Synthesis, characterization and properties of Ni2+-doped ZnAl2O4-based spinel-type solid acid catalysts: SO42−/Zn1-xNixAl2O4

Abstract

A novel series of SO42−/Zn1-xNixAl2O4 spinel-type solid acid catalysts were prepared simply by doping Ni2+ ions in the lattice of spinel ZnAl2O4 and applied in the synthesis of n-butyl acetate. The crystal structures, surface molecular structures, morphologies and acidic properties were characterized by the methods of XRD, Raman, FE-SEM, EDS and FT-IR (direct and NH3 adsorbed). The experimental results revealed that both Ni2+ content and the doping ion had a significant influences on the stability of spinel structure and the acidic properties of the catalysts. Doping Ni2+ played a significant role in stabling the spinel structure and consequently increasing the number of active acid centers. Compared with undoped SO42−/ZnAl2O4 and Cu2+-doped SO42−/Zn0.9Cu0.1Al2O4, Ni2+-doped SO42−/Zn0.9Ni0.1Al2O4 presented the higher catalytic activity with 92.0% conversion of acetic acid in the esterification of acetic acid with n-butanol. Moreover, Ni2+-doped SO42−/Zn0.9Ni0.1Al2O4 exhibited the better excellent reusability, which remained above 60% conversion of acetic acid even after being used six times in the esterification of acetic acid with n-butanol. Raman and FT-IR results proved that M-O bonds in tetrahedral (Td) and octahedral (Oh) sites were covalently linked with sulfate species to generate surface acid sites, which provided the main basis for the establishment of acid center formation model on AB2O4 spinel oxides. In addition, a possible Eley-Rideal model mechanism for the esterification of carboxylic acid with alcohol was proposed on this basis of both the characterization results and the results of the catalytic activities.