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The Origin of Metal Loading Heterogeneities in Pt/Zeolite Y Bifunctional Catalysts.

ABSTRACT: Preparing catalysts with highly dispersed metal nanoparticles and narrow particle size distribution has been in the focus of numerous studies. Besides size and size distribution, the location of metal nanoparticles within and local metal loading of the support can have significant impact on catalytic performance. This study revealed that great variations in Pt loading between individual Pt/zeolite Y crystals occurred irrespective of the metal deposition method, namely ion-exchange (IE) or incipient wetness impregnation (IWI). The variation in Pt loading was found to be directly related to different Si/Al ratios of individual zeolite crystals. Results indicate that this Si/Al variation was likely induced by post-synthesis treatments, commonly performed to introduce mesoporosity. In view of the great importance of zeolite-based catalysts for oil refining, understanding the origin of such metal loading heterogeneities may lead to further improvement of zeolite-based catalytic performance.

SUBMITTER: van der Wal LI

PROVIDER: S-EPMC6774382 | biostudies-literature | 2019 Aug

REPOSITORIES: biostudies-literature

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Project description:The preparation of zeolite-based bifunctional catalysts with low noble metal loadings while maintaining optimal performance has been studied. We have deposited 0.03 to 1.0 wt % Pt on zeolite H-USY (Si/Al ∼ 30 at./at.) using either platinum(II) tetraammine nitrate (PTA, Pt(NH3)4(NO3)2) or hexachloroplatinic(IV) acid (CPA, H2PtCl6·6H2O) and studied the nanoscale Pt loading heterogeneities and global hydroconversion performance of the resulting Pt/Y catalysts. Pt/Y samples prepared with PTA and a global Pt loading as low as 0.3 wt % Pt (n Pt/n A = 0.08 mol/mol, where nPt is the number of Pt surface sites and n A is the number of acid sites) maintained catalytic performance during n-heptane (T = 210-350 °C, P = 10 bar) as well as n-hexadecane (T = 170-280 °C, P = 5 bar) hydroisomerization similar to a 1.0 wt % Pt sample. For Pt/Y catalysts prepared with CPA, a loading of 0.3 wt % Pt (n Pt/n A = 0.08 mol/mol) sufficed for n-heptane hydroisomerization, whereas a detrimental effect on n-hexadecane hydroisomerization was observed, in particular undesired secondary cracking occurred to a significant extent. The differences between PTA and CPA are explained by differences in Pt loading per zeolite Y crystal (size ∼ 500 nm), shown from extensive transmission electron microscopy energy-dispersive X-ray spectroscopy experiments, whereby crystal-based n Pt/n A ratios could be determined. From earlier studies, it is known that the Al content per crystal of USY varied tremendously and that PTA preferentially is deposited on crystals with higher Al content due to ion-exchange with zeolite protons. Here, we show that this preferential deposition of PTA on Al-rich crystals led to a more constant value of n Pt/n A ratio from one zeolite crystal to another, which was beneficial for catalytic performance. Use of CPA led to a large variation of Pt loading independent of Al content, giving rise to larger variations of n Pt/n A ratio from crystal to crystal that negatively affected the catalytic performance. This study thus shows the impact of local metal loading variations at the zeolite crystal scale (nanoscale) caused by different interactions of metal precursors with the zeolite, which are essential to design and synthesize optimal catalysts, in particular at low noble metal loadings.

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The Origin of Metal Loading Heterogeneities in Pt/Zeolite Y Bifunctional Catalysts.

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