Heats of immersion in water have been determined for a number of outgassed porous crystals enriched by ion exchange in various cations zeolites X, Y, A, chabazite, and synthetic ferrierite , and for clinoptilolite and mordenite in their Na-forms, decationated, and in various stages of dealumination.
Finally, heats of immersion were determined in NaX, NaY, NaA, and Ca,Na chabazite in which the crystals initially contained various known loadings of zeolitic water. This result leads to the assumption that certain structures are formed between the NH3 molecules and the cations present in the zeolite cages. This behavior may be explained by cooperation between defect structures of the adsorbent and configuration effects of the hydrocarbon chain.
These measurements are more reliable than previous calorimetric results for sorption systems in that the calorimeter is not connected by a metal filling tube to the apparatus at room temperature. A Monte Carlo evaluation has been made of a "cell model, assuming Lennard-Jones potentials for all interactions. The isotherm and qst predictions for CH4 are good, but the Cs vs.
Equilibrium sorption isotherms, heats of sorption, and diffusivities have been calculated for carbon dioxide in type A zeolites, using an idealized model of the molecular potential field. Good agreement with practical results are obtained for heats of sorption, but results are poor for equilibrium isotherms, and the simple approximation used for diffusivity is quite inadequate. Nevertheless, useful insight is obtained on the basic assumptions. The self-diffusion of carbon dioxide in single pellets of commercial type 5A molecular sieve has been studied using C as a tracer.
Using a simple model of the pellet structure, it was possible to deduce effective diffusivities for both pore and crystal diffusion. Finite difference calculations, taking into account generation and loss of heat and changes in diffusivity equilibrium adsorption with temperature, reproduced pertinent features of the rate and temperature data. When the temperature maximum occurs late in the adsorption process, the rate curve is drastically different from expected for isothermal adsorption.
Results show clearly that intracrystalline diffusion is the rate-controlling process and that it is represented well by a Fick's law diffusion model. The contribution of specific interactions to the total energy of adsorption of ethylene on zeolites decreases from LiX to CsX. The curves of the heat of adsorption of water have a wavelike form, especially for KX zeolite.
Molecular sieves, 5A
At small filling, the heat diminishes from LiX to CsX zeolite but at intermediate fillings it goes through a maximum for KX. The increments of the heat of adsorption for the CH3, CH2, and OH groups in investigated homologous series are constant. The nature of the cations present in a zeolite can have a marked effect upon the rate of intracrystalline counterdiffusion, as shown by studies with several selected aromatic hydrocarbons in a series of ion-exchanged forms of the type Y zeolite.
For 1-methylnaphthalene diffusing from type Y into bulk cumene, the desorptive diffusion coefficients vary by 2 orders of magnitude over different ion-exchanged forms in the order:. The results are interpreted in terms of the size of the diffusing molecule and the effect of the cation upon the pore size of the zeolite. Counterdiffusion of the molecules studied occurs readily in the various forms of type Y zeolite, but molecule—molecule interactions between the counterdiffusing molecules have a pronounced effect upon the upon the diffusion rate.
Using the pure-component data, multicomponent isotherms were predicted and compared with experimental results. The more strongly adsorbed species completely overwhelm the lesser adsorbed components e. Wherever 2 species of approximately equal affinity are adsorbed e. For zeolites with 1-, 2-, and 3-charge cations, the specificity of adsorption interaction is determined by nature of the cations replacing sodium ions, by the degree of exchange, by population of single-cation positions, and by other factors.
A more pronounced influence of cations replacing sodium ions on sorption kinetics has been established for zeolites of type A. Chromatographic data show that when the number of cations in a unit cell of a faujasite type zeolite decreases, the values of retention volumes of hydrocarbon gases and carbon monoxide decrease. The nature of alkali metal cations of type X zeolites and their positions in the crystal lattice essentially influence values of the retention volumes of the compounds studied.
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On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite.
Dimethylnaphthalene concentrate contains significant amounts of 2,6-dimethylnaphthalene bound in a binary eutectic with 2,7-dimethylnaphthalene.
This eutectic cannot be broken by distillation or solvent crystallization. A practical method for separating this eutectic mixture of 2,7-dimethylnaphthalene and 2,6-dimethylnaphthalene has been achieved. Selective adsorption of 2,7-dimethylnaphthalene from a dimethylnaphthalene concentrate is obtained with sodium type Y molecular sieves.
Separation factors of 6 to 8 are obtained, indicating the high selectivity of these particular molecular sieves for this adsorption. Previous work in this area achieved a separation factor of 2. A continuous method has been developed for adsorption and desorption of 2,7-dimethylnaphthalene. Toluene has been selected as the optimum desorbent. This process makes 2,7-dimethylnaphthalene potentially available. Three modes of vapor diffusion in granular zeolites and some results are discussed.
Molecular sieves, 4A, mm | CAS | SCBT - Santa Cruz Biotechnology
The first mode involves diffusion of adsorbate through crystals and along secondary pores. On the basis of steady-state diffusion experiments, it was concluded that diffusion within the zeolite crystals is not significant as a contribution to total diffusion of organic vapors through the granule. In the second and the third modes, diffusion to the central parts of a granule is limited to secondary pores. For these modes which differ in the ratio of diffusion rates in the crystals and in the secondary pores, approximate equations of kinetics of adsorption are derived, and their agreement with experimental data is discussed.
The last 10 years have seen an extraordinary expansion in the catalytic applications of zeolites, particularly in the petroleum industry effort in catalytic cracking. Catalysis of a broad spectrum of "classical" organic reactions has been discovered for various base-exchanged forms of crystalline aluminosilicates, notably for the faujasite family.
In this review, these reactions are classified, and certain mechanistic and phenomenological features are highlighted. Also considered are hydride transfer processes, reactions on zeolite external surfaces, intracrystalline aging pathways, and systems involving inhibition of reaction by adsorbed reactant.
Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Caand La-exchanged Y zeolites is presented.
Cracking of the isomeric hexanes over alkali metal—exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. Catalyst preparations with controlled number and nature of acid sites were used to study the cracking of cumene, 2,3-dimethylbutane, and isomerization of xylenes.
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In all these cases, the Bronsted acid with its surface OH group was responsible for the catalytic activity. The cracking of 2,3-dimethylbutane required a small number of Lewis acid sites to initiate the reaction, presumably caused by abstraction of a hydride ion from the saturated hydrocarbon. The cracking of 2,3-dimethylbutenes was studied also. The hydrogen—deuterium equilibration is catalyzed by the Lewis acid sites. Introduction of palladium into the catalyst and the use of hydrogen as the gas carrier affects the cumene cracking favorably, but represses the branched chain hydrocarbon cracking.
The number of acid sites provided when calcium or lanthanum ions are exchanged depends mainly on the valency of the ion. The cracking of isooctane used to study catalytic properties of zeolites is enhanced considerably by hydrogen. Then a particular kind of sites has hydrogenative properties. Either acidic or catalytic properties are lowered by ions located near or in the supercage, and they increase when inner sites are exchanged. HY zeolite behavior compares well with the NH4Y zeolite properties.
The isomerization of n-butenes was used as a test reaction to follow the development of catalytic activity in Na-Y zeolite. A pure Na-Y zeolite containing no decationated sites was found catalytically inactive for this reaction. In contrast with silicaalumina catalysts, carbonaceous residues did not appear to play a role in the formation of the catalytic sites as long as H2O was used as cocatalyst.
Studies on ammonium Y zeolite back-exchanged with calcium and magnesium are reported. In the sodium system, calcium promotes the activity.
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