Energy & Fuels 2007, 21, 2827-2830
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Viscoelastic Properties and Adsorption Behaviors of Two Kinds of Pyridine Insoluble Fractions of Coal Extracts with Different Aggregated States Hengfu Shui* and Zhicai Wang School of Chemistry & Chemical Engineering, Key Laboratory of Anhui Educational Department, Anhui UniVersity of Technology, Ma’anshan 243002, Anhui ProVince, P. R. China ReceiVed February 27, 2007. ReVised Manuscript ReceiVed May 8, 2007
Two kinds of pyridine insoluble fractions (PI) of coal extracts with different solubilities in N-methyl-2pyrrolidinone (NMP) were characterized in this paper. PI-0 was obtained by the pyridine fractionation of Upper Freeport coal extracts with a CS2/NMP mixed solvent (1:1 by volume), and its solubility in NMP is 53 wt %. While PI-1, which was obtained by the removal of NMP and tetrabutylammonium acetate (TBAA) from a PI-0 solution in NMP containing TBAA, was almost completely soluble in NMP. Solid-state 13CNMR spectra indicated that the two PIs have the same chemical structure. The viscoelastic properties and methanol adsorption behaviors of the two PIs were measured. The dynamic viscoelasticities of the two PIs are similar, and the elastic modulus (G′) of PI-1 is lower before the softening temperature than that of PI-0, suggesting that the macromolecular network of PI-1 is looser compared to that of PI-0. The methanol sorption behaviors of PI-0 and PI-1 are also similar, and lines curve-fitted with the Langmuir-Henry equation were in agreement with the experimental data, suggesting that the bulk structure of the two PIs is similar. The methanol sorption for PI-1 is larger than that for PI-0. The constants of the Langmuir-Henry equation obtained by successive fitting for the two PIs indicated that the microporosity of PI-1 is larger than that of PI-0. The results obtained here suggested that the dissociation of molecular interactions is responsible for the high solubility of PI-1.
Introduction The phenomenon of coal molecular aggregation has been observed through coal extraction yield change,1,2 the observation of size exclusion chromatography of coal-derived materials in solution,3 swelling,4 and a small-angle X-ray scattering5 study of coal extracts. Coal aggregation behavior has a great effect on its physical and chemical properties, such as solubility,2 caking,6 and reactivity.7 In our previous works,8,9 we obtained two kinds of pyridine insolubles (PI and PI-1) from Upper Freeport (UF) coal with quite different solubilities in N-methyl2-pyrrolidinone (NMP), and the difference in solubility of the two PIs was considered to be due to their different aggregation states according to the X-ray diffraction analysis, heat capacity measurements, and thermomechanical analysis observations. PI1, which was a less aggregated state, had larger heat capacity and thermal deformation compared to PI. Aggregation kinetic research10 showed that the aggregation of PI-1 in solution was reaction limited, and the aggregation was quicker in the CS2/ NMP mixed solvent than in NMP solution due to the lower diffusibility (mobility) of PI-1 molecules in NMP. Viscosity * Corresponding author. E-mail:
[email protected]. (1) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. (2) Painter, P. C.; Opaprakasit, P.; Scaroni, A. Energy Fuels 2000, 14, 1115-1118. (3) Chen, C.; Iino, M. Fuel 2001, 80, 929-936. (4) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107-110. (5) Ho, B.; Briggs, D. E. Colloids Surf. 1982, 4, 285. (6) Shui, H.; Wang Z.; Li, X. Fuel 2007, 86, 1396-1401. (7) Larsen, J. W.; Azik, M.; Korda, A. Energy Fuels 1992, 6, 109-110. (8) Shui, H.; Norinaga, K.; Iino, M. Energy Fuels 2001, 15, 487-491. (9) Shui, H.; Norinaga, K.; Iino, M. Energy Fuels 2002, 16, 69-73. (10) Shui, H.; Zhou, H. Fuel Process. Technol. 2005, 86, 661-671.
changes of the PI-1 solutions with time were also observed due to the aggregation of the PI-1 molecules.11 Recently, the viscoelasticity of bituminous coals has been extensively studied, since this property is considered to be important for predicting coking behaviors.12,13 This technique was also applied to characterize the coal solvent interactions.14 The aggregation behavior of coal molecules affects its physical properties such as solubility2 and reactivity,7 as mentioned above, and also the viscoelasticity. Krzesinska15,16 measured the elastic dynamic moduli of a raw bituminous coal and its demineralized version using the molecular acoustics method (velocity of the ultrasonic wave). They found that the elastic dynamic modulus of the demineralized coal was greater than that of the raw coal, suggesting a more cross-linked structure for the demineralized coal. It is interesting and also useful to analyze the thermoplastic properties of coals for characterization of their aggregation behaviors in the solid state. Measurement of the adsorption behavior of methanol vapor is another effective means to characterize the micropore and bulk structure of coals. Green at al.17 have reported that the O-methylation of coals causes an increase in the sorption rate, probably as a result of the disruption of the hydrogen bonds that constitute the secondary structure of coals. One can (11) Shui, H.; Zhou, H. Fuel Process. Technol. 2004, 85, 1529-1538. (12) Nomura, S.; Kato, K.; Momaki, I.; Fujioka, Y.; Saito K.; Yamaoka, I. Fuel 1999, 78, 1583-1589. (13) Yoshida, T.; Takanohashi, T.; Iino, M.; Katoh, K. Energy Fuels 2001, 15, 170-175. (14) Takanohashi, T.; Yoshida, T.; Iino, M. Fuel 1999, 78, 865-866. (15) Krzesinska, M. Energy Fuels 2001, 15, 324-330. (16) Krzesinska, M. Energy Fuels 2001, 15, 930-935. (17) Green, T. K.; Ball, J. E.; Conkright, K. Energy Fuels 1994, 8, 213218.
10.1021/ef0701034 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
2828 Energy & Fuels, Vol. 21, No. 5, 2007
Shui and Wang
Figure 1. Preparation step of PI-0 and PI-1.
understand that the aggregation behaviors strongly affect the micropore structure of coals, which changes their adsorption behaviors. In this paper, we prepared two kinds of PIs with different solubilities in NMP following our previous method,8 and the viscoelastic properties and adsorption behavior of the two PIs were measured to characterize their aggregation behavior in the solid state.
Figure 2.
Experimental Section
sample
NMP
NMP+TBAA
C
H
N
S
Oa
Preparation of PIs with Different Solubilities in NMP. Original PI (referred to as PI-0) was prepared from the pyridine insoluble fractionation of UF coal extracts with a CS2/NMP mixed solvent (1:1 by volume). A total of 0.5 g of PI-0 was dissolved in 60 mL of NMP with an additive, tetrabutylammonium acetate (TBAA), under ultrasonic (38 kHz) irradiation for 30 min at room temperature. The amount of TBAA added was 0.25 mmol (0.075 g) per 1 g of PI-0. The removal of NMP and TBAA from the PI-0 solution in NMP gave PI-1. Figure 1 shows the individual steps, and the method for detailed preparation of the PIs was described elsewhere.8,9 13C NMR Measurements. Solid-state 13C NMR measurements were made by the single pulse excitation/magic angle spinning (SPE/MAS) method using a Bruker MSL-300 NMR. A sample of about 100 mg was packed in the sample rotor. The measuring conditions were as follows: 90° 13C pulse width, 4 µs; 13C frequency, 75.46 MHz; spinning rate of MAS, 10 kHz; pulse repetition time, 60 s. TG Analysis. A thermogravimetric (TG) analysis was carried out on a TG/DTA 32 analyzer (SII, Seiko Instruments). About 6 mg of the sample was placed in an aluminum pan and heated from 25 to 400 °C at a rate of 8 °C/min under a 60 mL/min helium gas flow. Dynamic Viscoelastic Measurements. Rheological measurements were performed using a rheometer (Rheometric Scientific Inc., ARES-2KSTD). Approximately 0.4 g of the sample was pressed under 100 MPa and evacuated into a sample pellet with a diameter of 13.5 mm. The pellets made from PI-0 and PI-1 were a homogeneous, continuous medium, and there was no crack observed on them. The sample pellet was placed between 25 mm parallel plates under 1 N of normal force. The measurement was carried out at intervals of 20 s from 50 to 500 °C at a heating rate of 8 °C/ min under a nitrogen flow of 80 L/min. Sorption Experiments. Methanol sorption was measured with an automatic adsorption apparatus (BELSORP 18, BEL Japan, Inc.) at 30 °C. Approximately 150 mg of the sample was placed in the sample tube and weighed. The deaeration treatment of methanol in the solvent tank was carried out with liquid nitrogen four times through freeze-thaw cycles. The samples were pretreated under a vacuum (
