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Abstract

A novel thermoregulated phase-separable Pt nanocatalyst stabilized by ionic liquid [CH₃(OCH₂CH₂)₂₂N⁺Et₃][PF₆⁻] (IL_PEG1000-PF6) was prepared and first explored for the selective hydrogenation of quinoline. The conversion of quinoline and selectivity of 1,2,3,4-tetrahydroquinoline were both >99% at 130 °C under 3 MPa for 1.5 h, and the IL_PEG1000-PF6 stabilized Pt nanocatalyst can be reused four times efficiently. In addition, the IL_PEG1000-PF6 stabilized Pt nanocatalyst was also extended to the selective hydrogenation of 3-methylquinoline, 6-methylquinoline, 8-methylquinoline, and 6-chloroquinoline with >99% conversion and selectivity, respectively.

Keywords

ionic liquids Pt nanocatalyst quinolines selective hydrogenation thermoregulated phase-separable catalysis

Introduction

Soluble transition-metal nanocatalyst (TMNC) has attracted much attention as a result of high activity and selectivity. However, it also suffers from the problem of separation from the reaction system, which is quite similar to the traditional homogeneous catalysts.¹ To solve this problem, a series of biphasic system were developed.^2
–4 Ionic liquids were increasingly investigated in the catalysis over the last few years^5
–7 and used to establish biphasic catalytic system involving TMNC, for examples, supported ionic liquid phase system^8,9 and ionic liquid biphasic system.¹⁰ In our previous studies, a thermoregulated phase-separable (TPS) catalysis system composed of ionic liquid [CH₃(OCH₂CH₂)_nN⁺Et₃][CH₃SO₃⁻] (n = 16 or 22) stabilized TMNC and the mixed solvent (toluene/n-heptane) was established and applied to various reactions.^11
–14 The catalysis process is as follows: the lower ionic liquid phase containing the TMNC is not miscible with the upper mixed solvent (toluene/n-heptane) including the substrate before the reaction. When the temperature is increased to the reaction temperature, the two phases become one phase, whereas the reaction proceeds in one phase under heating. After reaction, the system turns to be two phases again by cooling to room temperature (RT), which is conducive to the separation of TMNC from the product and reuse in next cycle.

Tetrahydroquinolines have a wide range of synthesis applications in medicines, pesticides and fine chemicals.^15,16 Selective hydrogenation of quinolines has been recognized as a feasible method for the preparation of tetrahydroquinolines owing to atom efficiency (Scheme 1).^17,18 Various TMNC were reported for selective hydrogenation of quinolines, such as Pd,¹⁹ Au,²⁰ Ru,²¹ Rh,²² and Pt^23
–29 (Table 1). Among these TMNC, the Pt nanocatalyst drew much attention due to its prominent capability for H₂ activation.^30,31 In our previous work, [CH₃(OCH₂CH₂)₂₂N⁺Et₃][CH₃SO₃⁻] (IL_PEG1000) stabilized Pt nanocatalyst was explored as catalyst for selective hydrogenation of quinolines, but the catalytic activity needs further improvement.³² Herein, a novel ionic liquid [CH₃(OCH₂CH₂)₂₂N⁺Et₃][PF₆⁻] stabilized Pt nanocatalyst (abbreviated as IL_PEG1000-PF6-Pt_nano), which possess the TPS property, was studied in the selective hydrogenation of quinoline as well as its derivatives with the aim to further improve the catalytic activity of Pt nanocatalyst.

Scheme 1.

Selective hydrogenation of quinolines.

Table 1.

The catalyst reported for the hydrogenation of quinoline in literatures.

Entry	Catalyst	Amount of catalyst(mmol%)	Time(h)	Yield(%)	Ref.
1	CCM/Pd	0.1	3	96	Zhu et al.¹⁹
2	1.2%Au@SBA-15-500	1.2	4	97	Zhao et al.²⁰
3	Ru@PGS	4	16	86	Zhang et al.²¹
4	Rh-PVP	1	2	94	Chaudhari et al.²²
5	Pt/TiO₂-ED	4	6	>98	Li et al.²³
6	Pt@JPS-FA	0.03	5	>98	Shaikh et al.²⁴
7	TBA-PWPt-250	0.72	0.67	94	Wei et al.²⁵
8	Pt@HA	2	6	97	Tan et al.²⁶
9	Pt/NR-CeO₂	0.25	5	>99	Zhang et al.²⁷
10	Pt-1.2	5	1.3	>98	Bai et al.²⁸
11	TPT-Pt_nano	0.1	3	>98	Xue et al.²⁹
12	IL_PEG1000-PF6-Pt_nano	0.08	1.5	>98	This work

Experimental section

Materials and reagents

The ionic liquid IL_PEG1000 was synthesized as reported method.³³ Toluene, n-heptane, and n-decane were obtained from Kermel. H₂PtCl₆·6H₂O were obtained from Alfa Aesar. Mercury (Hg) and KPF₆ were supplied by Aladdin. Quinoline, 3-methylquinoline, 6-methylquinoline, and 8-methylquinoline were got from Alfa Aesar. 6-Chloroquinoline were got from Aladdin.

Characterizations

The ¹H NMR and ³¹P NMR spectra were obtained from Bruker Avance II 400, the ¹³C NMR was recorded on Bruker Avance Ⅲ 500. Transmission electron microscopy (TEM) was measured by JEM-2000EX instrument (120 kV). GC was conducted on Tianmei 7900 GC equipped with OV-101 column and flame ionization detector (N₂ as carrier gas). GC-MS was conducted on Agilent 7000B instrument equipped with DB-1701 column (He as carrier gas). Inductively Coupled Plasma–Atomic Emission Spectrometry (ICP-AES) analysis was recorded on PerkinElmer Avio 220 instrument. The UV-Vis spectra were carried out by Flash SP-Max 2300A.

Preparation of IL_PEG1000-PF6

Typically, IL_PEG1000 (2.36 g, 2 mmol) and KPF₆ (0.37 g) were mixed with 30 mL deionized water in a flask (50 mL capacity). The mixture was extracted by 3 × 30 mL of CH₂Cl₂ after stirring for 12 h under RT and dried with MgSO₄. Then, CH₂Cl₂ was removed and the obtained IL_PEG1000-PF6 was dried under vacuum. ¹H NMR (400 MHz, CDCl₃): δ 1.32 (t, J = 8.0 Hz, 9H), 3.35-3.43 (m, 11H), 3.53-3.88 (m, 95H). ¹³C NMR (125 MHz, CDCl₃): δ 7.5, 53.9, 56.7, 59.0, 64.2, 70.2, 70.5, and 71.9. ³¹P NMR (162 MHz, CDCl₃) δ −131.28, −135.68, −140.07, −144.47, −148.86, −153.27, −157.66.

Preparation of IL_PEG1000-PF6-Pt_nano

In a typical procedure, an aqueous solution of H₂PtCl₆·6H₂O (1 mL, containing 7.4 × 10^-4 mmol of Pt) and 0.90 g IL_PEG1000-PF6 were added to a 75 mL Teflon-lined stainless-steel autoclave. The mixture was stirred at 70 °C for 2 h to evaporate water. After that, the autoclave was flushed and pressurized up to 4.0 MPa with H₂, and heated along with stirring at 120 °C for 2.5 h. After cooling and depressurization, a brown IL_PEG1000-PF6-Pt_nano was obtained.

Hydrogenation of quinoline and its derivatives

Take quinoline hydrogenation as an example, 0.115 g quinoline, 0.200 g n-heptane, 2.600 g toluene, and 0.100 g n-decane were transferred into the autoclave involving the as-prepared IL_PEG1000-PF6-Pt_nano. The autoclave was flushed and pressurized up to 3.0 MPa with H₂. Then, the hydrogenation was performed under 130 °C for 1.5 h. After cooling and depressurization, the upper organic phase including product was separated and directly examined by GC and GC-MS.

Hg poisoning experiment

Hg (0.15 g, Hg/Pt molar ratio = 1000) was added to the autoclave involving the as-prepared IL_PEG1000-PF6-Pt_nano and stirred under RT for 2 h, subsequently the hydrogenation was performed at 130 °C under 3 MPa for 1.5 h.

Results and discussion

The studies started with the preparation and characterization of the IL_PEG1000-PF6-Pt_nano. The IL_PEG1000-PF6-Pt_nano was prepared by the method of hydrogen reduction and analyzed firstly by TEM. As depicted in Figure 1, the newly obtained Pt nanocatalyst displayed a mean diameter of 2.4 ± 0.3 nm, indicating good stabilization of the IL_PEG1000-PF6. Furthermore, the UV-Vis absorption spectra of the IL_PEG1000-PF6-Pt_nano (Figure 2) showed that the characteristic peak disappeared completely after reduction, which also indicated the formation of Pt⁰.³⁴

Figure 1.

TEM image and particle size distribution of the newly prepared Pt nanocatalyst (the molar ratio of IL_PEG1000-PF6/Pt = 1000).

Figure 2.

UV-Vis absorption spectra of the IL_PEG1000-PF6-Pt_nano.

The schematic depiction of the structure of Pt with IL_PEG1000-PF6 was displayed in Figure 3. The PF₆⁻ anions interact with Pt nanocatalyst preferentially by electrostatic force and form a protective layer around the Pt surface, and the cations of IL_PEG1000-PF6 form the second protective layer to protect them from aggregation.^5,6

Figure 3.

Schematic depiction of the structure of Pt with IL_PEG1000-PF6.

After preparation and characterization of the IL_PEG1000-PF6-Pt_nano, the following experiments were carried out to certify that the IL_PEG1000-PF6-Pt_nano possessed the TPS property in the mixed solvent (toluene/n-heptane). As shown in Figure 4, the lower IL_PEG1000-PF6-Pt_nano phase is not miscible with the upper mixed solvent (toluene/n-heptane) at RT (left photo of Figure 4), then change to a single phase (central photo of Figure 4) as temperature rises. By cooling to RT, the IL_PEG1000-PF6-Pt_nano is separated from the mixed solvent (toluene/n-heptane) (right photo of Figure 4). The above experiments indicated that the IL_PEG1000-PF6-Pt_nano possessed the TPS property in the mixed solvent (toluene/n-heptane).

Figure 4.

TPS property of IL_PEG1000-PF6-Pt_nano in the mixed solvent (toluene/n-heptane).

With the above results in hand, the reactivity of IL_PEG1000-PF6-Pt_nano in the selective hydrogenation of quinoline was explored, and the results are displayed in Table 2. The influence of reaction temperature was first explored from 80 ℃ to 130 ℃. The >99% conversion was achieved as the temperature increased to 130 ℃ and the selectivity of 1,2,3,4-tetrahydroquinoline was >99% (Table 2, entry 4). Then the influence of time was studied. As the reaction time changed from 0.5 to 1.5 h (Table 2, entries 4–6), the conversion gradually increased to >99% and the selectivity of 1,2,3,4-tetrahydroquinoline remained >99%. Subsequently, the influence of H₂ pressure on the reaction was examined (Table 2, entries 4, 7, 8), >99% conversion of quinoline was achieved when the H₂ pressure was 3 MPa. This may be due to the fact that high pressure results in high hydrogen solubility in the organic phase, thus benefits the increasing of catalytic activity. Next, the influence of quinoline/Pt molar ratio was investigated (Table 2, entries 4, 9, 10). The conversion of quinoline decreased to 83% as the molar ratio of quinoline/Pt increased to 2000. The high molar ratio of quinoline/Pt may lead to a decreasing of H₂ at the Pt catalyst surface and therefore becomes unfavorable to the hydrogenation of quinoline. The influence of IL_PEG1000-PF6/Pt molar ratio was finally investigated. When the molar ratio of IL_PEG1000-PF6/Pt was 500, although the >99% conversion and selectivity could be achieved (Table 2, entry 11), the Pt nanocatalyst was agglomerated after the reaction because of the poor stability. Moreover, the conversion decreased to 90% when the IL_PEG1000-PF6/PF molar ratio was 1500 (Table 2, entry 12). The possible reason for this could be the diffusion barrier of quinoline. Therefore, the turnover frequency (TOF) value was 784 h⁻¹ under optimal reaction conditions (Table 2, entry 4). Considering that the anion of ionic liquids may influence reaction activity,^35,36 the reactivity of IL_PEG1000 stabilized Pt nanocatalyst was also investigated and showed a much lower catalytic activity (3% conversion of quinoline) compared with the IL_PEG1000-PF6-Pt_nano (Table 2, entries 4, 13). This may be due to the weakly coordinating effect of anion PF₆⁻,³⁷ which favors the absorption of H₂ and quinoline, therefore benefits the increasing of catalytic activity.

Table 2.

Selective hydrogenation of quinoline catalyzed by IL_PEG1000-PF6-Pt_nano^a.

Entry	T(℃)	t(h)	P (MPa)	Quinoline/Pt(molar ratio)	IL_PEG1000-PF6/Pt(molar ratio)	Conv.(%)^b	Sel.(%)^c	TOF(h⁻¹)^d
1	80	1.5	3	1200	1000	44	>99	348
2	100	1.5	3	1200	1000	88	>99	697
3	120	1.5	3	1200	1000	95	>99	752
4	130	1.5	3	1200	1000	>99	>99	784
5	130	0.5	3	1200	1000	63	>99	1497
6	130	1	3	1200	1000	94	>99	1167
7	130	1.5	1	1200	1000	87	>99	689
8	130	1.5	2	1200	1000	95	>99	752
9	130	1.5	3	1500	1000	98	>99	970
10	130	1.5	3	2000	1000	83	>99	1096
11	130	1.5	3	1200	500	>99	>99	784
12	130	1.5	3	1200	1500	90	>99	713
13^e	130	1.5	3	1200	1000	3	>99	24

Reaction conditions: Pt 7.4 × 10^-4 mmol, n-heptane (0.200 g), toluene (2.600 g), n-decane (0.100 g, as internal standard).

Conversion of quinoline, determined by GC and GC-MS.

Selectivity of 1,2,3,4-tetrahydroquinoline, determined by GC and GC-MS.

TOF: moles of 1,2,3,4-tetrahydroquinoline formed per mole of Pt per hour.

The IL_PEG1000 was used as stabilizer.

With the above experimental results in hand, the reusability of IL_PEG1000-PF6-Pt_nano was evaluated. Due to TPS property, the IL_PEG1000-PF6-Pt_nano can be simply separated from the product after reaction and reused in the next cycle. As shown in Figure 5, the IL_PEG1000-PF6-Pt_nano could be reused four cycles with >99% conversion and selectivity. The conversion was decreased at the fifth cycle. The possible reason for the decrease in conversion were further investigated. The TEM image indicated that the mean diameter of Pt nanocatalyst increased to 3.1 ± 0.6 nm after four cycles (Figure 6). Besides, the ICP-AES results indicated the Pt leaching in the organic phase gradually decreased in each cycle, the mean leaching of Pt was 0.68 wt% (Table 3). Therefore, the increasing of Pt nanocatalyst particle size and the leaching of Pt may lead to the decreased catalytic performance at the fifth cycle.

Figure 5.

The reusability of IL_PEG1000-PF6-Pt_nano in the selective hydrogenation of quinoline. Reaction conditions: Pt 7.4×10^-4 mmol, IL_PEG1000-PF6/Pt = 1000 (molar ratio), quinoline (0.115 g), n-heptane (0.200 g), toluene (2.600 g), n-decane (0.100 g, as internal standard), 130 °C, 1.5 h, 3 MPa.

Figure 6.

TEM image and particle size distribution of Pt nanocatalyst after four cycles (the molar ratio of IL_PEG1000-PF6/Pt = 1000).

Table 3.

The Pt leaching in the organic phase.

Cycle number	1	2	3	4
Pt leaching (wt%)	1.23	0.63	0.48	0.38

In order to get further insight in this catalytic reaction, Hg poisoning experiment was carried out to determine whether the IL_PEG1000-PF6-Pt_nano is homogeneous or heterogeneous. As shown in Table 4, the IL_PEG1000-PF6-Pt_nano was deactivated by adding Hg before the reaction, the conversion of quinoline was reduced from >99% to 2%. Therefore, the IL_PEG1000-PF6-Pt_nano might be heterogeneous in this catalytic process.³⁸

Table 4.

Hg poisoning experiment^a.

Entry	Hg/Pt (molar ratio)	Conversion (%)^b
1	1000	2
2^c	–	>99

Reaction conditions: Pt 7.4×10^-4 mmol, IL_PEG1000-PF6/Pt = 1000 (molar ratio), quinoline (0.115 g), n-heptane (0.200 g), toluene (2.600 g), n-decane (0.100 g, as internal standard), 130 ℃, 1.5 h, 3 MPa.

Conversion of quinoline, determined by GC.

Reaction without adding Hg.

To explore the generality of IL_PEG1000-PF6-Pt_nano, 3-methylquinoline, 6-methylquinoline, 8-methylquinoline, and 6-chloroquinoline were employed as substrates. As shown in Table 5, the above-mentioned substrates afforded the corresponding tetrahydroquinolines with >99% conversion and selectivity under the optimized reaction conditions.

Table 5.

Selective hydrogenation of quinoline derivatives catalyzed by IL_PEG1000-PF6-Pt_nano^a.

Reaction conditions: Pt 7.4 × 10^-4 mmol, IL_PEG1000-PF6 (0.60 g), n-heptane (0.200 g), toluene (2.600 g), n-decane (0.100 g, as internal standard), 130 °C, 4 MPa H₂, 1.5 h.

Conversion of substrate, determined by GC and GC-MS.

Selectivity of product, determined by GC and GC-MS.

Conclusion

A novel IL_PEG1000-PF6-Pt_nano with TPS property was synthesized and applied to catalyze the selective hydrogenation of quinoline and its derivatives. The IL_PEG1000-PF6-Pt_nano was easily reused four cycles that still maintained catalytic activity and selectivity. Further catalytic applications of IL_PEG1000-PF6-Pt_nano in hydrogenation of other substrates will be carried out in the future.

Footnotes

Author contributions

Mingyang Liu: Investigation, Writing—original draft, Writing—review & editing.

Yanhua Wang: Supervision, Writing—review & editing.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Data availability

The data generated and analyzed during the current study are available from the corresponding author on reasonable request.

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 work was supported by the National Natural Science Foundation of China (21173031).

Ethical consideration

Not applicable.

ORCID iD

Yanhua Wang

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A novel thermoregulated phase-separable Pt nanocatalyst for selective hydrogenation of quinoline and its derivatives

Abstract

Keywords

Introduction

Experimental section

Materials and reagents

Characterizations

Preparation of ILPEG1000-PF6

Preparation of ILPEG1000-PF6-Ptnano

Hydrogenation of quinoline and its derivatives

Hg poisoning experiment

Results and discussion

Conclusion

Footnotes

Author contributions

Consent to participate

Consent for publication

Data availability

Declaration of conflicting interests

Funding

Ethical consideration

ORCID iD

References

Preparation of IL_PEG1000-PF6

Preparation of IL_PEG1000-PF6-Pt_nano