The distribution of dissolved zinc (Zn) was investigated in the Atlantic sector of the Southern Ocean in the austral autumn of 2008 as part of the IPY GEOTRACES expedition ZERO & DRAKE. Research focused on transects across the major frontal systems along the Zero Meridian and across the Drake Passage. There was a strong gradient in surface zinc concentrations observed across the Antarctic Polar Front along both transects and high zinc levels were found in surface waters throughout the Southern Ocean. Vertical profiles for dissolved Zinc showed the presence of local minima and maxima in the upper 200 m consistent with significant uptake by phytoplankton and release by zooplankton grazing, respectively. Highest deep water zinc concentrations were found in the centre of the Weddell Gyre associated with Central Intermediate Water (CIW), a water mass which is depleted in O2, elevated in CO2 and is regionally a CFC minimum. Our data suggests that the remineralization of sinking particles is a key control on the distribution of Zn in the Southern Ocean. Disappearance ratios of zinc to phosphate (Zn:P) in the upper water column increased southwards along both transects and based on laboratory studies they suggest slower growth rates of phytoplankton due to iron or light limitation. Zinc and silicate were strongly correlated throughout the study region but the disappearance ratio (Zn:Si) was relatively uniform overall except for the region close to the ice edge on the Zero Meridian.

The distribution of dissolved zinc in the Atlantic sector of the Southern Ocean

The equilibrium and kinetics of the protonation of 5,10, 15,20-tetraphenylporphyrin (H2TPP) and the demetalation of 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) at a dodecane−aqueous acid interface were investigated by means of a new in situ spectrophotometric method, the centrifugal liquid membrane method, which can provide the ultrathin two-phase liquid membrane system in a rotating glass cell. The consumption of H2TPP in the bulk dodecane phase and the production of the diprotonated aggregate, (H4TPP2+)n, adsorbed at the liquid−liquid interface were directly measured from the spectral change. The equilibrium constants of the interfacial aggregation of H4TPP2+ and the demetalation of ZnTPP were determined as log(Ke1/dm6 mol-2) = 2.14 ± 0.07 and log(Ke2/dm9 mol-3) = −6.05 ± 0.04 at 298 K, respectively. The observed rate constant of the demetalation of ZnTPP depended upon the first order of the acidity function, and it was suggested that the rate-determining step is the formation of the monoprotonat...

Direct Spectrophotometric Measurement of Demetalation Kinetics of 5,10,15,20-Tetraphenylporphyrinatozinc(II) at the Liquid−Liquid Interface by a Centrifugal Liquid Membrane Method

Self aggregation of 4,4′,4″,4'″-(21H,23H-porphine-5,10,15,20-tetrayl)tetrakis[1-methylpyridinium] cation (TMPyP4+) in water has been verified by means of fluorescence and 1H NMR spectroscopy. The novel fluorescence behavior of TMPyP such as extraordinarily red-shifted Q(0-0) fluorescence band, blue shift of this band upon dilution, effects of added sodium chloride and sodium dodecyl sulfate micelles, and short fluorescence lifetime strongly suggests the formation of a stacking-type dimer of TMPyP4+ at the TMPyP concentrations above 1×10−6 mol dm−3. The 1H NMR signal due to the porphyrin ring protons was observed as a broad singlet even though the concentration of TMPyP in D2O was 1×10−5 mol dm−3 while the signal in DMSO-d6 was a sharp singlet, suggesting the spontaneous association of TMPyP4+ to form the face-to-face dimer having a considerably loose structure. The π–π interaction has been assumed to be enhanced upon photoexcitation.

Self Aggregation of Cationic Porphyrin in Water

Dimerization of tetracationic porphyrins: ionic strength dependence

The mesoporous molecular sieves containing different amounts of titanium were synthesized using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent and tetraethyl orthotitanate (TEOT) as the source of titanium. The heterogeneous catalysts were obtained by encapsulating cationic manganese-porphyrin, [ meso -tetrakis(4-trimethylammoniophenyl)porphyrinato]manganese(III) pentachloride (MnTAPP) into the titanium-substituted mesoporous molecular sieves. Both the type of catalysts, i.e., supported and unsupported were characterized by FTIR, TGA, low-angle X-ray diffraction, nitrogen adsorption isotherm, scanning electron microscopy and DRUV–vis spectroscopy. A combined study on the characterization and catalytic performance of MnTAPP supported systems suggests the successful encapsulation of MnTAPP inside the mesoporous molecular sieves. The catalytic activity of the manganese(III) porphyrin-encapsulated materials was tested in the epoxidation of cyclohexene and styrene using iodosylbenzene (PhIO) as oxygen source. When compared to the homogeneous system, the manganese(III) porphyrin-encapsulated materials show higher activity for the epoxidation reactions. The activity and selectivity of these heterogeneous catalysts were also compared with a heterogeneous catalyst containing no titanium and with unsupported molecular sieves. The former shows lower activity than the heterogeneous catalyst prepared by encapsulating MnTAPP onto titanium-substituted molecular sieves, whereas the latter was inactive in the oxidation of both cyclohexene and styrene. The selectivity of these heterogeneous catalysts does not change appreciably after three times of reusing, but their catalytic activity decreases marginally. The decrease in the activity may be due to some leaching and/or decomposition of MnTAPP complex under the reaction conditions.

Albert N. Thompson

Papers

Peripheral charge effects on the kinetics of Zn(II)-porphyrin system