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A kinetic study of the depolymerisation of poly(ethylene terephthalate) by phase transfer catalysed alkaline hydrolysis

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BACKGROUND: Chemical or tertiary recycling of waste polymers including PET, poly(ethylene terephthalate), leads to the formation of raw starting monomers by different depolymerisation routes. This work was focused on the identification of the
   9  2   ResearchArticle Received: 22 February 2008 Revised: 23 May 2008 Accepted: 11 June 2008 Published online in Wiley Interscience: 4 August 2008 ( DOI 10.1002/jctb.2011 Akineticstudyofthedepolymerisationof poly(ethyleneterephthalate)byphasetransfercatalysedalkalinehydrolysis R.L´opez-Fonseca, ∗ M.P.Gonz´alez-Marcos,J.R.Gonz´alez-Velascoand J.I.Guti´errez-Ortiz Abstract BACKGROUND: Chemical or tertiary recycling of waste polymers including PET, poly(ethylene terephthalate), leads to theformation of raw starting monomers by different depolymerisation routes. This work was focused on the identification of thecatalytic behaviour, if any, of a series of quaternary phosphonium and ammonium salts as phase transfer catalysts for thealkalinehydrolysisofPET,andonthedeterminationofthekineticsofthephasetransfercatalysedprocess.RESULTS:Amongthesaltsexaminedtributylhexadecylphosphoniumbromidewasfoundtobethemosteffectivecatalyst.Theproposedkineticmodelaccountedfortheuncatalysedandcatalysedreactionsandpredictedalinearcorrelationforthereactionrate with the concentration of the quaternary salt. The notable increase in the phase transfer catalysed reaction rate wasrelated mainlyto thegreater value of thepre-exponentialfactor whilethe value of theactivation energy was hardly modifiedby thepresenceof thequaternaryphosphonium salt,therebysuggestingasimilar mechanismfor thealkalinehydrolysiswithorwithoutphasetransfercatalyst.CONCLUSIONS:Theuse ofselectedphosphonium quaternarysaltsexhibitedaremarkablypositiveeffect on theexperimentalconditions under which the depolymerisation of poly(ethylene terephthalate) by alkaline hydrolysis can be carried out,especiallyintermsoflowoperatingtemperature.Kineticcorrelationsprovidedareliablemathematicalreactionmodelforthisrecyclingprocess,whichisinagreementwiththeprinciplesofsustainabledevelopment.c  2008SocietyofChemicalIndustryKeywords: chemical recycling; PET; alkaline hydrolysis; kinetics; phase transfer catalysts; quaternary phosphonium salts INTRODUCTION Poly(ethylene terephthalate) (PET) is a thermoplastic saturatedpolyester produced by reacting monoethylene glycol (EG) withterephthalic acid (TPA) (or dimethyl terephthalate). With a globalyearly production of 35 million tons PET is considered to be oneof the leading polymer resins. About 63% of PET is used as fibresin staple, filament and woven forms, while the remaining 37% isused as a packaging resin for bottles, containers, sheet and film.Globalgrowthratesarearound4%and8%peryear,respectively. 1  The wide application of PET in packaging industries is relatedmainly to its remarkable mechanical strength, low weight, lowpermeability to gases, good light transmittance, smooth surface,and the fact that it does not create a direct hazard to theenvironment (no side effects on humans). The interest in PETrecycling is increasing owing to its substantial production andvolume fraction in the waste stream and high resistance to bothatmospheric and biological agents.PET recycling is one of the most successful and widespreadexamples of polymer recycling. Petcore recently announced thatEuropean post-consumer PET collection rates reached 944,000tonnes in 2006, an 18.5% increase over the previous year. 2  The increase in PET collection continues to exceed growth inconsumption, with 38.6% of all PET bottles being collected forrecycling. Collection has continued to grow steadily in mostEuropeancountrieswiththegreatestincreaseinratesincountrieswhere national legislation is changing to allow single trip bottlesto replace refillable containers. TherecyclingofwastepolymersincludingPETcanbecarriedoutin many ways. 3 However, the only method acceptable accordingto the principles of sustainable development is the so-calledtertiary or chemical recycling, since it results in the yield of the corresponding monomers. The chemical recycling of PETcan be conducted by the following techniques: (i) glycolysis, 4,5 (ii) methanolysis, 6,7 (iii) hydrolysis 8,9 and (iv) aminolysis 10,11 orammonolysis. 12 AllthesemethodshavebeenreviewedrecentlybyPaszun and Spychaj, 13 Karayannidis and Achilias 14 and Lorenzetti etal  . 15  These processes are solvolitic reactions which, throughester bond cleavage, give various depolymerised products. ∗ Correspondence to: R.L´ opez-Fonseca, Chemical Technologies for Environ-mental Sustainability Group, Department of Chemical Engineering, Faculty of Science and Technology, Universidad del Pa´ıs Vasco/EHU, P.O. Box 644,E-48080, Bilbao,Spain.E-mail:ruben.lopez@ehu.esChemical Technologies for Environmental Sustainability Group, Department of Chemical Engineering, Faculty of Science and Technology, Universidad del Pa´ısVasco/EHU,P.O.Box644,E-48080,Bilbao,Spain  JChemTechnolBiotechnol   2009; 84 : 92–99 c  2008 Society of Chemical Industry   9   3   Phase transfer catalysed depolymerisation of PET www.soci.orgNowadays there is growing interest in hydrolysis for the chemicalrecycling of PET, since it is the only method that leads toterephthalic acid and ethylene glycol. These recovered productscan be used as feedstocks for the synthesis of virgin PET. Thisis connected with the trend in new factories for PET synthesisto produce it directly from TPA and EG, thus replacing dimethylterephthalate (the traditional monomer) from the technologicalprocess. 16 Hydrolysis canbecarriedoutunder(a) alkaline,(b) acidand (c) neutral conditions. This recycling process meets thecriteria of simplicity, low energy consumption, relatively lowenvironmental impact and recovery of materials that can bereadily assimilated into the polymerisation technology. This work has been particularly focused on analysing the PETdepolymerisation by means of alkaline hydrolysis. The specificobjectives of this study are, on one hand, to evaluate the catalyticperformanceofdifferentquaternaryphosphoniumorammoniumsalts as phase transfer catalysts (PTCs) for this process (allowingthe reaction to be carried out under mild conditions, especially interms of temperature) and, on the other hand, to determine thekinetics of the catalysed reaction in a batch reactor. EXPERIMENTAL Pure PET flakes (Aldrich Chemical Co., Steinheim, Germany) witha mean particle size of 2 mm were used in the experimentson depolymerisation in a sodium hydroxide solution. Flakeswere previously cut with a cryogenic rotary cutter (Retsch ZM2000, Haan, Germany) to reduce the particle size to 250 µ m. The experimental set-up for reaction experiments consisted of a4.5 cmi.d.,300mLcapacitystainlesssteelbatchreactor(AutoclaveEngineers, Erie, Pennsylvania, USA). 17  The tank was equippedwith a cooling coil, a thermometer pocket, a 4 cm diameter discturbineimpellerwithsixblades,locatedataheightof2.5 cmfromthe bottom of the reactor, an electric heating mantle, a digitaltemperature control system and a manometer. A nitrogen purgewas used to create an inertatmosphere inside the vessel.With the aim of decreasing the operating temperatures and/orthe reaction time required to achieve high conversions, a widenumber of PTCs, namelyquaternary phosphonium or ammoniumsalts (nine PTCs with varying alkyl groups, central cation (N or P)and anion (Cl − , Br − , I − , OH − )), were examined for their activity inthedepolymerisationprocess.Thesaltsusedinthisstudyarelistedin Table 1. All of them were supplied by the Aldrich Chemical Co.andwereusedwithoutfurtherpurification.Theselectedoperatingconditions for alkaline hydrolytic experiments were: stirring rate400 rpm; particle size 250 µ m; inert atmosphere 200kPa N 2 ;temperature 60–100 ◦ C, NaOH concentration 1.67mol L − 1 ; PETconcentration 0.29 mol L − 1 ; and PTC concentration 0–0.07 molL − 1 .PET flakes, aqueous sodium hydroxide solution, and PTC (whenused) were charged into the reactor at room temperature andthen heated to the selected temperature (60–80 ◦ C for catalyticexperiments and 60–100 ◦ C for non-catalytic experiments) forconducting the hydrolytic runs. The mixture was allowed to reactfor 1.5–4 h. Both temperature and pressure were kept constantduring each experiment. In all kinetic studies, reaction time zerowas taken to be the time at which the reactor vessel temperaturewas the predetermined reaction temperature. After the requiredtime interval for reaction was reached, the vessel was quicklyremovedfromtheheatingmantleandimmersedinanicebath.Thetemperatureofthevesselwasquenchedtoroomconditionssoastointerrupttheprogressofhydrolysis.Depolymerisationproducts Table1.  Quaternary phosphonium and ammonium salts used asPTCs in the alkaline hydrolysis of PETAbbreviation Quaternary salt4BuAB Tetrabutylammoniumbromide4OAB Tetraoctylammoniumbromide4BuACl Tetrabutylammoniumchloride4MPB Tetramethylphosphoniumbromide4BuPB Tetrabutylphosphoniumbromide4OPB Tetraoctylphosphoniumbromide3Bu6DPB Tributylhexadecylphosphoniumbromide4BuPCl Tetrabutylphosphoniumchloride4BuPH Tetrabutylphosphoniumhydroxide werecompletelysolubleintheaqueousalkalisolution.Theproductwasseparatedintosolidandaqueousphasesusingasinteredglassfilter(SchleiderandScheullGF6binderglassmicrofibrefilter,Kent,UK) under vacuum. The flakes were thoroughly washed withdeionised water, dried at 110 ◦ C for 1 h, and then weighed.PET conversion was calculated using the following equation: PET conversion  (%) = W  PET  ,0 − W  PET  W  PET  ,0 × 100 (1)where  W  PET  ,0  and  W  PET   refer to the initial weight of PET and theweightat a specific reaction time (unconverted PET), respectively. The melting thermogram of the residual PET was investigatedby differential scanning calorimetry (Mettler Toledo DSC 822e,Oberhausen, Switzerland) at a heating rate of 10 ◦ C min − 1 .Also, the size of partially converted PET flakes was measuredby laser scattering (Malvern Mastersizer X, Worcestershire, UK).After filtration disodium terephthalate, ethylene glycol, and thePTC were present in the liquid phase. Excess hydrogen chloridewas added into the filtrate in order to neutralise the sodiumhydroxide and provoke the precipitation of the terephthalic acid. Thissolidproductwasfilteredundervacuum,furtherwashedwithdeionised water, dried at 110 ◦ C and weighed. The yield of TPAwas estimated by applying the following expression: TPA yield   (%) = n TPA n PET  ,0 × 100 (2)where  n TPA  is the number of moles of TPA and  n PET  ,0  is the initialnumber of moles of PET (molecular weight of PET about 18000 gmol − 1 with 43 repeating units). The carboxylic acid concentration in the solid products wasdetermined by potential titration. 18  This analysis was used as ameasurementofthepurityoftheTPAprecipitated.Atotalof25 mLofdeuterateddimethylsulfoxidewasusedasthesolvent,inwhich0.1 g of the solid product (TPA) was dissolved. The solution wastitratedwitha0.1Npotassiumhydroxide/ethanolsolutionatroomtemperature. The potential of the solution versus the amount of the titrant recorded was recorded, and the plot was then used todeterminethephenolphthaleinendpointofthetitration.Alsothepurity of the separated TPA was determined by  1 H-NMR (RutinaBruker AC-250,Billerica, Massachusetts, USA).After acidification the remaining liquid was essentially com-posed of ethylene glycol and water. This phase was quantitativelyanalysed by gas chromatography in order to ascertain the pres-ence of products derived from secondary reactions (for example,  JChemTechnolBiotechnol   2009; 84 : 92–99 c  2008 Society of Chemical Industry   9  4 R L´opez-Fonseca  etal  .diethylene glycol, the dimer of ethylene glycol). Methanol wasused as an internal standard. The mixed solution was injectedinto a gas chromatograph (Agilent Technologies 6890N Network GC System, Santa Clara, California, USA) and separated by a 30 mcapillary column (DB-624) with an internal diameter of 3 µ m. The components were carried bya helium flow and detected by aflameionisationdetector.Moreover,thepresenceofotherorganiccompounds, in addition to ethylene glycol, was complementaryverified by  13 C-NMR (Rutina Bruker AC-250). RESULTSANDDISCUSSION PET flakes were hydrolysed with sodium hydroxide to yield thedisodium salt and ethylene glycol according to the followingliquid–solid chemical reaction: PET  ( s ) + 2 NaOH  ( l  ) → Na 2 − TPA ( l  ) + EG ( l  ) (3)Phasetransfercatalystsfindapplicationsinavarietyofreactions,mainly related to the synthesis of organic and fine chemicals. 19 Essentially the principle of PTC is based on the ability of certain‘phase-transfer agents (catalyst)’ to facilitate the transport of onereagent from one phase into another (immiscible) phase whereintheotherreagentexists.Quaternarysalts(QX)are,ingeneral,usedasphase-transfercatalysts.Withinthecontextofalkalinehydrolysisof PET a potentially active PTC should efficiently transport thehydroxide anion from the aqueous phase to the organic phase(external surface of solid PET particles), thereby accelerating thereaction rate. The use of quaternary salts was proposed in ordertocarryoutthereactionunderlessdemandingconditions,i.e.lowNaOH and PTC concentration and temperatures below 100 ◦ C.Recent studies have only evaluated the role of quaternary am-monium salts for this process; 20,21 however, little attention hasbeen paid to examining the behaviour of tetralkyl phosphoniumsaltsasPTCs(Table 1). 22  Themostimportantadvantageofquater-nary phosphonium salts with respect to quaternary ammoniumcounterparts is that the former are thermally more stable, asindicated by Van Krutchen (European PatentEP1140748). 23 Figure1. PETconversionofthePTCinvestigatedafter1hourreactiontime(temperature 80 ◦ C, NaOH concentration 1.67 mol L − 1 , PET concentration0.29 mol L − 1 , PTC concentration 0.07 mol L − 1 ).  The activity results of the PTCs investigated are shown in Fig. 1in terms of the conversion attained after 1 h with the same PTCconcentration (0.07 mol L − 1 ) and reaction temperature (80 ◦ C).It was clearly observed that six (4BuAB, 4BuACl, 4MPB, 4BuPB,4BuPCl,and4BuPH)outofninePTCsremainedcompletelyinactivein the reaction. In other words, conversion values were virtuallyidentical to those noticed in the absence of PTC. Interestingly thethreeremainingsaltsoutperformedallothercatalysts.Thus,itwasnoticed that 3Bu6DPB (tributylhexadecylphosphonium bromide)and4OPB(tetraoctylphosphoniumbromide)at80 ◦ CattainedPETconversion values of 84 and 75%, respectively, in 1 h while thenon-catalysed reaction gave only 16%conversion. The remarkable reactivity of these two quaternary salts wasconsidered to be because of a strong compatibility with theorganic phase and efficient anion transfer due to the highlylipophilic cation. Hence, the sufficient organic structure (largealkyl groups) was responsible for a substantial partition of thecation–anion (QOH) pair into the organic interphase. 24 In sum,thesetwoPTCsfulfilledtherequirementsofhavingenoughorganiccharacter to be lipophilic while small enough to avoid stericallyhinderingthereaction(toalargerextentfor4OPBthan3Bu6DPB).In contrast, four butyl or methyl groups in the quaternary salt(as in the case of 4BuAB, 4BuACl, 4MPB, 4BuPB, 4BuPCl, and4BuPH) appeared not to provide sufficient affinity for the organicphase to the resulting pair, thereby inhibiting efficient interphasetransport of the reactive anion. Further, in an attempt to analysethe influence of the chemical nature of the central cation in thequaternary salt on the catalytic performance, the activity of 4OPBwascomparedwiththatof4OAB(tetraoctylammoniumbromide).PET conversion results revealed that no significant differenceswere evident as conversion with 4OAB (73%) was quite similar tothat observed for 4OPB (75%), and that the PTC activity was thusgovernedmainlybythechemicalnatureofthealkylgroupsinthequaternary salt.It is hypothesised that the cationic part of the catalyst (alkylgroups)carriesthehydroxideanionintothesurfaceoftheorganicphase by means of an interfacial mechanism. Previously, an ionpair was required to be formd between the reaction anion (OH − )and the onium cation (Q + ). In this way the PET macromoleculeson the surface of the flakes can easily be attacked by the OH − group and subsequently depolymerised. The terephthalate anionproduced returns to the aqueous phase and forms the disodiumterephthalate salt with the Na + . The reaction proceeds untilcompletedepolymerisationofPETtoNa 2 -TPAandethyleneglycol,whilethecatalystremainsintheaqueousphase.PTCisregeneratedinitsaddedformandthePTCcyclecontinues.Theoverallreactionscheme is illustrated by Equations (4) and (5). NaOH  + QBr   QOH  + NaBr   (4)2 QOH  + PET  + 2 NaBr  → EG + Na 2 TPA + 2 QBr   (5)Figure 2 shows the influence of PTC (3Bu6DPB) concentration(0.02,0.04and0.07 molL − 1 )andreactiontemperature(60,70and80 ◦ C) on PET conversion. For 10 g of PET flakes depolymerised in150 mLofa6.7%aqueous sodiumhydroxide solution (NaOH:PETmolarratio = 5 . 76)itwasfound,asexpected,thatconversionwaspromotedwithincreasingcatalystconcentrationandtemperature.At 80 ◦ C about 83% conversion was attained after only 1.5 h witha PTC concentration as low as 0.04mol L − 1 ( C  PTC   : C  PET   =  0 . 125).In contrast, note that only 23% conversion was achieved forthe non-catalysed reaction, and the time required for completehydrolysis would be about 10 h. Figure 3 presents the conversion  c  2008 Society of Chemical Industry  JChemTechnolBiotechnol   2009;  84 : 92–99   9   5  Phase transfer catalysed depolymerisation of PET Figure2. PET conversion as a function of reaction time with varying PTC concentration at several reaction temperatures ((a) 60 ◦ C, (b) 70 ◦ C, (c) 80 ◦ C)with  C  NaOH   = 1 . 67 mol L − 1 and  C  PET   = 0 . 29 mol L − 1 . Solid lines representtheoreticalmodelling results. data corresponding to the alkaline hydrolysis carried out in theabsence of PTC at temperatures ranging from 60 to 100 ◦ C atintervals of 10 ◦ C. The concentration values in these experimentswere 1.67 and 0.29mol L − 1 for NaOH and PET, respectively.After removal of the unreacted flakes by filtration andsubsequent neutralisation of the liquid phase and filtration again,the liquid obtained was analysed by gas chromatography fororganicproducts.Onlyethyleneglycolwasfound.Thiswasfurtherconfirmed by  13 C-NMR. On the other hand, results from titrationand  1 H-NMR revealed that high purity TPA was the major solidproductobtainedafterneutralisation.GoodcorrelationwasfoundbetweentheyieldsofTPAandPETconversion.DSCanalysisoffreshand residual flakes suggested that remaining PET particles afterreaction maintained their srcinal structure, thus suggesting thatthe process of depolymerisation in sodium hydroxide solutionoccurred on the external surface of the flakes, and these werelamellarlydepolymerised. 25  Thiswasconsistentwiththeobserveddecrease in PET particle size with increasing conversion (Fig. 4). The kinetics of the PTC-assisted depolymerisation of PET underalkaline conditions is of considerable interest in determining theviabilityoftherouteforrecoveryofmonomersfromwastepolymermaterials. However, few reports are available in the literatureconcerningthisessentialknowledgeforchemicalreactordesign. 20 In this work an attempt to develop a kinetic model was madewithtributylhexadecylphosphoniumbromideasPTC.Thisreactionsystem consists of a solid reactant (PET flakes), a liquid reactant(OH − fromNaOH),andaliquidcatalyst(PTC)solubleintheaqueoussodium hydroxide solution. The phase transfer catalysed reactionsequence involves ion exchange, interphase mass transfer stepsandheterogeneousreactionbetweentheanionferriedacrossandthe PET repeating units. For modelling purposes the kinetics of ion exchange and external diffusion of the QOH catalytic entityfrom the liquid phase to the external surface of the solid reactantwere considered to be very fast, thereby the global reactivitybeing controlled by the reaction at the solid–liquid interface(rate controlling step). A simple theoretical power-law model was  JChemTechnolBiotechnol   2009; 84 : 92–99 c  2008 Society of Chemical Industry   9   6 R L´opez-Fonseca  etal  . Figure3. PET conversion as a function of reaction time in the absenceof PTC at several reaction temperatures (60, 70, 80, 90 and 100 ◦ C) with C  NaOH   =  1 . 67 mol L − 1 and  C  PET   =  0 . 29 mol L − 1 . Solid lines representtheoreticalmodelling results. Figure4. Evolution of PET particle size with increasing conversion(temperature: 70 ◦ C,  C  NaOH   =  1 . 67 mol L − 1 ,  C  PET   =  0 . 29 mol L − 1 , and C  PTC   = 0 . 07 mol L − 1 ). developed to predict the time evolution of conversion with andwithout PTC in the reaction mixture. The reaction rate ( − r  A  inmol L − 1 h − 1 )ofalkalinehydrolysisofPET(ortheproductioneitherof ethylene glycol or of disodium terephthalate) was defined bythemolarconsumptionofPETwithreactiontimeperunitreactionvolume,whichisstrictlyvalidonlyforbatchsystems.SincetheTPAproduced asthereactionproceededwasdissolved inthesolutionas Na 2 -TPA, the terephthalic salt (Na 2 -TPA) would be inactivein an eventual nucleophilic substitution for the esterification(the reverse reaction for the alkaline hydrolysis). 26  Therefore,the reaction of PET hydrolysis under alkaline conditions could beconsidered as an irreversible reaction. On the other hand, as PETconversion in the absence of PTC could not be  a priori   neglectedin the 60–80 ◦ C range, as evidenced by Figs 2 and 3, the reactionrate equation should include both the reaction rate associatedwith the non-catalysed process and the accelerated reaction ratedue to the addition of tributylhexadecylphosphonium bromide. Thus theoverallrate ofthe process could besimplyformulatedasthe following power form: − r   A  = k  NO − PTC  C  aPET  C  bNaOH  + k  PTC  C  aPET  C  bNaOH  C  c PTC   (6)where a , b and c  arethereactionorderswithrespecttoPET,sodiumhydroxide, and PTC (3Bu6DPB), respectively, and  k  NO − PTC   and  k  PTC  refers to the non-catalysed and catalysed-reaction rate constants,respectively. On the other hand,  c  PET  , c  NAOH   and  c  PTC   representthemolarconcentration(molL − 1 )ofPET,NaOHandPTC,respectively.For the sake of simplicity it was assumed that the kinetics of the process could be determined according to a homogeneouscatalytic system. Further, the reaction rate was considered to beproportional to the ester and alkali concentration ( a  =  b  =  1). Thus Equation (6) can be rearranged as follows: − r   A  = ( k  NO − PTC   + k  PTC  C  c PTC  ) C  PET  C  NaOH   = k   C  PET  C  NaOH   (7)where  k   is the apparent rate constant (L mol − 1 h − 1 ). Taking intoaccount that two moles of NaOH react with each mole of PETreactingunitandifaconstantvolumeforthereactionsisassumed(150 mL),the balanceequation intermsof PETconversion(  X  ) andNaOH:PET molar ratio ( M ) can be expressed as:12 − M  ln  M (1 −  X  ) M − 2  X   = k   C  PET  ,0 t   (8) This kinetic model was examined by fitting the experimentaldata recorded at temperatures ranging between 60 and 80 ◦ Cand varying PTC concentration (0–0.07 mol L − 1 ). The results areshowninFig. 5andindicatethatthissimplemodelprovidedgoodlinear relationships with linear correlation factors ( r  2 ) higher than0.98 in all cases. It could therefore be assumed that the reactionrate was first order with respect to the PET concentration and firstorderwithrespecttotheNaOHconcentration 27,28 inthepresenceor absence of PTC. The apparent rate constants at the selectedtemperatures could be estimated from the slopes in the plots.Note that experimental data corresponding to the non-catalysedprocess were also included. The linear relationship between thevaluesoftheapparentconstantrate( k   )andthePTCconcentrationallowed determination of the reaction order with respect to thephase transfer catalyst. Hence, results clearly evidenced a first-order reaction ( c   =  1) as revealed by the plot of   k   versus  C  PTC  shown in Fig. 6. Further, the reaction rate related to the catalysedreaction could be estimated from the slope of this plot. It wasobservedthatthevalues of  k  PTC   were130–190times greaterthanthose of   k  NO − PTC   (Table 2). Temperature effects on the hydrolysis rate constants ( k  NO − PTC  and  k  PTC  ) were also investigated. According to the relationship of the rate constants with the reaction temperature, the Arrheniusplots are shown in Fig. 7. As observed, data fell on straight lineswith linear correlation factors ( r  2 ) higher than 0.99. The activationenergy for the phase transfer catalysed alkaline hydrolysis of PETcalculatedfromtheslopewas75 ± 5 kJ mol − 1 .Thepre-exponentialfactor calculated from the intercept was 2 . 5 × 10 12 L 2 mol − 2 h − 1 with a confidence interval ranging from 3 . 6  ×  10 11 to 1 . 8  × 10 13 L 2 mol − 2 h − 1 . The value estimated for the activation energy  c  2008 Society of Chemical Industry  JChemTechnolBiotechnol   2009;  84 : 92–99
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