Catalytic Oxidation of Lignite to Carboxylic Acids in Aqueous

Mar 6, 2017 - State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China. ‡ Department ...
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Catalytic Oxidation of Lignite to Carboxylic Acids in Aqueous H5PV2Mo10O40/H2SO4 Solution with Molecular Oxygen Fan Yang,† Yucui Hou,‡ Muge Niu,† Ting Lu,† Weize Wu,*,† and Zhenyu Liu† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

‡

S Supporting Information *

ABSTRACT: Carboxylic acids (CAs) are considered as an important group of chemicals and are widely used in the chemical industry. The production of CAs via oxidation of lignite is a promising industrial process. In the present work, we found that H5PV2Mo10O40 (HPA) was beneficial to catalytic oxidation of lignite by O2 in aqueous H2SO4 solutions to produce CAs. The effects of HPA concentration, H2SO4 concentration, reaction temperature, initial O2 pressure, and reaction time on lignite conversion and CA yield were studied. The catalytic oxidation of lignite in aqueous HPA/H2SO4 solutions with O2 can yield 56.9 wt % CAs, including 33.5 wt % formic acid, 14.4 wt % acetic acid, 1.5 wt % succinic acid, 1.1 wt % oxalic acid, and 6.4 wt % benzene carboxylic acids (BCAs, including 12 types) at 170 °C and 3 MPa for 60 min. In this catalyst system, the existence of H2SO4 can change the catalytic activity of HPA, and the synergistic effect of HPA and H2SO4 can significantly promote the production of CAs. VV can oxidize lignite or intermediates to form VIV, which can be reoxidized by O2 to complete a redox cycle. In the catalytic oxidation, lignite was converted into water-soluble intermediates at first, and then the water-soluble intermediates were converted to CAs. The catalyst system was reused four times without significant decline in activity. Compared with the traditional alkali-oxygen oxidation, this method can decrease the usage of mineral acid and alkali and lower reaction temperatures. Franke et al.29 studied an alkali-oxygen oxidation of several coals at a NaOH/coal mass ratio of 6.71/1 and 250 °C and obtained about 45% (based on carbon) yield of water-soluble CAs. In our previous work,20,21 the oxidation of lignite was carried out in an aqueous alkali solution at an alkali/coal mass ratio of 3/1 and a temperature of 240 °C and yielded about 20 wt % BCAs and 40 wt % small-molecule fatty acids. Obviously, a large amount of alkali was consumed in the alkali-oxygen oxidation, and the reaction temperatures are high up to 270 °C. After oxidation, mineral acids are used to neutralize the alkali solution to obtain CAs.23,30 Consequently, the mineral acids are also consumed in a large amount. The alkali and mineral acid used in the process are very difficult to recover and result in serious environmental pollution. Furthermore, the high reaction temperature leads to high energy consumption. These disadvantages may limit its application. Therefore, it is necessary to search reusable catalysts that can efficiently oxidize lignite to CAs at lower temperature with a relatively high selectivity of products. In recent years, an efficient and environmentally friendly catalytic system using a heteropolyacid as a catalyst to produce formic acid from biomass was proposed. In 2011, Wolfel et al.31 proposed a new method to transform carbohydrate-based biomass to formic acid with oxidation by O2 in aqueous solution using a Keggin-type H5PV2Mo10O40 (HPA) polyoxometalate as catalyst. Some complex biomass mixtures, such as poplar wood sawdust, were transformed to formic acid with yields up to 19 wt %. In 2012, Li et al.32 used the same catalyst

1. INTRODUCTION As petroleum resources are drying up, coal as a substitute of petroleum is drawing more and more attention.1,2 Lignite (a low rank coal) has been identified as an inferior fuel because of its high oxygen content, high moisture content, high ash yield, low calorific value, and easy spontaneous combustion.3−6 Therefore, it is meaningful to develop a new method to utilize lignite based on its features. The high oxygen content of lignite is beneficial to the formation of carboxylic acids (CAs), including benzene carboxylic acids (BCAs, whose structures are shown in Scheme 1) and small-molecule fatty acids. CAs are important chemicals widely used in industrial manufacturing. Due to the significant potential for the oxidation of lignite to produce CAs, extensive studies have been carried out. Classified by the types of oxidants, the lignite oxidation methods mainly include hydrogen peroxide oxidation,7−9 permanganate oxidation,10,11 dichromate oxidation,12 oxidative acid oxidation,10,12−15 ruthenium-ion-catalyzed oxidation,16,17 sodium hypochlorite oxidation,16,17 and O2 oxidation.18−24 For the first six methods, oxidants are expensive and unsafe during the process, leading to the limitation of their application. Since O2 is inexpensive, easily available, and suitable as an oxidant, the oxidation of lignite in aqueous solutions using O2 was reasonably accepted. Coal oxidation in aqueous alkaline solutions by O2 (called alkali-oxygen oxidation) is the widely used method.24−28 Currently, in the processes of alkali-oxygen oxidation, alkali/ coal mass ratios of 2/1−7/1 are required with corresponding temperatures from 200 to 270 °C. For example, Zhang et al.23 carried out an alkali-oxygen oxidation of Huolinhe lignite at a temperature of 240 °C, a alkali/coal mass ratio of 3/1, and a reaction time of 30 min and obtained 22.5 wt % yield of BCAs. © XXXX American Chemical Society

Received: December 27, 2016 Revised: February 22, 2017 Published: March 6, 2017 A

DOI: 10.1021/acs.energyfuels.6b03479 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Scheme 1. Structure of BCAs Obtained from Selective Oxidation of Coala

a

Abbreviations: BPA, benzene pentacarboxylic acid; IPA, isophthalic acid; PMA, pyromellitic acid; TPA, terephthalic acid. 98.0%), and mellitic acid (AR, 98.0%) were purchased from Aladdin Chemicals Co., Ltd., Shanghai, China. Nitrogen (N2, 99.999%) and oxygen (O2, 99.995%) were supplied by Beijing Beiwen Gases Co., Ltd. (Beijing, China). All reagents were analytical grade and used without further purification. According to Tsigdions and Hallada’s work,37 HPA was prepared by using a standard method. The synthesized HPA was characterized in our previous work.36 2.2. Oxidation. The oxidation of lignite was carried out in a highpressure batch reactor made from Hastelloy (HC276), supplied by Haian Petroleum Scientific Research Co., Ltd., Jiangsu, China. The reactor has an inner volume of 25 cm3 and is equipped with a magnetic stirrer. A typical experiment included 0.1 g of lignite and 6.0 cm3 of catalyst solution. The reactor was purged with O2 or N2 and then filled with the same gas to a desired pressure. Then, the reactor was submerged into a heating furnace and heated at 8−12 °C/min to a desired reaction temperature controlled by a temperature controller and monitored by a K-type thermocouple with an accuracy of ±1 °C. When a desired temperature was reached, the reaction time was recorded. During the reaction, the mixture was stirred at a constant speed of 500 rpm. After the reaction, the reactor was transferred into a cool water bath to stop the reaction. When the reactor temperature reached room temperature, the reaction solution and residues remaining in the reactor were transferred into a beaker. The residues were filtered, and the liquid sample was diluted to 50 cm3 for further analysis. The residues were washed with distilled water and dried in air. 2.3. Recycle of the HPA/H2SO4 Catalyst System. The product solution was extracted with butanone of 3-fold volume to obtain CAs. HPA and H2SO4 did not dissolved in butanone. At last, a small amount of butanone dissolved in the liquid solution was swept out by N2. Due to a small amount of water dissolved in butanone during extraction, more water was added to the recovered catalytic solution to keep its original volume before the next reaction. 2.4. Analysis of Products. Liquid samples were injected into a high-performance liquid chromatograph (HPLC, Waters 2695, USA) for analysis of composition. BCAs and small-molecule fatty acids in samples were analyzed using different methods. A binary gradient elution procedure was used for HPLC analysis of BCAs. The mobile phase was acetonitrile and 0.1% (volume fraction) phosphoric acid aqueous solution, and the stationary phase was C18 bonded by silica gel (Waters Xbrige C18, 5 μm). A UV detector at 235 nm was used to quantify the products. The mobile phase flow rate was 1 cm3/min, and the column temperature was 35 °C. The gradient elution procedure was as follows: first, the volume ratio of acetonitrile to phosphoric acid aqueous solution was 5:95; the ratio was increased to 20:80 linearly over 10 min and then maintained for 2 min, and finally, the ratio was decreased to 5:95 over 2 min. The small-molecule fatty acids were determined with a Shodex SH 1011 column and a refractive index detector (Waters 4110, USA). The column oven temperature was 55 °C, and the mobile phase was an aqueous solution with 0.1 wt % H2SO4 at a flow rate of 0.5 cm3/min. The carbon content of liquid samples was determined with total organic carbon analyzer (TOC, Shimadzu TOC-L CPN, Japan).

(HPA) for the conversion of cellulose to yield formic acid. Zhang et al.33 reported that the oxidation of cellulose in an aqueous HPA solution with O2 yields 45% formic acid after 3 h. Khenkin et al.34 used HPA as a catalyst to oxidize phenethyl alcohol and obtained 41% yield of benzoic acid. Sarma et al.35 transformed benzene to phenol with HPA as the catalyst in the presence of O2. In our previous work,36 we found that a binary catalyst of Keggin-type heteropolyacid HPA/H2SO4 was efficient in oxidizing biomass cellulose to formic acid using O2 as an oxidant in aqueous solution. After 5 min at 180 °C, a 61% yield of formic acid could be obtained. The addition of H2SO4 increased the yield of formic acid and reduced the reaction time. The above studies indicated that HPA shows high catalytic activities for the conversion of biomass and the oxidation of aromatic compounds. The structure of lignite consists of large amounts of oxygen containing functional groups (like biomass structure) and aromatic structures. It is possible that HPA can be used in the oxidation of lignite to produce CAs. Therefore, in the present work, the oxidation of lignite was carried out using aqueous HPA solutions with O2. The effect of reaction parameters on lignite conversion and CA yield was studied. The reaction mechanism was explored. Lastly, the reusability of the catalyst system was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Xiaolongtan lignite (XL) from Yunnan, China, was used in this study. Its proximate and ultimate analyses are shown in Table 1. The lignite was pulverized to pass through a 200 mesh sieve

Table 1. Proximate and Ultimate Analyses of Lignitea proximate analysis (wt %)

ultimate analysis (wt %), in daf basis

lignite

Mad

Ad

Vdaf

C

H

Ob

N

S

XL

16.4

14.5

50.7

73.7

3.9

20.1

1.3

1.0

a

XL, Xiaolongtan lignite; ad, air-dry basis; d, dry basis; daf, dry-andash-free basis; M, moisture; A, ash; V, volatile matter content. bBy difference. before use. Sodium metavanadate (NaVO3, AR, 99.0%), disodium hydrogen phosphate (Na2HPO4, AR, 99.0%), concentrated sulfuric acid (H2SO4, AR, 98.0%), sodium molybdate dihydrate (Na2MoO4, AR, 99.0%), and butanone (AR, 99.0%) were purchased from Beijing Chemical Plant, China. Formic acid (AR, 98.0%), acetic acid (AR, 98.0%), succinic acid (AR, 99.5%), oxalic acid (AR, 99.5%), benzoic acid (AR, 98.0%), phthalic acid (AR, 98.0%), IPA (AR, 98.0%), TPA (AR, 98.5%), hemimellitic acid (AR, 99.0%), trimellitic acid (AR, 98.0%), trimesic acid (AR, 98.0%), PMA (AR, 96.0%), BPA (AR, B

DOI: 10.1021/acs.energyfuels.6b03479 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels The conversion of lignite and yields of carboxylic acids were calculated based on the organic matter in lignite using the following formulas.

yield (CAs) = (mass of CAs)/(mass of organic matter in lignite) × 100% conversion (lignite) = (1 − carbon mass in residue /carbon mass in lignite) × 100% where the mass of organic matter in lignite = mass of lignite − mass of ash − mass of moisture.

3. RESULTS AND DISCUSSION 3.1. Effect of HPA Concentration on Lignite Conversion and CA Yield. Figure 1 shows the effect of HPA

Figure 2. Effect of H2SO4 concentration on lignite conversion and CA yield. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %; temperature, 170 °C; initial O2 pressure, 3 MPa; reaction time, 60 min.

It also can be seen from Figure 2 that, with increasing H2SO4 concentration, the CA yield increases and the maximum value can be obtained at a H2SO4 concentration of 1.5 wt %. As we know, the reactivity and selectivity of HPA is related to the reduction potential and the participation of electron transfer from the substrate to HPA.36,42 The H2SO4 concentration influences the catalytic activity of HPA in two aspects. First, the increase in H+ concentration in aqueous solution is beneficial to the formation of protonated HPA, as shown in eq 1.43 H5PV2Mo10O40 + H3O+ ⇆ H6PV2Mo10O40+ + H 2O

(1)

In our previous work, we found that the reduction potential of protonated HPA is higher than that of HPA; namely, with the increase of H2SO4 concentration, the oxidation capability increases and promotes the generation of products.36 On the other hand, there exists an equilibrium system in aqueous HPA solution, as shown in eqs 2 and 3.44

Figure 1. Effect of HPA concentration on lignite conversion and CA yield. Reaction conditions: lignite, 0.1 g; H2SO4 concentration, 1.5 wt %; temperature, 170 °C; initial O2 pressure, 3 MPa; reaction time, 60 min.

[PV2 VMo10O40 ]5 − + H+ ⇆ [HPVVMo10O38]5 − + VO2+

concentration on lignite conversion and CA yield. In the absence of HPA, the conversion of lignite and the yield of CAs are both low. The lignite conversion and CA yield increase rapidly when HPA is employed. The result indicates that HPA can promote both the conversion of lignite and the production of CAs. With further increasing HPA concentration, the lignite conversion reaches 100%, and a maximum CA yield of 56.9 wt % (including 33.5 wt % formic acid, 14.4 wt % acetic acid, 1.5 wt % succinic acid, 1.1 wt % oxalic acid, and 6.4 wt % BCAs (the distribution of each BCA is shown in Figure S1)) is obtained when the HPA concentration is 1.25 wt %. It can be seen from Figure 1 that the yield of CAs decreases when the concentration of HPA exceeds 1.25 wt %. The possible reason is that the number of catalytic active species (the catalytic active species will be discussed in the following section) is increased with the increase of HPA concentration, causing the increase of oxidation capability. Thus, a deep oxidation of CAs occurs, resulting in lower CA yields. 3.2. Effect of H 2SO 4 Concentration on Lignite Conversion and CA Yield. In the process of homogeneous catalytic oxidation, pH of solution plays a key role on the oxidation.38−41 In the present work, the effect of H2SO4 concentration on lignite conversion and CA yield has been studied, and the result is shown in Figure 2. It can be seen in Figure 2 that the concentration of H2SO4 has no influence on lignite conversion in the studied H2SO4 concentrations. In other words, the presence of HPA makes the conversion of lignite easy.

(2) V

5−

[PV2 Mo10O40 ]

4−

V

⇆ [PV Mo10O37 ]

+

VO3−

(3)

In a strong acidic aqueous solution, there are large amounts of [HPVVMo10O38]5− and VO2+ instead of [PVVMo10O37]4− and VO3−.36,44 Therefore, based on eq 2 with the increase of H2SO4 concentration in aqueous solution, the amount of [PV2VMo10O40]5− decreases, while the concentration of VO2+ increases. In the aqueous solution, [PV2VMo10O40]5− and VO2+ can both act as active species of the HPA, but the VO2+ has a reduction potential higher than that of [PV2VMo10O40]5−.45 The corresponding pH of aqueous HPA solution as a function of H2SO4 concentration is shown in Table 2. It can be seen that Table 2. Corresponding pH Values of Aqueous HPA Solutions with Different H2SO4 Concentrations H2SO4 (wt %) pH

0 1.75

0.50 1.32

1.0 0.95

1.50 0.63

2.0 0.52

2.5 0.47

the pH of the aqueous solution decreases with an increase in H2SO4 concentration. Based on the above analysis, it can be concluded that a decrease in pH promotes the generation of protonated HPA and active species VO2+. Therefore, with an increase in H2SO4 concentration, the CA yield increases. With further increase of H2SO4 concentration (exceeding 1.5 wt %), the CA yield decreases. It can be seen that at a high H2SO4 C

DOI: 10.1021/acs.energyfuels.6b03479 Energy Fuels XXXX, XXX, XXX−XXX

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of lignite increases and reaches 100% at 3 MPa. At the same time, the yield of CAs reaches the maximum at 3 MPa. It was reported that the oxidation of coal in aqueous solutions was a first-order reaction with respect to O2 concentration in the aqueous solutions.48 As we know, O2 concentration in an aqueous solution is proportional to O2 pressure in the gas. Hence, an increase in O2 pressure can increase the oxidation rate of lignite. On the other hand, an increase in O 2 concentration can facilitate electron transfer of catalyst from the reduction state to the oxidation state and complete the catalytic oxidation cycle of the catalyst, promoting the generation of products.49 When the initial O2 pressure exceeds 3 MPa, the yield of CAs decreases with an increase in initial O2 pressure, especially in the yield of formic acid. In our previous work, with increasing initial O2 pressure, the stability of formic acid was reduced, resulting in a decreased yield of formic acid.38,47 3.5. Effect of Reaction Time on Lignite Conversion and CA Yield. Figure 5 shows the effect of reaction time on

concentration, there are a large amount of protonated HPA and VO2+, and the stability of CAs is reduced, which results in deep oxidation of CAs and then a decrease in the yield of CAs. 3.3. Effect of Reaction Temperature on Lignite Conversion and CA Yield. Figure 3 shows that both lignite

Figure 3. Effect of temperature on lignite conversion and CA yield. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %;H2SO4 concentration, 1.5 wt %; initial O2 pressure, 3 MPa; reaction time, 60 min.

conversion and CA yield are low at low temperatures, and they increase with increasing temperature. This result indicates that at low temperatures the catalytic activity of HPA is low and cannot promote the conversion of lignite and the generation of CAs efficiently. So a suitable temperature is necessary for the catalytic oxidation. With the increase in temperature, the catalytic activity of HPA increases, and lignite conversion and CA yield are both increased. A maximum yield of CAs is obtained at a temperature of 170 °C. With further increasing temperature, the stability of CAs is reduced, resulting in deep oxidation and decomposition. Therefore, the yield of CAs is decreased with further increase of temperature, as shown in Figure 3. 3.4. Effect of Initial O2 Pressure on Lignite Conversion and CA Yield. In oxidation with O2 as the oxidant, initial O2 pressure plays a key role in the reaction.21,38,46,47 In the present work, the initial O2 pressure was also investigated. Figure 4 shows the effect of initial O2 pressure on the conversion of lignite and the yield of CAs. At a low initial O2 pressure of 1 MPa, the lignite conversion and CA yield are both low. With increasing initial O2 pressure from 1 to 3 MPa, the conversion

Figure 5. Effect of reaction time on lignite conversion and CA yield. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %; H2SO4 concentration, 1.5 wt %; temperature, 170 °C; initial O2 pressure, 3 MPa.

lignite conversion and CA yield. It can be seen in Figure 5 that lignite conversion reaches 91.7 and 100% in 15 and 30 min, respectively. The result indicates that the oxidation of lignite in HPA/H2SO4 is very fast. The yield of CAs increases with an increase in time and reaches the maximum CA yield of 56.9 wt % in 60 min. The above result suggests that there exist watersoluble intermediates in the process of oxidation (water-soluble intermediates will be discussed in section 3.7). As the reaction time is further increased and extended to more than 60 min, the yield of CAs decreases (shown in Figure 5). The possible reason is that the decomposition of CAs occurred, leading to the decrease of the CA yield. 3.6. Reuse of the HPA/H2SO4 Catalyst System. In a catalytic reaction, the reusability of the catalyst system is important to evaluate the performance of the catalyst system. Figure 6 shows that the lignite conversion and CA yield of the HPA/H2SO4 system change little in four cycles, indicating that it has good reusability under the applied conditions in our work. 3.7. Reaction Pathways and Mechanism of Catalytic Oxidation. Figure 7 shows the carbon yields of products formed in oxidation of lignite as a function of time. In the figure, there are carbon balances of the products and residues for the reactions at various conditions; that is, the yield of carbon in reaction liquid + yield of carbon in CO2 + yield of carbon in residue is equal to 100%, where the yield of carbon in

Figure 4. Effect of initial O2 pressure on lignite conversion and CA yield. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %; H2SO4 concentration, 1.5 wt %; temperature, 170 °C; reaction time, 60 min. D

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converted into CAs. In the process of catalytic oxidation, CO2 is formed throughout the whole oxidation and increases with time (see Figure 7d). During the oxidation of lignite in aqueous HPA/H2SO4 solution, HPA, H2SO4, and O2 play important roles. In order to investigate the mechanism of catalytic oxidation of lignite in aqueous HPA/H2SO4 solution, the effects of HPA, H2SO4, and O2 on the conversion of lignite were studied. The results of lignite conversion to CAs in different conditions are shown in Table 3, where CA yields were calculated on a carbon basis. Table 3. Conversion of Lignite to CAs at Different Reaction Conditionsa

Figure 6. Reuse of aqueous HPA/H2SO4 solution. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %; H2SO4 concentration, 1.5 wt %; temperature, 170 °C; initial O2 pressure, 3 MPa; reaction time, 60 min.

entry atmosphere 1 2 3 4 5 6 7 8

the reaction liquid is equal to the yield of carbon in CAs + yield of carbon in water-soluble intermediates. Figure 7a shows lignite conversion in an aqueous HPA solution with O2 as oxidant, and it only needs 30 min to reach 100%. At this time, the carbon yield in liquid solution reaches the maximum value (see Figure 7b), but the yield of CAs does not reach the maximum value at 30 min, whereas it reaches the maximum yield at 60 min (see Figure 7c). The result indicates that, in the reaction system, water-soluble intermediates exist in the process of catalytic oxidation. Based on the results in the literature,4,8 we can speculate that water-soluble intermediates mainly include naphthalene carboxylic acids, biphenyl carboxylic acids, alkyl benzene carboxylic acids, alkanetricarboxylic acids or alkenetricarboxylic acids, and alkanedioic acids. In other words, at first, lignite is converted into water-soluble intermediates, and then the water-soluble intermediates are

N2 N2 N2 N2 O2 O2 O2 O2

H2SO4 (wt %)

HPA (wt %)

conversion (%)

CA yieldb (%)

selectivity (%)

0 1.5 0 1.5 0 1.5 0 1.5

0 0 1.25 1.25 0 0 1.25 1.25

11.4 23.8 21.5 30.4 48.4 65.7 100 100

2.1 3.1 7.1 9.4 17.5 27.7

9.6 10.1 14.7 14.4 17.5 27.7

Reaction conditions: lignite, 0.1 g; temperature, 170 °C; initial O2 pressure, 3 MPa; reaction time, 60 min. bCA yield was calculated on a carbon basis. a

When only N2 existed in the system (in absence of O2, HPA, and H2SO4), the lignite conversion was only 11.4% and no CAs were produced (entry 1, Table 3). When only H2SO4 was added into the reaction system in the presence of N2, the lignite conversion increased to 23.8% but no CAs were yielded (entry

Figure 7. Yield of carbon in different substances from the oxidation of the lignite: (a) coal residues, (b) liquid solution, (c) CAs, and (d) CO2. Reaction conditions: lignite, 0.1 g; HPA concentration, 1.25 wt %; H2SO4 concentration, 1.5 wt %; temperature, 170 °C; initial O2 pressure, 3 MPa. E

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of 8.0 and 9.6 wt %, respectively. Verheyen et al.14 investigated the oxidation of Australian Victorian brown coal with peroxytrifluoroacetic acid at conditions of 0.1 g coal and 1 cm3 of peroxytrifluoroacetic acid, and obtained about 30 wt % CAs. Zhang et al.11 studied the oxidation of a Chinese lignite with potassium permanganate at conditions of a potassium permanganate/coal mass ratio of 7 and an oxidation time of 36 h and obtained a BCA yield of 0.71 mmol/g, which is about 18 wt %. Based on the above analyses, it can be concluded that the above oxidants are expensive and unsafe during the process, and the CA yields are much lower than 56.9 wt % of CAs obtained in this work. Alkali-oxygen oxidation of lignite is widely used in the oxidation of lignite to produce CAs. The alkali-oxygen oxidation of lignite can yield about 55 wt % CAs at conditions of 240 °C, initial O2 pressure of 5 MPa, and alkali/coal mass ratio of 3.20 In the present method, the catalytic oxidation of lignite in aqueous HPA/H2SO4 solution can yield 56.9 wt % CAs at conditions of 170 °C, initial O2 pressure of 3 MPa, and 1.25 wt % HPA. Furthermore, there are no large amounts of mineral alkali and acid consumed. In summary, the present method can avoid the use of alkali and acids in a large amount and lower the reaction temperature and initial O2 pressure. Moreover, HPA/H2SO4 shows a good reusability.

2, Table 3), and the carbon yield in liquid is 22.5% (which is close to 23.8%). The result indicates that H2SO4 can promote the conversion of lignite to water-soluble intermediates but cannot promote the formation of CAs. It was also confirmed by our previous work that the presence of H2SO4 could promote the cleavage of weak bonds in lignite and promote the conversion of lignite into water-soluble intermediates.38 When only HPA was applied into the reaction in the presence of N2, the lignite conversion was 21.5% and CA yield was 2.1% (entry 3, Table 3). The result indicates that HPA can accelerate both the conversion of lignite and the generation of CAs, which is consistent with the result in section 3.1. When H2SO4 was added into the reaction system in the presence of HPA and N2, the conversion of lignite and the yield of CAs were 30.4 and 3.1%, respectively (entry 4, Table 3). This result suggests that H2SO4 can not only promote the conversion of lignite but also adjust the catalytic activity of HPA and promote the formation of CAs. When the reactions were performed with O2, the lignite conversion and the CA yield were both increased significantly (see entries 1 and 5, entries 2 and 6, entries 3 and 7, entries 4 and 8, Table 3). The results indicate that O2 can promote both the conversion of lignite and the formation of CAs. When only O2 existed in the reaction system with the absence of HPA and H2SO4, the lignite conversion was 48.4% and the CA yield was 7.1% (entry 5, Table 3). As H2SO4 was added into the reaction in the presence of O2, the conversion of lignite and the yield of CAs were increased to 65.7 and 9.4% (entry 6, Table 3), respectively. However, the selectivity of CAs with and without H2SO4 shows no significant difference (the selectivity of entries 5 and 6 in Table 3 was 14.7 and 14.4%, respectively). The result indicates that H2SO4 can promote the conversion of lignite but cannot promote the production of CAs, which is consistent with the result of entries 1 and 2 in Table 3. When HPA was added into the reaction system in the presence of O2, the lignite conversion increased to 100% and the selectivity of CAs increased to 17.5% (entry 7, Table 3). The result indicates that HPA can accelerate the conversion of lignite and the generation of CAs. When HPA and H2SO4 were both added into the reaction in the presence of O2, the lignite conversion and the selectivity of CAs were 100 and 27.7% (which corresponds to 56.9 wt % calculated on a weight basis), respectively. This result suggests that there is a synergistic effect of HPA and H2SO4, which can promote the conversion of lignite and the generation of CAs. After the conversion of lignite in the presence of N2 and HPA (entries 3 and 4, Table 3), the solution turned blue. The result indicates that in the aqueous solution vanadium species exist in a state of VIV.50 In other words, VV was reduced to VIV in the system, but VIV was not oxidized to VV at the same conditions.32,34,36 In the presence of O2 and HPA (entries 7 and 8, Table 3), the solution was yellow. The result indicates that in the aqueous solution vanadium species exist in a state of VV.50 According to the above analysis, it can be concluded that, in the process of catalytic oxidation with HPA, VV can be reduced by lignite or water-soluble intermediates to form VIV, which can be reoxidized by O2 to complete a redox cycle, processing the oxidation of lignite continuously. 3.8. Compared with Other Oxidation Methods. Kinney12 studied the oxidation of American North Dakota lignite and German brown coal to produce acetic acid at conditions of 1 g of lignite, 25 cm3 of concentrated nitric acid, and 20 g of sodium dichromate and obtained acetic acid yields

4. CONCLUSION In this work, we proposed a new method of lignite oxidation to produce CAs. The effects of HPA concentration, H2SO4 concentration, reaction temperature, initial O2 pressure, and the reaction time on the conversion of lignite and the generation of CAs were studied in detail. The results show that the oxidation of lignite in HPA/H2SO4 solution could yield 56.9 wt % CAs, including 33.5 wt % formic acid, 14.4 wt % acetic acid, 1.5 wt % succinic acid, 1.1 wt % oxalic acid, and 6.4 wt % BCAs at conditions of 1.25 wt % HPA concentration, a H2SO4 concentration of 1.5 wt %, a reaction temperature of 170 °C, an initial O2 pressure of 3 MPa, and a reaction time of 60 min. Moreover, the catalyst system shows a good reusability for the oxidation of lignite to produce CAs. In the catalytic oxidation, lignite was converted into water-soluble intermediates at first, and then the water-soluble intermediates were converted to CAs. The mechanism study showed that H2SO4 in the reaction system not only promoted the conversion of lignite but also promoted the generation of protonated HPA and active species VO2+. HPA in the reaction could promote both the depolymerization of lignite and the production of CAs. In the process of catalytic oxidation with HPA, lignite or watersoluble intermediates were oxidized by VV and the VV was reduced to VIV at the same time. Then, the VIV was oxidized to VV by O2, and a redox cycle of oxidation was completed. Compared with alkali-oxygen oxidation of lignite, the present method avoids the use of large amounts of alkali and acid and lowers the reaction temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03479. Figures S1−S6 (PDF) F

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 10 64427603. ORCID

Weize Wu: 0000-0002-0843-3359 Zhenyu Liu: 0000-0002-3525-273X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The project is financially supported by the National Natural Science Foundation of China (21676019), the National Basic Research Program of China (2014CB744301), and the longterm subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT).

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DOI: 10.1021/acs.energyfuels.6b03479 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b03479 Energy Fuels XXXX, XXX, XXX−XXX