K.K. Unger et ol. (Editors), Characterization of Porous Solids © 1988 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
MICROPOROSITY IN CARBONACEOUS MATERIALS: CHARACTERIZATION
139
DEVELOPMENT SURFACE COMPOSITION AND
H. MARSH and J. BUTLER Northern Carbon Research Laboratories, School of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NEl 7RU, U.K.
ABSTRACT The classification of carbons into non-graphitizable and graphitizable is presented. Non-graphitizable carbons are developed from woods, resins, etc. whilst the graphitizable carbons, from pitch materials and coals, owe their structure to the development of the intermediate liquid crystal phase or mesophase. Surface composition relates closely to that of the parent material. The non-graphitizable carbons have major importance because of their microporosity both in terms of equivalent surface areas and surface energetics. Currently, it is convenient to interpret isotherms in terms of distributions of potential energies of adsorption. Methods to characterise microporosity are critically assessed and recommendations made. INTRODUCTI ON Overall, carbons are classified as being either non-graphitizable or graphitizable. The latter carbons are usually anisotropic in terms of polarized light microscopy of polished surfaces and are derived from coals and pitch-like materials which pass through a fluid phase during carbonization. Such carbons by reason of this genesis have surface areas of <50 m2g-1, exhibit meso- and macroporosity without microporosity and cannot be chemically activated to increase their surface area significantly. On the other hand, the noncarbons which do not exhibit 3-dimensional X-ray graphitizable carbons (~. diffractions on heating to 3000°C) are optically isotropic and by reasons of their genesis have equivalent surface areas of hundreds of m2g-1 in microporosity. This can be developed further by chemical activation reactions. Such carbons are usually formed from naturally occurring materials as woods, cellulose, non-fusing coals as well as synthetic resins and polymers. These isotropic, microporous carbons have extensive commercial importance mainly for gas and vapour phase adsorption systems (1). The presence of mesoporosity extends their use into liquid phase adsorption systems. The subject area of physical adsorption of gases and vapours is well developed, experimentally and theoretically, a major effect being directed to an understanding of adsorption mechanism (2). There exists also a need to establish
140
methods to characterise and differentiate between porous carbons in a way which can be meaningful to industrial users (1). The approach of this paper therefore is to relate, where possible, the parameters of adsorption equations to the structure of porosity as it exists in carbons. The paper describes the origins of microporosity in carbons, aspects of chemical composition of surfaces and provides an overview of methods of characterization of microporosities. DEVELOPMENT To prepare a microporous carbon, the precursor must be polymeric either being heavily cross-linked initially or must develop cross-linkage in the early stages of carbonization (3. 4). Otherwise, carbonization results in either total volatilization as with linear polystyrene (5) or fusion of the polymer with subsequent formation of a non-porous residue (6). Early stages of carbonization involve the cleavage of bonds to give free radicals (7) which subsequently either combine with each other or abstract hydrogen from the polymer network (8). Monterra et al (4), in IR studies of the carbonization of cellulose, report that at 360°C, although the cellulose structure has collapsed, it is dominantly aljphatic and the beginnings of a polyaromatic network can be detected. Aliphatic side-chains are linked to the aromatic domains by mixed ketonic and ether linkages. At 5600~ the aliphatic CH residues are now converted to aromatic CH residues with loss of small molecules as volatiles. At 730°C residual surface structures are not detectable by IR and at 880°C a carbon has been formed. The IR spectra of a heat treated resin, similar to cellulose carbonizations, are reproduced in Figure 1 (9).
~
~Aliphatic
C-H Aromatic C-H
L..-,=---:_-:----..l
and water
~1
!
~gerprint
"-
~~~~:i C
C= 0 • C- 0
aromati c " WAVENUMBER ring stretch
region - OH
4000
Fig. 1. FTIR spectra of Phenodur carbons: Air oxidation 300°C: 48 h.
C-H
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The most important factor governing the development of microporosity is that a dense, three-dimensional, disordered array of sigma bonds is maintained throughout the carbonization process. Hence the volume elements generated by loss of aliphatic side-chains and oxygen-containing linkages remain vacant because the rigidity of the three-dimensionally cross-linked structures prevent large-scale re-ordering of the material. This convoluted, connected space within and between the macro-molecular network constitutes 'microporosity' of the carbon (10). The dimensions and surface composition of this microporosity are dependent upon the original polymer source, cellulose, lignin, phenolic resin, etc, conditions of carbonization and final heat-treatment temperature. Masters~. (11) give a detailed account of the development and closure of microporosity in cellulose carbon, and Neely (12) in carbonized sulphonated polystyrene. The surface of the microporosity is that of the defective, carbonaceous, macromolecular system and is chemically reactive at 800°C to water and carbon dioxide. In chemical activation processes these reactants remove carbon atoms, ring clusters or entire constituent lamellae as carbon monoxide and so create further microporosity and extend the dimensions of existing microporosity (13, 14). SURFACE COMPOSITION The shape, size and surface composition of the microporosity vary with adsorption site within a carbon. The surface composition varies in terms of proximity to atoms of hydrogen, oxygen, aliphatic and aromatic carbon, nitrogen, sulphur anions and cations. Heteroatoms are usually in the parent material and the activation p~ocess or ageing of carbons in air always forms surface oxygen complexes with a range of possible structures (15). The adsorption potential of a given site is a function of the number of atoms of the adsorbent surface influencing or creating the adsorption site and the intensity of this influence, the latter being determined by the polarity and polarizability of these atoms, ~. surface composition. Carbons free from chemisorbed heteroatoms probably do not exist. The roles of hydrogen and chemisorbed oxygen which exhibit a hydroxyl, carbonyl, carboxyl, ether, wide range of bonding characteristics, ~. etc, as well as sulphur and nitrogen are therefore crucial to the creation of adsorption characteristics. The presence of oxygen on microporous surfaces causes water to be adsorbed, often quite tenaciously. As a result, with inadequate outgassing prior to studying an adsorption isotherm, adsorption processes may take place over surfaces covered with water molecules. The problem is particularly acute when studying microporosity of coals which contain several wt% of heteroatoms. A knowledge of surface functionality is helpful when studying~. the adsorption of polar molecules such as water vapour and
142
methanol because of the temperature dependence of the adsorption process. Surface functionality can also contribute to molecular sieve effects in carbons. CHARACTERIZATION OF MICROPOROSITY The Basis of the Adsorption Isotherm The problem in characterization of microporosity in carbons of industrial importance is to devise a limited number of parameters which adequately describe the microporosity of a carbon, distinguish it from all other carbons and to provide a working specification. The nature of microporosity in carbons is that of a continuous, convoluted network, three-dimensional and slit-shaped. It must never be modelled as a series of independent drill-holes. Micropores may constitute about 10-20% by volume of the carbon offering of the order of 1020 adsorption sites to an adsorbate molecule per gram of carbon. A very basic, but often overlooked aspect of adsorption chemistry is that the experimentally determined adsorption isotherm has a very specific shape. Figure 2 illustrates this fundamental aspect where an experimental isotherm (curve A) is plotted together with two randomly drawn 'isotherm-like' curves, C' and C". Using Langmuir co-ordinates, only the experimentally determined isotherm is linearised. Such a basic consideration implies that the adsorption sites of microporosity are being filled progressively and according to some
Amount Adsorbed 'n a,
....
plpon a C', C" : Random curves A : Isotherm
C'''' ....
,,"
/
I
/
/
4." '"
"
.>
""
"
/
/"
.."
,,'....
",'
."
Isotherm Plots
0.2
0.4
.'"
e" . . . ·.. . ;' ,,'
0.6
--- ---
Langmuir Plots
0.2
RELATIVE PRESSURE/p/po
Fig. 2. The specificity of isotherm shape. in Langmuir co-ordinates.
.........
c~'"
0.4
0.6
Random curves are not linearised
143
distribution of adsorption potential which is regular and similar within the many microporous carbons which are currently manufactured. With something like 1020 g-1 adsorption sites it can be assumed that the range of properties of adsorption sites can be described by an appropriate statistical equation linking a frequency distribution to the adsorption behaviour of the carbon. That such continuous frequency distributions of adsorption potential exist in carbons is the very basis of the adsorption isotherm and all forms of adsorption equations used to interpret the isotherm, including Langmuir and BET. Adsorption Equations The two experimental approaches used in the main to study microporosity are adsorption (to give the isotherm) and calorimetry (to give molar enthalpies, etc. of adsorption). These are not unrelated techniques, the adsorption isotherm being a function of the enthalpies of adsorption. Again, interpretations of microporosity using Langmuir, BET, Dubinin-Radushkevich (D-R) or DubininAstakov (D-A) equations (1, 2) all use the same data base, ~. the adsorption isotherm, or sections of it. Langmuir and BET equations analyse the isotherm shape above p/po of -0.25. The D-R and D-A equations usually interpret data of p/po <0.1, but the entire isotherm is open to interpretation. Hence, these analytical approaches must never be considered as being totally independent of each other. The literature attempting to interpret the isotherm is impressive (2). Very recent considerations are published in Carbon 1987, Vol. 25(i) as a series of papers of a Workshop/Symposium. The Langmuir and BET equations are designed to produce a surface area value (or equivalent value (16)) with units of m2g-1. Differences in the way a given area may be constructed within a carbon are indicated by the constants 'b' and 'C' of these equations but have no precise meaning in terms of the dimensions of the microporous network. They do relate ultimately however, to the distributions of adsorption potential in carbons. Figure 3 is a diagram of such distributions and values of 'b' and 'C' increase as the distributions move to higher values of adsorption potential. But that is about as far as these equations can be taken. Although the theoretical basis of the D-R and D-A equations may be difficult to establish, precisely, the success and applicability of these approaches lies in the fact that they are taking advantage of the distributions of properties of these 1020 adsorption sites, per gram of carbon, ~. the equations have a statistical basis for considerations of adsorption potential and variations of extents of pore filling with relative pressure of adsorbate. The distributions can be complex, particularly if (i) polar adsorbates are used, (ii) activation processes have disturbed the original distributions of carbonization or the distributions are very narrow. In these situations,
144
Frequency .
U~tra
(~,
mtcropor-os tzy I
,
, J
I
I
I
/
I
I
I
1
Microporosity
\
I
I \
\ \
\ \
\
\ '" -
-","'''',
--"
- - .....
-..........
Open surface
' ....
HIGH Potential Energy for Adsorption LOW Fig. 3. Distributions of potential energies for adsorption. appropriately modified forms of the D-R equations are helpful (17). Characterization of Microscopy The ability to characterize and distinguish between microporosities must derive from a theoretical understanding of the complexities of physical adsorption in the microporosity. The industrial development and application of active carbons needs meaningful descriptive parameters (1). It is worthwhile to assess the value of existing parameters. Mention has been made above of the Langmuir and BET equations and the availability of surface areas and the constants 'b' and 'C'. The D-R and D-A equations probably have greater analytical power than Langmuir and BET equations but, in the end, the parameters available for characterization and differentiation are as unrelated to dimensions and structure of microporosity as the 'b' and 'C' values of Langmuir and BET. Consequently, it has to be accepted that only 'indirect' methods of characterization of microporosity will be available in the immediate future. The theoretical basis of the D-R and D-A equations (as below) is considered by Dubinin et al , (18-21), Kadlec (22), Stoeckli et al. (22-25) and Razwadowski with Wojsy (26). From the viewpoint of utilization of these studies, perhaps in routine analyses of microporous carbons, it is helpful to describe how isotherm shape relates to distributions of adsorption potential and the agreement with and deviations from the theoretical equation. The linear form of the DA equation is:log W= log W - 0' logn(p/p o) o 0'
2.303 n-1 (RT/E)n
E
is a characteristic free energy of adsorption.
145
Frequency
High.
log n
Potential Energy
Fig. 4. Examples of use of DR equation: Identical energy distributions but with different ranges of energy. Figure 4 (an idealized version of Figure 3) indicates how differences in the range of distributions of potential energies of adsorption reflect in the gradient of the D-R plots, assuming constant adsorption volume. The lower gradient, curve A', corresponds to adsorption in sites of highest potential energy. But note that adsorption does not continue to values of 10g2(p/po) of zero where the knee of the Type I adsorption isotherm becomes progressively sharper, before levelling off, in moving from curve C' to A'. Similar changes in gradient occur but with different relative positions within the D-R coordinates (curves C' to A) as the widths of the energy distribution widen, at constant adsorption volume. Figure 5 summarises possible interpretations of the use of D-R plots. In Figure 5(a), the plot shows filling of pores progressively within the potential energy distribution range to a value of p/po of unity. In Figure 5(b), pores of lowest adsorption potential are missing from the distribution range either as shown (Figure 5(b)) or as indicated in Figure 4 for materials with narrower distribution ranges. In Figure 5(c) adsorption occurs at the highest relative pressures above predicted values and is associated with mesoporosity and open surfaces. In Figure 5(d) adsorption occurs at the lowest relative pressures below predicted values and is associated with activated diffusion or molecular sieve effects. It is the pores of highest adsorption potential and possible smallest size which contribute to this effect. In Figure 5(e) the D-R plot is non-linear and associated with an irregular distribution of adsorption potential or a distribution which requires the D-A type of analysis (See Figure 7).
146
( a)
(c)
(e)
~l& ~ log2(po/p)
Adsorption Potential POSSIBLE INTERPRETATIONS Ads. Potential
Fig. 5. Possible distributions of adsorption potentials from D-R plots. Figure 6 sets out how isotherms and the corresponding D-R plots change with adsorption temperature (T1 < T2 < T3) for non-polar and polar adsorbates.
Fig. 6.
The variation of adsorption temperature for non-polar and polar adsorbates.
Figure 7 illustrates how the use of the D-R plot (logn(po/p)) which is linear for n = 2 indicates variations in distributions of adsorption potential. With a skewed distribution, values of n are <2 for linearization, giving a curve (concave upwards) in D-R co-ordinates. When values of n are >2 for linearization, the corresponding curves in D-R co-ordinates are convex upwards.
147
Frequency
Eo n> 2
:l\,•
I
I I
I
~ •
•
I I
\ n»2
'.-.,
Energy
Fig. 7.
Adsorption energy distributions and D-R plots.
An indication of the structural parameters available from General Adsorption Isotherms is given in Table 1. TABLE General Adsorption Isotherms
Structural parameters Distribution of adsorption energy
Assumptions Approach
DubininRadushkevich
DublnlnAstakhov
Local isotherm Step function A~RTlnpo/p Step
A=f~~~~~o~
p
Energy distribution function
Micropore volume
Position of maximum
Gaussian
W
Weibull
W
E
k or 8
0
0
DR equation
Gaussian (8 values!
W
8
RozadowskiWojsz
DA equation
Gaussian (E values!
W
E
0
0
n,(y)
0
Stoeckli
Dubinin: Stoeckli: Kadlec.
Spread (homogeneity)
!l
0
!l
0
Average half-width of slit-shaped mlcropores , x , x(nm! = kl Eo'
k = 9 to 13 nm kJ mol
-1
.
148
SUMMARY In the characterization of microporous materials, following assessments are available:1. Rates of adsorption. 2. Isotherm shape. 3. Shape of D-R plot. 4. General adsorption isotherms. Values of D, n, E. 5. Micropore volume/equivalent surface area (m 2g-1).
~.
carbons, the
6. 7. 8. 9. 10. 11. 12.
Values of 'b' (Langmuir): of 'C' (BET). Hg and He densities: open and closed porosity. Molecular sieve experiments. Isosteric heats of adsorption. Microcalorimetry: DSC: enthalpies of adsorption. High resolution phase contrast TEM. Small angle X-ray scattering (SAXS). Small angle neutron scattering (SANS). The cost of a comprehensive analysis of a single microporous carbon using the techniques listed above would not be less than £25,000 (in 1987). The need exists for suitable methods, on a routine basis, to characterise and differentiate microporosity. It is suggested that an information intensive approach is to adsorb carbon dioxide at 273 K; measure rates of adsorption and interpret the isotherm with the D-R equation. The iodine number is also useful (27). The cost of this approach is about £100 per sample. ACKNOWLEDGEMENTS The authors thank Miss Bridget A. Claw for assistance in the production of the manuscri pt. REFERENCES 1 R.T. Yang, "Gas Separation by Adsorption Processes" Butterworth, Stoneham, MA 02180, USA (1987). 2 S.J. Gregg and K.S.W. Sing, "Adsorption, Surface Area and Porosity", 2nd Ed. Academic Press, London, 1982. 3 R.E. Ehigiamusue, G.J. Howard, J. Appl. Polymer Sci., 1975, 19, 3327. 4 C. Morterra, M.J.D. Low and A.G. Severadia, Carbon, 1984, 2~ 5. 5 G.G. Cameron, J.R. MacCallum, J. Macromol. Sci., - Revs Macromol. Chem., 1967, Cl(2), 327. 6 S. Ota~. Yamada, T. Koitabashi, A. Yokoyama, Carbon, 1966, 4, 425. 7 I.C. Lewis, Carbon, 1982, 20, 519. 8 F.H. Winslow, W. Matreyek,~. Polymer Sci., 1956, XXII, 315. 9 H. Marsh and K. Clay, Unpublished results (1986). 10 M. Evans and H. Marsh, "Characterization of Porous Solids", Eds. S.J. Gregg, K.S.W. Sing and H.F. Stoecki, S.C.I. London (1979), p. 53. 11 K.J. Masters and B. McEnaney, Carbon, 1984, 22, 595. 12 J.W. Neely, Carbon, 1981, 19, 27. 13 E.M. Freeman and H. Marsh,-carbon, 1970, ~, 19.
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14 H. Marsh and B. Rand, Carbon, 1971, 9, 47; 79. 15 B.R. Puri, "Chemistry and Physics of-Carbon", Ed. P.L. Walker Jr., Marcel Dekker N. Y., 1970, 6, 201. 16 K.S.W. Sing, D.H. Everett, R.A.W. Paul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure and Appl. Chern. 1985, ~, 603. 17 H. Marsh, Carbon, 1987, 25, 49. 18 M.M. Dubinin, "Chemi stryjind Physics of Carbon, Ed. P.L. Walker Jr., Marcel Dekker N.Y. 1966, 2, 51, 19 M.M. Dubinin and V.A. Astakhov, Adv. Chern. Ser. (102), 1971, 69. 20 M.M. Dubinin, Carbon, 1980, 18, 355. 21 M.M. Dubinin and H.F. Stoeckll, J. Coll. Interface Sci., 1980, 75, 34. 22 O. Kadlec, Adsorption Science and Technology, 1984, 1, 133. -23 H.F. Stoeckli, Carbon, 1981, 19, 325. Carbon, 1981, 19, 353; 22, 297. 24 H.F. Stoeckli, F. Kraehenbueh~ 25 H.F. Stoeckli, F. Kraehenbuehl and D. Morel, Carbon, 1983, 21, 1983. 26 M. Rozwadowski and R. Wojsz, Carbon, 1984, 22, 363; 437 and~986, 24, 451. -27 A. Hill and H. Marsh, Carbon, 1968, ~, 31. --