Microporosity

In microporous solids all pores, or part of them, have diameters smaller than 2 nm. Well known examples are the pores of zeolites and in activated carbons. Type I isotherms are found (see Fig. 12.1) which are characterized by a large 

Methods for determining pore volume distributions in the micropore region have been published by Mikhail and coworkers [13], Medek [14] and by Horvath and Kawazoe [15]. 

Many microporous substances also contain meso and macropores. In such cases, isotherms are found which are a hybrid of isotherms type I and IV. As mentioned before (see Section 12.7), BET analysis is not allowed here. 

---

Fig. XVIII-26. A well-ordered pillared clay with almost exclusively zeolitelike microporosity. (From Ref. 299.)...

Fig. 2.28. The high-pressure branch is still linear (provided mesopores are absent), but when extrapolated to the adsorption axis it gives a positive intercept which is equivalent to the micropore volume. The slope of the linear branch is now proportional to the external surface area of the solid. Microporosity is dealt with in detail in Chapter 4.

Fig. 2.28 Effect of microporosity of the isotherm and t- (or a,-) plot, (a) (A) is the isotherm on a nonporous sample of the adsorbent (B) is the isotherm of the same solid when micropores have been introduced into it. [b) t- (or a,-) plots corresponding to the isotherms of (a). (Schematic only.)...

More often, however, microporosity is associated with an appreciable external surface, or with mesoporosity, or with both. The effect of microporosity on the isotherm will be seen from Fig. 4.11(a) and Fig. 4.12(a). In Fig. 4.11(a) curve (i) refers to a powder made up of nonporous particles and curve (ii) to a solid which is wholly microporous. However, if the particles of the powder are microporous (the total micropore volume being given by the plateau of curve (ii)), the isotherm will assume the form of curve (iii), obtained by summing curves (i) and (ii). Like isotherm (i), the composite isotherm is of Type II, but because of the contribution from the Type 1 isotherm, it has a steep initial portion the relative enhancement of adsorption in the low-pressure region will be reflected in a significantly increased value of the BET c-constant and a shortened linear branch of the BET plot. 

Thus, whilst a powder composed of nonporous particles gives rise to an isotherm of Type II, the converse is not necessarily true if a solid yields a Type II isotherm, it is not necessarily free of micropores. Similarly, though a Type IV isotherm signifies the presence of mesoporosity, it does not prove the absence of microporosity. - ... 

A high value of the BET constant c is a useful preliminary indication of the presence of microporosity, but it does not enable one to estimate the micropore volume itself, that is in effect to break down the composite isotherm (iii) into its components (i) and (ii). 

A number of methods which have been proposed for the evaluation of microporosity from Type II and Type IV isotherms will now be described. 

Perhaps the most direct method of evaluating microporosity is to fill up the micropores with some suitable adsorbate whilst leaving the mesopores, macropores and external surface free. The use of n-nonane as a preadsorbate was proposed by Gregg and Langford on the basis of earlier work on the adsorption of n-alkanes C, to C, on ammonium phos-phomolybdate, a microporous solid. This work had shown that the rate at... 

In the light of these results there is little doubt that the t-curve (or a,-curve] used in testing for microporosity should be based on the isotherm of a nonporous solid chemically similar to the substance under test and not on a reference isotherm which happens to have the same value of the c-constant. 

It follows that the applicability of the nonane pre-adsorption method for the evaluation of microporosity is restricted to adsorptives such as nitrogen which are used at temperatures far below ambient and which have negligible solubility in soUd or liquid nonane. 

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

Bohra, J. N. and Saxena, R. K., Microporosity in rayon-based carbonized and activated carbon fibers. Colloid Surf., 1991, 58(4), 375 383. 

Jaroniec, M., Gilpin, R. K., Kaneko, K. and Choma, J., Evaluation of energetic heterogeneity and microporosity of activated carbon fibers on the basis of gas adsorption isotherms, Langmuir, 1991, 7(1 1), 2719 2722. 

The development of microporosity during steam activation was examined by Burchell et al [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the Nj adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore sfructure characterization. 

---