Carboxymethyl β-Cyclodextrin Assistance for the 4-Nitrophenol Reduction Using Cobalt-Based Layered Double Hydroxides

Abstract

Cobalt-aluminum-layered double hydroxides containing carboxymethyl β-cyclodextrin (CMβCD) were synthesized by coprecipitation and evaluated as a cobalt source for the 4-nitrophenol reduction in an aqueous medium. Several physicochemical techniques (XRD, FTIR, TGA) indicated the intercalation of the anionic cyclodextrin without damages to the hydrotalcite-type structure. These lamellar cobalt-aluminum hybrid materials (CoAl_CMβCD) were evaluated in the 4-nitrophenol reduction and showed higher activities in comparison with the CMβCD-free standard material (CoAl_CO₃). To rationalize these results, a set of experimental controls going from physical mixtures of CoAl_CO₃ with different cyclodextrins to other cobalt-based materials were investigated, highlighting the beneficial effects of both the layered double hydroxide and CMβCD-based hybrid structures. CMβCD also showed a beneficial effect as an additive during the 4-nitrophenol reduction. CoAl_CO₃, dispersed in a fresh CMβCD solution could be re-used for five successive cycles without the loss of activity.

Keywords: carboxymethyl β-cyclodextrin; cobalt; layered double hydroxides; nitroaromatics.

Figure 1

XRD patterns of the CoAl LDH materials prepared with and without CMβCD at (A) [5–80] 2θ range and (B) [20–25] 2θ range. The diffraction peaks of the JCPDS card number 37-0630 are included for comparison purposes.

Figure 2

FTIR spectra of CoAl_CO₃ and CoAl_CMβCD[X] with X = 0.03 and 0.06. For comparison, the FTIR spectrum of CMβCD was also included.

Figure 3

TGA profiles of (a) the LDH-based materials (CoAl_CO₃; (b) CoAl_CMβCD [0.03]; (c) CoAl_CMβCD [0.06]) obtained under air flow. CMβCD profile has been given in the box.

Figure 4

UV-vis spectra of the 4-NPhOH reduction in presence of CoAl_CMβCD [0.06] at different reaction times (a = 0, b = 2, c = 4, d = 7, e = 10, f = 15, g = 20, h = 25 and i = 30 min).

Figure 5

4-NPhOH reduction using NaBH₄ in presence of LDH-based materials, followed by UV-Vis spectroscopy at 400 nm. Reaction conditions: 4-NP (2 µmol), NaBH₄ (0.2 mmol), LDH-based material (20 mg), H₂O (30 mL).

Figure 6

4-NPhOH reduction in a NaBH₄ aqueous solution in the presence of cobalt-based materials. Reaction conditions: 4-NP (2 µmol), NaBH₄ (0.2 mmol), cobalt-based material (20 mg), H₂O (30 mL).

Figure 7

Recycling attempts for the 4-NPhOH reduction in NaBH₄ aqueous solution in the presence of CoAl_CMβCD [0.06] or CoAl_CO₃ with additional free CMβCD in the reaction medium. After a 30 min test, the suspension was centrifuged, and the recovered solid was washed and allowed to react for an additional test. Reaction conditions: dark green bars: 4-NPhOH (2 µmol), NaBH₄ (0.2 mmol), H₂O (30 mL), CoAl_CMβCD [0.06] (initial weight = 20 mg); light green bars: 4-NPhOH (2 µmol), NaBH₄ (0.2 mmol), H₂O (30 mL), CoAl_CO₃ (initial weight = 15 mg), CMβCD (5 mg).

Figure 8

UV-Visible spectra of aqueous cobalt(II) solutions at pH = 10.5. Cobalt(II) nitrate solution at pH 6 (purple curve), cobalt(II) nitrate solution at pH 10.5 (green curve) and cobalt(II) nitrate solution at pH 10.5 with CMβCD (blue curve).

Figure 9

Proposed mechanism for the enhanced activity of CoAl_CMβCD.