Applied Catalysis A: General, Vol.575, 25-37, 2019
Methane dehydroaromatization over molybdenum supported on sulfated zirconia catalysts
The shale gas revolution has strongly impacted the world by providing a significant incentive for research in natural gas conversion. Natural gas is becoming an important feedstock for the production of valuable chemicals. Dehydroaromatization (DHA) is an important non-oxidative conversion route for methane conversion, and Mo doped on the H-ZSM-5 or H-MCM-22 have been well-studied for this reaction. In these catalysts, Mo sites activate CH4 to form dimeric species that are subsequently oligomerized on the strong BrOnsted acid sites. In the present study, Mo is doped on solid sulfated zirconia (SZ) to create a catalyst similar to Mo/H-ZSM-5, but with a different solid acid. Despite the logic of using SZ as the acid, we are aware of no systematic study of Mo/SZ catalysts for this reaction. These catalysts were characterized using Raman, XPS, DRIFTS, SEM-EDS, HRTEM, XRD, XANES and other temperature programmed techniques. Raman spectra confirmed the formation of Mo = O and O-Mo-O bonds on the surface of SZ support. DRIFTS confirmed that there was little difference in acid sites when Mo was doped on SZ, except at higher Mo loadings. XPS, XANES, and HRTEM analysis showed that MoO3 is converted to MoOxCy and is further converted to Mo2C as the reaction progresses. Further, these catalysts were evaluated for methane DHA reaction. All of these catalysts showed methane conversions of 5-20 % at temperatures of 600-700 degrees C. In each case, the catalysts deactivated steadily, attributable to strong coking on the surface, as confirmed with TPO. A comparison with literature showed that Mo/SZ has comparable activity to Mo/H-ZSM-5 at around 650 degrees C-675 degrees C temperature range. Further, Mo/SZ was more selective towards heavier aromatics such as naphthalene and coke as compared to Mo/H-ZSM-5 at 700 degrees C, which is more selective towards benzene.