Molecular traffic control

The use of cyclodextrins can allow stereo-, regio-, and optical selectivity, and thus molecular traffic control may be realized (Syamala et al. 1986). It has been claimed that the reaction of phenol with aqueous formaldehyde in the presence of cyclodextrins gives a 3 1 mixture of para to ortho at 38 % conversion in the absence of cyclodextrins, the ratio of para to ortho is around 2 3. 

Derouane and Gabelica16 proposed molecular traffic control as another type of shape-selectivity that could occur in zeolites having more than one type of intersecting pore system. Here, reactant molecules may preferentially enter into the catalyst through one pore system while the products diffuse out through the other, thereby minimizing counter diffusion and increasing the reaction rate. 

In Table 5.5, the effective diffusivity, De, for p-xylene plus o-xylene counterdiffusion in H-SSZ-24 and H-ZSM-11 zeolites at different temperatures and a concentration relation, cp x [%] = c x [%] = 50 [%], are reported [90], It is evident that the kinetics is governed by ordinary diffusion. Additionally, the study of the counterdiffusion of p-xylene + o-xylene and the reverse case o-xylene + p-xylene in a zeolite with a 10 member ring plus 12 member ring interconnected channel-like CIT-1 gives experimental evidence for the existence of molecular traffic control [125],... 

The concept of molecular traffic control was proposed by Derouane and Gabelica (49) to explain the unexpected absence of counter-diffusion effects during methanol conversion over zeolites such as MFI presenting interconnected channels with different sizes and tortuosity the smallest molecules (e.g. methanol) would diffuse through the sinusoidal channels while the bulkiest (e.g. aromatics) exit through the slightly larger linear channels. 

For instance, in the high-coverage form, p-xylene was shown to be localized both at the channel-intersections and in the sinusoidal channel-junctions of a B-ZSM-5 matrix, but to desorb only via the straight channels, and even more via one direction only (48). This might be related to the molecular traffic control (MTC) concept proposed several years ago by Derouane and Gabelica (49). 

Closely related to the shape selectivity on ZSM-5 is the effect of molecular traffic control. This results from the existence of two types of intersecting channels reactant molecules preferentially enter the catalyst through a given channel system while the products diffuse out through the other, so that counterdiffusion limitations are avoided. 

In the conversion of methanol to gasoline on ZSM-11, much less Ci-Cj products are observed than with ZSM5 while the Cg -aliphatics are more abundant. Whereas on ZSM-5 the aromatics fraction is mainly composed of xylenes, on ZSM-11 more aromatics are produced. Harrison et al. defined in this context product selectivity as the ratio of xylenes to trimethylbenzenes. The ratio varied from 2.5 to 0.5 for the conversion of methanol on ZSM-5 and ZSM-11, respectively. The difference in shape selectivity was also illustrated by the (m+p) o-xylene ratio, which was 13 on HZSM-5 but only 3 on HZSM-11. This was attributed to the relative dimensions of the channel intersections, 50% of which are 30% larger in ZSM-11, and to the difference in relative length of the straight (ZSM-5, ZSM-11) and tortuous channels (ZSM-5). Furthermore, since ZSM-11 contains only one type of channel, the molecular traffic control shape selectivity should, in principle, not occur in this catalyst. This provides a possible explanation for the higher yield of polysubstituted aromatics methanol reactant and aromatic products cannot avoid counterdiffusion as in ZSM-5 and this increases the probability of alkylation of the aromatics. 

The dominant deactivation process is coke formation, which explains why the catalyst can be regenerated by controlled coke combustion. The work done by Viswanadham, et al. ° and Echevsky, et al. on H/ZSM-5 catalysts indicated that coke tended to accumulate on the binder and external surface of the zeolite and occurred in such a way as to leave the active sites accessible. Deactivation occurred mainly via pore blockage. A decrease in acid strength was caused by the shifting of electron density from the condensed aromatic structures onto the active proton sites. Little loss of mean free pathlength was observed due to coke formation on H/ZSM-5 and coke did not increase diffusion resistance. Yet, the location of the coke deposits led Viswanadam, et al. to postulate that a molecular traffic control mechanism may be operative due to the position of the coke that forms in the zeolite itself. 

Molecular dynamics (MD) simulations have been used to simulate non-equilibrium binary diffusion in zeolites. Highly anisotropic diffusion in boggsite provides evidence in support of molecular traffic control. For mixtures in faujasite, Fickian, or transport, diffusivities have been obtained from equilibrium MD through appropriate correlation functions and used in macroscopic models to predict fluxes through zeolite membranes under co- and counterdiffusion conditions. For some systems, MD cannot access the relevant time scales for diffusion, and more appropriate simulation techniques are being developed. 

Reactivity enhancement by molecular traffic control - a consequence of released single-file constraints... 

Dynamic Monte Carlo simulation Molecular traffic control Random walk Single-file diffusion ... 

Molecular traffic control may occur in zeolites with more than one type of the pore system. Reactant molecules here may preferentially enter the catalyst through one of the pore systems, while products diffuse out by the other. 

E.G. Derouane, Z. GabeUca et al., A novel effect of shape selectivity molecular traffic control in zeolite ZSM-5. J. Catal. 65,486-489 (1980)... 

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