Natural Gas Conversion Into Value Added Chemicals Using Solid Acid Catalysts

Abstract

Direct, non-oxidative conversion of natural gas to value-added chemicals has been identified as one of the grand challenges of the 21st century. Circumventing indirect and costly reforming steps is highly sought after and solid acids can play an important role in that. The direct conversion of methane to higher hydrocarbons and hydrogen can be catalyzed using “superacids” at < 450 °C. Reported superacid catalysts in solid, liquid, and gas phase included sulfated zirconia (SZ), HF-SbF5, FSO3H-SbF5, and HBr-AlBr3. Liquid and gas phase superacids presented difficulties in separation while the solid ones provided low yields. Here, we report a new class of Br-based solid superacids, AlBrx/H-ZSM-5 (“ABZ-5”, x = 1 or 2). ABZ-5 is based on gas-phase HBr/AlBr3. This solid catalyst is synthesized using a vapor-phase process in which AlBr3 vapor is grafted on to solid H-ZSM-5. This catalyst is characterized using NH3-TPD, XRD, and DRIFTS. The results show that ABZ-5 is significantly more active than SZ and showed methane conversions of ~1% at 300 °C using ABZ-5. Hydrocarbon products observed in the temperature range of 200-400 °C include both C2-C6 hydrocarbons and aromatics. In another approach to methane activation, Mo is doped on solid SZ to create a catalyst similar to Mo/H-ZSM-5, but with a different solid acid for methane dehydroaromatization (DHA). 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 analyses showed that MoO3 is converted to MoOxCy and is further converted to Mo2C as the DHA 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 °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 650-675 °C temperature range.