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Desorption products from zeolites with medium (MFI) and small (CHA) pores and with and without ion-exchanged copper were studied during linear heating after the pre-adsorption of methanol using a chemical flow reactor with a gas phase Fourier transform infrared spectrometer. The methanol desorption profiles were deconvoluted and compared with those predicted from first-principles calculations. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to study the samples during methanol desorption following a step-wise increase of the sample temperature. It is shown that well-dispersed copper species in the Cu-zeolite samples interact more strongly with methanol and its derivatives as compared to the bare zeolites, resulting in methanol desorption at higher temperatures. Moreover, the introduction of Cu leads to CO formation and desorption in larger amounts at lower temperatures compared to the bare zeolites. The formation and desorption of dimethyl ether (DME) from pre-adsorbed methanol takes place at different temperatures depending on both the influence of Cu and the zeolite topology. The Cu sites in zeolites lead to higher DME formation/desorption temperatures, while a small shift of DME desorption towards higher temperatures is observed for the CHA framework structure compared to the MFI framework structure.

A large fraction of the global natural gas reserves is in the form of sour gas, i.e. contains hydrogen sulfide (H2S) and carbon dioxide (CO2), and needs to be sweetened before utilization. The traditional amine-based separation process is energy-intensive, thereby lowering the value of the sour gas. Thus, there is a need to find alternative processes to remove, e.g., hydrogen sulfide. Mo6S8 clusters are promising candidates for transforming methane (CH4) and hydrogen sulfide into methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), which are high-value sulfur-containing products that can be further used in the chemical industry. Here first-principles microkinetics is used to investigate the activity and selectivity of bare and promoted (K, Ni, Cl) Mo6S8. The results show that methanethiol is produced via two different pathways (direct and stepwise), while dimethyl sulfide is formed via a competing pathway in the stepwise formation of methanethiol. Moreover, there is an increase in activity and a decrease in selectivity when adding an electropositive promoter (K), whereas the reverse behaviour is observed when adding an electronegative promoter (Cl). When adding Ni there is also a decrease in activity and an increase in selectivity; however, Ni is acting as an electron donor. The results provide insights and guidance as to what catalyst formulation is preferred for the removal of hydrogen sulfide in sour gas.

Direct processing of sour, e.g. containing large amounts of acidic H2S and/or CO2 molecules, natural gas is of direct interest as vast amounts of it are available and accessible but are underutilized. While sour natural gas is still treated using energy-intensive amine absorption/desorption, here we propose and describe a first step in catalytically producing a value added chemical and energy carrier, CH3SH and H-2, respectively. For this purpose, we performed Density Functional Theory (DFT) and microkinetic modelling of CH4 and H2S reaction pathways to form CH3SH and H-2 as a first step in elucidating complex yet not explored pathways in oxygen-free sour gas reforming. For this purpose, we utilized bare unpromoted and K-or Nipromoted Mo6S8 clusters. CH4 dissociation was found to be the rate-determining step above 1100 K on Ni-promoted Mo6S8 while H-2 formation was the rate-determining step on the bare and K-promoted Mo6S8. At lower reaction temperatures between 800 and 1100 K, CH3SH formation becomes an important step, especially on Ni-Mo6S8. This method presents an interesting route of direct catalytic sour natural gas processing which potentially leads to high-value hydrocarbons, such as ethylene, using CH3SH as a reactive intermediate.

Direct methane-to-methanol conversion is a desired process whereby natural gas is transformed into an energy-rich liquid. It has been realised at ambient pressure and temperature in metal ion-exchanged zeolites, where especially copper-exchanged ZSM-5 has shown promising results. The nature of the active sites in these systems is, however, still under debate. The activity has been assigned to a [Cu-O-Cu]2+ motif. One remaining question is whether this motif is general and also active in other metal-exchanged zeolites. Herein, we use first-principles microkinetic modelling to analyse the methane-to-methanol reaction on the [Cu-O-Cu]2+ motif, for Cu and other metals. First, we identify the cluster model size needed to accurately describe the dimer motif. Starting from the [Cu-O-Cu]2+ site, the metal ions are then systematically substituted with Ni, Co, Fe, Ag and Au. The results show that activation of Ag and Au dimer sites with oxygen is endothermic and therefore unlikely, whereas for Cu, Ni, Co and Fe, the activation is possible under realistic conditions. According to the kinetic simulations, however, the dimer motif is a plausible candidate for the active site for Cu only. For Ni, Co and Fe, close-to-infinite reaction times or unreasonably high temperatures are required for sufficient methane conversion. As Ni-, Co- and Fe-exchanged ZSM-5 are known to convert methane to methanol, these results indicate that the Cu-based dimer motif is not an appropriate model system for these metals.