Capillary condensation in templated nanoporous materials

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkov~iand P. NachtigaU(Editors) 9 2005 ElsevierB.V. All rights reserved.

695

Capillary condensation in templated nanoporous materials K. Morishige Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan To elucidate the capillary condensation phenomena of vapor within mesoporous materials, we measured the adsorption-desorption isotherms of nitrogen and other simple gases onto templated nanoporous materials (TNM) with well-defined porous structures over a wide temperature range. The temperature dependences of the chemical potential of different fluids, which were adsorbed in TNM at the condensation and evaporation pressures, indicate that the adsorption branch of the isotherm corresponds approximately to the true equilibrium, irrespective of the type of adsorbents. The results for KIT-6 and SBA-16 indicate that the interconnections among pores of almost the same size do not have a significant effect on the adsorption hysteresis and pore criticality, whereas interconnections among cagelike pores lead to the occurrence of cavitation on capillary evaporation. 1. INTRODUCTION Capillary condensation within mesoporous adsorbents is often distinguished by a distinct step in the adsorption isotherm accompanied by a hysteresis loop. The mechanism of the adsorption-desorption hysteresis has been the subject of a great deal of interest over several decades [1 ]. The conventional mesoporous materials such as controlled pore glasses and silica gels have an interconnected network of pores of varying shape, curvature, and size. Capillary condensation within these materials must be affected by the complex porous structures and thus the elucidation of the capillary condensation phenomena is very difficult. On the other hand, templated nanoporous materials such as MCM-41 and SBA-15 possess well-defined porous structures and are regarded as the most suitable model adsorbents currently available for verification of the theoretical predictions for various idealized pores. For unconnected cylindrical pores of MCM-41 and SBA-15, it has become clear that the hysteresis results from the metastability of a confined phase and the critical temperature of vapor-liquid equilibrium (pore critical temperature, Tcp) is different from the temperature at which the adsorption hysteresis disappears (hysteresis temperature, Th) [2-4]. The shifts of Th relative to a bulk critical temperature (Tc) are inversely correlated to the radius of the cylindrical pores. Capillary condensation is accompanied by the hysteresis below Th, while it occurs reversibly between Th and Top. Above Top, adsorption takes place continuously without

696 a first-order capillary condensation. To examine the effects of pore shape and connectivity on capillary condensation and pore criticality, we performed measurements of a series of adsorption-desorption isotherms of nitrogen and other simple gases onto MCM-41, SBA-15, MCM-48, KIT-6, and SBA-16 templated nanoporous materials over a wide temperature range between a bulk triple point (Tt) and To. 2. EXPERIMENT All materials were prepared according to the literatures [5-7]. Adsorption isotherms were measured volumetrically on a homemade semiautomated instrument equipped with a Baratron capacitance manometer (Model 690A) with a full scale of 25000 Torr [4]. 3. RESULTS AND DISCUSSION Hysteresis loops are often used in determination of pore size distribution of the conventional mesoporous materials under several assumptions concerning the pore structure and the nature of adsorption and desorption branches. In most cases, it is assumed that the material is made up of a collection of unconnected cylindrical pores. 3.1. Nature of adsorption and desorption branches

Most of theoretical considerations, strongly suggest that for unconnected cylindrical pores capillary evaporation can occur via a meniscus formed at the pore mouth and thus takes place at a thermodynamical equilibrium transition, although in principle metastability is thermodynamically feasible on either branch. Cohan [8] first suggested this model. The view that experimental hysteresis in cylindrical pores occurs mainly on the adsorption branch, however, is not based on any direct experimental evidence. 30

N2/MCM'41

25

64.5K 68.4K 7~6K 78.9K 86.8K ~ ~ : ~ 90.7K

lll.2K 107K ~119.2 K

~20 ~15

10

0 1

1.5

2

2.5

3

3.5

4

4.5

logP(Torr) Fig. 1. Temperature dependence of the adsorption-desorption isotherm of nitrogen onto MCM-41.

697 The nature of the adsorption and desorption branches can be examined by plotting the condensation/evaporation pressure in a form of T ln(P/Po) against temperature over a wide temperature range including both regions of irreversible and reversible isotherms [9]. The principle of this method relies on the simple idea that the equilibrium phase-transition pressures in the hysteretic isotherms would be obtained by the extrapolation of the plot for reversible capillary condensation to lower temperatures. Fig. 1 shows the adsorption-desorption isotherms of nitrogen on MCM-41 with unconnected cylindrical pores of radius 2.2 nm. At lower temperatures, the isotherms exhibited hysteresis loops of type H 1 in the IUPAC classification [10]. When the temperature was increased, the hysteresis loop shrank and eventually disappeared a t - 7 9 K (Th). The adsorption steps were still nearly vertical above Th. This indicates that Th is lower than Tcp. Fig. 2 shows the plots of the capillary condensation and evaporation pressures against temperature for nitrogen on MCM-41. Here, the condensation and evaporation pressures were determined at the midpoint of the adsorption and desorption branches, respectively, and P0 is the saturated vapor pressure of the bulk liquid. At lower temperatures, T ln(P/Po) is the difference of the chemical potential with respect to the bulk liquid. The plot for capillary condensation forms an almost linear relationship over a wide temperature range spanning Th, whereas the same plot for capillary evaporation breaks at Th. We examined further the temperature dependence of the capillary condensation and evaporation pressures of argon, oxygen, and carbon dioxide onto MCM-41, nitrogen onto SBA-15 with larger cylindrical pores, and nitrogen, oxygen, argon, and carbon dioxide onto SBA-16 with cagelike pores. The plot for capillary condensation always connected smoothly into that for reversible capillary condensation measured above Th. On the other hand, the same plot for capillary evaporation did not connect smoothly into that for reversible condensation above Th. Therefore, the temperature dependence of capillary condensation and evaporation pressures of simple gases indicates that the adsorption branch of the hysteretic isotherm corresponds approximately to the true equilibrium, irrespective of the type of adsorbents. On the other hand, capillary ,

-10

,,

N2/MCM-41

-20 ~-30

Ads(. ) Oes(o )

~' -40 -60

L

Th

.

.

-80 -90

60

70

I

I

I

I

I

80

90

100

110

120

130

T(K) Fig. 2. Temperature dependence of the capillary condensation and evaporation pressures of nitrogen onto MCM-41.

698 evaporation from the cylindrical pores and the cagelike pores is always delayed. Use of the adsorption branch in pore size analysis is more appropriate than the desorption branch. 3.2. Pore-connectivity effects

The pore shape and connectivity of the KIT-6 ordered mesoporous silica can be represented by a pair of interpenetrating bicontinuous networks of channels as in the structure of MCM-48 [7]. Half the channels in a particle of MCM-48 and KIT-6 are interconnected with each other and thus filling and empting of a channel can trigger condensation and evaporation in the neighborhood, respectively, as opposed to the unconnected channels of MCM-41 and SBA-15. Fig. 3 shows the thermal behavior of the adsorption hysteresis of nitrogen onto KIT-6. At low temperatures, the adsorption and desorption branches are parallel to each other and also almost parallel to a bulk condensation line. When the temperature was increased, the hysteresis loop shrank and eventually disappeared at-~107 K(Th). The adsorption steps were still vertical above Th. This indicates that Th is lower than Tcp, in accord with the results on the templated nanoporous materials with unconnected cylindrical pores. Fig. 4 shows the plots of the capillary condensation and evaporation pressures against temperature for nitrogen on KIT-6. The plot for capillary condensation formed an almost linear relationship over a wide temperature range including Th , in accord with the results on MCM-41 and SBA-15 with unconnected cylindrical pores. This indicates that for the interconnected channels of KIT-6 capillary condensation in the hysteretic isotherms takes place near the equilibrium as for the unconnected channels of MCM-41 and SBA-15. The shape and thermal behavior of the adsorption hysteresis for KIT-6 were indistinguishable from those for SBA-15 with a mean pore diameter of 9.4 nm [4]. In order to compare further the behavior of a fluid confined to the interconnected pores 40 N2/KIT-6

35

79.3K

84.4K 89.2K

93.8K 99 l K 1 0

[

~

I 109.2K 117.7K

30 120.8K

25 20

15 10

0 2.5

3

3.5

4

4.5

LogP(Torr) Fig. 3. Temperature dependence of the adsorption-desorption isotherm of nitrogen onto KIT-6.

699

o N2/KIT-6

-5 -lO .-~ -15 -20

E- -25

.•

-30

0

(O)Des

-35

-40 6O

I

I

I

70

80

90

,1

100

'

I

110

120

130

T(K) Fig. 4. Temperature dependence of the capillary condensation and evaporation pressures of nitrogen onto KIT-6. of KIT-6 with that confined to the unconnected pores of SBA-15, we measured the adsorption and desorption scanning curves of nitrogen onto KIT-6 and SBA-15 at a liquid nitrogen temperature. In the independent domain theory of adsorption hysteresis, a "pore domain" is assumed as a region of porous space in which capillary condensation and evaporation occurs at well-defined vapor pressure values designated by the notations Xl2 and x2~, respectively [11 ]. Adsorption hysteresis arises by the fact that x~2>x2~ for every one of these domains. The main idea of the theory consists in presuming that pore domains constitute autonomous entities for which the filling with either liquid (condensation) or vapor (evaporation) takes place in accordance to their particular x12 and x2~ values, and irrespectively of the state (liquid- or vapor-filled) of their neighboring pore entities. Capillary condensation and evaporation in truly independent domain porous solids develops progressively according to a pore size distribution of the solid. Pore size distribution analysis by a gas adsorption method is usually based on this theory. Figs. 5 and 6 show primary desorption scanning curves of 800 700 - N2/KIT-6 600 ~0

":'. 500 ~- 400 E 300 200

100 [ I

0.5

0.6

I

I

0.7

0.8

P/Po Fig. 5. Primary desorption scanning curves for N2 on KIT-6 at 77 Iz.

0.9

700 700 N2/SBA-15

600

~-500 ~.400 [..

~-300 > 200 100 0

t

I

0.5

0.6

I

I

0.7

0.8

0.9

P/Po

Fig. 6. Primary desorption scanning curves for N2 on SBA-15 at 77 K. nitrogen on KIT-6 and SBA-15, respectively. In both solids, we obtained the results that were just expected from the independent domain theory. When the maximum amount of adsorption was decreased, the shape of the desorption scanning curve remained almost unchanged. In the case of the unconnected cylindrical pores of SBA-15, an ascending process will consist in the sequential filling of pore domains according to their diameters. When starting from a liquid-saturated porous structure, condensate desorption will occur successively from larger to smaller cylinders as the vapor pressure is decreased. In the case of the interconnected channels of KIT-6, half the channels in a particle that are interconnected with each other constitute a single domain. When the boundary scanning curves of both solids are compared, however, it is found out that the adsorption and desorption branches of KIT-6 are steeper than those of SBA-15. Such a difference may come from the fact that the size distribution of pores in a particle of SBA-15 leads to a finite range of condensation and evaporation pressures, whereas the size distribution of channels in a particle of KIT-6 is not recognized in condensation and evaporation processes. All the results for KIT-6 and SBA-15 indicate that interconnections among pores of almost the same size do not have a significant effect on the adsorption hysteresis and pore criticality. 3.3. Adsorption hysteresis in ink-bottle pore

The hysteresis loop of type H2 in the IUPAC classification, which is characterized by a steep desorption branch and a smoothly increasing adsorption branch, has frequently been interpreted as being connected with the existence of ink-bottle type pores, namely pores consisting of wide bodies fitted with narrow necks. At the heart of this mechanism lies the supposition that capillary condensation during adsorption in such pores is governed by the radius of curvature of the wide body of the pores, whereas capillary evaporation, during desorption, of the capillary condensate in the pores is obstructed by liquid remaining condensed in the narrow necks (pore blocking). The conventional mesoporous materials such

701

as porous glasses and silica gels giving rise to the hysteresis loop of type H2 consist of an interconnected network of pores of varying shape, curvature and size. As the relative pressure at which a pore empties is thought to depend on the size of the necks, the connectivity of the network, and the state of neighboring pores, the full understanding of the hysteresis phenomenon within such porous materials will be a very difficult task. The SBA-16 ordered mesoporous silica is one of the most suitable model adsorbents currently available for verification of the theoretical predictions for an ink-bottle pore. The structure of SBA-16 material consists of 9.5 nm spherical cavities arranged in a body-centered cubic array and connected through 2.3 nm openings [12]. Fig. 7 shows the adsorption-desorption isotherms of nitrogen on SBA-16. At lower temperatures, the isotherms exhibited large hysteresis loops of type H2 in the IUPAC classification. When the temperature was raised, the hysteresis loop shrank and eventually disappeared at ~98 K (Th). Fig. 8 shows the relative pressures p/po of the capillary condensation and evaporation as a function of reduced temperature T/Tc for Ar, N2, 02, and CO2 on SBA-16. At lower temperatures the relative pressures of At, N2 and CO2 are taken with respect to the bulk liquid. All data of the condensation and evaporation are represented by a common curve. A plot of p/p0 against T/Tc for capillary condensation connected smoothly into that for reversible capillary condensation measured above Th, whereas the same plot for capillary evaporation broke at Th. This clearly indicates that capillary condensation in the hysteretic isotherms takes place near the equilibrium while desorption from the large cavities is delayed. This delayed desorption is not concerned with emptying of the small channels. We calculated the temperature dependence of the capillary condensation and evaporation pressures of nitrogen under the assumptions that adsorption and desorption in an ink-bottle pore may be regarded as the process of the disappearance and formation of a gas bubble in a

19 N2/SBA-16 17

~15 m

57.2K 61.5K 6 7 . 5 K 77"6K87"6K 72.1K 93.2K 83.6K 96.2K

~13

E

>11

0

1

2

3

4

logP(Torr) Fig. 7. Temperature dependence of the adsorption hysteresis of nitrogen on SBA-16.

702 0.9 0.8 -

SBA-16

o~

0.7 0.6 ~7 0.5

~176176

0.4 0.2

02

~oo ~

0.1

[] Ar • CO2

9

0

0.3

I

I

I

I

I

I

0.4

0.5

0.6

0.7

0.8

0.9

1

T/Tc Fig. 8. Temperature dependence of the capillary condensation and evaporation pressures of Ar, 02, N2, ans CO2 on SBA-16. Full lines denote the transition pressures of N2 calculated on the basis of the simple phenomenological theory.

liquid droplet confined to the spherical pore [13]. A fit between the observed and calculated transition pressures in a wide temperature range was reasonable in light of several assumptions and approximations used. This clearly indicates that the energy barrier for the formation and disappearance of vapor bubbles in the liquid confined to the pores is responsible for the appearance of the adsorption hysteresis and the hysteresis temperature is not concerned with the so-called capillary criticality. REFERENCES

[1]

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