Towards recycling purpose: Converting PET plastic waste back to terephthalic acid using pH-responsive phase transfer catalyst

Yueqing Wang,Hongxing Wang,Hongmei Chen,Hantao Liu

1 School of Energy and Power Engineering,North University of China,Taiyuan 030051,China

2 State Key Laboratory of Coal Conversion,Institute of Coal Chemistry,Chinese Academy of Sciences,Taiyuan 030001,China

3 School of Instrument and Electronics,North University of China,Taiyuan 030051,China

Keywords:Polyethylene terephthalate Alkaline hydrolysis pH-responsive catalyst Terephthalic acid

ABSTRACT Converting polyethylene terephthalate (PET) wastes to its monomer and valuable chemicals via ecofriendly chemical method is still a challenge task.Previously,phase transfer catalysts used for alkaline hydrolysis were soluble in reaction media and hardly separated after reaction.Here,we reported several pH-responsive catalysts combined alkyl quaternary ammonium units with heteropolyacid anion for achieving stepwise product/catalyst separation and catalyst recycling.The properties of homogeneous/heterogeneous transfer behavior allow catalyst to be easily separated from reaction media by adjusting of pH value.Among them,[C16H33N(CH3)3]3PW12O40 (abbreviated as [CTA]3PW) exhibits the highest activity and the most suitable pH responsive values.Such a pH triggered switchable catalytic system not only shows good performance for depolymerization of pure PET,but also some real PET wastes such as coloured trays and PE/PET complex films could be completely degraded into terephthalic acid.Additionally,the reaction kinetics and activation energy of PET alkaline hydrolysis also studied with and without pH-responsive [CTA]3PW.

1.Introduction

Plastic plays important role in improving our daily lives with application range from food packaging,automotive to medicine and many others [1,2].Among them,polyethylene terephthalate(PET) represents 30% of global plastic demand due to its chemical resistance,impermeability,transparency and light weight [3,4].This commercial polymer is widely used for synthetic fibers,food packaging plastic films coupled with their low market price [5].The spontaneous degradation of PET generally takes over hundreds of years because its extreme stability and durability.The continuous accumulation of PET waste has become a serious environmental problem,and a large fraction of PET waste is end up in a landfill or ocean [6].Therefore,recycling of PET waste has gained great attention to mitigate the environmental impact.It is thus clear that there is need for developing the advanced technologies to degrade PET waste.Despite the waste PET can be reprocessed into fibers,sheets and strapping by mechanical method,several drawbacks like properties deterioration and lower reprocessing rate (～30%)are inevitable [7].Especially for the complex PET waste,such as laminated with polyolefins or coloured wastes,the mechanical method is even impossible,or only towards downgraded applications.Hence,converting PET wastes,especially post-consume complex wastes,back into its monomer is regarded as an economically efficient strategy to alleviate the PET waste pollution.Recovered terephthalic acid (TPA) can be used as chemical feedstock for high-value chemicals or reproduction PET by polymerization with ethylene glycol.

Towards recycling purpose,the significant endeavors,such as converting aromatic plastic to arenes by hydrogenolysis technology developed by Wang group [8,9],degrading PET to monomer using single-site molybdenum-dioxo complex reported by Marks and co-workers [10],and hydrolysis methods have been devoted to upgrade or depolymerize PET plastic waster [10-13].Among all them,hydrolysis is regarded as the most promising method with mild degradation conditions,high tolerance of contaminated post-consumer plastic and desirable reconversion of monomer[1,14,15].Hydrolytic depolymerization of PET involves chain scission of esteric bond by attacking of a molecule water,and this process generally categorized as neutral,acidic and alkaline hydrolysis[16].Specially,the ester bond hydrolysis is thermodynamically favorable due to denoting protons to the nucleophilic center of ester group in the acidic or alkaline reaction conditions.Acidic hydrolysis has to be conducted in strong acidic media such as 10 mol·L-1sulfuric acid or 13 mol·L-1nitric acid to enhance hydrolysis kinetics [17,18],but using highly amount acid makes process very costly and also affects the purity of reproduced TPA.Although the reasonable recovery rate was also obtained using solid acid as catalyst,the separation of water-insoluble TPA from solid catalyst is extremely complex [19].Therefore,alkaline hydrolysis,as an alternative method,gives dissolved disodium or dipotassium terephthalate salt and highly purified TPA can be easily recovered by acidification.Recently,ügdüler and coworkers [20] focused on the recycling of real PET plastic waste by alkaline hydrolysis using ethanol/water system.From ecofriendly perspective,using water as reaction media is more favorable than high ratio ethanol mixture.However,with a high ratio of aromatic terephthalate unites,PET is extremely difficult to hydrolyse in alkaline aqueous medium.High temperature and pressure are still employed to completely degrade PET in alkaline hydrolysis[21-23].Having employed phase transfer catalyst from amphiphilic quaternary ammonium salt is an efficient approach.The hydroxyl ion combined with phase transfer catalyst is more easily to attack ester bond than free hydroxyl ion.For instance,using tetrabutylammonium iodide [24,25],trioctylmethylammonium bromide and cetyltrimethylammonium bromide [16] as catalyst,the depolymerization of PET is much faster than the case of catalystfree.However,a serious obstacle for practical utilization of these homogenous phase transfer catalyst is the recycling and separation of catalysts after hydrolysis.Therefore,the design of high-efficient heterogeneous catalyst for depolymerization of PET waste plastic at mild conditions is of great importance in alleviation of environmental pollution.

Inspired by recovery of TPA from PET plastic waste always involves with tuning of pH value from alkalinity to acidity,we developed a pH-responsive phase transfer catalyst,the combination between H3PW12O40and quaternary ammonium,for multiphase alkaline hydrolysis.In this catalytic system,the active sites amphiphilic quaternary ammonium exhibits pH-controlled homogeneous/heterogeneous transfer behavior,it is dissolves in alkaline hydrolysis process (pH ＞ 10) and functions as highly active homogenous catalyst to enhance hydrolysis of PET waste.When tuning pH value to acidity,the homogeneous catalysts were precipitated again and it can be easily separated by simply centrifuge treatment.This pH-responsive behavior comes from assistance of large heteropolyacid group.Such a new strategy might provide opportunities to reduce PET waste using heterogenous catalysts.So far,few studies have been devoted to develop pH-responsive catalyst for the hydrolysis of PET waste into TPA.

In this work,different chain length of quaternary ammonium,including tetraethylammonium,tetrabutylammonium,dodecyltrimethylammonium and cetyltrimethylammonium,had been selected to vary the interfacial microenvironment between PET chain and aqueous phase.In addition,this efficient catalytic system also was expanded to post-consumer PET plastic waste disposal,which has very low recycling rate due to the broad range of coloured,multilayer structure,additives and other complexities.These post-consumer PET waste also completely depolymerized into TPA catalyzed by pH-responsive catalyst in mild conditions.

2.Experimental

2.1.Materials

PET pellets were purchased from Macklin Co.,Ltd.Quaternary ammonium salts with different chain length,namely tetraethylammonium bromide (TEAB,98.0%),tetrabutylammonium bromide(TBAB,99.0%),dodecyltrimethylammonium bromide (DTAB,99.0%) and cetyltrimethylammonium bromide (CTAB,99.0%) were supplied by Sinopharm Chemical Reagent Co.,Ltd.Acetonitrile(AR,≥99.5%),H3PW12O40(AR) and sodium hydroxide (AR,≥99.5%)were obtained from Shanghai Aladdin Co.,Ltd.All reagents utilized in this work were used as received without further purification.The post-consumer PET samples were prepared from single-use coloured trays and multilayer films.

2.2.Synthesis of catalysts

The pH-responsive catalysts were synthesized as follows.The organic cation and H3PW12O40(HPW) was respectively dissolved in a certain deionized water by mole ratio of 3:1.Take the preparation of [C16H33N(CH3)3]3PW12O40as example,5.0 g (1.74 mmol)H2PW12O40and 1.89 g (5.22 mmol) CTAB were dissolved into 30 ml DI water,respectively.Then the H2PW12O40aqueous solution was added into CTAB solution with stirring at room temperature.A white precipitate was formed and collected by filtration,washed by DI water to neutral and dried overnight at 100°C.Then the as obtained white powder was recrystallized twice by CH3CN and dried at 80 °C.While product [C16H33N(CH3)3]3PW12O40was obtained with yield of 67.13%,and abbreviated as [CTA]3PW.The other pH-responsive catalysts also prepared using the same pressure with different chain length of quaternary ammonium.

2.3.Catalysts characterization

The chemical compositions of prepared samples were determined by Elementar (Germany,Vario EL CUBE) and inductively coupled plasma atomic emission spectrometry (ICP-AES).X-ray diffraction (XRD) patterns were collected using BrukerAxs D2 diffractometer,using Cu Kα radiation from a Cu X-ray tube(λ=0.154 nm) at 30 kV and 10 mA.

FTIR spectroscopy was employed to distinguish characteristic peaks of synthetic catalysts.The FTIR spectra was recorded on Bruker VERTEX 70 FTIR spectrophotometer,equipped with deuterium triglycine sulfate(DTGS)detector.The power samples were mixed with KBr and pressed to translucent disks.The spectra were recorded between 4000 cm-1and 400 cm-1.

Liquid phase1H and13C nuclear magnetic resonance(NMR)was obtained by using Bruker AVANCE 400 spectrometer.The chemical shifts were referenced to an external standard of D6-dioxane.The13C NMR spectrum was recorded at 100 MHz,the relaxation delay was 10 s.The1H NMR spectrum was recorded in D6-DMSO operating at 400 MHz.Chemical shifts were given downfield from TMS.

2.4.Hydrolysis of PET waste

As shown in the Fig.1,PET granules were hydolyzed in aqueous alkaline (NaOH) medium at atmospheric pressure to produce disodium terephthalate (Na2TP) salt.Afterwards,the TPA monomer was precipitated by acidification of Na2TP solution.

All PET alkaline hydrolysis experiments were performed in a batch Teflon sealed reactor (30 ml) equipped with an agitator for stirring.Teflon reactor containing NaOH,PET granule,catalysts and water was placed into sand bath at selected temperature.Sand bath was preheated to set temperature in order to minimize the delays to reach specified temperature at atmospheric pressure.The reactor was removed from sand bath and quench at ice bath to stop the progress of PET hydrolysis at specified time interval.The residual PET was separated by centrifugation,after washed,dried at 70 °C and weighed.Sulfuric acid (1 mol·L-1) was added to transfer Na2TP to insoluble TPA and then separated by centrifugation.The conversion of PET was calculated by the following formula:

Fig.1.Alkaline hydrolysis of PET in presence of NaOH.

whererefer to the initial mass (g) of PET and PET mass (g) at a specified reaction time,respectively.

The yield of TPA was determined by1H NMR,and the standard solutions were used to obtained calibration curves to calculate the concentrations of TPA by external standard method.The yield was calculated using the following equation:

3.Results and Discussion

3.1.Characterization of catalysts

Generally,the protons of H3PW12O40could be completely or partly substituted by monovalent metal ion or ammonium ion.Therefore,the entire substitution was desired for enhancement of catalytic activity.In order to confirm the chemical compositions of synthetic catalysts,the elemental analyses results have been summarized in Table 1,the mole ratio of W : P was 11.0,12.5,12.8 and 10.9 for [TEA]3PW,[TBA]3PW,[DTA]3PW and [CTA]3PW,respectively.These results shown that the heteropolyacid anions kept a 1:12 Keggin structure in catalysts.Compared with calculated values of C,H and N,the experimental values were satisfactory to confirm that the protons of H3PW12O40have been entirely replaced by quaternary ammonium cation.

The FTIR spectrum was employed to further confirm structure of as-prepared catalysts.As shown in Fig.2,four mainly characterization peaks at 1096 cm-1,988 cm-1,902 cm-1and 806 cm-1were observed for H3PW12O40,reflecting the four different vibrations of oxygen atoms of Keggin structure of[26].The peak at 1096 cm-1was attributed the asymmetry vibration of internal oxygen connecting P and W(P-Oa).The other three peaks at 988 cm-1,902 cm-1and 806 cm-1could be assigned to asymmetry vibrations of terminal oxygen bonding to W atom (W-Od),edge-sharing oxygen connecting W (W-Ob) and corner-sharing oxygen connecting W3O13units (W-Oc),respectively [27,28].

Compared to pristine H3PW12O40,the same peaks were observed after substitution of protons by different quaternary ammonium cation.In addition,the peaks at 2923 cm-1,2853 cm-1assigned vibration of C-H bond for [CTA]3PW and[DTA]3PW [29],while vibration of C-H bond peak at 2967 cm-1and 2873 cm-1for[TBA]3PW.All of the synthesized samples exhibit the vibration peak of C-N bond at 1475 cm-1[30],showing the existence of quaternary ammonium in catalysts.These results clearly indicate that all these pH-responsive catalysts kept the pristine heteropolyacid Keggin structure after combination of different quaternary ammonium and H3PW12O40at certain ratio.Meanwhile,the elemental analysis results also demonstrate that there was not physical mixture between quaternary ammonium salt and H3PW12O40.Due to the ion radius of heteropolyacid anion was enough large,even some high-molecular-weight quaternary ammoniums could be entirely replaced three protons of phosphotungstic acid.

3.2.PET depolymerization by pH-responsive catalysts

Initially,the pH-responsive property was verified by blank experiments.All of the synthesized catalysts can be transformed between heterogeneous and homogeneous phase with alteration of pH values,but they were not responsive for temperature range from 80 to 110 ℃.

Then,the hydrolysis of PET was investigated with pHresponsive catalysts.As shown in the Fig.3,with increasing of alkyl chain length,the conversion percentage of PET was correspondingly increased.Using cetyltrimethylammonium bromide as a contrast catalyst,the comparable PET conversions were obtained on[DTA]3PW and [CTA]3PW under mild reaction conditions,indicating that the hydrolysis of PET can be significantly accelerated by pH-responsive catalyst,[DTA]3PW and [CTA]3PW could be considered as desirable catalyst.The remarkable performance of [DTA]3-PW and [CTA]3PW was derived from its stronger compatibility with the PET surface and efficient hydroxyl ion transfer.While lower PET conversions were observed over [TEA]3PW and [TBA]3-PW mainly arising from four ethyl and butyl appeared not to provide sufficient affinity for the solid PET phase.To easily obtained pure TPA,pH-responsive range of catalysts should not overlap with the value of TPA precipitation.As shown in Table 2,when pH valuewas adjusted to 5.0,the disodium terephthalate (Na2TP) can be completely transformed into TPA.The [TBA]3PW and [DTA]3PW should be firstly excluded due to the overlapped pH value of reprecipitation.The pH value of[TEA]3PW was not overlapped with TPA,but its activity was far lower than that other catalysts.Interestingly,pH value of reprecipitation could be finely tuned by change of alkyl chain length,leading the pH value of [CTA]3PW was not overlapped with TPA.

Table 1Elemental analyses of catalysts.

Table 2The pH value of reprecipitation.

In order to elucidate their different terminal pH value of reprecipitation,the crystal structure of these pH-responsive catalysts was investigated by X-ray diffraction patterns (as shown in ESI?,Fig.S1).There was the same terminal pH value for these catalysts such as[TEA]3PW and[DTA]3PW with the same characteristic peak at 8.9°.While the different terminal pH value for [TBA]3PW and[CTA]3PW with different characteristic peaks,indicating that the different pH value of reprecipitation may be ascribe from their different crystal structure.Moreover,the crystal structure also may be influenced by alkyl chain length of quaternary ammonium.After hydrolysis of PET,stepwise recovery of [CTA]3PW and TPA monomer can be successfully realized during adjusting of pH value from alkalinity to acidity.

Previous studies proposed that degradation rate of PET is influenced by amount of sodium hydroxide in case of catalyst free.Here,the different sodium hydroxide percentage also investigated with or without [CTA]3PW,the obtained PET conversions were plotted as function of reaction time as shown in Fig.4.In absence of [CTA]3PW,the PET conversions are no more than 30% even in high NaOH concentration.While the amount of PET converted to its monomer increases as addition of [CTA]3PW,and the PET conversions are distinctly increased from 7.8% to 80.0% in 2% (mass)of NaOH,indicating that the pH-responsive [CTA]3PW really plays important role in acceleration of PET degradation rate.

Additionally,the time of degradation was reduced from 5 h to 2 h as NaOH concentration increasing from 2%(mass)to 8%(mass),demonstrating that the NaOH concentration also is a crucial parameter in this catalytic hydrolysis.From the perspective of eco-friendly and cost-efficient,NaOH concentration was chosen 4% (mass) as the most optimal parameter.In order to confirm the role of alkane,(NH4)3PW was synthesized by replacing cetyltrimethyl ammonium bromide for NH4Cl.The degradation of PET only is approximately 16.8%,indicating that the alkane unit is necessary,while phosphotungstic acid anion is invalid for degradation of PET.Meanwhile,the longer alkyl chain is more efficient than shorter arise from its stronger affinity for PET surface.

Moreover,the other parameters such as catalysts dosage and reaction temperature also studied.As shown in Fig.5,even the dosage of [CTA]3PW reduced to 0.25% (mass),PET conversion still kept as high as 94.0%.As the temperature elevated from 90 °C to 110 °C,the PET conversion was distinctly increased from 50.0%to 98.4% (as shown in ESI?,Fig S2).

Fig.2.The FT-IR spectra of catalysts,HPW is abbreviation of H3PW12O40.

Generally,some transparent or uncontaminated PET waste plastics have been well recycled,while the recycling rate of opaque PET trays and films is significantly lower due to the broad range of coloured,multilayer structure and other complexities [31].Therefore,they are mainly disposed of in landfill because it is impossible to obtain secondary raw materials by using mechanical process.In addition to multilayer structure,coloured additives such as carbon black and others is one of the bottle neck due to its insolubility in aqueous media.The purity of monomer is decreased during precipitation of Na2TP mixing with carbon black pigments.Therefore,using [CTA]3PW as phase transfer catalyst,a potential process to obtain pure TPA from alkaline hydrolysis of food black PET trays and PE/PET complex film was shown in Fig.6.

First of all,conditioned post-consumer food tray was sieved to desired particle size(0.3 cm-0.5 cm)and then subjected to alkaline hydrolysis at the previously described optimal experimental conditions (catal.0.5% (mass),NaOH 4.0% (mass) and at 110 °C).To completely degrade these multilayer structure and bigger particle size PET,reaction time prolonged to 12 h.Afterwards,the black pigments could be removed by filtrating,and the obtained transparent solution was firstly acidified to separate pH-responsive[CTA]3PW.Then,pure TPA could be precipitatedviasecondary acidification step,and there are no impurities to be observed in1H NMR spectrum(as shown in ESI?,Fig.S3).Using PE/PET as substrate,the PET side could be also depolymerized under mild reaction conditions.As the lower reaction temperature,other polymer layer such as polyethylene are not affected by this process.Residual film was confirmed by FT-IR spectrum,as shown in Fig.7,the vibration peaks of PET are disappearance,while vibration peaks of PE are still existence,indicating that the PET layer has been completely depolymerized catalyzed by[CTA]3PW without destruction of PE layer.

Fig.3.The activity tests of different catalysts.Reaction conditions: 0.1 g PET granule with 250 μm particle size,10 g water with 4.0% (mass) NaOH,0.5% (mass)Catal.,110 °C,5 h.TEA=tetraethylammonium,TBA=tetrabutylammonium,DTA=dodecyltrimethylammonium,CTA=cetyltrimethylammonium.

Fig.4.Hydrolysis of PET in different sodium hydroxide percentage,a)with pH-responsive[CTA]3PW,b)without catalyst.Reaction conditions:0.1 g PET granule with 250 μm,catal.0.5% (mass),110 °C.Control experiments were conducted in each optimal reaction conditions.

Fig.5.The effects of [CTA]3PW dosage.

Compared with catalyst-free alkaline hydrolysis,pure or complex PET plastic waste could be completely degraded into its monomer catalyzed by pH-responsive [CTA]3PW with low sodium hydroxide concentration.

3.3.Analysis of reproduced TPA

The purification of obtained TPA was analyzed by NMR spectroscopy,and the1H NMR and13C NMR spectra have been shown in Fig.8(a) and (b),respectively.

In the1H NMR spectrum,a stronger resonance peak at 8.05 should be ascribed to aromatic protons (denoted as Ha).The other chemical shift of 13.2 was observed in the hydroxyl protons of TPA (denoted as Hb).It should be noted that there are only two resonance peaks were observed with no resonance peaks of methyl or methylene units,except for these arising from the NMR solvent indicating that the TPA and pH-responsive catalyst could be successfully recovered by stepwise acidification procedure.Similarly,in the13C NMR spectrum,three kinds of resonance peaks are observed,the chemical shift at 129.9 arising from aromatic carbon(denoted as Ca).The resonance peaks at 134.7 and 167.4 are ascribed to quaternary carbon(denoted as Cb)and carboxyl carbon(denoted as Cc),respectively.In addition to those peaks arising from NMR solvent,no further peaks were detected,also demonstrating that the recovered TPA is not mixed with [CTA]3PW.The NMR spectrum of recovered TPA matched well with previously reported1H and13C NMR spectra of commercial TPA.

Fig.6.Degradation of real post-consumer PET waste catalyzed by [CTA]3PW.

Fig.7.The FT-IR spectra of PET(blue line),PE(red line)and after degradation of PE/PET film (black line).

3.4.Kinetics studies on [CTA]3PW12O40 for PET depolymerization

To further understand the effect of pH-responsive catalyst on the PET hydrolysis,the kinetics studies were performed with and without[CTA]3PW.Generally,an assumption that cationic party of catalyst carries the hydroxide ion into the surface of PET flakes had been proposed to elucidate the catalytic behavior of homogeneous phase transfer catalysts.The external diffusion of catalytic species from liquid phase to solid reactant was considered to be very fast using soluble CTAB as catalyst,and the reactivity was mainly controlled by the reaction at the solid-liquid interface.Similarly,in this pH-responsive catalytic system,the reaction kinetics could be regarded as homogeneous catalytic process due to the homogeneous/heterogeneous transfer behavior of [CTA]3PW.Therefore,according with previously reported kinetic studies,TPA production from PET hydrolysis is followed pseudo 1st order reaction with a reaction kinetic constant (k) value that can be calculated using the following equations,derived by the integrated rate laws:

Fig.8.The NMR spectra of recovered TPA (a) 1H NMR and (b) 13C NMR.

Fig.9.Kinetic analyses (a) Time-ln(A) curves of the catalyst-free,CTAB and [CTA]3PW,(b) Arrhenius plots of PET depolymerization over [CTA]3PW catalyst (R2 is the correlation coefficient).

Fig.10.The reusability of [CTA]3PW.

Fig.11.(a) The XRD patterns of spent and fresh [CTA]3PW,(b) FTIR spectra of reused [CTA]3PW.

wherekis the reaction constant,tis the reaction time,CPET,tis the PET concentration at reaction timet,andCPET,0is the initial PET concentration.The-kvalue is calculated from the slope of the inverse PET concentration as a function of reaction time with and without catalyst.As shown in Fig.9(a),the calculatedkvalues of PET depolymerization were 0.089,0.4021 and 0.7145 for the catalyst free,[CTA]3PW and CTAB,respectively.The calculated reaction constant(0.4021)for hydrolysis catalyzed by[CTA]3PW is 4.5 times than the catalyst-free hydrolysis,indicating that the reaction rate is indeed accelerated by [CTA]3PW.

While the reaction constant is still slight lower than those hydrolysis with CTAB maybe due to the competitively combinate with quaternary ammonium between heteropolyacid anion and hydroxide ion in phase transfer process.

Moreover,apparent activation energy (Ea) of PET hydrolysis with [CTA]3PW was calculated using the Arrhenius equation as below:

wherekis the reaction constant,Ais the pre-exponential factor,Eais the apparent activation energy,Ris the gas constant,Tis the absolute temperature.The Arrhenius plots was obtained in Fig.9(b) shows good linear correlation with a correlation coefficient of 0.997.The apparent activation energy is 60.15 kJ·mol-1calculated from the slope of the curves of (1/T)versusln(k) for the [CTA]3PW,lower than alkaline hydrolysis without catalyst,96.27 kJ·mol-1.

3.5.Reused tests of [CTA]3PW12O40

The stability,recycling and reusability are important aspects of this pH-responsive catalyst and allow for the advancement of costeffective chemical process.The reusability of [CTA]3PW was also investigated under optimal reaction conditions,and the relative results had been shown in Fig.10.

Indeed,the pH-responsive[CTA]3PW exhibits high stability and is conveniently recycled and reused up to 5 times without significant loss of activity.The yield of TPA still maintained ＞90.0%over 5 cycles on continuous reuse.After 5 cycles,there was a 4.0%decrease of TPA yield,the proposed reason maybe is that weight loss of catalyst during each recycling processes.Meanwhile,high activity was maintained in each recycles,suggesting that the structure of [CTA]3PW could be recovered after transforming from homogeneous to heterogeneous phase.From first to fifth reaction cycle,pH responsive property of the systems could be still existed.

The structure of recycle [CTA]3PW was also investigated by X-ray diffraction patterns and FTIR spectrum.As shown in Fig.11(a),although distinctly decreasing of diffraction peak intensity,the same characteristic peak of spent[CTA]3PW is observed at XRD patterns indicating that spent catalyst possesses the same crystal structure with fresh [CTA]3PW.

Moreover,the Keggin structure of recovered[CTA]3PW also confirmed by IR spectra at peak 806 cm-1,902 cm-1,977 cm-1and 1077 cm-1,which could be assigned to asymmetry vibration of W (W-Oc),W (W-Ob),W(W-Od) and O(P-Oa) bonds (as shown in Fig.11(b)).The broad and shift of some peaks maybe results from hydrogen-bond interaction.Additionally,the residual P and W elements also were detected in solution phase.There are 0.00076% (mass) P and 0.017% (mass) W was quantified by ICP,indicating that the homogenous catalyst was nearly reprecipitated again.

4.Conclusions

In summary,several pH-responsive catalysts were synthesized by a simple precipitation method.Due to its transformation between heterogeneous and homogeneous,the solid [CTA]3PW exhibits significant catalytic performance in alkaline hydrolysis of PET.Using pure PET as substrate,the depolymerization rate was up to 99.0% at 110 ℃for 5 h.Moreover,some complex PET plastic wastes such as coloured trays and multilayer PET films also could be successfully degraded into its monomer under mild conditions.The separation of product and recycling of catalyst could be achieved only by adjusting the pH values of reaction system.Above results indicate that the pH-responsive catalyst [CTA]3PW are efficient,easily recoverable and reusable,making the process sustainable and eco-friendly.Therefore,it could be envisioned that this pH-responsive catalyst may be a promising strategy for alkaline hydrolysis of PET wastes.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work thanks to the support of the National Natural Science Foundation of China (22005276).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.10.028.