Energy & Fuels 1995,9, 1035-1037
1035
Examination of Change in Coal Gel Structure Due to Solvent Swelling by Size Exclusion Chromatography Jun-ichiro Hayashi, Shinobu Amamoto, Katsuki Kusakabe, and Shigeharu Morooka" Department of Chemical Science and Technology, Kyushu University, Fukuoka 812-81,Japan Received June 13, 1995. Revised Manuscript Received August 14, 1995@
An inverse liquid chromatography (ILC) technique was applied to evaluating the porous structure of coals. Swollen Morwell coal (C = 65 wt %, daf) or Pocahontas coal (C = 91 wt %, daf) particles were packed in a ILC column as the stationary phase, and n-hexane (HEX) or tetrahydrofuran (THF) was used as the mobile phase. For any combinations of coals and carrier solvents, n-alkanes of CS-CZZthat were used as the molecular probes eluted in the size-exclusion region. In the Pocahontas-HEX system, all probes eluted nearly at the size-exclusion limit. This indicates that there were few pores of 1-10 nm in that coal. In the Morwell-HEX and Morwell-THF systems, however, the elution volume was varied by the chain length of probes. Under the assumption that the elution was controlled by the steric-exclusion mechanism alone, the volume of pores into which pentane could penetrate was 0.04 and 0.83 cm3/g of coal for the Morwell-HEX and Monvell-THF systems, respectively. The swelling of Morwell coal by THF (swelling ratio = 2.15) created micro- and mesopores accessible to n-alkane molecules. Introduction Recently we developed an inverse liquid chromatography (ILC) technique that used a separation column packed with solvent-swollen coal particles. Pulses of hydrocarbons and alkylbenzenes are injected as molecular probes, and structural and interfacial properties of the coal are evaluated from their elution behavior. In the previous ILC was performed in the adsorption-controllingregime, where the elution volume of injected molecular probe was larger than that of carrier solvent. The interaction between coal and probes was then stronger than that between coal and carrier solvent (or between carrier solvent and probes). Aromatic rings were recognized more selectively than alkyl chains. For bituminous coals, the n-n interaction between aromatic planes of coals and molecular probes was responsible for the selective recognition of aromatic rings. For brown coals, on the other hand, the OH-n hydrogen bond between acidic OH groups in coals and aromatic planes of probes was important. It was also found that bulky alkylbenzenes were hindered from entering micropores of coal. Such a steric-exclusion effect was not precisely evaluated because the elution of these probes was decided not only by their molecular size and shape but also by their adsorptivity onto the coal surface. To examine the microporous structure of coal swollen in carrier solvent, the adsorption of probe molecules onto the coal surface should be eliminated and ILC should be conducted in the size-exclusion regime. In the present study, we applied the ILC t o evaluate the porous structure of solvent-swollen Morwell brown coal and Pocahontas No. 3 coal.
Experimental Section Monvell brown coal (C, 65;H, 4.9; N,0.6;0 + S, 29 wt
%,
daf) and Pocahontas No. 3 coal (APCS, C, 91; H, 4.5; N, 1.1;0 S,4 wt %, daf) were pulverized to smaller than 37 pm and dried under vacuum at 100 "C for 6 h. They were immersed n-hexane (HEX) or tetrahydrofuran (THF), which was used as the mobile phase, and packed in a n HPLC column (length = 150 mm, inner diameter = 4 mm, total column volume = 2.41 mL) as the stationary phase. As listed in Table 1, the amount of coal packed in the column was varied by solvent used, due to the difference in swelling ratio. The probe compounds were n-alkanes of c5-c~~ and were individually diluted in carrier solvent a t a concentration of 10-2-10-3 moll L. Pulses of 10 p L of the solution were injected into the carrier solvent, which was fed into the column a t a rate of 0.2-0.4 m u m i n . The elution volume of probe was detected with a differential refractometer. The theoretical plate number of ILC columns was determined from the elution profile of benzene and was in the range of 700-1000 m-l. Details are described elsewhere.lS2
+
Results and Discussion The total volume of the ILC column, Vt, is divided into three parts as follows:
v, = v, + vi + vg where VO,Vi, and V, are the interstitial volume of the packed bed, the volume of pores in swollen coal gel, and the volume of coal solid without pores, respectively. The elution volume of the probe, V,, is expressed by the following equation if the probe is eluted by the sizeexclusion mechanism.
* To whom all correspondence should be addressed.
Abstract published in Advance ACS Abstracts, September 15,1995. (1)Hayashi, J.4.;Amamoto, S.; Kusakabe, K.; Morooka, S. Energy Fuels 1993,7, 1112. (2) Hayashi, J.4.; Amamoto, S.; Kusakabe, K.; Morooka, S. Submitted for publication in Energy Fuels. @
where Kd is the partition coefficient. When small probes diffuse into the entire inner space of coal gel, & is unity. On the other hand, when probe molecules cannot enter
0887-0624/95/2509-1035$09.00/0 0 1995 American Chemical Society
1036 Energy & Fuels, Vol. 9, No. 6, 1995
Hayashi et al.
Table 1. Physical Properties of Coals and ILC Conditions physical property He density (g/cm3) Vln (cm3/g) Vzb (cm3/g) V3c (cm3/g) Rd/solvent ILC conditions carrier solvent Vt (cm3icolumn) mass of coal (gicolumn) volume of solvente (cm%olumn) Vd (cm3/column) Vjg (cm3/column) Vgh (cm3/column) flow rate of carrier solvent (cm3/min) column temperature ("C) probe compds and elution vol (VJ (cm3) pentane ((25) hexane (c6) heptane (C7) octane (CS) decane (Clo) dodecane (Cld tetradecane (C14) hexadecane (c16) octadecane (CIS) eicosane ( C ~ O ) docosane (CZZ)
Monvell
Pocahontas No. 3
1.374 0.727 0.093 0.022 1.02MEX
2.161l'HF
1.354 0.738 0.086 0.003 l.Ol/HEX
HEX 2.410 1.358 1.40 1.23 0.17 1.00 0.30 30
THF 2.410 0.599 1.98 1.36 0.62 0.44 0.30 30
HEX 2.410 1.618 0.81 1.06 0.16 1.20 0.30 30
1.284 n.d. 1.269 n.d. 1.248 n.d. 1.235 n.d. 1.228 n.d. 1.226
1.868 1.843 1.818 1.807 n.d. 1.763 n.d. 1.731 1.717 1.711 1.708
1.046 n.d. 1.045 n.d. 1.045 n.d. 1.046 n.d. 1.048 n.d. 1.049
a Specific volume of coal calculated from He density. Micropore volume determined from C02 isotherm at 25 "C based on DubininAstakhov equation. Total volume of pores with diameters of 1-100 nm, determined from Nz isotherm a t -196 "C. Volumetric swelling ratio, determined by the procedure proposed by Green et a1.l1 Swelling temperature was 30 "C. e Calculated from the density a t 30 "C. f Void volume (Le., interstitial volume). g Inner volume (volume of solvent that fills inner space of coal), calculated by assuming V, = R(V1 VZ V3) - V,. Volume of coal that does not contain the inner volume, V, = Vt - VO- V,.
+ +
the inner space of coal gel, & becomes zero. VOis often called the size-exclusion limit in the terminology of sizeexclusion chromatography. Separation of solutes by size exclusion ILC (SE-ILC) is explained by the following mechanisms. (i) Steric-Exclusion Me~hanism.~ Probe molecules diffuse into pores at an infinite rate, and the partition equilibrium is always attained. Kd is independent of the flow rate of carrier solvent. (ii) Restricted-Diffusion Me~hanism.~,~ In this case, the diffusion coefficient of probe in pores, Dint, is smaller than that in the interstitial space, D,a. The larger the size of the probe, the more significant the difference between Dint and De*. The partition equilibrium is not established. In general, both mechanisms affect the elution of solutes. Yau6and Cooper et aL7investigated the elution behavior of standard polystyrenes from a column of porous silica beads. The hydrodynamic diameter of the smallest polystyrene that eluted at the size-exclusion limit (V, = VO)agreed well with the pore diameter of the silica beads. Furthermore, porous silica beads possessing a pore diameter larger than 10 nm could not separate polystyrenes whose moleyular weights were smaller than lo3 (hydrodynamic diameter < 2 nm). All these polystyrenes eluted a t V, = VO Vi. In the present study, docosane (C22H46) was the largest molecular probe. Its extended chain length and hydrodynamic diameter were 2.65 and 1.8 nm, respectively. The
+
(3) Porath, J. Pure Appl. Chem. 1963, 6 , 233. (4) Laurent, T. C.; Killander, J. J . Chromatogr. 1964, 14, 317. (5) Ackers, G. K.; Steere, R. L. Biochim. Biophys. Acta 1962, 59, 137. (6)Yau, W. W. J . Appl. Polym. Sci. Part A - 2 1969, 7, 483. (7) Cooper, A. R.; Barrall, E. M. J . Appl. Polym. Sci. 1973,17,1253.
latter was determined from the Q factor (i.e., molecular mass per unit hydrodynamic diameter of polymer) of polyethylene.8 Thus the dependency of elution volume on molecular size reflects the information on pores smaller than 10 nm in diameter. Values of VO Vi and V, are determined by the following equation.
+
where Wtotal and Wcoalare the total mass of coal and included solvent and the mass of coal, respectively. Vi is defined as the total volume of pores whose diameters are smaller than 10 nm, and ps is the density of the solvent at 30 "C. The sum of Vi and V, corresponds to the volume of coal gel swollen in the carrier solvent. The volume of coal before swelling, Vci, is expressed as follows:
vci= VI + v, + v,
(4)
where VI is the volume of coal solid obtained from the helium density. V2 and V3 are the volumes of micropores (d, < l nm) and mesopores (1 < d, < 10 nm) in coal, respectively. d, is the pore diameter. The micropore volume was determined by analyzing a COS adsorption isotherm at 25 "C based on the DubininAstakhov e q ~ a t i o n ,and ~ the mesopore volume was decided from an N2 adsorption isotherm at -196 "C. Gan et al.1° evaluated the porosity of coals by a similar method. They determined total pore volume from helium and mercury densities, macropore volume (d, (8)Nakajima, N. J. Appl. Polym. Sci. 1971, 15, 3089. (9) Dubinin, M. M.; Astakhov, V. A. Adu. Chem. Ser. 1970,102,69. (10) Gan, H.; Nandi, S. P.; Walker, P. L. Fuel 1972, 51, 272.
Change in Coal Gel Structure
Energy & Fuels, Vol. 9, No. 6, 1995 1037
Morwell-THF
L
'
I
C-Size exclusion limit
0.1
-0.2
0
0.2
0.4
0.6
0.8
1.o
Partition coefficient, 4
Figure 1. Relationship between extended chain length of n-alkane probes and partition coefficient for Pocahontas-HEX, Monvell-HEX, and Monvell-THF. >
30 nm) by mercury porosimetry, mesopore volume (dp
= 1.2-30 nm) by NZadsorption, and micropore volume
(d, < 1.2 nm) by subtracting the macro- and mesopore volumes from the total pore volume. The total pore volume of the swollen coal, Vi, is expressed by the following equation.
Vi = RV,, - V , where R is the volumetric swelling ratio of coal. Calculated values of Vi are shown in Table l for combinations of Morwell-HEX, Morwell-THF, and PocahontasHEX. For Morwell coal, the Vo, Vi, and V, were greatly influenced by the volumetric swellingratio R, which was determined by the procedure of Green et al.ll at 30 "C. Figure 1shows the effect of extended chain length of probe molecule on the &. In the Pocahontas-HEX system, all probes eluted a t around Kd = 0. This indicates that the n-alkane probes used could not permeate into pores smaller than 10 nm in their elution period, about 3.3 min. Pocahontas coal was hardly swollen in HEX (R= 1.011,and its micro- and mesopore structure remained unchanged. As shown in Table 1, the value of VZ(=0.003 cm3/g)was negligible compared with VI. Alkane molecules do not normally diffuse into micropores ( d p < 1 nm) in minutes. Actually, several hours were needed to attain an adsorption equilibrium of gaseous n-butane onto Pocahontas coal at 25 "C. The elution behavior of the probes indicates that there were few pores of 1-10 nm diameter in Pocahontas coal. Results for the Morwell-HEX and Morwell-THF systems were different from that for the PocahontasHEX system. In the Morwell-HEX system, the & was in the range between zero (docosane and eicosane) and 0.33 (pentane). Judging from the extended chain length of the largest probes, the diameter of pores was several nanometers a t most. If the elution of probes is controlled by the steric-exclusion mechanism alone, the product of Kd and Vi (0.04 cm3/g of coal for pentane)
indicates the volume of pores into which probes can penetrate.
When the elution is controlled by the restricted diffusion mechanism, the following equation holds. The steric-exclusion mechanism is assured if & is independent of the flow rate of carrier solvent. In the present study, the independence was verified in the range of 0.2-0.4 mumin. Elution curves at a flow rate smaller than 0.2 mumin were not reproducible. Thus we could not fully guarantee the elimination of the restricted diffusion effect so far. Morwell coal was swollen more extensively in THF (R = 2.15) than HEX. Thus THF gave a & value larger than HEX at the same dp. The value of Vprobe for pentane in the Morwell-THF system was 0.83 cm3/gof coal and was 20 times larger than that for the MorwellHEX system. As mentioned above, the Vprobe may not directly correspond to the volume of pores where the probe can permeate. However, it is evident that swelling by THF created a larger number of micropores or mesopores accessible to n-alkane molecules in Morwell coal.
Conclusion The porous structure of Morwell and Pocahontas coals swollen in HEX or THF was investigated by the SEILC technique using n-alkanes as molecular probes. In the Pocahontas-HEX system, all probes tested eluted near the size-exclusion limit (& = 01, indicating the absence of pores into which probes could permeate. However, & was dependent on chain length of the probes in the Morwell-HEX and Morwell-THF systems. Under the assumption that the elution was controlled by the steric-exclusion mechanism, the pore volume accessible to pentane was 0.04 cm3/g of coal in the Morwell-HEX and 0.83 cm3/g of coal in the Morwell-THF system. Solvent swelling is useful for improving contact between the coal matrix and organic or inorganic reagents. Highly-dispersedliquefaction12J3or hydropyr~lysisl~ catalysts were prepared by impregnating organometallic or inorganic salt precursors into coals swollen in THF or methanol. Evaluation of the porous structure of solvent-swollen coal is thus essential for developing more efficient impregnation methods. The SE-ILC technique proposed in this study can provide the useful information on micro- and macroporous properties of swollen coals.
Acknowledgment. This study was supported by the Ministry of Education, Science and Culture, Japan, and the Plastic Waste Management Institute, Japan. EF9501105 (11)Green, T.K.; Kovac, J.; Larsen, J. W. Fuel 1984,63, 935. (12)Artok, L.;Davis, A,; Mitchell, G . D.; Schobert, H. H. Energy Fuels 1993,7, 63. (13)Song, C . ; Parfitt, D. S.; Schobert, H. H. Energy Fuels 1994,8, 813. (14)Miura, K.;Mae, K.; Morokawa, H.; Hashimoto, K. Fuel 1994, 73, 443.
