| Autor: |
Kovaleva EG; Institute of Chemical Engineering, Ural Federal University, 19 Mira St., Yekaterinburg, 620002, Russian Federation. gek1969@bk.ru., Molochnikov LS; Department of Chemistry, Ural State Forest Engineering University, 37 Siberian Highway, Yekaterinburg, 620100, Russian Federation. lsmolochnikov@gmail.com., Stepanova DP; Institute of Chemical Engineering, Ural Federal University, 19 Mira St., Yekaterinburg, 620002, Russian Federation., Pestov AV; Laboratory of Organic Materials, I.Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, Akademicheskaya / S. Kovalevskoi, 22/20, Ekaterinburg, 620990, Russian Federation., Trofimov DG; Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Akad. Lavrent'ev Av. 9, Novosibirsk, 630090, Russian Federation., Kirilyuk IA; Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Akad. Lavrent'ev Av. 9, Novosibirsk, 630090, Russian Federation.; Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russian Federation., Smirnov AI; Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, NC, 27695-8204, USA. Alex_Smirnov@ncsu.edu. |
| Abstrakt: |
Acid-base equilibria and interfacial electrostatic properties of hydrated mesoporous and nanostructured alumina powders are determining factors for the use of these materials in heterogeneous catalysis and as a sorption media for filtration and chromatographic applications including life sciences. Here spin probe electron paramagnetic resonance spectroscopy of pH-sensitive nitroxides was employed to evaluate the surface charge and interfacial acid-base equilibria at the pore surface of mesoporous powders of α-Al 2 O 3 , γ-Al 2 O 3 , Al 2 O 3 × nH 2 O, and basic γ-Al 2 O 3 and nanostructured Al 2 O 3 in the form of pristine materials and modified with aluminum-tri-sec-butoxide, hydroxyaluminum glycerate, and several phospholipids. A new pH-sensitive nitroxide probe, 4-dimethylamino-5,5-dimethyl-2-(4-(chloromethyl)phenyl)-2-ethyl-2,5-dihydro-1H-imidazol-1-oxyl hydrochloride semihydrate (nitroxide R1), has been synthesized and characterized. It was found that conditions of preparation of alumina powders exert strikingly large effects on the apparent pK a of nitroxides measured from electron paramagnetic resonance titration curves. Specifically, while the electron paramagnetic resonance titrations curves for the nitroxide R1 in mesoporous powders prepared from basic γ-Al 2 O 3 and Al 2 O 3 × nH 2 O were shifted by ΔpK a ≈ +0.6 and up to ≈ +1.2 pH units respectively, the shift for γ-Al 2 O 3 was found to be much higher: ΔpK a = +3.5. Assuming approximately the same ∆pH = 0.5-1.0 arising from a difference in the hydrogen ion activity between the bulk solution phase and that in a confined pore volume, the samples were ranked in the following order of descending magnitude of the effective surface electrostatic potential Ψ: mesoporous γ-Al 2 O 3 > Al 2 O 3 × nH 2 O > basic γ-Al 2 O 3 > α-Al 2 O 3 . Conditions of the Al 2 O 3 synthesis as well as the surface modification procedures were found to have profound effects on the interfacial electrostatic properties of hydrated samples that are likely related to the nature and concentration of the active sites on the alumina surfaces. |
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