Bifunctional Intimacy and its Interplay with Metal‐Acid Balance in Shaped Hydroisomerization Catalysts

Steering the Metal Precursor Location in Pd/Zeotype Catalysts and Its Implications for Catalysis

Bifunctional catalysts containing a dehydrogenation–hydrogenation function and an acidic function are widely applied for the hydroconversion of hydrocarbon feedstocks obtained from both fossil and renewable resources. It is well known that the distance between the two functionalities is important for the performance of the catalyst. In this study, we show that the heat treatment of the catalyst precursor can be used to steer the location of the Pd precursor with respect to the acid sites in SAPO-11 and ZSM-22 zeotype materials when ions are exchanged with Pd(NH3)4(NO3)2. Two sets of catalysts were prepared based on composite materials of alumina with either SAPO-11 or ZSM-22. Pd was placed on/in the zeotype, followed by a calcination-reduction (CR) or direct reduction (DR) treatment. Furthermore, catalysts with Pd on the alumina binder were prepared. CR results in having more Pd nanoparticles inside the zeotype crystals, whereas DR yields more particles on the outer surface of the zeotype crystals as is confirmed using HAADF-STEM and XPS measurements. The catalytic performance in both n-heptane and n-hexadecane hydroconversion of the catalysts shows that having the Pd nanoparticles on the alumina binder is most beneficial for maximizing the isomer yields. Pd-on-zeotype catalysts prepared using the DR approach show intermediate performances, outperforming their Pd-in-zeotype counterparts that were prepared with the CR approach.

Mechanistic classification and benchmarking of polyolefin depolymerization over silica-alumina-based catalysts

Carbon-carbon bond cleavage mechanisms play a key role in the selective deconstruction of alkanes and polyolefins. Here, we show that the product distribution, which encompasses carbon range and formation of unsaturated and isomerization products, serves as a distinctive feature that allows the reaction pathways of different catalysts to be classified. Co, Ni, or Ru nanoparticles immobilized on amorphous silica-alumina, Zeo-Y and ZSM-5, were evaluated as catalysts in the deconstruction of n-hexadecane model substrate with hydrogen to delineate between different mechanisms, i.e., monofunctional- (acid site dominated) or bifunctional-hydrocracking (acid site & metal site) versus hydrogenolysis (metal site dominated), established from the product distributions. The ZSM-5-based catalysts were further studied in the depolymerization of polyethylene. Based on these studies, the catalysts are plotted on an activity-mechanism map that functions as an expandable basis to benchmark catalytic activity and to identify optimal catalysts that afford specific product distributions. The systematic approach reported here should facilitate the acceleration of catalyst discovery for polyolefin depolymerization.

Product distributions have been used to classify the depolymerization pathways of polyolefins catalyzed by silica-alumina-based catalysts to construct an activity-mechanism map as a benchmarking tool to facilitate catalyst discovery.

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

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 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, in particular at low noble metal loadings.