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Langmuir 1997, 13, 2314-2317
Chemical Vapor Deposition of Heterocyclic Compounds over Active Carbon Fiber To Control Its Porosity and Surface Function Yuji Kawabuchi, Chiaki Sotowa, Masahiro Kishino, Shizuo Kawano, D. Duayne Whitehurst, and Isao Mochida* Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received October 29, 1996. In Final Form: February 19, 1997X The chemical vapor deposition (CVD) of some heterocyclic compounds was examined to control the porosity and surface functionality of the active carbon fiber (ACF). The deposition took place only on the pore wall of the ACF, when the heterocyclic compound as the precursor and the deposition temperature were selected carefully to be thermally stable and around 700 °C, respectively. Moderately activated ACFs modified with pyridine, pyrrole, and thiophene demonstrated molecular sieving activity for selective adsorptions of CO2/CH4 and O2/N2 through selective CVD. In contrast, furan decomposed at this temperature, failing to provide molecular sieving activity. The thermal stability of the depositing molecules is a key factor to obtain the molecular sieving performance after CVD. Pyridine, pyrrole, and thiophene produced amorphous carbon within the pore which appears to implant the nitrogen and sulfur atoms over the surface of the ACF, respectively.
Introduction 1-4
In previous studies, the pore size of an active carbon fiber (ACF) was controlled by the selective chemical vapor deposition (CVD) of benzene where the deposition temperature was selected carefully not to deposit carbon on the outer surface of the ACF. We postulated that benzene, one of the best organic compounds to confer molecular sieving capability, is adsorbed on the pore wall where it is condensed into nonvolatile substances within a short time by catalytic action at the ACF surface. The adsorbed species are carbonized into isotropic carbon because of limited rearrangement of adsorbed species into graphitic layer stacking. Under gas-flow conditions, adsorption of benzene on the outer surface is unfavorable, and if it happens, the residence time is not long enough for carbonization to occur. Thus, the carbonization continues to thicken the carbon coating on the wall as long as benzene can diffuse into the pore. When the pore width is reduced to less than the molecular thickness of benzene (0.37 nm),5 the carbonization ceases automatically. A pore width of 0.37 nm lies between the molecular sizes of CH4 (0.38 nm) and CO2 (0.33 nm) and can be distinguished them based on molecular sieving ability.6 It should be noted that CVD reagents maintain their molecular geometry to define the pore size according to their molecular size.7 Hence, pore size can be controlled by selecting a combination of temperature and compound which allows adsorption on the pore wall without pyrolysis in the gas phase. In the present study, CVD of some heterocyclic compounds (pyridine, pyrrole, furan, and thiophene) onto a ACF was tried in order to control their porosity and surface functionality. Compound selection was based on the thermal stability at around 700 °C, mimicking the CVD behavior of benzene. In addition, heterocyclic functions X
Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Kawabuchi, Y.; Kawano, S.; Mochida, I. Carbon 1996, 34, 711. (2) Jungten, H.; Knoblauch, K.; Harder, K. Fuel 1981, 60, 817. (3) Zhonghua, Hu.; Vansant, E. F. Carbon 1995, 33, 561. (4) Kawabuchi, Y. ; Kishino, M. ; Kawano, S. ; Whitehurst, D. D.; Mochida, I. Langmuir 1996, 12, 4281. (5) Eguchi, Y. J. Jpn. Petrol. Inst. 1970, 13, 105. (6) Mochida, I.; Yatsunami, S.; Kawabuchi, Y.; Nakayama, Y. Carbon 1995, 33, 1611. (7) Kawabuchi, Y.; Kishino, M.; Kawano, S.; Whitehurst, D. D.; Mochida, I. Proc. Carbon ’96 1996, 570.
S0743-7463(96)01052-9 CCC: $14.00
Table 1. Some Properties of the Pitch-Based Active Carbon Fiber mean pore diam, nm
sample as-received ACFa treated ACF by pyridineb treated ACF by pyrroleb
1.36
elem anal., % C
H
N
O
88.7 90.8 91.9
0.9 1.0 1.0
0.7 2.6 2.4
9.7 5.6 4.7
a Activated carbon fiber from coal tar pitch, fiber diameter 1015 µm. b Carbon deposition amount: 110 mg/g.
are expected to be introduced on the ACF surface. The treated ACFs were evaluated in terms of the gas separation of CO2/CH4 and O2/N2. Experimental Section ACF and Its Characterization. Some physical and chemical properties of a commercial pitch-based active carbon fiber used in the present study are listed in Table 1. After outgassing at 150 °C for 4 h, the BET surface area was measured by N2 adsorption at -196 °C, using a Simazu ASAP 2000 apparatus. CVD of Heterocyclic Compounds over Active Carbon Fiber. The pyrolysis of heterocyclic compounds was performed by flowing a helium stream containing controlled amounts of heterocyclic compounds at 150 mL/min over the ACF (200 mg) suspended in a quartz basket, in a microbalance (CAHN 1000). A thermocouple, in a glass tube, was inserted directly below the quartz basket. The sample was heated in the He flow to the fixed temperature at a programmed rate of 10 °C/min and maintained at that temperature for 1 h. Then, a prescribed amount of heterocyclic compound was supplied by a microfeeder into the He gas flow to be sent to the heated ACF for a fixed period. The weight uptake of the sample was recorded continuously by the balance. Adsorption of CO2/CH4 and O2/N2. Adsorptions of molecules on the ACFs were carried out separately using a volumetric adsorption apparatus. Each ACF was evacuated to 10-2 Torr at 150 °C for 1 h prior to the adsorption study. The gas volumes adsorbed by ACF within a given time interval at 30° C were determined from the change of pressure in the closed system. The initial pressures of each gas to be adsorbed were fixed at 760 Torr. The amount adsorbed vs time was plotted to determine the rate of adsorption. The kinetic adsorption capacity is defined in the present paper as the amount of gas adsorbed in 2 min, since the adsorption was almost saturated within that period. Kinetic selectivity is defined as the ratio of CO2(O2) adsorbed on
© 1997 American Chemical Society
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Figure 1. Weight increase of the ACF by contact with heterocyclic compounds at 725 °C. Heterocyclic compound concentration: 2% by volume in helium.
Figure 2. Adsorption profiles of CO2 and CH4 over as-received and treated ACFs. Initial pressure: 760 Torr. Final pressure: 500-600 Torr.
Table 2. Products of the Pyrolysis Reaction
Only pyridine was detected in the gas phase after the pyrolysis of pyridine at 725 °C, while a small amount of dimer was found on the reactor wall. Thiophene remained unchanged at this temperature, although very little of its dimer was also detected like pyridine. Pyrrole was also thermally stable, while any dimer was not detected. In contrast, furan was reactive to show 29% conversion, producing mainly butadiene (17% yield), cyclopentadiene, and benzene. Adsorption Profiles of CO2 and CH4 on AsReceived and Treated ACFs. Figure 2 illustrates adsorption profiles of CO2 and CH4 on as-received and treated ACFs. As-received ACF adsorbed both CO2 and CH4 very rapidly within 1 min, and the adsorptions of both CO2 and CH4 were apparently saturated by 2 min. After 2 min, the as-received ACF adsorbed 37 mL/g (STP) of CO2 and 16 mL/g (STP) of CH4, giving a CO2 selectivity (CO2/CH4) of 2.3. The ACF, modified by CVD of pyridine to 110 mg/g of the saturation level, showed an excellent CO2/CH4 adsorption selectivity of about 50, while the CO2 adsorption capacity decreased only slightly to 86% of that on the original ACF. The adsorption of CH4 was markedly reduced by CVD to less than 1 mL/g (STP), thus dramatically improving the selectivity. Pyrrole and thiophene provided excellent selectivity for CO2/CH4 adsorption as pyridine, when a 110 mg/g of weight increase was obtained at 700 and 780 °C, respectively. CVD of pyrrole and thiophene at 725 °C allowed a similar sieving ability when the same weight increase was obtained, although the time to obtain the weight increase varied with the temperatures. The ACF treated by furan was found to have poor CO2/ CH4 kinetic adsorption selectivity and substantively decreased both adsorptions. The CO2 adsorption was very slow, and no saturation was observed by 5 min. Adsorption Profiles of O2 and N2 on As-Received and Treated ACFs. Figure 3 illustrates the adsorption profiles of O2 and N2 on as-received and treated ACFs by pyridine, pyrrole, and thiophene. As-received ACF adsorbed both O2 and N2 very rapidly within 1 min, both O2 and N2 being saturated at 5.1-5.3 mL/g. In contrast, the ACF treated by pyridine to obtain a 110 mg/g weight gain (11% of saturated weight increase) showed significant kinetic selectivity, since N2 adsorption was reduced 1/10th at 2 min, although the O2 adsorption capacity was scarcely reduced by only 13%. The rate of N2 adsorption at 1 min was very slow. ACFs treated by pyrrole and thiophene showed also molecular sieving activity, although the initial rate of N2 adsorption was relatively fast on pyrrolemodified ACF. Introduction of Surface Function. ACFs designated by arrows in Figure 1, at amount gains of 110 mg/g
the ACF in 2 min to CH4(N2) adsorbed on the same ACF in 2 min.8 These parameters are more reflective of kinetic properties than equilibrium properties, as are appropriate for pressure swing adsorption (PSA) applications. Identification of the Organic Species Produced in the pyrolysis of Heterocyclic Compounds. The organic species produced by pyrolyzing the heterocyclic compounds were analyzed at the outlet of the reactor, using GC-MS (Simazu, GC-17AQP-5000). The detector was mainly used to detect heavy products (>50 amu). Products deposited on the reactor wall were recovered and analyzed. Characterization of the ACF Surface. The surface of the ACF was characterized by electron spectroscopy for chemical analysis (ESCA-1000, Simazu Co.) with Mg KR radiation. The operating pressure was below 4 × 10-5 Pa, and the acceleration voltage was 10 kV. The obtained spectra were corrected by the standard peak of C 1s at 284.6 eV.
Results Pyrolysis of Heterocyclic Compounds over ACF. The weight increases of ACF, during the pyrolysis of heterocyclic compounds at 725 °C, are illustrated in Figure 1. (The heterocyclic compound concentration was 2% by volume in He.) Pyridine increased the weight of ACF slowly up to 110 mg/g within 40 min when the weight gain ceased. Pyrrole, furan, and thiophene showed similar profiles of weight increase and saturation level. However, the time to achieve the saturation with pyridine was slightly longer than that with pyrrole but much shorter than that with thiophene. That with furan was similar to that with pyridine. Products of the Pyrolysis Reaction. Table 2 shows the GC-MS (only products heavier than 50 amu were detected) analysis of products at the outlet of the reactor. (8) Cabrera, A. L.; Zehner, J. E.; Coe, C. G.; Gaffney, T. R.; Farris, T. S.; Armor, J. N. Carbon 1993, 31, 969.
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Kawabuchi et al.
Figure 6. S 2p spectra of ACFs.
Discussion Figure 3. Adsorption profiles of CO2 and CH4 over as-received and treated ACFs. Initial pressure: 760 Torr. Final pressure: 500-600 Torr.
Figure 4. N 1s spectra of ACFs.
Figure 5. Schematic diagram of the pyridinic and quaternary nitrogen.
(saturation level), displayed a considerable increase of the nitrogen content after CVD treatment with pyridine and pyrrole (see Table 1). Figure 4 shows the N 1s ESCA spectras of those ACFs before and after CVD. The treated ACFs showed two peaks at 398 and 401 eV that are attributed to the pyridinic and quaternary nitrogens, respectively (see Figure 5),9 although the S/N ratio was too poor to quantify their contents. The S 2p ESCA specra of the ACF treated by thiophene are shown in Figure 6. Two peaks at 164 (p3/2) and 165 (p1/2) eV associated with sulfur atoms appeared clearly. Sulfur was also placed onto the ACF surface by CVD. (9) Grant, K. A.; Zhu, Q.; Thomas, K. M. Carbon 1994, 32, 883.
In the present study, pyridine, pyrrole, furan, and thiophene were used as the carbon precursor of CVD, because they possess similar molecular shape and thickness as those of benzene although the heteroatom is included in their ring. An activated pitch-based ACF was successfully controlled at the nanoscale by carbon deposition from pyridine, thiophene, and pyrrole at a specific range of temperatures (around 700 °C) to obtain a molecular sieving activity sufficient to separate CO2 from CH4. The modification by these reagents also improved the separation of O2 from N2 by kinetic adsorption. Pyrolysis and carbonization of thermally stable aromatic compounds during the adsorption continued as long as the reagents invaded the pores. When the width of the remaining pores is reduced to less than 0.37 nm, the diffusion of the aromatic compounds is prevented (see Figure 7). Smaller difference in their molecular sizes of O2 and N2 (0.346 and 0.364 nm, respectively10) forces their separation by purely kinetic adsorption. Thus, more precise control is necessary to achieve better selectivity. So far, the pore size is assumed to reflect the molecular thickness. However, the diffusion of a molecule into the pore is restricted by the van der Waals potential between the pore wall and electron cloud of the molecule.11,12 Hence, the real size of the pore is larger than that assumed above. Although furan showed a similar profile of weight increase of the ACF by carbon deposition, the molecular sieving selectivity was not provided at all. The kinetic adsorption capacity for both CO2 and CH4 was significantly diminished. Only furan was decomposed among the compounds examined at this temperature range. A variety of sizes and shapes of decomposed products fails to control the pore size, although the carbonization appears to be limited on the surface of the pore wall. The thermal stability of the aromatic compound is, thus, a key factor to get the molecular sieving activity by CVD. In this study, the surface function as well as pore size can be controlled by CVD of heterocyclic compounds. ACF treated by pyridine and pyrrole possessed pyridinic as well as quaternary nitrogen atoms. Since the starting compounds do not carry any quaternary nitrogen, condensation and carbonization are believed to form the rings to provide such nitrogen environments. After the CVD of pyrrole, pyrrolic nitrogen was not detected but pyridinic (10) Beck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; p 636. (11) Walker, P. L., Jr. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1969; Vol. 2, p 257. (12) Rao, M. B.; Jenkins, R. G.; Steele, W. A. Langmuir 1985, 1, 137.
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Langmuir, Vol. 13, No. 8, 1997 2317
quantify the extent of the conversion. Such a structural change may occur through decomposition and carbonization over the ACF surface, since pyrrole is unchanged in the gas phase. While thiophene provided cyclic sulfur functional groups on the ACF surface, the thiophene ring was clearly remains unchanged in the gas phase at around 700 °C. Thiophenic sulfur has been recognized as being thermally stable in coal-derived coke up to 900 °C.13 Thiophenic sulfur may be present selectively at the edge of the carbon plane with little condensation, maintaining its five-membered ring. A similar stacking thickness of aromatic planes may induce a similar molecular sieving activity. So far, the particular contribution of the sulfur atom in the five-membered ring is not identified. There have been several methods reported to introduce heteroatoms over the carbon surfaces. For example, the carbonization of the precursor containing a heteroatom,14-16 surface treatment with NH3, HNO3, H2SO4,17-19 and gas plasma.20,21 Among the methods, the CVD method can control the neighboring environment around the heteroatom on the surface, although the CVD reagents are certainly condensed to be fixed on the pore wall. Definite pyridinic nitrogen and thiophenic sulfur introduced by pyridine and thiophene are expected to develop pyridinic basicity and thiophenic coordination activity, respectively. Such properties are now under investigation. LA961052Y
Figure 7. Mechanism of CVD by heterocyclic compounds onto ACF.
and quaternary ones were detected over the ACF surface. Hence, the five-membered ring of the major pyrrole molecules on ACF may have collapsed into a six-membered ring, as derived from pyridine, although it is difficult to
(13) Calkins, W. H. Energy Fuels 1987, 1, 59. (14) Shindo, A. Japan Patent 37-4405. (15) Mochida, I.; An, K. H.; Korai, Y. Carbon 1995, 33, 1069. (16) Hatori, H.; Yamada, Y.; Shiraishi, M. Carbon 1994, 32, 359. (17) Saroni, A. W.; Abotsi, G. M. K. J.; Solar, M.; Derbyshire, F. J. Extended Abstracts 17th Biennial Conference on Carbon, Lexington, KY, 1985, p 120. (18) Golden, T. C.; Sircar, S. Carbon 1990, 28, 683. (19) Kawano, S.; Kisamori, S.; Mochida, I.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1993, 6, 694. (20) Pattabiraman, P.; Rodriguez, N. M.; Jang, B. Z.; Baker, R. T. K. Carbon 1990, 28, 867. (21) Nakajima, T. Tanso 1990, 145, 295.
