Applied Surface Science Volume 238 issue 1-4 2004 [doi 10.1016%2Fj.apsusc.2004.05.229] J.B. Parra; C.O. Ania; A. Arenillas; F. Rubiera; J.J. Pis -- High value carbon materials from PET recycling (1).pdf | Adsorption | Materials Science

Applied Surface Science 238 (2004) 304–308 High value carbon materials from PET recycling J.B. Parra*, C.O. Ania, A. Arenillas, F. Rubiera, J.J. Pis Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain Available online 19 July 2004 Abstract Poly(ethylene) terephthalate (PET), has become one of the m
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  High value carbon materials from PET recycling J.B. Parra * , C.O. Ania, A. Arenillas, F. Rubiera, J.J. Pis  Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 Oviedo, Spain Available online 19 July 2004 Abstract Poly(ethylene) terephthalate (PET), has become one of the major post-consumer plastic waste. In this work special attentionwas paid to minimising PET residues and to obtain a high value carbon material. Pyrolysis and subsequent activation of PETfrom post-consumer soft-drink bottles was performed. Activation was carried out at 925  8 C under CO 2  atmosphere to differentburn-off degrees. Textural characterisation of the samples was carried out by performing N 2  adsorption isotherms at  196  8 C.The obtained carbons materials were mainly microporous, presenting low meso and macroporosity, and apparent BET surfaceareas of upto2500 m 2 g  1 .The capacity of these materialsfor phenoladsorption andPAHs removal from aqueoussolutions wasmeasured and compared with that attained with commercial activecarbons. Preliminary tests also showed high hydrogen uptakevalues, as good as the results obtained with high-tech carbon materials. # 2004 Elsevier B.V. All rights reserved. Keywords:  Carbon materials; Plastic waste; Adsorption; H 2  storage 1. Introduction Plastics utilization has grown sharply over theyears. Particularly, PET consumption has recordedthe fastest growth rate in the global plastic marketduetoongoingexpansionofthePETbottlemarket[1].The usage of polyethylene and PET products, asthings stand right now, constitutes a relevant envir-onmentally unsustainable problem. The basic chal-lenge is for PET post-consumer reclamation to keeppace with growing consumption.The most common ways to deal with PET disposalproblems are incineration and chemical recycling[1,2]. Recycling PET waste is increasingly demandedfor both ecological and technological reasons. Inaddition, stricter regulation concerning the recoveryof waste is currently coming in force.In this work, the obtention of a high value carbonmaterial from PET waste was investigated as analternative to chemical recycling to minimise the evendisposal problems. In these lines, the aim of this work was to characterise the carbon products obtained frompyrolysis and subsequent activation in CO 2  of PETwaste, and to extend the field of the potential applica-tions of the final products as adsorbents. In this sense,the capacity of these materials for phenol adsorptionand PAHs removal from aqueous solutions was eval-uated and compared to that attained with commercialactivecarbons. On the other hand, preliminary tests onhydrogen uptake were also carried out and comparedwith those attained in other works with more expen-sive high-tech carbon materials [3,4]. 2. Experimental The experimental procedure followed during PETwaste pyrolysishasbeen previouslydescribed[5].The Applied Surface Science 238 (2004) 304–308 * Corresponding author. Tel.:  þ 34-985-119090;fax:  þ 34-985-297662. E-mail address: (J.B. Parra).0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2004.05.229  char obtained, P was ground, sieved and a particle sizefraction of 0.075 – 0.212 mm was selected for physicalactivation with CO 2 . P char was treated at 925  8 Cunder nitrogen atmosphere for 1 h (sample PC). Then,around 6 g of the carbon material was activated in10 mL min  1 of CO 2  at 925  8 C. Samples with differ-ent activation degrees (12, 35, 58 and 76% burn-off)were obtained and denoted as PC12, PC35, PC58 andPC76, respectively.Textural characterisation was carried out by mea-suring mercury porosimetry and N 2  adsorption iso-therms at   196  8 C. Different methods (i.e. BET,Dubinin-Radushkevich, DFT) were applied to theadsorption isotherms.For phenol and PAH adsorption from aqueoussolution, columns with 0.5 g of the carbon materialswere employed. The  fl ow rate of the solution(4 mL min  1 ) and the temperature (30  8 C) were keptconstant throughout the adsorption stage. In this work,breakthrough experiments were conducted and ana-lysis of the  fi nal concentration of the solutions wasdone spectrophotometrically (UV – Vis). 3. Results and discussion 3.1. Textural characterisation By application of the BET method to the N 2  adsorp-tion isotherms, the apparent surface area values of thesamples were obtained. Table 1 compiles the apparentBET surface areas along with the textural parametersby application of the Dubinin – Radushkevich (DR)equation.A signi fi cant and gradual increase of the apparentBET surface area with the degree of burn-off wasobserved, reaching values relatively high for the mostactivated sample PC76. An increase in the microporevolume and the accessible pore width (  L  ) with theburn-off degree was observed.The pore size distribution (cf. Fig. 1) was obtainedfrom the mercury porosimetry data and the DFTmethod applied to the N 2  adsorption isotherms [6].From mercury porosimetry, it was observed that thesamples studied are non-macroporous. Only the sam-ples activated with CO 2  at high burn-off degrees PC58and PC76 present a slight development of large pores.Onthe otherhand, an increase inthevolumeofnarrow Table 1Textural parameters obtained from N 2  adsorption isotherms and theBET and DR equationsSample  S  BET (m 2 g  1 )DR W  0 (cm 3 g  1 ) E  0 (kJ mol  1 )  L  (nm) S  mic (m 2 g  1 )PC12 668 0.25 27.8 0.66 629PC35 1405 0.52 21.8 1.04 909PC58 1920 0.67 19.0 1.42 930PC76 2468 0.90 16.3 2.19 870Fig. 1. Pore size distribution of the samples studied using the DFT method and mercury porosimetry.  J.B. Parra et al./Applied Surface Science 238 (2004) 304  –  308  305  micropores with the increase in the degree of burn-off was observed upto 35% of activation, while volume of the medium-sized micropores increases gradually asburn-off increases. This indicates a gradual broad-ening of the pores with the burn-off degree. Themesoporosity of the samples also increases graduallywith the burn-off degree. 3.2. PAH adsorption from an aqueous solution Preliminary tests for the evaluation of the ef  fi ciencyof these materials for the adsorption of aromatics fromliquid phase were carried out, using naphthalene andphenolasprobemolecules.Theadsorptioncapacitiesof the samples studied along with the uptake of commer-cialactivatedcarbons(Q,CMandF400)arecompiledinTable 2. The characteristics of the commercial adsor-bents selected can be found in the literature [7,8].A relationship between the surface area of theactivated materials and their adsorption capacitieswas observed. The most activated the sample, thehigher adsorption capacity was attained. The resultsobtained reached removal of both phenol and naphtha-lene comparable to the retention attained with com-mercial activated carbons. Consequently, samplesobtained by pyrolysis and subsequent activation of PET waste showed good properties as adsorbents of organic compounds. 3.3. Hydrogen adsorption One of the possible applications of these materialsis hydrogen storage. Hydrogen is currently of greatinterest because it is the cleanest vector of energy.Although carbon materials have been proposed as asolution for hydrogen storage [9,10], an adequate poresize distribution is necessary to assure a strong inter-action with hydrogen at room temperature and atmoderate pressures [11].As a preliminary study of the suitability of thesamples obtained from PET waste for hydrogen sto-rage, H 2 -adsorption measurements were performedwith a Micromeritics ASAP 2000 at   196  8 C in thepressure range 0 – 1 bar. It was observed that adsorp-tion – desorption experiments were reversible, suggest-ing that H 2  uptake path goes through physisorption.Modelling isotherms is a useful tool in adsorptionstudies, as practical information (i.e. adsorptive capa-city) can often be provided. The simplest approach fordescribing gas adsorption on surfaces is the idealisedtheory of Langmuir. Therefore, in this work, the H 2 adsorption data was  fi tted to this equation, written inthe form: V   ¼ V  m bP 1 þ bP where  V   is the volume adsorbed (cm 3 g  1 ),  P  theequilibrium pressure (mm Hg),  b  a parameter and V  m  the saturated amount adsorbed corresponding tomonolayer coverage.Table 3 shows the correlation of the experimentaldata to the Langmuir model. A good correlation wasfound for all the samples.Regarding the hydrogen adsorption capacities, theresults obtained for the samples studied in this work are comparable to or, in many cases, better than otherresults obtained for higher cost carbon materials[12,13].Furthermore, the apparent BET surface area, and thevolume of micropores obtained by the DR methodapplied to the N 2  adsorption isotherms, were correlated Table 2Phenol and naphthalene adsorption capacities obtained frombreakthrough curvesSample BET (m 2 g  1 ) Adsorption Capacity (mg g  1 )Phenol a Naphthalene b PC12 668 125 24PC35 1405 200 26PC58 1920 239 27PC76 2468 291 28Q 1149 289 30CM 849 263 24F400 1164 346 33 a Solution of phenol (2000 ppm). b Saturated solution of naphthalene (30 ppm).Table 3Langmuir parameters and H 2  adsorption capacities at   196  8 C forthe samples studiedSample  V  m (cm 3 g  1 ) b R 2 V  m (wt.%)H 2 (wt.%)PC12 176 0.010 0.999 1.6 1.4PC35 248 0.007 0.998 2.2 1.9PC58 309 0.004 0.999 2.8 2.1PC76 355 0.003 0.999 3.2 2.3306  J.B. Parra et al./Applied Surface Science 238 (2004) 304  –  308  to the hydrogen storage capacity at   196  8 C (Fig. 2). The increase in the surface area and the microporevolume gives rise to an increase of the H 2  adsorptioncapacity. This suggests that the H 2  adsorption on thecarbon materials depends primarily on their structuralproperties. Thus, it seems that hydrogen is basicallyphysisorbed (non-dissociatively adsorbed) on the car-bon structure of the samples studied.The above results also put forward the importanceof the pore size distribution in the hydrogen uptake. Inthis sense, large storage capacities are reached insamples containing a large volume of micropores.Regarding the suitable diameter of the micropores,it was observed that higher uptake is attained on thesamples with the largest pore width, given by the  L  value (cf. Table 1). Nevertheless, a contribution of theuptake might also be due to the generation of meso-porosity in most activated samples (PC58 and PC76).Future research will be focused on the optimumpore diameter of carbon materials and its effect onhydrogen uptake. 4. Conclusions The activation of PETwaste produced carbon mate-rials basically microporous, but with a low macro-porosity. Considerable mesoporosity is as welldeveloped in highly activated samples, and very highapparent BET surface areas upto 2468 m 2 g  1 wereachieved.Preliminary tests on potential applicability of these materials showed that removal of PAH andphenol was highly satisfactory and comparable tothe retention attained with commercial activatedcarbons.Regarding hydrogen adsorption, carbon materialsobtained from plastic waste, showed adsorption capa-cities higher than those of nanotubes, which are high-cost carbon adsorbents. Thus, carbon materialsobtained from PET pyrolysis and activation seem tohave textural properties (high surface area and largemicropore volume) adequate for hydrogen uptake.Consequently, these samples are good adsorbentsfor the purposes targeted, although more researchhas still to be done. References [1] Association of plastics manufacturer in Europe,  ‘‘ An analysisof plastics consumption and recovery in Western Europe ’’ ,Spring 2001.[2] S. Mishra, A.S. Goje, V.S. Zope, Polym. React. Eng. 11 (1)(2003) 79. 150200250300 Hydrogen uptake (cm 3 g -1 , STP) 5001000150020002500    A  p  p  a  r  e  n   t   B   E   T  s  u  r   f  a  c  e  a  r  e  a   (  m    2   g   -   1    ) 0.200.400.600.801.00    W  o   (  c  m    3   g   -   1    ) BETWo DR Fig. 2. Correlation of H 2  adsorption capacity with apparent BET surface area and micropore volume W 0 .  J.B. Parra et al./Applied Surface Science 238 (2004) 304  –  308  307
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