Abstract:
The present invention relates, in general, to the purification of boron trichloride (BCl 3 ). More particularly, the invention relates to a process for minimizing silicon tetrachloride (SiCl 4 ) formation in BCl 3  production and/or the removal of SiCl 4  in BCl 3  product stream by preventing/minimizing the silicon source in the reaction chambers. In addition, a hydride material may be used to convert any SiCl 4  present to SiH 4  which is easier to remove. Lastly freeze separation would replace fractional distillation to remove SiCl 4  from BCl 3  that has been partially purified to remove light boilers.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
     This application claims benefit of priority to U.S. Provisional Application No. 61/954,599, filed Mar. 18, 2014, the disclosure of which is fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved column reactor device and processes for the purification of boron trichloride (BCl 3 ). More particularly, the present invention relates to a device that minimizes silicon tetrachloride (SiCl 4 ) formation during BCl 3  production, and discloses processes for the removal of SiCl 4  from the BCl 3  product stream which may have been formed during the synthesis of BCl 3 . 
     2. Description of the State of the Art 
     Boron trichloride (BCl 3 ) is a highly reactive compound packaged as a liquid under its own vapor pressure that has numerous diverse applications. It is used predominantly as a source of boron in a variety of manufacturing processes. For example, in the manufacturing of structural materials, BCl 3  is the precursor for chemical vapor deposition (“CVD”) of boron filaments used to reinforce high performance composite materials. BCl 3  is also used as a CVD precursor in the boron doping of optical fibers, scratch resistant coatings, and semiconductors. Some of the non-CVD applications of BCl 3  are reactive ion etching of semiconductor integrated circuits and refining of metal alloys. In metallurgical applications, it is used to remove oxides, carbides, and nitrides from molten metals. In particular, BCl 3  is used to refine aluminum and its alloys to improve tensile strength. 
     There are known a number of processes for the production of BCl 3  for example, by chlorination of a borate ester, e.g., trimethyl borate, in a sealed tube. See, for example, U.S. Pat. No. 2,943,916. However, the most common technical process for the preparation of BCl 3  is the reaction of a boron compound, such as boron carbide (B 4 C) with chlorine. In this process BCl 3  can be prepared by passing chlorine over mixtures of boron carbide and optionally carbon, packed within a quartz column, which is heated to elevated temperatures of at least 800° C. to 1,200° C. Once the reaction is established, the reaction zone propagates slowly down the column generating BCl 3  at the reaction zone. The chlorination reaction results in the formation of BCl 3  having impurities such as unreacted chlorine (Cl 2 ), hydrogen chloride (HCl), and phosgene (COCl 2 ) which are generally removed from the raw BCl 3  stream through distillation and/or other purification methods. However, trace amounts of silicon tetrachloride (SiCl 4 ) are also produced and are much more difficult to remove from the product stream by the above described means due to its low volatility. The crude product, i.e., BCl 3  containing the SiCl 4  byproduct, is useful for some purposes; but, for many uses, SiCl 4  is an undesirable impurity, e.g., when BCl 3  is used as a precursor for high purity boron nitride. Therefore, its minimization during synthesis in the packed column reactor and its removal from the resulting BCl 3  product stream is highly desirable. 
     Boron trioxide (B 2 O 3 ) typically exists in boron carbide as an impurity with content varying from 1% (wt) to 5% (wt). Boron trioxide has a melting point temperature of about 450° C. or about 510° C. depending on its crystal structure. Hence, under the reaction condition as mentioned above, the impurity B 2 O 3  in B 4 C melts and forms a liquid in the B 4 C chlorination process. The liquidized B 2 O 3  in the process stream eventually forms deposits as the process temperature is below its melting point. The deposits may block the process stream flow as they are continuously accumulated after multiple reaction cycles. Typically, an activated carbon (such as charcoal) bed is loaded at the bottom of the reactor to adsorb liquidized B 2 O 3 . In the major section of the reactor, once the reaction is triggered, through induction heating, a porous carbon frame (graphite) is formed after boron is chlorinated and depleted from B 4 C. The presence of carbon (the carbon in the activated carbon bed and the carbon formed during the chlorination process) has a detrimental impact on BCl 3  purity, i.e., carbon can enhance the chlorination of quartz (SiO 2 ) at the B 4 C chlorination temperature of least 800° C. to 1,200° C. resulting in the formation of SiCl 4 , a highly undesirable impurity in BCl 3 , according to the reaction below:
 
SiO 2 +C+2Cl 2 =SiCl 4 +CO 2 ;ΔH° (1223 K)=−141.7 kJ/mol
 
     Glow Discharge Mass Spectrometry (GDMS) analysis indicates that silicon also exists in boron carbide (0.38% (wt) in one batch of boron carbide sampled). Hence, the SiCl 4  in the BCl 3  stream may also be attributed to the silicon impurity in B 4 C (the source material of BCl 3 ). 
     Therefore, there is a need to have a reactor for synthesizing BCl 3  in the absence of a silicon source and/or a process for the removal of any SiCl 4  impurities that may form during the synthesis processes. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, a process for the production of boron trichloride (BCl 3 ) by the reaction of boron carbide (B 4 C) with chlorine at a temperature of 800° C. to 1200° C. is disclosed herein using a reactor that either eliminates the silicon source resulting from the reactor by forming the reactor from a non-quartz material or applying a protective barrier to the quartz surface which is inert to chlorine attack at reaction conditions to prevent/minimize silicon tetrachloride (SiCl 4 ) formation. 
     Alternatively, in the case that quartz has to be used, employing an appropriate reactive material or adsorbent material to remove SiCl 4  and other silicon chlorides from the BCl 3  stream; 
     Alternatively, if the silicon contribution from the quartz reactor is minimized, and the formation of SiCl 4  is via the silicon impurity in B 4 C, similar to the above method, an appropriate reactive material or adsorbent material can be used to remove SiCl 4  from the BCl 3  product stream. In this instance, the present invention teaches the use of a hydride reducing agent to convert SiCl 4  to SiH 4 , which is easier to separate from BCl 3  than SiCl 4 . The hydride can readily be treated for disposal in the gas phase through controlled oxidation (e.g. exposure of low concentrations to air or burning in the presence of a fuel source), scrubbing with a liquid phase oxidizing medium (e.g. aqueous KMNO 4  or NaOCl), scrubbing with a solid phase medium (e.g. Cu(OH) 2 ), or other acceptable method. 
     In yet another embodiment, SiCl 4  may be further removed from a BCl 3  stream by freeze purification. Atmospheric pressure boiling points for BCl 3  and SiCl 4  are about 12.6° C. and 57.65° C., respectively. Freezing points are about −107.3° C. and −68.74° C. This suggests that solid SiCl 4  may be removed by condensing and cooling the BCl 3  product. 
     Additional embodiments and features are set forth in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed embodiments. The features and advantages of the disclosed embodiments may be realized and attained by means of the instrumentalities, combinations, and methods described in the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the description serve to explain the principles of the invention. 
       In the Drawings 
         FIG. 1  is a perspective view of the column reactor device of the present invention, with portions cut away to reveal the internal structure as assembled. 
         FIG. 2A  is an enlarged, detailed view of the protective coating of the column reactor device of  FIG. 1  indicated by dashed lines in  FIG. 1 . 
         FIG. 2B  is an enlarged, detailed view of the present invention similar to the view shown in  FIG. 2A , but having multiple protective coatings overlying one another. 
         FIG. 2C  is an enlarged, detailed view of the protective coating of the present invention similar to the view shown in  FIG. 2A  having an alternative protective barrier in contact with the surface of the protective coating. 
         FIG. 2D  is an enlarged, detailed view of the present invention as shown in  FIG. 2C  illustrating an alternative embodiment wherein the protective coating is absent and the protective barrier is in direct contact with the interior column reactor sidewalls and the reactive media. 
         FIG. 3  is a side, cross-sectional view of the filter of  FIG. 2D , with the addition of a filter bed. 
         FIG. 4  is a top plan view of the column reactor device shown in the position illustrated in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has now been discovered that the presence of the silicon tetrachloride (SiCl 4 ) impurity found in boron trichloride (BCl 3 ) can be minimized during the synthesis of BCl 3  and/or removed from the BCl 3  product stream thus producing a purified BCl 3 . 
     In accordance with the present invention, the silicon source attributable to the reactor is either eliminated by constructing a reactor column from a non-quartz material which is inert to chlorine attack at reaction conditions or minimized by inserting an protective coating or barrier between the interior sidewall of the column reactor and the reactive material used for the synthesis of BCl 3 , such as but not limited to boron carbide. This protective coating or barrier thereby minimizes the formation of impurities (such as SiCl 4 ) that are generated by the reaction of the interior quartz reactor walls with reactive chemical species within the reactor. It should be understood and appreciated that the embodiments and/or features of the present invention disclosed herein may be freely combined with one another. 
     In one embodiment, the reactor  10  of the present invention, as shown in  FIG. 1 , makes use of an inert non-reactive material  14  that shows resistance to chlorine attack, such as but not limited to graphite, graphene, or silicon oxynitride, or refractory ceramic materials to form a dense nonporous thin protective coating or layer on the inner surface  12  of quartz column  16 . The ceramics useful in this invention include but are not limited to silicon carbide, zirconium carbide, zirconium nitride, silicon nitride, or boron nitride. Such a protective coating  14  must have close thermal properties to quartz to avoid/minimize lamination and/or stress/tension due to thermal expansion at elevated temperatures. 
     The protective coating  14 , best seen in  FIG. 2A , is juxtaposed between the interior surface  12  of the interior reactor  10  wall and the reactive material R, such as, but not limited to a boron compound such as boron carbide (B 4 C). Typically, protective coating  14 , such as a graphene coating is formed by a chemical vapor deposition (CVD) process using methane or ethanol as a precursor at temperatures near 1000° C. on the interior surface  12  of quartz column  16  having an interior surface and an exterior surface. Alternatively, the protective coating  14  may be a silicon oxynitride layer formed on the interior surface  12  of the quartz material by rapidly flowing ammonia gas at 1200° C. A dense refractory ceramic coating typically is formed by a CVD process with appropriate precursors. For instance, a thin layer of boron nitride can be deposited on a quartz column surface by the chemical reaction between boron trichloride (or other boron compounds and ammonia. Alternatively, one or more additional coating layers (not shown) may be formed over the protective coating  14  on the interior surface  12  of the quartz column  16 . 
     If reactor  10  includes one or more different coatings  17  overlying and/or underneath the surface  15  of protective coating  14 ′ deposited on interior surface  12  of the quartz column  16 , as shown in  FIG. 2B , the various coatings may be formed as adjacent layers overlying one another sequentially, or one or more of the coatings may penetrate into or even through one or more of the other coatings. Accordingly, the various coatings may be fairly described as being formed generally “on” or “over” the column, regardless of how or to what extent any given coating contacts any of the other coatings and/or the column itself. Similarly, when a material is described as being applied generally to the column, the material may be applied directly to the quartz column, or the material may be applied to the quartz column over one or more coatings already present on the quartz column. 
     In another embodiment, shown in  FIG. 2C , the surface temperature of quartz column  16  can be reduced by packing a concentric ring of larger diameter, and/or less porous, and/or lower reactive particles, such as, but not limited to boron nitride, pure boron, etc. into quartz column  16  to form a barrier  18  between the protective coating  14  of quartz column  16  and reactive material R, such as boron carbide (B 4 C). Alternatively, barrier  18 ′ can be formed in direct contact with the interior surface  12  of column  16 , as shown in  FIG. 2D . This concentric ring of material, as best shown in  FIG. 4 , may be formed in juxtaposition with the interior sidewall surface  12  of quartz column  16  or it may have a graphite, graphene, or silicon oxynitride, or refractory ceramic materials interposed between it and the quartz surface, as shown in  FIG. 2C . In another embodiment, not shown, a barrier may be formed within column  16  using a non-reactive tube, as opposed to loose particles which are packed into column  16 . In this embodiment the exterior diameter of the tubular barrier would be slightly less than the inner diameter of column  16  so that when slid into place, the exterior surface of the tubular barrier would be in contact with the interior surface of column  16 . In the event the exterior diameter of the tubular barrier is significantly less than the interior diameter of column  16  then the concentric annular space or gap that is formed can further be pack with a ring of larger diameter, and/or less porous, and/or lower reactive particles such as those that were used to describe barrier  18  above. 
     By using an embodiment of reactor  10  as disclosed herein, BCl 3  can be prepared by introducing chlorine gas C through gas inlet  20  thus passing over boron carbide and optionally carbon, packed within quartz column  16 , which is heated, using inductive heating H ( FIG. 1 ), to elevated temperatures of at least 800° C. to 1,200° C. Once the reaction is established, the reaction zone propagates slowly down the column generating BCl 3  at the reaction zone. The chlorination reaction results in the formation of BCl 3  and the presence of the silicon tetrachloride (SiCl 4 ) impurity typically found in BCl 3  is minimized during the synthesis of BCl 3  as a result of the protective barrier that is established between the reactive materials and the quartz substrate. As will be disclosed in further detail below due to the presence of a silicon source in the reactive material R, any SiCl 4  that is eventually formed may be removed using the process described herein thereby producing a purified BCl 3  which exits reactor  10  by way of gas outlet  22 . 
     As discussed previously, regardless of the steps taken to eliminate the silicon source that results from the reactor, SiCl 4  impurities may still form due to the presence of a silicon source in the B 4 C. Therefore, the present invention further contemplates processes for purifying the BCl 3  product to remove any SiCl 4  impurity formed regardless of whether the interior sidewalls of the column are protected by a coating or a barrier as discussed above. The following embodiment as shown in  FIG. 3  contemplates having a protective coating or barrier; however, one skilled in the art will also recognize that the purification bed, as disclosed herein, could also be used in a standard quartz column (not shown). As shown in  FIG. 3  and in accordance with the present invention a thin pure boron zone  120  or purification bed is formed at the bottom of reactor  110  and maintained within quartz column  116  using a ceramic frit or alternatively boron is placed in a heated separate bed (not shown) to react with any SiCl 4  to form BCl 3  and solid Si, so that SiCl 4  impurity is removed from the BCl 3  stream. Alternatively, proper molecular sieve materials having good affinities for SiCl 4  and appropriate pore size to let SiCl 4  molecules diffuse into the pores and be adsorbed on the internal surfaces of the adsorbent materials but exclude BCl 3  molecule to enter the pores (the kinetic diameter of SiCl 4  and BCl 3  is 5.81 Å and 6.00 Å, respectively) can be utilized. 
     Purification bed  120  may also be formed using a reactive elemental material, mixed material or other compound to react with SiCl 4  to form, e.g. M x Cl y , wherein x=1-4 and y=1-8 and elemental silicon such that the SiCl 4  present as an impurity in BCl 3  is substantially removed. The reactive material is at least partially consumed and acts as a SiCl 4  getter. The byproduct or byproducts of reaction may need to be separated from BCl 3 , but this should be more convenient than separation of SiCl 4 . Preferred materials are elemental titanium (e.g. Ti sponge), 90% NaCl/10% elemental boron, elemental zinc (e.g. molten), and alumina (Al 2 O 3 ). 
     A hydride reducing agent may further be used to convert SiCl 4  to SiH 4 , which is easier to separate from BCl 3  than SiCl 4 . The hydride can readily be treated for disposal in the gas phase through controlled oxidation (e.g. exposure of low concentrations to air or burning in the presence of a fuel source), scrubbing with a liquid phase oxidizing medium (e.g. aqueous KMNO 4  or NaOCl), scrubbing with a solid phase medium (e.g. Cu(OH) 2 ), or other acceptable method. Without wishing to be bound by theory, general reaction schemes may include, for example:
 
SiCl 4 +4MH→SiH 4 +4MCl
 
SiCl 4 +2MH 2 →SiH 4 +2MCl 2  
 
SiCl 4 +MM′H 4 →SiH 4 +MCl+M′Cl 3  
 
SiCl 4 +4MR 2 H→SiH 4 +4MR 2 Cl
 
SiCl 4 +4MM′R 3 H→SiH 4 +4MCl+4M′R 3  
 
Where M comprises an alkaline earth metal, alkali metal or other main group metal or metalloid, and R comprises a hydrocarbyl group.
 
     The hydride reducing agent may include, but is not limited to, one or more of the following: LiH, NaH, KH, CaH 2 , LiAlH 4 , NaBH 4 , diisobutylaluminum hydride (DIBAL), and lithium triethylborohydride (LiB(Et) 3 H). Other hydride reducing agents not included in this list may also be effective. Ideally, the reducing agent will have a high selectivity for SiCl 4  over BCl 3  and will yield a byproduct or byproducts that do not have a significant negative impact on subsequent processing. 
     The best choice from the standpoint of reactivity and byproduct formation may be NaBH 4 , as this is less reactive than the alkaline and alkali earth hydrides and would generate NaCl and BCl 3 . DIBAL may also be a good choice, as the byproduct, diisobutylaluminum chloride, is a very high boiling liquid that would not generate solids in the process. 
     SiCl 4  may be further removed from a BCl 3  stream by freeze purification. Atmospheric pressure boiling points for BCl 3  and SiCl 4  are about 12.6° C. and 57.65° C., respectively. Freezing points are about −107.3° C. and −68.74° C. This suggests that solid SiCl 4  may be removed by condensing and cooling the BCl 3  product. 
     Without further elaboration it is believed that one skilled in the art can, using the description set forth above, utilize the invention to its fullest extent. 
     Having disclosed several embodiments, it will be recognized by those of skill in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosed embodiments. Additionally, a number of well known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention. 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included. 
     As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the dielectric material” includes reference to one or more dielectric materials and equivalents thereof known to those skilled in the art, and so forth. 
     Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.