Catalysis 2025 Issue

Maximising hydrocracker performance and middle distillate production

Combining high-quality materials and scalable low-cost catalyst manufacturing as a prerequisite to commercialise accessible USY zeolites

Danny Verboekend Zeopore

A ccessible (mesoporous) ultra-stable Y (USY) zeolites offer many benefits for refiners when applied as hydrocracking catalysts. However, their commercial - isation appears complicated by the quality of the accessible USY zeolite and the scalability and cost of their manufacture. The enhancement of accessibility via the introduction of mes - oporosity is typically associated with a reduction of intrinsic zeolitic properties, as illustrated in the case of acid strength. On the other hand, the inclusion of costly ingredients, such as organics and the to-be-treated parent USY zeolite itself, along with unscalable unit operations, is seemingly mandatory. Selection of the optimal mesoporisation route provides a unique combination of preserved acid strength and acces - sibility. Moreover, advanced descriptors featuring meso - porosity and acid strength and type guide catalyst design maximise benefits in hydrocracking. Finally, the integration of dealumination and mesoporisation post-synthetic steps enables combining high-quality materials with scalable and low-cost manufacture. Accessible USY zeolites The catalytic potential of accessible mesoporous zeolites has attracted a lot of attention over the last decade. A ‘rel - atively simple’ post-synthetic treatment complements the intrinsic micropores existing in conventional USY with a network of complementary connected mesoporous. The mesopores in the resulting hierarchical micro-mesoporous USY zeolite lead to increased access to active acid sites in the zeolite crystals and, at the same time, aid the evacua - tion of already-cracked species.1 A variety of benefits have been reported, most importantly the increased yield to middle distillates at the expense of gas formation. Other benefits include higher activity, reduced hydrogen consumption, higher plant capacity, higher stabil - ity and longer cycle time, increased tolerance to more diffi - cult feedstocks, higher quality of products, higher viscosity index, lower polycyclic aromatics, higher H-content, and an increased residue conversion.² , ³ However, despite this comprehensive list, the industrial adaptation of accessible USY zeolites has been underwhelming. Conventional USY zeolites USY zeolites have been used in hydrocracking for

several decades. They are high-silica faujasites derived from high-alumina NaY zeolites, which are obtained through crystallisation in autoclaves. USY are obtained from NaY zeolites by application of an established post-synthetic dealumination protocol featuring ion exchanges, steam treatments, and acid leaching steps. As a result, the mate - rial is made more hydrothermally stable, giving rise to the term ‘ultra-stable Y’ or USY zeolites. Importantly, although only low-cost ingredients are used, the efforts and losses incurred during this dealumination routine increase the price of a USY by a factor of five to 10 compared to the relatively cheap starting NaY zeolite. Mesoporous faujasites quality In an ideal scenario, the desired zeolitic properties are main - tained while introducing secondary porosity. However, the reality of mesoporous zeolites shows that upon the intro - duction of secondary porosity, the intrinsic zeolitic prop - erties are reduced.4 The latter is particularly true for USY zeolites, where the term ‘ultra-stable’ does not accurately reflect their behaviour upon mesoporisation. Compared to their relatively more stable ZSM-5 and beta zeolite coun - terparts, mesoporised USY zeolites typically feature signifi - cantly reduced micropore volumes, crystallinity, and acidity, which in turn can be related to the reduced activity of mes - oporous USY zeolites.5 An important descriptor in the design of mesoporous USY zeolites is their strong acidity. This acidity typically relates to acid sites that are able to retain a basic probe molecule (such as ammonia or pyridine) at higher temperatures. The acidity of mesoporised USY zeolites has been assessed on a number of accounts, systematically showing a reduction in acid strength for the mesoporous zeolite (see Figure 1 ). Consequently, one of the challenges in the design of meso - porous zeolites is to ensure that, in addition to the quantity of mesoporosity, the quality of the zeolite is preserved or even enhanced. Scalability and cost of manufacture Likely, the most notorious aspect involved in the synthesis of mesoporous USY zeolites is the use of organic mole - cules, typically tetraalkylammonium cations such as cetyl - trimethylammonium (CTA) or tetrapropylammonium (TPA).

Catalysis 2025