Control and Impact of Metal Loading Heterogeneities at the Nanoscale on the Performance of Pt/Zeolite Y Catalysts for Alkane Hydroconversion

Abstract

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(NH 3 ) 4 (NO 3 ) 2 ) or hexachloroplatinic(IV) acid (CPA, H 2 PtCl 6 ·6H 2 O) 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 n Pt 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, ...

Four-Carbon Segmented Discrete Hydrocracking of Long-Chain Paraffins in MTT Channels Following a Pore-Mouth Mechanism

“…The molar loss of cracking products should be attributed to the loss of light components carried away with the gaseous product. In comparison, during the cracking of n -hexadecane with a longer chain, some values of C/F were slightly higher than 2 at a high n -hexadecane conversion of >40%, which indicates the existence of secondary cracking . This was because the paraffin with a longer chain has higher cracking activity.…”

“…In comparison, during the cracking of n-hexadecane with a longer chain, some values of C/F were slightly higher than 2 at a high n-hexadecane conversion of >40%, which indicates the existence of secondary cracking. 54 This was because the paraffin with a longer chain has higher cracking activity. Overall, the degree of secondary cracking was low enough to be ignored when the substrate conversion was lower than 40%.…”

Maximizing noble metal utilization in solid catalysts by control of nanoparticle location

“…In the petrochemical industry, platinum (Pt) is frequently used in combination with acidic zeolites for the hydroconversion of linear alkanes to enhance the quality of liquid fuels (fig. S1), and its performance is influenced by the Pt nanoparticle (NP) size and distribution, metal-support interactions, and acidic properties (20)(21)(22)(23). To describe the required Pt loading sufficient to maintain the bifunctional metal-acid balance in hydroconversion catalysts, Guisnet and coworkers proposed a widely applied parameter of the surface Pt sites to Brønsted acid sites ratio (n pt /n a ) (24).…”

Hydroconversion of Octylcyclohexane over a Bifunctional Pt/USY Zeolite Catalyst

“…The PtCl 6 2– complex exhibits a strong ionic interaction with the alumina support and should be preferentially adsorbed on the latter (and not in the USY crystals) . We are aware that the impact of Pt location on activity and selectivity in bifunctional hydrocracking reactions is still subject to debate, but it is beyond the scope of the present article to fully address this issue. − A complete procedure of the impregnation process, as well as thermal treatments applied on the final catalysts, may be found in a previous work …”