Natural Gas Conversion using Sulfur Tolerant Catalyst

Natural gas is often sour, containing varying amounts of H2S. Moreover, the delayed release of H2S in shale gas reserves has also been reported, which stresses the importance of developing a versatile sour-gas-tolerant catalyst for natural gas monetization. While sulfur for decades has been regarded as catalyst poison, we seek to attain the catalytic routes in the presence of H2S. The dissertation's goal is to probe the effect of H2S on the reaction kinetics and develop the structure-activity relationship for the on-site valorization of natural gas and CO2 utilization. To achieve this, a holistic approach that combines catalyst synthesis, in-depth catalyst characterization, comprehensive kinetic evaluation, and modeling was adopted to unravel fundamental understanding of the H2S mediated catalytic pathway for CO2 and alkane activation. Two industrial relevant reactions have been evaluated as a model reaction â€" reverse water gas shift and propane dehydrogenation chemistry. Firstly, MoS2 is used in this work to investigate the reverse water gas shift (rWGS) chemistry to activate CO2 molecules in the presence of H2S. The work's objective is to study the interaction of molecules such as CO2, H2, and H2S and develop kinetic information on competing reactions. We have utilized a combination of reaction kinetics studies, electronic structure calculations to elucidate the nature of the active sites on MoS2 and their rWGS performance. The present work explains the effect of H2S co-feed on the reaction kinetics and the impact of varying H2S/H2 on the distribution of the sites using ab initio phase and Boltzmann studies. Our results suggest that the presence of H2S induces a change in catalyst structure (active sites), and it influences the activity and selectivity towards the reaction.The second part of the study focuses on developing a novel catalyst for propylene production via propane dehydrogenation (PDH) with co-feeding hydrogen sulfide (H2S). We report an unexpectedly high selectivity of ?-Al2O3 catalysts for propane dehydrogenation upon pretreatment and co-feeding of H2S. Specifically, we observe that selectivity of 94% (to propylene) and 16% propane conversion can be obtained at 560 °C for C3H8:H2:H2S ratio of 1.1:1:0.1 on ?-Al2O3. Our results indicate that H2S can irreversibly modify the active sites of ?-Al2O3, postulated to be defect sites on the 110 facet comprised of a tri-coordinated Al atom, such that the modified site was more active and selective towards propylene and less inhibited by H2S. Subsequently, the selectivity can be further enhanced to 94-98% on modification of Al2O3 via Sn modification at similar operating conditions. However, the initial activity decreases with Sn loading. In situ DRIFTS combined with AC-STEM, Raman, and XRD indicated that impregnation of tin leads to two types of Sn species, Sn anchored on alumina hydroxyl group and SnS upon H2S pretreatment¬. We postulate that the active sites are largely located on alumina (e.g., Al-O site pairs on 110 facets). Lastly, we have evaluated a spectrum of transition metal supported on alumina. We have determined that Fe impregnated Al2O3 can perform selective dehydrogenation in H2S at high conversion.We have determined that H2S can play a role as a promoter or inhibitor or a co-reactant. The insights from this work can, therefore, will be used to evaluate a spectrum of transition metal sulfide catalysts for upgrading C1-C4 hydrocarbons in the presence of H2S.