R-, â-, γ-, and δ-types, because the basic structural octahedral
unit [MnO6] can be linked in different ways, e.g., the R-type
is constructed from a double chain of [MnO6] octahedrons
with 2 × 2 tunnels.13 The crystallographic forms are generally
believed to be responsible for their properties and the
controlled synthesis of MnO2 has always been the focus of
material scientists.14 Numerous methods, such as simple
reduction,9,10 oxidation,15 coprecipitation,12 sol-gel,16 thermal
decomposition,11 etc., have been developed for the synthesis
of MnO2.
dried at room temperature (25 °C) under vacuum, and
employed as solid-phase catalysts for the hydrolysis of
nitriles.
The phase purity of the composite materials was studied
by X-ray diffraction (XRD), using a PW3040/60 X-ray
diffractometer with Cu KR radiation (λ ) 1.54178 Å) at a
scanning rate 0.1 deg/s in the 2θ range from 10° to 70°,
with the operation voltage and current maintained at 40 kV
and 30 mA, respectively. The diffraction pattern of the free
resin beads does not show any distinguishable peak, owing
to the amorphous nature of the polystyrene beads (see the
Supporting Information). Figure 1 shows the XRD patterns
Herein, we have explored for the first time a green
chemistry approach for in situ deposition of R-MnO2
nanorods onto the surface of functionalized polystyrene beads
through immobilization of metal precursor ions exploiting
the electrostatic field force of the charged resin beads,
followed by photochemical reduction in the presence of
visible light under alkaline conditions. The chemical com-
position and the morphology of the as-synthesized particles
were characterized by different physical methods. Finally,
the particles have been exploited as a solid-phase catalyst
for the one-step and facile synthesis of amide compounds
from their corresponding nitriles in the presence of visible
light in weakly basic medium.
The synthetic strategy involved for the deposition of MnO2
nanorods on resin surface is as follows: Anion-exchange
resin, SERALITE-SRA-400, a cross-linked polystyrene
containing quaternary ammonium groups as the integral part,
was purchased in the chloride form with ion-exchange
capacity 3.5 mmol g-1. First, 0.5 g of the anion-exchange
resin, [R+Cl-], was treated with 25 mL of 1 M aqueous
KMnO4 solution and the solution was stirred to complete
the exchange of Cl- ions with MnO4- ions. Within 1 h, the
purple solution became colorless indicating ready exchange
of MnO4- with Cl-. After that, the resin-bound permanganate
Figure 1. XRD pattern and EDX spectrum of the nanocomposite
material.
of R-MnO2 nanoparticles. All the diffraction peaks can be
readily indexed to a tetragonal phase of R-MnO2 with a space
group of I4/m (no. 87) having lattice constants a ) 9.7847
Å and c ) 2.8630 Å (JCPDS card, No. 44-0141). The sharp
diffraction peaks indicate the purity and crystallinity of the
product.
The surface property and the composition of the catalyst
were characterized from X-ray photoelectron spectroscopy
(XPS) and energy dispersive X-ray analysis (EDX). The
EDX spectrum (shown in Figure 1) further authenticates the
presence of Mn and oxygen in the nanocomposites. The Mn
2p core level spectrum (see the Supporting Information)
illustrates that the observed binding energy 642.37 and
653.71 eV corresponds to Mn2p3/2 and Mn2p1/2 electrons,
respectively, which is well in accordance with the literature
values for MnO2.17 Thus, the result suggests the photochemi-
cal deposition of tetravalent Mn in the form of MnO2 on the
surface of resin beads.
moiety [R+MnO4 ] was washed several times with distilled
-
water and completely reduced photochemically (24 h, 40 W
tungsten lamp, fluence ∼50 mJ/cm2) while stirring in weakly
alkaline conditions (0.1 M NaOH). Thus, the reduction of
resin bound precursor ions led to the deposition of MnO2
nanoparticles onto the resin surfaces (Scheme 1).
Scheme 1
The particle morphology and structural properties of
R-MnO2 nanoparticles were elucidated by transmission
electron microscopy (TEM). Figure 2 represents the TEM
image of the product, which consists of nanorods with
diameter ∼75 nm and length ∼235 nm. The HRTEM image
of R-MnO2 nanorods (Figure 2) shows that the nanorod is
structurally uniform with a fringe spacing of 0.311 nm, which
corresponds to the lattice spacing of the (110) plane. These
clear lattice fringes further confirm the single crystalline
nature of the nanorods.
The as-prepared shining black beads (see the Supporting
Information) were washed thoroughly with plenty of water,
(13) Thackeray, M. M. Prog. Solid State Chem. 1997, 25, 1.
(14) Xu, F.; Wang, T.; Li, W. R.; Jiang, Z. Y. Chem. Phys. Lett. 2003,
375, 247.
(15) Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880.
(16) AL-Sagheer, F. A.; Zaki, M. I. Colloids Surf., A 2000, 173, 193.
(17) Wanger, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.;
Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-
Elmer: Eden Prairie, MI, 1978.
2192
Org. Lett., Vol. 9, No. 11, 2007