Core-shell bifunctional catalysts: Controllable intimacy between metals and acids within nanometer-scale for n-alkane conversion

Owing to the important industrial value in hydro-isomerization/-cracking process, metal–acid bifunctional catalysts have attracted much attention. However, exploring the spatial distance namely the intimacy between the two active components remains a primary challenge in the development of a bifunctional catalytic system, as there are limited synthesis protocols for rationally organizing the two components to maximize the synergistic effect. Herein, a series of catalysts with different degrees of intimacy were prepared, which can be assorted into four scales: atomic-, nanometer-, µm-, and millimeter-scale. Specially, Pt/Y@nS core–shell composite with an adjustable shell thickness in a nanometer scale (∼15 to ∼ 60 nm) was designed and synthesized. Not only playing a crucial role in confining and promoting highly dispersed Pt nanoparticles, the acid-free mesoporous SiO₂ shell also finely regulates the distance (“L”) between acids and metals in a nanoscale, thereby facilitating a deep investigation into the spatial relationship between metals and acid sites. Three n-alkanes (n-heptane, n-dodecane, and n-hexadecane) with a different molecular chain length (“l”) were selected as probe molecules so as to investigate the effect of “L/l” ratio on the hydro-isomerization/-cracking reaction. The results indicated that a relatively small L/l (≤13.1) ratio is not in favor of the conversion of the alkane molecules, for example, the hydro-cracking/-isomerization process of n-dodecane, n-hexadecane, or n-heptane on 0.2Pt/Y, 0.2Pt/Y@0.5S. A smaller L/l ratio offers the catalyst with a low catalytic activity, here, isomerization and cracking yields are low. High selectivity towards mono-branched i-alkane accompanied with a little of cracked products with more carbon atoms can be obtained. On the contrary, a relatively large L/l (52.3 > L/l ≥ 26.5) ratio in the catalyst contributes to elevating the catalytic activity, for example, the hydro-cracking/-isomerization process of n-dodecane or n-hexadecane on catalysts 0.2Pt/Y@1.5S, n-heptane on 0.2Pt/Y@1S. A larger L/l ratio gives the catalyst a higher activity in hydrocracking, in which more cleavage products with lesser carbon atoms can be obtained, and with an enhanced L/l ratio, the selectivity towards corresponding i-alkane along with the selectivity ratio of mono-branched i-alkanes to multi-branched ones in the final products decreases significantly.