Isobutane Dehydrogenation over Bulk and Supported Molybdenum Sulfide Catalysts

Bulk and supported MoS_x materials have gained interest as alternative catalysts for light alkane dehydrogenation, but there is little kinetic data published with which rigorous comparisons can be made to other catalysts. Here, rates, selectivities, and activation barriers are collected under conditions of differential conversion for the dehydrogenation of isobutane over various morphologies of molybdenum sulfide. We find that a "rag-like" MoS₂ composed of small, disordered crystallites exhibits higher catalytic rates than layered, highly crystalline MoS₂ (52 vs 2.7 μmol ks^-1 g_cat ^-1 at 360 °C). This is in part not only due to increased surface area but also due to intrinsically more active edge and defect sites exposed by the rag-like structure, as shown by a decrease in apparent activation energy from 61 to 43 kJ mol^-1. Supporting MoS_x on metal oxides or metal organic frameworks gives small MoS_x clusters that have up to 7-fold higher rates per mass of MoS₂ than even the rag-like MoS₂. Rates are support-dependent, with the highest rates per mass of MoS₂ observed over MoS_x/TiO₂. Pt/Al₂O₃ has a 50-fold higher rate than the best MoS₂ catalysts (2700 μmol ks^-1 g_cat ^-1 at 360 °C), but it has an apparent activation energy of 41 kJ mol^-1, similar to that of the rag-like MoS₂. Therefore, the rates over MoS₂ appear to be limited by a small number of active sites on the surface, rather than intrinsically poor activity. Given the data provided in this manuscript and the enormous phase space available to metal sulfides, these materials warrant further investigation as alternative light alkane dehydrogenation catalysts, especially for use under conditions that would deactivate a precious metal catalyst.