In-depth study on propane dehydrogenation over Al2O3-based unconventional catalysts with different crystal phases

•The effect of crystal phase of Al2O3 on dehydrogenation performance was studied.•The type of the active sites is the same for Al2O3 with different phases.•Different performances result from various surface densities of the active sites.•The active sites for propane dehydrogenation are some specific Lewis acid sites.•Coordinatively unsaturated AlⅣ species are responsible for propane dehydrogenation. Al2O3 has been demonstrated to possess relatively high activity for alkane dehydrogenation, but there is short of in-depth understanding of the effect of its crystal phase on the dehydrogenation performance. In this study, γ-Al2O3, δ-Al2O3, θ-Al2O3 and α-Al2O3 samples have been synthesized and evaluated for propane dehydrogenation. With the phase transformation from γ-Al2O3 to α-Al2O3, propane conversion decreases gradually, while the selectivity to propene increases except for the inactive α-Al2O3. According to systemic characterization and deep analysis, phase transformation cannot change the type of active components for propane dehydrogenation, but influences its surface density, thus leading to the variation of the catalytic behaviors. Furthermore, the active sites have a close relationship with the difficultly removed hydroxyl groups on the surface of Al2O3, i.e. some specific Lewis acid sites (such as the coordinatively unsaturated AlⅣ species), but abundant acid sites will result in the secondary reactions of propene and decreased selectivity. From this perspective, the difference in propene selectivity with the phase transformation is probably caused by the change in the relative content of the active sites for propane dehydrogenation. [Display omitted]