Process of preparing nitrogen trifluoride by gas-solid reaction

NF.sub.3 is prepared with good yields by reaction between fluorine gas and an ammonium complex of a metal fluoride, such as (NH.sub.4).sub.3 AlF.sub.6, in solid phase. The metal flouride ammonium complex may be one additionally containing an alkali metal, such as (NH.sub.4).sub.2 NaAlF.sub.6. The gas-solid reaction is carried out preferably at temperatures above 80.degree. C. and at relatively low partial pressures of fluorine in the gas phase of the reaction system, so that the reaction is easy to control.

BACKGROUND OF THE INVENTION 
This invention relates to a novel process of preparing nitrogen trifluoride 
by reaction between fluorine gas and an ammonium complex of a metal 
fluoride. 
Nitrogen trifluoride NF.sub.3 is a colorless gas under normal conditions 
and has a boiling point of about -129.degree. C. and a melting point of 
about -208.degree. C. This compound is useful as a fluorine source 
material in the preparation of fluoroolefins and also as an oxidizer for a 
high-energy fuel. 
Nitrogen trifluoride is prepared usually by direct fluorination of ammonia 
in vapor phase or by electrolysis of ammonium hydrogenfluoride. Vapor 
phase reaction between hydrogen azide and oxygen difluoride is also known. 
U.S. Pat. No. 3,304,248 proposes to carry out reaction between nitrogen 
and fluorine by forcing nitrogen gas heated to a temperature above 
100.degree. C. to pass through a plasma arc and simultaneously introducing 
fluorine gas into a post-arc region very close to the anode. The reactions 
in these methods are vapor phase reactions which are relatively violent 
and not easy to control. Furthermore, in the popular methods it is 
necessary to take troublesome measures for the prevention of formation of 
a flammable or explosive gas atmosphere containing hydrogen. 
Japanese Patent Application Publication No. 55-8926(1980) proposes to 
prepare nitrogen trifluoride by reaction between ammonium hydrogenfluoride 
in molten state with fluorine gas. However, this method does not seem 
industrially favorable firstly because the gas-liquid reaction in this 
method is not so easy to control and causes significant corrosion of the 
apparatus and also because the yield of nitrogen trifloride is relatively 
low. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a novel process of 
preparing nitrogen trifluoride easily, safely and economically. 
In a process according to the invention, nitrogen trifluoride is formed by 
reaction between an ammonium complex of a metal fluoride in solid phase 
and fluorine gas. 
More specifically, the ammonium complex used in this invention is either an 
ammonium fluoride of a metal expressed by the general formula 
(NH.sub.4).sub.x MF.sub.y, where x is an integer from 1 to 3, y is an 
integer from 5 to 7, and M represents Fe, Al, Ti, V, Cr, Mn, Ni, Co, Cu, 
Zr, Nb, W, Si, Ge, Sb, Sn or Pb, or an alkali ammonium fluoride of such a 
metal M expressed by the general formula (NH.sub.4).sub.x MM'F.sub.y where 
M' represents Li, Na or K. An example of the former complex is 
(NH.sub.4).sub.3 FeF.sub.6, and an example of the alkali metal containing 
complex is (NH.sub.4).sub.2 NaAlF.sub.6. 
These metal fluoride ammonium complexes are usually in powdery form and 
readily react with fluorine gas to form nitrogen trifluoride together with 
hydrogen fluoride and metal fluoride, as represented by the following 
equation. 
EQU (NH.sub.4).sub.3 MF.sub.6 (s)+9F.sub.2 (g).fwdarw.3NF.sub.3 
(g)+12HF(g)+MF.sub.3 (s or g) (1) 
This gas-solid reaction takes place at temperatures over a wide range, so 
that the reaction in our process may be carried out at room temperature or 
at moderately elevated temperatures which are below the thermal 
decomposition temperature of the employed metal fluoride ammonium complex. 
For example, thermal decomposition of (NH.sub.4).sub.3 AlF.sub.6 proceeds 
roughly in two stages as represented by the following equations. 
##STR1## 
Accordingly, the reaction of this complex with fluorine is considered to 
proceed also in two stages. That is, the first stage of the reaction is to 
the extent of formation of NH.sub.4 AlF.sub.4. 
EQU (NH.sub.4).sub.3 AlF.sub.6 (s)+6F.sub.2 (g).fwdarw.2NF.sub.3 
(g)+8HF(g)+NH.sub.4 AlF.sub.4 (g) (4) 
The reaction at this stage starts at temperatures below about 100.degree. 
C., and the reaction temperature rises as the reaction proceeds to the 
extent of about 150.degree. C. 
At the second stage, ammonium aluminumfluoride reacts with fluorine to turn 
into aluminum fluoride, as represented by the following equation (5). The 
reaction at this stage starts at about 150.degree. C., and the reaction 
temperature rises as the reaction proceeds to the extent of about 
250.degree. C. 
EQU NH.sub.4 AlF.sub.4 (s)+3F.sub.2 (g).fwdarw.NF.sub.3 (g)+4HF(g)+AlF.sub.3 
(s) (5) 
The reactions of equations (4) and (5) are both useful in the present 
invention. 
A metal fluoride ammonium complex used as the starting meterial in the 
present invention is a solid and usually powdery material which is very 
convenient for industrial handling compared with the gaseous or liquid 
materials used in the known processes such as NH.sub.3 gas, N.sub.2 gas, 
NH.sub.4 F gas and NH.sub.4 HF.sub.2 melt. Furthermore, the metal fuloride 
ammonium complexes are available at relatively low prices. 
The gas-solid reaction according to the invention is a mild reaction 
compared with the conventional vapor phase or gas-liquid reactions. This 
gas-solid reaction smoothly proceeds even when the concentration of 
F.sub.2 in the gas phase of the reaction system is very low. Therefore, 
the reaction can be controlled very easily and can be accomplished very 
safely. As a further advantage of this invention, nitrogen trifluoride is 
obtained with high yields. 
The process of the invention gives a relatively large amount of hydrogen 
fluoride as a by-product. As is well known, hydrogen fluoride is an 
industrially valuable material and can be used for the preparation of a 
metal fluoride ammonium complex as the starting material in the present 
invention and also for the preparation of fluorine gas by electrolysis. A 
metal fluoride (which may be an intermediate complex comprising ammonium 
group) is obtained as an additional by-product. This material is useful 
for catalysts and also for the preparation of the metal fluoride ammonium 
complex employed as the starting material. Accordingly the process 
according to the invention is very favorable from an industrial or 
economical point of view. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As to the starting material in our process, (NH.sub.4).sub.3 FeF.sub.6, 
(NH.sub.4).sub.3 AlF.sub.6, (NH.sub.4).sub.2 TiF.sub.6, (NH.sub.4).sub.3 
VF.sub.6, (NH.sub.4).sub.3 CrF.sub.6, (NH.sub.4).sub.2 MnF.sub.5, 
(NH.sub.4).sub.2 NiF.sub.6, (NH.sub.4).sub.2 CoF.sub.6, (NH.sub.4).sub.3 
CuF.sub.6, (NH.sub.4).sub.2 ZrF.sub.6, NH.sub.4 NbF.sub.6, NH.sub.4 
WF.sub.7, (NH.sub.4).sub.2 SiF.sub.6, (NH.sub.4).sub.2 SnF.sub.6, 
(NH.sub.4).sub.2 PbF.sub.6, (NH.sub.4).sub.2 SbF.sub.5 and 
(NH.sub.4).sub.2 GeF.sub.6 are named as typical examples of metal fluoride 
ammonium complexes expressed by (NH.sub.4).sub.x MF.sub.y. Typical 
examples of the alkali metal (M') containing complexes expressed by 
(NH.sub.4).sub.x MM'F.sub.y are (NH.sub.4).sub.2 NaFeF.sub.6, 
(NH.sub.4).sub.2 NaAlF.sub.6, (NH.sub.4).sub.2 KFeF.sub.6, 
(NH.sub.4).sub.2 KAlF.sub.6, NH.sub.4 NaSiF.sub.6, and NH.sub. 4 
KSiF.sub.6. 
It is possible to carry out the gas-solid reaction according to the 
invention even at room temperature, but in that case it is necessary to 
maintain the concentration of F.sub.2 in the gas phase at a relatively 
high level. To carry out the reaction with F.sub.2 concentration below 10% 
by volume, the reaction temperature must be above about 80.degree. C. It 
is favorable to maintain the F.sub.2 concentration at such a low level 
firstly because the reaction proceeds mildly with little possibility of 
run-away reaction so that the control of the reaction becomes very easy, 
and also because the by-production of nitrogen fluorides other than 
NF.sub.3 is suppressed. Accordingly it is preferred to carry out the 
reaction at temperatures not lower than 80.degree. C. 
As to fluorine gas, it is optional whether to use a practically pure 
F.sub.2 gas or to dilute F.sub.2 gas with an inactive or unreactive gas 
such as Ar, N.sub.2 or air prior to the introduction of the gas into the 
reactor. In the latter case it is also possible to use the reaction gas 
produced by reaction between fluorine gas and the metal fluoride ammonium 
complex as the diluent. In practice, however, there is little need for 
intentional dilution of fluorine gas because fluorine gas introduced into 
the reactor is soon diluted with the gaseous reaction products such as 
NF.sub.3 and HF, so that the fluorine concentration in the gas phase of 
the reaction system lowers to a desirable level, even when pure F.sub.2 
gas is used, so long as the feed rate of F.sub.2 gas is adequate. 
The reaction according to the invention can be carried out in a 
conventional reactor for known solid-gas reactions. The metal fluoride 
ammonium complex employed as the starting material is charged into the 
reactor either batchwise or continuously. Also, the feed of fluorine gas 
and the discharge of the reaction gas may be either continuous or 
intermittent. The metal fluoride ammonium complex is subjected to the 
reaction in a suitably divided form such as powder or granules, but there 
is no strict limitations on the particle or granule size. 
For example, a batch of a powder of the selected metal fluoride ammonium 
complex is charged into a reactor of the compartment tray or plate tower 
type and preliminarily heated to a suitable temperature above 80.degree. 
C. After that fluorine gas is continuously introduced into the reactor at 
a rate suitable for a desirably low concentration of F.sub.2 in the gas 
phase of the reaction system. As mentioned hreinbefore, the temperature of 
the reaction system rises as the reaction proceeds. In principle the rate 
of the reaction according to the invention is very high so that the 
reaction time can be made very short. In practice it is favorable to carry 
out a mild and slow reaction by maintaining the F.sub.2 concentration in 
the gas phase at a sufficiently low level for the reasons described 
hereinbefore. Even under such reaction conditions, the reaction can be 
completed usually in 30-60 min. 
The gaseous product of the rection is a mixture of NF.sub.3, HF and 
possibly some nitrogen fluorides other than NF.sub.3. A large portion of 
HF contained in the reaction gas can be removed by a physical separation 
means such as a cold trap maintained at a temperature below the boiling 
point of HF. After that, almost complete removal of HF can be accomplished 
by treating the reaction gas with NaF. Then the reaction gas is liquefied 
by cooling with liquid air, liquid nitrogen or liquid argon, and the 
remaining impurities such as nitrogen fluorides other than NF.sub.3 are 
sucked out of the liquefied product by using a vacuum pump. If desired, 
the purity of the obtained nitrogen trifluoride can further be enhanced by 
treatment with KOH and/or by a molecular sieve treatment.

The invention will further be illustrated by the following nonlimitative 
examples. 
EXAMPLE 1 
A reactor of the forced circulation compartment tray type (two-stage) was 
used. The reactor was made of nickel and had an inner diameter of 300 mm 
and a length of 700 mm. Initially, 3000 g of (NH.sub.4).sub.3 AlF.sub.6 
powder was charged into the reactor and heated in N.sub.2 gas atmosphere 
up to 110.degree. C. by means of an external heater. After that 
practically pure F.sub.2 gas was continuously introduced into the reactor 
at such a rate that 3508 g (2.068 Nm.sup.3) of F.sub.2 was introduced in 
10 hr. 
The reaction gas discharged from the reactor was passed through a cold trap 
and then treated with NaF for the purpose of almost completely removing HF 
gas. The thus treated reaction gas was liquefied by cooling with liquid 
nitrogen, and the pressure was reduced by operating a vacuum pump to suck 
out unwanted substances other than NF.sub.3. 
The product obtained by the above process was 1660 g (0.524 Nm.sup.3) of 
nitrogen trifluoride which had a purity of 98.5%. At the end of the 10 hr 
reaction, the temperature of the solid material in the reactor was 
150.degree. C. The solid material remained in the reactor weighed 1860 g 
and was confirmed to be NH.sub.4 AlF.sub.4 by X-ray diffraction analysis. 
The yield of nitrogen trifluoride on the basis of fluorine: 
##EQU1## 
EXAMPLE 2 
In the reactor used in Example 1, 1680 g of NH.sub.4 AlF.sub.4 formed by 
the reaction of Example 1 was heated to 160.degree. C. in N.sub.2 gas 
atmosphere. After that fluorine gas was continuously introduced into the 
reactor at such a rate that 1752 g (1.032 Nm.sup.3) of F.sub.2 was 
introduced in 5 hr. The reaction gas was treated in the same manner as in 
Example 1. 
The product of this process was 709 g (0.214 Nm.sup.3) of nitrogen 
trifluoride of 98.1% purity. At the end of the 5 hr reaction, the 
temperature of the solid material in the reactor was 250.degree. C. The 
solid material remained in the reactor weighed 1290 g and was confirmed to 
be AlF.sub.3 by X-ray diffraction analysis. The yield of nitrogen 
trifluoride on the basis of fluorine was about 65%. 
EXAMPLE 3 
In the reactor mentioned in Example 1, 3000 g of (NH.sub.4).sub.2 
NaAlF.sub.6 powder was heated in N.sub.2 gas atmosphere up to 110.degree. 
C. by means of an external heater. After that F.sub.2 gas was continuously 
introduced into the reactor at such a rate that 3400 g (2.016 Nm.sup.3) of 
F.sub.2 was introduced in 10 hr. The reaction gas was treated in the same 
manner as in Example 1. 
The product of this process was 1434 g (0.484 Nm.sup.3) of nitrogen 
trifluoride of 98.8% purity. At the end of the 10 hr reaction, the 
temperature of the solid material in the reactor was 150.degree. C. The 
solid material remained in the reactor weighed 1890 g and was confirmed to 
be NaAlF.sub.4 by X-ray diffraction analysis. The yield of nitrogen 
trifluoride on the basis of fluorine was about 72%. 
EXAMPLE 4 
In the reactor mentioned in Example 1, 3000 g of (NH.sub.4).sub.3 FeF.sub.6 
powder was heated in N.sub.2 gas atmosphere up to 110.degree. C. by means 
of an external heater. After that F.sub.2 gas was continuously introduced 
into the reactor at such a rate that 4584 g (2.702 Nm.sup.3) of F.sub.2 
was introduced in 15 hr. The reaction gas was treated in the same manner 
as in Example 1. 
The product of this process was 2141 g (0.675 Nm.sup.3) of nitrogen 
trifluoride of 99.0% purity. At the end of the 15 hr reaction the 
temperature of the solid material in the reactor was 250.degree. C. The 
solid material remained in the reactor weighed 1512 g and was confirmed to 
be FeF.sub.3 by X-ray analysis. The yield of nitrogen trifluoride on the 
basis of fluorine was about 75%. 
EXAMPLE 5 
In the reactor mentioned in Example 1, 3000 g of (NH.sub.4).sub.2 SiF.sub.6 
powder was heated in N.sub.2 gas atmosphere up to 110.degree. C. by means 
of an external heater. After that F.sub.2 gas was continuously introduced 
into the reactor at such a rate that 3843 g (2.265 Nm.sup.3) of F.sub.2 
was introduced in 15 hr. The reaction gas was treated in the sme manner as 
in Example 1. In the reaction gas before the treatment the presence of HF 
and SiF.sub.4 was confirmed. At the last stage of the 15 hr reaction the 
temperature of the reaction system was 160.degree. C. 
The product of this process was 1627 g (0.513 Nm.sup.3) of nitrogen 
trifluoide of 97.8% purity. After the reaction no solid material remained 
in the reactor. The yield of nitrogen trifluoride on the basis of fluorine 
was about 68%.