Energy & Fuels 2007, 21, 2929-2934
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Adsorption of n-Heptane and 2-Methylheptane in the Gas Phase on Polyvinylidene Chloride-Based Microporous Activated Carbon Federico Jime´nez-Cruz,*,† J. A. Herna´ndez,‡ Georgina C. Laredo,† Maria Teresa Mares-Gallardo,† and Jose´ Luis Garcı´a-Gutierrez† Programa de InVestigacio´ n en Procesos y Reactores, Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Col. San Bartolo Atepehuacan C.P. 07730, Me´ xico, D.F. Me´ xico, and Centro de InVestigacio´ n en Ingenierı´a y Ciencias Aplicadas (CIICAP), UniVersidad Auto´ noma del Estado de Morelos (UAEM), AV. UniVersidad No. 1001. Col. Chamilpa. C. P. 62210, CuernaVaca Mor., Me´ xico ReceiVed February 7, 2007. ReVised Manuscript ReceiVed June 11, 2007
The selective adsorption of n-heptane and 2-methylheptane was tested on polyvinylidene chloride-based microporous carbon (CMS-IMP12) prepared from pyrolysis of poly(vinylidene chloride-co-vinyl chloride) (PVDC-PVC) copolymer. CMS-IMP12 was characterized by textural analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) and Raman spectroscopies. These results indicate the presence of graphene subunits inside the amorphous carbon and great quantity of sp2 carbon type in CMS-IMP12. It was found that 2-methylheptane is better adsorbed than n-heptane on the carbon sample by an inverse gas chromatography (IGC) adsorption study at 275-350 °C. Structural features of the carbonaceous material, entropy restrictions by steric parameters, diffusion conditions, and cooperative CH/π interactions between CH in n-heptane and 2-methylheptane and π electrons in the carbon walls determine the selective adsorption.
Introduction The Molex (UOP) process was developed in 1960 for separating linear paraffins from branched-chain and cyclic hydrocarbons by selective adsorption on zeolite 5A molecular sieves in a simulated moving bed (SMB) unit.1-3 This technology is usually employed in the production of linear alkanes for the manufacturing of linear alkylbenzenes.4 Molecular sieves have deserved great industrial interest because of their uniform pore structure and appropriate selective adsorptive properties.5 In this context, materials such as zeolites have the suitable surface area and porosity for the shape selectivity of molecular separations,5 while microporous activated carbon has a small pore size with a sharp distribution. The slit-shaped pore size in microporous carbons (about 2.910.0 Å on average) is comparable to the size and shape of small adsorbate molecules.6 The adsorbent properties of microporous carbons have been used in separations of nitrogen and oxygen by either pressure or temperature swing adsorption.5 An interesting source for microporous carbons preparation at suitable conditions of pyrolysis and/or activation are the polyvinylidene chloride (PVDC) copolymers.7-9 PVDC-based microporous carbons have been considered in separation * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Instituto Mexicano del Petro ´ leo. ‡ CIICAP, UAEM. (1) Wankat, P. C. Large-Scale Adsorption and Chromatography; CRC Press: Boca Raton, FL, 1986. (2) Ragil, K.; Jullian, S.; Durand, J.-P.; Hotier, G.; Clause, O. High octane number gasolines and their production using a process associating hydroisomerization and separation. US Patent 6,338,791 B1, 2002. (3) Broughton, K. B. Chem. Eng. Prog. 1968, 64, 60. (4) www.uop.com/objects/19%20Molex.pdf (accessed Aug 8, 2007). (5) Ruthven, D. M. Principles of adsorption and adsorption processes; Wiley-Interscience: New York, 1984. (6) Suzuki, M. Adsorption engineering; Elsevier: Amsterdam, 1990.
Figure 1. Nitrogen adsorption-desorption isotherm at 77 K of CMSIMP12.
technologies because of the adsorptive properties and molecular sieve discrimination, i.e., linear and branched alkanes.9,10 Earlier works reported the molecular sieVe effect resulting from pore constrictions in the selective adsorption of butane over isobutene or neopentane11 and other C5-C8 hydrocarbons using zeolites such as ZSM-5, MOR, HY, NaY, NaUSY, ZSM22, H-TON, and beta.12 (7) Pierce, C.; Wiley, J. W.; Smith, R. N. J. Phys. Chem. 1949, 53, 669683. (8) Lamond, T. G.; Metcalfe, J. E.; Walker, P. L. Carbon 1965, 3, 5963. (9) Barton, S. S.; Evans, M. J. B.; Harrison, B. H. J. Coloid Interface Sci. 1974, 49, 462-468. (10) Fernandez-Morales, I.; Guerrero-Ruiz, A.; Lo´pez-Garzon, F. J.; Rodrı´guez-Ramos, I.; Moreno-Castilla, C. Carbon 1984, 22, 301-304.
10.1021/ef070072c CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
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Jime´ nez-Cruz et al.
Figure 3. de Boer’s t-plot for CMS-IMP12.
in a Nicolet Magna-IR 560 spectrometer in a KBr disk. The Raman spectrum was recorded using Ybon Jobin Horiba (T64000) spectrometer equipped with a confocal microscope (Olympus BX41) with a laser beam of 514.5 nm at a 10 mW power level. X-ray diffraction was performed in a Siemens diffractometer (model D5000) with nickel-filtered Cu KR radiation (λ ) 0.15418 nm) at 35 kV and 35mA. Inverse Gas Chromatography (IGC). IGC experiments were carried out in a Gow Mac 750P gas chromatograph equipped with a flame ionization detector. Stainless steel tubing previously washed with acetone and dried (4.0 mm external diameter, 2.0 mm internal diameter, and 50 cm length) was packed with the microporous carbon (0.3742 g) and coupled to the injection and detection systems. The flow rate of the carrier gas (helium) was set at 30 mL/min for a head column pressure of 2.11 105 Pa. Small amounts (1-10 µL) of pure hydrocarbons were injected separately with a 10 µL syringe into the GC and were eluted isothermally. The retention times were determined by means of a PC with the LABQUEST software as an integrator. The value of the net moles of adsorbed hydrocarbon per gram of adsorbent (ns) was determined by eq 1:13-15 Figure 2. SEM (a) and TEM (b) micrographs of CMS-IMP12.
ns )
In an attempt to develop a new technology for selective separation of hydrocarbons, a study of the adsorption properties of n-heptane and 2-methylheptane on PVDC-based carbons prepared from pyrolysis of PVDC-PVC copolymer was performed. Additionally, the characterization of the synthesized material and the possible cooperative CH/π interactions between C-H bonding in the hydrocarbons and the π electron system in the carbon molecular structure is described. Experimental Section Materials. n-Heptane and 2-methylheptane of highest purity were purchased from Aldrich Co. A CMS-IMP12 microporous carbon sample was prepared from pyrolysis of cylindrical shaped extrudates of poly(vinylidene chloride-co-vinyl chloride) (PVDC-PVC; Saran, Dow Co.) at 700 °C for 6 h in helium atmosphere. The pyrolyzed material, CMS-IMP12, was characterized by scanning electronic microscopy (SEM) (Philips ESEM XL30 at 30 kV), transmission electronic microscopy (TEM) (TECNAI F30 with field emission gun at 300 kV), and surface area by nitrogen physisorption in a Micromeritics ASAP-2000 apparatus using the BET method at 77 K. The Fourier transform infrared (FTIR) spectrum was recorded
[
( )]
Tcol nc 1 (t - tm) j w R Tamb tR
(1)
where w ) weight of the adsorbent (g); tR ) retention time of the injected hydrocarbon (min); tm ) retention time of a nonadsorbing marker (methane) (min); Tcol ) column temperature (K); Tamb ) ambient temperature (K); nc ) injected hydrocarbon (mol); and j ) James-Martin factor correction of gas compressibility. The j factor was calculated according to eq 2: (11) Kitagawa, H.; Yuki, N. Carbon 1981, 19, 470-472. (12) Related studies are described in: (a) Webb, E. B.; Grest, G. S.; Mondello, M. J. Phys. Chem. B 1999, 103, 4949-4959. (b) Denayer, J. F.; Souverinjs, W.; Jacobs, P. A.; Martens, J. A.; Baron, G. V. J. Phys. Chem. B 1998, 102, 4588-4597. (c) Denayer, J. F.; Ocakoglu, A. R.; Huybrechts, W.; Martens, J. A.; Thybaut, J. W.; Marin, G. B.; Baron, G. V. Chem. Commun. 2003, 1880-1881. (d) Calero, S.; Smit, B.; Krishna, R. Phys. Chem. Chem. Phys. 2001, 3, 4390-4398. (13) Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography; Wiley: New York, 1979. (14) Kiselev, A. V.; Yashin, Y. I. Gas-chromatographic determination of adsorption and specific surface for solids. In Gas Adsorption Chromatography; Plenum: New York, 1969; pp 104-145. (15) Montes-Moran, M.; Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. J. Colloid Interface Sci. 2002, 247, 290-302.
Adsorption on PVDC-Based ActiVated Carbon
Energy & Fuels, Vol. 21, No. 5, 2007 2931
Figure 5. X-ray pattern of CMS-IMP12.
[ ]
carried out at different temperatures (285-350 °C). These data were adjusted to the Langmuir isotherm, Cµ ) CµsbP/(1 + bP), which is commonly employed for this type of exponential behavior and for the characterization of carbon adsorbents.16 Cµs (mmol/g) represents the maximum adsorbed concentration, b (1/kPa) is the affinity parameter or Langmuir constant, Cµ (mmol/g) is the adsorbed amount of hydrocarbon, and P is the pressure in kPa; Cµs and P were determined by IGC. The temperature effect study in the adsorption was performed by using the van’t Hoff equation, b ) b0 exp(-Qst/RT), in which b0 (1/kPa) is the affinity constant at zero loading, Qst (kJ/mol) represents the heat of adsorption, and R is the gas constant. Likewise, the Gibbs free energy (-∆G) was calculated according to the ∆G ) -nRT ln b equation. In general, the adsorption equilibrium isotherms’ experimental data were fitted to simple mathematic models with help of the software Matlab. The method of optimization used to fit the equilibrium constant is the Nelder-Mead Simplex Method of the Optimization Toolbox.17
Figure 4. FTIR (a) and Raman (b) spectra of CMS-IMP12.
( ) ( )
Pin P 3 out j) 2 Pin Pout
2
3
-1
(2)
-1
where Pin and Pout are the inlet and outlet column pressure (Pa). The pressure at the outlet was measured in triplicate as 2.09 105 Pa. Partial pressure (pHi, Pa) values were calculated by eq 3, in which R is the ideal gas constant (8.3145 106 cm3 Pa/mol K), T is the temperature of the column, Fc is the corrected flow rate (eq 4; cm3/min), and Fm is the uncorrected flow rate (cm3/min): pHi )
ncRT
CMS-IMP12 Characterization. The prepared sample CMSIMP12 showed a BET surface area of 814 m2/g. Nitrogen adsorption isotherm at 77 K shows a type I isotherm with a slight hysteresis loop (see Figure 1) indicating that these materials are predominantly slit shaped microporous with a very small amount of mesoporous character.16 In Figure 2a, the SEM image of CMS-IMP12 was obtained at 500 times magnification, showing the amorphous flakes morphology of the material. In Figure 2b, the TEM image for CMS-IMP12 shows the porous amorphous carbon and more ordered graphene subunits in which the interplanar distances are around of 0.34 nm which is slightly
(3)
(tR - tm)Fc
Fc ) jFm
Results and Discussion
( ) Tcol Tamb
(4)
Data Analysis. Experimental data for the adsorption equilibrium isotherm of n-heptane and 2-methylheptane in CMS-IMP12 were
Table 1. t-Plot Obtained Parameters in CMS-IMP12 Nitrogen Adsorption slope 1 (cc STP/g Å)
slope 2 (cc STP/g Å)
intercept (cc STP/g)
total area (m2/g)
external area (m2/g)
micropore volume (cc/g)
2t (nm)
54.757
0.3833
243.9500
844.33
5.9098
0.3772
0.9148
Table 2. Langmuir and van’t Hoff Adsorption Parameters for CMS-IMP12 Langmuir adsorbate n-heptane
2-methylheptane
van’t Hoff
T (°C)
Cµs (mmol/g)
b (kPa-1)
r2
-∆G (kJ/mol)
-Qst (kJ/mol)
b0 108 (1/kPa)
r2
285 300 325 350 275 300 325 350
0.4956 0.4861 0.5620 0.5351 0.3873 0.4292 0.4718 0.4666
20.2530 13.4055 4.8782 2.5505 85.8906 33.8273 13.6429 7.1389
0.9934 0.995 0.9965 0.9953 0.9861 0.9922 0.9936 0.9947
13.93 12.36 7.87 4.85 20.28 16.77 12.99 10.18
90.23
7.4041
0.9916
96.48
5.4709
0.9997
2932 Energy & Fuels, Vol. 21, No. 5, 2007
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Figure 6. Chromatographic separation of 2,2,4-trimethylpentane, n-heptane, and 2-methylheptane in CMS-IMP12.
higher than the interplanar spacing of (002) graphite planes (0.335 nm). de Boer’s t-plot can provide very useful information about pore structure.18 The thickness curve of the nitrogen isotherm for CMS-IMP12 is presented in Figure 3, the slopes of the two straight lines described therein, and the surface properties (total and external surface area and micropore volume) were calculated according the t-plot analysis (Table 1).16,19 The gradual inclination of the intercepts of the two lines shows the distribution of micropore radii in the t-plot. It is found that the 2t pore diameter value is 0.9148 nm.16,19 A similar result for the pore diameter value in PVDC-carbon is described by Endo et al.20,21 It is well-known that the pyrolysis of PVDCPVC undergoes a loss of weight by dehydrohalogenation and a polyene matrix (sClCdCHsClCdCHsClCdCHs) is produced, followed by intermolecular cross-linked cyclization into aromatic C6 and possible C5 rings and finally development of pores and graphene subunits as the temperature increases.20,22 The FTIR spectrum of CMS-IMP12 shows several bands (see Figure 4a), which can be assigned to PVDC-based carbons: the bands at 1342 and 1581 cm-1 can be attributed to aromatic Cd C bond23,24 and to aromatic ring stretching or coupled to highly conjugated carbonyls, respectively.23,25 According to early studies,23 the presence of the bands at 1437 (OH deformation vibration) and 1685 cm-1 (carbonyl stretching vibration in aromatic rings) suggest a low grade oxygenation of the surface. (16) Do, D. D. Adsorption analysis: equilibria and kinetics; Imperial College Press: London, 1998. (17) Demuth, H.; Beale, M. Optimization Toolbox for Matlab-Users Guide; Version 2.1, The MathWorks Inc.: Natrick, MA, 2000. (18) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319-323. (19) Nakai, K.; Sonoda, J.; Kondo, S.; Abe, I. Pure Appl. Chem. 1993, 65, 2181-2187. (20) Endo, M.; Kim, Y. J.; Ishii, K.; Inoue, T.; Takeda, T.; Maeda, T.; Nomura, T.; Miyashita, N.; Dresselhaus, M. S. J. Electrochem. Soc. 2002, 149, A1473-A1480. (21) Endo, M.; Kim, Y. J.; Takeda, T.; Maeda, T.; Hayashi, T.; Koshiba, K.; Hara, H.; Dresselhaus, M. S. J. Electrochem. Soc. 2001, 148, A1135A1140. (22) Hsieh, T.-H.; Ho, K. S. J. Polym. Sci., Part A 1999, 37, 32693276. (23) Starsinic, M.; Taylor, R. L.; Walker, P. L.; Painter, P. C. Carbon 1983, 21, 69-74. (24) Moreno-Castilla, C.; Lo´pez-Ramo´n, M. V.; Carrasco-Marı´n, F. Carbon 2000, 38, 1995-2001. (25) Zawadzki, J. Infrared spectroscopy in surface chemistry of carbons; Thrower, P. A., Ed.; Chemisrty and Physics of Carbon; Dekker: New York, 1989; Vol. 21.
Figure 7. Isotherm plots of n-heptane (a) and 2-methylheptane (b) for CMS-IMP12.
The Raman spectrum of CMS-IMP12 (Figure 4b) showed two bands: the first at 1331 cm-1, attributed to a D band, which is associated with the vibration of carbon atoms with dangling bonds in the plane of the disordered graphite (disorders and defects in carbon materials), and the second at 1584 cm-1, attributed to a G band, which is associated to sp2 vibrations in a two dimension hexagonal lattice, namely the graphite band. D and G bands in CMS-IMP12 have similar relative intensities,
Adsorption on PVDC-Based ActiVated Carbon
Energy & Fuels, Vol. 21, No. 5, 2007 2933
Figure 8. Possible CH/π interactions between n-heptane and 2-methylheptane C-H bonding and π orbitals in the carbon walls.
reflecting the discrete graphitization degree.26,27 The XRD pattern of CMS-IMP12 (Figure 5) presents a broad diffraction peak at 20.7° which can be attributed to (002) graphitic carbon and a weak peak at 43.2° associated to (100) planes of solid material carbons. Graphitized material inside the amorphous carbon is detected. These results are in agreement with those observed in TEM analysis. n-Heptane and 2-Methylheptane Adsorption. An illustrative chromatographic separation of linear NC7, monobranched 2MC7, and the multibranched 2,2,4-trimethylpentane (224TMC5) is shown in Figure 6. The retention time of 2MC7 is higher than that of NC7 on CMS-IMP12; in the case of 224TMC5, the retention time is not observed. The calculated adsorption parameters are depicted in Table 2, and isotherm plots of CMSIMP12 are shown in Figure 7. In these features, at lowering temperature, the increase of the maximum adsorbed concentration (Cµs) and the increase of the affinity constant (b) are observed in the carbonaceous sample; the adsorption of 2MC7 is higher than that of NC7 in the tested pressure range. Similar conclusions can be made from -∆G data. By comparing Cµs, saturation concentration is well achieved in CMS-IMP12 for both alkanes. The calculated van’t Hoff parameters are depicted in Table 2, and it is observed that the exothermic adsorption energy (-Qst) in 2MC7 is higher than that of NC7 by 6.25 kJ/mol. This may be attributable to the increasing in the carbon number as is reported for C5-C7 hydrocarbon adsorption on zeolites.12b,28 In the case of the affinity constant at zero loading (b0) is observed to have higher values for NC7 than for 2MC7. Because the pre-exponential factor (b0) is related to the entropy of adsorption,29 it is noticeable that this parameter can be controlled by the size of the alkane, the pore restrictions, and diffusion by loss of freedom degree. Thus, it is interesting to point out that the difference between 2MC7 (critical size: 0.591 nm)30 and NC7 (critical size: 0.45 nm)30 is only a methyl group. By considering an increase of entropy in NC7 (the greatest b0 values), the decrease in 2MC7 can be explained in terms of the restrictive pore size and additive weak CH/π interactions, in our case C-H bonds in the alkane and π bonds in the sp2 graphene subunits. The CH/π interactions are weak hydrogen bonds (4.18-12.56 kJ/mol) between CH groups (soft acid) and π systems (soft base) with a large dispersion, partly from charge-transfer and elec-
trostatic forces, which is orientation-dependent, additive in enthalpy, and entropically and cooperatively favored.31,32 This type of interaction has deserved great attention in molecular recognition as a crucial driving force in host-guest complexation in which C-H bonds and aromatic cycles are involved.31 The computational reported magnitude of CH/π in model probes as methane-benzene or ethane-benzene is 6.07 kJ/mol in CH4/ C6H6 and 9.21 kJ/mol for C2H6/C6H6.33 Experimentally, the magnitude of CH/π interactions of methane-benzene in the gas phase is around 4.31-4.73 kJ/mol determined by mass analyzed threshold ionization and well-supported by the quantum chemical calculations with electronic correlation (MP2/cc-pVTZ).34 In these features, these possible CH/π interactions between CH in n-heptane and 2-methylheptane and π electrons in the carbon walls can happen by either the methylene (2 C-H) or methyl (3 C-H) moieties. A model of these interactions is depicted in Figure 8. The additive C-H interactions can be accountable for the increase in the adsorption favoring (an 8-carbon alkane (2MC7) has more CH interactions than a 7-carbon alkane (NC7)). However, in addition to the cooperative CH/π interactions in determining the selectivity, complimentary factors may also be operating, such as the structural features of the carbonaceous material (sp2 over sp3 carbon), entropy restrictions by steric parameters (critical molecular size of the hydrocarbons), and diffusion conditions. Conclusions The PVDC-PVC-based carbon (CMS-IMP12) was wellcharacterized, showing graphene subunits inside the amorphous (26) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Po¨schl, U. Carbon 2005, 43, 1731-1742. (27) Poole, C. P.; Owens, F. J. Introduction to Nanotechnology; WileyInterscience: New York, 2003; pp 207-209. (28) Arik, I. C.; Denayer, J. F.; Baron, G. V. Microporous Mesoporous Mater. 2003, 60, 111-124. (29) Atkinson, D.; Curthoys, G. J. Chem. Educ. 1979, 56, 802. (30) Jime´nez-Cruz, F.; Laredo, G. C. Fuel 2004, 83, 2183-2188. (31) (a) Nishio, M.; Hirota, M. Tetrahedron 1989, 45, 7201-7245. (b) Nishio, M.; Hirota, M.; Umezawa, Y. The CH/π Interaction; Wiley-VCH: New York, 1998. (32) Umezawa, Y.; Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio, M. Tetrahedron 1999, 55, 10047-10056. (33) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K. J. Am. Chem. Soc. 2000, 122, 3746-3753. (34) Shibasaki, K.; Fujii, A.; Mikami, N.; Tsuzuki, S. J. Phys. Chem. A 2006, 110, 4397-4404.
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carbon and a great quantity of sp2 carbon type. It was found that 2MC7 is better adsorbed than NC7 in CMS-IMP12 and a best saturation is achieved. This may be explained by the increase in carbon number in the hydrocarbons. Additionally, structural features of the carbonaceous material, pore restrictions,
Jime´ nez-Cruz et al.
diffusion conditions, and cooperative CH/π interactions (CH/ sp2 carbon) may be related to the good adsorption selectivity in CMS-IMP12. EF070072C
