U.S. patent application number 11/260256 was filed with the patent office on 2006-05-18 for methods and apparatus for synthesis of metal hydrides.
Invention is credited to Michael T. Kelly.
Application Number | 20060102489 11/260256 |
Document ID | / |
Family ID | 36319661 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102489 |
Kind Code |
A1 |
Kelly; Michael T. |
May 18, 2006 |
Methods and apparatus for synthesis of metal hydrides
Abstract
An electrochemical process and apparatus for preparing metal
hydride compounds from metal salts under a hydrogen atmosphere are
disclosed. The electrochemical process may be integrated with
chemical reaction of a boron compound to produce borohydride
compounds. A metal salt and a borate are charged to the cathode of
an electrolytic cell wherein the borate reacts with the hydride, to
produce the borohydride compound.
Inventors: |
Kelly; Michael T.;
(Plainsboro, NJ) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L Street, NW
Washington
DC
20037
US
|
Family ID: |
36319661 |
Appl. No.: |
11/260256 |
Filed: |
October 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60622789 |
Oct 29, 2004 |
|
|
|
60662555 |
Mar 17, 2005 |
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Current U.S.
Class: |
205/357 ;
204/245; 204/246; 205/358 |
Current CPC
Class: |
C25B 1/00 20130101; C25C
3/02 20130101; C25B 1/14 20130101; C25B 9/19 20210101 |
Class at
Publication: |
205/357 ;
204/246; 204/245; 205/358 |
International
Class: |
C25C 3/00 20060101
C25C003/00; C25B 1/00 20060101 C25B001/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The invention was made with Government support under
Cooperative Agreement No. DE-FC36-04GO14008 awarded by the
Department of Energy. The Government has certain rights in this
invention.
Claims
1. A process for preparing a metal hydride compound, comprising:
providing an electrolytic cell containing anode and cathode
compartments separated by a separator which is permeable to ions;
supplying at least one metal salt in molten form to the cathode
compartment; applying an electric potential to the cell; and
providing hydrogen to the cathode compartment.
2. The process of claim 1, wherein the metal salt has the formula
MX.sub.n, wherein M is an active metal cation selected from the
group consisting of Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+,
Cs.sup.+, Be.sup.2+, Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+,
Sc.sup.3+, Ti.sup.3+, Ti.sup.4+, Zn.sup.2+, Al.sup.3+, Si.sup.4+,
Y.sup.3+, Y.sup.+, Zr.sup.2+, Zr.sup.3+, Zr.sup.+, Hf.sup.2+,
Hf.sup.3+, Hf.sup.4+, and lanthanides in the +3 oxidation state; X
is an anion selected from the group consisting of halides,
tosylate, sulfate, sulfonates, nitrate, phosphates,
hexafluorophosphate, phosphates, phosphinates, dicyanamide,
tetrafluoroborate, acetate, trifluoroacetate, borohydride,
benzoate, tetrachloroaluminate, thiocyanate, thiosalicylate,
methides, and imides; and n is the valence of the active metal
cation.
3. The process of claim 2, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+, K.sup.+ and Cs.sup.+; and X is
chloride or bromide.
4. The process of claim 1, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
5. The process of claim 1, wherein the separator is a NaSICON
membrane.
6. The process of claim 1, wherein the separator is a LiSICON
membrane.
7. The process of claim 1, wherein the separator comprises a
material selected from the group consisting of porous glass, porous
metals, porous ceramics, porous plastics, paper polymers,
fluorinated polymers, ion-conducting polymers, and fluorinated
ion-conducting polymers.
8. The process of claim 1, wherein the electrical potential is from
about 1 to about 10 volts.
9. The process of claim 7, wherein the electrical potential is from
about 1 to about 5 volts.
10. The process of claim 1, further comprising passing hydrogen or
a hydrogen containing gas to the cathode compartment through a gas
inlet means.
11. The process of claim 1, further comprising bubbling hydrogen
gas through the cathode compartment to agitate the catholyte.
12. The process of claim 1, further comprising providing hydrogen
to the cathode compartment from hydrogen absorbed in a metal.
13. The process of claim 1, further comprising providing hydrogen
to the anode compartment and electrooxidizing hydrogen at the
anode.
14. A process for producing a metal hydride compound, comprising:
providing an electrolytic cell containing anode and cathode
compartments separated by a separator which is permeable to ions;
supplying at least one metal salt to the cathode compartment,
wherein the metal salt is at least partially dissolved in an ionic
liquid; applying an electric potential to the cell; and providing
hydrogen to the cathode compartment.
15. The process of claim 14, wherein the metal salt has the formula
MX.sub.n, wherein M is an active metal cation selected from the
group consisting of Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+,
Cs.sup.+, Be.sup.2+ Mg.sup.2+, Ca.sup.2+, Sr.sup.2+ Ba.sup.2+,
Sc.sup.3+, Ti.sup.3+, Ti.sup.4+, Zn.sup.2+, Al.sup.3+, Si.sup.4+,
Y.sup.3+, Y.sup.+, Zr.sup.2+, Zr.sup.3+, Zr.sup.4+, Hf.sup.2+,
Hf.sup.3+, Hf.sup.4+, and lanthanides in the +3 oxidation state; X
is an anion selected from the group consisting of halides,
tosylate, sulfate, sulfonates, nitrate, phosphates,
hexafluorophosphate, phosphates, phosphinates, dicyanamide,
tetrafluoroborate, acetate, trifluoroacetate, borohydride,
benzoate, tetrachloroaluminate, thiocyanate, thiosalicylate,
methides, and imides; and n is the valence of the active metal
cation.
16. The process of claim 15, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+, K.sup.+ and Cs.sup.+; and X is
chloride or bromide.
17. The process of claim 14, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
18. The process of claim 14, wherein the separator is a NaSICON
membrane.
19. The process of claim 14, wherein the separator is a LiSICON
membrane.
20. The process of claim 14, wherein the separator comprises a
material selected from the group consisting of porous glass, porous
metals, porous ceramics, porous plastics, paper polymers,
fluorinated polymers, ion-conducting polymers, and fluorinated
ion-conducting polymers.
21. The process of claim 14, wherein the electrical potential is
from about 1 to about 10 volts.
22. The process of claim 21, wherein the electrical potential is
from about 1 to about 5 volts.
23. The process of claim 14, further comprising passing hydrogen or
a hydrogen containing gas to the cathode compartment through a gas
inlet means.
24. The process of claim 14, further comprising bubbling hydrogen
the cathode compartment to agitate the catholyte.
25. The process of claim 14, further comprising providing hydrogen
to the cathode compartment from hydrogen absorbed in a metal.
26. The process of claim 14, further comprising providing hydrogen
to the anode compartment and electrooxidizing hydrogen at the
anode.
27. The process of claim 14, wherein the ionic liquid is a salt
comprising a cation containing at least one carbon atom and having
a melting point between about -100.degree. C. to about 200.degree.
C.
28. The process of claim 14, wherein the ionic liquid comprises a
cation selected from the group consisting of mono-, di-, tri-, and
tetra substituted ammonium; mono-, di-, tri-, and tetra substituted
phosphonium, N-alkylpyridinium, 1,3-disubstituted pyridiniums,
1,4-disubstituted pyridiniums, 1,3-disubstituted imidazolium,
1,2,3-trisubstituted imidazolium, 1,1 disubstituted pyrrolidiums,
trialkylsulfonium, and trialkyloxonium cations.
29. The process of claim 14, wherein the ionic liquid comprises an
anion selected from the group consisting of halides, tosylate,
sulfate, sulfonates, nitrate, phosphates, hexafluorophosphate,
phosphates, phosphinates, dicyanamide, tetrafluoroborate, acetate,
trifluoroacetate, borohydride, benzoate, tetrachloroaluminate,
thiocyanate, thiosalicylate, methides, and imides.
30. A process for producing a boron hydride compound, comprising:
providing an electrolytic cell containing anode and cathode
compartments separated by a separator which is permeable to ions;
supplying at least one metal salt in molten form to the cathode
compartment; applying an electric potential to the cell; providing
hydrogen to the cathode compartment; and providing a boron compound
to the cathode compartment.
31. The process of claim 30, wherein the metal salt has the formula
MX.sub.n, wherein M is an active metal cation selected from the
group consisting of Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+,
Cs.sup.+, Be.sup.2+, Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+,
Sc.sup.3+, Ti.sup.3+, Ti.sup.4+, Zn.sup.2+, Al.sup.3+, Si.sup.4+
Y.sup.3+, Y.sup.+, Zr.sup.2+, Zr.sup.3+ Zr.sup.4+, Hf.sup.2+,
Hf.sup.3+, Hf.sup.4+, and lanthanides in the +3 oxidation state; X
is an anion selected from the group consisting of halides,
tosylate, sulfate, sulfonates, nitrate, phosphates,
hexafluorophosphate, phosphates, phosphinates, dicyanamide,
tetrafluoroborate, acetate, trifluoroacetate, borohydride,
benzoate, tetrachloroaluminate, thiocyanate, thiosalicylate,
methides, and imides; and n is the valence of the active metal
cation.
32. The process of claim 31, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+, K.sup.+ and Cs.sup.+; and X is
chloride or bromide.
33. The process of claim 30, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
34. The process of claim 30, wherein the separator comprises a
material selected from the group consisting of NaSICON and LiSICON
membranes.
35. The process of claim 30, wherein the separator comprises a
material selected from the group consisting of porous glass, porous
metals, porous ceramics, and porous plastics.
36. The process of claim 30, wherein the electrical potential is
from about 1 to about 10 volts.
37. The process of claim 36, wherein the electrical potential is
from about 1 to about 5 volts.
38. The process of claim 30, further comprising passing hydrogen or
a hydrogen containing gas to the cathode compartment through a gas
inlet means.
39. The process of claim 30, further comprising bubbling hydrogen
the cathode compartment to agitate the catholyte.
40. The process of claim 30, further comprising providing hydrogen
to the cathode compartment from hydrogen absorbed in a metal.
41. The process of claim 30, wherein the boron compound is an
oxidized boron compound.
42. The process of claim 41, wherein the oxidized boron compound is
sodium metaborate and the metal halide is lithium bromide.
43. The process of claim 30, further comprising maintaining the
cell at a temperature of about 70 to about 500.degree. C.
44. The process of claim 30, further comprising providing hydrogen
to the anode compartment and electrooxidizing hydrogen at the
anode.
45. The process of claim 30, wherein the electric potential is
removed before providing the boron compound.
46. The process of claim 30, wherein the boron compound is provided
before applying the electric potential.
47. The process of claim 30, further comprising separating the
boron hydride compound.
48. The process of claim 30, wherein the boron compound is a boron
halide.
49. The process of claim 30, wherein the boron compound is an alkyl
borate.
50. The process of claim 30, wherein the boron compound is a
borate.
51. The process of claim 30, wherein the boron compound is selected
from the group consisting of boric oxide and boric acid.
52. The process of claim 30, wherein the boron compound is an
alkali metal borate salt.
53. A process for producing a boron hydride compound, comprising:
providing an electrolytic cell containing anode and cathode
compartments separated by a separator which is permeable to ions;
supplying at least one metal salt to the cathode compartment,
wherein the metal salt is at least partially dissolved in an ionic
liquid; applying an electric potential to the cell; providing
hydrogen to the cathode compartment; and providing a boron compound
to the cathode compartment.
54. The process of claim 53, wherein the metal salt has the formula
MX.sub.n, wherein M is an active metal cation selected from the
group consisting of Li.sup.+, Na.sup.+, K.sup.+, Rb.sup.+,
Cs.sup.+, Be.sup.2+ Mg.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+,
Sc.sup.3+, Ti.sup.3+, Ti.sup.4+, Zn.sup.2+, Al.sup.3+, Si.sup.4+,
Y.sup.3+, Y.sup.+, Zr.sup.2+, Zr.sup.3+, Zr.sup.4+, Hf.sup.2+,
Hf.sup.3+, Hf.sup.4+, and lanthanides in the +3 oxidation state; X
is an anion selected from the group consisting of halides,
tosylate, sulfate, sulfonates, nitrate, phosphates,
hexafluorophosphate, phosphates, phosphinates, dicyanamide,
tetrafluoroborate, acetate, trifluoroacetate, borohydride,
benzoate, tetrachloroaluminate, thiocyanate, thiosalicylate,
methides, and imides; and n is the valence of the active metal
cation.
55. The process of claim 54, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+, K.sup.+ and Cs.sup.+; and X is
chloride or bromide.
56. The process of claim 53, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
57. The process of claim 53, wherein the separator comprises a
material selected from the group consisting of NaSICON and LiSICON
membranes.
58. The process of claim 53, wherein the separator comprises a
material selected from the group consisting of porous glass, porous
metals, porous ceramics, porous plastics, paper polymers,
fluorinated polymers, ion-conducting polymers, and fluorinated
ion-conducting polymers.
59. The process of claim 53, wherein the electrical potential is
from about 1 to about 10 volts.
60. The process of claim 53, wherein the electrical potential is
from about 1 to about 5 volts.
61. The process of claim 53, further comprising passing hydrogen or
a hydrogen containing gas to the cathode compartment through a gas
inlet means.
62. The process of claim 53, further comprising bubbling hydrogen
the cathode compartment to agitate the catholyte.
63. The process of claim 53, further comprising providing hydrogen
to the cathode compartment from hydrogen absorbed in a metal.
64. The process of claim 53, wherein the ionic liquid is a salt
comprising a cation containing at least one carbon atom and having
a melting point between about -100.degree. C. to about 200.degree.
C.
65. The process of claim 53, wherein the ionic liquid comprises a
cation selected from the group consisting of mono-, di-, tri-, and
tetra substituted ammonium; mono-, di-, tri-, and tetra substituted
phosphonium, N-alkylpyridinium, 1,3-disubstituted pyridiniums,
1,4-disubstituted pyridiniums, 1,3-disubstituted imidazolium,
1,2,3-trisubstituted imidazolium, 1,1 disubstituted pyrrolidiums,
trialkylsulfonium, and trialkyloxonium cations.
66. The process of claim 53, wherein the ionic liquid comprises an
anion selected from the group consisting of halides, tosylate,
sulfate, sulfonates, nitrate, phosphates, hexafluorophosphate,
phosphates, phosphinates, dicyanamide, tetrafluoroborate, acetate,
trifluoroacetate, borohydride, benzoate, tetrachloroaluminate,
thiocyanate, thiosalicylate, methides, and imides.
67. The process of claim 53, wherein the boron compound is a
boron-oxygen compound.
68. The process of claim 53, wherein the boron compound is an alkyl
borate.
69. The process of claim 53, wherein the boron compound is a
borate.
70. The process of claim 53, wherein the boron compound is selected
from the group consisting of boric oxide and boric acid.
71. The process of claim 53, wherein the boron compound is an
alkali metal borate salt.
72. The process of claim 53, wherein the boron compound is sodium
metaborate and the metal halide is lithium bromide.
73. The process of claim 53, further comprising maintaining the
cell at a temperature of about 70 to about 500.degree. C.
74. The process of claim 53, further comprising providing hydrogen
to the anode compartment and electrooxidizing hydrogen at the
anode.
75. The process of claim 53, wherein the electric potential is
removed before providing the boron compound.
76. The process of claim 53, wherein the boron compound is provided
before applying the electric potential.
77. The process of claim 53, further comprising separating the
boron hydride compound.
78. A process for producing borohydride anions comprising
dissolving a metal hydride and a boron compound in at least one
liquid salt.
79. The process of claim 78, wherein the metal hydride is selected
from the group consisting of alkali metal hydrides, alkaline earth
metal hydrides, aluminum hydrides and zinc hydrides.
80. The process of claim 78, wherein the metal hydride comprises a
metal characterized in that the standard reduction potential for
the reaction of the metal with oxygen is at least about 1.6 V.
81. The process of claim 78, wherein the metal hydride is formed by
the steps of: supplying at least one active metal salt in molten
form to a cathode compartment of an electrolytic cell containing
anode and cathode compartments separated by a separator which is
permeable to ions; applying an electric potential to said cell to
reduce the metal compound at the cathode; and passing hydrogen or a
hydrogen containing gas in the cathode compartment while the
compound is reduced at the cathode.
82. The process of claim 78, wherein the metal hydride is formed in
situ.
83. The process of claim 78, wherein the boron compound is an
oxidized boron compound.
84. The process of claim 78, wherein the liquid salt is a molten
active metal salt.
85. The process of claim 78, wherein the liquid salt is a mixture
of molten active metal salts.
86. The process of claim 78, wherein the liquid salt is an ionic
liquid.
87. The process of claim 78, wherein the liquid salt is a mixture
of at least one molten active metal salt and at least one ionic
liquid.
88. An apparatus for reducing boron compounds to produce boron
hydride compounds, comprising: an anode compartment containing on
anode; a cathode compartment containing a cathode; a separator
between the anode and cathode compartments, wherein the separator
is permeable to ions; at least one inlet for charging metal salt
and boron compounds to the cathode compartment; and a means for
supplying hydrogen to the cathode compartment.
89. The apparatus of claim 88, wherein the apparatus is configured
to maintain the cathode compartment at a temperature of about
70.degree. C. to about 500.degree. C.
90. The apparatus of claim 88, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
91. The apparatus of claim 88, wherein the separator is a NaSICON
membrane.
92. The apparatus of claim 88, wherein the separator is a LiSICON
membrane.
93. The apparatus of claim 88, wherein the separator comprises a
material selected from the group consisting of porous glass, porous
metals, porous ceramics, porous plastics, paper polymers,
fluorinated polymers, ion-conducting polymers, and fluorinated
ion-conducting polymers.
94. The apparatus of claim 88, further comprising a means for
bubbling hydrogen gas through the cathode compartment to agitate
the catholyte.
95. The apparatus of claim 88, wherein the cathode compartment
contains hydrogen absorbed in a metal adapted to release hydrogen
when heated.
Description
[0001] This application claims the benefit of U.S. Provisional
Application Ser. Nos. 60/622,789 filed on Oct. 29, 2004, and
60/662,555 filed on Mar. 17, 2005, the entire disclosures of which
are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to the electrochemical
reduction of active metal salt compounds with applications in
active metal hydride and active metal borohydride production.
BACKGROUND OF THE INVENTION
[0004] Sodium borohydride is a very versatile chemical and is used
in organic synthesis, waste water treatment, and pulp and paper
bleaching. The high hydrogen content of this compound also makes it
a good candidate for being a hydrogen carrier, and it could play a
major role as an enabler of a hydrogen economy if the cost of
producing this chemical can be reduced.
[0005] Today, sodium borohydride is produced by the so-called
Schlesinger process, which is a multi-step synthetic process,
wherein sodium borohydride is produced from the reaction of sodium
hydride and trimethyl borate in mineral oil. As none of the
reagents are soluble in mineral oil, it is necessary to ensure high
dispersions and the reaction must proceed at elevated temperatures,
typically around 250.degree. C. In addition, mineral oil evaporates
and can contribute to VOC emissions.
[0006] U.S. Pat. No. 3,734,842, U.S. Pat. No. 4,904,357, and U.S.
Pat. No. 4,931,154, the disclosures of which are incorporated by
reference herein in their entirety, refer to electrochemical
synthesis of sodium borohydride from aqueous sodium metaborate
solution. Such processes involve conversion of sodium metaborate
and water to form sodium borohydride and oxygen in an electrical
cell, as shown in the following half-cell reactions: Cathode:
B(OH).sub.4.sup.-+4H.sub.2O+8e.sup.-.fwdarw.BH.sub.4.sup.-+8OH.sup.-
(1a) Anode: 8O.sup.--.fwdarw.4H.sub.2O+2O.sub.2+8e (1b)
[0007] However, none of these processes has been implemented in
commercial practice.
BRIEF SUMMARY OF THE INVENTION
[0008] The invention is directed to electrochemical processes and
apparatus for preparing metal hydride compounds from active metal
salts.
[0009] In accordance with one aspect of the present invention,
molten active metal salts are electrolyzed under a hydrogen
atmosphere to produce active metal hydrides.
[0010] In accordance with another aspect of the present invention,
active metal salts are electrolyzed in ionic liquids under a
hydrogen atmosphere to produce active metal hydrides.
[0011] In accordance with another aspect of the present invention,
the electrochemical process is integrated with a chemical reaction
of a boron compound to produce boron hydride compounds.
[0012] In another aspect of the present invention, the
electrochemical process is integrated with an in situ chemical
reaction of an oxidized boron compound to produce boron hydride
compounds.
[0013] In another aspect of the present invention, oxidized boron
compounds are reduced by reaction with active metal hydrides in a
liquid salt to produce boron hydride compounds.
[0014] These and other features and advantages of the invention
will become apparent from the following detailed description that
is provided in connection with the accompanying drawings and
illustrated exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic view of an electrolytic cell in
accordance with one embodiment of the invention, where
hydrogen-containing gas is passed into the cathode compartment for
synthesis of an active metal hydride from molten active metal
salt;
[0016] FIG. 2 is a schematic diagram for producing borohydride
anions according to an exemplary process of the present
invention;
[0017] FIG. 3 is a schematic diagram for producing borohydride
anions according to another exemplary process of the present
invention; and
[0018] FIG. 4 is a view of an exemplary electrolytic cell suitable
for use in the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] In accordance with an exemplary embodiment of the present
invention, a metal salt or a mixture of metal salts are converted
into a metal hydride via electrolysis in the presence of hydrogen.
Without being limited by theory, it is thought that an
electrochemical reduction of the metal salt yields metal at the
cathode, and the metal formed then reacts chemically with hydrogen
to give metal hydride. The overall reaction is shown in Equation
(2) wherein X represents a halide anion (the reaction product of
the anion will depend on the anion, X, chosen),
MX.sub.n+n/2H.sub.2.fwdarw.MH.sub.n+n/2.times.2 (2)
[0020] where M is preferably selected from the group of metals and
semimetals wherein the potential of the reaction between the metal,
M, and oxygen to make a metal oxide is greater than about 1.6
volts, where the potential is defined as the negative of the free
energy of reaction (AG, measured in joules per mole of metal) at
standard conditions, wherein temperature is 298.15 K (25.degree.
C.) and pressure is 101.325 kPa (1 atm), divided by the number of
moles of electrons transferred per mole of metal (n), divided by
Faraday's constant (F) (Faraday's constant=96485 coulombs/mole of
electron), or Potential=-.DELTA.G/nF; X is chosen from the group of
anions comprising halides, tosylate, sulfate and sulfate
derivatives, trifluoromethanesulfonate and other sulfonates,
nitrate, phosphates, hexafluorophosphate, and other phosphate
derivatives, phosphinates, dicyanamide, tetrafluoroborate, acetate,
trifluoroacetate, borohydride, benzoate, tetrachloroaluminate,
thiocyanate, thiosalicylate, tris(trifuoromethylsulfonyl)methide
and other methides, and bis(trifluoromethylsufonyl)imide and other
imides; and n is the valence of the metal, preferably an integer
from 1 to 4. Metals and semimetals falling under this definition
are herein referred to as "active metals."
[0021] Active metals, include, but are not limited to, the alkali
metals, the alkaline earth metals, transition metals from Groups 3,
4, 12, and the lanthanide family. The active metals form cations
that include, but are not limited to, Li.sup.+, Na.sup.+, K.sup.+,
Rb.sup.+, Cs.sup.+, Be.sup.2+, Mg.sup.2+, Ca.sup.2+ Sr.sup.2+,
Ba.sup.2+, Sc.sup.3+, Ti.sup.3+, Ti.sup.4+, Zn.sup.2+, Al.sup.3+,
Si.sup.4+, Y.sup.3+, Y.sup.+, Zr.sup.2+, Zr.sup.3+, Zr.sup.4+,
Hf.sup.+, Hf.sup.3+, Hf.sup.3+, and lanthanides in the +3 oxidation
state. M is preferably chosen from the group of alkali metals, and
more preferably is lithium, sodium, potassium, and cesium; and X is
preferably chloride or bromide.
[0022] An exemplary two-compartment electrolysis cell 100 employed
in the process of the present invention is illustrated in FIG. 1.
The cell 100 comprises an anode compartment 104, anode 102, cathode
compartment 112, cathode 108, separator 106 which separates the
anode and cathode compartments but allows ionic transport, and an
optional gas inlet means 110 to supply a gas comprising hydrogen to
the cathode compartment. The anodes and cathodes may comprise any
suitable electrode material.
[0023] Separator 106 may preferably comprise a material such as
glass, polymer, or ceramic that allows ionic transport between the
cathodic and anionic compartments, but restricts reaction between
the active metal produced at the cathode and the product produced
at the anode. Porous separators such as porous glass, porous metal,
porous plastics, and porous ceramics are suitable separators.
Paper, polymer, polymer membranes, and perfluoronated
ion-conducting polymer membranes, are also suitable separators.
Nonlimiting examples of polymer separators include
polytetrafluoroethylene (PTFE), perfluoroalkoxy polymer,
perfluorosulfonated ionomers, polyamides, nylon polymers, and
polyethylene. Optionally, cationic conducting ceramics may be
employed as the separator. Nonlimiting examples of ceramic
separators include lithium-.beta.-aluminum oxide,
lithium-.beta.''-aluminum oxide, lithium-.beta./.beta.''-aluminum
oxide, lithium analogs of NaSICON ceramics, LiSICONs, and lithium
ion conductors with perovskite structure, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, NaSICON ceramics,
potassium-.beta.-aluminum oxide, potassium-.beta.''-aluminum oxide,
potassium-.beta./.beta.''-aluminum oxide, and potassium analogs of
NaSICON ceramics.
[0024] In a preferred embodiment of the method of the present
invention, one or more active metal salts of formula MX.sub.n are
charged to the cathode chamber 112 to prepare a metal hydride. The
cell is preferably maintained at temperatures from about 70.degree.
C. to about 500.degree. C., so that the one or more active metal
salts are in a liquid molten state. The active metal salts can be
used neat, i.e., without solvent, or a solvent may be included.
[0025] Alternatively, the one or more active metal salts can be
dissolved in an ionic liquid. Ionic liquids are defined herein as
salts with a melting point between about -100.degree. C. and about
200.degree. C., and preferably containing at least 1 carbon atom in
the cation. Typical ionic liquid cations include, but are not
limited to, mono-, di-, tri-, and tetra substituted ammonium;
mono-, di-, tri-, and tetra substituted phosphonium,
N-alkylpyridinium, 1,3-disubstituted pyridiniums, 1,4-disubstituted
pyridiniums, 1,3-disubstituted imidazolium, 1,2,3-trisubstituted
imidazolium, 1,1 disubstituted pyrrolidiums, trialkylsulfonium, and
trialkyloxonium cations. The anion in an ionic liquid can be any
anion. Some typical anions are halides, but other representative
and non-limiting examples include the group of common complex ions
such as tosylate, sulfate and sulfate derivatives,
trifluoromethanesulfonate and other sulfonates, nitrate,
phosphates, hexafluorophosphate, and other phosphate derivatives,
phosphinates, dicyanamide, tetrafluoroborate, acetate,
trifluoroacetate, borohydride, benzoate, tetrachloroaluminate,
thiocyanate, thiosalicylate, tris(trifuoromethylsulfonyl)methide
and other methides, and bis(trifluoromethylsufonyl)imide and other
imides. Preferably, neither the cation nor the anion of the ionic
liquid is easily reducible by strong hydrides. It is not necessary
that the liquid salt be a liquid at room temperature, but only that
at least a portion of the salt be liquid at the reaction
temperature.
[0026] Hydrogen is preferably supplied to the cathode chamber as a
gas stream via a gas inlet means. Suitable gas inlet means for
supplying a hydrogen or hydrogen-containing gas stream include a
pipe, a sparger, a hose, or a hydrogen gas diffusion material.
Alternatively, hydrogen can be absorbed in a metal or metal alloy
which can be released as the temperature increases. Such metals or
alloys can be impregnated with hydrogen and used as the cathode.
Preferably, a gas stream comprising hydrogen bubbles through or
otherwise agitates the catholyte.
[0027] Upon the application of an electric potential, preferably
from about 1.0 to about 10.0 V, preferably from about 1.0 V to
about 5.0 V, the active metal ions are reduced at the cathode to
the metal or semimetal as shown in Equation (3), and the anion is
oxidized at the anode as shown in Equation (4a) for a monovalent
anion such as a halide: Cathode: 2M.sup.y++ye.sup.-.fwdarw.M (3)
Anode: yX.sup.-.fwdarw.y/2X.sub.2+ye.sup.- (4a)
[0028] The active metal reacts with the hydrogen gas to form an
active metal hydride compound as shown in Equation (5):
M+y/2H.sub.2.fwdarw.MH.sub.y (5)
[0029] In Equations (3), (4), and (5), y is an integer from 1 to 4
and typically depends on the preferred (i.e. the most stable)
oxidation state of the active metal when it combines with oxygen to
make the active metal oxide. Some exceptions are known, such as
titanium, which preferentially forms a TiH.sub.2 hydride, rather
than TiH.sub.3 or TiH.sub.4.
[0030] Hydrogen may be supplied to the anode as well as to the
cathode to convert the anion oxidation product to a desirable or
valuable reaction product as shown in Equations (4b) and (4c)
wherein X is a halide, and halogen is converted to HX.
Electrochemically oxidizing H.sub.2 at the anode in preference to
X.sup.- will generally result in a lower cell potential than the
comparable electrochemical system that generates X.sub.2 depicted
in Equation (4a). Anode: 1/2H.sub.2.fwdarw.H.sup.++e.sup.- (4b)
X.sup.-+H.sup.+.fwdarw.HX+e.sup.- (4c)
[0031] In another aspect of the present invention, the
electrochemical-chemical process for obtaining metal hydrides
according to the present invention can be incorporated into a
process for producing boron hydride compounds. In this embodiment,
oxidized boron compounds are reduced by a hydride carrier in a
liquid salt to produce a boron hydride compound. The hydride
carrier may be, for example, derived from the electrochemical
reduction of active metal salts as described, wherein a molten
active metal salt or mixture of molten active metal salts, either
neat or in an ionic liquid, is converted into an active metal
hydride via electrolysis under an atmosphere of hydrogen. The
process of the present invention provides a ready "one-pot" means
to reduce boron compounds such as boron-oxygen compounds and boron
halide compounds, to boron hydride compounds including borohydride
anions (BH.sub.4.sup.-).
[0032] To produce a boron hydride compound, at least one active
metal salt is charged to the cathode compartment of an electrolytic
cell and an electric potential from about 1.0 V to about 10.0 V and
preferably from about 1.0 V to about 5.0 V is applied to form the
active metal as described above.
[0033] After the active metal has reacted with hydrogen gas to
produce the active hydride, the applied potential may be removed
and the cell maintained at a temperature such that the metal
hydride is at least partially dissolved in a liquid salt. Thus, for
neat metal salt systems, the cell is maintained at temperatures
above the melting point of the active metal salt or mixture of
active metal salts. For systems wherein the metal salt was
dissolved in an ionic liquid, the cell is maintained at a
temperature that allows the solvent to be liquid.
[0034] As shown schematically in FIG. 2, an oxidized boron species
is introduced to the liquid salt containing the active metal
hydride. The oxidized boron compound is selected from the group of
boron oxygen and boron halide compounds. The boron-oxygen compound,
collectively referred to as a "borate" in this application, is
preferably selected from the group comprising trialkyl borates of
formula B(OR).sub.3, where R is a straight-, branched-chain, or
cyclic alkyl group containing from 1 to 6, preferably from 1 to 4,
carbon atoms; boric oxide, B.sub.2O.sub.3; boric acid, B(OH).sub.3;
and the group of alkali metal borate salts represented by the
formula zM.sub.2O.xB.sub.2O.sub.3.yH.sub.2O, wherein z is 1/2 to 5;
x is 0.1 to 5, y is 0 to 10; and M is an alkali metal ion such as
sodium, potassium, or lithium. The boron halide compounds can be
chosen from the group of compounds represented by formula BX.sub.3,
where X is a halide, preferably chloride or bromide.
[0035] The oxidized boron compound reacts with the active hydride.
Equation (6a) illustrates the formation of borohydride from a
trialkyl borate and an active metal hydride such as MH:
4MH+B(OR).sub.3.fwdarw.MBH.sub.4+3MOR (6a)
[0036] The stoichiometry of the reduction reaction can be adjusted
as shown in Equations (6b) to (6d) to ensure the generation of a
borohydride compound from various active metal hydrides and other
oxidized boron compounds:
4MH.sub.2+2B(OR).sub.3.fwdarw.M(BH.sub.4).sub.2+3M(OR).sub.2 (6b)
4MH.sub.3+3B(OR).sub.3.fwdarw.M(BH.sub.4).sub.3+3M(OR).sub.3 (6c)
4MH.sub.4+4B(OR).sub.3.fwdarw.M(BH.sub.4).sub.4+3M(OR).sub.4
(6d)
[0037] Higher boron hydride compounds, such as diborane and
triborohydride compounds, can be prepared by varying the
stoichiometric ratio between the hydride and the oxidized boron
compound, as illustrated in Equation (6e) for the formation of the
diborane ion from a trialkyl borate:
6MH+2B(OR).sub.3.fwdarw.6M.sup.++.sup.-B.sub.2H.sub.6+6(OR).sup.- -
(6e)
[0038] The use of triborohydride compounds for hydrogen storage and
related methods for their preparation are described in co-pending
U.S. patent application Ser. No. 10/741,192, entitled
"Triborohydride Salts as Hydrogen Storage Materials and Preparation
Thereof," filed Dec. 19, 2003, the disclosure of which is
incorporated by reference herein in its entirety.
[0039] FIG. 3 schematically illustrates the process wherein the
oxidized boron compound is charged to the cathode compartment of an
electrolytic cell along with at least one active metal salt. For a
neat reaction, the metal salt may be used as the "solvent" for the
oxidized boron compound by heating the cell at temperatures from
about 70.degree. C. to about 500.degree. C. so that the one or more
active metal salts are in a liquid molten state. Alternatively, the
metal salt and oxidized boron compound may be dissolved in an ionic
liquid.
[0040] The oxidized boron compound/alkali metal salt mixture is
subjected to a potential from about 1.0 V to about 5.0 V to form
the active metal as described above. The oxidized boron compound
reacts with the active hydride as it is produced, to form a boron
hydride. In this case, the reactions illustrated in Equations
(6a)-(6d) occur continuously as the metal hydride is formed.
[0041] Alkali metal borates, B.sub.2O.sub.3, and trialkyl borates
such as B(OR).sub.3, may be reacted with alkali metal hydrides to
obtain borohydride compounds in suspension. For example, at
275.degree. C., NaH and B(OCH).sub.3 react in mineral oil to form
NaBH.sub.4, and NaOCH.sub.3. The present invention can achieve this
reaction in a liquid salt, a solvent system that supports ionic
conduction, and therefore electrochemical synthesis, wherein the
reactants and products are in a dissolved state. The liquid salts
include molten active metal salts and ionic liquids.
[0042] As an example of the process of this exemplary embodiment of
the present invention, borohydride anions are obtained from a
molten mixture of lithium bromide, potassium bromide, and cesium
bromide under a hydrogen atmosphere by the electrolytic process of
the present invention, where boric oxide is added to the melt
before the application of a potential. Without being limited to any
one particular theory, it is believed that electrolysis reduces the
metal ions in the melt to the corresponding metals, which then
react with hydrogen to make the metal hydrides. One or more of the
metal hydrides then react with the boric oxide to make borohydride
anions. A borohydride compound may then be isolated by suitable
separation and extraction steps.
[0043] For the particular case where the oxidized boron compound is
sodium metaborate, NaBO.sub.2, it is preferable that the active
metal hydride be lithium hydride. Lithium hydride can be formed in
situ according to the teachings herein by the electrolytic
reduction of lithium bromide, either as a liquid molten salt or
dissolved in an ionic liquid, under a hydrogen atmosphere to form
lithium hydride.
[0044] Hydrogen may be supplied to the anode as well as the cathode
to convert the anion oxidation product and to lower cell potential
according to the teachings herein.
[0045] In another embodiment of the invention, oxidized boron
compounds are converted to boron hydride compounds via reaction
with metal hydrides dissolved in liquid salts, wherein the metal
hydrides may be, for example, commercially available products
and/or not otherwise derived from the electrochemical reduction of
active metal salts as taught herein. The metal hydrides should
preferably be at least sparingly soluble in the liquid salt
solvent. The liquid salt may be a molten metal salt, or a mixture
of molten metal salts, or an ionic liquid.
[0046] The metal hydrides may be selected from, for example, the
group of alkali metal hydrides, alkaline earth metal hydrides,
aluminum hydrides including alane (AlH.sub.3), and zinc hydride. A
suitable metal hydride is chosen based on the standard reduction
potential of the metal. Any metal wherein the standard reduction
potential for the reaction of that metal with oxygen to yield the
most thermodynamically stable metal oxide is more than about 1.6 V
could be employed in this reaction.
[0047] The following examples further describe and demonstrate
features of the present invention. The examples are given solely
for illustration and are not to be construed as a limitation of the
present invention.
EXAMPLE 1
[0048] A schematic illustration of the reactions taking place in
the process is provided in FIG. 2. The working electrode (cathode)
is a nickel wire. The counter electrode (anode) is a platinum mesh
inside a glass sparging tube. The interior of the glass sparger
comprises the anode chamber, and the region external to the
sparging tube comprises the cathode chamber.
[0049] A mixture consisting of about 39.2 g LiBr, 18.1 g KBr, and
42.8 g of CsBr was charged to cathode compartment and was
electrolyzed at about 5 V for about 5 hours under a hydrogen
atmosphere to produce lithium metal at the cathode and bromine at
the anode. The tube impeded mixing of the bromine that formed at
the anode with the melt external to the tube, and thus slowed the
back-reaction of lithium and bromine to lithium bromide. The tube
also facilitated removal of gaseous bromine from the reactor under
a stream of flowing nitrogen. The reaction flask containing the
melt was maintained in a constant temperature bath at about
300.degree. C.
[0050] After about 5 hours, 587 mAh of current passed through the
cell. The nickel cathode and the sparging tube containing the anode
were both removed from the melt, and a cold-water condenser was
attached to the reaction flask. About 1.25 mL of tri-n-butyl borate
was injected directly into the melt using a syringe. The reaction
was allowed to proceed for about 15 minutes, and the reaction flask
was removed from the constant temperature bath and allowed to cool.
The melt solidified as it cooled. The cool, solid melt was
dissolved in 0.5 M NaOH aqueous solution. A 50 mL sample of the
solution was titrated using the iodate assay for borohydride as put
forth in the Sodium Borohydride Digest by Rohm and Haas Company.
The titration indicated that 3.15.times.10.sup.-4 mol
BH.sub.4.sup.- was formed, a yield of 5.7% based on the 587 mAh of
charge that passed through the cell. Boron NMR of the aqueous
solution confirmed the presence of borohydride anion (chemical
Shift=-40.85 ppm, Splitting=80.6 Hz).
EXAMPLE 2
[0051] Using the procedure described in Example 1, a melt
consisting of about 39.2 g LiBr, 18.1 g KBr, and 42.8 g of CsBr was
electrolyzed under an argon atmosphere at about 3 V for 34 minutes.
The potential, at 3 V, was too low to reduce the cations in the
melt to metal, and instead reduced the Ni surface of the cathode
and generated bromine at the anode. The reactor 400 was assembled
as shown in FIG. 4. The reaction flask 402 containing the melt in
cathode compartment 414 was maintained in a constant temperature
bath at about 275.degree. C. The working electrode (cathode) was a
nickel frit 404 connected to an inlet 406 through which a gas was
passed (the gas could exit the reactor via outlet 420). The counter
electrode 408 (anode) was a platinum mesh inside a glass sparging
tube 410 with a glass frit separator 412. After 34 minutes, argon
flowing through the cathode frit was replaced by flowing hydrogen.
Argon continued to flow over the anode to remove bromine. The
current was not interrupted as the gas changed. No changes were
observed in the current. After 74 minutes, about 0.2 grams of
B.sub.2O.sub.3 was added to the melt in chamber 414. No appreciable
changes in the current were observed after being allowed to run an
additional 126 minutes.
[0052] The electrolysis was reset to run for about 20 hours at
about 5 V. After 20 hours, 1975 mAh of current passed through the
cell. The nickel frit cathode and the sparging tube containing the
anode were both removed from the melt. The reaction flask was
removed from the constant temperature bath and allowed to cool. The
melt solidified as it cooled, and the melt was dissolved in 0.5 M
NaOH aqueous solution. A 50 mL sample of the solution was titrated
using the iodate method for borohydride. The titration indicated
that 2.34.times.10.sup.-4 mol BH.sub.4.sup.- anion was formed, a
yield of 1.3% based on the 1975 mAh of charge that passed through
the cell. Boron NMR of the aqueous solution confirmed the presence
of borohydride anion.
EXAMPLE 3
[0053] A melt consisting of about 9.8 g LiBr, 4.5 g KBr, and 10.7 g
of CsBr under a nitrogen atmosphere was heated to about 250.degree.
C. To this melt, 1.6 grams of B.sub.2O.sub.3 was added. With
stirring, 0.27 g of LiH was added to the melt. After adding LiH,
the temperature bath was turned off, but stirring was continued
until melt solidified. After dissolving the cooled melt in 100 mL
of 0.5 M NaOH, a 50 mL sample of the solution was titrated using
the iodate method for borohydride. The titration indicated that
5.3.times.10.sup.-3 mol BH.sub.4.sup.- was formed, a yield of 62%
based on the 0.27 grams of LiH added to the reactor. Boron NMR of
the aqueous solution confirmed the presence of the borohydride
anion.
EXAMPLE 4
[0054] A mixture of about 39.2 g of LiBr, 18.1 g of KBr, and 42.8 g
of CsBr, and 0.5 g of B.sub.2O.sub.3 were added to a 3-neck flask.
The solids were heated to about 300.degree. C., a temperature at
which this mixture is molten. A nickel metal sparging tube was
inserted into the solution of molten alkali bromides, and H.sub.2
gas passed through the sparger and bubbled through the solution.
This tube comprised the cathode. H.sub.2 gas was allowed to escape
from the cell through one of the necks of the flask. A glass tube
terminating in a porous glass sparger was also inserted into the
solution. Platinum wire and platinum gauze were inside the tube,
and the platinum comprised the anode. The porous glass of the
sparging tube acted as a separator between the anode compartment
(inside the glass tube) and the cathode compartment (outside the
tube). The application of about 5 V of potential led to the passage
of 1448 mAh of charge over 20 hours.
[0055] The net reaction at the cathode was the generation of alkali
metal and boron-hydride compounds. At the anode Br.sub.2 gas was
evolved. A stream of Ar gas helped carry the Br.sub.2 gas out of
the anode compartment.
[0056] After about 20 hours, the potential was removed, and the
cathode sparger and the anode tube were withdrawn from the molten
solution and the solution was allowed to cool to room temperature
and solidify. The resulting solid was dissolved in about 100 mL of
0.5 M aqueous NaOH solution. A small aliquot of solution was
submitted to .sup.11B-NMR analysis, which showed the presence of
borohydride (BH.sub.4.sup.-) anions in solution. Another sample of
the same aqueous solution was titrated, determining the yield of
boron-hydride anions to be 4%, with respect to the number of mAh of
current passed through the cell.
EXAMPLE 5
[0057] Using the process described in Example 4, about 39.2 g of
LiBr, 18.1 g of KBr, and 42.8 g of CsBr, and 1 g of B.sub.2O.sub.3
were added to a 3-neck flask. The solids were heated to about
300.degree. C., a temperature at which this mixture is molten. A
nickel metal sparging tube was inserted into the solution of molten
alkali bromides, and H.sub.2 gas passed through the sparger and
bubbled through the solution. This tube is the cell cathode.
H.sub.2 gas was allowed to escape from the cell through one of the
necks of the flask. A glass tube terminating in a porous glass
sparger was also inserted into the solution. Platinum wire and
platinum gauze were inside the tube, and the platinum comprised the
anode. The porous glass of the sparging tube acted as a separator
between the anode compartment (inside the glass tube) and the
cathode compartment (outside the tube). The application of about 5
V of potential led to the passage of 1000 mAh of charge over about
6 hours.
[0058] After the 6 hour period, the potential applied across the
anode and cathode was removed. The cathode sparger and the anode
tube were withdrawn from the molten solution and the solution was
allowed to cool to room temperature and solidify. The resulting
solid was dissolved in about 100 mL of 0.5 M aqueous NaOH solution.
A small aliquot of solution was submitted to .sup.11B-NMR analysis,
which showed the presence of both BH.sub.4.sup.- (borohydride) and
B.sub.3H.sub.8-- (triborohydride) anions in solution. Another
sample of the same aqueous solution was titrated, determining the
yield of boron hydride anions to be 8.3%, with respect to the
number of mAh of current passed through the cell.
EXAMPLE 6
[0059] Tetra-n-butylammonium bromide was heated to about about
120.degree. C., and about 1.5 mL of tri-n-butyl borate
(B(O-Bu).sub.3) followed by about 0.5 grams of sodium hydride was
added to the hot ionic liquid. The starting materials are only
sparingly soluble in the melt and fast stirring was necessary to
ensure adequate dispersion. After addition of the sodium hydride
was complete, the melt was cooled to room temperature and dissolved
in the minimum amount of aqueous 0.5 M NaOH. The presence of
borohydride in the aqueous solution was verified by NMR
spectroscopy.
[0060] The above description and drawings illustrate preferred
embodiments that achieve the objects, features and advantages of
the present invention. It is not intended that the present
invention be limited to the illustrated embodiments. Any
modification of the present invention that comes within the spirit
and scope of the following claims should be considered part of the
present invention.
* * * * *