18 The Effect of Reagent Access on the Coal Structure Downloaded from pubs.acs.org by SWINBURNE UNIV OF TECHNOLOGY on 08/07/18. For personal use only.
Reactivity of Coals JOHN W. LARSEN, THOMAS K. G R E E N , P. C H O U D H U R Y , and E. W. K U E M M E R L E Department of Chemistry, University of Tennessee, Knoxville, TN 37916
It is argued that diffusion of reagents through solid coal, as distinct from diffusion through pores in the solid coal, may be the rate-limiting step in many reactions of coals. Brief consideration is given to diffusion in polymeric systems. Examples of reactions that may be limited by reagent accessibility considerations are given. These include several methods of determining OH groups, depolymerization, and Friedel—Crafts acylation. Heat has rapid access to the interior of solid coal, and this is one of the reasons for the popularity of pyrolysis-based processes. Research into methods of increasing the rate and extent of diffusion of reagents into coal is an important and neglected area.
This article is an abecedarian inquiry into the relationship between accessibility of reactive sites i n coals to reagents and the rate and extent of reaction at those sites. As will become clear, so little attention has been given to this area [except by Peters and Fuentgen (1)] that definitive answers to the questons raised are not yet possible. Still, the questions are important and must be considered. It seems likely that accessibility to reagents is an important limiting factor i n many coal reactions. By writing this article we hope to convince coal chemists that the accessibility problem is one that should be considered when de signing experiments and interpreting results. Consider a reactive group located within solid coal, not at any sur face; neither an external surface nor an internal pore surface. If the coal is treated with reagents incapable of penetrating the solid coal, that group will not react unless it is brought to the surface. One way this can be done is by removing solid coal. When the reaction has proceeded far enough to uncover the group in question, it will react. However, if the 0065-2393/81/0192-0277$05.00/0 © 1981 American Chemical Society
278
COAL STRUCTURE
reagents are soluble in the solid coal, they then can diffuse through the solid until they reach the group and react. This diffusion may be a very slow process and may be the rate-controlling step in the reaction. If a reactive group is located in a variety of environments of differing ac c e s s i b i l i t i e s , the k i n e t i c s of its reaction w i l l be e x c e e d i n g l y complex. However, if diffusion into coal is quite rapid, then the normal activation energy for the reaction in question may be the rate-limiting factor. Thus it is clear that both the location of a reactive group in the coal and the rate of transport of the reagents into the coal may have significant effects on the reactivity of a group. This situation is caused by the coals insolubility. Most coals are nearly insoluble in nearly everything. Although selective solvents may dissolve as much as 20-25% of some coals without reaction, the bulk of the material remains insoluble. Thus the reagent must penetrate the coal to reach the reactive group. The fact that coals are highly porous certainly helps. W e will return to this point later. Now our concern is with reagent transport into the solid material as distinguished from trans port in the pore system. The character of the solid material will control that diffusion rate. A strong argument for the macromolecular character of coal has been made by Van Krevelen (2) and need not be repeated here. The nature of the macromolecular structure is important. Since it does not dissolve in solvents that can swell it to more than twice its original volume (see Table I), it must be a three-dimensionally cross-linked gel or consist of large molecules that are so entangled with each other that they cannot be separated. Coals are viscoelastic (3). When stretched, they snap back. They do flow but with high viscosities, approaching that of bakelite. A very careful study of this behavior lead Macrae and Mitchell and others to conclude that coals were highly cross-linked materials (3, 4). It seems improbable that a significant part of such cross-linking could be due to entanglements. X-ray data and most structural models show coals to be sheetlike molecules. This idea is supported by the asymmetry of the mechanical properties (5), indicating more cross-linking parallel to the Table I. Swelling of F o u r Pyridine-Extracted Coals with a Series of Organic Solvents; Ratio of Weight of Swollen Coal at Equilibrium to Weight of D r y Coal Coal
Toluene
Ethanol
Acetone
1,4-Dioxane
Pyridine
N . D . Lignite Wyodak Illinois No. 6 Bruceton
1.20 1.33 1.44 1.43
1.35 1.43 1.41 1.39
1.44 1.54 1.54 1.46
1.49 1.71 1.83 1.78
1.82 2.16 2.18 2.03
18.
LARSEN ET AL.
Reactivity of Coal
279
bedding plane than perpendicular to it. It is difficult to visualize large numbers of entanglements between sheetlike molecules. The view that the association of macromolecules in coals is due to forces other than covalent bonds has been forcefully expressed (6). W e prefer a threedimensionally cross-linked structure, but it is clear that the definitive answer is not yet available. Since coals are highly porous materials, diffusion will occur both through the pores and within the bulk solid. It is not easy to distinguish between them. Clearly, when two atoms that are part of the coal are in contact and must be separated by a diffusing molecule, this process is diffusion in a solid. Just as clearly, when the diffusing molecule can move through an open space without altering the position of any atoms in the coal, then that is diffusion through a pore. However, there will be cases in which the coal atoms are not touching, there is empty space between them, yet the diffusion of a molecule into this space increases their separation. This situation obviously will occur when the separation distance is smaller than the diffusing molecule. The pore structure of coals and the diffusion of small molecules within the pore systems of coals have been thoroughly studied (7-12). Certainly diffusion of larger molecules through this pore system may be slow and rate limiting. Our concern here is much more with diffusion of reagents into the solid coal. This diffusion process requires displace ment of atoms in the coal and will usually be a slow process at room temperature. There seems to be a general consensus that diffusion through the solid material itself, not an open pore, may be necessary and may be the limiting factor in chemical reactions. Diffusion in nonporous polymers has been extensively studied (13). Unfortunately for our present concerns, most attention has been given to permanent gases and light hydrocarbons. Nonetheless, the effects of many properties of polymeric systems on diffusion rates are known. It is worth describing them in a brief and qualitative manner. 1. Size of the diffusing molecule: the diffusion coefficient de creases as molecular size increases. Linear molecules dif fuse more rapidly than branched molecules of the same size. 2. Nature of the polymer: the greater the flexibility of the polymer chains, the greater will be the diffusivity. A n increase in the cohesive energy of the polymer reduces diffusivity. 3. Effect of the glass transition: this effect is small to non existent for the diffusivity of small molecules. 4. Effect of cross-linking: increasing the degree of cross-link ing increases both the activation energy and the pre-expon-
280
COAL STRUCTURE
ential factor for diffusion. The diffusion coefficient drops. This effect increases with increasing size of the diffusing molecule. 5. Effect of plasticizers: a plasticizer decreases the cohesive forces between polymer units and so increases diffusivity. 6. Effect of fillers: the presence of particles distributed in the polymer has complex and conflicting effects. In general, diffusion is hindered. However, if the filler is not thor oughly wet by the polymer, gaps can result, which are filled by the diffusing molecules, and the diffusion rate increases. 7. Presence of crystallites: diffusing molecules normally are not soluble in crystallites, and they act as fillers, inhibiting diffusion. A l l of the comments above apply to diffusion through solid coal, not through coal's pore structure. Diffusion through the pore network has been studied thoroughly and seems to be understood (8-12). The con siderations above are relevant to a reagent penetrating the solid coal to reach a functional group that is not located at a surface.
Consider a functional group, say, a phenolic O H group in coal. It may be found in three different regions. A n - O H group at the external surface of coal will obviously react quickly, at a rate similar to an - O H group on a molecule in solution. A n - O H group located at the surface of a pore will react more slowly, or perhaps not at all. To react, the reagents must penetrate into the pore. If they are large and the pore is small, this process will be an activated process. In fact, the reagent may not be capable of entering the pore, in which case the - O H group will not react unless the coal structure is altered to permit access. Reagent accessi bility to the - O H group located within the solid coal may be very slow. Only two routes are possible. Some coal can be removed so that the group is uncovered and is at the surface. In this case reaction rate will be affected strongly by the coal surface area that is accessible to the reagents. The other possibility is that the reagent diffuses through the solid coal to reach the site. This process may be very slow. Swelling
18.
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LARSEN ET A L .
281
coal in solvent vapors (Table I) required 2-3 days to approach equilibrium (5). Reagents with less affinity for coal than the swelling solvents would diffuse even slower, as would molecules of larger size. This situation sets a lower limit on the rate of reaction of a buried group with reagents whose diffusion rates are similar to those of these solvents. It is apparent from these considerations that mass transport may severely limit reactions of coals. W e next proceed to a brief discussion of some examples of mass transport limitations on reactions. A variety of techniques for analyzing - O H groups in coal have been developed, and a good selection is listed in Van Krevelen's book (2). From his table (12) we have abstracted the reactions used to determine - O H groups in coals and the recommended reaction times. In Table II these are compared with the reaction times recommended for these reactions in solution. Reaction times for coals are invariably much longer than those re quired for the same reaction between molecules in solution. Often reactions with coals are several thousand times slower. This reduction in rate occurs for a variety of reactions of varying charge character and type, suggesting that it is not a normal medium effect. It is most readily ascribed to rate limitations due to the inaccessibility of reactive sites to reagents. Table II. Reactions Used to Determine - O H Groups in Coals Reaction
Time Required with Coals
Time Required for Solution Reaction
Esterification with acetic anydride in pyridine at 90°C
24 hr
45 min (15)
Etherification with trimethylsilylchloride and hexamethyldisilazane in pyridine at 115°C
1-4 days
5 min at room temperature (16)
Exchange with D 0 (20% in pyridine) at 80°C
a few days
minutes
Etherification with C H N in E t 0 at 0°C
1 week
minutes (17)
Reaction with dinitrofluorobenzene in D M F / H 0 at 20°C
a few days
5 min (18)
Acetylation with ketene at room temperature
a few days to a week
minutes (19)
2
2
2
2
2
a few days Esterification with phthalic anhydride in pyridine at 90°C
1 hr (5)
282
COAL STRUCTURE
Another facet of this problem is revealed by the work of Maher. Using potentiometric titration with base in ethylendiamine solvent (20), Maher and O'Shea measured the phenolic - O H content of Greta seam coal (21) [82.4% C , 6.2% H , 1.7% N , 1.0% S, dry, ash-free basis (DAF)]. The coal was extracted with a variety of solvents, and then the phenolic - O H content was measured in both the residue and the material ex tracted from it. The results are shown in Table III. It is clear that the solvents used to extract the coal have an effect on the total number of O H groups found. Clearly this result must be due to the effect of the solvents on reagent access to the - O H groups, since the total number of - O H groups in the coal is fixed. Thus the various solvents differ in their abilities to expose O H groups to the reagents, as pointed out by Maher and O'Shea (21). Interestingly, there is no relationship between the ability of a solvent to extract material from the coal and its ability to uncover - O H groups. As shown by Hodek and Kolling (22), bituminous coals can be readily acylated. In connection with our studies of some Friedel-Crafts reac tions of coals, we carried out several acylations using Bruceton coal and octanoyl chloride under Hodek's conditions. Sufficient octanoyl groups were added to increase the weight of the coal by 40%. The H nuclear magnetic resonance (NMR) spectra of the extractable fraction (Figure 1) contained no absorption due to hydrogens on aromatic nuclei. Only the sharp singlet due to residual H in D C C 1 appears near the aromatic region. Hodek reported that the solubility of the acylated coal depended on the chain length of the acyl group, increasing as the chain length increased. This observation was the key to an explanation of the phe nomenon. It was postulated that reaction occurred primarily at the coal surface. The large octanoyl chloride molecules could not readily penetrate the coal but rather reacted at available surface sites. This reaction produced very tiny particles of coal, their surfaces covered with covalently bonded acyl chains. It is suggested that these units are suspended in a solvent by the favorable interactions between the octanoyl chains and the solvent. The situation is analogous to that of a reversed micelle. In this situation the aliphatic chains are in a liquid environment and will show nearly normal H N M R absorption. However, most of the aromatic hydrogens in the coal remain in a solid environment and will have absorptions characteristic of that state. The line widths for hydrogens in a solid are very broad and are not detected by a high-resolution N M R instrument designed for work with solutions. Thus the aromatic hydrogens in their solid environment are invisible to our N M R instrument, and only the aliphatic hydrogens appear. Two further observations supported this interpretation. The first was the C N M R spectrum of octanoylated Bruceton coal shown in Figure 2. The peaks in the octanoyl group were assigned by analogy l
!
3
l
1 3
h
a
fc
ND 71 7 40 42 66 4 1
ND" 3.21 1.73 2.51 2.82 2.88 0.92 0.30
2.2 22 4.1 16 15 23 4.7 2.0
Residue
97.8 78 95.9 84 85 77 95.3 98
1.99 1.35 1.86 1.75 1.53 1.24 3.03 2.32
Total acidity (meq/g) Weight (g) Mean
N D = Not determined. Approximate figure only, obtained from stirred extractor experiment where yield was 3.5%.
Iso-amyl alcohol Dimethyl formamide Diethylamine Pyridine Aniline Ethylenediamine Chloroform Dichloromethane
Solvent
Product (meq)
Total acidity (meq/g)
Weight (g)
Extract
195 105 178 147 130 95 289 227
Product (meq)
>195 176 185 187 172 161 293 228
Total meq
Table III. Sum of Phenolic Groups i n Extracts and Residues Basis: 100 g of D A F C o a l ; 160 meq of - O H (21)
CO
to oo
o
n
2
o
SB
00
284
COAL STRUCTURE
U Q U o o ss o s w CQ
o O
e I C O
LARSEN
ET AL.
Reactivity of Coal
286
COAL STRUCTURE
with octanoylphenone. Note that the two absorptions labeled C and C are broadened, while the other methylene groups in the chain have narrow, normal line widths. The absorptions labled C and C were assigned to the methylene groups a and P to the carbonyl, respectively. The line broadening is typical of that expected from hindered rotation. If extensive surface octanoylation of the coal had occurred, the chains near the surface would be crowded together, and the crowding would decrease as the distance from the surface increases. The rotation of the methylene groups near the surface, therefore, might be hindered because of this crowding, while the groups at the far end would be able to rotate freely. This situation is exactly what is observed. The methylenes near the far end of the chain and the methyl group seem to have normal line widths, while those caused by the methylenes close to the surface are broadened by hindered rotation. The second piece of evidence supporting the occurrence of a surface reaction is the molecular weight distribution of the products. In his original paper Hodek reported number average molecular weights for the acylated coals. For the octanoylated coals these were between 2300 and 3900, depending on coal and solvent. More recent work has revealed that the weight-average molecular weight is much higher and that much of the acylated coals have molecular weights greater than 10 (23). The N M R data can be explained without recourse to surface reactions if the H N M R is of poor quality and the broadening in the a and p methylene peaks in the C N M R is due to chemical shift differences arising from different groups in the coal having been octanolylated. However, considering the very high molecular weights of the products (above 10 ), the peculiar dependence of solubility on chain length, and the large numbers of octanoyl groups added that would be crowded at the surface, it seems quite likely that the acylation of Bruceton coal was dominated by a surface reaction, which still resulted in the production of extractable products. The obvious explanation for the dominance of the surface reaction is limited accessibility of the reagents to the interior of the solid coal. W e have undertaken a thorough study of the H e r e d y - N e u w o r t h procedure for solubilizing (depolymerizing) coals (24, 25, 26). The re action is an acid-catalyzed transalkylation, which, when carried out on coals, renders them soluble. a
a
p
5
l
1 3
5
Ar-(CH ) -Ar' + C H O H 2
n
6
5
HOTs
ArH + Ar'H + HOC H (CH ) C H OH + HOC H (CH ) Ar + HOC H (CH ) Ar' 6
4
2
n
6
4
2
n
6
4
2
n
6
p
4
18.
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287
Reactivity of Coal
The reaction is second order, first order in phenol and first order in the methylene-bridged aromatic (27). Presumably the mechanism involves ipso protonation of the aromatic followed by attack of phenol on the cationic intermediate (27). W h e n applied to coals, it renders them almost completely extractable into pyridine, and the number average molecular weights of the extractable material generally range between 300 and 750 (24). For the reaction to occur both phenol and a solvated proton must diffuse into the coal to reach an aromatic methylene bond. Phenol is a better solvent for bituminous coals than is pyridine (28) and is expected to interact strongly with coal and probably swell it. The penetration of coal by phenol at its boiling point should be rapid and extensive. How ever, the proton has little affinity for a hydrocarbon environment, and its diffusion into the hydrophobic coal is not expected to be facile. The way in which the solvation of the proton will affect its diffusion in coals is unknown. As a result of limitations on proton transport, this reaction may be subject to accessibility limitations. Bruceton coal was d e p o l y m e r i z e d in refluxing phenol using ptoluenesulfonic acid (HOTs) catalyst (29). The products were 91% ex tractable into pyridine and had a number average molecular weight of 400 in pyridine, measured by vapor pressure osmometry. Yet most of the materials in this solution have molecular weights above 2500. The fractionation scheme shown in Figure 3 was applied to the reaction products. The benzene-ethanol soluble fraction consisted primarily of a dozen individual compounds of low molecular weight, which probably come from condensation reactions of phenol. It is the large number, but small mass, of these low-molecular-weight materials that dominates the number average molecular weight M . Most of the mass of the product is contained in smaller numbers of very large mole cules. The molecular weight distribution of the pyridine solubles, after the very high molecular weight and colloidal material was removed by centrifugation, is shown in Figure 4. Of the material extractable into pyridine, about 8% will not pass a 7.5-|xm filter. After this material is removed, the solution still plugs a 0.5-|xm M i l l i p o r e filter. Clearly much colloidal material is present. Centrifugation at 360,000 X g gives a precipitate. Before centrifuga tion, evaporation of the pyridine gives a solid that will not redissolve. Once the pyridine solution has been centrifuged and the precipitate recovered, the pyridine can be removed from the remaining solubles, and they will redissolve. Clearly the principal coal-derived products from this reaction at the end of 24 hr are high-molecular-weight materials. Reaction for much longer periods of time does result in the very slow depolymerization of the precipitable material. It is reactive, but the reactions are quite slow. n
288
COAL STRUCTURE BRUCETON COAL 2g C H O H | TsOH 6
5
D€POLYMERIZED COAL SOLUBLE
27g
C H / E t O H (70/30 6
6
| 29%
v/v)
INSOLUBLE
71 7o SOLUBLE
PYRIDINE "^INSOLUBLE
6^% SOLUBLE
9%
ULTRACENTRIFUGATION | PRECIPITATE
37%
25% The Journal of Organic Chemistry
Figure 3.
Fractionation of depolymerized Bruceton coal (29)
One explanation for the persistence of colloidal material is limited reagent accessibility. Coals are known to contain aromatic-methylene carbon-carbon bonds. These bonds are known to be cleaved by the H e r e d y - N e u w o r t h reaction, yet large particles persist during the re action. If there are regions of the coal that the reagents cannot pene trate, then these bonds will not be cleaved and the regions will persist, w i l l not react. C o n t i n u e d investigation of this reaction w i l l show whether this explanation is correct. There are numerous other cases in the literature that could be used as examples of reagent access limitations. This review is not exhaustive, and those cases cited here serve to establish the point that the limited accessibility of reagents is an important factor in coal chemistry. Clearly it is worthwhile to attempt to improve the accessibility of coals to reagents. Perhaps in this way significant increases in coal reactivity can be obtained. It may be possible to increase accessibility by modifying either the coal or the reagent. First let us consider the coal modifications. Physi cal changes can be made. Extracting coals increases their surface area (30) and their reactivity (31). We know of no attempts to explore thor oughly or to maximize this effect. Any treatment that expands pores or increases surface area will help. Such a treatment can be as simple as grinding. It has been shown that going from 100-mesh to 325-mesh coal
18.
LARSEN ET A L .
Reactivity of Coal 1
3000
-
2000
-
1000
-
1
289
1
LU (Z
< 0.
CL
