Production of Benzene Carboxylic Acids and Small-Molecule Fatty

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Production of Benzene Carboxylic Acids and Small-Molecule Fatty Acids from Lignite by Two-Stage Alkali-Oxygen Oxidation Wenbin Li,† Yucui Hou,‡ Fan Yang,† and Weize Wu*,† †

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 are widely used in industry as a kind of important chemical materials, and it is very promising to produce carboxylic acids from lignite. The traditional alkalioxygen oxidation of lignite does not consider the complex structure of lignite, and the structures with different reactivities are treated under identical and harsh reaction conditions, resulting in a low yield of carboxylic acids. We propose a twostage oxidation process of lignite to increase the yield and mitigate the harsh reaction conditions. First, lignite was extracted using a NaOH aqueous solution to yield two parts (extract and residue) with different reactivities. The extract and residue then were oxidized separately with alkali-oxygen oxidation under different conditions. In this way, high yields of benzene carboxylic acids (BCAs, 12 types) and small-molecule fatty acids (SMFAs) could be obtained. The extraction conditions and the alkali-oxygen oxidation conditions of the extract and the residue were investigated. Compared with the oxidation of extract, the oxidation of residue requires higher reaction temperature, higher initial O2 pressure, higher alkali concentration, and longer reaction time. The structures of the extract and residue were characterized by 13C nuclear magnetic resonance (13C NMR) and Fourier transform infrared (FT-IR) spectroscopy, which indicates that most of aromatic structures in the extract are single aromatic rings with side chains containing many oxygen-containing functional groups. However, aromatic structures in the residue are mainly condensed aromatic rings with side chains containing many alkyl chains. These differences in their structures result in different yields of carboxylic acids from the extract and residue.

1. INTRODUCTION As petroleum resources are drying up, coal as an abundant energy and resource is drawing more and more attention all over the world.1,2 As a low-rank coal, lignite is abundant, but its usage is limited, because of low calorific value, high water content, high oxygen content, and easy spontaneous combustion. Lignite, which is rich in aromatic structures and side chains, forms carboxylic acids easily, including benzene carboxylic acids (BCAs, having 12 types) and small-molecule fatty acids (SMFAs), which are important industrial materials. For example, terephthalic acid (TPA) is broadly used for synthetic resin and fiber, pyromellitic acid (PMA) for polyimides, and trimesic acid, prehnitic acid, and mellophanic acid for the pharmaceuticals industry. As for SMFAs, formic acid, acetic acid, oxalic acid, and succinic acid are used in the chemical industry as solvents or feed materials. Therefore, it is recognized that the development of an efficient method to produce these carboxylic acids from lignite has significant potential economic impact for the chemical industry. The production of BCAs and SMFAs via the oxidation of lignite has been widely studied. The methods mainly include the following three: hydrogen peroxide oxidation,3,4 oxidative acid oxidation,5,6 and oxygen oxidation.7−12 The first two methods use high-cost oxidants and have safety problems, © 2017 American Chemical Society

which limit their applications. The third method using oxygen has more potential, because of its lower cost and high availability. Okuwaki et al.8 reported that coal could be oxidized with O2 in a 25 mol/kg NaOH solution, and yields of 20%− 24% (based on carbon) of volatile acids were obtained. Wang et al.9 oxidized lignite with O2 and a NaOH/coal mass ratio of 3/1 at a temperature of 240 °C, and achieved a BCAs yield of ∼20 wt % (including the 12 types of BCAs). However, these methods do not consider the differences in the structures of lignite, which are oxidized under the same conditions, despite having different reactivities. As a result, low yields of carboxylic acids are obtained, because of the overoxidation of easy oxidable structures in lignite to produce a large amount of CO2. In order to enhance the yield of carboxylic acids and reduce the yield of CO2, it is necessary to consider the basic structure of coal. We know that the molecular structure of coal is composed of condensed aromatic rings, hydrogenated aromatic rings, heterorings, and aliphatic rings, connected together by bridge bonds. Those structural units are connected to alkyl Received: Revised: Accepted: Published: 1971

November 25, 2016 January 10, 2017 January 28, 2017 January 29, 2017 DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research chains and various functional groups, including hydroxyl, carboxyl, methoxyl and carbonyl, etc.13 The current studies suggest that the cores of coal structure (aromatic rings) are relatively stable, and the side chains are active. Induced effects and conjugative effects are the main reasons why there are different reactivities in coal.14−17 Ge et al.18 reported that the order of reactivity of structures in coal is as follows: side chains of α-carbon atom with oxygen-containing functional groups > alkyl chains > aromatic rings. As we know, lignite has a high content of oxygen and a high content of activity groups. Therefore, active components (single aromatic rings with oxygen-containing functional groups) and inert components (condensed aromatic with alkyl chains) should be separated for oxidization to increase the yields of carboxylic acids. To separate different components from lignite, the extraction of lignite either uses organic solvent19,20 or alkali solution.21,22 Using the alkali solution is superior to the organic solvent, because the former is not only a good extraction solvent for lignite,21,22 but also a reaction medium in the subsequent alkalioxygen oxidation. It can extract polar components, which are single aromatic rings with oxygen-containing functional groups and more side chains, while the residue consists of condensed aromatic structures with alkyl chains that are nonpolar.23,24 By the above reasoning, we chose NaOH aqueous solutions as the solvent to extract lignite. The lignite was divided into extract (Ex) and residue (Re), which were then oxidized separately by the alkali-oxygen oxidation, under different conditions. The results indicate that the alkali-oxygen oxidation of Re required harsher reaction conditions than that of Ex. Our proposed two-stage alkali-oxygen oxidation of lignite achieves a higher yield of carboxylic acids and a lower consumption of alkali, compared with the traditional one-stage method.

Figure 1. Procedure of the two-stage alkali-oxygen oxidation of Huolinhe lignite and analysis techniques.

2.2.1. Extraction and Separation. Based on the previous studies,22,23,25−27 extraction experiments were conducted in a 250 cm3 three-necked flask with a magnetic stirrer. NaOH (4.00 g) was dissolved in 100 cm3 of distilled water. Then, 5.00 g of lignite was added into the above aqueous solution. After that, the flask was submerged into a constant-temperature oil bath at 100 °C and the mixture was stirred. At the same time, N2 was charged continuously into the flask to remove air. When the desired temperature of the mixture in the flask was reached, the starting time of extraction was recorded. After a desired time, the flask was transferred into a cool water bath. After the flask was cooled to room temperature, the mixed solution was centrifuged at a speed of 6000 rpm to obtain Ex and Re. The Ex then was diluted to a known volume (100 cm3) with distilled water. The Re was dried in a vacuum oven (100 °C) for 10 h and weighed. The total organic carbon (TOC) of the Ex and Re were determined with a total organic carbon (TOC) analyzer (Shimadzu, Model TOC-L CPN, Japan). The yield of Ex was calculated based on organic carbon, using the following formula:

2. EXPERIMENTAL SECTION 2.1. Materials. Huolinhe lignite (HLH) was supplied from Huolinhe Coal Mining Factory, Inner Mongolia, China. Table 1 Table 1. Proximate and Ultimate Analyses of Huolinhe Lignite Proximate Analysisa [wt %] Mad

Ad

Vdaf

25.20

16.84 Ultimate Analysis [wt %, daf]

49.17

C

H

Ob

N

S

71.20

5.78

21.44

1.05

0.53

yield of Ex (%) =

TOC(Ex) × 100 (TOC(Ex) + TOC(Re))

2.2.2. Oxidation of Extract and Residue. The oxidation of Ex 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 had an inner volume of 50 cm3 and was equipped with a magnetic stirrer. In the experiment, 20 cm3 extract solution was loaded into the reactor. Next, the reactor was sealed and purged with O2. After that, O2 was charged into the reactor to a desired pressure. The reactor then was submerged into a heating furnace and heated at 8−12 °C/min to a desired reaction temperature that was controlled by a temperature controller and monitored by a Type K 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 1000 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 mixture

a

Mad, moisture content based on air-dried coal; Aad, ash content based on air-dried coal; Vdaf, volatiles content based on dried ash-free coal. Subscripts: ad, air-dry basis; d, dry basis; and daf, dry-and-ash-free basis. bBy difference.

lists the proximate and ultimate analyses of the lignite sample. It was pulverized to pass through a 200 mesh sieve before use. Sodium hydroxide (NaOH, 96%) and concentrated sulfuric acid (H2SO4, 98%) were purchased from Beijing Chemical Plant (Beijing, China). Oxygen (O2, 99.995%) and nitrogen (N2, 99.999%) were supplied by Beijing Haipu Gases Co., Ltd. (Beijing, China). All reagents were analytical grade. 2.2. Apparatus and Procedures. Figure 1 shows the flowchart of the two-stage alkali-oxygen oxidation of Huolinhe lignite. 1972

DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research

extraction solvent/coal mass ratio of 20. The results are shown in Figure 2, showing that the yield of extract increased with the

in the reactor was transferred into a beaker and then was adjusted by titrating concentrated sulfuric acid to pH 1.5. Water-insoluble acids (WIAs) were precipitated after acidification and then separated by filtration. The filtrate was the water-soluble acid (WSA) solution, which was diluted to 500 cm3 before being injected into a high-performance liquid chromatography (HPLC) (Model 2695, Waters, USA) for analysis. Similar to the oxidation of Ex, the oxidation of Re was carried out in the same batch reactor. Typically, 1.00 g of Re, which was also pulverized to pass through a 200 mesh sieve before use, 2.40 g of sodium hydroxide, and 20.0 g of distilled water were loaded in the reactor. Other steps included features such as the oxidation of Ex. When the reactor temperature reached room temperature, the mixture in the reactor was transferred into a beaker. The unreacted residue 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 to 2000 cm3 before being injected into the HPLC for analysis. 2.3. Analysis of Products. Liquid samples were injected into a HPLC for analysis. BCAs and SMFAs 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 Xbridge 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 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, Model 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 yields of carboxylic acids were calculated based on organic carbon, using the following formula:

Figure 2. Effect of extraction time on the yield of extract. Reaction conditions: temperature, 100 °C; NaOH solution, 1 M; extraction solvent/coal mass ratio, 20:1.

increase of extraction time. It is consistent with the reports in the literature.25−27 As the extraction time reached 3 h or more, the extraction yield was not obviously changed. So we chose 3 h as the extraction time, at which time the yield of Ex was 25.4%. We speculate that the Ex yield of 25.4% is related to the lignite structures, which contain very complex and heterogeneous materials with macromolecular structures and low-molecularweight organic molecules.13,31−33 We now describe the oxidation of Ex and Re in the following sections. 3.2. Production of Carboxylic Acids from Ex and Re by Alkali-Oxygen Oxidation. 3.2.1. Effect of Reaction Temperature on the Yield of Carboxylic Acids. Figure 3 shows the effect of temperature on the conversion of Ex and Re and the yield of carboxylic acids, and the effect of temperature on detailed yields of BCAs are shown in Figure S1 in the Supporting Information. When Ex was oxidized, the conversion of Ex easily reached 100% at low temperatures. But the yield of carboxylic acids was low at low temperatures, and it increased with increasing temperature. When the temperature was 230 °C, the yield of total carboxylic acids peaked. The yield of SMFAs, including formic acid, acetic acid, oxalic acid, and succinic acid, also peaked; however, the yield of BCAs did not. When the temperature was further increased, the total yield of carboxylic acids decreased, which results from the overoxidation of carboxylic acids, but the yield of BCAs peaked at 240 °C. SMFAs are mainly generated from alkyl chains, which are active groups, while BCAs are mainly generated from aromatic rings, which are relatively stable.13−16 This explains why BCAs required higher reaction temperature than SMFAs. When Re was oxidized, as shown in Figure 3b, the conversion of Re reached 100% at high temperatures (≥270 °C), but the yield of carboxylic acids increased as the temperature increased and peaked at 260 °C. The maximum yields of SMFAs and BCAs were attained at reaction temperatures of 260 and 270 °C, respectively. Through the above analysis, it can be concluded that, when the yield of carboxylic acids peaked, the oxidation of Re requires higher reaction temperature than that of Ex. It also showed that two parts (Ex and Re) from lignite divided by the extraction of NaOH solution show different reactivities. 3.2.2. Effect of Initial O2 Pressure on the Yield of Carboxylic Acids. Figure 4 shows the effect of initial O2

yield of carboxylic acids (based on carbon) carbon mass of carboxylic acids = × 100 carbon mass of organic matter in Ex (or Re)

2.4. Solid-State 13C NMR and FT-IR Analysis. In order to study the structures of Ex and Re, we used solid-state 13C NMR and FT-IR to characterize the structures of Ex and Re.28 Solidstate 13C NMR spectra for Ex and Re were recorded by crosspolarization magic-angle spinning (CP/MAS) on a spectrometer (Model AV 300, Bruker, USA), following the method of Yoshida et al.29 FT-IR spectra for Ex and Re were recorded on the IR spectrometer (Model Nicolet 6700, Thermo Scientific, USA), which was obtained at a resolution of 4 cm−1 and collected in a wavenumber range of 4000−400 cm−1.

3. RESULTS AND DISCUSSION 3.1. Extraction of Lignite. The alkali solution extraction of lignite has been widely studied. Based on the previous studies,25−27,30 the extraction experiments were carried out at a temperature of 100 °C, an alkali concentration of 1 M, and an 1973

DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research

Figure 3. Effect of temperature on the conversion of (a) Ex and (b) Re and the yield of carboxylic acids. Reaction conditions (a): reaction time, 30 min; initial O2 pressure, 5 MPa; NaOH solution, 1 M; (b): reaction time, 30 min; initial O2 pressure, 5 MPa; NaOH solution, 3 M.

Figure 4. Effect of initial O2 pressure on the conversion of (a) Ex and (b) Re and the yield of carboxylic acids. Reaction conditions: (a) temperature, 230 °C; reaction time, 30 min; NaOH solution, 1 M; (b) temperature, 260 °C; reaction time, 30 min; NaOH solution, 3 M.

pressure on the conversion of Ex and Re and the yield of carboxylic acids, and Figure S2 in the Supporting Information shows the effect of initial O2 pressure on each BCA yield from Ex and Re. When Ex was oxidized, the conversion of Ex easily reached 100% at low initial O2 pressures, but the yield of carboxylic acids increased as the initial O2 pressure increased and peaked at 4.5 MPa. Further increasing the initial O2 pressure beyond this point caused the yield of carboxylic acids to decrease. O2 acts as the oxidant in the system, and O2 concentration has a great effect on the reaction rate. Taraba et al.34 reported that the oxidation of coal in water was a firstorder reaction, with respect to O2. Thus, the oxidation reaction rate was mainly decided by the O2 concentration. According to Henry’s law, a high O2 pressure means a high O2 concentration in aqueous solution. At low initial O2 pressures, the concentration of O2 in aqueous solution is low, resulting in a low reaction rate and a low yield of carboxylic acids. Obviously, an increase in the initial O2 pressure promotes the generation of carboxylic acids. Excessively low or high oxygen concentration results in unreached oxidation or overoxidation, respectively. As can be seen in Figure 4b, when Re was oxidized, the conversion of Re reached 100% at high O2 pressures of 5.5 MPa or more. The yield of carboxylic acids increased as the initial O2 pressure increased and also peaked at 5.5 MPa. The results suggest that when the yield of carboxylic acids peaked, the oxidation of Re requires a higher oxygen pressure and a higher oxidation temperature than that of Ex. It also

showed that the structure of Ex has higher reactivity than that of Re. 3.2.3. Effect of Reaction Time on the Yield of Carboxylic Acids. Figure 5 shows the effect of reaction time on the conversion of Ex and Re and the yield of carboxylic acids, and Figure S3 in the Supporting Information shows the effect of reaction time on the yield of each BCA from Ex and Re. When Ex was oxidized, the conversion of Ex reached 100% at 10 min, but the yield of carboxylic acids increased with the increase of reaction time and peaked at 40 min. Afterward, the overoxidation of carboxylic acids decreases the yield of carboxylic acids. When Re was oxidized, as shown in Figure 5b, the conversion of Re reached 100% at 30 min, but the yield of carboxylic acids reached a maximum value of 36.7% at 70 min. The alkali-oxygen oxidation of Ex and Re to produce BCAs and SMFAs is a consecutive reaction, and an increase of reaction time results in overoxidation. The above results indicate that the oxidation of Re requires higher initial O2 pressures, higher oxidation temperatures, and longer reaction time than that of Ex. It further demonstrates that the structures of Ex has higher oxidation activity than that of Re. 3.3. Production of BCAs and SMFAs from Lignite by Two-Stage Alkali-Oxygen Oxidation. Within the range of investigated conditions, the yield of carboxylic acids from Ex was 35.32%, including 5.60% formic acid, 6.15% acetic acid, 11.95% oxalic acid, 0.85% succinic acid, and 11.13% BCAs when the temperature, initial O2 pressure, reaction time, and 1974

DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research

comparison between the present two-stage alkali-oxygen oxidation and the traditional one-stage alkali-oxygen oxidation on the yield of each BCA and the yields of SMFAs. From the above comparison, it can be inferred that the two-stage alkalioxygen oxidation method can significantly increase the yield of BCAs and SMFAs. Moreover, the alkali/coal mass ratio based on lignite is 2.0 (alkali/coal mass ratio = amount of NaOH usage in Ex × yield of Ex + amount of NaOH usage in Re × (1 − yield of Ex)). Compared with the one-stage alkali-oxygen oxidation (alkali/coal mass ratio = 3.0), the two-stage alkalioxygen oxidation also decreases the amount of consumed alkali. Therefore, this work could increase the possibility of efficient utilization of poor-quality coal, lignite, to produce more valuable chemicals BCAs and SMFAs with yields of 17.31% (28.5% in mass) and 19.02% (50.2% in mass), respectively, which are increasingly desired for the chemical industry. 3.5. Study of the Structures of Ex and Re by SolidState 13C CP/MAS NMR and FT-IR. 3.5.1. Solid-State 13C− CP/MAS NMR. To quantify the relative proportion of different carbon types in Ex, Re, and HLH, 13C NMR was performed and the results are shown in Figures S5−S7 in the Supporting Information. The 13C NMR spectra can be divided into three main regions: aliphatic carbons (0−90 ppm), aromatic carbons (90−170 ppm), and carbonyl carbons (170−220 ppm).36 Peak fitting spectra of 13C NMR was broadly used to study various types of carbon and their contents.36−45 The chemical shifts of different carbons in 13C NMR were determined according to the literature,40−45 and they are shown in Table S1 in the Supporting Information. Peak fitting of the 13C NMR spectra was conducted using a NMR peak fitting program, and the fitting curves are shown in Figures S5−S7. The chemical shifts of different carbons in Ex, Re, and HLH through curvefitting are also shown in Table S1. Based on Table S1, structural parameters of carbons were calculated (see Table S2 in the Supporting Information) and the results of curve fitting are listed in Table 2. As shown in

Figure 5. Effect of reaction time on the conversion of (a) Ex and (b) Re and the yield of carboxylic acids. Reaction conditions (a): temperature, 230 °C; initial O2 pressure, 4.5 MPa; NaOH solution, 1 M; (b): temperature, 260 °C; initial O2 pressure, 5.5 MPa; NaOH solution, 3 M.

NaOH content were 230 °C, 4.5 MPa, 40 min, and 1 M, respectively. The yield of carboxylic acids from Re was 36.69%, including 0.35% formic acid, 5.05% acetic acid, 11.81% oxalic acid, and 19.48% BCAs when the temperature, initial O2 pressure, reaction time, and NaOH content were 260 °C, 5.5 MPa, 70 min, and 3 M, respectively. The total yield of carboxylic acids based on lignite was calculated using the following formula:

Table 2. Integration of Carbon Types in 13C NMR for Ex, Re, and HLH Mole Fraction on Carbon [%]

total yield of carboxylic acids = (yield of carboxylic acids from Ex) × yield of Ex + (yield of carboxylic acids from Rx) × [1 − (yield of Ex)]

The above yield of carboxylic acids was calculated based on carbon content of Ex or Re. The result indicates that the total carboxylic acids yield, based on lignite, is 36.28%, including 19.02% SMFAs and 17.31% BCAs. 3.4. Comparison with Traditional One-Stage AlkaliOxygen Oxidation. Alkali-oxygen oxidation is widely used for the selective oxidation of lignite to carboxylic acids. Wang et al.35 reported that the alkali-oxygen oxidation of lignite produced 12.6% BCAs and 14.05% SMFAs at the optimal conditions of 240 °C, alkali/coal mass ratio of 3.0, and 5.0 MPa of initial O2 pressure. In this work, the two-stage alkali-oxygen oxidation of lignite produced 17.31% BCAs and 19.02% SMFAs. Figure S4 in the Supporting Information shows a

carbon functionality

symbol

Ex

Re

HLH

aliphatic methyl aliphatic C(2) carbon aromatic methyl aromatic methylene quatermary oxy-aliphatic ortho-oxyaromatic protonated aromatic protonated bridging ring junction aromatic branched oxy-aromatic carboxyl carbonyl

fM al fBal fAal fHal fDal fOal fAar fHar fBar fCar fOar fCa fOa

1.2 3.8 2.3 8.1 4.9 1.8 7.3 17.7 5.8 21.2 13.8 7.7 4.2

2.2 4.2 4.5 21.9 9.8 3.5 2.9 11.7 8.2 15.7 10.2 1.9 3.3

2.1 4.0 3.2 21.6 6.6 2.3 3.4 15.7 7.3 16.6 10.3 3.2 3.7

Table 2, for Ex, the percentage of oxygen-containing functional groups, which is defined as percentage of oxygen‐containing functional groups = f alO + f arO + f aC + f aO 1975

DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research is 27.6%. Compared with the oxygen-containing functional groups of Re (18.8%) and HLH (19.6%), the order of oxygencontaining functional groups is Ex > HLH > Re. Furthermore, Table 2 also shows that the percentage of aliphatic carbons, which is defined as percentage of oxygen‐containing functional groups = falM + falB + falA + faH + faD + f aO

in Ex, Re, and HLH are 22.1%, 46.2%, and 39.7%, respectively, and the order is Re > HLH > Ex. The cores of carbon skeletal structures in coal are mainly aromatic rings, which are stable. The molar fraction of aromatic bridgehead carbon (Xb, Xb = farB/fa) can reflect the average size of aromatic rings in lignite structure, where fa is the fraction of aromatic carbon in coal:

Figure 6. FT-IR spectra of Ex, Re, and HLH.

The above analysis of FT-IR also shows that side chains of aromatic rings in Ex contain more oxygen-containing functional groups and less alkyl chains than those in Re, similar to the analysis of 13C NMR. Therefore, compared with Re, Ex has higher oxidation activity. This also leads to the conclusion that the alkali-oxygen oxidation of Ex requires milder reaction conditions than those of Re.

fa = farA + farH + farB + f arC + f arO

For example, for benzene, naphthalene and anthracene, Xb = 0, 0.2, and 0.286, respectively. This indicates that the size of the aromatic rings increases as Xb increases. Based on the results given in Table 2, the Xb values of Ex, Re, and HLH are 0.089, 0.169, and 0.137, respectively. Based on the previous studies,40,44,45 most of aromatic carbon structures in lignite are mainly single and double aromatic rings, with few triple rings or larger. Therefore, only single and double aromatic rings are considered. Based on the Xb value of Ex and Re, it can be calculated that the ratio of single to double aromatic rings in Ex is 2.08, and in Re is 0.306. This indicates that the carbon skeletal structure of Ex is mainly composed of single aromatic rings. The analysis of 13C NMR shows that most of the aromatic carbon structures in Ex are single aromatic rings and side chains of aromatic rings containing more oxygen-containing functional groups, whereas aromatic carbon structures in Re are mainly composed of condensed aromatic rings, side chains of aromatic rings containing more alkyl chains. According to previous studies, the oxygen-containing functional groups are active ingredients in lignite and have high oxidation activity; whereas the structures of condensed aromatic rings and alkyl carbon are inactive ingredients and have low oxidation activity. So it can be concluded that the structure of Ex has higher oxidation activity than that of Re. This result is consistent with the results in section 3.2 and proves that alkali-oxygen oxidation of Ex requires lower reaction temperature, initial O2 pressure and alkali concentration, and a shorter reaction time than those of Re. 3.5.2. FT-IR. FTIR has been widely used in the analysis of functional groups of coals. The FT-IR spectra of Ex, Re, and HLH are shown in Figure 6. The strong absorption peak at wavenumbers from 3600 cm−1 to 3200 cm−1 is assigned to phenolic hydroxyl Ar−OH stretching, which is intermolecular or intramolecular association absorption peak, and the peak intensity of Ex is the largest. The other typical and relatively broader peaks at 1710 cm−1 are contributed to CO stretching of carboxyl (e.g., acid, ester) and/or carbonyl (e.g., aldehyde, ketone), and the order of peaks intensity is Ex > HLH > Re. The aliphatic C−H stretching region occurs at 3000−2800 cm−1. In Figure 6, the two intense signal peaks at ∼2920 and 2850 cm−1 are attributed to the asymmetrical and symmetrical alkyl CH2 stretching, and the order of peaks intensity is Re > HLH > Ex.

4. CONCLUSIONS In this work, we proposed the production of benzene carboxylic acids (BCAs) and small-molecule fatty acids (SMFAs) from lignite by two-stage alkali-oxygen oxidation. The first stage divides lignite extracted with NaOH solution into two parts, extract (Ex) and residue (Re), which have different oxidation activities; then, in the second stage, they were oxidized separately to produce BCAs and SMFAs using alkali-oxygen oxidation under different conditions. The optimal oxidation conditions for Ex are a temperature of 230 °C, an initial O2 pressure of 4.5 MPa, a reaction time of 40 min, a NaOH concentration of 1.0 M, while those conditions for Re are a temperature of 260 °C, an initial O2 pressure of 5.5 MPa, a reaction time of 70 min, and a NaOH concentration of 3.0 M. Under the two optimal conditions, the total carboxylic acid yield based on lignite is 36.28%, including 19.02% of SMFAs and 17.31% of BCAs. The oxidation of Re requires a higher initial O2 pressure, a higher oxidation temperature and NaOH concentration, and a longer reaction time than that of Ex. Ex has a higher oxidation activity than Re. The analyses of 13C NMR and FT-IR show that most of aromatic carbon structures in Ex are single aromatic rings, and side chains of aromatic rings containing high oxygen-containing functional groups, which result in high activity. However, aromatic carbon structures of Re are mainly composed of condensed aromatic rings and side chains of aromatic rings containing more alkyl chains, which result in inactivity. This explains why Ex and Re have different oxidation conditions. The two-stage alkali-oxygen oxidation method can significantly increase the yields of BCAs and SMFAs and decrease the amount of alkali used, compared with the traditional one-stage alkali-oxygen oxidation method.

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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.iecr.6b04562. Effect of temperature, initial O2 pressure, and reaction time on the yield of each BCA from Ex and Re; comparison of the present two-stage alkali-oxygen 1976

DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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Industrial & Engineering Chemistry Research

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oxidation with the traditional one-stage alkali-oxygen oxidation on the yield of each BCA and the yield of SMFAs; types, symbols and chemical shifts of carbon analyzed with 13C NMR; and 13C NMR spectra and their fitting curves of Ex, Re and HLH lignite (PDF)

AUTHOR INFORMATION

Corresponding Author

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

Weize Wu: 0000-0002-0843-3359 Funding

The project is financially supported by the National Natural Science Foundation of China (No. 21676019), and the longterm subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT). 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. REFERENCES

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DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978

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DOI: 10.1021/acs.iecr.6b04562 Ind. Eng. Chem. Res. 2017, 56, 1971−1978