Graphene-encapsulated
nickel nanoparticles (Ni@C) were loaded on
Zeolite Socony Mobil (ZSM)-5 for lipid hydrogenation to produce biojet
fuel. Ni@C and ZSM-5 exhibit an efficient and synergistic catalytic
activity, and the strong chemical interaction between graphene and
nickel significantly improves the hydrogenation capacity of the catalyst.
The graphene layers strengthened the dispersion of nickel, reduced
the size of Ni@C to ∼3.6 nm, and kept the valence state of
nickel at 0. According to the hydrogenation tests, the conversion
of oleic acid to jet fuel could reach 98.59% with only 3 wt % nickel
loading amount. Furthermore, Ni@C/ZSM-5 shows excellent resistance
to nickel agglomeration and loss.
Graphene-encapsulated nickel nanoclusters are a feasible strategy to inhibit the nickel deactivation of nickel-based catalysts. In this work, graphene-encapsulated catalysts (Ni@C/HZSM-5) were prepared by a compression forming process, using pseudo-boehmite, Al2O3, and ZrO2 as binders. The pseudo-boehmite was gradually transformed from amorphous to crystalline alumina at high temperatures, which destroyed the nucleation of Ni@C. In contrast, the crystal-stabilized zirconia was more favorable for the nucleation of Ni@C. The extensive dispersion of alumina on the surface of HZSM-5 covers the acid sites of HZSM-5. In contrast, when zirconia was used as the binder, the binder existed in the form of the direct aggregation of ~100 nm zirconia spheres; this distribution form reduced better the damage of the binder to the acid site of the catalyst. Furthermore, the particle size of Ni crystals in the graphene-encapsulated catalysts decreased significantly (mostly <11 nm), and no evident agglomeration of nickel particles appeared. It was found that the stabilization of the metal interface delayed, to an extent, the accumulation rate of carbon deposits and, thus, postponed the deactivation of the acid sites. After 8 h of continuous reaction, the conversion of the traditional catalyst Ni/Z5+Zr dropped significantly to 60%. In contrast, the conversion of Ni@C catalysts prepared with ZrO2 remained above 90%. The regeneration test shows that air roasting could effectively remove carbon deposits and restore the catalyst activity.
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