Kinetics of Oxidative Dehydrogenation of n-Butane to C4-Olefins over a VO_x/CeO_2-Al₂O₃ Catalyst in Gas-Phase Oxygen-Free Conditions

In the present research, the phenomenologically based kinetics of the oxidative dehydrogenation (ODH) of n-butane to C4-olefins over a newly developed VOx/CeO2-gamma Al2O3 catalyst was investigated. The catalyst was formulated by impregnating 5 wt % V in a 0.2 wt % Ce-modified CeO2-gamma Al2O3 support. NH3-temperature-programmed desorption indicated the presence of both low- and high-temperature acid sites on the catalyst surface. Temperature-programmed reduction TPR/temperature-programmed oxidation (TPO) analyses showed that 80% of the loaded VOx was available for reduction. The kinetic experiments were carried out in a fluidized CREC Riser Simulator at different reaction temperatures (450-575 degrees C) and residence times (5-25 s). It was noticed that the highest C4-olefin selectivity of 62% was achieved at 450 degrees C and a 5 s reaction time. This value decreased with the increase of both reaction temperature and residence time. Two alternative Langmuir-Hinshelwood type kinetics models were formulated considering the cracking, the ODH, and the complete oxidation reactions. The availability of the catalyst oxygen was represented by an exponential decay function of n-butane conversion. The kinetic parameters of the developed models were estimated by fitting the experimental data using MATLAB. Based on goodness of prediction, thermodynamic consistency, and statistical analysis, it was found that the One Adsorption Site Type Langmuir-Hinshelwood model represented the experimental data adequately, with an Akaike information criterion (AIC) of -232. The estimated activation energy for the formation of C4-olefins (90.2 +/- 2.8 kJ/mol) was considerably lower than that for the n-butane cracking reaction (105.5 +/- 4.7 kJ/mol) as well as that for the complete oxidation to CO2 (121.6 +/- 4.2 kJ/mol). On the other hand, the complete oxidation of C2-lumps required a lower activation energy (55.00 +/- 2.1 kJ/mol) than the complete oxidation of C4-olefins (81.0 +/- 3.2). All these results were consistent with the product analysis data.