Micropores diameter

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. 

Pore size is also related to surface area and thus to adsorbent capacity, particularly for gas-phase adsorption. Because the total surface area of a given mass of adsorbent increases with decreasing pore size, only materials containing micropores and small mesopores (nanometer diameters) have sufficient capacity to be usehil as practical adsorbents for gas-phase appHcations. Micropore diameters are less than 2 nm mesopore diameters are between 2 and 50 nm and macropores diameters are greater than 50 nm, by lUPAC classification (40). 

Fig. 48. Experimental vanadium deposit distributions in a microporous catalyst (100 A micropore diameter/204 m2/g SA) macroporous catalyst (1300 A pore diameter/14.5 m2/g SA) and bimodal catalyst (120 A micropore, 25,000 A macropore diameters/200 m2/g SA) (Plumail e al., 1983).

For porous sorbents, most of the surface area is not on the outside of the particle but on the inside pores of the sorbent (Figure 2.20) in complex, interconnected networks of micropores (diameters smaller than 2 nm), mesopores (2 to 50 nm), also known as transitional pores, and macropores (greater than 50 nm) [57], Most of the surface area is derived from the small-diameter micropores and the medium-diameter transitional pores [56], Porous sorbents vary in pore size, shape, and tortuosity [58] and are characterized by properties such as particle diameter, pore diameter, pore volume, surface areas, and particle-size distribution. 

FIGURE 3.21 Plots of average micropore size against pressure applied during hot-pressing at 300°C. The average micropore diameter was calculated from the slope and intercept of an as plot of each sample. (From Hou, P.X., et al., Carbon, 45, 2011, 2007. With permission.)... 

The primary isomer distribution, which is the result of the disproportionation reaction, may deviate significantly from the thermodynamic equilibrium composition, for two reasons. First, the reaction may be controlled by the kinetics rather than the thermodynamics, i.e. mechanistic reasons may exist which cause the reaction to proceed along a certain path. Second, in the case that the reaction obeys a bimolecular mechanism, it may pass through a transition state which would presumably favor the (taller) para isomer. Hence, it is possible that the primary product contains an enhanced fraction of the para isomer. The departure from the equilibrium composition then gives the driving force for the subsequent (monomolecular) isomerization reaction. This will reduce the fraction of the para isomer, provided the formation of the bulky ortho and meta isomers are not inhibited by sterical effects, i.e. when the micropore diameter is sufficiently large or there is a chance for the isomerization reaction to take place at the outer surface of the crystallites. Thus, the secondary isomer distribution may approach the thermodynamic equilibrium composition, as a limiting case. 

Ctace a reactant molecule has adsorbed within the zeolite mouth, it needs to diffuse toward the active sites in contact of which reactions will occur. This diffusion can be very dependent on the size and shape of the zeolite micropores as well as on the size of the reactants or products. This becomes especially true when the reactants or products have similar size to the micropores diameter. After reaction, it is the turn of the products to diffuse away from the micropores. 

Catalysts Preparation. The silicoaluminophosphate (SAPO) molecular sieves employed in this study were synthesized in the laboratory of Professor Mark Davis in the Department of Chemical Engineering of the Virginia Polytechnic Institute, following the methods reported in U.S. Patent 4,440,871. The three different samples, distinguished by their microscopic structure, were the wide-pore SAPO-5, medium-pore SAPO-11, and the narrow-pore SAPO-34. Verification of their microscopic structure (through x-ray diffraction) and micropore diameters (by argon adsorption measurements) was performed at VPI. The SAPO molecular sieves were provided in the ammonium cation form. Ex situ calcination at 873 K for one hour in oxygen was performed on the SAPO samples prior to their use as catalysts for the propylene conversion. 

Experiments using Ar at 77K and 87 K lead to similar PSD and maxima of pore mean diameter, but resuts obtained with N2 are very different, with smaller mean pore diameters (particularly for NaY) and a bimodal PSD for USY zeolite centered on 0.46 nm and 0.58 nm. Aperture of Y zeolite micropore is 0.7-0.8 nm (cristallographic data [1]) and diameter of internal cage is -1.2 nm. N2 leads to underestimate Y zeolite micropore diameter, while Ar... 

Coke deposition on the walls of larger micropores (diameter > Dj), appearing predominant at the beginning of cracking reaction, e.g. for the lowest coke content. As a result, smaller micropores centered on D2 are created (argon experiments). 

MgO solid was used as adsorbent to simulate solid surface and micropore systems, because of its chemical inertness and stability. Solid surface system was constructed with the dimensions of a = 33.04A, b =16.62A and c = 42.HA, including 192 fluid molecules, 256 Mg atoms and 256 0 atoms in a unit cell. Solid surface and micropore (diameter = 4A) systems were constructed with the dimensions of a = 37.90A, b = 24.64A and c = 72.IOA, including 350 fluid molecules, 768 Mg atoms and 768 0 atoms in a unit... 

Fig. 5. Computed enthalpies of adsorption of linear hydrocarbons as a function of the average zeolite micropore diameter [70].

Figure 3.4.5 Condensed state of molecular adsorption into micropores.(Diameter and length ofl D channel decreases from left to right.)...

A high level of macropores (diameter more than 0.1J m) facilitates the intraparticular diffusion, as micropores (diameter less than 20 nm) are necessary to develop a high surface area. This double feature is know as bimodality, illustrated in fig. 4. 

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