Structure and catalytic activity correlation of CoAlPO4 in the synthesis of N,N-biphenyl urea via alkylation of aniline using dimethyl carbonate

Two sets of cobalt aluminophosphate materials, one at different pH (4.5, 7, 10, and 11.5) and the other containing different percentage of cobalt (0.5–20) were prepared by co-precipitation method. All the materials were characterized by XRD, SEM, ²⁷Al MASS NMR, TGA and DSC, FTIR, for their textural and structural properties. The surface area of the samples was determined by BET method and total surface acidity by n-butylamine titration method. The CoAlPO₄ obtained under acidic pH were found to be crystalline AlPO₄ with nonmicroporous tridymit structure associated with cobalt phosphate, whereas the one obtained under alkaline pH were amorphous aluminophosphate and also associated with cobalt phosphate. Tridymit-AlPO₄ crystals exhibited Al atom in a tetrahedral environment of phosphate groups. In the amorphous material the Al atoms were in a mixed tetrahedral and octahedral environment. The concentration of acid sites were found to be higher in the sample obtained under alkaline pH. This has been attributed due to the partial coordination of hydroxyl groups in the octahedral aluminum sites. The amorphous materials obtained under alkaline pH were found to be thermally stable up to 900 °C.The catalytic activity of all the materials was investigated in aniline alkylation using dimethyl/ethyl carbonates (DMC/DEC) under refluxing conditions. N-methyl/ethyl aniline (NMA/NEA) and biphenyl urea (BPU) were formed as the major products with the catalysts prepared at acidic and alkaline pH, respectively. A good correlation has been observed between the catalytic activity of CoAlPO₄ materials towards their selectivity for NMA/NEA and/or BPU and their textural and structural properties. Reaction conditions such as the amount of the catalyst, duration of the reaction, reusability and efficiency of the catalyst have also been evaluated. CoAlPO₄ prepared under alkaline pH with 5% of cobalt has been found to be the best catalyst for BPU synthesis from aniline and DMC/DEC under refluxing conditions.