Kinetics of the dehydration of glycerol over acid catalysts with an investigation of deactivation mechanism by coke

Kinetics for the dehydration of glycerol, taking deactivation into account, was investigated. HZSM-5 and ASPN-40 (aluminosilicophosphate nanosphere) catalysts were used in the studies. A kinetic model was established, based on the reaction mechanism, taking into account two parallel reactions (transformation of glycerol into acrolein or acetol) that occur over the acid sites. The results were consistent with experimental results using both catalysts. Based on the overall reaction, the apparent kinetic parameters were obtained and the results confirmed that the first dehydration step, which occurs over Brønsted acid sites, is the rate determining step. In addition, the structural features of catalysts appear to be one of the crucial factors that affect the kinetics of the reaction over acid sites. The HZSM-5 and ASPN-40 catalysts were deactivated to different degrees under same conditions. A kinetic equation for the deactivation, which is a function of product concentration, was found to be in good agreement with the experimental results. To elucidate the mechanism of catalyst deactivation, various characteristic analyses (BET, NH3-TPD, TPO, and 13C NMR) were employed. The changes in the properties of catalysts and the nature of the produced coke were analyzed. In the case of HZSM-5, the condensation of coke precursors is promoted within its narrow pore structures, which results in the rapid deactivation of the catalyst. In ASPN-40, less condensed carbonaceous compounds are observed and the deactivation is delayed due to the relatively uninterrupted diffusion of materials (reactants, products, or potential coke precursors).