Kresten Egeblad

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).

The Renewable Chemicals Industry

The possibilities for establishing a renewable chemicals industry featuring renewable resources as the dominant feedstock rather than fossil resources are discussed in this Concept. Such use of biomass can potentially be interesting from both an economical and ecological perspective. Simple and educational tools are introduced to allow initial estimates of which chemical processes could be viable. Specifically, fossil and renewables value chains are used to indicate where renewable feedstocks can be optimally valorized. Additionally, C factors are introduced that specify the amount of CO2 produced per kilogram of desired product to illustrate in which processes the use of renewable resources lead to the most substantial reduction of CO2 emissions. The steps towards a renewable chemicals industry will most likely involve intimate integration of biocatalytic and conventional catalytic processes to arrive at cost-competitive and environmentally friendly processes.

Aerobic oxidation of aldehydes under ambient conditions using supported gold nanoparticle catalysts

A new, green protocol for producing simple esters by selectively oxidizing an aldehyde dissolved in a primary alcohol has been established, utilising air as the oxidant and supported gold nanoparticles as catalyst. The oxidative esterifications proceed with excellent selectivities at ambient conditions; the reactions can be performed in an open flask and at room temperature. Benzaldehyde is even oxidised at a reasonable rate below −70 °C. Acrolein is oxidised to methyl acrylate in high yield using the same protocol.

Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold Nanoparticles†

Over the years, many strategies have been developed to address the problem of sintering of nanoparticle catalysts, including encapsulating metal nanoparticles in protective shells, and trapping nanoparticles in the cavities of certain zeolites in post-synthesis steps. In general, materials that contain metal nanoparticles that are only accessible via zeolite micropores are intriguing, specifically, but not exclusively, for catalytic applications. The encapsulation of carbon nanoparticles during zeolite crystallization is a well-known approach for making carbon–zeolite composites that afford mesoporous zeolites after combustion. Herein, we show that metal nanoparticles can also be encapsulated during zeolite crystallization, as exemplified by silicalite-1 crystals that are embedded with circa 1–2 nm-sized gold nanoparticles that remain stable and catalytically active after calcination in air at 550 8C. Moreover, we show that the encapsulated gold nanoparticles are only are accessible through the micropores of the zeolite, which makes this material a substrate-size selective oxidation catalyst. Currently, more than 175 different zeolite structures have been reported, and these can be tuned according to the desired acidity and/or redox properties. Expanding the scope from pure zeolites to hybrid materials, by combining the properties of zeolites with other components, significantly widens the field of zeolite materials design. Aside from posttreatment methods, two types of approaches have been pursued for preparing hybrid zeolite–nanoparticle materials. The first type of approach involves crystallization of the zeolite from a gel that contains metal ions that are immobilized in the zeolite during crystallization. With this kind of approach, it is very difficult to control the properties of the non-zeolite component in terms of, for example, particle size. The other type of approach is to first synthesize the nonzeolite component and subsequently encapsulate this in the individual zeolite crystals during crystallization. Indeed, this strategy is also well-known and an entire family of materials, known as mesoporous or hierarchical zeolite crystals, are based on the embedding of carbon nanoparticles, nanofibers, nanotubes, or other nanostructures during zeolite crystallization (and subsequent combustion) in a process known as carbon templating. 15, 16] Concerning the embedding of metal nanoparticles in zeolites, Hashimoto et al. reported a top down approach that features downsizing gold flakes to approximately 40 nm particles by laser ablation, and subsequent encapsulation of these particles during crystallization. A reduction in particle size by one order of magnitude is necessary for an efficient use of costly noble metals in catalytic applications. However, a reduction of the particle size enhances the tendency for sintering, owing to the increase in surface free energy. To mitigate this problem, we report herein a bottom-up approach for the preparation of hybrid zeolite-nanoparticle materials that contain small metal nanoparticles, dispersed throughout the zeolite crystals. This synthetic approach comprises three steps (Figure 1): First, a metal nanoparticle colloid is prepared with suitable anchor points for the generation of a silica shell. Second, the particles are encapsulated in an amorphous silica matrix. Third, the silica nanoparticle precursor is subjected to hydrothermal conditions in order for zeolite crystallization to take place. Using this approach, we successfully prepared a material that consisted predominantly of circa 1–2 nm sized gold particles that were embedded in silicalite-1 crystals. X-ray diffraction revealed that the material contained exclusively gold as well as MFI-structured material (generalized silicalite-1 crystal structure type). Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the hybrid material that consists of gold nanoparticles embedded in silicalite-1 crystals. The SEM image reveals that the material is mainly composed of circa 1–2 mm long coffinshaped crystals, with a minor fraction of intergrown coffin[*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen, S. Helveg, Prof. C. H. Christensen, K. Egeblad Haldor Topsøe A/S Nymøllevej 55, 2800 Kgs. Lyngby (Denmark) E-mail: chc@topsoe.dk kreg@topsoe.dk

Oxidations of amines with molecular oxygen using bifunctional gold–titania catalysts

Over the past decades it has become clear that supported gold nanoparticles are surprisingly active and selective catalysts for several green oxidation reactions of oxygen-containing hydrocarbons using molecular oxygen as the stoichiometric oxidant. We here report that bifunctional gold–titania catalysts can be employed to facilitate the oxidation of amines into amides with high selectivity. Furthermore, we report that pure titania is in fact itself a catalyst for the oxidation of amines with molecular oxygen under very mild conditions. We demonstrate that these new methodologies open up for two new and environmentally benign routes to caprolactam and cyclohexanone oxime, both of which are precursors for nylon-6.

Oxidation of glycerol and propanediols in methanol over heterogeneous gold catalysts

Aerobic oxidation of glycerol over metal oxide supported gold nanoparticles in methanol results in the formation of dimethyl mesoxalate in selectivities up to 89% at full conversion. The oxidative esterification takes place in methanol, acting both as solvent and reactant, and in the presence of base. Thus, it constitutes a direct transformation of the glycerol by-product phase from biodiesel production or from glycerol obtained e.g. by fermentation. Au/TiO2 and Au/Fe2O3 was found to have similar catalytic activity, whereas Au/C was inactive. 1,2-Propanediol was oxidized to methyl lactate with a selectivity of 72% at full conversion, while 1,3-propanediol yielded methyl 3-hydroxypropionate with 90% selectivity at 94% conversion. Methyl 3-hydroxy propionate can be easily converted into methyl acrylate, which is then a green polymer building block.

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.

Heterogeneous catalysis for production of value-added chemicals from biomass

Almost everything around us is in some way a product of controlled chemical processes. That is either chemical processes conducted in Nature or chemical processes conducted in the chemical industry. In the most developed parts of the World, it is in fact products from the chemical industry that comp...

Tailoring the Porosity of Hierarchical Zeolites by Carbon-templating

We report the synthesis and characterization of a series of hierarchical porous zeolite single crystal materials with a range of porosities made available by carbon-templating using differently-sized carbon particles as templates for the additional non-micropore porosity. The materials were prepared by adsorption of the required zeolite synthesis gel components onto various commercially available carbon black powders followed by crystallization of the zeolite crystals in the presence of the inert carbon matrix and subsequent removal of the carbon particles embedded in the zeolite crystals by combustion. It is shown that the additional porosity of the hierarchical zeolites can be tailored by encapsulation of the differently-sized carbon particles during crystallization.