Carbon Molecular Sieves Prepared from Polymeric Precursors

Jul 2, 2009 - H2 adsorption measurements on these CMS at cryogenic temperature (77 K) and ambient pressure (0.1 MPa) show that the hydrogen ...
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Ind. Eng. Chem. Res. 2009, 48, 7125–7131

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Carbon Molecular Sieves Prepared from Polymeric Precursors: Porous Structure and Hydrogen Adsorption Properties Ana Ma Silvestre-Albero,† Anass Wahby,† Joaquı´n Silvestre-Albero,*,† Francisco Rodrı´guez-Reinoso,† and William Betz‡ Laboratorio de Materiales AVanzados, Departamento de Quı´mica Inorga´nica, UniVersidad de Alicante, Ap. 99, E-03080 Alicante, Spain, and Supelco, 595 North Harrison Road, Bellefonte, PennsylVania 16823

The textural characterization of a series of carbon molecular sieves (CMS) prepared from different polymer precursors has been investigated using N2 adsorption at 77 K and CO2 adsorption at 273 K, together with immersion calorimetry into liquids of different molecular dimensions. Experimental results show that the carbon molecular sieves cover a wide range of porosity, from pure microporous CMS (pore diameter below 0.56 nm) to CMS combining larger micropores (above 0.7 nm) together with a certain proportion of mesoporosity. H2 adsorption measurements on these CMS at cryogenic temperature (77 K) and ambient pressure (0.1 MPa) show that the hydrogen adsorption capacity exhibits a linear correlation with the volume of narrow micropores (Vn). Furthermore, these results confirm experimentally the necessity of a tailored micropore size in order to achieve an optimum packing density of the adsorbed hydrogen molecules (micropore size around 0.6 nm). Introduction Carbon molecular sieves (CMS) are carbonaceous materials with a narrow pore size distribution, that is, effective pore diameter in the nanoscale, which enable them to discriminate molecules on the basis of a difference in adsorption equilibrium or on a difference in sorption rates (size selectivity and/or kinetic selectivity). Consequently, CMS require the presence of a specific porous network containing pore mouths of molecular dimensions, together with a relative high micropore volume, features that will provide them with capacity and selectivity into a given application. CMS are commonly prepared from a wide variety of carbonaceous raw materials such as cellulosic precursors,1,2 coals,3 carbon fibers,4,5 resins,6,7 and so on. The final porous structure depends both on the nature of the precursor material and on the pyrolisis conditions applied. Unfortunately, the achievement of the target pore size for a given application constitutes a difficult task because it requires a careful control of the carbonization conditions used. This drawback has been overcome by modification of the existing pore size using postsynthesis treatments. Among the different possibilities the most widely applied are (i) controlled activation of the carbonized materials at high temperatures,1,8,9 (ii) deposition of carbon on the wall of the pore mouth through carbonization of pitch or resin (i.e., cover method),6,10 (iii) chemical vapor deposition of the vapors of organic molecules such as benzene, propylene, and so on,1,4,5,11 (iv) thermal contraction,12,13 and (v) other postsynthesis treatments.2 In practice, commercial CMS are mainly manufactured from activated carbons by controlled deposition of pyrolytic carbon at the pore mouth. CMS exhibit important industrial applications in the separation and purification of binary gas mixtures, for example, separation of linear and branch hydrocarbons, removal of CO2 from gas/air streams, and separation of N2 and O2 from air and so on.1,2,4,9,11,14 Compared to conven* To whom correspondence should be addressed. E-mail: [email protected]. Tel.: +34 96590 9350. Fax: +34 96590 3454. † Universidad de Alicante. ‡ Supelco.

tional zeolites, CMS have some advantages such as higher hydrophobicity, higher resistance to both alkaline and acid media, thermal stability under inert atmosphere at higher temperatures and, due to the presence of slit-shaped pores, shape-selectivity toward planar molecules. Another potential application of CMS concerns their use in (i) hydrogen storage, (ii) hydrogen separation from methane, carbon monoxide and nitrogen in syngas ratio adjustment and (iii) hydrogen recovery from hydrocarbons in refineries.15,16 According to the literature, hydrogen adsorption on carbon-based nanomaterials (activated carbons,17-24 carbon nanotubes,18,19,21,22,24-26 carbon nanofibers,21-23 and so on) highly depends both on the textural characteristics of the sorbent and on the adsorption energy of the surface sites. Experimental17,19,20,23-25,27,28 and theoretical18,26,29 analysis suggest that hydrogen adsorption is affected by a high pore specificity, that is, both pore size and pore geometry (cylindrical or slit-shape pores) are postulated as critical parameter defining the total adsorption capacity. In this sense, Grand Canonical Monte Carlo calculations have shown that appropriate carbon materials for hydrogen storage require slit-shape pores with a pore size below 1 nm.18,29 However, some discrepancy exists between theoretical and experimental results regarding the optimum pore size (either a pore size able to accommodate one or two layers of adsorbed hydrogen). Furthermore, the real role of the nanostructure (specific surface area, total volume of micropores, etc.) on the adsorption behavior is still an open question. With this in mind, the aim of this work is to analyze the porous structure of newly developed carbon molecular sieves using a combination of N2 and CO2 adsorption at 77 and 273 K, respectively, together with immersion calorimetry measurements into liquids of different molecular dimensions. Additionally, hydrogen adsorption capacity at ambient pressure and cryogenic temperature will be analyzed and compared with the porous structure for the different CMS. The presence of a homogeneous and narrow pore size distribution on carbon molecular sieves will be very useful to asses experimentally the optimum pore size for hydrogen adsorption and to better

10.1021/ie900091n CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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understand the relationship between the textural parameters and the total adsorption capacity. Materials and Procedure Carbon molecular sieves (CMS) were prepared from polymer precursors. The CMS selected cover different micropore size distributions, and all are commercially available from Supelco. Carboxen-1012 (C-1012) is a spherical CMS prepared from a functionalized alkene polymer with a mesoporous structure, which was carbonized to possess a large micropore diameter and subsequent surface area. Carboxen-1003 (C-1003) and Carboxen-1000 (C-1000) are spherical CMS prepared from a functionalized aromatic polymer. C-1003 is post-treated (thermally) to increase/improve the hydrophobicity of the surface. Carboxen-1018 (C-1018) and Carboxen-1021 (C-1021) are spherical CMS prepared from functionalized aromatic polymers; changes in the polymer-based pore structure and carbon processing temperature provided for a larger average micropore diameter in the Carboxen-1018 particles compared to Carboxen C-1021. Finally, Carboxen-1016 is a graphitized polymer carbon prepared from a functionalized aromatic polymer, therefore no micropore region exists for the graphitized CMS. For all carbons, the carbonization temperature range was 973-1373 K, with the exception of C-1016 (>2773 K). The particle size distributions for the carbons are C-1016, C-1021, C-1018, C-1000: 60/80 mesh (175-250 µm) C-1003: 40/60 mesh (250-425 µm) C-1012: 80/120 mesh (125-175 µm) Textural characterization of the CMS was performed using several techniques. N2 and CO2 adsorption-desorption isotherms were measured in a homemade fully automated manometric equipment at 77 and 273 K, respectively. Before the experiment, the samples were outgassed at 423 K for 4 h under vacuum (10-9 MPa). The “apparent” surface area was obtained applying the BET method in the relative pressure range p/p0 ) 0.001-0.1. The micropore volume (V0) was deduced from the N2 adsorption data using the Dubinin-Radushkevich (DR) equation, while the mesoporous volume (Vmeso) was obtained as the difference between the total pore volume (Vt) adsorbed at p/p0 ≈ 0.95 and the micropore volume (V0). The pore volume corresponding to the narrow microporosity (Vn) was obtained after application of the DR equation to the CO2 adsorption data.30 Finally, Rs method was applied to the N2 adsorption data using a standard nonporous carbon black (no. 32B, Mitsubishi Chem.) as a reference sample.31 Immersion calorimetry measurements into liquids of different molecular dimensions (dichloromethane, 0.33 nm; benzene, 0.37 nm; 2,2-dimethyl-butane, 0.56 nm and R-pinene, 0.70 nm) were performed in a Setaram Tian-Calvet C80D calorimeter at 303 K. A complete description of the experimental setup can be found elsewhere.32 Briefly, previous to the experiment the sample was outgassed at 423 K for 4 h in a glass tube connected to a vacuum equipment. After the heat treatment, the bulb containing the sample was sealed in vacuum and then it was introduced into the calorimetric cell containing the immersion liquid. Once the thermal equilibrium was reached, the glass bulb tip was broken and the wetting liquid was allowed to contact the sample. The heat evolved as a result of this interaction was recorded as a function of time. The integration of the signal, after appropriate corrections (i) the breaking of the tip (exothermic) and (ii) the heat of evaporation of the immersion liquid to fill the empty volume of the bulb with the vapor at the

Figure 1. (a) N2 adsorption-desorption isotherms for the different carbon molecular sieves at 77 K, (b) detailed view of the nitrogen isotherms at 77 K for samples C-1018 and C-1021.

corresponding vapor pressure (endothermic), provides the total enthalpy of immersion (-∆Himm). Both corrections were previously calibrated using empty glass bulbs with different volumes. Experimental error associated with enthalpy measurements is below 3-4%. The total area of the solid accessible to the wetting liquid was estimated from the enthalpy of immersion (J/g) of the CMS using a nonporous graphitized carbon black (V3G; SBET: 62 m2/g) as a reference.32 H2 adsorption isotherms on the different CMS were measured in a homemade fully automated manometric equipment at cryogenic temperature (77 K) and atmospheric pressure (0.1 MPa). Previous to the adsorption measurement, the samples were outgassed at 423 K for 4 h under vacuum (10-9 MPa). Result and Discussion N2 and CO2 Adsorption Isotherms. Figure 1a shows the N2 adsorption-desorption isotherms for the different carbon molecular sieves. Additionally, Table 1 reports the “apparent” surface area (SBET), the volume of micropores (V0), the total pore volume (Vt), and the mesopore volume (Vmeso), obtained from the N2 adsorption data at 77 K, together with the volume of narrow micropores (Vn), obtained from the CO2 adsorption data at 273 K. As it can be observed in Figure 1, the N2 adsorption isotherms are quite different among the different samples, thus showing that these CMS exhibit completely different textural properties depending both on the polymer

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Table 1. Textural Characteristics of the Different Carbon Molecular Sieves Deduced from the N2 and CO2 Adsorption Isotherms at 77 and 273 K, Respectivelya sample

SBET (m2/g)

V0 (cm3/g)

Vmeso (cm3/g)

Vt (cm3/g)

Vn (cm3/g)

V 0 - Vn

C-1016 C-1021 C-1018 C-1000 C-1003 C-1012

77 695 757 1010 1260 2000

0.06 0.28 0.30 0.40 0.48 0.73

0.50 0.07 0.06 0.44 0.59 0.11

0.56 0.35 0.36 0.84 1.08 0.84

0.01 0.24 0.23 0.30 0.34 0.46

0.05 0.04 0.07 0.1 0.14 0.27

a SBET: “Apparent” surface area calculated using the BET method. Vo: Micropore volume calculated by applying the DR equation to the N2 adsorption data at 77K. Vt: Total pore volume obtained from the amount of N2 adsorbed at p/p0 ≈ 0.95. Vmeso: Mesopore volume obtained by subtracting Vt - Vo. Vn: Volume of narrow micropores calculated by applying the DR equation to the CO2 adsorption data at 273 K.

precursor and on the synthesis conditions used. In this sense, while samples C-1012, C-1018, and C-1021 exhibit a type I isotherm, characteristic of essentially microporous carbons, samples C-1000 and C-1003 exhibit both a highly developed microporosity together with a large nitrogen uptake at high relative pressures (above p/p0 ≈ 0.8). Finally, sample C-1016 exhibits a type III isotherm, indicative of weak adsorbentadsorbate interactions typically present on nonporous samples. A close inspection of the different adsorption isotherms permits us to obtain a more detailed description of the porous structure on these carbon molecular sieves. In this sense, although sample C-1012 exhibits a flat profile at high relative pressures, characteristic of a pure microporous sample, that is, there is no mesoporosity, the wide knee at low relative pressures (below p/p0 ≈ 0.2) clearly denotes the presence of a wide micropore size distribution. This finding is also confirmed by comparing the large difference between the volume of narrow micropores (Vn), deduced from the CO2 adsorption data, and the volume of total micropores (V0), deduced from the N2 adsorption data. It is noteworthy to mention that in the absence of kinetic restrictions the comparison of these two values (Vo, from N2 at 77 K, and Vn, from CO2 at 273 K) provides a good estimation of the micropore size distribution. In fact, these two values are very similar for carbons exhibiting narrow and homogeneous micropores, while they become different (Vo > Vn) with the broadening of the porosity.30 Samples C-1018 and C-1021 exhibit also a type I isotherm. However, the porous structure on these samples is slightly more complex (see Figure 1b). These two samples exhibit a small type H2 hysteresis loop in the relative pressure range p/p0 from 0.4 to 0.8, which reflects the additional contribution of a small proportion of large micropores-narrow mesopores. Comparing the adsorption data from these two samples it is clear that although the total development of porosity is quite similar, the difference between the volume of narrow and total microporosity (V0 - Vn) is larger on C-1018 as compared to C-1021, this suggesting the presence of slightly wider micropores in the former carbon. As described above, samples C-1000 and C-1003 exhibit, in addition to the high nitrogen uptake at low relative pressures, a large adsorption at relative pressures above p/p0 ≈ 0.8. A similar situation is observed on carbon C-1016, although without the large N2 adsorption observed at p/p0 < 0.1, that is, this graphitized polymer carbon is a nonmicroporous material. In the three cases, the high nitrogen uptake at high relative pressures is accompanied by a type H1-H3 hysteresis loop. In principle, the large adsorption uptake observed together with the hysteresis loop reflects the presence of secondary porosity on these samples, this complementary porosity being in the large

Figure 2. High-resolution Rs plots for the different carbon molecular sieves.

mesoporous range and ranging from Vmeso 0.4 to 0.6 cm3/g (see Table 1). However, the presence of capillary condensation in the interparticle space can not be excluded as responsible of the observed behavior, at least for sample C-1016. “Apparent” surface area of the synthesized CMS ranges from 77 m2/g, on the nonporous C-1016 sample, up to 2000 m2/g, on sample C-1012. A further analysis of the porous structure can be obtained by applying the Rs-method to the N2 adsorption data.33 The Rsmethod is purely experimental and it is based on the comparison of the adsorption data of a given sample with that of a nonporous reference sample of a similar chemical nature.34,35 Figure 2 shows the Rs-plots obtained for the different carbon molecular sieves using high resolution nitrogen adsorption data for a nonporous carbon black used as a reference sample. Except C-1016 sample, all carbon molecular sieves exhibit the characteristic swings at low Rs values, attributed to the presence of enhanced surface-molecule and molecule-molecule interactions in micropores. On samples C-1018 and C-1021, the upward deviation in the Rs plot occurs only at Rs values below 0.5, corresponding to the filling swing (FS) in narrow micropores (pores width below 0.6 nm).34 The absence of any significant deviation above Rs ) 0.5 clearly confirms the presence of narrow micropores on these samples, that is, a sharp micropore size distribution. A different situation occurs for samples C-1000 and C-1013. These samples exhibit the filling swing (FS) at Rs < 0.5 and the cooperative swing (CS) at Rs > 0.5, thus reflecting the presence of narrow and wide micropores, that is, bimodal pore size distribution. Furthermore, the upward deviation above Rs ) 1.5 on these samples denotes the additional presence of mesoporosity. Finally, Rs plot on sample C-1012 shows the two characteristic swings (FS and CS), although with a larger

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Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009 Table 2. Total Surface Area Available for Dichloromethane (0.33 nm), Benzene (0.37 nm), and 2,2-Dimethylbutane (0.56 nm) Obtained from Immersion Calorimetry Measurements at 303 K. BET Surface Area and Specific Surface Area (SSA) are Included for the Sake of Comparison sample

SBET (m2/g)

SSAa (m2/g)

SDCM (m2/g)

SBZ (m2/g)

S22DMB (m2/g)

C-1016 C-1021 C-1018 C-1000 C-1003 C-1012

77 695 757 1010 1260 2000

74 808 883 1182 1446 2143

64 836 919 1180 1265 1676

71 728 874 1092 1299 1678

56 95 129 907 1030 1457

a

Figure 3. Enthalpy of immersion (J/g) for the different carbon molecular sieves into liquids of different molecular dimensions: dichloromethane (DCM), benzene (Bz), 2,2-dimethyl-butane (2,2-DMB), and R-pinene.

contribution from the cooperative swing, as corresponds to a sample with a wide micropore size distribution. In summary, textural characterization results obtained by nitrogen adsorption show that carbon molecular sieves prepared from polymer precursors cover a wide range of pore sizes depending both on the nature of the raw polymer and on the synthesis conditions used. CMS studied range from pure microporous materials to CMS which combines a welldeveloped microporosity, together with secondary porosity in the large micropores-small mesopores range. Immersion Calorimetry Measurements. The heat of immersion of a porous solid into a certain liquid can be used to evaluate both the porous structure and the surface chemistry of the material.32,36 In the absence of specific interactions at the solid-liquid interface, the enthalpy of immersion can be regarded as an indirect measurement of the surface area available to a certain molecule. Thus, the appropriate selection of the immersion liquid (molecular dimensions) can be used to evaluate the surface area available to each molecule, that is, the pore size distribution (PSD). In this sense, Figure 3 shows the enthalpy of immersion (J/g) for the different carbon molecular sieves into liquids of different molecular dimensions, for instance, dichloromethane (0.33 nm), benzene (0.37 nm), 2,2-dimethyl-butane (0.56 nm), and R-pinene (0.70 nm). In accordance with adsorption measurements described before, sample C-1012 exhibits the largest enthalpy of immersion for all liquids studied. This result is in agreement with the larger surface area available on this sample. However, the presence of a similar enthalpy value independently of the size of the probe molecule clearly reflects the absence of important molecular sieving effects for molecules below 0.7 nm. A similar situation occurs for carbon molecular sieves C-1003 and C-1000, although in both cases the enthalpy of immersion follows a similar decreasing trend with an increase in the molecular size of the immersion liquid. Again, the large enthalpy of immersion observed for a large molecule such as R-pinene reflects the absence of important restrictions in both samples for the accessibility to the inner porosity of molecules below 0.7 nm. Interestingly, a close inspection of these profiles show that while both dichloromethane and benzene show a similar enthalpy of immersion on sample C-1003, that is, both molecules have a similar surface area accessible, the enthalpy of immersion into benzene exhibits a slight decrease when compared to dichloromethane in sample C-1000. These results suggest that, besides the similarity between these two samples,

SSA obtained from the high resolution Rs plots.

carbon molecular sieve C-1000 exhibits slightly thinner pores compared to sample C-1003, that is, sample C-1000 possesses pores below 0.37 nm while sample C-1003 does not. N2 and CO2 adsorption measurements described before showed that carbon molecular sieves C-1018 and C-1021 exhibit a narrow micropore size distribution, that is, the volume of narrow micropores, deduced from the CO2 adsorption, was quite similar to the total volume of micropores, deduced from N2 adsorption. This observation is clearly confirmed in Figure 3. The enthalpy of immersion into dichloromethane and benzene is quite significant in both samples, while a larger molecule such as 2,2-dimethyl-butane, with a kinetic diameter of 0.56 nm, is not able to access the microporosity. This result suggests that both samples are carbon molecular sieves with narrow pore mouths (pore diameter e 0.56 nm). Interestingly, even when both samples are prepared from the same polymer precursor, the enthalpy of immersion into benzene (0.37 nm) is slightly lower compared to dichloromethane (0.33 nm) on sample C-1021, while these two values are more similar on sample C-1018. The presence of kinetic restrictions for benzene to access the whole microporosity in sample C-1021 is in accordance with previous observations on these samples using H2 and Ar adsorption at 77 and 87 K, respectively.37 This behavior is attributed to the larger temperature used in the synthesis of sample C-1021, which causes a slight shrinkage of the micropores on this sample. The partial coalescence or thermal contraction of large pores after a high temperature treatment can have significant effects on the performance of carbon molecular sieves in practical applications where the presence of a specific pore diameter is required to discriminate between molecules with a similar kinetic diameter (e.g., chromatographic and gas separation applications).12,13 Finally, sample C-1016 exhibits a low enthalpy of immersion (∼7 J/g) independently of the immersion liquid, as it corresponds to the low “apparent” surface area of this sample. Specific Surface Area. In the absence of specific interactions, the enthalpy of immersion into liquids of different molecular dimensions is a powerful tool to evaluate the extent of porosity accessible for the different molecules, that is, the micropore size distribution. However, immersion calorimetry can also be used to estimate the surface area accessible to a certain molecule if the enthalpy of immersion into this molecule for a nonporous reference sample, with a surface chemistry similar to the sample under analysis, is known. For carbonaceous materials without a well-developed surface functionality, the use of nonporous carbon blacks as a reference material has been proposed.36,38 In this sense, Table 2 reports the total surface area available to dichloromethane (0.33 nm), benzene (0.37 nm), and 2,2dimethyl-butane (0.56 nm) in the different carbon molecular sieves using a graphitized carbon black, V3G, as a reference sample. The nonporous reference sample V3G possess a BET

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2

surface area of 62 m /g. The BET surface area obtained from the N2 adsorption data and the specific surface area (SSA) obtained from the Rs plots are also included for the sake of comparison. The total SSA has been determined from the slope of the straight line connecting the linear region between the filling swing and the cooperative swing (Rs ≈ 0.5 for all carbon molecular sieves) and the origin.34 The carbons described in Table 2 can be clearly classified into three different categories, depending on the microporous structure of the material. In this sense, for carbon molecular sieves with a very broad micropore size distribution (see Table 1), that is, samples exhibiting narrow and large micropores (e.g., C-1012), the surface area estimated using immersion calorimetry into a small molecule such as dichloromethane (DCM) (with a kinetic diameter slightly smaller than N2 (0.36 nm)) is lower than that estimated from the N2 adsorption data (using both the BET equation and the Rs method). In the case of samples with micropores of medium size (e.g., C-1000 and C-1003), the total surface area estimated using DCM is quite similar to that estimated from the adsorption data (only Rs method provides an overestimation for sample C-1003). Finally, samples with narrow micropores (e.g., C-1018 and C-1021) exhibit a larger surface area when using immersion calorimetry. In principle, nitrogen (0.36 nm) and benzene (0.37 nm) have similar molecular dimensions and, consequently, one would expect that both molecules should access the same porosity, thus providing a similar surface area available. However, as already mentioned, these two values exhibit a different behavior depending on the porous structure of the carbon materials under study. This discrepancy between adsorption measurements and immersion calorimetry has been already reported in the literature for activated carbons and it has been attributed to the uncertainty in the estimation of the BET surface area on microporous materials.38-40 Apparently, carbon materials with narrow micropores (e.g., samples C-1021 and C-1018) can only accommodate a single layer of adsorbed nitrogen molecules and, consequently, the BET method gives rise to an underestimation of the real surface area because it takes into account only one wall of the micropores. On the contrary, immersion calorimetry measures the heat of interaction of the benzene molecule with the two walls of the micropore, thus providing a more reliable value of the surface area. This discrepancy disappears for samples with larger micropores (e.g., samples C-1000 and C-1003) which are able to accommodate two layers of adsorbed nitrogen. In this case, the application of the BET equation provides a more realistic value of the surface area, this value being very similar to that obtained using immersion calorimetry. However, the situation becomes reversed for samples with highly developed microporosity (e.g., C-1012). The micropores in this sample are able to accommodate more than two monolayers of nitrogen, thus providing an overestimation of the BET surface area. Interestingly, on these samples with a high surface area and showing a highly developed cooperative swing (e.g., C-1012), the BET surface area estimated strongly depends on the p/p0 range used. In fact, the SBET for sample C-1012 varies from 1600 m2/g, when using the p/p0 range from 3 × 10-4 to 4 × 10-3, to 1810 m2/g, for the p/p0 range from 2 × 10-3 to 10-2 and finally 2000 m2/g, for the p/p0 range from 0.02 to 0.1. This increase in the BET surface area with the relative pressure range used is in accordance with previous observations of Kaneko et al. on mesophase carbon microbeads.41 The concepts of superhigh surface area and the subtracting pore effect (SPE) were proposed several years ago to determine

Figure 4. H2 adsorption isotherms at 77 K and up to 0.1 MPa for the different carbon molecular sieves (CMS).

the SSA for microporous carbons, avoiding drawbacks usually associated with the BET method.34,41 As described before, the slope of the straight line connecting the linear region between the FS and the CS processes and the origin was suggested to estimate the SSA. However, a close inspection of the values reported in Table 2 show that even a comparative method such as the high resolution Rs analysis fails for samples exhibiting either narrow (pore width below 0.6 nm) or wide micropores (pore width above 1.2-1.5 nm). For samples with narrow micropores, although the SSA is closer to the value of SDCM, the Rs-method still exhibits important limitations associated with the impossibility of these pores to accommodate two adsorbed nitrogen layers, thus providing an underestimation of the real surface area. On the other hand, for samples with a wide micropore size distribution (e.g., C-1003 and C-1012) the Rsmethod provides erroneous interpretations due to the difficulty in assessing the boundary region between the filling swing and the cooperative swing. In fact, the overlapping between these two regions (FS and CS) gives rise to an upward deviation of the experimental points, thus providing an overestimation of the SSA. In summary, the application of the BET equation and the Rs-method to the nitrogen adsorption data in order to estimate the surface area of porous carbon materials exhibits important limitations when dealing with samples with either narrow or wide micropores. In this sense, immersion calorimetry measurements into molecules of similar molecular dimensions (dichloromethane and benzene) can be suggested as a useful complement to nitrogen adsorption for a more accurate estimation of the specific surface area on these materials. However, care must be taken when using immersion calorimetry alone due to the possible enhancement of the adsorption energy in narrow micropores.42 Hydrogen Adsorption at 77 K. H2 storage on carbon materials by means of physical adsorption has created a high expectation due to the promising results reported in the literature.22 As described in the Introduction, both the nature of the carbonaceous material (activated carbon, carbon nanotubes, carbidederived carbon, etc.) and its porous structure (micropore volume, total surface area, pore size and shape, etc.) are important parameters defining the total adsorption capacity. To evaluate the potential performance of CMS on this topic, the H2 adsorption isotherms at 77 K and atmospheric pressure (0.1 MPa) were performed. As it can be observed in Figure 4, the shape of the H2 adsorption isotherms is very similar for the different carbon molecular sieves. Additionally, Figure 4 clearly shows that H2 adsorption requires the presence of microporosity. In fact,

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Table 3. H2 Adsorption Capacity and Average H2 Density on Narrow Micropores for the Different Carbon Molecular Sieves at 77 K and 0.1 MPa Sample H2 adsorption (mg/g) H2 density (g/cm3)

C-1016 C-1021 C-1018 C-1000 C-1003 C-1012 0.82

13.9 0.058

14.1 0.061

16.7 0.056

18.4 0.054

22.4 0.049

sample C-1016, which is a nonmicroporous sample, is not able to adsorb hydrogen even at 77 K. Taking a closer look to the other carbon molecular sieves, there is a clear increase in the amount adsorbed with the development of porosity up to a maximum in sample C-1012, with a total amount adsorbed of 22.4 mg/g (2.2 wt %) (see Table 3).37 Excluding sample C-1016, the calculated H2 density on these carbon molecular sieves ranges from 0.06 g/cm3 on samples with a narrow micropore size distribution (e.g., C-1018), to an average value of 0.05 g/cm3 on samples with larger micropores (e.g., C-1012), that is 70%86% of the density of liquid H2. The maximum packing density observed on carbon molecular sieves with narrow micropores (sample C-1018 with a pore diameter below 0.56 nm) confirms experimentally that the optimum pore size for hydrogen storage on porous carbons is ∼0.6 nm (i.e., pore size able to accommodate two layers of adsorbed hydrogen), in accordance with previous theoretical predictions.17,29 It is noteworthy to mention that the lower density observed on sample C-1021 compared to sample C-1018 must be attributed to the partial shrinkage of the microporosity in the former sample, that is, on sample C-1021 not all micropores are able to accommodate an optimum packing of the adsorbed hydrogen molecules. To better understand the effect of the porous structure in the adsorption of H2, we have tried to correlate the total amount adsorbed at atmospheric pressure and 77 K with the different textural parameters obtained from the adsorption measurements (BET surface area, total micropore volume (V0), total volume of narrow micropores (Vn), and the specific surface area (SSA) obtained using the Rs method). These analyses show that the hydrogen adsorption capacity at ambient pressure (0.1 MPa) and low temperature (77 K) exhibits a better correlation with the volume of narrow micropores (Vn), deduced from the CO2 adsorption data (see Figure 5). A detailed analysis of the H2 adsorption data reported in the literature for porous carbon materials shows the existence of some controversy concerning the correlation of the adsorption

capacity with the structural parameter. In fact, the hydrogen adsorption capacity has been proposed to correlate either with the specific surface area (SSA), the total volume of micropores, or the volume of the narrow micropores.20,24,28 As expected for a physisorption process, the role of the porous structure highly depends both on the adsorption temperature (77, 87, or 298 K) and the total pressure achieved (0.1-50 MPa).22,25 From the results described in Figure 5 it is clear that at cryogenic temperature (77 K) and atmospheric pressure (0.1 MPa) the narrow microporosity and not the total surface area available defines the total adsorption capacity on carbon molecular sieves. In summary, the better correlation observed between the hydrogen adsorption capacity at 77 K and atmospheric pressure (0.1 MPa) and the total volume of narrow micropores clearly confirms the importance of a characteristic type of porosity for hydrogen storage. Furthermore, our results confirm experimentally that the optimum packing density for hydrogen requires the presence of micropores around 0.60 nm. Consequently, the optimum carbon material for hydrogen storage at 77 K and atmospheric pressure (0.1 MPa) will require well-developed narrow micropores together with a narrow micropore size distribution. Conclusions A series of carbon molecular sieves (CMS) prepared from different polymer precursors have been characterized using adsorption of N2 (77 K) and CO2 (273 K) together with immersion calorimetry into liquids of different molecular dimensions. Experimental results shows that CMS cover a wide range of porosity going from pure microporous molecular sieves to samples containing large micropores together with a certain proportion of mesoporosity. H2 adsorption studies on these CMS at low temperature (77 K) and ambient pressure (0.1 MPa) show that hydrogen storage requires the presence of a well-developed microporosity associated with a high CO2 micropore volume (Vn). Additionally, narrow micropores (micropore size around 0.6 nm) seem to be the key factor in order to achieve an optimum packing density of the adsorbed hydrogen molecules. Acknowledgment Financial support from MEC (MAT2007-61734) and the Network of Excellence Insidepores (NMP3-CT2004-500895) is gratefully acknowledged. The authors also acknowledge Prof. Kaneko for supplying the high-resolution adsorption data on the nonporous reference carbon. J.S.A. acknowledges financial support from MEC, GV, and UA (RYC2137/06). Literature Cited

Figure 5. Correlation between the amount of H2 adsorbed (mg/g) and the volume of narrow micropores (Vn), obtained from the CO2 adsorption isotherms at 273 K (correlation coefficient R2 ) 0.9657).

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ReceiVed for reView January 19, 2009 ReVised manuscript receiVed May 21, 2009 Accepted June 12, 2009 IE900091N