Catalytic Transfer Hydrogenation of Furfural into Furfuryl Alcohol over

Research Article pubs.acs.org/journal/ascecg

Catalytic Transfer Hydrogenation of Furfural into Furfuryl Alcohol over Magnetic γ‑Fe2O3@HAP Catalyst Fan Wang and Zehui Zhang* Key Laboratory of Catalysis and Material Sciences of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Minyuan Road 182, Wuhan 430074, China ABSTRACT: Catalytic transfer hydrogenation of the CO bond has been considered to be one of the most important processes for the synthesis of fuels and chemicals. In this study, we have developed a metal-free transfer hydrogenation of furfural into furfuryl alcohol and some other valuable carbonyl compounds into alcohols as the hydrogen donors. Hydroxyapatite-encapsulated magnetic γ-Fe2O3 (γ-Fe2O3@HAP) acted as a magnetic base to promote this transfer hydrogenation with iso-propanol as the hydrogen donor. The catalytic performance of the γ-Fe2O3@HAP catalyst was greatly affected by the structure of alcohol. iso-Propanol was the best hydrogen donor for the transfer hydrogenation. In addition, the reaction temperature and the catalyst loading also affected this reaction. The highest yield of furfuryl alcohol reached 91.7% at a furfural conversion of 96.2%. Furthermore, this method was also useful for the transfer hydrogenation of other important carbonyl compounds into value-added chemicals and fuels. After the reaction, the γ-Fe2O3@HAP catalyst could be easily collected with the assistant of a magnet, and reused for six runs without the loss of its catalytic activity. KEYWORDS: Magnetic catalyst, Transfer hydrogenation reaction, Furfural, Furfuryl alcohol, Sustainable chemistry

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INTRODUCTION

temperature and high pressure required the use of specialized equipment, and has potential safety issues. Recently, catalytic transfer hydrogenation (CTH) of furfural into furfuryl alcohol has received great interest.22 This process can avoid the use of explosive H2; thus, it seems to be more economical and much safer. Among different hydrogen donors, alcohols are attractive, especially iso-propanol, as it is a noncorrosive alcohol and can serve as both hydrogen donor and solvent. Compared with the hydrogenation of furfural by H2, the CTH of furfural into furfuryl alcohol has been less studied, possibly due to low activity toward the hydrogen donors.23−25 Current methods for the CTH of furfural into furfuryl alcohol were mainly performed over metal catalysts, especially ruthenium catalysts.25 These reported methods for the CTH of furfural over metal catalysts demonstrated some drawbacks such as the high cost of high metal loading and low selectivity. For example, Fe2O3-supported Cu and Ni catalysts produced over 70% selectivity to furfuryl alcohol at moderate furfural conversion with iso-propanol as the hydrogen transfer reagent.26 Besides the metal catalysts, a few kinds of Lewis acidic zeolites (such as TiIV, SnIV, and ZrIV) were also reported to promote the CTH of furfural into furfural alcohol.23,27 On one hand, the preparative procedure of the Lewis acidic zeolites was tedious. On the other hand, the side reactions such as the etherification of furfuryl alcohol with the alcohols (hydrogen donors) and the ring-opening of the furan ring always occurred due to the presence of Lewis acid. Therefore, it is still desirable

Owing to the crisis of fossil resources and the growing environmental concerns, the search for renewable resources to supply chemicals and liquid fuels has received considerable attention.1,2 Biomass holds a promising potential as an alternative to fossil resources to supply chemicals and fuels.3,4 Thus, the conversion of biomass into fuels and chemicals is currently a very active issue. Among different methods for the transformation of biomass, catalytic routes are particularly of great importance, as many kinds of biomass-derived chemicals and fuels can be produced by the careful design of different catalytic systems.5,6 In this context, great effort has been devoted to develop different catalytic routes for the transformation of biomass resources into fuels and chemicals.7,8 Carbohydrates represent the major component of biomass. Catalytic dehydration of C5 carbohydrates can generate furfural,9 which can be transformed into a variety of chemicals and liquids fuels.10,11 Catalytic reduction of furfural can generate some valuable products, such as furfuryl alcohol, tetrahydrofurfuryl alcohol, and 2-methyl furan.12−14 Furfuryl alcohol is an important intermediate for the manufacture of lysine, ascorbic acid, and numerous lubricants, producing resins.15 It is commercially produced from the hydrogenation of furfural over a copper chromate catalyst at high temperature (130−200 °C) and high H2 pressure (30 bar).16,17 Obviously, this method had several drawbacks such as a high energy cost and the use of highly toxic Cr2O3. Later, a variety of supported single metal or bimetallic catalysts have been reported for the gas phase and liquid phase hydrogenation of furfural with H2.18−21 The use of explosive H2 at high © 2016 American Chemical Society

Received: September 20, 2016 Revised: November 23, 2016 Published: December 12, 2016 942

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furfural and furfuryl alcohol have maximum absorbance wavelengths of 270 and 212 nm, respectively. Recycling of the Catalyst. After the reaction, the γ-Fe2O3@HAP catalyst was collected by an external magnet and washed with water and ethanol 3 times, respectively. Then it was dried at 100 °C overnight. The spent catalyst was reused for the next run under identical conditions. These processes were repeated five times.

to develop new catalytic routes for the efficient CTH of furfural into furfuryl alcohol. The base catalysts were reported to have the ability to catalyze the transfer hydrogenation of carbonyl compounds or nitro compounds.28 In recent years, the magnetic catalysts have received great interest in the fields of catalysts.29−31 One of the unique properties of the magnetic catalysts is that the magnetic catalysts can be easily separated from the reaction mixture by an external magnet. Thus, it can avoid weight loss during the catalyst recycle, which was generally observed for the traditional filtration method. In our previous work, hydroxyapatiteencapsulated magnetic γ-Fe2O3 (γ-Fe2O3@HAP) was used as a magnetic base support to immobilize Ru and Pd catalysts for the oxidation of HMF.32,33 Thus, we wonder whether γFe2O3@HAP could be used as a magnetic base catalyst to promote the transfer hydrogenation of furfural into furfuryl alcohol (Scheme 1). In this study, the transfer hydrogenation of

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RESULTS AND DISCUSSION Catalyst Preparation and Characterization. γ-Fe2O3@ HAP was prepared and characterized as reported in our previous work (Scheme 2).32,33 Briefly, the coprecipitation of Scheme 2. Procedure of the Preparation of γ-Fe2O3@HAP

Scheme 1. Catalytic Transfer Hydrogenation of Furfural into Furfuryl Alcohol

Fe2+ and Fe3+ in alkaline solution under a N2 atmosphere produced Fe3O4 nanoparticles, which were then encapsulated by a layer of hydroxyapatite (HAP). The as-made material was then calcined at 300 °C for 3 h, generating a reddish-brown powder (γ-Fe2O3@HAP). Catalytic Transfer Hydrogenation of Furfural by Different Hydrogen Donors. ] Initially, some common alcohols were used as hydrogen donors for the transfer hydrogenation of furfural over the γFe2O3@HAP catalyst, and the results are summarized in Table 1. It is observed that the conversion of furfural and the

furfural together with other common carbonyl compounds was performed with iso-propanol as the hydrogen donor over γFe2O3@HAP catalyst. To the best of our knowledge, there have been no reports on the use of the base for the transfer hydrogenation of furfural into furfuryl alcohol with isopropanol.

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Table 1. Results of the Transfer Hydrogenation of Furfural with Different Hydrogen Donorsa

EXPERIMENTAL SECTION

Materials. FeSO4·7H2O (99.5%), FeCl3·6H2O (99.5%), Ca(NO3)2·4H2O (99.5%), (NH4)2HPO4 (99.8%), NaBH4 (98.0%), and all solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Furfural (99%), furfuryl alcohol (98%), 5hydroxymethylfurfural (≥95%), ethyl levulinate (99%), γ-valerolactone (98%), acetophenone (≥98%), α-phenylethyl alcohol (99%), cinnamaldehyde (≥95%), and cinnamyl alcohol (98%) were purchased from Aladdin Chemicals Co., Ltd. (Beijing, China). 2,5-Bis(hydroxymethyl)furan (99%) was purchased from Mclean Shanghai Biological Technology Co., Ltd. Benzaldehyde (≥99%) was purchased from the Shanghai ANPEL Scientific Instrument Co., Ltd. Benzyl alcohol (≥99%) was purchased from Alfa Aesar (Tianjin, China) Chemical Co., Ltd. Acetonitrile (HPLC grade) was purchased from Tedia Co. (Fairfield, OH). All of the solvents were used directly without treatment. γ-Fe2O3@HAP was prepared and characterization as described in our previous work.32,33 Typical Procedure for the Transfer Hydrogenation of Furfural. CTH of furfural was carried out in a stainless steel 40 mL Parr batch reactor. In a typical run, furfural (1 mmol), γ-Fe2O3@HAP (40 mg), and iso-propanol (15 mL) were charged in the reactor and sealed under N2 pressure (1.0 MPa). Then the reactor was heated from room temperature to 180 °C, and kept at 180 °C for 3 h. After cooling the reactor to room temperature, the magnetic γ-Fe2O3@HAP catalyst was collected by an external magnet, and the liquid solution was analyzed by HPLC. Analytic Methods. Analysis of furfural and furfuryl alcohol was conducted on a VARIAN ProStar 210 HPLC system. Samples were separated on a reversed-phase C18 column (200 mm × 4.6 mm). The mobile phase was constituted of acetonitrile and H2O (v/v = 55:45) at 0.8 mL min−1. The column oven temperature was kept at 25 °C. The content of furfural and furfuryl alcohol in samples was calculated by the external standard calibration curve method, which was constructed on the basis of the pure compounds. Each sample was run 2 times to determine the content of furfural and furfuryl alcohol, respectively, as

entry

hydrogen donors

conversion (%)

yield (%)

selectivity (%)

1 2 3 4 5 6 7b 8c

methanol ethanol n-propanol iso-propanol 2-butanol tert-butanol iso-propanol iso-propanol

40.3 30.2 36.3 40.8 40.1 3.8 7.1 18.2

1.8 21.4 29.6 36.7 34.4 1.3 2.3 10.2

4.5 70.9 81.5 90.0 85.7 34.2 32.4 56.0

Reaction conditions: furfural (1 mmol) and γ-Fe2O3@HAP (20 mg) were added into solvent (15 mL), and the reaction was carried out at 180 °C and 1 MPa N2 for 3 h. bThe reaction was carried out in the absence of catalyst. cγ-Fe2O3 (20 mg) was used for the same reaction. a

selectivity of furfuryl alcohol were greatly affected by the structure of alcohols. The lowest selectivity of furfuryl alcohol was observed at 4.5% when the transfer hydrogenation of furfural was performed in methanol (Table 1, entry 1). The major product was identified to be the acetal product, which was formed by the reaction of methanol with furfural. There were no other byproducts such as ethers and ethyl levulinate, which were often observed in the transfer hydrogenation of furfural over Lewis acid catalysts.23,27 Although the conversion of furfural was lower with ethanol than that with methanol, the selectivity of furfural alcohol greatly improved from 4.5% with methanol to 70.9% with ethanol (Table 1, entries 1 vs 2). The results from entries 1 and 2 suggested that the formation of acetal product was greatly inhibited with ethanol. This phenomenon was also observed when n-propanol was used as 943

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ACS Sustainable Chemistry & Engineering the hydrogen donor (Table 1, entry 3). To our delight, the highest furfural conversion of 40.8% and furfuryl alcohol selectivity of 90.0% were achieved using iso-propanol as the hydrogen donor (Table 1, entry 4). The higher conversion of furfural in iso-propanol revealed that the dehydrogenation of iso-propanol was much easier than the dehydrogenation of primary alcohols. In fact, acetone was detected as the dehydrogenation product of iso-propanol, which was much higher that other dehydrogenation products. Meanwhile, the selectivity of furfuryl alcohol was also higher than those obtained by primary alcohols. Similar results were also observed when 2-butanol was also used as the hydrogen donor (Table 1, entry 5). However, poor furfural conversion of 3.8% was observed when the tert-butanol was used as the hydrogen donor (Table 1, entry 6). The reason should be that there were no hydrogens close to the hydroxyl group in tert-butanol to produce the corresponding carbonyl compounds. Therefore, iso-propanol was the best hydrogen donor for the transfer hydrogenation of furfural into furfuryl alcohol over the γFe2O3@HAP catalyst. In addition, the CTH of furfural was carried out without catalyst at 180 °C (Table 1, entry 7). Interestingly, furfuryl alcohol was also produced with a low selectivity of 32.4% at furfural conversion of 7.1% after 3 h (Table 1, Entry 1). The GC−MS analysis of the product distribution showed that the acetal formed by reaction between furfural and iso-propanol was the main byproduct. In a comparison of the results from entry 3 and entry 7, it clearly indicated that the γ-Fe2O3@HAP catalyst with a base property had the ability to promote the transfer of the hydrogen from iso-propanol to furfural. Furthermore, the transfer hydrogenation of furfural was also performed in the presence of γ-Fe2O3. Furfural conversion increased from 7.1% without catalyst to 18.2% with γ-Fe2O3, but the conversion of furfural and the selectivity of furfuryl alcohol over the γ-Fe2O3 catalyst were much lower than those with the γ-Fe2O3@HAP catalyst (Table 1, entries 7 vs 8). These results indicated that the alkaline HAP in the γ-Fe2O3@HAP catalyst should be the active site for the transfer hydrogenation of furfural into furfuryl alcohol. Meanwhile, acetone as the dehydrogenation product of iso-propanol was also detected, which had almost the same amount of furfuryl alcohol. These results clearly suggested γFe2O3@HAP was an effective solid base catalyst for the transfer hydrogenation of furfural into furfuryl alcohol. Effect of the Catalyst Amount on the Transfer Hydrogenation of Furfural. The effect of γ-Fe2O3@HAP loading on the effect of the transfer hydrogenation of furfural was examined, and the results are shown in Figure 1. Furfural conversion was observed to increase with the increase of the γFe2O3@HAP loading, which was distinct especially at low catalyst loading. For example, 40.8% conversion of furfural was observed with the use of 20 mg of the γ-Fe2O3@HAP catalyst, and then it greatly increased to 64.9% with 40 mg of the γFe2O3@HAP catalyst. Upon further increasing the catalyst loading, furfural conversion increased slowly. The highest furfural conversion reached 89.0% after 3 h with 100 mg of the γ-Fe2O3@HAP catalyst. The higher furfural conversion with the higher loading of the γ-Fe2O3@HAP catalyst was due to the presence of many more catalytic sites, which accelerated the transfer hydrogen from iso-propanol to furfural. The selectivity of furfuryl alcohol did not change significantly, remaining stable around 90%. The stable selectively of furfuryl alcohol revealed that the catalyst loading showed no great effect on the catalyst selectivity, and the selectivity was influenced by other important

Figure 1. Results of the transfer hydrogenation of furfural into furfuryl alcohol with different amount of catalysts. Reaction conditions: furfural (1 mmol) and a setting amount of γ-Fe2O3@HAP were added into isopropanol (15 mL), and the reaction was carried out at 180 °C and 1 MPa N2 for 3 h.

parameters as discussed above such as the type of alcohols and the reaction temperature. Catalytic Transfer Hydrogenation of Furfural at Different Reaction Temperatures. The effect of the reaction temperature on the transfer hydrogenation of furfural over the γ-Fe2O3@HAP catalyst was then studied. As depicted in Figure 2, the reaction temperature demonstrated a significant effect on

Figure 2. Results of the transfer hydrogenation of furfural at different reaction temperatures. Reaction conditions: furfural (1 mmol) and γFe2O3@HAP (40 mg) were added into iso-propanol (15 mL), and the reaction was carried out at different temperatures under 1 MPa N2 for 3 h.

furfural conversion rate. Generally, the higher the reaction temperature was, the higher the reaction rate was. For example, only 15.1% of furfural conversion was obtained after 3 h at 120 °C, and a much higher furfural conversion of 64.9% was achieved at 180 °C. These results suggested that the dehydrogenation of iso-propanol was sensitive to the reaction temperature over γ-Fe2O3@HAP catalyst, and higher reaction temperature accelerated the dehydrogenation of iso-propanol. Interestingly, it was also observed that the reaction temperature showed a small effect on the furfuryl alcohol selectivity. Furfuryl alcohol selectivity slightly increased from 80.9% at 120 °C to 91.8% at 180 °C. Meanwhile, acetal product as the side product gradually decreased with the increase of the reaction temperature. These results indicated that higher temperature promoted the equibrilium shift from the acetal product to furfural. In addition, there were no other furfural hydrogenation 944

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ACS Sustainable Chemistry & Engineering products such as 2-methylfuran, and tetrahydrogenfuran. Thus, 180 °C was the most appropriate temperature for the transfer hydrogenation of furfural into furfuryl alcohol with iso-propanol as the hydrogen donor. To provide more insights into the transfer hydrogenation of furfural into furfuryl alcohol over the γ-Fe2O3@HAP catalyst, kinetics studies were performed at three reaction temperatures of 140, 160, and 180 °C. In our reaction system, iso-propanol is in great excess; thus, the reaction kinetics were not affected by the concentration of iso-propanol. The transfer hydrogenation of furfural can be assumed to proceed as a pseudo-first-order reaction, as a function of furfural concentration. Therefore, the reaction rate constant (k) was expressed as the following equation. Figure 3 depicts the plot of ln(Ct/C0) versus time for

Figure 4. Time course of the transfer hydrogenation of furfural into furfuryl alcohol. Reaction conditions: furfural (1 mmol) and γ-Fe2O3@ HAP (40 mg) were added into iso-propanol (15 mL), and the reaction was carried out at 180 °C and 1 MPa N2 for 3 h.

Substrate Scope and Comparison with Other Methods. Moreover, several representative carbonyl compounds were also used as the starting materials. Similar to the furfural example, HMF as the dehydration product of C6 carbohydrates is also a very important platform chemical. The selective hydrogenation of the CO bond in HMF can generate 2,5bis(hydroxymethyl)furan (BHMF), which is useful in the preparation of resins, polymers, and artificial fibers. Notably, the γ-Fe2O3@HAP catalyst can also successfully promote the transfer hydrogenation of HMF into BHMF with a high selectivity of 92.1% (Table 2, entry 2). There were few reports on the transfer hydrogenation of HMF into BHMF with isopropanol as the hydrogen donor. Ethyl levulinate was also smoothly converted into γ-valerolactone (GVL) with a selectivity of 75% yield (Table 2, entry 3), and the side product was the transesterfacition of ethyl levulinate with isopropanol. Besides the reduction of biomass-derived chemicals, the commodity representatives aldehyde (benzaldehyde) and ketone (acetophenone) were converted to their corresponding alcohols with high efficiency (Table 2, entries 4 and 5). In particular, cinnamaldehyde selectively reduced its CO bond and retained its CC bond (Table 2, entry 6). Therefore, the γ-Fe2O3@HAP catalyst shows great potential for the transfer hydrogenation of carbonyl compounds. Catalyst Recycling Experiments. Besides the high catalytic activity, stability was also very important for the heterogeneous catalysts. Therefore, the recycle of the γFe2O3@HAP catalyst was also studied. The CTH of furfural by iso-propanol at 180 °C was used as the model reaction. Compared with other catalytic systems, the γ-Fe2O3@HAP catalyst in our catalytic system can be easily separated from the reaction mixture by an external magnet, followed by washing with water and ethanol, respectively, and dried, and then used directly in next run under the same reaction conditions. As shown in Figure 5, furfural conversion and furfuryl alcohol selectivity were almost kept stable during the recycling experiments. Thus, it suggested that the γ-Fe2O3@HAP catalyst showed a high stability during the CTH of furfural at 180 °C without the significant loss of its catalytic activity.

Figure 3. First-order kinetic fit for the hydrogenation of furfural to furfuryl alcohol at different temperatures. Reaction conditions: the γFe2O3@HAP catalyst (20 g), temperature (140−180 °C), furfural (1 mmol), speed of agitation (1000 rpm), iso-propanol (15 mL).

the transfer hydrogenation of furfural over γ-Fe2O3@HAP catalyst, in which C0 is the initial concentration of furfural, and Ct represents the concentration of furfural at certain reaction time point. As shown in Figure 3, the reaction was observed to have pseudo-first order kinetics for all studied temperatures, and the reaction rate constant was determined to be 0.02772, 0.12425, and 0.17532 min−1 for the temperatures 140, 160, and 180 °C, respectively. The variation of the rate constant (k) with temperature is inserted in Figure 3. According to the Arrhenius plot, the activation energy (Ea) was calculated to be 47.69 kJ mol−1 over the γ-Fe2O3@HAP catalyst for the CTH of furfural into furfuryl alcohol with iso-propranol as the hydrogen donor. There were few studies on the calculation of Ea for the hydrogenation of furfural into furfuryl alcohol. Time Course of the Transfer Hydrogenation of Furfural. The time course of the transfer hydrogenation of furfural into furfuryl alcohol was recorded. As depicted in Figure 4, the molar percentage of furfural gradually decreased during the reaction process, while the molar percentage of furfuryl alcohol gradually increased. It is observed that the trend of the furfural conversion rate was sharper at an early reaction stage than that at the latter reaction stage. This is due to the higher concentration of furfural at an early reaction stage, thus with a higher reaction rate. During the whole reaction process, the acetal product was determined as the only byproduct, and its yield was kept at a low level. The selectivity of furfuryl alcohol remained around 90%. These results indicated that furfuryl alcohol was stable under our reaction conditions without further reduction to other products or degradation. After 10 h, furfural conversion reached 96.2% at 180 °C, and furfuryl alcohol was produced in a yield of 91.7%.

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CONCLUSIONS In conclusion, a new and effective method was developed for the transfer hydrogenation of furfural into furfuryl alcohols. The 945

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Table 2. Transfer Hydrogenation of Different Carbonyl Compounds by iso-Propanol over γ-Fe2O3@HAP Catalysta

Reaction conditions: substrate (1 mmol) and γ-Fe2O3@HAP (40 mg) were added into iso-propanol (15 mL), and the reaction was performed at 180 °C for a certain time.

a

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AUTHOR INFORMATION

Corresponding Author

*Fax: +8627-67842752. Phone: +8627-67842752. E-mail address: [email protected]. ORCID

Zehui Zhang: 0000-0003-1711-2191 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chenguang Youth Science and Technology Project of Wuhan City (No. 2014070404010212). Figure 5. Recycling of the γ-Fe2O3@HAP catalyst. Reaction conditions: furfural (1 mmol) and γ-Fe2O3@HAP (40 mg) were added into iso-propanol (15 mL), and the reaction was carried out at 180 °C and 1 MPa N2 for 3 h.

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DOI: 10.1021/acssuschemeng.6b02272 ACS Sustainable Chem. Eng. 2017, 5, 942−947