Benefits of simultaneous mesoporisation/metal incorporation

With the emerging shift towards a sustainable and renewable refining and petrochemical industry, the use of tailored metal-based catalysts is likely to intensify.

Danny Verboekend and Martin d’Halluin
Zeopore Technologies

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Article Summary

By combining mesoporisation with metal incorporation, a platform has been created with enormous synthetic and catalytic potential, applicable to most non-noble metals and any zeolite. The metal-containing mesoporous zeolites obtained display near 100% dispersions, unprecedented combinations of boosted Lewis acidity and preserved Bronsted acidity, and stability in aqueous solutions. Activity and selectivity benefits in dewaxing and methanol-to-olefins demonstrate the potential of this novel approach to catalyst manufacturing. The benefits of these materials in other catalytic applications are currently being explored, with the focus on commercialising this technology in established and emerging applications.

Mesoporous zeolites
Mesoporous zeolites complement intrinsic zeolitic micropores (0.5-1 nanometer) with a network of secondary mesopores (2-20 nanometer). This way, access and diffusion limitations are removed, yielding superior performance and attractive business cases in various applications, such as cracking, isomerisation, and alkylation.1

Within the discipline of mesoporisation, Zeopore’s technologies are scalable and commercially applicable, not relying on elaborate unit operations and/or exotic ingredients. Moreover, in-house scientific experience and the width of the technology toolbox facilitate obtaining the most out of any zeolitic material, from USY to ZSM-5 and from zeolite to SAPO.

Relevance of metal-containing zeolite catalyst
In many zeolite-based catalytic applications, a metal function is included to yield optimal performance. For example, in hydroprocessing reactions, such as cracking and isomerisation dewaxing, nickel is deposited on the catalyst to provide the required hydrogenation function. In other cases, a specific interaction with the to-be-converted substrate is achieved using metal-loaded zeolite powders, as is the case in selective catalytic reduction of NOx using Cu-based zeolites.

Yet in other cases, the zeolite facilitates sorption and combination of species first converted by the metal component, such as in Fischer-Tropsch chemistry. Metals can also be employed not for a direct role in the catalytic cycle but to tailor acidity and stability, such as the use of rare earths (RE) in fluid catalytic cracking.

With the emerging shift towards a sustainable and renewable refining and petrochemical industry, the use of tailored metal-based catalysts is likely to intensify. Many heavy biomass-derived oil streams benefit from qualitative hydroprocessing, first to lower the oxygen content (for example, using a Ni-ZSM-5-based hydrodeoxygenation process) and then to obtain the desired cloud and boiling points in dewaxing and hydrocracking (HDC) steps, respectively.

Moreover, with these emerging applications, the conversion of methanol, syngas, and aromatics predominates, often relying on specific metal-containing zeolites, such as Ga- and Zn-ZSM-5. Finally, especially for oxygen-rich streams, such as sugars, conversions towards desired products, such as lactic acid, are preferably achieved using metal-containing highly Lewis acidic zeolites, such as Sn-beta.

Simultaneous mesoporisation and metal deposition
The simultaneous mesopore formation and metal deposition occurs using an elegant process, in which both the porosity and composition can be easily tuned (see Figure 1). The obtained metal-containing zeolites can subsequently be shaped into a catalyst extrudate. Hence, the method of metal deposition is fundamentally different compared to standard metal-containing catalysts, in which first an extrudate is made, to be subsequently complemented with a metal using in an impregnation step.

The metal-containing mesoporous zeolites can be finely tuned towards any application. It has thus far proven particularly effective for alkaline earth and transition metals, such as Mg, Ca, Co, Fe, Cu, Ni, and Zn. Yet also metalloids, such as Sn and Ga, and lanthanoids, such as Ce, have proven suitable candidates.

Like metal-free mesoporous zeolites, the secondary porosity can be finely tuned towards applications. Typical mesopore surface areas range from 100-300 m2/g. These can be combined with metal contents ranging from very low, ca 100 ppm, to very high, ca. 25 wt%.

Traditionally, the deposition of metals on zeolite powder is quickly disregarded, as impregnation of powder in general is a challenging unit operation. Moreover, the metal-zeolite interaction is traditionally suboptimal, resulting in large metal particles (see Figure 2, top right). In contrast, Zeopore’s metal mesoporisation technology is highly controlled and bonds the metal to the zeolite, yielding a blanket of single metal atoms over the zeolite’s external surface. As the technology employs wet chemistry, it does not require energy-demanding evaporation steps, as is the case with state-of-the-art impregnations. This highly controlled metal mesoporisation also does not yield metal nitrates, avoiding undesired NOx emissions in subsequent metal activation steps.

On the spatial level, the deposition of the metal, specifically on the external surface of the zeolites, enables benefits on several levels (see Figure 2, bottom). First and foremost, the proximity of the metal to the acid site in the zeolite is hereby minimised, opening the door to pronounced catalytic benefits, as demonstrated in hydrocracking, for example.2

In addition, controlled dispersion of the metal on the zeolite enables complete avoidance of the complications from metal inhomogeneity on the extrudate level. Phenomena such as gravity-based settling of the metal and metal support interaction-based eggshell/yoke distributions are no longer concerns.

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