The
promoting effect of manganese on the Ni/Al2O3 catalyst for the hydrogenation of carbon oxides, in the presence
of light hydrocarbons, was studied. Ni/Al2O3 displayed a high activity for the complete conversion of CO and
CO2 to methane and C2+ hydrocarbons. Moreover,
over a discrete and relatively narrow temperature range, the net concentration
of light C2+ hydrocarbons was elevated, with the exit stream
containing a higher concentration of C2+ species than was
present in the feed stream and the product stream being virtually
free of carbon oxides. It is found that the addition of manganese
can enhance the selectivity toward the production of light hydrocarbons.
A series of Ni–Mn/Al2O3 catalysts, prepared
with different Ni/Mn ratios, were studied. Various characterization
techniques such as X-ray diffraction (XRD) analysis, CO and H2 chemisorption, in situ nitric oxide adsorption Fourier transform
infrared spectroscopy (NO-FTIR), and temperature-programmed reduction
(TPR) were performed to gain an insight into how the addition of Mn
to the primary catalyst enhances the yield of light hydrocarbons.
The origin of Mn promotion was demonstrated through density functional
theory (DFT) calculations, which revealed the favorable Mn substitution
at the Ni(211) step edge sites under reducing conditions. The affinity
of these Mn species toward oxidation stabilizes the CO dissociation
product and thus provides a thermodynamic driving force that promotes
C–O bond cleavage compared to the Mn-unmodified catalyst surface.
Exploratory studies on the vapor-phase oxidation and oxidative dehydrogenation of ethane and propane have been conducted in the presence of a countercurrent "rain" of noncatalytic dispersed particulate solids which carry the heat of reaction from the reactor and maintain essentially isothermal conditions. The manner in which the independent process variables affect the conversion levels and product yields is discussed for temperatures ranging from 31 0' to 560' c., pressures from 0 to 150 p.s.i.g.,oxygen-hydrocarbon molar ratios of 0.05 to 0.6, and contact times from 1 to 28 seconds, using air or oxygen as the oxidant.
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