Mechanistic Modeling of Polymer Pyrolysis:Polypropylene
Todd M.Kru,Hsi-Wu Wong,and Linda J.Broadbelt*
Department of Chemical Engineering,Northwestern University,Evanston,Illinois60208
Received June11,2003;Revid Manuscript Received September30,2003
ABSTRACT:The pyrolysis of polypropylene was modeled at the mechanistic level to predict the formation of low molecular weight products.Differential equations were developed that describe the evolution of the moments of structurally distinct polymer species.Unique polymer groups were devid that allowed the necessary polymeric features for capturing the pyrolysis chemistry to be tracked,while maintaining a manageable model size.The conversion among the species was described using typical free radical reaction types,including intermolecular hydrogen abstraction,midchain -scission,end-chain -scission, intramolecular hydrogen transfer,radical addition,bond fission,radical recombination,and dispropor-tionation.The model included over24000reactions and tracked213species(27products tracked with molecular weights below215amu).The intrinsic kinetic parameters(a frequency factor and activation energy for each reaction)were obtained from data in the literature and previous modeling work in our laboratory.1,2The model predictions for the evolution of the yields of five major alkenes and
five major alkanes compare well with experimental data obtained in our laboratory for the pyrolysis of polypropylene over a temperature range of350-420°C.In addition,literature data3for the evolution of the polypropylene molecular weight was captured by incorporating weak backbone links modeled as peroxide bonds.
Introduction
In recent years,emphasis has been placed on devel-oping techniques to recycle municipal solid waste(MSW). The amount of MSW generated in the United States each year continues to increa,where each person in the United States generated4.6lbs of waste per day in 2000,an increa from2.7lbs per person per day in 1960.4Due to this focus on recycling MSW,some success has been achieved as evidenced by an increa in the amount of MSW recycled from1999(16.4%)to2000 (23%).This increa is larger than the0.3%increa in the total amount of MSW generated from1999to2000.4 However,even with this increa in the amount of MSW recycled,the amount recycled of some components of MSW is significantly lagging behind that of other components.One gment of MSW that is of concern is plastic waste,where plastics compri10.7wt%(>30 vol%)of municipal solid waste,and only5.4%of plastic waste was recycled in the United States in2000.4 Adding to the concern,plastics are considered to be one of the most rapid
ly growing gments of municipal solid waste,4and for some types of plastic waste,the amount recycled is near zero.For instance,13.6wt%of plastic waste was compod of polypropylene in the United States in2000,and only about0.3%of this polypropyl-ene was recycled.4
Several options exist for recycling waste plastic,and the options can be lumped into the following four categories:5(1)reusing waste plastic products directly for other applications;(2)reprocessing waste plastic into condary products;(3)recovering valuable chemical resources from waste plastic;and(4)incinerating waste plastic to recover energy.The resource recovery strategy (category3above)involves converting polymers into fuels and chemicals and recovering monomer to produce new polymer.6In our lab,the thermal technique of pyrolysis was investigated as a resource recovery strat-egy.Pyrolysis is an attractive resource recovery process due to its simplicity,where plastic waste is thermally degraded in the abnce of oxygen.In addition,the pyrolysis of polymeric materials has grown as a resource recovery strategy in recent years7-9and has a high potential for growth in the coming years provided that economically feasible process can be developed.10 The main obstacles that have prevented the successful implementation of a resource recovery process bad on pyrolysis are the lack of a comprehensive understanding of the complex underlying reaction pathways and the difficulty in predicting the full product distribution of pyrolytic degradation.
For example,the pyrolysis of polypropylene results in a very diver product distribu-tion due to the high temperatures ud and the complex free radical reactions involved.To overcome the obstacles,population balance bad models have been developed using the method of moments to model molecular weight changes and small molecule evolution simultaneously.11-20In our previous work,2,21the ap-proaches for modeling polymer degradation and poly-merization were combined and extended to develop a detailed mechanistic model for polymer pyrolysis.In development of this polymer pyrolysis model,a balance was struck between a fully speciated model and one that does not posss sufficient detail to either capture the experimental measures of interest or differentiate the reactivities of chains with different structural features. Our previous work concentrated on developing the necessary framework for mechanistic modeling of the decomposition of polymers during pyrolysis by focusing on the pyrolysis of polystyrene,2,21and we have now extended this modeling work to investigate the pyrolysis of polypropylene.Detailed information about the de-composition of polypropylene was obtained from experi-ments in our laboratory,including work reported pre-viously.22The yields of specific low molecular weight products as a function of time were measured for polypropylene pyrolysis in a batch reactor.The pyrolysis of polypropylene at different temperatures(350,380, and420°C)was examined.
*To whom correspondence should be addresd:phone,847-
491-5351;fax,847-491-3728;e-mail,broadbelt@northwestern.edu.
9594Macromolecules2003,36,9594-9607
10.1021/ma030322y CCC:$25.00©2003American Chemical Society
Published on Web11/20/2003
The pyrolysis of polypropylene has been studied at different levels of detail in the literature.Overall,the major products from polypropylene pyrolysis were de-termined to be as follows(in decreasing order):2,4-dimethyl-1-heptene(trimer),n-pentane,2-methyl-1-pentene(dimer),and propylene.23Tsuchiya and Sumi propod a free radical mechanism analogous to the mechanism for polystyrene pyrolysis(e Kru et al. 2002)to account for the formation of the products.23 While the reactions propod are similar for polystyrene and polypropylene,their relative contributions are dependent on the backbone structure.For example,it has been postulated that hydrogen abstraction occurs more readily in polypropylene due to less steric hin-drance compared to polystyrene which has bulky phenyl substituent groups.24The reduction in steric hindrance due to the prence of methyl substituent groups results in more hydrogen abstraction,leading to more oligo-meric products.Other recent studies have examined the possible pr
ence of weak links within polypropylene chains.3It was propod that peroxide groups exist within polypropylene,but no weak links were detected using Fourier transform infrared spectroscopy(FTIR).3 More recent work has analyzed the pyrolysis of polypropylene at temperatures between500and900°C.25Compared to lower temperature studies,a greater conversion to products volatile at room temperature was achieved.In addition,yields of propylene as high as38% were obtained.Trimer was the product formed in the highest yield.The product distribution was propod to be most significantly influenced by the occurrence of backbiting reactions(intramolecular hydrogen abstrac-tion by end-chain radicals).The backbiting reaction to the carbon in the fifth position along the backbone leads to the production of trimer and n-pentane.However,it was also propod that intramolecular hydrogen ab-straction to the third,venth,and ninth carbon posi-tions is important during polypropylene pyrolysis,com-pared to just the third and fifth positions for polystyrene. This postulate was supported by the significant yields of tetramer(2,4,6-trimethyl-1-nonene)and pentamer (2,4,6,8-tetramethyl-1-undecene)which would be evolved from specific radicals at the venth and ninth carbon positions,respectively.Similar to other studies,the authors propod that backbiting reactions during polypropylene pyrolysis are more important and diver than tho that occur during polystyrene pyrolysis becau of the greater flexibility of polypropylene due to the small methyl substituent groups.
Up to this point,modeling studies in the literature have focud on either obtaining apparent rate param-eters for the overall decomposition of polypropylene26,27 or fitting an effective rate constant to model the polypropylene molecular weight distribution.28The primary challenge in modeling polypropylene pyrolysis is being able to capture the formation of all the specific low molecular weight products in the C1-C15size range (products below215amu),and this challenge has not been met in the literature thus far.
Therefore,we have developed a polypropylene model that quantifies both the evolution of the polypropylene polymer molecular weight distribution and the forma-tion of specific products in the C1-C15size range simultaneously.Differential equations tracking the moments of structurally distinct polymer species were developed,and the equations incorporated a wide range of free radical reactions,including reactions leading to specific low molecular weight products.Bond fission,chain-end -scission,midchain -scission,inter-molecular hydrogen abstraction,radical addition,radi-cal recombination,disproportionation,and intramolec-ular hydrogen transfer were included.Unique polymer groups were devid that allowed polymeric features to be tracked while maintaining a manageable model size. Polymer species were lumped into various class,as done previously,2,21,29-31to track the prence and location of radical centers,the position of double bon
ds, and the orientation of the“head”and“tail”ends of the monomer units.The model developed tracked all major low molecular weight products(27low molecular weight products in the C1-C15size range were tracked), consisted of over24000reactions,and tracked213 species.The intrinsic kinetic parameters(a frequency factor and activation energy for each reaction)were obtained from data in the literature and previous modeling work in our laboratory.1,2The approach to model construction,the detailed chemistry included,the quantitative parameters ud,and comparison to ex-perimental data are discusd below.
Experimental Section
The experimental data collected under isothermal conditions at350and420°C in our laboratory have been reported previously.22The same experimental procedure ud to collect that data was employed to gather data at380°C.Briefly,batch pyrolysis experiments were carried out by putting polypropyl-ene into a3.1-mL glass ampule(Wheaton),purging with argon, and flame aling.Loadings of20mg were studied for reaction times from5to180min.At least two replicates and in most cas three replicates were performed for each reaction time. The polypropylene was obtained from Aldrich(syndiotactic; M n)54000,M w)127000;unstabilized)in powder form.Note that the majority of commercial polypropylene samples are isotactic,but the pyrolysis kinetics of syndiotactic and isotacti
c polypropylene are similar.32While they afford different dia-stereomers becau of the different stereochemistry along the backbone,the amounts of a given alkene or alkane product are the same.Furthermore,as will be shown below,param-eters obtained from the literature for pyrolysis of isotactic polypropylene work well for the syndiotactic polypropylene studied here.Converly,the model we have developed bad on the pyrolysis of syndiotactic polypropylene can be ud without adjusting any parameters to capture pyrolysis results in the literature for isotactic polypropylene samples.
Products that were gaous at room temperature from the polypropylene pyrolysis were analyzed by placing the ampule inside a53-mL flask with a Tygon tube on one end and an injection port on the other.Both ends were then aled with pta.The flask was purged with helium for10min and,after the ampule was broken,the gas were allowed to equilibrate for30min.Gas samples were analyzed using gas chromatog-raphy.
Liquid and solid products were extracted with1.5mL of HPLC grade methylene chloride.The product solution was first filtered and then pasd through a gel permeation chromato-graph(GPC).Only polypropylene-derived products up to C25 were completely soluble in methylene chloride.Products with molecular weights less than400g/mol were collected using a fraction collector attached to the G
PC outlet.Products were then analyzed by gas chromatography and mass spectrometry. All details of the product analysis are provided in Wong and Broadbelt.22Error bars shown in the figures reprent the standard deviations of experiments that have been at least duplicated.
Model Development
Mechanistic Chemistry.The method of moments was ud to develop differential equations describing
Macromolecules,Vol.36,No.25,2003Mechanistic Modeling of Polymer Pyrolysis9595
the pyrolysis kinetics,and the mechanistic chemistry of interest was implemented by deriving the terms of the moment equations corresponding to each reaction type.2,21,30The terms of the moment equations were derived for the following reactions:(1)chain fission,(2)radical recombination,(3)allyl chain fission,(4)inter-molecular hydrogen abstraction,(5)midchain -scission,(6)radical addition,(7)end-chain -scission,(8)dispro-portionation,(9)1,3-end-hydrogen transfer,(10)1,4-end-hydrogen transfer,(11)1,5-end-hydrogen transfer,(12)1,6-end-hydrogen transfer,(13)1,3-mid-hydrogen trans-fer,(14)1,4-mid-hydrogen transfer,and (15)1,5-mid-hydrogen transfer.Examples of the reactions for polypropylene are pictured in Figure 1,and the moment equations describing thes
e reactions are given in our previous modeling work.2
One key to the formation of low molecular weight products was intramolecular hydrogen transfer reac-tions.The accessible positions along the polymer back-bone were lected bad on both theoretical data and experimental data in the literature reporting the rela-tive rates of the various intramolecular hydrogen trans-fer reactions.In our mechanism,the only backbiting reactions originating from an end-chain radical are the 1,3-,1,4-,1,5-,and 1,6-end-hydrogen transfers (reactions 9-12in Figure 1).Backbiting reactions from end-chain radicals to carbons further down the polymer chain (i.e.,venth carbon position or greater)have been found to be both energetically unfavorable 33,34and less probable 35compared to the 1,4-,1,5-,and 1,6-end-hydrogen trans-fers.However,it is clear from literature data 25and data collected in our laboratory 22that specific radicals at the venth and ninth carbon positions are being formed due to the significant yields of tetramer and pentamer obtained during polypropylene pyrolysis.For example,data in the literature 25and data collected in our laboratory 22show that tetramer and pentamer are among the five products with the highest mass yields.To account for the formation of radicals at positions further removed from the chain ends,1,3-,1,4-,and 1,5-mid-hydrogen transfers (reactions 13-15in Figure 1)following an end-hydrogen transfer were included.
For
Figure 1.Polypropylene reaction types incorporated into detailed mechanistic model.Reactions types are numbered in the preceding text.
9596Kru et al.Macromolecules,Vol.36,No.25,2003
mid-hydrogen transfer reactions,the midchain radicals produced from the initial1,3-,1,4-,and1,5-end-hydrogen transfers undergo an additional hydrogen transfer step,thereby forming specific radicals on the venth through the ninth carbons.The1,3-,1,4-,and 1,5-mid-hydrogen transfers are slower than the original 1,3-,1,4-,and1,5-end-hydrogen transfers due to entropic factors,but they are significant enough to produce midchain radicals further down the chain.Specific midchain radicals near the end of a polymer chain are also produced by the random intermolecular abstraction of hydrogen(reaction4in Figure1),and the reactions are included within the model.However,the amount of specific midchain radicals produced through inter-molecular hydrogen abstraction is very small compared to intramolecular hydrogen abstraction due to the large number of hydrogens available for abstraction along the backbone of the high molecular weight polymer chains. Overall,the mechanism we have propod only us conventional1,3-,1,4-,1,5-,and1,6-hydrogen tra
nsfers. By u of the free radical reaction types and keeping track of specific radicals,the evolution of27low mo-lecular weight products in the C1-C15size range (products below215amu)was followed in the model. The products are listed in Table1.Note that the dimer (C6d),trimer(C9d),tetramer(C12d),and pentamer (C15d)labels in Table1each reprent two isomers that are lumped together in the model results to obtain the yields of the species.The experimental data for all species larger than C5(including dimer,trimer, tetramer,and pentamer)reprent the sum of the yields of all the isomers for the species if multiple isomers were detected.Figures2,3,and4show all the specific midchain radicals formed by either1,3-,1,4-,1,5-,and 1,6-end-hydrogen transfers or1,3-,1,4-,and1,5-mid-hydrogen transfers and the products that are formed. Figure2shows the products formed from radicals on odd carbon numbers near the end of chain,which are the major products formed during polypropylene pyr-olysis.Figure3shows the products formed from radicals on even carbon numbers,which are all the minor products tracked.Figure4shows specific radicals near unsaturated chain ends that were required to track the formation of isobutylene,2-pentene,and low molecular weight dienes.
Branching reactions were also included in the model, where end-chain radicals were allowed to recombine with midchain radicals.The approach outlined by McCoy and Wang for describing random
and propor-tioned chain fission and -scission reactions11,17was implemented into the moment equations.Chain fission and midchain -scission of polymer chains were as-sumed to be random process.
Specification of Rate Constants.Incorporation of the free radical elementary step reactions in a model of polymer pyrolysis required the specification of the rate parameters.The same approach ud previously in our work to quantify rate constants was adopted.36 The rate parameters were dependent on not only the reaction type but also the structural characteristics of the reactants and products.Assuming the validity of the Arrhenius relationship,a frequency factor and an activation energy for each reaction were specified.Each reaction of a given ,bond fission)shared the same frequency factor.The activation energy for each specific reaction was calculated using the Evans-Polanyi relationship,37in which the activation energy is related linearly to the heat of ,E)E o+ R∆H
R泡脚几分钟最好
.The values of the heats of reaction were obtained from experimental polymerization data or bad on analogous reactions of molecular mimics of the polymer structure as ud previously.36,38,39
A summary of the frequency factors,Evans-Polanyi constants,and heats of reaction for the main pyrolysis reactions for polypropylene is provided in Table2.Note that no optimization of the frequency factors or activa-tion energies was carried out.The majority of the frequency factors and the parameters E o and R for each reaction type were taken directly from our previous work.1,2The frequency factor for -scission was obtained using thermodynamic data in the literature.40In addi-tion,the frequency factors for1,3-end-hydrogen transfer and1,5-end-hydrogen transfer were obtained by match-ing the model results to the ratios reported in the literature of trimer to monomer(5.5:1)and dimer to monomer(1.4:1)for the pyrolysis of polypropylene at 388°C.41The intrinsic barriers for the1,4-and1,6-end-hydrogen transfers were then estimated by adding the strain energy for five-membered and ven-membered rings,34respectively,to the intrinsic barrier for the1,5-end-hydrogen transfer reaction.The frequency factors for1,3-,1,4-,and1,5-mid-hydrogen transfers were obtained by matching the model results to the ratio of tetramer to monomer(1.5:1)for the pyrolysis of poly-propylene at388°C reported in the literature.41It was assumed that all ratios of the midchain hydrogen transfer frequency factors to their corresponding end-chain hydrogen transfer frequency factors were equal. The diffusion dependence of termination reactions was implemented using Smoluchowski’s equation42for the rate constant of a diffusion-controlled reaction.The termination rate constant was assumed to be inverly proportional to the chain length of the terminating radicals,and a termination rate constant was calculated
Table1.Labels of Products Tracked in Polypropylene
Model
product label species name
C1methane
C2ethane
C3d propene
C3propane
古代帝王
C4d isobutylene
C4isobutane
C5d2-pentene
C5n-pentane
南京必去景点排名C6d2-methyl-1-pentene数学板报
(dimer)4-methyl-2-pentene
C62-methyl-1-pentane
C7d d2,4-dimethyl-1,4-pentadiene
C7d2,4-dimethyl-1-pentene
C72,4-dimethylpentane
书空是什么意思C8d4-methyl-2-heptene
C84-methylheptane
C9d2,4-dimethyl-1-heptene
(trimer)4,6-dimethyl-2-heptene
C92,4-dimethylheptane
龚细水
C10d d2,4,6-trimethyl-1,6-heptadiene
C10d2,4,6-trimethyl-1-heptene
C102,4,6-trimethylheptane
C11d4,6-dimethyl-2-nonene
C114,6-dimethylnonane
C12d2,4,6-trimethyl-1-nonene
(tetramer)4,6,8-trimethyl-2-nonene
天使符文C13d d2,4,6,8-tetramethyl-1,8-nonadiene
C13d2,4,6,8-tetramethyl-1-nonene
C14d4,6,8-trimethyl-2-undecene
C15d2,4,6,8-tetramethyl-1-undecene
(pentamer)4,6,8,10-tetramethyl-2-undecene
Macromolecules,Vol.36,No.25,2003Mechanistic Modeling of Polymer Pyrolysis9597
for all termination reactions using the average length of all the polymer radicals prent in the polymer melt.In addition,hydrogen abstraction rate constants were assumed to be inverly proportional to the size of the abstracting radical.43
山刺玫Model Asmbly and Solution.To choo what polymer species to track,a lumping scheme was devid that lumped polymer species into groups bad on their end-chain structure.The different types of end-chain groups possible for polypropylene are shown in Figure 5,and all of the features were tracked within the model.In addition,four different types of radicals were tracked within the model.Both head and tail end-chain radicals were distinguished within the model since both of the radical types are formed during bond fission (reaction 1in Figure 1).In addition,both head and tail midchain radicals were tracked since both of the radical types are formed during hydrogen abstraction and intramolecular hydrogen transfer reactions.Ex-amples of head and tail midchain radicals are shown in Figures 2-4.However,the methyl substituent groups were assumed to be inert and not participate in the free radical chemistry since the abstraction of hydrogen from the methyl su
bstituent groups would result in the formation of unstable primary radicals.Instead,it is more energetically favorable that tertiary or condary hydrogen atoms are abstracted during intermolecular hydrogen abstraction and intramolecular hydrogen transfer reactions,which produce head and tail mid-chain radicals,respectively.The bond strength of the primary hydrogen atoms on the methyl substituent groups is 5.9kcal/mol stronger than the tertiary hydro-gen atoms and 2.8kcal/mol stronger than the condary hydrogen atoms on the backbone of the polypropylene
chains.
Figure 2.Midchain -scission reactions of specific midchain radicals located three,five,ven,and nine carbons from the end of a chain.The -scission of the specific midchain radicals leads to the formation of unsaturated oligomers and saturated low molecular weight radicals (labeled to the right of the products).The labels on the arrows reprent the type of midchain radical undergoing -scission,where the first letter reprents the type of midchain radical ((t)tail or (h)head saturated),the number reprents the distance in number of carbon atoms from the end of the chain,and the final letter reprents the type of end group ((t)tail or (h)head saturated).
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