Yangcheng Lu*,Tianyang Li,Rui Wang,Guangsheng Luo

State Key Laboratory of Chemical Engineering,Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

1.Introduction

Epichlorohydrin(ECH)is an organochlorine compound and an epoxide with high reactivity.The ring-open reactions of ECH may occur under attacks from nucleophiles,electrophiles or radicals due to the different electron densities of its three carbon atoms and oxygen atom[1].ECH and its ramifications are widely used in polymer science,either as crosslink reagents in polymer synthesis or as raw materials of epoxy adhesives,coatings and composite materials[2–4].The conventional ECH production process is originated from allyl chloride[5–7].Recently,with the fast development of biodiesel production,the main byproduct—glycerol— is becoming an important starting material[8,9].Although different starting materials correspond to various process routes,there exists a common step—the cyclization of dichloropropanol(DCP)through reacting with sodium hydroxide(NaOH)aqueous solution or calcium hydroxide(Ca(OH)2)slurry at 90°C or higher temperature.The DCP has two isomers,1,3-DCP and 2,3-DCP.The 1,3-DCP is almost the unique intermediate in the synthesis of ECH with glycerol as starting material.The cyclization of DCP abides an internal nucleophilic substitution mechanism,i.e.,the internal nucleophile(--O?)attacks the carbon with leaving group to form a C--O bond and break the C--Cl bond[10].The reactivity of DCP is strongly affected by the position of substituent groups and their electron withdrawing characters.Although there exists different reports on the reaction orders of the cyclization of DCP[11,12],researchers have common recognition that the synthesis from 1,3-DCP to ECH,much faster than that from 2,3-DCP to ECH,is easy to reach high conversion.But,the critical challenges still need to be met that excessive introduction of water leads to heavy burdens with respect to ECH separation and wastewater treatment[13],and high risk of ECH hydrolysis resulting in unsatisfied selectivity(lower than 95%)[14,15].

Solid base is an important branch of reagents and catalysts,particularly known as heterogeneous base catalysts.Reactions catalyzed by heterogeneous catalysts are highly preferred in industry for the convenience to separate the catalysts and products.The syntheses of some fine chemicals,such as transesterification of oil to give biodiesel,isomerization of unsaturated hydrocarbons and addition reactions,have been widely reported to use strong solid base catalysts like alumina coated with alkali and alkaline-earth metal oxides,or weak solid base catalysts like zeolites and hydrotalcites[16–19].In contrast,using solid base as reactant is rarely reported,of which main disadvantage is high cost without considerations of wastewater treatment and base regeneration.It should be noted that the production of NaOH by sodium chloride(NaCl)electrolysis is a well-known commercial process,in which considerable amount of energy is consumed by concentrating NaCl aqueous solution.Using solid NaOH instead of NaOH aqueous solution may greatly improve the economic feasibility of NaOH regeneration and near-zero emission of wastewater in ECH production.A nonaqueous environment is also worth expecting for enhanced selectivity.

In this work,the synthesis of ECH from 1,3-dichloropropanol(DCP)by using solid NaOH was carefully investigated.Inert organic solvent,1-octanol,was introduced to ensure reaction intensity under control.The reaction performances with respect to apparent kinetics and selectivity were determined to explore optimized reaction conditions and con firm potentials for enhancing productivity in one batch.The reaction mechanism was discussed to understand how the reaction occurs and how to establish a rational reaction system.Furthermore,a process design towards free additional water was proposed to manipulate solid NaOH,by-product,and unreacted starting materials to realize a nearly closed circuit.The usage of solid NaOH is meaningful for water saving,wastewater decrement,and selectivity enhancement in ECH production.

2.Materials and Methods

2.1.Chemicals

1,3-Dichloropropanol(DCP,99 wt%,Acros),NaOH(96.0 wt%,particle,less than 1 mm in diameter,Xilong),1-octanol(99.0 wt%,Fuchen),1-methyl-2-pyrrolidinone(NMP,TCI),solid calcium oxide(CaO,98.0 wt%,powder,Fuchen),and Ca(OH)2(95.0 wt%,powder,Fuchen)were of analytical grade and used as received.

2.2.Apparatus and procedure

A 100 ml three-necked balloon flask with condenser and magnetic stirrer was used to carry out batch reactions,which was placed in a water bath(DF-101S,Kexi)to control the reaction temperature.Before experiment,solid base,1-octanol,and DCP were weighed separately.One half of 1-octanol was mixed with DCP to obtain DCP solution.The DCP solution and remained 1-octanol were preheated to reaction temperature in advance.The experimental procedures included:(1)added the remained 1-octanol into the flask;(2)added the solid base into the flask quickly;(3)10 min later,added DCP solution to startup the reaction;(4)as preset time intervals reached,withdrew liquid phase samples by syringe with filter head instantly.Throughout the experiments,the magnetic stirrer was working at constant speed of 600 r·min?1.The sampling amounts were little enough to minimize the effects of sampling operation on reaction control.The sum of the weights of DCP and 1-octanol was kept at 50 g.

For the samples,gas chromatography(GC-2014,Shimadzu)was used to determine the mass fractions of DCP and ECH;elementary analyzer(Elementar Vario EL III)was used to determine the content of carbon,and the changing of liquid phase mass during reaction proceeding was considered based on the assumption of total carbon conversation in the liquid phase.Thus,the conversion of DCP(C)and the selectivity from DCP to ECH(S)were calculated as the following.

Fig.1.The time pro files of conversion(a)and selectivity(b)at various temperatures.[DCP]0=5 wt%,M=0.50.

wheremis the mass of liquid phase;[DCP]and[ECH]are the concentrations of DCP and ECH,respectively;128.9 and 92.5 are relative molecular masses of DCP and ECH,respectively;the subscripts 0 andtindicate the initial value and the value at sampling time point,respectively.

3.Results and Discussion

3.1.Reaction performances using solid NaOH

We first investigated the apparent kinetics in terms of DCP conversion under various temperatures.The initial concentration of DCP was set as low as 5 wt%to weaken the interference from reaction heat releasing on temperature control.Considering that water is a by-product of DCP conversion and an excess of NaOH may result in undesired ECH hydrolysis,the mole ratio of NaOH to DCP(M)was set as 0.5,far deviated from stoichiometric ratio.Fig.1(a)shows typical results,where the theoretical maximal conversion is about 50%.As seen,DCP conversion corresponding to the first sampling point(1 min)could directly jump to 15%at 298.2 K,23%at 308.2 K,and 39%at 323.2 K,respectively.Afterwards,the increasing of DCP conversion becomes smooth.Especially for 298.2 K and 308.2 K,the tendencies are similarly linear.In general,the temperature increasing could accelerate the conversion process much,and 10 min and 20 min could be regarded as total conversion durations for323.2 K and 308.2 K,respectively.Fig.1(b)shows the selectivities calculated from measurements for various sampling points.Selectivity over 97%could be guaranteed throughout experiments.

We further changed the initial DCP concentration to explore perhaps apparent kinetics changing.The temperature was set at 298.2 K to obtain a mild reaction process,which is favorite to figure out some details.Fig.2(a)indicates that these time pro files of conversion almost coincide each other.High initial DCP concentration,favorite in terms of productivity and operation costs for batch reaction,is compatible with fast DCP conversion at least.Fig.2(b)shows corresponding selectivities.No clear variance is found as changing initial DCP concentration individually.

3.2.Reaction mechanism

The apparent kinetics performances shown in Figs.1 and 2 reflect some discontinuity around the first sampling point.In details,to the left of the first sampling point,the abrupt jump of conversion implies a quite fast reaction even at 298.2 K;to the right of the first sampling point,the smooth increasing of conversion implies that the control step is not a reaction but mass transfer.In this heterogeneous system,decisive mass transfer steps may be the dissolution of solid NaOH or the transfer of DCP towards the surface of solid NaOH.As vigorous stirring was consistent in experiment,the latter could be excluded.Herein,we carried out dissolution experiments for NaOH in 1-octanol individually.All the procedures were the same with what described in Section 2.2 but without advanced mixing of solid NaOH and 1-octanol.The addition of solid NaOH was 2.0 g.As the time intervals reached,the solution samples were withdrawn,and NaOH concentration in the samples was determined by titration method(Titration Excellence T50,METTLER TOLEDO).

Fig.3 shows the time pro files of NaOH concentration in solution at various temperatures.Considering NaOH concentration changed little after 90 min,we supposed that NaOH concentration at90 min was a saturated concentration.As seen,after 10 min mixing,as done in reaction performance tests,NaOH concentration at 323.2 K was almost twice of that at 298.2 K.It approximately equal to their difference in the DCP conversion at the first sampling point.So,the abrupt jump of conversion might be attributed to that NaOH dissolved in liquid phase reacted with DCP instantly,and the smooth conversion rate at later stage was determined by NaOH dissolution rate.Based on the assumption of instant reaction(fast kinetics has been revealed for the reaction between 1,3-dichlopropanol and NaOH elsewhere[20]),we could derive that NaOH concentration in bulk is zero and apparent kinetics is determined by the rate of NaOH dissolution towards solute-free solution.From Fig.3,we can estimate such dissolution rates corresponding to the initial solid NaOH particles at various temperatures,which equal to,the slopes of dashed lines.Furthermore,we can infer that for each particle,the dissolution rate is only dependent on particle size,and independent on other particles.It provides an interpretation on the independence of DCP conversion with initial DCP concentration,since the dissolution process of each solid NaOH particle is not changed at all.

Fig.4 gives a proposed scheme of reaction process using solid NaOH.In details,solid NaOH dissolves into the liquid phase first,and then is consumed instantly by the reaction with DCP to generate NaCl.The solubility of NaCl in 1-octanol is far lower than that of NaOH,so most of NaCl will precipitate as particles.Besides,there is no evidence supporting that solid NaOH could be coated by NaCl.In other words,full conversion of NaOH is achievable.

According to the reaction mechanism,the dissolution of solid phase in non-aqueous solution is the prerequisite condition for the synthesis from DCP to ECH,and the dissolution rate of solid base will determine the apparent reaction kinetics.With the assumptions of instantaneous reaction and sufficient stirring in liquid phase,once we can obtain the proportional relationship between the dissolution rate and the surface area of solid base,the apparent reaction kinetics equation may be established to guide for a strict control on the reaction process.

Fig.2.The time pro files of conversion(a)and selectivity(b)at various initial DCP concentrations.M=0.50,T=298.2 K.

Fig.3.The apparent kinetics of NaOH dissolution in 1-octanol at various temperatures.

Fig.4.The proposed reaction mechanism using solid NaOH.

Herein,we further selected combinations of various solid bases and solvents to con firm our understanding on reaction mechanism.The results are shown in Table 1.As seen,in 1-octanol,both CaO and Ca(OH)2have no reaction activity;in NMP,a solvent with polarity higher than 1-octanol,NaOH has reaction activity as good as in 1-octanol,the reaction activity of Ca(OH)2also emerges,CaO has no reaction activity yet.As used for catalysis,CaO is traditionally dealt by dipping in active-hydrogen solutions(methanol,for instance)to alter the surface O2?into OH?(or OH),thus activating CaO[21].However,in this work,neither 1-octanol nor NMP activated CaO,and the reaction activity of Ca(OH)2in NMP may come from dissolving mechanism as well.On the other hand,the reaction activity of solid base is also dependent on the polarity of solvent,and a common criterion for solvent selection is of medium to high polarity as well as of chemical inertness.

Table 1Conversion and selectivity with different solid bases and solvents at 323.15 K after 20 min reaction.[DCP]0=5 wt%,M=0.5

3.3.Feasibility of productivity enhancement

The productivity is an important consideration in reactor or process evaluation.Fig.2 shows that the variance of DCP concentration does not change the time pro file of conversion,so enhancing the productivity of bath reactor may be realized directly by increasing the concentration and the limitation of reactant(s)conversion.According to the reaction mechanism shown in Fig.4,the mole ratio of NaOH to DCP(M)or 100%is the limitation of reactant(s)conversion.While,it is obviously not suitable to set anMover 1.0,because excessive NaOH will exist after DCP converts to ECH totally and promote ECH hydrolysis(ECH was almost undetected finally whenMwas 2.0 in a separate experiment).Even if NaOH is not excessive,ECH hydrolysis may also be preferred when ECH is overwhelmed than DCP in concentration.Therefore,the up limit ofMin practice needs careful consideration.Herein,we carried out experiments under variousM(between 0.5 and 1.0)to explore such a limitation.Fig.5 shows the results.As seen in Fig.5(a),20 min is sufficient duration to fulfill DCP conversion for every case,corresponding to a conversion approximately equal toM.Meanwhile,no obvious change in selectivity was found withMincreasing in Fig.5(b).Perhaps explanations are:(1)the reaction process is mild under the control of solid NaOH dissolution rate,and DCP has enough time to supply to the periphery of solid NaOH;(2)ECH hydrolysis in non-aqueous solution is far slower than the conversion from DCP to ECH.Therefore,it is feasible and reliable to setMat0.95 to guarantee high selectivity.

We further investigated the feasibility of increasing DCP concentration.Fig.6 shows the results.In the experiment using 100 wt%DCP,1-octanol was not introduced,step(1)in Section 2.2 was omitted,and solid NaOH was added into the pure DCP directly.Because NaOH dissolution did not occur before starting up the reaction,a little decrease of conversion could be found,as seen in Fig.6(a).However,the conversion after 20 min does not present clear difference.As for the selectivity,increasing DCP concentration may bring adverse effects since more water generated as by-product of reaction may promote ECH hydrolysis reaction.Fortunately,such situation does not take place,and the selectivity still reached over 97%as using 100 wt%DCP.Another noticeable phenomenon was that the reaction system using 100 wt%DCP was light dark in color,different from what's commonly transparent.The intensive NaOH dissolution process as well as lack of inert solvent as heat removal medium is a possible reason.From this point of view,the addition of inert solvent can help the control on reaction intensity,which is meaningful in engineering.

3.4.Process design

Fig.5.The responses of conversion(a)and selectivity(b)on changing the mole ratio of NaOH to DCP.[DCP]0=5%wt,T=323.2 K.

Fig.6.The responses of conversion(a)and selectivity(b)on changing initial DCP concentration.M=0.50,T=323.2 K.

Based on the understandings mentioned above,we can figure out a schematic flow sheet for the production of ECH from DCP by using solid NaOH,as shown in Fig.7.In details,select solvent(1-octanolin this work)to make up DCP solution as one feed of slurry reaction,and then feed solid NaOH(a little insufficient compared with DCP in reaction and almost equal molar compared with DCP input in the integrated system)under effective stirring.The reaction could fulfill with 20 min at around 323.2 K.The NaCl contained in slurry could be separated out by conventional solid–liquid separation method like filtration.The supernatant could be distillated to obtain ECH and H2O as light components.The heavy components including DCP and solvents as recycled stream could be mixed with added DCP to feed the slurry reaction.The addition of DCP could be carried by a direct extraction of DCP aqueous solution obtained from previous process like glycerol chlorination.Compared with conventional ECH production process using water as the carrier of base,this process extremely reduces the consumption of water and provides potential for near-zero wastewater emission.Besides,the byproduct,solid NaCl could be used to concentrate the depleted brine for recycled electrodialysis(organic impurities should be removed before feeding).The energy consumption saving for NaCl concentration could offset the energy consumption increasing for solid NaOH production to some extent.Another important advantage of this process is facile to complete DCP conversion as well as high selectivity over 97%at lower temperature.Therefore,the competitiveness of the production of ECH from DCP by using solid NaOH is worth expecting,whatever in terms of environmental compact and technical and economic evaluation.

Fig.7.Schematic flow sheet for the production of ECH from DCP by using solid NaOH.

4.Conclusions

Using solid NaOH and 1,3-dichloropropanol(DCP)as reactants,1-octanol as solvent,the production of ECH is successfully realized in a non-aqueous system.The apparent kinetics of this process is sensitive to temperature.Twenty minutes are enough to achieve complete conversion at 323.2 K,independent on the initial concentration of DCP.Meanwhile,high selectivity over 97%is always obtained as the molar ratio of NaOH to DCP(M)less than 1.0.The dissolution and liquid phase reaction mechanism and instant reaction assumption could be used to interpret all the apparent kinetics performances revealed in the experiments,and also be supported by apparent dissolution kinetics tests.Accordingly,the reaction activity of solid base is dependent on the polarity of solvent,and a common criterion for solvent selection is of medium to high polarity as well as of chemical inertness.Since the reaction process is mild under the control of solid NaOH dissolution rate,and ECH hydrolysis in non-aqueous solution is far slower than the conversion from DCP to ECH,it is feasible and reliable to setMat 0.95 to guarantee high selectivity.Besides,increasing the initial DCP concentration is appropriate for productivity enhancement as long as the accumulation of reaction heat at local could be instantly removed by solvent.Furthermore,we proposed a schematic process that solid NaOH and DCP are feedstock and 1-octanol is recycled in system for hot spot inhibition.This ECH production process has advantages including near-zero wastewater emission,economically possible NaOH regeneration from NaCl,high selectivity,mild,and robust operating condition window.

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