138 Chiang Mai J. Sci. 2014; 41(1)

Chiang Mai J. Sci. 2014; 41(1) : 138-147http://epg.science.cmu.ac.th/ejournal/Contributed Paper

PEG1000 -DAIL Enhanced Catalysis Activity: Oxidationof Ethylbenzene and its Derivatives byN-Hydroxyphthalimide and Oxime in 1000-BasedDicationic Acidic Ionic LiquidYuhang Wang*[a], Tingting Lu [b][a] Department of Chemistry and Chemical Engineering, Shaanxi Xueqian Normal University,

Xi’an,710100, China.[b] School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.*Author for correspondence; e-mail: sanbaoyisheng@126.com

Received: 30 August 2012Accepted: 10 April 2013

ABSTRACTN-Hydroxyphthalimide (NHPI)-oxime (DMG) catalyst system had better

performance in the PEG 1000 -based functional dicationic acidic ionic liquid (PEG1000 -DAIL)than in the conventional organic solvents for the aerobic oxidation of ethylbenzene and itsderivatives. PEG1000 -DAIL enhanced the catalytic ability of NHPI: 91.9% conversion ofethylbenzene with 84.8% selectivity for acetophenone could be obtained in 7 h, and 92.1%ethylbenzene was oxidized to benzoic acid with 89.3% selectivity in 12 h. Ethylbenzeneswith electron-rich groups were oxidized to different products when time was changed, butthe main product was only ketone when electron-withdrawing ethylbenzene was employed.Both the catalyst and PEG1000 -DAIL could be reused at least six times without significantlydecreasing the catalytic activity.

Keywords: ethylbenzene oxidation, catalysis activity, NHPI-DMG, PEG1000 -DAIL

1. INTRODUCTIONCatalytic oxidation of hydrocarbons like

ethylbenzene provides routes to a wide rangeof important functionalized moleculesincluding alcohols, ketones and carboxylicacids [1-2]. Traditional methods of oxidationoften involved the use of stoichiometricquantities or large excesses of poisonous highoxidation state chromium, manganese, andosmium reagents [3]. Environmental andeconomic factors make the use of thesereagents increasingly unacceptable. The

homogeneous catalytic systems which includelower oxidation state transition metals usedto be applied in industrial oxidation processes.However, the conditions are often harshand the chemistry is rarely selective [4].The heterogeneous oxidation catalystswith metalloporphyrins, zeolites and othermolecular sieves supported was alsodiscussed in considerable research, but itsuffered from drawbacks including theneed for expensive or hazardous co-oxidants

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and low conversions [5]. Recently,N-hydroxyphthalimide (NHPI) has beenrecognized as a valuable catalyst foroxidation various organic compounds suchas ethylbenzene in the presence of somemetallic compounds or non-metalliccompounds under mild conditions [6-11]. For example, combined with metal saltsCo(OAC)2, NHPI effectively catalyzedoxidation of ethylbenzene to acetophenone[12]. NHPI/sodium hydroxide was also auseful catalytic system for the oxidation ofethylbenzene to its hydroperoxide [13-14].

Ionic liquids (ILs) are a special class ofmolten salts composed of organic cationsand inorganic or organic anions. They havemany excellent advantages such as negligible

volatility, excellent thermal stability,remarkable solubility, and a variety ofavailable structures [15]. Recently, the novelPEG-1000-based dicationic acidic ionicliquid (PEG 1000 -DAIL), which exhibits atemperature-dependentphase behaviorwith toluene, is widely used in the organicreaction. And in the previous work,PEG1000 -DAIL was successful to replaceacetic acid as solvent in the oxidation ofalkylaromatics catalyzed by NHPI [16-17].

In this work, oxime (DMG) wasattempted as mediator in the NHPIcatalyzed oxidation of ethylbenzene withPEG1000-DAIL, and it was interesting thatthe product distribution was changed withreaction time.

Scheme 1. The preparation of the PEG1000-DAIL.

2. MATERIALS AND METHODS2.1 Materials

All materials were purchased fromcommercial sources and used without furthertreatment. 1H NMR (500 MHz) and 13C NMR(125 MHz) were recorded on a Bruker 500spectrometer using D2O as the solvent withtetramethylsilane (TMS) as an internal standard.IR spectra were recorded on a Bruker Vector22 infrared spectrometer. High performanceliquid chromatography (HPLC) experimentswere performed on a liquid chromatograph

(Dionex Softron GmbH, America), consistingof a pump (P680) and UV/Vis light detectorsystem (170U). The experiments wereperformed on a Diacovery C18 column;∅4.6*250 mm. The conversions of thesubstrates and the selectivitys of productswere estimated from the peak areas basedon the internal standard technique.The products were determined in somecases by comparison of their HPLC withthose of authentic samples.

140 Chiang Mai J. Sci. 2014; 41(1)

2.2 General Procedure for the OxidationThe PEG1000 -DAIL was prepared by the

procedure given in the literature (Scheme 1)[16]. Imidazole (0.5 mol) and sodium ethoxide(0.5 mol) were dissolved in ethanol (50 mL),stirred at 70°C for 8 h, then ethanol wasremoved by rotator evaporation underreduced pressure, the residue was washedwith diethyl ether three times and dried invacuum to give intermediate 1. Subsequently,intermediate 1 (0.5 mol) and compound 2(0.25 mmol, prepared by a reported method)were dissolved in ethanol (50 mL) and themixture was stirred at reflux for 10 h andfiltered, the filtrate was washed by etherseveral times then evaporated under reducedpressure to give intermediate 3 as a colorlessoil. And 1, 3-propane sultone was addeddropwise into the solution of 3 in toluene(30 mL) in 30 min and the mixture was stirredfor 8 h and evaporated under reducedpressure. Then H2SO4 was added dropwisein ethanol (30 mL) in 30 min. The final solutionwas stirred at 50°C for another 8 h andevaporated under reduced pressure to givePEG1000 -DAIL (4) as a viscous brown liquid.

1H NMR (D2O) δ 2.15 (t, 4H, J = 7 Hz,2×CH2), 2.77 (m, 4H, 2×CH2), 3.45-3.66(m, 90.3H, (OCH2CH2)n), 3.74 (4H, 2×CH2),4.21-4.30 (8H, 4×NCH2), 7.41 (s, 4H, 4×CH),8.71 (s, 2H, 2 ×CH); 13C NMR (D2O) δ 25.2,47.7, 47.9, 49.1, 68.6, 68.7, 69.6, 122.3, 123.1,136.0; IR (cm-1): 3849, 3400, 3020, 2883, 1732,1591, 1456, 1431, 1377, 1122, 1109, 972, 883,798, 677, 577.

Substrate (0.05 mol), PEG1000 -DAIL(2.8*10-4 mol) or acetic acid (10 mL), NHPI(5*10-3 mol, 10 mol %), and DMG (5*10-3

mol, 10 mol %) were placed in a three-neckedflask. O2 was bubbled into the flask at a flow

rate of 20 mL min-1. The reaction mixturewas stirred at a specific temperature for aspecific time, and the reaction progress wasmonitored by HPLC. After completion ofthe reaction, the mixture was cooled toroom temperature and extracted with etherthree times. After concentration of theether solution, the products were taken forGC measurement. Then only fresh substratewas recharged to the residual PEG1000 -DAILand the mixture was heated to react onceagain.

3. RESULTS AND DISCUSSION3.1 Influence of the Reaction Time onthe Oxidation of Ethylbenzene in PEG1000

-DAILThe effect of time on oxidation of

ethylbenzene catalyzed by NHPI withCo(OAc)2 in PEG1000-DAIL was investigatedin Figure 1. When NHPI-Co(OAc)2 wereused, the final conversion of ethylbenzenewas 99.9%, and the main product wasbenzoic acid(BAC) in 94.6% selectivity.The oxidation of ethylbenzene catalyzed byNHPI-Co(OAc)2 was done in the samecondition except that the solvent was aceticacid(10 mL). The conversion was 99.0% andthe main product was also BAC in 80.2%selectivity, which was similar with that inPEG1000-DAIL. Compared the result inPEG1000-DAIL with that in acetic acid, itwas indicated PEG1000-DAIL enhanced thecatalysis activity of NHPI. Lu et.al[17] alsoreported that PEG1000-DAIL can enhance thecatalysis activity of NHPI, as cumene wasoxidized to the main product BAC but notacetophenone catalyzed in PEG1000-DAIL.

And with the increase of time, theconversion and the selectivity of BAC

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Figure 1. Time-dependence curves for the oxidation catalyzed by NHPI-Co(OAc)2[a] [a]Reaction

conditions: 5*10-3 mol ethylbenzene, 0.5 mol% Co(OAc)2, 10 mol% NHPI, 2.8*10-5 molPEG1000-DAIL, 80°C, O2.

increased, and the selectivity of phenylethanol(PEA) increased rapidly in the initial 4.0 hbut then decreased due to the continuousoxidation of PEA (Figure 1). However, theconcentration of acetophenone (APO), whichwas usually the main product of the oxidationof ethylbenzene, was still low. The selectivitiesof APO were always below 4%, and thehighest value was 3.9% (6 h). In the previousworks, Xu [18] employed NHPI combinedwith nonmetallic mediators Hemin tocatalyze the oxidation ethylbenzene and themain product was APO. On the contrary,ethylbenzene was oxidized to BAC catalyzedby NHPI-Co(OAc)2 whether the solvent wasPEG1000-DAIL or acetic acid from aboveexperiments. And it was shown that theoxidation of ethylbenzene to APO was alwayscatalyzed by NHPI-nonmetallic mediators,and the product was acid in NHPI-metallicmediators.

The effect of time on oxidation ofethylbenzene catalyzed by NHPI with DMGin PEG1000-DAIL was investigated in Figure2. As DMG was employed, an interestingphenomenon appeared. The conversion ofethylbenzene increased with the time and then

became steady after 7 h. The selectivity ofAPO increased at first and peaked at about7 h, and then decreased to a very little amountat the end of reaction. BAC appeared after7h when the selectivity of APO reached top,furthermore, the selectivity of BAC increased.It was shown that the ethylbenzene inPEG1000-DAIL was first oxidized to APO butthen it converted to BAC with NHPI-DMGunder O2. This product distribution ofethylbenzene in PEG1000-DAIL was differentfrom that in traditional solvent [12].In CH3CN catalyzed by NHPI-DMG, theselectivity of APO continuously increased,and the selectivities of PEA decreased due tothe continuous oxidation of PEA to APO.However, after 10 h the conversion waschanged slowly and became stagnated near80%. Also, the product BAC did not appearin the reaction course. It also was indicatedPEG1000-DAIL enhanced the catalysis activityof NHPI. The reason was that ethylbenzenewas converted to BAC in PEG1000-DAIL butAPO in CH3CN, as the two catalystsystems were same (NHPI-DMG).

Then the effect of time on oxidationof ethylbenzene catalyzed by NHPI

142 Chiang Mai J. Sci. 2014; 41(1)

Figure 2. Time-dependence curves for the oxidation catalyzed by NHPI-DMG[a] [a]Reactionconditions: 5*10-3 mol ethylbenzene, 5 mol% DMG, 5 mol% NHPI, 2.8*10-5 mol PEG1000-DAIL, 80°C, O2.

Figure 3. Time-dependence curves for the oxidation catalyzed by NHPI- anthraquinone [a]

[a]Reaction conditions: 5*10-3 mol ethylbenzene, 5 mol% anthraquinone, 5 mol% NHPI,2.8*10-5 mol PEG1000-DAIL, 80°C, O2.

anthraquinone [19-20] which is anothernon-metal mediator for NHPI in PEG1000-DAIL was investigated in Figure 3. And inthe previous work, NHPI-anthraquinonewas used to catalyze the oxidation ofethylbenzene in CH3CN, and the mainproduct was APO although BAC appearedas a side product. It was shown that PEG1000-DAIL also enhanced the catalysis activity ofNHPI, because the main product was BAC.

3.2 Influence of the ReactionTemperature on the Oxidation

As the main product of the oxidationof ethylbenzene catalyzed by NHPI-DMGwas different at 7 h and 12 h, respectively,the oxidation of ethylbenzene catalyzedby NHPI-DMG was deeply studied. Theeffects of temperature on the oxidation ofethylbenzene catalyzed by NHPI-DMGwere investigated. No matter how to change

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the reaction temperature, the main productwas APO when the reaction time was7 h(Figure 4), but the main product was BACas the reaction time was 12 h (Figure 5).Regardless of reaction time, as the solubilityof ethylbenzene in PEG1000 -DAIL andthe activity of NHPI increased with the raiseof the temperature, the conversion ofethylbenzene first increased with the reactiontemperature rising(the conversion was 91.9%

at 7 h and 92.1% at 12 h), but graduallydecreased when the temperature was elevatedbeyond 90� C (80.5% at 7 h and 81.3% at12 h), which was probably because lots ofethylbenzene was exhausted by evaporationat higher reaction temperature. On thecontrary, the selectivity of APO (84.8% at7 h) and BAC (89.3% at 12 h) were almostunchanged while the reaction temperaturechanged. As the activity of NHPI was

Figure 4. Temperature-dependence curves for the aerobic oxidation at 7h[a] [a]Reactionconditions: 5*10-3 mol ethylbenzene, 5 mol% DMG, 5 mol% NHPI, 2.8*10-5 mol PEG1000-DAIL, O2, 8h.

Figure 5. Temperature-dependence curves for the aerobic oxidation at 12h[a] [a]Reactionconditions: 5*10-3 mol ethylbenzene, 5 mol% DMG, 5 mol% NHPI, 2.8*10-5 mol PEG1000-DAIL, O2, 12h.

144 Chiang Mai J. Sci. 2014; 41(1)

increased with temperature [21], theselectivity of PEA was decreased becausedue to the continuous oxidation of PEAto BAC or APO.

3.3 Influence of Different PEG BasedDicationic Acidic Ionic Liquid on theOxidation

Besides PEG1000-DAIL, we also tried touse other types of ionic liquids such as PEG200

-DAIL, PEG400-DAIL, PEG600-DAIL,PEG2000-DAIL as solvent in the oxidation

catalyzed by NHPI-DMG, and the mainproduct was changed with the reaction time.As shown in table 1, it was observed that theconversion and selectivity were highest forthe PEG1000-DAIL system, higher for thePEG600-DAIL, lower for the PEG400-DAIL,much lower for the PEG200 -DAIL, and thelowest for the PEG2000-DAIL. And from theexperiment, we found that the solubility ofNHPI in PEG ionic liquid was increased in80� C as the length of PEG was increased,but the solubility was lowest in PEG2000-

Table 1. Oxidation of ethylbenzene in different PEG based dicationic acidic ionic liquid [a].Run Time (h) Conversion (%) Main Products and Selectivity (%)

PEG-200 7 79.9 APO 70.112 80.3 BAC 81.7

PEG-400 7 84.6 APO 76.312 83.7 BAC 85.8

PEG-600 7 87.3 APO 80.312 87.6 BAC 87.1

PEG-1000 7 91.9 APO 84.812 91.1 BAC 89.3

PEG-2000 7 70.5 APO 67.812 71.1 BAC 73.2

[a]Reaction conditions: 5*10-3 mol ethylbenzene, 5 mol% DMG, 5 mol% NHPI, 2.8*10-5 molPEG based dicationic acidic ionic liquid, 80°C, O2

DAIL. As we did not find other reasons,we guess that the conversion and selectivitywas lowest for the PEG2000-DAIL systembecause the solubility of NHPI was worstin PEG2000-DAIL.

3.4 The Oxidation of SubstitutedEthylbenzene Catalyzed by NHPI-DMGin PEG1000 -DAIL

On the basis of these results, a series ofsubstituted ethylbenzene were allowed toreact under O2 in the presence of NHPI-DMG combined with PEG1000-DAIL(Table2). First, ethylbenzenes withelectron-rich substituted group were tested.The phenomenon was similar with that of

oxidation of ethylbenzene. When thereaction time was short, the oxidation ofp-ethyltoluene and p-ethylethylbenzenesoccurred to the formation the correspondingaldehyde or ketone (Run 1 and 3). As the timewas longer, the main products were acid.Because p-ethyltoluene was oxidized tocorresponding ketone catalyzed by NHPI-DMG in CH3CN, it was indicated thatPEG1000-DAIL enhanced the catalysis activityof NHPI [7]. On the contrary, the electron-withdrawing substituted ethylbenzene wasconverted into the corresponding aldehydeor ketone whether the reaction timechanged (Run 5 and 6). It was similar withthe result that catalyzed by NHPI-DMG in

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CH3CN [12]. It was shown that for electron-withdrawing substituted ethylbenzene, NHPIcan’t catalyze the oxidation to acid in despiteof the reaction time. As the oxidation catalyzedby NHPI always followed a radical pathway,and the benzyl radical was not steady, andit could become the substrate once againwhen the ethylbenzene with electron-withdrawing substituent (especial nitrocompound) [21, 22], so p-nitroethylbenzene

which had a strong electron-withdrawingsubstituent(especial nitro compound) hadno reaction under the present conditions.

3.5 The Cycles of Both the Catalyst(NHPI and DMG) and PEG1000 -DAIL

When the final reaction mixture wascooled to room temperature and extractedwith ether, the upper layer of ether, containingproduct, was removed by decantation. Then

Table 2. Oxidation of substituted ethylbenzene by NHPI-DMG in PEG1000 -DAIL[a].Run Substrate Time(h) Conversion(%) Products and Selectivity (%)12345[b]

67[c]

8131116

10(18)10(17)

24

90.389.578.278.2

79.1(81.7)75.5(77.6)

0

72.368.261.559.0

84.3(88.5)86.7(89.1)

26.230.136.839.0

14.8(10.5)13.0(8.6)

[a]Reaction conditions: 5*10-3 mol substituted ethylbenzene, 5 mol% DMG, 5 mol% NHPI,2.8*10-5 mol PEG1000-DAIL, O2, 80°C [b]: Both THICA and DMG were 10 mol%. [c]: Thetemperature was 140°C.

Figure 6. Repeating reaction using recovered PEG1000-DAIL[a] [a] we used the NHPI-DMGsystem for the recycles, and the reaction time was 12 h.

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only fresh substrate was recharged to theresidual PEG1000-DAIL and the mixture(including the catalyst NHPI and DMG) washeated to react once again. The results fromFigure 6 were shown that the oxidationprocedure of ethylbenzene was repeated sixtimes with no appreciable decrease in theconversion of ethylbenzene and the selectivityof BAC. The conversion and the selectivitydecreased obviously after six times becauseof the decomposition of NHPI.

4. CONCLUSIONIn summary, PEG1000-DAIL successfully

replaced the traditional solvent for NHPI-DMG catalyzed the oxidation of ethylbenzeneby oxygen, and it was found that the mainproducts were APO at 7 h and BAC at 12 h,respectively. The derivatives of ethylbenzenewere also tested and the main product ofoxidation changed with time when onlyethylbenzene with electron-rich substitutedgroup was used. The PEG1000-DAIL has theadvantages for enhanced the catalytic activityof NHPI-DMG, changed the product ofoxidation, operational simplicity, andenvironmentally benign.

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