Redistribution Mechanism of Chloromethylsilanes Catalyzed by HZSM-5 with Big and Small Apertures①

XU Wen-Yun LI Xio-Yn YANG Mei YANG Sho-Ming FANG Zhi-Li HONG Sn-Guo

a (College of Materials Science and Engineering,East China Jiaotong University, Nanchang 330013, China)

b (College of Chemistry, Nanchang University, Nanchang 330031, China)

1 INTRODUCTION

Organosilicone has been widely used in many fields for its electronic insulation, anti-radiation,temperature resistance and corrosion resistance[1,2].In addition, it has myriad forms to meet a wide range of industrial needs.(CH3)2SiCl2is the most important monomer that affects the development of organosilicone[3-5].In this regard, many techniques have been reported for the synthesis of (CH3)2SiCl2.Though the reported techniques are found suitable for the synthesis, it suffers from the residual problems of CH3SiCl3and (CH3)3SiCl[6-8].To tackle the issue some researchers prepared (CH3)2SiCl2by catalytic disproportionation using CH3SiCl3and(CH3)3SiCl as precursors, which can be economic and resolve the residual problems[9-11].However, the selection of disproportionation catalyst is in dire need[13].Tan et al[12]synthesized mesoporous MCM-type catalysts with amine by co-condensation to produce (CH3)2SiCl2with a 38.3% yield.Ao et al[13]produced (CH3)2SiCl2catalyzed by the bentonite after H2SO4activation.It is noticed that the HZSM-5 has a lot of cross-channel structure and abundant active protons (H) with identical properties on the surface[14-17].Hence, HZSM-5 as an acid catalyst is effective for these disproportionation and redistribution reactions[18-20].In our recent work, it was found that the surfaces of 3 T and 5 T cluster HZSM-5 had good catalytic activity towards the disproportionation production of (CH3)2SiCl2[21,22].This work deals the application of '7T HZSM-5 with small aperture' and '24T HZSM-5 with large aperture' for the disproportionation production of (CH3)2SiCl2.As per the guess 24T HZSM-5 with large aperture would be good for the disproportionation reaction compared to 7T HZSM-5 with small aperture due to the certain molecular volume of silane monomers.In order to support it, in this paper, we investigated the catalytic mechanism of HZSM-5 with two separate apertures of 24 T and 7 T.

2 METHODOLOGY

Fig.1 shows the optimized models of 7 T and 24 T cluster HZSM-5.Before optimization of 24 T and 7 T cluster HZSM-5, H atoms are used to saturate the dangling bonds, and the Si–H bond length is fixed as 0.150 nm in the intercepted models[23].The Si–H bonds are adjusted to line up with the direction of original Si–O bonds.

B3LYP/6-311++G (3df, 2pd) basis set was used to calculate the parameters of the reactants, transition states and products.In order to ensure the calculation accuracy, the frequency vibration by using the same basis set of each substance, the internals reaction coordinate (IRC) analysis of the spacing traces of the key atoms and localized orbital locator(LOL) of catalysts were considered.All the calculations were performed by Gaussian 09[24], Gaussion View 5.0[25]and Multiwfn 3.3[26].

In this study, the whole structures of 7 T (Fig.1a)and 24 T (Fig.1b) were calculated.The frequency vibration of 7 T and 24 T cluster HZSM-5 models was analyzed to ensure the correctness of the intercepted models.Wherein, the stretching vibration frequency of O2–H3was 3683 cm-1in 7 T and 3575.69 cm-1in 24 T, and the error was just less than 2% compared to the experiment result (3618 cm-1)[27].

Consequently, the intercepted models found correct and those can be used to simulate the acid point in the HZSM-5 structure.However, in order to illustrate the mechanism of their disproportionation in one figure, both 7 T and 24 T HZSM-5 were endowed with the same active sites (Fig.1c).Thus,the simplified model with key atoms was given in Fig.1 and Scheme 1 to represent the 7 T and 24 T cluster HZSM-5.

Fig.1. Models of 7T (a) and 24T (b) cluster HZSM-5

In this paper, the transition states were found by the combining synchronous transit and quasi-Newton (STQN) methods[28].There is just one negative value in all eigenvalues as per the frequency vibration analysis of reactants, transition states and products.All eigenvalues of reactants and products are positive while those of transition states have only one negative eigenvalue (a virtual frequency), and the minor frequencies caused by fixed bonds are small enough to be negligible.These results preliminarily show that the obtained transition states are valid.Then, by calculating IRC of each transition state, we find the steepest descent route to connect reactants and products, which not only corresponds with the reaction pathway, but also confirms the correctness of transition states.In addition, by analyzing the activation energies and the heat of reaction along the reaction pathways, the possibility of the reaction has been predicted.

3 RESULTS AND DISCUSSION

3.1 Reaction systems

The optimized geometric parameters (bond lengths, bond angles) of HZSM-5 are shown in Table 1.By comparing the parameters of 7 T and 24 T HZSM-5 in Table 1, we can see that the bond length of the active site (O2–H3) and the bond angle(∠O2–Al–O4) in 24 T are bigger than those in 7 T HZSM-5, indicating that the bond O2–H3in 24 T breaks easily than that in 7T HZSM-5, and 24 T HZSM-5 has a larger pore to accommodate the reactants.It preliminarily shows that 24T HZSM-5 has better catalytic activity.

Table 1. Optimized Geometric Parameters of HZSM-5 Catalyst

After calculating the IRC of each transition state,it is found that the transition states have only one negative eigenvalue (a virtual frequency) and they have the route to connect reactants and products.The virtual vibration modes oriented towards the products of TS are shown in Fig.2.However, those oriented towards reactants are omitted.Hence, the reaction pathway (Scheme 1) could be obtained.Depending on the different sequences of absorbing(CH3)3SiCl and CH3SiCl3into 7 T and 24 T HZSM-5, the reactions would proceed through channels 1 and 2.There are three steps in channel 1(Scheme 1).In the first step, (CH3)3SiCl absorbed in the catalyst reacts to produce product 1 (P1) through transition state 1 (TS1) and releases CH4, followed by the absorption of CH3SiCl3into P1 to produce P2 through TS2 and release (CH3)2SiCl2.In subsequent process the released CH4from the first step is absorbed into P2 to produce P3 through TS3.Lastly,P3 breaks down into (CH3)2SiCl2and HZSM-5.Similarly, Pattern was noticed in channel 2 (Scheme 1),in which the initially CH3SiCl3absorbed in the catalyst reacts to produce P4 through TS4.Then,(CH3)3SiCl is absorbed into P4 to produce P5 through TS5 and release SiCl4, followed by reducing P5 into P6 through TS6.Lastly P6 further breaks down into (CH3)4Si and HZSM-5.Hence, channel 1 is the main reaction channel when (CH3)2SiCl2is the goal product.The mechanism in Scheme 1 is accordant with the specific experimental results that(CH3)2SiCl2is the main product, whereas (CH3)4Si and SiCl4are the by-products[13].It shows that the reaction pathway in Scheme 1 is credible.

Fig.3 shows the spacing traces of the key atoms along the IRC analysis results of 7 T and 24 T HZSM-5 catalyzed disproportionation reaction.Here,we take TS1 as an example: when (CH3)3SiCl is absorbed with HZSM-5 to produce P1, the bond lengths of O2–H3and Si11–C10both widen to break.As a result, the distance between Si11and O4atoms was decreased, which facilitated the formation of new bonds, so did it between C10atom and H3atom.Based on IRC analysis of the catalyzed process, the variation trends of the distance between key atoms in 7 T and 24 T HZSM-5 are almost accordant.It matches with the result of the previous virtual vibration modes of TS, and proves that the mechanism shown in Scheme1 is credible.

3.2 Localized orbital locator(LOL) of the catalyst

LOL graphs of 7 T and 24 T HZSM-5 are shown in Fig.4, where the active sites are circled.By comparing to the LOL graphs of 7 T and 24 T HZSM-5, it can be seen that the electron locality of the active atoms (O2–H3) in 7 T is stronger than that in 24 T.That is to say, electrons of the active atoms in 24 T can be delocalized more easily.Hence, the active sites in 24 T can bond with (CH3)3SiCl and CH3SiCl3more easily, which means that the disproportionation activity of 24 T is higher than that of 7 T.

Scheme 1. Reaction process of the disproportionation catalyzed by HZSM-5 zeolite

Fig.2. Vibrated modes of transition states

Fig.3. Variation trends of the distance between key atoms along IRC in the catalytic system(——7 T, -----24 T)

Fig.4. LOL graphs of 7 T (a) and 24 T (b) cluster HZSM-5

3.3 Activation energy and reaction heat

Based on the density functional theory (DFT) and the energy gradient method, the energies of all stationary points in the potential energy surface are optimized, and the zero-point energies (ZPEs) are corrected using basis set of B3LYP/6-311++G (3df,2pd).Fig.5 provides the activation energies and the heat reaction along the reaction pathways.The energy of corresponding reactant is regarded as the zero point energy in each step, and the energies of transition states and products in subsequent steps are the gaps with their respective reactants.For example,the energy of R1 (R4) was assigned zero point energy of the first step to calculate the reaction enthalpies of P1 and the activation energies of TS1.

Fig.5. Data of active energies and reaction heats along the pathways

The reaction enthalpies (ΔH) and activation energies (Ea) are given in Table 2.In the disproportionation reaction catalyzed by 7T HZSM-5, the rate determining step of channel 1 produces P2 through TS2 (Ea2= 362.06 kJ/mol), whereas rate determining step of channel-2 absorbs CH3SiCl3in the catalyst through TS4 (Ea4= 394.35 kJ/mol).It can be seen that Ea2is lower than that of Ea4.Hence, the disproportionation in channel-1 proceeds more easily than that of channel-2.As a result channel-1 could be considered as a main reaction pathaway catalyzed by 7 T.However, in the disproportionation reaction catalyzed by 24 T HZSM-5, the rate determining step of channel 1 produces (CH3)2SiCl2through TS3 (Ea3= 220.05 kJ/mol), whereas the rate determining step of channel 2 absorbs (CH3)3SiCl in the catalyst through TS5 (Ea5= 289.07 kJ/mol).Obviously, the reaction through channel 1 proceeds more easily, wherein the activation energies of both the first and second steps found very small.As a result channel 1 could be considered as a main reaction pathaway catalyzed by 24 T, which is consistent with the reaction pathway shown in Scheme1 and the experimental results[13].

Table 2. Reaction Enthalpies and Activation Energies of Disproportion Catalyzed by HZSM-5

The activation energies of (CH3)3SiCl and CH3SiCl3absorbed in 7 T HZSM-5 are 145.72 and 394.35 kJ/mol, respectively.By contrast, the corresponding activation energies in 24 T HZSM-5 are 16.54 and 115.52 kJ/mol respectively, both of which are lower than those in 7 T.Hence, it is easier to form chemical bonds with two reactants in 24 T.The activation energies of rate-determining step in channel 1 of the disproportionation catalyzed by 24 T and 7 T HZSM-5 showed that the data of 24 T were far less than 7 T, indicating that the catalytic activity of 24 T was better than that of 7 T.This matches well with the structural analysis, LOL analysis and our expected results.

According to our previous research, the activation energies of rate-determining step of the disproportionation catalyzing the surfaces of 3 T and 5 T ZSM-5 are 155.64 and 181.68 kJ/mol[21,22].This indicates that the catalytic activity found in ZSM-5 channels is not as good as surfaces, which may be due to the repulsion of atoms that hinder the binding between the active sites on the catalyst and reactants.

4 CONCLUSION

In the present paper, B3LYP/6-311++G (3df, 2pd)basis set has been used to calculate the disproportionation reaction to produce dichlorodimethylsilane catalyzed by 7 T and 24 T HZSM-5.The electron locality of active site atoms in 24 T is weaker than that in 7 T HZSM-5.In 7 T HZSM-5 catalyzed disproportionation reaction, the rate-determining step generates (CH3)2SiCl2in channel 1 (Ea2=362.06 kJ/mol).When catalyzed by 24 T HZSM-5,the rate-determining step reduces the catalyst in channel 1 (Ea3= 220.05 kJ/mol).Hence, 24T HZSM-5 with large aperture would be good for the disproportionation reaction compared to 7 T HZSM-5 with small aperture.The results of structural analysis, reaction mechanism, LOL analysis and energy analysis are consistent with the expected results of calculation and the results of experiments.However, the catalytic activity of ZSM-5 channels is not as good as that of surfaces.

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