Functionalized multi-walled carbon nanotubes in an aldol reaction.

The covalent functionalization of multi-walled carbon nanotubes (MWCNTs) with a proline-based derivative is reported. Initially, MWCNTs were oxidized in order to introduce a large number of carboxylic units on their tips followed by N-tert-butoxycarbonyl-2,2'(ethylenedioxy)bis-(ethylamine) conjugation through an amide bond. Then, a proline derivative bearing a carboxylic terminal moiety at the 4-position was coupled furnishing proline-modified MWCNTs. This new hybrid material was fully characterized by spectroscopic and microscopy means and its catalytic activity in the asymmetric aldol reaction between acetone and 4-nitrobenzaldehyde was evaluated for the first time, showing to proceed almost quantitatively in aqueous media. Furthermore, several amino-modified MWCNTs were prepared and examined in the particular aldol reaction. These new hybrid materials exhibited an enhanced catalytic activity in water, contrasting with the pristine MWCNTs as well as the parent organic molecule, which failed to catalyze the reaction efficiently. Furthermore, the modified MWCNTs proved to catalyze the aldol reaction even after three repetitive cycles. Overall, a green approach for the aldol reaction is presented, where water can be employed as the solvent and modified MWCNTs can be used as catalysts, which can be successfully recovered and reused, while their catalytic activity is retained.

Photocatalytic applications with CdS • block copolymer/exfoliated graphene nanoensembles: hydrogen generation and degradation of Rhodamine B.

Amphiphilic block copolymer poly(isoprene-b-acrylic acid) (PI-b-PAA) was used to stabilize exfoliated graphene in water, allowing the immobilization of semiconductor CdS nanoparticles forming CdS â€¢ PI-b-PAA/graphene. Characterization using high-resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy proved the success of the preparation method and revealed the presence of spherical CdS. Moreover, UV-Vis and photoluminescence assays suggested that electronic interactions within CdS â€¢ PI-b-PAA/graphene exist as evidenced by the significant quenching of the characteristic emission of CdS by exfoliated graphene. Photoillumination of CdS â€¢ PI-b-PAA/graphene, in the presence of ammonium formate as a quencher for the photogenerated holes, resulted in the generation of hydrogen by water splitting, monitored by the reduction of 4-nitroaniline to benzene-1,4-diamine (>80 ± 4% at 20 min; 100% at 24 min), much faster and more efficient compared to when reference CdS â€¢ PI-b-PAA was used as the photocatalyst (<30 ± 3% at 20 min; 100% at 240 min). Moreover, Rhodamine B was photocatalytically degraded by CdS â€¢ PI-b-PAA/graphene, with fast kinetics under visible light illumination in the presence of air. The enhancement of both photocatalytic processes by CdS â€¢ PI-b-PAA/graphene was rationalized in terms of effective separation of holes and electrons, contrary to reference CdS â€¢ PI-b-PAA, in which rapid recombination of the hole-electron pair is inevitable due to the absence of exfoliated graphene as a suitable electron acceptor.

Photocatalytic application of nanosized CdS immobilized onto functionalized MWCNTs.

Nanosized semiconductor CdS immobilized onto modified multi-walled carbon nanotubes (MWCNTs) carrying poly(amidoamine) dendron units were visualized by HR-TEM. Evidently, spherical CdS nanoparticles 3-5 nm in diameter were identified. Moreover, EDX spectroscopy gave additional spectroscopic proof of the presence of CdS in the CdS-MWCNTs hybrid material. The photocatalytic activity of CdS-MWCNTs toward the decomposition of rhodamine B (RhB) was examined by monitoring spectral changes in the characteristic absorption band of RhB centred at 554 nm. The latter absorption band of RhB was found to continuously depress during visible light irradiation in the presence of CdS-MWCNTs, with faster kinetic rates as compared with the case when only reference CdS was present. The current result was rationalized in terms of efficient photoinduced electron-transfer from CdS to MWCNTs within the intrahybrid CdS-MWCNTs. In this frame, the suggested mechanism for the high and fast photocatalytic decomposition of RhB supports the accumulation of electrons in MWCNTs, which then react with molecular oxygen, thus reducing it to superoxide radical anion O2˙(-) responsible for the generation of the highly reactive species of HO˙ and HOO˙. The latter together with the holes generated in photoexcited CdS were responsible for the decomposition of RhB. Finally, the photocatalyst CdS-MWCNTs was recovered and efficiently reused for four consecutive catalytic cycles, thus highlighting its wider applicability in removing organic pollutants from water.