Metallocenes can catalyze the polymerization reaction of olefins. Prior to reaction, they require activation on the surface of solid activator particles. The active component in solid activators is methylaluminoxane (MAO). When MAO is heterogeneously dispersed over the particles, some particles have a homogeneous MAO distribution, whereas others only have an MAO-shell. Only the homogeneous particles are capable of activating the metallocenes. Using Energy-Dispersive X-Ray spectroscopy, the interparticle heterogeneity can be assessed and allowing for prediction of the catalytic activity. More information can be found in the Full Paper by B. M. Weckhuysen, et al. on page 11944.
Solid acids hold widespread applications in the field of heterogeneous catalysis. In this work, we present pyridine UV-Vis spectroscopy as a novel and promising tool to study the acidic properties of such solid acids. It was found that upon interaction with acid sites, the electronic properties of pyridine change significantly. Monitoring of consecutive adsorption and desorption of pyridine revealed absorption bands in the UV-Vis region characteristic for (a) pyridinium ions formed on Brønsted acid sites, (b) pyridine coordinated to Lewis acid sites, (c) pyridine hydrogen-bonded to surface hydroxyl groups, and (d) physisorbed pyridine. The classical pyridine FT-IR method probes the presence of different Brønsted and Lewis acid sites as well, but lacks sensitivity towards the differentiation between surface hydroxyl groups. In contrast, the pyridine UV-Vis spectroscopy method proves especially useful for the identification and distinction of different surface hydroxyl groups, since the band position in the UV-Vis spectrum strongly depends on the chemical environment of the hydroxyl group. Moreover, utilizing a slow desorption procedure under N2 flow, it was possible to study the differences in acidic strength of the hydroxyl groups. This method and related measurement protocols were developed for the study of acidic properties within solid acids with different silica/alumina ratios, but are, in our opinion, more generally applicable to any solid acid.
Abstract A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO) 4 ]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3–4 nm) were found to grow very large (20–200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO) 4 ] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis.
Matrix effects in a fluid catalytic cracking (FCC) catalyst have been studied in terms of structure, accessibility, and acidity. An extensive characterization study into the structural and acidic properties of a FCC catalyst, its individual components (i.e., zeolite H-Y, binder (boehmite/silica) and kaolin clay), and two model FCC catalyst samples containing only two components (i.e., zeolite-binder and binder-clay) was performed at relevant conditions. This allowed the drawing of conclusions about the role of each individual component, describing their mutual physicochemical interactions, establishing structure-acidity relationships, and determining matrix effects in FCC catalyst materials. This has been made possible by using a wide variety of characterization techniques, including temperature-programmed desorption of ammonia, infrared spectroscopy in combination with CO as probe molecule, transmission electron microscopy, X-ray diffraction, Ar physisorption, and advanced nuclear magnetic resonance. By doing so it was, for example, revealed that a freshly prepared spray-dried FCC catalyst appears as a physical mixture of its individual components, but under typical riser reactor conditions, the interaction between zeolite H-Y and binder material is significant and mobile aluminum migrates and inserts from the binder into the defects of the zeolite framework, thereby creating additional Brønsted acid sites and restoring the framework structure.
Abstract Methylaluminoxane (MAO) is an activator for single‐site olefin polymerization catalysts. Structural characterization of MAO, and in particular the influence of its heterogenization on a silica support, is pivotal for the understanding and optimization of this versatile co‐catalyst. In this work, we demonstrate that by varying the MAO loading on a silica support, we can tune the single particle characteristics in terms of MAO distribution and consequent activity. At low MAO loading we reveal two possible intraparticle MAO distributions: a homogeneous and an Al‐shell type distribution. With increasing MAO loading, the interparticle MAO distribution becomes more homogeneous. Acidity probing measurements on the single particle level explain how a different intraparticle MAO distribution results in a different activity in metallocene activation. Al‐shell particles contain remaining silanol groups and a negligible amount of acid sites and are, therefore, considered inactive for metallocene activation. Only the solid activator particles with a homogeneous intraparticle MAO distribution contain a sufficient number of acid sites, capable of activating the deposited metallocenes.
The preparation method greatly influences morphology, acid–base properties and performance of SiO2–MgO catalysts. Wet-kneaded catalysts possess an improved distribution, proximity and strength of acidic-basic sites, thus leading to higher butadiene yields.
Abstract Invited for the cover of this issue is the group of Bert M. Weckhuysen at Utrecht University. The image depicts an artistic impression of catalyst particles in full color: by combining multiple characterization techniques per single Fluid Catalytic Cracking (FCC) particle, insights are provided into the heterogeneity of an industrially used catalyst. Read the full text of the article at 10.1002/chem.201905880 .
Abstract Fluid catalytic cracking (FCC) is an important process in oil refinery industry to produce gasoline and propylene. Due to harsh reaction conditions, FCC catalysts are subject to deactivation through for example, metal accumulation and zeolite framework collapse. Here, we perform a screening of the influence of metal poisons on the acidity and accessibility of an industrial FCC catalyst material using laboratory‐based single particle characterization that is, μ‐XRF and fluorescence microscopy in combination with probe molecules. These methods have been performed on density‐separated FCC catalyst fractions, allowing to determine interparticle heterogeneities in the catalyst under study. It was found that with increasing catalyst density and metal content, the acidity and accessibility of the catalyst particles decreased, while their distribution narrowed with catalyst age. For example, particles containing high Ni level possessed very low acidity and were hardly accessible by a Nile Blue dye. Single catalyst particle mapping identifies minority species like the presence of a phosphated zeolite ZSM‐5‐containing FCC additive for selective propylene formation, catalyst particles without any zeolite phase and catalyst particles, which act as a trap for SO x .
In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe2+ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe2+, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.
