Energy & Fuels 1993, 7, 52-56
62
Quantitative Analysis of N-Methyl-2-pyrrolidinonein Coal Extracts by TGA-FTIR M. F. Cai and R.B.Smart' National Research Center for Coal and Energy and Department of Chemistry, West Virginia University, P.O.Box 6045, Morgantown, West Virginia 26506 Received August 20, 1992. Revised Manuscript Received October 26, 1992
N-Methyl-2-pyrrolidinone(NMP) is widely used in coal extraction. I t has been shown that NMP is strongly retained in coal extracts and the retained NMP affects the character of the extracts. In order to calculate the actual extraction yield and understand the interaction between NMP and coal, the amount of NMP retained by its coal extracts must be quantified. TGA-FTIR has been applied to the quantitative analysis of NMP. Neutral aluminum oxide with 4.74% NMP added was used as a standard, and a linear calibration curve for the area of the NMP profile and the retained NMP weight was obtained. Several NMP-soluble coal extracts were tested by this method. The percentage of the NMP retained by these extracts varied from 1.08 to 3.40% with deviations ( n = 2) within 0.25%. The detection limit can be as low as 5 pg with a S/N of 5 when the C=O stretching band of NMP is used. Introduction Solvent extraction is an important process in coal research. Among many solvents which have been applied to coal extraction, N-methyl-2-pyrrolidinone(NMP) gives a higher coal extraction yield than other NMPsoluble coal extracts retain some NMP even after a long period of drying, and elemental analysis has shown that the percentage of nitrogen contained in the NMP-soluble coal extracts is higher than that in the original coals and
residue^.^^ The amount of NMP retained in the extracts can affect the accuracy of the calculation of the extraction yield. It also changes the character of the extracts, and it will most certainly influence the end use of these extracts. Quantification of the NMP retained in the coal extracts is an important initial step in the understanding of its interaction with coal. The conventional quantitative analysis technique of solvent extraction followed by gas chromatography or IR is not practical in the present case because that approach is too time consuming. Moreover, the NMP is retained by some coal so strongly that in these extracts it can be retained a t temperatures as high as 490 OC and so complete removal of NMP may not be possible. In addition, as shown in the profiles of the evolution rate of NMP vs temperature of the furnace, the temperature at which NMP has its maximum evolution rate may vary from 150to 400 OC for extracts of different coals. Hence, the same thermal extraction temperature cannot guarantee the complete (1) Roy, J.; Banerjee, P.; Singh, P. N. Indian J. Technol. 1976, 14, 298-300. (2) Renganathan, K.; Zondlo, J. W.; Stiller, A. H.; Phillips, G.; Mintz, E.A. Coal Sci. Technol. 1987, 1 1 , 367-370 (Int. Conf. Coal Sci.). (3) Iino, M.; Takanohashi, T.; Ohsuga, H.;Toda, K. Fuel 1988, 67, 1639-1647. (4) Wei, X. Y.;Shen, J. L.; Takanohashi, T.; Iino, M. Energy Fuels 1989,3, 575-579. (5) Seki, H.; Ito, 0.; Iino, M. Energy Fuels 1990,4, 352-355. (6) Cagniant, D.;Gruber, R.; Lacordaire, C.; Jasienko, S.; Machnikowska, H.; Salbut, P. D.; Bimer, J.; Puttmann, W. Fuel 1990,69,902910. (7) Clements, J. L.; Dadyburjor, D. B. Fuel 1991, 70, 747-751.
0887-0624/93/2507-0052$04.00/0
desorption of the retained NMP from ita coal extract, which is the necessary condition for quantitative analysis. TGA or FTIR by themselves are also unreliable for quantification of the NMP retained by coal extracts because of the complicated composition of this coal-related material. During a conventional thermogravimetric analysis, the weight loss due to the desorption of the NMP cannot be distinguished from that due to other sources. Actually, there are very few cases where a percent weight remaining vs temperature curve of an NMP-soluble coal extract will show a distinguishable step corresponding to just one of the many processes occurring during TGA, such as desorption and decomposition. Attempts to quantify retained NMP by FTIR has similar difficulties. The FT-IR spectra of NMP-soluble coal extracts give only one band which corresponds to the retained NMP. This band is weaker and located on the shoulder of a strong absorption band due to coal. The shape of this strong coal band could be different for different types of coals. An accurate measurement of the area or height of this shoulder peak is either very difficult or impossible. In order to overcome these difficulties, the combination of TGA with FTIR was applied to determine the amount of the NMP retained in coal extracts. The integrated TGAFTIR system has been used previously to characterize various materials such as polymers, pharmaceutical samples, and inorganic chemicals.g"J It has also been applied to coal research and has provided a wealth of information about the thermal degration products of coal ~amp1es.ll-l~ The potential of TGA-FTIR as a quantitative tool has also been investigated and two examples are the quan(8) Lepharde, J. 0. and Fenner, T. A. Appl. Spectrosc. 1980,34,174185. (9) Johnson, D. J., et al. h o c . 17th North Am. Thermal Anal. SOC. Conf. 1988,2, 574-579. (IO) Compton, D. A. C. Int. Labmate, 1987, June, 37-43. (11) Carangelo, R. M.; Solomon, P. R.; Gerson, D. G. Fuel 1987, 66, 960-967. - - - - - .. (12) Whelan, J. K.; Solomon, P. R.; Deshpande, G. V.; Carangelo, R. M. Energy Fuels 1988,2, 65-69. (13) Solomon, P. R.; Serio, M. A.; Carangelo, R. M.; Basailakis, R. Energy Fuels 1990, 4,319-333.
0 1993 American Chemical Society
Energy & Fuels, Vol. 7, No. 1, 1993 53
NMP in Coal Extracts tification of the nitrocellulose and pulp in gunpowder14 and solvent retention in pharmaceutical ~amp1es.l~ In both of these cases, the matrix was much simpler than coal samples. Experimental Section The TGA-FTIR system used in this work is a combination of a PL-TGA 1500 (Omnitherm Corporation, Mundelein, IL) with an FTS 7 FTIR spectrometer (Bio-Rad, Digilab Division, Cambridge, MA). The TGA balance has a sensitivity of 1 pg a t 10 mg scale and is controlled by an electronic unit (Balance Control Unit, Omnitherm Corp.). The furnace has a maximum operating temperature of 1500 "C, and it can be programmed by computer through an interface (Computer Controlled Interface 11, Omnitherm Corp.) associated with the furnace. The furnace is interfaced with the FT-IR spectrometer by an external sample compartment (Bio-Rad, Digilab Division) adjacent to the main optical bench. This arrangement makes it possible for the FTS 7 unit to be used alone without detaching the TGA furnace. The entire system is under the direct control of a SPC 3200 workstation (Bio-Rad, Digilab Division), and data from both the IR and TGA can be simultaneously collected by the SPC 3200 data station. Helium (10 mL/min) is used as a sweep gas. The temperature of the furnace is first held a t 25 "C for 5 min and then heated to 650 "C a t a rate 20 "C/min for all experiments. The sample is placed in a platinum pan and, during heating, the evolved gas is carried to the IR sample cell through a heated transfer line. The temperature of the IR sample cell is kept a t 225 "C while the temperature of the transfer line is set 25 "C lower in order to prevent condensation in the cell. The Bio-Rad FTS 7 IR spectrometer uses a DTGS (deuterated triglycine sulfate) detector and is continuously collecting IR spectra while the sample is heating in the furnace. All spectra were collected a t 8 cm-l resolution over the range of 4000-1000 cm-I, coadding 16 scans per spectrum, which provided a time resolution of approximately 13 s. The standard NMP sample was made by adding 4.74 wt % NMP solvent to neutral aluminum oxide (Alumina Woelm N, Akt. 1,Woelm Pharma, Atlanta, GA). Prior to use the aluminum oxide was manually ground and then dried in a furnace a t 500 "C for 3 h. The NMP-soluble coal extracts were provided by the Department of Chemical Engineering a t West Virginia University and subsequently milled in a mortar until the size of the particles was less than 100 Km. The coal extraction process has been described elsewhere.* Briefly, about 10 g of coal was added to 100 mL of NMP and the mixture was refluxed under nitrogen for 60 min. The mixture was filtered and the filtrate was rotary evaporated a t 150 "C to remove the NMP solvent. The solid extract was dried in a vacuum oven at 100 "C and 0.1 Torr for 24 h. The diffuse reflectance IR spectra were measured by the FTS 7 FTIR spectrometer associated with a Digilab diffuse reflectance accessory a t resolution of 2 cm-l, coadding 256 scans. The sample was mixed with KC1 (1:19, w/w) and then further ground to a size of less than 20 pm. The KC1-diluted sample was placed into a stainless cup (4 mm diameter). The Kubelka-Munk scale was used for representing the diffuse reflectance.
Results and Discussion
An IR spectrum of the extract provides evidence for the retention of NMP in its coal extract. As shown in Figure 1, the diffuse reflectance IR spectrum of original coal is compared with the spectrum of NMP-soluble extract. The additional IR band at 1667 cm-* in the spectrum of the (14) Johnson, D. J.; Compton, D. A. C. Am. Lab. 1991, 23, 37-43. (15) Johnson, D. J.; Compton, D. A. C. Spectroscopy, 1988,3,47-50.
1
I
2000
3000
4000
1000
Wavenumber (cm?
Figure 1. Diffuse reflectance IR spectra of an original coal, WVGS No. 13425, (upper), the NMP-soluble extract (middle), and the original coal with NMP added (lower). The band indicated by an arrow belongs to NMP.
il
2-plrrolidinone, l-methyl-
I/ ~
I/ I
3400
I
I
3000
I
1
2600
1
1
-
2200
I
1
1800
1
1
1100
1
I
1000
Wavenumber (cm-')
Figure 2. IR spectrum of the evolved NMP (bottom) compared with the Digilab IR library spectrum.
extract is attributed to NMP since this band also appears in the spectrum of the original coal to which about 7% NMP has been added. Retention of NMP is further confirmed by thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR), where one of the evolved gases has an IR spectrum identical to the NMP spectrum as shown in Figure 2. A plot of percent weight remaining vs temperature for a TGA of a typical NMP-soluble coal extract is shown in Figure 3 together with its first derivative curve. The total percent weight loss from 25 to 650 "C varies for different coal extracts but is always more than 20% of the initial sample weight. On the other hand, the percentage of the NMP retained in the sample is less than 4 % . The percent weight remaining curve does not have any distinguishable individual weight loss steps. The main peak of the firstderivative curve (maximum at approximately 500 "C)is due to pyrolysis products of the coal extract. A weak shoulder peak was observed at 400 "C and is possibly attributed to the desorbed NMP.
54 Energy &Fuels,
Vol. 7,No.1, 1993
Cui and Smart
I
I
I
I
I
1
100
200
300
400
50c
600
Temperature (" C) Figure 3. Percent weight remaining and its first-derivative curves for the NMP-soluble extract of WVGS No. 13425.
-I
0.020
0.015
4000
I
1
1
3jOO
3000
2500
I
znnn
I
1500
1000
Wavenumber (cm.')
Figure 4. IR spectrum for the gases evolved from the NMPsoluble extract of WVGS No. 13425 at 427 "C. By continuously collecting IR spectra of the evolved gases during TGA, it was observed that the maximum evolution rate of the methane and other hydrocarbons always occurred at approximately 500 "C for extracts of different coals. On the other hand, as mentioned previously, the temperature for maximum evolution rate of NMP varied from 150 to 400 "C depending on original coal and, therefore, the shoulder peak of the derivative curve is very difficult to observe in some coals. I t is impossible to quantify the NMP based on this obscure shoulder peak. All of the NMP-soluble coal extracts evolved water, carbon dioxide, carbon monoxide, methane, various hydrocarbons, and NMP at temperatures below 650 "C.An IR spectrum of the gases evolved from an NMP-soluble coal extract at 427 "C is shown in Figure 4. The band at 2350 cm-l is attributed to carbon dioxide, the band at 2142 cm-l to carbon monoxide, and the bands between 2800 and 3200 cm-l indicate the methane and other hydrocarbon compounds produced by coal pyrolysis. A sharp and strong band at 1727 cm-l is due to the C=O stretching band of NMP, which is overlapped by one of the water bands. Another water band is located between 3600 and 4000 cm-1. In our application of the TGA-FTIR system, the TGA provides thermal extraction and the FTIR serves as a detector. The gases are evolved in different temperature
windows because of either different adsorption energies or a different pyrolysis mechanism; however, the temperature windows will most likely overlap and the evolved gases cannot be completely resolved. The FTIR provides selectivity since different gases have different IR bands and the selection of a unique IR band provides additional discrimination. Assuming Beer's law is obeyed, the integral of the selected IR absorbance over a certain frequency range for the gas of interest is proportional to ita evolution rate at that temperature. With a continuously scanning IR spectrometer, a profile of this integral vs the temperature (or time in the case where a constant temperature was used) can be obtained and this is referred to as a specific gas profile (SGP). The area of the SGP is proportional to the weight or the volume of the evolved gas. As shown in Figure 2, the NMP has four major absorbance bands or band groups in the region recorded. Any of these can be used to obtain the SGP of NMP, but it is obvious that the band at 1727 cm-l (strongest absorbance) and the band at 2922 cm-l (large area) would have a better signal-to-noise ratio (S/N) for the SGP than the other bands. Therefore, the ranges from 1680 to 1775 cm-l and 2816 to 3029 cm-l were chosen for comparison, and it was found that the SGP ranging from 1680 to 1775 cm-l had better S/N. Since NMP did not desorb from some of the extracts at temperatures below 300 O C and the hydrocarbons start to appear before 300 O C , the SGP of the range from 2816 to 3029 cm-l could contain contributions from both the NMP and hydrocarbons. This makes it difficult to determine the area of that profile attributed to the evolved NMP and so the other frequency range from 1680 to 1775 cm-' was used to obtain the SGP for the evolved NMP. As mentioned above, in the range from 1680 to 1775 cm-l, the SGP still contains some contribution due to the absorbance of water. In order to obtain an accurate area measurement of the evolved NMP profile, it is necessary to subtract that portion attributed to water. Water exhibits two bands in the IR spectra at 1596 and 3741 cm-'. The two bands are well correlated and, based on the profile of the water band at 3741 cm-', the profile of the water band at 1596 cm-l can be deduced as acorrection reference. The bandat 3741 cm-l actually covers the region from 3464 to 4000 cm-l; however, only the region from 3777 to 3952 cm-' was used as a reference for the water correction in order to avoid interference from carbon dioxide at 3711 cm-l. To get the correlation ratio for the profiles of the two water bands, both ranges were simultaneously scanned during a TGA-FTIR of aluminum oxide powder. Three scans at the same conditions were added together and the ratio of the areas of the water profiles was calculated as 1.09. This was used as a correction factor in all subsequent experiments. The aluminum oxide was chosen as the matrix for the standard samples because it is thermally stable and can retain NMP at room temperature. The SGP of the evolved NMP from the standard containing about 5% NMP exhibited a satisfactory shape. Too much NMP will cause evaporation to occur before the TGA-FTIR scanning starta as well as a tailing profile, both of which will affect the accuracy of the analysis. On the other hand, too little NMP makes the water correction more difficult since the area corresponding to the water band would be larger relative to that of NMP.
Energy & Fuels, Vol. 7, No. 1, 1993 55
NMP in Coal Extracts 0.30 3
;;
-
r-
M
_/i__ -/
0.20
1-
5 OJ5
e
z
%O.lOj I
I I I A
\
0.00 0.00
I 3500
I
I
I
I
I
3000
2500
2000
1500
1000
Wavenumber (cm")
Figure 5. Diffuse reflectance IR spectra of the standard before (bottom)and after TGA-FTIR (top). The spectrum of neutral aluminum oxide was used as the background. 0.135
0.115
0.095
i
0.274 mg 0.239 mg 0.204 mg
uc+o+
m+g++a
+ + + -
-c t-
*_
0.178 mg 0.136 mg 0.102 mg 0.077 mg 0.043 mg 0.016 mg
I---
I__x
Q)
I?
$ 0.075
0.10
0.20
0.30
0.40
0.60
0.70
0.80
0.90
Profile area
Figure 7. Calibrationcurve for the area of the NMP profile and the weight of NMP retained in the sample. Table I. Percentages of Retained NMP and Extraction Yields
sample No. 13407 No. 13421 No. 13422 No. 13423 No. 13424 No. 13425 No. 13426
NMPa
deviation on two tests
(%)
(%)
2.12 3.40
0.18 0.17 0.21 0.01 0.02 0.25 0.25
1.90
1.08 2.62 3.31 2.32
extraction yieldb ( % daf) 66.3 63.1 25.0 34.2 27.7 25.0 21.4
*
0 Average on two testa. Provided by Department of Chemical Engineering, West Virginia University.
the samples as shown in Figure 7. A linear least-squares fit of the all 14 points gave the formula of the calibration curve (correlation coefficient, R = 0.9990)
e 2s: 0.055
W 0.347A - 0.00072 where W is the weight of the NMP in the sample while A is the corrected profile area solely attributed to the adsorbed NMP, and
0.035
A = A,
0.015
-0.005
0.50
0.0
10.0
20.0
Time (min) Figure 6. Profiles of the evolved NMP for some standards. Fourteen standard samples (0.337-5.772 mg) were measured by TGA-FTIR. The IR spectra taken during the TGA did not indicate any decomposition of the NMP a t temperatures below 650 "C. The diffuse reflectance IR spectra of the standard sample before and after TGA were recorded as shown in Figure 5, using the spectrum of aluminum oxide as the background. It can be seen that all IR bands of NMP were absent in the spectrum after TGA, which indicates that no detectable NMP remained in the standard samples after the TGA. The SGPs of the NMP evolved from the standard samples are shown in Figure 6. Each curve has a hump on the right side of the curve which is caused by the water evolved in the TGA process. The calculated profile areas were first corrected by subtracting that portion due to the water band and then plotted against the NMP weights in
- 1.09A2
where A1 is the area of the profile corresponding to the range from 1680 to 1775 cm-l and A2 is the area of the profile ranging from 3777 to 3952 cm-*. Seven different coal NMP extracts were measured by this method and the results are summarized in Table I. The percentage of the NMP adsorbed by the seven extracts ranged from 1.1 to 3.4%,with a standard deviation ( n = 2) within 0.25%. There is no obvious correlation between the amount of NMP retained and the extraction yields. The reason for the different percentages of the NMP retained in the coal extracts is under investigation and could be due to slightly different drying conditions. The standard deviations of the slope and intercept of the calibration line were calculated as 4.38 X 10-3 and 1.71 x 10-3, respectively. An estimate of the standard deviation of the retained NMP is 3.9 pg. For a 9-mg coal extract sample this equals a deviation of 0.04%,which was much smaller than the most experimental values. In order to estimate the experimental error, an unextracted coal (WVGS No. 13425) with 5.66% NMP solvent added was tested by this method. The average result of three tests is 5.95% NMP with a standard deviation of 0.25 % ,which is in good agreement with the actual value. The major source of error is the correction for water vapor. The absorbance in the region 3777-3952 cm-l is
Cai and Smart
56 Energy & Fuels, Vol. 7,No. 1, 1993 0.0016
0.0012
8
0.0008
: 2
0.0004
o.oO0o I
3400
'
l
3000
'
l
2600
'
l
2200
'
l
1800
'
l
1400
'
l
inon
Wavenumber (cm") Figure 9. IR spectrum of a sample containing 5 rg of NMP. The band indicated by the asterisk is attributed to NMP. I
I
I
I
I
5
10
15
20
25
Time (min) Figure 8. Specific gas profile (upper) from 1680 to 1775 cm-' for a 0.160-mg standard sample containing 11 wg of NMP compared to its water correction reference profile.
weaker than that at the region 1680-1775 cm-l. In addition, the DTGS detector has a lower signal-to-noise ratio in the region of 3600-4000 cm-'; therefore, the ratio of S/N is worse in the water correction reference region than in the region from 1680 to 1775 cm-'. The portion of the water correction in the NMP profile of coal extract samples is also larger than for the standard samples. All of these caused a larger deviation for coal extracts than for standard samples. Improved accuracy could be obtained by coadding more scans for IR spectra. The sample of the NMP-spiked WVGS NO. 13425 was remeasured using 32 coadded scans per spectrum, which resulted in deviations of 0.24%, compared with 0.37 % from 16 coadded scans. An increase in the number of coadded scans would decrease the time resolution which would make the accuracy of the profile area measurement less accurate. To determine the detection limit of the TGA-FTIR method, an NMP standard sample of 0.160mg (containing 7.6 pg NMP) was examined and the result is shown in Figure 8. The upper trace is the profile of the NMP ranging from 1680 to 1775 cm-l. For comparison, the water correction reference profile of the range from 3777 to 3952 cm-l is shown on the bottom. The percentage NMP was calculated as 4.7296,in excellent agreement with the actual value of 4.74%. For the NMP-soluble coal extracts, the detection limit of NMP will be higher, since these samples have more water interference. For WVGS No. 13425,22.5 pg of NMP in a sample of 0.326 mg was clearly detected.
If the IR band of NMP at 1727 cm-l is used as the signal, the detection limit can be lowered to 5 pg, and an IR spectrum of the evolved gases of a coal extract sample containing 5 pg NMP in a weight of 0.475 mg is shown in Figure 9. The band corresponding to NMP can be easily detected at a S/N of about 5.
Conclusions The NMP retained by coal extracts can be quantitatively measured by the TGA-FTIR technique. The neutral aluminum oxide is suitable matrix for the preparation of NMP standards. The area of the SGP of the evolved NMP in the range from 1680 to 1775 cm-l vs the furnace temperature is linear with the weight of the NMP. The percentages of the NMP adsorbed in ita coal extracts are in the range between 1.1 and 3.4 96 in seven tested samples, and the temperature at which the NMP evolved at ita maximum rate varies with the extracts of the different coals. There is no obvious correlation between the percentages of NMP retained or the maximum evolution temperatures with the extraction yields, and this aspect is currently being investigated. Even in a very complicated matrix like coal extracts, a quantitative analysis can be successfully performed by selecting appropriate bands for the interested component and the correction background.
Acknowledgment. This work was partially supported by the US. Department of Energy Grant No. DE-FGO291NP00159. We thank Dr. Peter G. Stansberry, Department of Chemical Engineering, West Virginia University, for the NMP-soluble coal extract samples. Registry No. The following Registry number was supplied by the authors. N-Methyl-2-pyrrolidinone, 87250-4.
