The direct catalytic conversion of methane and hydrogen sulfide — the two problematic components of sour natural gas — into methanethiol (CH3SH) and dimethyl sulfide (DMS) is a candidate route that couples gas sweetening with sulfur valorization. A 2019 first-principles microkinetic study by Arvidsson, Taifan, Hellman and Baltrusaitis (Catal. Sci. Technol. 9, 4573–4580) established a Langmuir–Hinshelwood dual-site reaction network on bare and K-, Ni- and Cl-promoted Mo6S8 clusters that reproduces the promoter activity/selectivity trade-off, but left three elements unresolved: (i) quantitative inclusion and assessment of Eley–Rideal and Mars–van Krevelen mechanism alternatives, which were excluded by assumption rather than by calculation, (ii) first-principles treatment of the diffusion steps, which were included only parametrically with a 0.05 eV barrier, together with modeling of the higher-sulfur Mo6Sx (x>8) stoichiometries expected under sulfur-rich conditions, and (iii) the mechanistic electronic-structure origin of the Ni promoter anomaly — Ni is a net electron donor to the cluster yet over-stabilizes sulfur-containing intermediates so that the dual site becomes poisoned. All three elements remain unaddressed in the subsequent literature. The open problem is to complete the quantitative first-principles account of how promoters control activity and selectivity in Mo6S8-catalyzed methane thiolation.

Background

A large fraction of the world's natural gas reserves is 'sour', i.e. contains hydrogen sulfide (H2S) together with CO2, and must be sweetened before use because conventional amine-based separation is energy-intensive. One valorizing alternative is to react the H2S with the methane (CH4) in the gas itself: CH4 + H2S → CH3SH + H2, where methanethiol (CH3SH, also called methyl mercaptan) is a high-value organosulfur intermediate (e.g. for methionine synthesis), and the further methylated product dimethyl sulfide (CH3SCH3, DMS) is another commodity organosulfur compound. The reaction is thermodynamically limited and requires high temperature, so catalytic control of activity and of the selectivity between CH3SH and DMS is the central problem. Kinetic studies of CH4 + H2S over MoS2 date back to the 1990s (Megalofonos and Papayannakos, 1997), and molybdenum-sulfide catalysts related to the Mo6S8 unit are established for the related industrial thiolation of methanol with H2S ('Synthesis of Methanethiol from Methanol over Reduced Molybdenum Sulfide Catalysts Based on the Mo6S8 Cluster', J. Catal. 2002); Taifan and Baltrusaitis (2017) reviewed direct catalytic conversion of sour-gas components to chemicals. Mo6S8 is the building block of Chevrel-phase molybdenum sulfide: a face-capped octahedral cluster of six Mo atoms capped by eight S atoms, used here as a quantum-chemical cluster model of a sulfided Mo catalyst whose electronic structure can be tuned by adsorbed promoter atoms. Arvidsson, Taifan, Hellman and Baltrusaitis (2017) had already built a density-functional-theory (DFT) microkinetic model of CH4/H2S reforming on metal-promoted Mo6S8; a microkinetic model integrates a full set of elementary reaction steps, with rate constants derived from first-principles energetics, to steady state, and predicts turnover frequencies (TOF, product molecules per catalytic site per second) and product selectivities. The 2019 follow-up extended this to bare, K-, Ni- and Cl-promoted Mo6S8 with a dual-site mechanism: reactions occur on a –S–Mo– site pair denoted [A*,B*] (asterisks marking adsorbed species), with adsorbed sulfur species on S and carbon/hydrogen species on Mo. All steps were assumed to follow a Langmuir–Hinshelwood mechanism (both reaction partners adsorbed before reacting). Two alternatives were excluded by assumption rather than by calculation: an Eley–Rideal mechanism (a gas-phase molecule reacting directly with an adsorbed species) and a Mars–van Krevelen mechanism (lattice sulfur participating in the reaction via vacancy formation and re-sulfidation, here requiring formation of a Mo6S7 vacancy). Inter-site diffusion of adsorbates was treated phenomenologically with a parametric 0.05 eV barrier (sensitivity tests at 0.005–0.5 eV gave only minor TOF/selectivity changes). The model reproduced a clear promoter trend: the electropositive promoter K donates electron density (by Bader charge analysis, a spatial partitioning of the computed electron density into atomic charges) and raises activity while lowering CH3SH selectivity; the electronegative promoter Cl withdraws charge and does the reverse. Nickel is the anomaly: by Bader analysis Ni also acts as an electron donor (transferring about 0.2 electrons symmetrically to two coordinating Mo atoms), yet it stabilizes sulfur-containing intermediates — in particular adsorbed S* — so strongly that the dual site becomes poisoned, which lowers activity but sharply raises CH3SH selectivity at low temperature. The paper closes by stating that inclusion of Eley–Rideal and Mars–van Krevelen mechanisms 'is a future challenge', that high S* coverage is expected to form higher-sulfur clusters Mo6Sx with x > 8, which 'remains as a challenge for future investigation', and that the reason for Ni's behavior 'is not as clear'. The kinetic calculations used a CH4:H2S pressure ratio of 1:12 (chosen to suppress coke formation) over roughly 900–1500 K.

Problem Statement

Complete the first-principles microkinetic account of CH4 + H2S catalytic conversion to methanethiol (CH3SH) and dimethyl sulfide (DMS) on Mo6S8-based catalysts, to the level where promoter-controlled activity and selectivity are quantitatively understood. Relative to the Langmuir–Hinshelwood dual-site model of Arvidsson et al. (2019) on bare, K-, Ni- and Cl-promoted Mo6S8 clusters (CH4:H2S = 1:12, approximately 900–1500 K), the three elements that the study left open are: (1) Mechanism alternatives — include and quantitatively assess Eley–Rideal channels (direct reaction of gas-phase CH4 or H2S with adsorbed species) and Mars–van Krevelen channels (participation of lattice sulfur through Mo6S7 vacancy formation and re-sulfidation) alongside the Langmuir–Hinshelwood network, and determine whether they alter the predicted turnover frequencies or the CH3SH/DMS selectivity for each promoter; (2) Diffusion and higher-sulfur stoichiometries — replace the phenomenological 0.05 eV inter-site diffusion barrier with diffusion energetics computed from first principles for a representative set of inter-site paths, and model the higher-sulfur stoichiometries Mo6Sx with x > 8 expected under the sulfur-rich feed for a representative set of the low-energy structures; and (3) The Ni anomaly — identify the electronic-structure origin of Ni's anomalous promoter behavior (a net electron donor to the cluster by Bader analysis that nevertheless stabilizes sulfur-containing intermediates, in particular S*, strongly enough to poison the dual site, lowering activity while raising CH3SH selectivity) and determine whether it can be reconciled with the donor/acceptor descriptor trend established for K and Cl.

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