Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Brønsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.

In the reductive catalytic fractionation of lignocellulose, the choice of solvent significantly impacts the delignification efficiency, carbohydrate retention in the pulp and the macrostructure of the pulp.

One of the foremost challenges in lignocellulose conversion encompasses the integration of effective lignin valorization in current carbohydrate-oriented biorefinery schemes. Catalytic reductive fractionation (CRF) of lignocellulose offers a technology to simultaneously produce ligninderived platform chemicals and a carbohydrate-enriched pulp via the combined action of lignin solvolysis and metal-catalyzed hydrogenolysis. Herein, the solvent (composition) plays a crucial role. In this contribution, we study the influence of alcohol/water mixtures by processing poplar sawdust in varying MeOH/water and EtOH/water blends. The results show particular effects that strongly depend on the applied water concentration. Low water concentrations enhance the removal of lignin from the biomass, while the majority of the carbohydrates are left untouched (scenario A). Contrarily, high water concentrations favor the solubilization of both hemicellulose and lignin, resulting in a more pure cellulosic residue (scenario B). For both scenarios, an evaluation was made to determine the most optimal solvent composition, based on two earlier introduced empirical efficiency descriptors (denoted LFDE and LFFE). According to these measures, 30 (A) and 70 vol % water (B) showed to be the optimal balance for both MeOH/water and EtOH/water mixtures. This successful implementation of alcohol/water mixtures allows operation under milder processing conditions in comparison to pure alcohol solvents, which is advantageous from an industrial point of view.

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The wide range of liquid-phase reactions required for the catalytic conversion of biomass compounds into new bioplatform molecules defines a new set of challenges for the development of active, selective, and stable catalysts. The potential of bifunctional Ru/H-USY catalysts for conversions in hot liquid water (HLW) is assessed in terms of physicochemical stability and long-term catalytic performance of acid sites and noble metal functionality, as probed by hydrolytic hydrogenation of cellulose. It is shown that zeolite desilication is the main zeolite degradation mechanism in HLW. USY zeolite stability depends on two main parameters, viz., framework and extra-framework aluminum content. The former protects the zeolite lattice by counteracting hydrolysis of framework bonds, and the latter, when located at the external crystal surface, prevents solubilization of the zeolite framework which is the result of its low water-solubility. Hence, the hot liquid water stability of commercial H-USY zeolites, in contrast to their steam stability, increased with decreasing Si/Al ratio. As a result, mildly steamed USY zeolites containing a high amount of both Al species exhibit the highest resistance to HLW. During an initial period of transformations, Al-rich zeolites form additional protective extra-framework Al species at the outer surface, self-stabilizing the framework. A critical bulk Si/Al ratio of 3 was determined whereby USY zeolites with a lower Si/Al ratio will self-stabilize over time. Besides, due to the initial transformation period, the accessibility of the catalytic active sites is extensively enhanced resulting in a material that is more stable and drastically more accessible to large substrates than the original zeolite. When these findings are applied in the hydrolytic hydrogenation of cellulose, unprecedented nearly quantitative hexitol yields were obtained with a stable catalytic system.