More porous zeolitic products

Further, in agreement to increased CEC, this product also exhibits (i) remarkable increase in the meso-pores of size ranging from 2.5 to 10 nm and (ii) formation of many macro- pores (>50 nm, refer Table 6.15). Such results also confirm the formation of higher grades of more porous zeolitic products by the adopted fusion method. In addition, the dominant pore diameter in this product is noticed to be 30 nm followed by 6.5 nm, which are also noticed in its two micrographs (upper and lower images in Fig. 6.38 at resolutions of 300,000 and 30,000, respectively). 

The reason for the high selectivity of zeohte catalysts is the fact that the catalytic reaction typically takes place inside the pore systems of the zeohtes. The selectivity in zeohte catalysis is therefore closely associated to the unique pore properties of zeohtes. Their micropores have a defined pore diameter, which is different from all other porous materials showing generally a more or less broad pore size distribution. Therefore, minute differences in the sizes of molecules are sufficient to exclude one molecule and allow access of another one that is just a little smaller to the pore system. The high selectivity of zeolite catalysts can be explained by three major effects [14] reactant selectivity, product selectivity, and selectivity owing to restricted size of a transition state (see Figure 4.11). 

TTeterogeneous catalysts are usually high-area porous materials which may be amorphous or crystalline. An important aspect of all such materials is the rapidity with which reactant molecules reach active sites and products leave these sites. Apart from flow in gas or liquid phase, there may be surface migration into and from micropores, whether in amorphous catalysts or in crystalline ones, such as the zeolites. It is still an open question how important such migration processes are as ratecontrolling steps. However, it seems likely that active sites deep in a porous crystal will be less important than sites near the surface because many more unit diffusion steps will be needed to transport molecules to and from deeply buried sites. As corollaries, one would expect that only a limited volume fraction of a crystal of a zeolite such as sieve Y is catalytically effective, and that for best performance crystals in the catalyst support should be well exposed and as small as possible, in order to provide the largest surface-to-volume ratio. 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

It is now possible to prepare defect-free zeolite membranes for use in separations.221 (More details on separations can be found in Chap. 7.) A mordenite membrane on a porous alumina support had a separation factor for benzene over p xylene of more than 160.222 A ZSM-5 membrane on porous alumina separated /7-butane over isobutane by a factor of 31 at 185°C.223 The next step is to use them in membrane reactors to separate products as they form. 

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

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