Due to the increasing consumption of fossil fuels (coal, crude oil, and natural gas) to produce gasoline, diesel, jet fuel, propane, valuable chemicals, and other useful materials, many environmental issues ranging from air pollution, greenhouse gas emission, water pollution, land degradation to ocean acidification have resulted [[1], [2], [3]]. The ultimate use of lignocellulosic biomass as a renewable and promising feedstock to produce these products will help to reduce the negative impact of fossil fuels on our environment [[4], [5], [6], [7]].

Lignin is the second major component of lignocellulosic biomass and is comprised of three aromatic alcohols namely: the p-coumaryl alcohol (H), the coniferyl alcohol (G), and the sinapyl alcohol (S). They are connected by C-O-C and CC bonds [2,3]. There are considerable variations in their compositions, proportions, and substructures depending on the type of the species, as well as the type and quantity of linkages in the polymer and the number of methoxy groups present on the aromatic ring [8,9]. These factors (differences in chemical structures, and the extraction and purification processes) affect lignin conversion to value-added chemicals and products [1,[8], [9], [10]]. Lignin is made up of phenylpropane units and they are connected in three-dimensional form [[11], [12], [13]]. For proper valorization of lignin, depolymerization is usually the first step to break the high molecular weight compounds into low molecular weight compounds like liquid fuels or chemicals [2,12]. The major challenge for lignin depolymerization is the complex nature of its structure because of its irregular and diverse polymeric aromatic structure [11,13].

Although lignin has high carbon content, it is still a biodegradable material with high heat stability properties [14,15]. Based on this rigid structure of lignin, approximately 2 % of lignin is utilized on commercial scale and the remaining 98 % is used as a combustible fuel [1,16]. Utilizing lignin from biomass resources only for heat generation ultimately leads to poor utilization of this carbon-rich compound, which usually results in the emission of greenhouse gases [17,18]. Although lignin possesses valuable properties, its limited commercial utilization and excessive reliance on combustion raise concerns regarding inefficient use and potential environmental consequences. However, exploring alternative applications and efficient conversion methods can contribute to maximizing the benefits of lignin while minimizing its drawbacks.

There have been several reviews focusing on lignin catalytic hydrogenolysis reactions over monometallic catalysts [16,[19], [20], [21], [22]]. However, insufficient coverage has been given on lignin catalytic hydrogenolysis reaction over bimetallic catalysts to produce phenolic and aromatic monomers. Li et al. [23] used PdRu bimetallic catalysts to investigate the hydrogenolysis of lignocellulosic derivatives and obtained a high yield of aviation gasoline range olefins. Recent reviews show that the combinations of various metal catalysts such as Ni, Co, Ru, Pd, Pt, Mo, Re, Rh, etc. can also give a high catalytic activity because a single metallic catalyst generally have low catalytic selectivity and activity [15,22,23]. Yan et al. [15] also reported that the bimetallic NiMo (M = Ru, Rh, Pd) catalysts produced higher product yield in the lignin hydrogenolysis reaction than those of monometallic catalysts. The synergistic effect of these two metal catalysts helped increase the activity and selectivity of the bimetallic catalysts during the lignin hydrogenolysis reaction [18].

The research work focuses on the bimetal catalytic hydrogenolysis reaction method for the conversion of lignin into hydrocarbon liquid fuels. Lignin, as a major component of biomass, represents a vast and underutilized resource. Converting lignin into hydrocarbon liquid fuels is crucial for sustainable energy production and addresses the need for alternative feedstocks to traditional fossil fuels [[24], [25]]. The proposed bimetal catalytic hydrogenolysis reaction method aims not only for high yields but also for economic and environmental benefits. This implies that the research intends to optimize the conversion process for maximum efficiency, making it a potentially viable and competitive option in the energy landscape. The inclusion of economic and environmental aspects in the research rationale suggests a holistic approach. The economic feasibility of lignin conversion is essential for its practical implementation, while the environmental benefits align with the growing global emphasis on sustainable and green technologies. This work offers an in-depth overview of the most recent findings in catalytic hydrogenolysis reactions, accompanied by critical discussions. This indicates that the work will contribute to the existing body of knowledge, fostering a deeper understanding of the processes involved and potentially uncovering novel insights. This review also delves into modern reaction mechanisms, operating conditions, and developmental trends in lignin catalytic hydrogenolysis reactions over bimetallic catalysts. This comprehensive approach is essential for advancing the field, providing practical insights for researchers and industries alike. Finally, this work makes suggestions for future utilization of lignin as a raw material for the production of various lignin-based products. This forward-looking perspective extends beyond the immediate goal of lignin-to-fuel conversion, aiming to open avenues for diverse industrial applications.