Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

Abstract

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one‐dimensional zeolites (ZSM‐22 and mordenite) and a γ‐alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n‐heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low‐molecular‐weight reactants.

The Known and Overlooked Sides of Zeolite‐Extrudate Catalysts

“…The exclusive deposition of Pt particles on γ-Al 2 O 3 binder (Pt-inbinder) results in a nanoscale intimacy between metal sites and active sites, which is beneficial for the catalytic activity and yield to the target branched isomers. [45] As a comparison, the exclusive deposition of Pt particles inside zeolites (Pt-in-zeolite) is detrimental to the hydroisomerization despite its atomic scale intimacy in functional sites. As shown in Scheme 4, the Pt-inbinder catalysts enable the consecutive steps of "dehydrogenation on Pt-isomerization at zeolite pore mouth-hydrogenation on Pt", which minimize the undesirable cracking of the intermediates.…”

“…In fact, the apparent controversies for the hydroisomerization and hydrocracking reflect the diverse dependence on the size of molecules, dimensionality of the zeolites, and the reaction conditions. [43,45] The above elegant investigations depend on the well-controlled deposition of metallic sites on the zeolite or binder, which may be still a significant challenge for many of the supported zeolite-based catalysts. Such challenge is bounded to the competition between zeolite and binder for the active/promoter species.…”

“…Controlled location of Pt on zeolite or binder, and their schematic description of the catalytic consequence on hydroisomerization of n-heptane. [45] precursors) only occupy a specific portion of the apparent loadings, and tungsten oxide species share high similarity and complexity in the structural states. Therefore, the precise definitions of the activities associated species are still challenging.…”

“…[20] Given the challenges in the location and visualization of Lewis acid sites, such confirmations are inspiring and will enlighten the similar inspections into the zeolite extrudates. At the same time, the emerging techniques, including CFM, [54] focused-ion-beam scanning electron microscopy (FIB-SEM), [55] Xray tomography, [56] X-ray diffraction computed tomography (XRD-CT), [57] etc., [45,[58][59][60][61] have successfully driven the exploration of shaped catalysts. In particular, time-resolved in situ UV-Vis and CFM of fluorescence probes have enabled the visualizations of the acid sites inside the shaped zeolites, [18,29,62] which revolutionize several understandings and the overlooked sides of the zeolites extrudates.…”

“…However, a very recent report discloses a contractive principle for the catalyst design for the hydroisomerization of n ‐heptane, whose alkene intermediate is relative “bulky” and prone to the cracking compared with its C 5 ‐C 6 counterparts. The exclusive deposition of Pt particles on γ‐Al 2 O 3 binder (Pt‐in‐binder) results in a nanoscale intimacy between metal sites and active sites, which is beneficial for the catalytic activity and yield to the target branched isomers [45] . As a comparison, the exclusive deposition of Pt particles inside zeolites (Pt‐in‐zeolite) is detrimental to the hydroisomerization despite its atomic scale intimacy in functional sites.…”