Mar-2025

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

Article Summary

Accessible (mesoporous) ultra-stable Y (USY) zeolites offer many benefits for refiners when applied as hydrocracking catalysts. However, their commercialisation 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 mesoporosity 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 accessibility. Moreover, advanced descriptors featuring mesoporosity 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 ‘relatively 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 evacuation 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 stability and longer cycle time, increased tolerance to more difficult 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 material 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 maintained while introducing secondary porosity. However, the reality of mesoporous zeolites shows that upon the introduction of secondary porosity, the intrinsic zeolitic properties 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 counterparts, mesoporised USY zeolites typically feature significantly reduced micropore volumes, crystallinity, and acidity, which in turn can be related to the reduced activity of mesoporous 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 mesoporous 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 molecules, typically tetraalkylammonium cations such as cetyltrimethylammonium (CTA) or tetrapropylammonium (TPA). These species are used to scavenge dissolved silicon species, direct mesopore formation, and/or protect the fragile faujasite framework during the mesoporisation process (typically executed in alkaline media).¹ Illustratively, the state-of-the-art suggests that mesoporous USY zeolites of high crystallinity can only be made using organics (see Type D in Figure 2).

Such organic species are undesired as they not only complicate the wastewater treatment but also attach tightly to the zeolite. Accordingly, the removal of organics needs to be executed via combustion. This is a dangerous operation due to the release of explosive volatiles. It requires significant energy consumption and gives rise to carbon and nitrous oxide emissions.

Another undesired aspect is the unit operations associated with the manufacture of mesoporous zeolites. For example, the use of hydrothermal stages for extended periods is commonly reported. Moreover, using alkali cations to make alkaline media implies the need for complementary ion exchange treatments to restore the catalytically active protonic form. Furthermore, the use of organics carries the risk of foaming and separation difficulties. As a result, a ‘relatively simple’ mesoporisation treatment simply does not appear to exist.

Finally, it is crucial to highlight that one of the most overlooked expensive ingredients is the USY zeolite itself. As a precious zeolite, solid losses during mesoporisation should be minimised. The lack of coverage in the top right corner of Figure 2 is significant: not only will high-quality zeolites be unattainable without using organics, but achieving them also appears impossible without losing a sizable mass of the pristine parent USY zeolite. Thus, the underwhelming industrial adaptation may very well be attributed to the sub-optimal quality of the zeolite and a plethora of manufacturing challenges, complications, and costs (see Figure  3).

Limitations of conventional descriptors
In order to improve the understanding of mesoporous USY in hydrocracking, Zeopore has executed several hydrocracking campaigns. In each, industrial catalytic testing of a strategic variation of USY samples was applied. Zeolite powders were shaped into extrudates, impregnated with non-noble metals, and evaluated in the hydrocracking of vacuum gasoil in a fixed-bed reactor under both sweet and sour conditions.

In the quest for improved hydrocracking performance, the application of suitable descriptors to characterise the mesoporous USY zeolite is worthwhile. This becomes particularly relevant in cases where conventional descriptors fail. For example, take two seemingly similar mesoporous USY zeolites obtained via two different routes: ‘Route I’ and ‘Route II’. Both materials feature a largely retained micropore volume and a roughly doubled mesopore volume (see Figure 4, left). Additionally, both samples have an overall acidity of about 80% as compared to the conventional USY zeolite (see Figure 4, right).

Taking the overall Brønsted acidity into account, one would expect both mesoporous zeolites to be substantially less active than the conventional USY. Moreover, using mesoporosity as a descriptor for selectivity, both mesoporous zeolites should yield a similar benefit in middle distillates make. The above hypotheses are obviously not in line with the remarkably different catalytic results in Figure 4. A broader evaluation of the samples within this study confirms the sub-optimal correlation of established descriptors with the performance of mesoporous zeolites.