Molecules, adsorption

The borderlines between the different classes are not hard and fast, depending as they do both on the shape of the pores and on the nature (especially the polarizability) of the adsorptive molecule. Thus, the highest value of w (and therefore of p/p ) at which the enhancement of adsorption occurs, i.e. the upper limit of the micropore range, will vary from one adsorptive to another (cf. Chapter 4). 

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Although adsorption has been used as a physical-chemical process for many years, it is only over the last four decades that the process has developed to a stage where it is now a major industrial separation technique. In adsorption, molecules distribute themselves between two phases, one of which is a solid whilst the other may be a liquid or a gas. The only exception is in adsorption on to foams, a topic which is not considered in this chapter. 

Unlike absorption, in which solute molecules diffuse from the bulk of a gas phase to the bulk of a liquid phase, in adsorption, molecules diffuse from the bulk of the fluid to the surface of the solid adsorbent forming a distinct adsorbed phase. 

Pollutants Molecular weight Corrected dark adsorption (molecule/cm )... 

When an outgassed solid (the adsorbent) is confined to a closed space and exposed to a gas or vapor (the adsorptive) at a given pressure and temperature, an adsorption process takes place. The adsorptive molecules are transferred to, and accumulate in, the interfacial layer, as a consequence of an attractive force between the surface of the solid and the adsorptive (the adsorptive actually adsorbed by the adsorbent is named adsorbate). After some time, the pressure becomes constant and the thermodynamic equilibrium of adsorption is achieved. 

The film diffusion process provides a supply of adsorptive molecules at the adsorbent surface to engage in a chemical reaction leading to adsorption (Eq. 4.3). The rate law for this reaction is developed, for example, in conjunction with the adsorption step in the sequential reaction schemes that appear in Eqs. 3.46, 3.56, and 4.51. Prototypical expressions are in Eqs. 3.47, 3.57, and 4.52a a generic rate law for reaction-controlled adsorption is in Eq. 4.17. For the present example the rate of adsorption can be described by the equation... 

The term adsorption may also be used to denote the process in which adsorptive molecules are transferred to, and accumulate in, the interfacial layer. Its counterpart, desorption, denotes the converse process, in which the amount adsorbed decreases. Adsorption and desorption are often used adjectivally to indicate the direction from which experimentally determined adsorption values have been approached, e.g. the adsorption curve (or point) and the desorption curve (or point). Adsorption hysteresis arises when the adsorption and desorption curves do not coincide. 

The apparent adsorption may, alternatively, be defined by measuring the amount of gas which has to be added to the system at constant T, p to increase the volume V back to V°. The apparent adsorption is then equal to the extra amount of gas which can be accommodated in a volume V° at a given 7, p when the solid is introduced. It can, therefore, be expressed in terms of the local deviations of the concentration, c, of adsorptive molecules, from the bulk concentration c° ... 

The terminology of pore size has become somewhat confused because it has been customary to designate the different categories of pores in terms of exact dimensions rather than by reference to the particular forces and mechanisms operating with the given gas solid system (taking account of the size, shape and electronic nature of the adsorptive molecules and the surface structure of the adsorbent) as well as to the pore size and shape. 

To illustrate this problem, we note a possible difference between the surface area of a porous solid which is available for adsorption and the area (including that of closed pores) which can scatter low angle X-rays. Even in the former case, the extent of the adsorption is likely to be dependent on the size, shape and electronic nature of the adsorptive molecules in relation to the surface chemistry, roughness and porosity of the adsorbent. 

Helium is often used in adsorption manometry for the determination of the dead space volume (see Chapter 3), but this procedure is based on the presupposition that the gas is not adsorbed at ambient temperature and that it does not penetrate into regions of the adsorbent structure which are inaccessible to the adsorptive molecules. In fact, with some microporous adsorbents, significant amounts of helium adsorption can be detected at temperatures well above the normal boiling point (4.2 K). For this reason, the apparent density (or so-called true density ) determined by helium pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational temperature and pressure (Fulconis, 1996). 

We conclude that in the precipitation process, the trapping of water has resulted in the formation of a more open intraparticle microstructure than in the pyrogenic silicas. The internal surface remains hydroxylated, but is not easily accessible to most adsorptive molecules. Water molecules are able to undergo specific interactions with the internal OH groups which accounts for the abnormally high uptake of water vapour. 

A number of interesting features can be seen in Figure 11.21. First, the level of water adsorption at p/p° = 0.90 by Silicalite-I is only about 10% of the capacity available for nitrogen and other small adsorptive molecules (see Table 1 l.S). This is increased to about 18% for HZSM-5, when the Si/Al ratio is reduced to 90. The presence of the hysteresis loops in the capillary condensation range indicates that a high proportion of the water adsorption has occurred within the secondary pore structure or defect structure rather than in the zeolitic channel structure. Similar findings have been reported by Llewellyn et al. (1996). 

In the initial stages of adsorption, i.e, when a virgin surface is suddenly exposed to a gas of non-zero pressure, the surface starts to act as a sink for adsorptive molecules, then Pj(0)= 0 and the rate of supply is entirely determined by the second term on the r,h,s, of [1.1.15) ... 

Structure. At very low 6, i 1 may be higher because of attachment of adsorptive molecules to grain boundaries between the single crystals of graphite, whereas after completion of the monolayer, the enthalpy drops to (almost) the enthalpy of liquefaction (dashed line in the figure). In fig. 1.8b there Is no such plateau, although from the measurements with benzene It was deduced that the surface is homogeneous. The rise in I I was attributed to lateral interaction... 

Another idealization is illustrated in fig. 1.18. Here it is supposed that fractions and of the surface correspond to localized and mobile adsorption, respectively ). The adsorptive molecules may then "decide" where they are going. Results depend on the molecular parameters, as accounted for through and q. The example given is representative for non-rotating molecules with the mass and area of nitrogen molecules ). Figure 1.18a shows that at low 9 there is... 

These limits are somewhat arbitrary pore filling mechanisms also depend on the shapes of the pores and on the size of the adsorptive molecule. Despite this Inherent vagueness, the classification has its use as a first means of discrimination because it points to different pore filling mechanisms macropores are so wide that they behave as "virtually flat" surfaces, mesopores are mainly responsible for capillary condensation, whereas micropores are so narrow that one cannot speak of a macroscopic fluid in them. Because in micropore Jilling adsorbates are only a few layers thick, an adsorption plateau is found suggesting monolayer filling and applicability of the Langmuir or Volmer premises. This mechanism Is distinct from that in meso- and macropores. 

It may be good to note here that various molecular cross-sections have now been considered. In the treatment of adsorption on solid surfaces was introduced. Interpreting this area in terms of lattice models is not a property of the adsorptive molecule but of the adsorbent. It is possible to imagine a situation where greatly exceeds the real molecular cross-section. On the other hand, for mobile monolayers on homogeneous surfaces is the real molecular cross-section or, for that matter, it is the excluded area per molecule. To avoid an undue abundance of symbols we have used the same symbol for both situations, for instance in table 3.3 in sec. 3.4e. It is to be expected that a and a, obtained by compression of monolayers, are more similar to the a s for adsorbed mobile monolayers on homogeneous substrates than to those for localized monolayers. 

We are currently in the process of publishing reference data for various adsorptive molecules of different size and polarity which we hope other research groups will be able to evaluate. In this paper, we will present a summary of some of the results obtained with benzene, dichloromethane and methanol, and show how application of the reference data can be used to obtain useful information about the pore structure of molecular sieve carbons and superactivated carbons. 

The above-mentioned factors indicate that an ideal chromatographic adsorbent would exhibit a high surface area but a moderate pore volume and pore size (which is large enough to ensure good accessibility for the adsorptive molecules). As these three physical parameters are interlinked they cannot be independently chosen. Their interdependence is summarized in the Wheeler equation, which assumes a constant pore diameter within the particle. In Eq. 2.18, d, nr(, is the pore diameter (nm), Vp0re the pore volume (mlg-1) and SBet the specific surface area (m2g 1). 

Pore size is not the only parameter that affects adsorption. Molecules containing polar groups such as -OH, >CO, -NH2 and polarizable groups such as >CO=CO< and C6H5- strongly interact with the surface of zeolites the cations in the zeolites generate... 

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