Benjamin Frank

Metal-Free Heterogeneous Catalysis for Sustainable Chemistry

The current established catalytic processes used in chemical industries use metals, in many cases precious metals, or metal oxides as catalysts. These are often energy-consuming and not highly selective, wasting resources and producing greenhouse gases. Metal-free heterogeneous catalysis using carbon or carbon nitride is an interesting alternative to some current industrialized chemical processes. Carbon and carbon nitride combine environmental acceptability with inexhaustible resources and allow a favorable management of energy with good thermal conductivity. Owing to lower reaction temperatures and increased selectivity, these catalysts could be candidates for green chemistry with low emission and an efficient use of the chemical feedstock. This Review highlights some recent promising activities and developments in heterogeneous catalysis using only carbon and carbon nitride as catalysts. The state-of-the-art and future challenges of metal-free heterogeneous catalysis are also discussed.

Carbon-Catalyzed Oxidative Dehydrogenation of n-Butane: Selective Site Formation during sp3-to-sp2 Lattice Rearrangement

[liu, xi; frank, benjamin; zhang, wei; cotter, thomas p.; schloegl, robert; su, dang sheng] max planck soc, fritz haber inst, dept inorgan chem, d-14195 berlin, germany. [su, dang sheng] chinese acad sci, inst met res, shenyang natl lab mat sci, shenyang 110016, peoples r china.;su, ds (reprint author), max planck soc, fritz haber inst, dept inorgan chem, faradayweg 4-6, d-14195 berlin, germany;dangsheng@fhi-berlin.mpg.de

Oxidative Dehydrogenation of Ethane over Multiwalled Carbon Nanotubes

C-H activation; carbon nanotubes; dehydrogenation; oxidation; surface modification The suitability of nanocarbons as catalysts for oxida-tive dehydrogenation (ODH) reactions has been investi-gated for numerous hydrocarbon substrates, such as ethylbenzene, 1-butene, isobutene, n-butane, and pro-pane.

Oxidative Purification of Carbon Nanotubes and Its Impact on Catalytic Performance in Oxidative Dehydrogenation Reactions

Oxidative purification with mild diluted HNO3 followed by NaOH washing lowers the amount of amorphous carbon attached to multiwalled carbon nanotubes (CNTs). The graphitic structure improves remarkably by further annealing in argon at elevated temperatures, that is, 1173, 1573, and 1973 K. The influence of the purification treatment on the catalytic activity of the CNTs is investigated for the oxidative dehydrogenation (ODH) of ethylbenzene and propane as probe reactions. All samples tend to approach an appropriately ordered structure and Raman analysis of the used samples displays a D/G band ratio of 0.95-1.42. Oxygen functionalities are partly removed by the annealing treatment and can be rebuilt to some extent by oxygen molecules in the ODH reactant flow. The presence of amorphous carbon is detrimental to the catalytic performance as it allows for unwanted functional groups occurring in parallel with the formation of the selective (di)ketonic active sites.

Emission of Highly Activated Soot Particulate—The Other Side of the Coin with Modern Diesel Engines

increase in the proportion of new cars with diesel engines in the past 20 years in Western Europe is illustrated in Figure 1. [2] This trend is expected to further increase over the next few years [3] until the development of cheap and competitive hybrid technology brings the share of diesel and gasoline motors down to an expected 15–35 % in 2030. [4] The major disadvantage of diesel engines with regard to environmental and health protection is the typically enhanced production of black soot (more specifically: diesel particulate matter), which consists of unburned carbonaceous compounds. This is mainly caused by local cold spots, where the fuel is not fully oxidized. Relatively low temperatures appear at the walls of the combustion chamber and at the outside of poorly vaporized large fuel droplets. The surface of condensed fuel has less air to burn and partly pyrolizes to finally turn into a carbon deposit, which leads to the formation of soot. The presence of aromatic compounds in the diesel fuel typically enhances the soot emission through the facile condensation of aromatic units to form larger polyaromatic

Diesel Soot Toxification

E and environmental amenities of diesel engines such as high fuel efficiency led to a steady increase in popularity. However, their major disadvantage with regard to environmental and health protection is the typically enhanced production of diesel particulate matter (DPM) comprising soot and unburned carbonaceous compounds. Stricter emission levels, for example, the Euro I to VI standards in the European Union, and tax incentives are imposed. One strategy to lower this burden is the optimization of fuel combustion in the engine as typically realized by a turbocharger. Here, the drastically lowered fuel droplet size negatively influences the DPM nanostructure. Smaller particles may penetrate more deeply into the respiratory tract, where their large surface-to-volume ratio could allow for more biological interaction. It could turn out to be ironical history that huge effort made in this direction is driven by the increasing awareness of the public on the toxicity of diesel soot. What are the structural and chemical features of low-emission DPM on the nanoscale? The most evident change is in the size of primary soot particles as evidenced for common heavy-duty diesel engines fulfilling the Euro III, IV, and VI standard. Here, the average diameter steadily decreased from 30−40 nm down to 10−15 nm. The second significant trend is the more defective bulk and surface structure as the result of alteration of the fuel combustion process. The enhanced localization of conjugated π-electrons on graphitic surfaces generates favored anchoring points for surface functional groups by reaction with water or oxygen. The consequence is an abundance of chemically reactive oxygen functional groups on the highly defective modern low-emission diesel soot. The chemical activation of the DPM is astonishing. The carbon surface in its initial highly functionalized state shows an outstanding activity in heterogeneous catalysis. The oxidative dehydrogenation of ethylbenzene to styrene and the selective oxidation of acrolein to acrylic acid are large-scale chemical processes and are catalyzed by highly developed promoted (mixed) oxides with excess of steam, respectively. However, they were also proven to be successfully catalyzed by nanostructured carbon materials. We were surprised to see that the initial productivity of the untreated soot of a Euro IV test engine used as the catalyst in these reactions exceeds the data of well performing other carbonaceous materials. Especially the curvature of the outermost

Resin‐Derived Hierarchical Porous Carbon Spheres with High Catalytic Performance in the Oxidative Dehydrogenation of Ethylbenzene

Pre-shaped hierarchical porous carbon (HPC) spheres have been synthesized through a facile anion exchanged route. An industrial polymeric anion-exchange resin with a hierarchical pore structure was used as the carbon precursor. Its high porosity was conserved using an aluminate/silicate precursor forming a hard support to prevent the structural collapse during the carbonization process. Physicochemical bulk and surface properties of the obtained HPC spheres were characterized by X-ray diffraction, scanning and transmission electron microscopy, N(2) physisorption, and X-ray photoemission spectroscopy. Results obtained indicate that HPC keeps the abundant hierarchical porosity including meso- and macropores as well as the high surface area of the resin precursor. The as-synthesized HPC spheres were tested as a catalyst for oxidative dehydrogenation of ethylbenzene to styrene. The oxygen-rich catalyst surface formed under reaction conditions shows a high catalytic performance and stability, making HPC to a potential catalyst for this type of reaction.

Methane Activation over Cellulose Templated Perovskite Catalysts

A facile and rapid preparation method for a wide variety of medium surface area perovskite‐type catalysts on the laboratory scale is presented. The cellulose templating method allows for catalysts with high phase purity, even at the relatively low calcination temperatures. Among the versatile compositions of perovskites based on the SrCoOx system, straightforward modifications could be performed to optimize the catalytic performance in the oxidation of CH4. Substitutions in both the A and B positions in the ABO3 lattice can remarkably affect the catalytic activity. Compared to other preparation methods, the cellulose templating method is a rapid process and the catalytic performances obtained with SrCoOx and LaCoOx are at least as good as with materials prepared by conventional methods.

CNT supported MoxC catalysts: Impact of loading and carburization parameters

MoxC/CNT catalysts were prepared through carburization of an oxidic molybdenum precursor impregnated on multiwalled carbon nanotubes (CNTs). The effects of different carburization atmospheres, heating rates, and molybdenum loadings were tested. The catalysts were characterized by using CO temperature‐programmed desorption, XRD, N2 physisorption, SEM, and TEM. The catalytic performance in the steam reforming of methanol was used as a sensitive probe to indicate changes in the catalyst surface during the catalytic action. Contrary to the bulk MoxC catalysts, the heating rate during carburization has no effect on the catalysts. Instead, molybdenum loading and carburization atmosphere are the key factors for catalyst structure and performance. The molybdenum‐based activity decreases at loadings >10u2005wtu2009% at a constant product selectivity. The CO2/CH4 product ratio indicates changes in the catalyst properties at the loadings <20u2005wtu2009%, at which the activity is constant. Carburization in 20u2009% CH4/H2 yields 2u2005nm sized crystallites of cubic α‐MoC. Carburization in pure H2 and He yields hexagonal β‐Mo2C with a larger particle size. Both phases show different catalytic performances in terms of activity and CO2/CH4 selectivity. Thus, a multiparameter toolbox for fine‐tuning of catalyst properties is presented.

Active Sites in Olefin Metathesis over Supported Molybdena Catalysts

Metathesis of propene to ethene and 2‐butenes was studied over a series of MoOx/SBA‐15 catalysts (molybdenum oxide supported on mesoporous silica SBA‐15; Mo loading 2.1–13.3u2005wtu2009%, apparent Mo surface density 0.2–2.5u2005nm−2). The catalysts have been prepared by an ion exchange technique. Nitrogen adsorption, 1Hu2005MAS‐NMR, Raman, and FTIR spectroscopies were applied to characterize the catalysts. Adsorption of the reactant propene and the probe molecule NH3 was studied by inu2005situ FTIR spectrometry microcalorimetry and temperature‐programmed desorption. Irrespective of the loading, only ≈1u2009% of the Mo atoms in the MoOx/SiO2 catalysts transform into active carbene (Mo=CHR) sites catalyzing propene metathesis. Isolated, distorted molybdenum di‐oxo species in close vicinity to two silanol groups have been shown to be the precursor of the active site. Targeted active site creation by pretreatment with methanol resulted in an increase in initial catalytic activity by a factor of 800.