Journal of Catalysis p. 70 - 81 (1999)
Update date:2022-08-30
Topics:
Zhang, Wenmin
Smirniotis, Panagiotis G.
Various oxides and metal-supported oxides were studied for the oxidative transformation of acetonitrile to acrylonitrile with methane in the range 550 to 730°C. This reaction requires the activation of both methane and the α-carbon of the acetonitrile. Surprisingly, it was found that the effective coupling of the reactants to acrylonitrile does not occur over all the catalysts that are effective for oxidative coupling of methane reactions. Among all the catalysts tested, only Li/MgO with nominal composition of lithium in the range 15 to 30 wt% was effective in forming acrylonitrile. This was probably due to the maximum concentration of [Li+O-] surface centers which catalyze the abstraction of hydrogen from the reactants and simultaneously disfavor the undesired oxidation of the nitriles. It was concluded that the efficiency of Li/MgO could not be attributed to its basicity characteristics. Other oxides such as Sm2O3, La2O3, Bi2O3, and CaO either individually or loaded with monometallic and/or bimetallic combinations of alkali metals are not effective catalysts and oxidize acetonitrile to CO and CO2. Individual magnesia and MgO-loaded with alkali metals (other than lithium) are not effective for the coupling of methane and acetonitrile to form acrylonitrile and propionitrile. We also found that the decrease in the acrylonitrile selectivity for extented periods of operation is associated with the loss of lithium from the Li/MgO catalysts. Undoped samaria and lanthana at elevated temperatures (> 680°C) oxidize acetonitrile completely to CO and CO2. For the latter oxides, if the oxygen fed into the reactor is the limiting reactant, then lattice oxygen is used for the complete oxidation of acetonitrile. The loading of samaria or lanthana with an alkali metal decreases the extent of oxidation reactions. This behavior is in versed when CaO is used as support. FT-IR and temperature-programmed desorption experiments indicate that the increase in the number and strength of the basic sites of the catalysts plays a negative role in the coupling of methane and acetonitrile to acrylonitrile.
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