Linear 96-channel systems

Figure C2.12.7. Channel system of MFI (top) and MEL (bottom). The linear channels are interconnected by zigzag channels in ZSM-5 while exclusively straight miming channels are present in ZSM-11 - larger internal openings are present at the chaimel intersections - the arrows indicate the pathways for molecular transport tlirough the channel system.

Cyclohexene is oxidized very slowly in the presence of TS-1 little if any epoxide could be obtained under conditions of rapid oxidation of 1- and 2-alkenes to the corresponding epoxides. This low reactivity has been ascribed to the molecular dimensions of cyclohexene, which cannot enter the channel system of TS-1. Evidence for this suggestion was obtained by elution chromatography when TS-1 was loaded in a chromatographic column and a mixture of cyclohexene and 2-hexene injected, the retention time for cyclohexene was much less than that of linear 2-hexenes, despite the higher boiling point of cyclohexene (Tatsumi et al., 1990a). 

The high silica/alumina ratio zeolites ZSM-5 and ZSM-11 both contain two intersecting channel systems composed of 10-membered oxygen rings. The channels in these zeolites are elliptical, with a free cross-section of 5.5 x 5.1 for the linear channels, and a cross-section of 5.6 x 5.4 for the sinusoidal channels in ZSM-5. The channel structures of these two zeolites are shown in Figure 1. 

Temporal Devices. A temporal dispersive device uses a single channel which is scanned as a function or time to yield information on the intensities present in various resolution elements. Two basic approaches are possible (1) the detector may be scanned across a fixed spectrum or (2) the spectrum may be scanned across a fixed detector. In addition, these systems may be further differentiated on the basis of the manner in which the spectrum is scanned. Thus, linear-scan systems scan the spectrum at a constant, fixed rate. In contrast, programmed-scan systems have the capability of momentarily stopping at wavelengths of analytical interest, while spectral regions of little interest are rapidly scanned. For a complete review of the area of rapid-scanning spectrometry up to 1968, the interested reader should consult Volume T of Applied Optics which was entirely devoted to this subject. 

The delocalized state can be considered to be a transition state, and transition state theory [105], a well-known methodology for the calculation of the kinetics of events, [12,88,106-108] can be applied. In the present model description of diffusion in a zeolite, the transition state methodology for the calculation of the self-diffusion coefficient of molecules in zeolites with linear channels and different dimensionalities of the channel system is applied [88], The transition state, defined by the delocalized state of movement of molecules adsorbed in zeolites, is established during the solution of the equation of motion of molecules whose adsorption is described by a model Hamiltonian, which describes the zeolite as a three-dimensional array of N identical cells, each containing N0 identical sites [104], This result is very interesting, since adsorption and diffusion states in zeolites have been noticed [88],... 

During the first restructuration of the silicate or Si-rich aluminosilicate complexes prior to the zeolitic nucleation, the HM++ entities play a structure stabilizing role. By their particular linear shape, they favour the formation of channel systems. Simultaneously, they can also neutralize one AIO2- negative center of... 

D. P. Roelofsen (Technological University, Delft, Netherlands) In your adsorption experiments, you tried to reach equilibrium by working at 100°C. In view of the earlier paper presented by Satterfield and Katzer which showed a very slow counterdiffusion of molecules such as 1-methyl-naphthalene and cumene, I would imagine that the counterdiffusion of your 2,6- and 2,7-DMN would be even slower. This could well mean that you reach a pseudo-equilibrium in which the ratio 2,6 2,7 isomer is kinetically determined. In L sieve, one then would expect that 2,6-DMN, which is the more linear of the two, would be preferred by the onedimensional channel system, in agreement with your data. With X and Y sieves, one can imagine that the nonlinear 2,7-DMN has just the right... 

With this approach we can uncover, in an unprecedented detail, how a single fluorescent dye molecule travels through linear or strongly curved sections of the hexagonal channel system, how it changes speed, and how it bounces off a domain boundary with a different channel orientation. Furthermore, we can show how molecular travel is stopped at a less ordered region, or how lateral motions between leaky channels allow a molecule to explore different parallel channels within an otherwise well-ordered periodic structure. 

The pore diameters of MFI-type zeolites are comparable to the size of many commercially important molecules, such as aromatics or linear or branched hydrocarbons [1]. Thus, the study of the difiusion of reactive molecules in the channel system of zeolite catalysts is of considerable interest for the understanding of the catalyst performance. A variety of methods has been developed and applied to the measurement of diffii-sion coefficients, amongst others gravimetric techniques [2], neutron scattering [3], NMR [4] and Frequency Response [5]. The FTIR technique offers the possibility to study sorption and sorption kinetics under conditions close to those of catalytic experiments. By the use of an IR microscope, single crystals have become accessible to the FTIR technique. 

The zeolite A structure is produced by linking the ji-cages via their quadratic surfaces (over cubes). Linking over the. six-corned surfaces with hexagonal prisms leads to zeolite X and Y, which correspond to the mineral faujasite. The cationic sites are not shown in these figures. The complicated structure of ZSM 5 zeolite systems is characterized, see Fig. 5.1-7 by two crossing channel systems one linear and the other zig-zag. 

The gramicidin family of linear polypeptides represents a biologically viable channel system of related peptides in which specific changes in amino acid composition can be correlated with cation binding selectivity and transport. The parent molecule of this family of polypeptides, gramicidin A, has the amino acid sequence shown in Fig. 1. This relatively simple molecule is probably the best characterized ion channel (both structurally and functionally) and has, to date, been the principal proving-ground for many of our ideas about the molecular nature of ion conduction in membranes. ... 

Another property of zeolites is the high conversion rates in the channel system. It was also observed that with different spatial configurations of channels, cavities, windows, etc, the catalytic properties are changed and the selectivity orientates toward less bulky molecules due to limitation in void volume near the active sites or to resistance to diffusivity. This feature termed shape-selectivity, was first proposed by McBain (20) demonstrated experimentaly by Weisz et al (21) and reviewed recently (22). For instance CaA zeolite was observed to give selective dehydration of n-butanol in the presence of more bulky i-butanol (23) while CaX non selective zeolite converted both alcohols. In a mixture of linear and branched paraffins, the combustion of the linear ones was selectively observed on Pt/CaA zeolite (24). Moreover, selective cracking of linear paraffins was obtained from petroleum reformate streams resulting in an improvement of the octane number known to be higher for branched paraffins and for aromatics than for linear paraffins. Shape selectivity usually combines acidic sites within... 

It is possible to use surface modified plane wavefunctions to attempt to account for the effect of a surface on states of an ion in a channel conductor. Instead, we consider simply an abundance of ion states that can arise in a large, but finite channel system that is the analog of the molecular electronic problem for the large, finite linear polyene [40]. This approach easily could have been adopted above to... 

A similar experimental setup is used for bench-scale testing, with the Pellicon unit replaced by the Minitan (Millipore) small-scale serpentine flow system. Figure 8 shows the Minitan filter packet (right) and the linear channel retentate gasket (left). Serpentine flow is achieved by alternating left- and right-handed positions for the retentate path. This unit can hold 1/2 sq. ft. of filter area in 0.1 sq. ft. packets, and is useful for volumes to a maximum of 3-5 liters. 

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