Selective Conversion of Levulinic and Formic Acids to γ-Valerolactone

Article pubs.acs.org/Organometallics

Selective Conversion of Levulinic and Formic Acids to γ‑Valerolactone with the Shvo Catalyst Viktória Fábos,† László T. Mika,†,‡ and István T. Horváth*,†,§ †

Institute of Chemistry, Eötvös University, Pázmány Péter 1/A, H-1117 Budapest, Hungary Department of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Budafoki street 8, H-1111 Budapest, Hungary § Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong ‡

ABSTRACT: The selective transfer hydrogenation of levulinic acid (LA) with formic acid (FA) to 4-hydroxyvaleric acid (4HVA) and carbon dioxide followed by the intramolecular dehydration of 4-HVA to γ-valerolactone (GVL) are key steps of the conversion of carbohydrate-based biomass to GVL, which can be used for the production of both energy and carbon-based products. LA was converted to GVL in the presence of a small excess of FA and the Shvo catalysts {[2,5-Ph2-3,4-(Ar)2(η5C4CO)]2H}Ru2(CO)4(μ-H)]} (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). The reactions were performed at 100 °C with yields higher than 99% after a few hours. The formation of 1,4-pentanediol and 2-methyltetrahydrofuran remained below detection limits. The only side products were water and carbon dioxide, as expected, which were easily removed and separated from the product GVL under reduced pressure. The Shvo catalyst 1c was recycled four times without losing catalytic activity, and the product GVL was isolated each time as a colorless liquid of 99.9% purity with only trace amounts of water present.

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GVL occurs in fruits, is nontoxic, has been used by the food industry, and has very attractive physical and chemical properties.6 Its high solubility in water facilitates biodegradation and easy bioremediation. GVL’s low melting (−30 °C) and high boiling (204 °C) points make it one of the best candidates for a moveable liquid in pipes, trucks, and tankers. In addition, it is a safe material for large-scale use due to its very low vapor pressure, high flash point (96 °C), and high stability in water at neutral pH and in air. Also, GVL can be used as a platform chemical for the synthesis of 1,4-pentanediols, 6,8a,9 2MeTHF,8a,9,10 alkyl-4-alkoxyvalerates and tetraalkylammonium 4-hydroxyvalerates,11 butenes,12 mixtures of alkanes,8a alkylvalerates,13 4-hydroxypentane amides,14 and adipic acid via pentenoic acids15 (Scheme 2). Recently, numerous papers have been published on the catalytic conversion of LA to GVL. While heterogeneous supported Ru, Pd, Pt, Ir, and Re catalysts were used in the presence of different additives in the temperature range 25− 265 °C,16 homogeneous systems have been almost exclusively based on ruthenium-containing catalysts. The combination of a triphosphine-modified ruthenium catalyst with NH4PF6 was

he utilization of non-edible biomass as feedstock for the production of platform chemicals is becoming one of the enabling resources of sustainable development.1 Since carbohydrates are the most abundant components of biomass,2 the acid-catalyzed conversion of fructose, glucose, and sucrose as well as the oligomers and polymers of C5- and C 6carbohydrates to 5-(hydroxymethyl)-2-furaldehyde (HMF)3 and/or levulinic acid (LA) and formic acid (FA)4 has been the focus of intense research (Scheme 1). The hydrogenation of LA to 4-hydroxyvaleric acid (4-HVA) opens the way to the facile formation of γ-valerolactone (GVL),5 which has been proposed as a sustainable liquid6 and the key molecule of a GVL economy.7 Scheme 1. Conversion of Carbohydrates to GVL

Received: September 19, 2013 Published: January 2, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/om400938h | Organometallics 2014, 33, 181−187

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can be used for the transfer hydrogenation of LA to 4-HVA.8b The latter approach could be an inherently safer strategy, as the process is independent of hydrogen gas and readily usable even in remote parts of the world. Formic acid has been used for the transfer hydrogenation of a broad range of aldehydes and ketones.21 We have reported the first example for the transfer hydrogenation of LA with FA using [(η6-C6Me6)Ru(bpy)(H2O)][SO4],22 resulting in 25% GVL, 25% 1,4-pentanediol, and 1% 2-MeTHF at 70 °C after 18 h.8a Since the dehydration of 1,4-pentanediol leads to the formation of 2-MeTHF, which can readily form peroxides under air,23 we have continued our search for a much more selective catalyst. The Shvo catalyst precursors {[2,3,4,5-Ar4-(η5-C5O)]2H}Ru2(CO)4(μ-H) have been used for the hydrogenation of various aldehydes and ketones and transfer hydrogenation of ketones using alcohols, sodium formate, and formic acid as the reducing agents.24−26 The selective conversion of LA to GVL was achieved by using formic acid in the presence of {[2,5-Ph23,4-(p-MeOPh)2(η5-C5O)]2H}Ru2(CO)4(μ-H)] (1a).8b Concerning the currently accepted mechanism, the reversible dissociation of {[2,5-Ph2-3,4-Ar2(η5-C5O)]2H}Ru2(CO)4(μH)] (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)) leads to the formation of two mononuclear forms of the active species in solution: the hydroxycyclopentadienyl ruthenium hydride complex {[2,5-Ph2-3,4-Ar2(η5-C5OH)]Ru(CO)2H} (2a−c) and the coordinatively unsaturated {[2,5-Ph2-3,4-Ar2(η4C4CO)]Ru(CO)2} (3a−c) (Scheme 3).24,25 Intermediate 2 has both acidic and hydridic functions, which could readily transfer a proton from the hydroxide group and a hydride from the ruthenium atom to the CO double bond of LA, resulting in the formation of 4-HVA and 3. While 4-HVA could readily undergo dehydration to give GVL, the coordinatively unsaturated 3 could activate FA27 to yield 2 and CO2 or react with hydrogen (if available) to form 2. In addition, the reversible dimerization of intermediate 3 could result in {[2,5-Ph2-3,4-Ar2(η4-C4CO)]Ru(CO)2}2, another dimeric resting form of the Shvo catalyst.25a

Scheme 2. Conversion of GVL to Chemicals

reported by Leitner for the selective conversion of LA to GVL, 1,4-pentanol, or 2-MeTHF.9 Although the water-soluble tris-msulfonated triphenylphosphine (TPPTS) modified Ru catalyst was active for the conversion of LA to GVL under biphasic conditions, the catalyst activity decreased during recycling.17 The more basic water-soluble butylbis(m-sulfonatophenyl)phosphine-modified Ru catalyst was very efficient for the conversion of neat LA to GVL.18 It should be noted that the Ru-catalyzed continuous hydrogenation of LA was also tested using molecular hydrogen or formic acid as a hydrogen source.18b,c A “hydrogen free” domino process was recently reported starting from furfural and involving two transfer hydrogenation steps using 2-butanol as the hydrogen source.19 While this is an interesting approach, the origin of 2-butanol and/or the fate and reuse of 2-butanone was not addressed. If the 2-butanone has to be reduced back to 2-butanol, this approach also needs molecular hydrogen. Since the products of the hydration of HMF are an equimolar mixture of LA and FA,20 there are two possible strategies for their conversion to GVL. The catalytic decomposition of FA to CO2 and H2 opens the way to catalytic hydrogenation of LA to 4-HVA, which can undergo ring closure via dehydration to form GVL.8a Alternatively, FA

Scheme 3. Shvo Catalyst Precursors (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)) and Proposed Mechanism for Transfer Hydrogenation of Levulinic Acid with Formic Acid

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the intramolecular dehydration of the initial product 4-HVA to GVL is a facile process, much faster than formate formation. The formation of 2a could be also observed in the reaction of 1a with FA at 80 °C (Figure 2), as a very slow or no reaction

We report here the use of the Shvo catalyst for the selective conversion of LA to GVL using FA as the hydrogen source. The formation of 1,4-pentanediol and 2-methyltetrahydrofuran, the typical side products of the reduction of GVL with molecular hydrogen, was not observed. The reactions were performed with yields up to >99%, and the only byproducts were water and carbon dioxide, which were easily eliminated. The Shvo catalyst was recycled several times without losing catalytic activity.

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RESULTS AND DISCUSSION The orange powders of the dinuclear ruthenium Shvo catalyst precursors (1a−c) were stable in air for years. Their light yellow solutions in d8-toluene were also stable for several days at room temperature in air. When the temperature was raised to 80 °C in air, the solutions turned dark brown within minutes, indicating rapid decomposition. The most likely mechanism of the decomposition involves the dissociation of 1 to 2 and 3 followed by their reaction with oxygen. It should be noted that the elimination of hydrogen from 2b could also lead to the formation of 3b (Scheme 3), which opens the way to (1) reaction with 2b to re-form 1b, (2) dimerization to {[2,5-Ph23,4-Ar2(η4-C4CO)]Ru(CO)2}2, or (3) completion of the decomposition by reaction with oxygen. In contrast, in the presence of hydrogen or formic acid, the coordinatively unsaturated 3b could react either with H2 to form 2b or with HCOOH to yield 2b and CO2, respectively. Accordingly, when the dimer 1b was placed under 13 bar of hydrogen at 21 °C for 5 h, only trace amounts of 2b were observable by NMR (Figure 1). When the reaction mixture was kept under 13 bar of

Figure 2. 1H and 13C{1H} NMR spectra of 1a treated with FA in THF-d8 at 100 °C after 30 min.

was observed at lower temperatures. In addition, the formation of small amounts of H2 and CO2 was also detectable by NMR, confirming that the Shvo catalyst can catalyze the conversion of FA to H2 and CO2. While this reaction was at least 10 times slower than the transfer hydrogenation of LA with FA to 4HVA, it has an unexpected impact: a slight excess of FA must be used for full conversion of LA. Also, an in situ IR study has shown the rapid disappearance of the bands of 1a at 2035, 2002, and 1973 cm−1 with a shoulder at 1962 cm−1 and the simultaneous appearance of bands at 2012 and 1953 cm−1 for 2a (Figure 3). Since only 2a

Figure 1. Treatment of {[2,5-Ph2-3,4-(p-MePh)2(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1b) in d8-toluene with 13 bar of H2 at different temperatures.

Figure 3. In situ IR spectra of the reaction of 1a with HCOOH in THF at 80 °C.

hydrogen at 80 °C for 24 and 36 h, the ratio between 1b and 2b was practically the same, clearly demonstrating that 1b and H2 are in equilibrium with 2b. After the tube was depressurized and the H2 was replaced with N2, 2b was converted slowly to 1b at 21 °C and much more quickly at higher temperatures. It is important to note that the original procedure of the transfer hydrogenation of ketones by Shvo used HCOONa and H2O to prevent the formation of formates.27 In the case of levulinic acid, neat formic acid could be used efficiently because

was formed, we propose that in the presence of FA the dissociation of 1a to 2a and 3a was followed by the activation of FA by 3a, resulting in 2a via 4a (Scheme 3). Since {[2,5-Ph 2 -3,4-(p-MeOPh) 2 (η 5 -C 4 CO)] 2 H}Ru2(CO)4(μ-H) (1a) was the most effective Shvo catalyst for the reduction of ketones, 1a was used in the catalytic experiments. The transfer hydrogenation of neat LA with FA in the presence of the Shvo catalyst was performed by heating 12 mmol of LA and 20.4 mmol of FA in the presence of 0.01 mmol of 1a at 100 °C for 5 h. The only observable product was GVL in up to 99.9% purity by NMR (Figure 4) and GC. 183


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from FA. The retarding effect of CO2 has been shown by performing the reaction in a high-pressure sealed tube. While full conversion was achieved in an open vessel at an FA/LA ratio of 2 and an LA/1a ratio of 2400 at 100 °C after 12 h, the sealed tube gave only 52% conversion of LA under the same conditions. In order to establish the influence of the substituents of the phenyl ring of the cyclopentadienyl ring on the catalyst performance, two other Shvo catalyst precursors, 1b,c (Figure 5), were used in the reduction of LA. Table 2 clearly shows that

Figure 4. Transfer hydrogenation of LA with FA (neat) in the presence of 1a at 100 °C.

Although the dehydration of 4-HVA produces water, the latter can easily be eliminated by distillation, as GVL does not form an azeotropic mixture with water.6 Also, the other byproduct formed from FA, carbon dioxide, does not cause any difficulties, as it bubbles out of the solution. The catalytic performance of the Shvo catalyst 1a was evaluated using LA/FA as the solvent under various conditions. The reaction does not take place in the absence of formic acid or the catalyst, as expected (Table 1). Table 1. Transfer Hydrogenation of 8.6 mmol of LA with FA in the Presence of 1a at 100 °C for 8 h

a

T (°C)

n(FA)/n(LA)

n(LA)/n(Shvo catalyst)

GVL (%)

100 100 60 80 100 100 100a 100 100 100 100b 100 100

0 1.5 1.0 1.0 1.0 1.0 1.5 1.5 1.5 2.0 2.0 2.5 3.5

400 0 400 400 1200 2400 400 400 2400 2400 2400 2400 2400

0 0 0 20.0 94.3 70.1 40.3 99.9 99.9 99.9 52.0 97.2 96.3

Figure 5. Transfer hydrogenation of 8.6 mmol of LA with FA in the presence of different Shvo catalysts at 100 °C, n(LA)/n(FA) = 2, and n(LA)/n(Shvo catalyst) = 1200.

Table 2. Transfer Hydrogenation of LA with FA in the Presence of 1a−c at 100 °C for 1 h catalyst precursor

LA (mmol)

FA (mmol)

[LA]/[1a]

GVL (%)

1a 1b 1c 1a 1b 1c

8.6 8.6 8.6 8.6 8.6 8.6

12.9 12.9 12.9 17.2 17.2 17.2

1200 1200 1200 1200 1200 1200

59.2 56.6 44.8 61.5 59.2 48.1

Na formate was use instead of FA. bIn a sealed vessel after 12 h.

the highest conversion level was achieved when p-MeOPh was used as the ligand on the cyclopentadienyl ring, after 1 h. This observation is consistent with the trends observed previously by Shvo23 and Williams.28 Although the reaction is slower using the precursors 1b,c, the full conversion of LA was observed after 4−5 h with all three catalyst precursors (Figure 5). The recyclability of the Shvo catalyst 1c, which has been the cheapest and easiest to prepare, was investigated in a batch reactor. An 85.7 mmol portion of LA was treated with 172.7 mmol of FA in the presence of 87.4 mg (0.0805 mmol) of {[2,3,4,5-Ph4-(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c) at 95 °C for 6 h. The rapid evolution of CO2 from the orange solution was observed during the reduction (Figure 6a). The volatile compounds were removed by vacuum transfer at 85 °C/1 mmHg, resulting in a clear and colorless distillate containing 99.9% of the product GVL, the side product water, and the unreacted FA. The orange viscous residue (Figure 6b) was mixed with 85.6 mmol of LA and 174.7 mmol of FA under nitrogen, and the mixture was heated to 95 °C for 6 h.

No reaction was observed at 60 °C, as the conversion of 1a to the catalytically active species requires at least 80 °C. While 1 equiv of FA with respect to LA was not enough for the full conversion of LA, the variation of the FA concentration did not have a significant influence between 1.5 and 2.5 equiv of FA to LA but resulted in lower conversion in the case of a higher excess of FA. The highest turnover number of 3400 was achieved at an FA/LA ratio of 2 and an LA/1a ratio of 3600 at 100 °C. The use of aqueous sodium formate as the H donor instead of formic acid resulted in much lower conversion. It has to be emphasized that GVL does not react further with FA or the catalyst to produce unwanted byproducts such as 1,4pentanediol and 2-methyltetrahydrofuran. This was confirmed in a separate experiment by refluxing GVL with FA in the presence of the precursor 1a for 20 h, wherein no hydrogenation of GVL was observed. It is also important to note that the reaction vessel was opened to the air via a capillary tube to release the CO2 formed 184


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with a recycle delay of 80 s. Quantitative 13C NMR was carried out using inverse-gated decoupling with a recycle delay of 50 s. High pressure NMR experiments were performed using a single-crystal sapphire NMR tube equipped with a titanium head.31 Synthesis of {[2,5-Ph 2 -3,4-(p-MeOPh) 2 (η 5 -C 4 CO)] 2 H}Ru2(CO)4(μ-H) (1a). A solution of 0.4 g (0.63 mmol) of Ru3(CO)12 and 0.83 g (1.88 mmol) of 3,4-bis(4-methoxyphenyl)-2,5-diphenylcyclopentadienone in 80 mL of methanol was refluxed for 40 h. After the reaction mixture was cooled to room temperature, 0.96 g (85%) of an orange solid was filtered, washed with water and methanol, and dried. Synthesis of {[2,5-Ph2-3,4-(p-MePh)2(η5-C4CO)]2H}Ru2(CO)4(μH) (1b). A solution of 0.4 g (0.63 mmol) of Ru3(CO)12 and 0.77 g (1.88 mmol) of 3,4-bis(4-tolyl)-2,5-diphenylcyclopentadienone in 80 mL of methanol was refluxed for 40 h. After the reaction mixture was cooled to room temperature, 0.96 g (85%) of an orange solid was filtered, washed with water and methanol, and dried. Synthesis of {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c). A solution of 0.4 g (0.63 mmol) of Ru3(CO)12 and 0.72 g (1.88 mmol) of 2,3,4,5-tetraphenylcyclopentadienone in 80 mL of methanol was refluxed for 40 h. After the reaction mixture was cooled to room temperature, 0.96 g (85%) of an orange solid was filtered, washed with water and methanol, and dried. Treatment of {[2,5-Ph 2 -3,4-(p-MePh) 2 (η 5 -C 4 CO)] 2 H}Ru2(CO)4(μ-H) (1b) with H2. A solution of 67.5 mg (0.06 mmol) of 1b in 1.5 mL of d8-toluene was placed in a high-pressure singlecrystal sapphire NMR tube at room temperature, and NMR spectra were recorded. The tube was charged with 13 bar of H2 at room temperature, and NMR spectra were recorded at room temperature again. Treatment of {[2,5-Ph 2 -3,4-(p-MeOPh) 2 (η 5 -C 4 CO)] 2 H}Ru2(CO)4(μ-H) (1a) with Formic Acid. (a) For the NMR experiment a solution of 50 mg (0,042 mmol) of 1a in 1.7 mL of THF-d8 was placed into a high-pressure single-crystal sapphire NMR tube at room temperature and 15 mg (0.31 mmol) of formic acid was added. After the tube was sealed, it was heated at 100 °C for 30 min. The tube was cooled to room temperature, and NMR spectra were recorded. (b) For the in situ IR experiment a solution of 25 mg (0.021 mmol) of 1a in 0.9 mL of THF was placed in a 50 mL three-necked flask equipped with a reflux condenser and a SiComp probe of a ReactIR 1000 instrument. After the solution was heated to 80 °C for 20 min, 0.025 mL (0.55 mmol) of formic acid was added and IR spectra were recorded. Transfer Hydrogenation of Levulinic Acid with Formic Acid in the Presence of {[2,5-Ph2-3,4-Ar2(η5-C5O)]2H}Ru2(CO)4(μ-H)] (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). (a) A typical smallscale experiment was performed by heating a mixture of 3 g (25.84 mmol) of levulinic acid, 1.78 g (38.8 mmol) of formic acid, and 0.022 mmol of 1a−c in a 10 mL glass tube in an oil bath at 100 °C. Samples were taken at different times and analyzed by GC-MS or NMR. (b) A typical larger scale experiment was performed by heating a mixture of 15 g (129 mmol) of levulinic acid, 11.9 g (258 mmol) of formic acid, and 0.36 mmol of 1a−c in a 150 mL Hasteloy-C reactor at 100 °C. Samples were taken at different times and analyzed by GC-MS or NMR. Transfer Hydrogenation of Levulinic Acid with Formic Acid in the Presence of {[2,3,4,5-Ph4(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1c) with Catalyst Recycling. An 87.4 mg portion of 1c was placed in a 250 mL Schlenk flask followed by the addition of 9.96 g (85.7 mmol) of levulinic acid and 7.95 g (172.7 mmol) of formic acid. The Schlenk tube was connected to a bubbler containing oil, and the mixture was heated to 95 °C. After 1 h a clear homogeneous solution was observed. After 6 h, the reaction mixture was cooled and all volatile compounds were removed under reduced pressure (1 mmHg) at 85 °C, resulting in 13.96 g of a clear solution as distillate and 0.431 g of an orange viscous residue. Due to the carbon dioxide evolution, 3.55 g weight loss was measured. The calculated mass balance was 99.8%. The orange residue was mixed with the next dose of LA and FA (Table 3), and the protocol was repeated.

Figure 6. (a) Reaction mixture of LA, FA, and {[2,3,4,5-Ph4(η5C4CO)]2H}Ru2(CO)4(μ-H) (1c) during reaction and (b) orange viscous residue of the catalyst before recycling.

After a similar workup of the initial run the yield was 99.7% GVL. The yields of the next two recycling experiments were 99.6% and 97.8%, respectively (Table 3). Table 3. Transfer Hydrogenation of LA with FA in the Presence of 1c at 95 °C after 6 h entry

LA (mmol)

FA (mmol)

conversn (%)

TON

TOF (h−1)

∑TON

1 2 3 4

85.7 85.6 87.4 86.4

172.7 174.7 174.4 174.6

99.9 99.7 99.6 97.8

1065 1063 1085 1046

177 177 181 174

1065 2128 3213 4259

The combined distillates (55.1 g) were fractionated by atmospheric distillation using 10 cm Sulzer EX-20 HC packing to yield 35.5 g of GVL and 19.5 g of aqueous FA, as expected.

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CONCLUSIONS We have shown that levulinic acid (LA) and a small excess of formic acid (FA) can be converted to γ-valerolactone (GVL) in the presence of the Shvo catalysts {[2,5-Ph2-3,4-Ar2(η5C4CO)]2H}Ru2(CO)4(μ-H)]} (Ar = p-MeOPh (1a), p-MePh (1b), Ph (1c)). The reactions were performed at 100 °C in an open vessel with yields higher than 99% after a few hours. The formation of 1,4-pentanediol and 2-methyltetrahydrofuran, the typical side products of the reduction of GVL with molecular hydrogen, was not observed. The only byproducts were water and carbon dioxide, which were easily eliminated, and the Shvo catalyst was recycled several times without losing catalytic activity.

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EXPERIMENTAL SECTION

1,3-Diphenyl-2-propanone (99%, Aldrich), 4,4′-dimethoxybenzil (98%, Aldrich), RuCl3 (Aldrich), H2SO4 (95−97%, Sigma-Aldrich), concentrated HCl (37%, Sigma-Aldrich), ammonium hydroxide (25%, Sigma-Aldrich), levulinic acid (98%, Aldrich), formic acid (98%-100%, Merck), biphenyl (99.5%, Sigma-Aldrich), p-anisaldehyde (98%, Aldrich), d8-toluene (>99.5+ atom % D, Sigma-Aldrich), d6-dimethyl sulfoxide (DMSO-d6, >99.5+ atom % D, Sigma-Aldrich), and deuterium oxide (>99.8 atom D %, Armar) were all used as received. Literature methods were used for the synthesis of Ru3(CO)12,29 3,4bis(4-methoxyphenyl)-2,5-diphenylcyclopentadienone,30 and the Shvo catalyst precursors.25a NMR spectra were collected using a Bruker AV III 400 instrument at 20 °C. Quantitative 1H NMR was carried out using 90° flip angle 185


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AUTHOR INFORMATION

Corresponding Author

*E-mail for I.T.H.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support of the Hungarian Scientific Research Fund (OTKA-T047207 and OTKA-CNK 78065). Portions of this work were funded by the City University of Hong Kong under project number 9380047 and the Budapest University of Technology and Economics under project number KMR_12-1-2012-0066.

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