Dispersed platinum supported on ultrastable Y (USY) zeolites are used for a variety of hydrocarbon conversion applications. The reaction properties of such catalysts depend strongly on (i) their chemical compositions, e.g., Si/Al ratio, exchangeable cations, and binder (if present), (ii) both meso- and nanoscale pore structures of the zeolite support, and (iii) the types and locations of metal species and acid sites within the zeolite nanopores. Reaction pathway, X-ray scattering, electron microscopy, and solid-state NMR spectroscopy analyses yield quantitative atomic-scale insights on the performances of dual-function Pt,H+/USY catalysts with different silica-to-alumina molar ratios (SAR = 60 or 108) and correspondingly different acidities. Using n-hexadecane as probe molecule, a sequential reaction network was deduced for the more acidic Pt,H+/USY catalyst (SAR = 60), in which n-hexadecane, mono-branched-, and multi-branched-isomers form in series and with cracking products subsequently formed from the latter. Pseudo first-order rate coefficients for the isomerization and cracking reactions were determined and cracking rates by carbon number were determined, a novel feature of this work. For Pt,H+/USY catalyst with lower acidity (SAR = 108), a parallel set of reaction pathways were identified in which n-paraffin cracking products are formed directly from n-hexadecane. The different n-hexadecane hydroisomerization activities of the two Pt-H+/USY catalysts were correlated with different local Si and Al environments of the USY zeolite framework and Bronsted acid sites, as characterized by 29Si, 27Al, and 1H NMR. By comparison, a Pt/SiO2 catalyst was less active for catalyzing the formation of n-paraffin cracking products. Direct hydrocracking of n-hexadecane to n-paraffin products appears to be characteristic of Pt-containing low-acidity zeolites and could provide a new route for hydrogenolytic cleavage of aliphatic carbon-carbon bonds.

Probing of Pt/USY catalyst performance using n-hexadecane reaction pathways and advanced spectroscopic techniques

ACS Spring 2022

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