Zeolite channel effect

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

Nitrogen adsorption/desorption isotherms on Zeolite and V-Mo-zeolite are very similar and close to a type I characteristic of microporous materials, although the V-Mo-catalysts show small hysterisis loop at higher partial pressures, which reveals some intergranular mesoporosity. Table 1 shows that BET surface area, microporous and porous volumes, decrease after the introduction of Molybdenum and vanadium in zeolite indicating a textural alteration probably because of pore blocking by vanadium or molybdenum species either dispersed in the channels or deposited at the outer surface of the zeolite. The effect is far less important for the catalysts issued from ZSM-5. 

Specific structure-directing effects of some organic bases or cations When in the procedure BT Pr N is replaced by other organics, various pentasil-type zeolitic precursors are formed. It appears that specific zeolites are formed only when quaternary ammonium salts are used, their nature (structure) being essentially dependent on the length of the alkyl chains pure ZSM-8, ZSM-5 and ZSM-11 are obtained respectively with Et N+, Pr N+ and Bu N cations. TG data indicate that the latter fill nearly completely the zeolitic channel system (Table VIII). 

The zeolite provides the environment for shape selective chemistry and is also a high surface area support on which to disperse platinum in a relatively confined environment. The small platinum crystals within the zeolite channels and the orientation effect of the channel window are responsible for the high efficiency of the Pt-KL catalyst to convert linear paraffin to aromatics. Zeolite KL also provides an electron rich environment to enhance stronger platinum-substrate interaction via stronger platinum-support interaction. A review on the subject can be found in the article written by Meriasdeau and Naccache [85]. 

FIGURE 7.16 Effect of calcium exchange for sodium in zeolite A on hydrocarbon adsorption. Replacement of four sodium ions by two calcium ions permits easy diffusion of n-alkanes into the zeolite channels. 

Activated diffusion of the adsorbate is of interest in many cases. As the size of the diffusing molecule approaches that of the zeolite channels, the interaction energy becomes increasingly important. If the aperture is small relative to the molecular size, then the repulsive interaction is dominant and the diffusing species needs a specilic activation energy to pass through the aperture Similar shape-selective effects are shown in both catalysis and ion-exchange, two important applications of these materials. 

Nitrogen physisorption of the Ge-ZSM-5 sample revealed a considerable contribution of mesopores to the total pore volume, accompanied by a drop in micropore volume of 20%. In a study of the catalytic activity of these materials it was found that the increased mesoporosity of Ge-ZSM-5 had a beneficial effect on the catalytic activity in a series of acid-catalysed reactions.1771 It was observed that the presence of germanium in the framework does not change the strength of the acid sites but, instead, decreases the extent of deactivation from coke residues formed upon reaction. The microporous domains only have short diffusional lengths, but the shape selectivity ascribed to the zeolitic channels is still fully... 

Pre-admitting of a noble gas influence the adsorption, while admitting a noble gas subsequently to butane adsorption has no influence at all. This suggests that the observed phenomena is a kinetic effect. Likely, the noble gasses are present in the zeolite channels but no adsorbed, seriously hamper the diffusion of both n- and isobutane. 

The separation of xylene isomers on MFl zeolite membranes can be considered one example of intracrystaUine size exclusion and competitive adsorption (strongly dependent on coverage). The difference in their kinetic diameters ( 0.58 nm for p-xylene and 0.68 nm for o- and m-xylene) indicates the possibility of an effective separation using MFl membranes (see Table 10.1). The kinetic diameter of p-xylene is close to one of the MFl channels ( 0.55 nm) whereas o- and m-xylene might be excluded. Therefore, MFl zeolite channels and crystal grain boundaries determinate the permeation characteristics [27]. 

Molecules with a cross section diameter greater than about 0.60 nm caimot diffuse into the TS-1 channels and are, therefore, not oxidized. This size restriction limits this system essentially to the oxidation of linear molecules and monocyclic aromatic rings with at most, small substituents. Even with these limitations, TS-1, and to a lesser extent TS-2, is an effective catalyst for the selective oxidation of a number of different types of organic compounds. Thirty percent hydrogen peroxide is the most commonly used oxidant. The more bulky alkyl peroxides are not effective because of their inability to diffuse into the zeolite channels to react with the titanium sites. While the oxidation of most primary and secondary alcohols occurs with reasonable ease, methanol is sufficiently inert to oxidation under the common reaction conditions that it is the solvent of choice for most TS-1 catalyzed reactions. 2 26,27... 

The reaction of n-butane at different WHSV (1.5 h to 5.5 h ) over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts resulted in the same type of products as those formed at different temperatures, indicating that the reaction products formed at longer contact time are not adsorbed on the surface of zeolite or trapped in the zeolite channel system. The n-butane conversion and selectivity to aromatics over H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts decreased with increase in the space velocity. The effect of space velocity on n-butane conversion and selectivity to aromatic hydrocarbons is given in Figures 6 and 7. 

A similar shape selective effect was observed in the liquid phase. Those catalysts with the smaller pore and channel openings were more selective for para-nitrotoluene. However, in the liquid phase, no induction period was observed. Rather, all catalysts exhibited significant deactivation throughout time on stream and after 5 hrs. little of the original activity remained. As shown in Fig. 3, the para selectivity was found to decrease with time on stream. This would indicate that deactivation occurs within the pore channels effectively reducing the preferential capacity of the catalyst to generate the para isomer. The decrease in para selectivity was not evident on Beta zeolite, which has larger pores and may allow for a more uniform production of... 

In 1990, Canham reported the room-temperature luminescence of porous silicon.11111 This important discovery has attracted much attention towards an investigation in silicon with the quantum-size effect. Through chemical-vapor deposition, it is also possible to load silicon nanoparticles into zeolite channels and pores. First, disilane is treated with the protons in a zeolite and is grafted to the zeolite walls [Equation (9.13)] ... 

Thermal dealumination also tends to decrease the effective pore diameter either by shrinkage of the unit cell and/or by generating amorphous materials in the zeolite channels. This results in an increase of the resistance to diffusivity and an increased shape selectivity. This has opened several new potential applications for zeolite matrices with controlled selectivity. 

Furthermore the influence of acetone as a solvent was examined. For the homogeneously catalyzed epoxidation reaction in general, lower yields and selectivities are obtained with acetone as solvent. However for the transport of substrates and oxidant in and out of the zeolite channels, acetone could have a beneficial effect. This could not be confirmed by our experiments. 

Recently, interest has grown in the modification of zeolites to effect the conversion of methanol (refs. 1-4) or toluene (ref. 1,5) into specific types of hydrocarbons. The introduction of modifiers, such as P and B, increased the yield of olefins in the former case and the relative distribution of para aromatics in both cases. However, the fundamental question remains open as to whether the role of modification consists only of blocking the channels and thus creating diffusional hindrances, or also in altering the concentration and strength of acid sites. In this work, Mo-zeolites (ZSM-5, mordenite and Y) were used to investigate the modification effect of Mo on these reactions. 

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