Georges Marie Karel Mathys

Organo-Ruthenium Supported Heteropolytungstates: Synthesis, Structure, Electrochemistry, and Oxidation Catalysis

The reaction of [Ru(arene)Cl(2)](2) (arene = benzene, p-cymene) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb(III), Bi(III)) in buffer medium resulted in four organo-ruthenium supported heteropolytungstates, [Sb(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (1), [Bi(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (2), [Sb(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (3), and [Bi(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (4), which have been characterized in solution by multinuclear ((183)W, (13)C, (1)H) NMR, UV-vis spectroscopy, electrochemistry, and in the solid state by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Polyanions 1, 2, and 4 crystallize in the triclinic system, space group P1 with the following unit cell parameters: K(5)Na(5)[Sb(2)W(20)O(70)(RuC(6)H(6))(2)] x 22 H(2)O (KNa-1), a = 12.1625(2) A, b = 13.1677(2) A, c = 16.0141(3) A, alpha = 78.9201(7) degrees, beta = 74.4442(8) degrees, gamma = 78.9019(8) degrees, and Z = 1; Cs(2)Na(8)[Bi(2)W(20)O(70)(RuC(6)H(6))(2)] x 30 H(2)O (CsNa-2), a = 11.6353(7) A, b = 13.3638(7) A, c = 16.7067(8) A, alpha = 79.568(2) degrees, beta = 71.103(2) degrees, gamma = 80.331(2) degrees, and Z = 1; Na(10)[Bi(2)W(20)O(70)(RuC(10)H(14))(2)].35H(2)O (Na-4), a = 15.7376(12) A, b = 15.9806(13) A, c = 24.2909(19) A, alpha = 92.109(4) degrees, beta = 101.354(4) degrees, gamma = 97.365(3) degrees, and Z = 2. Polyanions 1-4 consist of two (L)Ru(2+) (L = benzene or p-cymene) units linked to a [X(2)W(20)O(70)](14-) (X = Sb(III), Bi(III)) fragment via Ru-O(W) bonds resulting in an assembly with idealized C(2h) symmetry. Polyanions 1-4 are stable in solution as indicated by the expected (183)W, (13)C, and (1)H NMR spectra. The electrochemistry of 1-4 is described by considering the reduction and the oxidation processes. The nature of the arene in Ru(arene) has practically no influence on the formal potentials of the W-centers, which are more sensitive to the Sb or Bi hetero atoms. The results suggest that the respective Sb- and Bi derivatives have very different pK(a) values, with the reduced form of 1 being the most basic, thus permitting the observation of two well-developed voltammetric waves at pH 6. In contrast, the identity of the arene influences the oxidation processes, thus permitting to distinguish them. A strong electrocatalytic water oxidation peak is observed that is more positive than the one corresponding to the Ru(arene) oxidation process. Also a stepwise oxidation of the Ru(benzene) group could be observed at pH 3. The catalytic efficiency, on the other hand, of 1-4 toward the oxidation of n-hexadecane and p-xylene illustrated the effect of ruthenium substitution on the polyanion catalytic performance.

Heterogeneous Wheel-Shaped Cu20-Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− Catalyst for Solvent-Free Aerobic Oxidation of n-Hexadecane

The selective oxidation of alkanes as a green process remains a challenging task because partial oxidation is easier to achieve with sacrificial oxidants, such as hydrogen peroxide, alkyl hydroperoxides or iodosylbenzene, than with molecular oxygen or air. Here, we report on a heterogeneous catalyst for n-hexadecane oxidation comprised of the wheel shaped Cu20-polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- anchored on 3-aminopropyltriethoxysilane (apts)-modified SBA-15. The catalysts were characterized by powder X-ray diffraction (XRD), N2-adsorption measurements and Fourier transform infrared reflectance (FT-IR) spectroscopy. The heterogeneous Cu20-polyanion system catalyzed the solvent-free aerobic oxidation of n-hexadecane to alcohols and ketones by using air as the oxidant under ambient conditions. The catalyst exhibits an exceptionally high turn over frequency (TOF) of 20,000 h(-1) at 150 degrees C and is resistant to poisoning by CS2. Moreover, it can be easily recovered and reused by filtration without loss of its catalytic activity. Possible homogeneous contributions also have been examined and eliminated. Thus, this system can use air as oxidant, which, in combination with its good overall performance and poison tolerance, raises the prospect of this type of heterogeneous catalyst for practical applications.

Design of a Cobalt–Zeolite Catalyst for Semi‐Linear Higher‐Olefin Synthesis

Olefins that contain 6–16 carbon atoms are important chemical intermediates in the synthesis of plasticizers and surfactants. They are produced on an industrial scale in oligomerization processes, mainly starting from ethylene, propylene, and butenes. The value of a long olefin depends on its skeletal structure; linear olefins are the most valuable. In industrial olefin-oligomerization processes, dissolved nickel complexes and supported-nickel catalysts are used to produce semi-linear oligomers. Homogeneous cobalt catalysts show better selectivity for linear oligomers than nickel catalysts and are more resistant to poisons, such as thiols, thiophenes, and dihydrogensulfide impurities that are contained in industrial feedstocks. Solid catalysts allow reactions to be performed in fixed-bed reactors. Heterogeneous cobalt catalysts have been reported in the literature dating back to the 1960s. The development of a solid cobalt catalyst for olefin-oligomerization is, however, not straightforward. By building on the knowledge of the placement of transition-metaland alkaline-earth-metal cations in zeolites and by exploiting the concept of framework-stabilization, we designed the first zeolite-based cobalt catalyst for olefin-oligomerization and demonstrated its excellent selectivity for the synthesis of quasi-linear octene from butenes. The active site, in which alkyl chains are attached to the cobalt atom, was uniquely visualized by X-ray diffraction and structure-refinement. Zeolite catalysts have shown great potential in the olefin-oligomerization reaction. ZSM-57 zeolite in its acid form is an excellent butene-dimerization catalyst; the octene products are mainly comprised of dimethylhexenes and the number of chain branches (NCB) of the dimer fraction is around 2. ZSM-22 zeolite, with its narrow tubular pores, has shown the ability to reduce the NCB of propene oligomers, but it is less active in the conversion of butenes. Oligomers with low degrees of branching can be obtained by using a heterogeneous nickel catalyst in a process called OCTOL. Here, the NCB of the octene-product fraction out of n-butenes is about 1.1. The mechanism of coordinative oligomerization by nickel on homogeneous as well as heterogeneous catalysts involves Ni species and can be divided into three steps: 1) the formation of a p-complex between the olefin and the transitionmetal ion followed by transformation into a s-bonded metal alkyl complex; 2) insertion of olefin molecules, in which a pbonded olefin is inserted into the metal carbon bond of the metal alkyl complex; and 3) the termination of chain-growth by b-H-transfer or b-elimination. Starting from terminal alkenes, this mechanism leads to linear and monobranched dimerization products. On the targeted heterogeneous cobalt catalyst, on analogy with nickel, a similar reaction mechanism could be followed provided that Co atoms with free coordination sites can be generated at the surface of a solid support. The synthetic faujasite zeolites, known as zeolite X and Y, are common catalyst and adsorbent materials. This type of zeolite can conveniently be described by considering the three types of cavities that it comprises: hexagonal prisms, sodalite cages, and supercages. The incorporation of cobalt ions into zeolites X and Y causes structural stress and damage. Hydrolysis of the hexa-aquo complexes leads to de-alumination and eventual destruction of the zeolite framework. Furthermore, Co cations are known to distort the six-membered rings of these zeolites by twisting the three outer oxygen atoms of the six-membered ring towards the center, thereby introducing considerable strain. Therefore, stabilization of the faujasite framework is advised. Such stabilization can be achieved by the introduction of Ca ions. Denayer et al. observed a stabilizing effect of Ca ions in the presence of Co ions in zeolite X. The cation-exchange of the parent Na–X zeolite with Ca ions and a first calcination step led to the occupancy of the sodalite cages and hexagonal prisms by Ca ions. After subsequent ion-exchange with Co ions, followed by calcination, calcium was confined inside the hexagonal prisms, which forced the Co ions into the supercages, where they were available for adsorption. This strategy was adopted herein on a Y-type zeolite. First, Na–Y zeolite was exchanged with Ca ions, calcined, and exchanged with Co ions. The structure of the resulting zeolite, with the composition Co9.2Na9.1Ca12.2Al52Si140O384, was analyzed by Rietveld refinement of the powder X-ray diffraction data (see the Supporting Information). The location of the different cations is shown in Figure 1; the cation sites were numbered as according to Breck. As expected, Ca ions were found on the six-membered rings in the hexagonal prisms and in the sodalite cages at sites SI and SI’, respectively, thereby stabilizing the zeolite framework. The remaining Na ions were assigned to electron density in the six-membered rings of the sodalite cages (SII’ sites). Co ions were exclusively observed in the supercages on the six-membered-ring windows (SII’ and SII* sites) and, to a small extent, in 12-membered rings that connected two supercages, at SV sites. Structure and data refinement are discussed in more detail in the Supporting Information. [a] Dr. J. Franken, Prof. C. E. A. Kirschhock, Prof. Dr. J. A. Martens Centre for Surface Chemistry and Catalysis Faculty of Bio-Science Engineering, KU Leuven Kasteelpark Arenberg 23, 3001 Heverlee (Belgium) Fax: (+ 32) 16-32-19-98 E-mail : johan.martens@biw.kuleuven.be [b] Dr. G. M. Mathys ExxonMobil Chemical Europe Inc Hermeslaan 2, 1831 Machelen (Belgium) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200267.