Production of Benzene Poly(carboxylic acid)s and Small-Molecule

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Production of Benzene Poly(carboxylic acid)s and Small-Molecule Fatty Acids from Lignite by Catalytic Oxidation in NaVO3/H2SO4 Aqueous Solution with Molecular Oxygen Fan Yang,† Yucui Hou,‡ Muge Niu,† Weize Wu,*,† Dongyue Sun,† Qian Wang,† 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

‡

ABSTRACT: Carboxylic acids are widely used in industry and are considered an important type of chemicals. The production of carboxylic acids through oxidation of lignite is very promising. The traditional alkali−oxygen oxidation of lignite can produce high yields of carboxylic acids, but the process consumes a great deal of alkali and acids and the high reaction temperature increases the energy consumption. In the present work, we found that carboxylic acids, including small-molecule fatty acids and benzene poly(carboxylic acid)s, could be obtained by catalytic oxidation of lignite in NaVO3/H2SO4 aqueous solution with molecular O2. The effects of NaVO3/coal mass ratio, H2SO4 content, reaction temperature, initial O2 pressure and reaction time on the conversion of lignite and yield of carboxylic acids were investigated. In the process of reaction, lignite is first converted into water-soluble intermediates, which are then converted into carboxylic acids. The second step is the rate controlling step. It has been found that in the catalytic system, sulfuric acid not only promotes the degradation of lignite, but also changes the activity of vanadium species. Vanadium species promote both the conversion of lignite and the generation of carboxylic acids. The presence of oxygen makes vanadium species complete redox cycle, keeping the reaction ongoing. Compared with the alkali− oxygen oxidation, this catalytic oxidation method can reduce the usage of acid and alkali, and lower reaction temperature, while keeping the same yield. The catalytic system was reused four times without decline in activity. NaOH/coal mass ratio of 3/1 at a temperature of 240 °C, and achieved ∼20 wt % yields of BPCAs (including 12 types). It is clear that the alkali−oxygen oxidation technique consumes a large amount of alkali and the reaction temperatures are high, up to 250 °C. After the oxidation, alkali in the product mixture has to be directly neutralized by acid to obtain carboxylic acids.16,18,22 Consequently, the acid used to neutralize the alkali solution is also consumed in a large amount. Since the alkali and acid used are very difficult to recover, this technique results in serious pollution to the environment. The high reaction temperature increases its energy consumption. These disadvantages limit its application and call for reusable catalysts that can efficiently oxidize lignite to carboxylic acids. Vanadium-based catalysts were widely used in catalytic oxidation reactions because of its excellent catalytic performance.23−28 Shreve et al.27 studied catalytic oxidation of 1-methyl naphthalene and 2-methyl naphthalene to phthalic acid with V2O5 as the catalyst. Using vanadate catalyst, xylene could be oxidized to toluic aldehyde and phthalic anhydride.26 Emerson et al.24 transformed the p-toluic acid to terephthalic acid with a high yield using potassium metavanadate as catalyst. Jian et al.29 studied catalytic oxidation of benzene to phenol with sodium metavanadate and the catalyst showed a high selectivity. In our previous work, biomass was oxidized to formic acid in a NaVO3/H2SO4 aqueous solution with molecular O2, and the

1. INTRODUCTION As petroleum resources drying up, coal as an abundant energy and resource is drawing more and more attention all over the world.1,2 Lignite is a kind of low rank coal and not favorable for gasification and electric power generation due to its high oxygen content, high water content, low calorific value, and easy spontaneous combustion. Therefore, it is meaningful to develop an efficient method to utilize lignite. Due to its low rank property, lignite was used as nonenergy and nonfuel applications, such as sorbents3 and fertilizers.4,5 The high oxygen content of lignite indicates an inherent advantage in manufacturing oxygen-containing carboxylic acids, including benzene poly(carboxylic acid)s (BPCAs, whose structures are shown in Scheme 1) and small molecule fatty acids (SMFAs). The production of BPCAs and SMFAs by oxidation of lignite has been widely studied. It mainly includes hydrogen peroxide oxidation,6−9 oxidative acid oxidation10−12 and oxygen oxidation.13−21 As for the first two methods, oxidants are expensive and safety problems exist in the process, which limit their application. Since oxygen is inexpensive, easily available and suitable as oxidant, the oxidation of lignite in aqueous solutions using O2 was reasonably accepted. The traditional oxidation of lignite in aqueous solutions using O2 is alkali−oxygen oxidation. In 1984, Okuwakl et al.13 reported that coal could be oxidized with O2 in 25 mol/kg sodium hydroxide solution, and 20−24% yields (based on carbon) of volatile acids were obtained. Franke et al.19 investigated alkali−oxygen oxidation of several coals in an alkali/coal mass ratio of 6.71/1 at a temperature of 250 °C, and obtained a high total yield of water-soluble poly(carboxylic acid)s (about 45% in carbon yield). Wang et al.16,22 oxidized lignite with O2 in a © 2015 American Chemical Society

Received: Revised: Accepted: Published: 12254

August 25, 2015 November 3, 2015 November 19, 2015 November 19, 2015 DOI: 10.1021/acs.iecr.5b03127 Ind. Eng. Chem. Res. 2015, 54, 12254−12262

Article

Industrial & Engineering Chemistry Research Scheme 1. Structure of BPCAs Obtained from Selective Oxidation of Coala

a

Abbreviations: BPA, benzenepentacarboxylic acid; IPA, isophthalic acid; PMA, pyromellitic acid; TPA, terephthalic acid.

0.5 g, 0.833 g, 1.167 g, 1.667 g, 2.5 g) was mixed with NaVO3 solution. Lastly, the mixed solution was diluted to a known volume (100 cm3) with distilled water. The oxidation of lignite was carried out in a high-pressure 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. In a typical experiment, 0.1 g of lignite, 6.0 cm3 of NaVO3/H2SO4 aqueous solution were loaded into the reactor. Next, the reactor was sealed and purged with O2 or N2. After that, O2 or N2 was charged into the reactor 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 the 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 reaction. When the reactor temperature reached room temperature, the gases in the reactor were collected into a gas bag and the mixture remained in the reactor was transferred into a beaker. The residues were filtered, and the liquid sample was diluted to 100 cm3 for further analysis. The residues were washed with distilled water and dried in air. 2.3. Recycle of the Catalytic System. After the oxidation of lignite, the residues in the product solution were separated by filtration. BPCAs and SMFAs in the product solution were extracted with butanone and ethyl ether in sequence. First, BPCAs in the product solution were extracted with butanone (V butanone/V product solution = 2/1) for twice and SMFAs remained in the product solution. Then, SMFAs in the product solution were extracted with ethyl ether (V butanone/V product solution = 2/1) for twice. NaVO3 and H2SO4 did not dissolved in either butanone or ethyl ether. At last, ethyl ether dissolved in the liquid solution with a small amount was swept by N2. Due to a small amount water dissolved in butanone and ethyl ether during extraction, water was added to the recovered catalytic liquid solution to keep the original volume before the next reaction. 2.4. Analysis of Products. Liquid samples were injected into a high-performance liquid chromatography (HPLC, Waters 2695) for analysis. BPCAs and SMFAs in samples were analyzed using different methods. A binary gradient elution procedure was used for HPLC analysis of BPCAs. The mobile phase was acetonitrile and 0.1% (volume fraction)

catalyst system was found to be efficient and the yield of formic acid was 65% (based on carbon).30−32 The above studies indicate that vanadium-based catalysts show high catalysis for selective oxidation of aromatic compounds and biomass. Because the structure of lignite also contains many aromatic units like biomass, it is possible that vanadium-based catalysts can be used to catalyze the oxidation of lignite to produce carboxylic acids and the process can avoid using alkali and acid. NaVO3/H2SO4 catalysts are easily prepared and show a high efficiency for oxidization of biomass. Therefore, in this work, the oxidation of lignite in NaVO3/ H2SO4 aqueous solution with molecular O2 as oxidant was studied. The effect of oxidation conditions on the conversion of lignite and yield of carboxylic acids, and the reaction pathway and reaction mechanism were investigated. Lastly, the reusability of the catalytic system was investigated. The results indicated that this method could reduce the use of acid and alkali, and lower reaction temperature, and the catalytic system had good reusability.

2. EXPERIMENTAL SECTION 2.1. Materials. Xiaolongtan lignite (XLT) was collected from Xiaolongtan coal mine in Yunnan province of China. Table 1 shows its proximate and ultimate analyses. The lignite Table 1. Proximate and Ultimate Analysis of Lignitea proximate analysis/wt %

ultimate analysis/wt %, daf

lignite

Mad

Ad

Vdaf

C

H

Ob

N

S

XLT

16.4

14.5

50.7

73.7

3.9

20.1

1.3

1.0

a

XLT: Xiaolongtan lignite. ad: air-dry basis; d: dry basis; daf: dry-andash-free basis. M: moisture; A: ash; V: volatile matter content. bBy difference.

sample was pulverized to pass through a 200 mesh sieve before use. Sodium metavanadate (NaVO3, AR), concentrated sulfuric acid (H2SO4, 98%), ethyl ether (AR), and butanone (AR) were purchased from Beijing Chemical Plant. Oxygen (O2, 99.995%) and nitrogen (N2, 99.999%) were supplied by Beijing Haipu Gases Co., Ltd. (Beijing, China). All reagents were analytical grade and used without further purification. 2.2. Experimental of Oxidation. A series of NaVO3/ H2SO4 aqueous solutions with different mass fractions were prepared before use. A known amount of NaVO3 (0 g, 0.167 g, 0.333 g, 0.45 g, 0.5 g, 0.667 g) was dissolved in distilled water with ultrasonic heating. Then, a known amount of H2SO4 (0 g, 12255

DOI: 10.1021/acs.iecr.5b03127 Ind. Eng. Chem. Res. 2015, 54, 12254−12262

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Industrial & Engineering Chemistry Research 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 at 35 °C. The gradient elution procedure was as follows: first, the volume ratio of acetonitrile to phosphoric acid aqueous 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 SMFAs were determined with a Shodex SH 1011 column. The refractive index detector (Waters 4110) was employed to analyze SMFAs. The column oven temperature was 55 °C and the mobile phase was diluted H2SO4 aqueous solution with a concentration of 0.1 wt % and 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). The gases after reaction were analyzed by gas chromatography (GC, Agilent Technologies 7890A) using a TCD detector with a Poropak Q column, with helium used as carrier gas. CO2 was absorbed by NaOH aqueous solution, and then CO2 was quantitatively analyzed using the above TOC analyzer. The yields of carboxylic acids were calculated based on the organic matter in lignite using the following formulas. yield(carboxylic acids) = (mass of carboxylic acids)

Figure 1. Effect of NaVO3/coal mass ratio on (a) the conversion of lignite and the yield of carboxylic acids and (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; initial O2 pressure, 3 MPa; H2SO4 content, 1.167 wt %.

/(mass of organic matter in coal) × 100%

where, mass (organic matter in coal) = mass (coal) − mass (ash) − mass (moisture).

3. RESULTS AND DISCUSSION Analysis of liquid product shows that carboxylic acids in the products include the 12 BPCAs as shown in Scheme 1 and SMFAs (formic acid, acetic acid, oxalic acid, succinic acid). 3.1. Effect of NaVO 3 /Coal Mass Ratio on the Conversion of Lignite and the Yield of Carboxylic Acids. Figure 1(a) shows the effect of NaVO3/coal mass ratio on the conversion of lignite and the yield of carboxylic acids. The conversion of lignite and the yield of carboxylic acids are low in the absence of NaVO3 but increase rapidly when NaVO3 is employed. The result indicates that NaVO3 can promote the conversion of lignite and the generation of carboxylic acids. The maximum carboxylic acids yield is 54.33 wt % when the NaVO3/coal mass ratio is 0.27. When the mass ratio of NaVO3/coal exceeds 0.27, the yield of carboxylic acids decreases as NaVO3 was further added. In the process of catalytic oxidation with NaVO3, reactant was oxidized first and the oxidation state of vanadium species was reduced. Then, the reduction state of vanadium species was oxidized by oxygen and a redox circle of oxidation was completed.32 It could be explained as follows: the number of active vanadium species is increased with further adding NaVO3, which causes an increase of oxidation capability. Thus, carboxylic acids are further oxidized, leading to low yields and selectivity. Effect of NaVO3/coal mass ratio on the yield of each BPCA is shown in Figure 1(b). To BPCAs of benzoic acid, phthalic acid, terephthalic acid and hemimellitic acid, their yields increase with an increase in NaVO3/coal mass ratio. The yields of other BPCAs reach maxima at an NaVO3/coal mass ratio of 0.27. An excess amount of NaVO3 in the system is unfavorable

because of the increase of oxidation capability as mentioned above. 3.2. Effect of H2SO4 Content on the Conversion of Lignite and the Yield of Carboxylic Acids. Figure 2(a) indicates the effect of H2SO4 content on the conversion of lignite and the yield of carboxylic acids. As can be seen in the figure, with the addition of sulfuric acid, lignite conversion and carboxylic acids yield are increased significantly. In aqueous solution, the vanadium species formed are related to pH of the aqueous solution as shown in Scheme 2.29,33 It was reported that the lower the pH, the stronger the reduction potential of vanadium species.34 Table 2 shows the corresponding pH under the experimental conditions of the NaVO3 aqueous solution. When the amount of sulfuric acid added reached 1.167 wt %, the pH of solution was less than 1. It can be seen from Scheme 2 that the V−O species of solution was VO2+ and the yield of carboxylic acids at this time reached the maximum value. As sulfuric acid was added further, the vanadium active species maintained as VO2+. Meanwhile, the yield of carboxylic acids had no obvious change. Thus, we deduce that VO2+ species have played a key role in the catalytic oxidation of lignite and sulfuric acid has the function of cocatalyst in the process. Effect of H2SO4 content on the yield of each BPCA is shown in Figure 2(b). For BPCAs with four, five, six carboxyls and trimesic acid, their yields increase with H2SO4 content. But the yields of other BPCAs reach maximum values at H2SO4 content of 1.167 or 1.667 wt %. The parent structures that form BPCAs with more carboxyls are likely to be more complex than that 12256

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Figure 3. Effect of temperature on (a) the conversion of lignite and the yield of carboxylic acids and (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; initial O2 pressure, 3 MPa; NaVO3/coal mass ratio, 0.27; H2SO4 content, 1.167 wt %.

Figure 2. Effect of H2SO4 content on (a) the conversion of lignite and the yield of carboxylic acids and (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; initial O2 pressure, 3 MPa; NaVO3/coal mass ratio, 0.27.

Scheme 2. Relationship between Vanadium Species and Solution pH Value

acids decreases, which results from overoxidation of carboxylic acids. Effect of temperature on the yield of each BPCA is illustrated in Figure 3(b). All of BPCAs reach a maximum at 160 or 180 °C except for PMA and prehnitic acid. The yields of PMA and prehnitic acid increase with increasing temperature. 3.4. Effect of Initial O2 Pressure on the Conversion of Lignite and the Yield of Carboxylic Acids. In oxidation reactions with O2 as oxidant, initial O2 pressure played a crucial role in the oxidation reactions.16,36 In this work the initial O2 pressure was also investigated. Effect of initial O2 pressure on the conversion of lignite and the yield of carboxylic acids are shown in Figure 4(a). With the increase of initial O2 pressure from 1 to 3 MPa, the conversion of lignite increases and reaches 100% at ≥3 MPa. The yield of carboxylic acids increases with the increase of initial O2 pressure and reaches a maximum at 3 MPa. Figure 4(b) shows the effect of initial O2 pressure on the yield of each BPCA. Yields of all BPCAs reach a maximum at an initial O2 pressure of 3 or 4 MPa. Taraba et al.37 reported that oxidation of coal in aqueous solution was a firstorder reaction with respect to O2. Thus, the concentration of O2 in the aqueous solution played a vital role for the reaction. According to Henry’s law, a high O2 pressure means a high concentration of O2 in aqueous solution. At low initial O2 pressures, the concentration of O2 in aqueous solution is low, resulting in a low oxidation reaction rate of lignite and the lignite is not completely converted in the given time of 1 h. Obviously, the increase of initial O2 pressure promotes the conversion of lignite and the generation of carboxylic acids. On the other hand, the increase of O2 concentration can accelerate electron transfer of catalyst from reduction state to oxidation

Table 2. pH Values of Sodium Metavanadate Aqueous Solution with Different H2SO4 Contents H2SO4/wt % pH

0 8.39

0.5 1.41

0.833 1.10

1.167 0.87

1.667 0.70

2.5 0.50

with less carboxyls BPCAs. The increase of H2SO4 content may promote cleavage of the complex structure. 3.3. Effect of Temperature on the Conversion of Lignite and the Yield of Carboxylic Acids. Figure 3(a) shows the effect of temperature on the conversion of lignite and the yield of carboxylic acids. The conversion rate of lignite and the generation rate of carboxylic acids are low at low temperatures, and they increase with increasing temperature. When the temperature was 160 °C, the yield of carboxylic acids attained a maximum value of 54.33 wt %, including 28.66 wt % formic acid, 12.41 wt % acetic acid, 1.53 wt % oxalic acid, 1.16 wt % succinic acid and 10.82 wt % BPCAs. When the temperature was further increased, the total yield of carboxylic acids decreased, especially the yield of formic acid. It was reported that at reaction temperatures more than 160 °C, the stability of formic acid was reduced, resulting from excessive oxidation and decomposition.35 Thus, it can be concluded that with further increasing the temperature, the yield of carboxylic 12257

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Figure 4. Effect of initial O2 pressure on (a) the conversion of lignite and the yield of carboxylic acids and (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; NaVO3/coal mass ratio, 0.27; H2SO4 content, 1.167 wt %.

Figure 5. Effect of reaction time on (a) the conversion of lignite and the yield of carboxylic acids and (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; temperature, 160 °C; initial O2 pressure, 3 MPa; NaVO3/coal mass ratio, 0.27; H2SO4 content, 1.167 wt %.

especially formic acid. The oxidation of formic acid was carried out to study the stability of formic acid, and the decomposition rates of formic acid were 13.7% and 23.0% in 1.5 and 2 h, respectively. 3.6. Proposed Reaction Pathways and Reaction Mechanism of the Catalytic Oxidation. Figure 6 shows the amount of products formed in the oxidation of lignite, including residues, total organic carbon in liquid solution, carboxylic acids and CO2, all in carbon yield. It can be seen from Figure 6(a) that the lignite conversion is very fast, and it only needs 0.5 h to reach complete conversion. When the lignite is completely converted in 0.5 h, carbon yield in the liquid solution reaches a maximum as shown in Figure 6(b). At this time the yield of carboxylic acids does not achieve the maximum, while it reaches a maximum value at 1 h (see Figure 6(c)). The result indicates that there exist water-soluble intermediates in the process of catalytic oxidation. Only CO2 is detected in the gas phase after oxidation reaction. Figure 6(d) shows that the yield of CO2 increases with time, suggesting that it is formed throughout the entire oxidation reaction. Comparing Figure 6(a)−(c), it is found that in the catalytic oxidation process, lignite is converted into water-soluble intermediates first and then the water-soluble intermediates is converted into products. The first step is faster than the second step, causing the accumulation of water-soluble intermediates. In other words, the second step that water-soluble intermediates are converted into carboxylic acids is the ratecontrolled step in the process of catalytic oxidation. In order to study the mechanism of lignite conversion, effects of O2, H2SO4 and NaVO3 on the conversion were investigated. When only N2 existed in the reaction system (in the absence of O2, H2SO4, and NaVO3), the conversion of lignite was only

state, and then accelerate the catalytic oxidation cycle of catalyst, finally promoting the formation of products.38 As can be seen from Figure 4(a), the yields of formic acid and acetic acid decline noticeably when the initial O2 pressure exceeds 3 MPa. The oxidation of pure formic acid and acetic acid in the same reaction system as that of lignite in Figure 4(a) was carried out to study the stability of the products in the reaction system. At the initial O2 pressure of 4 MPa, decomposition rates of formic acid and acetic acid were 15.33% and 3.44%, respectively. The decomposition rates of formic acid and acetic acid were increased to 17.78% and 5.09%, respectively, as the initial O2 pressure was increased to 5 MPa. The results suggest that at initial O2 pressures greater than 3 MPa, overoxidation of products occurs, and the rate of acids formation are lower than that overoxidation rate, which results in low yields of carboxylic acids. 3.5. Effect of Reaction Time on the Conversion of Lignite and the Yield of Carboxylic Acids. Figure 5(a) indicates the effect of reaction time on the conversion of lignite and the yield of carboxylic acids. The catalytic oxidation of lignite is very fast, and the conversion reached 100% in 0.5 h. The total yield of carboxylic acids increases with time and reaches a maximum of 54.33 wt % in 1 h. The results suggest that there are intermediates between lignite and products (carboxylic acids), and the intermediates existing in the conversion of lignite. The information about intermediates will be discussed in section 3.6. Figure 5(b) shows the effect of reaction time on the yield of each BPCA. The tendency of the yield of each BPCA as a function of time is similar to that of total carboxylic acids in Figure 5(a). As oxidation time is extended more than 1 h, the overoxidation of carboxylic acids occurs, which causes the decrease of carboxylic acids yield, 12258

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Figure 6. Amount of the products from the oxidation of lignite as a function of time. (a) coal residues; (b) total organic carbon in liquid solution; (c) carbon in carboxylic acids; (d) carbon in CO2. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; initial O2 pressure, 3 MPa; NaVO3/coal mass ratio, 0.27; H2SO4 content, 1.167 wt %.

Table 3. Conversion of Lignite at Different Reaction Conditionsa

a

entry

atmosphere

NaVO3/g

H2SO4/wt %

conversion/%

carboxylic acids/%

selectivity/%

1 2 3 4 5 6 7 8

N2 N2 N2 N2 O2 O2 O2 O2

0 0 0.027 0.027 0 0 0.027 0.027

0 1.167 0 1.167 0 1.167 0 1.167

8.68 15.92 14.47 19.64 36.58 59.62 65.32 100

0 0 2.54 4.28 6.42 10.8 13.74 24.17

0 0 17.55 21.79 17.6 18.1 21.03 24.17

Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; initial O2 pressure, 3 MPa.

increased to 19.64% and 4.28%, respectively (Table 3, entry 4). The result indicates that H2SO4 can not only promote the conversion of lignite, but also change the activity of vanadium species (as mentioned in section 3.2). Therefore, the conversion of lignite and the yield of carboxylic acids were higher than that in the presence of either H2SO4 or NaVO3. When the reactions were performed under O2 condition (comparing entries 1 and 5, 2 and 6, 3 and 7, 4, and 8 in Table 3), the conversion of lignite and the yield of carboxylic acids were increased significantly. The result indicates that O2 can accelerate the conversion of lignite and the production of carboxylic acids. It was observed that the reaction solutions containing NaVO3 is blue in color in anaerobic condition (Table 3, entries 3 and 4), which indicates that vanadium species exist in a state of VIV. The color of the reaction solutions is yellow in the presence of O2 (Table 3, entries 7 and 8), which indicates that vanadium species exist in a state of VV. The results are consistent with that in the literature.32 The

8.68% in 1 h, and no carboxylic acids were produced (Table 3, entry 1). When only sulfuric acid was added into the reaction system in the presence of N2 instead of O2, the conversion of lignite was 15.92%, and also no carboxylic acids were produced (Table 3, entry 2). It can be seen that sulfuric acid can promote the conversion of lignite but cannot promote the production of carboxylic acids. The reason may be that the presence of sulfuric acid can promote the cleavage of weak bonds in lignite, and then promote the conversion of lignite to water-soluble intermediates. When only NaVO3 was added into the reaction system in the presence of N2 instead of O2, the conversion of lignite and the yield of carboxylic acids were 14.47% and 2.54%, respectively (Table 3, entry 3). This result indicates that NaVO3 can promote both the conversion of lignite and the production of carboxylic acids, which is consistent with the conclusion in section 3.1. When H2SO4 and NaVO3 both exist in the reaction system in the presence of N2 instead of O2, the conversion of lignite and the yield of carboxylic acids were 12259

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Industrial & Engineering Chemistry Research results suggest that O2 can make vanadium species catalyst complete redox cycle, progressing the oxidation reaction continuously. When only O2 existed in the reaction system (in the absence of H2SO4 and NaVO3), the conversion of lignite is 36.58% and the yield of carboxylic acids is 6.42% (Table 3, entry 5). As H2SO4 was added into the reaction system, the conversion of lignite was increased to 59.62%, and the yield of carboxylic acids was increased to 10.8% (Table 3, entry 6). But the selectivity of carboxylic acids with and without H2SO4 are almost identical (17.6% and 18.1% in entry 5 and entry 6, respectively). It indicates that H2SO4 can only promote the conversion of lignite but cannot promote the production of carboxylic acids, which is similar to the result obtained from entries 1 and 2 in Table 3. When NaVO3 was added into the reaction system in the presence of O2, the conversion of lignite and the yield of carboxylic acids were 65.32% and 13.74%, respectively (Table 3, entry 7), which were higher than that without NaVO3 (Table 3, entry 5). The selectivity to carboxylic acids was also increased from 17.6% to 21.03%. The result indicates that vanadium species can promote both the conversion of lignite and the production of carboxylic acids, which is in accordance with the results obtained from entries 1 and 3, and the section 3.1. When NaVO3 and H2SO4 were both added into the aqueous solution in the presence of O2, the conversion of lignite and the yield of carboxylic acids were 100% and 24.17%, respectively (Table 3, entry 8). It suggests that there is a synergistic effect of NaVO3 and H2SO4, which can significantly promote the conversion of lignite and the production of carboxylic acids. The above results indicate that H2SO4 not only changes the activity of vanadium species (as discussed in section 3.2) but also promotes the conversion of lignite into water-soluble intermediates. In the reaction system, NaVO3 accelerates both the transformation of lignite into intermediates and the oxidation of the intermediates into carboxylic acids. In the process of catalytic oxidation, the main vanadium species is VO2+. Active oxygen species was provided from VO2+ and used for the oxidation of lignite and intermediates. At the same time, VO2+ was reduced to VIV. Then, VIV was oxidized to VV by O2, which completes the catalytic cycle. Figure 7 shows the proposed pathways and reaction mechanism of catalytic oxidation of lignite to carboxylic acids.

Figure 8. Reuse of NaVO3/H2SO4 aqueous solution. (a) the conversion of lignite and the yield of carboxylic acids; (b) the yield of each BPCA. Reaction conditions: lignite, 0.1 g; reaction time, 1 h; temperature, 160 °C; initial O2 pressure, 3 MPa; NaVO3/coal mass ratio, 0.27; H2SO4 content, 1.167 wt %.

3.8. Comparison with Alkali−Oxygen Oxidation. Alkali−oxygen oxidation is widely used for selective oxidation of lignite to carboxylic acids. Wang et al.22 reported that the alkali−oxygen oxidation of lignite produced 55 wt % carboxylic acids at the optimal conditions of 240 °C, alkali/coal mass ratio of 3, and 5 MPa of O2. For this method, the catalytic oxidation of lignite in NaVO3/H2SO4 aqueous solution produced 54.33 wt % carboxylic acids only at conditions of 160 °C and 3 MPa O2, and there are no alkali and no large amount acids consumed. By the above comparison, it can be known that the catalytic oxidation method can avoid the use of alkali and acid, and lower the reaction temperature. Moreover, the catalytic oxidation method shows a good reusability of catalytic reaction system, and overcomes the disadvantages of alkali−oxygen oxidation.

4. CONCLUSIONS Catalytic oxidation of lignite to carboxylic acids was carried out in NaVO3/H2SO4 aqueous solution using O2 as oxidant. The effects of NaVO3/coal mass ratio, H2SO4 content, temperature, initial O2 pressure and reaction time on the conversion of lignite and the yield of carboxylic acids were investigated in detail. The results showed that the catalytic oxidation of lignite in NaVO3/H2SO4 aqueous solution with O2 could produce high yields of carboxylic acids. The carboxylic acids yield was 54.33 wt %, including 28.66 wt % formic acid, 12.41 wt % acetic acid, 1.53 wt % oxalic acid, 1.16 wt % succinic acid, and 10.82 wt % BPCAs (including 3.56 wt % mellitic acid, 1.38 wt % benzenepentacarboxylic acid, 1.10 wt % prehnitic acid, 1.04 wt % PMA, 0.85 wt % mellophanic acid) when the NaVO3/coal mass ratio, H2SO4 content, temperature, initial O2 pressure and

Figure 7. Proposed reaction pathways and reaction mechanism for the oxidation of lignite in an aqueous NaVO3/H2SO4 solution under O2 pressure.

3.7. Reuse of NaVO3/H2SO4 Reaction System. The reusability of NaVO3/H2SO4 reaction system for lignite oxidation with O2 was studied, and the results are shown in Figure 8. In four cycles, the conversion of lignite and the yield of carboxylic acids show no obvious change. In other words, the catalytic system has a well reusability in the oxidation of lignite. 12260

DOI: 10.1021/acs.iecr.5b03127 Ind. Eng. Chem. Res. 2015, 54, 12254−12262

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Industrial & Engineering Chemistry Research reaction time were 0.27, 1.167 wt %, 160 °C, 3 MPa, and 1 h, respectively. In the process of catalytic oxidation, lignite was converted into intermediates quickly and then the intermediates were converted slowly into carboxylic acids. The second step is the rate-controlled step in the process of catalytic oxidation. The research of reaction mechanism shows that H2SO4 in the catalytic reaction system not only promotes depolymerization of lignite but also affects the activity of vanadium species. Vanadium species can accelerate both the transformation of lignite into intermediates and oxidation of the intermediates into carboxylic acids. At first, the vanadium species is reduced into reduction state, and then is oxidized by O2 into oxidized state, completing the redox cycle, which progresses the oxidation reaction continuously. Compared with alkali−oxygen oxidation of lignite, this method can avoid the use of alkali and acid in large amounts, lower reaction temperature, and maintain good reusability. Thus, the production of carboxylic acids from lignite by catalytic oxidation is of high efficiency and low cost.

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +86 10 64427603; e-mail: [email protected]. cn. Funding

The project is financially supported by the Natural Science Foundation of China (21076138) and the National Basic Research Program of China (2011CB201300). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Qingya Liu and Dr. Shuhang Ren for their helpful discussion and suggestions. REFERENCES

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DOI: 10.1021/acs.iecr.5b03127 Ind. Eng. Chem. Res. 2015, 54, 12254−12262