Production of γ-valerolactone from biomass-derived compounds using formic acid as a hydrogen source over supported metal catalysts in water solvent

Abstract

γ-Valerolactone (GVL) is a key intermediate for production of fuels and chemicals. In this research, GVL is synthesized from biomass-derived compounds using formic acid (FA) as a hydrogen source over various supported metal catalysts which are prepared by a simple impregnation or co-precipitation method. Under optimum conditions, levulinic acid (LA) is almost converted to GVL by Ru/C, Ru/SBA, Au/ZrC and Au/ZrO₂ catalysts with above 90% yield in water solvent. Especially, the Au/ZrO₂ showed excellent activity and recyclability; the Au/ZrO₂ catalyst can decompose completely FA to CO₂ and H₂, which gives high yield of GVL (ca. 97%) from hydrogenation of LA, and can retain its activity for at least 5 recycle runs. GVL is also obtained from one-pot dehydration/hydrogenation reaction of fructose in water solvent. In this reaction, FA plays two roles: an acid catalyst for dehydration of fructose to LA, and a hydrogen source for hydrogenation of the obtained LA over supported metal catalysts. The Au/ZrO₂ is the best catalyst for dehydration/hydrogenation reaction with overall GVL yield of 48% and can be reused several times.

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1. Introduction

The rapid developments of industry and transportation all over the world have led to a drastic increase in the demand for fuels. Currently, over 84% of this demand is based on the burning of fossil fuels – non-renewable resources (oil 34%, gas 28% and coal 22%), but these resources are finite and becoming more expensive. Furthermore, the combustion of fossil fuels for the production of heat and power is associated with a worldwide increase in greenhouse gas levels which is considered the main cause of climate change.^1–4

Diminishing fossil resource reserves and degradation of the environment are strong driving forces for the search for sustainable and renewable resources.⁴ Various alternative energy sources have been developed, such as hydroelectric energy, wind power, geothermal energy, solar energy, and so on. Since the application of those energy sources might take longer than they are expected, the development of efficient processes to convert biomass resource into liquid fuels and valuable chemicals is a key research area in the next few decades.^5–11 Biomass is an abundant and renewable alternative resource that is the best candidate to replace fossil fuels for the sustainable production of energy^1–3,12,13 as well as chemicals.^12,14–17

Some of the most important characteristics for an expected ideal platform chemical include the possibility to use it for production of both energy and carbon-based products, renewable, easy and safe to store and move in large quantities, low melting point, high boiling and flash points, low or non-toxicity, and easy for bio-degradation.⁶ Because of its versatile properties, γ-valerolactone (GVL) is considered as one of the most promising platform molecules that satisfy above requirements. GVL can be converted into liquid fuels, fuel additives,^18,19 green solvents,^12,20 food additives, and intermediates for chemicals and pharmaceutical industries.^2,21,22

One of the most effective methods that can be used for production of GVL is the catalytic hydrogenation of levulinic acid (LA) which can be obtained from the hydrolysis followed by dehydration of carbohydrate compounds in acidic media.^23,24 The hydrogenation of LA in vapor phase takes place at an atmospheric pressure of hydrogen and gives high yield of GVL.^25–27 Nevertheless, vapor phase reaction needs huge amount of energy for vaporization of reactants. Hydrogenation of LA in liquid phase is more common; however, the reaction usually requires high pressured H₂ gas (1.2–5.5 MPa). In liquid phase, hydrogenation reaction of LA can occur in the presence of either homogeneous or heterogeneous catalysts. The homogeneous catalysts are usually complexes of Ru and Ir.^28–31 These homogeneous catalysts have high activities but are difficult to separate from the reaction mixture.

Many supported metal catalysts have been developed for liquid phase hydrogenation of LA under high pressured H₂.^32–34 In these works, various metals such as Ir, Rh, Pd, Ru, Re, Ni, etc. on different supports such as carbon and metal oxides have been screened. The results showed Ru catalyst supported on carbon gave the highest yield of GVL; however, a weak point of Ru/C is its recyclability. The reaction of carbohydrate compounds usually generates huge amount of undesired solid known as humins that deactivate the catalysts. The reuse of catalyst, therefore, needs calcination at high temperature of catalyst to combust humins. Under this condition, the Ru/C is destroyed completely.

Up to now, most studies have been used hydrogen gas that is a non-sustainable source for hydrogenation of LA, there have been few researches focusing on the utilization of the alternative hydrogen source. These are some advantages when using formic acid (FA) as the hydrogen resource compared with hydrogen gas because FA is cheaper, safer, giving higher atomic efficiency, and needing less instrumental requirements.^35–39 The first purpose of this work is to synthesize GVL from LA using FA as a hydrogen source and heterogeneous catalysts in water solvent. We found that Ru/SBA-15 had good activity as well as Ru/C. Besides Ru/SBA, Au/ZrO₂ and Au/ZrC acted as acid-tolerant catalysts with excellent catalytic activities for production of GVL from LA using FA as a hydrogen donor. The second topic of this work is one-pot dehydration/hydrogenation of fructose to GVL over supported metal catalysts in water solvent. To the best of our knowledge, this is the first report on production of GVL via dehydration/hydrogenation of fructose in the absence of mineral acid catalysts.

2. Experimental

2.1. Chemicals

Metal supported catalysts, metal oxides, and salts used in present work including 5 wt% Ru/C, 5 wt% Ru/Al₂O₃, 5 wt% Pt/C, 5 wt% Pd/C, RuCl₃·3H₂O, HAuCl₄·4H₂O, Cu(NO₃)₂·3H₂O, Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Ni(NO₃)₂·6H₂O, activated carbon, mordenite zeolite (hydrogen form), formic acid (FA), and naphthalene were supplied from Wako Pure Chemical Ind., Ltd. Fructose, acetone, TiO₂, ZrO₂, Al₂O₃, ZrOCl₂·8H₂O, and Na₂CO₃ were obtained from Kanto Chemical Co., Inc. Levulinic acid (LA) was bought from Tokyo Chemical Industry Co., Ltd. Tri-block co-polymer poly(ethylene oxide)-poly(propylene oxide)-poly(ethyleneoxide) (P123, (EO)₂₀(PO)₇₀(EO)₂₀, M = 5800), tetraethylorthosilicate (98%, TEOS), and γ-valerolactone (GVL) were purchased from Sigma Aldrich.

2.2. Catalyst preparation

Preparations of supported metal catalysts were performed by impregnation method. This method is used for preparation of Ru, Pd, Pt or Au supported on mesoporous silica (SBA-type, see ESI†), TiO₂, ZrO₂, Al₂O₃, C, mordenite zeolite (hydrogen form), and ZrC (as-prepared zirconium carbonate according to the reference,⁴⁰ see ESI†). Typically, 0.2 g of supporting material and 20 mL of water were introduced into a round-bottom flask, and then the mixture was stirred (500 rpm) at room temperature. Thereafter, a desired amount of metal precursor was added, and the mixture was kept at same conditions for 2 h (in case of Au catalysts, the pH of mixture was adjusted to 10 by an ammonia solution). Subsequently, the mixture was refluxed at 140 °C for 4 h. After cooling to room temperature, the obtained solid was collected by filtration, and washed with 1 L of water followed by drying at 100 °C overnight.

Metals (Ru, Au, Ag, Fe, Co, Cu, and Ni) supported on ZrO₂ were prepared by co-precipitation method. Typically, a 50 mL of 0.25 M Na₂CO₃ solution was added slowly (1 mL min⁻¹) into a round-bottom flask containing a 20 mL solution of 0.5 M ZrOCl₂·8H₂O including the desired amount of metal precursor. The obtained gel was kept at room temperature under a violent stir for 2 h, followed aging at 140 °C for 12 h. Solid was recovered by filtration, extensively washed with 2 L of distilled water, and dried at 100 °C. After ground to fine powder, the solid was calcined at 500 °C for 4 h.

2.3. Procedure of catalytic reaction and product analysis

Hydrogenation of LA. Typically, 2 mmol (0.232 g) of LA, 4 mmol (0.184 g) of FA, 1 mL of water, and 20 mg of supported metal catalyst were introduced into a sealed glass tube (5 mL vol.). Reactor was heated and kept at desired temperatures (110–170 °C), followed by cooling to room temperature.

One-pot dehydration/hydrogenation of fructose to GVL. 2 mmol (0.36 g) of fructose, 4 mmol (0.184 g) of FA, 1 mL of water, and 20 mg of supported metal catalyst were introduced into a sealed glass tube (5 mL vol.). Dehydration stage of fructose to LA was carried out at 120 °C for 3 h. In this step, FA acted as an acid catalyst for dehydration of fructose to HMF and consecutive rehydration of HMF to LA. After that, reaction temperature was elevated to 150 °C in order to decompose FA over supported metal catalyst into CO₂ and H₂ that participated in hydrogenation of LA to generate GVL.

Analyses. The reaction mixture after reaction was diluted by acetone and analyzed by GC with naphthalene used as an internal standard. GC-17A instrument (Shimadzu) was equipped an Agilent DB-1 column (30 m × 0.32 mm × 025 μm), a FID detector, and an injection port working at 280 °C with split ratio 100 : 1. The temperature program for a column is described as follows: 40 °C isothermal for 0.5 min, 20 °C min⁻¹ to 200 °C, 7 °C min⁻¹ to 250 °C, and isothermal at 250 °C for 2 min. Retention times of acetone, GVL, LA, and naphthalene are observed at 1.5 min, 3.7 min, 4.6 min, and 5.7 min, respectively. The amounts of fructose and FA were determined by a high performance liquid chromatograph (HPLC, Waters Co., Ltd.) equipped with an Aminex HPX-87H column (Bio-Rad Laboratories, Inc.) and a refractive index detector. 10 mM H₂SO₄ aq. was used as an eluent at a flow rate of 0.5 mL min⁻¹. Both column and detector were operated at 50 °C. Typically, fructose, FA, and HMF were obtained in the HPLC chart at 12.3 min, 17.2 min, and 39.5 min, respectively. All data were based on repeated runs and the error was less than 2%. Metal content in the catalyst was measured by inductively coupled plasma-atomic emission spectroscopy (ICPS-7000 Ver.2, Shimadzu Co., Ltd.). All samples was dissolved in aqua regia before ICP measurement.

3. Results and discussion

3.1. Hydrogenation of LA to GVL

The reaction to GVL by LA hydrogenation is shown in Scheme 1. In this pathway, FA is decomposed to CO₂ and H₂ by supported metal catalyst. Subsequently, H₂ takes part in the hydrogenation of LA to generate GVL.

Scheme 1 Hydrogenation of LA toward GVL using FA as a hydrogen source.

Firstly, the reaction was carried out with 1/1 mole ratio of LA/FA at 150 °C using Ru-supported carbon as catalyst because Ru/C is the most common catalyst for hydrogenation of LA using H₂ gas as reductant.^{32,34,41–43} The conversion and yield were monitored as functions of time (Fig. 1).‡ The reaction occurred quite fast and almost reached maximum performance after 5 h. Therefore, 5 h of reaction time was applied for next experiments.

Fig. 1 LA conversion (black diamond) and GVL yield (open circle) as a function of reaction time. Reaction conditions: LA (2 mmol), FA (2 mmol), 5 wt% Ru/C (0.02 g), water (1 mL), temperature (150 °C).

In order to estimate the effect of support materials on the activity, the impregnation method was used for grafting Ru on various supports such as C, SBA-15 (ordered mesoporous SiO₂), TiO₂, ZrO₂, and Al₂O₃ to afford Ru/C, Ru/SBA-15, Ru/TiO₂, Ru/ZrO₂, and Ru/Al₂O₃, respectively. The activities of these catalysts were listed in Table 1. Obtained results showed the similar activities of C and SBA-15 supported Ru catalysts (21–22% GVL yield, entries 1 and 2), while metal oxide supported Ru catalysts had much lower activities with trace amount of product (GVL yields were below 3%, entries 3–5). Because of good support for Ru catalyst, C support was also used for preparation of other catalyst such as the Pt/C, Pd/C, and Au/C; nevertheless, the activities of these catalysts were much lower than that of the Ru/C (1–2% GVL yield, entries 6–8).

Table 1 Hydrogenation of LA toward GVL using various supported metal catalysts^a

Entry	Catalyst	LA conv. (%)	GVL yield (%)	GVL sel. (%)
a Reaction conditions: LA (2 mmol), FA (2 mmol), 5 wt% supported metal catalyst (0.02 g), water (1 mL), temperature (150 °C), time (5 h).
1	Ru/C	29	21	73
2	Ru/SBA-15	31	22	71
3	Ru/Al2O3	16	3	17
4	Ru/TiO2	10	2	20
5	Ru/ZrO2	11	2	18
6	Pt/C	13	2	13
7	Pd/C	9	2	17
8	Au/C	13	1	9

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Interestingly, Ojeda and Iglesia reported that well-dispersed Au species could selectively decompose gaseous FA.⁴⁴ The obtained small Au clusters had very high activity in FA dissociation reaction and even better than some well-known catalysts such as Pt or Ru. In this report, various supported Au catalysts were also examined for hydrogenation of LA to GVL using FA as a hydrogen source (Fig. 2). Among tested catalysts, the Au/ZrO₂ and Au/ZrC possessed the highest activity with 77% yield of GVL. Activities of the other metals on ZrO₂ were quite low compared with that of Au (1–2% GVL yield, Table S1, ESI†). These results indicated that both of the noble metal and the nature of the support played important roles in the catalytic activity of LA hydrogenation to GVL using FA as a hydrogen source.

Fig. 2 GVL yield from LA hydrogenation over 5 wt% Au catalyst on various supports. Reaction conditions: LA (2 mmol), FA (2 mmol), 5 wt% Au-supported catalyst (0.02 g), water (1 mL), temperature (150 °C), time (5 h).

The influence of FA/LA mole ratio on LA conversion and GVL yield was investigated on 5 wt% Ru/C and 5 wt% Au/ZrO₂ catalysts (Fig. 3). In case of 5 wt% Ru/C catalyst, the hydrogenation reaction is proportional to the initial amount of FA. With FA/LA mole ratio of 1, GVL yield was only 21%, while LA was converted completely giving 90% GVL yield when the FA/LA mole ratio changed to 3. At every FA/LA mole ratios, the Au/ZrO₂ catalyst was more efficient than the Ru/C for hydrogenation of LA. 77% GVL yield was obtained over 5 wt% Au/ZrO₂ at mole ratio of FA/LA = 1, while there was no significant difference in reaction performances with FA/LA mole ratios of 2 and 3 (94–97% GVL yield). Therefore, the best FA/LA mole ratio for 5 wt% Ru/C and 5 wt% Au/ZrO₂ were 3 and 2, respectively. Higher activity of the Au/ZrO₂ than Ru/C was due to the differences in their decomposition ability of FA (Table 2 and S2, ESI†); FA was decomposed completely at all investigated amounts with the Au/ZrO₂ catalyst, whereas Ru/C converted only 56%, 49%, and 43% FA when using 2 mmol, 4 mmol, and 6 mmol of FA, respectively. It seems that the Au/ZrO₂ was very stable in presence of FA, but the Ru/C was deactivated gradually by FA during the reaction. Indeed, Ru/C almost lost its activity after the first run and could not be recycled, in contrast to Au/ZrO₂ (vide infra).

Fig. 3 Effect of FA/LA mole ratio on the hydrogenation performance. Reaction conditions: LA (2 mmol), catalyst (0.02 g), water (1 mL), temperature (150 °C), time (5 h).

Table 2 Hydrogenation of LA to GVL at different reaction temperatures^a

Entry	Catalyst	React. Temp. (°C)	LA conv. (%)	GVL yield (%)	GVL sel. (%)	FA Conv. (%)
a Reaction conditions: LA (2 mmol), FA (^a 6 mmol, ^b 4 mmol), catalyst (0.02 g), water (1 mL), time (5 h).
1	5 wt% Ru/C^a	100	7	2	29	13
2		110	16	5	30	16
3		120	30	12	39	19
4		130	35	22	62	33
5		140	50	32	63	37
6		150	100	90	90	43
7	5 wt% Au/ZrO2^b	100	7	2	28	10
8		110	19	11	56	24
9		120	21	14	67	32
10		130	40	26	67	52
11		140	73	66	91	100
12		150	100	97	97	100

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The hydrogenation of LA using FA as a hydrogen source strongly depended on reaction temperature. Two represented catalysts for this experiment were Ru/C and Au/ZrO₂ (Table 2). At temperatures lower than 120 °C, GVL yield was less than 15% in both cases (entries 1–3, 7–9) but increased gradually when rising reaction temperature higher than 130 °C. At 150 °C, GVL yield suddenly achieved more than 90%. These results showed that at low temperatures (below 130 °C), FA was not decomposed significantly, but violently decomposed when reaction temperature was higher than 140 °C to release sufficient amount of hydrogen for the hydrogenation reaction. LA conversion only reached 100% when reaction temperature was elevated to 150 °C (entries 6 and 12). Therefore, the suitable temperature for decomposition of FA and consecutive hydrogenation of LA was 150 °C.

The activities of various contents of Au catalyst on ZrO₂ were tested (Table S3, ESI†). The increase in percentage of Au led the rise of product yield. However, above 3 wt% of Au, the active sites on ZrO₂ support became saturation leading to the unchanged GVL yield with 96–97%. The highest TON value of 630 was obtained with 3 wt% Au/ZrO₂ catalyst.

In order to investigate the recycling properties of the catalyst, the Au/ZrO₂ was removed from the reaction mixture by centrifugation, washed with acetone, dried at 80 °C for 2 h before subjected to further catalytic reaction. As a result, we found that the Au/ZrO₂ could be recycled at least five times without significant loss of catalytic activity (Fig. 4).

Fig. 4 Recycling properties of 3 wt% Au/ZrO₂ in the production of GVL via hydrogenation of LA with FA as hydrogen source. Reaction conditions: LA (2 mmol), FA (4 mmol), 3 wt% Au/ZrO₂ catalyst (0.02 g), water (1 mL), temperature (150 °C), time (5 h).

3.2. One-pot dehydration/hydrogenation reaction of fructose to GVL

The transformation of fructose to GVL undergoes two main steps: (i) acid-catalyzed dehydration of fructose yielding LA and (ii) hydrogenation of obtained LA to GVL (Scheme 2). The latter process is promoted by supported metal catalysts. As mentioned above, FA is decomposed slowly below 130 °C by supported metal catalysts. Fortunately, this temperature is suitable for dehydration of fructose to LA.⁴⁵ Therefore, FA can serve as homogeneous acid catalyst for dehydration of fructose to LA at 120 °C. In next step, the temperature is elevated to 150 °C or higher to facilitate the decomposition of FA to H₂, followed by hydrogenation of LA obtained in previous step to GVL. Consequently, it is expected that this multi-step reaction can be done in a one-pot manner. In this work, instead of utilization of mineral acids (H₂SO₄, HCl, etc.), FA was attempted to use as both an acid catalyst for dehydration of fructose step at low temperature (120 °C) and a hydrogen source for hydrogenation step that occurred over supported metal catalysts at higher temperature (150 °C).

Scheme 2 Dehydration/hydrogenation reaction of fructose to GVL using FA as both of an acid catalyst and a hydrogen source.

A preliminary experiment was carried out to estimate the performance of dehydration of fructose by FA: a mixture including fructose (16 mmol) and FA (48 mmol) was heated at 120 °C for 3 h. HPLC analysis showed the complete conversion of fructose affording 50% LA yield and huge amount of undesired solid compounds, known as humins as side products in this reaction.

Four catalysts were selected for one-pot dehydration/hydrogenation of fructose included Au/ZrO₂, Au/ZrC, Ru/SBA-15, and Ru/C. For direct conversion of fructose to GVL, the Au/ZrO₂ exhibited an excellent catalytic activity with 48% yield of GVL (TON = 317). Result from Table 3 also showed that Au/ZrC might be another potential candidate of catalyst for this reaction, since activity of the Au/ZrC could be comparable to that of the Au/ZrO₂. Ru catalysts had lower activities in this one-pot reaction (entries 3 and 4). The recyclability of Au/ZrO₂ and Ru/SBA-15 catalysts were also tested. After the reaction, reaction mixture was separated by centrifugation to collect solid that contained both catalyst and humins. To remove humins and other products deposited on the catalysts, the solid was calcined at 500 °C for 4 h. Calcined Au/ZrO₂ and Ru/SBA-15 were used for the next run. After 3 runs, activity of the Au/ZrO₂ decreased gradually from 48 to 37%, while the Ru/SBA-15 lost its activity faster than the Au/ZrO₂ catalyst (only 9% GVL yield was obtained for the 3rd run) (Fig. S1, ESI†). The Au/ZrO₂ catalyst was still reused for several times though it was partly deactivated.

Table 3 One-pot transformation of fructose to GVL over supported metal catalysts^a

Entry	Catalyst	Actual metal content (wt%)^a	GVL yield (%)	TON^b
a Reaction conditions: Fructose (2 mmol), FA (4 mmol), 3 wt%, supported metal catalyst (0.02 g), water (1 mL), temperature (150 °C), time (5 h). ^a Determined by ICP. ^b Turnover number (TON) is defined as mole of formed GVL per actual mole of supported metal.
1	Au/ZrO2	3.02	48	317
2	Au/ZrC	2.81	47	307
3	Ru/SBA-15	2.63	26	87
4	Ru/C	2.68	21	71

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4. Conclusion

γ-Valerolactone (GVL) can be obtained from hydrogenation of levulinic acid (LA) using formic acid (FA) as hydrogen source and supported metal catalysts in water. For Ru catalyst, the best supports include carbon or mesoporous silica (SBA-15). Under optimized reaction conditions, the reaction with these catalysts can give at least 90% GVL yield. Au supported on zirconia and zirconium carbonate (denoted as Au/ZrO₂ and Au/ZrC) are also found to be excellent catalysts for decomposing FA and hydrogenating LA to produce GVL with 97% yield. The Au/ZrO₂ is found to be a highly stable, acid-tolerant catalyst and can be reused for 5 recycles without significant loss of its activity. In the one-pot transformation of fructose to GVL, FA acts as an acid catalyst for dehydration reaction of fructose to LA at 120 °C before participating in the hydrogenation stage as a hydrogen source at higher temperature (150 °C) to afford GVL. In this reaction, the Au/ZrO₂ is also the best catalyst for one-pot synthesis of GVL from fructose in water which provides 48% yield of GVL.

Acknowledgements

This work is supported by Scientific Research (C) (no. 25420825) under Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. P.A.S. is thankful to 322 Project of Ministry of Education and Training (MOET) from Vietnam Government for the scholarship.

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Footnotes

† Electronic supplementary information (ESI) available: Preparation method, hydrogenation of LA over various metal supported ZrO₂, decomposability of FA, and recylability. See DOI: 10.1039/c3ra47580h

‡ The difference between conversion and yield may be caused by the formation of intermediates or unidentified compounds formed during the reaction.

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