Chlorination of trichlorosilane/chlorodimethylsilane using metal chlorides: experimental and mechanistic investigations

Abstract

Removal of carbonaceous impurities from trichlorosilane (SiHCl₃) reduces the carbon content of solar grade polysilicon produced with the improved Siemens method. The separation of chlorodimethylsilane (CH₃)₂SiHCl from SiHCl₃ by distillation remains challenging due to the small difference in their boiling points. Herein, the chlorination of (CH₃)₂SiHCl/SiHCl₃ with metal chlorides (WCl₆, MoCl₅) were studied. The aim was to convert (CH₃)₂SiHCl into (CH₃)₂SiCl₂, increase the relative volatility of (CH₃)₂SiHCl and SiHCl₃ and facilitate the distillation. The optimum reaction conditions were 60 °C, 60 min and n(WCl₆ or MoCl₅): n(SiHCl₃ or (CH₃)₂SiHCl) = 0.7 at 0.8 MPa. Under these conditions, and when WCl₆ and MoCl₅ were used as the chlorine sources, the extents of (CH₃)₂SiHCl conversion were 22.7 and 18.5 times higher than those of SiHCl₃, respectively. In addition, a mechanistic study showed that the difference between the reactions of SiHCl₃ and (CH₃)₂SiHCl resulted from the different energy barriers for the reactions of the and (CH₃)₂SiCl· radicals with WCl_x or MoCl_x, and the barrier for the reaction was higher than that for the (CH₃)₂SiCl· reaction.

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1. Introduction

Solar energy is a renewable and clean energy that will play an important role in solving the energy crisis.^1–3 Therefore, polysilicon has rapidly developed as a raw material for solar photovoltaic cells. It is important to limit the impurities contained in solar-grade polysilicon to obtain higher photoelectric conversion efficiencies.

The standard for solar-grade polysilicon requires the carbon atom concentration to be less than 5 × 10¹⁸ atoms per cm³.⁴ Currently, the main process used in producing polysilicon is the improved Siemens method.^1,5 Since the final polysilicon product is obtained by reacting SiHCl₃ with H₂ in a bell jar furnace,⁵ carbon impurities in SiHCl₃ should be strictly limited. Therefore, the removal of carbonaceous impurities in SiHCl₃ is a key step in the improved Siemens method.

The carbonaceous impurities in SiHCl₃ were mainly methylchlorosilanes [(CH₃)_nSiCl_4−n, n = 1–3].⁶ Moreover, the boiling points of CH₃SiHCl₂ (41.9 °C) and (CH₃)₂SiHCl (34.7 °C) are close to the boiling point of SiHCl₃ (32.0 °C).⁷ Azeotropes are easily formed during SiHCl₃ purification via distillation. Thus, purifying SiHCl₃ by distillation is difficult. One possible method is to convert CH₃SiHCl₂ and (CH₃)₂SiHCl into methylchlorosilanes with high boiling points and high chlorine contents by chlorination. This would result in higher relative volatility and make distillation easier.

Typical chlorine sources for the chlorination reactions of CH₃SiHCl₂ are chlorine gas,⁸ chlorinated hydrocarbons^6,9 and chlorosilane.^4,10,11 Wan et al.⁸ proposed photochlorination of CH₃SiHCl₂ with Cl₂ in a continuous microchannel reactor. The results showed that the removal rate of CH₃SiHCl₂ was as high as 99.67% under the optimal reaction conditions. But this method is currently in the laboratory research stage. In addition, Zhang and Huang⁹ reported catalytic chlorination of CH₃SiHCl₂ with carbon tetrachloride (CCl₄) over a Pd/Al₂O₃ catalyst. However, the introduction of new carbon impurities cannot be avoided, and the high price of Pd limits its utilization in industry. Additionally, silicon tetrachloride, a byproduct of polysilicon production via a modified Siemens process, can also be used as a chlorine source for chlorination of CH₃SiHCl₂.⁴ However, silicon powder is easily formed, blocking the pores of the activated carbon catalyst during this process. Therefore, the catalytic performance and stability of the activated carbon catalyst are poor.

Chlorination of (CH₃)₂SiHCl with LiCl as the chloride source and B(C₆F₅)₃ as the catalyst was reported to occur in a mixture of ethyl ether and toluene.¹² Obviously, many researchers are studying chlorination reactions of CH₃SiHCl₂. However, the boiling point of (CH₃)₂SiHCl is closer to that of SiHCl₃, which makes separation more difficult. Furthermore, SiHCl₃ and (CH₃)₂SiHCl may be chlorinated at the same time. Therefore, it is important to study the competitive relationship between the SiHCl₃ and (CH₃)₂SiHCl during chlorination reactions.

Metal chlorides are also used as catalysts in chlorination reactions. For instance, (a) chlorination of methylphenyldichlorosilane to chlorinated methylphenyldichlorosilanes with gaseous chlorine has been catalysed by FeCl₃, SbCl₅, SnCl₄ and AlCl₃;¹³ (b) chlorination of 1,3-dithiolanes and 1,3-dithianes with CH₂Cl₂ has been catalysed by WCl₆;¹⁴ and (c) chlorination of allyl groups in terpenic olefins (β-pinene and carvone) with NaClO has been catalysed by MoCl₅, AlCl₃, FeCl₃ and FeCl₂.^15,16 Metal chlorides can be used not only as catalysts but also directly as chlorine sources in reactions, such as in the photochemical chlorination of methane mediated by FeCl₃.¹⁷ In addition, the boiling points of WCl₆ (346.7 °C) and MoCl₅ (268.0 °C) are very high and they are easily removed by distillation.

Therefore, this work will focus on the thermal chlorination reaction of SiHCl₃ and (CH₃)₂SiHCl by using WCl₆ and MoCl₅ as chlorine donors. The effects of the metal chloride type, molar ratio of reactants, reaction temperature and reaction time were investigated in detail. Finally, a reaction mechanism was proposed and explored in detail with density functional theory calculations.

2. Experiments and calculations

2.1 Materials used

Analytical standard dimethylchlorosilane ((CH₃)₂SiHCl, 99.0%) and trichlorosilane (SiHCl₃, 99.0%) were purchased from Sigma-Aldrich. Silicon tetrachloride (SiCl₄, 99.0%) was kindly supplied by Xinjiang Daqo New Energy Co. Additionally, dichlorodimethylsilane ((CH₃)₂SiCl₂, 99.0%), chloromethyldichloromethylsilane ((CH₂Cl)CH₃SiCl₂, 99.0%), tungsten hexachloride (WCl₆, 99.5%) and molybdenum pentachloride (MoCl₅, 99.6%) were purchased from Adamas.

2.2 Experimental setup

The specific experimental operations were as follows: first, to prepare stock solutions, SiHCl₃ or (CH₃)₂SiHCl was dissolved in SiCl₄ at a concentration of 0.5 mol L⁻¹. In addition, a given amount of WCl₆ or MoCl₅ and 10 mL of stock solution were added to a 36.5 mL batch reactor. WCl₆ and MoCl₅ were easily soluble in SiCl₄ and formed homogeneous reaction systems, and the molar ratios of reactants [n(WCl₆/MoCl₅) : n(SiHCl₃/(CH₃)₂SiHCl)] were 0.3, 0.7 and 1.2. Next, a nitrogen stream was introduced into the reactor three times to replace the air. To maintain a homogeneous reaction system, the N₂ pressure was raised to a higher reaction pressure (0.8 MPa). Then, the reactor was heated in a water bath and cooled with an ice-salt bath (−20 °C) after the reaction. Finally, the reactor was opened after slowly relieving the pressure. The samples were then removed from the reactor and distilled to remove metal impurities.

The mole fraction of the chlorosilane solution was determined with a 9790 Plus gas chromatograph equipped with a thermal conductivity detector (TCD), and H₂ was used as the carrier gas. The gas chromatography (GC) detection conditions were as follows: a 3 m 25% DC-550/Chromo packed column; an injection temperature of 150 °C; and a detector temperature of 150 °C. In addition, a programmed temperature rise was used for the oven temperature, i.e., it was first held at 60 °C for 2.5 min, then raised to 120 °C at a rate of 30 °C min⁻¹, and finally held at 120 °C for 2.5 min. The identities of the products were determined from the retention times of standard samples.

The conversion of SiHCl₃ was calculated according to eqn (1), and it was expressed as X(SiHCl₃). The inlet and outlet mole fractions of SiHCl₃ were expressed as x(SiHCl₃)_in and x(SiHCl₃)_out, respectively. The formula used for calculation of the (CH₃)₂SiHCl conversion rate was the same as that used for SiHCl₃.

To further identify the components in the reaction product, samples were dissolved in deuterated chloroform (CDCl₃) and qualitatively analysed with nuclear magnetic resonance (NMR) spectroscopy using a Bruker Avance III [¹H (400 MHz),¹³C (101 MHz)].

2.3 Theoretical method

The reactions discussed herein are free radical reactions, including chain initiation reactions, chain propagation reactions, and chain termination reactions. All the calculations were performed with the Gaussian 16 program.¹⁸ The geometric configuration of each stationary point for a reactant, transition state, or product along the reaction pathway was studied with B3LYP calculations by using the def2-TZVP basis set for the metal atoms and the 6-311G++(2d,p) basis set for the remaining atoms. In addition, frequency analyses were performed to ensure that the structure determined for a reactant or product was at a local minimum (all frequencies were positive) or in a transition state (only one negative frequency). The intrinsic reaction coordinates (IRCs) were used to evaluate whether the structures of the transition states were correctly connected to the products and reactants.¹⁹ The energy (E) in the entire reaction process was taken from the Gibbs free energy in the output file, that is, EE + thermal free energy correction (T = 298.15 K). The energy barrier (EB), the energy change (ΔE) and the dissociation energy (DE) were calculated with eqn (2), (3), and (4).

EB = E(transition state) − E(reactant) (2)

ΔE = E(product) − E(reactant) (3)

DE = ∑E(free radicals) − E(molecular) (4)

3. Results and discussion

3.1 Experimental research on chlorination of SiHCl₃/(CH₃)₂SiHCl with WCl₆

3.1.1 Effect of reaction temperature. The influence of reaction temperature on the conversion rate for the reaction of WCl₆ and SiHCl₃/(CH₃)₂SiHCl was investigated over the temperature range of 40–80 °C with a reaction pressure of 0.8 MPa and n(WCl₆) : n[SiHCl₃/(CH₃)₂SiHCl] = 0.3. The SiHCl₃ or (CH₃)₂SiHCl conversion rate as a function of temperature is shown in Fig. 1. These results showed that the conversion rates for (CH₃)₂SiHCl and SiHCl₃ both increased with increasing temperature and time. The conversion rate of SiHCl₃ was extremely low, between 0.8% and 6.3% at 40 °C to 80 °C for 10 min to 120 min. The (CH₃)₂SiHCl conversion was also low, between 2.0% and 9.5%, at 40 °C. However, when the reaction temperature was 60 °C, the conversion of (CH₃)₂SiHCl increased substantially from 3.3% to 59.9% with an increase in the reaction time from 10 min to 30 min. By prolonging the reaction time to 60 min, the conversion of (CH₃)₂SiHCl gradually increased to 75.2%. And the conversion of (CH₃)₂SiHCl increased slightly with increasing temperature and time. Therefore, the optimum reaction conditions were 60 °C for 60 min.

Fig. 1 Conversions of (a) SiHCl₃/(b) (CH₃)₂SiHCl in chlorination reactions run with WCl₆ at different reaction temperatures.

In addition, a solid precipitated from the reaction products obtained with conversion rates greater than 50%. The metal chloride is highly moisture-sensitive, and it was difficult to analyse it further. Since the high-valent tungsten chloride was highly soluble in silicon tetrachloride, (CH₃)₂SiHCl is thought to react with WCl₆ to form a low-valent tungsten chloride or elemental tungsten.

3.1.2 Effect of reactant ratio. The conversions of SiHCl₃ or (CH₃)₂SiHCl observed for chlorination reactions run with WCl₆ at different reactant ratios are shown in Fig. 2. Herein, the experimental conditions included a reaction pressure of 0.8 MPa, a reaction temperature of 60 °C and a reaction time of 60 min. When WCl₆ and (CH₃)₂SiHCl were reacted, with increases in the molar ratio of WCl₆ to (CH₃)₂SiHCl from 0.3 to 0.7 and 1.2, the conversion of (CH₃)₂SiHCl increased from 75.2% to 100.0% and 100.0%. The conversion rate of SiHCl₃ was far lower than that of (CH₃)₂SiHCl. Obviously, the optimum molar ratio of WCl₆ to (CH₃)₂SiHCl/SiHCl₃ was 0.7. The ratio of the two conversion rates was 22.7.

Fig. 2 Conversions of SiHCl₃/(CH₃)₂SiHCl during chlorination of SiHCl₃/(CH₃)₂SiHCl with WCl₆ at different reactant ratios.

The product from the 100% conversion reaction was analysed by GC. It contained a large amount of (CH₃)₂SiCl₂ and a small amount of (CH₂Cl)CH₃SiCl₂. To further confirm the composition of the sample, the product was qualitatively analysed by ¹H NMR and ¹³C NMR, as shown in Fig. 3. The main product (CH₃)₂SiCl₂ (¹H NMR: 0.81 ppm; ¹³C NMR: 6.83 ppm) was identified from the NMR spectrum. Therefore, the main product of the reaction between (CH₃)₂SiHCl and WCl₆ was (CH₃)₂SiCl₂, and the byproduct was (CH₂Cl)CH₃SiCl₂.

Fig. 3 (a) ¹H NMR and (b) ¹³C NMR spectra for the product of (CH₃)₂SiHCl chlorination by WCl₆.

3.2 Experimental research on chlorination of SiHCl₃/(CH₃)₂SiHCl with MoCl₅

3.2.1 Effect of reaction temperature. The conversion rates for SiHCl₃/(CH₃)₂SiHCl in chlorination reactions run with MoCl₅ at different temperatures are shown in Fig. 4. Here, the reaction conditions were the same as those used for SiHCl₃/(CH₃)₂SiHCl and WCl₆. Similarly, the conversion rates for (CH₃)₂SiHCl and SiHCl₃ both increased with increases in temperature and time. The conversion rate of SiHCl₃ was very low. (CH₃)₂SiHCl hardly reacted at 40 °C but reacted rapidly at 60 °C and 80 °C. Apparently, the conversion levels for the MoCl₅ reaction with (CH₃)₂SiHCl were much higher than those with SiHCl₃ at 60 °C and 80 °C. In addition, the conversion of (CH₃)₂SiHCl was as high as 64.0% at 60 °C for 60 min. Therefore, we will continue to explore the effect of the reactant ratio on the conversion of SiHCl₃/(CH₃)₂SiHCl in the chlorination reactions with MoCl₅ under these reaction conditions. Additionally, when the products were formed with conversion rates greater than 40%, a solid deposit formed during the reaction of MoCl₅ with (CH₃)₂SiHCl. The reaction was also presumed to yield a low-valent molybdenum chloride or elemental molybdenum.

Fig. 4 Conversions of (a) SiHCl₃/(b) (CH₃)₂SiHCl in chlorination reactions run with MoCl₅ at different reaction temperatures.

3.2.2 Effect of reactant ratio. Fig. 5 shows the conversion of SiHCl₃/(CH₃)₂SiHCl during chlorination with MoCl₅ at different reactant ratios. Herein, the experiment was also carried out at 0.8 MPa, 60 °C and 60 min. Fig. 5 shows that high molar ratios of MoCl₅ to SiHCl₃/(CH₃)₂SiHCl favoured conversion of SiHCl₃ or (CH₃)₂SiHCl. As expected, the conversion rate of (CH₃)₂SiHCl was still much higher than that of SiHCl₃. When the molar ratio of MoCl₅ to SiHCl₃/(CH₃)₂SiHCl was 0.7, the ratio of the two conversion levels was the largest at 18.5. Therefore, the optimum conditions were 60 °C, 60 min and n(MoCl₅) : n(SiHCl₃ or (CH₃)₂SiHCl) = 0.7 for chlorination of SiHCl₃/(CH₃)₂SiHCl with MoCl₅ at 0.8 MPa. Furthermore, the product formed with 100% conversion was also analysed by GC, ¹H NMR and ¹³C NMR, which showed that the main product of the reaction between (CH₃)₂SiHCl and MoCl₅ was (CH₃)₂SiCl₂ and that the byproduct was (CH₂Cl)CH₃SiCl₂.

Fig. 5 Conversions of SiHCl₃ or (CH₃)₂SiHCl in chlorination reactions run with MoCl₅ at different reactant ratios.

3.3 Mechanism calculation of chlorination of SiHCl₃/(CH₃)₂SiHCl with WCl₆

To verify the results of the calculations, the main geometric parameters of SiHCl₃, (CH₃)₂SiHCl, WCl₆ and MoCl₅ were compared with the reported experimental results. It can be seen from the data in Table 1 that the calculated geometric parameters have small errors compared with the experimental values.

Table 1 Calculated and experimental geometric parameters of SiHCl₃, (CH₃)₂SiHCl, WCl₆ and MoCl₅ (bond lengths/Å and bond angles/deg)

Compound	Parameter	Value
		Calculated	Experimental^20–22
a Equatorial plane.b Axial direction.
SiHCl3	Si–H	1.462	1.464
	Si–Cl	2.052	2.020
	Cl–Si–Cl	109.582	109.4
	Cl–Si–H	109.360	109.5
(CH3)2SiHCl	Cl–Si	2.101	2.0604
	C–Si	1.868	1.8542
	Cl–Si–C	108.37	108.43
	C–Si–C	112.83	112.32
WCl6	W–Cl	2.316	2.26 ± 0.02
MoCl5	Mo–Cl	2.247^a, 2.324^b	2.27 ± 0.02

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3.3.1 Chain initiation reaction. The dissociation energies for the bonds in WCl_x (x indicates the number of chlorine atoms in the metal chloride, and 1 ≤ x ≤ 6), SiHCl₃ and (CH₃)₂SiHCl are shown in Table 2 and 3. By comparing the dissociation energies, it was found that the bond energy of the W–Cl bonds is the smallest in SiHCl₃ and WCl_x (4 ≤ x ≤ 6), with values of 155.6 kJ mol⁻¹, 202.0 kJ mol⁻¹ and 314.2 kJ mol⁻¹ respectively. Moreover, the Si–H bond, with an energy of 333.3 kJ mol⁻¹, is the weakest bond in SiHCl₃, WCl₃, WCl₂, and WCl. Therefore, chain initiation reaction can be divided into cleavage of the Si–H bond and cleavage of the W–Cl bond. Chain initiation involves a decomposition reaction in which Cl· is released from WCl_x (eqn (5)) for the SiHCl₃ reaction with WCl₆, WCl₅, and WCl₄. In contrast, in the reactions between SiHCl₃ and WCl₃, WCl₂, and WCl, the chain initiation reaction is cleavage of the Si–H bond of SiHCl₃ (eqn (6)).

WCl_x = WCl_x−1 + Cl· (5)

Table 2 Dissociation energies of the W–Cl bonds in WCl_x

Compound	Parameter	DE/kJ mol^−1
WCl6	W–Cl	155.6
WCl5		202.0
WCl4		314.2
WCl3		374.4
WCl2		389.5
WCl		368.6

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Table 3 Dissociation energies for SiHCl₃ and (CH₃)₂SiHCl

Compound	Parameter	DE/kJ mol^−1
SiHCl3	H–Si	333.3
	Si–Cl	379.6
(CH3)2SiHCl	H–Si	343.5
	H–C	378.4
	Si–Cl	411.9

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Correspondingly, in reactions of (CH₃)₂SiHCl with WCl₆, WCl₅ and WCl₄, the chain initiation reaction is cleavage of the W–Cl bond in WCl_x (eqn (5)). In the (CH₃)₂SiHCl reactions with WCl₃, WCl₂ and WCl, the chain initiation reaction involves breakage of the Si–H bond in (CH₃)₂SiHCl (eqn (7)).

(CH₃)₂SiHCl = H· + (CH₃)₂SiCl· (7)

3.3.2 Chain propagation reaction. In the SiHCl₃ reactions with WCl₆, WCl₅ and WCl₄, the chlorine atoms from WCl_x continue to react with SiHCl₃. Then, chain growth could occur via two reactions: ① in the substitution reaction, Cl· attacks the silicon atom in SiHCl₃ to generate H· and SiCl₄, and the energy barrier for this step is 144.8 kJ mol⁻¹. The structure of transition state TS1 involved in this reaction is shown in Fig. 6. ② In the hydrogen abstraction reaction, Cl· abstracts the hydrogen atom on silicon to generate HCl and , and the energy barrier is so small that it can be treated as no energy barrier. DeSain et al. also considered the reaction to be a barrierless hydrogen abstraction reaction.²³

Fig. 6 Structures of transition states formed during the reactions of SiHCl₃/(CH₃)₂SiHCl with WCl₆.

Therefore, when WCl₆, WCl₅ and WCl₄ react with SiHCl₃, the chain growth reaction is hydrogen transfer between SiHCl₃ and Cl· (eqn (8)). The generated continues to react with WCl₆, WCl₅ and WCl₄ (eqn (9)). In the SiHCl₃ reactions with WCl₃, WCl₂, and WCl, from SiHCl₃ continues to react with WCl_x (eqn (9)). The relative energies for the reactions of SiHCl₃ with WCl₆, WCl₅, WCl₄ and WCl₃ are shown in Fig. 7. The structures of the transition states involved in each reaction are shown in Fig. 6.

Fig. 7 (a–d) Relative energies for the reactions of SiHCl₃ with WCl_x.

In the (CH₃)₂SiHCl reactions with WCl₆, WCl₅ and WCl₄, the Cl· from WCl_x decomposition continues to react with (CH₃)₂SiHCl. There are also two possible chain growth reactions. ① Substitution reaction: Cl· directly attacks the silicon atom in (CH₃)₂SiHCl, passes through transition state TS6, and generates H· and (CH₃)₂SiCl₂, and the reaction energy barrier is 85.6 kJ mol⁻¹. TS6 is shown in Fig. 6. ② Hydrogen abstraction reaction: Cl· attacks the hydrogen atom on silicon to generate (CH₃)₂SiCl· and HCl. This reaction step is also regarded as having no energy barrier.

Therefore, the pathway followed in the reaction of Cl· with (CH₃)₂SiHCl is hydrogen transfer from silicon to the chlorine atom (eqn (10)). The generated (CH₃)₂SiCl· continues to react with WCl₆, WCl₅, and WCl₄ to form (CH₃)₂SiCl₂ (eqn (11)). In the (CH₃)₂SiHCl reactions with WCl₃, WCl₂, and WCl, (CH₃)₂SiCl· and WCl_x react to form (CH₃)₂SiCl₂ (eqn (11)). The relative energies for the (CH₃)₂SiHCl reaction with WCl₆, WCl₅, WCl₄, WCl₃, WCl₂ and WCl are shown in Fig. 8. The structures of the transition states involved in each reaction are shown in Fig. 6.

(CH₃)₂SiHCl + Cl· = HCl + (CH₃)₂SiCl· (10)

WCl_x + (CH₃)₂SiCl· = (CH₃)₂SiCl₂ + WCl_x−1 (11)

Fig. 8 (a–f) Relative energies for the reactions of (CH₃)₂SiHCl with WCl_x.

Fig. 7 and 8 show that the difference in the reactions of SiHCl₃ and (CH₃)₂SiHCl with WCl_x lies in the energy barriers of the and (CH₃)₂SiCl· reactions with WCl_x. The energy barriers and Gibbs free energy changes of the reactions of WCl_x with and (CH₃)₂SiCl· are shown in Table 4. The data show that the energy barriers for the reactions of WCl₆, WCl₅, WCl₄ and WCl₃ with are higher than those for the (CH₃)₂SiCl· reactions, with differences of 86.9 kJ mol⁻¹, 62.6 kJ mol⁻¹, 71.6 kJ mol⁻¹ and 75.2 kJ mol⁻¹. These results also show that ΔG > 0 for the reactions of WCl₃, WCl₂, and WCl with and that ΔG < 0 for the reactions of WCl₃, WCl₂, and WCl with (CH₃)₂SiCl·. Obviously, WCl_x reacts more readily with (CH₃)₂SiHCl. These results are consistent with the experimental data, which demonstrates the practicality of the calculations.

Table 4 Energy barriers and Gibbs free energy changes for the reactions of WCl_x with and (CH₃)₂SiCl·

Reactant		EB (kJ mol^−1)	ΔG^a (kJ mol^−1)
a The formula used for calculation of ΔG was the same as that for ΔE (eqn (3)).
WCl6	·SiCl3	86.9	−211.2
	·(CH3)2SiCl	0	−247.2
WCl5	·SiCl3	102.9	−164.7
	·(CH3)2SiCl	40.3	−200.8
WCl4	·SiCl3	159.5	−52.5
	·(CH3)2SiCl	87.9	−88.6
WCl3	·SiCl3	168.0	7.7
	·(CH3)2SiCl	92.8	−28.4
WCl2	·SiCl3	—	22.7
	·(CH3)2SiCl	79.5	−13.4
WCl	·SiCl3	—	1.8
	·(CH3)2SiCl	96.3	−34.2

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3.3.3 Chain termination reaction. At the chain termination stage, free radicals combine with each other to form the bond with the largest bond energy. According to a comparative analysis using the data in Tables 5 and 6, in the reactions of SiHCl₃ and (CH₃)₂SiHCl with WCl₆, WCl₅, and WCl₄, chain termination involves reactions of both and (CH₃)₂SiCl· with Cl· (eqn (12) and (13)). The calculated dissociation energy for H₂ is 436.9 kJ mol⁻¹. Hence, in the reactions of SiHCl₃ and (CH₃)₂SiHCl with WCl₃, WCl₂, and WCl, the chain termination reaction involves the combination of two H· (eqn (14)).

(CH₃)₂SiCl· + Cl· = (CH₃)₂SiCl₂ (13)

2H· = H₂ (14)

Table 5 Free radicals and dissociation energies in the chlorination reaction of SiHCl₃

DE/kJ mol^−1	·Cl	·SiCl3
·Cl	193.6	366.8
·SiCl3	—	216.4

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Table 6 Free radicals and dissociation energies in the chlorination reaction of (CH₃)₂SiHCl

DE/kJ mol^−1	·Cl	·(CH3)2SiCl	·CH2CH3SiCl2
·Cl	193.6	402.9	264.0
·(CH3)2SiCl	—	234.6	275.4
·CH2CH3SiCl2	—	—	250.9

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In summary, the overall process for SiHCl₃ and (CH₃)₂SiHCl reactions with WCl₆, WCl₅ and WCl₄ is shown in eqn (15) and (16), and the overall process for reactions with WCl₃, WCl₂ and WCl is shown in eqn (17) and (18).

HSiCl₃ + 2WCl_x = SiCl₄ + 2WCl_x−1 + HCl (15)

(CH₃)₂SiHCl + 2WCl_x = (CH₃)₂SiCl₂ + 2WCl_x−1 + HCl (16)

2HSiCl₃ + 2WCl_x = 2SiCl₄ + 2WCl_x−1 + H₂ (17)

2(CH₃)₂SiHCl + 2WCl_x = 2(CH₃)₂SiCl₂ + 2WCl_x−1 + H₂ (18)

3.4 Mechanism calculation of chlorination of SiHCl₃/(CH₃)₂SiHCl with MoCl₅

3.4.1 Chain initiation reaction. The calculation process was the same as that in the previous section. By comparing the bond energies of Si–H bonds and Mo–Cl bonds in Tables 3 and 7, we can also divide the chain reaction into two reaction paths. In the processes involving MoCl₅ and MoCl₄ reactions with SiHCl₃ and MoCl₅, MoCl₄, MoCl₃ and MoCl reactions with (CH₃)₂SiHCl, Mo–Cl bond cleavage serves as the chain initiation reaction (eqn (19)). In the processes involving MoCl₃, MoCl₂, and MoCl reactions with SiHCl₃ and MoCl₂ reaction with (CH₃)₂SiHCl, chain initiation involves cleavage of the Si–H bonds (eqn (6) and (7)).

MoCl_x = MoCl_x−1 + Cl· (19)

Table 7 Dissociation energies of Mo–Cl in MoCl_x

Compound	Parameter	DE/kJ mol^−1
MoCl5	Mo–Cl	126.5
MoCl4		281.5
MoCl3		336.5
MoCl2		347.2
MoCl		341.3

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3.4.2 Chain propagation reaction. When the chain initiation reaction involves the release of Cl· from molybdenum chloride, chlorine atoms capture the hydrogen on the silicon to generate or (CH₃)₂SiCl· (eqn (8) and (10)). When the chain initiation reaction involves cleavage of Si–H bonds, or (CH₃)₂SiCl· is formed. The or (CH₃)₂SiCl· generated in the two reactions reacts with MoCl_x (eqn (20) and (21)). The relative energies for the reactions of SiHCl₃ and (CH₃)₂SiHCl with MoCl_x are shown in Fig. 9 and 10. The structures of the transition states involved in these reactions are shown in Fig. 11.

MoCl_x + (CH₃)₂SiCl· = (CH₃)₂SiCl₂ + MoCl_x−1 (21)

Fig. 9 (a–c) Relative energies for the reactions of SiHCl₃ with MoCl_x.

Fig. 10 (a–e) Relative energies for the reactions of (CH₃)₂SiHCl with MoCl_x.

Fig. 11 Structures of transition states during the reactions of SiHCl₃/(CH₃)₂SiHCl with MoCl₅.

The main difference between the reactions of MoCl_x with SiHCl₃ and (CH₃)₂SiHCl lies in the energy barriers for the reactions of ·SiCl₃ and ·(CH₃)₂SiCl with MoCl_x. Fig. 9 and 10 show the energy barriers and Gibbs free energy changes for MoCl_x reactions with and (CH₃)₂SiCl·, and these are summarized in Table 8. The energy barriers for the reactions of MoCl₅, MoCl₄, and MoCl₃ with are higher than those for the (CH₃)₂SiCl· reactions, and the differences are 75.6 kJ mol⁻¹, 72.7 kJ mol⁻¹, and 69.3 kJ mol⁻¹, respectively. The ΔG values for the MoCl₅, MoCl₄, MoCl₃, MoCl₂ and MoCl reactions with are all larger than the ΔG values for the corresponding (CH₃)₂SiCl· reactions. The above results reveal that MoCl_x reacts more easily with (CH₃)₂SiHCl. In addition, the energy barrier difference between the reactions of MoCl₅ with SiHCl₃ and (CH₃)₂SiHCl and the reactions of WCl₆ can be explained by the presence of only five Cl atoms in MoCl₅. WCl₆ is more conducive the chlorination of (CH₃)₂SiHCl in SiHCl₃. Obviously, the calculated results are in good agreement with the experimental results.

Table 8 Energy barriers and Gibbs free energy changes for MoCl_x reactions with and (CH₃)₂SiCl·

Reactant		EB (kJ mol^−1)	ΔG (kJ mol^−1)
MoCl5	·SiCl3	75.6	−240.3
	·(CH3)2SiCl	0	−276.3
MoCl4	·SiCl3	140.3	−85.3
	·(CH3)2SiCl	67.6	−121.3
MoCl3	·SiCl3	143.6	−30.3
	·(CH3)2SiCl	74.3	−66.4
MoCl2	·SiCl3	—	−19.6
	·(CH3)2SiCl	46.6	−55.6
MoCl	·SiCl3	—	−25.5
	·(CH3)2SiCl	89.2	−61.6

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3.4.3 Chain termination reaction. In the same way, in the SiHCl₃ reactions with MoCl₅ and MoCl₄ and the (CH₃)₂SiHCl reactions with MoCl₅, MoCl₄, MoCl₃ and MoCl, chain termination occurs via a combination of and (CH₃)₂SiCl· reactions with Cl· (eqn (12) and (13)). In the processes for SiHCl₃ reactions with MoCl₃, MoCl₂ and MoCl and (CH₃)₂SiHCl reaction with MoCl₂, chain termination occurs via the combination of two hydrogen atoms (eqn (14)).

Therefore, the overall equation for the reactions of SiHCl₃ with MoCl₅ and MoCl₄ is shown as eqn (22), and the overall equation for the reactions with MoCl₃, MoCl₂ and MoCl is shown as eqn (23). The overall equation of the reactions of (CH₃)₂SiHCl with MoCl₅, MoCl₄, MoCl₃ and MoCl is shown as eqn (24), and the overall equation of the reaction with MoCl₂ is shown as eqn (25).

HSiCl₃ + 2MoCl_x = SiCl₄ + 2MoCl_x−1 + HCl (22)

2HSiCl₃ + 2MoCl_x = 2SiCl₄ + 2MoCl_x−1 + H₂ (23)

(CH₃)₂SiHCl + 2MoCl_x = (CH₃)₂SiCl₂ + 2MoCl_x−1 + HCl (24)

2(CH₃)₂SiHCl + 2MoCl_x = 2(CH₃)₂SiCl₂ + 2MoCl_x−1 + H₂ (25)

4. Conclusions

Chlorination reactions of SiHCl₃/(CH₃)₂SiHCl were carried out with two metal chlorides (WCl₆ and MoCl₅) as the chlorine sources. The conversion rates for (CH₃)₂SiHCl and SiHCl₃ both increased with increases in the reactant ratio, temperature and time. The conversions for the reactions of (CH₃)₂SiHCl with WCl₆/MoCl₅ were much higher than those of SiHCl₃. Furthermore, the use of WCl₆ as the chlorine source showed higher conversion of the (CH₃)₂SiHCl than MoCl₅. The optimum conditions for the reaction of WCl₆ with (CH₃)₂SiHCl were as follows: a reaction pressure of 0.8 MPa, a reaction temperature of 60 °C, a reaction time of 60 min and n(WCl₆) : n(SiHCl₃ or (CH₃)₂SiHCl) = 0.7. The conversion rate for (CH₃)₂SiHCl was 22.7 times that for SiHCl₃ in the reactions of SiHCl₃/(CH₃)₂SiHCl with WCl₆.

The mechanisms for the reactions of SiHCl₃/(CH₃)₂SiHCl with WCl₆/MoCl₅ were explored in detail with density functional theory calculations. The differences in the reactions of SiHCl₃ and (CH₃)₂SiHCl with WCl₆ or MoCl₅ were found to lie in the energy barriers of the and (CH₃)₂SiCl· reactions with WCl_x/MoCl_x. The energy barriers for the reactions of WCl_x (3 ≤ x ≤ 6) with were higher than those for the (CH₃)₂SiCl· reaction. The same is true of MoCl_x (3 ≤ x ≤ 5). On the whole, the energy barrier differences for WCl₆ reactions with SiHCl₃ and (CH₃)₂SiHCl were higher than those for MoCl₅ reactions with SiHCl₃ and (CH₃)₂SiHCl. The experimental results were in good agreement with the calculation results. (CH₃)₂SiHCl is converted to (CH₃)₂SiCl₂ in a chlorination reaction, which is conducive to the removal of carbonaceous impurities from SiHCl₃ by distillation in the improved Siemens method.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Major Science and Technology Project of Xinjiang Bingtuan (No. 2017AA007, 2020AA004) and the Major Science and Technology Project of Shihezi (No. 2020ZD02).

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