Ruthenium nanoparticle-intercalated montmorillonite clay for solvent-free alkene hydrogenation reaction

Abstract

Well-characterized, ruthenium nanoparticle-intercalated montmorillonite clay was used as a catalyst in solvent-free alkene hydrogenation reactions and the corresponding products were obtained in good yields. The catalytic activity of ruthenium nanoparticle-intercalated montmorillonite clay was successfully tested with 16 different functionalized and non-functionalized alkenes. Apart from alkene reduction, the ruthenium nanoparticle-intercalated montmorillonite clay was also tested in Wittig-type reactions for obtaining dehydrobrittonin A, an important intermediate for the synthesis of brittonin A. Ruthenium nanoparticle-intercalated montmorillonite clay was found to be active in the synthesis of dehydrobrittonin A and brittonin A. The ability to recycle the catalyst nine times, together with low catalyst loading, high catalytic activity and catalytic selectivity were noteworthy advantages of the proposed protocol.

---

1. Introduction

Ru nanoparticles are well-known catalysts for a wide range of organic reactions, such as hydrogenation, hydrogenolysis, nitrile hydration, epoxidation, Fischer–Tropsch synthesis, and so on.^1,2 Various organic–inorganic supports, such as activated carbon,³ SiO₂,^4,5 Al₂O₃,^6,7 zeolite,^8,9 ionic liquids^10,11 and polymers,¹² have been reported as supporting materials for Ru nanoparticles in their application as effective heterogeneous catalysts. Montmorillonite clay (MMT) is an important material for supporting catalysts, due to its unique phytochemical properties, such as high cation exchange capacity and good swelling properties.^13–15 Furthermore, reactions that are catalyzed by MMT are extremely easy to “work up”, since the clay does not dissolve in the reaction medium (solvent)—it can simply be filtered away when the reaction is complete. MMT has been the most frequently used support to intercalate diverse metal ions and has been further tested as a catalyst in various organic transformations such as oxidation, reduction, transesterification, coupling reactions, and so on.¹⁵ In particular, metal-intercalated MMTs are well-documented catalysts for the selective hydrogenation reactions of various organic compounds. The complete exchange of Rh³⁺, Ru³⁺ or Pd³⁺ ions for the interlayer cations of MMT clay is still difficult to achieve, however¹⁶; few reports are available describing the exchange of Ru³⁺ ions within the interlayer spacing of MMT clay. Recently, Liu et al.¹⁷ reported a synthetic protocol for Ru nanoparticle-immobilized MMT clay using a task-specific ionic liquid [IL].¹⁷ In this report, the ionic liquid was first exchanged with the exchangeable ions of the MMT and then Ru³⁺ ions were loaded onto the IL-exchanged MMT clay. This system was successfully tested as a catalyst for the benzene hydrogenation reaction.

Hydrogenation of alkenes is an important synthesis tool for achieving a diversity of biologically important molecules in good yields.^16–21 Several hydrogenating reagents, such as NiCl₂–NaBH₄, PdCl₂–NaBH₄, polyethylene glycol (PEG)–CH₂Cl₂, and CoCl₂–NaBH₄, have been tested as hydrogenation systems;²² however, in addition to toxicity, these systems were characterized by long reaction times, high catalyst loadings, costly starting materials and tedious reaction protocols.

In this paper, we describe the controlled synthesis of Ru metal nanoparticles (Ru MNPs) via the exchange of [Ru (NH₃) ₆]⁺³ ions for interlayer Na⁺, K⁺, or Ca²⁺ ions of MMT. The exchanged and highly dispersed Ru³⁺ ions were then successively reduced by sodium boron hydride to Ru MNPs, which were intercalated into MMT. The resulting ruthenium nanoparticle-intercalated montmorillonite clay (Ru MMT) was used to hydrogenate cyclic as well as acyclic olefins under different reaction conditions, such as temperature, pressure, solvent systems, etc., to obtain corresponding reaction products in good yields.

2. Experimental

Reagent Plus®-grade ruthenium(III) chloride hydrate was purchased from Aldrich. Reagent Plus® and extra pure-grade alkenes were purchased from Aldrich and Acros Chemicals. Lindlar catalysts, 5% Pt on carbon and Renney nickel were supplied by Vinneth Chemicals (India) as free samples. Nuclear magnetic resonance (NMR) spectra were recorded on a standard Bruker 300WB spectrometer with an Avance console at 400 and 100 MHz for ¹H and ¹³C NMR, respectively. Z/E ratio was determined from the crude product ¹H NMR spectroscopy. The commercial sodium montmorillonite clay (Cloisite® Na) used in the present study was supplied by Southern Clay Products (Gonzales, Texas, USA). The cation exchange capacity (CEC) of sodium montmorillonite (Na-MMT) is 92.6 meq per 100 g as reported by suppliers. All hydrogenation reactions were carried out in a 100 mL stainless steel autoclave (Amar Equipment, India). The Ru MMT catalyst material was characterized by TEM (Hitachi S-3700N) and energy-dispersive X-ray spectroscopy (EDX) (Perkin Elmer, PHI 1600 spectrometer). The specific surface area (BET) of the catalyst was determined on a Micrometrics Flowsorb III 2310 instrument. The catalyst was pre-treated at 120 °C under vacuum for over 2 h to desorb contaminating molecules (mainly water) from the catalyst surface for the determination of BET.

2.1 Catalyst preparation

The [Ru (NH₃)₆] Cl₃ solution was prepared by reacting a solution containing equimolar amounts of water and ammonia with RuCl₃·3H₂O.^23,24 The solvent was evaporated under mild reduced pressure on a steam bath until only a faint odor of ammonia gas was noticeable. The resulting pale yellow solution was filtered and allowed to cool in an ice bath (5–10 °C). The neat MMT clay (without any treatment or modification) was dispersed in 250 mL deionized water and stirred to facilitate complete dispersion for 2 h at room temperature. Then, 40 mL hexamine ruthenium(III) solution (0.92 meq g⁻¹ of MMT) were added under constant stirring at room temperature (303 K) with an addition rate of 0.5 mL min⁻¹ to obtain [Ru(NH₃)₆]-MMT, which was subsequently reduced by NaBH₄ at room temperature. A grayish black dispersion was formed. Ru MMT was filtered, washed, and dried at 313 K for the next 3 h in a vacuum oven and ground to a fine powder using a pestle and mortar.

2.2 Hydrogenation of alkenes and catalyst recycling

The autoclave (1 mL) was charged with alkenes (2 mL) and catalyst (0.1 g). Solvent, additive, and hydrogen source (except hydrogen gas) were added as required. After closing the autoclave, the air was replaced with hydrogen gas (10 bar), and then the reactants were allowed to react at 80 °C for 1 h. After that, the autoclave was cooled using cold water (2–5 °C). The reaction product was isolated with diethyl ether (5 × 2 mL). The isolated product was further purified by column chromatography using an ethyl acetate/hexane solvent system (15 : 85). The catalytic system was further dried under reduced pressure at 50 °C for 30 min and recycled for the next run.

2.3 One-pot Wittig olefination reaction

A 50 mL, round-bottom flask was charged with 1.6 M n-BuLi (6.25 μL, 1.0 mmol) and methoxylated benzyltriphenylphosphonium halide (1.5 mmol) in dry THF (2 mL), at 0 °C under argon atmosphere. After 20 min, Ru MMT (0.1 g) was added to the combined reaction mass comprising the (3,4,5-trimethoxyphenyl) methanol (1 mmol) and the initially prepared suspension. The reaction mixture was allowed to stir for the next 1 h at 80 °C. The resulting reaction product was isolated by diethyl ether washing (5 × 2 mL) and further purified with column chromatography using an ethyl acetate/hexane solvent system (10 : 90) to obtain the product dehydrobrittonin A.

3. Results and discussion

A small- to medium-angle X-ray scattering (SAXS) analysis was carried out to study the change between the basal spacing of Ru MMT with respect to neat MMT (Fig. 1). The basal spacing of neat MMT was d₀₀₁ = 12.95 Å, while after the exchange of Ru ions within the interlayer spacing of MMT, a significant increase was observed and the Ru MMT was enlarged to d₀₀₁ = 14.96 Å. Such an increase between the basal spacing of MMT represents the presence of Ru ions between the basal spacing of MMT. It is worth noting here that the small characteristic peak for Ru nanoparticles in the XRD data represents the existence of Ru MNPs (either very small or amorphous) (Fig. 2).

Fig. 1 Small- to medium-angle X-ray scattering (SAXS) data for MMT and Ru MMT.

Fig. 2 XRD patterns of MMT, Ru MMT (fresh), Ru MMT (used).

The average TEM particle size of Ru is 20.5 ± 2 nm in Ru MMT. The TEM image clearly reveals the agglomeration of Ru MNPs on MMT (Fig. 3). The presence of Ru species within the interlayer spacing of MMT clay was also confirmed by using high-resolution transmission electron microscopy (HRTEM) analysis (Fig. 4). We embedded Ru MMT in epoxy resin in order to get a uniform solid specimen and cut the specimen using an ultramicrotome to achieve an ultrathin section of Ru MMT. Ruthenium wt% in the catalyst was calculated using an inductively coupled plasma atomic emission spectrometer (ICP-AES; ARCOS MS, Spectro, Germany). 00.1 g of sample was digested in a minimum amount of concentrated HNO₃ with heating, and the volume was made up to 10 mL. A theoretical (cation exchange capacity) and an experimental (ICP-AES) method were used to calculate the amount of Ru species in MMT. Both theoretical and experimental values were found to be in good agreement, at 2.5 wt% Ru MMT.²² This protocol also minimized the loss of Ru nanoparticles during the synthesis of Ru MMT.

Fig. 3 TEM images of MMT (left) and RU MMT (right).

Fig. 4 HRTEM analysis.

The specific surface area of Ru MMT was measured by nitrogen adsorption using the BET method. This is an important characteristic of heterogeneous catalysts and liable to change during preparation, conditioning, and use, particularly if exposed to excessive temperature. The specific surface area of neat MMT (82 m² g⁻¹) was increased up to 85 m² g⁻¹ for Ru MMT clay.

A primary study to check the catalytic activity of Ru MMT was carried out using hydrogenation of allylbenzene as a model test reaction to optimize the reaction conditions and all corresponding results are summarized in Table 1, entries 1–19. The best test result was obtained while stirring the allylbenzene and Ru MMT without solvent at 80 °C for 1 h under hydrogen atmosphere (Table 1, entry 1).

Three different hydrogen sources were screened to replace the hydrogen gas, but as per the corresponding results, high yield was only obtained with hydrogen gas (Table 1, entries 1, 15 and 16). On the basis of the experimental results (Table 1, entries 1–19), the Ru MMT-catalyzed alkene hydrogenation reaction was totally dependent on temperature, reaction time, and pressure of the hydrogen gas because if any of these factors was altered, the lower catalytic response of Ru MMT was observed in the corresponding product yield (Table 1, entries 1–14). We also checked the catalytic behavior of Ru MMT in the presence of additives; results are documented in Table 1, entries 18 and 19. Raney nickel and other catalysts were also evaluated under optimized reaction conditions, but no noteworthy results (Table 1, entries 20–22) were obtained.

Table 1 Reaction optimization for alkene hydrogenation

Entry^a	Catalyst (0.100 g)	Solvent (1 mL)	Temperature (°C)	Time (h)	Additive	Hydrogen source	Yield^b (%)
a Allylbenzene (5mL) was allowed to stir with hydrogen source at 80 °C for 1 h under high pressure autoclave.b Isolated yield after column chromatography.c EDA = ethylene diamine.d Triphenyl phosphine.
1	Ru-MMT	—	80	1	—	H2 gas (10 bar)	87
2	Ru-MMT	—	100	1	—	H2 gas (10 bar)	85
3	Ru-MMT	—	40	1	—	H2 gas (10 bar)	30
4	Ru-MMT	—	80	2	—	H2 gas (10 bar)	86
5	Ru-MMT	—	80	0.3	—	H2 gas (10 bar)	38
6	Ru-MMT	—	80	1	—	H2 gas (20 bar)	87
7	Ru-MMT	—	80	1	—	H2 gas (5 bar)	50
8	Ru-MMT	Methanol	80	1	—	H2 gas (10 bar)	70
9	Ru-MMT	Ethanol	80	1	—	H2 gas (10 bar)	73
10	Ru-MMT	DMF	80	1	—	H2 gas (10 bar)	68
11	Ru-MMT	THF	80	1	—	H2 gas (10 bar)	71
12	RuCl3·3H2O	—	80	1	—	H2 gas (10 bar)	75
13	Ru-MMT (0.05 g)	—	80	1	—	H2 gas (10 bar)	42
14	Ru-MMT (0.200 g)	—	80	1	—	H2 gas (10 bar)	85
15	Ru-MMT	—	80	1	—	NaBH4	64
16	Ru-MMT	—	80	1	—	LiAlH4	61
17	Ru-MMT	—	80	1	—	HCOOH/Et3N (50%, 1 mL)	58
18	Ru-MMT	—	80	1	EDA^c	H2 gas (10 bar)	55
19	Ru-MMT	—	80	1	PPh3^d	H2 gas (10 bar)	58
20	Lindlar catalyst (5% w/w Pd)		80	1	—	H2 gas (10 bar)	59
21	5% Pt on carbon		80	1	—	H2 gas (10 bar)	43
22	Raney Ni	—	80	1	—	H2 gas (10 bar)	67

---

Ru MMT was further evaluated for the hydrogenation of 13 different functionalized and non-functionalized alkenes (Table 2, entries 1–13). Terminal alkenes were easily reduced to the corresponding alkanes in quantitative yields (Table 2, entry 1). The same behavior was observed with trans-stilbene, as well as with α-methylstyrene, and we easily isolated their corresponding products in good yield (Table 2, entries 2 and 3). Furthermore, we studied the hydrogenation of functionalized olefins (entries 4–13). Ru MMT was found to be catalytically effective for both terminally as well as internally unsaturated esters, and yielded the corresponding products in high yields. Interestingly, no transesterification was detected while performing the hydrogenation reaction of ethyl 6-heptenoate (Table 2, entry 4). In the presence of Ru MMT, not only electron-rich aromatic 3,4-dimethoxystyrene and isosafrole were nicely reduced in high isolated yields (entries 7 and 8), but also allylic alcohols, with either a monosubstituted or geminal C=C bond, were properly reduced without any isomerization of corresponding reaction products (Table 2, entries 9 and 10). It is noteworthy that transition-metal catalysts, during catalytic hydrogenation, encourage such types of isomerization. Monoterpene (±)-linalool,²⁵ which contains both a mono- as well as tri-substituted C=C bond, was easily altered into the saturated tertiary alcohol 3,7-dimethyloctan-3-ol-(tetrahydrolinalool) in moderate yield (Table 2, entry 11). The reduction of allyl benzyl ether and N-allylcyclohexylamine was easily accomplished by Ru-MMT. For both substrates, the desired products were obtained in quantitative yields (Table 2, entries 12 and 13).

Table 2 Hydrogenation of functionalized and non-functionalized alkenes

Entry	Alkene	Product	Yield^a (%)
a Isolated product after column chromatography and confirmed ^1H NMR and ^13C NMR analyses.^26–28
1			95
2			93
3			94
4			93
5			92
6			95
7			94
8			93
9			90
10			89
11			68
12			92
13			93

---

The flexibility of Ru MMT was also checked for the synthesis of two unique biologically important products. First, we performed the synthesis of brittonin A, which was isolated from Frullania brittoniae subsp. truncatifolia (F. muscicola)²⁶. The brittonin A was obtained as a hydrogenated product of dehydrobrittonin A in high yield.²⁷ To the best of our knowledge, this is the first solvent-free, Ru MMT catalyzed, Wittig-type reaction between substituted benzyl alcohols and phosphorus ylides, in which no standard redox step was used. We synthesized dehydrobrittonin A in moderate yield and the process also displayed low diastereoselectivity, mainly in favor of the E diastereoisomer.²⁸ In our study, a maximum ∼1 : 4 Z/E ratio of diastereomeric dehydrobrittonin A was obtained, while nickel nanoparticles offered the same reaction product with comparatively low diastereomeric Z/E ratio (46 : 54) (Scheme 1).

Scheme 1 Synthesis of brittonin A.

After obtaining excellent results from Ru MMT-catalyzed hydrogenation of different alkenes, we also tested the stability of Ru MMT in a catalyst recycling experiment with our model test reaction (hydrogenation of allylbenzene), and recycled Ru MMT for up to nine runs without any significant loss of yield (Fig. 5). Potential Ru leaching into the reaction mixture was also analyzed, using ICP/OES analysis. For this purpose, samples were taken through a syringe filter (Whatman Puradisc 4, 4 mm diameter, 0.45 μm PTFE) during our model test reaction (hydrogenation of allylbenzene). Volatile impurities were evaporated, and the residue was dissolved in HNO₃. Analysis of these samples with ICP-AES showed that the Ru concentration in the reaction solution was less than the detection limit (i.e., 50 ppb). The same result was obtained when the complete reaction mixture of the model test reaction was filtered, the solvent evaporated, and residue dissolved in HNO₃. Both findings indicated that virtually no Ru leached from the Ru MMT into the solution.

Fig. 5 Recycling test of Ru MMT for allylbenzene hydrogenation reaction.

4. Conclusion

In this manuscript, a synthetic protocol for Ru nanoparticles intercalated into MMT is presented. The physiochemical parameters of Ru MMT were investigated using a range of sophisticated analytical techniques. TEM images showed agglomeration of Ru nanoparticles in MMT, and the average particle size of Ru nanoparticles was 20.5 ± 2 nm. The catalytic activity of well-characterized Ru MMT was tested for 13 types of functionalized and non-functionalized alkenes. All the alkenes were nicely hydrogenated under solvent-free conditions, with good yields of the corresponding reaction products. The catalytic activity of Ru MMT was also explored for the synthesis of the natural product brittonin A, as well as for Wittig-type reactions to obtain dehydrobrittonin A. Both products were obtained in quantitative yields. Apart from the above findings, low catalyst loading, lack of use of additives, a wide substrate scope, a solvent-free approach, easy product isolation and catalyst recycling for up to nine times were the major advantages of the proposed protocol.

References

1. P. Lara, K. Philippot and B. Chaudret, ChemCatChem, 2013, 5, 28–45.
2. J. Gavnholt and J. Schiøtz, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 1–10.
3. E. J. García-Suárez, P. Lara, A. B. García and K. Philippot, Recent Pat. Nanotechnol., 2013, 2, 247–264.
4. A. I. Carrillo, L. C. Schmidt, M. L. Marínab and J. C. Scaiano, Catal. Sci. Technol., 2014, 4, 435–440.
5. R. B. N. Baig and R. S. Varma, ACS Sustainable Chem. Eng., 2013, 1, 805–809.
6. I. Balinta, A. Miyazakib and K.-I. Aikab, J. Catal., 2002, 207, 66–75.
7. I. Balint, A. Miyazakib and K.-I. Aikab, Chem. Commun., 2002, 630–631.
8. J. Sun, X. Li, A. Taguchi, T. Abe, W. Niu, P. Lu, Y. Yoneyama and N. Tsubaki, ACS Catal., 2014, 4, 1–8.
9. D. K. Mishra, A. A. Dabbawala and J.-S. Hwang, J. Mol. Catal. A: Chem., 2013, 376, 63–70.
10. L. M. Rossi, J. Dupont, I. G. MachadoI, P. F. P. Fichtner, C. Radtke, I. J. R. Baumvol and S. R. Teixeira, J. Braz. Chem. Soc., 2004, 15, 904–910.
11. G. Salas, P. S. Campbell, C. C. Santini, K. Philippot, M. F. C. Gomesd and A. A. H. Pádua, Dalton Trans., 2012, 41, 13919–13926.
12. D. K. Mishra, A. A. Dabbawala and J.-S. Hwang, Catal. Commun., 2013, 41, 52–55.
13. A. Jhaa, A. C. Garadea, M. Shiraib and C. V. Rode, Appl. Clay Sci., 2013, 74, 141–146.
14. J. M. Adams, T. V. Clapp and D. E. Clement, Clay, 1983, 18, 411–421.
15. V. Srivastava, Bulletin of the Catalysis Society of India, 2012, 11, 56–77.
16. A. B. Boricha, H. M. Mody, H. C. Bajaj and R. V. Jasra, Appl. Clay Sci., 2006, 31, 120–125.
17. H. Xu, K. Wang, H. Zhang, L. Hao, J. Xua and Z. Liu, Catal. Sci. Technol., 2014, 4, 2658–2663.
18. C. Morina, D. Simonb and P. Sautet, Surf. Sci., 2006, 600, 1339–1350.
19. M. Guerrero, N. T. T. Chau, S. Noël, A. Denicourt-Nowicki, F. Hapiot, A. Roucoux, E. Monflier and K. Philippot, Curr. Org. Chem., 2013, 17, 364–399.
20. M. Cano, A. M. Benito, W. K. Masera and E. P. Urriolabeitia, New J. Chem., 2013, 37, 1968–1972.
21. C. P. Ruas, D. K. Fischer and M. A. Gelesky, J. Nanotechnol., 2013, 2013, 1–6.
22. P. K. Sharma, S. Kumar, P. Kumar and P. Nielsen, Tetrahedron Lett., 2007, 48, 8204–8708.
23. K. B. Sidhpuria, H. A. Patel, P. A. Parikh, P. Bahadur, H. C Bajaj and R. V. Jasra, Appl. Clay Sci., 2009, 42, 386–390.
24. L. Jiao and J. R. Regalbuto, J. Catal., 2008, 260, 329–341.
25. V. R. Coelhoa, J. Gianesinia, R. Von Borowskia, L. Mazzardo-Martinsb, D. F. Martinsb, J. N. Picadaa, A. R. S. Santosb, L. F. S. Bruma and P. Pereira, Phytomedicine, 2011, 896–901.
26. Y. Asakawaa, K. Tanikawaa and T. Aratanib, Phytochemistry, 1976, 15, 1057–1059.
27. F. Alonso, P. Riente and M. Yus, Tetrahedron, 2009, 65, 10637–10643.
28. F. Alonso, P. Riente and M. Yus, European Journal of Organic Chemistry, 2009, 2009, 6034–6042.

---