Two-dimensional (2D) materials, including graphene oxides and transition metal dichalcogenides (TMDCs), have emerged as promising catalysts for biomass conversion due to their unique structural and electronic properties. These materials offer advantages in acid/base site engineering, hydrothermal stability, and selectivity, making them suitable for processes such as lignin depolymerization and sugar dehydration. Their performance contrasts with traditional catalysts like zeolites and enzymatic systems, particularly in terms of tunability and robustness under harsh reaction conditions.

Graphene oxides (GO) and reduced graphene oxides (rGO) are widely studied for biomass conversion due to their high surface area, oxygen-containing functional groups, and ease of modification. The presence of carboxyl, epoxy, and hydroxyl groups on GO surfaces provides active sites for acid-catalyzed reactions. For example, in lignin depolymerization, these acidic sites facilitate the cleavage of β-O-4 linkages, a major challenge in lignin valorization. The density of these functional groups can be controlled through chemical reduction or thermal treatment, allowing precise tuning of catalytic activity. In sugar dehydration, GO catalysts have demonstrated high selectivity toward platform chemicals like 5-hydroxymethylfurfural (HMF), a key intermediate for biofuels and polymers. The selectivity arises from the balanced distribution of acidic and basic sites, which minimizes side reactions such as humin formation.

TMDCs, such as MoS2 and WS2, exhibit distinct catalytic properties due to their sulfur-rich edges and tunable electronic structure. The edges of TMDCs possess unsaturated metal and sulfur atoms, which can act as Lewis acid and base sites, respectively. These sites are highly effective for hydrolytic reactions, such as the conversion of cellulose to glucose. Unlike graphene-based materials, TMDCs can also facilitate hydrogenation and dehydrogenation reactions, making them versatile for multi-step biomass upgrading. For instance, MoS2 has been shown to catalyze the hydrodeoxygenation of lignin-derived phenols into aromatic hydrocarbons, a critical step in biofuel production. The sulfur vacancies in TMDCs can be engineered to enhance catalytic activity, offering another dimension for optimization.

Hydrothermal stability is a critical factor for biomass conversion catalysts, as many processes occur in aqueous or high-temperature environments. 2D materials generally exhibit superior stability compared to zeolites and enzymes. Graphene oxides maintain their structural integrity under hydrothermal conditions up to 200°C, while TMDCs can withstand even higher temperatures due to their strong covalent bonding. In contrast, zeolites often suffer from framework collapse or dealumination under similar conditions, leading to deactivation. Enzymatic catalysts, while highly selective, are limited by their narrow operational pH and temperature ranges, as well as susceptibility to denaturation.

Selectivity in biomass conversion is another area where 2D materials excel. The ability to tailor surface chemistry allows for precise control over reaction pathways. For example, in glucose dehydration to HMF, GO catalysts with a higher density of carboxyl groups favor the fructofuranose pathway, reducing unwanted byproducts. TMDCs, on the other hand, can be optimized for selective C-O bond cleavage in lignin without excessive C-C bond scission, preserving valuable aromatic units. This level of control is difficult to achieve with zeolites, which often exhibit broad pore size distributions and non-specific active sites. Enzymes, while inherently selective, are limited by substrate specificity and slow reaction kinetics.

Comparisons with zeolites and enzymatic catalysis highlight the trade-offs between these systems. Zeolites are widely used in industrial catalysis due to their high acidity and shape selectivity. However, their microporous structure can lead to diffusion limitations, particularly for bulky biomass molecules. Enzymes offer unparalleled selectivity under mild conditions but are expensive to produce and recycle. 2D materials bridge this gap by combining the robustness of inorganic catalysts with the tunability of biological systems. Their layered structure allows for facile mass transport, while their surface chemistry can be tailored to mimic enzymatic precision.

Recent advances in 2D material catalysts have focused on enhancing their performance through defect engineering and heteroatom doping. Nitrogen-doped graphene, for instance, introduces basic sites that improve selectivity in sugar isomerization. Similarly, transition metal-doped TMDCs can activate inert C-H bonds in lignin, enabling milder depolymerization conditions. These modifications further expand the scope of 2D materials in biomass conversion, offering pathways to greener and more efficient processes.

In summary, 2D materials like graphene oxides and TMDCs represent a versatile and robust platform for biomass conversion. Their tunable acid/base sites, hydrothermal stability, and high selectivity address key challenges in lignin depolymerization and sugar dehydration. While zeolites and enzymes remain important benchmarks, 2D materials offer distinct advantages in terms of adaptability and performance under demanding conditions. Continued research in surface engineering and mechanistic understanding will further solidify their role in sustainable chemical production.