Synthesis and Characterization of Co–Mn Nanoparticles Immobilized on a Modified Bentonite and Its Application for Oxidation of <i>p</i>-Xylene to Terephthalic Acid

Abstract

This study describes the synthesis and characterization of a new supported bentonite catalyst prepared by the two-step modification of Na-bentonite. In the first step, Na-bentonite is modified with cetyl pyridinium bromide (CPB) to give a monolayer bentonite (CP-bentonite) in which surfactant ions are adsorbed by cation exchange with the counterions of bentonite. This gives a disordered liquid-like monolayer arrangement of alkyl chains within the gallery. This monolayer bentonite is then treated with the second generation of an amidoamine cascade known as 3,3′-(dodecylazanediyl)bis(N-(2-(2,3,3′-(dodecylazanediyl)bis(N-(2-(2-aminoethyl(2-hydroxybenzenimine))ethyl)propanamide) (DAEP), which has a long aliphatic tail (C12) and a hydrophilic head, to form a modified bilayer bentonite (Ben-DAEP-modified). The solid/liquid interface layer of this architecturally designed bentonite (Ben-DAEP-modified) was used as a pincer-type ligand for the synthesis of a new host–guest catalyst by immobilization of Co, Mn, or Co/Mn on it. The structure, specific surface area, and porosity of bentonite are significantly altered by the incorporation of nanoparticles. The resulting solid catalysts were characterized by UV–vis, Fourier transform infrared (FT-IR), inductively coupled plasma (ICP), scanning electron microscopy (SEM), N2 adsorption, and transmission electron microscopy (TEM) techniques. The oxidation of p-xylene produces five products: 4-methyl benzaldehyde, 4-methyl benzoic acid, 1,4-benzene dialdehyde, 4-carboxybenzaldehyde, and 1,4-benzenedioic acid. The reaction conditions for oxidation of p-xylene were optimized by varying the temperature, pressure, amount of catalyst, time, and catalyst-to-oxidant ratio. The optimum conditions for oxidation of p-xylene to 1,4-benzenedioic acid (terephthalic acid) were 0.15 g of Ben-DAEP-modified-Co/Mn (Co/Mn with a ratio of 10/1), 190 °C, 3% of Br– as promoter, 17 atm, and 3 h.