Roles of texture and acidity of acid-activated sepiolite catalysts in gas-phase catalytic dehydration of glycerol to acrolein

Abstract

In the search for novel solid acid catalysts for the gas-phase catalytic dehydration of glycerol to acrolein, sepiolite clay minerals were activated with hydrochloric acid to produce acid-activated sepiolite catalysts. The effects of the activation process on the texture and acidity of acid-activated sepiolite, and their roles in gas-phase catalytic dehydration of glycerol to acrolein, were then investigated. A series of acid-activated sepiolite catalysts were made from purified natural sepiolite treated with hydrochloric acid of different concentrations (0–8mol/L) at 80°C, followed by drying at 100°C. The acid-activated sepiolite catalyst samples were characterized using X-ray diffraction, X-ray fluorescence analysis, scanning electronic microscopy, thermogravimetric analysis, N2 adsorption-desorption isotherms, NH3-temperature-programmed desorption. Fourier transform-infrared spectroscopy and pyridine adsorption followed by in situ infrared spectroscopy. The catalytic performances in gas-phase catalytic dehydration of glycerol to acrolein were studied using a vertical fixed-bed reactor. Typically, a glycerol conversion of 92.9%, an acrolein selectivity of 59.4%, and a yield of acrolein of 55.2% were achieved when the gas-phase catalytic dehydration of glycerol was conducted over HCl (2mol/L) acid-activated sepiolite catalyst at 320°C with an aqueous glycerol solution (20wt.%) at a rate of 0.10ml/min as the feedstock and air as the carrier gas at a flow rate of 20ml/min. The activation of the sepiolite with hydrochloric acid followed by drying can eliminate a portion of the zeolitic water in the tunnels, remove part of the magnesium and aluminum cations in the octahedral sheets of sepiolite, unbundle aggregated sepiolite fibers, and partly break the Si-O-Si in the tetrahedral sheets of sepiolite. These factors increased the porosity, the specific surface area and the acidity of the sepiolite. The partial leaching of magnesium and aluminum ions in the octahedral sheet generated octahedral vacancies and created MgOAl+ sites in the octahedral sheets, thereby increasing the amount of Lewis acid sites. Along with penetration of H+ cations into the interlayered space of the sepiolite by an ion-exchange reaction and physicochemical surface adsorption, some breakage of the SiOSi bonds in the tetrahedral sheets generated more SiOH+, thus increasing the quantity of Brønsted acid sites. Furthermore, the strength of acidity of the hydrochloric acid-activated sepiolite was significantly changed after the acid activation. The medium-strong Brønsted acid sites (SiOH+ and H+ adsorbed on the surface) appeared to be beneficial for the high conversion of glycerol and the selectivity to acrolein. Lewis acid sites could facilitate the formation of acetol. Under the reaction conditions mentioned above, the coking on the hydrochloric acid-activated sepiolite catalysts readily occurred, rapidly leading to the decrease of the yield of acrolein.