Yinbin Wang,Senjun Yao,Wei Wang,Chenglong Qiu,Jing Zhang,Shengwei Deng,*,Hong Dong*,Chuan WuJianguo Wang,*

1 Institute of Industrial Catalysis,State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology,College of Chemical Engineering,Zhejiang University of Technology,Hangzhou 310014,China

2 College of Computer Science and Technology,Zhejiang University of Technology,310014,China

3 Key Laboratory of Organosilicon Chemistry and Material Technology,Ministry of Education,Hangzhou Normal University,310012,China

Keywords:Pyrolysis ReaxFF Molecular simulation Vulcanized Styrene-butadiene rubber Sulfur products

ABSTRACT Styrene-butadiene rubber (SBR) is widely used in tires in the automotive segment and vulcanization using sulfur is a common process to enhance its mechanical properties.However,the addition of sulfur as the cross-linking agent usually results in impurities in pyrolysis products during rubber recycling,and thus the desulfurization during tire pyrolysis attracts much attention.In this work,the pyrolysis of vulcanized SBR is studied in detail with the help of ReaxFF molecular dynamics simulation.A series of crosslinked SBR models were built with different sulfur contents and densities.The following ReaxFF MD simulations were performed to show products distributions at different pyrolysis conditions.The simulation results show that sulfur products distribution is mainly controlled by sulfur contents and temperatures.The reaction mechanism is proposed based on the analysis of sulfur products conversion pathway,where most sulfur atoms are bonded with hydrocarbon radicals and the rest transfer to H2 S.High sulfur contents tend to the formation of elemental sulfur intermediate,and temperature increase facilitates the release of H2 S.

1.Introduction

Styrene-Butadiene Rubber (SBR) is a general-purpose synthetic rubber and a main component in passenger car tires.The production of scrap tires increases with the development of automotive industry.However,only a small portion of scrap tires was recycled or reused,and the large part was treated unprofessionally and caused ‘‘black pollution”.Pyrolysis of scrap tires is an efficient and eco-friendly process that allows producing fuels and chemicals from scrap tires [1,2],and also provides heat and electrical energy due to the high calorific value [1–5].The products from thermal decomposition of scrap tires include gas and liquid products which can be used as energy sources,and char that can be transformed into carbon black after treatment.Additionally,sulfur is widely used in tires manufacture as an additive[1,6].The addition of sulfur forms a network structure by bringing in poly-,di-,and monosulfide bonds [7]to enhance the mechanical properties of rubber.In the pyrolysis process,the sulfur which is originally in organic forms releases into pyrolysis products such as oil,tar and gases.The removal of sulfur during rapid pyrolysis is of high importance for industrialization of scrap tires.

The sulfur usually remains in the oil after pyrolysis process resulting in low quality products [8,9].Thus,the desulfurization during pyrolysis is necessary to recycle vulcanized rubber effectively [10].Previous studies indicate that sulfur is involved in a serial of reactions in the pyrolysis process [9],and sulfur removal is accomplished by H2S release [11].The pyrolysis product is strongly dependent on the temperature [12],heating rate,residence time and mass transfer,which are related with the reactor configurations [9,13].Among these factors,the temperature is a common factor discussed in their works.However,it is still a challenge to examine the effect of a factor(independent variable)in the pyrolysis experiment.During the last decade,rapid development of computer power has made molecular simulation indispensable in the study on many chemical processes such as the thermal decomposition of organic materials.

Reactive force field [14,15]molecular dynamics (ReaxFF MD)simulation as an efficient computation tool has been used in simulating the oxidation of hydrocarbons.Importantly,single factor experiment is able to be performed with the help of this theoretical method.The bond order is employed in this force field to determine the bond formation and breakage.Unlike other simulation methods,ReaxFF MD can simulate the formation of radicals,the bonding and breaking dynamically.The accuracy of ReaxFF force field was insured by quantum mechanics (QM) data,and ReaxFF MD can simulate large-scale systems with lower computa-tional expense than QM methods [16].The application of ReaxFF has been expanded constantly,which has been used in simulating the pyrolysis of complex compound such as coal [17–19]and biomass [20].Lately,the natural rubber and its pyrolysis mechanism was studied through ReaxFF MD combined with thermogravimetry-infrared spectroscopy experiments [21].The simulation results are in good agreement of experimental pyrolysis data.Therefore,ReaxFF MD simulation is practicable to explain the pyrolysis mechanism of vulcanized SBR at various reaction conditions.

Table 1 Cross-linked S number and bonding rate of all the four systems

Fig.1.Time evolution of pyrolysis product distributions at different temperatures:(a)S-1 at 2500 K,(b)S-1 at 3000 K,(c)S-2 at 2500 K,(d)S-2 at 3000 K,(e)S-4 at 2500 K,(f)S-4 at 3000 K,the Cn represents molecules that contain n carbon atoms.

Our previous work[22]used density function theory(DFT)and ReaxFF MD simulation to study bond dissociation energies of SBR polymer chain and the pyrolysis process of pure SBR.In this work,we build the SBR models with cross-linking structures firstly.And the ReaxFF MD simulation is used to investigate the pyrolysis mechanism of vulcanized SBR at different temperatures and densities.Especially,we focus on the transfer of sulfur during the pyrolysis which is key to the devulcanization [7],and the sulfur products distribution are well concluded.We intend to provide a detailed theoretic understanding and yield guidelines on the devulcanization during pyrolysis of vulcanized SBR.

2.Computational Method

The bond dissociation energies of C-S and S-S bonds are carried out using the Gaussian 09 series of programs [23].All simulations were performed by using the MD software Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software.Inspired by the cross-linking of polymers by united-atom model in MD simulation [24,25],we built cross-linked models based on all-atom SBR chains.The hydrogen atoms must be taken into calculation to meet the need of pyrolysis simulations.And thus,the Universal Force Field (UFF) is adopted to describe chemical bonding and van der Waals interactions in the cross-linking process.

ReaxFF[14,15]MD simulations are then performed in the pyrolysis simulations.This method employs the concept of bond order to describe bond formation and breaking.The empirical ReaxFF force field was well trained against QM (mostly DFT) data and experiments to insure its high accuracy.Recently,ReaxFF MD simulation has applied to pyrolysis and combustion of hydrocarbon with nitrogen [26],oxygen [27],and sulfur [28].In this work,the C/H/N/S/Si ReaxFF force field [29]is employed to study the pyrolysis of vulcanized styrene-butadiene rubber,where the force field parameters are successfully applied in thermal decomposition of hydrocarbons [30 31,32].The system energy including multiple interaction energies among atoms is described as follows.

where the Ebond,Eover,Eunder,Eval,Epen,Econj,Etors,EvdWaals,Ecoulomband EH-bondrepresent bond energy,over-coordination energy penalty,under-coordination stability,valence angle energy,penalty energy,conjugate effect of the molecular energy,torsion angle energy,van der Waals energy Coulomb energy,and H-bond energy,respectively.

2.1.Construction of vulcanized SBR model

The initial SBR chain was built based on 4 kinds of repeat units,which are styrene unit and three units of butadiene (－CH2－CH(CH＝CH2)－,cis－CH2－CH＝CH－CH2－,and trans－CH2－CH＝CH－CH2－),and more details are described in our previous work [22].Each single chain contains 32 units including 4 kinds of repeat units.The sulfur atoms are randomly bonded with carbon atoms in polymer chains.If sulfur atom is connected to the carbon atom in C－C double bond,double bond is converted to single bond which is saturated with hydrogen.For each vulcanized SBR sample,we first put 10 sulfur-anchored SBR polymer chains in the simulation box.To avoid bond-breaking during the crosslinking process,the Universal force field (UFF) is used to describe the atomic interaction and simulation is performed in the LAMMPS molecular dynamics code[24,25].The sulfur atoms on the polymer chains are regarded as the reactive sites,S－S bonds would be formed between these sites.The S atoms were bonded with carbon atom in the main chain of SBR initially.The principle to choose the bonded carbon atoms is based on the reactivity between carbon atoms and the sulfur.Sulfur atoms are more likely to bond with carbon atoms of C－C double bond than other carbon atoms due to addition reactions.Therefore,most of bonded carbon atoms were chosen from the C－C double bond on the main polymer chain.To avoid intramolecular ring formation,sulfur atoms were only allowed to bonded between two chains.The cross-linking process was accomplished in canonical (NVT) ensemble at 600 K for 500 ps with a timestep of 0.1 fs.The cutoff radius for reactions to occur was set to 5 ? (1 ?=0.1 nm),and the probability of bonding was set as 0.5 (same as 50 %) and distances of reactive sites were checked for every 0.1 ps.The choice of 0.5 would avoid too many bonds from being created at the beginning of the simulations[25].

Fig.2.Snapshots of the pyrolysis product distribution of S-2 system at 3000 K(a)and partial enlarge image(b).Different colors represent different element,bule:hydrogen,black:carbon,yellow:sulfur.

Fig.3.Time evolution of sulfur products of S-2 system at four temperatures,(a) 2000 K,(b) 2500 K,(c) 3000 K,(d) 3500 K.(e) Time evolution of H2 S.

2.2.Pyrolysis simulation

The vulcanized SBR samples are served as the initial structures.To change the density,the simulation is performed in the isothermal-isobaric(NPT)ensemble at a temperature of 300 K with a time step of 0.25 fs.And then systems with densities of 0.2 and 0.5 g·cm-3are obtain by changing pressures.An equilibration process is carried out before the pyrolysis.The pyrolysis process was performed in NVT ensemble at 2000,2500,3000 and 3500 K for 400 ps.Note that the temperatures in ReaxFF MD are usually much higher than those in real experiments,however,the temperature range in this work may correspond to the range of 700–1100 K in experiment if comparing pyrolysis product distributions.The detail of ReaxFF used in this work can be found in Ref.[19,33].All the snapshots of simulations were accomplished in OVITO program [34].

3.Results and Discussion

3.1.The analysis of vulcanized SBR structures

The vulcanized SBR models were accomplished by a crosslinking process (Section 2.1) in MD simulations with UFF.The initial density of all samples is set as 0.1 g·cm-3.Note that the rubber is not fully filled in the pyrolysis reactor in the industrial production process,and thus,we prepare the samples with different densities to simulate different solid contents at the beginning.The SBR chains contain different number of sulfur atoms ranging from 1 to 8 for different vulcanized SBR samples.After 500 ps simulations,cross-linking structures were formed and the bonding rates were concluded in Table 1.The bonding rate was defined as the number of S－S bond divided by half of the number total S atoms,which can be regarded as percentage.The bonding rate increases along with the quantity of initial sulfur atoms.More sulfur atoms in the chain result in higher frequently of new bond formation.S-1,S-2,S-4 and S-8 represent cross linked systems with initial SBR chains bonded with 1,2,4 and 8 sulfur atoms on each chain,respectively.The structures of S-1,S-2,S-4 and S-8 are showed in Fig.S1 and the S-2 before cross linking are showed as well.Comparing Fig.S1a with c,it shows that 10 polymer chains are closely packed through S-S cross-link bond.And as showed in Fig.S2,the MSDs of crosslinked system is much lower than those of free systems where only Van der Waals force drives polymer chains to aggregate,it means that the chain mobility is severely weakened after cross-linking process.After removing the unbonded S atoms,the final sulfur contents(mass percent)of the system are also counted in Table 1.The sulfur content of S-2 system is close to that of real scrap tires[1,9].The bonding rates are positively related to the numbers of sulfur atoms.

3.2.Pyrolysis products distributions

3.2.1.Formulations of short-chain hydrocarbon

To show the effect of vulcanized process on the distribution of main pyrolysis products,ReaxFF MD simulations were performed for different samples to compare the pyrolysis results for uncross-linked system [22].The pyrolysis oil and gas are the needed products which are reused as fuel or chemicals.In the industrial application,the pyrolysis products are classified into several phases according to the C atoms number of products,gas(C1-C4),tire pyrolysis oil (C5-C11),char and so on.Thus,we classified the products into different Cnspecies instead of CnHm(or CnHmSl).Short-chain hydrocarbons ranging of C1to C8are the most important pyrolysis products and discussed in this part.Here,we abbreviate hydrocarbon radicals to R.High temperature pyrolysis simulations were carried out on all the systems after equilibrium at 300 K.The time evolution of C1-C8products are showed in Fig.1.The pyrolytic reaction is continuing unless the temperature is lower than the minimum pyrolysis temperature.However,it is possible to cease the simulation when most polymer chains are decomposed into small molecules.C4and C8species reach the peaks at the very beginning,and the number of C4is much more than that of C8.Then C2species increase gradually and become the main product,accompanied by the decrease of C4and C8species.Notably,high temperature accelerates the conversion of C4and C8into C2species (Comparing the results of 2500 K to 3000 K).And C4species reach a higher peak value in the S-2 and S-4 systems than S-1 system.The results of C1-C8at 2000 K and 3500 K are showed in Fig.S3.Similar to our previous work [22],the generation pathway to C4and C8can be explained by the βscission in the main chain[35].C4and C8correspond to butadiene monomers and styrene monomers,respectively.And the high selectivity of monomer products has been proved by experiments[36].Both C4and C8are generated through the breaking of weak intermonomer bond.Every polymer chain contains butadiene monomers three times than the styrene monomers in the initial models.As consequence,the C4curves are much higher than the C8as showed in the Fig.1.Temperature plays a promotive effect during pyrolysis process.Compared to weak intermonomer bonds,the break of intramonomer bonds requires relatively high bond dissociation energies.Therefore,high temperature helps C4and C8species break into shorter C species like C1,C2,and C3.In all the systems,C-S and S-S bonds are weaker than C-C bond.The bond dissociation energies of C-S and S-S bonds are carried out using the Gaussian 09 series of programs in Table S1.The bond dissociation energies via ReaxFF MD simulation show the same trend as those of QM calculation results.The C-C bond dissociation energies on main chain range from over 700 kJ·mol-1to 210 kJ·mol-1.Compared to the dissociation energies of C－C bond,the C－S and S－S bonds are weaker.The breakage of C－S bond leads to the unsaturated carbon atoms forming on the chains.And then,these active sites caused by bond breakage initiate the crack of carbon chains [8].The system with high sulfur content like S-2 and S-4 would contain more active sites than S-1 system,which lead to faster bond breakage on chains,e.g.the formation of C4in S-4 is much faster than S-2,S-1 or uncross-linked system[22].In summary,the distribution and evolution of Cnproducts mainly depend on the chain structures,the vulcanization is benefit for bonds breakage.

3.2.2.Distributions and conversions of sulfur products

The tracing of sulfur during the pyrolysis process is key for removal of sulfur-containing components and achieve high quality pyrolysis products.Experimental results show that only a small amount of sulfur release as H2S gas,and main part of sulfur transfer into sulfides,sulfates,thiol and thiophenes[1,9]existing in the oil or the solid products.We simulated the thermal cracking of aforementioned system at four temperatures,2000 K,2500 K,3000 K,and 3500 K.After 400 ps simulation,the results of S-2 pyrolysis at 3000 K are pictured in Fig.2.H2S,thiol and thioether can be found as the final sulfur products.The H2molecules were released from the polymer chain by dehydrogenation reactions.The sulfur atoms are likely to capture H2and then transform into H2S molecules,or bonding with hydrocarbon radicals forming thiol and thioether.

Fig.4.Snapshots and partial enlarged drawings of the pyrolysis of S-4 system at 3000 K:(a) 50 ps,(b) 200 ps,(c) 400 ps.

The detailed evolution of sulfur products is further investigated at various conditions.Plenty of sulfur products are generated during the pyrolysis process and result in difficulty for analysis.Therefore,according to the composition,we classified the products into three types:CHS (hydrocarbon sulfide),HS (hydrosulfide) and S.Usually,during the pyrolysis,S would be transformed from solid state into H2S and then removed by adsorbents.Therefore,we took H2S into consideration.Both CHS/HS/S distribution and H2S evolution of S-2 systems are showed in Fig.3.In Fig.3a–d,the CHS products account for the largest proportion at all temperatures.The proportion of HS increases with the increase of simulation temperature.Combined with Fig.3e,it concludes that all the HS products are in the form of H2S molecule.The S products which represent elemental sulfur arise for a short time at different simulations.It seems that the elemental sulfur here is an intermediate product as a consequence of C－S bond breakage.The major pyrolysis products are the CHS species and the amount of H2S is significantly lower than CHS.This distribution of sulfur products is also observed in previous experimental works [9].The degradation of rubber is a free radical mechanism reaction [13].The C－H and C－C bond dissociation energies are significantly higher than C－S and S－S bond dissociation energies [8].The specific bond dissociation energies depend on molecular structure.During the pyrolysis process,the C－S bond broke firstly[37]on account of its relatively weak dissociation energy compared with C－C and C－H bonds to produce H2S and linear chains.The element sulfur appears as an intermediate due to the break of C－S bonds.The free sulfur atoms are likely to capture H2forming H2S.But the element sulfur is only a small part.The main part of sulfur is still bonded with polymer chains after S－S bond breakage.And then these atoms would transform into short chain products during the cracking of polymer chains.Finally,most sulfur atoms are in the forms of R-S/R-SH.

Fig.5.Time evolution of sulfur products and H2 S of S-4 system at four temperatures:(a) 2000 K,(b) 2500 K,(c) 3000 K,(d) 3500 K,(e) time evolution of H2 S.

Fig.6.Time evolution of H2 S of different systems at 2500–3500 K,(a)S-2 at 2500 K,(b)S-4 at 2500 K,(c)S-2 at 3000 K,(d)S-4 at 3000 K,(e)S-2 at 3500 K,(f)S-4 at 3500 K.

Fig.7.Time evolution of sulfur products of different system density at 3000 K,(a)S-2 at density of 0.2 g·cm-3,(b)S-2 at density of 0.5 g·cm-3,(c)S-4 at density of 0.2 g·cm-3,(d) S-4 at density of 0.5 g·cm-3.

To study the effect of sulfur content on the final sulfur product distribution,ReaxFF MD simulations are performed on S-4 system with the comparison of pyrolysis results of S-2 system.The S-4 system contains 3.8 wt% sulfur content which is twice more than S-2 system.While the sulfur content of S-2 is close to the sulfur content of real vulcanized rubber.The snapshot results of S-4 systems at 3000 K are shown in Fig.4.And the results for other systems were investigated as well,the snapshots of S-1 and S-8 can be found in Fig.S4.Out of expectation,a cluster containing over 10 sulfur atoms appears in the final snapshot of Fig.4c.And the growth of sulfur cluster can be verified through comparing Fig.4a–c.Actually,the sulfur aggregation is considered as the intermediate for dense sample at the beginning during the pyrolysis.Due to the lack of in situ electron microscope image at the very beginning during the pyrolysis,it is difficult to build the connection between simulation results and experimental observation.The time evolution of sulfur products and H2S are showed in Fig.5.The significant fluctuation is related to the sample size.Larger simulation system usually results in more accurate description.The products distribution of S-4 system showed in the Fig.5 is different to S-2.At 2000,2500,and 3000 K,evolution curves of S show high ratios,and the three curves fluctuate frequently.Combined with the Fig.5e,it reveals that only a small amount of H2S belong to the HS products.Actually,there is a sulfur clusters growth process during the pyrolysis process.Firstly,a small sulfur cluster containing 4 atoms has formed after 50 ps which initiated next reactions(Fig.4a),And then,the initial sulfur cluster would capture other sulfur atoms from H2S or CHS,which gives rise to the transformation between S and HS (or CHS).As the hydrogen (or R) deprived from the cluster,sulfur cluster is grown.Finally,a cluster showed in Fig.4c is formed by repeating this growing process many times.In consequence,HS curves rise while the S curves drop at Fig.5a–c.Comparing to S-2 system,the S2or another element sulfur in S-4 system is more likely to contact and combine with each other at the very beginning.But high temperature can eliminate the formation of element sulfur,which is showed in Fig.5d.The reason is that relatively high temperature can promote the S－S bond breaking and H2S formation.The initial sulfur cluster cannot exist during the pyrolysis process.On the other hand,more H2gas would release at high pyrolysis temperature [4]due to intensive dehydrogenation reactions.The amount of H2also boosts the possibility of forming H2S [26].Generally,the sulfur cluster formation was found in these simulations and this mechanism could be restrained at high pyrolysis temperatures.

The release of H2S is of high importance during the pyrolysis of scrap tires.The influence of temperature and pressure(density)on H2S production is compared in Figs.6 and 7,respectively.Results show that temperature plays a more important role on H2S production than density.More H2molecules release at higher pyrolysis temperatures which facilitate the formation of H2S,this result is also proved by experiments.As showed in Fig.7,high density would restrain H2S releasing at relatively low temperature,but at high temperatures,density rarely influenced the H2S formation.As for the low sulfur content systems such as S-2,density has only slight impact on the pyrolysis process.However,in the relatively high sulfur content systems such as S-4,high density contributes to the enlargement of HmSn.Based on above results,temperature has a remarkable impact on the H2S production during the pyrolysis process,but the impact of pressure is limited.

Generally,the pyrolysis of cross-linked SBR can be divided into two stages.In the primary pyrolysis,the C－S and S－S bonds break firstly which promote the crack of main polymer chains.Meantime,C4and C8generate due to β-scission.In the secondary pyrolysis,primary pyrolysis products (e.g.C4and C8) further transform into low molecular hydrocarbons C1－C3as showed in Fig.1.Temperature accelerates the whole pyrolysis process.There are two different pathways about the sulfur products conversion.At low sulfur content,C－S and S－S bond break up first attaching to the polymer chains or forming S radicals.And then S radicals may combine with hydrocarbon radicals R which break away from the main polymer chains,and form thiol or thioether belonging to the CHS products.A small percentage of sulfur in CHS is transformed into H2S.High temperature facilitates the generation of H2.The other pathway is forming a sulfur cluster.At relatively high sulfur content,sulfur atoms are released from the polymer chains forming a tiny sulfur cluster intermediate.The H2S molecules and short chains of hydrocarbon sulfide are bonded with sulfur clusters intermediate.And then,the hydrogen (or R) is separated from the S cluster which is accompanied by the growth of S clusters.And such cluster intermediate would finally transfer into sulfide.However,enough high temperature like 3500 K for S-4 system can change the pathway towards H2S formation.

4.Conclusions

In this study,we built the cross-linked SBR model and performed the ReaxFF molecular dynamics simulation to study the pyrolysis process of the vulcanized SBR.The effect of temperature and pressure on the pyrolysis product distribution is greatly emphasized,especially the formation and transfer of sulfur containing compounds.Based on simulation results and comparison with corresponding experiments,the main conclusions are as follows:

1.Sulfur as a cross-linking agent decreases the mobility of polymer chains and promotes the crack of polymer chain during pyrolysis process.

2.High pyrolysis temperature facilitates the sulfide to transfer from solid/liquid states to the gas state such as H2S.

3.The sulfur cluster intermediate is observed at high sulfur content systems,where the small cluster captures sulfur atoms from CHS/HS intermediates.However,high temperature changes this pathway towards H2S formation.

4.At low sulfur content system,hydrocarbon sulfide (CHS) is the main sulfur-containing products.

The ReaxFF molecular dynamic simulations on the pyrolysis of vulcanized SBR provides an in-depth analysis of pyrolysis product distribution,which may provide theoretical guidance for the devulcanization during the pyrolysis of scrap tires.

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

The authors would like to express appreciation for the support of National Key Research and Development Program of China(Grant No.2018YFC1902601).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.10.033.