10	9	80	45	25	85	>99	28
11	9	80	75	25	98	>99	19
12	9	80	60	35	75	>99	26
13	9	80	60	50	73	>99	36

TPT-Ru_nano catalyst: thermoregulated phase-transfer ruthenium nanocatalyst; TOF: turnover frequency.

Reaction conditions: Ru (3.76 × 10⁻⁶ mol) dissolved in water (4 mL), 1-pentanol (2 mL), n-decane (100 mg, as an internal standard), and H₂ balloon.

Determined by gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS).

Selectivity for dihydrochalcone.

TOF is calculated from the number of moles of dihydrochalcone generated per mole of Ru per hour.

The lifetime of noble transition-metal nanocatalysts is an important aspect for evaluating catalyst performance. Thus, the recyclability of the TPT-Ru_nano catalyst for the selective hydrogenation of chalcone was examined under the optimal conditions. After reaction, by simple phase separation, the aqueous phase containing the TPT-Ru_nano catalyst was reused directly in the next cycle. The results are shown in Figure 4. It can be seen that the TPT-Ru_nano catalyst could be recycled three times with the conversion remaining almost the same and the selectivity being >99%. However, in the fourth cycle, the conversion of chalcone dropped to 86%. In order to investigate the factors that resulted in the decrease of the conversion in the fourth cycle, we first detected Ru leaching in the upper 1-pentanol phase through inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results indicated that the average leaching of Ru was 2.5 wt% (weight percentage). The TPT-Ru_nano catalyst after three cycles was thus examined by TEM, and the image indicated that the average particle size was 1.9 ± 0.2 nm (Figure 5), which is larger than that of the newly prepared TPT-Ru_nano catalyst. Based on the above results, the leaching of Ru and the particle size increase of the TPT-Ru_nano catalyst may be the reason for the decrease of conversion in the fourth cycle.

Figure 4.

The reusability of the TPT-Ru_nano catalyst for the selective hydrogenation of chalcone under atmospheric hydrogen pressure.

Figure 5.

The TEM image and particle size histogram of the TPT-Ru_nano catalyst after the third cycle.

In order to further investigate the application scope of the TPT-Ru_nano catalyst, the atmospheric hydrogenation of different α,β-unsaturated ketones was evaluated. The experimental results are shown in Table 2. Under the optimal reaction conditions, the conversion of the substrate and the selectivity for hydrogenation of the C=C bond were all >99%.

Table 2.

Atmospheric hydrogenation of different α,β-unsaturated ketones catalyzed by the TPT-Ru_nano catalyst.

Entry	Time (min)	Conversion (%)^a	Selectivity (%)^b
1	60	>99	>99
2	60	>99	>99
3	60	>99	>99
4	60	>99	>99
5	180	>99	>99

TPT-Ru_nano catalyst: thermoregulated phase-transfer ruthenium nanocatalyst.

Reaction conditions: Ru (3.76 × 10⁻⁶ mol) dissolved in water (4 mL), 1-pentanol (2 mL), n-decane (100 mg, as an internal standard), substrate/Ru = 25 (molar ratio), L_P1000/Ru = 9 (molar ratio), temperature = 80 °C, and H₂ balloon.

Determined by gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS).

Selectivity for hydrogenation of the C=C bond.

Mercury-poisoning experiments are commonly used for checking whether a catalyst is homogeneous or heterogeneous.³⁷ Thus, to obtain further insight into the TPT-Ru_nano catalyst-catalyzed selective hydrogenation of chalcone, mercury-poisoning experiments were carried out. The results are listed in Table 3. When Hg was added before the reaction, the conversion of chalcone was only 4% (Table 3, entry 1). Compared with the reaction without Hg (Table 3, entry 2), the addition of Hg almost stopped the reaction completely. In another experiment, Hg was added to the system when the reaction had proceeded for 30 min. In this case, the conversion of chalcone was almost brought to a standstill despite extending the reaction time to 60 min (Table 3, entries 3 and 4). These results indicated that adding Hg to the system led to poisoning of the TPT-Ru_nano catalyst, which implies that the atmospheric hydrogenation of chalcone catalyzed by the TPT-Ru_nano catalyst may be heterogeneous.

Table 3.

Mercury-poisoning experiments on the atmospheric hydrogenation of chalcone catalyzed by the TPT-Ru_nano catalyst.

Entry	Hg/Ru (molar ratio)	Time of addition of Hg (min)	Total time (min)	Conversion (%)^a
1	1000	0	60	4
2	–	–	60	98
3	–	–	30	53
4	1000	30	60	55

TPT-Ru_nano catalyst: thermoregulated phase-transfer ruthenium nanocatalyst.

Reaction conditions: Ru (3.76 × 10⁻⁶ mol) dissolved in water (4 mL), 1-pentanol (2 mL), n-decane (100 mg, as an internal standard), chalcone/Ru = 25 (molar ratio), L_P1000/Ru = 9 (molar ratio), temperature = 80 °C, and H₂ balloon.

Determined by gas chromatography (GC) and gas chromatography–mass spectrometry (GC-MS).

Conclusion

In summary, a TPT-Ru_nano catalyst has been prepared and employed for the first time for the selective hydrogenation of α,β-unsaturated ketones under atmospheric hydrogen pressure. The catalyst was found to exhibit high conversion and selectivity. Furthermore, the catalyst could be easily separated from the product and recycled up to three times.

Experimental section

Materials

Ruthenium(III) chloride hydrate (RuCl₃·xH₂O, 38% Ru), chalcone, benzylideneacetone, and 3-methyl-2-cyclohexen-1-one were obtained from Alfa Aesar (Haverhill, MA, USA). 1-Pentanol was purchased from Macklin (Shanghai, China), and n-decane was purchased from Kermel (Tianjin, China). Mercury was obtained from Aladdin (Shanghai, China). 2-Cyclohexen-1-one was purchased from Aike (Chengdu, China). 3-Hepten-2-one was purchased from TCI (Tokyo, Japan). 1-Octen-3-one was purchased from Innochem (Beijing, China). All these chemicals were of analytical grade and were used without further purification. L_P1000 was synthesized according to the reported literature.^38–40 ¹H NMR (400 MHz, CDCl₃): δ 7.47-7.27 (m, 10H), 3.73-3.53 (m, 95H), 3.38 (s, 3H), 2.40 (t, J = 7.7 Hz, 2H). ¹³C NMR (101 MHz, CDCl₃): δ 137.4-127.4, 71.0-69.1, 67.5, 58.0, 27.8. ³¹P NMR (162 MHz, CDCl₃): δ −22.29.

Characterization

The NMR data were recorded with a Bruker Avance II 400 spectrometer (Bruker, Switzerland). The UV-Vis analyses were carried out on an Implen N50 spectrophotometer (Implen, Germany). The XPS were measured with an ESCALAB XI+ spectrometer (Thermo Fisher Scientific, Inc., UK). TEM data were obtained from a JEM-2000EX instrument (JEOL, Japan). Gas chromatography (GC) was performed with a Tianmei 7900 GC (Techcomp, China) fitted with an OV-101 column (50 m × 0.25 mm, film thickness 0.5 μm) and a flame-ionization detection (FID) detector; the carrier gas was nitrogen. Gas chromatography–mass spectrometry (GC–MS) was performed on an Agilent 7890A gas chromatograph (Agilent, USA) interfaced with an Agilent triple quadrupole 7000B mass detector (Agilent, USA). The GC separation was carried out on an Agilent DB-1701 column (30 m × 320 μm, film thickness 0.25 μm); the carrier gas was helium and the mass spectrometer mode was electron ionization (70 eV) using the scan mode. ICP–AES analyses were conducted on an AVIO 500 instrument (PerkinElmer, Waltham, MA, USA).

Preparation of the TPT-Ru_nano catalyst

An aqueous solution of RuCl₃·xH₂O (4 mL, containing 3.76 × 10⁻⁶ mol of Ru), L_P1000, and 1-pentanol (2 mL) were added to a Teflon-lined stainless-steel autoclave (75 mL). Then, the autoclave was filled and evacuated three times with 1 MPa H₂ and then filled with 5 MPa H₂. After the mixture had been stirred at 100 °C for 6 h, the autoclave was cooled to room temperature and depressurized. The aqueous phase exhibited a brownish-black color, which indicated the formation of the TPT-Ru_nano catalyst.

Atmospheric hydrogenation of α,β-unsaturated ketones

In a typical experiment, n-decane (100 mg, as an internal standard) and chalcone (0.0196 g, 9.40 × 10⁻⁵ mol) were added to an autoclave, which was charged with the TPT-Ru_nano catalyst. The autoclave was filled and evacuated three times with 1 MPa H₂ and then filled with H₂ (balloon). The obtained mixture was then stirred at 80 °C for 60 min. After the reaction was complete, the autoclave was cooled to room temperature. The 1-pentanol phase was removed and analyzed by GC and GC–MS. The lower aqueous phase containing the TPT-Ru_nano catalyst could be recycled.

Supplemental Material

sj-docx-1-chl-10.1177_17475198231189838 – Supplemental material for A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones

Supplemental material, sj-docx-1-chl-10.1177_17475198231189838 for A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones by Bin Gao and Yanhua Wang in Journal of Chemical Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (no. 21173031).

ORCID iD

Yanhua Wang

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Entry	L_P1000/Ru (molar ratio)	Temperature (°C)	Time (min)	Chalcone/Ru (molar ratio)	Conversion (%)^a	Selectivity (%)^b	TOF (h⁻¹)^c
1	2	80	60	25	67	>99	17
2	4	80	60	25	71	>99	18
3	8	80	60	25	93	>99	23
4	9	80	60	25	98	>99	24
5	10	80	60	25	92	>99	23
6	9	50	60	25	21	>99	5
7	9	70	60	25	78	>99	19
8	9	90	60	25	94	>99	23
9	9	80	30	25	53	>99	26
10	9	80	45	25	85	>99	28
11	9	80	75	25	98	>99	19
12	9	80	60	35	75	>99	26
13	9	80	60	50	73	>99	36

Entry	L_P1000/Ru (molar ratio)	Temperature (°C)	Time (min)	Chalcone/Ru (molar ratio)	Conversion (%)^a	Selectivity (%)^b	TOF (h⁻¹)^c
1	2	80	60	25	67	>99	17
2	4	80	60	25	71	>99	18
3	8	80	60	25	93	>99	23
4	9	80	60	25	98	>99	24
5	10	80	60	25	92	>99	23
6	9	50	60	25	21	>99	5
7	9	70	60	25	78	>99	19
8	9	90	60	25	94	>99	23
9	9	80	30	25	53	>99	26
10	9	80	45	25	85	>99	28
11	9	80	75	25	98	>99	19
12	9	80	60	35	75	>99	26
13	9	80	60	50	73	>99	36

A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones

Abstract

Keywords

Introduction

Results and discussion

Conclusion

Experimental section

Materials

Characterization

Preparation of the TPT-Runano catalyst

Atmospheric hydrogenation of α,β-unsaturated ketones

Supplemental Material

sj-docx-1-chl-10.1177_17475198231189838 – Supplemental material for A thermoregulated phase-transfer ruthenium nanocatalyst for the atmospheric hydrogenation of α,β-unsaturated ketones

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Supplemental material

References

Supplementary Material

Preparation of the TPT-Ru_nano catalyst

Entry	L_P1000/Ru (molar ratio)	Temperature (°C)	Time (min)	Chalcone/Ru (molar ratio)	Conversion (%)^a	Selectivity (%)^b	TOF (h⁻¹)^c
1	2	80	60	25	67	>99	17
2	4	80	60	25	71	>99	18
3	8	80	60	25	93	>99	23
4	9	80	60	25	98	>99	24
5	10	80	60	25	92	>99	23
6	9	50	60	25	21	>99	5
7	9	70	60	25	78	>99	19
8	9	90	60	25	94	>99	23
9	9	80	30	25	53	>99	26
10	9	80	45	25	85	>99	28
11	9	80	75	25	98	>99	19
12	9	80	60	35	75	>99	26
13	9	80	60	50	73	>99	36