The study of an alcoholysis reaction of silicon tetrafluoride with alcohols and magnesium to prepare tetraalkoxysilanes and magnesium fluoride

Article abstract of DOI:10.1080/10426507.2014.965812

An efficient alcoholysis reaction of silicon tetrafluoride with alcohols in the presence of magnesium can directly prepare tetraalkoxysilane and magnesium fluoride. The reaction can afford tetraethoxysilane, tetramethoxysilane, and magnesium fluoride in good yields with the aid of a catalytic amount of iodine.

Full text of DOI:10.1080/10426507.2014.965812

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Phosphorus, Sulfur, and Silicon and the Related
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The study of an alcoholysis reaction of silicon
tetrafluoride with alcohols and magnesium to prepare
tetraalkoxysilanes and magnesium fluoride
Zongfan Zhanga, Zhong Huanga, Qiang Tiana, Yuye Lia, Xia Lia, Xuesong Lianga & Lusha Du
a Fine Chemical Industry Center, Yunnan Chemical Research Institute, Yuntianhua Group
Corporation Limited, 1417 Dianchi Road, 650228, Kunming, Yunnan, China
Accepted author version posted online: 16 Oct 2014.
To cite this article: Zongfan Zhang, Zhong Huang, Qiang Tian, Yuye Li, Xia Li, Xuesong Liang & Lusha Du (2014): The study
of an alcoholysis reaction of silicon tetrafluoride with alcohols and magnesium to prepare tetraalkoxysilanes and magnesium
fluoride, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2014.965812
To link to this article: http://dx.doi.org/10.1080/10426507.2014.965812
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THE STUDY OF AN ALCOHOLYSIS REACTION OF SILICON TETRAFLUORIDE WITH
ALCOHOLS AND MAGNESIUM TO PREPARE TETRAALKOXYSILANES AND
MAGNESIUM FLUORIDE
Zongfan Zhang, Zhong Huang*, Qiang Tian, Yuye Li, Xia Li, Xuesong Liang, and Lusha
Du 1
Fine Chemical Industry Center, Yunnan Chemical Research Institute, Yuntianhua Group
Corporation Limited, 1417 Dianchi Road, 650228, Kunming, Yunnan, China
Abstracts An efficient alcoholysis reaction of silicon tetrafluoride with alcohols in the presence
of magnesium can directly prepare tetraalkoxysilane and magnesium fluoride. The reaction can
afford tetraethoxysilane, tetramethoxysilane and magnesium fluoride in good yields with the aid
of a catalytic amount of iodine.
Mg
F
F Si
F
OR
RO Si OR
OR
I
2
Mg*
O
+
+
F
MgF2
R
H
Keywords Alcoholysis; Silicon tetrafluoride; Tetraalkoxysilane; Magnesium fluoride
* Corresponding author at: Fine Chemical Industry Center, Yunnan Chemical Research Institute, Yuntianhua Group Corporation
Limited, 1417 Dianchi Road, 650228, Kunming, Yunnan, China.
Tel.: +86(871)64313269; Fax: +86(871)64327017.
E-mail address: pifushu@163.com (Zhong Huang )
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INTRODUCTION
Silicon tetrafluoride (SiF4) is one of the key by-products of the phosphate fertilizer industry,
and can efficiently meet the shortage of fluorspar, which is the dominant source of fluorine. But
the use of SiF4 is very limited by technology. It is well known to us that the traditional method to
utilize SiF4 is generating fluorosilicic acid, which is usually used to prepare HF or other
fluorides. One example of utilizing fluorosilicic acid is adding NH3 into fluorosilicic acid to
prepare SiO2 and NH4F, and then NH4F reacts with concentrated sulfuric acid to give HF and
(NH4)2SO4.1 The method has some disadvantages such as that by-products (SiO2 and (NH4)2SO4)
cannot be sold at a reasonable price and must be forced to be stored, enhancing the
manufacturing cost of such enterprises and giving potential contamination to the environment.
Moreover, SiF4 could be used to prepare tetraalkoxysilanes. The reported normal method is to
add NH3 into the system of SiF4/ROH to give tetraalkoxysilanes.2,3 However, this method can
only transfer 1/3 of silicon atoms into tetraalkoxysilanes (the theoretical yield is only 33.3%),
and the remaining into (NH4)2SiF6, and thus the efficiency is very low.
Therefore, it is necessary to find a new route, by which the fluorine atoms and the silicon
atom of SiF4 can be transformed into valuable products such as fluorides (AlF3, MgF2) and
silicate esters in one step. MgF2, as one of products in this study, has a significant application in
many fields,4-6 such as in optical lens coating, ceramics, catalyst, the electronics industry, the
electrolytic magnesium industry. Silicate esters such as tetraethoxysilane and its polymers are
widely used as coupling agents, crosslinking agents and plasticizers. However, except for some
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reports on the structure of SiF4,7-13 there are very few reports focused on an efficient method
which can transfer fluorine atoms of SiF4 into fluorides and silicon atoms into silicones.
Herein, we explored a new alcoholysis of SiF4 in the presence of magnesium powder (Mg)
to produce MgF2 and tetraalkoxysilane (Si(OR)4) with the aid of a catalytic amount of iodine
(Scheme 1). After the optimization of conditions, the reaction can give both MgF2 and Si(OR)4
in good yields from methanol and ethanol without producing unnecessary by-products.
RESULTS AND DISCUSSION
The alcoholysis of SiCl4 in EtOH at room temperature can give HCl and tetraethoxysilane.14,
15 However, according our experiments, the alcoholysis can not be directly applied in the
reaction of SiF4 with EtOH (or MeOH) for obtaining HF and tetraethoxysilane (or
tetramethoxysilane).
We found that when SiF4-EtOH solution and Mg were added into a flask, some bubbles
(hydrogen gas) were released. The liquid of the reaction system was tested by GC-MS, and an
EI-MS (m/z) peak was appeared at 208, which is the relative molecular mass of Si(OEt)4. The
isolated solid was washed with EtOH and ethyl acetate (EA), and then calcined at 300 oC for 2 h
to give a white powder which was then examined with a XRF spectrometer. The result showed
that the powder was mainly composed of Mg, O and F. The content of F was about 9.5%, which
meant that the powder may contain MgF2.
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The reaction of SiF4, EtOH and Mg
Building on the result above, the reaction of SiF4, EtOH and Mg was investigated. Initially,
the reaction was examined at room temperature for 24 h (Table 1, Entry 1). The yields of
Si(OEt)4 and MgF2 were only 38% and 61%, respectively. Subsequently, the reaction
temperature was enhanced to 40 oC for 24 h, and the yields of both products were only slightly
increased. The two results indicate that the reaction can occur, but is very slow. Then, a minute
quantity of I2 was added into the reaction system. The result was that iodine could efficiently
accelerate the reaction of magnesium. In the comparison of Entry 1 and Entry 3 (Table 1), after a
minute quantity of I2 was added, the reaction rate was nearly enhanced by 8-fold.
We assumed that the activator played an important role in activating Mg as in the
preparation of Grignard reagent, but not on SiF4 and EtOH. In order to verify this hypothesis, we
did three groups of experiments, which contained the reagents as follows: (1a) EtOH and (1b)
EtOH, I2; (2a) SiF4, EtOH and (2b) SiF4, EtOH, I2; (3a) Mg, EtOH, SiF4 and (3b) Mg, EtOH,
SiF4, I2. In the experiments of (1a) and (1b) or (2a) and (2b), no reaction occurred. Otherwise,
there were hydrogen that escaped from both (3a) and (3b), but the escape velocity of hydrogen
from (3a) was much slower than (3b). These results defined our hypothesis that it was Mg that
was activated by I2. With I2 as the activator, the reaction time was extended to 24 h, but the
yields were not very high (Entry 5, Table 1). The result indicates that room temperature is not
suitable for obtaining good product yields in the reaction. Thus, the reaction temperature was
enhanced to 40 oC (Entry 7, Table 1), and compared with Entry 4, the yield of Si(OEt)4 was
increased to 54%.
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In order to enhance the product yields, the process was improved, in which the mixture was
stirred under room temperature for 3 h before being heated to 40 oC, 60 oC and reflux (Entry 8-
10, Table 1). The highest yield of Si(OEt)4 was 71% (Entry 10, Table 1). After the reaction time
was prolonged to 10 h, the yields of Si(OEt)4 and MgF2 were up to 78% and 89%, respectively.
However, there is still a disadvantage in the selected process above, which is that without the
addition of NH3, we can not obtain liquid Si(OEt)4 in the post-treatment process, but a gel which
is the polymer of Si(OEt)4 (Entry 6, Table 1). This may be due to the excess SiF4 that causes the
reaction system to be very acidic, and under this condition, tetraethoxysilane can easily condense
to polysilicate.
The reaction of SiF4, MeOH and Mg
The substrate scope was enlarged to MeOH. The result gave that the reaction of SiF4, MeOH
and Mg could be conducted without iodine (Entry 1, Table 1), and occurred much faster than that
of EtOH (Entry 1, Table 2). The reason is that the proton of MeOH is more acidic than that of
EtOH and can react with Mg more easily (pKa of MeOH is 15.54 and pKa of EtOH is 16).16
Iodine can also efficiently accelerate the reaction. The experiment showed that the reaction
completed in 2 h with the addition of a catalytic mount of I2, giving Si(OMe)4 and MgF2 with
good yields of 82% and 93%, respectively. However, when the reaction temperature was
enhanced to 40 oC (Entry 3, Table 2), the yields of Si(OMe)4 and MgF2 were not increased. After
the temperature was enhanced to reflux (Entry 4, Table 2), the yields were slightly decreased.
.
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The improved reaction process
The optimized conditions described for EtOH and MeOH can give the target products, but
there are still at least three disadvantages as followings. First, the yields are not very satisfactory.
Second, NH3 must be added. Third, it is very difficult to control the process of adding NH3,
because the quantity of NH3 must be sufficient to make the system neutral, and too much or too
little NH3 can lead to the condensation of Si(OR)4. During the study, it is found that without the
presence of SiF4, both EtOH and MeOH can react with Mg quickly with the aid of I2. The
process was improved to be that ROH reacted with Mg to give [(RO)nMg], which then reacted
with SiF4 to give the target products with good yields (Entry 1-2, Table 3). The first advantage of
the improved process is that for both of EtOH and MeOH, the yield of MgF2 is more than 98%.
The second advantage is that the addition of NH3 is avoided. By experiments under the same
conditions of PrOH and BuOH for 3 d, we did not obtain Si(OPr)4 and Si(OBu)4 (Entry 3-4,
Table 3).
In order to prove that the solid product is MgF2, the solid product was tested by X-ray
powder diffraction and X-ray fluorescence. X-ray diffraction shows that the patterns of the solid
correspond to the standard files of MgF2 from ICDD, which belongs to P42/mnm space group
(Figure 1). The X-ray fluorescence shows that the quality of MgF2 could be up to the standard in
industry (special grade of YS/T 691-2009 in China, the specific data of which can be seen in
Table S 1 in Supplemental Materials), and the content of MgF2 is 98%.
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CONCLUSIONS
In summary, we found a new alcoholysis reaction of SiF4 with ROH in the presence of Mg.
The reaction afforded the products (MgF2 and Si(OR)4) in good yields when methanol and
ethanol were used as the substrate and iodine was used as the activator. More importantly, the
study may provide an effective method to overcome the difficulty of the use of SiF4 for
phosphate fertilizer industry.
EXPERIMENTAL
The fluoride, magnesium and silicon and other elements in MgF2 were measured on a
Bruker S4 Pioneer XRF spectrometer. The tetraalkoxysilanes were determined on an Agilent
6890 series GC-MS spectrometer. X-ray diffraction (XRD) analysis was carried out by using a
1
D/MAX 2500VB2+/PC and Cu Kα radiation (λ=1.54056 Å). H and 13C NMR were recorded
with a Bruker spectrometer at 500 MHz using TMS as internal standard. The yields of
tetraalkoxysilane were reported with the silicon dioxide after hydrolysis. SiF4, ammonia,
aluminum powder and anhydrous ROH were used as purchased without purification.
Characteristic spectra obtained from the reactiosn are presented in the Supplemental Materials
(Figures S 1 – S 8)
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Procedure for the preparation of SiF4-alcohol solution
A typical procedure to produce SiF4-MeOH solution: Anhydrous MeOH (1 kg) was charged
in a glass flask of 2 L in an ice bath. Then, SiF4 gas was passed into the cooled MeOH from a
cylinder through a teflon tubule until SiF4 (317.5 g) was solved in MeOH to be SiF4-MeOH
solution with a mass concentration of 24.1%. During the process, MeOH was stirred. The weight
of SiF4 was confirmed by gravimetric method. It’s noted that (1) the whole system must be
closed and the outlet of the glass flask was linked to an absorption bottle with NaOH-water
solution with a mass concentration of 10%; (2) the preparation must be carried out in a fume
hood; (3) operators must be protected by tightly fitting safety goggles, gloves, and complete suit
protecting against chemicals; (4) the prepared solution was stored in a closed teflon bottle at < 10
oC.
Typical procedure for the alcoholysis of SiF4 in MeOH
The reaction was performed in a flask of 50 mL volume. Before materials were charged,
the flask was filled with dried N2, making sure that during the reaction process, water in the
atmosphere was avoided. The flask was charged with MeOH (15 mL), Mg powder (1.5 g) and a
minute quantity of I2 (0.39% of magnesium mass). The mixture was stirred at room temperature
for 1 h and then heated to reflux for 0.5 h. After the reaction system was cooled to r.t., SiF4-
MeOH solution (13.4 g, 24.1%) was added to make the system neutral using a syringe.
Subsequently, the mixture was filtered, and the cake was washed with EA (3×15 mL). The two
filtrates were merged, and after removal of the solvents of MeOH and EA by distillation method,
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tetramethoxysilane (3.98 g) was obtained. The washed cake went through calcination at 300 oC
for 2 h to provide MgF2 (3.77 g).
Tetramethoxysilane: yield 85%; colorless oil; boiling point (800 mbar): 113-116 oC; boiling
point (1 atm): 121-122 oC;17 Density (20 oC): 1.03; 1H NMR (500 MHz, CDCl3): 3.56 (s, 12H);
13C NMR (500 MHz, CDCl3): 51.1; EI-MS (m/z): 152 (M+), 137, 121, 107, 91, 59, 31.
Magnesium fluoride from MeOH: yield >98%; white solid; MgF2: 98%; Ca: not detected;
SiO2:0.13%; Fe2O3: 0.014%; H2O: 0.005%.
Typical procedure for the alcoholysis of SiF4 in EtOH
The reaction was performed in a flask of 100 mL volume. Before materials were charged,
the flask was filled with dried N2, making sure that during the reaction process, water in the
atmosphere was avoided. The flask was charged with EtOH (30 mL) and Mg powder (1.5 g).
The mixture was stirred at room temperature for 0.5 h and then a minute quantity of I2 (0.39% of
magnesium mass) was added, followed by stirring for 1.5 h. The system was heated to reflux for
1.5 h. After the system was cooled to r.t., SiF4-EtOH solution (14.8 g, 22%) was added in 1 h
using a dropping funnel with stirring. Then, the system was heated to reflux for 1.5 h, and the
system was cooled to r.t. to be neutral. Subsequently, the mixture was filtered, and the cake was
washed with EA (15 mL×3). The two filtrates were merged, and after removal of the solvents of
EtOH and EA by distillation under ordinary pressure, tetraethoxysilane (5.26 g) was obtained.
The washed cake went through calcination under 300 oC for 2 h to provide MgF2 (3.78 g).
During the process, water must be avoided.
Tetraethoxysilane: yield 82%; light yellow oil; boiling point (800 mbar): 155-157 oC; boiling
point (1 atm): 168 oC;17 Density (20 oC) 0.94; 1H NMR (500 MHz, CDCl3): 3.86 (q, J = 7.2 Hz,
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8 H), 1.24 (t, J = 7.2 Hz, 12 H); 13C NMR (500 MHz, CDCl3): 59.0, 17.9; EI-MS (m/z): 152
(M+), 208, 193, 179, 163, 149, 135, 119, 107, 91, 79, 63, 45, 31.
Magnesium fluoride from EtOH: yield >98%; white solid; MgF2: 98%; Ca: not detected;
SiO2:0.15%; Fe2O3: 0.015%; H2O: 0.009%.
ACKNOWLEDGMENTS
The project is generously supported by Research Institutes Foundation of Yunnan
Provincial Science and Technology Department (Grant No. 2012CF028) and the Science and
Technology Innovation Project of Yuntianhua Group Corporation Limited (Grant No.
2013YTH04).
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REFERENCES
[1] Li, Z. X.; Ming D. Z.; Zong Y. Phosphate. Compd. Fertilizer, 2008, 23, 64-66.
[2] Ennan, A. A.; Kats, B. M.; Skorichenko, A. V. Zh. Neorg. Khim. 1971, 44, 1429-1431.
[3] Hannover, K. S.; Hannover-Kirchrode, H. W. S. U.S. Patent 2 814 635, 1959.
[4] Kim, L. G.; Park, H. G.; Han, J. J.; Han, J. M.; Seo, D. S. IEEE Electr. Device L. 2013, 34,
283-285.
[5] Wojciechowska, M.; Zielinsky, M.; Piotrowski, M. J. Fluor. Chem. 2003, 120, 1-11.
[6] Kemnitz, E.; Wuttke, S.; Coman, S. M. Eur. J. Inorg. Chem. 2011, 4773-4794.
[7] Atoji, M.; Lipscomb, W. N. Acta Cryst. 1954, 7, 597.
[8] Khoroshilov, A. V.; Cherednichenko, S. A.; Lizunov, A. V.; Fedorova, S. N. Theor. Found.
Chem. Eng. 2010, 44, 569-573.
[9] Nikitin, V. I.; Ennan, A. A. Zh. Neorg. Khim. 1980, 25, 3391-3394.
[10] Shimazu, M.; Suzaki, Y. Japan J. Appl. Phys. 1965, 4, 819.
[11] Ostapchuk, L. V.; Gavrilova, L. A.; Ennan, A. A. Zh. Neorg. Khim. 1982, 27, 344-347.
[12] Guertin, J. P.; Onyszchuk, M. Can. J. Chem. 1963, 41, 1477-1484.
[13] Gel’mbol’dt, V. O.; Koroeva, L. V.; Ennan, A. A. Russ. J. Coord. Chem. 2007, 33, 160-167.
[14] Xie, R. Z.; Mao, W. J. Chem. World 1958, 512-513.
[15] Wlodzimierz, R. Roczniki Chemii 1955, 29, 284-286.
[16] Ballinger, P.; Long, F. A. J. Am. Chem. Soc. 1960, 82, 795-798.
[17] The boiling points of tetramethoxysilane and tetraethoxysilane are from the Material Safety
Data Sheet of Sigma-Aldrich: http://www.sigmaaldrich.com/china-mainland.html.
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I2
SiF4 + ROH + Mg
R = Me; Et
MgF2 + Si(OR)4 + H2
Scheme 1 The reaction of SiF4, ROH and Mg.
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Figure 1. The powder XRD of product MgF2
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Table 1 The reaction of SiF4, EtOH and Mg a
Entry Temperature Time Addition Addition Yield of
Yield of
(oC)
(h)
24
24
3
of I2
No
of NH3
Yes
Yes
Yes
Yes
Yes
No
Si(OEt)4(%) MgF2(%)
1
r.t.
38
61
63
57
63
78
58
64
65
74
80
84
89
2
40
No
45
3
r.t.
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
35
4
r.t.
5
47
5
r.t.
24
3
70
6
r.t
A gel form
7
40
5
Yes
Yes
Yes
Yes
Yes
Yes
54
58
65
71
75
78
8 b
9 b
10 b
11 b
12 b
r.t. to 40
r.t. to 60
r.t. to reflux
r.t. to reflux
r.t. to reflux
5
5
5
8
10
a All reactions were performed in a flask of 50 mL. Unless otherwise stated, the reaction was performed as follows.
Before materials were charged, the flask was filled with dried N2, making sure that during the reaction process,
water in the atmosphere was avoided. The flask was charged with SiF4-EtOH solution (17.0 g, 22%), Mg powder
(1.5 g) and a minute quantity of I2 (0.39% of magnesium mass). Then, the mixture was stirred and heated to a certain
temperature. After the stated reaction time, a quantity of NH3-EtOH solution (10%) was added until the mixture was
neutral. Subsequently, the mixture was filtered, and the cake was washed with EA (3×15 mL). The two filtrates were
merged, and after removal of the solvents of EtOH and EA under vacuum by distillation method, tetraethoxysilane
was obtained. The washed cake went through calcination under 300 oC for 2 h to provide MgF2.
b The mixture was stirred at room temperature for 3 h, and then at the set temperature.
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Table 2 The reaction of SiF4, MeOH and Mg a
Entry Temperature Time Addition Yield of
Yield
Si(OMe)4 of
(oC)
(h)
of I2
(%)
MgF2
(%)
85
1
r.t.
24
2
No
53
82
81
78
2
r.t.
Yes
Yes
Yes
93
3 b
4 b
r.t. to 40
r.t. to reflux
2
93
2
90
a All reactions were performed in a flask of 50 mL. Unless otherwise stated, the reaction was
performed as follows. Before materials were charged, the flask was filled with dried N2, making
sure that during the reaction process, water in the atmosphere was avoided. The flask was
charged with SiF4-MeOH solution (14.8 g, 24.1%), Mg powder (1.5 g) and a minute quantity of
I2 (0.39% of magnesium mass). Then, the mixture was stirred and heated to a certain
temperature. After the stated reaction time, a quantity of NH3-MeOH solution (6%) was added
until the mixture was neutral. Subsequently, the mixture was filtered, and the cake was washed
with EA (3×15 mL). The two filtrates were merged, and after removal of the solvents of MeOH
and EA by distillation method, tetramethoxysilane was obtained. The washed cake went through
calcination under 300 oC for 2 h to provide MgF2..
b The mixture was stirred at room temperature for 0.5 h, and thenat the set temperature
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Table 3 Redefinition of reaction process of SiF4, ROH and Mg
Entry ROH Addition Addition Yield of Yield
Si(OR)4(%) of
of I2
of NH3
MgF2
(%)
1 a
2 a
3
Me
Et
Yes
Yes
Yes
Yes
No
No
No
No
85
>98
>98
82
Pr
Not obtained
Not obtained
4
Bu
a About the procedure, please see the part of Experimental.
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