Langmuir 1995,11, 3964-3969
3964
Sieving Properties of Carbons Obtained by Template Carbonization of Polyfurfuryl Alcohol within Mineral Matrices Teresa J. Bandosz, Jacek JagieHo,+Karol Putyera,i and James A. Schwarz" Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244-1190 Received March 9, 1995. I n Final Form: June 26, 1995@ Carbons obtained by carbonization of polyfurfuryl alcohol within the smectite and taeniolite matrices were studied by sorption methods. The sorption of nitrogen at 77 K showed only small differences in the pore structure ofthese materials related to the properties ofthe inorganic matrix. More specific information about pore structure was obtained from inverse gas chromatography, where molecules of different sizes were used to study sieving properties. Micropore structure was also studied by sorption of methane, carbon tetrafluoride, and sulfur hexafluoride at supercritical temperatures; at these temperatures they are adsorbed mainly in small pores. Thermodynamic quantities, such as adsorption energy distribution, micropore size distribution, and isosteric heat of adsorption,were calculated assuming the energy distribution is governed by pore size distribution due to adsorption potential enhancement in small pores. The results obtained demonstrate the presence of sieving effects for molecular sizes between 3.6 and 6 A.
Introduction The need for microporous sorbents with a relatively uniform pore structure has resulted in the search for new methods of modification of classical materials with the goal to increase their sorption properties. It is well-known that the presence of small pores enhances adsorption energy' and, as a consequence, the storagelseparation capacity, Recently new methods have been introduced to optimize the sorption properties by decreasing the pore sizes of activated carbon^.^,^ These methods are based on the carbonization of small hydrocarbon molecules inside the pores ofmole+-ir sieving carbons with a pore diameter smaller than 15 A, which leads to caTbon molecular sieves (CMS) with pores smaller than 5 A. Another method for the preparation of microporous carbons is based on carbonization of organic precursors within clay mineral interlayer pace.^,^ It was shown elsewhere t h a t the pore structure of the carbons obtained is governed by the properties of the inorganic matrix. Such factors as the size of the interlayer spacing, water content of the matrix, and carbonization temperature have a significant influence on the properties and the porous structure of the final products.6 The aim ofthis paper is to describe the sieving properties and the subtle differences in the porous structure of carbons obtained by the template carbonization method. This is accomplished by the study of the sorption of nonspecifically interacting gases of different molecular sizes. Information about the presence of sieving effects
* Author to whom correspondence should be addressed.
Permanent address: Institute of Energochemistry of Coal and Physical Chemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakow, Poland. t Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovakia. Abstract published in Advance ACS Abstracts, September 15, +
can be obtained from inverse gas chromatography using normal and branched Also, physical adsorption of nonspecifically interacting gases of different molecular sizes can provide information about the pore structure of the sorbents. The gases chosen for volumetric adsorption experiments were methane (CH4), carbon tetrafluoride (CFd), and sulfur hexafluoride (SF6). Their molecular diameters are 3.6,4.7,and 5.4A. At ambient temperature these gases are adsorbed only in small pores the size of which is slightly larger than the size of the adsorptive molecules. The assumption t h a t the energy distribution is governed by the pore size distributiong-ll provides a full characterization of the structural and energetic heterogeneity of these material within the range of the molecular diameters of the probes.
Experimental Section Materials. The method of obtaining microporous carbon by carbonization of polyfurfuryl alcohol within the intercalated clay matrix was described elsewhere.5s6J2In this study we used raw bentonite (Wyoming) as a matrix.'O The minimal sample was saturated for 3 days with a 80%solution of furfuryl alcohol (FA) in benzene. Polymerizationwas carried out by heating the sample under Nz flow at 353 K for 24 h and then at 423 K for 6 h. Bentonite, with incorporated polymer in the interlayer space, was heated at 973 K for 3 h under Nz flow to carbonize the organic compound. The inorganic matrix was removed by extraction with hydrochloric and hydrofluoric acids. The carbon derived from an unmodified bentonite matrix was designated as C-3. Another set of carbon samples was prepared on the basis of the synthetic mineral taeniolite provided by Topy Ind. Co.l3 The lithium form of this mineral has the following formula: Li(Mg2Li)8(Si4)401oFznHzO. The sample of mineral supplied by the manufacturer was intercalated with hydroxyaluminum and hydroxyaluminum-zirconium cations from solutions of Chlorhydrol (TL-1) and Rezal (TL-2), respectively. A part of the
(7) Jagiello, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. In Characterization ofPorous Solids-III;Rouquerol, J.;Rodrigez-Reinoso, 1995. F., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1994; p (1)Everett, D. H.; Powl, J. C. Powl J . Chem. Soc., Faraday Trans. 679. I1976, 72,619. ( 8 ) JagieBo, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994,32,687. (2) Verma, S. K.; Walker Jr., P. L. Carbon 1992, 30, 829. (9) Jagiello, J.; Schwarz, J. A.Langmuir 1993, 9, 2513. (3) Gaffney,T.R.;Fanis,T.S.;Cabrera,A.L.;Armor,J.N.U.S.Pat.(10) Jagiello, J.; Bandosz, T. J.; Putyera, K.; Schwarz, J. A. J . Am. 5,098,88, 1992. SOC. Faraday Trans. in press. (11)Putyera, K.; Jagiello, J.; Bandosz, T. J.; Schwarz, J. A.Carbon, (4)Bandosz, T. J.; Jagiello, J.; Andersen, B.; Schwarz, J. A. Clays Clay Min. 1992, 40, 306. in press. (5) Bandosz. T. J.: Jaeielio. (12) Bandosz, T. J.; Putyera, K.; Jagiello, J.; Schwarz, J. A. Mi. J.; Putvera. - , K.:, Schwarz. J. A. Carbon 1994, 32, 659. croporous Materials 1993, l , 73. (13)Bandosz, T. J.; Jagiello, J.; Putyera, K.; Schwarz, J. A. Clays (6) Bandosz, T. J.; Gomez-Salazar, S.; Putyera, K.; Schwarz, J. A. Clay Min. submitted. Microporous Materials 1994, 3, 177. @
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0743-746319512411-3964$09.00/0 0 1995 American Chemical Society
Sieving Properties of Carbons
Langmuir, Vol. 11, No. 10, 1995 3965
samples (TL-1and TL-2) was saturated with the 80% solution of furfuryl alcohol. Polymerization and carbonization were carried out under the same conditions as described above, and inorganic matrices were removed by acid treatment. The carbons obtained from taeniolite matrices were designated as LC-1 and LC-2, depending on the constitution of the matrix. The content of elemental carbon in the carbon samples was over 70%. Methods. Volumetric Sorption Experiments. Nitrogen isotherms were measured by a GEMINI I11 2375 surface area analyzer (Micromeritics) at 77 K. Before the experiment the samples were heated for 10 h at 473 K and then outgassed at this temperature under a vacuum of mmHg. The isotherms were used to calculate the BET surface (p/poranges between 0.05 and 0.3); micropore volume, Vmic(DR method);14 and mesopore size di~tributi0n.l~ Methane, carbon tetrafluoride, and sulfur hexafluoride adsorption isotherms were also measured on the GEMINI I11 2375. The pretreatment of the samples was the same as in the case of nitrogen adsorption, and the isotherms were measured around room temperature (about 250,270, and 290 K). The apparatus was equipped with a homemade thermostated system controlled by a Fisher Scientific Isotemp Refrigerated Circulator, Model 900. From the adsorption isotherms the micropore size distributions, adsorption energy distributions, and heats of adsorption were cal~ulated.~-l~ Inverse Gas Chromatography. Inverse gas chromatography experiments were conducted at infinite dilution. Detailed descriptions of the method and derived parameters are given elsewhere.16J7 Briefly, the thermodynamic parameters characterizing gas-solid interactions are obtained from the basic quantity measured in gas chromatography, the net retention volume, vN.18319 It is related to the standard free energy of adsorption, AG, by the equation
0
AGO = -RT In -
+C
(1)
s m
0.6
0.8
1
Relative pressure
Figure 1. Nitrogen adsorption isotherms at 77 K.
a 0.004 1 0.003 U
z9
0.002
-
0.001
-
0 4
0
VN
0.4
0.2
20
40
60
80
100
Pore radius [A]
Figure 2. Mesopore size distributions (calculated from the adsorption branch of isotherm).
where R, T, m, and S are the gas content, temperature, mass, and specific surface area of the adsorbent, respectively; C is a constant related to the standard state ofgas and adsorbed phases. From the measurement of retention volumes at different temperatures the enthalpy of adsorption can be calculated: (2)
The above thermodynamic quantities are related only togassolid interactions since, under the conditions of infinite dilution, interactions between adsorbed molecules can be neglected.16 The chromatographic results were obtained using an ANTEK 3000 gas chromatograph. Stainless steel columns 20 cm long and 2.17 mm in diameter were used. The experiments were performed at different temperatures between 573 and 673 K. Granulation ofcarbonswas 0.2-0.4 mm. Before the experiments, the samples were conditioned at 673 K for 15 h under helium flow that was used as a carrier gas. Branched alkanes (2,2-dimethylalkanesand 2-methylalkanes) in conjunctionwith n-alkanes were used as molecularprobes.lOJ1 They represent systematic structural changes and their critical sizes range from 3.6 A (n-alkanes)to 4 A (2-methylalkanes) to Quantities referenced to 2-methyl6 A (2,2-dimethylalkane~).~O alkanes and 2,2-dimethylalkanes adsorbates are denoted in the text by single and double primes, respectively. (14)Dubinin, M. M. In Chemistry and Physics of Carbon; P. L., Walker, Jr., Ed.; M. Dekker: New York, 1966;Vol. 2. (15)footnote not given (16) Jagielo, J.; Bandosz, T. J.; Schwarz, J . A. J. Colloid Interface Sci. 1992,151, 433. (17)Jagielo, J.; Bandosz, T. J.; Schwarz, J. A.Carbon 1992,30,63. (18)Kiselev, A.V.; Yashin, Y. I. In Gas Adsorption Chromatography; Plenum, New York, 1969. (19)Conder, J. R.; Young, L. C. In Physical Measurements by Gas Chromatography; Wiley: New York, 1979. (20)Roberts, R.A,; Sing, K. S.; Tripathi, V. Langmuir 1987,3,331. (21)Ross, S.;Olivier, J. P. In On Physical Adsorption; Interscience Publishers: New York, 1964.
Table 1. Structural Parameters Calculated from Nitrogen Sorption Isotherms sample SN.(m2/d Vmie (cm3/d LC-1 460 0.120 LC-2 0.080 330 0.148 c-3 490 Scanning Electron Microscopy. Morphological observations were made with a scanning electron microscope (SEMI JEOL JSM-35G, operated at 24 kV after samples were coated in a Hummer I1 apparatus using Au-Pd electrodes.
Results and Discussion The analysis of the nitrogen adsorption isotherms at 77 K is a classical method to assess porous properties of materials. The isotherms obtained are presented in Figure 1. From the nitrogen measurements, surface areas and micropore volumes (DR) were calculated (Table 1). The carbons obtained are characterized by surface areas below 500 m2/g and micropore volumes of about 0.100 cm3/g. The calculated specific surface areas and micropore volumes decrease in the order C-3, LC-1, LC-2. This ordering is in agreement with that of a previous study5J3 since the carbons studied in this paper were obtained from matrices with different water content, which is an important factor in the process of micropore creation. Water released during simultaneous dehydroxylatiod carbonization is an activation agent for the carbonaceous deposit present between the mineral layers. It can create small pores the sizes of which depend on the extent of water available and the thickness of the carbon deposit. Both factors are responsible for the subtle differences in the final materials. In the case of the carbon derived from the Wyoming bentonite matrix host (C-3),structural water is released from the mineral (smectite) layers during the
Bandosz et al.
3966 Langmuir, Vol. 11, No. 10, 1995
--
0
100
200
400
300
500
600
700
7
1
I 0.5
2 1
-
--
LC-1
.i - LC-2
\
c-3
I. I
3
4
5
c-3
800
Pressure [mm Hg] Figure 3. Adsorption isotherms of CHI, CFI, and SFs on LC-2 carbon measured at 297 K. 0.6
I
6
Molecular size [A] Figure 4. Amount adsorbed (760 mmHg, 295 K) versus molecular size. carbonization process, whereas in the case of taeniolites the hydroxyaluminum (LC-1) or hydroxyaluminumzirconium pillars (LC-2)introduced are the only source of an activation agent. In the latter case the amount ofwater released is smaller13 and results in lower surface area and smaller micropore volume. The differences in the mesopore size distributions (Berrett- Joyner-Halenda method) provide some information about the microstructure. The distribution curves for the carbons are collected in Figure 2. An conclusion about the micropore sizes (smaller than 20 ) cannot be drawn, but in the range of pores having radii between 20 and 100 A, the samples differ in the same order that was described above, i.e., the C-3 sample has the largest number of pores in the mesopore range compared to the LC-1 and LC-2 samples. The differences in mesoporosity are probably related to the different spatial arrangement of carbon crystallites extracted from the matrices due to the different morphology of the parent minerals. Analysis of the total sorption capacity ofthe adsorbents a t a pressure of 760 mmHg provides some additional information. Figure 3 shows the isotherms measured a t
1
- 0
4
5
6
7
0
9
Number of carbon atoms Figure 5. Variation of the net retention volume of n-alkanes (n),2-methylalkanes (01, 2,2-dimethylalkanes (A) measured at 573 K, versus their number of carbon atoms. 297 K on LC-2 carbon. The total uptake decreases with an increase in the molecular size ofthe sorbate. The same relationship was observed in the cases of LC-1 and C-3 carbons. The decrease, in this case, is probably be due to the fact that not all pores accessible for methane are accessible for either carbon tetrafluoride or sulfur hexafluoride. The phenomena described above indicate the sieving properties of the carbons which, under certain conditions, can be used to separate small gas molecules. In Figure 4we present the relationship of the total amount adsorbed (298 K, 760 mmHg) versus the size of the adsorbant. In all cases, a decrease compared to the methane sorption capacity is observed. This decrease is the most pronounced in the case of the LC-2 sample; here the smallest pores are probably present due to the lowest water content of the matrix compared to the other template ~ a r b o n s . ~ J ~ To further confirm the existence of sieving properties for small molecules, we performed an inverse gas chromatography study a t infinite dilution, where alkanes together with their branched homologues were used as molecular probes. The results obtained are presented in Figure 5, and the linear relationship between In VN and the number of carbon atoms in the alkane molecule is in accordance with expectation. The free energies of interaction of one CH2 segment of the alkane, AGcH~(slopes of the lines), are collected in Table 2. High values indicate the presence of small pores.16 Table 2 also contains the average shift of branched isomer lines from their corresponding parent alkane line. They are denoted as A‘ and A” for 2-methylalkanes and 2,2-dimethylalkanes, respectively. The shift of In VN lines for the isomers is due to the fact that part of the branched molecule cannot be accommodated in the same plane parallel to the surface, and the total contribution to the interaction energy of the branched molecule is smaller than that ofthe more flexible n-alkane molecule^.^,^ When microporous materials that contain pores not accessible for branched molecules are considered, an additional shift in the In VN lines occurs since VNis proportional to the accessible surface or number
Table 2. IGC Results at 673 K -AGcH~ sample LC- 1
LC-2 c-3
(kJ/mol)
6.8 7.0 6.6
-AGcH~ (kJ/mol) 7.0 6.5 6.6
-AGCH?
-W(C6)
-W(C6’)
-A.?Io(C6)
(kJ/mol)
(kJ/mol)
(kJ/mol)
(kJ/mol)
-C
6.4 6.3 6.4
72.4 74.0 74.3
67.7 64.1 71.6
46.6 45.1 55.0
0.88 1.35 0.79
-A” 3.27 3.53 2.22
S’(%)
35 59 29
S(%) 91 93 76
Langmuir, Vol. 11, No. 10, 1995 3967
Sieving Properties of Carbons Table 3. Results of Fitting Equation (6) to the Experimental Methane Adsorption Data sample (A) s [AI ao (mmoVg) uo (cm3/g) 6 (mmoVg) LC-1 LC-2 C-3
5.02
0.91
4.95 5.34
1.08
3.26 2.39 3.19
1.04
0.109 0.079 0.107
24
0.003 0.013 0.003
--.._
--
18.1
0.8
E E
8
I
4
/- - -
LC-2
~
u
OXi 0.4
5 0
0.2
0
0
100
300
200
500
400
600
700
800
Pressure [mm Hg]
Figure 6. Methane adsorption isotherms on LC-1carbon. Solid lines indicate the goodness of the fit. 0.5
0.4
-
0.3
E. 0.2
0.1
0 2
3
4
6
5
7
8
9
h [AI
Figure 7. Micropore size distributions calculated from CHI adsorption isotherms. 0.2
1
0.15
h
k!
X
0.1
0.05
0 10
12
14
16
18
20
22
24
Adsorption energy [kJ/mol]
Figure 8. Methane adsorption energy distributions.
of adsorption sites (see eq 1).We have previouslyreported the analysis of data obtained on a nonporous carbon In the case of that considered as a reference carbon the shift ofln VNoccurs only due to different spatial orientations of the alkane molecule. The A' and A" values obtained on that reference carbon, at the temperature used in this work (573 K) were 0.45 and 0.80, respectively. Taking the difference in A' and A" values for the porous carbons and a nonporous reference, we can calculate the
Where a, is the limiting adsorption capacity and O(c,p,T)
3968 Langmuir, Vol. 11, No. 10, 1995
Bandosz et al.
A
B
-
4 p
-
zpm
Figure 10. SEM micrographs of C-3 (A) and LC-1(B) samples.
is the so-called local adsorption isotherm. Here the HilldeBoer(HDB) isotherm [211, which describes the mobile adsorption model and takes into account interactions
between adsorbed molecules, was used as a local isotherm. The above equation, under the assumption of the dependences of energetic heterogeneity on pore hetero-
Langmuir, VoL. 11, No. 10, 1995 3969
Sieving Properties of Carbons geneity, can be written as
where xminis the minimum pore size accessible for the gas molecule and q ( x ) is the pore size distribution. The pore size and energy distributions are related by the following equation
av avac
a€
q ( x ) = - = -- = "&)ax ax a€ a3c
In this work we assume that the pore size distribution is Gaussian.lo A detailed discussion of the parameters used for calculation, limitations of the method, and the windows of sensitivity for the molecules used are described elsewhere.1° The interpretation presented here is based on the factors discussed previously. Since in the case of our carbons the sieving effects were observed for molecules larger than n-alkanes, the micropore size distributions and other thermodynamic quantities such as adsorption energy distributions and isosteric heats of adsorption were calculated only from methane adsorption isotherms. In the case of other gases used in the volumetric sorption experiments, the results obtained can be misleading due to pore inaccessibility and a significant contribution of sorption on the flat surface due to the sieving effect. In reporting and discussing tbe results, we use the quantity h defined as h = x - 3.4 A, which represents the available pore width. The numerical results of the fitting procedure are collected in Table 3, where h and s describe the calculated mean value of h and the standard deviation of the pore size distribution. Limiting adsorption capacities, a, and vo, are given in terms of number of modes and volume per gram of carbon. The value v, was obtained from a,assuming the liquid density of methane. The root mean square error of the fit is presented as 6. Examples of isotherms on LC-1 carbon are shown in Figure 6, where solid lines indicate the goodness of the fit. The error in all cases was about 5%. From the analysis of the 6 values collected in Table 3 it is clearly seen that the values of the mean width increase in the order LC-2, LC-1, and C-3; the ordering is the same as that observed from the IGC study when surface exclusion (Table 2) is taken into account. Examples of micropore size distributions obtained are shown in Figure 7. The positions of peak maxima are very similar; however, subtle differences can be distinguished, and the highest contribution of small pores exists in the case of the LC-2 carbon. The conclusioncan be drawn from the adsorption energy distributions collected in Figure 8. These distributions can be calculated independently, without any assumption about pore structure, and are less model dependent than pore size distributions. I t is clearly seen that the highest energy centers (small pores) are present in the structure of LC-2 and LC-1 carbons, whereas the C-3 sample distribution is slightly shifted to a lower energy. Self-consistency is also provided by the relationship between the isosteric heats of methane adsorption; they are also related to pore size distribution. The isosteric heats of CHI adsorption are presented in Figure 9. The smallest pores enhance the adsorption energy and are in the structure ofthe LC-2 and LC-1 samples, while the C-3
carbon shows a higher level of heterogeneity caused by the presence of larger pores. Additional consistency between the methods used here and the classical nitrogen adsorption measurements is found in the values of the limiting sorption capacity, vo (Table 3). The methane molecule is adsorbed only in small ores, smaller than double the size of the molecule (7.2 , so its limiting sorption capacity can be considered as equivalent to its micropore volume. Indeed, the comparison of vo and V,,, shows good agreement, which indicates that all microstructural features in the samples are available for methane adsorption (Table 1 and 2). SEM analysis provides information about surface texture. Figure 10 presents the SEM images of the C-3 and LC-1 samples. Differences are easily recognizeable and they are related to the differences in the inorganic matrices, the features of which are preserved in the resulting carbon materials. The image of the C-3 carbon shows small flakes, similar to montmorillonite crystallites with a lack of any spatial orientation (Figure 10A). On the other hand, the carbon obtained from polyfurfuryl alcohol carbonized within taeniolite has a texture similar to its host mineral, with large crystallites and a preserved layered structure (Figure 10B). The differences in the crystallites' organization are responsible for the shape of the nitrogen isotherms, where the influence of mesoporosity and external surface can be seen in the case of the C-3 sample. Taking into account the completely different texture of the carbons as seen by SEM analysis and small differences, however significant for gas separation of small molecules, in micropore size distributions, we conclude that the nature of the matrix (size of crystallites) is not a significant factor which can influence the properties of the material for gas separation. On the other hand, the differences in the microporous structure of the carbons obtained are related to the water content and stability of the matrices, two factors which can influence the chemistry of the processes occurring during carbonization.
XI
Conclusions Based on the findings presented above and on the conclusions of previously published paper^,^,^ the results presented here support the hypothesis that the micropore structure of carbons synthesized within mineral matrices is dependent on the size and structure of pillars and the water content and thermal stability of the matrix. The smaller content ofwater in the case of LC-2 carbon resulted in the presence of very small pores. The differences are not big, however, they are self-consistent and are corroborated by the thermodynamic characteristics of the materials. A difference of 0.5 A in the micropore size distribution becomes a significant quantity when molecular interactions are considered. Materials obtained by the method of template carbonization are characterized by sieving properties. They can separate molecules with small differences in their sizes such as methane, carbon tetrafluoride, and sulfur hexafluoride as well as normal and branched hydrocarbons.
Acknowledgment. The authors would like to thank Ms. Marta Dana Frtalova from the Institute of Inorganic Chemistry in Bratislava for performing the SEM study. T.J.B. thanks Dr. Takashi Kyotani for his help in obtaining the taeniolite samples. LA950188D
