F. M. Bobonich

The Relation between the Efficiency of an Adsorbent in a Refrigeration System and Its Hydration Isotherm

The efficiencies of various types of silica gels, zeolites, and crystalline microporous aluminophosphates in adsorption systems of refrigeration have been determined. On the basis of the results obtained a method is proposed for the determination of the efficiency of an adsorbent from its hydration isotherm without experimental determination of the temperature dependence of the adsorption equilibrium parameters.

Effect of Hydration–Dehydration Processes on the Structure and Porosity of Detemplated Silica Fibers

A study was carried out on the conditions of template removal (calcination or extraction by solvents) and hydration–dehydration processes on the hexagonal structure and porosity of detemplated fibers. The hydration–dehydration cycle leads to a decrease in fiber porosity, especially in fiber detemplation by extraction with surfactant solvents such as ethanol and acetic acid.

RELATIONSHIP BETWEEN ADSORPTIVITY OF ZEOLITES AND THE DENSITY AND COMPOSITION OF THEIR FRAMEWORK

This article presents a summary and correlation of results from an investigation of the adsorptivity of Na zeolites in relation to the physicochemical properties of the adsorbate and the density and composition of the molecular sieve framework. Examples are presented to illustrate the possibilities of using the correlations that had been established as a means for calculating the energy characteristics and isotherms for the adsorption of various molecules by zeolites, and for predicting the adsorption properties of zeolites. It is shown that zeolites that differ in structure and chemical composition of the framework but have very similar values of the cation density of the crystals are quite comparable in the strength of the acid centers.

Relationship between hydration energy of zeolites and cation distribution in their crystals

A method is proposed for calculating the heat of immersion of zeolites in water from data on the composition and density of the zeolite framework. The phenomenon of localization of some of the zeolite cations in the unhydrated crystallographic positions is explained. For ferrisilicates with the ZSM-5 structure, the fraction of iron atoms in the framework has been estimated from data on the energy of hydration.

Increased n-Hexane Hydroconversion upon Poisoning of External Acid Sites of the Catalyst PdHZSM-5

Substitution of protons on external acid sites of the bifunctional catalyst PdHZSM-5 by tetrabutylammonium cations is accompanied by a substantial increase in n-hexane hydroconversion at T = 498 to 548 K. Based on analysis of the distribution of the catalytic reaction products, we suggest an explanation for this effect.

Features of the Distribution of Aluminum in Mordeniteclinoptilolite Catalysts for the Hydroisomerization of n-Hexane

The content of tetrahedral, extraskeletal, and ion-exchanged aluminum in zeolites consisting of natural and decationized forms of mordeniteclinoptilolite rocks after acidic activation were studied. The results were compared with acidity and catalytic activity properties of modified samples of rocks and synthetic mordenite in the hydroisomerization of n-hexane.

Characteristics of molecular sorption on template containing mesoporous molecular sieves of type MCM-41

Template containing materials of type MCM-41 have properties of microporous substances. On absorption into the micropores of such materials the dependence of the characteristic absorption energies on the molecular mass of the sorptive is extreme, and the specific absorption volume decreases with increased molar volume of the sorptive. A model is proposed to explain the peculiarities of the absorption properties of the template containing materials.

EFFECT OF REHYDRATION ON THE POROSITY OF MCM-41 MESOPOROUS MOLECULAR SIEVES

In distinction from aluminosilicates, the porosity of the detemplated silicous MCM-41 is significantly reduced after several cycles of dehydration-rehydration. The stability of Si-MCM-41 is not changed by absorption-desorption of nonpolar hexane. An explanation is proposed for the effect of rehydration on the porosity of MCM-41.

Siliconizing as a method for the production of acid-resistant modifications of faujasite

It was shown that acid-resistant modifications of faujasite can be produced by its thermal treatment in silicon tetrafluoride vapor if the ratio of the atoms in the zeolite framework is increased as a result of such treatment to Si/Al ≥ 12. At lower degrees of isomorphous substitution of the atoms. the faujasite crystal lattice is destroyed in a 5 M solution of hydrochloric acid as a result of the low acid resistance of the aluminum-oxygen tetrahedra of the zeolite and of the imperfections in its structure.

Crystallization of zeolites as formation of inclusion compounds

We consider the process of crystallization of aluminosilica gels under hydrothermal conditions as formation of inclusion compounds. We show that the possibility and direction of crystallization of zeolites, like formation of other types of inclusion compounds, are determined by the principle of maximum utilization of space and the energy factor. We establish the limits on the variation in the packing density of water molecules and organic templates occluded within the zeolite cavities. Using the classical theory of crystal formation, we explain the interconnection established in this paper between the characteristics of the reaction mixture (the nature of the cations, the alkalinity, the water content) and the integral heat of hydration of the zeolite crystals formed upon hydrothermal treatment of aluminosilica gels. For the example of ZSM-5-type zeolites, we consider the combined effect of the geometric and energy factors on the process of zeolite formation.