Christina Hviid Christensen

Hierarchical zeolites: Enhanced utilisation of microporous crystals in catalysis by advances in materials design

The introduction of synthetic zeolites has led to a paradigm shift in catalysis, separations, and adsorption processes, due to their unique properties such as crystallinity, high-surface area, acidity, ion-exchange capacity, and shape-selective character. However, the sole presence of micropores in these materials often imposes intracrystalline diffusion limitations, rendering low utilisation of the zeolite active volume in catalysed reactions. This critical review examines recent advances in the rapidly evolving area of zeolites with improved accessibility and molecular transport. Strategies to enhance catalyst effectiveness essentially comprise the synthesis of zeolites with wide pores and/or with short diffusion length. Available approaches are reviewed according to the principle, versatility, effectiveness, and degree of reality for practical implementation, establishing a firm link between the properties of the resulting materials and the catalytic function. We particularly dwell on the exciting field of hierarchical zeolites, which couple in a single material the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. The carbon templating and desilication routes as examples of bottom-up and top-down methods, respectively, are reviewed in more detail to illustrate the benefits of hierarchical zeolites. Despite encircling the zeolite field, this review stimulates intuition into the design of related porous solids (116 references).

Aluminum-rich mesoporous MFI-type zeolite single crystals

Zeolites are crystalline materials, which are widely used as solid acid catalysts and supports in many industrial processes. Recently, mesoporous MFI-type zeolite single crystals were synthesized by use of carbon particles as a mesopore template and sodium aluminate as the aluminum source. With this technique, only zeolites, with relatively low A1 contents were reported (Si/Al ratio about 100). In this work, the preparation of aluminum-rich mesoporous MFI-type zeolite single crystals (Si/Al∼16–50( using aluminum isopropoxide as the aluminum source is reported for the first time. All samples are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), ammonia temperature programmed desorption (NH 3 -TPD), and N 2 adsorption measurements. The obtained zeolites combine the high crystallinity and the characteristic micropores of zeolites with an intracrystalline mesopore system having mesopore volumes varying between 0.37–0.40 cm 3 /g.

Hierarchical zeolites: progress on synthesis and characterization of mesoporous zeolite single crystal catalysts

Abstract Recently, a new family of crystalline zeolitic materials was reported, the so-called mesoporous zeolite single crystals featuring individual zeolite single crystals with an additional noncrystalline mesopore system interconnected with the usual micropore system of the zeolite, resulting in a hierarchical pore size distribution. In this work, the preparation of mesoporous ZSM-12 single crystal catalysts using a new improved procedure for directly introducing carbon in the reaction mixture is reported. The microwave heating technique is also applied for the synthesis of mesoporous silicalite-1 single crystals using this direct introduction of carbon into the reaction mixture. All samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-programmed desorption of ammonia (NH 3 -TPD), and N 2 adsorption measurements. Additionally, the results of diffusion of n -hexadecane in conventional and mesoporous zeolites are presented. Isomerization and cracking of n- hexadecane was chosen as model test reaction for these materials. All results support that mesoporous zeolites are superior catalysts due to improved mass transport. Importantly, the mesoporous zeolites show significant improved resistance to poisoning by carbon formation.

Ammonia Synthesis: State of the Bellwether Reaction

Catalytic ammonia synthesis has been judged to be one of mankinds greatest scientific achievements during the twentieth century. The socioeconomic implications of producing ammonia industrially have been a strong driving force, and this development has spurred a range of new discoveries within physics, chemistry, and chemical engineering. In this chapter, we describe how it has been possible in recent years to provide a full understanding of the catalytic ammonia synthesis reaction at the atomic level through the combined use of experiments and quantum mechanical electronic structure calculations, thus clearly showing many of the reasons why ammonia synthesis has been, and still is, the bellwether reaction in heterogeneous catalysis.