Selective hydrogenation of α-pinene to cis-pinane over Ru nanocatalysts in aqueous micellar nanoreactors

Abstract

D-α-Tocopheryl polyethylene glycol 1000 succinate (TPGS-1000) stabilized Ru(0) nanoparticles were prepared and characterized. These nanoparticles were employed to selectively hydrogenate α-pinene to cis-pinane. With a small amount of Na₂CO₃ present, reaction rates could be increased significantly, and the reaction medium could be readily recycled. TEM, CLSM, IR and leaching experiments were employed to quantify the advantages of the catalytic system. The procedure is environmentally friendly. It offers a reference for the catalytic hydrogenation of other hydrophobic natural products.

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1. Introduction

Metal nanoparticles have attracted a growing interest in the international scientific community in the last few decades. Such nanoparticles exhibit many novel chemical and physical properties that differ from their bulk counterparts.¹ These novel properties are attributed to the surface effect, quantum size effect and quantum tunneling effect. They are widely used in various kinds of catalytic reactions and fuel cells due to their large surface area, small size and high activity,² such as hydrogenation, hydrogenolysis, oxidation, biomass conversion and Fischer–Tropsch synthesis reaction.³ Therefore, the synthesis of metal nanoparticles has attracted a tremendous amount of attention from researchers over the past two decades. The micellar method is one of the most important and widely used ways to prepare nano-metal catalysts with accurately controlled particle sizes and shapes.⁴

Micelles are formed by the addition of amphiphiles to an aqueous reaction medium. The hydrophilic chains extend into the water and the lipophilic chains point to the interior. Then the surfactant is assembled into micelles with a lipophilic ‘core’.⁵ For the fabrication of metal nanoparticles, the use of micelles for nanoparticle synthesis has attracted much interest because of its potential advantages. The reaction is restricted to the lipophilic core, the growth of the metal nanoparticles obtained can be controlled by the size of the nonpolar core, and the metal nanoparticles can be stabilized. Metal nanoparticles protected by micelles can be thought of as micellar catalysts.⁶ Micellar catalysis in water can play an important contributory role in green chemistry. Early review papers on the subject of micellar catalysis date back to the late 1970s.⁷ Since that time, this area has witnessed rapid growth. Today, many valuable reactions take place within the lipophilic core of self-aggregated nanomicelles. This type of catalyst, which combines the advantages of both homogeneous and heterogeneous catalysts, can be regarded as a “semi-homogeneous catalyst”: the metal nanoparticles are heterogeneous in nature but, similar to homogeneous catalysts, their high degree of dispersion in the solvent facilitates the access of reactant molecules.⁸ Furthermore, a high local concentration of the reactants in the nanometer-sized micelles leads to accelerated transformations and increased selectivities.

Compared to traditional “soaps”, the vitamin E derived amphiphiles TPGS offer outstanding properties in terms of reactivity, selectivity, and catalyst recycling in many transition-metal-catalyzed reactions:⁹ such as asymmetric hydrosilylation reactions, cross-couplings, trifluoromethylation of heterocycles, and asymmetric intramolecular hydrocarboxylations. These could all be carried out in ≤5 wt% TPGS/water micellar catalyst at room temperature.¹⁰ However, the preparation of nanoparticles using TPGS micelles as a stabilizer has seldom been reported. In this paper, we have designed a catalyst of Ru(0) nanoparticles protected by TPGS-1000 micelles in aqueous solution. The catalytic system was used in the hydrogenation of α-pinene to produce cis-pinane, which is a highly desirable material for fine chemistry, because numerous derivatives, such as linalool, dihydromyrcenol and other terpene series spices have been prepared from such intermediates.¹¹

2. Results and discussion

2.1 Impact of surfactant concentration

The results from varying the amount of TPGS-1000 from 0 to 1.5 weight percent for the reaction are shown in Table 1. The conversion and selectivity were poor in the control experiment performed ‘on water’ (entry 1). This confirmed the importance of micellar catalysis in facilitating hydrogenation in aqueous media. The conversion of α-pinene increased with the weight percent of TPGS-1000 (entries 2 and 3), and decreased with further increase in TPGS-1000 weight percent (entries 4 and 5). This could be ascribed to a combination of the following factors: on the one hand, Ru(0) particle size decreased with an increase in the amount of TPGS-1000, and the catalyst activity increased. The Ru(0)/TPGS-1000 micelle size also decreased from 17.3 nm to 13.6 nm, and the zeta potential of the Ru(0)/TPGS-1000 micelles increased with an increase in the amount of TPGS-1000. These factors led to the Ru(0) nanoparticles becoming more stable.¹² Furthermore, the number of micelles increased when the amount of TPGS-1000 rose further, and the interfacial area between the two phases also increased.¹³ As a result, the reaction was accelerated. However, decreased activity was observed as the weight percent of TPGS-1000 exceeded a certain value. This was probably due to the micelle sizes decreasing, which left less room for the substrate to diffuse into the micelles. Furthermore, the viscosity of the mixture increased as the amount of TPGS-1000 increased (1.25 cP to 1.50 cP), which may have interfered with the impingement of the reactant molecules.¹⁴ Hence, we selected 0.5% TPGS-1000/H₂O for subsequent experiments.

Table 1 Screening of the weight percent of TPGS-1000 for the conversion of α-pinene^a

Entry	Weight percent (%)	Average diameter^b (nm)	Size^c (nm)		Viscosity^d (cP)	Zeta potential (mV)	Conv. (%)	Select. (%)
			TPGS	Ru(0)/TPGS
a Conditions: substrate (2 mmol), catalyst (0.01 mmol), H2 (0.5 MPa), temperature (50 °C), time (1.5 h).b Measured using TEM, the results represent an error margin of 95% confidential intervals. TEM images are shown in Fig. S3.c Measured using DLS.d Measured at 16 °C.
1	0	—	—	—	1.12	—	6.0	96.7
2	0.25	2.63 ± 0.15	12.3	17.3	1.28	12.2	52.5	98.5
3	0.50	2.47 ± 0.07	10.6	15.1	1.32	23.5	61.3	99.2
4	1.00	2.19 ± 0.09	10.5	14.2	1.43	29.3	59.0	99.0
5	1.50	2.63 ± 0.16	10.0	13.6	1.50	31.2	50.7	98.9

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2.2 Effects of metal salts on α-pinene hydrogenation

It is known that the size of TPGS-derived micelles can be greatly increased in the presence of a metal salt, leading to faster reactions in cross-coupling and ring-closing olefin metathesis.¹⁵ To establish whether this effect can also be utilized in a hydrogenation reaction catalyzed by a Ru(0) nanoparticle catalyst, dynamic light scattering (DLS) measurements were carried out on a Ru(0)/TPGS-1000 solution (Table 2). Interestingly, the addition of salt to the Ru(0)/TPGS-1000 solution made the micellar sizes larger. But the impact on the conversion was different. When a small amount of acidic salts or neutral salts was added, the conversion decreased (entries 1, 2, 3 compared with entry 3 in Table 1). However, the limited conversion could be increased dramatically by adding 2 mg of Na₂CO₃ (entry 5) or NaOH (entry 7). With a further increase in the amount of Na₂CO₃, the conversion and selectivity declined to some degree (entry 6). In an attempt to figure out the cause of the increased conversion, we designed a series of experiments to explore this phenomenon. The research started with an evaluation of pH effects on the hydrogenation of α-pinene over a Ru catalyst (as the Ru catalyst was resistant to corrosion and oxidation, it provided reliable results over the whole pH range).¹⁶ Experiments were carried out at a pH range from 0.51 to 12.9, adjusted with HCl to obtain an acidic and with NaOH for a basic reaction medium. The results are presented in Fig. 1. Without the addition of acid or base the reaction mixture has an initial pH of 2.1. Decreases in the pH to 0.95 and 0.51 led to a decrease in the conversion from 61.3% to 47.5% and 43.1%, respectively. By a sharp contrast, changing the pH to higher values of 10.2 and 11.2 induced a marked increase in the conversion of α-pinene. It was also observed that even higher pH values of 11.5 and 12.9 led to a gradual reduction in conversion. Nevertheless, the conversion of α-pinene was almost unchanged when the reaction system was free of TPGS-1000 micelles. These results indicated that high pH had no direct impact on Ru(0) nanoparticles and α-pinene, but was favorable to hydrogenation of α-pinene in the micellar system. The Ru(0) nanoparticles before the reaction (Fig. S3†) and after the reaction (Fig. 4B) were measured using TEM. The results showed that there was almost no change in the size of the Ru(0) nanoparticles after adding 2 mg Na₂CO₃ in the hydrogenation (pH = 11.2). Furthermore, infrared spectroscopy was also used to characterize the change in Ru(0) nanoparticles under higher pH (Fig. 2). The appearance of the shoulder peak around 1600 cm⁻¹ suggested that the interactions between Ru(0) and TPGS-1000 were enhanced, and that Ru(0) nanoparticles became more stable in the micelles.¹⁷ Hence, the acceleration by Na₂CO₃ could be ascribed to a combination of the following factors: on the one hand, TPGS-1000 micelles were enlarged, and the internal volume capacity of the lipophilic ‘core’ increased. This could accommodate more substrate in the reaction. On the other hand, Na₂CO₃ was added to the reaction system and provided an alkaline environment. An alkaline environment was favorable to the hydrogenation of α-pinene in the micellar system. Furthermore, alkaline conditions could improve the mobility of the lipophilic chain. The mass transfer rate between the aqueous phase and the lipophilic ‘core’ was accelerated, which affected both the reaction rate and the product separation procedure.¹⁸ In addition, after Na₂CO₃ was added to the hydrogenation reaction, Ru(0) nanoparticles were uniformly dispersed in the aqueous solution (Fig. S4†), providing an effective catalytic system. Moreover, α-pinene was also dispersed evenly in the semi-homogeneous catalyst during hydrogenation (Fig. S5†). The biphasic interface area was significantly increased, and the energy barrier to phase transfer was effectively minimized.¹⁹ The combination of the aforementioned factors created highly advantageous conditions for an acceleration of the reaction.

Table 2 Effects of metal salts on the conversion of α-pinene^a

Entry	Salt	Weight (mg)	Size^b (nm)	Conv. (%)	Select. (%)
a Conditions: substrate (2 mmol), catalyst (0.01 mmol), H2 (0.5 MPa), temperature (50 °C), time (1.5 h).b Measured using DLS.
1	H3O40PW12	2.5	15.9	50.3	99.0
2	CoCl2	2.0	16.3	51.5	98.8
3	NaCl	2.0	17.1	58.0	99.1
4	Na2CO3	2.0	19.7	99.7	98.8
5	Na2CO3	10.0	39.8	99.1	98.3
6	NaOH	2.0	18.7	95.6	97.5

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Fig. 1 Conversion of the hydrogenation of α-pinene over Ru(0) nanoparticles as a function of pH. Reaction condition: H₂ (0.5 MPa), temperature (50 °C), time (1.5 h), 0.5 mol% Ru.

Fig. 2 IR spectra of the Ru(0)/TPGS-1000.

2.3 Recyclability of the catalyst

Another characteristic of this process was the opportunity to recycle the contents of the entire reaction mixture. When the reaction was complete, the product was extracted with a single organic solvent (ethyl acetate) in a reactor (5 × 0.5 mL). The organic phase was removed with a syringe to ensure that the product and any starting materials were removed as completely as possible (Fig. S5†). Remaining in the water were the surfactant, Na₂CO₃, Ru(0) catalyst and a thimbleful of ethyl acetate. With the addition of fresh α-pinene (0.2730 g) and H₂ (0.5 MPa), a new reaction could be conducted as described for the initial run. As illustrated in Fig. 3, this process could be repeated more than ten times with a small decrease in conversion. However, when there was no Na₂CO₃ in the mixture, this process could be repeated for only one cycle. The reduced conversion may be related to the particle size of the Ru(0) nanoparticles. TEM was employed to measure particle size and shape. As shown in Fig. 4, after one catalytic cycle, the particle size of the Ru(0) nanoparticles increased, and aggregation occurred without Na₂CO₃ in the mixture. Some of the Ru(0) nanoparticles gathered into clumps (Fig. 4A), presumably leading to the decrease in catalytic activity during the recycling experiments. Ru(0) nanoparticles gathering into clumps also could be observed in the catalyst phase after reaction (Fig. S4†). Nevertheless, when Na₂CO₃ was added to the mixture and the reaction system was in an alkaline environment, Ru(0) nanoparticles displayed a homogeneous distribution after one catalytic cycle (Fig. 4B). Even after fifteen cycles, only a small part of the Ru(0) nanoparticle aggregation occurred (Fig. 4C). Furthermore, this gradual decrease in conversion was probably due to the diffusion of ethyl acetate into the micelles, which left less room for the substrate.²⁰ This problem can be solved either by removal of excess ethyl acetate under reduced pressure or by extending the time for phase separation. We also tested the catalyst for Ru(0) leaching, which is an important criterion in terms of recyclability. 1 g of extract was dissolved in aqua regia and tested by ICP-AES analysis. Over the first cycle, no Ru(0) leaching was observed. Only 0.2% of the Ru(0) catalyst was lost over the 17 cycles. This result demonstrated the high stability of the Ru(0) catalyst in hydrogenation of α-pinene under an alkaline environment.

Fig. 3 Recycling of the aqueous reaction mixture for the hydrogenation of α-pinene.

Fig. 4 TEM images of Ru(0) nanoparticles in various recycle stages. (A) After 1 cycle without Na₂CO₃. (B) After 1 cycle. (C) After 15 cycles.

2.4 Hydrogenation of α-pinene at room temperature

Moreover, unlike most hydrogenation of α-pinene that occurs best at high temperatures and high H₂ pressure,²¹ these procedures could also proceed smoothly at normal pressure and room temperature. The results of α-pinene hydrogenation at ambient temperature are summarized in Table 3. The conversion of α-pinene was poor at ambient temperature (entry 1), but increased significantly with addition of Na₂CO₃ (entries 2 and 3). Some examples previously studied by Nowicki and co-workers were directly compared (entries 4 and 5).²² The TOF were improved, while reaction times are comparable. We also compared a classical catalyst such as Pd/C (entry 6). Ru(0)/TPGS-1000 had a better result on selectivity for cis-pinane than Pd/C. This illustrated that Ru(0)/TPGS-1000 was an efficient catalyst for the hydrogenation of α-pinene under an alkaline environment.

Table 3 Direct comparisons with other catalysts at room temperature^a

Entry	Catalyst	Salt	Reaction medium	P (MPa)	T (°C)	Time (h)	Conv. (%)	Select.^b (%)	TOF^c (h^−1)
a Conditions: substrate (2 mmol), catalyst (0.01 mmol), Na2CO3 (2 mg).b Selectivity: the percentage of cis-pinane in the product.c Turnover frequency defined as number of mol of converted substrate per mol of ruthenium per hour.d The surfactant was dissolved in water.e The data were referenced from literature.^22f 2 mg of Pd/C substituted for Ru(0)/TPGS-1000 in the hydrogenation.
1	Ru(0)	—	TPGS-1000^d	0.1	20	12	22.0	98.6	3.7
2	Ru(0)	Na2CO3	TPGS-1000^d	0.1	20	5	64.3	98.5	25.7
3	Ru(0)	Na2CO3	TPGS-1000^d	0.1	20	9	99.5	98.7	22.1
4^e	Ru(0)	—	Me-γ-CD^d	0.1	20	9	>99.9	95	11.1
5^e	Ru(0)	—	HEA16Cl^d	0.1	20	30	>99.9	88	3.3
6^f	Pd/C	—	Water	0.1	20	9	57.2	80.8	—

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2.5 The mechanism of reaction

During the preparation of the Ru(0) nanoparticles, an amphiphilic surfactant TPGS-1000 was dissolved in aqueous medium. The hydrophilic heads extended into the water. The lipophilic ends pointed to the interior and were shielded from water. Then, the amphiphilic surfactant TPGS-1000 was assembled into spherical micelles with lipophilic cores (Scheme 1(a)). When the hydrophilic Ru(III) was reduced to lipophilic Ru(0), Ru(0) nanoparticles were embedded into the lipophilic cores and protected by micelles (Scheme 1(b)). In the procedure of hydrogenation of α-pinene, α-pinene and H₂ were easily dissolved in the lipophilic cores.⁵ In this micro-circumstance, micelles could be considered to be nanoreactors. Reactions were divided and limited to the nanoreactors (Scheme 1(c)).²³ These nanoreactors were beneficial for promoting the interaction of the substrate with the Ru(0) catalyst, and the reaction was accelerated.²⁴ Furthermore, α-pinene was dispersed evenly in the semi-homogeneous catalyst (Fig. S5†), and the biphasic interface area was significantly increased. Under such conditions, the energy barrier to phase transfer was effectively minimized, and the reaction rate was greatly improved.

Scheme 1 Mechanism of the hydrogenation of α-pinene.

In an effort to determine the source of the hydrogen atoms in the product pinanes, hydrogenation of α-pinene was carried out under our standard conditions, replacing water with D₂O (Scheme S1†). Hydrogenation in D₂O afforded nondeuterated cis-pinane and trans-pinane. This indicated that both hydrogen atoms arose from H₂, and the aqueous medium plays no role in providing H₂. In addition, this also suggested that hydrogenation of α-pinene took place in the lipophilic core between the metal and the hydrogen-containing micelles (Scheme 1(c)).⁵

2.6 The scope of the catalyst

Lastly, to further demonstrate the potential for this mild, green catalyst to be used in other hydrogenation reactions, an expanded study was undertaken examining several types of arene derivatives and terminal olefins. The results are summarized in Table 4. Remarkably, all the hydrogenation reactions were accelerated by Na₂CO₃. Cyclohexene (entries 13, 14) and octene (entries 1, 2) were completely transformed in a very short time. Some interesting results in terms of selectivity were obtained in the hydrogenation of styrene: the aromatic ring was hydrogenated after the addition of Na₂CO₃ (entry 6). This indicated that the regioselective hydrogenation of the exocyclic C–C double bond in the case of styrene was possible (entry 5 and entry 6). In the hydrogenation of nitrobenzene, Ru(0) (entries 7, 8) nanoparticles were inert to these hydrogenation conditions. Pd(0) (entries 9, 10) has much higher activity than Ru(0) nanoparticles.²⁵ Interestingly, the aqueous phase was clear and colorless after hydrogenation of phenol (entry 11). This was presumably due to the phenol destroying the structure of the micelles. Ru(0) nanoparticles were absolutely aggregated into clumps and deposited at the bottom of the reactor. However, this damage was greatly reduced after addition of Na₂CO₃ (entry 12). The Ru(0) nanoparticles were dispersed evenly in the aqueous phase after the reaction.

Table 4 Scope of the hydrogenation in TPGS-1000 micelles^a

Entry	Substrate	Salt	Conv.^b (%)	Product^b (select.%)		Time (h)	TOF (h^−1)
a Conditions: substrate (2 mmol), Ru(0) catalyst (0.01 mmol), H2 (0.5 MPa), temperature (50 °C), Na2CO3 (2 mg).b Determined by GC-MS.c Ru(0) was replaced with Pd(0), the detailed information was placed in S1.2.
1	Octene	—	84.1	Octane (>99)		1.0	168.2
2		Na2CO3	99.9	Octane (>99)		1.0	199.8
3	Toluene	—	81.2	Methylcyclohexane (>99)		1.5	108.3
4		Na2CO3	99.9	Methylcyclohexane (>99)		1.5	133.2
5	Styrene	—	95.6	Ethylbenzene (91.3)	Ethylcyclohexane (8.7)	1.5	127.5
6		Na2CO3	99.9	Ethylbenzene (6.4)	Ethylcyclohexane (93.6)	2.0	99.9
7	Nitrobenzene	—	5.2	Aniline (>99)		1.5	6.9
8		Na2CO3	10.6	Aniline (>99)		1.5	14.1
9^c	Nitrobenzene	—	85.8	Aniline (98.8)	Cyclohexylamine (1.2)	1.5	114.4
10^c		Na2CO3	98.9	Aniline (>99)		1.5	131.9
11	Phenol	—	89.5	Cyclohexanol (98.5)	Cyclohexanone (1.5)	1.5	119.3
12		Na2CO3	99.9	Cyclohexanol (>99)		1.5	133.2
13	Cyclohexene	—	77.3	Cyclohexane (>99)		1.0	154.6
14		Na2CO3	99.9	Cyclohexane (>99)		1.0	199.8

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3. Conclusions

In summary, the hydrogenation of α-pinene could be performed in water typically at room temperature by employing nanoreactors formed from the commercially available surfactant TPGS-1000. This procedure was environmentally friendly, limited amounts of water were used as the reaction medium, and there was an in-reactor extraction with only a minimal amount of a single, recoverable organic solvent. Additionally, with as little as 2 mg of Na₂CO₃ present, reaction rates could be increased significantly, and the reaction medium could be readily recycled. Lastly, these reactions took place at high yields and with stereoselectivity, thereby offering considerable potential for applications to other hydrophobic natural products in hydrogenation reactions.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 31270615) and Taishan Scholar Program of Shandong.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13099b

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