Waste management in zinc promoted allylation of aldehyde

Article abstract of DOI:10.1039/c5nj03569d

By using an efficient, sustainable and green procedure, the waste zinc material in Zn(0) promoted Grignard-Barbier type allylation of aldehydes has been successfully utilized as a reusable material for the adsorption of various dyes and also converted int

Full text of DOI:10.1039/c5nj03569d

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Waste management in zinc promoted allylation
of aldehyde†
Cite this:DOI: 10.1039/c5nj03569d
Sanjay Pratihar,* Anindita Kakoty and Kasturi Sarmah
By using an efficient, sustainable and green procedure, the waste zinc material in Zn(0) promoted
Grignard–Barbier type allylation of aldehydes has been successfully utilized as a reusable material for the
adsorption of various dyes and also converted into the corresponding hexagonal wurtzite phase of ZnO.
The need for aqueous NH4Cl was justified by experimental and theoretical evidence, which suggests that
the probable coordination of H2O or NH3 to the Zn center in both the intermediate and transition states
is responsible for a lower activation energy and consecutively higher yield of products in both the cases.
The conversion of the corresponding Zn(0) to its hydroxide or ZnO was studied by XRD analysis, which
showed the facile conversion of Zn(OH)2 to its corresponding ZnO after calcination of the sample at
300 1C. The reported method is a cost-effective chemical route for producing the ZnO nanomaterial at
the gram level. The as-obtained zinc waste from allylation reaction shows an excellent ability as a reusable
material for the adsorption of Congo red, methyl red and methylene blue and is expected to be useful in
many other applications. The easy synthesis, effortless separation, efficient recycling and good adsorption
capability of the waste zinc nanomaterial toward various dye molecules make it a competent candidate
for wastewater treatment.
Received (in Montpellier, France)
15th December 2015,
Accepted 1st April 2016
DOI: 10.1039/c5nj03569d
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useful material. Amongst the various metal oxides, zinc oxide
provides one of the greatest assortments of varying particle
Introduction
In the past few decades, various useful methodologies have structures and is widely used in numerous materials and pro-
been successfully developed for the generation of allyl metals ducts including plastics, ceramics, glass, ointments, etc.5 ZnO is
and the exploitation of the metal–carbon bond reactivity for the present in the earth’s crust as the mineral zincite; however most
synthesis of various important molecules.1 In particular, the of the commercial ZnO is produced synthetically.6 In the past
addition of allyl metals to carbonyl compounds leads to syntheti- few decades, various synthetic methods have been established
cally important homoallylic alcohols, which can participate in based on solvothermal, precipitation, chemical vapour deposi-
various synthetically useful transformations: cycloaddition, dihydr- tion, hydrothermal, sol–gel, and mechanochemical techniques.7
oxylation, epoxidation, hydroboration, hydroformylation, hydro- In this work, the utilization of the corresponding zinc waste in
genation, hydration, olefin metathesis, ozonolysis, etc.2 The zinc(0) promoted allylation of aldehydes was shown to produce a
corresponding allyl metals may be generated either via the ZnO nanomaterial. The role of water, additives, and aldehydes in
oxidative addition of an allyl electrophile to a main group or zinc(0) promoted allylation of aldehydes was investigated from
transition metal through a Grignard-like protocol or via the experimental and theoretical evidence (Fig. 1).
oxidative addition of an allyl electrophile to a transition metal
followed by coupling with a main group metal salt in tandem.3
Although allyl metals are considered to be important reagents in
many aspects, their metal counterparts are treated as waste
materials after the completion of the reaction. Amongst the
various metal mediated Grignard type allylation reactions, we
were particularly interested in Zn(0) promoted allylation4
because we wanted to utilize the corresponding waste zinc as a
Department of Chemical Sciences, Tezpur University, Napaam, Asaam-784028,
India. E-mail: spratihar@tezu.ernet.in, spratihar29@gmail.com
Fig. 1 Conversion of waste zinc in zinc promoted Barbier-type allylation
† Electronic supplementary information (ESI) available: Experimental and opti-
reaction to a useful zinc oxide nanomaterial.
mized coordinates. See DOI: 10.1039/c5nj03569d
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The conversion of the waste zinc material to the corres-
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Table 1 Effect of different additives on Zn(0) promoted allylation reaction
ponding ZnO nanomaterial was monitored by XRD analysis,
which showed the facile conversion of the Zn(OH)2 waste to
the corresponding ZnO nanomaterial after calcination of the
sample at 300 1C. The reported method is a cost-effective
chemical route for producing the ZnO nanomaterial at the
gram level with a band gap of 3.15 eV. The as-obtained zinc
waste from the Zn(0) promoted allylation of aldehydes is found
to be active for the adsorption of dyes like Congo red, methyl
red and methylene blue with good adsorption capacity.
Additive
Time (h)
Yield (%)
NH4Cl
NH4OH
HCl
6
12
6
72
24
16
Reaction condition: 1 mmol aldehyde, 0.4 ml of allyl bromide, Zn0
(70 mg), and additive (3 mmol) were added to 2 ml of THF and 3 ml of
water and stirred at room temperature.
Results and discussion
Role of additives and water
In most of the zinc promoted Barbier type allylation reactions,
the corresponding zinc salts were thrown away as waste pro-
ducts after the isolation of the corresponding homoallylic
alcohols from aldehydes (Scheme 1).
As an important contribution, Luche et al. showed the zinc
mediated effective allylation of aldehydes and ketones in aqueous
media using THF as a cosolvent under magnetic stirring or
sonication conditions.8 They observed enhanced efficiency in zinc
mediated allylation of aldehydes in the case of replacing water by
an aqueous saturated ammonium chloride solution.9 On the
other hand, the replacement of ammonium chloride with sodium
chloride (NaCl) does not show any product formation. So, the
higher efficiency in the case of aqueous saturated ammonium
chloride solution is due to both the increased acidity of the media
and the formation of complexes between the metal ion and
ammonia. Recently, by using the Hammett plot, kinetic isotope
effects, and theoretical studies, Madsen et al. showed that Zn(0)
promoted Barbier type allylation is feasible through a six-
membered transition state between allyl zinc and aldehydes.10
For the model studies, zinc promoted room temperature allylation
of 4-chlorobenzaldehyde was chosen as a model reaction in the
THF/water solvent system. To investigate the effect of pH on
the model reaction, the model reaction was performed in the
presence of NH4Cl, HCl, and NH4OH (Table 1). The allylation
reaction proceeds smoothly in NH4Cl, while a low yield of the
corresponding homoallylic alcohol (1a) was achieved under both
acidic and basic conditions. The oxidative addition of allyl
bromide to Zn(0) leads to an allyl zinc reagent, which further
reacts with the aldehyde to give the corresponding homoallylic
alcohol (1a) with the release of the corresponding Zn(OH)Br
(Scheme 2). Under acidic conditions, there is a chance of probable
Scheme 2 Probable mechanism of Zn(0) promoted allylation reaction of
aldehydes.
oxidation of Zn(0) to Zn(II), which will inhibit the first step of the
reaction by decreasing the available Zn(0).
On the other hand, the presence of an acid will promote
step-3 of the allylation reaction. So, a slightly acidic pH in
NH4Cl medium (4.6 to 6) promotes the overall allylation
process. The role of water and NH4Cl was further studied from
theoretical analysis.
We have also optimized the probable intermediate and six-
membered transition states between allyl zinc bromide and
aldehydes at the B3LYP level of theory using the effective core
potential (ECP) along with valence basis sets (LANL2DZ) for
zinc and the 6-31G* basic set for other atoms.11 The free energy
of activation of zinc promoted allylation of benzaldehyde is
found to be 6.2 kcal molÀ1. The probable coordination of water
or ammonia with the intermediate and transition states will
facilitate the reaction. In fact, the calculated free energy of
activation is found to be lower in both the cases, which justifies
the requirement of aqueous reaction medium and saturated
ammonium chloride in zinc promoted allylation reactions. The
activation energy in both the gas phase and THF solvent shows
similar results.
Effect of aldehydes
Further to check the substituent effect on the allylation reac-
tion, the Zn(0) promoted allylation reaction was performed with
different aldehydes. The Zn(0) promoted allylation reaction in
the case of electron withdrawing aldehydes like 4-chlorobenz-
aldehyde and 4-(trifluoromethyl)benzaldehyde leads to the
corresponding homoallylic alcohols in 72 and 86% yield,
Scheme 1 Conversion of waste Zn to useful ZnO and its utilization.
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Fig. 2 Gibbs free energy (in kcal molÀ1) profile for Zn(0) promoted
allylation of benzaldehyde under different reaction conditions in gas phase
and in tetrahydrofuran (in parentheses).
Fig. 3 Gibbs free energy (in kcal molÀ1) profile for Zn(0) promoted
allylation of three different para substituted benzaldehydes in gas phase
and in tetrahydrofuran (in parentheses).
respectively, after 6 h. On the other hand, the yield of the
corresponding homoallylic alcohols in the case of electron
rich aldehydes like 4-methylbenzaldehyde and 4-methoxybenz-
aldehyde is found to be 58 and 52% after 12 and 14 h,
respectively (Fig. 2).
Synthesis and characterization of waste zinc
Now, we are interested in checking the fate of the Zn metal after
the reaction. For this, we isolated the corresponding white zinc
material after the completion of the Zn(0) promoted allylation
reaction and washed it with water and methanol several times.
The as-prepared white material was calcined at 100 1C for
different time intervals and the samples were collected for
XRD analysis. Most of the peaks in XRD analysis of the freshly
prepared white material match with Zn(OH)2 (JCPDS 38-385),
which indicates the formation of Zn(OH)2 after hydrolysis of
the corresponding allyl zinc reagent (Fig. 4).
In terms of yield, electron withdrawing aldehydes are better
substrates compared to electron rich aldehydes because the
higher electrophilicity at the carbonyl centre in the former case
will facilitate the formation of a six-membered transition state
(Table 2). For better understanding, we have optimized both the
intermediate and transition states of three different para sub-
stituted aldehydes. In fact, the calculated free energy of activa-
tion (DG#) in the case of electron withdrawing aldehydes like
4-chlorobenzaldehyde is found to be lower compared to benz-
aldehyde in both gas phase and THF (Fig. 3). On the other
hand, the highest DG# was achieved in the case of 4-methoxy
benzaldehyde, which also explains the lower yield of the
corresponding homoallylic alcohol in this case (Table 2).
When the white material was calcined at 100 1C for 6 h,
the transformation of Zn(OH)2 to ZnO (JCPDS 36-1451) was
Table 2 Substrate scope of Zn(0) promoted allylation reaction
R
Time (h)
Product
Yield (%)
Cl
CF3
H
Me
OMe
6
6
10
12
14
1a
1b
1c
1d
1e
82
85
66
58
52
Reaction condition: 1 mmol aldehyde, 0.4 ml of allyl bromide, Zn0
(70 mg), and 300 mg of NH4Cl were added to 2 ml of THF and 3 ml of
water and stirred at room temperature.
Fig. 4 The XRD pattern of the zinc material after the Zn(0) promoted
allylation reaction, (a) as-prepared white material; (b) white material after
calcination at 100 1C for 6 h; white material after calcination at 100 1C for 24 h.
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Fig. 6 TEM/HR-TEM image of ZnO-1.
Fig. 5 The XRD pattern of the as-obtained white material (ZnO-1) after
calcination at 300 1C for 6 h.
hanging bonds and charged ions/groups. Generation of
enhanced surface charge is possible after dispersion in an
aqueous medium due to the possible surface ionization and
adsorption of ions. To obtain insight into the surface charge of
the synthesized ZnO nanomaterials, the zeta potentials of the
powdered samples suspended in nanopure water were mea-
sured. The zeta potential of the ZnO-1 material is found to be
+10.6 mV, which suggests a slightly positive charge at the ZnO
surface. Based on the Derjaguin–Landau–Verwey–Overbeek (DLVO)
model, agglomeration of uncapped nanocrystals depends upon
the repulsive interaction arising from electrostatic force and the
van der Waals force of attraction.13 As the surface charge of the
nanoparticles (NPs) mainly influences the electrostatic repulsive
force, NPs with a larger zeta potential will generally reduce the
hydrodynamic size. In this case, a slightly positive charge on the
ZnO surface will contribute less to the electrostatic repulsive
force, so a larger hydrodynamic size of the synthesized ZnO-1
material is expected. In fact, the measured average hydro-
dynamic size of the ZnO samples suspended in nanopure water
is found to be 600 nm. On the other hand, the zeta potential of
the ZnO-2 material is found to be +1.2 mV, which suggests a
observed as the XRD peaks matched well with the standard
XRD pattern of both the samples. Interestingly, higher conver-
sion of Zn(OH)2 to ZnO was achieved by heating the sample for
a longer time. Although we successfully utilized the waste zinc
material in the allylation reaction as a useful ZnO material, we
failed to obtain a good crystalline ZnO material even after
calcination of the white material at 100 1C for a prolonged
time (Fig. 4). So, the white waste material was further calcined
at 300 1C to obtain a good crystalline ZnO material. Gratifyingly,
the XRD analysis of the white material after calcination at
300 1C for 6 h reveals the hexagonal wurtzite structure of ZnO
as all the diffraction peaks match with the standard data for the
hexagonal wurtzite phase of ZnO (JCPDS 36-1451). Finally, we
were successfully able to convert the corresponding waste
material of Zn(0) promoted allylation reaction to a useful
hexagonal wurtzite phase of ZnO. The XRD data show the
presence of both Zn(OH)2 and ZnO in the material as most of
the peaks matched well with the standard XRD pattern of both
the samples (Fig. 5).
The as-obtained waste Zn material after heating at 100 1C for
24 h (hereafter ZnO-1) and after calcination at 300 1C (hereafter
ZnO-2) was used for further characterization and application.
The HR-TEM images of the synthesized ZnO-1 at different
magnifications revealed fibrous morphology with several
nanorods wedged into a bundle. We also analyzed the lattice
fringes of the nanocrystal (Fig. 6); the lattice spacing between
two planes was observed to be 0.28 nm, corresponding to the
distance of two (100) planes of ZnO. The optical properties of
both the nanomaterials were investigated by DRS experiments.
The band gap calculation of the synthesized materials was done
using the plot of (aEp)2 versus Ep based on the direct transition
and the extrapolated value of Ep at a = 0.
The observed band gap values of ZnO-1 and ZnO-2 nano-
materials are found to be 3.22 and 3.15 eV, which are slightly
smaller compared to bulk ZnO (3.3 eV), maybe due to the
oxygen vacancies or crystal lattice defects in the synthesized
materials.12 The nanomaterials have large surface to volume
ratios, and their surface may consist of several uncompensated
Fig. 7 Nitrogen adsorption–desorption isotherm of ZnO-1.
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Fig. 8 Nitrogen adsorption–desorption isotherm of ZnO-2.
Fig. 10 ZnO-1 promoted adsorption of methyl red (MR) at room
temperature.
slightly positive charge at the ZnO surface. The nitrogen adsorp-
tion–desorption isotherm and the pore size distribution of ZnO-1
and ZnO-2 nanomaterials are shown in Fig. 7 and 8.14 ZnO-1 is
found to be more porous compared to ZnO-2 because high
temperature calcinations transform the amorphous ZnO-1 to
more crystalline ZnO-2 with a reduced surface area and pore
volume. Next, the adsorption activity of both the materials was
investigated using various dyes.
biodiversity. Therefore, understanding and developing effective
technologies for wastewater treatment is environmentally
important. To date, various physical and chemical paths
including adsorption on activated carbon, reverse osmosis,
coagulation by chemical agents, visible or UV light photo-
catalysis, chlorination, ozonolysis, and oxidation processes for
wastewater treatment have been reported.17 Towards the search
for a cost effective route for the removal of dyes from waste-
water, the waste ZnO-1 nanomaterial was successfully utilized
for the color dye removal.18 The as-obtained ZnO-1 nanomaterial
was found to be active towards the adsorption of methylene blue
(MB), Congo red (CR), and methyl red (MR). The aqueous solution
of Congo red shows three peaks at 495, 340, and 237 nm with
extinction coefficients (e, molÀ1 L cmÀ1) of 20 300, 14 000, and
17 000, respectively. Upon addition of waste ZnO-1 in aqueous
solution of CR, a gradual decrease in the absorbance of all the
three peaks was observed, which indicates the adsorption of CR
on the surface of the ZnO-1 nanomaterial.
Application of waste zinc for adsorption of dye molecules
The textile, paper, plastic, tannery, and paint industries con-
sume large quantities of water and produce large volumes of
wastewater in different steps of the dyeing and finishing
processes.15 The wastewater is often rich in color, contains
various dyes and chemicals, and poses a direct threat to water
bodies and the ecosystem by causing non-aesthetic pollution
and eutrophication.16 In addition, dyes are commonly toxic and
some are carcinogenic and mutagenic and if discharged into
water, they can cause deterioration in water quality and some-
times food chain contamination resulting in adverse effects on
Although we observed faster adsorption initially, over time
the adsorption process became slower, which may be due to the
low availability of the active pores and repulsion between
adsorbed CR and incoming CR. The deep red color CR solution
faded away with time and the colorless ZnO-1 adsorbent turned
Fig. 11 Digital images of Congo red (CR), methyl red (MR), and methylene
blue (MB) with increasing adsorption time and the color of the ZnO-1
nanomaterial after adsorbing different dyes.
Fig. 9 ZnO-1 promoted adsorption of Congo red (CR) at room
temperature.
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Table 3 Adsorption activity of both ZnO and Zn(OH)n for various dyes
deep red, which also indicates the complete adsorption of
negatively charged CR molecules on the slightly positive surface
of the synthesized ZnO-2 nanomaterial (Fig. 9).19 The adsorp-
tion activity of the synthesized ZnO-1 for MR was found to be
slightly sluggish, maybe due to the less electrostatic interaction
between neutral MR (Fig. 10) and the slightly positively charged
ZnO-1 surface compared to CR and ZnO-1. On the other hand, it
took almost 20 days for complete adsorption of aqueous MB
solution on the ZnO-1 surface. This negligible adsorption
activity of ZnO-1 for MB is due to the electrostatic repulsion
between cationic MB and positively charged ZnO-1 (Fig. 11).
The adsorption capacities of both the synthesized ZnO-1 and
ZnO-2 nanomaterials for various dyes were calculated and are
summarized in Table 3. The observed adsorption capacity of
ZnO nanomaterials for anionic CR is found to be higher
compared to that for neutral MR and cationic MB, which also
justifies the more electrostatic attraction between two oppo-
sitely charged CR and ZnO surfaces. On the other hand, the
adsorption activity of the as-obtained waste ZnO-1 nanomaterial is
found to be higher compared to ZnO-2. The higher adsorption
activity of ZnO-1 compared to ZnO-2 is attributed to the larger
surface area and surface volume and the slightly more positive
charge in the former case, which make it feasible for more attraction
between ZnO-1 and dye molecules. Both the nanomaterials were
Congo Methyl Methylene
#
red
red
blue
Adsorption capacity (mg gÀ1) of ZnO-1 53.7
Adsorption capacity (mg gÀ1) of ZnO-2 38.7
45.5
30.7
29.0
21.2
Fig. 12 Plot of percentage of adsorption versus number of cycles for
ZnO-1 and ZnO-2 for the adsorption of CR.
Fig. 13 Linear plot for the adsorption of CR onto ZnO-1: (a) pseudo-first-order kinetic plot and (b) pseudo-second-order kinetic plot for the adsorption
of CR onto ZnO-1, (c) Langmuir and (d) Freundlich isotherms.
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Fig. 14 Non-linear plot for the adsorption of CR onto ZnO-1: (a) pseudo-first-order kinetic plot and (b) pseudo-second-order kinetic plot for the
adsorption of CR onto ZnO-1, (c) Langmuir and (d) Freundlich isotherms.
found to be inactive for the adsorption of other dyes like then performed using a fresh CR solution under optimized
rhodamine B, crystal violet, and methyl green. For practical reaction conditions.
applications of such heterogeneous ZnO nanomaterials for
In terms of adsorption activity, the ZnO-1 nanomaterial
wastewater treatment, the lifetime of the materials and their could be reused at least 6 times without any change in activity.
reusability are very important factors. For this, the adsorption However, a slight drop in the activity of the ZnO-1 nanomaterial
of aqueous CR solution promoted by the as-obtained ZnO-1 was observed in cycles 7–9. The XRD analysis of the used ZnO
nanomaterial was studied. After the completion of the first nanomaterial after the 7th cycle was performed. No appreciable
reaction, the ZnO nanomaterial was carefully separated change was observed in the XRD spectrum, which suggests the
through centrifugation, washed with acetone and methanol robustness of the material and it can be used potentially for
3–4 times to remove the adsorbed dye from the ZnO surface wastewater treatment as an adsorbent.20 On the other hand, in
and dried for reutilization as an adsorbent. A new reaction was terms of reusability, ZnO-2 is found to be less suitable for the
Table 4 Linear and non-linear adsorption kinetics parameters of ZnO-1 with Congo red (CR)
Linear
Value
Non-linear
Value
Model
Standard error
Standard error
Pseudo first order
K1 (minÀ1
)
9.4 Â 10À4
0.03769
0.30746
—
1.56
52.16
0.999
3.33753
1.28528
—
qe (mg gÀ1) [cal]
49.76
R2
0.9977
Pseudo second order
K2 (minÀ1
)
6.62 Â 10À4
38.85
0.9797
0.58784
0.27761
—
0.341
43.15
0.9802
0.68867
0.29636
—
qe (mg gÀ1) [cal]
R2
Experimental qe
53.72 mg gÀ1
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Table 5 Comparison between two isotherm models for Congo red (CR) adsorption onto ZnO-1 using linear and non-linear fits
Linear
Value
Non-linear
Value
Model
S.E.
S.E.
Langmuir isotherm
b (L mgÀ1
)
0.171
74
0.9950
0.20655
0.00344
—
0.2202
28.8
0.9917
0.32466
0.00765
—
qm (mg gÀ1
)
R2
Freundlich isotherm
KF ((mg1Àn Ln) gÀ1
)
0.894
7.02
0.9764
0.00833
0.455252
—
2.3125
7.02
0.98044
0.02065
0.89521
—
N
R2
adsorption of CR. Further to know the reaction mechanism, the is related to binding strength, KF is a rough indicator of the
kinetics of ZnO-1 nanomaterial promoted dye adsorption was adsorption capacity, and n is the adsorption intensity. The accuracy
studied using CR (Fig. 12).
of adsorption parameters is compared in both linear and non-linear
isotherm models to establish a suitable equilibrium correlation.
All the fitting parameters derived from both the Langmuir and
Kinetics of dye adsorption
Both the pseudo-first-order (1) and pseudo-second-order (2) Freundlich adsorption isotherms are shown in Table 5. The stan-
kinetic models were employed to study the adsorption kinetics dard errors and correlation coefficient (R2) of each plot were used to
of the ZnO promoted CR removal process. In the following measure the goodness-of-fit (Table 5 and Fig. 14). When judged in
equations, qe and qt (mg gÀ1) are the amount of the adsorbate terms of correlation coefficient (R2) and standard error of each plot,
adsorbed at equilibrium and at any time t (min), respectively, a better correlation was found in the Langmuir model compared to
whereas k1 (minÀ1) and k2 (g mgÀ1 minÀ1) are the pseudo-first- the Freundlich model in both non-linear and linear analyses,
order and pseudo-second-order adsorption rate constants (Fig. 13): possibly confirming the occurrence of monolayer adsorption of
CR molecules onto the internal and external surfaces of ZnO-1.
ln(qe À qt) = ln qt À k1t
(1)
(2)
2
t/qt = 1/K2qe + t/qe
Conclusions
The adsorption kinetics of CR with the ZnO nanomaterial was
monitored in 5 ml aqueous solution with varying amounts of In summary, the reaction mechanism and the effect of water,
CR and ZnO at different time intervals by UV-vis spectroscopy. reaction pH, additives, and aldehydes in zinc(0) promoted Barbier
The linear analysis of isotherm data in isotherm models is an type allylation of aldehydes were investigated from suitable experi-
alternative mathematical approach to predict the overall mental and theoretical studies, which suggested that (i) a slightly
adsorption behaviour. However, linear analysis for modelling acidic pH in saturated aqueous NH4Cl and probable coordination of
of isotherm data might cause inconsistency between the pre- water or ammonia with the intermediate and transition states will
dictions and experimental data. So, to obtain a better set of facilitate the reaction by lowering the free energy of activation (DG#)
results, both the linear and non-linear isotherm models have and (ii) DG# in electron withdrawing aldehydes is found to be lower
been studied for describing the number of adsorption parameters. compared to electron rich aldehydes. The as-obtained waste Zn
The collected data were fitted in both the pseudo-first-order and material from the allylation reaction was successfully utilized as a
pseudo-second-order kinetic models using both linear and non- reusable material for the adsorption of dyes like Congo red, methyl
linear methods (Fig. 14).21 When judged in terms of correlation red, and methylene blue. At the same time, the conversion of the
coefficient (R2), a better correlation has been found in the pseudo- as-obtained waste Zn material to the hexagonal wurtzite phase of
first-order (1) model in both linear and non-linear fittings ZnO at the gram scale was performed using a cost effective,
(Table 4). However, the calculated qe from the non-linear pseudo- sustainable, and green procedure. The kinetic study on waste Zn
first-order model is found to be closer to the experimentally promoted dye adsorption shows monolayer adsorption of CR
observed qe, which also indicates that the adsorption process of molecules onto the internal and external surfaces of waste Zn.
CR follows the pseudo-first-order model (Table 4). Further, to study The as-obtained waste Zn nanomaterial is a competent candidate
the interaction between adsorbates and adsorbents and the mecha- for wastewater treatment because of its easy synthesis, effortless
nism involved in the process both the Langmuir (3) and Freundlich separation, efficient recycling and good adsorption capability for
(4) adsorption models are employed (Fig. 14).22
various dye molecules.
1/qe = 1/qm + 1/Ceqmb
(3)
(4)
Experimental section
ln qe = ln Kf + (1/n)ln Ce
In the above equations, qm (mg gÀ1) is the maximum adsorption
capacity corresponding to a complete monolayer coverage, Ce and qe
Typical procedure of Zn(0) promoted allylation reaction of
aldehydes and isolation of the ZnO-1 material
are the equilibrium concentration and the amount adsorbed at In a 25 ml round bottom flask, 4 mmol 4-chlorobenzaldehyde,
equilibrium (mg gÀ1), b is the equilibrium constant (L mgÀ1) and it zinc dust (260 mg), and 0.8 ml of allyl bromide are taken in a
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mixture of 5 ml of water and 5 ml of THF. To this, 350 mg of References
ammonium chloride is added and the mixture is subjected to
1 (a) Y. Yamamoto, Acc. Chem. Res., 1987, 20, 243; (b) C. J. Li,
Chem. Rev., 1993, 93, 2023; (c) Y. Masuyama, Adv. Met.-Org.
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vigorous stirring overnight at room temperature. The gray color
zinc metal turns into a milky white material after stirring
overnight. The milky white precipitate is washed with distilled
water and THF 4–5 times under centrifugation, and dried in an
oven for 24 h at 100 1C and collected for further study.
General procedure of Zn(0) promoted allylation reaction with
different aldehydes
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room temperature for the required time. After completion of
the reaction (vide TLC monitoring), the product is isolated with
10 ml of ethyl acetate and the conversion of the desired product
is analyzed by the GC-MS study.
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Computational details
All the calculations were performed using the Gaussian 09 suite
of programs.23 The geometries of all the reactants and transi-
tion states (TSs) were optimized at the B3LYP level of theory
employing the effective core potential (ECP) along with valence
basis sets (LANL2DZ) for the Lewis acidic transition or main
group metal and the 6-31G* basic set for other atoms. The free
energy of activation of all the reactions was calculated at the
same level of theory with zero-point energies (ZPE) and thermal
corrections at 298 K. The free energy of activation in tetra-
hydrofuran (THF) was determined by frequency calculations in
a single point run of the optimized gas phase geometry in
solution phase using a polarized continuum model (PCM)24
employing THF as the solvent.
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Acknowledgements
Financial support of this work by DST-Inspire (Grant no. IFA-12-
CH-39) is gratefully acknowledged. We are also thankful to
Professor R. C. Deka and Dr P. Bharali for access to the BET
analyser. SP is highly grateful to Professor M. K. Chaudhuri for
all the help, support and inspiration.
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