No abstract

No abstract

In a hybrid catalyst process, syngas is converted, via a methanol intermediate, into a mixture of short-chain hydrocarbons over a combined methanol synthesis catalyst and molecular sieve. It operates under process conditions where the hydrogen partial pressure is relatively high vs the (intermediate) methanol partial pressure. We show that, under these conditions, the lifetime of the molecular sieve (SAPO-34) is greatly enhanced and the amount and type of carbon on the spent catalyst are vastly different as compared to those in a methanol to olefins (MTO) process. The impact of the hydrogen partial pressure, temperature, and methanol partial pressure is studied, leading to a conceptual model of coke deposition and removal. This allows the identification of process conditions that lead to a stable and long lifetime of SAPO-34 in a direct syngas-to-hydrocarbons process.

The influence of carbon on the adsorption of CO from a Rh(100) single crystal has been studied by a combination of experimental techniques: Temperature Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED), and High Resolution Electron Energy Loss Spectroscopy (HREELS). These experimental techniques were combined with a computational approach using Density Functional Theory (DFT). Using this combination of techniques, we have shown that surface carbon greatly influences adsorbed CO and we have determined the exact magnitude of this interaction. Furthermore, we have demonstrated that carbon does not remain fully on the surface; at higher coverage it diffuses partially to subsurface positions. The presence of these subsurface species significantly influences the adsorbates on the surface.

No abstract

A hybrid catalyst consisting of a Cu-ZnO/Al2O3 methanol synthesis catalyst and a SAPO-34 molecular sieve was shown to convert syngas into a narrow mixture of short chain paraffins centered at C2–C3 with little methane and C5+ made. This product mix is expected to be an excellent cracker feedstock for the production of olefins. The hybrid catalyst system closely couples sequential reactions on each of the two independent catalysts. The function of each of the components was unraveled by performing experiments in which the reactor was loaded with discrete layers of the individual catalysts. Methanol (MeOH) synthesis over Cu-ZnO/Al2O3 is followed by methanol conversion to dimethyl ether (DME) + steam over the acidic SAPO-34 or Cu-ZnO/Al2O3 component. Simultaneously, the so generated water feeds into the water gas shift (WGS) reaction over Cu-ZnO/Al2O3, producing H2 + CO2, and the DME/MeOH undergoes methanol to olefins (MTO) conversion over SAPO-34. The olefins are subsequently hydrogenated to paraffins over the Cu-ZnO/Al2O3 and SAPO-34 catalysts using hydrogen from the feed or hydrogen produced in the WGS reaction. Comparison of fully mixed vs layered catalyst bed systems indicated that a mixed catalyst bed is preferred to give a high CO conversion and selectivity to desired paraffin products. This scheme enables high single stage conversion by consuming methanol, thereby removing the thermodynamic constraint on that reaction step. The interactions between all the reactants and catalysts in this system create a complex relationship that is probed in this paper.

No abstract

The decomposition of ethylene on a Rh(100) single crystal has been studied by a combination of experimental techniques: static secondary ion mass spectrometry (SSIMS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS), to gain insight into the nature of the reaction intermediates during the decomposition process. These experimental techniques were combined with a computational approach using density functional theory (DFT). Ethylene adsorbs irreversibly on the Rh(100) surface and eventually decomposes to atomic carbon and gas phase hydrogen. The type of intermediate species depends strongly on the initial surface coverage of ethylene. At low coverage, ethynyl (CCH) is the main intermediate species, whereas at high coverage a mixture of ethynyl, acetylene (CHCH), and ethylidyne (CCH 3 ) forms. The rate of decomposition is significantly slower at higher coverages, indicative of lateral interactions between coadsorbed species and site-blocking effects.