N,N-dimethylformamide assisted facile hydrothermal synthesis of boehmite microspheres for highly effective removal of Congo red from water

The boehmite hollow microspheres with excellent adsorption performance for CR were successfully prepared by N,N-dimethylformamide (DMF) assisted facile hydrothermal method using Al2(SO4)3·18H2O as the aluminum source. Their crystallinities and hollow degrees increase with accelerating hydrolysis rates of the amides as precipitants. Especially, dimethylamine as the hydrolysis product of DMF can act as structure directing agent to make the pore size distribution of the microspheres center at 3–4 nm which are beneficial to their adsorption for CR. Furthermore, the boehmite microspheres not only show fast adsorption kinetic with an adsorption amount of 484.1 mg/g at 60 min, but also show the high maximum adsorption capacity of 847.5 mg/g fitted by Langmuir isotherm model. [Display omitted] •DMF assisted hydrothermal preparation of boehmite hollow/solid microspheres.•Dimethylamine can promote the formation of boehmite microspheres.•Dimethylamine is beneficial to forming pore sizes distribution centering at 3–4 nm.•The pore sizes distribution centering at 3–4 nm is beneficial to their adsorption.•boehmite hollow microspheres show excellent adsorption performance for Congo red. A series of boehmite microspheres with highly effective adsorption performance for Congo red (CR) were successfully prepared via amides assisted hydrothermal method at 180 °C. Effects of dosages and hydrolysates of N,N-dimethylformamide (DMF), amides species including DMF, N-methylformamide (MF) and formamide (FA), and reaction times on their physicochemical properties were studied in detail. It was found that increase on their crystallinity and shell thickness results from the different hydrolysis rates of the amides; amorphous alumina hydrate, boehmite core-shell structure and hollow microspheres were obtained at hydrothermal times of 60, 140 and 360 min, respectively due to the Ostwald ripening. Especially, dimethylamine (DMA) as a hydrolysate of DMF, can effectively regulate the morphologies of the boehmites together with the sulfate ions, and make their pore sizes distribution (PSD) centering at 3–4 nm. Importantly, the boehmite microspheres with specific surface area of 221.3 m2/g shows the maximum adsorption capacity of 847.5 mg/g for CR calculated from Langmuir isotherm model, and its adsorption amount reached a high value of 484.1 mg/g at 60 min due to the mentioned PSD. This template-free hydrothermal method using DMF as precipitant provides an alternative approach for preparing