Paolo P. Pescarmona

Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction

Carbon materials such as graphite, graphene, carbon nanotubes and ordered mesoporous carbon have attracted a lot of attention for their use in fuel cells, due to beneficial properties like high conductivity, high mechanical and chemical stability and, for the latter, high surface area. Doping these materials with nitrogen or, less commonly, other elements alters their (electronic) properties, making them particularly suitable for application as electrocatalysts for the oxygen reduction reaction (ORR) in a fuel cell. This paper reviews the synthesis methods employed for the doping of these different types of carbon materials with various elements and the characterization techniques used to investigate their physicochemical properties such as degree of graphitization, dopant content, dopant configuration and surface area. Furthermore, their application as electrocatalysts for the oxygen reduction in a fuel cell is reviewed. Finally, the possible mechanisms for the ORR on N-doped carbon materials are critically discussed and compared to the mechanisms of commercial Pt/C electrocatalysts.

Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional Carbon–Silica Catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.

Challenges in the catalytic synthesis of cyclic and polymeric carbonates from epoxides and CO2

The addition of carbon dioxide to epoxides to produce either cyclic carbonates or polycarbonates is an important reaction allowing the conversion of a renewable, inexpensive and non-toxic feedstock such as CO2 into useful products with many potential applications. In this perspective article, an overview of the type of catalysts used for this reaction, the mechanisms with which they operate and the parameters influencing their activity and selectivity are presented and discussed critically. In this context, the main challenges to be tackled in this vibrant area of research are highlighted.

Zeolite-catalysed conversion of C3 sugars to alkyl lactates

The direct conversion of C3 sugars (or trioses) to alkyl lactates was achieved using zeolite catalysts. This reaction represents a key step towards the efficient conversion of bio-glycerol or formaldehyde to added-value chemicals such as lactate derivatives. The highest yields and selectivities towards the desired lactate product were obtained with Ultrastable zeolite Y materials having a low Si/Al ratio and a high content of extra-framework aluminium. Correlating the types and amounts of acid sites present in the different zeolites reveals that two acid functions are required to achieve excellent catalysis. Bronsted acid sites catalyse the conversion of trioses to the reaction intermediate pyruvic aldehyde, while Lewis acid sites further assist in the intramolecular rearrangement of the aldehyde into the desired lactate ester product. The presence of strong zeolitic Bronsted acid sites should be avoided as much as possible, since they convert the intermediate pyruvic aldehyde into alkyl acetals instead of lactate esters. A tentative mechanism for the acid catalysis is proposed based on reference reactions and isotopically labelled experiments. Reusability of the USY catalyst is demonstrated for the title reaction.

High-Resolution Single-Turnover Mapping Reveals Intraparticle Diffusion Limitation in Ti-MCM-41-Catalyzed Epoxidation†

Microand mesoporous materials offer unique opportunities for catalysis thanks to their large surface area. By introducing active elements inside the pore walls of such materials, a wide range of acid–base or redox catalysts has been developed. For example, incorporation of Ti sites in silicalite resulted in the TS-1 catalyst, which is known for its high performance in the selective oxidation and epoxidation of hydrocarbons. However, the small (0.55 nm) micropores of this catalyst hinder the uptake of larger olefins as substrates for the epoxidation. To circumvent this limitation of TS-1, titanosilicates with larger pores, such as Ti-Beta and Ti-MWW zeolites, have been synthesized. Even mesoporous titanosilicates such as Ti-MCM-41 were developed with the aim of faster diffusion of more bulky substrates towards the inner active sites. 8] MCM-41 materials are characterized by a hexagonal array of pores with a uniform diameter that can be tuned between 1.5 and 10 nm. Despite the relatively large pore size, maximal utilization of the Ti sites in diffusion unlimited conditions remains a major challenge. Typically, TiMCM-41 is prepared in the form of particles with sizes of a few micrometers. It was recently demonstrated that a decrease in particle size to about 100 nm was accompanied with a relevant increase in selectivity and reaction rate for the epoxidation of cyclohexene and cholesterol. 11] It was reasoned that intraparticle diffusion limitations in the mesopores of the large particles hindered an optimal use of the active titanium sites, similarly as previously described for the microporous TS-1 catalyst. The kinetics of a catalytic process are often governed by the interplay between diffusion and reaction. Such insights are classically gathered by macroscopic kinetic experiments, for example, by comparing reaction rates using crystals with different sizes, by varying space velocities of the feed, or by measuring apparent activation energies. Pulsed-field gradient NMR spectroscopy has been used to determine intraparticle diffusion coefficients during catalysis, but this technique is restricted to extremely large particles (> 10 mm) and only yields ensemble-averaged results. Recent technological evolutions in optical microscopy now offer the opportunity to confront these insights with in situ observations for single catalyst particles. The high spatiotemporal resolution (submicrometer and milliseconds) of (single-molecule) fluorescence microscopy has proven to be extremely useful to study catalysis at the level of individual particles or even at the level of individual reaction events, as well as to investigate diffusion processes in mesoporous materials. However, so far these two phenomena, catalytic conversion and diffusion in porous materials, were treated separately in single-molecule studies; no direct information on the interplay between these two processes has been obtained. Moreover optical microscopy is subject to the laws of diffraction, limiting the spatial resolution to a few hundred nanometers, whereas the interesting processes related to catalysis within porous particles typically occur on smaller length scales. The present contribution circumvents the resolution discrepancy by applying a single-turnover-based strategy in fluorescence microscopy to provide diffractionunlimited resolution. This approach allows mapping the catalytic activity with nanometer-scale spatial resolution, that is, in the order of 10 to 30 nm, which is competitive with the most recent, but more complex nanoscopy tools such as PALM, STORM, STED, and related techniques. The high spatial resolution provides the direct visualization, and thus the immediate localization of active sites within individual particles, while recording the catalytic process under realistic conditions. By exploiting the milliseconds time resolution of the technique, the direct evaluation and quantification of the kinetics is within reach with a very limited number of experiments, as will be demonstrated below for epoxidation over Ti-MCM-41. Typical parameters such as the Thiele modulus and the related effectiveness [*] G. De Cremer, E. Bartholomeeusen, Dr. K. Lin, Prof. Dr. P. P. Pescarmona, Prof. Dr. P. A. Jacobs, Prof. Dr. D. E. De Vos, Prof. Dr. B. F. Sels Department of Microbial and Molecular Systems Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Heverlee (Belgium) Fax: (+ 32)16-321-998 E-mail: bert.sels@biw.kuleuven.be

Structure and stability of oxygen vacancies on sub-surface, terraces, and low-coordinated surface sites of MgO:: an ab initio study

We have performed ab initio Hartree–Fock cluster model calculations on the electronic structure and properties of neutral and charged oxygen vacancies, the F color centers, at various sites of the MgO(001) surface. Sub-surface, surface, step, and corner sites have been considered. For each site we have determined the optimal structure, the relative stability of neutral versus charged vacancies, the formation energy, and, for the paramagnetic F+ centers, the spin distribution as given by the isotropic hyperfine coupling constants of the unpaired electron with the surrounding Mg2+ nuclear spin. The barrier for diffusion of oxygen ions in the presence of F2+ centers has been estimated for the migration from the sub-surface to the surface, from a terrace site to another terrace site, and from a step to a terrace site.

Highly-efficient conversion of glycerol to solketal over heterogeneous Lewis acid catalysts

The acetalization of acetone with glycerol to yield 2,2-dimethyl-1,3-dioxolane-4-methanol (solketal) was successfully catalyzed by mesoporous substituted silicates including the novel Hf-TUD-1 material. This reaction offers an attractive path for the conversion of glycerol, which is the main side-product in the synthesis of biodiesel, to a valuable compound with potential for industrial applications. The most promising among the heterogeneous catalysts employed in this work, Zr- and Hf-TUD-1 and Sn-MCM-41, display mainly Lewis acid properties as demonstrated by characterization with FT-IR analysis of pyridine adsorption, and achieve superior results compared to a reference solid acid catalyst such as Ultrastable zeolite Y. Especially the newly synthesized Hf-TUD-1, showing the highest conversion and turnover among the screened materials, is a promising catalyst for the acetalization of acetone with glycerol in a sustainable process. The excellent performance of these mesoporous catalysts is ascribed to their combination of acidity, wide pores, large specific surface area and relatively hydrophobic surface.

High activity and switchable selectivity in the synthesis of cyclic and polymeric cyclohexene carbonates with iron amino triphenolate catalysts

Iron(III) amino triphenolate complexes were studied as catalysts for the reaction of carbon dioxide (CO2) with cyclohexene oxide, which can lead to the formation of cyclic carbonate and/or polycarbonate products. Both types of compound are relevant, but for their practical application it is crucial to be able to control the selectivity of the reaction. By working under solvent-free, green conditions with CO2 in the supercritical state and by tailoring the nature and the relative amount of the co-catalyst (Bu4NX or PPNX, where X is a halide) used in combination with the iron(III) complex, we have been able to enhance the catalytic efficiency and achieve a selective and high-yield synthesis of either the cyclic or the polymeric product. The studied reaction is relevant in the context of green chemistry as it provides an atom-efficient route for the conversion of CO2, which is an inexpensive, widely available, renewable and non-toxic feedstock, into valuable products.

A highly active Zn(salphen) catalyst for production of organic carbonates in a green CO2 medium

Zn(salphen), in combination with Bu4NI, was studied as a binary catalyst system for CO2-fixation in the context of organic carbonate formation. The catalytic potential of this binary catalyst system was considerably improved by working in a solvent-free, CO2-rich environment, thereby increasing the overall contact between the reagents and catalyst. Under these green conditions, excellent conversion and selectivity towards the cyclic carbonate product were obtained with epoxides that are generally less prone to undergo cycloaddition with carbon dioxide. The effect of the reaction conditions and the type of co-catalyst employed together with Zn(salphen) were systematically investigated and optimised.

Multilayered supported ionic liquids as catalysts for chemical fixation of carbon dioxide: a high-throughput study in supercritical conditions

Multilayered, covalently supported ionic liquid phase (mlc-SILP) materials were synthesized by using a new approach based on the grafting of bis-vinylimidazolium salts on different types of silica or polymeric supports. The obtained materials were characterized and tested as catalysts in the reaction of supercritical carbon dioxide with various epoxides to produce cyclic carbonates. The material prepared by supporting a bromide bis-imidazolium salt on the ordered mesoporous silica SBA-15 was identified as the most active catalyst for the synthesis of cyclic carbonates and displayed improved productivity compared with known supported ionic liquid catalysts. The catalyst retained its high activity upon reuse in consecutive catalytic runs. This is the first report of the application of mlc-SILP materials as catalysts in a reaction for the fixation of carbon dioxide. Rapid, parallel screening and comparison of the catalysts was performed by means of high-throughput experimentation.