Designing and engineering catalysts for hydrotreating
Advanced analytical and experimental techniques combine in the development of a new hydrotreating catalyst.
KARL KRUEGER, MAX OVCHINNIKOV and THOMAS WEBER
Criterion Catalysts & Technologies
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There are few areas of research that are more mature than hydroprocessing catalyst development. This has been an area of active research for over seven decades, involving academic, business and government institutions, with thousands of patents granted and many major breakthroughs in activity and stability along the way. The reason for this persistent interest is driven by the enormous value that a better catalyst solution can offer. As an example, an improved hydrocracker pretreat catalyst with higher aromatic saturation (HDA) and hydrodenitrogenation (HDN) activity will produce millions of dollars in extra revenue over the course of a commercial run. The benefit of higher activity catalysts is not unique to first stage hydrocracking. Indeed, similar opportunities exist across the whole spectrum of hydrotreaters in a refinery complex, making the case for advanced catalyst technology clear.
Oil refiners worldwide are being challenged to provide clean transportation fuels in order to meet stricter environmental regulations while moving to more difficult crudes. In particular, removal of sulphur in diesel fuels to ultra low levels continues to be implemented throughout the world following an early adoption of ultra low sulphur diesel (ULSD) standards in Europe, Japan, and the US in the early 2000s. The implementation of Tier 3 gasoline sulphur regulations in the US, starting in 2017, has refiners focused on changes in both pretreat and post-treat catalyst systems. Finally, the new lower marine fuels specifications dropping to 0.5% sulphur in 2020 represents yet another upcoming challenge. Refiners that are investing in residue conversion units in combination with sophisticated hydrotreating and hydrocracking will be further ahead, with all refiners looking for hydroprocessing catalyst solutions to handle heavier and more difficult feeds as 2020 approaches.
The task of developing these advanced catalyst technologies falls to catalyst suppliers and there are various strategies that can be employed to accomplish this task. Criterion Catalysts & Technologies (Criterion) has over 29 years of catalyst research and development, applying customised catalyst systems and operating strategies to improve days on stream, generate higher yields and facilitate the processing of more challenging feeds.
The traditional workflow that has been utilised in the industry for decades is best characterised as highly linear (see Figure 1). It entails a sequential movement through the design of catalyst formulations, the testing of those formulations, the analysis of those test results, and the application of those learnings to the next batch of catalyst formulations. The linear nature of this process misses opportunities for parallel work which ultimately slows innovation and invention. This traditional workflow is also one that is filled with barriers. Barriers such as limited testing capability, poor data management tools, and classical methods of catalyst characterisation are major roadblocks to discovery.
Over the past 10 years, Criterion has moved away from this old fashioned process to one that embraces parallel workflows and one that destroys barriers. In this new workflow, ideation, screening, and analysis all take place simultaneously, resulting in a more efficient workflow where no process is beholden to another. This parallel workflow is enabled by several key technologies: computational modelling, advanced characterisation, high throughput experimentation, and advanced data analytics (see Figure 2). In the past decade, these techniques have proven their ability to produce step-out innovations compared to other methodologies. For this reason, they are widely employed in industries such as pharmaceuticals, electronics, and aerospace where innovation is the driver of success. When applied together, these technologies affect a paradigm shift in the R&D pipeline where innovation becomes easier and scientists can focus on new technology development.
Computational modelling is where it all starts. It is hardly possible to engineer technology platforms exceeding today’s activity by trial and error methodologies alone. New synthetic strategies must be based on a precise understanding of which structural aspects of the complex catalyst system should be optimised. This starts with the carrier by adjusting it to the right surface area and pore size distribution, both of which influence surface properties that enable the formation of highly active Type II catalysts.
Among the surface properties that are crucial in generating Type II particles is the right distribution of aluminium and oxygen at the carrier surface. In order to fundamentally understand this property, its influence in the active phase generation, as well as how to engineer this surface, Criterion is increasingly performing quantum chemical calculations in a high performance computing facility. These ab initio or ‘first principles’ methods allow calculation of structural properties, simulation of their functioning during catalyst preparation, and elucidation of their catalytic action. Figure 3 shows the results of density functional theory (DFT) calculations, targeted at optimising the aluminium and oxygen distribution on two prominent surfaces of γ alumina (111 and 100 surfaces). This structural setting enables a balanced deposition of metal components during catalyst synthesis, ensuring formation of particles with the right morphology and structure.
Another technology that goes hand in hand with computational modelling is advanced catalyst characterisation. By applying the most cutting edge surface science techniques, we are able to gain new insights into the structure and function of catalysts. This insight is fundamental in driving the development of the next generation of catalyst platforms. One critical physical feature for all hydroprocessing catalysts is the catalytically active MoS2 edge responsible for sulphur, nitrogen, and aromatics removal. Achieving the right active site architecture is crucial for high performance so we have put a major effort towards the challenging task of characterising this feature in alumina supported, non-model catalyst systems operating on real feedstocks.
Aberration corrected scanning transmission electron microscopy (STEM) has been applied to reveal the exact position of individual molybdenum atoms in real catalysts. Figure 4 shows data from a commercial NiMo Centera catalyst pulled from ULSD service on a straight-run gas oil. The top image shows the edge region of a particle in top view (white arrow) where the catalytic reaction takes place; every yellow spot represents one metal atom. The structure of the metal atoms in the particle is largely determined by the regular arrangement of the bulk MoS2 structure shown in the inset. In this perfect regularity, however, catalytic activity would be relatively low; alteration is required to switch active sites on. The image shows this clearly – distortions engineered into the arrangement of metal atoms close to the edge enable the high performance of Centera catalysts. An edge-on view of the MoS2 slab can be seen in the bottom image (white arrow); again, each spot represents a metal atom in the edge. This concept of engineering the active edge sites has the advantage that activity remains high regardless of the particle size. We are thus able to move away from the traditional credo in catalysis which requires generation of very small particles for high activity. Through this we were able to accommodate structural functions for both direct desulphurisation and hydrogenation into one larger particle.
Equally important to idea generation is catalyst screening. Perhaps the most important improvement in catalyst testing has been the utilisation of high throughput experimentation. This technique, with roots in combinatorial chemistry, utilises miniaturisation, robotics, and automation to activity test a huge number of catalyst formulations in a short period of time. Historically, catalysts were tested in micro-flow units that contained only one catalyst. Operation of these units was largely manual, requiring significant human intervention for most tasks. This was improved somewhat with the invention of multi-barrel units that combined two to four reactor tubes in a common heating bath. While some process parameters such as temperature and treat gas rate were computer controlled, these systems were only minimally programmable. These old systems have now been completely surpassed by high throughput experimentation. These units typically test a minimum of 16 reactors at a time utilising one-hudredth the volume of catalyst used traditionally. These systems are fully automatic and utilise robotics for tasks such as product collection. They are also infinitely programmable and can run unmanned for days at a time. The result is a laboratory that tests catalysts five times faster than previously possible with the same manpower requirements.
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