You-Kyong Seo

Synthesis and catalytic properties of MIL-100(Fe), an iron(III) carboxylate with large pores

The large-pore iron(III) carboxylate MIL-100(Fe) with a zeotype architecture has been isolated under hydrothermal conditions, its structure solved from synchrotron X-ray powder diffraction data, while Friedel-Crafts benzylation catalytic tests indicate a high activity and selectivity for MIL-100(Fe).

Selective Removal of N‐Heterocyclic Aromatic Contaminants from Fuels by Lewis Acidic Metal–Organic Frameworks

Fossil fuels, such as diesel or gasoline, are blends of aromatic and aliphatic compounds that contain significant levels of heterocyclic aromatic contaminants. These contaminants have to be removed for environmental reasons. One of the most important issues is the presence of sulfur compounds, such as thiophene (TPH), benzothiophene (BT), and dibenzothiophene (DBT) in fuel feeds, which lead to the formation of SOx exhaust gases and eventually to acid rain. As environmental legislation becomes more stringent on SOx exhaust levels, it is imperative to keep lowering the sulfur concentrations to currently 10 ppmw S (parts per million by weight of sulfur) or less. The main industrial process is hydrodesulfurization (HDS) in which sulfur compounds are hydrogenated to hydrocarbons and H2S over typically a CoMo catalyst. However, nitrogen compounds, such as (substituted) indoles and carbazoles, which are also present in fossil fuels, compete for the active sites on these HDS catalysts, preventing a deep HDS. In the absence of nitrogen compounds, deep HDS can easily produce fuels with sulfur levels well below 10 ppmw, for instance by using the newest generations of materials based on Mo-W-Ni, which can lower sulfur levels to 5 ppmw. As the eventual aim is to have sulfur-free fuel, even these low concentrations will have to be removed. A promising way to selectively remove nitrogen contaminants would be adsorption on a microporous material. Efficient purification can be performed by adsorption as long as the interaction between the adsorbate and the adsorbent is relatively strong. A CuY zeolite has been described as a potential adsorbent for the removal of nitrogen compounds by p complexation, but the maximal capacity at saturation only amounted to 3 mg N per gram of adsorbent, and moreover sulfur compounds are adsorbed as well. An ideal adsorbent for such application should be easy to synthesize, stable in the given feed compositions, possess pores that are large enough to accommodate bulky organic molecules, such as carbazoles, have a sufficient capacity, and be highly selective for nitrogen over sulfur compounds. Metal–organic frameworks (MOFs) are an emerging class of highly porous materials, formed of inorganic subunits and organic linkers that bear multiple complexing functions (for example, carboxylates, phosphonates, and others), which enables a unique variety of potential interactions inside the pores. To date, they have been successfully used as adsorbents for the capture of greenhouse gases, such as CO2 and CH4, and in liquid-phase separations such as those of alkylaromatics and styrene, olefins and paraffins, and for fuel and water purification by adsorption of organic pollutants. Herein, we propose the use of mesoporous metal carboxylates with different topologies and compositions for the selective adsorption of nitrogen contaminants. These heterocyclic contaminants are found in fuel feeds that are typically aliphatic with a minor aromatic fraction. This system is simulated herein by using a solvent composed of heptane/toluene in a volumetric ratio of 80:20 (labeled hereafter as H/T). Specifically, the adsorptive removal of indole (IND), 2-methylindole (2MI), 1,2-dimethylindole (1,2DMI), carbazole (CBZ), and N-methylcarbazole (NMC) as well as of TPH, BT, and DBT has been studied. These molecules are the most important heterocyclic contaminants in fuel feeds. To study the influence of the toluenecontaining solvent on the adsorption and on the interaction strength between the host and the adsorbate, the adsorption of the contaminants has also been studied using a toluene/ [*] M. Maes, S. Schouteden, F. Vermoortele, Dr. L. Alaerts, Prof. Dr. D. E. De Vos Centre for Surface Chemistry and Catalysis Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Leuven (Belgium) Fax: (+ 32)16-321-998 E-mail: dirk.devos@biw.kuleuven.be

Intracrystalline diffusion in Metal Organic Framework during heterogeneous catalysis: Influence of particle size on the activity of MIL-100 (Fe) for oxidation reactions

Three MIL-100 (Fe) samples differing in average crystal size (from 60-70 to >400 nm) have been synthesized by microwave heating using three HF/Fe(3+) ratios. Oxidation of diphenylmethane with tert-butylhydroperoxide (TBHP) and thiophenol with oxygen are catalyzed by three MIL-100 (Fe) samples with similar reaction rates regardless of its average particle size. In contrast, the activity of the three MIL-100 (Fe) samples for the oxidation of bulky triphenylmethane by TBHP largely depends on the average crystal size of the sample: the smaller the average particle size, the larger the initial reaction rate of triphenylmethane oxidation. These results show that diffusion limitation takes place on MOF catalysis depending on the substrate size and provides indirect evidence that these reactions take place inside the intracrystalline space of the porous catalysts.

Discovering the active sites for C3 separation in MIL-100(Fe) by using operando IR spectroscopy.

A reducible MIL-100(Fe) metal-organic framework (MOF) was investigated for the separation of a propane/propene mixture. An operando methodology was applied (for the first time in the case of a MOF) in order to shed light on the separation mechanism. Breakthrough curves were obtained as in traditional separation column experiments, but monitoring the material surface online, thus providing evidences on the adsorption sites. The qualitative and quantitative analyses of Fe(II) and, to some extent, Fe(III) sites were possible, upon different activation protocols. Moreover, it was possible to identify the nature and the role of the active sites in the separation process by selective poisoning of one family of sites: it was clearly evidenced that the unsaturated Fe(II) sites are mainly responsible for the separation effect of the propane/propene mixture, thanks to their affinity for the unsaturated bonds, such as the C=C entities in propene. The activity of the highly concentrated Fe(III) sites was also highlighted.