Energy & Fuels 1995,9, 1023-1027
1023
Evaluation of Interaction between Aromatic Penetrants and Acidic OH Groups of Solvent-Swollen Coals by Inverse Liquid 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 1, 1995. Revised Manuscript Received August 14, 1 9 9 P
The objective of the present study is to clarify the role of OH-n interaction in selective recognition of aromatic rings by Monvell brown coal. Low-volatile Pocahontas No. 3 bituminous coal was used as a reference. The microstructure of Monvell coal was modified by swelling in various polar and nonpolar solvents. The interaction between coal and aromatic penetrants was evaluated with an interaction index, S,, that was determined by an inverse liquid chromatography technique. Swelling ratio and Sa, for Monvell coal were strongly affected by solvents, while for Pocahontas coal they were influenced little. The lowest value of Sa, for Monvell coal in n-hexane, which did not swell the coal (swelling ratio Q = 1.021, shows that aromatic molecules hardly penetrated into the micropores and therefore could not access self-associated OH groups. Sa, showed a maximum in acetonitrile (Q= 1.25) and methanol (Q= 1.401,both of which formed moderate hydrogen bonds with acidic OH groups in the coal. Then the swelling induced by the disruption of hydrogen bonds was essential for the selective recognition of aromatic rings by Monvell coal. THF gave a higher swelling ratio Q (=2.0) than acetonitrile and methanol, but it lowered Sa,. However, excessively strong hydrogen bond between OH groups and the solvent interfered with the interaction of aromatic molecules with OH groups. Pyridine added to acetonitrile reduced Sa, significantly and obstructed the OH-n interaction by forming stable hydrogen bonds with acidic OH groups in the coal. Chemical removal of OH groups by O-butylation also greatly decreased Sar. These results indicate that selective recognition of aromatic rings by Monvell coal is ascribed to the formation of OH-n interaction between coalOH and aromatic plane.
Introduction and Background Coal contains various functional groups that interact mutually and with other molecules. Phenolic hydroxyls and carboxylic groups are abundant especially in low rank coals and are capable of forming hydrogen bonds that result in the three-dimensional network structure of coal. Hydrogen bonds also play an important role in retrogressive reactions such as low-temperature crosslinking' in coal conversion processes. Thus, understanding of the interaction between acidic hydroxyl groups and swelling reagents or hydrogen-donor solvents is indispensable to control the structure and reactivity of coal. We have developed a novel technique of inverse liquid chromatography (ILC) that uses swollen coal particles as the stationary phase and have investigated structural and interfacial properties of coal swollen in polar solvent such as methanol and acetonitrile.2 In the ILC technique, physical interaction between swollen coal and penetrant molecules is described by the capacity ratio k defined as follows:
where Vt and Vo are the elution volume of penetrant * To whom all correspondence should be addressed.
@Abstractpublished in Advance ACS Abstracts, September 15,1995. (1)Solomon, P.R.; Serio, M. A.; Deshpande, G. V.; Kroo, E.Energy Fuels 1990,4, 42. (2)Hayashi, J.4.;Amamoto, S.; Kusakabe, K.; Morooka, S. Energy FueZs 1993,7,1112.
and carrier solvent, respectively. The penetrant functions as a molecular probe and gives microstructural and interfacial properties of swollen coal packed in the LC column. In the previous study,2 alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), and alkyl alcohols were utilized as the molecular probes. For various combinations of coals and probes, the logarithm of the capacity ratio increases linearly with the increase in molecular volume of the probes for each homologue of alkylbenzene or PAH. The capacity ratio of PAHs was much larger than that of alkylbenzenes when the molecular volume of the probes was equal. Based on the linear relationship between log k and the molecular volume of probes, the following coalprobe interaction index S was introduced.
S = d(1og k)/d(molecular volume)
(2)
Indexes Sa, and Sa& indicate the affinity between aromatic ring and coal and alkyl chain and coal, respectively. They were not affected by the packing density of coal particles in the LC column and were characteristic for particular combinations of coal and solvent.2 Sa, and Salk were determined for six coals of different ranks, and Sa, was always much larger than Salk. This result indicates that aromatic rings were recognized by swollen coals more selectively than alkyl chains. From the coal-probe interaction index and the adsorption enthalpy of probe molecules, it was found
0887-0624/95/2509-1023$09.00/00 1995 American Chemical Society
1024 Energy & Fuels, Vol. 9, No. 6, 1995 Table 1. Elemental Composition (in wt % daf) of Coals coal C H N o+s Morwell 65 4.9 0.6 30 Yallourn 66 4.9 0.6 29 Illinois No. 6" 78 5.5 1.9 14 Pocahontas No. 3 91 4.5 1.1 4 Reference 2.
that OH--7~interaction314was responsible for selective recognition of aromatic planes by brown coals and that z-n interaction was important for the recognition by bituminous coals. In the present study, we applied the inverse liquid chromatography technique to a low-volatile bituminous coal and a brown coal swollen in polar and nonpolar solvents, and clarified the contribution of the OH-n interaction to selective recognition of aromatic planes by lower rank coals. Polar solvents having various heats of hydrogen-bond formation were employed. Acidic OH groups in the brown coal were physically or chemically modified, and the effects of environmental and structural changes in the coal matrix on the coal-probe interaction index were determined. Experimental Section Coal Sample and Chemical Modification. Monvell brown coal and Pocahontas No.3 coal were used in the present study. The coal samples were pulverized, sized to smaller than 0.037 mm, and dried under vacuum at 70 "C for 24 h. The elemental compositions are listed in Table 1. The dried samples were ultrasonically extracted at 30 "C with the solvent that was used in the subsequent liquid chromatography. To convert acidic OH groups into butyl ethers or esters, Monvell coal was butylated by the procedure of L i ~ t t a First, .~ 7 g of the coal sample was swollen in THF (70 mL) for 1h at 20 "C, and then a methanol solution of tetrabutylammonium hydroxide (TBAH, 70 mmol) was added to the slurry. After stirring for 24 h under nitrogen atmosphere, a THF solution of alkylating agent, iodobutane (105 mmol), was dropped into the slurry. After further stirring for 72 h, THF was removed by evaporation. TBAH and unreacted iodobutane were removed by washing the butylated coal with a 1 m o m HCl solution, a 50 vol % aqueous methanol solution and finally distilled water. The amount of 0-butyl groups introduced in Morwell coal was calculated as 4.8 mol per 1kg of the initial coal from the increase in atomic WC ratio and the weight gain after butylation. The total amount of acidic OH groups was determined as 6 mo&g.6 Thus 80% of OH groups were butylated by this procedure. Inverse Liquid Chromatography. The carrier solvents used were n-hexane (HEX), acetonitrile (ACN), tetrahydrofuran (THF), a mixture of pyridine and ACN and a mixture of pyridine and HEX. The concentration of pyridine in the mixed solvents was varied in the range of 0.3-3.5 wt %. The molecular probes used are listed in Table 2 with their molecular weights and volumes. Details of the ILC technique were reported in the previous paper.2 Figure 1 shows a typical chromatogram of mixed PAH probes. The number of theoretical plates per unit length of separation column was calculated as 800-1500 m-l. When the interaction between coal and carrier solvent or between molecular probe and carrier solvent is stronger than that between coal and molecular probe, elution of the probe is controlled by the size-exclusion mechanism. In other cases, elution is controlled by the adsorption of probe molecules onto (3) Cairns, T.; Eglinton, G.Nature 1962,196, 535. (4) Bellamy, L. J.; Pace, R. J. J . Spectrochim. Acta 1966,22, 525. ( 5 ) Liotta, R. Fuel 1979,58, 724. (6) Hayashi, H.-i.; Kawakami, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993,7, 1118.
Hayashi et al. Table 2. Molecular Volume of Probe Compounds probe compound benzene toluene ethylbenzene m-xylene n-propylbenzene 1,2,4-trimethylbenzene n-butylbenzene n-pentylbenzene n-hexylbenzene n-heptylbenzene n-decylbenzene n-tridecylbenzene naphthalene anthracene chrysene
mol vol (nm3) 0.0974 0.1190 0.1409 0.1404 0.1623 0.1601 0.1835 0.2054 0.2273 0.2483 0.3131 0.3781 0.1463 0.1946 0.2429
B
C
I
I
1
20
40
60
Elution time [min] Figure 1. Typical adsorption chromatogram of benzene and PAH probes onto 0-butylated Monvell coal at 30 "C. (A)
benzene, (B)naphthalene, (C) anthracene, chrysene.
(D)pyrene, (E)
the microporous structure of swollen coal, and the elution volume of probe molecules is larger than that of carrier solvent. The VOof ACN and THF was determined from the elution time of the corresponding deuterated solvent, acetonitrile-& or THF-de. It was confirmed that the VOof ACN or THF was equal to that of each deuterated solvenL2 Since deuterated HEX was not obtainable, the VOof HEX was determined by the following method: The Vt of n-alkane homologues C s (npentane) t o Clo (n-decane) except HEX was measured using the HEX carrier. When Monvell coal was used as the stationary phase, Vt decreased monotonously with increasing carbon number of the probes. For Pocahontas coal, on the other hand, Vt was nearly independent of the carbon number. From these results, the VOof HEX was determined interpolatively . The net enthalpy change for adsorption of probe molecules was determined by a van't Hoff plot of k . For this purpose, the ILC was carried out at 20, 30, 40, and 50 "C and the interaction index was calculated from eq 2 for PAHs (Sar) and monoalkylbenzenes (Salk). Figure 2 shows the relationship between log k and molecular volume of probes at 30 "C when ACN was used as the carrier solvent. S a l k for Monvell and Yallourn coals2 could not be simply determined because log k
Energy & Fuels, Vol. 9, No. 6, 1995 1025
Acidic OH Groups of Solvent-Swollen Coals
I
1.5
i
0 n-Alkylbenzene
* 0.5 2 0.0 cl
=e
20
E
y 1 / Pocahontas
1
2 -1.51.
l.o 0.5
t
Lqz
I
10
Yallourn
n
U
Naphthalene
Anthracene
Naphthalene
Anthracene
HEX
ACN
Figure 3. Effect of carrier solvent on net adsorption enthalpy of naphthalene and anthracene. 1.0 0.5 4
2
Illinois.No.6
P'
,Y
-
.
gJ0.0 -
-
1
-0.5 -1.0
'
-
1
0.10
0.20
0.30
0.40
Molecular Volume [nm3]
Figure 2. Relationship between log k and molecular volume. Solvent: ACN, temperature: 30 "C. Table 3. Effect of Solvent on Swelling Ratio and S , at 30 "C coal
solvent
swelling ratio at 30 "C
Sa,
Morwell
HEX ACN MeOH THF HEX ACN
1.02 1.25 1.40 2.01 1.0 1.0
11.2 19.0 19.0 14.0 15.0 14.6
Pocahontas
for alkylbenzenes was not linearly correlated with the molecular volume. Such nonlinearity was observed only for brown coals.
Results and Discussion Effect of Carrier Solvent on Sar. Table 3 shows that the swelling ratio and S a r of Morwell coal are strongly affected by solvents used, while they are influenced little by solvents in the case of Pocahontas coal. Our previous results2 indicate that the selective recognition of aromatic rings over alkyl chains by Morwell coal is attributable to the formation of specific OH-n interaction between the OH groups of coal and the aromatic plane of probes. The lowest value of Sa, in HEX, which does not swell Morwell coal, shows that aromatic probes hardly penetrate into the micropores of coal and thus cannot access OH groups that are selfassociated7 in the poor solvent atmosphere. S , shows a maximum in ACN and MeOH which form moderately strong hydrogen bonds with phenolic OH (7) Painter, P. C.; Sobkowiak, M.; Youtcheff, J. Fuel 1987,66, 973. ( 8 )Amett, E. M.; Joris, L.; Mitchell, E.; Murty, T. S. S. R.; Gorric, T. M.; Schleyer, P. v. R. Am. Chem. SOC.1970,92, 2365.
group^.^ The heat of hydrogen-bond formation between solvent and p-fluorophenol, AHf, is a useful index for hydrogen-bond formation ability of the solvent.8 Larsen et al.9 found that the disruption of coal-coal hydrogen bonds was a primary contributor to the solvent swelling of Illinois No. 6 coal. The degree of hydrogen-bond breakage by swelling is well correlated with AHf of the solvent. The AHf of ACN is 16 kJ/mol,8 much lower than the 24 kJ/mol of THF8 but enough to break a portion of hydrogen bonds in Morwell coal as shown in Table 3. The larger Sa, values in ACN and MeOH compared to HEX reveal that swelling induced by hydrogen bond disruption is necessary for the selective recognition of aromatic ring by Morwell coal. Addition of aromatic compounds into ACN and MeOH enhances breakage of OH-OH hydrogen bonds in brown coals and as a result increases the swelling ratio.2J0 Thus, aromatic probes contribute to the formation of hydrogen bonds between OH groups in coal and ACN or MeOH molecules. However, the variation of S a , in ACN, MeOH, and HEX cannot be explained by this reason alone. The selective recognition of aromatic rings should also be attributed to the specific OH-n interaction. THF possesses a pronounced ability for disrupting coal-coal hydrogen bonds. The swelling by THF is mainly due to the breakage of coal-coal hydrogen bonds.g Actually, THF swelled Morwell coal much more extensively than ACN and MeOH, but it lowered the Sa,. The small Sa, value in THF compared with ACN and MeOH shows that the access of aromatic probes to OH groups is more restricted in THF than in ACN and MeOH. Thus, swelling is not directly related to the selective recognition of aromatic rings by Morwell coal. The solvent effect is further discussed later. Figure 3 shows the net adsorption enthalpy EN of naphthalene and anthracene. EN is expressed by the following equation when no physical or chemical change other than adsorption occurs in coal. -
(3) where Ecp,Eps,and E,, are the affinities between coalprobe, probe-solvent, and solvent-coal, respectively. For Pocahontas coal, the EN of anthracene in HEX was 1.5 kJ/mol higher than that in ACN. Nearly the same EN
=
-
(9) Larsen, J. W.; Green, T. K.; Kovac, J. J . Org. Chem. 1985, 50, 4279. (10) Kawano, S.; Abe, M.; Shimizu, K.; Ogino, K.; Honda, H. Nippon Kagaku Kaishi 1987,2301.
1026 Energy & Fuels, Vol. 9, No. 6,1995
Hayashi et al. Table 4. Capacity Ratio and Reversibly Adsorbed Amount of Pyridine on Morwell Coal" in Pyridine/HEX Mixed Solvent Carrier
20
Pyiidine concentration [wt%]
pyridine concn (wt %)
amount of reversibly adsorbed pyridine ( g )
1.77 3.45
1.268 0.0029 0.723 0.0032 a Amount of Morwell coal packed in the column was 1.1g.
F
2 2
k
10
0
Morwell
Pocahontas
Morwell Pocahontas
ACN
HEX
Figure 4. Effect of pyridine added to ACN and HEX on Sa, at 30 "C.
difference was observed with naphthalene. E,, and E,, were solvent-dependent but E,, was not always so. For Pocahontas coal, the E,, in HEX was equivalent to that in ACN because the coal was hardly swollen in ACN and HEX. The initial micropore structure was therefore maintained in both solvents. Furthermore, absence of any solvent effect on Sa, suggests that the adsorption of anthracene and naphthalene onto Pocahontas coal was caused by the same mechanism in ACN and HEX. Thus, the difference in EN between ACN and HEX is roughly explained by that in (E,, ICsc). The EN of anthracene and naphthalene for Morwell coal in HEX is 5 and 3 kJ/mol, respectively, and is much smaller than that for Pocahontas coal in HEX. The adsorption of these PAH probes onto Morwell coal in HEX is explained by van der Waals interaction, because OH groups in the coal are deactivated by self-association via hydrogen bonds. It is evident that the contribution of the van der Waals interaction of PAHs with Morwell coal is much weaker than that with Pocahontas coal. On the other hand, the EN of anthracene in ACN is 17 kJ/mol higher than that in HEX. The EN of naphthalene in ACN is also 11kJ/mol higher than that in HEX. This remarkable difference in EN between ACN and HEX cannot be explained by the difference in (Eps Esc)alone and should be attributed to the change in Ecp. This result supports the importance of OH-n interaction. Effect of Capping of OH Groups by Pyridine on Sar.The results in the above section reveal that strong hydrogen-bond breaking solvents such as THF diminish the selective recognition ability of Morwell coal toward aromatic rings. Pyridine is a well-known good solvent for extraction and swelling of coal. Its "coal-philic" property are based on its strong hydrogen-bond forming ability with OH groups. The AHf of pyridine is 30 kJ/ mol, which is large enough to break all hydrogen bonds in low-rank coalsg by replacing coal-coal hydrogen bonds by coal-pyridine ones. When Morwell coal is swollen in pyridine and ACN, OH groups in the coal form hydrogen-bonded complexes with pyridine rather than ACN. Figure 4 shows Sa, of Morwell and Pocahontas coals swollen in pyridine-ACN mixtures. As expected, Sa, of Morwell coal was greatly decreased by addition of a small amount of pyridine but was nearly constant at a pyridine concentration of 0.59-3.45 wt %. The decrease
+
+
in Sa, is due to the formation of strong hydrogen-bonded complexes between pyridine and OH groups in Morwell coal. Aromatic probes cannot interact with OH groups after this reaction. As shown in Figure 2, Sa, is the slope of the straight line that correlates log k and the molecular volume of benzene and unsubstituted PAHs. The addition of pyridine to ACN carrier hardly changed log k of benzene but decreased that of larger PAHs such as naphthalene and anthracene. The greater the ring size (i.e., molecular volume), the greater the decrease in log k. The contribution of OH-n interaction to the adsorption of aromatic probes increases with increasing ring size.8 The larger reduction of log k for larger PAHs does not contradict with this rule. The addition of pyridine to HEX decreased Sa, of Morwell coal, but the decrement was much smaller than that when pyridine was added to ACN. Fundamentally, pyridine penetrates the coal and replaces coal-coal hydrogen bonds with stronger coal-pyridine hydrogen bonds. The OH-n interaction between aromatic probe molecules and OH groups in coal is impossible in pyridine/HEX and pyridine/ACN mixtures because aromatic probes cannot access OH groups that are tightly capped by pyridine, while OH groups are mutually capped in pure HEX. A portion of pyridine mixed in ACN was reversibly adsorbed (mobile) and the rest was irreversibly adsorbed (mobile) on the surface of Morwell coal. To determine the capacity ratio of pyridine in mixed carrier systems, an ACN solution that contained slightly more pyridine than that in the mixed carrier solvent was prepared and injected into the ILC column of Morwell coal swollen in the same carrier. The k in Table 4 means the capacity ratio for reversibly adsorbed pyridine. The amount of reversibly adsorbed pyridine is then calculated from k = (number of reversibly adsorbed pyridine molecules)/ (number of pyridine molecules in mobile phase) = (number of reversibly adsorbed pyridine molecules)/
(amount of mobile phase)(concentration of pyridine) (4) The amount of mobile phase was exactly determined from the elution volume of ACN-d3. As listed in Table 4, the amount of adsorbed pyridine was little affected by the pyridine concentration in the mobile phase and was only about 0.003 g per 1.1 g of dry coal (0.28% of dry coal mass) packed in the column. Nelsonll determined pyridine uptake by various coals from the gas phase. The pyridine uptake by lignites was more than 30% of initial dry coal mass at 25 "C where the saturated vapor pressure of pyridine was only 21 mmHg. Thus, most pyridine adsorbed on the surface of Morwell coal was immobile. Sa,of O-Butylated Morwell Coal. Figure 5 shows the relationship between log k and molecular volume for O-butylated Morwell coal. The O-butylation re(11) Nelson, R. Fuel 1983, 62, 112.
Acidic OH Groups of Solvent-Swollen Coals
Energy & Fuels, Vol. 9, No. 6, 1995 1027 was affected by the micropore structure of coal. The
EN of alkylbenzenes smaller than n-heptylbenzene was
Molecular volume [nm3] Figure 5. Changes in relationship between log k and molecular volume of probe compounds at 30 "C by 0-butylation of Morwell coal.
M W raw
--
30 -
.
z : 5- ' E : 20-
10
-
01 0
" " " " " " " ' l 0.10
0.20
0.30
0.40
Molecular volume [nm'] 0 0 0 Monvell raw coal m 0 0-butylated Mowell coal
W
0 0 n-Alkylbenzene
~~methylbcnzene
Figure 6. Comparison of net adsorption enthalpy of aromatic molecular probes between original and 0-butylated Morwell coals. Solvent: ACN, temperature: 20-50 "C.
moved a t least 80% of the initial OH groups serving as sites for OH-n interaction. This agreed with a remarkable decrease in Sa,after the 0-butylation. As reported previously2 for bituminous coals, log k of monoalkylbenzenes increased with increasing molecular volume of probes. For raw Monvell coal, however, log k decreased with increasing molecular volume for alkylbenzenes smaller than n-propylbenzene and then increased with increasing molecular volume. The interaction of benzene ring with OH groups normally increases with increasing size of probe, but the alkyl chain attached t o the benzene ring hinders the access of the probe molecules to adsorption sites in coal micropores. The profile of log k is composed of these two effects and shows a minimum at n-propylbenzene. On the other hand, the 0-butylated Monvell coal gave a linear log k profile even for alkylbenzenes with a longer chain. The removal of OH groups and the introduction of bulky butyl groups expanded the network structure of Morwell coal in ACN and minimized the hindrance by alkyl substituents. The 0-butylation remarkably decreased the net adsorption enthalpy EN. As shown in Figure 6 , the EN for unmodified Monvell coal was not correlated simply with the molecular volume and was affected by the chemical form of probes. This suggests that adsorption
evidently lower than the prediction based on the result with larger alkylbenzenes,although interaction between coal and alkylbenzene became stronger for a smaller alkyl chain. Furthermore, the ENof PAHs was smaller than that of alkylbenzenes having the same molecular volume. These results can be explained by the reduction of the net adsorption enthalpy by the following endothermic processes occurring simultaneously; (1)disruption of coal-coal hydrogen bonds caused by formation of OH-n bonds between coal and aromatic probes and (2) diffusion of aromatic probes into micropores.2 The relationship between E N and the molecular volume of probes was significantly changed by 0butylation for Monvell coal. The ENof all alkylbenzenes and PAHs was greatly decreased. Meanwhile, the E N for each series of alkylbenzenes and PAHs increased linearly with increasing molecular volume. The slope of EN with respect to the molecular volume of PAHs, SEN,ar, was 127 kJ/(mol nm3)and much larger than that of alkylbenzenes, S E N & , which was 50 kJ/(mol nm3). This result is quite different from that for Pocahontas and Illinois No. 6 coals, for which ENwas correlated with a single straight line2 and S E N , a r was nearly equal to SEN,&. The value of SEN was 75-80 kJ/(mol nm3) for Pocahontas coal and 50-55 kJ/(mol nm3)for Illinois No. 6 coal.2 The effect of the size of aromatic ring on EN was more marked for the butylated Monvell coal than for Pocahontas and Illinois No. 6 coals, even after 80% of the initial OH groups were removed from Monvell coal. The large SEN,^^ value of the butylated Monvell coal can not be clearly explained at the present moment. However, a specific interaction between aromatic ring and residual OH groups may be a major factor because no z-z interaction is expected for the butylated Monvell coal. Conclusions The specific OH-n interaction between aromatic ring and OH groups in Monvell brown coal swollen in polar solvent was examined by an inverse liquid chromatography technique. The swelling of Monvell coal, caused by disruption of hydrogen bonds, was important for the selective recognition of aromatic rings by the coal. However, the excessive swelling by strong solvent such as THF resulted in a reduction of selectivity, because aromatic probes could not interact with OH groups which had already formed hydrogen-bonded complexes with the solvent. This "capping effect" of OH groups was confirmed by chromatography with mixed solvents of pyridine and HEX. Chemical elimination of OH groups by 0-butylation also significantly decreased the Sa, value. 0-Butylation expanded the network structure of Monvell coal and minimized the hindrance effect of alkyl substituents.
Acknowledgment. This work was financially supported by the Ministry of Education, Science and Culture, Japan (Grant-in-Aidfor Scientific Research No. 07455309). EF950 101W
