Mesoporous dimensions

These results highlight the emerging potential of the mesoporous dimension in regio-and enantioselective catalysis. 

Based on the lUPAC nomenclature, microporous molecular sieves have pores with dimensions up to 2.0 nm in dimension. The mesopore dimension is between 2.0 and 50 nm, while macropores exhibit pores larger than 50 nm. Shown in Fig. 2 are the pore size and schematic structure of several zeolites, zeotypes, and mesoporous molecular sieves. Evidently, a large number of different zeolite or zeotype structures and mesoporous materials are available from which the optimum one for the given application can be selected. 

LayCTed manganese oxide mataials have porous structures ranging from ultramicropore to mesopore dimensions and cation exchange and molecular adsorptive properties. Tha-efore, they can be used as ion sieves, molecular sieves, and catalysts, similar to zeolites [61]. Spinel-type ion sieves have shown good adsorptive properties for lithium from seawate and other dilute solutions. Repeated adsorp-... 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

A number of attempts have been made to demonstrate the connection between mesoporosity and the Type IV isotherm by comparing the isotherm of a vapour on a nonporous powder before and after it has been formed into a compact. The process of compaction produces pores in the form of interstices between the particles of the original powder such pores will tend to have dimensions of the same order as those of the constituent particles, and it can be arranged that these shall fall within the mesopore range of size. 

Now, in principle, the angle of contact between a liquid and a solid surface can have a value anywhere between 0° and 180°, the actual value depending on the particular system. In practice 6 is very difficult to determine with accuracy even for a macroscopic system such as a liquid droplet resting on a plate, and for a liquid present in a pore having dimensions in the mesopore range is virtually impossible of direct measurement. In applications of the Kelvin equation, therefore, it is almost invariably assumed, mainly on grounds of simplicity, that 0 = 0 (cos 6 = 1). In view of the arbitrary nature of this assumption it is not surprising that the subject has attracted attention from theoreticians. 

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. 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Sol-gel processing forms the basis for various routes employed for the fabrication of a wide diversity of functional materials. To impart a structural organization at various length scales, the syntheses are performed using templates. Most consist of a self-organized ensemble of surfactants and co-polymers [1-10]. They have been successfully applied to control the geometry and dimensions of pores that are periodically arranged as in the initial structures. Mesoporous silica materials of the MCM family, which were first synthesized by a team from the Mobil oil company [11,12], are a well-known example. 

IUPAC classification, mesoporous materials are defined as porous materials with diameters in the range 2-50 nm, which is rather dose to the dimensions of functional biomolecules such as proteins. Therefore, unexplored phenomena and functions could be observed for biomolecules confined in mesopore channels due to their restricted motion and orientation. In this chapter, we briefly introduce recent developments on the immobilization of biomolecules in mesoporous media, where the use of mesoporous silica and mesoporous carbon are mainly discussed. 

Mesoporous silicas have characteristics of high specific surface areas and pores with defined dimensions and uniform distribution. These features make mesoporous systems ideal candidates as host materials to guest bio-molecules. Protein stability may be enhanced due to reduced autolysis in the case of protease enzymes, and more generally reduced protein aggregation, as a result of the separation of the molecules adsorbed on the surface. 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

Detailed accounts of fibers and carbon-carbon composites can be found in several recently published books [1-5]. Here, details of novel carbon fibers and their composites are reported. The manufacture and applications of adsorbent carbon fibers are discussed in Chapter 3. Active carbon fibers are an attractive adsorbent because their small diameters (typically 6-20 pm) offer a kinetic advantage over granular activated carbons whose dimensions are typically 1-5 mm. Moreover, active carbon fibers contain a large volume of mesopores and micropores. Current and emerging applications of active carbon fibers are discussed. The manufacture, structure and properties of high performance fibers are reviewed in Chapter 4, whereas the manufacture and properties of vapor grown fibers and their composites are reported in Chapter 5. Low density (porous) carbon fiber composites have novel properties that make them uniquely suited for certain applications. The properties and applications of novel low density composites developed at Oak Ridge National Laboratory are reported in Chapter 6. 

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

Textural mesoporosity is a feature that is quite frequently found in materials consisting of particles with sizes on the nanometer scale. For such materials, the voids in between the particles form a quasi-pore system. The dimensions of the voids are in the nanometer range. However, the particles themselves are typically dense bodies without an intrinsic porosity. This type of material is quite frequently found in catalysis, e.g., oxidic catalyst supports, but will not be dealt with in the present chapter. Here, we will learn that some materials possess a structural porosity with pore sizes in the mesopore range (2 to 50 nm). The pore sizes of these materials are tunable and the pore size distribution of a given material is typically uniform and very narrow. The dimensions of the pores and the easy control of their pore sizes make these materials very promising candidates for catalytic applications. The present chapter will describe these rather novel classes of mesoporous silica and carbon materials, and discuss their structural and catalytic properties. 

Bulk silicon constitutes an optical long-pass filter, as shown in Fig. 7.6. The same is true for micro- and mesoporous silicon, for which the effective medium approximation (EMA) is valid in the visible regime. The dimensions of macroporous silicon are in the visible regime and the EMA becomes invalid. 

Materials with controlled pore sizes and functionality, particularly in three dimensions would have many uses (53-59). Numerous totally inorganic microporous and mesoporous materials have been subject of thousands of papers, and applications of the former (e.g., zeolites) have a sizable impact on the global economy at present (myriad uses from production of gasoline to a host of chemicals) (60-66). However, the use... 

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