Titanate nanotubes covalently bonded sulfamic acid as a heterogeneous catalyst for highly efficient conversion of levulinic acid into n-butyl levulinate biofuels

Covalently bonded sulfamic acid on the titanate nanotubes (TNTs-N(SO 3 H)-(CH 2 ) 2 -NHSO 3 H) was designed and successfully fabricated by the post-synthesis modification method. The as-prepared catalyst was characterized by a variety of characterization techniques such as FT-IR, TEM, XRD, XPS, and TG-DTG. The catalytic activity of TNTs-N(SO 3 H)-(CH 2 ) 2 -NHSO 3 H was evaluated for the synthesis of n-butyl levulinate (BL) from levulinic acid (LA). It was found that the tube-like structure of the TNTs still maintained during the modification process, and Brønsted and Lewis acid sites can be efficiently immobilized on the TNTs. The effects of reaction factors such as reaction time, reaction temperature, and catalyst dosage on the esterification of LA were systematically studied. Under the optimal reaction conditions, a relatively high yield of BL (95.3%) was achieved for esterification of levulinic acid owing to the strong Brønsted acidic sites. Furthermore, the hybrid catalyst TNTs-N(SO 3 H)-(CH 2 ) 2 -NHSO 3 H was used at least five times without significant decrease in activity, and no leaching of the active species in hot filtration test was observed. Graphical abstract