CO2 absorption in C60 solid

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CHEMICAL PHYSICS LETTERS

CO2 absorption in ChOsolid * Yatsuhisa

Nagano,

Tetsu Kiyobayashi

Microcalorimetry Research Center, Faculty of Science, Osaka University, Toyonahz 560, Japan

and Tomoshige

Nitta

Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka 560, Japan

Received 8 September 1993; in final form 6 November 1993

A large amount of CO2 absorption in Cso solid was found under a supercritical CO* treatment. Absorption kinetics show that the process is significantly slower than normal physical adsorptions and is accelerated with increasing temperature. The COa infrared absorptions are observed at 2329 and 65 1 cm-‘, suggesting a strong interaction between CO2 and Cm. DSC thermograms show a remarkable effect of the CO2 absorption on the orientational phase transition at 250 K of C6,,crystals.

1. Introduction A characteristic property of fullerenes is their affinity to various molecules. They form polymorphic solvated crystals with several inorganic and organic compounds [ 1- 13 1. Solvent molecules such as benzene and toluene are trapped in solid fullerenes [ 2,3,9]. These properties are probably attributable to a small difference in the stabilities between the polymorphic structures. Another notable property related to solvated crystals is that the trapped molecules in solid fullerenes can hardly be removed by the usual thermal treatments in vacua [ 141. Although some of the structures of the solvated crystals have already been determined, kinetic and thermodynamic studies are required in order to know how and why such strong interactions occur between fullerenes and solvent molecules. Recent studies have ensured that the CeO solids have become well-established materials. However, some of the properties of these materials are still uncertain because of the difficulty of obtaining non-solvated and well-characterized CeO solids. Cso solid is * Contribution Center. 186

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known to be a plastic crystal, which shows an orientational phase transition at 250 K [ 15,161. The transition depends strongly on the solvent under which the crystal grows. In fact, the solids formed from toluene and hexanelbenzene solutions show heat capacity anomalies with twin peaks [ 14,171. While the origin of the splitting has not yet been clarified, the heat capacity anomalies are useful for the characterization of C& solids. Originally we tried to remove the solvent molecules from Ceo solids by a supercritical fluid treatment. On the way, we found that a large amount of CO* was absorbed in C6,, solids, which seemed to form a solvated crystal with CO*. In the present Letter, we report on the kinetics of the CO2 absorption and on a drastic effect that CO2 absorption has on the orientational phase transition.

2. Experimental The CeO sample was prepared at the Institute for Molecular Science, Okazaki. The soot containing fullerenes was generated by a plasma discharge [ 18 1. ChOwas separated by a large-scale preparative HPLC system (Shodex, AUTOPREPwith KP-801) after

0009-2614/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved.

SSDI 00009-2614(93)E1384-S

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soxhlet extraction. Toluene was used for the eluent. Contaminating CT0 and higher fullerenes in the Cso fraction were determined to be less than 0.5% by means of high-resolution HPLC (Hewlett-Packard, HP1090LC with Bucky Clutcher 1 (Regis) ) and mass spectrometry. Before the first COZ treatment, the Cso powder was kept at 120°C in vacua for 12 h in order to remove adsorbed toluene. However, the proton NMR spectra show that the Chosample is still solvated with toluene by as much as 5 mol% even after this procedure. After the CO* and thermal treatments, W spectra in cyclohexane solution and HPLC were measured to make sure that no significant degradation occurred. The apparatus for supercritical COZ treatment was equipped with an HPLC pump and a column oven. The Ceopowder was installed in a column of l/4 inch SUS tube with 0.5 urn filters. At 0.1 and 1 MPa, the amount of absorbed CO2 was determined by volumetry [ 19 1. At 20 MPa, it was determined by weighing the column. The absorption gravimetry at high pressure requires the exact weight of the COZ fluid in the column, because it is much larger than the amount of absorbed COz. However, the volume expansion of the solid due to the CO* absorption prevented us from precisely determining the fluid volume in the column. Therefore, the weight of the column was measured after discharging the COZfluid. Because rapid gas expansion chilled the column, it was weighed 10 min after the discharge. In order to inhibit the desorption of CO1 from the solid, the column oven was cooled before discharging the CO2 fluid. Actually, both the absorption and desorption rates were extremely slow at room temperature compared with the weighing time scale. High-purity CO* (Sumitomo Seika, 99.9%) was used without further purification. In the volumetry, the density of supercritical CO, was calculated by using the virial coefficients (B= -74.6 cm3/mol, Cc4138 cm6/mol at 97.61 “C, B= - 111.4 cm’/mol, C=4944 cm6/mol at 39.23”C; B= - 124.0 cm3/mol, Cc5140 cm6/mol at 25.00°C), which were evaluated using literature data [ 20 1. Calorimetric measurement was carried out by a DSC (Perkin-Elmer, DSC7 ) with a scanning rate of 10 K min- ‘. The C6o powder was sealed in an aluminum pan under dry Nz or CO* gas. The calorimeter was calibrated with cyclohexane of spectral grade,

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which showed a solid/solid phase transition at - 87.05 ‘C and melting at 6.67’ C [ 2 11. Infrared absorption was measured with an FT-IR (Shimadzu, FTIR-8500) by a KBr pellet method under dry air conditions.

3. Results 3.1. Gravimetry and volumetry of CO, absorption The volumes of the column and the C6o powder were volumetrically determined with He gas at 0.1 and 1 MPa. The volume of the C6Dpowder, 0.61 cm3/ g at 39°C agreed well with that calculated using the lattice constant of the fee Ceo crystal [22]. Fig. 1 shows the amount of absorbed CO* determined volumetrically and gravimetrically as a function of the time of the CO, treatment at 39 and 98°C under a CO2 pressure of 0.1, 1, and 20 MPa. The initial absorption rate increased with CO2 pressure. The absorption was slow compared with the usual gas adsorption on activated carbons and zeolites. The total amount of absorbed COZ, 50 mg per 1 g CeOafter 25 h CO2 treatment, corresponds to 0.82 mol COZ per 1 M C6o. After CO2 absorption, a slight volume expansion of the CeOsolid was observed. The absorption curve shows a steep increase in the

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Fig. 1. Temporal variation of absorbed CO2 in the 1 g CsOsolid under a CO2 pressure of 20 MPa at 39°C (open circles) and 98°C (closed circle). The temperature was increased after 5 h. The amount at 25 h is indicated by a bold arrow. The dashed line is the expected increase for a temperature of 39°C after 5 h. Closed and open squares indicate CO* absorption at 39°C under CO2 pressures of 0.1 and 1 MPa, respectively.

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first 1 h followed by a slow linear increase. It apparently consists of two different kinetic processes, which, however, may not be real due to the constant period between the discharge and the weighing. The column temperature was increased up to 98 ‘C after a 5 h treatment at 39°C. The amount of absorbed COz after 3 h treatment at 98°C (closed circle) is obviously larger than the expected amount at 39°C (dashed line). Therefore, the absorption is accelerated by heating. These kinetic features strongly suggest that CO2 absorption in the C6,,solid is different from the physical adsorption onto surfaces or micropores. 3.2. Infrared spectra Figs. 2a and 2b show the infrared spectra before and after CO* treatment, respectively. Four peaks at 1427, 118 1, 575 and 527 cm-’ for the Cso molecule agree with literature values [ 231. Two single peaks at 2329 and 651 cm-’ are attributed to CO2 vibrations, whereas the band origins of free CO2 are at 2349 and 667 cm-‘, respectively [ 241. The large redshift suggests a strong interaction between CO2 and the C6,, solid. Within the resolution of 4 cm-‘, the frequencies of the absorbed CO2 vary with neither the time after the supercritical CO2 treatment nor the thermal treatment, while the thermograms show drastic successive changes.

3.3. DSC measurements

Fig. 3 shows the DSC thermograms in the vicinity of the orientational phase transition of Cc0 at 250 K. Thermogram (a) was obtained before the COz treatment. The heat capacity anomaly is composed of a sharp peak at - 18 “C and a broad peak at - 29’ C. The double peak structure of this anomaly agrees with those reported by Atake et al. [ 141. Thermograms (b)-(f) were obtained after the supercritical COz treatment. Thermograms (b)-(d) show the heat capacity anomalies of the CsOsolid, which were kept under atmospheric COz pressure at room temperature for 4.5 h, 55 h, and 12 days, respectively, after the supercritical CO1 treatment. After storage for 2 weeks at room temperature, the ChOwas annealed at 110 ’ C for 2 h and at 160 oC for a day. Thermograms

-_A :a)

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Fig. 2. Infrared spectra of the CW solid (a) before and (b) after the supercritical COs treatment. The vibrational absorptions of CO2 are marked with asterisks at 2329 and 651 cm-‘.

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Fig. 3. DSC thermograms of the Cso solid before (a) and after the supercritical CO* treatment (b)-(f). Thermograms (b)-(d) show temporal variations of the heat capacity anomalies at 4.5 h, 55 h and 12 days after the treatment. After 2 weeks storage, the CsOwas annealed at 110°C for 2 h (e) and at 160°C for a day (f).Thescalesof(b)-(d)and(e)areenlarged4timesandtwice, respectively.

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(e) and (f ) show the anomalies after these respective thermal treatments.

4. Discussion Lately, the adsorptions on the Cso solid have been investigated for several gases [ 25-271. Kaneko et al. measured the Nz adsorption isotherm, which shows a typical type I curve suggesting a micropore structure of the adsorbent [ 25 1. The total surface area was determined to be 24 m* g-’ at 77 K, while those for the activated carbons are 600 to 800 m* g-i [28]. If the molecular cross-sectional area of CO2 is assumed to be 0.22 nm*, the surface area yields 8 mg CO2 per 1 g ChOfor the CO2 capacity of the monolayer, which is much less than the total amount of CO2 absorbed in the Cso solid, 50 mg/g, obtained from the present gravimetric measurements. In addition, the CO2 desorption rate is extremely slow as well as the absorption rate, while the rates increase with temperature. Therefore, the present CO2 absorption is different from physical gas adsorption on the C6,, solid. The entire COZ absorption is composed of fast and slow processes. The former process is the normal physical adsorption into the micropores. The CO* diffusion in the micropores would be completed much faster than the present experimental time scale. Such normal adsorbates would immediately be spouted by reducing the pressure, so the first process does not contribute to the present results. The absorbed COZ in the C6c solid is predominantly composed of that due to the second process. Similar slow absorption is observed in the case of the chemical adsorption of COZ in zeolites [ 29 1. However, the infrared spectra clearly indicate that the COZ molecule is not chemically changed, while the significant red-shift suggests a strong intermolecular interaction. The rate of the second process is accelerated by increasing the temperature. This property suggests that the rate is governed by some activation process. In the fee lattice of the Ceo crystal, we can find octahedral voids, which may accommodate CO2 molecules between Cm molecules. However, no molecules can till these positions because there is no channel through which they can penetrate from the outside to the void. Therefore, it is necessary to de-

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form the crystal lattice to allow CO2 molecules to diffuse in the crystal. An activation energy is possibly required for such lattice deformation, which suggests that the polymorphic structures of the solvated C6e crystals would vary with moderate thermal treatment under CO2 pressure. Actually, the C& solid has a lot of dislocations which provide rapid pathways for CO2 diffusion in the crystal. The dislocations may form a micropore structure, as determined by the N2 adsorption isotherm. In other words, the dislocations contribute to the first process of the absorption. The second process, which has been observed in the present study, would be slow penetration into the CGO crystalline lattice along the pathways. Thermogram ( f ) in fig. 3, which was obtained after baking out COZ from the Cso solid, looks similar to thermogram (a) measured before the CO2 treatment. However, the peak temperatures of the anomalies are 6°C lower than those of thermogram (a). These temperature shifts are attributable to the size effect of the crystalline domains which are broken up by the CO2 absorption. This shift also suggests that CO2 absorption accompanies lattice deformation. The heat capacity anomaly (b) occurring soon after the COZ treatment is much smaller than those of (a). The magnitude of the anomalies is recovered by leaving the ChOsolid under 1 atm CO2 (c, d) and by annealing at high temperatures (e, f ). Possibly, there is a 1: 1 C60-C02 phase, which may not have an orientational phase transition. Soon after the CO2 treatment, a major part of the solid is dominated by such a phase filled with COZ. Consequently, the broad anomaly in thermogram (b) would be attributed to the original phase transition, which is related to the remaining fee domains. As CO2 is discharged, the other anomaly appears at - 65 ‘C (c). It grows slowly and the peak position moves to higher temperature. Following this peak, another anomaly appears at - 37°C (d). Finally, these anomalies mostly become a single peak at -42°C after annealing at 110°C (e). As the low-temperature anomalies develop, the magnitude of the original anomaly in the vicinity of - 28’ C decreases. However, the original peak recovers the amplitude after annealing at 160” C (f ), where the thermal motions of CeO molecules would help to construct the original packing structure. Observation of the variation from (e) to (f > indicates that the peak at -42°C of (e) converges to 189

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the peak at - 35 “C of (f ), which originally appears under the effect of the toluene molecules trapped in the CbOsolid. Therefore, the temporal variation of the crystal structure after COZ treatment may provide information on how the trapped molecules contribute to the orientational phase transition. An Xray diffraction study is now in progress. At present, it should be emphasized that the variation of the thermograms indicates that structural changes accompany the phase transition and there are several metastable structures. The variation could be due to transformations between polymorphic structures.

Acknowledgement YN and TK are grateful to Professor Yusei Maruyama for his kind offer to use the fullerene preparation system at the Institute of Molecular Science. They also thank Dr. Hiroshi Kitagawa and Mr. Masaaki Nagata for their kind help at IMS. They thank Dr. Ken-ichi Lee for the NMR measurement and Mr. Hiroshi Adachi for the mass spectrometric measurement. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas No. 04238105 from the Ministry of Education, Japan.

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