Capillary condensation of adsorbates in porous materials

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Abstract

Hysteresis in capillary condensation is important for the fundamental study and application of porous materials, and yet experiments on porous materials are sometimes difficult to interpret because of the many interactions and complex solid structures involved in the condensation and evaporation processes. Here we make an overview of the significant progress in understanding capillary condensation and hysteresis phenomena in mesopores that have followed from experiment and simulation applied to highly ordered mesoporous materials such as MCM-41 and SBA-15 over the last few decades.

Graphical abstract

Highlights

► Advances of the modern tools to analyse capillary condensation and evaporation. ► Molecular simulation is a useful in describing condensation and evaporation. ► Better understanding of cavitation and pore blocking.

Introduction

Investigations of adsorption in porous materials date back over 100 years, nevertheless many problems remain unresolved and new challenges arise as new materials and new industrial and environmental requirements emerge. Capillary condensation of adsorbate in porous materials is a well-documented phenomenon that is observed in pores whose sizes are typically in the mesopore range between 2 and 50 nm. The enduring interest in this phenomenon arises from several sources: (1) condensation in mesopores always exhibits hysteresis between the adsorption and desorption branches of an adsorption isotherm when temperature is less than the critical hysteresis temperature; (2) the characteristics of the hysteresis loop depend on the material examined, (3) the adsorbate and the temperature; thus for a given adsorbate and temperature the adsorption and desorption branches of the isotherm are associated with the distribution of pore size, and shape and the connectivity in the material. The links between the properties of a material and the macroscopic observation are complex, and decades of research have shown that the tantalizing source of information about porous materials is difficult to extract in an unambiguous way from the experimental data because of the complex dependence of hysteresis and its associated properties on a wide range of parameters and operating conditions. The aim of this review is to present a survey of contributions from both experiment and simulation to the study of capillary condensation and hysteresis over the last few decades.

Gas adsorption is a widely used technique to investigate the morphology of porous materials. In discussions of adsorption in these materials it is customary to distinguish between micropores (now often described as nanopores) and mesopores. Broadly speaking, in the latter the capillary condensation phenomena are observed (preceded by a molecular layering on the pore walls), while in the former, isotherms do not exhibit this transition, and micropore filling (and cooperative filling) is the dominant mechanism. It is important to note that real materials contain pores ranging from micropores to macropores, and there are ranges of pore sizes where there is an overlap between these two mechanisms of adsorption.

We can obtain a great deal of information about porous materials from adsorption isotherms; surface area, pore volume, pore size distribution, etc. which are now routinely obtained from the volumetric adsorption method. This method is implemented in many automated commercial instruments, which are now equipped with a bank of sensitive transducers that allow isotherms to be measured at extremely low pressures, the range of which gives us much valuable information about the ultra-micropores and strong surface sites.

The mechanism of adsorption of simple gasses in mesoporous materials begins with the formation of a monolayer on the surface followed by multilayer formation (unless the temperature is low enough, adsorption on higher layers occurs before the lower layers are completed and this is due to the molecular thermal energy) and finally by capillary condensation. At temperatures below the critical point, the amount of gas adsorbed in a porous material increases with increasing relative pressure. Isotherms (plots of adsorbed amount versus pressure) are classified into six types according to the International Union of Pure and Applied Chemist (IUPAC) classification. Fig. 1 [1] shows the six main adsorption isotherm types.

Types IV and V, show a hysteresis loop associated with the filling and emptying of the mesopores by capillary condensation and evaporation, respectively. Historically, the first report on capillary condensation was by van Bemmelen in 1896, who studied the adsorption of water in silica gel [2]. Since this pioneering work, there have been many studies reporting different isotherms, many of which exhibit a hysteresis loop. These hysteresis loops were subsequently classified into four types by IUPAC, and these are shown in Fig. 2 [1]. It is interesting to make a note that this classification is different from the one suggested earlier by de Boer [3]. One of the earliest attempts to exploit the adsorption–desorption hysteresis for analysis of the pore size distribution of mesoporous materials was due to Barrett, Joyner and Halenda (BJH) [4]. Their paper has received over 3500 citations and the method is still widely in use for characterization of mesoporous solids.

In Section 2, we present the commonly used classical theories of capillary condensation, and describe how they are applied to determine the pore size distribution of porous materials using the adsorption isotherms. In Section 3, we survey a number of highly ordered materials with different pore shapes, e.g. materials with cylindrical pores such as MCM-41 and SBA-15, and those with spherical pores such as SBA-1, SBA-6, and KIT-5 etc. In Section 4, we discuss the behavior of the adsorption isotherm: Section 4.1 focuses on adsorption hysteresis as observed in experimental and simulation studies, and in Section 4.2 we present comparisons between the experimental and simulation results in isotherm hysteresis; specifically we discuss the shape of the hysteresis loop, the hysteresis and pore critical temperatures, the pore-blocking and cavitation effects for pores having narrow necks, scanning curve behavior in the hysteresis loop, and the isosteric heat of adsorption.

Section snippets

Classical theory of condensation

Zsigmondy, in 1911, proposed the earliest theory of adsorption hysteresis phenomena, attributing it to the difference between the contact angles of the condensing and evaporating liquids [5]. Subsequently the classical Cohan and Kelvin equations were invoked to describe condensation and evaporation in cylindrical spaces [1]. The Kelvin equation is derived from the Laplace equation which expresses the pressure difference across the curved surface of an unconfined liquid (i.e. one in which the

Ordered mesoporous materials

The study of capillary condensation in mesopores was changed drastically by the discovery in the 1990s of a class of highly ordered mesoporous materials known as the M41S family [19], [35] such as MCM-41 [19] and SBA-15 [20]. Since these discoveries, many similar materials with the highly ordered pore structures; e.g. cylindrical, interconnected sphere etc., have been developed. We shall discuss some of these briefly in the following section. A survey of these mesoporous materials can be found

Adsorption isotherms

An abundance of experimental adsorption isotherms on many porous materials have been measured with a wide variety of adsorbates. Excellent reviews prior to 2004 have been presented in the literature [52], [53], [54].

The shapes of the adsorption hysteresis loop classified by IUPAC (see Fig. 2 [1]) have been attributed to particular pore shapes. Type H1 loop, which is narrow with very steep and parallel adsorption and desorption branches, is associated with adsorbents with a narrow and uniform

Conclusion

We have reviewed the literature on experimental and theoretical studies of capillary condensation and hysteresis with particular emphasis on the newer materials with well-defined pore structure that have appeared in the last two decades and which have led to. There has been a significant progress in our understanding of the nature of the hysteresis in capillary condensation. Computer simulation has provided significant insight into the microscopic behavior of condensation and evaporation.

While

Acknowledgment

We acknowledge the JSPS for its financial support in the form of Excellent Young Researcher Overseas Visit Program to TH. Support from the Australian Research Council is gratefully acknowledged.

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