Titanate nanotubes as acid catalysts for acetalization of glycerol with acetone: Influence of the synthesis time and the role of structure on the catalytic performance

A series of titanate nanotubes (TNTs) have been prepared by a simple hydrothermal procedure (e.g.,sodic and protonic TNTs). Investigations on the influence of the material type as well as the synthesis time conditions were examined. The as-synthesized and spent catalysts were characterized by XRD, Raman spectroscopy, TEM, SEM-EDS, N-2 adsorption-desorption isotherms and acidity measurements through thermoprogramthed desorption of pyridine (pyridine-TPD). Catalytic results towards acetalization of glycerol with acetone was determined by varying the reaction conditions. A screening of the physicochemical properties revealed differences in the tubular structures of the TNTs with interwall distances varying from 1.07 to 1.11 nm, depending on the synthesis time applied. Sodic TNTs synthesized with times as high as 24 h possessed the Na2Ti3O7 phase whereas the protonic solids had H2Ti3O7 ones. These phases changed, depending on the synthesis time. Longer hydrothermal treatment times such as 72 h at 160 degrees C exerted potent effects of decreasing the structural order of the TNTs, but lead to high textural properties and acidities of the solids as well. Most sodic TNTs did not show glycerol conversion, under relatively mild reaction conditions. On the contrary, glycerol was readily converted to solketal and acetal products with high selectivity, depending on the synthesis time of the prepared material. The best performance towards glycerol conversion was achieved by the HTNT synthesized at 72 h, whose glycerol conversion was 44.4% and selectivity toward the desired products more than 98% and only 2% side products (mostly cyclic products) at 50 degrees C and acetone glycerol molar ratio of 1. (C) 2016 Elsevier B.V. All rights reserved.