| The manufacture of ethylene from biomass-derived ethanol is an environmentallyfriendly and sustainable technological route. At present for the industrial manufactureof ethylene via ethanol dehydration, the γ-Al2O3catalyst is applied. The deep analysisof the catalyst could provide useful guidance to develop high selective catalyst used atlower temperature. In this work, by using molecular simulation approach, theappropriate γ-Al2O3（100） surface was choosed and the thermostability and catalyticproperties were studied. The adsorptive capabilities, as well as the reaction kineticswere also investigated. And the acidity and basicity of γ-Al2O3and reactionmechanism on γ-Al2O3were studied through TPD and DRIFTS.The f model of Al terminal surface was the most stable γ-Al2O3（100） surfacemodel. The reconstruction of surface would take place at temperature of1000K; thesurface reactive sites were the Al2a cidic site and O2a alkalic site; the moderate aciditywas required for ethanol dehydration; and according to the geometric adaptability rulein multi-site theory, it was reasonable to deduce that ethylene from ethanoldehydration at lower temperature might comply to the E1mechanism, as with thetemperature was increased, the ethylene formation would comply to E1ã€E2or E1cBmechanism. After the hydroxylation of γ-Al2O3（100） surface, the characteristics ofsurface was changed dramatically, and the hydroxyl groups were consistentlyadsorbed on the surface of catalyst, which could inhibit the adsorption and activationof ethanol molecules.At temperature743K and pressure1001500kPa, the corresponding sequence ofadsorption amount of single components, such as ethanol, ethylene, diethyl ether andwater, was as following: diethyl ether>ethanol>ethylene>water; all of the fourcomponents could be chemically adsorbed on γ-Al2O3（100） surface, and thecorresponding sequence of adsorption heat was as following: ethylene>ethanol>water>diethyl ether; when above components co-adsorbed on γ-Al2O3（100） surface,diethyl ether was the strongest adsorption specie with the adsorption heat246.9kJ mol-1, which generated ethylene on adsorption site. It was the importantreason why γ-Al2O3catalyst performanced high dehydration selectivity.On the γ-Al2O3（100） surface, the sequence of reaction barriers were as following:β-dehydrogenation of ethanol to form carbanions> direct dehydration of ethanol toform ethylene>dehydroxylation of ethanol to form carbonium ions. With the harmonic transition state theory, the reaction rate constant for each elementary step wascalculated using the reaction barrier and the vibration frequencies of reactant andtransition state. When temperature under500K, the sequence of reaction rate constantwas as following: dehydroxylation of ethanol to form carbonium ions>directdehydration of ethanol to form ethylene>β-dehydrogenation of ethanol to formcarbanions. When temperature above500K, the sequence of reaction rate constantswere as following: β-dehydrogenation of ethanol>direct dehydration of ethanol toform ethylene>dehydroxylation of ethanol to form carbonium ions.The result of TPD and DRIFTS experiment showed that there were acid andalkali centers with different intensity on the real γ-Al2O3surface, and the presence ofa small amount of B acid center, showed that there was hydroxylation phenomenon onthe real γ-Al2O3surface. The result of DRIFTS-MS experiment showed that, with therise of temperature, the ethylene production increases and diethyl ether production isreduced from ethanol dehydration on γ-Al2O3. The result of ethanol pulse feedingexperiment showed that when ethanol feeding is stopped, acetylene was detected aftera short time, and ethylene may be activated to generate acetylene on γ-Al2O3catalystsurface. Polymerization of acetylene may cause carbon deposition on catalyst surfacearea. |
