Effect of chloralkanes on the phenyltrichlorosilane synthesis by gas phase condensation

Tong Liu,Yunlong Huang,Chao Wang,Qiang Tang,Jinfu Wang*

Beijing Key Laboratory of Green Reaction Engineering and Technology,Department of Chemical Engineering,Tsinghua University,Beijing 100084,China

Keywords:Phenyltrichlorosilane Gas phase condensation Chloralkane Reaction mechanism Radical Synthesis

ABSTRACT To enhance the process of phenyltrichlorosilane synthesis using gas phase condensation,a series of chloralkanes were introduced.The influence of temperature and chloralkane amount on the synthesis was studied based on the product distribution from a tubular reactor.The promoting effect of chloralkane addition was mainly caused by the chloralkane radicals generated by the dissociation of C-Cl bond.The promoting effect of the chloromethane with more chlorine atoms was better than those with less chlorine atoms.Intermediates detected from the reactions with isoprene and bromobenzene demonstrated that both trichlorosilyl radical and dichlorosilylene existed in the reaction system in the presence of chloralkanes.A detailed reaction scheme was proposed.

1.Introduction

Phenyltrichlorosilane(C6H5SiCl3)is an important organosilicon monomer used as a silane coupling agent and a precursor of siloxane polymers[1-7].Industrially,phenyltrichlorosilane is mainly produced through thermal gas phase condensation with trichlorosilane(HSiCl3)and chlorobenzene(C6H5Cl)as reactants[8,9].However,the conversion of trichlorosilane is poor at lower temperature(＜400 °C),while byproducts increase at higher temperature.In order to improve the yield of phenyltrichlorosilane,an initiator or catalyst which can enhance the reaction at lower temperature would be invaluable.Goetze et al.reported that alkanes,diazenes and organodisilanes can be used as a free radical initiator in the synthesis of organosilanes[10].Krasnova investigated the pyrolysis of trichlorosilane in the presence of chloroform(CHCl3)and concluded that chloroform enhanced the conversion of trichlorosilane and the yield of desired products[11].However,few researches have reported the role of chloroform and other chloralkanes in this process.In this investigation,the synthesis of phenyltrichlorosilane in the presence of a chloralkane,such as CH2Cl2,CHCl3and CCl4,was studied.The influence of temperature and chloralkane amount on the product distribution was analyzed.

In this reaction system,there are two major reactions,namely,the condensation reaction[Eq.(1)]and the reduction reaction[Eq.(2)][8,9],which give four main products.With respect to the reaction mechanism,most researchers believe that the initial step is the pyrolysis of trichlorosilane. The trichlorosilyl radical (·SiCl3) and dichlorosilylene(:SiCl2)are the main intermediates formed in this step[12-15].Regarding the role of these two active species,Heinicke reported that dichlorosilylene can be generated at above 670°C via the combination of two trichlorosilyl radicals and that α-elimination from hexachlorodisilane and direct α-elimination from HSiCl3required a high temperature[14].Heinicke concluded that without a free radical initiator,the trichlorosilyl radical was the main intermediate.Krasnova[11]reported that the introduction of chloroform favored the generation of dichlorosilylene.It is dubious whether dichlorosilylene serves as another important intermediate.In this work,the role of dichlorosilylene in the mechanism of phenyltrichlorosilane synthesis and the promoting effect of chloralkanes were discussed.

2.Experimental

2.1.Materials

All reagents used in this study were of analytical grade.Trichlorosilane and chlorobenzene were purchased from Hycegas Co.,Ltd.and Beijing Modern Eastern Fine chemical Co.,Ltd.,respectively.All the chloralkanes and benzene used were from Beijing Modern Eastern Fine Chemical Co.,Ltd.Isoprene used to trap dichlorosilylene was purchased from Alfa Aesar Co.,Ltd.and bromobenzene and fluorobenzene were purchased from Sinopharm Chemical Reagent Co.,Ltd.For the product analysis,chromatographically pure phenyltrichlorosilane was bought from TCI Chemical Industry Co.,Ltd.

2.2.Experimental apparatus

The phenyltrichlorosilane synthesis is performed in a quartz tube reactor with inner diameter of 20 mm and length of 300 mm,as illustrated in Fig.1.The reactor is composed of four sections:feeding and vaporizing system,reacting and quenching system,condensing and collecting system,and pressure stabilizing system.Trichlorosilane and chlorobenzene are pumped into the vaporizers,respectively.The flow rate of the feedstock is controlled to fit the value of space time and the molar ratio of trichlorosilane and chlorobenzene(always 1:1 unless otherwise emphasized).The promoter chloralkane is pre-mixed with chlorobenzene.High purity nitrogen is used to dilute the vapor of the reaction gas,to quench the outlet mixture and also to purge the system to avoid the blockade of the vaporizer and reactor.The outlet gas is water-cooled to below 20°C and most of the products are condensed.Some light components such as trichlorosilane are subsequently condensed by a cold trap.Afterward,the non-condensable gas is vented through a pressure transmitter and an electromagnetic valve,so as to maintain the system pressure.

2.3.Analytical method

A Tech comp 7890II gas chromatograph(GC)equipped with a thermal conductivity detector(TCD)was used to analyze the condensed mixture from the water-cooler and the cold trap.According to the composition of condensate,the yield of phenyltrichlorosilane(Y)was defined as the ratio of the flow rate(F)of phenyltrichlorosilane produced to that of trichlorosilane in the feed:

The selectivity of phenyltrichlorosilane S was defined as the ratio of the flow rate of phenyltrichlorosilane in the condensate to the flow rate of trichlorosilane converted:

In the case of trapping experiment,a GC-MS system(Agilent 6890 GC and 5973N MS)was used to identify the products whose standard samples were hard to obtain.

3.Results and Discussion

3.1.Effects of chloralkanes in the low temperature region

When a chloralkane is added to the reacting system,its effects on the yield and selectivity ofphenyltrichlorosilane below 400°C(low temperature region)are different from those above 500°C(high temperature region).This suggests that the promoting mechanisms of chloralkanes are different in the two temperature regions.

In the low temperature region,as the reaction temperature is increased from 240 to 400°C,the yield and selectivity of phenyltrichlorosilane increase monotonically.The reaction initiates beyond the threshold of 240°C as illustrated in Fig.2.

The effects of chloralkanes can be correlated to the number of chlorine substituent,n.The more chlorine atoms in the chloromethane,CH4?nCln,the higher yield of phenyltrichlorosilane is obtained.As the temperature is increased,the difference between the chloromethanes becomes more obvious.As shown in Table 1,the bond dissociation energy of the C—Cl bond DC—Clis much lower than that of the C—H,chemical bond DC—H,and the former decreases with increasing n[16].So,it can be concluded that the increased yield of phenyltrichlorosilane is due to the dissociation of the C—Cl bond in the chloromethanes.As compared with the Si—Cl bond in trichlorosilane,the homolytic cleavage of the C—Cl bond in chloromethanes is easier,generating the chlorine radical·Cl.As a matter of fact,chloromethanes are used as free radical initiators in this reaction system.However,the chloromethanes favor not only the synthesis of phenyltrichlorosilane,but also the generation of side products,such as tetrachlorosilane and benzene,leading to the decreased selectivity to phenyltrichlorosilane.Only CH4-nClnwith n＞1 was considered because methyl chloride(n=1)was in gas phase at room temperature[17].

The chloromethanes participated in the phenyltrichlorosilane synthesis reaction.It experienced dechlorination and hydrogenation process,and its amount decreased during this reaction.Take tetrachloromethane as an example.The chloromethane distribution in the product after adding tetrachloromethane(τ=60 s)is listed in Table 2.Tetrachloromethane is first converted into chloroform and then dichloromethane.As the reaction temperature is increased,the conversion of trichlorosilane increases and tetrachloromethane is finally completely converted.The hydrogen needed in the conversion is from trichlorosilane that has a hydrogen atom.

Fig.1.Schematic of the experimental apparatus.1—high purity nitrogen;2,4—feed tank;3,5—single cylinder plunger pump;6—pressure reducing valve;7,8,10,11—one way valve;9,15—mass flow controller;12,13,14—vaporizer;16—tubular oven and tube reactor;17—water-cooler;18—product collection tank;19,21—corrosion resistance valve;20—cold trap;22—buffer tank;23—electromagnetic valve;24—pressure transmitter;25—thermocouple.

Fig.2.Effect of chloralkanes on the yield and selectivity of phenyltrichlorosilane in the low temperature region(p=101,325 Pa,τ =60 s).■ no chloralkane;● add CH2Cl2;▲add CHCl3;▼add CCl4.

Table 1 Bond dissociation energies of chloromethanes[16]

3.2.Effects of chloralkanes in the high temperature region

In the high temperature region,adding a chloralkane also favors the synthesis of phenyltrichlorosilane,as seen in Fig.3.As reaction temperature isincreased,the yield and selectivity of phenyltrichlorosilane increase until reaching their maximum at 600 and 540°C,respectively.The decrease in the yield of phenyltrichlorosilane is believed to be caused by its self-pyrolysis[18].

In the high temperature region,bond dissociation not only occurs for the C—Cl bond but also with the C—H bond,so the advantage of more chlorine atoms in the promoter is diminished at high temperature.The promotion by CH2Cl2is more obvious than thatby CCl4at600°C.On comparing C2H2Cl4and CCl4that have the same Cl atom number,the former shows a more obvious promotion effect.This indicated that some otherbond dissociation also favors phenyltrichlorosilane synthesis besides that of the C—Cl bond.C2H2Cl4and C2Cl6showing the similar promoting effect indicate that excess Cl atom will not favor phenyltrichlorosilane synthesis anymore.

Table 2 Chloromethane distribution in the product after adding tetrachloromethane at τ=60 s

3.3.Effect of the chloralkane amount

Chloroform was used to study the influence of the promoter amount on the yield and selectivity of phenyltrichlorosilane.The conditions were 540°C,101,325 Pa and the space time of60 s.The resultis illustrated in Fig.4.As the molar fraction of chloroform is increased from 0 to 2%,the phenyltrichlorosilane yield increases and then it decreases slightly beyond the range.The selectivity to phenyltrichlorosilane decreases monotonically.After the reaction,it is observed that there are some cokes deposited on the reactor wall.The amount of coke increases with increasing molar fraction of chloroform,xCHCl3.This indicates that chloralkanes enhance coke production.

3.4.Proposed reaction mechanism

It is widely accepted that the initial step of phenyltrichlorosilane synthesis is the pyrolysis of trichlorosilane.There are two possible routes of trichlorosilane pyrolysis,which generate two active species:trichlorosilyl radical[Eq.(5)]and dichlorosilylene[Eq.(6)].Heinicke investigated the copyrolysis of trichlorosilane with chlorobenzene and 1,3-butadiene without a free radical initiator and detected no dichlorosilylene[14],while Krasnova showed that the introduction of chloroform favored the generation of silylenes[11].Several researches used conjugated dienes to react with dichlorosilylenes[19-22].In this work,two experiments were applied to verify whether dichlorosilylene was generated in the reaction,especially in the presence of chloralkanes.

3.4.1.Trapping dichlorosilylene by a free radical inhibitor isoprene

Trichlorosilane,chlorobenzene and isoprene were mixed in the molar ratio 1:1:1,and an amount of chloroform that was 2%of the total amount of trichlorosilane and chlorobenzene was used.The mixture was fed into the reactor at 540°C,101,325 Pa and 60 s.The product is identified with GC-MS.About 5%1,1-dichloro-3-methyl-sila-cyclo-3-pentene,which is regarded as the 1,4-cycloaddition product of dichlorosilylene and isoprene as shown in Eq.(7),is detected.This result confirms the presence of dichlorosilylene in the reactor.Meanwhile,trichlorosilyl-1-butene and trichlorosilyl-2-butene were also detected,which could be the direct 1,4-addition products of trichlorosilane and isoprene or ring opening addition products of hydrogen chloride and 1,1-dichloro-1-sila-cyclo-3-pentenes.

3.4.2.Reaction product of trichlorosilane and bromobenzene

Fig.3.Effect of chloralkanes on the yield and selectivity of phenyltrichlorosilane in the high temperature region.(The molar ratio of HSiCl3 and C6H5Cl was 1:1.The molar fraction of chloralkanes was 2%.)

The different routes of trichlorosilane pyrolysis suggest the different sources of the chlorine atoms in the final product.It is proposed that chlorobenzene can be replaced with bromobenzene,and thus the chlorine atoms are labeled and the reaction paths are discriminated.In the experiments,trichlorosilane and bromobenzene were mixed in the molar ratio 1:1,and an amount of chloroform that was 2%of the total amount of trichlorosilane and bromobenzene was added.The mixture was fed into the reactor at 540°C,101,325 Pa and the space time of 60 s.The product was analyzed with GC-MS.Phenyltrichlorosilane and phenyldichlorobromosilane were detected in the ratio of 5:1.These would be produced in the reaction as shown in Eq.(8).This result indicates that the two intermediates,trichlorosilyl radical and dichlorosilylene,exist at the same time.

Fig.4.Effect of the chloroform amounton the yield and selectivity ofphenyltrichlorosilane(T=540 °C,p=101325 Pa,τ =60 s).

3.4.3.Reaction scheme

The reactions shown in Eqs.(7)and(8)indicate that in the presence of a chloralkane,both trichlorosilyl radical and dichlorosilylene exist and are two important intermediates that enhance the formation of phenyltrichlorosilane.Singlet dichlorosilylene was believed to insert into aryl-halo bond of chlorobenzene in the way a dichlorocarbene does[22].From these observations,a reaction scheme for phenyltrichlorosilane synthesis in the presence of chloralkanes can be proposed as illustrated in Fig.5.The bold lines represent the reactions promoted by chloralkanes.

Reactions from①to⑤show the initial step in which the homolytic dissociation of the C-Cl bond in the chloromethanes occurs.In this step,two of the important free radicals,trichlorosilyl radical and chlorine radical,are generated.Meanwhile,the transformation from tetrachloromethane to dichloromethane takes place,showing the dechlorination and hydrogenation step mentioned in Section 3.1.On the one hand,the chlorine free radical facilitates the transformation of trichlorosilane to the trichlorosilyl radical(Step?);on the other hand,it accelerates the transformation of the trichlorosilyl radical to tetrachlorosilane(Step ?),so the selectivity to phenyltrichlorosilane decreases.Steps⑥to?show the most probable step for dichlorosilylene generation[11].Some other possible steps of dichlorosilylene generation are shown as Steps⑩and?.

Besides the insertion of dichlorosilylene into the C-Cl bond of chlorobenzene(Step?),the combination of the trichlorosilyl and phenyl radicals(Step?)is another way to form phenyltrichlorosilane.This is indicated by that when switching from chlorobenzene to bromobenzene,Step?is changed to generated phenyldichlorobromosilane.The phenyl radical can easily react with trichlorosilane to form the undesired product benzene(Step?).

Fig.5.Reaction scheme in the presence of chloralkanes.

4.Conclusions

A series of chloralkanes were introduced to promote the process of phenyltrichlorosilane synthesis by gas phase condensation.The chloralkanes acted as free radical initiators that increased the conversion of trichlorosilane and yield of phenyltrichlorosilane.The promoting effect was mainly caused by the chloralkane radicals from dissociation of the C-Cl bond.The chloromethanes underwent dechlorination and hydrogenation reactions.Isoprene and bromobenzene were employed to confirm the existence of both trichlorosilyl radical and dichlorosilylene at the same time.A reaction scheme was developed to interpret the experimental results.

Nomenclature

D bond dissociation energy,kJ·mol?1

n number of chlorine atoms in the chloromethane

p reaction pressure,Pa

S selectivity of phenyltrichlorosilane,%

T reaction temperature,°C

x molar fraction,%

Y yield of phenyltrichlorosilane,%

τ space time,s