Chemical, Structural, and Morphological Changes of a MoVTeNb Catalyst during Oxidative Dehydrogenation of Ethane

Mo'VTeNb mixed oxide, a highly active and selective catalyst for the oxidative dehydrogenation of ethane to produce ethylene, exhibits the so-called M1 and M2 crystalline phases. The thermal stability of the MoVTeNb catalytic system was assessed under varying reaction conditions; to this end, the catalyst was exposed to several reaction temperatures spanning from 440 to 550 degrees C. Both the pristine and spent materials were analyzed by several characterization techniques. The catalyst was stable below 500 degrees C; a reaction temperature of >= 500 degrees C brings about the removal of tellurium from the intercalated framework channels of the M1 crystalline phase. Rietveld refinement of X-ray diffraction patterns and microscopy results showed that the tellurium loss causes the progressive partial destruction of the M1 phase, thus decreasing the number of active sites and forming a MoO2 crystalline phase, which is inactive for this reaction. Raman spectroscopy confirmed the MoO2 phase development as a function of reaction temperature. From high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses it was noticed that tellurium departure occurs preferentially from the end sides of the needlelike M1 crystals, across the [001] plane. Detailed analysis of a solid deposited at the reactor outlet showrf that it consisted mainly of metallic tellurium, suggesting that the tellurium detachment occurs via reduction of Te4+ to Te-0 due to a combination of reaction temperature and feed composition. Thus, in order to sustain the catalytic performance exhibited by MoVTeNb mixed oxide, hot spots along the reactor bed should be avoided or controlled, maintaining the catalytic bed temperature below 500 degrees C.