The high selectivity of bifunctional catalysts in the hydroisomerization of long-chain n-paraffins is a key factor for producing high-quality fuels. However, it is directly dependent on a complex balance between the topology, acidity, morphology, and crystal size of the zeolite support. To address this challenge, this work presents a comparative study of medium-pore molecular sieves (ZSM-5, ZSM-23, ZSM-48, and SAPO-11) with different structural and acidic properties. The physicochemical characteristics of the materials were investigated using a set of analytical techniques, including XRD, SEM, low-temperature N2 adsorption, and NH3-TPD. It was shown that the pore topology (3D in ZSM-5 vs. 1D in the others), chemical composition, and crystal morphology (ranging from ~50 nm nanoprisms for ZSM-48 to 200–300 nm needles for ZSM-23-80 and 1 μm needles for ZSM-23-100) determine both the textural properties and the strength and concentration of acid sites. In the hydroisomerization of n-hexadecane, the Pt/ZSM-5 catalyst, with its high concentration of strong acid sites and 3D channel pore structure, exhibited the highest activity but an extremely low isomer selectivity (7%) due to intense cracking. The best results were demonstrated by the Pt/SAPO-11 and Pt/ZSM-48 samples, which provided isomer yields of 80% and 73%, respectively. Their high efficiency is attributed to the combination of a one-dimensional pore structure, moderate acidity, and nanocrystalline size, which minimizes diffusion limitations and suppresses side reactions. Thus, controlling the topology and acidity in combination with nanocrystalline morphology is an effective strategy for the rational design of highly selective hydroisomerization catalysts.

The rational design of highly efficient bifunctional SAPO-11 catalysts for hydroisomerization of n-C₁₆ requires unprecedented control over both acidic properties and diffusion characteristics. This work systematically investigates the influence of the SiO₂/Al₂O₃ molar ratio (0.1–0.4) in the initial gel on the physicochemical and catalytic properties of SAPO-11. Using a combination of characterization techniques (XRD, SEM, TEM-SAED, ²⁹Si MAS NMR, and IR-Py), it was established that this parameter serves as a simple tool for crystal engineering. The concentration of Brønsted acid sites and the external surface area demonstrate a non-linear dependency, reaching their maximum at SiO₂/Al₂O₃ = 0.3. Further increase in silicon content reduces both crystallinity and acidity due to the transition to the dominant SM2 + SM3 incorporation mechanism and the formation of silicon islands. Notably, varying the SiO₂/Al₂O₃ ratio enables control over crystal morphology—progressing systematically from truncated cones (SiO₂/Al₂O₃ = 0.1) to flat prismatic platelets (SiO₂/Al₂O₃ = 0.2) and ultimately hierarchical spherical aggregates (SiO₂/Al₂O₃ = 0.4). In n-C₁₆ hydroisomerization, the Pt/SAPO-11(0.2) catalyst demonstrated the highest yield of i-C₁₆ compared to other samples reaching 81%. The platelet morphology ensures a minimal diffusion path (<100 nm), effectively suppressing secondary hydrocracking. This finding underscores that morphology optimization is more critical than maximizing acidity for achieving high selectivity in the context of n-C₁₆ hydroisomerization over Pt/SAPO-11.

Catalytic systems based on SAPO-11 molecular sieves are known for their superior selectivity in the hydroisomerization of higher (C₁₆₊) n-paraffins. However, further improvement of SAPO-11-based catalysts is impeded by the lack of effective tools for controlling the morphology and size of silicoaluminophosphate crystals. The present study investigates the effects of the DPA/Al₂O₃ molar ratio in reaction gels on the nature of intermediate phases and on the physicochemical properties of SAPO-11 molecular sieves. SAPO-11 has been found to crystallize through the formation of different phases depending on the DPA/Al₂O₃ ratio: an AlPO₄·2H₂O intermediate at DPA/Al₂O₃ = 1.0, a mixture of AlPO₄·2H₂O and a layered silicoaluminophosphate at DPA/Al₂O₃ = 1.4, and a layered silicoaluminophosphate alone at DPA/Al₂O₃ = 1.8. Proper adjustment of this ratio provides an effective tool to control the crystal morphology, crystal size, and the porous structure characteristics of the SAPO-11 molecular sieves. It has been identified that, at DPA/Al₂O₃ = 1.0, cubic and lamellar SAPO-11 nanocrystals 200–400 nm in size are formed. These nanocrystals have the following textural properties: S_BET = 286 m²/g; V_micro = 0.06 cm³/g; and V_meso = 0.21 m³/g. Furthermore, the effects of the morphology and size of SAPO-11 molecular sieve crystals on their catalytic performance in the hydroisomerization of n-hexadecane have been demonstrated. Promising catalytic systems based on SAPO-11 nanocrystals have been proposed for the hydroisomerization of C₁₆₊ n-paraffins.

In this paper, we discuss options for the synthesis of granular free-binder ZSM-5 zeolites using synthetic aluminosilicates prepared by sol-gel technology with organic and inorganic silicon sources. It has been shown that the properties of the amorphous aluminosilicate used to prepare the initial granules influence the crystallization conditions, as well as the morphology and size of the crystals formed from granular ZSM-5 zeolite. The granular free-binder Pt/ZSM-5 with a developed secondary porous structure showed higher activity in the hydrocracking of hexadecane than the granular binder Pt/ZSM-5. At a reaction temperature of 220 °C, the conversion of n-hexadecane in the granular free-binder sample was 59.1%. At the same time, the selectivity for hexadecane isomers was 15.7%.