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Ozone Initiated Ni/Metal Oxide Catalyzed Conversion of 1,2Dichlorobenzene to Mucochloric Acid in Aqueous Solutions Estelle C. Chetty,a S. Maddila,a C. Southway,b and S. B. Jonnalagaddaa,* a

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

b

S Supporting Information *

ABSTRACT: In aqueous systems, chlorinated hydrocarbons exhibit low chemical reactivity and nonbiodegradability due to its toxicity. Relative efficiencies of 1% and 5% Ni loaded on alumina, silica and titania as catalysts for conversion of 1,2dichlorobenzene to muchochloric acid (MCA) were investigated at (20 ± 1) °C and ∼1 atm as function of time. All nickel loaded metal oxides gave good conversions and product selectivity toward MCA, with little mineralization. Ni/SiO2 showed superior conversion with selectivity of 89.9−100% toward MCA. The ozonation products were characterized by 1H NMR, IR, Mass and GC-MS, while the catalyst materials were characterized by BET, SEM, FT-IR, ICP, and XRD methods.

1. INTRODUCTION Decomposition of organics by ozone aeration is one of the most promising processes in water and wastewater treatments and its effectiveness toward oxidation of organics is well proven.1 During ozonation, intermediates that retain the aromatic ring of the pollutant are formed. Successive degradation of such intermediates, by reaction with ozone and •OH radicals, leads to aromatic and aliphatic C ± C and C ± Cl bond scission, and to the formation of low molecular weight compounds such as aldehydes and simple organic acids.2 Typically, ozonation rarely produces complete mineralization to CO2 and H2O, but leads to partially oxidized products such as organic acids, aldehydes, and ketones, where oxygen is introduced into many of the carbonaceous sites within the product’s molecules.3 Although ozonation does not remove pollutants to a high extent from water, it leads to removal of color, changes in the structure of the pollutant, and the formation of organic compounds of lower molecular weight, which are thought to biodegrade more easily than the initial substances.4 Many compounds, such as chlorinated organics do not have any strong nucleophilic sites and are not readily oxidized by ozone.5 In order to improve the ozonation efficiency, advanced oxidation processes (AOPs) such as O3/ H2O2, O3/OH−, and homogeneous or heterogeneous catalytic ozonation have been studied by several workers. The aim of such processes was to increase the production of highly reactive species, hydroxyl radicals (•OH), through enhancement of ozone decomposition. Hydroxyl radicals are highly reactive, but unselective and thereby may react with nontarget compounds and may lead to undesirable oxidation byproduct. In addition, AOPs are considered expensive, which is a limiting barrier for their industrial application.6 In an attempt to solve some of these problems, new approaches of novel ozonation processes were attempted that combined ozone with a third medium. The third medium can be either a solid material capable of adsorbing ozone or a water insoluble liquid solvent capable of absorbing ozone. Upon contact of the ozone loaded third medium with water, high ozone concentrations in the aqueous © 2012 American Chemical Society

phase can then be achieved, which would enable faster oxidation rates. Better and effective ozone utilization can also be expected. Ozone can be adsorbed at high concentrations from an oxygen or air stream on to a silica-based adsorbent (e.g., silica gel adsorbs 710 g/m3 O3 at 20 °C).7 The ozone was found to be stable for longer times on the silica-based adsorbents in comparison to many other adsorbents. The other possibility is the use of a commercially available polydimethylsiloxane (PDMS) ozone-solvent that dissolves ozone 10 times higher than water.8 Again, the reaction region can be either in the aqueous phase or in the PDMS phase, if the pollutant is soluble in PDMS or in both phases with potentially useful results. Dong et al. have reported the remarkable catalytic activity and stability of Y zeolite for phenol degradation in the presence of ozone.9 Previously, we have reported the goodness of Ni/SiO2 as catalysts in ozone initiated oxidation of saturated long chained hydrocarbons, for oxidation of various substituted benzyl alcohols with peroxide and for Knoevenagel condensation reactions.10−12 Considering the scope of silica, alumina, etc. to improve the dissolution probability of ozone in water systems and their possible role as catalysts, in the present study the efficiency of metal oxides with or without Ni loading have been investigated for their efficiency in ozone initiated oxidation of 1,2-dichlorobenzene (DCB).

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Catalysts were prepared by wet impregnation method by dissolving appropriate amount of nickel nitrate nona-hydrate in distilled water (20.0 mL) and adding it to 10 g of γ-Alumina (γ-Al2O3, Aldrich), silica gel (Aldrich) or titania (Aldrich) and stirring for 2 h using a magnetic stirrer at room temperature (20 ± 1 °C) and aging at room temperature for overnight. The excess water is removed Received: Revised: Accepted: Published: 2864

July 19, 2011 December 10, 2011 January 10, 2012 January 10, 2012 dx.doi.org/10.1021/ie202570e | Ind. Eng.Chem. Res. 2012, 51, 2864−2873

Industrial & Engineering Chemistry Research

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mL/min for 60 min and the temperature was thereafter brought down to 80 °C. A 4.1% ammonia in helium gas mixture was passed (30 mL/min) over the catalyst for 60 min. The excess ammonia was removed by passing helium with a flow rate of 30 mL/min over the catalyst for 30 min. The temperature was raised gradually to 950 °C by means of a temperature ramp of 10 °C/min under the flow of helium and the desorption was recorded. 2.3.3. Elemental Analysis. The elemental composition of the catalysts was determined by ICP-OES using an Optima 5300 DV Perkin-Elmer Optical Emission Spectrometer, showed that the experimental values were in close agreement with the theoretical values confirming the composition of the materials and adsorption of the Ni on the three support materials. 2.3.4. Surface Area Analyzer. BET surface area of Ni loaded metal oxide catalysts were measured by nitrogen physisorption isotherms at 77 K using the standard multipoint method on a Micrometrics Gemini instrument. Prior to the analysis samples were degassed in a stream of nitrogen at 473 K for 24 h. 2.3.5. SEM Studies. The SEM images summarize the main features of the prepared catalysts. Magnification of micrographs ranges from 1200X to 3500X with accelerator voltages at 12 and 20 kV. All samples were viewed in a Phillips XL30 Electron Scanning Microscope interfaced with EDAX version 3.2. 2.3.6. XRD Studies. X-ray diffraction analyses were performed to characterize the catalyst. The powder diffraction patterns were recorded by X-ray diffraction using a Bruker AXS, D8 Advance Diffractometer with Cu Kα radiation with wavelength 0.15406 nm at 40 kV and 30 mA. Data were collected over the range 10°−90° with a step size of 0.5 per second. 2.3.7. GC-MS Spectrometry of Catalyst Materials. GC-MS analysis of catalyst samples were carried out on a ThermoFinnigan Trace GC with AS 3000 autosampler. Samples mass range was 30−400 (full scan) by using Alltech EC-5 Column. 2.3.8. 1H NMR Analysis. NMR spectra were recorded on a Bruker 400 MHz spectrometer for 1H NMR. 2.3.9. Product Characterization. All of the experiments were carried out under the controlled conditions of room temperature using 20 mL aqueous solution 10% v/v of 1,2dichlorobenzene (DCB) at fixed ozone concentration and flow rate, unless otherwise specified. All reactions were done in duplicate. Ozonation experiments were conducted by aerating the samples with ozone for different durations, viz. 2, 4, and 5 h. After ozonation, the organic portion of the reaction mixture was extracted and analyzed to estimate the extent of conversion and to identify and quantify the reaction products in each run.

by heating the mixture on water bath and using a rota-vapor under mild vacuum to evaporate the water. The catalyst material is dried in an oven at 100 and 140 °C for 12 h.13,14 2.2. Material and Method for Catalyst Evaluation. All experiments were carried out under ambient conditions (20 ± 1) °C at a gas flow rate of 1.0 LPM and ozone concentration of 1.094 mg/L. The ozone generator was calibrated by maintaining the flow rate. Ozone concentration was determined by the iodometric titration method, using starch as indicator and standard sodium thiosulfate as titrant.15 Ozone containing compressed oxygen was bubbled into a 50 cm3 reactor through a sintered porous diffuser with porosity 2 (Figure 1a,b). A

Figure 1. (a) Diagram showing experimental setup of ozonatio/pn reaction. (b) Dimensions of reactor used in ozonation reactions.

magnetic stirrer was placed at the bottom of the reaction vessel to ensure maximum contact of the substrate with ozone by continuous stirring. The outlet at the top of ozone reactor allowed excess gas to flow from the reactor into three 200 mL flasks containing 2% potassium iodide solution. All reactions were carried out in aqueous phase at pH 5, 7, and 11 for 2 h, 4 and 5 h using 20 mL aqueous solution 10% v/v of (DCB) and 0.2 g catalyst at the fixed ozone concentration. Ni/Al2O3, Ni/ TiO2 and Ni/SiO2 (1% w/w or 5% w/w Ni) were employed. 2.3. Instrumentation. 2.3.1. FT-IR Spectroscopy. FT-IR spectroscopy of catalyst samples were carried out on a PerkinElmer BX model spectrometer in the 400−4000 cm−1 region. Samples were using the KBr disk technique. Dry KBr was added to the samples and thoroughly mixed to ensure homogenization. Transparent discs were formed and analyzed for IR spectral measurements. 2.3.2. Temperature Programmed Desorption (TPD). In the TPD experiments, the catalyst was pretreated at 350 °C under the stream of helium which was used as a carrier flowing at 60

3. RESULTS AND DISCUSSION 3.1. Fourier Transform Infrared Spectroscopy (FT-IR). An absorption band at 1384 cm−1 is exhibited by 1% and 5% Ni loaded alumina, which is not present in the IR spectrum of Al2O3. This can be assigned to the stretching vibration of Ni−O bond.16 which is more intense in the 5% Ni/Al2O3 catalyst than 1% Ni/Al2O3 catalyst. The vibrational stretching frequency of *hydroxyl groups appeared at 1630, 1622, and between 3500 and 3400 cm−1 (Figure 2a,b).17−19 With 1% and 5% Ni on TiO2, the absorption band exhibited at 1384 cm−1 (Figure 2c,d) was not present in pure TiO2, can be assigned to Ni−O vibrational stretching.16 The absorption bands at 3434 cm−1 as well as 1622 cm−1 and 3433 cm−1 represented the existence of *hydroxyl group vibrational stretching.17−19 IR spectra of 1% 2865

dx.doi.org/10.1021/ie202570e | Ind. Eng.Chem. Res. 2012, 51, 2864−2873


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Figure 2. Infrared Spectra of catalyst materials. (a,b): 1% Ni and 5% Ni on Al2O3 support; (c,d): 1% Ni and 5% Ni on SiO2 support; and (e,f): 1% Ni and 5% Ni on TiO2 support.

and 5% Ni/SiO2 catalysts (Figure 2e,f) showed bands at 1099 cm−1 and 1098 cm−1 (asymmetrical Si−O−Si) which are due to formation of silicates and an absorption band at 1630 cm−1, showing the presence of Si−O−Ni bonds.14 The absorption band at 1384 cm−1 shows the presence of Ni−O vibrational stretching.16 3.2. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). An examination of the results summarized in Table 1, shows that the experimental values were in close agreement with the theoretical values confirming the composition of the materials and adsorption of the Ni on the three support materials. 3.3. Brunauer−Emmett−Teller Surface Area Analysis (BET). Ni loaded on Al2O3 and TiO2 support material exhibited marginally lower surface area than pure metal oxides. Surface area marginally decreased with increased loading of Ni on both Al2O3 and TiO2 supports. A possible reason for marginal change could Ni is finely spread with minimal surface area

Table 1. Theoretical and Observed wt % of Ni on Metal Oxides Using ICP-OES catalyst

theoretical wt % of catalyst

observed wt % of catalyst

Ni/Al2O3

1 5 1 5 1 5

1.12 5.08 1.09 5.19 1.15 5.10

Ni/TiO2 Ni/SiO2

changes. BET surface area marginally increased, when Ni loaded on silica support. (Table 2). Lin et al. have investigated the activities of a series of oxide supports and dozens of supported metal catalysts toward the decomposition of aqueous ozone. They reported that Al2O3, SiO2, SiO2·Al2O3, and TiO2 have showed no or negligible activity, five noble metals, and many other metals loaded on supports exhibited high activity (mg O3 min−1 g−1 catalyst) in the order M/SiO2 (0.22), M/ 2866



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and titania supports, the specific acidity of the loaded catalyst increased with Ni loading. In case silica support, there marginal increase in surface area, still specific acidity recorded an increase with Ni loading. Among the three catalysts, silica supported catalyst is proved to be highly acidic in nature, followed by alumina and titania with lower acidity. 3.5. Scanning Electron Microscopy (SEM). A comparison of Figure 3a,b shows a significant difference in the presence of Ni (white glow) on Al2O3 surface with an increase of Ni loading from 1% to 5%. As shown in Figure 3c,d, Ni particles are evenly distributed on silica surface with 1% Ni, whereas with 5% Ni loading on silica the Ni particles are closer together and seem to start assembling on each other therefore the decrease in surface area measurements as shown in Table 2. From Figure 3e,f, it is observed that Ni (white fluffy substance) sparsely occupies the TiO2 surface. An increase from 1% to 5% Ni loading shows that Ni occupies titanium dioxide surface more densely resulting in the larger surface area (BET values) observed in Table 2. The morphology shows that the crystalline Ni is well distributed over 1% Ni loaded support material, i.e., alumina, titania, and silica surfaces (Figures 3a and 2c,e). As the amount of Ni loading is increased from 1% to 5% Ni, it is still widely distributed over support catalysts. Reactivity of Ni on support material could be due to the metal and support interactions as well as the consequential changes in surface properties of the

Table 2. BET Surface Area and Acidity of Ni Loaded Supports catalyst (%) γ-Al2O3 SiO2 TiO2 1% Ni/ Al2O3 5% Ni/ Al2O3 1% Ni/ SiO2 5% Ni/ SiO2 1% Ni/ TiO2 5% Ni/ TiO2

surface area (m2/g)

acidity (mmol NH3/g)

specific acidity (mmol NH3/m2)

210 165 41 200

850 3800 558 875

4.1 23.0 13.6 4.4

190

905

4.8

170

3990

23.5

174

4130

23.7

40

600

15.0

38

615

16.2

Al2O3 (0.16) and M/TiO2 (0.09) and M/SiO2·Al2O3, and rate constants in parentheses values are for M is Ni.20 3.4. Temperature Programmed Desorption (TPD). The TPR results show that as the % Ni loading increased, the acidity as measured by mmol of ammonia adsorbed enhanced. It shows Ni loading enhances the acidity of support. With increased loading of Ni and resulting decrease in surface area for alumina

Figure 3. SEM images. (a,b): 1% Ni and 5% Ni on Al2O3 support; (c,d): 1% Ni and 5% Ni on SiO2 support; and (e,f): 1% Ni and 5% Ni on TiO2 support. 2867



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reactive sites. Urbano et al.,21 reported that Ni on supported catalysts prepared by Ni(NO3)2 using impregnation method, exhibit wide size distribution compared with the preparation by more controlled and deposition method precipitation route, which generates low metallic Ni particles. The low metallic surface of nickel on the support encourages the nickel oxide crystallized formation. The presence of weak interaction between metal and support, due to little distribution of Ni entailing the formation of the layer of impregnated Ni, probably contribute to the increase of catalytic activity.22 3.6. X-ray Diffraction (XRD). Diffraction bands (Supporting Information Figure S1a,b) due to Al2O3 and NiO are identified and confirmed by literature.23−26 The characteristic peaks which are present for NiO suggest that nickel oxide is not highly dispersed on the alumina surface.27 Figures (Supporting Information Figure S1c,d) illustrate the XRD pattern of Ni/ SiO2. Characteristic bands of nickel oxide and support were observed.24,26 The characteristic peak of silica appears as the broad band peak at 2θ = 22°. The XRD pattern for Ni/TiO2 catalyst is represented in (Supporting Information Figure S1e,f). The X-ray diffraction peaks for NiO and TiO2 are identified.24,26−28 3.7. Gas Chromatography−Mass Spectrophotometry (GC−MS). In all of the experiments, the organic layer was extracted by using 3 × 5.0 mL diethyl ether in a separating funnel and excess water was removed by the addition of anhydrous sodium sulfate (Na2SO4). The product was filtered and the solvent is allowed to evaporate. The product was characterized by GC and GC−MS. The amount of DCB was consumed in the reaction are expressed as a percentage of original amount and it referred reaction conversion. The percent selectivity refers to the amount of product formed divided by the reactant consumed. 3.8. Product Characterization. After ozonation, the reactions products were identified and quantified after each experiment (Supporting Information Figures F1−F3). On the basis of the characterization of IR, 1H NMR, and mass data for the oxidation products, 3,4-dichloro-5-hydroxy-2(5H)-furanone (DHF) (Supporting Information Figures F4−F6) and mucochloric acid (MCA) (Supporting Information Figures F7−F8) were found to be the major species. The product of DHF was also proved by its spectral analyses. The IR spectra showed characteristic absorption bands at 3312.87, 1774.70, 1635.71, and 1225.42 cm−1 corresponding to the OHstr, C Ostr, CCstr, and C−Ostr functional groups in the structures. 1 H NMR spectra showed proton regions at δ 6.76 ppm due to CH proton as a singlet and at δ 2.45 ppm due to OH proton as a singlet. The compound MCA was also characterized by 1H NMR spectrum. It showed the region at δ 13.40 ppm due to COOH proton as a broad peak and at δ 9.02 ppm due to CHO proton as a singlet. Further, the limewater test confirmed the release of CO2 during all the ozonation reactions, suggesting some mineralization of the DCB. Literature reports suggest that ozonation of organic compounds in water usually produces oxygenated organic products and low molecular weight acids that are more biodegradable.29−32 The byproducts are often observed to consist of aldehydes, small carboxylic acids33−36 and ketones. The unidentified products (UnP) may constitute such products. For stoichiometric balance, the DCB conversion to DHF or MCA consumes four moles of ozone. For mass balance and carbon accountability, for each mole of DHF or MCA formed, which contains 4 C, two moles of CO2 are generated.

In the conversion and selectivity tables, CO2 is not included. The approximate stoichiometric equation showing the major products can be expressed as follows:

C6H4Cl2 + 4O3 = C4 H2Cl2O3 + 2CO2 + 2O2 + H2O 1,2-Dichlorobenzene (DCB); 3,4-Dichloro-5-hydroxy-2(5H)-furanone (DHF); and Mucochloric acid (MCA).

The presence of MCA in drinking water was reported in literature, which is as a result of chlorine bleaching and chlorine-disinfection. MCA, an aldehyde, is also used as an intermediate to make dyes, pigments, pesticides, pharmaceuticals, and photo chemicals. This substance has no considerable potential for bioaccumulation and is partially biodegradable.37 MCA is capable of undergoing ring−chain tautomerism, and its molecule possesses several reaction centers.38 MCA is among such chemically and biologically active compounds. In recent time, it has attracted considerable attention as an accessible starting material,39−42 so that it may be regarded as α,βunsaturated acid, α,β-unsaturated aldehyde, tetra substituted Zolefin, vinyl halide, latent hemiacetal, or pseudolactone.43 With prolonged ozonation, DHF was found to tautomerize to MCA. 3.9. Uncatalyzed and in Presence of Metal Oxides. An observation of the results presented in the Table 3 and Figure 4 show that uncatalyzed had lowest and silica had highest Table 3. Percentage Conversion of DCB and Product Selectivity for Ozonation Reactions of at pH 7.0a,b % selectivity ozonation time/h 2 4 5 2 4 5 2 4 5 2 4 5 2 4 5

C%

DHF

uncatalyzed 34.4 37.5 63.3 21.2 94.3 9.4 with Al2O3 only 73.6 45.0 94.4 27.3 96.8 11.9 with SiO2 only 89.0 40.0 89.9 23.8 99.4 15.7 with TiO2 only 75.4 17.5 89.5 12.0 90.9 8.9 with Ni only 75.2 40.1 93.5 31.7 95.5 14.3

MCA

UnP

2 4 5

34.4 63.3 94.3

2 4 5

73.6 94.4 96.8

2 4 5

89.0 89.9 99.4

2 4 5

75.4 89.5 90.9

2 4 5

75.2 93.5 95.5

a

Reaction mixture =20 mL (10% v/v of DCB in water); Catalyst =0.20 g/20 mL. Flow rate =1 LPM. [O3] = 1.094 mg/L. Temperature (20 ± 1 °C) (n = 2). bn = 2. 2868



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Figure 4. Percentage conversion of DCB and product selectivity for ozonation reactions of at pH 7.0 (Uncatalyzed, in presence of Al2O3, SiO2, TiO2 or Ni as catalysts. Reaction mixture = 20 mL (10% v/v of DCB in water); Catalyst = 0.20 g/20 mL; Flow rate =1 LPM. [O3] = 1.094 mg/L; Temperature (20 ± 1 °C) (n = 2).

Figure 5. % Conversion of DCB after 2 h with 1 and 5% nickel loaded catalysts at pH 5.0, 7.0, and 11.0.

and selectivity is measured by dividing the amount of product formed by the amount of reactant consumed, expressed as a percentage.44 At pH 5, 7, and 11, Ni on Al2O3 had 92.5, 89.8, and 90.9% conversion, Ni/SiO2 had 78.2, 82.6, and 80.6%, while Ni/TiO2 had 67.0, 78.2, and 75.2% respectively, suggesting optimum pH for the conversion of DCB using Ni/Al2O3 is 5, with Ni/SiO2 and Ni/TiO2 is pH 7. Hence, the further studies to oxidize DCB in aqueous phase were conducted at pH 5.0 with Ni/Al2O3 catalyst and at pH 7.0 with Ni/Al2O3, Ni/TiO2, and Ni/SiO2. An examination of the results tabulated in Table 4 and Figure 6 shows that after 2 h ozonation, Ni/Al2O3 catalyst had higher conversions, but after a 5 h period, Ni/SiO2 and Ni/TiO2 catalysts recorded almost 100% conversion. Further, Ni on Silica catalysts gave higher selectivity, 97.8%, followed by Ni on titania, 95.9%, and Ni on alumina had 79.6% toward MCA. While Ni on SiO2 and TiO2 had no residual DHF, at 5 h with Ni/Al2O3 significant amounts of DHF and UnP were present. The improvement in efficiency of 5% Ni on supports relative to

conversions relative to others at 2 h. At 5 h ozonation, conversion was from 99.4 to 90.9% in the order SiO2 > Ni > Al2O3 > Uncat > TiO2, and the selectivity toward MCA was 87.9 to 49.1% with Uncat > Ni > SiO2 > TiO2 > Al2O3. During the ozonation period, the decrease in the percentages of DHF and UnP clearly suggest that during the reaction progress, DHF gets isomerized to MCA and some the unknown intermediates too get converted to MCA. 3.10. With Ni Loaded Metal Oxides. Studies by Ernst et al.,45 revealed that the catalyst exhibited different adsorption abilities to different model compounds and that the adsorption of organic model substances on the catalyst’s surface actually inhibits the catalytic effect. The initial reactions were carried out with 5% w/w nickel loaded on three oxide supports at pH 5, 7, and 11 for 2 h ozonation, to identify the pH optimum for maximum conversion of DCB for each catalyst (Figure 5). The conversion and selectivity percentages were calculated for all reactions. Conversion is the amount of reactant consumed in the reaction expressed as a percentage of the original amount 2869



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Figure 6. Percentage conversion of DCB and product selectivity for ozonation reactions of at pH 7.0 (with 1% Ni/Al2O3, 5% Ni/Al2O3, 1% Ni/SiO2, 5% Ni/SiO2, 1% Ni/TiO2, or 5% Ni/TiO2). Reaction mixture = 20 mL (10% v/v of DCB in water); Catalyst = 0.20 g/20 mL; Flow rate = 1 LPM. [O3] = 1.094 mg/L; Temperature (20 ± 1 °C) (n = 2).

selectivity of 82.4−97.6% toward MCA, while conversions of 98.6−100% and selectivity of 86.8−97.8% was achieved with 5% Ni loaded catalysts. The reaction investigated is found to be sensitive to the nature of the catalyst material. While Ni and Silica on their own registered poor catalytic activity, the modified surface properties obtained by loading 1% Ni showed significant improvement in catalytic efficiency. The further increase in loading of Ni did not contribute to increased catalytic efficiency either in the form of conversion or selectivity. This suggests that active sites provided by 1% Ni on silica surface are adequate to achieve the optimum catalytic efficiency and more Ni on silica surface was unsuitable to accelerate the oxidation process of DCB. The selectivity obtained with each set of ozonation reactions indicate that some mineralization was achieved with Ni-loaded catalysts (Table 5). As Ni on Al2O3 showed a better conversion % at pH 5, all experiments (uncatalyzed, Ni or Al2O3 alone, 1% and 5% Ni on Alumina) were repeated at that pH. A perusal of results at pH 5 reveals that relative to pH 7, reaction had higher percentage of conversion and selectivity toward MCA as function of time. Individually, both Ni and Al2O3 proved to be poor catalysts, but 1% Ni on Al2O3 gave good conversion and selectivity toward MCA at pH.5 than at pH.7. The increased loading of Ni, 5% Al2O3 further improved the efficiency of the catalyst relative to 1% loading and to pH 7 conditions. In the Ni/Al2O3 catalyzed ozonation, the oxidation efficiency of DCB was higher at low pH, 5, than that at high pH conditions, 7 and 11 (Figure 5). Due to the amphoteric nature of alumina, it behaves as a strong base under acidic conditions causing nickel to remain active as a metal ion and releasing a large quantity of OH groups which react with nickel.46 At pH 5, with Ni/Al2O3 catalyst, reaction molecular ozone with DCB molecule is anticipated. Ni/Al2O3 catalyst showed an improvement in DCB degradation with complete conversion after 4−5 h, compared to the 95.9% conversion achieved with uncatalyzed reaction and reactions with activated charcoal, which achieved complete conversion after 5−6 h at pH 5. There was a conspicuous increase in selectivity toward MCA with the combined use of ozone and

Table 4. Percentage Conversion of DCB and Product Selectivity for Ozonation Reactions of at pH 7.0a % selectivity ozonation time/h with 2 4 5 with 2 4 5 with 2 4 5 with 2 4 5 with 2 4 5 with 2 4 5

C%

1% Ni on Al2O3 84.1 88.4 92.3 5% Ni on Al2O3 89.2 93.5 95.5 1% Ni on SiO2 78.2 93.4 99.3 5% Ni on SiO2 79.5 91 100 1% Ni on TiO2 77.5 91.2 100 5% Ni on TiO2 82.6 94.6 100

DHF

MCA

UnP

38 30.2 15.5

24 50.1 76.5

38 19.7 8

40 31.7 14.3

26 53.3 79.6

34 15 6.1

40 23.4 0

30 51.1 97.6

30 25.5 2.4

42.2 25.2 0

28.9 50.3 97.8

28.9 24.5 2.2

40.7 20.9 0

33.3 43.5 89.9

26 35.6 10.1

43.4 17.5 0

35.9 55 95.9

20.7 27.5 4.1

a

Reaction mixture = 20 mL (10% v/v of DCB in water); Catalyst = 0.20 g/20 mL. Flow rate = 1 LPM. [O3] = 1.094 mg/L. Temperature (20 ± 1 °C) (n = 2).

1% loading was small, suggesting that 1% loading was sufficient enough to achieve optimum conversion and selectivity. Although similarities exist in % conversion in all Ni-loaded catalysts, a difference in selectivity toward product with time was observed. Efficient oxidation of DCB was achieved with the 1% Ni loaded catalysts with conversions of 96.3−100% and 2870



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weakly reactive and the ozone molecule attacks mainly on the least deactivated meta position. The initial attack of the ozone molecule on the substrate attached to catalyst surface leads to the formation of the hydroxylated product, which will be highly susceptible to further oxidation. An ozonide is generated and added to the CC double bond. As the reaction progresses, O 3 decomposes into • OH radicals, which attack the intermediate to form 3,4 dichloro-5-hydroxy-2(5H)-furanone (DHF). Under oxidizing conditions, this intermediate undergoes rearrangement to form the final product, mucochloric acid (MCA). In addition to the proposed mechanism scheme, the direct pathway for the formulation of MCA without going through DHF as an intermediate may not be ruled out. That possibly may happen without the ring closure of DCB after the ozone attack on the double bond resulting in the breaking of the six membered ring.

Table 5. Percentage Conversion of DCB and Product Selectivity for Ozonation Reactions of at pH 5.0Reaction mixture =20 mL (10% v/v of DCB in water); Catalyst =0.20 g/20 mL. Flow rate =1 LPM. [O3] = 1.094 mg/L. Temperature (20 ± 1 °C) (n = 2) % selectivity ozonation time/h

C%

uncatalysed 2 84.3 4 91.8 5 99.0 with Ni 2 66.3 4 89.3 5 97.4 with Al2O3 2 66.5 4 75.3 5 88.8 with 1% Ni on Al2O3 2 85.2 4 92.5 5 100 with 5% Ni on Al2O3 2 92.9 4 97.7 5 100

DHF

MCA

UnP

40.0 23.3 12.3

2 4 5

84.3 91.8 99.0

40.0 24.4 8.8

2 4 5

66.3 89.3 97.4

14.3 26.2 11.4

2 4 5

66.5 75.3 88.8

37.5 30.1 7.0

2 4 5

85.2 92.5 100

47.4 25.4 5.3

2 4 5

92.9 97.7 100

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CONCLUSIONS

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ASSOCIATED CONTENT

Oxidation of 1,2-dichlobenzene by ozone in presence nickel loaded metal oxides gives good conversions and product selectivity toward mucochloric acid as the stable oxidation product and with little mineralization. The experimental results indicate that pH 7 is ideal for the reactions. The selectivity of 89.9−100% toward MCA is achieved. The percent conversion efficiency of Ni on support was Al2O3 > SiO2 > TiO2.

Ni/Al2O3 catalyst of 82.4% (1% w/w) and 86.8% (5% w/w), which compares favorably to the 62.9% in the absence of the catalyst. 4.11. Proposed Mechanism Scheme. Ozone is capable to act as either a nucleo- or electrophile, due to its resonance structure. The organic substrate adsorbs on the catalyst active site. Due to the hydrocarbon-catalyst interactions and the latent polarity δ−, δ+, etc. on each subsequent carbon atom, attack by ozone either as an electrophile or nucleophile on different sites is facilitated. Aromatics with electron withdrawing groups are

S Supporting Information *

Figures F1−F8 are available in the form of 1H NMR, IR, GC, GC-MS, and Mass Spec. data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding author: S.B. Jonnalagadda Ph: + 27 32 260 7325; Fax: + 27 31 2603091; Email: [email protected].

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ACKNOWLEDGMENTS

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REFERENCES

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Authors sincerely thank the University of KwaZulu-Natal and the National Research Foundation for the facilities and financial assistance to the project and Mr. V.B. Dasireddy for his help in characterization of the catalyst materials.

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