Polymerization of rac-lactide using schiff base aluminum catalysts: Structure, activity, and stereoselectivity

A series of aluminum ethyls and isopropoxides based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis and H-1 NMR confirmed that these Schiff base aluminum ethyls and isopropoxides were all monomeric species with a five-coordinated central aluminum in their solid structures. Compared to the aluminum ethyls which all retain their monomeric structure in the solution, the dinucleating phenomenons of aluminum isopropoxides with less steric hindered substituents in the solution have also been observed. The activities and stereoselectivities of these complexes toward the ring-opening polymerization of rac-lactide have been investigated. Polymerization experiments indicated that (SB-2d)(AlOPr)-Pr-i [(SB-2d) = 2,2-dimethyl-1,3-propylenebis(3,5-di-tert-butylsalicylideneiminato)] exhibited the highest stereoselectivity and (SB-3b)(AlOPr)-Pr-i [(SB-3b) = 2,2-dimethyl-1,3-propylenebis(3,5-dichlorinesalicylideneiminato)] possessed the highest activity among these aluminum isopropoxides. The substituents and the mode of the bridging part between the two nitrogen atoms both exerted significant influences upon the progress of the polymerizations, influencing either the tacticity of isolated polymers or the rate of polymerization.

Novel highly selective anion chemosensors based on 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole

2-(2-Hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole 1 and 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole 2 were used as anion fluorescent and colorimetric chemosensors with high selectivity for H2PO4- and F- over Cl-, while 2 can even distinguish H2PO4- from F-. (C) 2002 Elsevier Science Ltd. All rights reserved.

Novel polyoxometalate-templated, 3-D supramolecular networks based on lanthanide dimers: Synthesis, structure, and fluorescent properties of [Ln(2)(DNBA)(4)(DMF)(8)][Mo6O19] (DNBA=3,5-dinitrobenzoate)

The spherical Lindquist type polyoxometalate, Mo6O192-, has been used as a noncoordinating anionic template for the construction of novel three-dimensional lanthanide-aromatic monocarboxylate dimer supramolecular networks [Ln(2)(DNBA)(4)(DMF)(8)][Mo6O19] (Ln = La 1, Ce 2, and Eu 3, DNBA = 3,5-dinitrobenzoate, DMF = dimethylformamide). The title compounds are characterized by elemental analyses, IR, and single-crystal X-ray diffractions. X-ray diffraction experiments reveal that two Ln(III) ions are bridged by four 3,5-dinitrobenzoate anions as asymmetrically bridging ligands, leading to dimeric cores, [Ln(2)(DNBA)(4)(DMF)(8)](2+); [Ln(2)(DNBA)(4)(DMF)(8)](2+) groups are joined together by pi-pi stacking interactions between the aromatic groups to form a two-dimensional grid-like network; the 2-D supramolecular layers are further extended into 3-D supramolecular networks with 1-D box-like channels by hydrogen-bonding interactions, in which hexamolybdate polyanions reside. The compounds represent the first examples of 3-D carboxylate-bridged lanthanide dimer supramolecular "host" networks formed by pi-pi stacking and hydrogen-bonding interactions encapsulating noncoordinating "guest" polyoxoanion species. The fluorescent activity of compound 3 is reported.

Novel, highly active binuclear 2,5-disubstituted Amino-p-benzoquinone-nickel(II) ethylene polymerization catalysts

Reaction of salts of the 2,5-disubstituted amino-p-benzoquinone bridging ligand (la-e) with trans-bis(triphenylphosphane)phenylnickel(II) chloride results in the binuclear complexes 2a-e, which show high activities for ethylene polymerization without any cocatalysts. High-molecular-weight, moderately branched polyethylene of broad molecular-weight distribution was obtained.

Comparative study of Nickel-based perovskite-like mixed oxide catalysts for direct decomposition of NO

The mixed oxides LaNiO3, La0.1Sr0.9NiO3, La2NiO4 and LaSrNiO4 were prepared and used as catalysts for the direct decomposition of NO. The catalysts were characterized by means of XRD, XPS, O-2-TPD, NO-TPD and chemical analysis. By comparing the physico-chemical properties and catalytic activity for NO decomposition, a conclusion could be drawn as follows. The direct decomposition of NO over perovskite and related mixed oxide catalysts follows a redox mechanism. The lower valent metal ions Ni2+ and disordered oxygen vacancies seem to be the active sites in the redox process. The oxygen vacancy plays an important role favorable for the adsorption and activation of NO molecules on one hand and on the other hand for increasing the mobility of lattice oxygen which is beneficial to the reproduction of active sites. The presence of oxygen vacancies is one of the indispensable factors to give the mixed oxides a steady activity for NO decomposition.

On the selectively catalytic reduction of NOx with methane over Ag-ZSM-5 catalysts

The catalytic performance of silver-modified ZSM-5 catalysts in the selectively catalytic reduction (SCR) of NOx with methane was investigated. NO was selectively reduced by CH4 to N-2 in the presence of excess O-2, and the catalytic activity depended on both the activation of CH4 and the adsorption properties of NOx. Silver incorporated in ZSM-5 zeolite activated CH4 at low temperatures and lowered the "light-off" temperature for the CH4-SCR of NOx. Temperature-programmed (TP) spectroscopy studies depicted that surface nitrosyl species directly decomposed to N-2 in the absence of O-2. CH4 could not effectively reduce surface nitrosyl species, but might facilitate the direct decomposition of NO through the removal of surface oxygen. Surface nitrates were formed in NO and O-2 coexisting system and could be effectively reduced by CR4 to nitrogen. The priority of surface nitrates to O-2 in the reaction with CH4 clearly demonstrated that CH4 selectively and preferentially reduced the surface nitrate species to N-2 in the excess of oxygen. (C) 2002 Elsevier Science B.V. All rights reserved.

Butene catalytic cracking to propene and ethene over potassium modified ZSM-5 catalysts

Catalytic cracking of butene over potassium modified ZSM-5 catalysts was carried out in a fixed-bed microreactor. By increasing the K loading on the ZSM-5, butene conversion and ethene selectivity decreased almost linearly, while propene selectivity increased first, then passed through a maximum (about 50% selectivity) with the addition of ca. 0.7-1.0% K, and then decreased slowly with further increasing of the K loading. The reaction conditions were 620 degrees C, WHSV 3.5 h(-1), 0.1 MPa 1-butene partial pressure and 1 h of time on stream. Both by potassium modification of the ZSM-5 zeolite and by N(2) addition in the butene feed could enhance the selectivity towards propene effectively, but the catalyst stability did not show any improvement. On the other hand, addition of water to the butene feed could not only increase the butene conversion, but also improve the stability of the 0.7%K/ZSM-5 catalyst due to the effective removal of the coke formed, as demonstrated by the TPO spectra. XRD results indicated that the ZSM-5 structure of the 0.07% K/ZSM-5 catalyst was not destroyed even under this serious condition of adding water at 620 degrees C.

On the correlation between microstructural changes of Ag-H-ZSM-5 catalysts and their catalytic performances in the selective catalytic reduction of NOx by methane

The silver catalyzed, selective catalytic reduction (SCR) of nitrogen oxides (NOx) by CH4, is shown to be a structure-sensitive reaction. Pretreatment has a great affect on the catalytic performances. Upon thermal treatment in inert gas stream, thermal induced changes in silver morphology lead to the formation of reduced silver species of clusters and particles. Catalysis over this catalyst indicates an initially higher activity but lower selectivity for the CH4-SCR of NOx Reaction induced restructuring of silver results in the formation of ill-defined silver oxides. This, in turn, impacts the adsorption properties and diffusivity of oxygen over silver catalyst, results in the decrease in activity but increase in selectivity of Ag-H-ZSM-5 catalyst for the CH4-SCR of NO.. (c) 2004 Elsevier B.V. All rights reserved.

Achiral, selective CCK2 receptor antagonists based on a 1,3,5-benzotriazepine-2,4-dione template

Novel, achiral 1H-1,3,5-benzotriazepine-2,4(3H,5H)-diones have been prepared and structurally characterized. These compounds are potent CCK2 receptor antagonists that display a high degree of selectivity over CCK1 receptors.