Molecular Shape Selectivity Research Articles

CVD zeolites with controlled pore-opening size

Chemical vapor deposition (CVD) techniques developed for the fine control of pore-openings of zeolite are reviewed. Silicon methoxide is used to deposit ultra thin layers of silica on the external surface of zeolites. The silica deposit narrows the pore-openings very precisely, and enhances the molecular sieve properties. The preparation, characterization, and molecular shape-selectivity of the CVD zeolite will be described and exemplified by applications to mordenite and ZSM-5. The background for the present study, i.e. the novel modification method of zeolites and the advantage of vapor-phase methods in the preparation of novel materials, is also discussed.

CVD zeolite: Preparation, characterization and molecular shape-selectivity

Preparation method, characterization and molecular shape-selectivity of CVD (Chemical Vapor Deposition) zeolite is reviewed in detail. An ultra-thin layer of silica deposited to cover the external surface of zeolites is effective in controlling the pore-opening size without changing the internal structure. Because of its fine-controlled pore-opening size, extremely high shape-selectivity is observed on the CVD zeolites. Cracking of octane isomers on mordenites and methanol conversion on ZSM-5 were explained as examples of the improved shape-selectivity.

Hydrogenation of benzene, mono‐, di‐, and trimethylbenzenes over nickel catalysts supported on porous glass

AbstractThe hydrogenation of benzene and methyl‐benzenes was performed over a nickel catalyst supported on the porous glass prepared from borosilicate glass with an alumina content ranging from 3% to 5%. The micropore size of the porous glass decreased with increasing alumina content. Although the overall reaction rates of benzene and toluene did not significantly change with the alumina content in the borosilicate glass, the rate of 1,3,5‐trimethylbenzene, which is the bulkiest molecule among the hydrocarbons tested, decreased as the content of the alumina in the support increased. This result indicates that a catalyst support with molecular shape selectivity can be prepared from borosilicate glass containing alumina.

Transition-state selectivity of H-ZSM—5 in isomerization of n-butenes

The isomerization of n-butenes took place over H-ZSM—5 and H-Y zeolite catalysts. The rates of the reaction and the activation energies were measured. It was found that H-ZSM—5 acted as a solid Brönsted acid which appeared to suppress the cis-trans isomerization. It is strongly suggested that this suppression is due to a kind of molecular shape-selectivity, transition-state selectivity.

Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

Transport properties of eastman cellulose acetate membranes: Influence of diffusant size and shape on permeability

AbstractEastman cellulose acetate membranes (acetyl content = 40%) have been studied by means of dialysis rate experiments with uncharged permeants of selected sizes and shapes. The experimental results show that the high flux membranes exert no molecular size or shape selectivity on the transport of permeants whose molecular weights are less than 1152. The membranes used in desalination, however, are selective as to molecular size and shape. Desalination membranes, therefore, may be useful in separations where differences in size and shape are present.