Energy & Fuels 1989,3,362-365
362
Electrochemical Hydrogenation of Coal with Active Hydrogen Generated from Water in a Mediator/Nickel Powder System under U1trasonic Irradiation Mikio Miyake,* Maki Hamaguchi, and Masakatsu Nomura" Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565 Japan Received December 5, 1988. Revised Manuscript Received February 21, 1989
A new electrochemical system was shown to hydrogenate coal with active hydrogens generated from water. The system consists of electron transfer from a P b cathode to a Ni powder suspension via a Cr3+/Crz+mediator dissolved in an aqueous electrolytic solution. Active hydrogens produced on the Ni powder, which has a large surface area, hydrogenate coal chemically by contacting coal particles. The electrolysis was carried out under ultrasonic irradiation to prevent adhesion of coal-derived material on the P b cathode and to agitate the system efficiently. The current efficiency attained by the system was 11.2% a t 1440 C and 13.3 hydrogen atoms per 100 carbon atoms were added by electrolysis with 17 280 C. Several electrolytic parameters were investigated. Analyses of the hydrogenated coal indicate that cleavage of ether bonds and hydrogenation of aromatic rings result from the reaction.
Introduction Electrochemical methods have begun to draw increasing attention as a new and potentially useful approach to coal treatment, conversion, and analyses. Several research groups'-1o are currently investigating anodic oxidations of coal coupled with proton reduction in aqueous solutions, resulting in both coal gasification and hydrogen production. In this system, coal is used as a reducing agent to save electricity for water electrolysis. Electrochemical systems have been applied successfully for sulfur removal from coal"J2 or from coal gas.I3J4 Electrochemical methods have also been applied to the analyses of constituents in The advantages of electrochemical (1) Coughlin, R. W.; Farooque, M. Nature 1979,279,301. (2) Farooque, M.; Coughlin, R. W. Fuel 1979,58,70&712.
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(3) Coughlin, R. W.; Farooque, M. Ind. Eng. Chem. Rocess Des. Deu. 1980,19, 211-219. (4) Baldwin, R. P.; Jones, K. F.; Joseph, J. T.; Wong, J. L. Fuel 1981, 60.739-143. . -(5) Okada, G.; Guruswamy, V.; Bockris, J. OM. J. Electrochem. S O ~ . 1981,128, 2097-2102. (6) Dhooae. P. M.: Stilwell, D. E.: Park. S.-M. J. Electrochem. SOC. 1982,129,1?19-1724. (7) Dhooge, P. M.; Park, S.-M. J. Electrochem. SOC.1983, 130, 1029-1036. (8) Dhooge, P. M.; Park, S.-M. J. Electrochem. SOC.1983, 130, 1539-1542. (9) Taylor, N.; Gibson, C.; Bartle, K. D.; Mills, D. G.; Richards, D. G. Fuel 1985,64, 415-419. (10) Kawakami, K.; Okumura, T.; Kusunoki, K.; Kusakabe, K.; Morooka, S.; Kato, Y. J. Chem. Eng. Jpn. 1986, 19, 134. (11) Lalvani, S.; Pata, M.; Coughlin, R. W. Fuel 1983, 62, 427-437. (12) Kusakabe, K.; Nishida, H.; Morooka, S.; Kato, Y.; Kawakami, K.; Kusunoki, K. Fuel 1987,66, 271-275. (13) Lim, H. S.; Winnick, J. J. Electrochem. SOC. 1984,131,562-568. (14) White, K. A., III; Winnick, J. Electrochim. Acta 1985, 30, 511-519. (15) Given, P. H.;Schola, J. M. J. Chem. SOC. 1958, 2680-2684. (16) Bartle, K. D.; Gibson, C.; Mills, D.; Mulligan, M. J.; Taylor, N.; Martin, T. G.; Snape, C. E. Anal. Chem. 1982,54, 1730-1733. (17) Santos, L. M.; Baldwin, R. P. J. Appl. Electrochem. 1986, 16, 203-212.
(18) Mebrahtu, T.; Berry, G. M.; Soriaga, M. P. J. Electroanul. Chem. Interfacial Electrochem. 1988, 247, 241-251.
Scheme I. Schematic Representation of a Mediator/Ni Powder System for Electrochemical Reduction of Protons to Active Hydrogens Followed by Chemical Hydrogenation of Coal
Cathode
1
e- (
R
X
Coal, H-Coal
H
+
ox
H*
Mediator
methods have recently been r e v i e ~ e d .However, ~ ~ ~ ~ only a few reports have appeared for the electrochemical hydrogenation of coal. Cathodic reductions of coal in dimethylformamidezl and in aminezzsolvents followed by protonation have been reported where electricity was used in place of alkali metal for the Birch reduction. In the present study, a new system is reported for coal hydrogenation with active hydrogens produced by the electrochemical reduction of protons in aqueous solution. Usually the current efficiency of such a system will be very low since solid coal particles must contact active hydrogens having a short lifetime on the cathode surface. In this respect, the following system, shown in Scheme I, was adopted in the present study: electrons from a cathode are transferred to a Ni powder suspension via a mediator dissolved in an electrolytic aqueous solution. On the Ni powder, a proton is reduced electrochemically to produce active hydrogen. The Ni powder has a very high surface area compared with a conventional plate electrode. Therefore, coal particles contact active hydrogen efficiently on the powder surface to chemically hydrogenate coal. Pb was selected as the cathode because of its high hydrogen overvoltagez3to avoid proton reduction on the cathode. (19) Kreysa, G. Nachr. Chem., Tech. Lab. 1986, 34, 967. (20) Mebrahtu, T.; Berry, G. M.; Soriaga, M. P. J. Electroanul. Chem. Interfacial Electrochem. 1988,247, 241-251. (21) Given, P. H.; Peover, M. E. Fuel 1960,39,463. (22) Sternberg, H.W.; Delle Donne, C. L.; Markby, R. E.: Wender, I. Fuel 1966,45,469-482.
OSS7-0624/S9/2503-0362$01.50/~ 0 1989 American Chemical Society
Energy & Fuels, Vol. 3, No. 3, 1989 363
Electrochemical Hydrogenation of Coal
expt no. 1 2 3 4 5 6
mediator Cra+/Cr2+ no Yes Y88
no Yes Yes
Table I. Effects of Electrolytic Conditions on Coal Hydrogenation electrolvtic conditiona hydrogenated coal ultrasonic Ni duration of electricity atomic no. of H added current powder irradiation electrolysis, h passed, C efficiency, % H/C per 100 C noC 48 8616 no* 0.829 1.0 0.7 yes noc 24 4320 0.835 1.6 2.4 4320 yes Yes 24 0.860 4.1 6.3 4320 yes Yes 24 0.848 2.9 4.4 no Yes 24 4320 0.832 1.3 2.0 d yes yes 72d 0.835 1.6
'A coal sample (1 g) and aqueous THF (water:THF = 6:l volume ratio) (42 cm3) containing 1 M LiCl were added to a cathodic cell. A constant current of 50 mA was passed in the presence or absence of 1 M CrC13and/or Ni powder (1 g) with or without ultrasonic irradiation. b A Ni plate cathod was used in place of a Pb cathode. cAgitatedwith a magnetic stirrer. dNo external electricity was passed. Sonicated for 72 h.
expt no. 3 7 8 9
electrolyte solution HzO-THF HzOb HzO-THF H&THF
Table 11. Effects of Electrol~ticConditions on Coal Hydrogenation electrolytic conditiona hydrogenated coal current, duration of electricity atomic no. of H added current supporting electrolyte mA electrolysis, h per 100 C efficiency, % H/C passed, C LiCl 50 24 4320 0.860 4.1 6.3 50 21 3753 0.841 2.2 7.2 LiCl 100 12 4320 0.838 1.9 2.9 LiCl 0.857 3.8 2.9 100 48 17280 H&30dc
"Electrolysis with a constant current (50 or 100 mA) was carried out in the mediator (1 M CrC13)/Ni powder (1 g)/ultrasound system. Detailed conditions were described in Table I. bAn aqueous solution without THF was used. c0.5 M HzSOI was used in place of 1 M LiCl.
Nickel powder was adopted because this material has a low hydrogen overvoltage and a high exchange current density for hydrogen production.29 The mediator used was C?+/CP+ with the standard oxidation/reduction potential of -0.424 V vs NHE.= Furthermore, the electrolysis was carried out under ultrasonic irradiation to prevent adhesion of coal-derived material to the electrode surface and to agitate the coal particles and the Ni powder effectively. Sonication has been applied to coal and related materials for rapid solvent extractionui26 and for effective anionization during reductive alkylation reactions.26 An intensity of ultrasonic energy adopted in the present study (0.5 W/cm2, 45 kHz) has been reported to cause little degradation of covalent bonds in coal but to affect weak secondary bonds, such as van der Waals and hydrogen bonds.24*26
Experimental Section Materials. Japanese Yubari coal (-200 mesh) was used as the coal sample. The ultimate analysis data (dry basis) were 81.6% C, 5.61% H, and 1.88% N; the atomic H/C was 0.819. The electrolyte solutions were double-distilled water and tetrahydrofuran (THF) distilled just prior to use. Lithium chloride, concentrated sulfuric acid, CrCl8-6H20,and Ni powder (-100 mesh) were reagent grade chemicals (Wako Pure Chemicals Industry), which were used as received. Lead plate (1 X 1 cm2)and nickel plate (1 X 1 cm2) cathodes were chemically etched by immersion into a hydrogen peroxideacetic acid solution (1:4 volume ratio) for 30 s and a nitric acid-sulfuric acid-phosphoric acid-acetic acid solution (3:1:1:5 volume ratio) for 2 min at 90 "C,respectively.29 A platinum foil anode (3 x 4 cm2)was dipped into aqua regia for 30 min. These treated electrodes were used after being washed with a stream of deionized water for 30 min. Pyridine used as a coal extraction solvent was purified by distillation. Electrolytic Cell Configuration. Figure 1 shows the configuration of an electrolytic cell. The cell was of a double cylindrical type with the inner and outer cell compartments made (23)Handbook ofJ3lectrochemistr-y;EledrochemicalSociety of Japan, Ed.; Maruzen: Tokyo, 1986. (24)Cooke, N. E.;Gaiward, R. P. Can. J. Chem. Eng. 1983, 61, 697-702. (26) Jackson, W. R.; Larkins, F. P.; Thewlis, P.; Watkins, L. Fuel 1983, 62,606-607. (26)Miyake, M.; Uematsu, R.; Nomura, M. Chem.Lett. 1984,636-638.
Gasout A
Silicon stopper Gas in
I\
I
Fritted glass filter
Figure 1. Electrolytic cell configuration. of Teflon (8 mm 0.d. X 70 mm) and Pyrex glass (35 mm 0.d. X 100 mm), respectively. The electrolyte solutions in both compartments were separated by a fritted glass filter fixed a t the bottom of the anodic cell. The Pt anode and the Pb or Ni plate cathodes were fixed by silicon stoppers. Typical Procedure for the Electrolysis. The coal sample (1g), the Ni powder (1 g), and 42 cm3of aqueous THF (water:THF = 6:l volume ratio), which contained 1 M CrC13 and 1 M LiC1, were added to the cathodic cell. In the anodic cell, 6 cms of the electrolyte solution with the same composition as the catholyte but without the coal sample and the Ni powder were added. One Pt anode and two Pb cathodes were set in the cell. Nitrogen gas was bubbled through the cathodic solution during reaction to remove the dissolved oxygen. The cell was immersed in a water bath of an ultrasonic cleaner (Branson 220;45 kHz, 100 W,0.5 W/cm2). The temperature of the bath was maintained at 20 "C during reaction. A constant current of 50 mA was passed for 24 h (electricitypassed was 4320 C) by using a Takasago power supply (GP 05001). The amount of electricity passed was measured by a coulomb/(ampere hour) meter (Hokuto Denko, HF-201). A constant current of 50 or 100 mA was maintained during the whole period of the electrolysis; however, a concomitant increase in voltage between the plate electrodes up to around 100 V was observed. After electrolysis, the contents in the cathodic cell were poured into 1 L of a 1 M aqueous HC1 solution. The solution was agitated vigorously for several hours to dissolve most of the Ni powder. The recovered solid was washed repeatedly with deionized water until no halide was detected in the filtrate and the product was dried to a constant weight a t 90 "C under 100 Pa for 48 h. The product obtained was referred to as the hydrogenated coal. Analyses of Hydrogenated Coal. The ultimate analyses of the hydrogenated coals were used to estimate the number of hydrogen atoms added per 100 carbon atoms by the increase in
Miyake et al.
364 Energy & Fuels, Vol. 3, No. 3, 1989
expt no. 10 3 11
Table 111. Effects of Amounts of Electricity Passed on Coal Hydrogenationa duration of electricity no. of H added per 100 C atomic H/C passed, C electrolysis, h 0.843 2.4 1440 8 0.860 4.1 4320 24 0.952 13.3 17280 96
current efficiency, % 11.2 6.3 5.1
nElectrolysisat 50 mA was carried out in the mediator (1 M CrCl,)/Ni powder (1 g)/ultrasound system in an aqueous THF solution containing 1 M LiCl. Detailed conditions were described in Table I. the atomic H/C. The number of hydroxyl groups was assessed by the acetylation method using pyridine.n The IR spectra were obtained by using KBr pellets.2s The pyridine-soluble portion of the product was analyzed by proton NMR spectroscopy in pyridine-ds. The resonances were assigned as described in a previous report.29 The current efficiency was calculated based on Faraday’s law from the current passed and the number of hydrogen atoms added. In the calculation, one proton was assumed to be reduced by one electron to produce an active hydr~gen.~~ Results a n d Discussion Electrolysis under Various Conditions. The results of the electrochemical hydrogenation of coal are summarized in Tables 1-111. The effects of fundamental electrolytic parameters, such as the mediator (Cr3+/Cr2+),the Ni powder, the ultrasonic irradiation, and electricity, are given in Table I. At first, simple electrolysis by the use of two Ni plate cathodes was investigated in the absence of the mediator, the Ni powder, and ultrasonic irradiation (experiment 1). Only one hydrogen atom was added per 100 carbon atoms after electrolysis for 2 days (8616 C), resulting in extremely low current efficiency (0.7%). In this system, active hydrogens generated a t the Ni plate cathodes do not contact with the coal particles efficiently but recombine to form hydrogen gas. This observation suggests that dissolved hydrogen in the electrolytic solution, if any, plays minor role in coal hydrogenation. Alternatively, electrolysis with P b cathodes in the presence of mediator and Ni powder for 24 h (4320 C) enhanced the process to 1.6 hydrogens added per 100 carbon atoms and the current efficiency to 2.4% (experiment 2). However, black solids covered the P b cathode surface during electrolysis for the system with mechanical agitation (experiment 2). An electrolysis was carried out under ultrasonic irradiation to prevent this adhesion and to agitate sufficiently the solution that contained the Ni and coal powders. A mediator/Ni powder/ultrasound system was very effective (experiment 3); both the number of added hydrogen atoms and the current efficiency increased to more than 2.5 times those for the system without ultrasonic irradiation (experiment 2). The current efficiency attained (6.3%) was about 9 times greater than that of the simple Ni cathode system (experiment 1). Under ultrasonic irradiation, coal hydrogenation proceeded to a considerable extent even in the absence of the mediator (experiment 4) because the effective agitation allowed more contact of the Ni powder directly with the P b cathodes. However, hydrogenation proceeded to a rather limited extent without the Ni powder (experiment 5). Thus, the Ni powder plays a very important role in coal hydrogenation. Interestingly, coal hydrogenation occurred without passage of external electricity in the mediator/Ni powder/ultrasound system (experiment 6). The number of hydrogen atoms added was (27)Bloom, L.;Edelhausen, L.; van Krevelen, D. W. Fuel 1957,36, 135.
(28)Miyake, M.; Sukigara, M.; Nomura, M.; Kikkawa, S. Bull. Chem. SOC.Jpn. 1984,57,840-843. (29)Miyake, M.; Sukigara, M.; Nomura, M.; Kikkawa, S. Fuel 1980, 59, 637-640.
Scheme 11. Schematic Representation of Oxidation of Ni and Reduction of a Proton by a Local Cell System Followed by Chemical Hydrogenation of Coal
H-Coal
4an
Jooo
Po0
l6w
1200
800
wave n u m b , cm-1
Figure 2. IR spectra of original and hydrogenated coals. A hydrogenated coal sample was prepared by the same conditions as experiment 11 described in Table 111.
1.6 after 72 h of ultrasonic irradiation. The standard oxidation/reduction potentials of Ni2+/Ni and H+/H are -0.236 and 0 V vs NHE, r e s p e c t i ~ e l y .Therefore, ~~ simultaneous oxidation of the Ni powder and reduction of the proton will proceed by a local cell system,3oas shown in Scheme 11. Active hydrogens produced by such an electrochemical system will contact coal particles, resulting in chemical hydrogenation of coal. This result predicts a potential that with the selection of a suitable reducing agent with a negative standard oxidation/reduction potential enables us to reduce coal without external current. The results of electrochemical hydrogenation in the mediator/Ni powder/ultrasound system by changing the types of electrolytic solutions, supporting electrolytes, and the amount of current density are given in Table 11. In the present study, T H F was added to the aqueous electrolyte solution to provide affinity for the coal particles to the aqueous solution. There is a possibility that an increase in the hydrogen content of coal after electrolysis is due to the addition of THF-derived materials. To clarify this problem, electrolysis was conducted in an aqueous solution that contained no THF (experiment 7). The electrolysis with a constant current a t 50 mA was stopped at 21 h, before the standard period of 24 h, because most of the coal particles floated to the surface of the solution during electrolysis. A high current efficiency of 7.2% was attained (experiment 7), which was comparable to 6.3% in aqueous THF (experiment 3), indicating that the contamination by THF derivatives was insignificant. Elec(30) Miyake, M.; Yoneyama, H.; Tamura, H. J.Catal. 1979,58,22-27.
Electrochemical Hydrogenation of Coal
Energy & Fuels, Vol. 3, No. 3, 1989 365
Table IV. Change of Elemental ComDosition and OH before and after Electrolvsis ____ ultimate anal.; w t % sample C H N Ob atomic H/C 0/100 c 2.0 5.5 0.819 4.8 original coal 86.5 6.0 2.0 5.4 0.852 4.8 hydrogenated coalc 86.4 6.1 ~
a
~~
~
~
~
~
_
_
OH/100 C 2.0 4.7
Dry, ash-free basis. *By difference. c A sample was prepared by the same electrolytic conditions as experiment 9 in Table 11.
Table V. Hydrogen Distributions of Pyridine-Soluble Materials before and after Electrolysis no. of H per 100 C" atomic sample t0t.H H, H, H, H, H/C 88.9 22.8 27.8 25.4 12.9 0.819 original coal hydrogenated coalb 96.0 19.8 26.2 36.8 13.2 0.960
aNotations were described in the text. bThe same sample as experiment 11in Table 111. Scheme 111. Plausible Hydrogen Addition Reaction
trolysis a t 100 mA caused high hydrogen gas evolution, resulting in low current efficiency (experiment 8). Higher concentrations of protons in the electrolyte solution, prepared by the addition of 0.5 M sulfuric acid as a supporting electrolyte in place of 1M LiC1, was also disadvantageous and led to hydrogen gas evolution (experiment 9). Electrolysis in the mediator/Ni powder/ultrasound system was carried out a t 50 mA by changing the electrolytic period (Table 111). By prolonged electrolysis, the quantity of hydrogen atoms added was increased. After electrolysis for 4 days (17280 C), 13.3 hydrogen atoms were added (experiment 11). However, a decrease in the current efficiency was observed by prolonged electrolysis. This trend suggests that the sites in the coal have different reactivities toward hydrogenation; hydrogenation occurs sequentially from the more reactive sites to the less reactive ones. It should be noted that the current efficiency of 11.2% was attained a t 1440 C (experiment 10).
Analyses of Hydrogenated Coal. The original coal and the hydrogenated coal obtained by experiment 11were examined by IR spectroscopy (Figure 2). The electrolysis led to an increase in the absorption band around 3300-3500 cm-' and a slight decrease in the absorption band near 870 cm-'. These changes in the IR spectra suggest that cleavage of ether bonds and hydrogenation of aromatic rings occur by the present system. In order to confirm the occurrence of the cleavage of ether bonds, the concentration of OH groups in the original and hydrogenated coals was estimated (Table IV). A definite increase in the number of OH groups was observed after electrolysis. The pyridine soluble material obtained from the original coal and the hydrogenated coal (experiment 11)were analyzed by 'H NMR in order to characterize the aromatic ring hydrogenation. The estimated distributions of various types of hydrogens are summarized in Table V. The number of hydrogens attached to aromatic carbons (H,) decreased from 22.8 to 19.8 after electrolysis, as expected from the IR spectra analyses. The number of hydrogens attached to p-carbons increased considerably whereas those attached to a-and y-carbons were roughly similar before and after electrolysis. This observation may be attributed to the addition of hydrogen to aromatic carbons that have side chains because of the high stability of the radical a t these positions, as represented in Scheme 111. Therefore, the present electrochemical process leads to the cleavage of ether bonds and the hydrogenation of aromatic rings. Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture, Japan (No. 60750784). Registry No. LiC1, 7447-41-8; HzS04, 7664-93-9; CrC13, 10025-73-7; Ni, 7440-02-0; Hz, 1333-74-0.
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