Kinetics of Oxidative Dehydrogenation of n-Butane to C4-Olefins over a VO_x/CeO₂–γAl₂O₃ Catalyst in Gas-Phase Oxygen-Free Conditions

Abstract

In the present research, the phenomenologically based kinetics of the oxidative dehydrogenation (ODH) of n-butane to C4-olefins over a newly developed VO x /CeO2–γAl2O3 catalyst was investigated. The catalyst was formulated by impregnating 5 wt % V in a 0.2 wt % Ce-modified CeO2–γAl2O3 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 VO x was available for reduction. The kinetic experiments were carried out in a fluidized CREC Riser Simulator at different reaction temperatures (450–575 °C) and residence times (5–25 s). It was noticed that the highest C4-olefin selectivity of 62% was achieved at 450 °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.

Kinetics of Oxidative Cracking of n‐Hexane to Light Olefins using Lattice Oxygen of a VOx/SrO‐γAl2O3 Catalyst

“…The opposing view is that surface lattice oxygen and sub‐surface lattice oxygen play critical roles in the formation of the desired and undesired products. Previous studies by present research group showed that the latter case usually involves the anaerobic conversion of paraffins to light olefins [40] . In this study, the oxygen involved in the oxidative conversion of n‐hexane mainly results from the VO x component of the catalyst.…”

“…The hydrogen temperature program reduction (H 2 ‐TPR) analysis was conducted to study the reducibility of the catalyst samples. This analysis specified the catalysts’ active temperature range as well as the available lattice oxygen for the oxidative conversion of n‐hexane [40] …”

“…Catalytic deactivation mainly occurs due to the depletion of the lattice oxygen. Recently, the present research group explored various catalyst decay models to estimate the catalyst activity and the amount of lattice oxygen by using experimental data from hydrogen‐TPR and catalyst evaluation data from the riser simulator [40] . Considering these two types of deactivations, the catalyst activity function is defined as follows. …”

“…The result showed an accurate prediction of their experimental studies against the developed model. The same research group recently reports the implementation of VO x /CeO 2 ‐Al 2 O 3 catalysts for the kinetics study of n‐butane to C 4 olefins [30] . Here, the researchers used the one site adsorption Langmuir‐Hinshelwood model to predict the kinetics experiments.…”