1517-49-3Relevant academic research and scientific papers
Reaction Mechanism and Kinetics of Olefin Metathesis by Supported ReOx/Al2O3 Catalysts
Lwin, Soe,Wachs, Israel E.
, p. 272 - 278 (2016/01/12)
The self-metathesis of propylene by heterogeneous supported ReOx/Al2O3 catalysts was investigated with in situ Raman spectroscopy, isotopic switch (D-C3= → H-C3=), temperature-programmed surface reaction (TPSR) spectroscopy, and steady-state kinetic studies. The in situ Raman studies showed that two distinct surface ReO4 sites are present on alumina and that the olefins preferentially interact with surface ReO4 sites anchored at acidic surface sites of alumina (olefin adsorption: C4= > C3= > C2=). The isotopic switch experiments demonstrate that surface Re?CH3 and Re?CHCH3 are present during propylene metathesis, with Re? representing activated surface rhenia sites. At low temperatures (3=][Re?]2. At high temperatures (>100 °C), the rate-determining step is the recombination of two surface propylene molecules (rate ≈ [C3=]2[Re?]). To a lesser extent, the recombination of surface Re?CH3 and Re?CHCH3 intermediates also contributes to self-metathesis of propylene at elevated reaction temperatures.
Active Sites in Olefin Metathesis over Supported Molybdena Catalysts
Amakawa, Kazuhiko,Kr?hnert, Jutta,Wrabetz, Sabine,Frank, Benjamin,Hemmann, Felix,J?ger, Christian,Schl?gl, Robert,Trunschke, Annette
, p. 4059 - 4065 (2015/12/26)
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 2.1-13.3 wt %, apparent Mo surface density 0.2-2.5 nm-2). The catalysts have been prepared by an ion exchange technique. Nitrogen adsorption, 1H MAS-NMR, Raman, and FTIR spectroscopies were applied to characterize the catalysts. Adsorption of the reactant propene and the probe molecule NH3 was studied by in situ FTIR spectrometry microcalorimetry and temperature-programmed desorption. Irrespective of the loading, only ≈1 % 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. Targeted active site creation: Only ≈1 % of isolated distorted Mo di-oxo species that are in close vicinity to two silanol groups have been shown to be the precursor of the active carbene (Mo=CHR) sites on MoOx/SiO2 catalysts for propene metathesis. Targeted active site creation resulted in an increase in initial catalytic activity by a factor of 800.
In situ generation of active sites in olefin metathesis
Amakawa, Kazuhiko,Wrabetz, Sabine,Kroehnert, Jutta,Tzolova-Mueller, Genka,Schloegl, Robert,Trunschke, Annette
supporting information; experimental part, p. 11462 - 11473 (2012/09/05)
The depth of our understanding in catalysis is governed by the information we have about the number of active sites and their molecular structure. The nature of an active center on the surface of a working heterogeneous catalyst is, however, extremely difficult to identify and precise quantification of active species is generally missing. In metathesis of propene over dispersed molybdenum oxide supported on silica, only 1.5% of all Mo atoms in the catalyst are captured to form the active centers. Here we combine infrared spectroscopy in operando with microcalorimetry and reactivity studies using isotopic labeling to monitor catalyst formation. We show that the active Mo(VI)-alkylidene moieties are generated in situ by surface reaction of grafted molybdenum oxide precursor species with the substrate molecule itself gaining insight into the pathways limiting the number of active centers on the surface of a heterogeneous catalyst. The active site formation involves sequential steps requiring multiple catalyst functions: protonation of propene to surface Mo(VI)-isopropoxide species driven by surface Bronsted acid sites, subsequent oxidation of isopropoxide to acetone in the adsorbed state owing to the red-ox capability of molybdenum leaving naked Mo(IV) sites after desorption of acetone, and oxidative addition of another propene molecule yielding finally the active Mo(VI)-alkylidene species. This view is quite different from the one-step mechanism, which has been accepted in the community for three decades, however, fully consistent with the empirically recognized importance of acidity, reducibility, and strict dehydration of the catalyst. The knowledge acquired in the present work has been successfully implemented for catalyst improvement. Simple heat treatment after the initial propene adsorption doubled the catalytic activity by accelerating the oxidation and desorption-capturing steps, demonstrating the merit of knowledge-based strategies in heterogeneous catalysis. Molecular structure of active Mo(VI)-alkylidene sites derived from surface molybdena is discussed in the context of similarity to the highly active Schrock-type homogeneous catalysts.
Coupling reactions in aldehydes adsorbed on V(100) single-crystal surfaces
Shen, Min,Zaera, Francisco
experimental part, p. 8708 - 8713 (2009/10/23)
The thermal chemistry of formaldehyde on vanadium (100) single-crystal surfaces was characterized under ultrahigh vacuum (UHV) conditions by using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) in combination with isotope-labeling experiments. Particular emphasis was placed on establishing a mechanism for the formation of ethylene, which was observed to desorb in two temperature regimes, at 290 and 540 K. The low-temperature reaction was determined to occur via the coupling of methylene groups formed on the surface upon dissociation of the C-O bond in adsorbed formaldehyde. The high-temperature ethylene, on the other hand, was proven to require the prior formation of a diolate, -OCH2CH2O-, intermediate. This chemistry was shown to be quite general, also occuring in cross-coupling mode between two different coadsorbed aldehydes.
Hydroformylation of olefins in the presence of dicobalt octacarbonyl: some considerations
Piacenti, Franco,Bianchi, Mario,Frediani, Piero,Menchi, Gloria
, p. 77 - 88 (2007/10/02)
New data for the deuteroformylation of propene and hydroformylation of deuteropropenes are presented, together with revised data for the hydroformylation of but-1-ene-4-d3.The mechanism of formation of isomeric aldehydes is discussed, and it is concluded that several reaction paths may be followed depending on the reaction conditions and the structure of the substrate.
Effect of a Hydrogen Pretreatment on the Mechanism of Deuterium Addition Exchange of Propene over Ni-Cu Alloy Catalysts
Naito, Shuichi,Tanimoto, Mitsutoshi
, p. 1253 - 1260 (2007/10/02)
The effects of a hydrogen pretreatment on the activity and reaction intermediate in a C3H6-D2 reaction over a Ni-Cu alloy as well as Ni metal catalysts were investigated in detail by applying an isotope tracer technique.Ni metal and Ni-rich alloy catalysts, cooled down to the reaction temperature under a hydrogen atmosphere (D-surface), exhibited low activity compared to that of a surface evacuated before cooling (E-surface).For Cu-rich alloy catalysts, however, the E-surface exhibited a lower activity and a higher activation energy than did the D-surface.A comparison of the TPF spectra of adsorbed hydrogen with the dependence of the activity upon the evacuation temperature suggests that strongly adsorbed hydrogen retards the reaction over Ni and Ni-rich alloy catalysts at lower temperature.Microwave spectroscopic analysis demonstrated that the reaction intermediates over D- and E-surfaces of Cu-rich alloy catalysts were similar to those over Cu and Ni metals, respectively.This result suggests that a hydrogen pretreatment of Cu-rich alloys forms a specific surface structure which is destroyed by evacuation at elevated temperatures.
IR SPECTRA OF DEUTERATED PROPENE ANALOGUES OF ZEISE'S SALT
Rericha, Roman,Zdrazil, Miroslav
, p. 2647 - 2655 (2007/10/02)
The IR absorption spectra (4000-250 cm-1) of anhydrous K using propene-2-d1 and propene-1,1-d2 as the ligands, were measured in KBr discs.A new weak band for the fundamental deformation frequency of the C-C=C skeleton softened by the coordination of the propenes was found around 400 cm-1, namely at 420, 417, and 375 cm-1 for d0, d1, and d2 species, resp.Vibrational couplings of the so-called ν(C=C) with other modes of the ligands is discussed.
Metallacarboranes in Catalysis. 6. Kinetics and Mechanism of Alkene Hydrogenation and Isomerization Catalyzed by Rhodacarborane Clusters. A Search for Cluster Catalysis
Behnken, Paul E.,Belmont, James A.,Busby, David C.,Delaney, Mark S.,King, Roswell E.,et al.
, p. 3011 - 3025 (2007/10/02)
In a search for cluster-catalyzed reactions the rhodacarboranes (1), (II), (III), and (IV) were employed as catalyst precursors in a study of 1-hexene (B) isomerization.Precursors I, III, and IV were examined in the hydrogenation of 3-methyl-3-phenyl-1-butene (A).Complete rate laws were developed for isomerization and hydrogenation.Studies with D2 demonstrated the presence of reversible alkylrhodium formation during hydrogenation.The use of D-labeled catalyst precursors, etc., proved that the Rh-H ligand of the closo precursors was not directly involved in either alkene isomerization or in hydrogenation.Competitive isomerization and hydrogenation of 1-hexene catalyzed by precursors I and IV suggested the presence of a common intermediate for these two reactions.Extensive intermolecular D scrambling was observed in equilibration experiments which employed propene-1,1,1-d3 and propene-2-d with precursor I and isotopically normal propene.The slow regiospecific transfer of deuterium from carbon in A to the 9, 10, and 12 boron vertices in I was observed and is believed to proceed via H-Rh-B bonded intermediates.The mechanistic implications of these and other observations are integrated into a mechanistic scheme which is based upon the prior equilibrium of Rh(3+) closo- and Rh(1+) exo-nido-rhodacarboranes which, in the presence of alkene, produce an equilibrium concentration of a key (phosphine)(alkene)Rh(1+) exo-nido intermediate regardless of the closo or exo-nido nature of the catalyst precursor used.Alkene isomerization is thought to involve η3-allylic intermediates produced from the exo-nido alkene complex.Hydrogenation appears to proceed via oxidative addition of H2 to this same complex followed by rate-determining decomposition of the hydridorhodium alkyl produced by this means.These kinetic characteristics may have their origin in the weak electron-donor properties of the chelated exo-nido-C2B9H12(1-) ligands which are attached to Rh(1+) or Rh(3+) in the exo-nido intermediates by a pair of B-H-Rh three-center, two-electron bonds.
Direct and Regioselective Transformation of α-Chloro Carbonyl Compounds into Alkenes and Deuterioalkenes
Barluenga, Jose,Yus, Miguel,Concellon, Jose M.,Bernad, Pablo
, p. 2721 - 2726 (2007/10/02)
The successive treatment ethyl chloroacetate or chloroacetyl chloride with Grignard reagents and lithium powder leads to symmetrical terminal olefins in a regioselective manner.The best results are obtained with acid chlorides.The influence of the temperature and the reaction time on overall yield of the process are studied; in general, yields are increased by working at low temperature (-60 deg C).Internally substituted olefins are obtained from α-chloro acid chlorides through a similar process.The treatment of α-chloro aldehydes, ketones and carboxylic acid derivatives (esters or acid chlorides) with lithium aluminium hydride or lithium aluminium hydride/aluminium chloride and lithium powder at low temperature (-60 deg C) leads in a regioselective manner to olefins with the same carbon skeleton as the starting carbonyl compound.Reactions with lithium aluminium deuteride lead to incorporation of deuterium at predetermined positions in the alkene.
