Pores, molecular ordering

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

In this chapter we will focus on molecular ordering and confinement effects in pores. Diffusion experiments with the pulse-field gradient method ([162-165] and references therein) and characterization of the surface properties using NMR of noble gases such as 129Xe ([166-171] and references therein), or 83Kr [172], will be omitted due to excellent reviews that have appeared quite recently in these areas. 

More complex reactors, like packed-bed reactors or catalytic monoliths, consist of many physically separated scales, with complex nonlinear interactions between the processes occurring at these scales. Figure 3 illustrates scale separation in a packed-bed reactor. The four length (and time) scales present in the system are the reactor, catalyst particle, pore scale, and molecular scale. The typical orders of magnitude of these four length scales are as follows reactor, lm catalyst particle, 10 2m (1cm) macropore scale, 1pm (10 6m) micro-pore/molecular scale, 10 A (10 9m). The corresponding time scales also vary widely. While the residence time in the reactor varies between 1 and 1000 s, the intraparticle diffusion time is of the order of 0.1s and is 10 5s inside the pores. The time scale associated with molecular phenomenon like adsorption is typically less than a microsecond and could be as small as a nanosecond. 

The Kelvin equation takes into account molecule/solid and intermolecular interactions using contact angle and surface tension, respectively. However, the Kelvin approach is not appropriate for de.scription of adsorption on small mesopores. Saam and Cole developed the thermodynamic theory with the average molecular potential for liquid helium in a cylindrical pore in order to understand unusual properties of liquid helium[19,20]. Findenegg et al have applied the Saam-Cole theory to elucidate fluid phenomena near the critical temperature[21]. The Saam-Cole theory includes the molecule/solid interaction in a form of the sum of the dispersion pair interactions. The Saam-Cole theory is fit for description of adsorption phenomena in regular mesopores[22j. 

Table 2 Pseudo-first-order rate small-pore molecular sieves constants for n-butane cracking on, 19,20 ...

As with propylene, the medium pore SAPO s are again quite active and also quite selective. Thus, sApO-11 gave more than 90% 1-hexene conversion and more than 90% selectivity to isomerized products, half of which were skeletal isomers. The medium pore SAPO-11 is several orders of magnitude more coke resistant than the large pore SAPO-5. This large pore molecular sieve was already catalytically inactive after 30 minutes, whereas isomerization activity with SAPO-11 remained unchanged throughout the three-hour... 

The discovery of ordered mesoporous silica materials by scientists from Mobil Corporation in 1992 was a significant breakthrough in the field of porous materials. This class of materials, whose most prominent representatives are MCM- and SBA-type materials, offers unique potential for the immobilization of catalysts. Their unprecedented properties, such as large pore spaces, ordered tuneable pore sizes and large surface area open up new possibilities for the catalytic conversion of substrates with larger molecular size. The physical properties of their inner surfaces, such as surface acidity, can be chemically modified, affbrding additional flexibility in catalyst design. ... 

Recent progress in the theory of adsorption on porous solids, in general, and in the adsorption methods of pore structure characterization, in particular, has been related, to a large extent, to the application of the density functional theory (DFT) of Inhomogeneous fluids [1]. DFT has helped qualitatively describe and classify the specifics of adsorption and capillary condensation in pores of different geometries [2-4]. Moreover, it has been shown that the non-local density functional theory (NLDFT) with suitably chosen parameters of fluid-fluid and fluid-solid interactions quantitatively predicts the positions of capillary condensation and desorption transitions of argon and nitrogen in cylindrical pores of ordered mesoporous molecular sieves of MCM-41 and SBA-15 types [5,6]. NLDFT methods have been already commercialized by the producers of adsorption equipment for the interpretation of experimental data and the calculation of pore size distributions from adsorption isotherms [7-9]. 

Randomly constrained orientational order has been investigated by H NMR in porous Vycor glass [217] and aerogels [218] of various pore size distributions. The isotropic-nematic transition was found to be replaced by a continuous evolution of orientational order in the pores. The reason for this is that in contrast to classical liquids the surface effect on the orientational molecular order in liquid crystals is of long range. 

Kapoor et al [131] demonstrated that the synthesis of hybrid organosilicas with mesoscopically ordered pores and molecularly ordered pore walls was not confined to symmetrically linear bridged organosilane precursors, but a similar arrangement could also be obtained with nonlinear symmetric bridged organosilane precursors, such as 1,3-BTEB. 

Synthesis of periodic mesoporous phenylenesilica under acidic conditions with novel molecular order in the pore walls. Chem, Mater., 15,4886-4889. 

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