Energy & Fuels 1996, 10, 243-249
243
Effects of LiAlH4 Reduction and/or O-Methylation on Lignite Conversion under CH3OH-NaOH Solubilization and Pyrolysis Conditions J. P. Boudou* CNRS, Laboratoire de Ge´ ochimie et Me´ talloge´ nie, Universite´ Pierre et Marie Curie, Case 124, 4, Place Jussieu, 75252 Paris Cedex 05, France
J. Bimer and P. D. Salbut Institute of Organic Chemistry, Polish Academy of Sciences, 44 Kasprzaka St., 01-224 Warsaw, Poland
P. N. Kuznetsov Institute of Chemistry and Chemico-Metallurgical Processes, Russian Academy of Sciences, 42 K. Marx St., Krasnoyarsk 660049, Russia Received July 10, 1995. Revised Manuscript Received September 27, 1995X
The chemistry of the reaction of coal with methanol and sodium hydroxide involves not only hydrogenation, mainly through the mechanism of hydride transfer from alcohol anion, and alkylation of aromatic systems but also several other types of coal structure transformations: hydrolytic cleavage of bonds, mainly ether bridges, and elimination and chemical blocking (Oalkylation) of oxygen functional groups. Coal solubilization by reaction with methanol and sodium hydroxide is a result of combined effects of the above transformations. In this study, the effects of a preliminary reduction of carbonyl groups with lithium aluminium hydride and/or of a preliminary O-methylation with dimethyl sulfate on the conversion of the Kansk-Achinsk lignite under methanol-sodium hydroxide solubilization were investigated by means of structural characterization, sequential solvent extraction, and programmed pyrolysis of the lignite and of its solubilization byproducts. In spite of a decrease in susceptibility to hydrogenation and alkylation reactions, which otherwise play a critical role during the CH3OH-NaOH treatment of bituminous coal, blocking of the numerous free carboxyl and hydroxyl groups of the lignite increases the conversion yields, as a result of a prevention of repolymerization reactions. O-Methylation of the hydroxyl groups released by hydrolytic cleavage would also stabilize the reactive oxygen groups of the lignite.
1. Introduction In recent years, much attention has been paid to the effect of selective chemical modifications on the behavior of coal in its various conversion processes. Knowledge of this dependence is of great importance for improving overall understanding of the chemistry of coal conversion under the investigated process conditions. In previous papers, Bimer, Kuznetsov, and Cagniant1-4 showed that pretreatments involving mainly the carboxyl and hydroxyl groups greatly affect the solubilization of coal by reaction with methanol and sodium hydroxide. These selective modifications also influence the formation of tar and gas during pyrolysis.5-7 These earlier findings dealt mainly with bituminous coals which were oxidized with performic acid, in order to Abstract published in Advance ACS Abstracts, November 1, 1995. (1) Bimer, J.; Salbut, P. D.; Berlozecki, S. Fuel 1993, 72, 1063-1068. (2) Kuznetsov, P. N.; Sukhova, G. I.; Bimer, J.; Salbut, P. D.; Korniyets, E. D.; Belskaya, N. A.; Ivanchenko, N. M. Fuel 1991, 70, 1031-1038. (3) Cagniant, D.; Bimer, J.; Salbut, P. D.; Gruber, R.; Henrion, P.; Krzton, A. Fuel 1994, 73, 871-879. (4) Kuznetsov, P. N.; Bimer, J.; Salbut, P. D.; Sukhova, G. I.; Korniyets, E. D.; Dje´ga-Mariadassou, G.; Brodzki, D.; Sayag, C.; Gruber, R. Fuel 1994, 73, 901-906. X
0887-0624/96/2510-0243$12.00/0
increase their carboxyl and hydroxyl group contents. In the present study, lignite from the Kansk-Achinsk basin was used and the effects of O-methylation with dimethyl sulfate and reduction with lithium aluminum hydride on the conversion of the pretreated lignite under methanol-sodium hydroxyde solubilization (M treatment) and pyrolysis conditions were investigated. 2. Experimental Section The Kansk-Achinsk lignite,8 in its original state, was used in this study. The elemental analysis and free oxygen group contents in the starting and pretreated samples are presented in Table 1. In its original state, the lignite contains carboxyl groups in acid form (Table 1), but also in Ca and Mg salt forms. The lignite demineralized by treatment with hydrochloric acid shows an increased content of carboxyl groups in the acid form (OCOOH ) 6.4%). However, pretreatment of the original lignite (5) Boudou, J. P.; Bimer, J.; Salbut, P. D.; Cagniant, D.; Gruber, R. Fuel 1994, 73, 907-917. (6) Boudou, J. P.; Bimer, J.; Salbut, P. D.; Cagniant, D.; Gruber, R. Fuel 1995, 74, 846-852. (7) Boudou, J. P.; Espitalie´, J.; Bimer, J.; Salbut, P. D. Energy Fuels 1994, 8, 972-977. (8) Kuznetsov, P. N.; Sharypov, V. I.; Beregovtsova, N. G.; Rubaylo, A. I.; Korniyets, E. D. Fuel 1990, 69, 911-916.
© 1996 American Chemical Society
244
Energy & Fuels, Vol. 10, No. 1, 1996
Boudou et al.
Table 1. Characteristics of the Original and Pretreated Lignite Samples composition (% daf) sample initial O-methylated reduced with LiAlH4 reduced and O-methylated a
L LM LR LRM
yielda (% daf)
ash (% dry)
C
H
OCOOH
OOH
100.0 108.1 96.3 99.8
4.3 1.8 1.3 0.4
69.7 72.8 71.9 71.1
4.9 5.8 6.1 6.5
2.8 0.4 1.1 0.4
7.2 1.1 9.0 1.4
Based on the initial lignite.
converts both acid and salt forms into the appropriate derivate. The high content of hydroxyl groups after LiAlH4 reduction confirms such a view. Analytical determinations, pretreatments and experiments of solubilization and pyrolysis were performed according to the procedures previously described.1,2,5 The FTIR spectra were recorded using Perkin-Elmer 1640 spectrometer. The 1H NMR data were obtained with a Varian Gemini 200 spectrometer. 2.1. Pretreatments. The reaction with dimethyl sulfate was used as a method of O-methylation9 of the lignite or of the product of LiAlH4 reduction. LiAlH4 reduction of carboxyl groups to carbinols was realized in two stages: 1. Esterification of Coal. Approximately 10 g of dried coal was mixed with 30 mL of triethyl orthoformate and placed in a flask equipped with a Vigreux column. The mixture was heated on an oil bath (160-170 °C) under argon. Over a period of 3 h, 4 g of the distillate (ethyl alcohol and ethyl formate) were collected. After cooling of the reaction mixture, esterified coal was filtered off, thoroughly washed with methanol, and dried under vacuum at 80 °C. The soluble fraction of the esterified product was isolated by evaporation of the filtrate and washings. 2. At the next stage, the esterified coal (the combined fractions of the products) was mixed with 100 mL of freshly purified tetrahydrofuran and placed in a flask equipped with a magnetic stirrer, reflux condenser, and an inert gas flow system. Lithium aluminum hydride (1.3 g) was introduced in portions. The reaction mixture was heated on an oil bath for 7 h, whereupon it was left overnight with stirring in argon. This procedure, comprising the addition of LiAlH4, heating, and overnight stirring, was repeated twice more. To cool the reaction mixture, 15 mL of ethyl acetate was introduced dropwise; then 450 mL of water and finally 50 mL of concentrated hydrochloric acid were added. The product was filtered off, washed with water until free from chloride ions, and dried under vacuum at 80 °C. Reduction of the lignite with lithium aluminum hydride produces new structural moieties. It involves the hydrogenation of the carboxylic carbonyl groups (-COOH f -CH2OH) and of other ketonic or quinonic carbonyl groups. The effectiveness of this method with respect to hydrogenation of carboxyl groups has been shown by analytical determinations and differential IR spectra.3 The transformation of -COOH group into -CH2OH group is observed by IR spectroscopy: almost complete elimination of the carboxyl carbonyl band at the 1714 cm-1 band, with co-occurrence of the alcoholic band at 1034 cm-1; functional analysis of the carboxyl and hydroxyl group contents: a substantial decrease in carboxyl group content in reduced coal and a proportional increase in hydroxyl group content. The reduction of quinone structures with LiAlH4 is somewhat more complex as it may lead not only to phenols but also to hydroxyl derivatives of the hydroaromatics. One cannot compare the properties of the hydroxyl and carboxyl groups of coal with the hydroxyl groups formed by reduction with LiAlH4 (-COOH f CH2OH). These last groups are artificially formed and have a quite different chemical (9) Stock, L. M. In Coal Science; Gobarty, M. L., Larsen, J. W., Wender, I., Eds.; Academic Press: New York, 1982; Vol. 1, pp 161282.
character (alcoholic as opposed to phenolic character of regular -OH groups in coal). O-Methylation of coal reduced with LiAlH4 involves the O-methylation of the new alcoholic OH groups formed by the hydrogenation of carboxyl groups. 2.2. Reaction of Coal with Methanol and Sodium Hydroxide. A 10 g portion of dried coal was mixed with 210 mL of a solution of sodium hydroxide in methanol. The resultant suspension was placed in a 2 L autoclave. The reactor was flushed twice with nitrogen and then heated, without stirring, at the desired temperature (300, 325 or 350 °C) for 1 h. The total pressure arising from methanol vapour and reaction gases amounted to about 12, 13, and 14 MPa, depending on the reaction temperature. After the completion of reaction and cooling, the combined reactor content and washings were concentrated on a Rotavap, treated with large amount of water, and acidified with hydrochloric acid. The water-insoluble product was filtered off, thoroughly washed with water, and dried at 80 °C under vacuum. The chemistry of the reaction of coal with methanol and sodium hydroxide involves hydrogenation and several other reactions.10-16 It is generally accepted that the main mechanism of hydrogenation under alcohol-sodium hydroxyde treatment consists in the hydride transfer from alcohol anion to coal substance. This idea was first presented by Ross and Blessing.10 They found that a significant amount of acetone is formed when coal is treated with 2-propanol and sodium hydroxide:
〉CHO- + coal f 〉CdO + coalHcoalH- + 〉CHOH f coalH2 + 〉CHOcoalH- and coalH2 are respectively an anionic intermediate and reduced coal. Apart from the fact that methanol reactivity is much lower compared to that of other alcohols, it was found to be the only alcohol which does not form any nonvolatile byproducts in the reaction with coal with NaOH.10 Formaldehyde (H2CdO) deriving from methanol is disproportionated under the reaction conditions into CH3OH and HCOOH. HCOOH gives NaCOOH which decomposes in Na2CO3, CO, and H2. Other mechanisms of hydrogenation would also be involved in the solubilization process. Coal can be solubilized to some degree in the reaction with alcohol alone (without NaOH).11-14 Under such reaction conditions, the model compounds give the products of alkylation, reductive cleavage of some bonds, and hydrogenation of aromatic systems.11 It has been also assumed that R-C-H bond undergoes the attack of hydrocarbon radicals,14 as confirmed by isotope effect of ethanol and 2-propanol deuterated at the R-position.13 Coal -CH2OH (10) Ross, D. S.; Blessing, J. E. Fuel 1979, 58, 433-442. (11) Makabe, M.; Ouchi, K. Fuel Process. Technol. 1982, 6, 307316. (12) Kuznetzov, P. N.; Sharypov, V. I.; Rubaylo, A. I.; Korniyets, E. D. Fuel 1988, 67, 1685-1690. (13) Kuznetzov, P. N.; Beregovtsova, N. G.; Rubaylo, A. I.; Korniyets, E. D.; Pavlenko, N. I. Fuel 1991, 70, 559-563. (14) Tegay, F.; Aliulin, V. V.; Plopsky, E. J.; Kirilets, V. M. Chim. Tverd. Topl. 1983, 5, 92-98. (15) Makabe, M.; Ouchi, K. Fuel Process. Technol. 1979, 2, 131141. (16) Bimer, J.; Salbut, P. D. Erdo¨ l Kohle 1988, 41, 155-156.
Lignite Conversion groups from LiAlH4 reduction could participate in the reaction as hydride donor reagent. It is obvious that the reactivity of the alcohol of this type (e.g., benzyl alcohol) would be lower as compared with methanol or 2-propanol, but such a contribution cannot be excluded. We could also expect an oxidation of -CH2OH groups leading finally to -COOH groups, as pointed out by Makabe and Ouchi.15 As the reaction of coal with methanol and sodium hydroxide is performed at rather high temperature, the effects of thermal decomposition of CH3OH in CO and H2 must be taken into account. It was experimentally shown that the reaction gases contain mainly hydrogen, carbon monoxide, carbon dioxide, and methane. A mixture of carbon monoxide with hydrogen, water, and sodium hydroxyde is an effective hydrogenating agent.10 2.3. Continuous Detection of Total Hydrocarbons and of H2, CH4, and CO. Pyrolysis was performed in IFP with a Rock-Eval equipped with a carbon module.7 Total hydrocarbons (oil) were continuously detected by means of a FID detector. A finely crushed (
