Production of Benzene Polycarboxylic Acids from Lignite by Alkali

Article pubs.acs.org/IECR

Production of Benzene Polycarboxylic Acids from Lignite by AlkaliOxygen Oxidation Wenhua Wang,† Yucui Hou,‡ Weize Wu,*,† Muge Niu,† and Weina 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: The oxidation of coal to produce high-valued benzene polycarboxylic acids (BPCAs), which are obtained currently from diminishing petroleum reserves, is a promising industrial process of the future. Up to now, the yield distribution of BPCAs has not been studied in detail and the mechanism of coal oxidation to BPCAs remains unclear. In this study, Huolinhe lignite was oxidized in a batch reactor by alkali-oxygen oxidation. All 12 kinds of BPCAs obtained were quantified by a new established method. Effects of alkali/coal mass ratio, reaction temperature, initial oxygen pressure, and reaction time on the yield distribution of BPCAs were studied for the first time. The results indicate that BPCAs with four or five carboxyls are the predominant products, and BPCAs with one or two carboxyls are formed in a relatively short time; moreover, the formation of BPCAs with more carboxyls is relatively more sensitive to the salting out effect. CP/MAS 13C NMR spectra and oxidation of model compounds show that phenolic, ether-substituted aromatic, ether, and aldehyde groups are easily converted and that water-soluble acids (WSA) are formed rapidly and largely due to the breakage of these bonds. The step from WSA to BPCAs is relatively slow mainly due to the inertia of the aromatic clusters with attached carboxyls or carboxylate. On the whole, the BPCAs are derived from aromatic clusters through the oxidation of condensed benzene rings, bridges, or peripheral groups that are attached to the aromatic clusters. Possible mother units for BPCAs in the lignite are suggested based on the generally agreed lignite structure.

1. INTRODUCTION The oxidation of coals is not only an important way to study the chemical structure of coals but is also a way to produce useful chemical products, e.g., acetic acid, oxalic acid, and benzene polycarboxylic acids (BPCAs, Scheme 1). It is wellknown that the chemical structure of coal, unlike that of petroleum, consists of many benzene-ring units. This feature determines that coal has inherent advantages to produce benzene-ring based chemicals such as BPCAs, which are increasingly desired for fundamental use in daily life; examples include terephthalic acid (TPA) for synthetic resin and fiber, pyromellitic acid (PMA) for polyimides, and trimesic acid for the pharmaceuticals industry.1 However, almost all these chemicals are derived from ever-shrinking petroleum reserves. For instance, p-xylene or durene are separated through many steps from the cut fraction of petroleum at first and then oxidized to TPA or PMA under catalysis.2,3 The world’s coal resource is relatively inexpensive, and its proven reserve is much more abundant than that of petroleum. Thus, it is important to explore methods to synthesize these chemicals from coals as opposed to the traditional source petroleum.1 The oxidation of coal is a degradative process. Coal macromolecules are attacked by the oxidant, resulting in micromolecular chemicals that can convey structural information about the coal. There have been alot of research studies on the oxidation reaction of coals, e.g., oxidant type, oxidation medium, mechanism, kinetic study, etc.4,5 The studied oxidants for coal oxidation include hydrogen peroxide,6,7 ozone,8 oxidative acids,9 sodium hypochlorite,10,11 and oxygen or air.12,13 Regarding the use of the former four kinds of oxidants, small molecular acids are inclined to be formed, such as oxalic © 2012 American Chemical Society

acid, malonic acid, etc. BPCAs can be obtained via oxidation of coal by oxygen or air in alkaline aqueous solutions.14,15 This method has shown the possibility to directly afford aromaticbased carboxylic acids from coal, and many research groups have been involved in this field. The agreed view of the coal oxidative degradation process in alkaline aqueous solution with oxygen/air as oxidant is shown in Scheme 2; that is, coal is oxidized into water-insoluble acids (WIA) and water-soluble acids (WSA) successively, both of which are called humic acids, and then into BPCAs and finally into CO2 and H2O. The oxidation of coal with oxygen or air, the least expensive oxidant mentioned above, to produce BPCAs is considered to have the most potential for industrial applications. In the 1940s, several groups investigated the effect of coal range, alkali/coal mass ratio, and reaction temperature on WSA yields.16−20 However, limited by the utilization of WSA in the industry, progress in this field was delayed. In the 1980s, Kamiya et al.14,21 found that materials in the WSA mainly consisted of 12 kinds of high-value BPCAs. This was a very exciting prospect to obtain so many kinds of BPCAs from coal as a substitute for petroleum. Although the process of alkali-oxidation of coal is fascinating, there are many problems. First and foremost, the effect of reaction parameters on BPCA distribution, which is crucial for industrialization processes, is not clear. This might be due to the difficulty of the analysis and quantification of the products, Received: Revised: Accepted: Published: 14994

April 13, 2012 October 11, 2012 October 22, 2012 October 22, 2012 dx.doi.org/10.1021/ie3021297 | Ind. Eng. Chem. Res. 2012, 51, 14994−15003

Industrial & Engineering Chemistry Research

Article

Scheme 1. Structures of BPCAs Obtained from Alkali-Oxygen Oxidation of Coal

some model compounds were studied under the same conditions as those for coal oxidation.

Scheme 2. Stepwise Degradation of Coal by Alkali-Oxygen Oxidation

2. EXPERIMENTAL SECTION 2.1. Materials. Huolinhe lignite was supplied from Huolinhe coal field, Inner Mongolia, China. Table 1 shows

and the analytical method used at the time was very complicated, comprising three steps: butanone extraction of the products, esterification of the products by diazomethane, and GC analysis of the BPCA esters.22,23 The butanone extraction was not complete, and the esterification reaction was not quantitative.22,24 Thus, the yields of BPCAs obtained were not entirely accurate, and there was no detailed study on the effect of reaction parameters on the coal oxidation process. Besides, the understanding of the mechanism of coal alkalioxygen oxidation is very limited and seldom reported so far. Lignite is a not a well-utilized coal in the world but is easily oxidized because of a low coalification degree.1 Chart 1 shows a

Table 1. Proximate and Ultimate Analysis (wt %) of Huolinhe Lignite proximate analysis Ad

Vdaf

C

H

N

O

S

16.95

7.09

37.24

58.87

2.81

0.89

13.19

0.20

the proximate and ultimate analysis of the coal sample. Sodium hydroxide (96%) and concentrated sulfuric acid (98%) were purchased from Beijing Chemical Plant, benzoic acid (>99.5%) from Tianjin Fuchen Chemical Company (Tianjin, China), acetaldehyde (99%), ethyl ether (99%), IPA (99%), phthalic acid (98.5%), TPA (99%), trimellitic acid (98%), trimesic acid (98%), hemimeltic acid (98%), PMA (96%), mellophanic acid (96%), prehnitic acid (96%), naphthol (98%), dibenzofuran (98%), and fluorene (99%) from Aladdin Chemistry Co. Ltd., pentacarboxylic acid (99%) and mellitic acid (99%) from Tokyo Chemistry Co. Ltd., and naphthalene (>98%), anthracene (>98%), phenanthrene (>98%), and toluene (≥99.5%) from Tianjin Fuchen Chemical Company (Tianjin, China). All reagents and solvents were analytical reagents. 2.2. Apparatus and Procedures. Figure 1 shows the flowchart for the alkali-oxygen oxidation of Huolinhe lignite. The oxidation of lignite was carried out in a high-pressure batch reactor made from Hastelloy alloy (HC276), supplied by Haian Petroleum Scientific Research Co. Ltd., Jiangsu, China. The inner volume of the reactor was 50 cm3 (30 mm inner diameter and 71 mm height), and a magnetic driving rabble was equipped from the top of the reactor to stir the mixture inside. Typically, 1.0 g of coal, 0.5−6 g of sodium hydroxide, and 20 g of distilled water were loaded in the reactor, air was purged by oxygen, and then oxygen was charged into the reactor to a desired pressure monitored by a pressure gauge composed of a pressure transducer (KLP-800KG) and an indicator (Beijing Tianchen Instrument Company). Next, the reactor was submerged into the heating furnace and then heated at 8−10 °C/min to a desired reaction temperature controlled by a temperature controller (XTD-7000) and monitored by a Ktype thermocouple within an accuracy of ±1 °C. During the reaction, the mixture was stirred by the magnetic rabble at a constant speed of 200 r/min. After the reaction, the reactor was transferred into a cool water bath for rapid cooling to stop

Chart 1. Hypothetical Lignite Structure Model,23 in Which Five Aromatic Clusters Are Illustrateda

a

ultimate analysis

Mad

Circles A and B indicate other parts of the molecule.

hypothetical structure model of lignite.23 It is generally agreed that the aromatic clusters are small, i.e., predominantly based on benzene, naphthalene, and three-ring structures. These structures are highly cross-linked by bridges and attached to lots of peripheral groups such as methyl. There is a variety of oxygen-containing functional groups, including phenolic hydroxyl group, aldehyde group, ether group, and carboxyl group. In the present study, Huolinhe lignite, a very abundant coal reserve, was chosen as the raw material, an accurate quantitative method for BPCAs was developed, and the effect of the reaction parameters on BPCA distribution was investigated. To clarify the process and mechanism of the lignite alkali-oxygen oxidation, CP/MAS 13C NMR of the oxidized coal at different oxidation times and the oxidation of 14995

dx.doi.org/10.1021/ie3021297 | Ind. Eng. Chem. Res. 2012, 51, 14994−15003


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To understand the oxidative degradation process of the lignite, the residues, WIA, WSA, and CO2 generated in the oxidation process were quantified at different reaction times that ranged from 10 to 50 min. The residues, WIA, and WSA were quantified using an electrical balance (BS224S Sartorius) with an accuracy of ±0.0001 g after drying at 70 °C for 24 h in a vacuum drying oven. WSA was obtained by exhaustive extraction of WSA aqueous solution with butanone, which was dried via rotary evaporation. CO 2 was quantified by determining the weight difference induced by acidification because the CO2 that had reacted into sodium carbonate in the oxidation process could be released by acidification. The residues, WIA, and WSA were mixed together uniformly. The mixture is referred to as the oxidized coal hereafter. The oxidized coal was characterized by CP/MAS 13C NMR analysis to measure the distribution of carbon types. All the CP-MAS spectra were obtained on a spectrometer (AV-300) following the method of Yoshida et al.25 The gases after reaction were analyzed by GC (Agilent Technologies 7890A) using a TCD detector with a Poropak Q column, and helium was used as the carrier gas. The stability experiments for 12 kinds of BPCAs were carried out using the same device as for the coal oxidation to learn whether any decarboxylation reaction had occurred. Oxidation of model compounds, naphthalene, anthracene, toluene, naphthol, dibenzofuran, phenanthrene, acetaldehyde, ethyl ether, and fluorene, was also carried out under the same conditions as for the coal oxidation. As a typical experiment, 0.1 g of sample, 3 g of sodium hydroxide, 20 cm3 of H2O, and 5 MPa O2 were loaded into the reactor, and the reaction parameters were the same as for the coal oxidation. The yields of BPCAs were calculated based on the organic matter in the coal using the formulas below:

Figure 1. Procedure of the alkali-oxygen oxidation of Huolinhe lignite.

yield (BPCAs) =

further reaction. The gases were collected into a gas bag when cooled, and the mixture that remained in the reactor was transferred into a beaker. The unreacted residues were filtered, and the filtrate acidity was adjusted by titrating with concentrated sulfuric acid to pH = 1.5. WIA was precipitated after acidification, followed by filtration to separate it. The filtrate was WSA solution, which was diluted before being injected into the HPLC for analysis. We performed three time parallel experiments at each set of conditions, and the results reported herein represent the mean values. The reproducibility of BPCA yields was estimated as better than an average relative deviation of 5%. A binary gradient elution procedure was used for the HPLC (Waters 2695, Milford, MA) analysis of BPCAs. 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 UVdetector at 235 nm was used for the quantification of 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 BPCAs were identified by HPLC-MS (mirOTOF-QII, Bruker), and the retention times were contrasted with those of standard substances.

mass of BPCAs × 100% mass of organic matter in coal

mass (organic matter in coal) = mass (coal) − mass (water) − mass (ash)

3. RESULTS AND DISCUSSION 3.1. Effect of the Alkali/Coal Mass Ratio on the Products. Effect of the alkali/coal mass ratio on the total yield of BPCAs is shown in Figure 2. The total yield of BPCAs is

Figure 2. Effect of alkali/coal mass ratio on the total yield of BPCAs. Conditions: temperature, 240 °C; initial oxygen pressure, 5 MPa; time, 30 min. 14996



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3.2. Effect of Reaction Temperature on the Products. Figure 4 shows the effect of temperature on the total yield of

very low when alkali is absent; however, the total yield of BPCAs increases dramatically when alkali is employed in the system. The highest total BPCA yield of 20.5% is obtained when the alkali/coal mass ratio is 3. The variation trend of total BPCA yield with the alkali/coal ratio from lignite in this study is consistent with that of other coal ranks reported in the literature.23,26 The role of alkali in the system is of great importance. In fact, many reports discuss the role of the alkali, mainly involving protection of intermediates and products by neutralizing the formed carboxyls, thus pulling the intermediate into the water phase and away from the coal surface and catalysis. The decrease in BPCA yield with high alkali amounts is due to the salting out effect of humic acids (WIA and WSA) formed on the coal surface, thus resulting in the decrease in oxidation rate.14,17,27 This point of view, however, has not yet been demonstrated. In our experiments, it was found that the solution color became darker when more alkali was used, indicating that the oxidation procedure was actually delayed. Moreover, when more alkali was added to the mixtures after reaction, some colored substances would precipitate, which could dissolve in water again after recycling by filtration. Thus, the salting out effect is indeed present in the system. Besides, because stronger ionic strength will decrease oxygen solubility in water when more alkali is used, the oxidation rate would be slowed by the decrease in oxygen concentration in the water. Effect of alkali/coal mass ratio on the yield of each BPCA is shown in Figure 3. For BPCAs with one, two, and three

Figure 4. The effect of temperature on the total yield of BPCAs. Conditions: alkali/coal mass ratio, 3; time, 30 min; initial oxygen pressure, 5 MPa.

BPCAs. The degradation reaction rate of coal is low at low temperatures, and it increases with temperature. In the tested reaction time, the optimum reaction temperature is 240 °C. Some BPCAs will be overoxidized at temperatures higher than 240 °C. The detailed variation trend of BPCA yields with temperature is shown in Figure 5. BPCAs with one or two

Figure 5. Effect of temperature on the yield of BPCAs. Conditions: alkali/coal mass ratio, 3; time, 30 min; initial oxygen pressure, 5 MPa.

Figure 3. Effect of the alkali/coal mass ratio on the yield of BPCAs. Conditions: temperature, 240 °C; initial oxygen pressure, 5 MPa; time, 30 min.

carboxyls, which may be derived from simple structures of coal, decrease continuously in the initial stage, but when the temperature exceeds 250 °C, yields of two-carboxyl BPCAs seem to rebound. For BPCAs with more than two carboxyls, most reach a maximum yield at 240 °C, but trimellitic, pyromellitic, and pentacarboxylic acid are exceptions, which increase or decrease constantly with temperature. The oxidation of 12 kinds of BPCA standard substances was carried out to show the stability of the products and also the probable existing decarboxylation reactions. All of them exhibited high stabilities, and more than 88% could be recovered after reaction even at 260 °C. Some decarboxylation reactions actually existed when the temperature was higher than 240 °C, as shown in Scheme 3. Pyromellitic acid decarboxylation to trimellitic acid, and three-carboxyl BPCA decarboxylation to two-carboxyl BPCAs, were observed slightly. This

carboxyls, the yields increase with the alkali/coal mass ratio, except for hemimeltic acid. The salting out effect seems to have no effect on them in the investigated range. The yields of BPCAs with four carboxyls, however, reach a maxima at an alkali/coal mass ratio of 3 or 4. An excess amount of alkali in the system is unfavorable because it resulted in the salting out effect. Particularly, the yields of pentacarboxylic and mellitic acids decrease significantly when the alkali/coal mass ratio is higher than 3. The mother structures that formed BPCAs with more carboxyls are likely to be more complex than that for lesscarboxyl BPCAs in the lignite. Because the salting out effect is more influential toward complex mother structures,28 the formation of BPCAs with more carboxyls is affected by alkali concentration to a greater extent. 14997



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Scheme 3. Decarboxylation Reaction of BPCAs at Temperatures More Than 240 °C

can partly explain the yield variation of BPCAs with temperature as shown above. 3.3. Effect of Initial Oxygen Pressure on the Products. Figure 6 shows the effect of initial oxygen pressure on the total

Figure 7. Effect of initial oxygen pressure on the yield of BPCAs. Conditions: temperature, 240 °C; alkali/coal mass ratio, 3; time, 30 min.

Figure 6. Effect of initial oxygen pressure on the total yield of BPCAs. Conditions: temperature, 240 °C; alkali/coal mass ratio, 3; time, 30 min.

yield of BPCAs. At first, the BPCA yield increases with the initial oxygen pressure and reaches a maximum at 5 MPa, and then it decreases slowly. Oxygen acts as the oxidant in the system, whose concentration has a great effect on the reaction rate. Taraba et al.29 reported that the oxidation reaction of coal under water was a first-order reaction with respect to oxygen. Thus, the oxygen concentration mainly decides the oxidation reaction rate. According to Henry's law, a high oxygen pressure means a high oxygen concentration in water. Excessively low or high oxygen concentration results in unreached oxidation or overoxidation, respectively. Figure 7 shows the effect of initial oxygen pressure on the yield of each BPCA. Yields of BPCAs with one or two carboxyls and pyromellitic acid decrease with the initial oxygen pressure in the tested range; however, others reach a maximum yield at an initial pressure of 5 or 5.5 MPa. The effect of oxygen concentration on coal oxidation is not like the temperature effect because it does not seem to change the reaction route or product selectivity; however, from the perspective of a radical mechanism for the coal oxidation, the chance that free radicals formed in the process bond to each other or to the oxygen molecules is affected by the oxygen concentration, and so the oxidation process of coal is related to the oxygen concentration to some extent. 3.4. Effect of Reaction Time on the Products. The effect of reaction time on the yield of BPCAs is shown in Figures 8 and 9. The total BPCA yield increases rapidly in the initial 30 min and proceeds to decrease slowly afterward. The slow rate of decrease of BPCA yield mapped against time or initial oxygen pressure, as shown above, is due to their high stability, and as a result, only a small amount of BPCAs is overoxidized.

Figure 8. Effect of reaction time on the total yield of BPCAs. Conditions: temperature, 240 °C; alkali/coal mass ratio, 3; initial oxygen pressure, 5 MPa.

Yields of BPCAs with one or two carboxyls decrease continuously with time in the tested range, which is similar to that from the effect of initial oxygen pressure. For BPCAs with more carboxyls, the yields reach a maxima in 20 min, such as trimesic acid, and at 30 min, such as trimellitic acid and pentacarboxylic acid. The yields of pentacarboxylic and mellitic acid, however, increase continuously with time. BPCAs with a few carboxyls, such as benzoic acid, may be formed faster because their mother structures in coal may be simpler than those of BPCAs with many carboxyls, whose formation needs more oxidative breakage of bonds attached to the benzene ring. Therefore, the maximum yield of BPCAs with a few carboxyls can be obtained within a short time or at a low initial pressure. 3.5. Reaction Process and Mechanism. Figure 10 shows the amount of products including coal residues, WIA, WSA, 14998



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slow. The yield of CO2 keeps increasing with time, suggesting that it is formed throughout the entire oxidation reaction. CP/MAS 13C NMR spectra of the raw coal and the oxidized coal from 10 to 40 min are shown in Figure 11. From the

Figure 9. Effect of reaction time on the yield of BPCAs. Conditions: temperature, 240 °C; alkali/coal mass ratio, 3; initial oxygen pressure, 5 MPa.

and CO2 in the oxidation of Huolinhe lignite at different reaction times. Only CO2 is detected in the gas phase after the reaction. Figure 10a shows that the coal conversion rate is very fast; only 10 min is needed, and the organic matter in the coal is almost completely converted. The amount of WIA is very small while the amount of WSA is very large in the lignite degradation process, and their maxima appear at 10 min. After 10 min, their amounts decrease continuously, and the amount of WSA decreases slower than that of WIA. These results indicate that the conversion of coal to WIA and the conversion of WIA to WSA in the lignite oxidation process are very fast, and the conversion of WSA to BPCAs is relatively

Figure 11. The CP/MAS 13C NMR spectra of the raw and oxidized coal at different oxidation times: (a) raw Huolinhe coal; (b) 10 min oxidation; (c) 20 min oxidation; (d) 30 min oxidation; (e) 40 min oxidation.

Figure 10. The amount of the products from the oxidation of Huolinhe lignite as a function of time: (a) coal residues; (b) WIA; (c) WSA; (d) CO2. Conditions: coal, 1.0 g; temperature, 240 °C; alkali/coal mass ratio, 3; initial oxygen pressure, 5 MPa. The effect of reaction time on the total yield of BPCAs is shown in Figure 8. 14999



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Figure 12. Oxidation reactions of typical model compounds.

reactive bonds in pyrolysis or hydrogenation play a critical role in the coal pyrolysis or coal hydrogenation process because their chemical changes significantly affect the two processes mentioned.30,31 Hayashi et al.13,32 studied the oxidation of lignite at low temperatures (under 100 °C) in alkaline aqueous solution, and they observed that the phenolic structures were easily oxidized under that condition. The oxidation of the model compounds 1-naphthol, dibenzofuran, and ethyl ether in our experiments also shows that they are easily converted (conversion rate >90%) in only 10 min under the same

spectrum of the raw lignite, carboxyl and carbonyl carbons at 175 ppm, phenolic carbons or ether-substituted aromatic carbons at 154 ppm, and carbons in ether bonds at 52 ppm can be observed. The big peak from 0 to 60 ppm is attributed to aliphatic carbons, and the peak from 100 to 160 ppm is attributed to aromatic carbons.25 After 10 min of reaction, the peaks corresponding to phenolic carbons, ether-substituted aromatic carbons, and ether-bond carbons disappear. This suggests that they are easily converted, and these structures may be more reactive to oxidation in the lignite structure. The 15000



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Figure 13. The possible mother units for the BPCAs in the lignite structure in which the aromatic systems are assumed predominantly based on benzene, naphthalene, and three-ring structures. R, R1, R2, R3, R4, R5, and R6 represent aliphatic chains or aliphatic chains with oxygen-containing groups.

the investigated time range, which indicates that the oxidation reaction is proceeding during the oxidation time. The change in aromatic carbon/aliphatic carbon ratio does not follow a continuous increasing or decreasing trend, as indicated in Figure 11. It decreases in the first 10 min and then increases with time. The oxidative opening reactions of phenolic and ether-substituted aromatic structures probably result in the initial decrease in the aromatic carbon/aliphatic carbon ratio. After the oxidation of phenolic and ethersubstituted aromatic structures, the active aliphatic structure would possibly undergo oxidation, especially groups such as the α-methylene or methyl attached to the benzene ring, which are relatively oxidative active. On the other hand, the aromatic hydrocarbon clusters are relatively inert. This may be the reason that the aliphatic carbon structure decreases so fast compared to the aromatic structure after 10 min. As the oxidation reaction proceeds, carboxyls or carboxylates would be formed that are attached to the aromatic clusters by oxidation of phenolic, ether-substituted aromatic, and aliphatic structures. It is known that a carboxyl or carboxylate substituent is a type of strong inactivating group for aromatic rings; therefore, the attachment of carboxyls or carboxylates to the aromatic rings will decrease the oxidation rate of the aromatic-ring groups, which may be the rate-controlling step in the lignite oxidation to BPCAs. This may be the reason that the degradation rate of

oxidation conditions as those used for the lignite. 1-Naphthol was oxidized to be phthalic acid exclusively, the acetic acid was detected as the product of ethyl ether oxidation, and the benzoic acid was derived from dibenzofuran as shown in Figure 12 (1−3). Besides, the aldehyde groups should receive attention because they are sufficiently reactive.33 Acetaldehyde could be oxidized completely within 10 min. Furthermore, the oxidation rate of lignite could be accelerated when acetaldehyde (0.1g) was added to the lignite oxidation system, and the maximum BPCA yield (20.6%) was obtained with 20 min oxidation (30 min without acetaldehyde). This might be due to the formation of free radicals, which can initiate the oxidation reaction of the coal structure during the oxidation of aldehyde groups. The lignite structures consist of a large amount of oxygen-containing groups which are reactive in the oxidation process. This may be one reason that lignite is more easily oxidized than bituminous coal or anthracite. Besides, lignite has smaller aromatic ring clusters, higher porosity, and greater pore sizes, making oxidant access easier. When this information is combined with the data shown in Figure 10, it can be inferred that oxidative breakage of these reactive bonds for oxidation in lignite possibly leads to the formation of WIA or WSA quickly. From the increase in the intensity of the peak at 125 ppm (Figure 11), it is known that the carboxyls kept increasing in 15001



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WSA to BPCAs is very low, and it is also the reason why BPCAs are so stable in the system. It would be interesting to know from which kind of structure in the lignite the BPCAs are derived, and on the basis of the BPCA distribution, the structures of lignite can be investigated. The study of model compounds is a conventional method to understand a very complicated system, such as lignin and biomass.34,35 The oxidation of model compounds is shown in Figure 12 (1, 4−8). Naphthalene is converted to phthalic acid (52% carbon yield) exclusively. In the oxidation reaction of fused-ring compounds, typically they are oxidized to phenol and then to quinone before ring-opening,25,36 and it is believed that the oxidation of fused ring structures in the alkali-oxygen system shares the same process. For anthracene, however, two kinds of BPCAs are detected, including phthalic acid (23% carbon yield) and PMA (6.8% carbon yield), and the mole ratio of phthalic acid to PMA is nearly 4/1. That is probably because the hydrogen in the γ position is more active than that in the α position or β position. For phenanthrene, two kinds of BPCAs are also detected, including phthalic acid (18% carbon yield) and prehnitic acid (22.1% carbon yield), with a mole ratio of nearly 1/1. For fluorene, phthalic acid (21% carbon yield) is the exclusive BPCA in the product. Toluene is oxidized to benzoic acid (75% carbon yield). Thus, the oxidative opening of benzene rings and chains attached to the aromatic clusters is really the important oxidation route in the process of BPCA production from coals. It is irrefragable that the BPCAs are derived from the aromatic clusters in coals. Thus, the structure and amount of the clusters and the chain types attached to them will determine the distribution of the BPCAs. The carboxyls attached to the BPCAs are derived from the oxidation of condensed benzene rings, bridges, or peripheral groups. Therefore, the possible mother units for the BPCAs are shown in Figure 13 based on the generally agreed lignite structure and according to which the lignite structure could be reflected to some extent. From the BPCA distribution in the lignite oxidation products, it can be seen that four-carboxyl BPCAs and pentacarboxylic and mellitic acids are predominant. So it can be speculated that the mother units that could evolve into these BPCAs are dominant in the lignite structures.

suggested, which is instructive for the understanding of lignite structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], tel/fax: +86 10 64427603. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professors Zhenyu Liu and Qingya Liu for their helpful discussion and suggestions. The project is financially supported by the Natural Science Foundation of China (21076138) and the National Basic Research Program of China (2011CB201303).

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4. CONCLUSIONS The effect of alkali/coal mass ratio, reaction temperature, reaction time, and initial oxygen pressure on BPCA distribution from lignite oxidation has been studied for the first time. It was found that BPCAs with four or five carboxyls predominated in the products, and BPCAs with one or two carboxyls were formed in a relatively short time; moreover, the formation of BPCAs with more carboxyls were more sensitive to the salting out effect. CP/MAS 13C NMR spectra of the raw coal and oxidized coal at different reaction times showed that phenolic, ether-substituted aromatic, and ether groups were easily oxidized, and the water-soluble acids were formed rapidly and largely due to the oxidative breakage of these weak bonds. Oxidation of BPCA standard substances indicated that the BPCAs were very stable under the same reaction parameters with coal oxidation, and some slight decarboxylation was observed. Oxidation of model compounds showed that the fused rings and benzene rings with aliphatic chain groups were probably mother structures in coal that could evolve into BPCAs, and on the basis of this information, the possible mother structures for every kind of BPCA in the lignite are 15002



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Production of Benzene Polycarboxylic Acids from Lignite by Alkali

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