Synthesis, modification, characterization and catalytic isomerization studies of NaX, B/Al-ZSM-11 and ZSM-22 zeolites

Date of Completion

January 1995

Keywords

Chemistry, Inorganic|Engineering, Chemical

Degree

Ph.D.

Abstract

Faujasite zeolites have been modified by ion-exchange of transition metal and rare earth lanthanide ions resulting in changes in the chemical, physical, and catalytic properties of these materials. These materials were studied by a variety of physical methods, including temperature programmed reduction and desorption techniques.^ NaX zeolites were ion-exchanged with Ni$\sp{2+}$ ions and Eu$\sp{3+}$ ions, resulting in significantly higher acidities than the unmodified sodium form of NaX zeolite. Desorption studies have shown that as the concentrations of Eu$\sp{3+}$ ions increase, an increase in the population and types of acid sites is observed. Multiple acid sites of varying strengths are observed in materials containing high Eu$\sp{3+}$ concentrations. Reduction of Ni$\sp{2+}$ in zeolites results in an increase in the acidity of these materials.^ Apparent activation energies, reaction rates, and rates of catalyst deactivation have been determined for cyclopropane isomerization reactions over Ni$\sp{2+}$/NaX and Eu$\sp{3+}$/NaX materials. Catalytic data show that increased Eu$\sp{3+}$ and Ni$\sp{2+}$ concentrations give much lower activation energies and faster reaction rates. This increase in catalytic activity has been attributed to increased Bronsted acidity in the ion-exchanged form of NaX.^ Sodium vapor was applied to these materials, resulting in an elimination of Bronsted acidity and loss of catalytic activity. These studies suggest that Bronsted sites are exclusively responsible for catalytic activity in these systems and that cyclopropane isomerizes by way of a carbenium ion intermediate. Sodium vapor applied to Na$\sp+$/NaX zeolite results in the formation of sodium ionic clusters arranged in octahedral symmetry. These materials were found to be active in cyclopropane ring opening and it has been suggested that electron deficient defect sites are responsible for catalytic activity.^ A second portion of this dissertation deals with n-butene isomerization reactions over B/Al-ZSM-11 and ZSM-22 zeolites. B/Al-ZSM-11 zeolites were utilized for their weaker acidities, while ZSM-22 zeolites were utilized for their unique shape-selectivities. This dissertation discusses the synthetic approaches for these materials as well as a variety of physical methods used to characterize these systems. Kinetic information was obtained by varying the temperature, feed rate, space velocities, and partial pressure of reactant. Deactivation and regeneration have been monitored in these reactions. Yields and selectivities for isobutene formation are reported. ^

Link to Full Text

Share

COinS