Acid-Catalyzed Depolymerization of Coals of Different Rank under

Energy & Fuels 1999, 13, 197-203

197

Acid-Catalyzed Depolymerization of Coals of Different Rank under Mild Conditions with HF and Superacid HF/BF3 Kiyoyuki Shimizu,* Ikuo Saito, and Hiroyuki Kawashima National Institute for Resources and Environment, AIST, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

Shinsuke Sasaki Science University of Tokyo, Noda, Chiba 278-8510, Japan Received September 9, 1998. Revised Manuscript Received November 9, 1998

Hydrogen fluoride and a mixture of hydrogen fluoride/boron trifluoride were examined as acid catalysts for solubilization of coals of different coal rank under mild reaction conditions. Hydrogen fluoride alone accelerated solubilization of the coals with toluene at 150 °C to some extent. Mixtures of hydrogen fluoride and boron trifluoride in the presence of toluene at 150 °C for 3 h under autogenous pressure were found to solubilize all coals nearly completely. Miike bituminous coal readily produced highly soluble products in all reaction conditions, whereas the extent of solubilization of Yallourn lignite depended on reaction conditions such as temperature, stabilizer, and acidity of acid catalyst. Solubilization of Miike bituminous coal was mainly the result of depolymerization by cleavage of methylene bridges, while in Yallourn lignite, deoxygenation as well as depolymerization was required for solubilization. Most of the remaining BF3 probably formed complexes with the organic matter, but no fluoride compounds were detected in the soluble fraction with the exception of BF3. Concentration of the main ash component, Si, in the original coal greatly decreased after the reaction, but a small proportion of another major ash component, Al, remained as AlF3.

1. Introduction Conventional coal liquefaction, which relies basically on the thermal cracking of internuclear bonds, requires severe reaction conditions: high temperature (420-450 °C) and high hydrogen pressure (15-20 MPa) to prevent the radicals produced from recombination, leading to retrogressive reactions. Such severe conditions raise the costs of facilities and operations. Acid-catalyzed coal depolymerization has been widely studied as a way to liquefy coal under milder conditions.1-7 Coal depolymerization has been studied in the presence of strong acids or superacids such as AlCl3-HCl, AlBr3-HBr, SbF5-HF, SbF5-FSO4H, and trifluoromethanesulfonic acid.5-9 However, they are usually very difficult to remove and recover completely from resulting solids or products. In contrast, mixtures of HF/BF3, which has been recognized as a Bro¨nsted/Lewis superacid catalyst (1) Heredy, L. A.; Neuworth, M. B. Fuel 1962, 41, 21. (2) Ouchi, K.; Imuta, K.; Yamashita, Y. Fuel 1965, 44, 205. (3) Larsen, J. W.; Kuemmerle, E. W. Fuel 1976, 55, 162. (4) Shabtai, J.; Oblad, H. B.; Katayama, Y.; Saito, I. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1985, 30 (3), 495. (5) Kumagai, H.; Shimomura, M.; Sanada, Y. Fuel Process. Technol. 1986, 13, 97. (6) Farcasiu, M. Fuel Process. Technol. 1986, 14, 161. (7) Shimizu, K.; Karamatus, H.; Inaba, A.; Suganuma, A.; Saito, I. Fuel 1995, 74, 853. (8) Low, J. Y.; Ross, D. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1977, 22, 118. (9) Amendola, S. C. U.S. Patent 4,202,757, 1980.

for Friedel-Crafts reactions, isomerization and separation of m-xylene, and formylation of aromatic compounds on an industrial scale (Mitsubishi Gas Chemical Co., Ltd.), are fully recoverable from the products by distillation and can be reused because their boiling points are very low (HF, 19.9 °C; BF3, -101 °C). Olah studied coal liquefaction using the HF-BF3H2 system and HF-BF3-isopentane.10,11 We also reported that HF and HF/BF3 in the presence of toluene depolymerized coal more efficiently at 100-150 °C through acid-catalyzed transalkylation reactions in subbituminous coal.12,13 In that study, however, recovery of the HF/BF3 mixture was not complete after the reaction despite their low boiling point, and the forms of the remaining BF3 and HF in the treated coal were not clear. Acid-catalyzed reactions are usually carried out at a low temperature, which favors depolymerization of some types of coals. In the present study, solubilization of coals of different rank was carried out at 50-150 °C in order to evaluate the use of recyclable superacid HF/BF3 as a catalyst for efficient depolymerization via an ionic reaction of coal of various ranks. The depolymerization of coals in the (10) Olah, G. A.; Bruce, M. R.; Edelson, E. H.; Husain, A. Fuel 1984, 63, 1130. (11) Olah, G. A.; Surya Prakash, G. K.; Husain, A. Fuel 1984, 63, 1427. (12) Shimizu, K.; Saito, I. Energy Fuels 1998, 12, 115. (13) Shimizu, K.; Saito, I. Energy Fuels 1998, 12, 734.

10.1021/ef980185j CCC: $18.00 © 1999 American Chemical Society Published on Web 12/17/1998

198 Energy & Fuels, Vol. 13, No. 1, 1999

Shimizu et al.

Table 1. Elemental Analyses of Original Coals elemental analyses (wt %, daf base) run

C

H

N

Odiff

ash (wt %)

Yallourn Taiheiyo Miike

65.6 73.3 82.0

4.6 6.4 6.7

0.6 1.2 1.2

29.2 19.1 10.1

1.3 13.7 14.7

Table 2. Reaction Conditions in the Coal Treatmentsa run

temp (°C)

HF (g)

Y-1 Y-2 Y-3 Y-4 Y-5 Y-6

150 50 100 150 150 150

5.8 5.8 5.8 5.8 5.8 5.4

T-1 T-2 T-3 T-4 T-5 T-6

150 50 100 150 150 150

5.10 5.10 4.92 5.8 5.5 5.82

M-1 M-2 M-3 M-4 M-5 M-6

150 50 100 150 150 150

5.82 5.46 5.46 5.82 5.82 6.06

BF3 (g)

stabilizer

WI (%)

toluene toluene toluene toluene isopentane H2

28 37 44 97 32 -7


3 18 35 71 24 -14


12 11 9 113

Yallourn 1.10 1.18 1.26 1.24 1.38 Taiheiyo 1.38 1.28 1.39 1.38 1.38 Miike

a

1.4 1.4 1.4 1.4 1.25

-3

WI ) Weight increase.

acid-catalyzed reaction process was considered in terms of the chemical structure of coals and the behavior of oxygen-containing functional groups. The forms of the remaining HF/BF3 in the treated coals were characterized by 19F NMR and XPS (X-ray photoelectron spectroscopy). 2. Experimental Section Yallourn lignite, Taiheiyo sub-bituminous, and Miike bituminous coal were ground to a diameter of less than 0.25 mm, dried in a vacuum at 110 °C for 24 h, and used as feedstock. Elemental analyses of original coals are summarized in Table 1. Liquefaction was carried out in a hastelloy-C microautoclave of 100-mL capacity. Coal (5 g) and toluene or isopentane (20 mL) were placed in the dry ice-methanol cooled autoclave. First, the reactor of the autoclave was evacuated by vacuum pump; then the HF (5-6 g/g of coal) and BF3 (0-1.4 g/g of coal) were introduced into the coal-solvent slurry while dry ice-methanol cooling continued. Gaseous hydrogen (5.1 MPa) was introduced to the autoclave after BF3 instead of toluene or isopentane. The autoclave was then heated to 50-150 °C at a heating rate of 1.5 °C per min for 3 h under autogenous pressure with vigorous stirring. After the reaction, gaseous HF/BF3, stabilizer (toluene or isopentane), and the volatile fraction from stabilizer in the autoclave were depressurized and absorbed into ice-water at 150 °C under flowing nitrogen gas (100-150 mL/min) with stirring (300 rpm) for 20-120 min. Part of the contents of the autoclave was slowly poured into cold water, filtered, and washed in water with sonication. The remainder was gradually neutralized with a cool aqueous solution (5 wt %) of Na2CO3 and then washed with an aqueous methanol solution (30 to 50 vol %) 3 times in order to remove the small amount of neutralized NaF product and dimer of toluene. All solid products were vacuum-dried at 110 °C for 24 h. Reaction conditions are summarized in Table 2. Toluene and isopentane were used as a stabilizer to prevent fragments (carbenium ions) from combining and recombining.

Consequently, toluene and isopentane were incorporated into the subsequent carbenium ions from cleavage of coal molecules, resulting in a weight increase of the feed coal. The product derived from toluene and isopentane itself by the polymerization reaction during the reaction was negligibly small (0.7-2.3 wt %) in the products after 3 treatments of bubbling nitrogen gas at 90-110 °C, filtration with an aqueous methanol solution (30-50 vol %), and vacuum-drying. Calculations used to obtain the yield of the soluble fraction and weight increase have been described in detail elsewhere.12 The products were extracted with benzene, THF, and pyridine in a sequential Soxhlet extractor. The average molecular weights of the soluble fractions (benzene-soluble, BS; benzene-insoluble-THF-soluble, BI-THFS; pyridine-soluble, PS) were measured by vapor pressure osmometry (KNAUER Vapor Pressure Osmometer). The 13C-CP/MAS solid-state NMR spectra were measured with CHEMAGNETICS at 75.58 MHz. The following operating parameters were used: a 90° proton pulse of 5 µs, a pulse repetition time of 4 s, and an accumulation of 4000 scans. Oxygen-containing functional groups were determined by the method of Hatami et al.14 The treated coal samples were vacuum-dried at 110 °C for 20 h. Determination of carboxyl, hydroxyl, and carbonyl groups was carried out by ion exchange in aqueous calcium acetate ((CH3COO)2Ca); acetylation in an acetic anhydride ((CH3CO)2O)-pyridine solution; and oximation with hydroxylamine hydrochloric acid (NH2OH‚HCl) in pyridine, respectively. Recovery of HF/BF3 was calculated from analyses of the neutralization products in aqueous solution, HCl and H3BO3, when HF and BF3 were fixed with CaCl2. H3BO3 was hydrolyzed with mannitol (HOCH2(CHOH)4CH2OH) before titration with a 0.1 N NaOH aqueous solution.15 The THF-soluble fraction was analyzed by 19F NMR in the solvent THF-d1 using hexafluorobenzene (C6F6) as an internal standard. The 19F NMR spectrum was obtained under the following conditions: pulse width 9 µs, spectral width 282 MHz, pulse repetition 3.262 s, and proton decoupling by a Bruker AC-300P FT-NMR. The F and B contents of the original coal and products were measured by ion chromatography and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Samples were pretreated with Na2CO3 at 550 °C for 2 h, and the residue produced was dissolved in water for instrumental analysis. The content of the inorganic component in ash of the original coals and treated coals was measured by ICP-AES. Samples were decomposed with mixed acid (HF and HNO3) and dried, and the ash produced was dissolved in NaOH solution for instrumental analysis. XPS was measured using Al KR radiation (1486.6 eV) from an SSX-100 model 1206 (Surface Science Instruments Co., Ltd.). The bonding energy was corrected to the carbon (1S) peak at 284.6 eV. Spectra were obtained at a pass energy of 155.625 eV in the wide-scan spectra and 54.750 eV in the narrow-scan spectra. The full-width at half-maximum (fwhm) of the Au 4f7/2 signal was 1.71 eV in the wide-scan spectra and 1.12 eV in the narrow scan.

3. Results 3.1. Coal Solubilization with HF Alone or with HF/BF3. The degree of solubilization of the original coal and coal treated with HF/toluene is shown in Figure 1. The original coals were almost insoluble in benzene but slightly soluble in THF and more soluble in pyridine. Miike bituminous coal showed higher extractability than the others. Addition of anhydrous HF of ap(14) Hatami, M.; Osawa, Y.; Sugimura, H. J. Fuel Soc. Jpn. 1967, 46, 819. (15) Lourijsen, S. Bull. Soc. Chem. Fr. 1956, 893.

Acid-Catalyzed Depolymerization of Coals

Figure 1. Extractability of original and treated coals with HF in the presence of toluene at 150 °C. (4) Benzene-soluble fraction of original coal. (0) THF-soluble fraction of original coal. (O) Pyridine-soluble fraction of original coal. (2) Benzenesoluble fraction of treated coal. (9) THF-soluble fraction of treated coal. (b) Pyridine-soluble fraction of treated coal.

Figure 2. Extractability of original and treated coals with HF/BF3 in the presence of toluene at 150 °C. (2) Benzenesoluble fraction of treated coal. (9) THF-soluble fraction of treated coal. (b) Pyridine-soluble fraction of treated coal.

proximately 5.1-5.8 g/g of coal in the reaction improved the extractability of products. Taiheiyo sub-bituminous and Miike bituminous coal showed higher extractability than did Yallourn lignite. Extractability of the treated coals with a mixture of HF and small proportions of BF3 (7 mol %) in the presence of toluene is shown in Figure 2. HF/BF3 provided much higher extractability: more than 90 wt % in pyridine, more than 80 wt % in THF, and about 60-75 wt % in benzene. HF/BF3 greatly increased the extractability of products from Yallourn lignite also. 3.2. Effects of Stabilizer. The extractability of products from coals treated with HF/BF3 and different kinds of stabilizer is shown in Figure 3. Extractability of products from Yallourn lignite depended very much upon the type of stabilizer. Yallourn coal was solubilized almost completely in pyridine by the reaction with toluene, but it showed low extractability, 49 and 20 wt %, in the reactions with isopentane and hydrogen, respectively. Differences in extractability of Taiheiyo

Energy & Fuels, Vol. 13, No. 1, 1999 199

Figure 3. Effect of stabilizer on coal solubilization. Reaction temperature: 150 °C. (b) Pyridine-soluble fraction of treated coal. (2) Benzene-soluble fraction of treated coal. (9) THFsoluble fraction of treated coal.

Figure 4. Effects of reaction temperature on coal solubilization with HF/BF3/toluene. (O) Original coal. (2) 50 °C. (9) 100 °C. (b) 150 °C.

sub-bituminous coal in reactions with their stabilizers were smaller than those with Yallourn lignite. Products from Miike bituminous coal did not change significantly with stabilizer type. 3.3. Effects of Reaction Temperature. The effects of reaction temperature on coal solubilization in the presence of HF/BF3 (7 mol %) are shown in Figure 4. The reaction at 50 °C showed higher extractability than in the original coals. The reaction at 100 °C greatly increased extractability, especially in Miike bituminous coal, which was almost completely solubilized. The pyridine-soluble yield of products from coal treated at 50 and 100 °C increased with increasing carbon content in the coal. At these temperatures the order of extractability was the same as that of the original coals. However, at 150 °C, the extractability of products from all coals was similar. 3.4. Average Molecular Weight of Soluble Fractions. The average molecular weights of the soluble fractions are shown in Table 3. That of the PS fraction of the original coals was around 830-1000. The BS fractions of the treated coal had lower molecular weights,


Shimizu et al.

Table 3. Average Molecular Weights of Soluble Fractions of Treated Coals reaction condition run Yallourn Y-1 Y-2 Y-4 Y-5 Y-6 Taiheiyo T-1 T-2 T-4 T-5 T-6 Miike M-1 M-2 M-4 M-6 a

temp (°C)

HF (g)

150 50 150 150 150

5.8 5.8 5.8 5.8 5.4

150 50 150 150 150

5.8 5.10 5.8 5.5 5.82

150 50 150 150

5.82 5.46 5.82 6.06

average molecular weight

BF3 (g)

stabilizer

BS (content wt %)

BI-THFS (content wt %)

1.10 1.26 1.24 1.38

toluene toluene toluene isopentane H2

(3) 395(18) 334(66) 390(27) 506(14)

479(22) 1068(5) 1190(30) 1295(12)

1.38 1.39 1.38 1.38

toluene toluene toluene isopentane H2

448(36) 465(21) 347(58) 521(41) 396(13)

1131(14) 1168(12) 876(22) 1134(8) 606(15)

1.4 1.4 1.25

toluene toluene toluene H2

385(23) 439(22) 304(75) 492(31)

557(31) 863(12) 372(11) 545(43)

PS (content wt %) 985(9)

893(14)

832(25)

BS: benzene soluble. BI-THFS: benzene insoluble-THF soluble. PS: pyridine soluble. Table 4. Distribution of Oxygen-Containing Functional Groups in the Original and Treated Coals reaction conditions run

Yallourn Y-1 Y-2 Y-3 Y-4 Y-5 Y-6 Taiheiyo T-1 T-2 T-3 T-4 T-5 T-6 Miike M-3 M-4 M-6

temp (°C)

HF (g)

BF3 (g)

wt % stabilizer

150 50 100 150 150 150

5.8 5.8 5.8 5.8 5.8 5.4

1.10 1.18 1.26 1.24 1.38


150 50 100 150 150 150

5.8 5.10 4.92 5.8 5.5 5.82

1.38 1.28 1.39 1.38 1.38


100 150 150

5.46 5.82 6.06

1.40 1.40 1.25

toluene toluene H2

COOHa

OH

CdO

Orestb

total

4.1 4.1 1.9 2.3 0.6 0.8 2.5 0.6 1.3 1.0 0.5 0.7 1.2 3.0 0.1

5.7 5.0 5.0 2.9 1.8 3.9 5.9 5.3 3.8 5.1 6.1 3.4 4.6 5.7 3.3 1.7 0.7 3.2

4.5 3.8 2.8 4.2 4.5 4.5 4.1 3.4 2.8 2.8 2.9 2.8 3.4 2.6 1.1 1.9 2.5 0.7

14.9 13.2 12.8 8.4 6.5 9.0 8.4 9.8 4.9 3.7 3.0 1.7 4.3 2.5 3.6 3.8 3.4 3.8

29.2 26.1 22.5 17.8 13.4 18.0 20.9 19.1 12.8 12.6 12.5 8.6 13.5 13.8 8.1 7.4 6.6 9.3

1.6

a

Content of oxygen functional groups and total oxygen was corrected for weight increases during the reactions. b Orest: ether or ester groups.

around 350-600. The reaction with HF/BF3 and toluene resulted in products with a lower average molecular weight than those from the reaction with HF and toluene, indicating that HF/BF3 accelerated more depolymerization. The average molecular weights of soluble fractions from products obtained under gaseous hydrogen, isopentane, and toluene decreased in that order. The average molecular weights of soluble fractions obtained from reactions at 50 °C were higher than those from reactions at 150 °C. 3.5. Oxygen-Containing Functional Groups. The distribution of oxygen-containing functional groups in the original coals and the treated coals is summarized in Table 4. Oxygen-containing functional groups were divided into four groups (carboxylic, hydroxyl, carbonyl groups, and Orest). Orest were mainly ether groups such as Ar-CH2-O-R, Ar-O-Ar, and Ar-O-R. The content of all functional groups was highest in Yallourn lignite and lowest in Miike bituminous coal. All groups in the original coals decreased with increasing coal rank. Most of the oxygen-containing functional groups in Yallourn lignite and Taiheiyo sub-bituminous coal decreased after reaction. HF in the presence of toluene slightly decreased the content of oxygen-containing

functional groups, while HF/BF3 significantly reduced them. Reductions in ether bonds and in carboxylic and hydroxyl groups increased with increasing reaction temperature. The reaction with isopentane or H2 under HF/BF3 at 150 °C retained more hydroxyl groups and ether bonds in the products than did the reaction with toluene. There were slightly more hydroxyl and carboxylic groups in products from Yallourn coal obtained under H2 than under isopentane, indicating that these oxygen-containing functional groups remained and restricted extractability. Miike bituminous coal has few oxygen-containing functional groups, and there was little decrease in them after reaction. 3.6. Solid CP/MAS-13C NMR Spectra. Table 5 summarizes the carbon distribution of the original and treated coal from the reaction with HF/BF3 under gaseous hydrogen. The carbon atoms were classified into seven categories as shown. Original Yallourn lignite had more aromatic carbons bound to oxygen (phenolic-OH), oxygen functional groups CdO, COOH, and ether bonds (-O-CH2-) but fewer methylene bridges and terminal CH3 content than did the other coals. The carbon distribution of Taiheiyo coal was similar to that of Miike coal, but the former had slightly more ether bonds and


Energy & Fuels, Vol. 13, No. 1, 1999 201

Table 5. Carbon Distribution in Original and Treated Coals by NMR (ppm) COOH, CdO (220-171)

phenolic C-O (171-149)

Ar-C (149-128)

Ar-H (128-93)

-CH2-O(75-50)

-CH2-, CH (55-22)

-CH3 (22-0)

7.1 8.5 4.3 3.7 4.3 3.6

12.9 14.6 8.2 9.6 8.3 10.0

17.2 18.7 15.3 18.4 15.7 21.7

27.2 26.0 24.4 29.7 24.4 31.5

9.4 6.6 7.0 7.8 5.9 6.5

22.3 18.8 34.1 22.9 32.5 18.6

3.9 6.8 6.7 7.9 8.9 8.1

Yallourn Y-6 Taiheiyo T-6 Miike M-6

Table 6. Recovery of HF and BF3 wt % run

HF

Y-1 Y-4 Y-5 T-1 T-4 M-4

95 81 86 95 93 78

BF3 26 16 81 26

Table 7. Elemental Analyses of Original and the Treated Coalsa elemental analyses (wt %, daf base) run

C

H

N

Yallourn Y-4 Taiheiyo T-4 Miike M-4

65.6 85.7 73.3 85.7 82.0 85.5

4.6 6.4 6.4 6.4 6.7 6.9

0.6 0.3 1.2 0.9 1.2 0.5

B

F

0.08

0.80

0.31

2.13

0.3

3.0

Odiff

ash (wt %)

29.20 6.72 19.1 4.56 10.1 3.8

1.3 1.2 13.7 5.8 14.7 7.2

B:F ) 0.08:0.80 ) 1:5.71. B:F ) 0.31:2.13 ) 1:3.85. B:F ) 0.30: 3.00 ) 1:5.50. Ash content was calculated by considering weight. Increase by stabilizer (toluene) during the reaction. a

methylene bonds and fewer terminal methyl groups. After reaction, the reduction in methylene bridges was the lowest (less than 5%) in the treated Yallourn coal. The treated Yallourn coal had more aromatic carbons bound to oxygen and oxygen-functional groups than did those of products from the other coals, indicating that products from this coal still contained a significant amount of oxygen functional groups. 3.7. Recovery of HF and BF3. Recovery rates of HF and BF3 are shown in Table 6. The recovery of HF was around 80-95%; the recovery rate of BF3 was much lower except for that from the T-3 group. 3.8. Content of Boron and Fluorine in Product. Elemental analyses of original coals and products from the reaction under HF/BF3/toluene at 150 °C are summarized in Table 7. Boron and fluorine were observed in all products. The molar ratios of boron to fluorine in the total products from Taiheiyo and Miike coal were not 1:3; excess fluorine was found. These results indicate that both BF3 and HF remained in the product. 3.9. 19F NMR Spectrum. The 19F NMR spectrum of the benzene-insoluble-THF-soluble fraction in THF-d1 with C6F6 as the standard reagent is shown in Figure 5. The peak around 150 ppm is due to BF3 only; no other peaks were observed. 3.10. Content and Characterization of Inorganic Components. The composition of the main ash in original Miike and treated coal (M-6) is summarized in Table 8. After reaction, the Si content was drastically decreased and the content of Ca and Na was also reduced to almost one-third the original level. M-6 had a slightly lower Al content and more Fe content. The increase in Fe was probably due to a little corrosion

Figure 5. 19F NMR spectrum of benzene-insoluble-THFsoluble fraction in M-6.

Figure 6. XPS spectra of Al2p in original and treated coal (M-6): (s) Original coal; (- - -) M-6. Table 8. Composition of the Main Ash in the Original and Treated Miike Coal ash composition % Miike coal M-6

Si

Al

Fe

Ca

Na

1.66 0.28

1.04 0.82

0.40 0.78

0.90 0.32

0.24 0.08

during the reaction; four valves which connected with the hastelloy-C autoclave were made from stainless steel. XPS spectra of Al2p in the original and treated coal (M-6) are shown in Figure 6. AlF3 was the main compound in M-6. The major ash component in coal, Si and Al, has been recognized as mainly aluminasilicate, which exchanged with metal cation (e.g., Na+, Ca2+, Mg2+). The peak in the original coal was ascribed to Al-O (73.5-75.5 eV).16 In contrast, the peak in the product was shifted to around 76.3 eV after the reaction,


and its peak is probably mainly due to Al-F (76.1-76.3 eV).16

Shimizu et al.

The test coals were solubilized by reaction with HF/ toluene at 150 °C; the extractability of product Y-1 from Yallourn lignite was lower than that of the others. The average molecular weight of the soluble fraction in Y-1 was higher than that of the products (T-1 and M-1) from Taiheiyo sub-bituminous and Miike bituminous coals, indicating that the extent of Yallourn lignite depolymerization was lower. Deoxygenation is an important factor in coal solubilization and liquefaction as are depolymerization and hydrogenation. The degree of deoxygenation of Yallourn lignite in the reaction with HF alone (Y-1) was very low. The ether groups in Y-1 were nearly all intact, indicating that cleavage of the -CH2-O- bond did not take place in this reaction. Taiheiyo sub-bituminous coal lost more oxygen functional groups, while original Miike bituminous coal inherently has a low oxygen content. Thus, inhibition of solubilization by oxygen-containing functional groups in both Taiheiyo and Miike coals would be lower than that in Yallourn lignite. The mixture of HF and BF3 in the presence of toluene at 150 °C almost completely solubilized all coals. The BS fractions resulting from the reaction with HF/BF3 (Y-4, T-4, and M-4) had a limited range of average molecular weights, around 300-350, lower than those in the soluble fractions in products from the reaction with HF alone. HF/BF3/toluene removed COOH/CdO groups and cleaved -CH2-O- bonds even in Yallourn lignite, indicating that HF/BF3 in the presence of toluene promoted deoxygenation and depolymerization of coal, leading to high extractability. It is well-known that the acidity of Bro¨nsted acids such as HF can be greatly enhanced by combination with Lewis acids such as BF3 and SbF5; HF/BF3 (7 mol %) has a higher acidity (H0 ) -16.6) than HF alone (-11 to -15.1).17 This high acidity of HF/BF3 would promote the generation of both carbenium ions as fragments from the cleavage of ether as well as of methylene bridges in coal. Yallourn coal is a strong base because it has more heteroatom-containing functional groups, which would weaken the acidity of HF alone by the formation of complexes. In contrast, the strong acidity of HF/7mol % BF3 would be sufficient to depolymerize and deoxygenate Yallourn lignite. We described in a previous paper how extractability of products depends on the stabilizer in the reaction.12 Transalkylation of coal with toluene occurred preferentially to hydride transfer from isopentane or gaseous hydrogen to produce carbenium ions, promoting depolymerization and desulfurization.12,13 The low extractability of products obtained by treatment with HF/BF3/ H2 suggests that abstraction of hydride from H2 was more difficult than from isopentane. The reaction with isopentane or H2 under HF/BF3 at 150 °C retained more

hydroxyl groups and ether bonds in the products than did the reaction with toluene, indicating that with isopentane or H2 these oxygen-containing functional groups remained in the products and restricted their extractability. However, the high-ranking Miike bituminous coal was not affected by stabilizers in terms of the pyridine-soluble yield. As mentioned, Miike bituminous coal has few oxygen-containing functional groups, such that its deoxygenation in this acid-catalyzed reaction did not take place to a significant extent. This result suggests that changing of oxygen-containing functional groups in Miike bituminous coal did not significantly contribute to the solubilization reaction, so this coal could be solubilized easily without significant deoxygenation by reaction with hydrogen or isopentane. Strausz et al. reported that superacid-catalyzed hydrocracking for heavy oils with HF/BF3 in the presence of methylcyclohexane and its product was of high reactivity for oxygenation on exposure to air.19,20 They explained that a conjugated alkene was produced from the polymerization of methylcyclohexane itself. The mechanism of stabilization of the carbenium ion with methylcyclohexane is the same with isopentane. However, the increasing oxygen content in the product was not observable in this study. Isopentyl cation after abstraction of hydride would be reacted with an aromatic ring and converted to alkyl groups as previously reported.18 We found that the extractability of products from the reactions at 50 and 100 °C with HF/BF3 in the presence of toluene was in proportion with that of the original coals, and only Yallourn lignite was not almost completely solubilized to PS at this reaction temperature, but the reaction at 150 °C almost completely solubilized all coals. The average molecular weight of the soluble fractions from all products obtained at 150 °C was lower than that from the reaction at 50 °C, and removal of oxygen-containing functional groups increased to a great extent at 150 °C when compared with that at 50 and 100 °C. These results indicated that a temperature of 150 °C is required for Yallourn lignite solubilization and liquefaction by depolymerization and deoxygenation, even in this acid-catalyzed reaction. Products Y-6, T-6, and M-6 from the reaction with hydrogen reflect the chemical structure of the original coals because only a small amount of stabilizer was incorporated in these products. Product Y-6 from Yallourn lignite contained more oxygen-containing functional groups than the other products and showed less reduction in methylene bridges, leading to lower extractability. The methylene bridges were more reduced in Miike and Taiheiyo coals, and their products were highly soluble, having lower oxygen-containing functional groups. These results indicate that the improvement in extractability of Taiheiyo and Miike coals in the reaction is due to cleavage of methylene bridges; Yallourn coal requires removal of oxygen-containing functional groups as well as cleavage of methylene bridges. The coal constituent molecules which contain hydroxyl groups and ether bonds are highly reactive for

(16) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mullenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy (A Reference Book of Standard Data For Use In X-ray Photoelectron Spectroscopy); Perkin-Elmer Co., 1978. (17) Olah, G. A.; Surya Prakash, G. K.; Sommer, J. SUPERACIDS; Wiley: New York, 1985; p 51.

(18) Shimizu, K.; Miki, K.; Saito, I. Fuel 1997, 76, 23. (19) Strausz, O. P.; Mojelsky, T. W.; Payzant, J. D.; Olah, G. A.; Prakash, G. K. S. U.S. Patent 5,290,428. (20) Strausz, O. P.; Mojelsky, T. W.; Payzant, J. D.; Olah, G. A.; Prakash, G. K. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43, 526.

4. Discussion


Energy & Fuels, Vol. 13, No. 1, 1999 203 Scheme 1

recombination with coal fragments (carbenium ion) through the electrophilic reaction during acid-catalyzed reaction because their functional group is an electrondonating group.11,18 Yallourn lignites have more hydroxyl groups and ether bonds and probably produce hydroxyl groups in intermediates by the cleavage of the ether-methylene bond (-O-CH2-), as shown in Scheme 1. Consequently, Yallourn lignite would comparatively readily recombine during the reaction and its lower depolymerization induce inhibition of deoxygenation. Yallourn lignite, with a higher content of oxygencontaining functional groups, depends on the conditions of its reaction, such as stabilizer type, acidity of catalyst, and temperature, for the nature of its products. In contrast, Miike bituminous coal readily produces highly soluble products in any reaction conditions due to the low content of oxygen-containing functional groups in the original coal. The recovery rate of BF3 was low regardless of coal rank. Yallourn lignites have more oxygen-containing functional groups, and high-rank coals such as bituminous coal have a more condensed aromatic unit. These results indicate that BF3 easily forms strong complexes with basic aromatic rings as well as heteroatomcontaining functional groups in coals; this is because BF3 is an electron-deficient molecule. Fluorine was not observed in the BI-THFS fraction from Y-4 by 19F NMR except in BF3, indicating that HF hardly reacted with the organic matter in the coals. However, the molar ratios of boron to fluorine in the total products from Taiheiyo and Miike coal were not 1:3; excess fluorine was found (Table 7). These results indicate that HF reacted with ash and produced inorganic fluorides. The major ash component, Al, was slightly less after the reaction, although most components were greatly decreased. The main form of Al in coal has been recognized as aluminasilicate, and it is probably mostly converted to AlF3 by the reaction with HF. These results show that the excess content of fluorine in the treated coal was in the form of inorganic fluoride such as AlF3 produced by the reaction with HF. The forms of Fe and Ca present in M-6 were not clear

because it is difficult to differentiate between their oxides, which have almost the same binding energy. The Si in the main ash component and Na and Ca in the original coals were greatly reduced in the reactions. This is because Si reacts with HF and produces SiF4, which is a gas and is easily removed from the products by depressurizing with N2 bubbling and stirring at 150 °C. Thus, this superacid-catalyzed reaction also contributes to deashing, although the reason reduction of Na and Ca occurs is not clear. These results indicated that this superacid-catalyzed reaction contributes to deashing, but it produces inorganic fluorides such as AlF3 that release HF during combustion. SiF4 is a feedstock for hexafluorosilicate (Na2SiF6), which is used for water purification. AlF3 is widely used as a flux for aluminum refining (around 30 kg/ton of Al). Effective separation processes of these inorganic fluoride compounds from the organic matter in the product are required for the safe utilization of the products as fuel and useful metal fluorides for the industry. 5. Conclusions Reaction with HF/BF3/toluene at 150 °C solubilized all coals nearly completely regardless of rank. Solubilization of Yallourn lignite depended more on the reaction conditions such as temperature, stabilizer, and acidity of acid catalyst than did the other coals. Solubilization of Miike bituminous coal was mainly due to depolymerization by cleavage of methylene bridges, while Yallourn lignite was solubilized by deoxygenation as well as depolymerization. Miike bituminous coal easily produced highly soluble products, even when reacting with hydrogen or isopentane as stabilizers because of the low content of oxygen-containing functional groups in this coal. This superacid-catalyzed reaction with HF/BF3 also contributes to deashing. Only BF3 formed complexes with the organic matter in the coals, but HF reacted with ash in the coals and produced inorganic fluorides such as AlF3. EF980185J

Acid-Catalyzed Depolymerization of Coals of Different Rank under

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