molecules Article
High Catalytic Efficiency of Nanostructured β-CoMoO4 in the Reduction of the Ortho-, Meta- and Para-Nitrophenol Isomers Fahd Al-Wadaani 1 , Ahmed Omer 1 , Mostafa Abboudi 1 , Hicham Oudghiri Hassani 1,2, *, Souad Rakass 1 , Mouslim Messali 1 and Mohammed Benaissa 3 1
2 3
*
Chemistry Department, College of Science, Taibah University, Almadinah 30002, Saudi Arabia;
[email protected] (F.A.-W.);
[email protected] (A.O.);
[email protected] (M.A.);
[email protected] (S.R.);
[email protected] (M.M.) Département Sciences de la Nature, Cégep de Drummondville, 960 Rue Saint-Georges, Drummondville, QC J2C 6A2, Canada LMPHE, Département de Physique, Faculté des Sciences, Université Mohammed V, B.P. 1014 RP, Rabat 10000, Morocco;
[email protected] Correspondence:
[email protected]; Tel.: +966-543-549-454
Received: 8 January 2018; Accepted: 7 February 2018; Published: 9 February 2018
Abstract: Nanostructured β-CoMoO4 catalysts have been prepared via the thermal decomposition of an oxalate precursor. The catalyst was characterized by infrared spectroscopy (FTIR), X-ray diffraction (XRD), Brunauer-Emmett-Teller method (BET), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). The efficiency of these nanoparticles in the reduction of ortho- and meta-nitrophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding aminophenols was tested using UV-visible spectroscopy measurements. It was found that, with a β-CoMoO4 catalyst, NaBH4 reduces 3-NP instantaneously, whilst the reduction of 2-NP and 4-NP is slower at 8 min. This difference is thought to arise from the lower acidity of 3-NP, where the negative charge of the phenolate could not be delocalized onto the oxygen atoms of the meta-nitro group. Keywords: nanostructures; β-CoMoO4 ; nitrophenol reduction; nanoparticles
1. Introduction Aminophenols are implicated in the synthesis of pharmaceutically active compounds [1]. For instance, p-aminophenol is an important intermediate in the preparation of several analgesic and antipyretic drugs, such as paracetamol, acetanilide, and phenacetin [2]. In addition, aminophenols are key ingredients in the production of metal-complex dyes and are used in corrosion inhibition, hair-dying agents, and polymer production [1,2]. Due to their diverse applications, the development of efficient processes for the synthesis of aminophenols has gained increasing attention. One method of synthesizing aminophenols is via the reduction of nitrophenols. Nanostructured transition metal molybdate materials have shown promise as catalysts for this process [3,4]. In particular, nanostructured CoMoO4 provides a non-toxic and inexpensive candidate for this process [5]. CoMoO4 , which may exist in several phases including a low temperature α-phase and a high temperature β-phase, [3,5–7] is used as a catalyst in many chemical [8] and petrochemical processes such as cracking, hydrogenation, dehydrogenation, and hydrodesulfurization (HDS) [6,9]. The catalytic properties of CoMoO4 depend on the structure of the material, which in turn depends on the method of preparation [10]. Various methods have been developed for the synthesis of CoMoO4 including precipitation, sol–gel, solid state reaction, hydrothermal, complete evaporation of a polymer-based metal-complex precursor solution, microemulsion and sonochemical [11–16]. Molecules 2018, 23, 364; doi:10.3390/molecules23020364
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Several materials were tested in the reduction reaction of the nitrophenol isomers. The ferrites materials CuFe2 O4 and NiFe2 O4 were studied by Goyal et al. and found that the copper containing ferrite was a better catalyst for these reactions [17]. Moreover, Nandanwar et al. investigated the efficiency of CuO/γAl2 O3 , which showed good catalytic activity [18]. Ni/C black composite material was also tested as a catalyst for this reaction by Xia et al., and low efficiency was observed [19]. Better results were found in a study conducted by Oudghiri on the iron molybdate Fe2 (MoO4 )3 [20]. Ghorai et al. worked with a mixed molybdate phosphate of cobalt, copper, and chromium that was were found to efficiently reduce 4-NP [21]. In recent years, the application of CoMoO4 to the catalytic degradation of organic dyes has started to receive increasing attention. For instance, CoMoO textsubscript4 nanoparticles prepared via the microemulsion method have been shown to be quite effective at eliminating reactive black (RB) 8 dye from aqueous solution, with a decolorization efficiency of 91.47% [12]. In this study, nanostructured β-CoMoO4 catalysts were prepared via the thermal decomposition of an oxalate precursor prepared in the solid state using a new method. The obtained powder was found to be formed of nanoparticles. The freshly synthesized β-CoMoO4 nanoparticles were tested as catalysts in the reduction of three aminophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding aminophenol isomers. 2. Material and Methods 2.1. Materials Cobalt nitrate (Co(NO3 )2 ·6H2 O), ammonium molybdate ((NH4 )6 Mo7 O24 ·4H2 O), and oxalic acid (H2 C2 O4 ·2H2 O) were obtained from Aldrich (St. Louis, MO, USA). All the chemicals were of analytical grade and used without further purification. 2.2. Catalyst Preparation The cobalt molybdate catalyst (CoMoO4 ) was prepared by thermal decomposition of an oxalate precursor in a tubular furnace (open on both sides) at 550 ◦ C for 2 h. The oxalate precursor was obtained by the solid state reaction of a well-ground mixture of cobalt nitrate (Co(NO3 )2 ·6H2 O), ammonium molybdate ((NH4 )6 Mo7 O24 ·4H2 O), and oxalic acid (H2 C2 O4 ·2H2 O) in the molar ratio 1:1/7:5, respectively, on a hotplate at 160 ◦ C for 1 h [22]. During heating, an orange/brownish gas is evolved, indicating the formation of NO2 gas due to the reduction of the nitrate anions. In the meantime, the mixture takes a dark blue coloration confirming the reduction of the molybdenum (+VI) to lower oxidation states (+V or +IV). The oxalic acid acted not only as a reducing agent but also as a complexing agent leading to the formation of the oxalate complexes of the existing metals [23,24]. The advantage of this preparation method is that it occurs in the solid state without adding any solvent. There is no factor to control other than the temperature of the precursor formation at 160 ◦ C and the heating temperature at 550 ◦ C. No strict conditions such as higher temperatures or pressures are needed. Moreover, the time of the preparation of the nanoparticles is limited to a few hours with a yield of almost 100%. All synthesis methods encountered in the literature require the use of solvents for the filtration and drying stages. In the solid-state reaction between MoO3 and Co3 O4 , higher temperatures as well as regrinding mixture and annealing stages are needed to obtain improved homogeneity. 2.3. Characterization In order to study the decomposition with temperature of the prepared oxalate precursor, thermal analyses were conducted on an SDT Q600 instrument (Ta Instruments, Hayesville, NC, USA). Infrared spectroscopy studies were conducted on a Shimadzu 8400S (Shimadzu, Tokyo, Japan), to confirm the oxalate complex formation. To verify the crystallinity and the purity of the prepared cobalt molybdate, the X-ray diffraction patterns were recorded on a Shimadzu X-ray diffractometer ´ equipped with a Ni filter. The patterns 6000 (Shimadzu, Tokyo, Japan), using Cu Kα radiation (1.5406 Å
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were recorded in the range of 10–80◦ in 2θ. Crystallites size was calculated using the Scherer formula DXRD = 0.9 λ/(B cosθ), where λ is the Cu Kα wavelength, B is the full width at half maximum (FWHM) expressed in radians, and θ is the Bragg angle. Particle sizes could also be estimated using the equation DBET = 6000/d.S, where d is the density and S is the specific surface area calculated from the adsorption/desorption isotherms that were obtained from a Micrometrics ASAP 2020 (Micromeritics, Norcross, GA, USA), surface area and porosity analyzer. The variation of the nitrophenol isomer (2-NP, 3-NP and 4-NP) concentrations during the catalytic transformation to their corresponding aminophenol isomers was measured using a Varian Cary 100 UV-visible spectrophotometer (Varian Inc., Palo Alto, CA, USA). Transmission electron microscopy (TEM) was performed using a Tecnai-12 electron microscope (FEI, Hillsboro, OR, USA), operated at 120 kV to investigate the shape and size of the particles. For the TEM measurement, the sample was prepared by simply grinding the powder between two glass plates and bringing the fine powder into contact with a carbon-coated copper TEM grid. Energy dispersive X-ray (EDX) microanalyses were obtained using an EDAX detector coupled to a TEM instrument. 2.4. General Procedure for the Reduction of Nitrophenol Isomers The prepared cobalt molybdate powder was tested as a catalyst in the reduction of three nitrophenol isomers (2-NP, 3-NP, and 4-NP) to their corresponding aminophenol isomers. In a typical test, 40 mL of an 8 × 10−4 M aqueous solution of sodium tetrahydroborate (NaBH4 ) were added to 40 mL of a 4 × 10−4 M aqueous solution of the nitrophenol isomer under continuous stirring at ambient temperature. The solution color immediately became dark yellow, with an absorption located at 415, 393, and 401 nm for the ortho-isomer (2-NP), the meta-isomer (3-NP), and the para-isomer (4-NP), respectively, due to the resonance stabilization of the formed nitrophenolate anions. One hundred milligrams of the cobalt molybdate catalyst was then added to the mixture under continuous stirring. The reaction was monitored via UV visible spectroscopy measurements. Absorbance intensity decreases as nitrophenolate anion concentration decreases, thus allowing us to use this relationship as a measure of the efficiency of the catalyst in this reduction reaction. 3. Results and Discussion 3.1. Complex Precursor Characterizations As the first step, the product of the solid state reaction between the mixture of Co(NO3 )2 ·6H2 O and the ammonium molybdate ((NH4 )6 Mo7 O24 ·4H2 O) with the oxalic acid was studied by infrared spectroscopy. The FTIR spectrum of the resulting complex is shown in Figure 1. The spectrum is similar to previous studies for oxalate complexes [25–28]. The bands at 920 and 963 cm−1 are assigned to the Mo=O stretching, the 1360 and 1317 cm−1 region can be assigned to υ(C-O) and δ(OCO), and the 1402 cm−1 band to the C-O stretching [28]. The spectrum also reveals the presence of a large band in the range of 1800–1550 cm−1 . The deconvolution of this band shows bands at 1731 and 1675 cm−1 , which can be assigned to the C=O vibration of the oxalate group [27,28] and a band at 1635 cm−1 corresponding to δ(H2 O) [26]. At high frequency, the spectrum reveals a broad intense band in the 2850–3750 cm−1 range. The deconvolution of this broad band reveals bands at 3385 cm−1 , which can be attributed to the OH bridging group between two metal ions [29], and at 3170 cm−1 , which can be attributed to the stretching vibration of NH4 + ion [29,30]. Furthermore, a band at 1240 cm−1 can be attributed to δ(NH4 + ) and confirm therefore the presence of the NH4 + group in the precursor [30]. These results permit the conclusion that an oxalate complex, containing oxo molybdenum units, hydroxyl (-OH), water, and NH4 + ion, was formed. The results from the thermal analysis (TGA and DTA) studies are presented in Figure 2. The TG curve is divided in four parts with a total weight loss of 44.1% between room temperature and 380 ◦ C. The first weight loss of 4.4% can be attributed to the crystallization water. Moreover, the second and third weight losses located between 170 and 300 ◦ C can be attributed to the decomposition of the complex. Finally, a last
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small loss of 2.1% is observed between 300 and 375 ◦ C. A similar loss in the same range was obtained in a previous study of bismuth oxalate complex containing an OH group and can be attributed to this OH group [31,32]. By compiling the results obtained by FTIR, TGA, and the possible Molecules 44of Molecules2018, 2018,23, 23,xx of10 10 oxidation degree of cobalt and molybdenum, a formula of the oxalate complex can be suggested as (NH )2 (OH) ·H2 O. The total loss value observed is 44.1% in with the loss observed isis 44.1% comparison with the theoretical for suggested 4 )CoMoO(C 2 O4in loss observed 44.1% in comparison with theweight theoretical value of of 45.3% 45.3% for the thecomparison suggested formula. formula. theoretical value 45.3% for the suggested formula. This wastemperature subsequently This was subsequently thermally decomposed in at of 550 This complex complex wasof subsequently thermally decomposed in air aircomplex at aa chosen chosen temperature ofthermally 550 °C °C to to ◦ C to obtain the resulting oxide phase. decomposed in air at a chosen temperature of 550 obtain the resulting oxide phase. obtain the resulting oxide phase.
70 70
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Transmitance (%) (%) Transmitance
80 80
1800 1800 1600 1600 1400 1400 1200 1200 1000 1000 -1
-1 Frequency Frequency (cm (cm ))
Figure 1.FTIR oxalate precursor. precursor. Figure1. FTIRspectrum spectrumof ofthe theas-prepared as-preparedoxalate precursor.
Weight loss loss // % % Weight
TGA TGA DTA DTA
4.4% 4.4%
95 95 90 90 85 85 80 80 75 75 70 70
37.6% 37.6%
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2.1% 2.1%
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Temperaturevariation variationΔΔTT//(a.u.) (a.u.) Temperature
105 105 100 100
50 450 50 100 100150 150200 200250 250300 300350 350400 400 450500 500550 550600 600 o o Temperature Temperature // C C
Figure Figure 2. TGA and DTA thermogramsrecorded recordedunder underair. air. Figure2. 2.TGA TGAand andDTA DTAthermograms thermograms recorded under air.
3.2. 3.2.Cobalt CobaltMolybdate MolybdateCharacterization Characterization 3.2. Cobalt Molybdate Characterization 3.2.1. 3.2.1. X-ray X-ray Diffraction Diffraction The pattern of the powder prepared via this new and original method TheX-ray X-raydiffraction diffractionpattern patternof ofthe thepowder powderprepared preparedvia viathis this new and original method isgiven given The X-ray diffraction new and original method is is given in in Figure 3. A pure phase CoMoO 4 was obtained. All the observed peaks could be indexed in in Figure A pure CoMoO 4 was obtained. All the observed peaks could be in Figure 3. A3.pure phasephase CoMoO All the observed peaks could be indexed in indexed accordance 4 was obtained. accordance with J.C.P.D.S. file ## 21-0868. CoMoO 44in crystallizes in the monoclinic cell in the accordance with the the J.C.P.D.S. file 21-0868. The CoMoO crystallizes in the monoclinic cell in the with the J.C.P.D.S. file # 21-0868. The CoMoOThe crystallizes the monoclinic cell in the space group 4 space C2/m with 10.21, 9.268, cc == 7.022, and == 106.9°. The space group group C2/m with the the parameters: parameters: 10.21,cbb===7.022, 9.268,and 7.022, and◦β.βThe 106.9°. The crystallites crystallites C2/m with the parameters: a = 10.21, baa===9.268, β = 106.9 crystallites size was size using the equation, assuming the sizewas wascalculated calculated using theDebye–Scherer Debye–Scherer equation, assuming that thecrystallites crystallites haveaaspherical spherical calculated using the Debye–Scherer equation, assuming that thethat crystallites have a have spherical shape. shape. A diffraction for calculation (2θ == 23.22°). shape. A separate separate diffraction peak (021) was was considered considered for this this(2θ calculation (2θ crystallites 23.22°). The The A separate diffraction peak (021)peak was (021) considered for this calculation = 23.22◦ ). The size crystallites size was found to be 17.6 nm. crystallites size was found to bewas 17.6found nm. to be 17.6 nm.
Molecules 2018, 23, 364 Molecules 2018, 23, x Molecules 2018, 23, x
10 10
20 20
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(400) (400) (040) (040) (003) (003) (222) (222) (-422) (-422) (113) (113) (241) (241) (-133) (-133)
(-131) (-131) (022) (022) (-222) (-222)
CoMoO CoMoO44 JCPDS # 21-0868 JCPDS # 21-0868
(-112) (-202) (-112) (-202) (-311) (-311)
(-201) (-201)
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Intensity Intensity/ /(a.u.) (a.u.)
5 of 10 5 of 10 5 of 10
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30 35 2θθ 25 degrees 2 // degrees
45 45
50 50
Figure 3. XRD pattern of the β-CoMoO nanoparticle powder obtained after calcination of the oxalate Figure 3. 3. XRD XRD pattern pattern of of the the β-CoMoO β-CoMoO444nanoparticle nanoparticlepowder powderobtained obtained after after calcination calcination of of the the oxalate oxalate Figure ◦ precursor at 550 °C. precursor at at 550 550 °C. C. precursor
3.2.2. Specific Specific Surface Area Measurement Measurement 3.2.2. Specific Surface Surface Area The synthesized synthesized cobalt cobalt molybdate molybdate(CoMoO (CoMoO44)) has has a BET surface surface of of 28.35 28.35m m222/g, /g, which which corresponds The molybdate (CoMoO of 28.35 m /g, whichcorresponds corresponds 4 has a BET surface to a particle size of 46.3 nm calculated using the equation D BET = 6000/d.S, where d is the density and to a particle size of 46.3 nm nm calculated calculated using usingthe theequation equationDDBET BET = 6000/d.S, 6000/d.S,where wheredd is is the the density density and and is the the specific specific surface surface area. area. It It is is aa higher higher value value than than that that calculated calculated from from the the XRD XRD pattern. pattern. On On the SS is On the other hand, as depicted in Figure 4, the adsorption/desorption isotherms are type IV exhibited by other the adsorption/desorption adsorption/desorption isotherms other hand, as depicted in Figure 4, the isotherms are are type type IV IV exhibited exhibited by by mesoporous solids and exhibit the characteristic H1 hysteresis loop (IUPAC classification), which is mesoporous mesoporous solids solids and and exhibit exhibit the the characteristic characteristic H1 H1 hysteresis hysteresis loop loop (IUPAC (IUPAC classification), which is consistent with mesoporous or nanoporous materials [33,34]. The pores are cylindrical or slit shaped. consistent with mesoporous mesoporous or or nanoporous nanoporous materials materials [33,34]. [33,34]. The The pores pores are arecylindrical cylindricalor orslit slitshaped. shaped. The pore pore size size was was found found to to be be of of 18.8 18.8 nm, nm, and and the the total totalpore porevolume volumefound foundto tobe be0.1336 0.1336cm cm333/g /g using using The 18.8 nm, and the total pore volume found to be 0.1336 cm /g the BJH BJH (Barrett-Joyner-Halenda) (Barrett-Joyner-Halenda) method. method. the
Quantity QuantityAdsorbed Adsorbed/ (mmol/g) / (mmol/g)
4.5 4.5
Adsorption Adsorption Desorption Desorption
4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0
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Relative Pressure / (P/P0 ) Relative Pressure / (P/P )
0.8 0.8
0.9 0.9
1.0 1.0
Figure 4. 4. Adsorption and and desorption desorption curves curves obtained obtained from from the BET BET measurements measurements of of the the β-CoMoO β-CoMoO44 Figure Figure 4.Adsorption Adsorption and desorption curves obtainedthefrom the BET measurements of the nanoparticles. nanoparticles. β-CoMoO4 nanoparticles.
3.2.3. Transmission Transmission Electron Microscopy Microscopy 3.2.3. 3.2.3. Transmission Electron Electron Microscopy Micrographs of of the the β-CoMoO β-CoMoO44 nanoparticles nanoparticles were were taken taken at at low low magnification magnification showing showing agglomerated agglomerated Micrographs Micrographs of the β-CoMoO nanoparticles were taken at low magnification showing 4 nanoparticles (Figure (Figure 5a). 5a). The The agglomerates agglomerates are are about about one one micron micron in in size. size. At At higher higher magnification, magnification, nanoparticles agglomerated nanoparticles (Figure 5a). The agglomerates are about one micron in than size. the particle particle size size found found is is in in the the range range between between 20 20 and and 40 40 nm nm (Figure (Figure 5b,c). 5b,c). This This value value is is lower lower the than At higher magnification, the particle size found is in the range between 20 and 40 nm (Figure 5b,c). obtained from from BET BET study study (46.3 (46.3 nm). nm). This This result result can can be be explained explained by by the the fact fact that that in in BET, BET, agglomerated agglomerated obtained This value is lower than obtained from BET study (46.3 nm). This result can be explained by the size fact particles offer less surface for the adsorption-desorption phenomena, so the calculated particle particles offer less surface particles for the adsorption-desorption phenomena, so the calculated particle size that in BET, agglomerated offer less surface for the adsorption-desorption phenomena, so the is higher. higher. The The calculated calculated particle particle size size using using the the Scherer Scherer formula formula from from XRD XRD study study (17.6 (17.6 nm) nm) was was found found is lower than than those those found found in in either either BET BET or or TEM TEM observation. observation. This This is is because because what what was was calculated calculated was was lower the crystallite size, which was smaller than the particle size. the crystallite size, which was smaller than the particle size.
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calculated particle size is higher. The calculated particle size using the Scherer formula from XRD study (17.6 nm)x was found lower than those found in either BET or TEM observation. This is because Molecules 66of Molecules2018, 2018,23, 23, x of10 10 what was calculated was the crystallite size, which was smaller than the particle size.
aa
bb
200 200nm nm
50 50nm nm
cc
Figure 5. The TEM micrographs of the as-prepared β-CoMoO4 powder (a) at low magnification (×100,000), Figure Figure 5.5. The The TEM TEM micrographs micrographs of of the the as-prepared as-prepared β-CoMoO β-CoMoO44powder powder (a) (a) at at low low magnification magnification scale bar 200 nm; and (b) at high magnification ( × 500,000), scale bar 50 nm; (c) Measured particle size (×100,000), (×100,000),scale scalebar bar200 200nm; nm;and and(b) (b)at athigh highmagnification magnification(×500,000), (×500,000),scale scalebar bar50 50nm; nm;(c) (c)Measured Measured at high magnification. particle particlesize sizeat athigh highmagnification. magnification.
In 44 nanoparticles, aa order to confirm the atomiccomposition compositionand andproportion proportionin inthe theβ-CoMoO β-CoMoO nanoparticles, 4nanoparticles, In order order to to confirm confirm the the atomic atomic composition and proportion in the β-CoMoO study with EDX spectroscopy was performed. Figure 6 shows the results. The abundance of oxygen, astudy study with EDX spectroscopy was performed. Figure 6 shows the results. The abundance of oxygen, with EDX spectroscopy was performed. Figure 6 shows the results. The abundance of oxygen, cobalt, 67.71%, 15.47%, and 16.82% compared the theoretical cobalt,and andmolybdenum molybdenumwere werefound foundto tobe beof of67.71%, 67.71%,15.47%, 15.47%,and and16.82% 16.82%compared comparedto tothe thetheoretical theoretical values respectively. values66.67, 66.67,16.67, 16.67,and and16.67 16.67respectively. respectively.
Element Element OO Co Co Mo Mo Total Total
Weight Weight%% 30.02 30.02 25.26 25.26 44.71 44.71 100.00 100.00
Atom Atom%% 67.71 67.71 15.47 15.47 16.82 16.82 100.00 100.00
and its atomic abundance. Figure Figure6.6.EDX EDXspectrum spectrumof ofthe theas assynthesized synthesizedβ-CoMoO β-CoMoO444and andits itsatomic atomicabundance. abundance.
3.3. 3.3.Reduction ReductionofofNitrophenol NitrophenolIsomers Isomers The The results results of of the the catalytic catalytic reduction reduction study study are are shown shown in in Figure Figure 7. 7. As As isis clear, clear, the the efficiency efficiency of of the β-CoMoO 4 catalyst is remarkable. An instantaneous reduction happened in the case the β-CoMoO4 catalyst is remarkable. An instantaneous reduction happened in the case of of 3-NP 3-NP (Figure (Figure7a). 7a).In Incontrast, contrast,the thereduction reductionreaction reactionwas wasslower slowerin inthe thecase caseof of2-NP 2-NPand and4-NP 4-NP(8 (8min). min).This This
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3.3. Reduction of Nitrophenol Isomers The results of the catalytic reduction study are shown in Figure 7. As is clear, the efficiency of the β-CoMoO is remarkable. An instantaneous reduction happened in the case of 3-NP (Figure 7a). 4 catalyst Molecules 2018, 23, x 7 of 10 In contrast, the reduction reaction was slower in the case of 2-NP and 4-NP (8 min). This difference in efficiencyinisefficiency probably is due to a higher of 3-NP, where mesomeric conjugation is difference probably duebasic to a character higher basic character ofthe 3-NP, where the mesomeric more important. conjugation is more important. 1.4
Absorbance
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Figure 7. Successive UV-vis spectra of the reduction of (a) 2-nitrophenol solution (4 × 10−4 M); (b) 3Figure 7. Successive UV-vis spectra of the reduction of (a) 2-nitrophenol solution (4 × 10−4 M); nitrophenol solution (4 × 10−4 M); (c) 4-nitrophenol solution (4 × 10−4 M) in the presence of 8 × 10−4 M (b) 3-nitrophenol solution (4 × 10−4 M); (c) 4-nitrophenol solution (4 × 10−4 M) in the presence of 4 4using β-CoMoO4 nanocatalyst (0.1 g); (d) Plot of ln(C/C0) against reaction time for the 8NaBH × 10− M NaBH4 using β-CoMoO4 nanocatalyst (0.1 g); (d) Plot of ln(C/C0 ) against reaction time 4 and determination of the reduction reaction rate nitrophenol isomers in the in presence of β-CoMoO for the nitrophenol isomers the presence of β-CoMoO 4 and determination of the reduction reaction app. constant k rate constant kapp .
An experiment was conducted to study the influence of the NaBH4 concentration on the rate An experiment was conducted to study the influence of the NaBH4 concentration on the rate constant of the reduction reaction. Different concentrations were taken for NaBH4 maintaining the same constant of the reduction reaction. Different concentrations were taken for NaBH4 maintaining the conditions as described in the protocol. When the concentration was increased to NaBH4/NP ratios of same conditions as described in the protocol. When the concentration was increased to NaBH4 /NP 50:1, 25:1, 12.5:1, and 5:1, the catalyst was decomposed by the hydride. However, when a 2:1 ratio was ratios of 50:1, 25:1, 12.5:1, and 5:1, the catalyst was decomposed by the hydride. However, when a used, the catalyst did not react with NaBH4. These were the optimal conditions for this reaction. 2:1 ratio was used, the catalyst did not react with NaBH4 . These were the optimal conditions for On the other hand, the catalyst recycling was studied for 3-NP, where a faster reduction time this reaction. was observed. After the reduction test, the solution was filtered and the powder recuperated. After On the other hand, the catalyst recycling was studied for 3-NP, where a faster reduction time thorough washing with water, the powder was left to dry at 80 °C overnight and used in the next was observed. After the reduction test, the solution was filtered and the powder recuperated. recycling test, conserving the same experimental conditions operated in the first reduction test. The After thorough washing with water, the powder was left to dry at 80 ◦ C overnight and used in recycling was repeated four times, and the results are represented in Figure 8. The reduction time of the next recycling test, conserving the same experimental conditions operated in the first reduction test. 1 min did not change in any the recycling tests, showing the good stability of the β-CoMoO4 The recycling was repeated four times, and the results are represented in Figure 8. The reduction nanocatalyst in this type of reduction reaction. time of 1 min did not change in any the recycling tests, showing the good stability of the β-CoMoO4 It is important to compare the relative reaction kinetics of the 2-NP, 3-NP, and 4-NP reduction nanocatalyst in this type of reduction reaction. for the catalysts synthesized in this work and those reported in the literature for other catalysts. The It is important to compare the relative reaction kinetics of the 2-NP, 3-NP, and 4-NP reduction results are summarized in Table 1. for the catalysts synthesized in this work and those reported in the literature for other catalysts. The Importantly, this is the first time that such a high catalytic efficiency has been observed for a results are summarized in Table 1. low-cost nanocatalyst and highly concentrated solutions of nitrophenol isomers. The observed rate constant for the simultaneous reduction of the 3-NP isomer for the β-CoMoO4 catalyst is relatively high and is of 1.310 min−1. However, definite conclusions cannot be reached from these comparisons due to the differences in the experimental conditions, such as the NaBH4/NP/catalyst equivalent ratio and the temperature or the reactants concentration. On the other hand, the reaction time is also a
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Importantly, this is the first time that such a high catalytic efficiency has been observed for a low-cost nanocatalyst and highly concentrated solutions of nitrophenol isomers. The observed rate constant for the simultaneous reduction of the 3-NP isomer for the β-CoMoO4 catalyst is relatively high and is of 1.310 min−1 . However, definite conclusions cannot be reached from these comparisons due to the differences in the experimental conditions, such as the NaBH4 /NP/catalyst equivalent ratio and the temperature or the reactants concentration. On the other hand, the reaction time is also a good criterion of efficiency and in our case the reaction is instantaneous. Goyal et al. have reported a Molecules 2018, 23, x 8 of 10 similar efficiency [17] but for a lower concentration of the nitrophenol. One important point is that the reduction of the 3-NP isomer is faster than the reduction of the 2-NP isomer by approximately the reduction of the 3-NP isomer is faster than the reduction of the 2-NP isomer by approximately one order of magnitude (8/1) situationisisobserved observed work of Goyal al. This one order of magnitude (8/1)and andthe thereverse reverse situation in in thethe work of Goyal et al.etThis suggests that that the the reduction reaction mechanism forthe theβ-CoMoO β-CoMoO CuFe suggests reduction reaction mechanismis is different different for 4 and CuFe 2O4 catalysts. 4 and 2 O4 catalysts.
Reduction Time (min)
1.0 0.8 0.6 0.4 0.2 0.0 Test
First Recycling
Second Recycling
Third Recycling
Fourth Recycling
Figure 8. Recycled catalyst against reduction time reductionofof3-NP 3-NPwith withNaBH NaBH4 catalyzed catalyzed with Figure 8. Recycled catalyst against reduction time in in thethe reduction 4 with β-CoMoO4 nanoparticles. β-CoMoO4 nanoparticles. Table 1. Pseudo-first order rate constants for the reduction of 2-NP, 3-NP, and 4-NP by CoMoO4 with
Tableother 1. Pseudo-first order rate in constants for the reduction of 2-NP, 3-NP, and 4-NP by CoMoO4 with nanocatalysts reported the literature. other nanocatalysts reported in the literature. Catalyst
Catalyst CoMoO4
CoMoO4 CuFe2O4
Type
Type Nanoparticles
Nanoparticles Nanoparticles
CuFe2 O4
Nanoparticles
CuO/ NiFe 2 O4
Nanoparticles Nanocomposites
NiFe2O4
γAl2O3
Ni/C black CuO/γAl 2 O3
Fe2(MoO4)3
Ni/C black
Nanoparticles
Nanocomposites Nanocomposites
Nanoparticles
Nanocomposites
Concentration of NP (mol/L)
Reaction Rate Constant (min−1) References Time (min) Concentration of Reaction Time Rate Constant 8 0.273 for 2-NP References −1 NP (mol/L) −4 2 × 10 1 (min) 1.310 for(min 3-NP ) This work 8 0.2880.273 for 4-NP 8 for 2-NP 3 3.6761.310 for 2-NP 1 for 3-NP This work 2 × 10−4 3.6 × 10−5 1.5 0.9830.288 for 3-NP [17] 8 for 4-NP 3 0.846 for 4-NP 3 3.676 for 2-NP 20 0.327 for 2-NP 1.5 0.983 for 3-NP [17] 3.6 × −510−5 3.6 × 10 12 0.062 for 3-NP [17] 3 0.846 for 4-NP 16 0.118 for 4-NP 20 for 2-NP 15 ---- 0.327 for 2-NP −5 12 for 3-NP 3.6× × 2.9 10−510 20 ---- 0.062 for 3-NP [18][17] 16 for 4-NP 12 0.1740.118 for 4-NP 15 0.594 for 15 —-2-NP for 2-NP 5.0 10−410−5 15 0.594 for [19][18] 20 —-3-NP for 3-NP 2.9× × 15 0.5970 for 4-NP 12 0.174 for 4-NP 12 0.160 for 2-NP 15 0.594 for 2-NP 2 × 10−4 −4 4 0.427 for 3-NP [20] 15 0.594 for 3-NP [19] 5.0 × 10 9 0.323 for 4-NP
15
0.5970 for 4-NP
12 0.160 for 2-NP 4 0.427 for 3-NP [20] 9 in the reduction 0.323 for 4-NP β-CoMoO4 nanostructures with a high catalytic efficiency of ortho-, meta-, and
4. Conclusions Fe2 (MoO4 )3
Nanoparticles
2 × 10−4
para-nitrophenol isomers to their corresponding aminophenols have been successfully prepared via the thermal decomposition of an oxalate precursor. The reduction was instantaneous in the case of 4. Conclusions 3-NP but quite a bit slower in the cases of 2-NP and 4-NP (8 min). This difference in efficiency is β-CoMoO withofa 3-NP. high Acatalytic the reduction ortho-, 4 nanostructures probably due to the higher basicity recyclingefficiency test on thein reduction of 3-NP of showed themeta-, and para-nitrophenol isomers to their corresponding aminophenols have been successfully prepared stability of the nanocatalyst and its high efficiency even after four cycles. Acknowledgments: This work was supported financially by the Research Deanship of Taibah University under Grant No. 435/6426). Author Contributions: M.A. and H.O.H. conceived and designed the experiments; all the authors performed the experiments and analyzed the data; M.A., H.O.H. and A.O. wrote the paper.
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via the thermal decomposition of an oxalate precursor. The reduction was instantaneous in the case of 3-NP but quite a bit slower in the cases of 2-NP and 4-NP (8 min). This difference in efficiency is probably due to the higher basicity of 3-NP. A recycling test on the reduction of 3-NP showed the stability of the nanocatalyst and its high efficiency even after four cycles. Acknowledgments: This work was supported financially by the Research Deanship of Taibah University under Grant No. 435/6426). Author Contributions: M.A. and H.O.H. conceived and designed the experiments; all the authors performed the experiments and analyzed the data; M.A., H.O.H. and A.O. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds cobalt molybdate (β-CoMoO4 ) are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
