Effects of compression on the textural properties of porous solids

Microporous and Mesoporous Materials 126 (2009) 291–301

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Effects of compression on the textural properties of porous solids J. Alcañiz-Monge *, G. Trautwein, M. Pérez-Cadenas, M.C. Román-Martínez Grupo de Materiales Carbonosos y Medio Ambiente, Dpto. Química Inorgánica Facultad de Ciencias, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain

a r t i c l e

i n f o

Article history: Received 9 December 2008 Received in revised form 10 June 2009 Accepted 13 June 2009 Available online 21 June 2009 Keywords: Adsorption Microporosity Mesoporosity Packing density Compression

a b s t r a c t This work deals with the analysis of the effects of compression in the textural properties, the crystallinity and the packing density of porous solids. It has been found that compression produces the decrease of both, the pore volume and the interparticle voids of porous solids. The reduction of the pore volume depends on the mechanical strength of the material. Activated carbons and inorganic porous oxides with a high mechanical strength, show a relatively low reduction of their pore volume, while porous solids in which the pore walls are constituted by organic frameworks, like MOF-5, have a lower mechanical strength and thus, their porous texture is largely affected by compression. In general terms, an increasing compressive pressure produces the removal of pores in the following sequence: mesopores, broad micropores and narrow micropores. Thus, compression of porous materials could be considered as a procedure to tailor the pore volume and the pore size distribution of porous solids. The results obtained in this work allow affirming that, in the preparation of adsorbent materials, together with the development of a high specific pore volume or a specific surface area, the behaviour upon compactatation must, undoubtedly, be taken into account. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction The void space existing within a solid defines its porous texture. The porous texture is behind the capability of some solids to adsorb molecules, that is, of being adsorbents, a very interesting property in industrial fields like the treatment of liquid and gaseous effluents [1,2], separation processes and gas storage. The number of natural solids with a developed and open porous texture is quite reduced (coals, clays, zeolites, Kieselgur, perlite), and thus, almost all of the industrial adsorbents must be prepared using different precursors and synthetic procedures [3]. Activated carbons are prepared by thermal treatment of carbonaceous precursors, either mixed with chemicals (alkaline hydroxides, ZnCl2, H3PO4), or under an oxidising atmosphere (H2O, CO2) [4]; zeolites are obtained by the sol–gel method and hydrothermal treatments [5]; mesoporous silica is produced by a template assisted method [6]; and the preparation of metal organic frameworks (MOFs) involves a solvothermal method [7]. When the adsorbents are going to be used for gas storage or as catalysts supports, the synthetic procedures focus on the development of a high specific surface area with a regular porous network of certain dimensions, and a narrow distribution of pore sizes. In this area of Materials Science, the literature shows that it is common to make a competitive comparison between the different kinds of adsorbents. Usually, this comparison is based on the spe* Corresponding author. Fax: +34 965903454. E-mail address: [email protected] (J. Alcañiz-Monge). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.06.020

cific values (i.e. per adsorbent gram) of pore volume or surface area. However, it must be considered that in the practical application, the adsorbent must be packed inside a definite space (a column or a cartridge) and depending on its packing density the amount of solid used can be different. Because of that, it is more correct to make the comparison in volumetric basis, that is, taking into account the available porosity per container volume. Usually, as a common industrial practice, the adsorbent powders are compacted under external pressure into pellets or monoliths in order to increase their packing density (qpaq). It must be remembered that the pore texture is related with the void space inside the solids, which could be affected by this compression step. This important aspect has been scarcely analysed in the literature, almost all the published studies related with the synthesis of adsorbents overlook this point. The present work analyses the effect of the compactation pressure on the following properties of several adsorbents: the packing density, the porous texture and the volumetric adsorption capacity. The selected adsorbents show significant differences in the skeletal chemical composition and in the pore morphology. 2. Experimental 2.1. Materials The adsorbents used in this study are: the commercially available powdered activated carbon West (from Westvaco SA-30), the activated carbon fibre A20 (from Osaka Gas Co.) and five sam-

292

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

ples prepared in our laboratory using published synthetic procedures. These samples are: a zeolite NaA (Z NA) [8], a mesoporous silica (MCM-41) [6], ammonium phosphomolybdate ((NH4)3PMo12O40 – POM) [9] and two porous metal organic frameworks (MOF-5 [10] and HKUST-1 [11]). For each material, a series of compacted samples was prepared by uniaxial compression of a given amount (around 0.25 g) in a mould (1.45 cm diameter) at applied forces from 0 to 68 KN. The nomenclature of the compressed samples includes the applied pressure (i.e. sample West compacted using a pressure of 180 MPa is named West 180). The packing density (qpaq) of each compacted sample was determined. Measurements were repeated three times and the obtained values have an error lower than 5%. 2.2. Characterization The analysis of the porous texture was carried out by N2 adsorption at 77 K and CO2 adsorption at 273 K. Adsorption isotherms were determined using an Autosorb 6 equipment. The samples were degassed at 523 K (excepting MOFs and POM that were degassed at 423 K) under vacuum (1 Pa) (4 h). The distribution of specific pore volumes was calculated as follows [13]: (i) the volume of narrow micropores (pore size < 0.7 nm) was calculated by applying the Dubinin–Radushkevich (DR) equation [12] to the CO2 adsorption data at relative pressures <0.015; (ii) the total micropore volume (pore size < 2 nm), which includes the volume of the narrow micropores and of supermicropores, was calculated by applying the DR equation to the N2 adsorption data at relative pressures <0.14 and (iii) the volume of mesopores was determined by applying the BJH-method to the N2 adsorption data [14]. The specific surface area was determined using the BET [15] (Brunauer–Emmett–Teller) equation. The pore size distribution (PSD) was calculated using the density functional theory (DFT) method, applied to the N2 adsorption data [16]. The skeletal density (qHe) was determined by helium pycnometry (AccuPyc 1330 Pycnometer; Micromeritics). The envelope (particle) density (qHg) was measured by mercury pycnometry. In both cases, 0.5 g of sample previously degassed at 400 K, under vacuum (1 mmHg) overnight, were used. Bulk density (qbk) was measured using a graduated cylinder to determine the bulk volume of 2–3 g of the dry adsorbents. The crystalline structure of the samples (original and compressed) was examined by X-ray powder diffraction (XRD; Seifert diffractometer JSO Debye-Flex 2002, with Cu Ka radiation). The pieces resulting from the compression of adsorbent particles were observed by SEM (Hitachi S-3000N). The crystalline structure of sample MCM-41 and the corresponding compacted sample at 420 MPa were also observed by TEM (JEOL; JEM-2010). The compressive strengths of the adsorbent pieces were determined from the pressure at which they completely fail in compression tests (Instrom 4411).

3. Results and discussion 3.1. Densities Table 1 shows the values of the skeletal, particle and bulk densities, the specific pore volume and the specific interparticle void volume, determined for the different adsorbents used in this work. First of all, it must be noted that the skeletal densities determined are very close to those found in the literature for these substances. This means that the samples do not contain closed porosity. The POM sample has a skeletal density noticeably higher than that of the rest of the samples, which show very similar values of the skeletal density. The high skeletal density of POM ((NH4)3PMo12O40) is due to both, its chemical composition, as it contains a heavy element (Mo), and its crystalline structure, formed by large condensed units (Keggin structures) [17]. Particle density is quite different for the different samples. It must be remembered that the volume used to determine the particle density includes the skeletal volume and the pore volume. Thus, the total specific pore volume (mp) can be calculated from the values of specific skeletal and particle densities, according to the expression:

mp ¼ ð1=qHg Þ  ð1=qHe Þ The specific total pore volume calculated in this way is included in Table 1. The obtained values reveal that the selected adsorbents show noticeable differences in this aspect. It can be observed that there are samples with a low specific total pore volume (POM and Z NA), samples with intermediate values of specific total pore volume (HKUSK-1), and samples with a quite developed specific total pore volume (West, A20, MOF-5, MCM-41). The first inspection of the bulk density values indicates that they seem to be somehow arbitrary. Thus, samples with a similar particle density (A20 and MOF-5) have very different bulk density. Contrarily, samples with different particle density (Z NA and HKUST-1) show a similar bulk density. This is because the volume used to calculate the bulk density includes apart of the skeletal volume and the total pore volume, the interparticle void volume (Vip), which is closely related to the geometry and size of the particles. The specific interparticle void volume of powdered or fibrous (A20) adsorbents can be calculated from the values of the bulk density and the particle density according to the expression:

V ip ¼ ð1=qbk Þ  ð1=qHg Þ Sample POM shows the highest bulk density due to the high particle density and the low specific interparticle void volume (0.34 cm3/g), that is in agreement with the low particle size (50 nm) determined by TEM. On the other hand, sample MCM-41 shows the lowest bulk density, that is consequence of its low particle density and its high specific interparticle void volume (2.7 cm3/g). This aspect will be analysed in Section 3.4.

Table 1 Density, specific total pore volume and interparticle void volume of the adsorbents.

Skeletal densitya Particle densityb Bulk densityc Specific pore volumed Interparticle void volumee a b c d e

qHe (g/cm3). qHg (g/cm3). qbk (g/cm3). mp = (1/qHg)  (1/qHe) (cm3/g). Vip = (1/qbk)  (1/qHg) (cm3/g).

POM

Z NA

MCM-41

West

A20

MOF-5

HKUST-1

3.60 2.23 1.26 0.17 0.34

2.23 1.47 0.61 0.23 0.96

2.11 0.69 0.24 0.97 2.73

2.00 0.52 0.34 1.43 1.01

1.99 0.66 0.24 1.02 2.66

1.92 0.61 0.32 1.11 1.49

1.90 0.84 0.55 0.67 0.63

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

Fig. 1 shows the values of the packing density of the adsorbents compressed at different pressures, up to 420 MPa, versus the compression pressure. The packing density of the non-compressed sample (P = 0) is its bulk density. As expected, compression produces an increase of the packing density of the samples. It can be observed that the evolution of the packing density with the compression pressure is different for the different samples. Thus, in the case of samples West and A20, the increase of the packing density with compression pressure is very low and for samples HKUST-1 and Z NA there is a slight positive slope. For the rest of the samples, a marked increase of the packing density in a certain pressure range can be observed: for MOF-5 from 60 to 180 MPa, for MCM-41 from 180 to 300 MPa and for POM from 300 to 420 MPa. The increase of the packing density must be due to two effects: reduction of the specific interparticle void and decrease of the specific total pore volume. The increase of the packing density at low pressure (60 MPa) must be related to the reduction of the specific interparticle void volume, while the changes at higher pressures must be related to the mechanical strength of the porous solid, that is different for the different materials. The specific interparticle void volume and the specific total pore volume were determined in a different way for compressed and not compressed samples. As it has been commented above, in the case of uncompressed adsorbents the specific particle density determined by mercury pycnometry was used. For compressed samples, the precise specific total pore volume should be determined ‘‘in situ” (that is, when the sample is submitted to pressure). This cannot be experimentally made. Carbonaceous materials that do not consolidate, that is, they remain as a powder after compression, can be analysed by mercury pycnometry, assuming that the particle size does not change when pressure is released. POM adsorbent, which gives a brittle disc-like pellet that breaks easily, is in a similar situation. In the case of adsorbents that consolidate during compression (samples MOFs, Z Na and MCM-41), they form a piece with certain mechanical consistence (this point is commented in Section 3.3), it can be assumed that the particle size does not change after pressure release. However, mercury pycnometry results are not reliable due to the uncertain access of mercury to the whole piece. Because of these differences between the samples and the above considerations about the difficulties in mercury pycnometry measurements in some cases, the specific total pore volume (Vtotal) of compressed samples has been obtained from the N2 adsorption isotherms, presented in the next section.

POM A20

Paking density (g/cm3)

2.5

Z NA MOF-5

MCM-41 HKUST-1

West

2 1.5 1 0.5 0 0

60

120

180

240

300

360

Pressure (MPa) Fig. 1. Packing density versus compression pressure.

420

293

3.2. Pore texture characterization of original and compacted samples The analysis of the pore texture of original and compacted samples was carried out by N2 adsorption at 77 K and CO2 adsorption at 273 K. Table 2 collects the results of the characterization of the porous texture derived from the N2 and CO2 adsorption isotherms. Figs. 2–4 show the N2 adsorption–desorption isotherms obtained at 77 K for the original adsorbents and some representative examples of the series of compacted samples obtained from them. It must be pointed out that, for the sake of clarity, Figs. 2– 4 contain only isotherms of samples that show appreciable differences respect to samples of the same series obtained at different compression pressure. The important details of these isotherms are the magnitude of adsorption at P/P0 < 0.3, which is related to the specific micropore volume, the sharpness of the isotherm knee, related with the micropore size distribution, the slope at P/P0 > 0.3 and the presence of a hysteresis loop, which is due to the presence of mesopores. Fig. 2 shows the isotherms of the original mesoporous silica MCM-41 and powdered activated carbon West and of some of the compressed samples obtained from them. Both adsorbents show a high adsorption capacity at low P/P0 values, indicating their microporous character [18]. They have, as well, a high specific mesopore volume (Table 2): sample MCM-41 with a well defined mesopore size [19], as deduced from the sharp step of the isotherm in the region 0.31–0.38 P/P0, and sample West with a heterogeneous mesopore size distribution, denoted by the high slope of and the H4 type hysteresis loop, that is characteristic of adsorbents containing slit-shaped pores [18]. I can be mentioned that the absence of hysteresis loop in the N2 adsorption isotherm of sample MCM41 is indicative of the complete reversibility of the adsorption in this sample, in agreement with the results reported in the literature [19]. Results presented in Fig. 2 show that compression produces a noticeable modification of the pore texture of the adsorbents, and that it is different for the two samples. It can be observed that in the case of the activated carbon West a compressive pressure higher than 420 MPa is necessary to produce a relatively low decrease of its pore volume (about 23% reduction, Table 2), with a slight variation of the hysteresis loop. These results contrast with those obtained by Hou et al. [20] dealing with the compactation of activated carbons prepared by a template method, that report a micropore volume reduction around 40–55%. This high micropore volume reduction upon compression is due to a low mechanical strength, which is related with the low particle size (around 500 nm) and the preparation method. Activated carbon West was obtained by chemical activation of a relatively hard wood-based char of particle size close to 0.5 mm. Carbonization of the raw material gives a graphitic microstructure [3,4] which, together with the relatively large particle size, provides a high mechanical strength, and thus, mechanical stability of the porous texture. Sample MCM-41 shows a considerable mechanical strength up to 180 MPa. By increasing the compressive pressure up to 300 MPa, a sudden and important modification of the porous texture is observed: 41% reduction of the specific micropore volume and 60% reduction of the specific mesopore volume (Table 2). A further increase of the pressure produces a less pronounced effect on the porous texture (the reduction of the total specific pore volume is about 67%, Table 2). It must be noted that, in spite of the considerable reduction of the specific mesopores volume, the N2 adsorption isotherms of the compressed samples do not exhibit either a hysteresis loop. This fact suggests that compression does not affect the structure of the remaining mesopores. Fig. 3 shows the N2 adsorption isotherms of samples POM and Z NA, that contain both micro and mesopores. The mesoporous texture of sample POM can be defined by the H3 type hysteresis loop,

294

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

Table 2 Porous texture properties of original and compacted adsorbents in a gravimetric basic. Sample

SBET (m2/g)

V DRCO2 a (cm3/g)

V DRN2 b (cm3/g)

VMesopore (cm3/g)

Vtotalc (cm3/g)

POM POM 60 POM 180 POM 300 POM 420 Z NA Z NA 60 Z NA180 Z NA 300 Z NA 420 MCM-41 MCM-41 60 MCM-41 180 MCM-41 300 MCM-41 420 West West 60 West 180 West 300 West 420 A20 A20 60 A20 180 A20 300 A20 420 MOF-5 MOF-5 60 MOF-5 180 MOF-5 300 MOF-5 420 HKUST-1 HKUST-1 60 HKUST-1 180 HKUST-1 300 HKUST-1 420

204 211 177 152 125 55 43 44 45 44 1065 1064 1035 590 555 1576 1583 1567 1560 1430 2017 2014 2013 2011 2014 2160 1830 275 61 45 1145 1070 470 420 425

0.088 0.088 0.087 0.089 0.049 0.171 0.149 0.122 0.137 0.153 0.27 0.27 0.27 0.19 0.19 0.33 0.34 0.34 0.33 0.34 0.56 0.55 0.54 0.55 0.55 0.25 0.21 0.07 0.03 0.03 0.66 0.63 0.34 0.29 0.28

0.088 0.091 0.083 0.072 0.060 0.028 0.022 0.022 0.022 0.022 0.48 0.48 0.48 0.28 0.26 0.71 0.72 0.70 0.70 0.64 1.01 1.00 0.99 1.00 1.00 1.06 0.88 0.14 0.03 0.02 0.55 0.50 0.24 0.21 0.22

0.082 0.095 0.091 0.091 0.052 0.058 0.039 0.045 0.043 0.042 0.39 0.38 0.31 0.16 0.11 0.49 0.50 0.47 0.46 0.38 – – – – – – – – –

0.17 0.18 0.17 0.16 0.11 0.23 0.19 0.17 0.18 0.20 0.97 0.94 0.84 0.45 0.39 1.43 1.41 1.37 1.34 1.13 1.02 1.00 1.00 1.00 1.01 1.11 0.92 0.16 0.04 0.04 0.66 0.63 0.34 0.29 0.28

a b c

– – – – –

Narrow micropore volume (<0.7 nm). Total micropore volume. Total pore volume.

1000 West

900

N2 ads (cm3 /g) STP

800

West 300

700

West 420

600

MCM-41

500 MCM-41 180

400 MCM-41 300

300 200

MCM-41 420

100 0

0

0.1

0.2

0.3

0.4

0.5 P/Po

0.6

0.7

0.8

0.9

1

Fig. 2. Isotherms of N2 adsorption at 77 K of the original adsorbents MCM-41 and West, and some compacted samples obtained from them.

characteristic of aggregates of platy particles [18]. In the case of sample Z NA, the N2 adsorption isotherm also shows a hysteresis loop, type H2, characteristic of an interconnected network of pores, usually found in inorganic oxide gels, like Z Na. The relatively low

adsorption capacity of these two adsorbents, in particular of Z NA, is indicative of a low specific pore volume. Sample Z NA shows a very low decrease of the adsorption capacity even after compression at 420 MPa. There are no varia-

295

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

120

N2 ads (cm3/g) STP

100

80

60

POM POM 300

40 POM 420 Z NA

20

Z NA 180 & Z NA 420 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

P/Po Fig. 3. Isotherms of N2 adsorption at 77 K of the original adsorbents POM and Z NA and some compacted samples obtained from them.

800 MOF-5 700

A20 A20 420

600

N2 ads (cm3 /g) STP

MOF-5 60 500 400

HKUST-1

300

HKUST-1 60 HKUST-1 180

200

HKUST-1 420 MOF-5 180

100

MOF-5 300 0 0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 4. Isotherms of N2 adsorption at 77 K of the original adsorbents A20, MOF-5 and HKUST-1 and some compacted samples obtained from them.

tions in the hysteresis loop, which allow discarding any possible source of mesoporosity related with the interparticle void. In the case of sample POM, 300 MPa are needed to produce an appreciable modification of the porous texture and it is mainly located in the micropore region (around 20% of the total specific micropore volume is lost, see Table 2). The specific mesopore volume even increases (around 10%). Compression of POM at 420 MPa produces an important decrease of the specific meso and micropore volumes (37% and 31%, respectively, Table 2), with some modification of the hysteresis loop. These results indicate that due to a high mechanical strength, the porous texture of these two samples is quite stable. The N2 adsorption isotherms of the essentially microporous adsorbents: activated carbon fibre A20, MOF-5 and HKUST-1

and the corresponding compressed samples are shown in Fig. 4. As expected, the isotherms of the original materials are type I, following the IUPAC classification [18], indicative of microporosity. The isotherms are parallel to the x axis at P/ P0 > 0.3, confirming the absence of meso and macropores in the samples. It must be pointed out that samples A20 and MOF-5 show a very high specific adsorption capacity, indicative of a high specific micropore volume. The analysis of the sharpness of the isotherms knee indicated that the micropore size distribution is narrow in sample HKUST-1, whereas it is broader in the case of sample A20. Regarding the stability of the porous texture, the following comments can be outlined: MOF-5 is the adsorbent with the lowest mechanical strength, at 60 MPa an important decrease of the

296

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

specific micropore volume (around 20%, Table 2) is observed and a higher compression pressure produces the collapse of the porous texture. This point was supported by the helium density of the compacted samples, that is similar to that of the uncompressed sample, indicating a scarce closed porosity. On the other side, sample A20 does not show any decrease of the pore volume under the highest compression pressure used in this study (420 MPa). This is particularly relevant taking into account that this sample has a high specific micropore volume (1.01 cm3/g). Finally, sample HKUST-1, that is also a MOF adsorbent like MOF-5, has a porous texture with a good stability, since a compressive pressure of around 180 MPa is needed to produce a decrease of the pore volume. And, contrarily to what happens with sample MOF-5, under a compressive pressure of 420 MPa the porous texture is similar to that of the sample compressed at 180 MPa. The pore size distribution of the original adsorbents and the compacted samples has been determined by applying the Density Functional Theory method to the adsorption data [16]. The pore size distribution (PSD) obtained from the adsorption isotherms presented in Figs. 2–4 is included as Supplementary material, Figs. S1–S5. Fig. S1 shows the pore size distribution of the series of MOF-5 samples (derived from N2 adsorption isotherms of Fig. 4). The pore size distribution of the original sample and of the sample compacted at 60 MPa are very similar, with a shift of the main peak from 1.2 nm on original MOF-5 to 1.4 nm in the sample compacted a 60 MPa. For the sample compressed at 180 MPa the profile of the pore size distribution shows a low intensity, in agreement with the collapse of the porosity mentioned above. The PSD obtained for the series of samples HKUST-1 is shown in Fig. S2. Compression at 60 MPa does not modify the pore size distribution of the original HKUST-1, compression at 180 MPa produces an important decrease of the intensity of the PSD curve, but the main peak remains almost at the same position (shift from 1.02 nm in the original sample to 1.07 nm in HKUST-1 180); and compression at 420 MPa produces the removal of pores of about 1.0 nm and the new maxima is located at about 1.29 nm. Fig. S3 shows the pore size distribution curves for the MCM-41 series. In this case a bimodal pore size distribution with two main peaks located at about 1.40 nm (related with micropores) and at 3.17 nm (corresponding to mesopores) has been found. With compression, the intensity of both peaks is reduced and their relative intensity changes: the second one (at 3.17 nm) is more affected, whereas the first peak shows a lower variation. This indicates that compression of MCM-41 reduces the mesopore volume more than the micropore volume. Interestingly, these variations of the intensity of main peak are not associated with variations of the pore size. Figs. S4 and S5 show the pore size distribution curves obtained for the series of samples POM and West. In these samples compression, even at the highest pressure used in this work (420 MPa), produces small changes in the profiles of the pore size distribution curves. That is, as previously commented, the porous texture of these samples is scarcely affected by compression. Further information about the size of the pores that are modified by the effects of compression can be obtained from the analysis of the specific pore volumes determined from N2 and CO2 adsorption data (Table 2). It is important to remind that the specific micropore volume obtained from N2 adsorption data corresponds to the whole range of micropores (pore size up to 2 nm), while CO2 adsorption provides only information about narrow micropores (pore size < 0.7 nm) [13,21,22]. The decrease in the capacity for N2 adsorption of most of the compressed samples (Figs. 2–4) is well correlated with the decrease of the specific volume of narrow micropores, total micropores and mesopores (see also Figs. S1–S5). In the case of sample

A20, almost not affected by compression, data of Table 2 show that the compressed samples have similar pore volumes. Data presented in Table 2 indicate that the type of pores affected by compression depends on the adsorbent. Thus, in the case of MCM-41, the specific mesopore volume decreases after compactation at 180 MPa, while the specific narrow and total micropore volume are not modified (see also Fig. S3). This fact must be remarked in relation with the controversy around the presence of micropores in these materials. Thus, some researches indicated that MCM-41 is an essentially mesoporous solid which does not contain micropores [19,23], and some others consider that is the material contains both meso and micropores [24]. In the present work it has been observed that a further increase of the compressive pressure produces a reduction of the whole porosity of sample MCM-41, but the specific mesopore volume suffers a larger decrease than the specific micropore volume (72% versus 45%). The different effect of compression in these two types of pores together with the fact that the pore size distribution remains unchanged upon compression, clearly shows the independence of the micropore and mesopore networks of sample MCM-41. Hence, these results support the presence of micropores in MCM-41. Sample POM suffers a decrease of the total specific micropore volume at compressive pressures of 300 MPa, whereas under this pressure the specific narrow micropore volume and the specific mesopore volume are unaffected (see also Fig. S4). Reduction of this type of pores takes place at 420 MPa. In the case of activated carbon West, a decrease of the total specific micropore and mesopore volume occurs at the highest compressive pressure used, while the specific narrow micropore volume is unaffected (see also Fig. S5). Sample Z NA shows an interesting behaviour: a continuous decrease of the specific total pore volume and specific narrow micropore volume as the compressive pressure increases up to 180 MPa, and an unusual increase of the narrow micropore volume with a further rise of the compressive pressure up to 420 MPa. This fact will be analysed in Section 3.3. Additionally, the results of Table 2 explain the evolution of the packing density with increasing pressure in the different samples (Fig. 1). Thus, the initial increase of the packing density, upon compression at 60 MPa, is clearly related with the removal of the interparticle void, since, excepting the case of sample MOF-5 which shows a considerable reduction of the total specific pore volume (0.2 cm3/g), the reduction of the total specific pore volume is very low (around 0.04–0.02 cm3/g). The constant packing density shown by the carbonaceous adsorbents A20 and West is in agreement with the light decrease of their total specific pore volume. On the other hand, as it has been commented in the previous section, the characteristic increase of the packing density shown by each adsorbent at a certain value of compressive pressure correlates well with the decrease of a particular specific pore volume. Thus, the considerable increase of the density of sample POM after compression at 420 MPa is related with the decrease of its specific micropore volume and specific mesopore volume. The sudden increase of the packing density of sample MCM-41 by raising the compressive pressure from 180 to 300 MPa, agrees with the notable decrease of the total specific pore volume. The high increase of the packing density of sample MOF-5 when the compressive pressure increases from 60 to 180 MPa is clearly related with the collapse of the micropore network. 3.3. Structural characterization of original and compacted samples Although the objective of this work was not to analyse the consolidation of porous powders, it seems appropriated to indicate, from a semi-quantitative point of view, the mechanical strength of the obtained pieces. As previously commented (Section 3.1), carbonaceous adsorbents do not consolidate. Sample POM gives a

297

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

100 Z NA Z NA 60 Z NA 180 Z NA 420

90 80

Intensity (%)

70 60 50 40 30 20 10 0 10

20

30 40 2θ θ (degrees)

50

60

Fig. 6. XRD of the original Z NA and samples compacted at 60, 180 and 420 MPa.

100 90 70 60 MOF-5 180

50 40 30 20

Intensity (a.u.)

80 Intensity (%)

brittle disc-like pellet, which breaks totally after a Drop shatter test (ASTM D440-07) cycle. The rest of samples give hard disc-like pellets, that do not break after 10 cycles of the Drop shatter test. The compressive strength of pieces compacted at 180 MPa is: MOF-5 2 ± 0.5 MPa, HKUST-1 5 ± 0.7 MPa, MCM-41 32 ± 5 MPa and Z NA 40 ± 5 MPa. As commented in the previous section, the compression of porous solids produces a modification of their porous structure. In this sense, it is interesting to know if the compression affects also the crystalline structure. In order to asses this point, porous samples (original and compacted) have been analysed by XRD. Figs. 5–8 contain the XRD patterns of original and compacted samples of the POM, Z NA, MOF-5 and MCM-41 series. It must be pointed out that samples West, A20 and HKUST-1 have been also analysed by this technique, but no clear XRD peaks were observed, what is a consequence of their disorganized structure. According to the results shown in Fig. 5, compression does not produce any change in the crystalline structure of sample POM. Neither the position nor the intensity of the diffraction peaks are affected. Similarly, the compression of Z NA at 60 MPa leaves the XRD pattern of this sample unchanged. However, higher compression pressures lead to a clear decrease of the intensity of the XRD peaks at 2h lower than 30°. The higher the pressure, the lower the intensity (Fig. 6). The intensity of the diffraction peaks depends on the nature of the atoms, on their location in the unit cell and on the cells orientation in the crystalline structure. It is clear that the compression carried out in this work does not affect neither the chemical composition, nor the crystalline cell. Hence, the observed variation of the XRD peaks intensity must be related with a slight change in the orientation of the unit cell that configures the crystalline structure of sodalite cages on Z NA [5,8,25]. It is reasonable to think that this light movement must be related with the direction in which compression exerts a higher pressure on the crystalline structure. This modification of the crystalline structure of Z NA with pressure can explain the unusual increase of the specific narrow micropore volume that has been observed in the sample compressed at 420 MPa. Translation of atoms could open the microstructure of Z Na in some directions, making it more accessible to CO2 molecules. In the case of sample MOF-5, compression at 180 MPa produces a more disorganized solid and broad bands appear in the XRD pattern (Fig. 7). These results, together with those of the porous texture characterization (Fig. 4 and Table 2), are in agreement with a collapse of the microstructure of the original MOF-5 upon compression.

MOF-5

10 0 5

15

25

35 2θ θ (degrees)

45

55

Fig. 7. XRD of the original MOF-5 and sample compacted at 180 MPa.

Fig. 8 contains the XRD patterns of the original and compacted MCM-41 samples. Original MCM-41 shows a clear peak around 2.1°, that is related to mesopores and remains after compression at 180 MPa [19]. However, after compression at 420 MPa, a drastic decrease of the intensity of the mentioned peak is observed. These results, together with those obtained by adsorption (Fig. 4 and Table 2), are in agreement with a collapse of the mesopore network upon compression at pressures higher than 180 MPa.

100 100

90

POM

80

POM 180

MCM-41

MCM-41 180

MCM-41 420

80

POM 300

Intensity (%)

Intensity (%)

70

90

60 50 40 30

70 60 50 40 30

20

20

10

10

0 10

20

30

40

50

60

2θ θ (degrees) Fig. 5. XRD of the original POM and samples compacted at 180 and 300 MPa.

0

1

2

3 4 2θ θ (degrees)

5

6

Fig. 8. XRD of the original MCM-41 and samples compacted at 180 and 420 MPa.

298

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

SEM analysis was carried out to analyse the morphology of the compacted samples. It has to be remembered that, with the exception of carbonaceous materials, the compacted samples form a consolidate piece. The analysis by SEM of HKUST-1 and POM pieces showed that they are compacted solids, without appreciable porosity (in the lm range) (Figs. 9 and 10, respectively). Sample HKUST1 compressed at 180 MPa shows a very smooth surface, while sample POM shows a rough surface, due to the presence of crystalline particles (mean particle size around 50 nm). Some of these particles, are still visible after compression at 180 MPa, which is in agreement with the mechanical stability of this sample deduced form adsorption and XRD measurements. The MOF-5 series shows, as well, a quite smooth surface. On the other hand, pieces of MCM41 and Z NA series, show an appreciable porosity (Figs. 11 and 12, respectively). Thus, even after having been compressed under high pressure, an important interparticular void remains in these samples. This is related with the high mechanical strength of the silicate and aluminosilicate particles.

Fig. 13 shows TEM images of the original and compacted (at 420 MPa) MCM-41 samples. This Fig. gives a good visual example of the collapse of the main part of the mesoporosity in the compacted MCM-41, without change in the mean size of the remaining mesopores. This is in agreement with the discussion of the adsorption data presented in Section 3.2. and the XRD results commented above. 3.4. Volumetric adsorption capacities of original and compacted samples Taking into account the skeletal density (qHe, Table 1), the packing density (qpaq, Fig. 1) and the total specific pore volume (Vtotal, Table 2), the specific interparticle void volume (Vip) of the original and compacted adsorbents can be calculated according to the expression:

V ip ¼ ð1=qpaq Þ  ð1=qHe Þ  V Total

Fig. 9. SEM images of HKUST-1 compacted at 180 MPa.

Fig. 10. SEM images of POM compacted at 180 MPa.

Fig. 11. SEM images of MCM-41 compacted at 420 MPa.

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

299

Fig. 12. SEM images of Z NA compacted at 420 MPa.

Fig. 13. TEM images of MCM-41 samples: (a) original, (b) compacted at 420 MPa.

The Vip values calculated for each series of compacted samples are represented in Fig. 14. Fig. 14 shows that, with the exception of the POM sample, all adsorbents have a high specific interparticle void volume when they are packed without compression inside a container (deposit or column). The specific interparticle void volume is particularly high in the case of samples MCM-41 and A20. This is drawback for them to be used as adsorbents since it implies that an important volume fraction of the container is not occupied. Hence, a first

objective of the compactation is that the adsorbent occupies more efficiently the container volume. As it can be appreciated in Fig. 14, compression at 60 MPa is enough to produce a considerable decrease of the specific interparticle void volume in samples POM, HKUST-1 and MOF-5. However for adsorbents MCM-41, A20 and Z NA a higher compressing pressure is needed to reduce the specific interparticle void volume. This different behaviour is related with differences in the mechanical strength, the geometrical form and/or the synthetic processing of the adsorbents. In the case of A20 it is a consequence of its fibrous morphology, for which a good packing is difficult to obtain. As the pressure increases, fibres break and the fragments occupy the voids. In the case of MCM-41 and Z NA samples, the hydrothermal synthesis conduces to the development of aggregates of nanocrystals, with voids inside, that contribute, as well, to the interparticle void [26]. Thus, to decrease the intraparticle void on these samples, the crystalline particles must break. This is in agreement with the high compressive pressure needed to reduce specific interparticle void volume of samples MCM-41. The last step in the preparation of MCM-41 is a calcination treatment at 823 K [6] that produces a strong interaction between the particles and a high mechanical resistance. Because of that, an important specific interparticle void volume remains after compression (Figs. 11 and 12). Obviously, the goal of the compactation process must be to reduce the specific interparticle void volume without destroying the porous texture. In the present work it has been found (part 3.2) that after compactation the reduction of the specific pore volume and the type of pores destroyed is different for the different adsorbents investigated. That is, the compactation pressure for which the reduction of specific interparticle void volume and the modification of the porous texture are optimal is different for the different adsorbents. This aspect can be better analysed if the specific pore volume (Table 2) is expressed in a volumetric basis, that is as volumetric adsorption capacity (Table 3). Data of Table 3 indicate that after compactation the volumetric adsorption capacity increases, for all the samples, between 40% and 170% respect the value corresponding to the original sorbent. As an example, the original samples MCM-41 and A20 show volumetric micropore volumes of 0.12 cm3/cm3 and 0.24 cm3/cm3, respectively, that after compactation at 180 MPa rise to 0.31 cm3/cm3 and 0.59 cm3/cm3, respectively. On the other hand, as commented in the introduction, the volumetric basis seems to be the most appropriated one to quantify the adsorption capacity of the adsorbents. In this way, the suitability of the adsorbents to be used for gas storage or as catalyst supports can be better checked. The most illustrative example is sample POM. It has low specific micropore and mesopore volumes (Table 2), but when these values are expressed in a volumetric basis, they are very close to those corresponding to mesoporous samples like West and MCM-41 (Table 3). Hence, an important aspect

300

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301

Interparticle void volume (cm3 /g)

3 MCM-41

A20

MOF-5

West

Z NA

HKUST-1

POM

2.5 2 1.5 1 0.5 0 0

50

100

150

200

250

300

350

Pressure (MPa) Fig. 14. Specific interparticle void volume versus compressive pressure.

Table 3 Pore volume of original and compacted samples in a volumetric basis. Sample

V DRCO2 a (cm3/cm3)

V DRN2 b (cm3/cm3)

VMesopore (cm3/cm3)

Vtotalc (cm3/cm3)

POM POM 60 POM 180 POM 300 POM 420 Z NA Z NA 60 Z NA 180 Z NA 300 Z NA 420 MCM-41 MCM-41 60 MCM-41 180 MCM-41 300 MCM-41 420 West West 60 West 180 West 300 West 420 A20 A20 60 A20 180 A20 300 A20 420 MOF-5 MOF-5 60 MOF-5 180 MOF-5 300 MOF-5 420 HKUST-1 HKUST-1 60 HKUST-1 180 HKUST-1 300 HKUST-1 420

0.11 0.17 0.19 0.20 0.12 0.11 0.13 0.14 0.18 0.21 0.06 0.12 0.17 0.19 0.21 0.11 0.15 0.17 0.17 0.20 0.13 0.30 0.32 0.32 0.35 0.08 0.13 0.12 0.07 0.05 0.36 0.57 0.39 0.39 0.42

0.11 0.17 0.18 0.16 0.15 0.02 0.02 0.02 0.03 0.03 0.12 0.21 0.31 0.28 0.29 0.25 0.33 0.35 0.35 0.37 0.24 0.50 0.59 0.59 0.64 0.34 0.56 0.22 0.05 0.05 0.30 0.45 0.27 0.29 0.33

0.10 0.18 0.20 0.20 0.13 0.04 0.04 0.05 0.06 0.06 0.09 0.16 0.20 0.16 0.12 0.17 0.31 0.24 0.34 0.22 – – – – – – – – – – – – – – –

0.21 0.35 0.38 0.37 0.28 0.14 0.17 0.19 0.23 0.27 0.23 0.41 0.54 0.45 0.44 0.49 0.64 0.70 0.69 0.68 0.25 0.50 0.60 0.60 0.65 0.35 0.59 0.26 0.07 0.07 0.36 0.57 0.39 0.39 0.42

a b c

Narrow micropore volume (<0.7 nm). Total micropore volume. Total pore volume.

to be considered in the preparation of an adsorbent, apart of developing a high specific pore volume or specific surface area, is to accomplish an effective compactation. 4. Conclusions Compression of the porous solids investigated in this work produces the decrease of both, the pore volume and the interparticle

void. The effects of compression are different for the different adsorbents and depend on their mechanical strength, the particle size and the occurrence or not of consolidation. Activated carbons and inorganic porous oxides, with a high mechanical strength, show a relatively low reduction of the specific pore volume, while the porous texture of solids made of organic frameworks, with a lower mechanical resistance, like MOF-5, is largely affected by compression. In general terms, as the compressive pressure increases, the specific pore volume decreases and the destruction of pores with increasing pressure is as follows: first the mesopores, then the micropores, and finally the narrow micropores. This trend is quite interesting in the case of sample MCM-41, as it reveals that in this sample the micropore network is independent of the mesopore network. The effect of compactation in the specific interparticle void of the different adsorbents can be related to the mechanical strength of the adsorbent, to the particle size and to the preparation method. The volumetric basis has been shown to be the most appropriated to quantify the adsorption capacity of the adsorbent. A balance of the interparticle void and the pore volume resulting after compactation (either in gravimetric or volumetric basis), allows to determine an optimal compression pressure, which is different for the different adsorbents. Adsorbents developed for industrial applications must have a high specific pore volume or specific surface area and to show a suitable compactation. Furthermore, compression of porous materials could be considered as part of the synthetic procedure used to tailor the pore volume and pore size distribution of porous solids. Acknowledgment The authors would like to thank the Generalitat Valenciana, projects GV/2007/144 and ARVIV/2007/063, for financial support.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2009.06.020. Pore size distribution (PSD) curves obtained by applying the Density Functional Theory method to the N2 adsorption data. Data for the series of compacted samples A20 and Z NA have not been included because these samples show small differences in N2 adsorption upon compression. References [1] A. Dabrowski (Ed.), Studies in Surface Science and Catalysis, vol. 120, Elsevier, Amsterdam, 1999 (2 vols.). [2] R.T. Yang, Adsorbents: Fundamentals & Applications, Wiley-Interscience, 2003. [3] F. Schüth, K.S.W. Sing, J. Weitkamp (Eds.), Handbook of Porous Solids, vol. 2, Wiley-VCH, Weinheim, 2002. [4] R.C. Bansal, M. Goyal, Activated Carbon Adsorption, Taylor & Francis, Boca Raton, FL, 2005. [5] D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry, and Use, John Wiley & Sons, Inc., USA, 1974. [6] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuti, J.S. Beck, Nature 359 (1992) 710. [7] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Nature 423 (2003) 705. [8] H. Robson, Verified Synthesis of Zeolitic Materials, second ed., Elsevier, Amsterdam, 2001. [9] D. Lapham, J.B. Moffat, Langmuir 7 (1991) 2273. [10] U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J. Pastré, J. Mater. Chem. 16 (2006) 626. [11] J.L.C. Rowsell, O.M. Yaghi, J. Am. Chem. Soc. 128 (2006) 1304. [12] M.M. Dubinin, in: P.L. Walker (Ed.), Chemistry and Physics of Carbon, vol. 2, Dekker, New York, 1966, p. 51. [13] F. Rodríguez-Reinoso, A. Linares-Solano, in: P.A. Thrower (Ed.), Chemistry and Physics of Carbon, vol. 21, Dekker, New York, 1988, p. 1. [14] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373.

J. Alcañiz-Monge et al. / Microporous and Mesoporous Materials 126 (2009) 291–301 [15] [16] [17] [18]

S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. C. Lastoskie, K.E. Gubbins, N. Quirke, J. Phys. Chem. 27 (1993) 4786. J.F. Keggin, Proc. Roy. Soc. Lond. A 144 (1934) 75. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [19] P.J. Branton, P.G. Hall, K.S.W. Sing, H. Reichert, F. Schüth, K.K. Unger, J. Chem. Soc., Faraday Trans. 90 (1994) 2965. [20] P. Hou, H. Orikasa, H. Itoi, H. Nishihara, T. Kyotani, Carbon 45 (2007) 2011. [21] D. Cazorla-Amorós, J. Alcañiz-Monge, A. Linares-Solano, Langmuir 12 (1996) 2820.

301

[22] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. de la Casa-Lillo, A. Linares-Solano, Langmuir 14 (1998) 4589. [23] S. Storck, H. Bretinger, W.F. Maier, Appl. Catal. A: Gen. 174 (1998) 137. [24] (a) M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267; (b) Y. Long, T. Xu, Y. Sun, W. Dong, Langmuir 14 (1998) 6173; (c) A. Berenguer-Murcia, J. García-Martínez, D. Cazorla-Amorós, A. MartínezAlonso, J.M.D. Tascón, A. Linares-Solano, Stud. Surf. Sci. Catal. 144 (2002) 83. [25] L.B. McCusker, F. Liebau, G. Engelhardt, Pure Appl. Chem. 73 (2001) 381. [26] C.G. Sonwane, S.K. Bhatia, Langmuir 15 (1999) 2809.