28

WET OXIDATION OF MALEIC ACID BY A PUMICE SUPPORTED

COPPER (II) SHIFF BASE CATALYST

JEG Mdoe1*

RAJ Minja2 and L Daniel

2

1Chemistry Department, University of Dar es Salaam, P. O. Box 35061, Dar es Salaam, Tanzania2Department of Chemical and Mining Engineering, University of Dar es Salaam, P. O. Box 35131,

Dar es Salaam, Tanzania*Corresponding author: james_mdoe@yahoo.co.uk

ABSTRACT

Pumice supported Cu (II) Schiff base catalysts were prepared by surface chemical modification

followed by complexation with Cu (II) acetate. The resulting materials were characterised by

Diffuse Reflectance Fourier Transform Spectroscopy (DRIFTS) to confirm the modification. The

materials were tested in a wet oxidation of maleic acid using air or hydrogen peroxide as an

oxidant. Results indicate that up to 80% degradation of maleic acid could be achieved in the

presence of the catalyst using hydrogen peroxide as an oxidant. The degradation was found to

depend on the type of oxidant, temperature and whether the parent pumice was acid pre-treated or

not prior to the preparation of the catalyst.

Keywords: Supported Cu (II) Schiff base, maleic acid degradation, pumice

INTRODUCTION

Water pollution is a serious problem in theglobal context. It has been suggested that itis the leading cause of deaths and diseasesworldwide and that it accounts for the deathsof more than 14,000 people daily (West2006). Tanzania is not an exception. It isreported that among the potential sources ofground and surface water pollutants inTanzania include discharges from industrialprocesses, domestic wastewaters andagricultural activities (Sheya and Nyamsika1996). One among the pollutants found inthe discharged wastewater, especially fromtextile, pharmaceuticals, paper and pulpindustries are phenols and other aromaticcompounds such as toluene. Of the availabletechnologies for degrading toxic wastes, themost common is biodegradation wherebymicroorganisms play an important role indegrading the organic wastes. However, thisprocess is limited when pollutants are toxicto the microorganisms. In addition, theresidence time for biodegradation reaction islong, sometimes taking several days. One ofthe alternative technologies for oxidizing

toxic organic wastes that are currently underserious investigation is the catalytic wetoxidation (CWO) process. This processinvolves contacting an oxidizing agent, anorganic pollutant and a catalyst in aqueousmedium. The process is reported to havehigher pollutant conversion levels, lowenergy consumption and i t i senvironmentally friendly (Masende 2004).However, one of the challenging issues onthe application of this process is the highcost of the catalyst. It is envisaged however,that if the catalyst is obtainable at low cost,the process can go a long way in solvingenvironmental problems caused bydischarge of organic wastes.

Tanzania is endowed with abundant amountof pumice soils that are found in areas suchas Mbeya, Arusha and Moshi. Pumice isnaturally porous and hence possesses highparticle surface area which can be utilized asreaction sites. Due to its abundantavailability as well as good physicochemicalproperties, it is potentially a good catalystsupport. Indeed it has been used before as a

Tanz. J. Sci. Vol 38 (2) 2012

29

support for palladium catalysts in thehydrogenation of 1,3-cyclooctadiene andphenyl acetylene (Duca et al. 1996).Similarly, Ni/pumice catalyst has beendeveloped and used on a laboratory scale tostudy reactivity in carbon monoxidehydrogenation (Brito et al. 2004).Furthermore, pumice modified withhexadecyltrimethyl ammonium bromide andbenzyldimethyl tetradecylammoniumchloride has been used for sorption ofphenol and 4-chlorophenol from solutions(Akbal 2005). On the other hand, pumicesupported copper catalysts are also reportedto have been used in degradation of cyanidefrom aqueous solutions (Kitis et al. 2005). Inthe later case it was observed that thespecific surface area as well as the surfacechemistry of particular pumice does affectboth the rate as well as the extent of cyanidedestruction.

As far as CWO process is concerned, anumber of researches have been conductedto find active and stable catalysts for theprocess (. For instance, Valkaj et al. (2007)investigated different types of Cu/ZSM-5catalysts for catalytic wet peroxide oxidationof phenol. In a different study, coppersupported onto micelle templated silica wasinvestigated in the catalytic wet peroxideoxidation of maleic acid in aqueous phase(Daniel and Katima 2009). Recently, a novelheterogeneous copper(II) Schiff basecatalyst was successfully prepared using aCashew Nut Shell Liquid (CNSL)templating agent (Hamad et al. 2011). Thecatalyst was tested on the oxidation ofmaleic acid at room temperature using H2O2

as an oxidant. The catalysts showed verygood catalytic efficiency, with a yield of upto 90% and a turn over number of about1000 in ten minutes (Hamad et al. 2011).

Despite a number of active catalytic systemsthat have been developed so far for theCWO process, most of them are developedby impregnation method and hence are

hampered by leaching of the activecomponent (Wu et al. 2001), or they aredeveloped from expensive materials andhence are not viable economically. Thisstudy has explored the possibility ofutilizing low cost pumice soils in producingheterogeneous copper catalysts that aredeveloped through covalent attachment ofthe active component. The catalysts weretested in wet oxidation of maleic acid, whichis one of the oxidation intermediates ofphenol.

EXPERIMENTAL

Preparation of the Pumice Samples

The pumice soil samples were collectedfrom Mbeya (MB) and Moshi (MS) regionslocated in the southern and northern parts ofTanzania, respectively. After collection thesamples were cleaned using deionised waterto remove soluble impurities, then dried andground to a fine powder. The samples werethen tested for their thermal stability throughcalcinations at various temperatures. Beforeusing the pumice samples as catalystsupport, acid leaching was carried out. Theprocedure involved treatment with 10 ml of3.0 M HCl for every gram of pumice for 10h at 70 oC. This was repeated with fresh acidat least three times to ensure maximumleaching of exchangable cations. Afterthorough washing with deionized water, thesamples were dried in air at 110 oC, cooledand stored for futher use.

Preparation of the Pumice Supported

Copper Catalysts

The anchoring of a copper Schiff base ontopumice involved a number of steps. First, apre-treated pumice sample was modified byattaching 3-aminopropyltrimethoxysilaneonto its surface (Macquarrie et al. 1997).This was followed by reacting the modifiedpumice with 2-hydroxy-1-naphthaldehyde toproduce a Schiff base ligand (Figure 1).Then a solution of copper acetate ofpredetermined concentration was added andthe mixture refluxed in toluene for 3 h. The

Mdoe et al. - Wet Oxidation of Maleic Acid By a Pumice Supported Copper (II) Catalyst …

30

resulting material was then dried in air at 110 oC.

Figure 1: Preparation of supported copper catalyst by a covalent bonding method

Characterization of the Catalyst Support

and Pumice Supported Copper Catalyst

A number of techniques were applied tocharacterize the catalyst support as well asthe supported copper catalyst. The porosityproperties of the materials were studied bynitrogen physisorption. This was performedat Brown University, USA using anAutosorb-1 system from QuantachromeCorp. Prior to the analysis the samples wereoutgassed at 300oC. Nitrogen gas was usedas an adsorbate at 77.4 K whereas 8571Alumina, supplied by U.S. Department ofCommerce, National Institute of Standardsand Technology (NIST), was used as areference material. Scanning electronmicroscopy (SEM) images were alsoperformed at Brown University, USA usingHitachi SEM S2400 instrument at electrongun energy of 8 keV. On the other hand,Diffuse Reflectance Infrared FourierTransform Spectroscopy (DRIFTS) analysesof the materials were done at the Departmentof Physics, University of Dar es Salaam,Tanzania by using a Perkin-Elmer 2000Fourier Transform Infrared (FTIR). Thesamples were initially dried at 100 oC prior

to analysis and then run at room temperatureas mixtures with KBr. X-ray Fluorescence(XRF) analysis was also conducted atBrown University, USA to determine thechemical composition of the pumicesamples.

Experimental Set-up for Catalytic Tests

The catalytic tests were performed at theDepartment of Chemical and MiningEngineering, University of Dar es Salaam,Tanzania for maleic acid oxidation in abatch stirred reactor at a temperature rangefrom 30 to 100 °C at atmospheric pressure.The procedure involved adding 0.07 g ofmaleic acid into the reactor and making thesolution to 350 ml using distilled water.Thereafter 3 g of the copper catalyst and 7ml of 30% hydrogen peroxide wereintroduced. The stirrer speed was setbetween 80 and 150 rpm.

The samples from the reactor were drawn ata fixed interval of 10 minutes, and theircompositions were determined using anHPLC set (Merck-Hitachi) which consists ofa HPLC pump (L-6200A Intelligent Pump),

Tanz. J. Sci. Vol 38 (2) 2012

31

sampler (AS-2000A Autosampler), columnheater (Merck T-6300), detector (L-4250UV-VIS Detector) and an integrator (D-2500 Chromato-Integrator). The separationof the components was achieved by a 300 x8 ID mm Shodex, Rspak KC-811 columnpacked with ion-exchange resin gels of

sulphonated rigid styrene-divinylbenzeneco-polymer. Elution was performed at a flowrate of 1 ml/min at a temperature of 40 oC.Determination of residual maleic acidconcentration was done by DAXLITEsoftware on a PC by calculating the areaunder the chromatogram peaks.

RESULTS AND DISCUSSION

Physicochemical Properties of the Pumice Samples

Figure 2: Composition of pumice samples as determined by XRD

The two samples that were collected fromMbeya and Moshi were characterized byXRF, nitrogen physisorption and SEManalysis in order to establish their physico-chemical properties. Figure 2 gives thecomposition of the two samples as analysedby the XRF. As seen from the figure themost abundant component in both samples isSiO2 (42 -55%). Other components include

Al2O3 (13 - 20%), CaO (2 - 14%), Fe2O3 (4 –15%), Na2O (2%), K2O (1 - 4%), MgO(6%), TiO2 (1 - 4%) and other minorcomponents. It is also noted that the Moshisample is richer in Fe2O3 than the samplefrom Mbeya. Treating the samples with theacid removes the components by about 2 –10%. As expected, the treatment does notremove SiO2 because it is located at the

Mdoe et al. - Wet Oxidation of Maleic Acid By a Pumice Supported Copper (II) Catalyst …

32

centre of the crystal structure (Brito et al.

2004). Since the other components areremoved by this process while SiO2 is notremoved, the resulting product containshigher percent of SiO2 than the untreatedsample.

The porosity properties of the pumicesamples were determined by analysis ofnitrogen adsorption isotherms using the BETmethod. The shapes of the isothermsobtained for the two materials are type IV(Rouquerol & Sing 1999), withcharacteristic hysteresis loops (Figure 3).This is typical for mesoporous materials

(Rouquerol & Sing 1999). The adsorptionshows an initial gradual increase of nitrogenuptake at low relative pressure (< 0.1)attributed to formation of nitrogenmonolayer on the adsorbent surfaces. This isfollowed by a plateau and then a sharp riseof nitrogen uptake starting from about P/Po =0.85. The sharp rise can be associated withthe formation of multilayers of theadsorbate, causing capillary condensation inthe pores. Comparisons of nitrogen uptakefor the two materials reveal that the samplefrom Moshi has a higher capacity than thesample from Mbeya.

Figure 3: Adsorption-Desorption isotherms of the pumice samples from Mbeya (MB) andMoshi (MS).

Figures 4 and 5 give the scanning electronmicrographs of the original pumice and 500oC calcined samples from Mbeya and Moshi,respectively. As seen in Figure 4 the samplefrom Mbeya contains loosely packedprimary particles of mixed shapes which donot change upon calcinations to 500 oC. Onthe other hand, the micrographs of original

pumice and 500 oC calcined samples fromMoshi (Figure 5) show particles that haveirregular shapes and are packed together intobig blocks. The particles disintegrate uponheating to 500 oC. In both cases, however,the particle sizes are in the micron-sizedrange.

Tanz. J. Sci. Vol 38 (2) 2012

33

a

b

Figure 4: Micrographs of pumice samples from Mbeya (a) original, (b) calcined at 500 oC.

a

Mdoe et al. - Wet Oxidation of Maleic Acid By a Pumice Supported Copper (II) Catalyst …

34

b

Figure 5: Micrographs of pumice samples from Moshi (a) original (b) calcined at 500 oC

The development of the pumice-supportedcatalyst was monitored stepwise usingDRIFTS technique. Figure 6 gives thespectra of the material at various stages ofdevelopment. It is evident from the spectrathat the aminopropyl groups weresuccessfully introduced onto the pumicesurface due to the N-H bending mode at1630 cm-1, the C-H stretching vibrationfrequency at 2950 cm-1, and the C-Hbending mode at 1300 cm-1. These vibrationbands were absent in the spectrum ofunmodified pumice. Upon reacting theaminopropyl-functionalised pumice with 2-hydroxy-1-naphthaldehyde the resultingmaterial gave a Schiff base whose spectrumshowed a band at 1640 cm-1 attributed toN=C stretching vibration. Other new bandswere observed at 1550 cm-1 and 1560 cm-1

corresponding to the aromatic ring breathingmode. Upon complexation of copper withthe supported Schiff base, the N=Cstretching vibration shifted to a lower

frequency. The presence of the azomethinevibration band in the spectrum of thesupported Schiff base and its shift to lowerfrequencies in the spectra of the supportedcopper Schiff base catalysts confirms thecoordination of the azomethine nitrogenwith copper (Figure 6).

The pumice supported copper (II) Schiffbase was tested in the catalytic wet oxidationof maleic acid. The study was conducted atdifferent temperatures using air or hydrogenperoxide as an oxidant. The catalysts wereprepared using original or acid pretreatedpumice samples collected from Mbeya andMoshi. Preliminary results indicated that theperformance of the catalysts prepared fromthe samples collected from the two differentareas were nearly identical. In that case thereported results were obtained using thepumice sample collected from Moshi.

Tanz. J. Sci. Vol 38 (2) 2012

35

Figure 6: Spectra of the material at different stages of preparation (A= original pumice, B =aminopropylsilyl-functionalized pumice, C = pumice supported Schiff base, D =pumice supported copper (II) Schiff base catalyst).

Figure 7 shows the extent to which maleicacid is catalytically degraded to carbondioxide and water at room temperature,using air or hydrogen peroxide as oxidants.As seen from the figure it can be seen thatspontaneous degradation of maleic acid atroom temperature was very low, notexceeding 13% in 100 minutes. Thedegradation tendency increased whenpumice supported copper (II) Schiff baseswere used as catalysts. However, the extentof the oxidation was variable depending onthe type of oxidant used, and whether thecatalyst was prepared using acid pretreatedor non acid pretreated pumice. Further,results indicate that the acid pretreatedpumice-based catalyst is more active thanthe non acid pretreated-based catalyst in thetest reaction. In addition, hydrogen peroxide

causes a higher and faster oxidation ofmaleic acid than air, especially when an acidpretreated pumice-based catalyst was used.It can further be seen that whereas peroxidewet oxidation of the maleic acid using theacid pretreated pumice-based catalystachieves about 80% degradation in 60minutes, air oxidation achieves only about45% degradation in 80 minutes. When thetemperature was increased to 50 oC,degradation of maleic acid also increased(Figure 8). The effect of temperature was thesame in both non acid pretreated pumice-based catalyst and the acid pretreated one.This means that pumice could be utilizedwithout acid treatment where temperaturecan be raised but in cases where roomambient temperatures are employed, acidtreated pumice would be beneficial.

Mdoe et al. - Wet Oxidation of Maleic Acid by a Pumice Supported Copper (II) Catalyst

36

Figure 7: Effect of acid treatment and H2O2 on conversion of maleic acid at room temperature.(NB: OPBC = original pumice-based catalyst, TPBC = acid treated pumice-basedcatalyst)

Figure 8: The effect of temperature on conversion of maleic acid

Tanz. J. Sci. Vol 38 (2) 2012

37

Generally, the performance of the developedcatalysts was comparable to that ofPt/graphite catalyst which was found todegrade maleic acid between 10 - 90%(Sheldon 1996). However, the laterdegradation was conducted at 120 to 157 oCwhereas in this case it was between roomtemperature and 50 oC.

CONCLUSION

Pumice samples collected from Mbeya andMoshi areas in Tanzania were studied fortheir potential as supports for copper (II)Schiff bases. XRF studies of the pumiceparent materials indicated that they werecomposed of SiO2 (42 -55%), Al2O3 (13 -20%), CaO (2 -14%), Fe2O3 (4 – 15%) andother minor components. The materials hadmicron-sized loosely packed primaryparticles of irregular shapes. The parentmaterials were used to prepare supported Cu(II) Schiff base catalyst by a stepwisesurface chemical modification. Prior to themodification some samples were acidpretreated to remove physisorbed materials.The success of the modification process wasconfirmed by a diffuse reflectance infraredFourier transform spectroscopy. Thematerials were tested as catalysts in wetoxidation of maleic acid using air orhydrogen peroxide as oxidants. Resultsindicate that the materials could catalyse upto 80% degradation of the maleic acid attemperatures ranging from roomtemperature to 50 oC using hydrogenperoxide as an oxidant. Under theexperimental conditions, increase oftemperature showed only a slight increase inthe degradation. When air was used as anoxidant the degradation was up to 45% only.Acid pre-treatment of the pumice before thepreparation of the catalyst also affected theperformance of the catalyst. The acidpretreated pumice-based catalysts showed ahigher activity than the non acid pretreatedones especially at ambient temperature.

ACKNOWLEDGEMENTS

The authors would like to thankSida/SAREC-core support and theUniversity of Dar es Salaam for financialsupport. We are also thankful to BrownUniversity for assisting in the analysis of oursamples.

REFERENCES

Akbal F 2005 Sorption of phenol and 4-chlorophenol onto pumice treated withcationic surfactant. J. Environ. Manage.

74: 239-244.Brito A, Garcı´a FCÄ lvarez A, Arvelo R,

Fierro JLG and Dı´az C 2004 Highsurface area support/catalyst derivedfrom natural pumice. Study ofpretreatment variables. Ind. Eng. Chem.

43: 1659-1664Daniel L and Katima JHY 2009 Factors

influencing catalytic wet peroxideoxidation of maleic acid in aqueousphase over copper/micelle templatedsilica-3-aminopropyltrimethoxysilanecatalyst. Water Sci. Tech. 60: 2621-2627

Duca D, Frusteri F, Parmaliana A andDeganel lo G 1996 Select ivehydrogenation of acetylene in ethylenefeedstocks on Pd catalysts. Appl. Catal.

146: 269-284Hamad FB, Mubofu EB and Makame YMM

2011 Wet oxidation of maleic acid bycopper(II) Schiff base catalysts preparedusing cashew nut shell liquid templates.Catal. Sci. Technol. 1: 444-452

Kitis M, Karakaya E, Yigit NO, CivelekogluG and Akcil A 2005 Heterogeneouscatalytic degradation of cyanide usingcopper-impregnated pumice andhydrogen peroxide. Water Res. 39: 1652-1662

Macquarrie DJ Clark JH Lambert A MdoeJEG and Priest A 1997 The use ofaminopropylsilica as a base catalyst inthe Knoevenagel reaction. React. Funct.

Polymers 35: 153-158.Masende ZPG 2004 Catalytic wet oxidation

of organic wastes using platinum

Mdoe et al. - Wet Oxidation of Maleic Acid by a Pumice Supported Copper (II) Catalyst

38

catalysts. PhD Thesis, TechnischeUniversiteit Eindhoven.

Rouquerol F and Sing K 1999 Adsorption byPowders and Porous Solids: PrinciplesMethodology and Applications.Academic Press, London.

Sheldon RA 1996 Synthesis of oxiranes. In:Cornils B and Herrmann WA (ed)Applied homogeneous catalysis by

organometallic compounds VCH,Weinheim etc. pp 411-423.

Sheya MS and Nyamusika SSM 1996Environmentally sound technologies:some policy perspectives. In:Mwandosya MJ, Luhanga ML andMugurusi EK (ed) Env i ronmen ta l

protection and sustainable development,CEET, Dar es salaam.

Valkaj KM, Katovic A and Zrncevi_ S 2007Investigation of the catalytic wetperoxide oxidation of phenol overdifferent types of Cu/ZSM-5 catalyst. JHazard Mater. 144: 663-667

West L 2006 World Water Day: A BillionPeople Worldwide Lack Safe DrinkingWater.http://environment.about.com/od/environmentalevents/a/waterdayqa.htm.

Wu Q, Hu X, Yue PL, Zhao XS, Lu GQ.,2001 Copper MCM-41 as catalyst for thewet oxidation of phenol. Appl. Catal. B:

Environmental 32: 151-156