No abstract

MoVTeNb mixed oxide is a highly active and selective catalyst for oxidative dehydrogenation of ethane to produce ethylene, which exhibits the so-called M1 and M2 crystalline phases. Thermal stability of MoVTeNb catalytic system was assessed under reactions conditions, for this end the catalyst was exposed to several reaction temperatures spanned from 440 to 550°C and the pristine and spent materials were thoroughly analyzed by several characterization techniques. The catalyst's stability limit to operate at reactions temperatures <500°C, since, when reaction temperature is ≥500°C brings about the removal of tellurium from the intercalated framework channels of M1 crystalline phase. Rietveld refinement of XRD patterns and microscopies results point out that the tellurium loss cause the progressive partial destruction of M1 phase, thus decreasing the active sites amount and forming a MoO 2 crystalline phase, which is inactive in the employed reactions conditions. Raman spectroscopy confirms the MoO 2 phase development in function of reaction temperature. From HRTEM and EDS analyses it was noticed that tellurium depart occurs preferentially from the end sides of the needle-like M1 crystals, across the [001] plane. A detailed analysis of the solid deposited at the reactor outlet displays that it consists mainly in metallic tellurium; hence, the tellurium detaching is carried out by means of a reduction process of Te 4+ to Te 0 due to a combination of reaction temperature and feed composition. In order to keep the catalytic performance exhibited by MoVTeNb mixed oxide, hot spots along the reactor bed should be avoided or controlled for maintaining the catalytic bed temperature <500°C.

A sol−gel method for the synthesis of MgAl layered double hydroxides is presented. The sols thus obtained were submitted to several hydrothermal treatments, in presence or absence of water, in order to examine the thermal evolution of specific physicochemical properties, such as morphology, crystal size, basicity, and textural properties. The morphological evolution was followed by transmission electron microscopy (TEM) and scanning electron microscopy techniques. TEM analyses revealed unique nanocapsular morphology; the nanocapsules grow with increasing hydrothermal treatment time and aggregate in the presence of water. X-ray diffraction confirms the expected increase in crystal size. The nitrogen adsorption−desorption surface analysis (Brunauer−Emmett−Teller) of the calcined solids indicates that surface areas range from 251 to 289 m2 g−1. Furthermore, CO2 thermoprogrammed desorption and Fourier-transform infrared analyses using CDCl3 as a probe molecule indicate that basic strength is inversely proportional to crystal size.

No abstract