Channel intersections

In the as-synthesized MFI-crystals the tetrapropylammonium (TPA) ions are occupying the intersections between the straight (parallel) and the sinusoidal channels of the zeolite, thus providing an efficient pore filling. The detailed structure of as-synthesized MFI-TPA has been elucidated by X-ray single crystal analysis (ref. 3). Also the combination tetrabutyl-Ztetraethylammonium can be applied as template in MFI-synthesis. A 1 1 build-in is found then (Fig. 1). When only tetrabutylammonium is available as template, the MEL (ZSM-11) lattice is formed with another distance between the channel intersections. 

From the comparison of the results, it can be inferred that copper ions exchanged in the ZSM-5 zeolites assumes a bidentate (sites 12 and II) or tridentate coordination (sites M5, Z6, and M7). These two groups differ also in the molecular properties (Table 2.2). The I-centers are characterized by lower values of the valence index and greater partial charges, QCu, in comparison to the M and Z centers, which is associated with the deeper laying HOMO and LUMO levels. In the M5, Z6, and M7 sites Cu1 ions exhibit more covalent character, and the frontier orbitals have less negative energies. As a result, the chemical hardness of the I-centers, located at the channel intersections, is smaller than those located on the walls of the ZSM-5 zeolite. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

Large molecules or ions can also be trapped inside the zeolitic framework during cristallization. Tetrapropy1ammonium ions in ZSM-5 zeolite and tetrabutylammonium ions inZSM-11 zeolite (intact in their respective frameworks) occupy the channel intersections and their alkyl chain extend in the linear and zig-zag channels in ZSM-5 zeolite or in the perpendicular linear channels of ZSM-11 zeolite. 

In ZSM-5 zeolite, almost every channel intersection is occupied by a tetrapropylammonium ion. Oppositely, tetrabutylammonium ions occupy preferentially the large cavities in ZSM-11 zeolite, the small cavities being only partially occupied. Their alkyl chains extend in the channel system in order to fill completely the available channel length (11). 

When tripropylamine or tributylamine is used, mixed ZSM-5/ZSM-11 phases are formed. Their nature seems to be determined more by the zeolitic channel filling than by the location of the organic molecules at the channel intersections. 

Zeolite structures are designated by a three capital-letter code, for example, FAU stands for the faujasite structure, to which the well-known X and Y zeolites belong. A very useful short notation is used for the description of the pore system(s) each pore network is characterized by the channel directions, the number of atoms (in bold type) in the apertures, the crystallographic free diameter of the apermre (in A), asterisks (1, 2, or 3) indicating whether the systems is one-, two-, or three-dimensional. To completely specify the pore system, the eventual presence of cages (or channel intersections) should be indicated, along with their... 

Processes such as UOP s Tatoray [2] process are used to increase xylene yield in an aromatics facility by converting two moles of toluene into one mole of xylenes and one mole of benzene. As the reaction is thought to be a bimolecular process, zeolites with side pockets or channel intersections are the most desired (Table 12.12). 

The molecules were predicted to travel through the center of the channel, avoiding the channel intersections. This result is in agreement with the similar calculations of Demontis et al. (23) and June et al. (11). While the centers of mass of molecules translate around the channel centers, libration also occurs to permit a closer approach of some C and H atoms to the zeolite walls (68). 

In Silicalite. A variety of papers are concerned with sorption of methane in the all-silica pentasil, silicalite. June et al. (87) used a Metropolis Monte Carlo method and MC integration of configuration integrals to determine low-occupancy sorption information for methane. The predicted heat of adsorption (18 kJ/mol) is within the range of experimental values (18-21 kJ/ mol) (145-150), as is the Henry s law coefficient as a function of temperature (141, 142). Furthermore, the center of mass distribution for methane in silicalite at 400 K shows that the molecule is delocalized over most of the total pore volume (Fig. 9). Even in the case of such a small sorbate, the channel intersections are unfavorable locations. 

In an MD study of methane sorption and diffusion in silicalite, Nicholas et al. (67) identified favorable sites for sorption. From the MD calculations, the time-averaged position of the center of mass of the methane molecule was plotted. Energy minimization calculations were then performed, locating the methane molecule at positions where the MD calculations predicted they spent the most time. Each channel intersection region was found to contain two sites that are minima for methane-zeolite interactions. These two sites are separated by a translation parallel to the straight channel... 

From the solid-state 2H NMR spectra of tert-butyl alcohol sorbed within samples of HZSM-5 catalysts, it was concluded that the geometry of the isolated molecule is unperturbed when bound to the active site (8g, h). Since the critical dimension of the ter/-butyl alcohol (ca. 6.8 A) significantly exceeds the channel diameter of the catalyst (ca. 5.5 A), it follows that this reactant molecule must be accommodated at channel intersections [where there is a free diameter of ca. 9 A (70)], and that, therefore, the Bronsted active sites are themselves located at such intersections, as illustrated in Figs. 4 and 5. 

Using the pulse flow catalytic reactor mentioned earlier, we were able to create a pentamethylbenzenium (Figure 1) in zeolite HZSM-5 (16). The cation was synthesized in the channel intersections or pore mouths by alkylating benzene or toluene with methanol at 523 K. This cation cannot be prepared in detectable concentration when the reaction is carried out in a sealed rotor, as is commonly done in in situ NMR studies. In contrast, when alkylation is carried out in a flow reactor, the co-product—water—is removed, and the cation accumulates as a significant product in the zeolite. 

This micro device contains one zig-zag-like main micro channel. In the direction of the flow, four smaller channels intersect this main flow path [160], These smaller channels have all the same width, but differ in length. The two confluent streams enter the micro mixer in parallel fashion. 

To create liquid access, holes were punched through the replica. Then it was placed on another thin slab of PDMS for sealing. Because an anisotropic KOH-based etching method was employed to etch the Si master without corner compensation, the channel intersections in the replica were limited in shape by the <111> plane of the Si master [159]. 

FIGURE 5.3 Electropherograms of fluorescein and BODIPY separations (a) without and (b) with field-amplified sample stacking. Fluorescence signal is normalized with exposure time in both plots. The position of the detector is 10 mm from the downstream channel intersection of the chip. The signal increase is 1100-fold for the stacked case, and resolution increases from 3 to 120 [584]. Reprinted with permission from Wiley-VCH Verlag. 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

The formation of coke requires therefore the possibility for (a) reactant(s) to undergo bimolecular reactions and for the reaction products to be retained in or on the zeolite. This retention occurs either because the products are not volatile enough to be eliminated from the zeolite under the operating conditions or because their size is greater than the pore aperture (hence a steric blockage in the cavities or at channel intersections). Obviously the first mode of retention concerns not only the coke molecules deposited within the micropores but also those deposited on the outer surface of the crystallites. 

At 450°C the formation of coke compared to that of the other products is very slow. Whatever the reactant the main coke components are alkylpyrenes. However these alkylpyrenes result probably through different reaction paths one involving aromatics and alkene would be responsible for coke formation from the propene-toluene mixture and from propene, the other involving only reactions of aromatics would be responsible for coke formation from toluene [8]. The same coke molecules are formed through both paths because their size and their shape are imposed by the size and the shape of channel intersections. 

Olefins resulting from n-heptane cracking (step 1) are transformed through various reactions (oligomerization, cyclization, hydrogen transfer etc...) into soluble coke molecules sterically blocked in the cavities or at channel intersections (step 2), The same reactions transform soluble coke molecules into non soluble molecules (step 3) that overflow onto the outer surface of the zeolite crystallites. Non soluble coke molecules could also overflow in the mesopores created during zeolite... 

---