Gerald P. Niccolai

New insights into the encapsulation and stabilization of heteropolyacids inside the pore walls of mesostructured silica materials

A method of synthesizing stable and reactive modified mesoporous hybrid materials with intact heteropolyacid (HPA) Keggin units is reported. The materials were synthesized by a one-pot method using a mixture of structure directing agents (CTAB and Pluronic 123). The key to obtaining stable HPA containing materials was the sequential calcination and extraction steps, and the authors present a working hypothesis for this phenomenon. Studies of several other key factors concerning synthetic parameters are also reported. All materials were characterized by a wide variety of physical and spectroscopic methods at each step of the synthesis. The kinetics of the esterification of acetic acid by butanol mediated by several of the materials and relevant homogeneous acid catalysts showed that the HPA containing hybrid material was a highly effective catalyst.

Oxide supported surface organometallic complexes as a new generation of catalysts for carbon–carbon bond activation

Abstract This paper reviews recent applications of well-defined silica-supported hydrides of the group 4 and 5 transition metals in the field of carbon–carbon and carbon–hydrogen bonds activation of alkanes. The synthesis and characterization of the zirconium and tantalum hydrides is presented. (SiO) 3 M–H complexes (M=Ti 1 , Zr 2 , Hf 3 ) are obtained by hydrogen treatment at c.a. 150°C of Si–O–MNp 3 (Np=CH 2 C(CH 3 ) 3 ). These three surface complexes are formally 8 electrons species and are consequently very electrophilic. (SiO) 2 Ta–H 4 is obtained by hydrogen treatment at c.a. 150°C of (Si–O) x Ta(CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 3− x ( x =1,2) and is also highly electrophilic. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. The complexes 1 – 3 can achieve the hydrogenolysis of alkanes at low temperature. With 1 a simultaneous reaction of skeletal isomerization occurs. With 2 and 3 the mechanism of C–C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with 1 passes also by an elementary step of β-alkyl transfer but in this case the β-H elimination–olefin reinsertion occurs quite rapidly so that a skeletal isomerization also occurs. Complexes of type 2 or 3 were found to catalyze under olefin pressure the olefin polymerization, and under hydrogen pressure, the polyolefin hydrogenolysis. Here the equilibrium between the olefin insertion into a metal–alkyl and the β-alkyl transfer is shown to occur with the same catalyst in agreement with the concept of microreversibility. A new catalytic reaction, called “alkane metathesis” has been discovered with complex 4 . By this reaction alkanes are catalytically transformed into higher and lower alkanes. The mechanism by which this reaction occurs is not fully understood. The products distribution, especially with labeled alkanes is explained by a concerted mechanism by which a Ta–C bond and a C–C bond of the alkane can be cleaved and reformed simultaneously via a kind of four centered σ-bond metathesis which has no precedent in classical organometallic chemistry.

Primary selectivity in the activation of the carbon-hydrogen bonds of propane by silica supported zirconium hydride

Abstract The stoichiometric reaction between propane and (≡ SiO) 3 ZrH leads to the formation of a mixture of (≡ SiO) 3 ZrCH(CH 3 ) 2 and (≡ SiO) 3 ZrCH 2 CH 2 CH 3 . The relative amounts of each complex resultant from the reaction was determined by (a) reaction of (≡ SiO) 3 ZrC 3 H 7 with D 2 O to produce quantitatively a mixture of CH 3 CH 3 CH 2 D and CH 3 CHDCH 3 which in turn were discriminated by mass spectroscopy, and (b) reaction of (≡ SiO) 3 ZrC 3 H 7 with O 2 followed by HCl yielding 1-propanol and 2-propanol in low yield. Each of these methods indicated that (≡ SiO) 3 ZrCH 2 CH 2 CH 3 was the major product of propane activation. The catalytic deuteration of propane by deuterium at room temperature catalysed by (≡ SiO) 3 ZrH showed that the primary C H bonds of propane reacted seven times faster than the secondary C H bonds. A mechanism of degenerate carbon-hydrogen bond activation is invoked for the latter reaction.

Formation of bridging acylium and nitrilium complexes by reaction of carbon monoxide and tert-butyl isocyanide with a bridging diiron methylidyne complex. Evidence for strong electron donation from the Fe2C core onto the .mu.-CHC.tplbond.O and .mu.-CHC.tplbond.NR ligands.

Structure cristalline du complexe [C 5 H 5 (CO)Fe] 2 (μ-CO)[μ-CHCNC(CH 3 ) 3 ] + PF 6 − , obtenu par reaction du complexe [C 5 H 5 (CO)Fe] 2 (μ-CO)(μ-CH) + PF 6 − avec CNC(CH 3 ) 3

Group emotions: the social and cognitive functions of emotions in argumentation

The learning sciences of today recognize the tri-dimensional nature of learning as involving cognitive, social and emotional phenomena. However, many computer-supported argumentation systems still fail in addressing the socio-emotional aspects of group reasoning, perhaps due to a lack of an integrated theoretical vision of how these three dimensions interrelate to each other. This paper presents a multi-dimensional and multi-level model of the role of emotions in argumentation, inspired from a multidisciplinary literature review and extensive previous empirical work on an international corpus of face-to-face student debates. At the crossroads of argumentation studies and research on collaborative learning, employing a linguistic perspective, we specify the social and cognitive functions of emotions in argumentation. The cognitive function of emotions refers to the cognitive and discursive process of schematization (Grize, 1996, 1997). The social function of emotions refers to recognition-oriented behaviors that correspond to engagement into specific types of group talk (e. g. Mercer in Learning and Instruction 6(4), 359–377, 1996). An in depth presentation of two case studies then enables us to refine the relation between social and cognitive functions of emotions. A first case gives arguments for associating low-intensity emotional framing, on the cognitive side, with cumulative talk, on the social side. A second case shows a correlation between high-intensity emotional framing, and disputational talk. We then propose a hypothetical generalization from these two cases, adding an element to the initial model. In conclusion, we discuss how better understanding the relations between cognition and social and emotional phenomena can inform pedagogical design for CSCL.

The comparison of two classes of bifunctional SBA-15 supported platinum–heteropolyacid catalysts for the isomerization of n-hexane

Mono- and bifunctional catalysts composed of silicotungstic acid (H4SiW12O40) and/or platinum nanoparticles supported on SBA-15 mesostructured silica were comparatively tested for the isomerization of n-hexane. Monofunctional catalysts were produced by impregnating the SBA-15 support with either H4SiW12O40 or the platinum particle precursor, H2PtCl6. In the bifunctional catalysts, both metallic and acidic functions were combined either by mechanically mixing the monofunctional solids yielding a multiphase bifunctional material (HSiW/SBA-15 + Pt/SBA-15), or by dual impregnation of H4SiW12O40 and Pt within a single material leading to the monophasic bifunctional catalyst, HSiW/Pt/SBA-15. These hybrid materials, with the exception of the monofunctional platinum catalyst, were all active for the gas-phase isomerization of n-hexane. The two bifunctional catalyst systems showed high activity and selectivity for branched isomers with no catalyst degradation over 3 days of reaction.

Hydrogenolysis of 1,4-dimethylcyclohexane on silica supported iridium catalyst: influence of time on stream on activity and selectivity

Abstract Opening cyclic or polycyclic alkanes by carbon–carbon bond cleavage can be a good route for the valorization of certain alkanes. A key point of any proposed catalytic system would be the catalyst poisoning. In this paper, we determine experimentally the influence of the time on stream on the distribution of products when 1,4-dimethylcyclohexane is introduced together with hydrogen over a Ir/SiO 2 catalyst. A simplistic model of selective catalyst poisoning is proposed to explain the influence of the time on stream on the activity and product selectivity.

New Processes for Carbon-Carbon Bond Activation Catalysed by Oxide Supported Surface Organometallic Complexes

The expanding field surface organometallic chemistry (referred to as SOMC, or by its French acronym, COMS) offers new possibilities to heterogeneous catalysis. The basic philosophy of this new area of heterogeneous catalysis is based on the concept that the supported catalyst is a kind of molecular entity. Ideally, one ceases to speak of “the immobilization of a catalyst” or of “the modification of a surface” but rather one considers the entire continuum - support, metal, and ligands - as a quasi-molecular species responding to both the fundamental rules derived from organometallic chemistry and to the rules of surface science. Surface organometallic chemistry preserves some of the advantages of traditional supported catalysis such as the ease of separation of the catalyst from the substrate/product and the heightened stability with respect to homogeneous analogs. As with traditional techniques, one can vary such parameters as the surface area, porosity, and electrophilicity of the support, and the identity of the metal. Surface organometallic chemistry offers much more precise control of other metal-centered factors such as the oxidation state and coordination geometry of the catalytic site. The stoichiometric nature of COMS synthesis often leads to catalytic systems with very high percentages of relatively well defined active catalytic sites.