Catalytic Deoxygenation of Xylitol to Renewable Chemicals: Advances on Catalyst Design and Mechanistic Studies

Xylitol is commonly known as one of the top platform intermediates for biomass conversion. Catalytic deoxygenation of xylitol provides an atomic and energetic efficient way to produce a variety of renewable chemicals including ethylene glycol, 1,2-propanediol, lactic acid and 1,4-anhydroxylitol. Despite a few initial attempts in converting xylitol into those products, improving catalyst selectivity towards C-O and C-C cleavage reactions remains a grand challenge in this area. To our best knowledge, there is lack of comprehensive review to summarize the most recent advances on catalyst design and mechanisms in deoxygenation of xylitol, offering important perspective into future development of xylitol transformation technologies. Therefore, in this mini-review, we have critically discussed the conversion routes involved in xylitol deoxygenation over solid catalyst materials, the nanostructures of supported metal catalysts for C-H, C-C and C-O bond cleavage reactions, and mechanistic investigation for xylitol conversion. The outcome of this work provides new insights into rational design of effective deoxygenation catalyst materials for upgrading of xylitol and future process development in converting hemicellulosic biomass.

Nanostructured Metal Catalysts for Selective Hydrogenation and Oxidation of Cellulosic Biomass to Chemicals

Conversion of biomass to chemicals provides essential products to human society from renewable resources. In this context, achieving atom-economical and energy-efficient conversion with high selectivity towards target products remains a key challenge. Recent developments in nanostructured catalysts address this challenge reporting remarkable performances in shape and morphology dependent catalysis by metals on nano scale in energy and environmental applications. In this review, most recent advances in synthesis of heterogeneous nanomaterials, surface characterization and catalytic performances for hydrogenation and oxidation for biorenewables with plausible mechanism have been discussed. The perspectives obtained from this review paper will provide insights into rational design of active, selective and stable catalytic materials for sustainable production of value-added chemicals from biomass resources.

Controllable hydrothermal synthesis of Ni/H-BEA with a hierarchical core-shell structure and highly enhanced biomass hydrodeoxygenation performance

Ni based catalysts are wildly used in catalytic industrial processes due to their low costs and high activities. The design of highly hierarchical core-shell structured Ni/HBEA is achieved using a sustainable, simple, and easy-tunable hydrothermal synthesis approach using combined NH₄Cl and NH₃·H₂O as a co-precipitation agent at 120 °C. Starting from a single-crystalline hierarchical H⁺-exchanged beta polymorph zeolite (HBEA), the adjustment of the precipitate conditions shows that mixed NH₄Cl and NH₃·H₂O precipitates with proper concentrations are vital in the hydrothermal synthesis for preserving a good crystalline morphology of HBEA and generating abundant highly-dispersed Ni nanoparticles (loading: 41 wt%, 5.9 ± 0.7 nm) encapsulated onto/into the support. NH₄Cl solution without an alkali is unable to generate abundant Ni nanoparticles from Ni salts under the hydrothermal conditions, whereas NH₃·H₂O seriously damages the pore structure. After studying the in situ changes in infrared, X-ray diffractometry, temperature-programmed reduction, and scanning electron microscopy measurements, as well as variations in the filtrate pH, Si/Al ratios, and solid sample Ni loading, a two-step dissolution-recrystallization process is proposed. The process consists of Si dissolution and no change in elemental Al, and after the dissolved Si(iv) concentrations have promoted Ni phyllosilicate nanosheet solubility, further growth of multilayered Ni phyllosilicate nanosheets commences. The precursor Ni phyllosilicate is changeable between Ni₃Si₂O₅(OH)₄ and Ni₃Si₄O₁₀(OH)₂, because of competition in kinetically-favored and thermodynamically-controlled species caused by different basic agents. The superior catalytic performance is demonstrated in the metal/acid catalyzed biomass derived bulky stearic acid hydrodeoxygenation with 90% octadecane selectivity and a promising rate of 54 g g^-1 h^-1, which highly excels the reported rates catalyzed by Ni catalysts. Significant improvements in activity and selectivity are related to the highly dispersive Ni nanoparticles onto/into intra-mesopores of hierarchical HBEA, hence enhance the accessibility of bulky substrates to metal sites and mass transfer capacity.