Synthesis and characterization of heteropoly acids and related materials.

摘要

Heteropoly materials have received attention in the last decade because of their unique structure and catalytic activity. Many are used industrially as oxidation and hydrogenation catalysts. The heteropoly acid of interest in this study, H{dollar}sb5{dollar}(PMo{dollar}sb{lcub}10{rcub}{dollar}V{dollar}sb2{dollar}O{dollar}sb{lcub}40{rcub}{dollar}), is an example of the now familiar Keggin structure.; Many chemical substitutions and derivatives of heteropoly acids have been synthesized in an attempt to improve stability and enhance the catalytic and electronic properties. Typical substitutions have included the replacement of one or more of the Mo atoms with vanadium, or replacement of the central atom with borate, silicate, aluminate, or permanganate ions. Many salts of these acids have been prepared as well, including simple metal, transition metal, and ammonium complexes.; In this study two of the molybdenum atoms have been replaced with vanadium. This substitution of vanadium causes an intrinsic reduction in the Keggin ion without compromising the structure. The V = O centers in the structure are of particular importance since their paramagnetic activity allows them to be studied by Electron Paramagnetic Resonance (EPR); they essentially become a "probe" into any electronic and structural changes. Other techniques used in characterization include chemical analysis, Thermal Gravimetric Analysis (TGA), and X-ray Diffraction (XRD).; The research has also included synthesizing several ammonium salts of the Keggin acid including the NH{dollar}sb4{dollar}-, Nethyl{dollar}sb4{dollar}-, and Npentyl{dollar}sb4{dollar}-salts. When heteropoly acids are used industrially as catalysts, they are frequently supported on oxides such as silica, alumina, or titania. This study also examines the effect of these supports on the Keggin ion by comparing neat, supported, and sol-gel incased materials.; The catalytic ability of these complexes has been examined by studying their reactivity in hydrocarbon cracking reactions. A flow reactor system equipped with a tube furnace was used for these reactions. Analysis was performed by gas chromatography with comparison to standards. The reaction chosen for study was the cracking of cumene.