Hollow hierarchical silicalite-1 zeolite encapsulated Pt and PtNi bimetals (Pt@HS-1 and PtNi@HS-1) were successfully synthesized via a ship-in-bottle strategy and employed for hydroconversion of methyl stearate into aviation fuel range alkanes in a fixed-bed reactor. Their morphologies, texture, and acidity were characterized by field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray diffraction, nitrogen adsorption and desorption, hydrogen temperature-programmed reduction, X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectrometry, ammonia temperature-programmed desorption, and CO pulse chemisorption. The bimetallic catalysts (PtNi/S-1 and PtNi@HS-1) showed higher catalytic activity and selectivity to aviation fuel owing to the stronger cleavage ability related to the introduction of Ni species. Moreover, aviation fuel selectivity was significantly improved on the hollow hierarchical encapsulated catalysts than their precursors, ascribed to the extended contact time between reactants and active sites. A maximum aviation fuel selectivity of 45% was obtained for the PtNi@HS-1 catalyst at 360 °C under 3 MPa. Furthermore, the PtNi@HS-1 catalyst exhibited superior catalytic stability and selectivity than the PtNi/S-1 catalyst during the stability test. This indicates that the hollow hierarchical encapsulated structure plays crucial roles in enhancing aviation fuel selectivity and catalytic performance.

Hollow Silicalite-1 and ZSM-5 zeolites with hierarchical porous shells have been synthesized by using a dissolution-recrystallization method. The morphology, structure, and acidity of these zeolites supported Pt catalysts were characterized by XRD, FT-IR, MAS-SSNMR, FE-SEM, FE-TEM, N2-BET, XPS, NH3-TPD, and CO pulse chemisorption. Compared to the conventional ZSM-5 supported Pt catalyst, the special structure in hollow ZSM-5 zeolite significantly promotes the dispersion of metallic Pt and the synergistic effect between metal active sites and acid sites. These boost the catalytic activity, selectivity of guaiacol hydrodeoxygenation toward cycloalkanes and long-term stability over the Pt/hollow ZSM-5 catalyst combined with improved mass transfer of products and reactants derived from the hierarchical hollow porous structure. Moreover, the Pt/hollow ZSM-5 catalyst exhibits excellent low temperature catalytic activity to completely transform guaiacol into cycloalkanes with the cyclohexane selectivity of more than 93% at 220 °C, suggesting that hollow ZSM-5 zeolite is a promising support for upgrading of bio-oils.

Ultrathin Ni/ZSM-5 nanosheet catalysts with different acid concentrations, B/L (Brønsted/Lewis acid sites) ratios, and nanosheet thicknesses are designed for the hydroconversion of oleic acid into aviation-fuel-range alkanes (AFRAs). The role of acid concentration/distribution and nanosheet thickness was investigated. High acid concentration enhances the hydrodeoxygenation (HDO) reaction and intrinsic deoxygenation activity because of the synergistic effect between acid sites and metal sites. The selective cracking reaction can be enhanced by tailoring nanosheet thickness and acid distribution. The thin nanosheet favors isomerization, the selective cracking reaction, as well as the generation of central branched isomers. External acid sites dominate the primary cracking and central C−C cracking of the deoxygenated products, but internal acid sites promote the deeper cracking and terminal C−C cracking. Formation of aromatics is thoroughly suppressed at low reaction temperature (≤533 K). A high AFRA yield of 41.4% is achieved over the 3 nm nanosheet catalyst with Si/Al = 300 and B/L = 0.18.

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