# Patent

## Title
Method for making catalyst compositions of alkali metal halide-doped bivalent metal fluorides and process for making fluorinated olefins

## Abstract

There is provided methods for making a catalyst composition represented by the formula MX/M′F2 wherein MX is an alkali metal halide; M is an alkali metal ion selected from the group consisting of Li+, Na+, K+, Rb+, and Cs+; X is a halogen ion selected from the group consisting of F−, Cl−, Br−, and I−; M′F2 is a bivalent metal fluoride; and M′ is a bivalent metal ion. There is also a method for making a fluorinated olefin.

## Background
CROSS-REFERENCE TO A RELATED INVENTION
The present application claims priority from U.S. Provisional Patent Application No. 61/012,566, filed Dec. 10, 2007, which is incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for making catalyst compositions of alkali metal halide-doped bivalent metal fluorides. The present invention also relates to a process for making fluorinated olefins with the catalyst compositions.
2. Description of the Related Art
2,3,3,3-tetrafluoropropene (1234yf), a hydrofluoroolefin that exhibits low global warming potential, can be used in a variety of applications, for example, as a refrigerant, a blowing agent, a solvent, a cleaning agent, and a monomer for macromolecular compounds.
One process for making 1234yf entails the dehydrochlorination of 1,1,1,2-tetrafluoror-2-chloropropane (244bb). U.S. Provisional Application 60/958,468, filed Jul. 6, 2007, discloses a process for making 1234yf by dehydrochlorinating 244bb in the presence of catalysts of bivalent metal fluorides doped with alkali metal halides.
There is a need for commercially viable methods for preparing catalysts of bivalent metal fluorides doped with alkali metal halides.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method for making a catalyst composition. The catalyst composition is represented by the formula MX/M′F2. MX is an alkali metal halide. M is an alkali metal ion selected from the group consisting of Li+, Na+, K+, Rb+, and Cs+. X is a halogen ion selected from the group consisting of F− Cl−, Br−, and I−. M′F2 is a bivalent metal fluoride. M′ is a bivalent metal ion. The method has the following steps: (a) dissolving an amount of the alkali metal halide in an amount of solvent sufficient to substantially dissolve or solubilize the alkali metal halide to form an alkali metal halide solution; (b) adding an amount of the bi-valent metal fluoride to the alkali metal halide solution to form a slurry of the alkali metal halide and bi-valent metal fluoride; and (c) removing substantially all of the solvent from the slurry to form a solid mass of the alkali metal halide and bi-valent metal fluoride.
According to the present invention, there is provided a method for making a catalyst composition. The method has the following steps: (a) an amount of hydroxide, oxide, or carbonate of an alkali metal is added to an aqueous solution of a hydrogen halide and reacted to form an aqueous solution of an alkali metal halide; (b) an amount of a hydroxide, oxide, or carbonate of a bivalent metal is added to an aqueous solution of hydrogen fluoride and reacted to form a precipitate of a bivalent metal fluoride therein; (c) admixing the alkali metal halide solution and the precipitate of the bivalent metal fluoride to form an aqueous slurry; and (d) removing water from the aqueous slurry to form a solid mass.
Still further according to the present invention, there is provided process for making a fluorinated olefin. The process has the step of dehydrochlorinating a hydrochlorofluorocarbon having at least one hydrogen and at least one chlorine on adjacent carbons in the presence of a catalytically effective amount of a catalyst composition represented by the formula MX/M′F2. MX is an alkali metal halide. M is an alkali metal ion selected from the group consisting of Li+, Na+, K+, Rb+, and Cs+. X is a halogen ion selected from the group consisting of F−, Cl−, Br−, and I−. M′F2 is a bivalent metal fluoride. M′ is a bivalent metal ion.


DETAILED DESCRIPTION OF THE INVENTION
Catalyst compositions that are useful products of the methods of the present invention are combinations/admixtures of an alkali metal halide(s) and a bivalent metal fluoride(s) that can be represented by the following:

MX/M′F2 

wherein MX is an alkali metal halide; M is an alkali metal ion selected from the group consisting of Li+, Na+, K+, Rb+, and Cs+. X is a halogen ion selected from the group consisting of F, Cl−, Br−, and I−. X is preferably F− and Cl−. M′F2 is a bivalent metal fluoride. M′ is a bivalent metal ion. M′ is preferably selected from the group consisting of Mg2+, Ca2+, Sr2+, Ba2+, Ni2+, Fe2+, Co2+, Cu2+, and Zn2+. M′ is most preferably Mg2+.
The catalyst compositions can alternately be represented by the following:

n % MX/M′F2 

wherein n % is the weight percentage of alkali metal halide in the composition based upon the total weight of the composition. The alkali metal halide is preferably from about 0.05 wt % to about 50 wt %, more preferably about 5 wt % to about 15 wt %, and most preferably about 7.5 wt % to about 12.5 wt % of the catalyst composition based on the total weight of the catalyst composition.
Examples of alkali metal halides include LiCl, NaCl, KCl, RbCl, CsCl, LiF, NaF, KF, RbF, and CsF. Preferred alkali metal halides include KCl, CsCl, KF, and CsF. Examples of bi-valent metal fluorides include MgF2, CaF2, SrF2, BaF2, NiF2, FeF2, CoF2, CuF2, and ZnF2. Preferred bi-valent metal fluorides include MgF2 and NiF2.
The alkali metal halide is added to an amount of solvent sufficient to substantially dissolve or solubilize the alkali metal halide. The preferred solvent is one in which the alkali metal halide is readily soluble. The choice of solvent may vary depending on the particular alkali metal halide(s). Examples of solvents that can be used for the preparation of the catalyst compositions of the present invention include water, alcohols, ethers, and mixtures thereof. Useful alcohols include monohydric and polyhydric alcohols. Most preferred alcohols are those that are monohydric and have 1 to 5 carbon atoms. A most preferred solvent is water.
In one embodiment of the method of the invention, the bi-valent metal fluoride is added to the solution of the alkali metal halide to form a slurry. After formation of the slurry, substantially all of the solvent is removed to form a solid mass of a mixture of the alkali metal halide and bi-valent metal fluoride. Although the solvent can be removed in one step, a preferred method is to drive off a portion of the solvent from the slurry to form a paste and then follow by drying the paste to form the solid mass. Any conventional technique can be used to drive off the solvent. Examples of such techniques include vigorous stirring at room or elevated temperatures, evaporation, settling and decanting, centrifugation, and filtration. It is preferred to evaporate off a desired amount of solvent to form the paste. The paste is then dried by any suitable method to form a free-flowing, substantially solvent-free powder. Preferred methods for drying include oven drying, most preferably at temperatures from about 110° C. to about 120° C., and spray drying. Being solvent free means that less than 1 wt. %, preferably about 0.5 wt % or less, more preferably about 0.1 wt % or less, and most preferably no solvent will remain with the powder after solvent removal/drying. Upon removal of solvent, the powder will take the form of a solid mass (or powder) of a mixture of particles of the alkali metal halide and the bi-valent metal fluoride.
In another embodiment of the method of the present invention, a slurry of the alkali metal halide and the bivalent metal fluoride is prepared by a different, reactive technique. In a first step, a hydroxide, oxide, or carbonate of an alkali metal is added to an aqueous solution of a hydrogen halide and reacted to form an aqueous solution of an alkali metal halide. In a second step, a hydroxide, oxide, or carbonate of a bivalent metal is added to an aqueous solution of hydrogen fluoride and reacted to form a precipitate of a bivalent metal fluoride therein. In a third step, the alkali metal halide solution and the bivalent metal fluoride precipitate are then admixed to form an aqueous slurry. In a fourth step, water is then removed from the aqueous slurry in the manner described herein to form a solid mass.
Optionally, the solid mass of the mixture of the alkali metal halide and the bi-valent metal fluoride powder is then calcined. Calcination is preferably carried out at a temperature of about 100° C. to about 750° C., more preferably at a temperature of about 200° C. to about 600° C., and most preferably at a temperature of about 300° C. to about 600° C. Calcination may further optionally be carried out in the presence of an inert gas, such as nitrogen or argon.
After calcination, the powder is optionally further grinded such that it becomes more finely-divided. The powder is further optionally pelletized in order to form pellets. The pellets then provide catalyst surfaces to use in actual process application.
The catalyst compositions of the present invention may afford performance advantages over compositions that are obtained by simple dry mixing of components. A more complete degree of intermixing may be achieved. The complete degree of mixing may manifest itself in higher selectivity to the target product, such as 1234yf (and less to the formation of a dehydrofluorinating product, such as 1233xf).
The catalyst compositions are useful in converting hydrochlorofluorocarbons to fluorinated olefins. Useful hydrochlorofluorocarbons have at least one hydrogen and at least one chlorine on adjacent carbons.
Table 1 sets forth examples of fluorinated olefins and precursor hydrochlorofluorocarbons from which they can be obtained (precursor hydrochlorofluorocarbon in left column and corresponding product fluorinated olefin in the right column).








TABLE 1





Hydrochlorofluorocarbon
Fluorinated Olefin(s)







chlorotetrafluoropropane
tetrafluoropropene


chloropentafluoropropane
pentafluoropropene


chlorohexafluoropropane
hexafluoropropene


1,1,1,2-tetrafluoro-2-chloropropane
2,3,3,3-tetrafluoropropene


CF3CFClCH3 (244bb)
CF3CF═CH2 (1234yf)


1,1,1,2-tetrafluoro-3-chloropropane
2,3,3,3-tetrafluoropropene


CF3CHFCH2Cl (244eb)
CF3CF═CH2 (1234yf)


1,1,1,3-tetrafluoro-3-chloropropane
1,3,3,3-tetrafluoropropene


CF3CH2CHFCl (244fa)
CF3CH═CHF (trans/cis-



1234ze)


1,1,1,3-tetrafluoro-2-chloropropane
1,3,3,3-tetrafluoropropene


CF3CHClCH2F (244db)
CF3CH═CHF (trans/cis-



1234ze)


1,1,1,2,3-pentafluoro-2-chloropropane
1,2,3,3,3-pentafluoropropene


CF3CFClCH2F (235bb)
CF3CF═CHF (Z/E-1225ye)


1,1,1,2,3-pentafluoro-3-chloropropane
1,2,3,3,3-pentafluoropropene


CF3CHFCHFCl (235ea)
CF3CF═CHF (Z/E-1225ye)


1,1,1,3,3-pentafluoro-3-chloropropane
1,1,3,3,3-pentafluoropropene


CF3CH2CF2Cl (235fa)
CF3CH═CF2 (1225zc)


1,1,1,3,3-pentafluoro-2-chloropropane
1,1,3,3,3-pentafluoropropene


CF3CHClCHF2 (235da)
CF3CH═CF2 (1225zc)


1,1,1,2,3,3-hexafluoro-2-chloropropane
1,1,2,3,3,3-hexafluoropropene


CF3CFClCHF2 (226ba)
CF3CF═CF2 (1216)


1,1,1,2,3,3-hexafluoro-3-chloropropane
1,1,2,3,3,3-hexafluoropropene


CF3CHFCF2Cl (226ea)
CF3CF═CF2 (1216)









The following are examples of the invention and are not to be construed as limiting.
EXAMPLES
In the following examples, 244bb is dehydrochlorinated to 1234yf in the presence of the catalysts of combinations of alkali metal halides and bivalent metal fluorides.
Example 1
244bb Dehydrohalogenation Over CsCl/MgF2 Catalysts Having Various CsCl Loadings
A series of CsCl/MgF2 catalysts with various loadings of CsCl were tested to determine the effect of CsCl loading on reactivity. 20 cc of catalyst pellets was typically used. A mixture of 97.2%/2.0% 244bb/1233xf was passed through catalyst bed at a rate of 6 g/h (grams/hour) at a temperature ranging from 470° C. to 520° C. The temperatures at the bottom and top of the catalyst bed were measured. As shown in Table 2 below, activity remained at about the same level regardless of loading, while the selectivity to 1233xf (a non-desired dehydrofluorination product) decreased as CsCl loading increased to 5.0 wt %. No 1233xf was formed over the 10 wt % CsCl/MgF2 catalyst.







TABLE 2







(Effect of CsCl loading on the performance of CsCl/MgF2


catalysts during 244bb dehydrohalogenation*)














Temp.




Selec-


CsCl
Bottom-

Conversion,
Selectivity
Selectivity 
tivity


loading
Top
time
(%)
(%)
(%)
(%)


(wt %)
(°)
(h)
244bb
1234yf
1233xf
others
















0.0
475-506
1
48.2
76.9
17.7
5.4



475-509
2
52.9
79.8
14.6
5.6



475-509
3
53.3
80.7
12.9
6.4



475-507
4
52.4
81.4
11.9
6.7



475-509
5
54.2
83.0
10.9
6.1



475-510
6
54.1
83.6
10.2
6.2



475-508
7
54.7
84.7
9.6
5.7



475-509
8
53.7
85.4
9.2
5.4



475-510
9
54.9
86.0
8.6
5.5



475-509
10
53.5
86.7
8.2
5.1


2.5
500-514
1
48.4
88.7
5.2
6.1



500-514
2
48.1
88.5
5.2
6.3



500-514
3
49.5
89.1
5.0
5.9



500-507
4
46.9
89.3
4.8
5.9



500-509
5
48.5
89.9
4.6
5.5



500-513
6
48.5
89.6
4.7
5.7



500-514
7
49.6
89.9
4.6
5.5


5.0
490-510
1
49.0
94.8
0.5
4.7



490-511
2
51.0
94.5
0.4
5.1



490-510
3
49.2
95.3
0.5
4.2



490-505
4
48.7
95.0
0.4
4.6



490-507
6
49.8
95.4
0.4
4.2



490-503
8
49.2
95.7
0.4
3.9


10.0
475-511
1
49.6
96.9

3.1



475-510
2
51.2
97.0

3.0



475-511
3
51.8
96.9

3.1



475-508
4
50.4
96.9

3.1



475-510
5
51.4
97.0

3.0





*Reaction conditions: 20 ml of catalyst, 6 grams organic/hour, 97.2% 244bb/2.0% 1233xf, 1 atm pressure






Example 2
244bb Dehydrohalogenation Over 10 wt % Alkali Metal Chloride/MgF2 Catalysts
10 wt % KCl/MgF2 and 10 wt % CsCl/MgF2 catalysts were tested. 20 cc of catalyst pellets was used. A mixture of 99.1%/0.4% 244bb/1233xf was passed through the catalyst bed at a rate of 6 g/h at a temperature ranging from 380° C. to 480° C. The temperature at the bottom and top of the catalyst bed were measured. As shown in Table 2, both catalysts exhibited about the same activity (20%), while the 10 wt % CsCl/MgF2 catalyst provided a higher selectivity to 1234yf without generation of 1233xf over the catalyst.







TABLE 3







(Reactivity of 10 wt % KCl/MgF2 and 10 wt %


CsCl/MgF2 Catalysts during 244bb Dehydrohalogenation*)














Temp.

Conversion






Bottom-Top
time
(%)
Selectivity (%)
Selectivity (%)
Selectivity (%)


Catalyst
(°)
(hour)
244bb
1234yf
1233xf
others
















10 wt % KCl/MgF2
405-477
1
21.9
89.1
0.4
10.5



405-480
2
17.8
95.2
0.6
4.2



405-480
4
20.0
95.8
0.6
3.6



405-480
6
21.2
96.0
0.6
3.7



405-480
8
20.1
96.1
0.6
3.3



405-480
10
21.5
96.2
0.6
3.1



405-480
12
20.9
96.2
0.6
3.2



405-479
14
20.5
96.3
0.6
3.1



405-479
16
20.2
96.2
0.6
3.2



405-479
18
20.1
96.3
0.6
3.1



405-480
20
20.5
96.4
0.6
3.0



405-478
22
20.4
96.4
0.6
3.0



405-478
24
19.9
96.3
0.6
3.1


10 wt % CsCl/MgF2
380-481
1
10.3
91.1
0.0
8.9



380-481
2
14.0
95.9
0.0
4.1



380-482
4
16.8
96.7
0.0
3.3



380-484
6
19.6
97.4
0.0
2.6



380-482
8
20.0
97.5
0.0
2.5



380-481
10
20.5
97.5
0.0
2.5



380-481
12
20.6
97.8
0.0
2.2



380-479
14
19.9
97.7
0.0
2.3



380-478
16
20.0
97.8
0.0
2.2



380-481
18
21.0
97.8
0.0
2.2



380-483
20
21.8
98.0
0.0
2.0



380-481
22
20.7
97.7
0.0
2.3



380-481
24
19.7
97.6
0.0
2.4





*Reaction conditions: 20 ml of catalyst, 6 grams organic/hour, 99.1%/0.4% 244bb/1233xf, 1 atm pressure






Example 3
244bb Dehydrohalogenation Over 10 Wt % CsCl/Bi-Valent Metal Fluoride Catalysts
10 wt % CsCl/NiF2 and 10 wt % CsCl/MgF2 catalysts were tested. 20 cc of catalyst pellets was used. A mixture of 99.1%/0.4% 244bb/1233xf was passed through a catalyst bed at a rate of 6 g/h at a temperature ranging from 380° C. to 480° C. The temperature at the bottom and top of the catalyst bed were measured. As shown in Table 4, both catalysts exhibited about the same selectivity to 1234yf (97% to 98%), while the 10 wt % CsCl/MgF2 catalyst provided higher activity.







TABLE 4







(Reactivity of MgF2 and Alkaline Metal Chloride-Doped


MgF2 Catalysts during 244bb Dehydrohalogenation*)














Temp.

Conversion






Bottom-Top
time
(%)
Selectivity (%)
Selectivity (%)
Selectivity (%)


Catalyst
(°)
(hour)
244bb
1234yf
1233xf
others
















10 wt % CsCl/NiF2
410-482
1
5.6
86.3
0.0
13.7



410-482
2
8.3
90.4
0.0
9.6



410-483
4
9.9
93.6
0.0
6.4



410-480
6
10.1
95.2
0.0
4.8



410-481
8
10.9
95.9
0.0
4.1



410-480
10
12.0
96.2
0.0
3.8



410-481
12
13.2
96.8
0.0
3.2



410-482
14
14.2
97.1
0.0
2.9



410-481
16
14.4
97.3
0.0
2.7



410-481
18
14.7
97.3
0.0
2.7



410-480
20
14.8
97.4
0.0
2.6



410-481
22
15.1
97.8
0.0
2.2



410-480
24
15.4
97.6
0.0
2.4


10 wt % CsCl/MgF2
380-481
1
10.3
91.1
0.0
8.9



380-481
2
14.0
95.9
0.0
4.1



380-482
4
16.8
96.7
0.0
3.3



380-484
6
19.6
97.4
0.0
2.6



380-482
8
20.0
97.5
0.0
2.5



380-481
10
20.5
97.5
0.0
2.5



380-481
12
20.6
97.8
0.0
2.2



380-479
14
19.9
97.7
0.0
2.3



380-478
16
20.0
97.8
0.0
2.2



380-481
18
21.0
97.8
0.0
2.2



380-483
20
21.8
98.0
0.0
2.0



380-481
22
20.7
97.7
0.0
2.3



380-481
24
19.7
97.6
0.0
2.4





*Reaction conditions: 20 ml of catalyst, 6 grams organic/hour, 99.1% 244bb/0.4% 1233xf, 1 atm pressure






Example 4
244bb Dehydrohalogenation Over Alkaline Metal Chloride-Doped MgF2 Catalysts
In Example 4, a series of alkaline metal chlorides were investigated as an additive to MgF2 with a purpose of improving the selectivity to 1234yf. For comparison purpose, the results obtained over MgF2 catalyst were also reported. 20 cc of catalyst pellets was used in a typical run. A mixture of 97.2% 244bb/2.0% 1233xf was passed through catalyst bed at a rate of 6 g/h (grams/hour) at a temperature ranged from 470° C. to 520° C. The temperatures at the bottom of catalyst bed and at the top of catalyst bed were measured.
As shown in Table 5, the MgF2 provided a 244bb conversion of 53-55%, a 1234yf selectivity of 80-87%, and a 1233xf selectivity of 8-15%; the 10% LiCl/MgF2 provided a 244bb conversion below 45%, a 1234yf selectivity of about 90%, and a 1233xf selectivity of about 5%; the 10% KCl/MgF2 provided a 244bb conversion below 50%, a 1234yf selectivity of about 96%, and a 1233xf selectivity of about 1%; and the 10% CsCl/MgF2 provided a 244bb conversion of 50-52%, a 1234yf selectivity of about 97%, and no formation of 1233xf. CsCl exhibited the best results. The 10% CsCl/MgF2 catalyst provided activity comparable to MgF2 and the highest 1234yf selectivity while generating no 1233xf.







TABLE 5







(Reactivity of alkaline metal chloride-doped MgF2


catalysts during 244bb dehydrohalogenation*)














Temp.

Conversion
Selectivity
Selectivity
Selectivity,



Bottom-Top
t
244bb
1234yf
1233xf
Unknowns


Catalyst
(°)
(h)
(%)
(%)
(%)
(%)





10 wt % LiCl/MgF2
475-490
1
29.4
89.1
5.3
5.6



475-506
2
38.8
89.6
5.3
5.0



475-505
3
40.4
89.9
5.2
4.9



475-507
4
42.9
90.5
4.8
4.7


10 wt % KCl/MgF2
475-514
1
38.3
95.1
0.9
4.0



475-515
3
47.2
95.6
0.8
3.6



475-515
5
47.5
95.8
0.7
3.5



475-509
6
43.7
95.8
0.6
3.5



475-514
7
47.1
95.8
0.7
3.5


10 wt % CsCl/MgF2
475-511
1
49.6
96.9

3.1



475-510
2
51.2
97.0

3.0



475-511
3
51.8
96.9

3.1



475-508
4
50.4
96.9

3.1



475-510
5
51.4
97.0

3.0





*Reaction conditions: 20 ml of catalyst, 6 grams-organic/hour, 97.2% 244bb/2.0% 1233xf, pressure = 1 atm






Additional teachings to catalyst compositions having mixtures/combinations of alkali metal halides and bivalent metal fluorides and use of same in dehydrochlorinating hydrofluorocarbons to fluorinated olefins is shown in U.S. Provisional Patent Application No. 60/958,468, filed Jul. 6, 2007, which is incorporated herein by reference.
It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.

## Claims

1. A method for making a catalyst composition, comprising:
(a) adding an amount of hydroxide, oxide, or carbonate of an alkali metal to an aqueous solution of a hydrogen halide and reacting to form an aqueous solution of an alkali metal halide;
(b) adding an amount of a hydroxide, oxide, or carbonate of a bivalent metal to an aqueous solution of hydrogen fluoride and reacting to form a precipitate of a bivalent metal fluoride;
(c) admixing the alkali metal halide solution and the bivalent metal fluoride precipitate to form an aqueous slurry; and
(d) removing water from the aqueous slurry to form a solid mass, wherein the alkali metal halide is represented by the formula MX; M is an alkali metal ion selected from the group consisting of Li+, Na+, K+, Rb+, and Cs+; X is a halogen ion selected from the group consisting of F−, Cl−, Br−, and I−; the bivalent metal fluoride is represented by the formula M′F2; and M′ is a bivalent metal ion.

2. The method of claim 1, wherein the water is removed by driving off a portion of the water from the slurry to form a paste following by drying or evaporation of the paste to form the solid mass.

3. The method of claim 2, further comprising calcining the solid mass to form a calcined mass.

4. The method of claim 3, further comprising grinding the calcined mass to form a powder.

5. The method of claim 4, further comprising pelletizing the powder to form pellets.

6. The method of claim 2, wherein the calcination is carried out at a temperature of about 100° C. to about 750° C.

7. The method of claim 6, wherein the calcination is carried out at a temperature of about 200° C. to about 600° C.

8. The method of claim 1, wherein M′ is selected from the group consisting of Mg2+, Ca2+, Sr+, Ba2+, Ni2+, Fe2+, Co2+, Cu2+, and Zn2+.

9. The method of claim 1, wherein the alkali metal halide is from about 0.05 wt % to about 50 wt % of the catalyst composition based on the total weight of the catalyst composition.

10. The method of claim 1, wherein the alkali metal halide is from about 5 wt % to about 15 wt % of the catalyst composition based on the total weight of the catalyst composition.

11. The method of claim 1, wherein the alkali metal halide is from about 7.5 wt % to about 12.5 wt % of the catalyst composition based on the total weight of the catalyst composition.

12. The method of claim 7, wherein the calcination is carried out at a temperature of about 300° C. to about 600° C.

13. The method of claim 1, wherein M is selected from the group consisting of potassium and cesium, wherein X is selected from the group consisting of F− and Cl−, and wherein M′ is selected from the group consisting of Mg2+ and Ni2+.

14. The method of claim 13, wherein M is cesium, X is Cl− or F−and M′ is Mg2+.