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Chemical Science & Engineering Research
Research Article
Catalytic Reduction of p-Nitrophenol and Carbonyl Compounds by NiO-Nanoparticles Fastened Graphene Oxide
Somasundaram Saravanamoorthy,a Elayappan Vijayakumar,b Savariraj Jemimahc and Andivelu Ilangovan*a
*Corresponding author E-mail address: ilangovan@bdu.ac.in (A. Ilangovan)
Abstract: Owing to high activity and cost-effectiveness, NiO nanocomposites have been utilized in wide range of catalytic applications. In the present study, ultrafine NiO nanoparticles with average size of 7.5 nm were uniformly dispersed on graphene oxide by simple mix and heat method (Ni-oxide/GOSs). TEM and AFM results revealed the uniform dispersion of the Ni-oxide nanoparticles on the graphene oxide surface. Interaction between NiO nanoparticles and GO sheets were studied by Raman. SEM-EDS and XRD were used to study the Ni-loading and crystalline properties of Ni-oxide/GOSs. After being optimized, the Ni-oxide/GOSs catalyst was used for the reduction of 4-nitrophenol and carbonyl compounds. The reduction of 4-nitrophenol by Ni-oxide/GOSs was found to be very rapid and selective. An excellent kapp value of 60.8 ×10-3 min-1 was determined from the plots of ln[Ct/C0] versus reaction time for the reduction of 4-nitrophenol over 1.0 mg of Ni-oxide/GOSs. To our delight, the Ni-oxide/GOSs showed excellent activity in transfer hydrogenation of carbonyl compounds. Results confirmed that the Ni-oxide/GOSs catalyst is highly reusable in both reductions of 4-nitrophenol and carbonyl compounds.
Keywords: Graphene oxide; NiO-nanoparticles; Reduction; Nitrophenol; Carbonyl compounds
Publication details: Received: 19th July 2019; Revised: 31st July 2019; Accepted: 31st July 2019; Published: 01st August 2019
1. Introduction
In recent years, graphene-based nanocomposites have gained huge attention due to their extensive applications in various fields, including drug delivery, energy, adsorption, sensors, electrochemistry, environmental protection and catalysis.[1] Graphene is a 2D-sheet of sp2-hybridized conjugated carbon atoms with an extended honeycomb network structure.[2,3] High surface area, chemical inertness, fine size, conductivity and great mechanical strength make graphene an ideal ‘raising star’ material for catalysis, energy storage and conversion.[4,5] Recently, many researchers have focused on the graphene-based composites for catalysis, especially for graphene-transition metal oxide composites because they can combine the advantages of two components and offer special properties through the reinforcement or modification of each other.[6,7] Generally, due to high stability and reusability, metal-oxides are often preferred to prepare composites with carbon nanotubes, graphene, polymers and organic materials and so on.[8-12] In recent years, the graphene based nanocomposites are being frequently tested as catalyst for the removal or conversion of organic pollutants present in the wastewater. With economic globalization, phenolic wastewater pollution has become one of many strategic issues.[13,14] Without any doubt, the nitrophenols are a common organic pollutant found in environmental wastewater.[15] Carcinogenesis, hepatotoxicity and mutagenesis nature of nitrophenols cause various side effects to the human.[16,17] Since the nitrophenols are very stable, it is very important to remove from the environmental wastewater. Hence, the removal or conversion of nitrophenols has gained increasing attention over the past few decades. Traditionally, the nitrophenols were removed from wastewater by several environmental unfriendly or expensive methods such as electrocoagulation, adsorption, microbial degradation, electrochemical treatment, electro-Fenton method, and so on.[18-20] Conversion of nitrophenols to aminophenols in the presence of metal catalysts with NaBH4 is a highly efficient method. Aminophenols can be utilized for the manufacture of photographic developers, hair-dyeing agents, and analgesics.[21,22] Moreover, aminophenol is an important intermediate in the preparation of dyes, pigments, photographic developers, agrochemicals and pharmaceuticals.[23] Till date, a lot of efforts have been devoted to the development of the catalysts for efficient reduction of nitrophenol under mild conditions.[24] Nickel oxide (NiO), one of the most important transition metal oxides, has been intensively studied and widely used for various applications.[25-27] NiO-catalysts prepared via hydrothermal and solvothermal methods, precipitation, sol–gel method have been reported.[28,29] Very recently, a simple and reducing or capping agent free ‘mix and heat’ method for the preparation of metal-oxide/graphene composites is developed.[27]
It was found that the nanocomposites prepared via mix and heat method showed excellent catalytic activity in various organic reactions. Hence, we presume that NiO/Graphene composite prepared via mix and heat method would show superior catalytic activity in reduction of nitropehnol reaction. In the present study, a NiO-supported graphene oxide nanosheet (Ni-oxide/GOSs) was prepared by mix and heat method. The Ni-oxide/GOSs was characterized by transmission electron microscopy (TEM), atomic force microscopes (AFM), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), Raman, X-ray diffraction (XRD), brunauer–emmett–teller (BET) and X-ray photoelectron spectroscopy (XPS). Remarkable catalytic activity of the Ni-oxide/GOSs in the reduction of 4-nitrophenol was found out. Similarly, catalytic reduction of carbonyl compounds was also demonstrated. Reusability of Ni-oxide/GOSs in both reductions of 4-nitrophenol and carbonyl compounds was tested.
2. Experimental Section
2.1. Materials and Characterization
Graphene oxide nanosheets (GOSs) were purchased from Sigma-Aldrich, USA. Carbonyl compounds, Ni(acac)2 (97%), sodium borohydride (NaBH4) and 4-nitrophenol were purchased from Sigma Aldrich or Wako Pure Chemicals, Japan, and used as received.
Transmission electron microscope (TEM, JEOL JEM-2100F) was operated at the accelerating voltage of 200 kV to record the TEM images of Ni-oxide/GOSs catalyst. The 1D atomic force microscopic image of Ni-oxide/GOSs catalyst was captured using Park System model XE100 AFM in a non-contact mode. Scanning electron microscope-energy dispersive spectroscope (SEM-EDS, Hitachi 3000HSEM), Raman spectroscopy (Hololab 5000, Kaiser Optical Systems Inc., USA), powder X-ray diffraction (XRD) Rotaflex RTP300 (Rigaku Co., Japan) and inductively coupled plasma-mass spectrometer (ICP-MS, 7500CS, Agilent) were used to study the Ni-oxide/GOSs catalyst. XRD experiment was performed at 50 kV and 200 mA. X-ray photoelectron spectroscopy (XPS) was recorded on Kratos Axis-Ultra DLD, Kratos Analytical Ltd, Japan. Ultraviolet-visible (UV-vis, Shimadzu UV-2600 spectrophotometer) was used to test the catalytic efficacy of the Ni-GO composites. Gas chromatograph (GC) with a Shimadzu GC-2014 was performed to evaluate the yield of the catalytic products.
2.2. Preparation of Ni-oxide/GOSs Catalyst
To our delight, the present preparation method is very simple and required no reducing agent or capping agent. In a typical preparation method, a mixture of Ni(acac)2 (100 mg), GOSs (200 mg) and methanol (10 mL) was stirred under air atmosphere at 50°C for 3 h.
Then the mixture was dried and the slurry was mixed will with the help of mortar-pestle for 30 min. Finally, the mixture was calcinated under N2 atmosphere at 400 °C for 3 h. The obtained Ni-oxide/GOSs catalyst was further characterized.
2.3. Reduction of 4-Nitrophenol
A mixture of 4-nitrophenol (80 μL, 0.01 M) and NaBH4 (4 mL, 0.015 M) was stirred at 27 °C. Followed by, 0.5 mg or 1 mg of Ni-oxide/GOSs catalyst was added to the above mixture and stirred under open air atmosphere for 30 sec. The absorption spectra were recorded and the conversion of 4-nitrophenol was calculated.
2.4. Reduction of Carbonyl Compounds
In a typical procedure, a mixture of Ni-oxide/GOSs catalyst (5.0 mg), substrate (1.0 mmol), 2-propanol (4 mL), and NaOH (2 mmol) was refluxed at 82 °C. The yield of the catalytic products was calculated by using GC. After the reaction, the Ni-oxide/GOSs catalyst was separated out from the reaction mixture to test its reusability nature. Yield of the catalytic product, conversion and selectivity were calculated by using the equations (1), (2) and (3), respectively.
GC yield (%) = % of product formed (1)
GC conversion (%) = 100 – % of reactant remains (2)
Selectivity (%) = 100 – (conversion – yield) (3)
3. Results and Discussions
3.1. Catalyst Characterization
The prepared Ni-oxide/GOSs were analyzed by TEM and AFM in order to study its surface morphology. Fig. 1 shows the TEM and AFM images of Ni-oxide/GOSs. The TEM results show that the NiO nanoparticles are spherical in shape, homogenously dispersed, and strongly attached with the GOSs surface. The average size of the NiO nanoparticles was found to be 7.5 nm. Moreover, the TEM images confirm that the GO sheets are not aggregated and highly maintained the single layer nature of the GO.[30,31] AFM 1D image and its corresponding three-dimensional (3D) projection are shown in Fig. 1d and 1e. Well incorporation of NiO on the GOSs surface was confirmed by AFM images. AFM images of Ni-oxide/GOSs confirmed the decoration of two different sizes of NiO nanoparticles; smaller nanoparticles with ~2 nm and the bigger nanoparticles with ~10 nm. The results are in good agreement with the TEM images.
The Raman spectra of pure GOSs and Ni-oxide/GOSs are shown in Fig. 2. It can be clearly observed that there are two intense peaks at 1365 and 1580 cm–1 in both samples, corresponding to D and G bands, respectively. The calculated ID/IG value of GOSs and Ni-oxide/GOSs was 0.86 and 0.93, respectively. The higher ID/IG value of Ni-oxide/GOSs compared to GOSs confirmed the presence of more defect sites in Ni-oxide/GOSs.[32] The high ID/IG value may be due to two main reasons: (a) creation of more defect sites due to the mechanical grinding, and (b) strong attachment of NiO nanoparticles. Taking TEM and AFM results into the discussion, it is very fair to claim that the defect site is mainly due to the strong attachment of NiO with GOSs.
Fig. 4 presents the representative SEM and corresponding EDS spectrum of Ni-oxide/GOSs. Weight percentage (wt %) of C, O and Ni in NiO/GOSs was found to be 65.1, 25.5, and 9.4 respectively. Interestingly, no other elements or impurities except C, O and Ni were detected, which show the reliability of the present method and high purity of the present catalyst Ni-oxide/GOSs.[33] The elemental mapping of C, O and Ni demonstrate the homogenous dispersion of Ni-oxide nanoparticles on the surface of GOSs.
X-ray photoelectron spectroscopy (XPS) was performed to gain information of elements present in the Ni-oxide/GOSs composite. As shown in Fig. 3, the survey spectrum of Ni-oxide/GOSs mainly confirms the presence of C, O, and Ni species (Data not shown). The major peak of the Ni 2p XPS spectra can be assigned to the Ni 2p3/2 (~ 856.0 eV). The deconvoluted Ni 2p3/2 XPS peak at around 854.2 eV is from Ni2+ and is associated with the Ni-O octahedral bonding of cubic rock salt NiO. Moreover, satellite peaks were noticed at 861 eV (Ni 2p3/2 peak) which is due to shakeup process in the NiO structure. The information’s clearly revels that NiO is well supported on the GOSs surfaces.
The XRD pattern of Ni-oxide/GOSs is showed in Fig. 5. The two diffraction broad peaks at 2q = ~25° & 44° which attributed to the (002) plane and (100) planes are indicates hexagonal graphite structure.[35] In general, peaks at 2θ = ~45° and 2θ = ~52° can be assigned to Ni-oxides such as NiO, Ni2O3 and NiO2. However, there is no peaks were noticed corresponding to NiO which may be due to the nano-crystalline nature of the NiO nanoparticles in Ni- oxide/GOSs.
3.2. Catalytic Conversion of 4-Nitrophenol to 4-Aminophenol
The catalytic activity of Ni-oxide/GOSs in the reduction of 4-nitrophenol was tested. The 4-nitrophenol reduction reaction in the presence of NaBH4 is very simple and stable process.[36] Conversely, the reduction process is highly restricted in the absence of metal catalysts due to the high kinetic barrier between the negative ions, nitro group of and BH4- ions.[37] The pure 4-nitrophenol demonstrated an intense UV adsorption band at ~315 nm.[38] Upon the addition of NaBH4, the adsorption bands of 4-nitrophenol were noticed to be red-shifted due to the formation of nitrophenolate ions. Similarly, the catalytic activity of the GO was also tested. But, unfortunately the reaction it was not going. To our pleasure, in the presence of NiO/GOSs, the 4-nitrophenol reduced very rapidly (Fig. 6a & b). The reduction products were confirmed by UV-vis (Fig. 6a and 6b). It was found that even a very low amount of catalyst (0.5, and 1 mg) is enough for the complete reduction of nitrophenols with high reaction rate. The rate of reaction increased with the amount of catalyst and reaction time. The NiO/GOSs showed an excellent activity towards the reduction of 4-nitrophenol. With 1 mg of the NiO/GOSs, the system required only 0–25 min for the reduction of both 4-nitrophenol. Increasing the amount of NiO/GOSs from 0.5 mg of catalyst, the NiO/GOSs required only 4 to 5 min of the reaction time to achieve the 100% conversion of 4-nitrophenol to 4-aminophenol.
The time-dependent UV-Vis spectra was used to study the reaction kinetics of the Ni-oxide/GOSs mediated reduction of 4-nitrophenol (Fig. 6c). In the present case, the support, graphene oxide, is inactive and the NaBH4 was used in excess concentration, hence, the adsorption of nitrophenol molecules on NiO is only considered. In was found that the ln(Ct/C0) and time plot is liner and obeys pseudo-first-order reaction kinetics. The kapp value was derived from slope of ln(Ct/C0) versus time liner curve. It was found that the NiO/GOSs is highly efficient with the rate constant (kapp) value of 60.3 × 10-2 mins-1. Tian and co-workers[39] prepared Ni nanoparticles supported reduced graphene oxide (Ni–RGO) catalyst was tested for the reduction of 4-nitrophenol and 2-nitrophenol. The Ni–RGO system required longer reaction time of 9 min for the complete conversion of 4-nitrophenol to 4-aminophenol (with kapp value of 5.57 x 10-3 s-1) when compared to the present Ni-oxide/GOSs system. Similarly, the catalytic activity of the present Ni-oxide/GOSs system can be compared over other heterogeneous catalysts such as Pt–Ni/RGO,[40] PdNiP/RGO,[41] NiNPs/Silica,[42] Ni/SNTs,[43] Ni/MC-950,[44] RGO/Ni,[45] Ru/C and Cu/C.[46]
3.3. Catalytic Reduction of Carbonyl Compounds
Metal catalyst mediate reduction of carbonyl compounds to their corresponding alcohols is an important reaction in both in industrial and fine chemical processes.[47] Although several systems are available to achieve this reaction, transition metals-catalyzed reduction of carbonyl compounds by using 2-propanol as H donor. It is very simple and these reactions can be performed under mild reaction conditions.[48] In the present study, the Ni-oxide/GOSs was used for the reduction of carbonyl compounds by using 2-propanol as H donor. In order to obtain best reaction conditions, several reactions under different conditions were carried out. Reduction of benzaldehyde to benzyl alcohol was taken as a model reaction.
Without catalyst, the system gave very low amount of the catalytic product. Among different catalyst amount, 10 mg was found to the optimal amount of the catalyst since it gave maximum yield of 93% of benzyl alcohol. Reaction temperature was also optimized. At room temperature, the system afford very low amount of the product. The temperature of 82°C was found to be the optimal temperature. Similarly, base played a crucial role in the catalytic conversion in the presence of Ni-oxide/GOSs. Various bases such as KOH, NaOH and K2CO3 were tested. However, the system with use of NaOH afford excellent yield of the product.
Using the optimized reaction conditions, scope of the system was extended. Table 1 shows the scope of the Ni-oxide/GOSs system catalyzed wide range of carbonyl compounds to their corresponding alcohols. Benzaldehyde was reduced in the presence of Ni-oxide/GOSs to yield 93% of benzyl alcohol with 100% selectivity (Table 1, entry 1). Alike, 95% of 1-phenyl ethanol was obtained from the reduction of acetophenone in the presence of 10 mg of Ni-oxide/GOSs (Table 1, entry 3). Aliphatic ketones were also worked well with the Ni-oxide/GOSs. The 2-octanal was reduced to 2-octaol in excellent yield of 90% (Table 1, entry 3). In the reduction of cyclohexanone to cyclohexanol, Ni-oxide/GOSs afford a good yield of 87% with high selectivity (Table 1, entry 4). Heterocyclic carbonyl compound, 2-acetylthiophene, was reduced to corresponding alcohol in good yield of 85% with high selectivity (Table 1, entry 5). Chemoselective nature of Ni-oxide/GOSs system was also tested. 4-Acetylbenzaldehyde was reduced selectively to 1-(4-(hydroxymethyl)phenyl)ethanone in the present catalytic system (Table 1, entry 6).
Overall, the present Ni-oxide/GOSs system found to be highly efficient and selective. This is due to its excellent morphology, high surface area, fine dispersion in the reaction medium, small NiO particle size and a good NiO-GOSs interaction.
Entry | Substrate | Product |
Time (min) |
Conversionb (%) |
Selectivityb (%) |
Yieldb (%) |
1 | 60 | 93 | 100 | 93 | ||
2 | 90 | 95 | 100 | 95 | ||
3 | 120 | 95 | 95 | 90 | ||
4 | 120 | 91 | 96 | 87 | ||
5 | 90 | 89 | 96 | 85 | ||
6 | 90 | 83 | 76 | 59 | ||
a Reaction conditions: Substrate (1 mmol), Ni-oxide/GOSs (10 mg), NaOH (2 mmol), i-PrOH (5 mL), 82°C. b Determined by GC analysis. |
3.4. Reusability of Ni-oxide/GOSs
Reusability is one of the hallmarks of the heterogeneous catalyst as it is very important in industrial applications. The present Ni-oxide/GOSs catalyst was reused in both reductions of 4-nitrophenol and carbonyl compounds. Fig. 7 shows the reuse test of Ni-oxide/GOSs catalyst in both reductions of 4-nitrophenol and benzaldehyde. About 98% of the catalytic product of 4-aminophenol was achieved even after 7th cycle. Similarly, after 7th cycle, the Ni-oxide/GOSs catalyst yielded a good 83% of benzyl alcohol. Moreover, the selectivity of the catalytic products was highly maintained.
4. Conclusions
In conclusion, Ni-oxide/GOSs nanocomposites was successfully prepared and used as catalyst for the organic reduction reactions. The simple ‘mix and heat’ method was found to be very efficient method to obtain the Ni-oxide/GOSs with excellent morphology. The merit of the Ni-oxide/GOSs was confirmed by its superior catalytic performance in the reduction of 4-nitrophenol to 4-aminophenol and reduction of carbonyl compounds. In the reduction of 4-nitrophenol catalyzed by 1.0 mg of Ni-oxide/GOSs, an excellent kapp value of 60.8 ×10-3 min-1 was determined. Similarly, the Ni-oxide/GOSs transformed a wide range of carbonyl compounds to their corresponding alcohols in an excellent yield and selectivity. The Ni-oxide/GOSs was found to be highly reusable in the reduction of 4-nitrophenol to 4-aminophenol and reduction of carbonyl compounds.
Acknowledgements
Dr. S. Saravanamoorthy is thankful to University Grant Commission (UGC-India) for providing Kothari Fellowship.
Conflicts of Interest
The authors declare no conflict of interest.
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