ARTICLE pubs.acs.org/JPCC
Catalytic Aerobic Oxidation of Renewable Furfural with Phosphomolybdic Acid Catalyst: an Alternative Route to Maleic Acid Huajun Guo and Guochuan Yin* School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China ABSTRACT: Developing new technologies to obtain chemicals from biomass in place of the fossil feedstock have attracted attention in academic and industrial communities. In this work, using renewable furfural as the feedstock, catalytic aerobic oxidation of furfural to maleic acid was investigated with phosphomolybdic acid catalyst in the aqueous/organic biphase system. The oxidation happens in the aqueous phase, and the organic phase serves as the reservoir to release the substrate gradually through phase equilibrium. Under the optimized conditions, 34.5% yield of maleic acid could be obtained with 68.6% of selectivity, and the conversion of furfural is 50.4%. Because furfural and maleic acid dominantly exist in two different phases, the product separation and reactant recycle would be very simple in its potential application. The FT-IR and 31P NMR technologies were applied to characterize the phosphomolybdic acid catalyst, and the pathway of maleic acid formation was also discussed based on obtained mechanistic information. This work demonstrates an alternative, renewable route to maleic acid, and the mechanistic information from this study also provides clues to improve the catalyst for efficient oxidation of furfural to maleic acid.
’ INTRODUCTION Biomass is the largest, annually renewable carbon resources on the earth. With diminishing fossil resources, developing new technologies to utilize versatile biomass as the alternative feedstock of energy and chemical sources has been attracting more and more attention than ever.14 In deed, biofuel, manufactured from the transesterification of plant and/or animal oil, has been added as additives in gasoline for automobiles in many countries. Due to that its productions are competitive with foods of human beings and animals, current interests have been focused on exploring new technologies to obtain the replacements of the fossil feedstock from the nonfood biomass of human beings, and versatile catalysts have been tested to obtain the chemical building blocks from cellulose, lignin and hemicellulose.511 Furfural, a C5 compound, is industrially manufactured for a long time through hydrolysis of pentose which comes from agricultural raw materials including corncobs, oat, wheat bran, sawdust, etc.12 Importantly, these materials are annually renewable and not competitive with human beings. Thus, exploring the subproducts from furfural as the replacements of fossil resources is greatly attractive. In fact, utilization of furfural as the starting material could synthesize a variety of chemicals including more than 1600 commercial products. Compared with furfural, maleic acid and fumaric acid are the C4 starting materials which are extensively applied in resins, surface coatings, lubricant additives, plasticizers, copolymers, and agricultural chemicals. Currently, both maleic acid and fumaric acid are manufactured from aerobic oxidation of benzene, butane, or butadiene, which are typically fossil oil products.13 In 1949, Neilsen even introduced a gas vapor oxidation of furfural to maleic acid at elevated temperature (above 270 °C).14 However, r 2011 American Chemical Society
its commercialization to replace the current fossil oil routes has not been reported until now, and no further investigation was reported. Recently, we demonstrated an example of maleic acid synthesis through liquid oxidation of furfural using oxygen as the oxidant. Using the combination of copper(II) salts with phosphomolybdic acid, aerobic oxidation of furfural in aqueous solution could provide maleic acid as the expected product.15 Due to the seriously competitive polymerization of furfural to resins which makes the color of the reaction mixture generally turn dark, the selectivity of maleic acid product is low to modest, even though a large amount of catalyst was used. Importantly, the mechanism of maleic acid formation is unknown until now. In the present work, we introduce a biphase system for furfural oxidation in which phosphomolybdic acid catalyst alone could provide maleic acid with improved selectivity, and the catalytic mechanism has been discussed.
’ MATERIALS AND METHODS All of the reagents are analytic purity grade, and were purchased from local Sinopharm Chemical Reagent, except furan from TCI. FT-IR analysis was performed on Bruker EQUINOX 55, 31P NMR was performed on Bruker AV 400 with H3PO4 (85%) as external standard, and the product identifications by HPLC-MS were performed on Agilent 1100 LC/MSD. General Procedure for the Furfural Oxidation with Phosphomolybdic Acid Catalyst. In a general oxidation reaction,
0.24 mmol of phosphomolybdic acid was dissolved in 2 mL of Received: June 11, 2011 Revised: August 1, 2011 Published: August 03, 2011 17516
dx.doi.org/10.1021/jp2054712 | J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C
ARTICLE
Table 1. Catalytic Efficiency of Furfural Oxidation with Phosphomolybdic Acid in Biphase Systema percentage of furfural in the aqueous phase conversion yield entry
cosolvent
(%)b
1 2 nitrobenzene
21.9
3 tetrachloromethane
Figure 1. Percentage distribution of furfural in water/tetrachloroethane biphase system (v/v, 2/0.8) at different temperatures. Conditions: H3PMo12O40 3 xH2O, 0.2 mmol; furfural, 2.4 mmol; H2O, 2 mL; temperature, 383 K; O2, 20 atm; time, 14 h.
water and then mixed with 2.4 mmol of furfural in 0.8 mL of tetrachloroethane in a glass tube equipped with a glass cap having holes. The glass tube was put into a 50 mL stainless autoclave. Then the autoclave was charged with 20 atm of oxygen. The resulting reaction mixture was magnetically stirred at 383 K for 14 h in an oil bath. The quantitative product analysis with an internal standard sample (acrylic acid) was performed by HPLC equipped with a UV detector and a C18 column (250 4.6 mm), and the products were further confirmed by HPLC-MS. The mobile phase was methanol with water (v/v 10%: 90%) containing KH2PO4 buffer (0.01 M) at 1 mL/min. The pH of the mobile phase was adjusted to pH 2.4 with H3PO4. Determination of the Ratio of Furfural in the Water/ Tetrachloroethane Biphase System. In a glass tube, 2.4 mmol of furfural in 0.8 mL of tetrachloroethane was mixed with 2 mL of water. The mixture solution was magnetically stirred at a designed temperature in the oil bath for 1 h. Then the solution was left in the oil bath for an additional 0.5 h without stirring. Next, the tetrachloroethane phase was separated from the biphase system with a 1 mL syringe, and the quantitative analysis of furfural in the tetrachloroethane solution was performed by HPLC. Pretreatment of the Phosphomolybdic Acid Catalyst for FT-IR Analysis. After a typical oxidation reaction, the autoclave was cooled and oxygen was released. Then, the phosphomolybdic acid catalyst in the aqueous solution was extracted with ethyl ether. The generated heteropoly etherate was separated, and ethyl ether was removed with a blow drier. Then the obtained solid sample was analyzed by FT-IR using the KBr pellet method.
’ RESULTS AND DISCUSSION Distribution of Furfural in Aqueous/Organic Biphase System. Because furfural is relatively unstable when exposed to air,
its competitive polymerization to resins is the dominant side reaction for its selective oxidation to maleic acid; thus, one may hypothesize that the decrease of the concentration of furfural substrate in the catalytic solution may help to reduce the furfural polymerization. To efficiently control the concentration of furfural in the reaction solution, in present studies, an aqueous/organic biphase system was applied for furfural oxidation with phosphomolybdic acid as the catalyst and oxygen as the
selectivity
(%)
(%)
(%)
86.2
38.1
44.2
66.9
37.8
56.4
74.5
38.3
51.4
4 toluene
27.9
73.4
37.1
50.0
5 nitromethane
36.1
54.2
14.7
27.2
6 p-xylene
27.8
66.2
34.8
52.6
7 cyclohexane 8 tetradecane
35.4
84.6 82.4
38.2 38.3
45.1 46.5
9 tetrachloroethane
16.4
50.7
30.7
60.6
a
Conditions: H3PMo12O40 3 xH2O, 0.2 mmol; furfural, 2.4 mmol; H2O, 2 mL; co-solvent, 0.8 mL; temperature, 383 K; O2, 20 atm; reaction time, 14 h. b The percentage distribution of furfural in the biphase system was measured at 383 K; due to their low boiling points, the data for tetrachloromethane and cyclohexane cosolvents are not measured.
terminal oxidant. Since phosphomolybdic acid is water-soluble but insoluble in organic phase like tetrachloroethane, the catalytic oxidation happens in the aqueous phase. With the oxidation proceeding in the biphase system, the exhaustion of furfural substrate in the aqueous phase would drive the mass transfer of furfural from the organic phase to the aqueous phase gradually, and the concentration of furfural in the aqueous phase always remains at a low level. As shown in Figure 1, in the water/ tetrachloroethane biphase system, furfural will dominantly exist in the organic phase with a low concentration in the aqueous phase. Increasing the temperature just slightly improves the concentration of furfural in the aqueous solution. For example, at 298 K, the percentage of furfural in the organic phase is 90.1%, and it is only 9.9% in the aqueous phase, whereas at 371 K, they are 83.6% in the organic phase and the remaining 16.4% exists in the aqueous phase. The low concentration of furfural in the aqueous phase of the biphase system could be expected to compress the polymerization of furfural during catalytic oxidation, and it does as demonstrated below. Catalytic Oxidation of Furfural in Different Biphase Systems. Using phosphomolybdic acid as the catalyst, the catalytic oxidations were carried out in a stainless reactor in an oil bath at 383 K with charged 20 atm of oxygen, and the results from a list of the biphase combinations are summarized in Table 1 with the percentage of furfural substrate occurring in the aqueous phase. Without the organic cosolvent, the yield of maleic acid is 38.1% with 44.2% of selectivity, and the conversion of furfural is as high as 86.2%. When adding organic cosolvent to generate a biphase system, the selectivity of maleic acid was apparently improved with the reduced conversion of furfural in most cases. In the water/nitrobenzene system, the yield of maleic acid is 37.8% with 56.4% of selectivity, and conversion of furfural is 66.9% which is about 20% lower than that in the purely aqueous medium. Apparently, the addition of nitrobenzene does not affect the catalytic oxidation of furfural to maleic acid but substantially compress its polymerization due to the low concentration of furfural in the aqueous phase (the percentage of furfural in the aqueous phase is 20.9%). In the water/tetrachloroethane system, the selectivity of maleic acid could be improved up to 60.6% with 17517
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C
Figure 2. Influence of tetrachloroethane content on furfural oxidation. Conditions: H3Mo12O40P 3 xH2O, 0.2 mmol; furfural, 2.4 mmol; H2O, 2 mL; temperature, 383 K; O2, 20 atm; time, 14 h.
30.7% of yield, and the conversion of furfural is 50.7% (the percentage of furfural in the aqueous phase is only 16.4%), supporting the fact that the low concentration of the furfural substrate in the reaction medium does help to improve maleic acid selectivity as expected. The extremely low selectivity of maleic acid (14.7%) with nitromethane as cosolvent is possibly related to the fact that it is partially soluble in water which may have changed the properties of the aqueous medium. After the reaction, the color of the reaction mixture is deep-blue, and no black precipitate (resins) with other oxidation products such as fumaric acid and 2-furioc acid was observed. When using other heteropoly acids including phosphotungstic acid, silicomolybdic acid, and silicotungstic acid as catalyst, only a minor amount of maleic acid was obtained. In the control experiment without catalyst in water/tetrachloroethane, the conversion of furfural is 26.1%, and the yield of maleic acid is only 4.6% with 17.5% selectivity, implying that the phosphomolybdic acid catalyst is critical for selective oxidation of furfural to maleic acid. The higher catalytic efficiency of phosphomolybdic acid could be attributed to its much higher redox potential than other heteropoly acids.16 In the control experiment using pressured argon in place of oxygen, phosphomolybdic acid alone provides 1.4% of maleic acid, and after the reaction, the bright yellow color of the fresh phosphomolybdic acid turns to blue, a typical color of heteropoly blue,17 implying that phosphomolybdic acid alone could serve as an oxidant to oxidize maleic acid and then reduce itself to heteropoly blue. Catalytic Oxidation of Furfural under Different Conditions. The ratio of water/tetrachloroethane in the biphase system was next investigated for the phosphomolybdic acid catalyzed furfural oxidation. According to the furfural distribution in the water/tetrachloroethane biphase system (Figure 1), increasing the content of tetrachloroethane in the reaction media would decrease the content of furfural in the aqueous phase, which would lead to a reduction in the furfural conversion with the improvement of maleic acid selectivity. Indeed, as shown in Figure 2, with the change of the ratio of water/tetrachloroethane from 2/0.2 to 2/1.2 (v/v), the selectivity of maleic acid could be improved from 48% to 62.2%, whereas the conversion of furfural drops from 73.6% to 37.1%. Since the temperature of the reaction media would affect the furfural distribution in the biphase system as shown in Figure 1, raising the reaction temperature would increase its fraction in the
ARTICLE
Figure 3. Influence of the reaction temperature on furfural oxidation. Conditions: H3PMo12O40 3 xH2O, 0.24 mmol; furfural, 2.4 mmol; H2O, 2 mL; tetrachloroethane, 0.8 mL; O2, 20 atm; time, 14 h.
Figure 4. Influence of the catalyst content on furfural oxidation. Conditions: catalyst H3PMo12O40 3 xH2O; furfural, 2.4 mmol; H2O, 2 mL; tetrachloroethane, 0.8 mL; temperature, 383 K; O2, 20 atm; time 14 h.
aqueous phase. Figure 3 displays the influence of the reaction temperature on the catalytic efficiency. With the reaction temperature goes up, all of the furfural conversion, maleic acid yield, and selectivity increase at the beginning. When the temperature goes up to above 383 K, the furfural conversion and maleic acid yield keep increasing, whereas the selectivity of maleic acid decreases. At 383 K, the conversion of furfural and the yield and selectivity of the maleic acid are 50.3%, 34.5%, and 68.6%, respectively, whereas they are 87.6%, 47%, and 53.3% at 403 K (at this temperature, 3% of fumaric acid was observed), respectively, indicating that the competitive polymerization becomes slightly serious at the elevated temperature due to the increased furfural concentration in the aqueous solution. The influence of the catalyst content was further investigated, and the results are demonstrated in Figure 4. At 383 K, all of the furfural conversion, maleic acid yield, and selectivity increase parallelly with the increase of the catalyst content, indicating that the phosphomolybdic acid catalyst is not involved in furfural polymerization and solely serves as the oxidation catalyst for maleic acid formation, otherwise, if phosphomolybdic acid was involved in furfural polymerization, the selectivity of maleic acid would decrease with the increase of the catalyst loading. For example, with the catalyst loading of 0.12 mmol, the conversion of furfural is 35.2% and yield of maleic acid is 16.7% with 47.3% of 17518
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C selectivity, representing that 18.5% of the furfural substrate was polymerized to resins under the reaction conditions, whereas at the catalyst loading of 0.24 mmol, the conversion of furfural is 50.3%, and the yield of maleic acid is 34.5% with 68.6% of selectivity, representing 15.8% of furfural substrate was polymerized to resins, which is slightly lower than that at lower catalyst loading. Thereby, these results strongly support that phosphomolybdic acid is an excellent oxidation catalyst for selective oxidation of furfural to maleic acid, even that the noncatalyzed polymerization of furfural, which is independent of catalytic oxidation, still occurs under the current reaction conditions. The time course of furfural oxidation is demonstrated in Figure 5. With the reaction time changing from 2 to 14 h, the selectivity of maleic acid keeps stable with obvious improvement. At the reaction time of 2 h, the yield of maleic acid is 3.3% with selectivity of 55.4%, and the conversion of furfural is 5.9%, while, after 14 h of reaction, the yield and selectivity of maleic acid are 34.5% and 68.6%, respectively, and the conversion of furfural is 50.3%. The improved selectivity of maleic acid with the reaction time extending could be rationalized by that the concentration of furfural substrate would gradually decrease with the conversion increase, which would compress the competitive polymerization of furfural as well as the application of the organic cosolvent. The constant catalytic selectivity also suggests that the phosphomolybdic acid catalyst remains stable during the reaction period.
Figure 5. Time course of furfural oxidation with phosphomolybdic acid catalyst. Conditions: H3PMo12O40 3 xH2O, 0.24 mmol; furfural, 2.4 mmol; H2O, 2 mL; tetrachloroethane, 0.8 mL; O2, 20 atm; temperature, 383 K.
ARTICLE
In the independently quantitative analysis of furfural and maleic acid in the aqueous and tetrachloroethane phases after the reaction, it was found that maleic acid product dominantly exists in the aqueous phase because of its high solubility, whereas the furfural substrate dominantly exists in the organic phase. For example, at the furfural conversion of 50.3% with maleic acid yield of 34.5% and 68.6% of selectivity, the remaining furfural in the organic phase is 48.6% with only 2.8% occurring in the aqueous phase, whereas the maleic acid in the aqueous phase is 34% with only 0.5% in the organic phase. Therefore, this biphase system becomes extremely attractive for its advantages in product separation and reactant recycling in its potentially industrial application. As shown in Figure 6, after the reaction, through simple oilwater separation, the remaining furfural substrate can be directly recycled in the organic phase and then pumped back to the reactor for next reaction cycle; meanwhile, after separation of maleic acid from the aqueous phase, the left aqueous solution containing catalyst may also be pumped back to the reactor for the next cycle. FT-IR and 31P NMR Characterization of Phosphomolybdic Acid. The IR spectra of phosphomolybdic acid before and after the reaction are displayed in Figure 7. Before the reaction, the IR spectra of the fresh phosphomolybdic acid demonstrates the characteristic bands of Keggin structure at 1062 cm1 representing the stretching of the PdO, at 961 cm1 representing the stretching of the ModOd, at 869 cm1 representing the stretching of the MoObMo bridges (“inter” oxygen bridges between corner-sharing octahedra), and at 781 cm1 representing the stretching of the MoOcMo bridges (“intra” oxygen bridges between edge-sharing octahedra).18,19 After the reaction, the relative intensities of the PdO at 1062 cm1 and the MoObMo bridges at 869 cm1 sharply decrease in comparison with those of the ModOd and the MoOcMo bridges (the stretching of the MoOcMo bridges shifts from 781 cm1 to 805 cm1 after reaction). The interpretation of the decreased strength of the PdO and MoObMo stretching are controversial in the literature. In some works it was explained as the removal of the bridge oxygen, but later it was proposed as being due to the reduction of the heteropoly acid to heteropoly blue and the Keggin structure was not changed.2023 The 31P NMR spectra of the present phosphomolybdic acid catalyst before and after reaction are displayed in Figure 8. Before the reaction, the chemical shift at 3.8 ppm is a typical NMR signal of H3PMo12O40 3 xH2O;24 the presence of a few peaks with low intensity can be attributed to the little impurities in the commercial
Figure 6. Simplified flow line of catalytic oxidation of furfural with phosphomolybdic acid catalyst in the biphase system. 17519
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C
ARTICLE
Scheme 1. Proposed mechanism for furfural oxidation catalyzed by phosphomolybdic acid
Figure 7. FT-IR spectrum of phosphomolybdic acid catalyst before and after reaction.
Figure 8. reaction.
31
P NMR of phosphomolybdic acid catalyst before and after
phosphomolybdic acid. After the reaction, the peak at 3.8 ppm almost disappears, and a new peak occurring at 6.5 ppm can be attributed to the reduction of Mo6+ to Mo5+ in heteropoly blue. A small peak at ca. 12.8 ppm may be related with little degradation of the Keggin structure. In the case of the Keggin structure, the framework decomposed to form smaller active intermediates in, for example, olefin epoxidations, several peaks of 31P NMR could be observed and assigned to the different subunits.2528 Thus, the decreased intensities of the PdO stretching and the MoObMo bridge stretching does not suggest the decomposition of phosphomolybdic acid but represents its reduction to the corresponding heteropoly blue, which is also consistent with the stable selectivity of maleic acid during the long time of catalytic performance (see Figure 5). Mechanistic Discussion of the Furfural Oxidation. In the literature, the peroxomolybdate species are frequently proposed in phosphomolybdic acid catalyzed oxidation,25,26 and hydrogen peroxide was even reported as oxidant for furfural and furan oxidations.27,28 In this work, hydrogen peroxide was also tested in place of oxygen as the terminal oxidant for furfural oxidation; however, the yields of maleic acid are only 1.8% at room temperature after 39 h of reaction, 3.3% at 343 K after 14 h, or 1.4% at 383 K after 14 h under 20 atm argon, which are much lower than these from the reaction using oxygen as the oxidant. On the other side, as stated above, the fresh phosphomolybdic acid alone can convert furfural to maleic acid under argon and reduces itself to heteropoly blue. Apparently, the peroxomolybdate species
does not serve as the key active intermediates in the furfural oxidation with phosphomolybdic acid catalyst. In oxidation of furfural to maleic acid, one carbon atom needs to be removed from reactant (eq 1). There are two potential pathways to lose one carbon atom through decarboxylation: one is decarboxylation prior to furan ring-opening, and the other is vice versa. In the literature, several publications have introduced that oxidation of furan with hydrogen peroxide or oxygen could provide maleic acid as one of the oxidation products.2931 To test whether 2-furioc acid and furan are the oxidation intermediates for the maleic acid formation in this work, 2-furioc acid and furan are separately used as the substrate for oxidation under the identical conditions of furfural oxidation. The obtained yields of maleic acid are only 1.0% from 2-furioc acid and 2.0% from furan, respectively. Therefore, neither 2-furioc acid nor furan is the intermediate of maleic acid formation under this phosphomolybdic acid mediated furfural oxidation; thus, the furan ringopening should be prior to the decarboxylation. C5 H4 O2 þ 2O2 f C4 H4 O4 þ CO2
ð1Þ
In the autopolymerization of furfural when exposed to oxygen, oxygen first abstracts a hydrogen atom from the 5-position of furfural to generate a furfural radical which attacks the CdO bond of another furfural molecule to initialize the polymerization.12 Similarly, the maleic acid formation through the furan ringopening pathway could start from the same furfural radical intermediate as it in polymerization, and a plausible mechanism has been proposed in Scheme 1. After the first hydrogen abstraction by either oxygen or phosphomolybdic acid to generate the furfural radical, compound 1, it can initialize polymerization to resins or proceed to further electron transfer to phosphomolybdic acid to generate the furfural cation, compound 2. Next, attacking on the furfural cation by H2O forms compound 3 17520
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C which may proceed through 1,4 rearrangement to generate compound 4 or 1,2 rearrangement to form compound 6. Compound 4 is next hydrolyzed to form compound 5 which can be feasibly oxidized to maleic acid. In the other route, hydrolysis of compound 6 forms compound 7 which proceeds through next 1,2 rearrangement to form the compound 8. Oxidation of compound 8 would generate succinic acid. However, in the present studies, no succinic acid was observed in product analysis, thus, 1,4 rearrangement of compound 3 to form compound 4 would be much more energetically preferable than the corresponding 1,2 rearrangement to compound 6. Hydroquinone is a common radical scavenger to terminate the reaction having the radical intermediate.32,33 Interestingly, here, the yield of maleic acid (34.1%) was not affected by addition hydroquinone as radical scavenger, and the conversion of furfural even increased up to 61.5%. The invalidity of hydroquinone could be rationalized by the fact that phosphomolybdic acid could rapidly oxidize hydroquinone and reduce itself to heteropoly blue as observed in experimental tests. As shown in Scheme 1, the furfural radical is the shared intermediate for both polymerization and maleic acid formation. To improve the yield of maleic acid, efficient trapping of the furfural radical intermediate to generate the next furfural cation is most crucial. Copper(II) cation is a well-known carbon radical trapper frequently applied in oxidation reactions, and heteropoly acids are also a good electron transfer agent.3437 Thus, in our earlier studies performed in the purely aqueous solution, copper(II) plus phosphomolybdic acid serves as a good catalyst candidate for furfural oxidation to maleic acid; however, due to the high concentration of furfural in the purely aqueous system, the polymerization of furfural was not efficiently prohibited, which leads to the low selectivity of maleic acid. Here, using a biphase system to reduce the furfural concentration in the aqueous phase where the catalytic oxidation happens, phosphomolybdic acid alone can serve as the efficient catalyst to provide better maleic acid selectivity than the previous copper(II) / phosphomolybdic acid system. The much higher efficiency of phosphomolybdic acid than other heteropoly acids including phosphotungstic acid, silicomolybdic acid, and silicotungstic acid in furfural oxidation is consistent with its higher redox potential which facilitates the electron transfer from the furfural radical to phosphomolybdic acid.
’ CONCLUSIONS An aqueous/organic biphase system has been explored for furfural oxidation, and the catalytic oxidation happens in the aqueous phase when phosphomolybdic acid was used as the catalyst. The low concentration of furfural in the aqueous phase helps to improve the selectivity of maleic acid. Under the optimized conditions, oxidation of furfural could provide 34.5% yield of maleic acid with 68.6% of selectivity, and the conversion of furfural is 50.3%. Although the conversion of furfural is relatively low, the dominant existence of the furfural substrate in the organic phase with the same dominant existence of maleic acid product in the aqueous phase could benefit the feasible product separation and reactant recycle after the reaction. During the oxidation process, the Keggin structure of phosphomolybdic acid keeps stable as evidenced by FT-IR and 31P NMR. In maleic acid formation, the furan ring-opening is prior to decarboxylation, which has been supported by independent 2-furioc acid and furan oxidation tests. A plausible mechanism based on our
ARTICLE
experimental results has been proposed for this phosphomolybdic acid catalyzed furfural oxidation. The present studies have provided new clues on exploring novel catalytic technologies to obtain maleic acid with its derivatives from the renewable furfural in place of the current fossil feedstock.
’ AUTHOR INFORMATION Corresponding Author
*Fax: 86-27-87543632. Phone: 86-27-87543732. E-mail: gyin@ hust.edu.cn.
’ ACKNOWLEDGMENT The fund from the Project of the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is deeply appreciated. The 31P NMR experiments and the product identification through HPLC-MS were performed in Analytical and Testing Center, Huazhong University of Science and Technology. ’ REFERENCES (1) Collinson, S. R.; Thielemans, W. Coord. Chem. Rev. 2010, 254, 1854–1870. (2) Bridgwater, A. V. Appl. Catal. A: Gen. 1994, 116, 5–47. (3) Climent, M. J.; Corma, A.; Iborra, S. Green Chem. 2011, 13, 520–540. (4) Alonso, D. M.; Bond, J. Q.; Dumesic, J. A. Green Chem. 2010, 12, 1493–1513. (5) Huber, G. W.; Chheda, J. N.; Barrett, C. J.; Dumesic, J. A. Science 2005, 308, 1445–1450. (6) Binder, J. B.; Raines, R. T. J. Am. Chem. Soc. 2009, 131, 1979–1985. (7) Ji, N.; Zhang, T.; Zheng, M.; Wang, A.; Wang, H.; Wang, X.; Chen, J. Angew. Chem., Int. Ed. 2008, 47, 8510–8513. (8) Yan, N.; Zhao, C.; Luo, C.; Dyson, P. J.; Liu, H.; Kou, Y. J. Am. Chem. Soc. 2006, 128, 8714–8715. (9) Fukuoka, A.; Dhepe, P. L. Angew. Chem. 2006, 118, 5285–5287. (10) Roy, D.; Subramaniam, B.; Chaudhari, R. V. Catal. Today 2010, 156, 31–37. (11) Du, Z.; Ma., J.; Wang, F.; Liu, J.; Xu, J. Green Chem. 2011, 13, 554–557. (12) Zeitsch, K. J., Ed.; The Chemistry and Technology of Furfural and Its Many By-products; Elsevier: Amsterdam, 2000. (13) Felthouse, T. R.; Burnett, J. C.; Horrell, B.; Mummey, M. J.; Kuo, Y. Kirk-Othmer Encycl. Chem. Technol. (5th Ed.) 2005, 15, 481-523. (14) Nielsen, E. R. Ind. Eng. Chem. 1949, 41, 365–368. (15) Shi, S.; Guo, H.; Yin, G. Catal. Comm. 2011, 12, 731–733. (16) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171–198. (17) Varga, G. M., Jr.; Papacoxstantinou, E.; Pope, M. T. Inorg. Chem. 1970, 9, 662–667. (18) Rocchiccioli-deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Inorg. Chem. 1983, 22, 207–216. (19) Venkateswara Rao, K. T.; Rao, P. S. N.; Nagaraju, P.; Sai Prasad, P. S.; Lingaiah, N. J. Mol. Catal. A: Chem. 2009, 303, 84–89. (20) Tsuneki, H.; Niiyama, H.; Echigoya, E. Chem. Lett. 1978, 7, 645–648. (21) Akimoto, M.; Echigoya, E. Chem. Lett. 1981, 10, 1759–1762. (22) Eguchi, K.; Toyozawa, Y.; Furuta, K. Chem. Lett. 1981, 10, 1253–1256. (23) Mizuno, N.; Katamura, K.; Yoneda, Y.; Misono, M. J. Catal. 1983, 83, 384–392. (24) Rob van Veen, J. A.; Sudmeijer, O.; Emeis, C. A.; de Wit, H. J. Chem. Soc., Dalton Trans. 1986, 1825–1831. (25) Gao, J.; Chen, Y.; Han, B.; Feng, Z.; Li, C.; Zhou, N.; Gao, S.; Xi, Z. J. Mol. Catal. A: Chem. 2004, 210, 197–204. 17521
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
The Journal of Physical Chemistry C
ARTICLE
(26) Chen, Y.; Zhuang, J.; Liu, X.; Gao, J.; Han, X.; Bao, X.; Zhou, N.; Gao, S.; Xi, Z. Catal. Lett. 2004, 93, 41–46. (27) Salles, L.; Piquemal, J.; Thouvenot, R.; Minot, C.; Brkgeault, J. J. Mol. Catal. A: Chem. 1997, 117, 375–387. (28) Karimi, Z.; Mahjoub, A. R.; Aghdam, F. D. Inorg. Chim. Acta 2009, 362, 3725–3730. (29) Poskonin, V. V.; Badovskaya, L. A.; Povarova, L. V. Chem. Heterocycl. Compd. 1998, 34, 900–906. (30) Poskonin1, V. V. Chem. Heterocycl. Compd. 2009, 45, 1177–1183. (31) Badovskaya, L. A.; Povarova, L. V. Chem. Heterocycl. Compd. 2009, 45, 1023–1034. (32) Budnik, R. A.; Kochi, J. K. J. Org. Chem. 1976, 41, 1385–1389. (33) Wu, S.; Liu, W.; Jiang, X. J. Org. Chem. 1994, 59, 854–857. (34) Yin, G.; Piao, D.; Kitamura, T.; Fujiwara, Y. Appl. Organomet. Chem. 2000, 14, 438–442. (35) Kochi, J. K.; Subramanian, R. V. J. Am. Chem. Soc. 1965, 87, 4855–4866. (36) Jenkins, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1972, 94, 856–865. (37) Weinstock, I. A. Chem. Rev. 1998, 98, 113–170.
17522
dx.doi.org/10.1021/jp2054712 |J. Phys. Chem. C 2011, 115, 17516–17522
