Metal-Oxide-Catalyzed Efficient Conversion of Cellulose to Oxalic Acid

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Research Article pubs.acs.org/journal/ascecg

Metal-Oxide-Catalyzed Efficient Conversion of Cellulose to Oxalic Acid in Alkaline Solution under Low Oxygen Pressure Zhiwei Jiang, Zhanrong Zhang, Jinliang Song, Qinglei Meng, Huacong Zhou, Zhenhong He, and Buxing Han* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Conversion of cellulose into value-added chemicals and/or fuels has attracted worldwide attention due to the dwindling fossil fuel reserves and concerns over global warming. Herein, the conversion of microcrystalline cellulose into oxalic acid in homogeneous NaOH solution catalyzed by metal oxides under low oxygen pressure was reported. The effects of metal oxides, reaction temperature, reaction time, and oxygen pressure on the yields of the major products were studied. The results showed that a high yield of organic acids, mainly including oxalic acid, formic acid, glycolic acid, lactic acid, and acetic acid, could be obtained. Catalytic amounts of CuO could effectively improve the yield of oxalic acid. The yield of the oxalic acid could be as high as 41.5% with catalytic amount of CuO at oxygen pressure of 0.3 MPa and 200 °C for 2 h. A tentative reaction pathway for the selective oxidation of cellulose into small molecular organic acids in aqueous NaOH solution was investigated and proposed. KEYWORDS: Cellulose conversion, Oxalic acid, Metal oxides, NaOH aqueous solution

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INTRODUCTION In the pursuit of sustainable development and to address the increasingly excessive consumption of fossil fuel reserves, the effective valorization of naturally abundant renewable biomass represents a worldwide grand research challenge.1 Cellulose is the most abundant renewable carbon resource on the planet and has great potential to be used as an alternative feedstock for the production of biobased valuable platform molecules.2 The conversion of cellulose into value-added fuel and chemicals has received increasing attention.3−6 For instance, sorbitol and gluconic acid were produced from direct conversion of cellulose through catalytic hydrogenation and oxidation, respectively.7,8 5-Hydroxymethylfurfural (HMF) and its derivatives have been produced from catalytic dehydration of cellulose.9 Various metal catalysts, such as Zn, Ni, CuO, Pb2+,10−12 were employed to generate lactic acid from cellulose in aqueous solutions. Moreover, it has been reported that hydrothermal oxidation of cellulose could yield formic acid (FA) and glycolic acid under oxidative conditions.13,14 Cellulose exists mainly as aggregates due to its complex interand intramolecular hydrogen bonding network, which renders its depolymerization and conversion very difficult in most processes. Dissolution of cellulose in suitable solvents could promote the conversion of cellulose into low molecular weight compounds, as cellulose chains are well-extended and -dispersed in such conditions. In this context, ionic liquids and alkaline aqueous solutions are generally employed for dissolution of cellulose.15−17 For example, ionic liquids have been used as solvent to convert cellulose into HMF.18 Oxalic © XXXX American Chemical Society

and glycolic acids have been obtained from cellulose by alkali fusion or nitric acid oxidation processes.19 However, this process requires high concentrations of nitric acid (ca. 40 wt %), which were very costly and corrosive and require special reaction apparatus in terms of scaling up for industrial production. Traditionally, industrial production of oxalic acid from cellulose was performed by immersing cellulose in alkaline solutions (e.g., >16 N NaOH) at high temperatures.20,21 Also, it is known that homogeneous aqueous solution of cellulose could be formed in 6−10 wt % NaOH solution through a freezing− thawing process. This offers a potential opportunity to produce organic acids (e.g., oxalic acid, glycolic acid, lactic acid) through hydrothermal degradation of cellulose in its homogeneous solution under relatively mild reaction conditions. Oxalic acid (OA) is an industrially important chemical with various applications, such as the production of celluloid and rayon, leather manufacture and dressing, extraction of rare earths from monazite, etc.22 It was reported that OA could significantly promote the hydrolysis of lignocellulosic materials due to the presence of the two carboxylic functional groups, which mimics the structure of active sites in cellulase enzymes. Also, unlike sulfuric acid, oxalic acid was thermally decomposed into nontoxic molecules (i.e., CO2 and formic acid) in this process.23 Received: October 6, 2015 Revised: November 20, 2015

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DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Metal oxides, such as CuO and NiO,24,25 have been widely used to produce lactic acid and other organic acids from cellulose in alkaline medium. In these processes, the metal oxides were generally reduced, resulting in decreased catalytic reactivity and recycling abilities. Oxygen or air has been extensively explored as the oxidant for the copper-promoted or copper-catalyzed oxidation of cellulose.26,27 Many methods for pretreatment of lignocellulosic biomass have been widely employed due to its improvement of cellulose conversion and relatively low cost, including dilute acid, dilute alkali, and organosolvent methods.28−30 In this contribution, cellulose was dissolved in alkaline aqueous solution through a freezing− thawing process. A series of transition metal oxides were investigated as catalysts for the production of OA from microcrystalline cellulose (MCC) under oxidative conditions. In addition, the influences of metal oxides, reaction temperature, and reaction time on the yield of oxalic acid were investigated. Furthermore, a possible mechanism for the conversion of cellulose into oxalic acid is proposed.

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Research Article

RESULTS AND DISCUSSION Catalyst Screening. Microcrystalline cellulose was used as feedstock, and it was dissolved in aqueous NaOH solution to form a homogeneous solution. After that, the oxidative conversion of MCC was carried out at an oxygen pressure of 0.3 MPa and at 190 °C, resulting in a clear colorless solution. The HPLC analysis reveals that OA, LA, GOA, FA, and AA were the major products, as shown in Figure S1. Among these, FA is the most abundant product with yield of 20.3%, and the yields of OA, GOA, LA, and AA are 10.7%, 6.4%, 8.8%, and 9.8%, respectively. The total yield of the five acids is 56.1%. This indicated the successful conversion of cellulose under low oxygen pressure in its alkaline aqueous solution. In order to enhance the efficiency of cellulose oxidation and increase the yields of organic acids, the catalytic properties of various transition metal oxides including CuO, Fe2O3,Co2O3, MnO2, ZrO2, Bi2O3, La2O3, CeO2, Ni2O3, Cr2O3, Nb2O5, and ZnO were investigated for their effect on the production of organic acids, as summarized in Figure 1. It was found that both

MATERIALS AND METHODS

Materials. Microcrystalline cellulose (MCC), lactic acid (LA) (1.0 N), glucose (99%), and Nb2O5 were purchased from Alfa Aesar. Formic acid (FA) (>98%) was obtained from Fluka. Oxalic acid (0.1 N) was purchased from Acros. Acetic acid (1 N) was provided by Aladdin. NaOH, H2SO4 (98%), glycolic acid (GOA), acetic acid (AA) (99%), CuO, Fe2O3,Co2O3, MnO2, ZrO2, Bi2O3, La2O3, CeO2, Ni2O3, Cr2O3, Cu(OH)2, and ZnO were obtained from Sinopharm Chemical Reagent Co., Ltd. Cellulose (cotton linter pulp) with DP (degree of polymerization) = 410 was provided by Hubei Chemical Fiber Group Ltd. (Xiangyang, China). All these reagents were used as received without further purification. Catalytic Oxidative Conversion of Microcrystalline Cellulose. The dissolution procedure of cellulose in NaOH aqueous solution has been described in detail elsewhere.31,32 Basically, a certain amount of microcrystalline cellulose was subjected into aqueous NaOH solution (8 wt %), and the mixture was sharply stirred for 1 h at room temperature, resulting in a aqueous suspension of cellulose. This suspension was then transferred and kept in a freezer until it became a frozen solid. This frozen solid was thawed at room temperature with gentle stirring, resulting in a clear cellulose solution (ca. 0.01 g mL−1). Then 2 mL of the solution and desired amount of metal oxides were sealed into an autoclave of 10 mL equipped with a magnetic stirrer. The air in the reactor was removed by flashing O2 of 1 MPa three times, and then O2 pressure was charged to the desired pressure. The reaction mixture was stirred at a given temperature in an oil bath for a known time. After reaction, the reactor was immediately quenched in an ice−water bath. Analytical Methods. The liquid samples were collected by removing the solid catalyst via centrifugation, and analyzed without pretreatment by high performance liquid chromatography (HPLC, Shimadzu, LC-15C) equipped with UV and RID detectors, using a HPX-87H column and 5 mmol L−1 sulfuric acid aqueous solution as the mobile phase with a 0.6 mL min−1 flow rate at 55 °C. Solid samples were also collected and washed with distilled water several times, dried, and then characterized by X-ray diffraction (XRD, Rigaku D/max-2500) using Cu Kα radiation (λ = 0.154 06 nm) at a scanning rate of 5° min−1. The total organic carbon (TOC) of liquid products was quantified using a total organic carbon analyzer (TOC-L; Shimadzu). The concentration of transition metal ions was measured by inductively coupled plasma (ICP, PerkinElmer, Optima 2100DV). The yields of organic acids were calculated on the basis of the following equation. The reported values are averages of three times of reaction, and relative error was less than 5%. yield =

moles of carbon in organic acid × 100% moles of carbon in feedstock

Figure 1. Effect of various metal oxides on the yields of organic acids (reaction conditions: 0.02 g of MCC, 0.25 mmol of metal oxides, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h, 0.3 MPa O2).

CuO and Cr2O3 were effective for catalytic conversion of MCC into organic acids. In contrast, other metal oxides could not significantly increase and could even decrease the total organic acid yield. For the reaction catalyzed by CuO, the OA yield was significantly increased to 37.7% from 10.7% (without catalyst). The yields of LA and GOA decreased, and that of FA remained approximately unchanged. It has been reported that CuO can produce a hydroxo complex under alkaline hydrothermal conditions.33 We suggest that NaOH under hydrothermal conditions could improve the solubility of CuO to form a hydroxo complex that may enhance the formation of more organic acids from MCC conversion.25 In order to further explore the effect of Cu2+ on the conversion of MCC, Cu(OH)2 was used to catalyze the MCC conversion under the same conditions. The yield of oxalic acid is around 24.3%, indicating that Cu2+ could effectively promote the conversion of MCC into oxalic acid. The different yields of oxalic acid using CuO and Cu(OH)2 indicate that the crystal form of the Cu2+ compound also affects the conversion of MCC into oxalic acid. Hence, CuO was selected as catalyst for further optimization. Optimization of Reaction Conditions for Attaining High Yield of Oxalic Acid. With the aim of establishing

(1) B

DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Table 1. Effect of Reaction Temperature on the Yields of Organic Acids Derived from Microcrystalline Cellulosea yield, %

a

entry

temp, °C

conv, %

OA

GOA

LA

FA

AA

total

1 2 3 4 5 6 7 8

160 170 180 190 200 210 220 230

100 100 100 100 100 100 100 100

26.9 30.1 30.2 37.7 41.5 40.9 36.6 36.9

8.9 7.4 5.6 3.2 0.2 N.D.b N.D. N.D.

6.1 4.7 4.5 4.2 3.0 1.8 1.2 0.5

19.4 20.5 20.6 20.6 20.1 18.9 18.6 15.0

3.9 4.4 5.5 8.2 6.6 6.3 6.6 7.2

65.1 67.1 66.5 73.9 71.5 68.0 63.1 59.7

Reaction conditions: 0.02 g of MCC, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 2 h, 0.3 MPa O2. bN.D.: not detected.

optimum reaction conditions, effects of reaction temperature, time, O2 pressure, and amounts of CuO on the yield of organic acids were studied. Table 1 summarizes the yields of small molecular organic acids and their total yield obtained at various temperatures ranging from 160 to 230 °C. Initially, the total yield of organic acids increased with reaction temperature. Highest yield (73.9%) of total organic acids was obtained at 190 °C with OA of 37.7%, GOA of 3.2%, LA of 4.2%, FA of 20.6%, and AA of 8.2%. This result is possibly attributed to the significant changes in water properties such as ion product (kw) at higher temperature, but may also due to the further decomposition of carboxylic acid through decarboxylation at higher temperature.25 This hypothesis will be further discussed later. The yield of oxalic acid increased significantly from 26.9% at 160 °C to a maximum value of 41.5% at 200 °C. In contrast, the yields of lactic and glycolic acid decreased with reaction temperature. This indicates that higher temperature benefits the formation of oxalic acid. Because the formation of OA, FA, and AA from the decomposition of intermediates can compensate their decompositions, the highest yield of oxalic acid and no significant change in the total acid yield were observed from 190 to 200 °C. Both the yield of OA and that of total organic acids dropped with further increasing the reaction temperature from 200 to 230 °C. This may be attributed to the decomposition of generated oxalic acid. Figure 2 illustrates the effect of reaction time on the conversion of MCC into organic acids. The reactions were conducted in NaOH (2 N) with CuO (0.25 mmol) at 190 °C. It is obvious that the yield of oxalic acid increased with the reaction time and reached equilibrium after about 2 h (ca. 37.7%). This effect suggests that oxalic acid could be hardly degraded under this reaction condition. The yields of glycolic acid (15.8%) and lactic acid (7.7%) were relatively high after 0.5 h and decreased gradually with reaction time, indicating that these intermediates were converted to oxalic acid and/or other organic acids. However, as shown in Figure 2, the quantified total organic acids (OA, GOA, LA, FA, and AA) were about 20% lower than the TOC yield, indicating that there were other products present in the aqueous solution. These products may be attributed to side reactions. In the context of this paper the possible side reactions are not considered further. The effect of oxygen pressure ranging from 0 to 1.0 MPa on the yields of small molecular organic acids was studied, as summarized in Table 2. Reactions were conducted at 190 °C for 2 h. The yield of oxalic acid was only around 3.6% in the absence of oxygen and increased sharply to around 31.8% (0.1 MPa, O2), suggesting oxygen plays a key role in the conversion of MCC into oxalic acid. These results indicate that some

Figure 2. Effect of reaction time on the yields of organic acids (reaction conditions: 0.02 g of MCC, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 0.3 MPa O2).

intermediates, such as GOA and LA, could further convert into OA in the present of oxygen. It is noteworthy that the most abundant product was lactic acid in the absence of O2, which was consistent with the results reported by Wang et al.25 However, in our work, the yields of lactic acid (20.8%) and total organic acids (43.1%) are much higher than those reported previously (around 12.5% for lactic acid and 35.2% for total acids).25 This confirms that the dissolution of cellulose in aqueous sodium hydroxide could significantly improve the accessibilities of cellulosic chains for the catalysts, as the hydrogen bond network in the compact cellulose molecules was disrupted and the chains were well-dispersed in the solution.34 Further increasing oxygen pressure resulted in declined oxalic acid yield, around 34.5% at 0.5 MPa and 31.3% at 1.0 MPa. This effect indicates that oxalic acid probably degraded under high oxygen pressures. A similar trend was observed for glycolic acid and lactic acid. For instance, when the oxygen pressure is 0.1 MPa, the yields for glycolic acid and lactic acid are around 12.1% and 8.9%, respectively. At higher pressures (e.g., 1.0 MPa), the yields for these acids are only around 3.4−4.8%. As discussed above, these intermediates such as glycolic acid and lactic acid were further degraded to small molecular organic acids (e.g., FA and AA) under high oxygen pressure. To investigate the effect of CuO amount on the yields of organic acids, we carried out MMC conversion with NaOH (2 N) at 190 °C for 2 h in oxygen (0.3 MPa), with various amounts of CuO (Figure 3). The yield of oxalic acid was only C

DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 2. Effect of Oxygen Pressure on the Yields of Organic Acidsa yield, %

a

entry

PO2, MPa

conv, %

OA

GOA

LA

FA

AA

total

1 2 3 4 5

0 0.1 0.3 0.5 1.0

100 100 100 100 100

3.6 31.8 37.7 34.5 31.3

7.8 12.1 3.2 1.9 3.4

20.8 8.9 4.2 4.4 4.8

8.5 14.5 20.6 22.5 16.3

2.4 8.2 8.2 5.7 7.4

43.1 75.4 73.9 68.9 63.2

Reaction time: 0.02 g of MCC, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h.

Figure 4. XRD patterns of CuO before and after the reaction (reaction conditions: 0.02 g of MCC, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h, 0.3 MPa O2).

Figure 3. Effect of the amount of CuO on the yields of organic acids (reaction conditions: 0.02 g of MCC, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h, 0.3 MPa O2).

catalyst was reused 3 times (Figure 5). This indicates that the CuO was quite stable as catalyst for converting cellulose under the given reaction conditions.

around 10.7% in the absence of CuO. Addition of catalytic amounts of CuO into the reaction system could significantly promote the formation of oxalic acid. A maximum yield around 37.7% was obtained with 0.25 mmol of CuO. Further increase in the amounts of CuO results in a slightly lower yield of OA. In addition, the yield of GOA gradually decreased from 6.4% in the absence of CuO to around 1.5% (0.5 mmol CuO). The yield of lactic acid decreased from 8.8% (0 mmol CuO) to 4.8% (0.125 mmol), and remained constant afterward. The yields of formic acid and acetic acid fluctuate with various amounts of CuO, since CuO could not only catalyze the conversion of MCC, but may also affect the decomposition of these acids under the given reaction conditions. However, this hypothesis needs to be further studied. During the reaction, CuO could be reduced into Cu2O or Cu in aqueous NaOH solution in the absence of oxygen.25 To investigate the CuO behavior in the presence of oxygen in our study, the structures of collected solid residues and CuO were characterized with XRD (Figure 4). No significant structural changes before/after the reaction could be observed, suggesting that CuO was not reduced to Cu or Cu2O. In the presence of oxygen, the crystal structure of CuO was maintained, and oxygen prevented it from reducing. Further, the concentration of Cu2+ ions in the solution was less than 1 ppm by the ICP characterization. It is noteworthy that, without CuO and oxygen, oxalic acid cannot be formed. Hence, Cu2+ is crucial for the generation of oxalic acid from MCC in this reaction. To investigate the reusability of CuO, the solid residue was collected, washed, and dried in oven at 100 °C. The decrease in the yield of total organic acid was not considerable after the

Figure 5. Reusability of CuO for MCC conversion (reaction conditions: 0.02 g of MCC, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h, 0.3 MPa O2).

Mechanism Investigation. In this work, various sugar derivatives including monosaccharaides (i.e., glucose and fructose) and disaccharide (i.e., cellobiose), were used as substrates under the same reaction conditions used for MCC conversion (Table 3). Similar yields of oxalic acid were obtained from glucose (34.1%), fructose (35.4%), and D

DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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carboxylic acids were used as starting materials under the same reaction conditions. According to the literature, there are two main degradation pathways for glucose, C3−C4 scission, and C2−C3 scission.36 Glyceraldehyde, an important intermediate formed by C3−C4 scission of glucose, has good selectivity toward lactic acid in alkali solution. Besides, glycolaldehyde as another intermediate via C2−C3 scission could be further oxidized to glycolic acid. The organic acid intermediates derived from both reaction pathways such as glycolic and lactic acid could be further decomposed into smaller molecular organic acids under alkaline conditions.37 As shown in Table 3, under the given reaction conditions, lactic and glycolic acids were degraded to smaller acids. The conversion of LA and GOA reached 51.7% and 84.5%, respectively. In addition, only FA and OA could be derived from glycolic acid, whereas LA could also generate AA. From previous discussion, oxalic and formic acids accounted for the major portion of degradation products of MCC under the same reaction conditions. This result indicates that it is relatively easy to generate glycolic acid from alkaline solution of MCC. Pyruvic acid, as a possible derivative of lactic acid, shows a similar reaction profile to that of lactic acid (Table 3, entry 3). The stabilities of the five small molecular organic acids derived from MCC under the given reaction condition were investigated (Table 3, entries 1, 2, 7, 8, 9). For these produced mainly organic acids, they can further decompose into smaller organic acids or gases by decarboxylation under hydrothermal conditions.37 It was demonstrated that the decomposed order was glycolic acid > lactic acid ≫ acetic acid > oxalic acid > formic acid, indicating glycolic acid could be more effectively converted in comparison with the others. On the basis of the above results, a plausible pathway for MCC conversion into these five organic acids is proposed, as shown in Figure 6. Cellulose was initially hydrolyzed into

Table 3. Yields of Organic Acids Derived from Various Substratesa yield, % entry

substrates

conv, %

OA

GOA

1 2 3 4 5 6 7 8 9 10

LA GOA pyruvic acid cellobiose glucose fructose FA AA OA celluloseb

51.7 84.5 100 100 100 100 0.3 4.0 1.0 100

25.0 51.1 31.4 34.4 34.1 35.4 N.D. 0.5

N.D.c

35.1

N.D. 1.8 0.6 1.0 N.D. N.D. N.D. 3.7

LA N.D. 2.0 11.7 21.9 23.1 N.D. N.D. N.D. 7.5

FA

AA

3.4 15.0 4.8 16.5 13.4 13.3

6.1 N.D. 26.5 7.3 7.5 7.2 N.D.

0.5 N.D. 20.1

0.2 7.9

a

Reaction conditions: 0.02 g of substrates, 0.25 mmol of CuO, 2.0 mol L−1 NaOH, 2.0 mL of water, 190 °C, 2 h, 0.3 MPa O2. bCellulose (cotton linter pulp) with DP = 410. cN.D.: not detected.

cellobiose (34.4%) in comparison with that derived from cellulose (37.7%). Interestingly, the yields of glycolic and formic acid increased from monosaccharide, to disaccharide, to polysaccharide (cellulose), whereas the yield of lactic acid decreased from monosaccharide to polysaccharide. These results suggested that the degree of polymerization may affect the reaction pathway and the yields of organic acids. A kind of regular cellulose (cotton linter pulp) with DP = 410 as the starting material was also investigated and a similar result compared with MCC listed in Table 3, entry 10. Cellulose chains were hydrolyzed and depolymerized to form cellobiose followed by further decomposition into glucose and/ or fructose under hydrothermal conditions.34,35 To investigate the mechanism of cellulose conversion in NaOH aqueous solution, some possible intermediates from cellulose such as

Figure 6. Plausible reaction pathway for cellulose conversion into organic acids (R1, retro-aldol condensation; R2, keto−enol tautomerization; R3, elimination of water; R4, benzilic acid rearrangement; R5, base-catalyzed oxidation by water). E

DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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glucose under alkaline conditions. Because NaOH under hydrothermal conditions can enhance the solubility of CuO by the formation of hydroxo complex,33 dissociated Cu2+ ions from the hydroxo complex may coordinate with hydroxyl oxygen atoms of glucose to form a comparatively stable coordination compound.25 Subsequently, glucose was further converted into glyceraldehyde (C3−C4 scission of glucose, pathway 1) and glycolaldehyde (C2−C3 scission of glucose, pathway 2). Besides, glyceraldehyde could convert to formaldehyde and glycol aldehyde through retro-aldol condensation. In pathway 1, glyceraldehyde could generate pyruvaldehyde through elimination of water, which could rapidly convert into LA through benzilic acid rearrangement with CuO.11 LA was then oxidized under alkaline conditions to pyruvic acid and 2hydroxyacrylic acid. 2-Hydroxyacrylic acid is an isomerization product of pyruvic acid formed by the Lobry de Bruyn−Alberda van Ekenstein (LBAE) transformation. Subsequently, 2hydroxyacrylic acid could be converted to glyceric acid under alkali conditions,36 which then undergoes base-catalyzed oxidation to form 2-hydroxy-3-oxopropanoic acid. 2-Hydroxy3-oxopropanoic can further convert into formaldehyde and glycoxylic acid via retro-aldol condensation. Finally, formaldehyde was oxidized to FA, and glycoxylic acid was oxidized to OA in the present with oxygen. Pyruvic acid could also be directly converted to acetic acid. In pathway 2, glycolic acid was derived from glycol aldehyde via base-catalyzed oxidation in NaOH solution. GOA could be further oxidized to glycoxylic acid with CuO in the presence of oxygen, which could produce FA and OA via two different pathways. This is consistent with the experimental results discussed above (see Table 3, entry 2).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China and Chinese Academy of Sciences for financial support. This work was supported by the National Natural Science Foundation of China (21003133, 21321063), and the Chinese Academy of Sciences (KJCX2.YW.H30).

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(1) Tuck, C. O.; Pérez, E.; Horváth, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of biomass: Deriving more value from waste. Science 2012, 337 (6095), 695−699. (2) Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A. Cellulose: Fascinating biopolymer and sustainable raw material. Angew. Chem., Int. Ed. 2005, 44 (22), 3358−3393. (3) Op de Beeck, B.; Dusselier, M.; Geboers, J.; Holsbeek, J.; Morre, E.; Oswald, S.; Giebeler, L.; Sels, B. F. Direct catalytic conversion of cellulose to liquid straight-chain alkanes. Energy Environ. Sci. 2015, 8 (1), 230−240. (4) Wang, A.; Zhang, T. One-pot conversion of cellulose to ethylene glycol with multifunctional tungsten-based catalysts. Acc. Chem. Res. 2013, 46 (7), 1377−1386. (5) Wang, F.-F.; Liu, J.; Li, H.; Liu, C.-L.; Yang, R.-Z.; Dong, W.-S. Conversion of cellulose to lactic acid catalyzed by erbium-exchanged montmorillonite K10. Green Chem. 2015, 17 (4), 2455−2463. (6) Zhou, C.-H.; Xia, X.; Lin, C.-X.; Tong, D.-S.; Beltramini, J. Catalytic conversion of lignocellulosic biomass to fine chemicals and fuels. Chem. Soc. Rev. 2011, 40 (11), 5588−5617. (7) Lucilia, S. R.; Orfao, J. J. M.; Manuel Fernando, R. P. Enhanced direct production of sorbitol by cellulose ball-milling. Green Chem. 2015, 17 (5), 2973−2980. (8) An, D.; Ye, A.; Deng, W.; Zhang, Q.; Wang, Y. Selective conversion of cellobiose and cellulose into gluconic acid in water in the presence of oxygen, catalyzed by polyoxometalate-supported gold nanoparticles. Chem. - Eur. J. 2012, 18 (10), 2938−2947. (9) Nandiwale, K. Y.; Galande, N. D.; Thakur, P.; Sawant, S. D.; Zambre, V. P.; Bokade, V. V. One-pot synthesis of 5-hydroxymethylfurfural by cellulose hydrolysis over highly active bimodal micro/ mesoporous H-ZSM-5 catalyst. ACS Sustainable Chem. Eng. 2014, 2 (7), 1928−1932. (10) Zhang, S.; Jin, F.; Hu, J.; Huo, Z. Improvement of lactic acid production from cellulose with the addition of Zn/Ni/C under alkaline hydrothermal conditions. Bioresour. Technol. 2011, 102 (2), 1998−2003. (11) Choudhary, H.; Nishimura, S.; Ebitani, K. Synthesis of highvalue organic acids from sugars promoted by hydrothermally loaded Cu oxide species on magnesia. Appl. Catal., B 2015, 162, 1−10. (12) Wang, Y.; Deng, W.; Wang, B.; Zhang, Q.; Wan, X.; Tang, Z.; Wang, Y.; Zhu, C.; Cao, Z.; Wang, G.; Wan, H. Chemical synthesis of lactic acid from cellulose catalysed by lead(II) ions in water. Nat. Commun. 2013, 4, 2141. (13) Xu, J.; Zhang, H.; Zhao, Y.; Yang, Z.; Yu, B.; Xu, H.; Liu, Z. Heteropolyanion-based ionic liquids catalysed conversion of cellulose into formic acid without any additives. Green Chem. 2014, 16 (12), 4931−4935. (14) Zhang, J.; Liu, X.; Sun, M.; Ma, X.; Han, Y. Direct conversion of cellulose to glycolic acid with a phosphomolybdic acid catalyst in a water medium. ACS Catal. 2012, 2 (8), 1698−1702. (15) Zhang, L.; Ruan, D.; Gao, S. Dissolution and regeneration of cellulose in NaOH/thiourea aqueous solution. J. Polym. Sci., Part B: Polym. Phys. 2002, 40 (14), 1521−1529. (16) Cai, J.; Zhang, L. Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions. Macromol. Biosci. 2005, 5 (6), 539−548.

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CONCLUSION In summary, it was demonstrated that cellulose could be efficiently converted into organic acids including oxalic acid, formic acid, glycolic acid, lactic acid, and acetic acid, in its homogeneous alkaline solution under low oxygen pressure. It was found that CuO was much more active than the other metal oxides, and the yield of OA could reach 41.5% in the homogeneous alkaline solution of microcrystalline cellulose with catalytic amount of CuO (0.25 mmol) at 200 °C and 0.3 MPa oxygen with a reaction time of 2 h. The presence of CuO and oxygen is crucial for the generation of high yields of OA. Also, no significant structural changes of CuO were observed before and after the reaction, as evidenced by the XRD analysis. According to the obtained results, we proposed a plausible pathway for the conversion of microcrystalline cellulose into the five major organic acids (OA, FA, GOA, LA, and AA). In particular, OA was mainly generated through oxidation of GOA.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01212. Typical HPLC chromatograms of the liquid sample after the reaction (PDF)

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*Phone: +86-10-62562821. E-mail: [email protected]. F

DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.5b01212 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX