An investigation on the synthesis of borazine
Graphical abstract
The reaction of sodium borohydride and ammonium sulfate for synthesis of borazine has been studied. The reaction proceeds in different way depending on temperature.
Research highlights
► The reaction of NaBH4 and (NH4)2SO4 produces mainly ammonia borane below 60 °C. ► The yields of borazine reach the maximum around 110 °C. ► Further increase of temperature leads to the polymerization of borazine.
Introduction
Boron nitride has a lot of important applications as high temperature structural and functional materials, due to its unique properties such as high thermal stability, excellent chemical, thermal-shock, corrosion and oxidation resistances, low dielectric constant and loss, high electrical resistivity and thermal conductivity [1]. However, it is not feasible to be prepared in forms of fibers, coatings, and complicatedly shaped bodies by conventional high temperature sintering process. Precursor derived ceramic routes provide an approach to this issue [2], [3], [4]. Borazine (B3H3N3H3) is reported to be a promising precursor for boron nitride [5], [6], [7], [8].
Borazine was originally prepared in 1926 by pyrolysis of the diammoniate of diborane. Subsequently it was observed as product in several other reactions [9]. However, all those reactions involved the use of diborane and gave borazine in small quantity. Later a procedure suitable for laboratory preparation of borazine was reported, which involved the initial preparation of trichloroborazine (TCB) and its subsequent reduction by metal borohydrides [10]. Yet, this approach has some drawbacks such as difficult purification and handling of diborane. In 1979 a method for synthesizing borazine by thermal decomposition of ammonia borane in glymes was described [11]. However, ammonia borane is expensive and can be purchased only in small quantity. Thomas Wideman et al. reported a procedure for the laboratory preparation of borazine by the reaction of (NH4)2SO4 and (NaBH4) in tetraglyme at 120–140 °C in 1995 [12]. This approach was a one-step procedure, starting from inexpensive materials and using standard laboratory equipments. However, they didn’t study the effects of conditions on the reaction.
Investigation on the reaction of (NH4)2SO4 and (NaBH4) will help understanding of the formation of borazine, and hence improve yields of borazine. However, there are no reported papers to date about study on the reaction of (NH4)2SO4 and (NaBH4) for synthesis of borazine. Herein, it makes sense to study the reaction under various temperatures.
Section snippets
Raw materials
Sodium borohydride, ammonium sulfate and triglyme, all of which were reagent grade, were purchased from commercial resource. Sodium borohydride was used as received. Ammonium sulfate was dried under vacuum at 120 °C for 12 h and grinded to pass 200 mesh sieve before use. Triglyme was vacuum distilled from molten sodium shortly before use.
Preparation of borazine
All manipulations were carried out under an argon atmosphere using standard Schlenk techniques. It is noteworthy that a significant amount of hydrogen was
Reactions at temperatures below 60 °C
The yields of borazine at various temperature were summarized in Table 1, which showed that the temperature has a significant influence on the yields of borazine. No borazine was collected at temperatures below 60 °C. Fig. 1 shows the 11B NMR spectra of the reaction mixture at 40 °C and 60 °C, respectively after completion of the reaction. Both of the spectra show a strong signal (δ = −25.2 ppm, quartet, JBH = 94 Hz) of ammonia borane [13]. There appears no signal of borazine in the spectrum of the
Conclusions
In summary, the reaction of NaBH4 and (NH4)2SO4 in triglyme developed very differently depending on the temperature. The reaction produced mainly ammonia borane (AB), but not borazine at temperatures below 60 °C. Yields of borazine increased with the increase of temperature, and reached the maximum (34%) around 110 °C. However, further increase of temperature (up to 120 °C) caused severe polymerization of borazine, and hence decreased the yield of borazine. Hydrogen, ammonia borane and
Acknowledgment
We are grateful for the support by Chinese National Natural Science Foundation (Nos. 50902150 and 90916019).
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