A deactivation mechanism of sulfated titania in the esterification of acetic acid and n-butanol

The present work systematically investigated the deactivation of sulfated titania in the esterification of acetic acid and n-butanol. Under the set conditions, sulfated titania was used 20 times and for 10 h accumulatively. The catalyst had the largest catalytic activity of 95.69 % at the second cycle and then the catalytic activities gradually dropped, and the last-cycle conversion of acetic acid was 46.15 %, suggesting a serious deactivation of the catalyst of sulfated titania after use in 20 cycles. XPS, FTIR, XRD, BET, TG–DSC, TEM and NH3-TPD were employed to characterize the new and deactivated catalysts, to systemically study the deactivation phenomena of sulfated titania and to find some reasons for the considerable deactivation of sulfated titania in the esterification. Some deactivation phenomena were observed on the deactivated catalyst when compared with the new one. (1) The deactivated catalyst decreased its acidity, based on the IR and NH3-TPD characteristic results. (2) The deactivated catalyst increased its surface specific area and its pore volume, and decreased its pore diameter, based on the BET characteristic results. (3) Some carbon deposits appeared on the surface of the deactivated catalyst, and some originally active sulfate groups may have been lost, poisoned and turned into less-catalytic and non-catalytic sulfur species based on the XPS and TG–DSC results. (4) The deactivated catalyst decreased its crystallinity based on the XRD results. (5) The deactivated catalyst diminished in particle aggregation, based on the SEM results. Thus, a deactivation mechanism is tentatively proposed. Namely, some originally active surface sulfate groups may have been gradually turned into the undesired free sulfuric acid and organic sulfate esters (OR)–(O=S=O)–O or (OR)–(O=S=O)–(OR) [R = C4H9, etc.] due to Ti4+ cations’ hydrolysis by H2O and their alcoholysis by n-butanol, which may lead to a gradual acidity degradation of the catalyst so as to lead to a gradual deactivation of sulfated titania. All the characteristic results more or less support the proposed deactivation mechanism.