Energy & Fuels 1992,6, 694-701
694
Articles Pyrolysis-Field Ionization Mass Spectrometry of N-Methyl-2-pyrrolidinoneExtracts of Coals D. Cagniant,; R. Gruber, and C. Lacordaire-Wilhelm Laboratoire de Chimie Organique, PICS Carbochimie PAN-PIRSEM-CNRS, Universitg de Metz, Ile du Saulcy, 57045 Metz, France
H.-R. Schulten Department of Trace Analysis, Fachhochschule Fresenius, Dambachtal, 20-6200 Wiesbaden, Germany Received November 26, 1991. Revised Manuscript Received July 16, 1992
The examination of the structural characteristicsof the substances extracted from four coal samples, using N-methyl-2-pyrrolidinone, was carried out by pyrolysis-field ionization mass spectrometry (Py-FIMS). The extracts are complex mixtures of relatively easily distillable components and of larger entities, supposed to be “oligomeric”parts of the coal structure, which are pyrolyzed above 400 “C with the production of hydroxylated derivatives. Polycondensed structures predominate (4 to 7-8 nuclei) in a wide range of mlz values, mainly 250-350. This work can serve as a basis for further studies on the carbonization process of coals and the role of “the extractible phase” in the first stages of the plastic state and the process of coal softening.
Pyrolysis-field ionizationmass spectrometry(Py-FIMS) has been used in a wide range of investigation of biomass and fossil fuels. The method was applied to the characterization of coals and c~alextractsl-~ and the identification of products obtained by coal hydrogenation4v5and liquef a ~ t i o n . ~Some t ~ mechanistic aspects of coal chemistry, such as the bimolecularand radical hydrogenation of coal? and the determination of the coal reactive sites involved in hydrogen transfer from H-donor solvents8 were also investigated by Py-FIMS. Besidescharacteristic mass spectra fingerprints of whole coals, Py-FIMS gives additional informations on the coal pyrolysisprocess: on the one hand, thermogramsobtained by plotting the total ion current (TIC) or the total ion intensity (TII) against temperature provide an indication of the chemical nature of products volatilized from the samples, and, on the other hand, the Py-FI mass spectra can be integrated for the whole temperature or for distinct temperature intervals. Two devolatilization steps during heating of high- and low-volatile bituminous coals were observedS13 corre(1) Schulten, H.-R. Fuel, 1982,61, 670. (2) Vanderborgh, N. E.; Williams, J. M. Jr.; Schulten, H.-R. J.Anal. Appl. Pyrol. 1985,8, 271. (3) Schulten, H.-R.; Marzec, A. Fuel, 1986, 65, 885. (4) Marzec, A.; Schulten, H.-R. Fuel, 1987, 66, 844. (5) Schulten, H.-R.; Marzec, A. Fuel Process. Technol. 1987,15,307. (6) Marzec, A.; Schulten, H.-R. Fuel, 1988, 67, 584. (7) Schulten, H.-R.; Simmleit, N.; Marzec, A. Fuel 1988, 67, 619. (8) Marzec,A.; Czajkowska,S.;Simmleit, N.;et al.FuelPr0ces.s. Techn. 1990, 26, 53. (9) Meuzelaar,H. L. C.; Yun, Y.;Simmleit,N.;etal.Prepr.Pap.-Am. Chem. SOC.,Diu. Fuel. Chem. 1989, 34, 693. (10) Yun, Y.; Meuzelaar, H. L. C.; Simmleit, N.; etal. Recent Aduances
in Coal Science; ACS Symposium Series; American Chemical Society: Washington, DC, 1991; pp 89-110.
sponding respectively to two maxima in the thermograms, M I and MZ (cf. Table I). The low-temperature devolatilization steps (MI)were attributed‘+ll to a vacuum desorption and distillation process rather than to a pyrolytic process, as the temperature ( t < 400 “C in the case of a high-volatile bituminous coal) is too low to induce significant pyrolysis. Conversely, the high-temperature devolatilization steps corresponding to MZ imply the probability of pyrolysis. These two steps were related to the ‘mobile” and “nonmobile”phases of coal.l2J3 Nevertheless, other authors14J5do not agree with the two-phase model for coal, thinking that a monophase model is more suitable, the mobile phase consisting largely of partial constituents of macromolecular compounds released by heat treatment. By solvent extraction, according to the rank of the coal, the solvent power, and the experimental conditions, larger and larger molecules can be extracted from the coals and it is often difficult to determine the delineation between either a physical action involving a decrease in noncovalent interaction or a chemical action involving the cleavage of covalent bonds. In this respect by utilization of N-methyl-2-pyrrolidinone (NMP) alone16J7 at its boiling point or mixed with (11) Yun, Y.; Meuzelaar, H. L. C.; Simmleit, N.; et al. Energy Fuels 1991, 5 , 22. (12) Xiangming, W.; Marzec, A.; Schulten, H.-R. Fresenius 2.Anal. Chem. 1989,333, 793. (13) Simmleit, N.; Schulten, H.-R.; Yun, Y.;Meuzelaar, H. L. C. Advances in Coal Spectroscopy; Meuzelaar, H. L. C., Ed.; Plenum Press: New York, 1992. (14) Nishioka, M. F’repr. Pap.-Am. Chem. SOC.,Diu.Fuel. Chem. 1989, 34, 685. (15) Nishioka, M.; Gorbaty, M. L. Energy Fuels, 1990,4, 70. (16) Lacordaire-Wilhelm, C. Doctoral Thesis, University Metz, 1990.
0887-0624/92/2506-0694$03.00/00 1992 American Chemical Society
Py-FIMS of NMP Extracts of Coals
Energy & Fuels, Vol. 6,No. 6, 1992 695
Table I. Py-FIMS Data of Coals. total heating range ("C) 50-750 37-39 (coal a)b total yields of volatilization (% ! ) 50 (coal b)b 9 (coal C)b recorded mass range 50-900 thermograms ("C) Mi a (ref 16) b (ref 16) c (ref 16) d (ref 10)
340 420 490 38oC (hump)
Mz 425-435 480 530 480
a From refs 10 and 16. Coal a, high-volatile bituminous; coal b, low-volatile bituminous; coal c, semianthracite, coal d, hvAb (Pittsburgh No. 8). The low-temperature hump corresponds to 25-30% of the total integrated FI signal intensity.
CSz at room temperature,18 it is commonly accepted that the substance extracted by this kind of solvolysis is mainly present in the original coal and not formed by a depolymerization process occurring by the breaking of some' covalent bonds (methylene bridges, ether, and thioether linkages) between the structural units of coal. Owing to the predictable dissolvingeffect of this extracted substance in the early stages of the plastic state through the process of coal softening, it is of interest to have more information about ita main characteristics according to the rank and to the maceral composition. In this way, several analytical methods were used, such as l3C CP/MAS NMR,19-20 EPR,21transmission electronic microscopy (TEM),22and thermodesorption on line with gas chromatography/mass spectrometry('I'D/ GC/MS).16J7 We report here on the results of Py-FIMS analyses, applied to the products extracted from coals. Another part of this study, concerning the macerals, is published in the following article in this issue. Experimental Section 1. Coal Samples. Three Polish coking coals from Silesia (flamecoal FC, gas coal GC, orthococking coal DC) and one French coal from Lorraine (flame coal from Merlebach (FCM)) were examined. FCM, FC, and GC are high-volatile coals, whereas OC is a medium-volatile coal. Their approximate, ultimate and petrographic analyses are shown in Tables I1 and 111. 2. Solvent Extraction. The procedure for extracting by N-methyl2-pyrrolidinone (NMP), a modification of the method applied by RenganathanF3 was described re~ent1y.l~Up to 10 extractions were made, using in all cases 10-gsamples, previously ground to 100-200 pm according to Scheme I. The NMP ertract is obtained after dilution of the filtrate F with 1L of acidified water (HC1) (24 h at 85 "C with stirring) followed by heat filtration under vacuum. The extract is purified by stirring in acidified water (1 L), by filtering at room temperature and then washing with water. (17) Cagniant,D.; Gruber, R.; Lacordaire-Wilhelm,C.;et al. Fuel 1990, 69, 902. (18)Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel, 1988,67,
1639.
(19) Cagniant,D.; Gruber, R.; Lacordaire-Wilhelm,C.; et al. Fuel 1991,
70, 675. (20) Sderi, D.; Tougne, P.; Legrand, A. P.; et al. Int. Symp. Carbon
Tsukuba (Jpn.) 1990,6 PA-15, 802. (21) Cagniant, D.; Duber, S.; Lacordaire-Wilhelm, C. Coal Struct. React., 6-7 Sept., Cambridge 1990,65. (22) Darif, M.; Gruber, R.;Lacordaire-Wilhelm,C.; et al. Coal Struct. React., 5-7 Sept., Cambridge ISSO,66. (23)Renganathan, K.; Zondlo, J. W.; Stiller, A. H.International Conference on Coal Science; Moulijn, J. A., et al., Eds.; Elsevier Science Publishers: Amtersdam, 1987; p 367.
Table 11. Proximate and Ultimate Analyses of Coals proximate analyses moisture ash (wt % dry) volatile matter (VM) (wt % d& ultimate analysesa (wt % daf) C H N Ob H/C atomic ratio O/C atomic ratio
FCM
FC
GC
OC
2.4 3.9 37.6
9.1 5.4 35.5
2.6 4.1 35.1
1.0 5.4 30.5
83.3 5.4 1.0
78.5 4.5 1.4
8.8 0.78 0.08
0.69 0.10
84.3 5.2 1.6 9.2 0.73 0.08
86.1 5.0 2.2 5.7 0.70 0.05
11
From Service Central of Microanalyses, CNRS. By direct determination. Table 111. Petrographic Analysis of Coals (vol %) macerals vitrinite exin ite inertinite (1) fusinite micrinite semifusinite mineral matter PRV
FCMa 73.4 6.8 15.6 4.2 11.4
FCb 52.9 12.0 33.0 22.7 3.2 7.1 2.1 0.58
4.2 0.78
GCb 69.6 6.9 23.0
OCb 71.1 7.4 18.2 9.4 0.8 8.0 3.3 1.01
8.8
3.9 10.3 0.5 0.87
Data from Centre de Pyrolyse de Marienau (France). b Data from Institute of Chemistry and Technology of Wroclaw (Poland).
Scheme I NMP extraction
coal (10 g)
1 NMP ( l o o mL)
( 1 ) reflux 1 h (N2,202 " C ) ( 2 ) filtration in a heated Buchner funnel
solid
I
washing in the Buchner lunnel with 100 mL of NMP (202 "C)
t
I
1 solid R I The NMPresidue is obtained from solid Rafter two successive suspensions in acidified water (1 L) followed by filtration and then washing with ether. A Whatman No. 1 filter paper was used for all heat filtrations and a No. 540 paper was used in all other cases. The residues R and the extracts E were vacuum dried a t 100 "C for 2-3 days and then weighed. The extraction yields, as determined from the weights of the residues and on a daf basis" are recalled here (mean values): FCM (24%), FC (30%), GC (29%), and OC (42%). These results are in agreement with the literature data concerning the variation of extract yields, with the carbon contents, with NMP,l7vZ4NMP-CS2 mixtures,18 pyridineF5 or DMF,12 a maximum of yields being observed for about 87% C. Some experiments carried out on model compounds (dibenzyl ether, phenyl and benzyl benzoates, dibenzofuran, diphenyl sulfide, diphenylmethane, phenol, and anthracene) verified that no chemical reactions were involved in the NMP treatment. Indeed only the starting products were detected by gas chromatography and quantitatively recovered.l6J7 From the compilation of all analytical data on E-NMP and R-NMP samples, no significant nitrogen increase (less than 1%) (24) Fortin, F. Doctoral Thesis, University
Orleans, 1991.
(25) Van Krevelen,D. W. Coal;Elsevier Publishers: Amsterdam, 1961.
Cagniant et al.
696 Energy & Fuels, Vol. 6, No. 6,1992
?'om
/I
I I I::l
g
100,
I~I
*
145
I
- 410 O C
,I
410 - 500
435
OC
- 620 O C
1
m/z
-
Figure 1. NMP extract of gas coal (GC). (a) Integrated pyrolysis-FI mass spectrum, temperature range 50-750 O C ; see thermogram upper right. Partial mass spectra of selected temperature ranges: (b) 145-410 O C , and (c) 410-500 O C . can be established in comparison with the starting For a given sample, extractions were repeated about 10 times and an abnormal excess of nitrogen was always related to incomplete NMP elimination, as shown by IR spectroscopy. 3. Pyrolysis-Field Ionization Mass Spectrometry. For Py-FIMS, about 100pg of each sample was transferred to a quartz microoven and heated linearly in the direct inlet system of the mass spectrometer from 50 to 750 O C at a rate of 1 OC/s. All substances supplied by this heating program are considered, disregardingthe kind of physical or chemical bond in the authentic materials. A double-focusing Finnigan MAT 731 mass spectrometer (Finnigan MAT, Bremen, Germany) was used. The ion source was kept at a pressure below 1 mPa and at a temperature of 250 "C. To avoid condensation of the volatilized products during the recording of the FI mass spectra, the emitter was flash heated to 1500 O C between magnetic scans. Between 35 and 40 spectra were recorded in the mass range mlz 50-1200. The FI signals of all spectra were integrated and plotted with the aid of a Finnigan 55200data system to produce summed spectra. A detailed description of the methodology was given by Schulten et aLZ6
m/z
_.___o
Figure 2. NMP extract of orthocoking coal (OC). (a) Integrated pyrolysis-FI mass spectrum, temperature range 50-750 O C ; see thermogram upper right. Partial mass spectra of selected temperature ranges: (b) 145-415 "C,and (c) 43i5-620 "C. 1001
c
80
60
40
t
2o
145
8
:
-
410 O C
40
C
20
$
Rssults 1. V o l a t i l i z a t i o n of the Extracts in the M a s s Spectrometer. The integrated FI mass spectra of the four extract samples (Figures la-4a) display molecular ions in the mass range mlz 50 to mlz 800, when only significant masses are considered (when a large extending scale is used, the limit of the detected masses is about mlz 900). Each even and odd nominal mass is detected in this range, the maximum of mass distribution being obtained, in each case, with values going about from mlz 250 to mlz 450 (structures with 5-8 aromatic nuclei if we consider only condensed polycyclic aromatic hydrocarbons). (26) Schulten, H.-R.; Marzec, A.; Simmleit, N.;e t al. Energy Fuels 1989, 3, 481.
m/z
-
F i g u r e 3. NMP extract of flame coal (FC). (a) Integrated pyrolysis-FI mass spectrum, temperature range 50-750 "C; see thermogram upper right. Partial mass spectra of selected temperature ranges: (b) 145-410 O C , and (c) 410-590 "C.
2. Thermograms. In all cases, the thermograms display two maximaM1 and M Z(Table I, and Figures 1-4, inseta), the first and the second maximum being respectively in the narrow ranges 310 10 O C (MI) and 480 10 oc (MZ).
*
*
Py-FIMS of NMP Extracts of Coals
Energy & Fuels, Vol. 6,No. 6,1992 697 Table IV. Py-FIMS Data of Coal NMP Extracts. total heating range ( O C ) 50-750 total yields of volatilization nd recorded mass range 50-900 thermograms (OC)
115
- 350
FCM FC GC
OC
oc b)
I
d
1001
t
11. 350
- 420
420
- 585 OC
OC
20
100
,1 ,I
m/z
D
Figure 4. NMP extract of flame coal of Merlebach (FCM). (a) Integrated pyrolysis-FI mass spectrum, temperature range 50750 O C ; see thermogram upper right. Partial mass spectra of selected temperature ranges: (b) 115-350 O C , (c) 350-420 O C , and (d) 420-585 OC.
3. Mass SpectraRecorded in SelectedTemperature Ranges. The partial mass spectra are selected in each thermogram (see Figures 1-4) in relation to the temperature ranges. In order to discuss their main features and to provide a means to compare the various samples, we selected several fundamental series of homologous ions: Series A, starting from mlz 128. The first terms might correspond to naphthalenic compounds. Series B , starting from mlz 178,180,and 182. The first terms might correspondto three ringed aromaticstructures (phenanthrene-anthracene mlz = 178 and their dihydro and tetrahydro derivatives). Series C, starting from mlz 228,230,and 232. The first terms might correspond to four ringed aromatic compounds and their dihydro and tetrahydro derivatives. Series D , starting from mlz 94. The first terms might correspond to hydroxy derivatives of the phenol group. These series are plotted on each partial mass spectrum. Taking into account the similarity of some spectra, only typical examples will be reported. It is obvious that the only goal of this presentation is to compare samples and, for each of them, to assess the development of the homologous ions in relation to the devolatilization temperature. Our point is not to make any structural assignments. Indeed, the complexity of the products obtained from coal by all thermal processes (pyrolysis, hydroliquefaction, etc.) is well established. It was recently emphasized by the high-resolution analysis of a Polish bituminous coal that at least three or more species provide a nominal high-mass signal recorded in the low-resolution mode.26 Furthermore, if series C, for example, really started with four ringed structures, then other structuresshould develop from five to seven or eight (27) Nishioka, M.; Larsen, J. W. Energy Fuels, 1990,4, 100.
a
325 316 320 297
520 465 480 493
From ref 17.
nuclei, more or less hydrogenated, together with the increase in mlz value. Finally, in each case, the superposition of hydrocarbons and oxygenated compounds is probable, but with a predominance of aromatic hydrocarbons and their partially hydrogenated derivative^.^
Discussion The first part of the discussion refers to the Py-FIMS analyses of the NMP extracts. The temperature ranges of devolatilization, the mass mlz ranges of the molecular ions, and the thermogram data are summarized in Table IV. The second part of the discussion is devoted to the developmentof the partial mass spectra of NMP extracts as the temperature of devolatilization increases. 1. Py-FIMS Analysis of NMP Extracts. The most interesting features concern the comparison between the thermogramsof coals and those of NMP extracts (Figures 1-4) (Table IV). As in coals, two devolatilization steps (maximaM1and M2 of the thermograms)are found in the NMP extracts. The total devolatilization step in the lowtemperature range is accomplished at approximately400 OC. The maxima M Iappear at temperatures that are somewhat lower than in the case of highly volatile bituminous coals. From the partial mass spectra corresponding to MI (Figures 1-4) the mass ranges, besides minor components of low MW (mlz < 150), extend significantly from mlz 150 to 550,with a predominanceof mlz 250 to 360. These distillable fractions of NMP extracts correspond to components easily devolatilized without pyrolysis. As far as the high-temperature step is concerned, the temperature ranges of the maxima M Zare, as a rule, in the same range as in the case of coals and the total devolatilization step is accomplished at t < 750 "C. The mass ranges from mlz 50 to 550, with a nearly equal distribution without predominant molecular ions between mlz 150and 400. The most important molecular ions correspond to homologous ions of hydroxylated compounds (see series D, below), in agreementwith the occurrence of a pyrolysis process. Nevertheless, it must be borne in mind that a part, not quantified here, of the extracts could not be volatilized. In the case of coals, it was assumed that 25-3012 or 3750%BJ0of the coal organic matter was volatilized. It is likely that, in the case of NMP extracts, greater values are obtained. We conclude that the NMP extracts are made up of components devolatilized at 460 OC, probably by cleavage of ether bonds. The same serieswere also found in the high-temperature coal pyrolysis pr0ducts+~lJ3besides dihydroxybenzenes ( m l z 110-166). In the case of the NMP extracts, the latter compounds are also present (110,124,138,152) but in a much lesser amount than the phenolic derivatives. Other series of hydroxylated compounds could also be detected, such as hydroxynaphthalenes (mlz 144,158,172, and 1861,dihydroxynaphthalenes ( m l z 160,174,188,202), hydroxyindenes ( m l z 132, 146, 160). They were not reported in Figure 8 for two reasons: they are present in very low amounts in comparison with phenolic derivatives and they could overlap with other structures (i.e., with series C beyond mlz 228 in the case of hydroxynaphthalenes and with alkyltetralins and dihydroxynaphthalenes in the case of hydroxyindenes). The detection of the alkylphenolicderivatives is a very good way to test the occurrence of pyrolysis. A narrower fractionation of the thermograms could lead to an accurate determination of the inception of pyrolysis. It is interesting to cross check the information obtained by Py-FIMS and by other analytical methods carried out with the same extract ~amples.'~J~ Each sample was fractionated into four FI fractions by extrography in a given set of eluting solvents; the residue remained on the adsorbant. Each sample was fractionated by sonication into four fractions: hexane soluble (HS), toluene soluble (TS), tetrahydrofuran soluble and insoluble (THF-S and THFI)* The mass balances, reported elsewhere, vary slightly according to the coal sample, but the general trends are similar. We report, for example, about the case of the FCMINMP extract (in w t % of the extract): by extrography F, 3.5;F25.5; F314.3;F430.4; residue 46.3 The F1and FZfractions are mainly composed of mono-, di-, and triaromatic hydrocarbons; more condensed polycyclic aromatic hydrocarbons, polar compounds, and heterocycles appeared in the F3 fraction. Large coal fragments are probably the main constituents of the F4 fraction and residue. by sonication: HS 0.2;TS 1.2;THF-S 44.4;THF-I 54.2 As in the other samples, the F1 and FZfractions and the HS and TS fractions represent only a small percentage of the extracts. It could be suggested that the FI+ FZ+ F3 fractions (or HS + TS + a part of THF-S) are involved in the maximum MI in the thermograms and that pyrolysis occurs mostly with the F4 + residue (or the other part of THF-S, and THF-I). In this respect, the Py-FIMS results with Pittsburgh No. 8 coal indicate that the percentage of the "distillable" phase (68%) lies between the yields of the TS (5.7%) and THF-S (12.7%) fractions.10 Conclusions The Py-FIMS analyses emphasize the great heterogeneity of the NMP extracts whose main features appear to be as follows:
Py-FIMS of NMP Extracts of Coals
The extracts are a complex mixture, with a wide range of m/z values (50-900), mainly 250-350, in which polycondensed structures predominate. The devolatilization behavior of the extracts displays two steps, a low- and a high-temperature step, as proved by the thermograms. On the basis of the development of some typical homologous ions in relation to the rising temperature, it can be considered that the first step corresponds to componentseasily devolatilized at temperature below 400 OC and refers mostly to homologous n-paraffins, alkylsubstituted aromatics, and hydroaromatics thermally distillable without pyrolysis. The second step corresponds to the devolatilization of larger entities, with ether oxide bonds, which are cleaved at t > 450OC giving rise to predominant alkylhydroxylated compounds. In addition, it is suggestedthat the increasing amount of light n-paraffins (C~S-CM)in the high-temperature step evidences also the occurrenceof the pyrolysis process. It can be noticed that, besides the alkylhydroxylated compounds,the m/z values of the other components are the same as in the low-temperature steps but with a sizable decrease in intensity of the FI signals. These results can be explained if the coals could be pictured as a continuum of basic structural unities
Energy & Fuels, Vol. 6, No. 6,1992 701
("oligomers") of larger and larger sizes but with the same functionalities, bound by noncovalent interactions and dispersed in a solvatingmedium made up of smaller entities and free molecules. According to ita solvatingpower, NMP can extract some of the basic structuresand free molecules by disruption of noncovalent interactions. By heating in the conditions of Py-FIMS experiments, the most easily distillable compounds (free molecules and macromolecules of smaller sizes) are volatilized during the low-temperature step. It is also possible that during this step a part of the extract undergo cross-linking and participate in the hightemperature devolatilization step.
Acknowledgment. This work is apart of a cooperative research program (PICS) between the Polish Academy of Sciences (PAN) and the French Centre National de la Recherche Scientifique (CNRS). The financial support granted to H.R.S. by the Deutsche Forschungsgemeinschaft, Bonn-BadGodesberg (GermanyProject Schu 416/ 15-1)is gratefullyacknowledged. We thank the PIRSEMCNRS for the financial support provided by PICS "Carbochimie" (PAN-CNRS) and Professor S. Jasienko (Wroclaw) for a gift of the samples of Polish coals. Registry No. NMP, 872-50-4.
