Catalysis Communications 9 (2008) 2417–2421

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Catalysis Communications

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Selective oxidation of p-nitrobenzyl alcohol to p-nitrobenzaldehydewith 10% Ni silica with 30% H2O2 in acetonitrile solvent

Ateeq Rahman, V.S.R. Rajasekhar Pullabhotla, S.B. Jonnalagadda *

School of Chemistry, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, Private Bag 54001, Durban 4000, South Africa

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 November 2007Received in revised form 2 June 2008Accepted 9 June 2008Available online 17 June 2008

Keywords:Hydrogen peroxideNi–SiO2 catalystp-Nitrobenzyl alcoholOxidation

1566-7367/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.catcom.2008.06.004

* Corresponding author. Tel.: +27 31 2607325; fax:E-mail address: jonnalagaddas@ukzn.ac.za (S.B. Jo

p-Nitrobenzyl alcohol is oxidized to p-nitrobenzaldehyde with good selectivity using 30% H2O2 and 10%Ni/SiO2 as catalyst in acetonitirle, as refluxing solvent. A 95% conversion and single product are achieved.Using the same system, benzhydrol was selectively converted to benzophenone with 100% conversion.Cinnamyl alcohol, menthol and cyclohexanol were also converted to their corresponding aldehydes orketones, but with less efficiency.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Selective oxidation of alcohol is an important transformation inorganic synthesis and several methods are known for this particu-lar conversion [1]. Aldehydes are important class of compoundsused as food additives and in fragrances and as intermediates in or-ganic syntheses such as Aldol, Micheal, Cannizaro and Perkins reac-tions. To achieve selective oxidations, various homogeneouscatalysts, such as salts of Cr, Mn, Ru, Re, HNO3, CrO3 in H2SO4,pyridinium chlorochromate salts, some in combination with basesand TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) as co catalystshave been reported in the literature [1–5]. Many of the processescall for large amounts of the catalyst materials leading to heftytoxic waste. In some cases additives have to be employed whichmust be disposed or can only be recycled with difficulty [6].

The use of heterogeneous catalysts in the liquid phase offersseveral advantages over homogeneous ones such as ease ofrecovery and recycling, atom utility, and enhanced stability inthe oxidation of alcohols. In the literature, many heterogeneouscatalysts have been proposed for oxidation of alcohols to alde-hydes or ketones. To mention a few, Kockritz et al. [7] reportedRu–TiO2 for oxidation of primary alcohols to aldehydes withNaOCl or tertiarybutyl hydroperoxide (TBHP). The presence ofbases as additives leads to side products, which pose difficultyin their separation from the reaction mixture. Clay cop catalystswere used for oxidation of primary alcohols in the presence ofhydrogen peroxide in microwave irradiation, yielding good con-

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+27 31 260 3091.nnalagadda).

versions, but with less selectivity [8]. Zhao et al. [9] have re-ported oxidation of benzylic alcohols to benzoic acids withmesoporous modified catalysts in hydrogen peroxide under sol-vent free conditions and found poor selectivity towards aldehydeformation. Due to the acid formation, it is also a tedious proce-dure to isolate the product from the reaction medium. Uranylimmobilized on mesoporous catalysts [10] were used for oxida-tion of alcohols employing TBHP as oxidant, but the reaction wasnot controlled and yielded acids, which again becomes cumber-some to isolate the product from the reaction mixture. The oxi-dation of alcohols with Ti-Silicate [11] showed less selectivity toaldehydes. Oxidation of alcohols in TBHP, with copper bentonitecatalysts was reported by Alizadeh et al. [12] which demandsthe preparation of catalysts incorporating organic ligands. Oxida-tion of p-nitrobenzyl alcohol with TBHP as oxidant and zeolites[13] requires longer reaction durations. Choudary et al. [14] re-ported that Ni–Al hydrotalcite showed good activity for the oxi-dation of allylic alcohols, ketones, substituted benzylic alcoholsto benzaldehyde by O2. Similarly, Kawabata et al. [15] reportedthat Ni–Mg–Al hydrotalcite also showed catalytic activity foroxidation of benzyl alcohol to benzaldehyde with O2. Oxidationof alcohols with hydrotalcites catalysts in TBHP under solventfree conditions were also reported with good selectivity, butwith low conversions [16]. This prompted us to investigate anddesign an oxidation route for conversion of alcohol to aldehydeor ketone with improved selectivity and conversion efficiency.In this communication, to explore the scope of selective oxida-tion of alcohols using peroxide as oxidant, the catalytic efficien-cies of different loadings of Ni on silica and titania areinvestigated in detail.

2418 A. Rahman et al. / Catalysis Communications 9 (2008) 2417–2421

2. Experimental

2.1. Materials

All chemicals were synthesis grade reagents available from(Merck). Silica gel (Merck) particle size 63–200 lm, 70–230 mesh,pore size 100 Å, BET surface area 480–540 m2 g�1 was used.

2.2. Preparation of catalysts

The catalysts were prepared by the impregnation method bytaking nickel nitrate nonahydrate in distilled water and adding toit silica gel and stirring for 2 h at room temperature and agitatingat room temperature for overnight. The solvent is removed by dry-ing the material at 100 �C for 12 h.

2.3. Typical oxidation procedure

p-Nitrobenzyl alcohol (2 mmol) and catalyst (0.4 g) in aceton-itirle (15 ml) and 30% hydrogen peroxide (1 ml) are mixed at roomtemperature and heated at 90 �C under stirring conditions, for therequired duration. Then, it was filtered and to the filtrate smallamounts of MnO2 was added to decompose the unreacted H2O2.The treated reaction mixture was filtered to remove solid MnO2,and the products were extracted with ethyl acetate, dried overanhydrous Na2SO4 and evaporated in vacuum to afford the p-nitro-benzaldehyde as a pale yellow solid, m.p. 105 �C, 1H NMR(400 MHz, CDCl3, 25 �C, TMS): d = 8.1 (d, 3J (H,H) = 8.3 Hz, 2H,aryl-H), 8.4 (d, 3J(H,H) = 8.3 Hz, 2H, aryl-H), 10.2 (s, 1H, CHO); IR(KBr pellets) = 1700 cm-1 (sh,C@O; MS (70 eV) m/z (%): 151 (64)[M+], 150 (61) [M+ -H], 105 (20) [C7H5O+], 77 (93) [C6H5+], 51(100) [C4H3+]. The reaction was monitored by thin-layer chroma-tography. The product was characterized by 1H NMR (400 MHz-Bruker) and IR spectroscopy. Based on comparison with the stan-dard sample, the product was identified as p-nitrobenzaldehyde.

3. Results and discussion

Among the industrial oxidants with the exception of oxygen,H2O2 is unique due to its only byproduct, H2O. H2O2 is increasinglybecoming a preferred oxidant in the industrial processes. Conse-quently the production of H2O2 is expected to increase in the nextfew years. Therefore H2O2 is a viable alternative, as chemical com-panies face strict guidelines on environmental pollution. [16–17].Using hydrogen peroxide as the oxidant, the catalytic efficienciesof the materials with 5, 10 and 15% Ni loaded on silica or titaniasupports were investigated. All the experiments were conductedin duplicate. The conversion and selectivity efficiencies for thesix investigated catalyst materials, on the oxidation of p-nitroben-zyl alcohol to p-nitrobenzaldehyde by hydrogen peroxide are sum-marised (Table 1).

Table 1Selective oxidation of p-nitrobenzyl alcohol to p-nitrobenzaldehyde in acetonitrile*

Support Ni loading (%) Reaction duration Conversion (%) Selectivity (%)

TiO2 5 6 3 –5 17 10 90

10 6 5 –10 17 20 90

SiO2 5 6 4 905 10 5 905 17 80 95

10 6 38 9510 10 45 9510 17 90 P95

p-Nitrobenzyl alcohol = 2.0 mmol, 30% H2O2 = 1 ml and catalyst = 400 mg.* Mean of duplicate runs.

An examination of the results in the Table 1 indicate that using30% H2O2 as oxidant, with 5 and 10% Ni loaded catalysts on titania,although the selectivity is good, conversions are poor even withlong reaction times.

With 15% Ni loaded titania, negligible reaction progress was ob-served even after 17 h reaction duration. The weak activity of Ni–TiO2 for oxidation of p-nitrobenzyl alcohol to p-nitrobanzaldehydecould be possibly due the weaker metal to support interactions notwell suited for catalysis. With commercial TiO2, the reaction gave30% less conversions and these results confirm that Ni in associa-tion with SiO2 and TiO2 is the active species for the hydrogen per-oxide initiated oxidation of alcohols to aldehydes and ketones.Kawabata et al. [15] during the oxidation of alcohols using molec-ular oxygen and Ni loaded on hydrotalcite support have observedthat at the active sites, Ni(II) actions were probably coordinatedby either Mg(II) or Al(III) through oxygen bonding in which Mg(II)as a base can donate electron to the Ni atoms through oxygen,whereas Al(III) as a Lewis acid can activate alcohols as alkoxide an-ions by the deprotonation. The catalyst inactivity with higher Ni isresulting from the orientation of the Ni spread on support surfaceand due to the uneven distribution of the metal on the supportadjacent to the active sites on the support.

Using 30% H2O2 and with 5% Ni silica catalyst, with reactiontime, % conversion improved from 4% in 6 h to 80% by 17 h, withhigher selectivity of 95% for later reaction. With the 10% Ni silicacatalyst, reaction was even faster and the 6 h reaction gave 38%conversion. With increased reaction time, both the conversionand selectivity percentages have improved. With 17 h reaction, itgave 90% conversion and selectivity >95%. The increased reactionduration has positively contributed to higher conversion and selec-tivities. With 15% Ni on SiO2, negligible conversion was observedeven after 17 h of reaction time.

3.1. The characterization part is moved prior to discussion of catalysts

3.1.1. Characterization of catalystsNi silica catalysts were characterized by following techniques:

XRD, BET, IR spectroscopy SEM methods.

3.1.2. X-ray diffraction (XRD) patternsThe XRD studies on the 10% Ni silica were carried out on Phil-

lips-PW 1830 Powder X-ray diffraction instrument. The XRD spec-tra showed the characteristic bands of the nickel phase andsupport, but no mixed nickel oxide support phases were identified.The pattern for the supports was that of crystallized materials withthe well defined bands except for the amorphous structure, as onlyone broad peak around 2h = 25 appeared, which is characteristic ofsilica (Fig. 1). A perusal of the figure shows the silica dioxide peakgets broadened with increased Ni loading. Zhou et al. [18] too re-ported that when nickel is incorporated into SiO2, the sharp peakat 22� broadens corresponding to an amorphous silica matrix.

3.1.3. BET surface area analysisBET surface area analysis of 10% Ni/silica was carried using the

Brunnauer–Emmett–Teller Equation. As higher catalytic activitywas observed with 10% Ni/SiO2, surface area of only 10% Ni/SiO2

catalyst is estimated. The BET surface area of the catalysts is180 mg�1. During the impregnation stage of the preparation, sur-face hydroxyl groups of the silica where consumed by reactionwith the active phase precursor. Such a surface reaction may havecaused the decrease of available surface area of the support, prob-ably by closure of the pores.

3.1.4. FT-IR spectraThe results of both 10 and 15% Ni/SiO2 were similar hence only

the 10% Ni/SiO2 result are discussed. The examination of IR spectra

0

100

200

300

0 20 40 60 80 100

cps

a

b

c

Fig. 1. XRD patterns of (a) 5% Ni–SiO2 (b) 10% Ni–SiO2 (c) and (d) 15% Ni–SiO2.

A. Rahman et al. / Catalysis Communications 9 (2008) 2417–2421 2419

of 5 and 10% Ni loaded SiO2 using FT-IR spectroscopy showed thatthe band at 1100 cm�1 (asymmetrical Si–O–Si) is very perceptiveto formation of silicates [20]. The vibrational stretching frequencyof the hydrogen atom in hydroxide catalysts 5 and 10% Ni/SiO2 ap-pears at 3422 cm�1. The vibrational stretching frequency of thehydrogen atom in hydroxide at wavenumber 3422 cm�1 indicatesthat it has ordered cation distribution [14]. In the IR results of 5%Ni/SiO2, the strong and intense absorption band between 1078–1050 cm�1, shows the presence of Si–O–Ni bonds [20].

3.1.5. SEM characterizationMorphology and location of metallic species on the surface of

the catalyst were examined by scanning electron microscopy(SEM) using a JEOL JSM-6100 microscope equipped with an en-ergy-dispersive X-ray analyzer (EDX). The images were taken withan emission current = 100 lA by a Tungsten (W) filament and anaccelerator voltage = 12 kV (Fig. 2). The SEM figures of the 5 and10% Ni loaded silica are shown in Fig. 2a and b, respectively. Themorphology shows crystalline Ni of 2–4 lm is well distributedover the silica surface. The Fig. 2c and d show the scans of the15% Ni loaded silica, which indicates that the distribution of Nion support is either in conglomerates or in layers, thus hinderingthe participation of both Ni and silica active sites in the reaction.

SiO2 or Ni2+ individually had no catalytic effect on the reaction.The observed reactivity of Ni supported on silica material can bepossibly attributed to the metal and support interactions, and theresultant changes in surface properties of the reactive sites. Earlierstudies by Urbano et al. [19] have revealed that supported Ni cat-alysts prepared by Ni(NO3)2 by impregnation exhibit wide size dis-tribution when compared with prepared by more controlled anddeposition method precipitation route which generates low metal-lic Ni particles.

The low metallic surface of nickel on the silica support encour-ages the nickel oxide crystallized formation. The presence of weakinteraction between metal and support, due to little distribution ofNi entailing the formation of the layer of impregnated Ni, probablycontribute to the increase of catalytic activity [20–21]. The ob-served non- reactivity with 15% Ni loaded material, could be possi-bly due to the multilayer of Ni loading on the support resulting inloss of activity as suggested by Choudary et al. [16]. This is also

supported by the SEM figures of the different amounts of Ni loadedsilica in the current study (Fig. 2). An examination of SEM figuresshow that Ni particles are well distributed in 5 and 10% Ni loadedmaterial with fine particles and the 15% loaded material shows Nimultilayers (Fig 2c) and conglomeration of Ni particles (Fig. 2d) onthe silica surface. Further, the broadened silica peak with the 15%Ni loading relative to lower Ni- loaded surfaces in the XRD patterns(Fig. 1) also supports the surface characteristics illustrated by SEMfigures.

Recently from the TEM characterization studies of Ni/SiO2 pre-pared from nickel nitrate, Houi et al. have reported activity of Ni onsilica surface depended on the particle size of the metal and lowercatalytic activity is observed with increased Ni particle size. Activ-ity with 15% Ni loaded silica they observed that support surface isalmost completely covered by Nickel particles, [22] suggesting thatthe Ni could be unevenly distributed which has contributed to thelow activity of 15% Ni/SiO2.

CO is widely used as probe molecule to detect the presence andthe nature of lewis acidic sites as well as Bronsted sites (in thepresent case acidic hydroxyls), to which it may have H– bond.When the interaction between CO and absorbing sites has basicallyan electrostatic nature (as in the foregoing cases), a hypsochromicshift occurs with respect to the free CO molecule (2143 cm�1).From their study of the surface acidity of Ni/SiO2 catalysts bymeans of FT-IR measurements of CO, Bonelli et al. reported the hy-droxyl stretching mode (OH) 3800–3400 cm�1 and that of the COstretch mode (v(CO), 2250–2050 cm�1 [23]. They observed thatwith increasing CO pressure, a negative band develops at3744 cm�1, whereas a broad adsorption forms centred at3666 cm�1 and with a shoulder at 3600 cm�1. These features aredue to H-bonding among CO molecules and OH species; those orig-inally absorbing 3745 cm�1 isolated silanols shift to 3666 cm�1 asdo silanols at the surface of dehydroxylated silicas indicating thatthe acidity of silanols is not altered by the presence of Ni species[24].

The plausible reaction pathway involves the initial conversionof alcohol to alkoxide on the silanol site (acidic site) and formationof nickel peroxide upon reaction of hydrogen peroxide with Ni.From the alkoxide on the catalyst surface transfers hydride to per-oxide ensuing the aldehyde formation. The formation of nickel per-

Fig. 2. SEM images of (a) 5% Ni–SiO2 (b) 10% Ni–SiO2 (c) and (d) 15% Ni–SiO2.

Table 2Selective oxidations Aromatic alcohols to aromatic aldehydes or ketones inacetonitrile*

Starting material Product Conversion (%) Selectivity (%)

Benzhydrol Benzophenone 100 100Cinnamyl alcohol Cinnamaldehyde 20 100Menthol Menthone 20 100

Aromatic alcohols = 2.0 mmol, 30% H2O2 = 1 ml and catalyst = 400 mg, Reflux: 17 h.* Mean of duplicate runs.

2420 A. Rahman et al. / Catalysis Communications 9 (2008) 2417–2421

oxide followed by the abstraction of proton from the alkoxide re-sults in the formation of the carbonyl product. The proposed reac-tion pathway can be justified from IR spectral results from thecurrent studies. The samples are outgassed at 723 K in the OHstretch region (3800–3400 cm�1), which also gives a spectrum ofthe commercial silica used as a support. In 10% Ni/SiO2 a promi-nent band present at about 3745 cm�1 is due to isolated silanolsinvariably observed at the surface of the silica. With pure silica, acomponent is seen at 3742 cm�1 assigned to isolated/geminal sila-nols groups. When Ni is present, the peak at 3742 cm�1 decrease inintensity and disappears at higher Ni loading representing to thenominal monolayer coverage and only the band of free silanols isseen. The disappearance of the 3742 cm�1 band is probably relatedto the slightly more acidic nature of the species, i.e. the free silanolsgroup’s band present that have facilitated the hydrogen peroxideinitiated oxidation of alcohol to aldehyde.

In the preliminary experiments, the scope of different Ni loadedcatalysts on the titania or silica supports for the selective oxidationof menthol and cyclohexanol were also investigated. As conver-sions were poor (10%), those reactions were not studied further.The suitability of 10% Ni loaded silica support catalyst for the con-trolled oxidative. The conversion of other alcohols in acetonitrilesolvent was also investigated. The results are summarised in Table2.

Benzhydrol was absolutely oxidized to benzophenone with100% conversion in 17 h. Cinnamyl alcohol was oxidized to cinna-maldehyde with 100% selectively, but with 20% conversion, withno affect on the double bond. Cinnamyl alcohol an allylic alcoholis also investigated in this study. Results in this study were foundimproved compared with cinnamyl alcohol oxidation using NiAlHtlc catalysts [14]. Menthol is oxidized to menthone with 100%

selectivity and again only a 20% conversion could be achieved in17 h reaction duration.

Instead of hydrogen peroxide as oxidant, when either molecularoxygen or air was bubbled through p-nitrobenzyl alcohol contain-ing 10% Ni loaded silica, even after 17 h there was no noticeablereaction. This shows that hydrogen peroxide initiated oxidationof benzylic alcohols with 10% Ni loaded silica in acetonitrile is idealsystem for the selective transformation to aldehydes. These resultscategorically demonstrate that 10% Ni silica catalyst is ideally sui-ted for selective hydrogen peroxide initiated oxidation of benzylalcohols to corresponding aldehydes, with good conversion andhigh selectivity.

4. Conclusion

With 90% conversion and >95% selectivity, the 10% Nickel onsilica catalyst with 30% H2O2 as oxidant, proved excellent systemfor selective and controlled oxidation of p-nitrobenzyl alcohol top-nitrobenzaldehyde. Cinnamyl alcohol is also selectively oxi-dized to cinnamaldehyde, but with 20% conversion. Benzhydrol

A. Rahman et al. / Catalysis Communications 9 (2008) 2417–2421 2421

can be converted to benzophenone with 100% conversion andselectivity. In conclusion, Ni supported silica works as an idealcatalyst for hydrogen peroxide initiated selective oxidation ofbenzylic alcohols to aldehydes under mild conditions with goodselectivity.

Acknowledgements

Authors thank the National Research Foundation, Pretoria andthe University of KwaZulu-Natal for financial support of thisresearch.

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