Layered metal sulfides, particularly molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have emerged as highly effective catalysts for the hydrodeoxygenation (HDO) of biomass-derived oxygenates. These materials exhibit unique structural and electronic properties that make them suitable for cleaving C–O bonds while minimizing over-hydrogenation, a critical requirement for producing high-value aromatic and phenolic compounds from lignocellulosic feedstocks. The catalytic performance of these sulfides is influenced by several factors, including edge site activation through promoter atoms (Co or Ni), sulfur vacancy engineering, and the choice of support material (γ-Al2O3 or carbon). Each of these aspects plays a crucial role in determining activity, selectivity, and stability during HDO reactions.

The active sites in MoS2 and WS2 for HDO are primarily located at the edges of the layered structure, where unsaturated sulfur and metal atoms facilitate the adsorption and activation of oxygen-containing molecules. The basal planes, in contrast, are largely inert. To enhance the density of active edge sites, transition metals such as cobalt or nickel are often incorporated as promoters. These promoters occupy the edges of MoS2 or WS2 layers, forming the so-called Co–Mo–S or Ni–Mo–S phases. The presence of Co or Ni increases the electron density around molybdenum or tungsten, weakening the metal-sulfur bonds and creating more labile sulfur species. This electronic modification enhances the catalyst's ability to remove oxygen via direct deoxygenation or hydrogenation-dehydration pathways. For instance, Co-promoted MoS2 has been shown to increase the turnover frequency for guaiacol HDO by a factor of two compared to unpromoted MoS2, with a selectivity toward benzene and phenol exceeding 80% under optimized conditions.

Sulfur vacancies are another critical feature governing HDO activity. These vacancies act as Lewis acid sites, polarizing C–O bonds and facilitating their cleavage. Controlled generation of sulfur vacancies can be achieved through reduction treatments or by adjusting the sulfidation temperature during catalyst preparation. However, excessive vacancy formation can lead to catalyst deactivation via coking or irreversible structural collapse. Optimal vacancy concentrations are typically achieved under moderate sulfidation conditions (300–400°C), where the balance between active site creation and structural stability is maintained. For example, WS2 with a sulfur vacancy density of approximately 0.2 vacancies per nm² has demonstrated high activity in the HDO of anisole, yielding a selectivity of 75% toward toluene at 300°C.

The choice of support material significantly influences the dispersion and stability of MoS2 or WS2 catalysts. γ-Al2O3 is widely used due to its high surface area and ability to stabilize small sulfide clusters. The acidic nature of γ-Al2O3 can also promote dehydration steps, enhancing the production of aromatic hydrocarbons. However, strong metal-support interactions may lead to partial oxidation of sulfide phases under reaction conditions, reducing HDO activity. Carbon supports, on the other hand, provide a neutral and hydrophobic environment that minimizes unwanted side reactions, such as polymerization of reactive intermediates. Carbon nanotubes or graphene-based supports have been shown to improve the accessibility of edge sites and reduce mass transfer limitations, leading to higher reaction rates. For instance, MoS2 supported on carbon nanofibers exhibited a 30% higher conversion of vanillin compared to γ-Al2O3-supported counterparts, with a selectivity shift toward phenolic products due to weaker adsorption on the carbon surface.

Selectivity in HDO reactions is highly dependent on the nature of the oxygenate feedstock and the catalyst's ability to balance hydrogenation and deoxygenation steps. Phenolic compounds, such as guaiacol and catechol, tend to undergo partial deoxygenation to phenol or complete deoxygenation to benzene, depending on the hydrogen pressure and temperature. Aromatic production is favored at higher temperatures (300–350°C) and lower hydrogen pressures, where direct C–O cleavage is dominant. In contrast, higher hydrogen pressures promote hydrogenation of the aromatic ring, leading to cyclohexanol or cyclohexane as byproducts. The presence of Co or Ni promoters can further tune selectivity by favoring hydrogenolysis over hydrogenation. For example, Ni-promoted MoS2 catalysts have been reported to enhance the production of benzene from phenol with a selectivity of over 90% at 325°C, whereas non-promoted MoS2 yields a more balanced mixture of phenol and benzene.

The stability of layered metal sulfides under HDO conditions is another critical consideration. Sulfur loss due to reduction or leaching can lead to deactivation over time. Strategies to mitigate this include the use of sulfur-containing feedstocks (e.g., H2S co-feeding) or the development of sulfur-rich catalyst formulations. Additionally, carbon-supported sulfides often exhibit better resistance to deactivation compared to oxide-supported systems, as carbon does not participate in redox reactions that could destabilize the sulfide phase. Long-term stability tests have shown that carbon-supported Co–Mo–S catalysts retain over 80% of their initial activity after 100 hours on stream during guaiacol HDO, whereas γ-Al2O3-supported catalysts show a more pronounced activity decline due to sulfur loss and coke deposition.

In summary, layered metal sulfides such as MoS2 and WS2 offer a versatile platform for the HDO of biomass-derived oxygenates. The rational design of these catalysts—through promoter incorporation, sulfur vacancy engineering, and support selection—enables precise control over activity and selectivity. Co or Ni promotion enhances edge site activity, while careful management of sulfur vacancies ensures optimal C–O bond cleavage. The choice between γ-Al2O3 and carbon supports depends on the desired balance between acidity and hydrophobicity, with carbon supports generally offering better stability. Selectivity toward phenolic or aromatic products can be tuned by adjusting reaction conditions and promoter identity, making these catalysts adaptable to a wide range of biomass conversion processes. Future advancements in sulfide catalyst design will likely focus on further improving stability and selectivity through nanostructuring and advanced characterization techniques.