Pore size of zeolites

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

The molecular sieve behaviour of zeolites can be controlled by a hydrolytic process at elevated temperatures. Water vapour in contact with zeolite crystals at elevated temperatures results in a variation of the zeolitic adsorption characteristics. The amount of water vapour, the pretreatment temperature and the pretreatment time, can control the effect pore size of zeolites. It appears, that a steam treatment causes a cation migration and a cation hydrolysis of the exchangeable cations. However, the effect of steam on the adsorption behaviour of zeolites is influenced by the nature of the initial exchangeable cations. 

Furthermore, the implantation of various boron-nitrogen compounds inside the zeolite framework can reduce, in a controlled way, the effective pore size of zeolites. When NH3 is added to the boranated zeolite (before hydrolysis reaction), at room temperature, the formation of amine-boranes can be detected, which changes the molecular sieving properties of the zeolite. 

MFI type) zeolites have straight channels and zigzag channels which connect them. The cross sections of these pores have the sizes of 0.51 X0.55 nm and 0.54 x 0.56 nm, respectively (3). The intersections of these channels have space which just two butene molecules can occupy (4). Mordenite has a porous structure, in which the supercages and their windows are connected in a straight line. The pore sizes of zeolites are close to the minimum molecular sizes of light hydrocarbons. Hence, zeolites have the ability to sieve molecules (shape selectivity). 

The thermodynamically more stable trowx-isomer, which is the sole product of homogeneous MPV reduction in presence of propane-2-ol, was not detected in this case. This remarkable selectivity is attributed to the pore size of zeolite BEA. 

An important type of porous carbons is activated carbons. Granular activated carbons are prepared from different precursors and used in a wide range of industries. Their preparation, structure and applications were reviewed in different books and reviews [3,4,71-75]. In Table 8, some properties of different adsorbents (activated carbons, silica gel, alumina gel and zeolite) are compared with each other. High BET surface area and light weight are the main advantages of activated carbons. Usually activated carbons have a wide ran of pore sizes from micropores to macropores, which shows a marked contrast to the definite pore size of zeolites. 

Hong et al. [29] exploited silylation reactions with proper silane precursors to engineer the pore size of zeolite membrane for hydrogen separation. As a result, H2/CH4 separation selectivity can be increased more than 50% from 35 to 59. 

The percent volume occupied by host or guest provides a measure of the porosity of a network. In this respect, many of the networks such as 11 (Fig. 1), 23 (Fig. 7), 30 (Fig. 9), 38 (Fig. 10), 42 (Fig. 11), and 45 (Fig. 12) are highly porous, i.e. >50% of the total volume of the adduct is occupied by the guest or only > 50% of that is occupied by the host framework. Some of the guest-binding channels have a cross-section as wide as 10 A. For comparison, typical pore sizes of zeolites (molecular sieves 4 A, 5 A, and 13X) fall in the range 4-13 A and void volvunes are generally <50% [63]. 

Table 7-2 Molecular diameters and pore sizes of zeolites [7,T32]...

Frilette et al. proposed a simple test reaction for estimating the effective pore size of zeolites. The determination of the constraint index is done by continuously passing a mixture of hexane and 3-methylpentane over a zeolite at atmospheric pressure. The constraint index is defined as follows. 

An 8-membered ring zeolite, erionite, gives a constraint index of 38, 10-membered ring zeolites, ZSM-5 and ZSM-11, give values of 8.3 and 8.7, respectively, while 12-membered ring zeolites, mordenite and REY, give values of 0.5 and 0.4. In this way, the constraint index is a very useful tool for estimating the pore size of zeolites of unknown structures. 

Mitani at al., in their work related to the investigation on the acidity of faujasites with different framework aluminium content, reported that the extent of the decrease in acid sites concentration varies with the kind of basic probe used in microcalorimetry experiments (see Fig. 9.6) [92]. The experimental results were explained by differences in molecular diameters of ammonia and pyridine, and the fact that pore size of zeolite becomes widely distributed as the dealumination proceeds so OH groups in supercages have become available for adsorption of such large molecule as pyridine is. 

Fig. 7.4 Pore size of zeolites and the molecular size of various gases suitable for adsorption...

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