Temperature programmed reduction of unsupported molybdena and vanadia catalysts by ammonia and hydrogen

Temperature-programmed reduction (TPR) is a valuable tool for the characterisation of catalysts. The reductant mostly used is hydrogen. Hydrogen reduces the catalyst by producing water. The amount of hydrogen reacted is measured by menas of a thermal conductivity detector. In this paper the temperature-programmed reduction of vanadia and molybdena catalysts by ammonia is emphasized. During the temperature-programmed reduction ammonia is consumed. The compounds formed in this process are detected by means of a mass spectrometer. It was found that almost exclusively N₂, H₂O and H₂ were formed. Hydrogen formed by the decomposition of ammonia over vanadia and molybdena above 600° produces hydroxyl groups, which are a source for the formation of water at temperatures above 600°.NH₃-TPR gives more relevant information than H₂-TPR for processes in which ammonia is used as a reactant.     

Modeling and conformation analysis of β-cyclodextrin complexes

Summary A series of -cyclodextrin complexes containing various guest molecules was studied using computer-aided molecular modeling and conformation analysis techniques. The geometry of each complex was studied using crystallographic data. The positions of the glycosidic O4 atoms indicate that the -cyclodextrin molecules are elliptically distorted. This distortion can be related to the van der Waals volume of the guest molecules. This correlation is different for aromatic and non-aromatic guest compounds. Rigid body docking experiments demonstrated that in crystal structures the guest molecule occupies a position in the cavity of nearly minimum interaction energy when there are no other molecules having interactions with the guest molecule. From the crystallographic data several rules could be deduced which seem to determine the conformation of -cyclodextrin molecules in complexes. A procedure was developed to construct -cyclodextrin molecules that are able to encompass guest molecules having a given van der Waals volume.     

A new oxidative alkoxylation reaction The preparation of 5-alkoxy-4,4-dialkyl-1-aryl-3-pyrazolidinones

A new oxidative alkoxylation method is described. According to this method alkoxy groups are introduced in the 5-position of 4,4-dialkyl-1-aryl-3-pyrazolidinones by oxidation with HgO or SeO₂ in alcohols. The mechanism, the scope and limitations of the reaction are discussed. The 5-alkoxy-4,4-dialkyl-1-aryl-3-pyrazolidinones may be converted to 1-aryl-4,4-dialkyl-5-(1-aryl-3-oxo-4,4-dialkyl-5-alkoxy-5-pyrazolidinyl)-3-pyrazolidinones. The structure of the reaction products was studied by IR and NMR analyses.     

A Missing Reaction Step in Dithiobenzoate-Mediated RAFT Polymerization

The debate on the mechanism of dithiobenzoate-mediated RAFT polymerization may be overcome by taking the so-called “missing step” reaction between a highly reactive propagating radical and the three-arm star-shaped product of the combination reaction of an intermediate RAFT radical and a propagating radical into account. The “missing step” reaction transforms a propagating radical and a not overly stable three-arm star species into a resonance-stabilized RAFT intermediate radical and a stable polymer molecule. The enormous driving force behind the “missing step” reaction is estimated via DFT calculations of reaction enthalpies and reaction free enthalpies.