1560-60-7Relevant articles and documents
A Comparative Analysis of the CO-Reducing Activities of MoFe Proteins Containing Mo- and V-Nitrogenase Cofactors
Lee, Chi Chung,Tanifuji, Kazuki,Newcomb, Megan,Liedtke, Jasper,Hu, Yilin,Ribbe, Markus W.
, p. 649 - 653 (2018/04/16)
The Mo and V nitrogenases are structurally homologous yet catalytically distinct in their abilities to reduce CO to hydrocarbons. Here we report a comparative analysis of the CO-reducing activities of the Mo- and V-nitrogenase cofactors (i.e., the M and V clusters) upon insertion of the respective cofactor into the same, cofactor-deficient MoFe protein scaffold. Our data reveal a combined contribution from the protein environment and cofactor properties to the reactivity of nitrogenase toward CO, thus laying a foundation for further mechanistic investigation of the enzymatic CO reduction, while suggesting the potential of targeting both the protein scaffold and the cofactor species for nitrogenase-based applications in the future.
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.
