Selective nitration of phenol to: O -nitrophenol in the presence of metal-free reduced graphene oxide at room temperature

Article abstract of DOI:10.1039/d0nj00885k

We report here a reduced graphene oxide (RGO)-catalyzed ultra-efficient, highly selective and low temperature synthesis of o-nitrophenol using phenol and nitric acid in an equimolar ratio. The effects of various parameters such as concentration of reactants, type of catalyst, weight of the catalyst, solvent, temperature and time of reaction on the catalytic reaction have been explored. The optimum conditions for the catalytic nitration reaction have been observed using a 1 : 1 molar ratio of nitric acid to phenol in the presence of 50 mg RGO in dichloroethane at room temperature for 3 h with a turn over frequency (TOF) of 10.96 × 1017 molecules g-1 s-1 and a reaction rate of 12.17 × 1014 molecules mL-1 s-1. In order to explore the decrease in efficiency of the catalyst during reuse, both the parent catalyst and used catalyst have been characterized by XRD, Raman, SEM, EDX, BET and FT-IR. The large surface area of RGO promotes the catalytic activity of aromatic compounds via smooth and high conversion of products with fine regioselectivity. So this protocol opens up a new avenue for the selective synthesis of o-nitrophenol using reduced graphene oxide at room temperature. This journal is

Full text of DOI:10.1039/d0nj00885k

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Selective Nitration of Phenol to o-Nitrophenol in the Presence of
Metal Free Reduced Graphene Oxide at Room Temperature
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Sourov Mondal, Jaydeep Singh, Shikha Singh, Sambhav Vishwakarma, Kheyanath Mitra,
Archana Kumari, Rajshree Singh, Susanta K. Sen Gupta, Biswajit Ray*
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Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221005,
India (*Corresponding author’s email: biswajitray2003@yahoo.co.in)
Abstract
We report here reduced graphene oxide (RGO)-catalyzed an ultra-efficient, highly selective and
low temperature synthesis of o-nitro phenol using phenol and nitric acid in equimolar ratio. The
effects of various parameters such as concentration of reactants, types of catalyst, weight of the
catalyst, solvent, temperature and time of reaction on the catalytic reaction have been explored.
The optimum condition for catalytic nitration reaction has been observed using 1:1 molar ratio of
nitric acid to phenol in the presence of 50 mg RGO in dichloroethane at room temperature for 3 h
with the turn over frequency (TOF) of 10.96 x 1017 molecules g-1 s-1 and the reaction rate of
12.17 x 1014 molecules mL-1 s-1. In order to explore decrease in efficiency of catalyst during
reuse, both the parent catalyst as well as used catalyst has been characterized by XRD, Raman,
SEM, EDX, BET and FT-IR. Large surface area of RGO has promoted the catalytic activity in
aromatic compounds via smooth and high conversion of products with fine regioselectivity. So
this protocol opens up a new avenue for the selective synthesis of o-nitro phenol using reduced
graphene oxide at room temperature.
Keywords: Nitric acid, Reduced graphene oxide (RGO) as Catalyst, Nitration of phenol, Room
temperature
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Introduction
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Nitration method has been exploited extensively in the industries for the production of dyes,
pharmaceutical chemicals, agrochemicals, highly explosive materials etc1,2. Nitration of phenol
is one of the most studied basic nitration reaction.3-15 Different nitrating agents in the presence
and absence different catalysts have been reported for this reaction. As for example, mixture of
sulphuric acid and nitric acid was used as common nitrating reagent to produce a mixture of 2-
nitrophenol and 4-nitrophenol in a ratio of 1.42 apart from the formation of large amount of
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wastes3,4. Many metal nitrates such as Ca(NO3)3,5 Bi(NO3)2 .5H2O,6 Cu(NO3)2,7 NaNO3,
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Mg(NO3)2,9 VO(NO3)3, Fe(NO3)3,11 Zr(NO3)4,12 Y(NO3)3 etc. in combination with an extra
reagent such as solid acid or, ionic liquid or, phase transfer catalyst14,15 were also used for the
nitration of phenol via nitronium ion formation. Several other nitrating agents such as organic
nitrating compounds as well as polymer supported nitration salt14, concentrated nitric acid, acid
anhydrides/triflates16, nitrogen oxides17,have also been reported by several investigators. Use of
ionic liquids18, surfactants19, microemulsions20, ultrasound21 has also been demonstrated to
control the selectivity of the reaction. Nitration of phenol was also carried out using nitric acid in
the presence of different catalysts such as tetra butyl ammonium bromide21, sulfated titania22-23
,
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N2O5-ClNO2 , p-toluene sulfonic acid24, ZSM-5 Zeolite25, SnO2
etc. Little work has been
done over alumina-supported catalyst28. There is a necessity for the development of new class of
catalysts that could operate at room temperature. Recently, Prasad et al. introduced silica-
supported vanadium incorporated tungsto-phosphoric acid for nitration of phenol at room
temperature15.
Nontoxicity, abundance, recyclability, low cost and minimal ecological impact are
desirable property of green catalyst29. Bielawski et al. have elegantly reviewed the wide synthetic
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utility of heterogeneous carbon materials30. Carbocatalyst are those catalysts whose elemental
compositions are predominantly carbon on the basis of sustainability. Carbocatalysts like
graphene, fullerene, activated carbon etc. have extensive application in organic reactions as
active support.31 Nanocarbon has large specific surface area making surely most of consisting
atoms accessible for catalyst and it is ever been support to deposite the transition metal which is
more relevant catalytic center31. Main limitation with such carbocatalyst used in the synthesis of
organic compound is the highly unstable surface and for this reason recycle is minimum.
Another limitation is the very poor selectivity of the product. Moreover, it is difficult to explain
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the mechanism in support of the catalytic reaction31,32
.
In the present communication we report here the study of the catalytic property of the
reduced graphene oxide (RGO) for the nitration of phenol to ortho-nitrophenol at room
temperature (RT) using nitric acid as nitrating agent. The effect of different process parameters
such as type of catalyst, weight of catalyst, reaction time, reaction temperature, reactant
concentration, type of solvent has been studied comprehensively. In order to explore the catalytic
activity of RGO, surface properties of RGO have also been studied before and after catalytic
reaction.
Experimental
Materials
Sulfuric acid (98% assay with 99% purity Merck, India), nitric acid (70% assay with 99% purity,
Qualigens, India), phenol (Merck, India), phosphoric acid (Merck, India), 1,2-dichloroethane
(DCE) (SRL, India), dichloromethane (SRL, India), acetonitrile (Merck, India), ethanol (SRL,
India), ethyl acetate (SDFCL, India), carbon tetrachloride (Merck, India), hydrogen peroxide
(Merck, India), hydrazine hydrate (Loba Chemie, India), ammonium solution (Merck, India),
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were used as received. Natural graphite powder was received as gift from Dr. Sanjay Dakate of
National Physical Laboratory, New Delhi.
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Methods
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FT-IR spectra were recorded using a PERKIN ELMER spectrum version 10.03.05 Spectrometer
in the range of 400 to 4000 cm-1. The diffraction scattering patterns of the powdered samples
were recorded by a Bruker D8 Advance X-ray Diffractometer. Samples were grinded well before
measurement and diffractograms were recorded from 5o to 80o using Cu Kα radiation.
Morphology studies of the composites were performed using Scanning electron microscopy
(SEM) EVO 18 Research, Zeiss. Elemental analyses were performed using Energy Dispersive X-
ray Spectroscopy (EDX) Oxford instrument (X – act) connected to SEM. Surface area, pore
volume, pore size distribution and pore diameter were measured using a BET
QuantachromeAutosorb AS1WIN instrument (Quantachrome Instruments, USA). To analyze the
pore size, the sample is outgassed at 100°C for 5 h.
Synthesis of Graphene Oxide (GO)
Graphen oxide was synthesized by oxidation of graphite powder using an improved method.33 At
first 3.0 g graphite powder was taken in a 500 mL 2 neck round bottom flask fitted with a
condenser and a pressure equalizer. Typically, 400 mL 9:1 (v/v) mixture of concentrated H2SO4
/ H3PO4 was added drop wise via the pressure equalizer under vigorous stirring at 0⁰C
temperature. KMnO4 (18 g, 6 wt equivalents) to graphite) was added to the resultant mixture
producing a slight increase in temperature to 40⁰C. The reaction mixture was then heated to
50⁰C and stirred for 12 h. The final reaction mixture was cooled to room temperature and poured
into 400 mL ice cold water containing 3 mL 30% H2O2 .The resultant paste like product was then
washed with double distilled water and centrifuge successfully with 200 mL 5% HCl and then
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with deionized water for several times till the supernatant become neutral. After neutralization it
was washed with ethanol, followed by washing with deionized water and finally dried under
vacuum at 40⁰C. Yield = 4.2 g.
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Synthesis of Reduced Graphene Oxide (RGO)
100 mg GO was taken in 250 mL round bottom flask fitted with a water cooler condenser and
300 mL deionized water was added to it and placed in sonication bath for the exfoliation of
graphene oxide until it became no visible particulate matter. Hydrazine hydrate (1 mL, 32.1
mmol) was then added to this dispersion and heated in an oil bath maintained at 100⁰C for 24 h.
The resultant RGO was collected by filtration, washed with deionized water and finally with
acetone and dried under vacuum for 12 h at 40⁰C.
Typical Experimental Procedure for Nitration of Phenol (Table 1, Run 1)
Initially 50 mg RGO and 45 mL of 1,2-dichloro ethane (DCE) was taken in a 100 mL of beaker.
The beaker was then placed in a sonication bath and sonicated for 2 h to disperse RGO properly
in DCE. Thus, exfoliation was achieved, which is necessary to increase the surface area of RGO.
Then the properly dispersed RGO was transfered to 100 mL jacketed reaction vessel maintained
at 25oC. 0.09413 g (1 mmol) of phenol was added to the reaction mixture with vigorous stirring
for 5 min. To it, 32 L (1 mmol) of 71% nitric acid with added under starring and reaction
continued for 3h. Reaction of nitric acid with phenol was clearly noticeable via the change of the
color of the reaction mixture from blackish to pale yellow. After the completion of the reaction,
reaction mixture was then filtered through Nylon66 membrane filter paper (pore size 0.4 μm).
Finally product was obtained via the evaporation of solvent under reduced pressure. Formation
of the product was further confirmed by 1H NMR.
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OH
OH
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NO2
Catalyst
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Nitric Acid, DCE
o-Nitrophenol
Phenol
Scheme 1 Nitration of phenol
Regeneration of the Spent Catalyst
After the catalytic reaction, resultant mixture was filtered through a nylon filter paper and
catalyst left over filter paper was washed with deionized water 3 times using each time 50 mL
followed by washing with acetone 3 times using each time 10 mL. Finally, it was dried at 40oC
under vacuum for 24 h. This dried catalyst has been used as catalyst in recycling reaction.
Results and Discussions
Selection of Catalyst
In order study the efficacy of graphene oxide (GO) and reduced graphene oxide (RGO) as
catalyst for the nitration of phenol, three reactions were initially carried out using 1 mmol phenol
and 1 mmol nitric acid in 45 mL 1,2-dichloro ethane (DCE) solvent at 25oC for 3 h in the
absence of catalyst and in the presence of 50 mg of each GO and RGO, respectively, as catalyst.
The corresponding yields of nitrated phenol were 71, 15 and 98.3%, respectively (Figure 1). The
corresponding 1H NMRs of the products (Figures 2, S1 and S2, Supporting information) revealed
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that only selectively o-nitro phenol has been formed in all cases. HPLC study also supports this
result. Interestingly, reduced graphene oxide (RGO) showed highest (98.3%) catalytic activity,
while blank reaction and reaction with GO yielded 71 and 15 % coversion, respectively. So, we
have performed the rest of the nitration reactions of phenol using RGO for standardization.
Table 1. Effect of solvent type on the nitration of phenola
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Run
Solvent
DCE
Conversion (%)
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98.3
100
DCM
CH3COOET
ETOH
No reaction
No reaction
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CH3CN
CCl4
45
a using 50 mg RGO catalyst at 25⁰C for 3 h in 45 mL DCE
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RGO
Blank
Different catalyst
Figure 1. Effect of different catalysts
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a
OH
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NO2
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e'
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Figure 2. 1H NMR spectrum (in CDCl3 at RT) of the resultant reaction mixtures of the nitration
of phenol in the presence of RGO as catalyst.
Effect of the reaction time
To standardize the nitration of phenol using RGO as catalyst, we have further carried out the
nitration of phenol by decreasing the reaction time to 2 h keeping other conditions same like run
1 of Table 1 (Figure 3). The yield of o-nitro phenol decreases to 40%. So, the rest of the nitration
reactions were carried out for 3 h.
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Time(h)
Figure 3. Effect of reaction time
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Effect of the amount of catalyst
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Since the reaction conversion depends upon the amount of catalyst, we have also performed the
nitration of phenol using 10, 20, 30 and 40 mg RGO catalyst keeping other conditions remained
unchanged like run 1 of Table 1 (Figure 4). In the reaction using 10 mg of RGO, the conversion
was 44%, but with the increase in the weight of catalyst to 20 mg, the percentage of conversion
decreased to 35% and almost remained same up to 40 mg. Interestingly, the conversion increased
suddenly to 98.3% using 50 mg of RGO showing the highest catalytic activity. So, the rest of the
reaction was carried out using 50 mg RGO catalyst for 3 h.
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Conversion (%)
TOF
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Catalyst amount (mg)
Figure 4. Effect of the amount of catalyst
Effect of Solvent
In order to explore the effect of solvent, nitration reactions of phenol were carried in 45 mL of
different solvents e.g., dichloroethane (DCE), dichloromethane (DCM), ethyl acetate, ethanol,
acetonitrile and carbon tetrachloride using 50 mg RGO catalyst at room temperature for 3 h
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keeping other conditions same like run 1 of Table 1. The best solvent for the highest conversion
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(100%) of phenol into nitro phenol was found using DCM. Next best result (98.3%) was
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o
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obtained using DCE. As DCE has higher boiling point (~82 C) than DCM (~39.6 C), so the
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remaining reactions were carried out in DCE keeping other conditions same.
Effect of Reactant Concentration
The effect of nitric acid and substrate phenol molar ratios on its conversion using 50 mg RGO
catalyst at 25⁰C for 3 h in DCE is shown in Table 2. It clearly indicates that maximum
conversion (98.99%) is achieved at equal molar ratio of nitric acid to phenol followed by the
occurrence of 80% conversion using 1.50 molar ratio of the nitric acid to phenol. Therefore, the
rest of the reactions were carried out at 1:1 molar ratio of nitric acid to phenol.
Table 2. Effect of molar ratio of nitric acid to phenola
Nitric acid/Phenol(mole ratio)
Phenol Conversion (%)
0.50
22.12
24.23
98.99
80.01
69.66
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1.00
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2.00
a using 50 mg RGO catalyst at 25⁰C for 3 h in 45 mL DCE
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Effect of temperature
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In order to study the effect of temperature, we have also performed the nitration of phenol at 15
and 35oC keeping other conditions same like run 1 of Table 1 (Figure 5). It is observed that the
maximum conversion was observed at 25oC.
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Figure 5. Effect of Temperature
Effect of Recyclability
While studying catalytic recyclability under the same experimental conditions like run 1 of
Table 1, we have observed that product yields dropped to 47.1% conversion during the
second use (first recycle) of the catalyst in comparison to the fresh load with 98.3 %
conversion (Figure 6). When we reduced the used catalyst (after first time use) following the
reduction method of GO to make RGO and used it for catalytic study, we observed that the
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conversion become less (34%) than that (47.1%) observed in 1st recycle. Therefore, the
reduction of catalyst after use did not bring back its catalytic activity, instead, worsen it.
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1st recycle
1st recycle after reduction
Cycle number
Figure 6. Number of recycle test
Calculated Catalytic Efficiency
Turn over number, TON, by definition, is the number of moles/molecules of reactant
converted per unit weight of catalyst before becoming deactivated. For the present
heterogeneous catalytic reaction, the catalyst is becoming deactivated after single use of a
certain time interval. So, turn over frequency, TOF, which is TON per unit time, will give the
actual information about the efficiency of the catalyst.34
o
The calculated TOFs of this catalytic reaction of phenol (1mM) at the temperature of 25 C
considering 3h reaction time and assuming that catalyst become deactivated within this time
(although we have checked the reuse catalytic activity for only reaction using 50 mg catalyst)
and using 10, 20, 30, 40 and 50 mg RGO as catalyst are 24.50 x 1017, 9.76 x 1017, 6.51 x
1017, 4.88 x 1017, and 10.96 x 1017 molecules g-1 s-1, respectively (Table 3). The
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corresponding plot of TOF vs. catalyst amount has been shown in the Figure 4. It is clear
from the graph that although conversion is highest at highest catalyst loading (here, at 50 mg
RGO), the TOF is largest at the lowest catalyst loading (here, at 10 mg RGO). However, the
rate of the reaction, which is the product of TOF and catalyst concentration, was remained
constant at around 5 x 1014 molecules mL-1 s-1 at the use of less than 50 mg catalyst and
become 12.17 x 1014 molecules mL-1 s-1 at the use of 50 mg catalyst (Table 3). Therefore,
faster catalytic reaction was observed using 50 mg RGO as catalyst under the experimental
conditions. For GO catalyst, using 50 mg of it in place of RGO keeping other experimental
conditions the same, the calculated TOF and reaction rate are found at 1.67 x 1017 molecules
g-1s-1 and 1.86 x 1014 molecules mL-1 s-1. Both values for GO catalyst are considerably lower
than those of RGO catalyst.
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Table 3 Calculated TOFs and Reaction rates using different concentrations of catalyst
RGO Catalyst
amount (mg)
Conversion
(%)
TOFa
Reaction rate
(molecules g-1s-1)
(molecules mL-1s-
1
)
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20
30
40
50
44
35
35
35
98.3
24.5 x 1017
9.76 x 1017
6.51 x 1017
4.88 x 1017
10.96 x 1017
5.44 x 1014
4.34 x 1014
4.34 x 1014
4.34 x 1014
12.17 x 1014
a Calculated using the equation:
TOF= number of moles of phenol used x fraction conversion x 6.023 x 1023 / (catalyst
amount in g) x (reaction time in sec)
b Calculated using the equation:
Rate of the reaction = TOF x catalyst concentration in g/mL
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Characterization of catalyst
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In order to investigate the cause behind the decrease of activity after 1st use, we have
characterized the RGO catalyst before (RGO) and after its use (RGOU) using X-ray
diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy
(FESEM), BET, and FTIR studies [Figures 7 - 10 and Tables 4 - 5] . X-Ray diffraction study
reveals little shifting of the two characteristic peaks of RGO observed at 2 = 23.96 and
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42.97o to the higher values at 2 = 25.61 and 43.08o, respectively after use indicating
decrease in the interlayer displacing in used RGO [Figure 7(a)]. Raman spectroscopy study
shows the slight decrease in the ID/IG ratio from 1.08 to 1.06 [Figure 7(b)] after use
indicating the decrease in the ratio of sp3 to sp2 carbon owing to the occurrence of the
catalytic reaction.
Figure 7. (a) XRD of RGO before (RGO) and after use (RGOU); and (b) Raman Spectra of RGO
and RGOU
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FESEM study reveals the decrease in its roughness after use [Figure 8(a)-(b)]. The
corresponding EDX study reveals the decrease of carbon element content from 93.36 to
81.09% and increase of oxygen element content from 6.64 to 18.91% after use (Table 3).
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Figure 8. FESEM image of RGO (a) before and (b) after use.
Table 4 EDX results of reduced graphene oxide (RGO) before and after use (RGOU)
Compound
RGO
Element
C K
Weight%
93.36
O K
6.64
RGOU
C K
81.09
O K
18.91
Isothermal and pore size distribution plots [Figure 9(a)-(d)] related to the BET study of
RGO before and after use have confirmed that the BET surface area and pore volume
decreases considerably after its use. Surface area and pore volume of RGO changes from
265.2 m2/g and 0.089 cc/g to 9.6 m2/g and 0.013 cc/g after use although the average pore
size almost remained unchanged around 1.5 nm (Table 4).
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Figure 9. Nitrogen adsorption/desorption isotherm [(a) vs. (b)] and pore size distribution [(c)
vs. (d)] for RGO and RGO after use (RGOU), respectively.
Table 5 BET Surface areas and pore volumes of RGO before and after use (RGOU)
Sample
RGO
Surface
Pore
Volume(CC/g)
Pore size(A⁰)
15.287
area(m2/g)
265.213
0.089
RGOU
9.623
0.013
15.278
Moreover, FTIR study indicates suppression of the peaks of RGO at 3427, 1630, 1204, and
1084 cm-1 corresponding to stretching frequency of –OH of carboxyl group, CO stretching
frequency of carboxyl group, epoxy CO stretching, and CO stretching vibrations of COC
respectively after use (Figure 10). All these results indicate the changes in chemical composition,
architecture, and surface properties of RGO after catalytic use. However, the observed fall in
catalytic efficiency of RGO is presently not clear to us. A possible explanation is that C-C bond
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is breaking or undergoing oxidizing reaction under experimental condition and these are
responsible for the change in all these physical properties and its dramatic low catalytic activity.
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26
1204
1084
25
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RGO
4000
3500
3000
2500
2000
1500
1000
500
-1
)
Wave number (cm
Figure 10. FTIR of RGO before (RGO) and after use (RGOU).
A comparative table (Table S1, supporting information) showing the results of the
nitration of phenol using different catalyst systems has been made. It shows that almost 100%
selectivity of o-nitrophenol (o-NP) formation has been reported earlier by only metal-based
catalysts. Our present work using reduced graphene oxide is a metal-free catalyst system which
showed also such selectivity for o-NP. Thus, our present system is low cost and superior to other
catalyst systems reported earlier.
Conclusion
In conclusion, we have introduced here an eco-friendly catalyst RGO for the regioselective and
quantitative nitration of phenol to o-nitro phenol at room temperature. The optimum condition
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for catalytic nitration reaction is using 1:1 molar ratio of nitric acid to phenol in the presence of
50 mg RGO in dichloroethane at room temperature for 3 h with the turn over frequency (TOF) of
10.96 x 1017 molecules g-1 s-1 and the reaction rate of 12.17 x 1014 molecules mL-1 s-1. So this
protocol opens up a new avenue for the selective synthesis of o-nitro phenol using reduced
graphene oxide at room temperature.
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Acknowledgment
BR acknowledges the financial support from DBT (Govt. of India) through grant no.
BT/PR889/NNT/28/570/2011 and the Council of Science and Industrial Research, Government
of India, through Grant no. 02(0002)/11/EMR-II. SM gratefully acknowledges the financial
support from UGC-NET-fellowship. SV and AK gratefully acknowledge the financial support
from Banaras Hindu University through UGC-CRET-fellowship.
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Reduced graphene oxide catalyzed selective synthesis of o-nitro phenol from equimolar
mixture of phenol and nitric acid in dichloroethane at room temperature.