Ethylene In Absence Research Articles

VOx-Nb/La-γAl2O3 catalysts for oxidative dehydrogenation of ethane to ethylene

The present study deals with oxidative dehydrogenation (ODH) of ethane to ethylene in absence of gas phase oxygen using VOx-Nb/La-γAl2O3 catalysts. In catalyst formulation, La is used to modify γ-Al2O3, stabilizing its bulk phase transformation. Nb is introduced as a promoter of VOx species. The prepared catalysts are characterized using various physicochemical techniques in order to demonstrate reducibility, oxygen carrying capacity, stability, metal–support interaction and acidity of the catalysts. TPR shows that VOx-Nb/La-γAl2O3 is highly active and stable over repeated reduction and oxidation cycles. XRD analysis indicates that VOx appears both as amorphous and crystalline phases. The presence of Nb minimizes the formation of crystalline V2O5 phases, which is undesirable for ODH reaction. The NH3-TPD analysis reveals an intermediate acidity of the VOx-Nb/La-γAl2O3 catalysts. NH3-TPD kinetics analysis shows that the addition of Nb increases the activation energy of ammonia desorption, suggesting increased V–support interaction and minimized V–V interaction. The ODH of ethane activity and stability is evaluated in a CREC fluidized riser simulator. The addition of Nb increased ethylene selectivity (85.7%) at 20.1% conversion comparatively at low reaction temperature (550°C) and short contact time. Thus, Nb enhances VOx isolation and forms a secondary oxide, resulting a superior catalyst activity.

Hydrocarbon reactions on MoS 2 revisited, I: Activation of MoS 2 and interaction with hydrogen studied by transient kinetic experiments

MoS 2 was prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at 723–773 K and activated by procedures involving evacuation at 723 K and/or reduction at different temperatures, predominately 573 K. Bulk and surface properties of these samples were studied by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, oxygen chemisorption (273 K, pulse mode), and isotope exchange with D 2 to determine the quantity of exchangeable hydrogen. In addition, the quantity of hydrogenation-active surface hydrogen was determined by hydrogenation of ethylene in absence of gas-phase hydrogen. No obvious relationship was found between the S/Mo stoichiometry and the degree of Mo exposure; depending on the pretreatment, exposed Mo ions can be detected by oxygen chemisorption at widely varying quantities while the S/Mo ratio remains ⩾2. At more severe reduction treatments to remove additional sulfur, the oxygen chemisorption capacity decreases, likely due to migration of defects into the bulk. The reactivity of MoS 2, as expressed by the temperatures required to expose Mo ions and the quantity of exchangeable hydrogen, varies significantly between batches prepared by similar routes. Therefore, thorough surface characterization by a set of techniques including reactivity studies is an indispensable prerequisite for any meaningful study investigating the relationship between catalytic properties with preparation procedures. After activation, MoS 2 holds hydrogen, which is able to hydrogenate ethylene in absence of gas-phase H 2; the quantity of this hydrogenation-active surface hydrogen is not correlated with the hydrogenation activity, however.